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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559

ISIJ International Vol. 61 (2021), No. 1

  • Formation, Evolution and Removal of MgO·Al2O3 Spinel Inclusions in Steel

    pp. 1-15

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    DOI:10.2355/isijinternational.ISIJINT-2020-352

    MgO·Al2O3 spinel inclusions are generally found in steel, and required be controlled during steelmaking process due to the influences on continuous casting and the quality of final products. The present review aims to give an overall picture on the formation, evolution and removal of spinel inclusions. In this review, the reported formation mechanisms are reclassified, and the effects of Ca, Ti and Mn as well as rare earth on the evolution of spinel inclusions are summarized. Different methods with thermodynamic data sheets are also introduced for the calculation of phase stability diagram. To explain the evolution route of inclusions in industry, the sources and the formation kinetics of dissolved Mg and Ca are discussed. In addition, the agglomeration, separation and dissolution behaviors of spinel inclusions are considered, and the advantages of spinel inclusions are also addressed. Based on literature survey, some suggestions on the control of inclusions are given as well.
  • Low Density Fe–Mn–Al–C Steels: Phase Structures, Mechanisms and Properties

    pp. 16-25

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    DOI:10.2355/isijinternational.ISIJINT-2020-467

    This review introduces the structural phases, microstructural characteristics, and the most relevant room and cryogenic properties of low density Fe–Mn–Al–C steels. The combination of outstanding physical and mechanical properties while offering a weight reduction of up to 18% make low density Fe–Mn–Al–C steels attractive structural materials as lightweight crash-resistant car body structures and structural components in the cryogenic industry. In this review, the latest alloy design strategies are introduced. In particular, the novel aspects of the phase structures and their deformation behavior, in particular, those related to L’12 (Fe, Mn)3AlC carbides (κ-carbides) and B2-type Ni/Cu-rich precipitates, are critically summarized. Future scientific and technical challenges are provided to establish these steels as structural materials for industrial applications.
  • Determination of Thermal Diffusivity and Its Temperature Dependence of Fe1−xO Scale at High Temperature by Electrical-Optical Hybrid Pulse-Heating Method

    pp. 26-32

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    DOI:10.2355/isijinternational.ISIJINT-2019-635

    Thermal diffusivity of Fe1−xO scale formed on iron sheets have been measured using an electrical-optical hybrid pulse-heating method, which can avoid decomposition of Fe1−xO scale even at elevated temperatures by executing the experiment rapidly. The samples were 50 µm-thick Fe1−xO scale, which had been obtained by oxidation of a 0.5 mm-thick iron coupon at 1123 K in the air followed by sandblasting to remove the outer oxide layers of Fe3O4 and Fe2O3. In the experiment, the sample was heated by a large current pulse supplied to the iron layer of the coupon, and the Fe1−xO scale was indirectly heated up to experimental temperature from room temperature within 0.2 s. The temperature was maintained at the experimental temperature, and the laser flash method was conducted to measure the effective thermal diffusivity of the coupon. The laser irradiation position was adjusted by two ceramics blocks to make the temperature profile better. The effective thermal diffusivity produced the value for Fe1−xO scale based on a three-layered analysis for the Fe1−x O/iron/Fe1−xO structure. Thermal diffusivities of Fe1−xO scale were around 4.8 × 10−7 m2s−1, and there can be seen no obvious temperature dependence from 600 K to 900 K. X-ray diffraction analysis confirmed that phase transformation did not occur in the Fe1−xO scales during the experiment and x value was calculalted to be 0.09. Non-stoichiometry is supposed to have a significant effect on thermal diffusivity of Fe1−xO scale and its temperature dependence in this research.
  • Preparation of HCl Gas Scavenger from Blast Furnace Slag through Alkali Fusion

    pp. 33-41

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    DOI:10.2355/isijinternational.ISIJINT-2020-193

    Blast furnace (BF) slag, one of the byproducts of iron- and steel-making plants, was converted to a product, including a hydrogrossular, through the alkali fusion method for HCl gas removal. BF slag was transformed to the alkali-fused slag with reactive phases via alkali fusion, and then, the fused slag was added to distilled water and stirred at room temperature to prepare the precursor for the synthesis of the product including a hydrogrossular by heating. The effects of the mixing ratio of NaOH to slag (NaOH/slag ratio), fusion temperature, ratio of the fused slag mass to distilled water volume (W/V ratio), stirring time, heating time, and heating temperature of the product phase were investigated, and the HCl gas removal ability of the obtained product was determined. The optimal conditions for hydrogrossular synthesis are NaOH/slag ratio of 1.6, fusion temperature of 600°C, W/V ratio of 125 g/L, stirring time of 24 h, heating temperature of 80°C, and heating time of 3–6 h. The product removed more HCl gas than the BF slag and showed higher Cl fixation than lime. These results suggest that a novel scavenger for HCl gas removal at high temperature can be synthesized from the BF slag through alkali fusion.
  • Effective Removal Zone of Inclusions in a Horizontal Channel under A.C. Magnetic Field Imposition

    pp. 42-48

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    DOI:10.2355/isijinternational.ISIJINT-2020-306

    A channel type horizontal induction heating tundish compensates for the heat loss of the molten steel due to Joule loss generated by an A.C. magnetic field. It also exhibits another function of inclusions removal because the A.C. magnetic field generates an electromagnetic pinch force. For the inclusions below the center of the horizontal channel, the direction of the electromagnetic pinch force and the buoyancy force acting on them are opposite. Thus, there is a possibility of the existence of the balanced position where the magnitudes of the electromagnetic pinch force and the buoyancy force are same. Around there the net time average force acting on the inclusions is almost zero, and there is a dead zone where the removal time of the inclusions under the imposed A.C. magnetic field is longer than that without it. In this study, non-dimensional models of the force balance and the inclusion trajectory were established and numerically solved to find out the relationship between the dead zone and the A.C. magnetic field parameters because the dead zone range should be reduced for effective removal of the inclusions. Consequently, the dead zone range decreased with the increase in the magnetic field intensity. Furthermore, the shielding parameter of 5–10 is one of the optimum conditions to reduce the dead zone range under the constant magnetic field condition because the dead zone range has the local and/or global minimum at this parameter.
  • Migration Behavior of Components in Converter Slag during Smelting Reduction Process Using Aluminum Dross

    pp. 49-54

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    DOI:10.2355/isijinternational.ISIJINT-2020-436

    In order to efficiently recycle valuable element from the molten converter slag and enhance utilization ratio and added value of the slag, a novel aluminothermic smelting reduction process using aluminum dross as reductant was investigated from component migration and reduction kinetics, meanwhile the reduction mechanism of smelting reduction process of molten converter slag using aluminum dross was discussed. The results showed that the reduction of FeO firstly occurred with the Al/(FeO+MnO+P2O5) mass ratio≤0.27, and MnO began to be reduced with the ratio increasing to 0.33. Further increasing the ratio to 0.40, P2O5 could be reduced from the molten slag. Moreover, the contents of FeO, MnO and P2O5 in molten slag decreased sharply within the first 4 min, 6 min, and 6 min respectively and stabilized thereafter, and the Al2O3 content was increased dramatically over the first 6 min and followed by a continuity increase. Recovery of metal was increased to a maximum of 99.32% with the mass ratio increasing, and the crude alloy content containing Fe, Mn, and P was up to 93.31%, 1.98%, and 4.72%, respectively.
  • Optimization Analysis of Mechanical Performance of Copper Stave with Special-shaped Tubes in the Blast Furnace Bosh

    pp. 55-61

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    DOI:10.2355/isijinternational.ISIJINT-2019-799

    Based on the theory of heat transfer, parametric modeling is established for the heat transfer model of copper cooling stave, which appears in recent years, with special-shaped tubes (elliptical, rectangular, double circular, three circular and ortho hexagonal) in a blast furnace (BF) bosh and the optimal tube for the cooling pipe is selected on the basis of the heat transfer characteristics of the stave. The heat transfer model of the hot end of stave embedded bricks which are not covered by slag, is analyzed using thermal-structural coupling method at the initial stage of blow-in under the normal working condition. The mutual influence of various parameters on the mechanical properties of copper stave is obtained using the response surface method. This method is combined with NSGA-II genetic algorithm to optimize the structure parameters and longevity technology of the bosh. The optimized structure of the furnace bosh is improved in heat transfer characteristics and mechanical properties, which proves the model and parameterized calculation program can be used as an optimized design and evaluation of the longevity technology of the bosh structure.
  • Numerical Study of Combustion and Air Supply Characteristics and Structural Optimization of Top Combustion Hot Blast Stoves

    pp. 62-70

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    DOI:10.2355/isijinternational.ISIJINT-2020-119

    The high-temperature hot air of hot blast stoves has an important effect on blast furnace ironmaking; it is one of the crucial factors that are used to assess the performance of hot blast stoves. In this study, a three-dimensional fluid flow heat transfer model combining turbulence, heat transfer, combustion, heat radiation, and heat exchange models was developed to assess the combustion and air supply characteristics of a new type of top combustion hot blast stove. The results indicate that nozzles that are alternately arranged in the same layer of the new hot blast stove caused rapid combustion reactions. In addition, it caused the high-temperature flue gas in the pre-combustion chamber to accelerate toward the combustion chamber, thereby eliminating the “eccentric swirl” of the traditional hot blast stove and improving the heat transfer efficiency and heat storage capacity. However, the “attachment effect” of the fluid still occurred in the new stove type, which led to an unreasonable temperature distribution inside the combustion chamber and regenerator. Therefore, an improved design of a top combustion hot blast stove was proposed in this paper. Using the developed numerical model, the performance of the new design was evaluated and compared with the original one.
  • Melting Erosion Failure Mechanism of Tuyere in Blast Furnace

    pp. 71-78

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    DOI:10.2355/isijinternational.ISIJINT-2020-138

    In this paper, the common damage types of tuyere were sampled and analyzed. Specifically, the element content in tuyere was measured by Inductively Coupled Plasma Source Mass Spectrometer, Nitrogen-Hydrogen-Oxygen Analyzer, and Carbon-Sulfur analyzer. Then, Scanning Electron Microscope was used to analyze the microstructure of tuyere damage, and the element distribution of the damaged area was observed by Energy Dispersive Spectrometer. Finally, a metallographical analysis of the damaged location was carried out by an optical microscope. On account of those above analyses, the following results were obtained: firstly, the tuyere damage was mainly caused by erosion. After that, the grains at the hot surface and melting area of the tuyere were large, while those in the middle region were small. The content of the impurity element in tuyere nose increased, and the content of copper decreased. Moreover, there were two interfaces of slag-copper and iron-copper in the damaged area, and the Cu–Fe alloy was formed. At last, the failure mechanism of blast furnace tuyere erosion was explained in the paper.
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    Readers Who Read This Article Also Read

    1. Kinetic Behaviors of Coke Gasification with CO2 and H2O ISIJ International Vol.61(2021), No.1
    2. Gasification Behaviors of Coke in a Blast Furnace with and without H2 ISIJ International Vol.61(2021), No.1
    3. Evaluation and Prediction of Blast Furnace Status Based on Big Data Platform of Ironmaking and Data Mining ISIJ International Vol.61(2021), No.1
  • Effect of Oxygen Enrichment on Top Layer Sinter Properties

    pp. 79-85

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    DOI:10.2355/isijinternational.ISIJINT-2020-125

    Properties like shatter index and yield of the top layer of iron ore sinter in a Dwight-Lloyd sintering machine have always been a bottleneck for improvement of overall sinter quality due to the practical limitation of achieving high sintering temperature and dwell time in the top layer. To improve the sinter yield and shatter index of top layer, various technologies like usage of additional coke breeze, gaseous fuel injection and oxygen enrichment have been reported. In this paper, effect of oxygen enrichment of the incoming air on top part of the sinter bed was investigated using pot sinter experiments with varying oxygen percentage of 0 to 12 vol.% in the incoming air for an initial duration of 1/3rd of the total sintering time. It was observed that with increasing oxygen percentage from 0 to 9 vol.% in the incoming air, top layer sinter yield increased from 76.1 to 80.6% and shatter index from 69.0 to 74.8%. The improvement in sintering properties were mainly attributed to the promotion of sintering reaction with improved time-temperature profiles specifically at the top layer which results in increase of phase fraction of Silico ferrites of calcium and aluminum (SFCA-I). However, beyond 9 vol.% oxygen enrichment, change in sinter properties was not significant. It is considered that excessive increase of oxygen enrichment does not contribute much to the increase of top layer peak temperature.
  • Effects of Liquidus Temperature and Liquid Amount on the Fluidity of Bonding Phase and Strength of Sinter

    pp. 86-92

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    DOI:10.2355/isijinternational.ISIJINT-2020-243

    Fluidity is one of the important properties of bonding phase in sintering process, because better fluidity is beneficial for improving the strength of sinter. In this work, the effects of liquidus temperature and liquid amount on the fluidity of bonding phase and the strength of sinter were investigated. The experimental results in SiO2–Fe2O3–CaO system indicated that, for both SiO2=5% and SiO2=10%, with increasing Fe2O3 content (decreasing CaO content), the fluidity indices of samples first increased and then decreased. When the liquidus temperature was lower and the liquid amount was more, the fluidity index of SFC sample was higher, and vice versa. The sinter pot experimental results showed that, (1) for the iron ore with SiO2=4.30%, the major phases in the sinter were hematite and SFCA, and the liquid SFCA phase was evenly distributed in sinter. The tumble strength of sinter was higher than 60% in a wider basicity range of 1.8–2.2. (2) For the iron ore with SiO2=12.42%, the olivine was another major phase, and was unevenly distributed in part of sinter. There was a peak value for tumble strength of sinter when the basicity was 2.0. The basicity was higher or lower, the tumble strength sharply decreased. The reasonable basicity of sinter with high-SiO2 content was difficult to determine, and was not proposed to be used in an actual sintering production. The outcomes of the present work may provide guidelines for better understanding the properties of bonding phase and improving the strength of sinter.
  • Extraction of Iron from Refractory Titanomagnetite by Reduction Roasting and Magnetic Separation

    pp. 93-99

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    DOI:10.2355/isijinternational.ISIJINT-2020-251

    The abundant refractory titanomagnetite (TTM) provides a cheap alternative source of iron, but this ore contains impurities and is difficult to process to make suitable concentrates for the blast furnace. In this study, reduction roasting of a primary TTM concentrate followed by magnetic separation was investigated to understand the effects of reduction time, coal dosage, and CaF2 addition on the reduction behavior of TTM and growth mechanism of iron particles. The phase composition of reduced samples was characterized by X-ray diffraction. The size distribution of iron particles was quantitatively examined using image analysis. Results showed that CaF2 can help improve the reduction degree and particle size of metallic iron. The metallization degree increased from 85.5% to 89.5% when the CaF2 dosage increased from 0% to 4%, while a minor increase was observed when the CaF2 dosage exceeded 4%. Accordingly, the TTM samples were treated by reduction roasting with 4% CaF2 and 25% coal at 1200°C for 60 min followed by magnetic separation. A magnetic concentrate with an iron content of 91.1% and a recovery of 92.9% was achieved. In addition, the relationship between the size distributions of iron particles and grinding fineness was also studied. The size distribution using data from the diameter of iron particles was found to be close to the actual grinding fineness.
  • Influence of B2O3 on the Oxidation and Induration Process of Hongge Vanadium Titanomagnetite Pellets

    pp. 100-107

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    DOI:10.2355/isijinternational.ISIJINT-2020-272

    To develop a novel and clean smelting process for the comprehensive utilization of Hongge vanadium titanomagnetite (HVTM), this work determined the influence of B2O3 on the oxidation and induration process of HVTM pellets (HVTMP). The oxidation degree, compressive strength, porosity, crystalline phase, microstructure, and induration degree of HVTMP were comprehensively investigated, and the relevant mechanisms were discussed. The results indicated that B2O3 decreased the oxidation degree and porosity of HVTMP, while the compressive strength was clearly enhanced. Increasing the amount of B2O3 did not significantly affect the crystalline phase, but it decreased the XRD peak intensity. The induration degree appreciably increased after the addition of B2O3 because it favored the generation of liquid phases and increased the average grain size by creating a dense and continuous bonding structure. Additionally, B-rich phases were mainly uniformly distributed in the liquid phases along the grain boundaries. Based on the experimental results, the induration mechanism of HVTMP with different amounts of B2O3 was proposed. This study provides a theoretical and technical foundation for the effective production of HVTMP.
  • Evaluation and Prediction of Blast Furnace Status Based on Big Data Platform of Ironmaking and Data Mining

    pp. 108-118

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    DOI:10.2355/isijinternational.ISIJINT-2020-249

    The applications of big data in the steel industry are widely developed. Ironmaking is a multi-sectoral joint-operation production process that generates massive data constantly. It is required to build the big data platform to efficiently organize and fully utilize the production data of the ironmaking. In this work, we build a comprehensive status evaluation and prediction system for the blast furnace (BF) to achieve the goal of high production, low consumption, high quality and long life of the BF. The evaluation system is based on the big data platform and equipped with the factor analysis method, which can define and extract the hidden common factors in the production index of the BF by considering 19 state parameters and can calculate the comprehensive BF status index as well. The prediction system employs the AdaBoost model which can accurately predict the BF status index 3 hours in advance. Evaluation results show that the proposed BF status index is highly consistent with the actual status of the BF in the selected time period. The coincidence degree between BF status index in different time periods and the actual situation is also verified by factor analysis. Although the evaluation and prediction system demonstrates high accuracy in current production environment, it may still need calibrate and update regularly due to the changing of the BF production in the long run. The online comprehensive evaluation and prediction system for BF can effectively assist operators to optimize the BF operation and maintain the stabilization of BF.
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    Readers Who Read This Article Also Read

    1. Gasification Behaviors of Coke in a Blast Furnace with and without H2 ISIJ International Vol.61(2021), No.1
    2. Kinetic Behaviors of Coke Gasification with CO2 and H2O ISIJ International Vol.61(2021), No.1
    3. Melting Erosion Failure Mechanism of Tuyere in Blast Furnace ISIJ International Vol.61(2021), No.1
  • Effect of Changes in Mechanical Properties of Coke Matrix Caused by CO2 or H2O Gasification Reaction on the Strength of Lump Coke

    pp. 119-128

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    DOI:10.2355/isijinternational.ISIJINT-2020-292

    In this study, we evaluated the effect of the CO2 or H2O gasification reaction on the mechanical property of the coke matrix by measuring the elastic modulus of the coke matrix before and after the gasification reaction. We also investigated the effect of the distribution of the elastic modulus in the coke matrix on the strength of the lump coke by conducting the fracture analysis for the coke model with porosity of 0–0.4 in which the distribution of the elastic modulus obtained by the experiment was reflected. The nanoindentation measurements of the elastic modulus of the coke matrix before and after the gasification reaction implied that the distribution of the elastic modulus in the coke matrix differs depending on the gasification agent. In the case of the CO2 gasification reaction, both the coke matrices with high and low elastic moduli were consumed by the gasification. On the other hand, in the case of the H2O gasification reaction, only the coke with an elastic modulus of over 30 GPa before the reaction was consumed by the gasification reaction. Also, the numerical results showed that the distribution of the elastic modulus in the coke matrix affects the strength of the coke model with low porosity whereas the one did not affect that with high porosity.
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    Readers Who Read This Article Also Read

    1. Flow Stress of Duplex Stainless Steel by Inverse Analysis with Dynamic Recovery and Recrystallization Model ISIJ International Vol.61(2021), No.1
    2. Gasification Behaviors of Coke in a Blast Furnace with and without H2 ISIJ International Vol.61(2021), No.1
    3. Kinetic Behaviors of Coke Gasification with CO2 and H2O ISIJ International Vol.61(2021), No.1
  • Theoretical Yield of Metallic Fe by COG in a Shaft Furnace in the Case of O2 Pyrolysis

    pp. 129-137

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    DOI:10.2355/isijinternational.ISIJINT-2020-303

    Reduction shaft furnace is an effective process to produce Direct Reduced Iron (DRI), in which natural gas is used as reducing agent and heat source. Recently, in some countries lacking natural gas resources, Coke Oven Gas (COG) was proposed to be used in shaft furnace process instead of natural gas. In the present work, the effect of COG consumption on the yield of metallic Fe in shaft furnace was thermodynamically calculated. Both the chemical equilibrium and heat balance were considered. The main findings include, in shaft furnace process, the COG consumption as heat source is more than that as reducing agent. In the case of directly supplying reformed COG, the yield of metallic Fe decreases with increasing formation temperature of metallic Fe. For a coke oven with capacity of 600000 tons, as the formation temperature are 850°C and 900°C, the corresponding annual yields of shaft furnace are 253.58 × 103 tons/year and 249.48 × 103 tons/year. In the case of ZR technology followed by supplying extra COG, the yield of metallic Fe first increases and then decreases, and there is a peak value. For a coke oven with capacity of 600000 tons, as the formation temperature are 845°C and 900°C, the corresponding annual yields of shaft furnace are 283.44 × 103 tons/year (maximum value) and 278.38 × 103 tons/year. The findings from this work may provide guidelines for choosing optimal parameters for an actual shaft furnace process.
  • Characterization of Ti(C,N) Superstructure Derived from Hot Metal

    pp. 138-145

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    DOI:10.2355/isijinternational.ISIJINT-2020-319

    Ti(C,N) in BF hearth plays an important role in protecting the BF lining and prolonging the life of BF. The characterization and the crystallization process of the Ti(C,N) obtained from one dead BF were clarified in the paper. It is found that the precipitated Ti(C,N) exhibits annual-ring shape features and different colors. The annual-ring shape topography is caused by the change of the C/N ratio. The precipitated Ti(C,N) is composed of one or more large grains. Ti(C,N) presents as a superstructure or mesocrystal and evolves from Ti(C,N) nanoparticles, which are self-assembled nanomaterials with highly ordered structures. Ti(C,N) deposit forms continuously with layer-by-layer grain growth because the mesocrystal has a large specific surface area. The phase interface of Ti(C,N)-Fe presents as a shape of jagged step, and the phase interface is inlaid by many random mesocrystal structures. The Ti(C,N) deposit derived from hot metal is accompanied by layer-by-layer mesocrystal structures. The results of phase transition and morphology of Ti(C,N) provide guidance for the regulation of Ti(C,N) behavior in BF hearth.
  • Interface Behavior and Interaction Mechanism between Vanadium-Titanium Magnetite Carbon Composite Briquette and Sinter in Softening-Melting-Dripping Process

    pp. 146-157

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    DOI:10.2355/isijinternational.ISIJINT-2020-309

    As an innovative and promising BF iron-bearing burden, the vanadium-titanium magnetite carbon composite briquette (VTM-CCB) charging significantly affects the softening-melting-dripping characteristics and cohesive zone of the mixed burden. In this study, the interface behaviors between VTM-CCB and sinter were investigated by conducting interrupted softening-melting experiments to elucidate intrinsic structure evolution and interaction mechanisms. During softening, when the FC/O ratio of VTM-CCB ranges from 0.8 to 1.0, the molten slag-metal coexisting structure formed at the interface, thereby promoting the shrinking and decreasing T4 and T40. However, with increasing FC/O ratio higher than 1.0, the interface slagging and bonding would be suppressed due to the unconsumed carbon particles. During melting, the increasing of FC/O ratio would lower the FeO content and decrease the molten slag, and the interface layer transformed from molten slag-iron coexisting structure to dense metallic iron shell, suppressing the collapse of molten mixtures and increasing Ts. In the dripping process, increasing the FC/O ratio appropriately could promote the interface iron carburization and the aggregation of molten iron, thereby decreasing TD and improving the dripping performance. Besides, the VTM-CCB, acting as skeleton in the molten mixtures, could provide more gas channels to improve the permeability of packed bed. However, as the FC/O ratio exceeds 1.2, the Ti(C,N) would precipitate at the slag-metal interface and deteriorate the fluidity of molten mixtures, thereby deteriorating the gas permeability and increasing TD notably. Fully considering the softening-melting-dripping characteristics and the gas permeability, the appropriate FC/O ratio of VTM-CCB should not be higher than 1.2.
  • Gasification Behaviors of Coke in a Blast Furnace with and without H2

    pp. 158-166

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    DOI:10.2355/isijinternational.ISIJINT-2020-372

    The gasification behaviors of coke in a blast furnace with and without H2 were studied by thermodynamic calculations and high-temperature simulation experiments, and the change in the coke porosity was also studied. The results show that with the decrease in φ(CO)/φ(H2), the temperature range of C gasification decreases and moves to the low-temperature zone. In the absence of H2, the increase in φ(CO) increases the Ri, RC. In the presence of H2, φ(CO) and Ri increase, whereas the RC, decreases. With the increase in φ(CO) and φ(H2), the reduction of iron oxide tends to be carried out in the low-temperature zone, and the φ(CO2) and φ(H2O) produced in the high-temperature zone decrease, which is conducive to reducing the consumption of coke. The presence of H2 intensifies the gasification of coke. The presence of H2 aggravates the increase of coke porosity in the low temperature region, but it reduces the internal porosity in the high temperature region.
  • Kinetic Behaviors of Coke Gasification with CO2 and H2O

    pp. 167-173

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    DOI:10.2355/isijinternational.ISIJINT-2020-401

    In the temperature range of 1173–1573 K, the constant temperature weight-loss experiment of coke reaction with CO2 and H2O was carried out by thermogravimetry. The gasification kinetic behaviors of coke reaction with CO2 and H2O were compared and analyzed, and the mechanism of the difference of the kinetic behaviors was discussed. The results show that the internal diffusion condition and interfacial reaction condition of coke gasification in H2O are better than those in CO2, and the difference of diffusion property is greater than that of interface reaction property. The activation energies of internal diffusion and interface reaction of coke gasification in H2O are 80.36 kJ/mol and 36.97 kJ/mol lower than that in CO2, respectively. In H2O, the controlling region of interfacial reaction is larger than that in CO2, and the effect of temperature on controlling region of the interfacial reaction is also greater than that in CO2. The energy required for the chemical adsorption of H2O on the coke surface to form a stable intermediate configuration is 40.22 kJ/mol less than that of CO2, and the energy barrier that needs to cross during the coke gasification in H2O is 29.11 kJ/mol less than that in CO2. The chemisorption capacity of CO2 on coke surface is weaker than that of H2O.
  • Numerical Analysis of Blast Furnace with Injection of COREX Export Gas After Removal of CO2

    pp. 174-181

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    DOI:10.2355/isijinternational.ISIJINT-2020-384

    Blast furnace (BF) injection of COREX export gas after removal of CO2 (CEG) displays many ecological and environmental advantages. A static model of BF operation of CEG was developed according to mass and heat balance. The effect of CEG injection on the raceway adiabatic flame temperature (RAFT), the amount and composition of bosh gas, and the shape of raceway were studied. The acceptable injection volume of CEG under different thermal compensation measures was investigated. The results show that under no thermal compensation, with the increase of CEG injection, the RAFT decreases but the volume of bosh gas increases. The content of CO and H2 increases with the increase of CEG injection. Based on the standard of maintaining the RAFT and volume of bosh gas, addition of oxygen, reducing blast humidity and increasing blast temperature are effective measures of thermal compensation to increase the quantity of CEG injection. The characteristics of high temperature zone of BF under different suitable CEG injection volumes were also studied. The findings of this work can be used as a theoretical basis to guide plant operations for CEG injection in BF.
  • Steam Reforming of Methane on Sponge Iron: Influence of Gas Composition on Reaction Rate

    pp. 182-189

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    DOI:10.2355/isijinternational.ISIJINT-2019-477

    Direct Reduction processes use gases (CO and H2) for iron reduction and production of sponge iron or direct reduced iron (DRI). The generation of this gas occurs through methane reforming, which can be done in a reformer or inside the reduction shaft with the sponge iron as a catalyst. The latter occurs in the auto-reforming processes. The kinetics of steam reforming of methane catalyzed by sponge iron was studied at temperatures between 875°C and 1050°C. Results showed that sponge iron acts as a catalyst and methane conversion is increased in higher temperatures and with higher H2/H2O ratio in the inlet gas. The inlet gas composition like one of the industrial auto-reforming processes led to intense carburization and hindered the catalytic reforming reaction.
  • Melting Characteristics of Multipiece Steel Scrap in Liquid Steel

    pp. 190-199

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    DOI:10.2355/isijinternational.ISIJINT-2020-269

    Based on the analysis of single-bar melting experiments in a previous article,1) the effect of spacing between the steel bars on the agglomeration of steel shells around the steel bars and the melting rate of steel scrap samples was studied in this article. In addition, a calculation model of melting time of steel scrap in the electric arc furnace under different bulk densities and random stacking conditions was also established. The two-bar melting experimental results show that the thermal simulation results are basically consistent with the numerical simulation results. And an increase in the spacing between the steel bars up to 6 mm and the preheating temperature of the steel scrap samples up to 1073 K, can greatly reduce or eliminate the adverse effect of the agglomeration of steel shells around the steel bars on the melting process, thus greatly reducing the melting time. The multibar melting experimental results show that an increase in the porosity is beneficial to the melting of steel scrap samples, and when the porosity reached 0.84 and above, the melting time of multibar samples is close to that of an individual steel bar. In addition, the calculation model can accurately predict the melting time of the steel scrap in the electric arc furnace.
  • Dissolution Rate and Interfacial Behaviors of Alumina Particle in Molten Slag Studied by Single Hot Thermocouple Technique

    pp. 200-208

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    DOI:10.2355/isijinternational.ISIJINT-2020-310

    In view of the deficiencies of traditional methods to study the dissolution kinetics of solid oxides in molten slag, a novel method based on single hot thermocouple technique (SHTT) is proposed in this paper. The feasibility of this method is verified and the controlling means of experiment reproducibility is explored. And the effect of slag basicity and Li2O content on the dissolution behavior of Al2O3 in mold flux is investigated via SHTT. The results show that: 1) Under the condition that the density of the Al2O3 particle is slightly higher than that of slag and the mass ratio of Al2O3 particle to liquid slag is less than 2%, the relative standard error of Al2O3 dissolution rate is within 10%. 2) The effect rule of slag basicity on the Al2O3 dissolution rate studied via SHTT is same as that of rotating cylinder method. The dissolution rate of Al2O3 increases with the increase of slag basicity. When the basicity increases from 1.0 to 1.2, the dissolution rate increases significantly, which is due to the formation of xCaO∙yAl2O3 or xCaO∙yAl2O3∙zSiO2 intermediate compounds with low melting point at the Al2O3 boundary layer. 3) With the increase of the Li2O content in mold flux, the dissolution rate of Al2O3 first increases and then decreases. The decrease in dissolution rate is caused by the formation of high-melting MgAl2O4 at the boundary of Al2O3 particle.
  • Interfacial Reactions and Inclusion Formations at an Early Stage of FeNb Alloy Additions to Molten Iron

    pp. 209-218

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    DOI:10.2355/isijinternational.ISIJINT-2020-353

    Nb is an important microalloying element in steelmaking. Its interaction with liquid Fe during an early stage of the alloying process has a considerable influence on the Nb recovery. In the present work, the inclusions in FeNb alloys were characterized using the electrolytic extraction method combined with SEM-EDS. The interfacial reactions between FeNb alloy and liquid Fe, as well as inclusion formations, were studied during an early stage of an alloy addition using a liquid-metal-suction method. The results revealed that a diffusion zone consisting of different regions of Fe–Nb phases was formed and that the thickness of the zone increased with time. Based on the experimental findings, the mechanism of the early dissolution process of FeNb alloys in liquid Fe was discussed. Moreover, the Nb rich regions formed after the alloy contacted with liquid Fe could modify the existing inclusions in the alloy, also their evolution mechanisms were studied. The addition of FeNb alloys can introduce inclusions, such as Al–O and Al–Ti–Nb–O inclusions to the liquid steel. Overall, this study has contributed to the understanding the behaviour of impurities from the FeNb source at the early dissolution process during the microalloying process of steels containing Nb.
  • Desulfurization Behavior of Low-sulfur Plastic Die Steel during ESR Process under Different Atmospheres

    pp. 219-228

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    DOI:10.2355/isijinternational.ISIJINT-2020-421

    Experimental investigation and kinetics model ware carried out to study the effect of the atmosphere on the desulfurization of low-sulfur plastic die steel during the electroslag remelting process. 55Cr17Mo1VN plastic die steel was applied as the electrode and remelted with two different kinds of atmospheres using a laboratory-scale ESR furnace. It was found that the sulfur content of 50 ppm in the electrode decreased to 8–12 ppm in the air atmosphere, while reduced to 9–14 ppm in a protective atmosphere. The desulfurization rates were 82% and 78%, respectively. Correspondingly, the sulfur content of 0.12% in initial slag increased to 0.125% and 0.15%. The coupled desulfurization kinetics model was established, the oxygen activity (aO) and sulfur distribution coefficient (Ls) are taken into consideration, and they change with the remelting time during the calculation. The results show that the calculated values are in good agreement with the experimental values. The desulfurization effect at the electrode tip is significantly better than the positions where the droplet passes through the slag layer and the slag pool/molten pool interface. The Ls and comprehensive mass transfer coefficient of sulfur (kS*) decrease with the remelting time, while the aO at each reaction position increases. Compared with the protective atmosphere, Ls and kS* have larger values during the air atmosphere ESR process, but the aO value is equal. Under the different atmospheres, the most types of inclusions in the steel are MnS, and the refining atmosphere has no significant effect on the types of inclusions.
  • Precipitation Behavior of Nitride Inclusions in K418 Alloy under the Continuous Unidirectional Solidification Process

    pp. 229-238

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    DOI:10.2355/isijinternational.ISIJINT-2020-345

    Adopting effective routs to control the precipitation and size of nitride inclusions in superalloys during the solidification is a very interesting subject for metallurgists. The precipitation behavior of nitride inclusions in K418 alloy under the continuous unidirectional solidification process was investigated by scanning electron microscopy, ASPEX Explorer, and LECO ONH-836. The results show that there were two types of nitride inclusions in the K418 alloy ingot: TiN and complex inclusion of Al2O3–TiN. There were gradient distributions of the number density, average and max sizes of nitride inclusions along the casting direction, as well as the contents of Ti and N. Based on the thermodynamic and kinetic calculations, the precipitation time of TiN inclusion changed from mushy to liquid zones under different initial contents of Ti and N. Al2O3 inclusion began to precipitate in liquid zone and acted as the nucleation site for TiN inclusion. The radius of TiN inclusion increased from 3.2 µm at 0.36 K/s to 8.6 µm at 0.08 K/s when the fraction of solid approached 1. The nitride inclusions can be refined and reduced in the K418 alloy ingot under the continuous unidirectional solidification process compared with those in revert K418 alloy. The methods to control the precipitation and size of nitride inclusions were reducing the contents of N and O and increasing the cooling rates.
  • Torque Model in Plate Rolling Process with Biting Impact Considered

    pp. 239-247

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    DOI:10.2355/isijinternational.ISIJINT-2020-230

    In this paper, the research work of the torque model in plate rolling process, with biting impact considered, is carried out based on mechanical dynamics and the rolling process technology. The five-degree-of-freedom mechanical dynamic model was established for the main drive system of the actual heavy plate mill, considering the clearance between parts. The biting peak torques under different rolling process conditions were calculated. The influence of the biting time and the steady-state torque were analyzed: the biting peak torque decreases with the biting time increasing, and increases with the steady-state torque increasing. The biting time calculation model was established based on the rolling process parameters. The steady-state torque model was improved by rebuilding level arm ratio model. The influence of deformation area arithmetic average aspect ratio and reduction rate was considered. The calculation model of biting peak torque is built with biting time and steady-state torque influence. The model accuracy is verified by comparing the calculated data with actual data. The average deviation of steady-state torque and peak biting impact torque is within ± 8%, and ± 10%. The accuracy of these models can be improved by offline intelligent method and online learning function, subsequently.
  • Automatic Ultrasonic Testing of Non-metallic Inclusions Detectable with Size of Several Tens of Micrometers Using a Double Probe Technique along the Longitudinal Axis of a Small-diameter Bar

    pp. 248-257

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    DOI:10.2355/isijinternational.ISIJINT-2020-248

    An improvement of automatic ultrasonic testing through a double probe technique along the longitudinal bar axis used in a round bar, with a diameter of several millimeters, is proposed. Non-metallic inclusions of several tens of micrometers in the cross-sectional length can be detected using this novel technique, whereas the detectability in a conventional normal beam technique is limited to 100–150 µm. The main advantages of this technique are an increased working sensitivity owing to a decreased surface echo width and the use of a shear wave with a shorter wavelength as compared with a conventional normal beam technique. As another advantage of this technique, malfunctions caused by air bubbles in the coupling medium can be eliminated. Further, the beam paths of the surface echo and the bottom echo are discussed herein using the propagation time difference between both echoes in Appendix.
  • A Novel Genetic Simulated Annealing Algorithm for No-wait Hybrid Flowshop Problem with Unrelated Parallel Machines

    pp. 258-268

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    DOI:10.2355/isijinternational.ISIJINT-2020-258

    This paper studies the problem of scheduling N jobs in a hybrid flowshop with unrelated parallel machines at each stage. Considering the practical application of the presented problem, no-wait constraints and the objective function of total flowtime are included in the scheduling problem. A mathematical model is constructed and a novel genetic simulated annealing algorithm so-called GSAA are developed to solve this problem. In the algorithm, firstly a modified NEH algorithm is proposed to obtain the initial population. A two-dimensional matrix encoding scheme for scheduling solutions is designed and an insertion-translation approach are employed for decoding in order to meet no-wait constraints. Afterwards, to avoid GA premature and enhance search ability, an adaptive adjustment strategy is imposed on the crossover and mutation operators. In addition, a SA procedure is implemented for some better individuals from the GA solutions to complete re-optimization, where five neighborhood search structures are constructed including job based exchange, gene based exchange, gene based insertion, job based mutation, and gene based mutation. Finally, various simulation experiments in two scales of small-medium and large are established. Computational results show that the presented algorithm performs much more effectively compared with several heuristic algorithms reported in the literature.
  • Experimental Analysis of Image Dehazing Algorithms for Pelletization Process Images

    pp. 269-279

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    DOI:10.2355/isijinternational.ISIJINT-2020-295

    The product quality of pelletization process in steel industry is usually monitored by machine vision system. However, the image quality deteriorates significantly by haze generated during pelletization. Current image dehazing algorithms mainly concentrate on natural haze in outdoor or synthetic hazy images. Whether these algorithms can be directly adopted in solving haze removal problem in industrial process images, needs to be studied. In the present work, experiments are performed to compare the performance of five state-of-the-art image dehazing algorithms, using the image dataset PELLET that consists of real hazy images captured from pelletization process in a local steel company. For a comprehensive comparative study of the image dehazing algorithms, both qualitative and quantitative evaluation criteria are adopted, including visual perceptual evaluation, no-reference image quality assessment, and task-driven comparison. Our experimental analysis demonstrates that Boundary Constrained Context Regularization (BCCR) and Non-local (NLD) image dehazing algorithms generally achieve better quality of restored image than the other three algorithms (Dark-Channel Prior, Optimized Contrast Enhancement, and AOD deep learning network) in dealing with pelletization process images with different haze levels. The computing time needed by BCCR algorithm is only half of that by NLD (2.5 vs. 5 seconds) in processing a hazy image of size 656 × 490. Nevertheless, the performance of these algorithms needs to be improved in the future to deal with pelletization process images with dense haze, as well as to meet with the real-time requirement of pelletization process monitoring.
  • Flow Stress of Duplex Stainless Steel by Inverse Analysis with Dynamic Recovery and Recrystallization Model

    pp. 280-291

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    DOI:10.2355/isijinternational.ISIJINT-2020-122

    To obtain the flow stress in duplex stainless steel, a duplex flow model is proposed that applies a rule of mixtures with the relationship between the volume fractions of austenite and ferrite. The model includes the saturated stress ratio λ and the volume fractions of austenite and ferrite at various temperatures. It considers the mechanical deformation and microstructural evolution with dynamic recrystallization (DRX) and dynamic recovery (DRV) of the two phases during hot working. To confirm the validity of the proposed model and new inverse analysis method, hot compression experiments were performed at deformation temperatures of 1050, 1150, and 1250°C and strain rates of 0.1, 1, and 10 s−1 with SUS329J4L, which is an austenite-ferrite duplex stainless steel. According to the flow curves, the softening rate from the peak stress was steeper with decreasing temperature from 1250 to 1050°C, corresponding to estimated austenite volume fractions from 33% (1250°C) to 61% (1050°C). Microstructural heterogeneity between DRX in the austenite and DRV in the ferrite was observed at deformation temperatures from 1050 to 1250°C, confirming that a clearly different restoration mechanism occurred in the two phases.
  • Effect of Carbon Content on Machinability of Steel in Gear Cutting

    pp. 292-301

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    DOI:10.2355/isijinternational.ISIJINT-2020-334

    Machinability of steels containing different carbon contents is evaluated in cutting with a fly tool of TiAlN coated high speed steel, as performed in gear cutting. In order to investigate the effect of carbon content on the cutting process, 0.2, 0.4 and 0.6 mass% C steels are prepared with controlling nearly the same hardness. The cutting tests are conducted to measure the cutting forces, observe the chip formations and analyze the damage on the rake and flank faces of the tools. The machinability of the tested steels is compared each other in terms of the cutting model in the cutting force simulation. The orthogonal cutting data are identified to minimize the discrepancies between the measured and the simulated forces. The shear stress on the shear plane becomes large at high carbon contents, and thus the cutting force increases with the carbon content. On the rake face of the tool, substrate softening and cracking in the coated thin layer occur in a certain cutting length. In cutting of the 0.6 mass% C steel, the cracks initiate rapidly in the coated thin layer on the rake face due to large cutting forces and cutting heat. Small flank wear is observed in the cutting of 0.2 and 0.4 mass% C steels, while in the 0.6 mass% C steel thermal wear with adhesion is promoted at high cutting temperatures.
  • Optimization of Heating Process for Bearing Rings in a Vacuum Furnace Based on Numerical Analysis

    pp. 302-308

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    DOI:10.2355/isijinternational.ISIJINT-2020-464

    With the aid of a FEM software COMSOL Multiphysics the transient radiation heat transfer model has been established to simulate the vacuum heating process of M50NiL bearing rings. The heating curves obtained from the numerical model compare well with the experimental data. Based on the model, effects of heating process parameters on the radiative heat transfer in a vacuum furnace are investigated. The simulation results indicate that reducing heating rate or increasing preheating temperature can improve the temperature uniformity, thereby reducing the soaking time of bearing rings. Accordingly, an optimized heating process with high preheating temperature and low heating rate is proposed. The new process cuts the soaking time by a quarter and reduces the proportion of grade 3 and coarser grains from 12% to 2%.
  • Effect of S and Si on the Formation of Intragranular Ferrite and Inclusions in Ultra-low Oxygen Weld Metal of Low Carbon Steel

    pp. 309-316

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    DOI:10.2355/isijinternational.ISIJINT-2020-112

    It is well-known that the toughness of carbon steel weld metal can be improved when the oxygen content is generally about 300 to 400 ppm in arc weld metal, because of high fraction of intragranular ferrite (α) nucleated and formed on oxides. It is generally difficult to nucleate intragranular α from oxides in electron beam weld metal which is extremely low oxygen content at about 10 ppm. In this study, the influence of S and Si contents on inclusions and intragranular α formation in the extremely low O content weld metal are investigated. Specifically, inclusions as nucleation sites of intragranular α are examined and their role in the nucleation mechanism is discussed. Adding S and Si is very effective in promoting the nucleation of the intragranular α in weld metals. The intragranular α is nucleated by complex inclusion of Si–Mn oxide and MnS. The rise of the Ae3 transformation temperature around the inclusions along with the formation of the manganese-depleted zone (MDZ) is considered as a principal intragranular α nucleation mechanism. The S addition has the effect of increasing the α nucleation ability of the inclusions by coarsening the inclusions and forming MDZs around the inclusions. The Si addition has the effect of increasing the SiO2 content of the inclusions and increasing the intragranular α nucleation probability of the inclusions.
  • Effect of Gas Pressure on the Formation Mechanism of Welds Based on Local Dry Underwater Welding

    pp. 317-325

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    DOI:10.2355/isijinternational.ISIJINT-2020-231

    During the local dry underwater welding process, the gas pressure in the micro drainage cover plays a key role in the formation of welds, but the relationship among the stability of the underwater arc, the formation of welds and the gas pressure has not been clarified. In this study, a micro drainage cover with dual-gas curtain based on the Lafar tube was used in the local dry underwater MIG welding experiments at a depth of 20 cm to explore the influence of gas pressure on the formation of underwater welds. The results showed that when the arc shielding gas was only 0.1 MPa in the micro drainage cover, the welding process was interrupted and the width of welds was not uniform; while the drainage gas was added, both the welding process and formation effect were improved. However, when both the shielding gas and the drainage gas pressure reached 0.4 MPa, the process and formation effect performed worse. The photos of arc combustion and metal transfer under different gas pressure were shot using high-speed photography. It turned out that the most ideal condition was both 0.2 MPa of the shielding gas pressure and drainage gas pressure at the water depth of 20 cm.
  • Valorization of Coal Gangue and Vanadium-titanium Slag into Glass-ceramic Coating for Oxidation Resistance of 60Si2Mn Spring Steel at High Temperature

    pp. 326-334

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    DOI:10.2355/isijinternational.ISIJINT-2020-008

    Glass coatings and ceramic coatings are usually used to protect the slabs from oxidation during the reheating process of 60Si2Mn spring steel before hot rolling. In view of the characteristic that the primary glass is able to transform from amorphous phase to ceramic phase during the reheating process, glass-ceramics are potential to become a new type of coating materials with both the advantages of the glass coatings and ceramic coatings. In this paper, glass-ceramic was firstly prepared with solid wastes including vanadium-titanium slag (VTS) and coal gangue (CG). With the increasing content of VTS, the main crystalline phase of the glass-ceramic transformed from cordierite to spinel and hercynite, and the crystallization activation energy (Ek) increased. New glass-ceramic coatings were prepared with the as-prepared primary glass powders with different proportion of VTS. When the proportion of VTS within the primary glass powders was 6% by weight, the glass-ceramic coatings possessed the best anti-oxidation effect of 74.4% at 1100°C for 2 h. The protective mechanism of glass-ceramic coatings was discussed with the results of SEM and EDS. Before crystallization, the amorphous phase within primary glass contributed to good sintering performance, and thus the coatings formed a dense film, slowing down the oxidation caused by the direct contact between oxygen and the slabs. With the increasing temperature, amorphous phase gradually transformed to ceramic phase, preventing the oxidation resulted from the diffusion of Fe ions.
  • Determination of Facet Plane and Cleavage Fracture Plane of the Top Dross Formed in a Molten Zinc Bath

    pp. 335-342

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    DOI:10.2355/isijinternational.ISIJINT-2020-361

    In a molten zinc bath of a continuous galvanizing line, top dross particles crystallize as Fe2Al5, an intermetallic compound containing Zn. These particles readily adhere to the steel sheets, causing surface defects. Therefore, controlling the top dross particles is a key issue. The present study focused on determining the facet plane of top dross particles via three-dimensional analysis of morphology by serial sectioning and electron back scattering diffraction (EBSD). Furthermore, the crystallographic plane of the cleavage fracture surface of the top dross was determined by EBSD, after a cleavage crack was introduced by Vickers hardness indentation. The facet planes of the top dross consist of two planes of (001), four planes of {110}, and eight planes of {111}. In addition, the top dross particles grow fastest in the [001] direction. Consequently, the top dross particle was concluded to possess a polyhedron structure with 14 facet planes. Finally, the cleavage fracture surface of the main crack in the top dross is the (100) plane.
  • Ferrite Transformation from Fe-0.3N Austenite

    pp. 343-349

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    DOI:10.2355/isijinternational.ISIJINT-2020-166

    Ferrite transformation behavior of Fe-0.3 mass%N binary alloy was studied in the temperature range between 500°C and 750°C. By isothermal holding in the α + γ two phase region, two morphologies of Allotoriomorphic ferrite (AF) and Widmanstetten ferrite (WF) formed from the prior γ grain boundary. On the other hand, in the case of isothermal holding at 500°C, nitride-free bainitic ferrite (BF) was formed at the beginning, and changed to bainite accompanied with γ’ precipitation in further holding. The retained γ was obtained by decreasing the transformation temperature in the two phase region, and maximum volume fraction of 9% was obtained at 600°C. AF had near K-S OR with one side of the adjacent prior γ and grew into the grain without K-S OR. On the other hand, WF and BF had near K-S relationship with the matrix. Both WF and BF had significantly higher energy dissipation than AF, and the energy dissipation of AF is due to interfacial friction. On the other hand, the strain energy associated with the transformation was dominant in WF.
  • Phase-field Simulation of Recrystallization in Cold Rolling and Subsequent Annealing of Pure Iron Exploiting EBSD Data of Cold-rolled Sheet

    pp. 350-360

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    DOI:10.2355/isijinternational.ISIJINT-2020-226

    A unified theory for continuous and discontinuous annealing phenomena based on the subgrain growth mechanism was proposed by Humphreys around twenty years ago. With the developments in the unified subgrain growth theory, a number of Monte Carlo, vertex, and phase-field (PF) simulations have been carried out to investigate the nucleation and growth mechanisms of recrystallization by considering the local alignment of the subgrain structure.In this study, the effects of the microstructural inhomogeneities created in the deformed state on recrystallization kinetics and texture development were investigated. Numerical simulations of static recrystallization were performed in three-dimensional polycrystalline structures by coupling the unified subgrain growth theory with PF methodology. To prepare the initial microstructures, two-dimensional electron back scattering diffraction (EBSD) measurements were carried out on 90% and 99.8% cold-rolled pure iron. Our previous experimental study has shown that there are large differences in the texture formation processes during the recrystallization of cold-rolled iron samples.In cold-rolled iron with 90% reduction, the simulated texture exhibited nucleation and growth of γ-fiber (ND//<111>) grains at the cost of α-fiber (RD//<011>) components, where ND and RD denote normal direction and rolling direction, respectively. In contrast, the simulation results for cold-rolled iron with 99.8% reduction reproduced the high stability of the rolling texture during recrystallization. As a result, we conclude that the simulation results agreed with the experimentally observed textures in both the samples.
  • Microstructure Evolution and Tempering Transformation Kinetics in a Secondary Hardened M50 Steel Subjected to Cold Ring Rolling

    pp. 361-371

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    DOI:10.2355/isijinternational.ISIJINT-2020-293

    The microstructure evolution and tempering transformation kinetics of the M50 steel subjected to cold ring rolling (CRR) have been investigated. The results indicate that the brass R{110}<110> texture is weakened with the enhancement of the <111>//ND texture during CRR. Due to the increased low angle boundaries by CRR, the Ac1 temperature decreases while the carbon content and volume fraction of RA increase. During tempering, the activation energy of carbon atoms segregation and transition carbide precipitation decrease, while the activation energy of retained austenite (RA) decomposition increases after CRR. The kinetic analysis shows that the CRR is beneficial to the carbon atoms segregation during the beginning of tempering. Then, the CRR leads to the delay of the onset of transition carbide precipitation, but decreases the whole reaction time, which has been verified by the transmission electron microscopy (TEM) and hardness results. The lagging of transition carbide precipitation in the early stage is caused by the increased segregation trapping of carbon atoms, while the higher nucleation rate is responsible for the enhanced precipitation of transition carbide during the later stage. For the cementite formation, there are no significant changes in the predictive kinetics after the applied CRR. However, the kinetic transformation of RA decomposition is inhibited by the CRR, which is attributed to the higher carbon content and smaller grain size of RA. Additionally, the alloy carbides precipitation is also enhanced by the CRR process during secondary hardening.
  • Effect of Carbon Concentration in Austenite on Cementite Morphology in Pearlite

    pp. 372-379

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    DOI:10.2355/isijinternational.ISIJINT-2020-325

    To understand the formation mechanism of degenerate pearlite, the effect of carbon concentration on the cementite morphology in pearlite was investigated in hypoeutectoid C–Mn steels with fully pearlite and ferrite–pearlite duplex microstructures. The carbon concentration in untransformed austenite was enriched by the precipitation of proeutectoid ferrite during isothermal holding after austenitization and could be controlled based on local equilibrium theory. Consequently, it was found that the morphology of cementite in the pearlite formed by the decomposition of the untransformed austenite continuously changed from lamellar to fine rod or spherical shapes by decreasing the carbon concentration. The critical carbon concentration for the cementite morphology transition was evaluated at approximately 0.42 mass% at 773 K. The ferrite growth rate increases with decreasing carbon concentration in austenite, which leads to non-cooperative growth between ferrite and cementite in the eutectoid reaction, resulting in the formation of degenerate pearlite. The critical carbon concentration and its temperature dependence for lamellar and degenerate pearlite transition can be estimated by a simple competition model of growth kinetics between ferrite and pearlite formations. In addition, it was found that the softening of degenerate pearlite during annealing after the decomposition was much faster than that of lamellar pearlite because the constrictions of cementite lamella do not exist for Ostwald ripening.
  • Quantification of the Severity of Ridging in Ferritic Stainless Steel Sheets Using a Profilometric Technique

    pp. 380-386

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    DOI:10.2355/isijinternational.ISIJINT-2020-137

    A new method to quantify the ridging phenomenon in ferritic stainless steels has been developed based on the evaluation of surface profiles after the tensile elongation of 100 mm wide sheet specimens. The ridging components of the surface profiles are extracted by a tailored spline filtering procedure. A ridging index is proposed to quantify the severity of the surface defect based on surface profile height and spacing parameters. The procedure is independent of the type of profilometer used as long as unfiltered raw profiles can be recorded. The reproducibility of the measurement method and its correlation with the visual assessment of strained specimens is discussed.
  • Effects of NiAl Precipitation on the Stability of Retained Austenite and Mechanical Properties of a Quenching-partitioning-tempering Treated Medium-manganese Steel

    pp. 387-395

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    DOI:10.2355/isijinternational.ISIJINT-2020-185

    The effect of NiAl-type nanoparticles on the austenite stability was investigated during quenching-partitioning-tempering (QPT) processes in a cold rolled medium-manganese steel. A good combination of ductility (total elongation: 37.9%) and strength (yield strength: 995 MPa/ultimate tensile strength: 1260 MPa) is obtained after the first step of partitioning treatment at 630°C/1 h (P630) due to appropriate austenite stability and multiple strengthening mechanisms. Moreover, the yield strength and ductility increase further after the second step of tempering treatment at 500°C/2 h by about 113 MPa and 8.5% respectively. It was found that high density of intragranular NiAl-type nanoparticles precipitated during tempering improve the ductility from two aspects. NiAl-type nanoparticles could provide a harder and work-hardenable martensite matrix, which is beneficial for the stability and sustainability of the retained austenite during tensile deformations.
  • 1011 Gigacycle Fatigue Properties of High-strength Steel

    pp. 396-400

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    DOI:10.2355/isijinternational.ISIJINT-2020-212

    Fatigue tests were conducted up to 1011 cycles on high-strength steel to clarify a fatigue limit. The fatigue limit of the high-strength steel was not confirmed by gigacycle fatigue tests up to 1010 cycles, while our previous study suggested that the fatigue limit was probably confirmed by those up to 1011 cycles. However, the 1011 cycles fatigue testing was challenging since it took 2 months even by using ultrasonic fatigue testing at 20 kHz. In this study, 3 specimens were tested beyond 1010 cycles. Although a test on a specimen was terminated at around 5 × 1010 cycles, 2 specimens reached 1011 cycles without failure. In other word, no specimen failed above 1010 cycles. These results demonstrated the fatigue limit on high-strength steel in a gigacycle region. The fractured specimens below 1010 cycles revealed internal fractures originating from oxide-type inclusions. When the specimens failed in long-life regions, clear ODAs (Optically Dark Areas) were observed on the fracture surfaces at around the internal fracture origin, while the ODAs were obscure in case of failure in short-life regions. The runout specimens up to 1011 cycles were forcibly fatigue-fractured at higher stress amplitudes in the short-life regions. As the result, the ODA was observed on the forcibly fatigue-fractured surface. This meant that small internal cracks existed in the runout specimens since the ODA was a trace of small internal crack growth. Namely, non-propagating cracks were the mechanism of the appearance of the fatigue limit.
  • Estimation of Lankford Coefficients of Austenitic and Ferritic Stainless Steels using Mean Grain Orientations from Micro-texture Measurements

    pp. 401-407

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    DOI:10.2355/isijinternational.ISIJINT-2020-256

    A new method to calculate the plastic anisotropy r-values of austenitic and ferritic stainless steels has been developed. The mean orientation of individual grains is obtained from SEM-EBSD data and r-values for individual grains are calculated by weighting all slip systems according to their Schmid factors. Calculated and measured r-values are in good agreement for austenitic stainless steels. However, in ferritic stainless steels, which are highly anisotropic, good agreement requires the introduction of a Schmid factor threshold below which slip systems are inactive. The present method can be used to estimate local differences in r-values in ferritic stainless steels showing the local variations in texture responsible for ridging.
  • Relationship between Creep Strength and Magnetic Properties of Cobalt-bearing High Chromium Ferritic Steel

    pp. 408-416

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    DOI:10.2355/isijinternational.ISIJINT-2020-315

    In this study, the relationship between changes in the magnetic properties and creep strength with the addition of 3 or 6 mass% Co was investigated for ferritic steel containing 15 mass% Cr. Co addition up to 6 mass% hardly contributed to solid solution strengthening or precipitation strengthening at room temperature. However, in the range of 650 to 750°C, the steel with the larger amount of Co exhibited higher creep strength, which is explained by a reduction in the diffusion rate associated with a change in magnetic properties by Co addition. An increase of the volume magnetization of the steel with increasing Co content in the range from room temperature to about 800°C was confirmed. Comparing the difference in volume magnetization and the ratio of the creep strain rate for steels with different amounts of Co, a clear correlation was found. That is, at the temperature at which the difference in volume magnetization reached a maximum, the peak of the creep strain rate ratio was also observed. This result is explained as follows. In a low temperature region where the magnetization is large or in a high temperature region above the Curie point of both steels, the steels exhibit no significant difference in the creep strength. However, at a temperature where one steel loses its ferromagnetism but the other steel retains it, a significant difference in the creep strength is observed.
  • Prediction Method of Void Distribution near Punched Surface of Medium-Carbon Steel Sheet using Scrap

    pp. 417-423

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    DOI:10.2355/isijinternational.ISIJINT-2020-329

    The objective of this study was to confirm the validity of the prediction method of void distribution near a punched surface of a blank (holder side) by observing the void distribution of scrap (hole side). It is important to know the exact void number density and void area fraction through an appropriate evaluation method such as that mentioned, just where a crack occurs owing to stretch-flange deformation in the same individual sample because the crack and the void behaviors fluctuate from sample to sample and with position, even under the same punching condition. This study investigated the correlation of void distribution near punched fracture surfaces of the blank and scrap in medium-carbon steel. The voids near the punched fracture surfaces of the blank and the scrap were measured using SEM images. The voids of the blank and the scrap were distributed point-symmetrically with respect to the center of the fracture surface. The equivalent plastic strain and the stress triaxiality that were analyzed with FE analysis were also distributed point-symmetrically. The void distribution of scrap was shifted to the sheared surface side, compared to that of the blank. To predict the void distribution of the blank using the scrap, the void distribution of the scrap with the burr side as a reference point was approximated by a cubic function. Furthermore, the void distribution of the scrap shifted to the burr side. The prediction method of void distribution near punched surface by scrap was validated by considering stress and strain during punching.
  • Contributions of Grain Size and Crystal Orientation to Fatigue Crack Deflection and Branching Behavior in Low Carbon Steel Plates

    pp. 424-433

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    DOI:10.2355/isijinternational.ISIJINT-2020-311

    In-situ observation of fatigue crack growth behavior under ΔK-constant cyclic loading in Paris regime for two Thermo-Mechanical Control Process (TMCP) steels with different grain size was carried out. It was found from the results that crack closure dominantly contributed to fatigue crack growth resistance in the fine grain material, while crack tip stress shielding induced by crack deflection and crack branching dominantly contributed to fatigue crack growth resistance in the coarse grain material. The electron back scatter diffraction (EBSD) analysis revealed that crack deflection and crack branching in the coarse grain material were induced in the grain with (101) crystal orientation intensively under co-existence of the grain with (001) crystal orientation and also along the grain boundary between the grains with (101) and (001) crystal orientations. Hence, higher content ratio of the grains with (001) crystal orientation could contribute to higher crack growth resistance.
  • Microstructure and Wear Properties of a Bainite/Martensite Multi-phase Wear Resistant Steel

    pp. 434-441

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    DOI:10.2355/isijinternational.ISIJINT-2020-327

    The hardness, tensile properties, low temperature impact toughness and wear resistant performance in a low-carbon bainite/martensite multi-phase wear resistant steel were investigated. The results show that the multi-phase wear resistant steel with less amount of bainite exhibits better wear resistant performance at lower applied load, while the steel with more bainite displays better wear resistant performance at higher applied load. It is interesting to find that the wear rate varies with amount of baintie. Both the impact toughness and fracture elongation increase with the increase of the amount of bainite, while the hardness and strength decrease. The present work aims to provide guidance for the optimization of wear property, mechanical properties and microstructure of low carbon wear resistant steels.
  • Structure-Property Correlations of a Medium C Steel Following Quenching and Isothermal Holding above and below the Ms Temperature

    pp. 442-451

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    DOI:10.2355/isijinternational.ISIJINT-2020-355

    The processing of advanced multiphase high strength steels often includes isothermal treatments around the martensite start temperature (Ms) for achieving a refined microstructure comprising bainite-austenite and/or bainite-martensite-austenite phase constituents. The objective of this research work was to investigate the structure-property relationship for a medium carbon, high-silicon DIN 1.5025 steel (Fe-0.529C-1.67 Si-0.72Mn-0.12Cr (in wt.%)) following isothermal holding close to the Ms temperature (~275°C) to enable low temperature austenite decomposition. For realizing multiphase microstructures, DIN 1.5025 steel samples were austenitized at 900°C for 5 min and then quenched to the isothermal holding temperatures 350 and 250°C for various times ranging from 5 to 3600 s. Microstructural investigation corroborated the formation of multiphase microstructure comprising tempered martensite, bainite, retained austenite, and fresh martensite in both the samples isothermally held above (350°C) and below the Ms (250°C) temperature. The sample isothermally held at 250°C showed a much more refined microstructure in comparison to that held at 350°C due to the presence of a fraction of initial martensite laths which acted as potential sites for bainite nucleation. Also, the evaluation of mechanical behaviour showed that the best tensile properties in terms of high tensile strength and good ductility were achieved in samples with high volume fractions of both interlath and blocky retained austenite, particularly those isothermally treated at 350°C for 200 s and at 250°C for 600 s, respectively.
  • Effects of Distributions of Constituent Phases on Mechanical Properties of C–Si–Mn Dual-phase Steel

    pp. 452-462

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    DOI:10.2355/isijinternational.ISIJINT-2020-435

    Two kinds of ferrite were produced due to slow cooling after intercritical annealing, one being intercritically-annealed ferrite (αa phase), and the other transformed ferrite (αc phase). The effects of distributions of the αa and αc phases on the mechanical properties of a Fe–0.17C–1.5Si–1.7Mn dual-phase steel were examined experimentally at room temperature. Two types of intercritical annealing (IA) were conducted to control the distribution. The IA temperature and time are Ta = 800°C (1073 K) and ta = 0.5 h (1.8 ks), respectively, for the first type, and Ta = 740°C (1013 K) and ta = 4 h (14.4 ks), respectively, for the second one. For both types of IA, the steel was slowly cooled to 400°C (673 K) at a cooling rate of 10°C/s, followed by water quenching. While the total volume fraction fα of the αa and αc phases is close to 0.68–0.69 for both Ta = 800 and 740°C (1073 and 1013 K), the combination of the volume fraction of each α phase is different for Ta =800 and 740°C. The volume fraction fαa of the αa phase and fαc of the αc phase is 0.33 and 0.36 for Ta = 800°C, respectively, and 0.68 and 0 for Ta = 740°C, respectively. The ultimate tensile strength su is about 970 MPa for fα = 0.68–0.69 independent of combination of fαa and fαc values. Thus, the effect of the α phase on su is close to each other between the αa and αc phases. In contrast, both the uniform strain eu and the local strain el increase with increasing volume fraction fαc of the αc phase. Such increase in eu is attributed to larger values of the strain hardening rate dσ/dε in the large strain region. Misfit strain stored at the boundary between the αa and αc phases causes to the larger values of dσ/dε. On the other hand, suppression of void formation deduces the increase in el. Acceleration of dynamic recovery of the α phase adjacent to martensite (M phase) and decrease in the hardness of the M phase adjacent to the α phase due to formation of the αc phase can suppress void formation. Consequently, formation of the αc phase contributes to improvement of the mechanical properties of the dual-phase steel.
  • Simultaneous Improvement of Toughness and Fatigue Life in a Typical Ultrahigh Strength Steel by a New Deep Cryogenic Treatment Process

    pp. 463-472

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    DOI:10.2355/isijinternational.ISIJINT-2020-397

    As a supplemental process of heat treatment, deep cryogenic treatment (DCT) is believed to have favorable effects on the mechanical properties of steels. In this study, three different sequences of DCT and tempering, including QCTT (Q: quenching, C: DCT, T: tempering), QTC, and QTCT, were investigated to discover an effective method to improve the impact toughness and fatigue life of a typical ultrahigh strength steel simultaneously. Compared with the traditional heat treatment process, the impact toughness, fracture elongation, and fatigue life were improved by 10%, 17.8%, and 13.7%, respectively, after the QTC treatment process. The microstructures were characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and electron probe micro-analysis (EPMA). The results indicate that after QTC process, the formation of cracks was restricted due to the decrease of carbides and disappear of twinned martensite, both of which would serve as the sources of cracks. Besides, the propagation of cracks is hindered because the decrease of blocky retained austenite and increase of filmy retained austenite. As a result, the impact toughness, fracture elongation and fatigue life are improved simultaneously. This work provides a new and effective method to simultaneously improve the toughness and fatigue life of ultrahigh strength steels.
  • Analysis of Nano-hardness Distribution Near the Ferrite-martensite Interface in a Dual Phase Steel with Factorization of Its Scattering Behavior

    pp. 473-480

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    DOI:10.2355/isijinternational.ISIJINT-2020-546

    Herein, we investigated the local preliminary hardening of ferrite near the ferrite–martensite interfaces in a dual-phase (DP) steel. Geometrically necessary dislocations (GNDs), generated due to interfacial misfit between different phases, may cause preliminary hardening of ferrite around such interfaces. However, for nano-hardness distribution, the hardened zone was not evidently detected by scattering measurement. Thus, we factorized nano-hardness scattering to estimate the actual ferrite hardness near ferrite–martensite interfaces.First, nano-hardness was measured around a martensite island using a conical nano-indenter in the DP steel containing 10% martensite by volume. Taking into account the scattering, the nano-hardness measurement converged to the hardness of ferrite, exceeding the distance corresponding to the nano-indenter radius. Thus, a preliminary hardening zone was not detected. Subsequently, the surface of the nano-indented microstructure was polished and observed using scanning electron microscopy (SEM) by analyzing electron back scattering diffraction (EBSD). This analysis confirmed the presence of the nano-indented microstructure under ferrite. Moreover, it established that the majority of the irregularly higher nano-hardness was caused by the buried martensite under ferrite. The value of the kernel average misorientation (KAM), which is proportional to the GND density for other irregularly higher nano-hardness points, was higher for the nano-indented microstructure as compared to that of the buried martensite. On the other hand, the ferrite was expanded under the nano-indented points for the majority of the irregularly lower nano-hardness, with some exceptions. Further, soft martensite was observed to induce irregularly lower nano-hardness locally around the interface.
  • Strain Distribution Analysis of Two Perpendicular Planes in SUS310S Austenitic Stainless Steel Using Digital Image Correlation

    pp. 481-486

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    DOI:10.2355/isijinternational.ISIJINT-2020-305

    We performed a strain distribution analysis of two perpendicular planes in SUS310S austenitic stainless steel using digital image correlation (DIC). Strain bands in both the rolling direction, (RD)-transverse direction and RD-normal direction planes, were connected at an edge, and the formation of a strain plane surrounded by the strain bands in each plane was confirmed. Furthermore, we found that the three-dimensional strain distribution can be inferred by analyzing the strain distribution of two perpendicular planes using DIC. The finite element method (FEM) was used to verify the formation of macro and micro strain bands. The FEM analysis also revealed that the connection of micro strain bands led to the formation of macro strain bands. The angle of macro strain bands to tensile loading direction was dependent on preferential deformation site distribution. In contrast, the angle of micro strain bands was mainly determined by preferential deformation site distribution when the angle of preferential deformation site to tensile loading direction was <50° and by maximum shear stress when it was >50°. From these results, we concluded that (i) the angle of the macro strain bands observed by DIC to tensile loading direction was mainly determined by the preferential deformation site distribution; and (ii) the angle of the micro strain bands was determined by both the preferential deformation site distribution and maximum shear stress.
  • Modelling of Effects of Cold Work on Sigma Phase Growth Rate in Super Duplex Stainless Steel

    pp. 487-492

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    DOI:10.2355/isijinternational.ISIJINT-2020-481

    By the modelling of the precipitation and growth behaviour of the sigma phase the quantitative effects of amounts of cold work was investigated in super duplex stainless steels. The cold work promotes sigma phase precipitation. The equation to describe the growth rate of sigma phase proposed formerly by the authors was employed. That is based on the classical nucleation theory and J-M-A-K formula.The effects of cold work was incorporated in that equation as two parameters. Those parameters were determined using the experimental data. Experimental data was obtained by measuring the growth of sigma phase in the isothermally heated specimen at 1173 and 1223 K. The specimens consisting of 25mass%Cr-mass7%Ni-3mass%Mo-2mass%W-0.3mass%N super duplex stainless steel were prepared with various reduction levels of 3 to 10% in cold rolling process. As result the model to described quantitatively the effect of the amount of cold work on the acceleration of sigma phase was proposed. The validity of the model was confirmed by clarifying to have good fit to the experimental data referred from another work published.
  • Faster Generation of Nanoporous Hematite Ore through Dehydration of Goethite under Vacuum Conditions

    pp. 493-497

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    DOI:10.2355/isijinternational.ISIJINT-2020-403

    Goethite (FeOOH)-based ore has become attractive to be utilized in ironmaking. As mildly dehydrated to remove its high combined water (CW), it changes to a nanoporous hematite ore. The nanopore contributes a significant increase in ore reduction reactivity due to the nanocontact between iron oxide and reducing agents such as C, CO, or H2. However, the long dehydration time of goethite ore is still one problem. This study revealed the effect of vacuum condition on the mild-dehydration of goethite-based ore significantly reduces the ore dehydration time. The dehydration is finished within one hour at 300°C under high-vacuum condition (P=1.7 Pa), producing nanoporous hematite ore that is similar to under atmospheric one for 24 h. However, no significant decrease in the dehydration temperature under the high-vacuum condition. In contrast, in-situ heating TEM observation reveals that nanopore generation can occur at low temperatures under ultra-high vacuum conditions (P = 5.6×10−6 Pa). Slit pore generates at low temperatures that then eventually disappear by merging to bigger pores at the higher temperatures.
  • Potential Influences of Impurities on Properties of Recycled Carbon Steel

    pp. 498-505

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    DOI:10.2355/isijinternational.ISIJINT-2020-377

    Contamination with tramp elements is a major concern in steel recycling. This study aimed to identify the influence of impurity elements on the major properties of carbon steel. The content of impurity elements in recycled steel was determined via elemental analysis of randomly sampled steel bars. Of the 23 impurity elements considered, 15 were found to be mixed in the recycled steel. Industrial standards stipulate that the six major properties of carbon steel are tensile strength, elongation, yield point or proof stress, soundness in the welding area, fracture toughness, and bendability. The influence of the fifteen impurity elements on all of these properties except bendability was investigated by reviewing previous academic reports. Properties related to strength and stress were found to be enhanced by the presence of almost any impurity, while elongation and welding soundness were often compromised. The influence of impurities on the other properties remains largely unknown. The negative effects of impurities on the workability and weldability can be minimized by adjusting the steelmaking, casting, and annealing processes, or rethinking the design of products and manufacturing processes. Further, the incorporation of impurities may be prevented during the recovery of steel scrap from end-of-life products. A useful approach to the prevention of the negative influence of impurities is a new concept termed R-PSPP, which stands for recovery, process, structure, property, and performance.
  • Sulfated Steelmaking Slags as Se(IV) Adsorbents: Effects of Preparation Conditions on Adsorption Performance

    pp. 506-512

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    DOI:10.2355/isijinternational.ISIJINT-2020-420

    Steelmaking slags treated with aqueous H2SO4 were characterized as Se(IV) adsorbents, and notable adsorption ability was shown to develop at H2SO4 concentrations of 0.18–0.20 mol/L. The enhanced adsorption performance of slags sulfated in the presence of H2O2 suggested that their Se(IV) adsorption sites contained Fe(III) and resembled those of schwertmannite. Langmuir analysis of Se(IV) adsorption isotherms was used to determine the corresponding adsorption capacities and adsorption constants. Thus, the obtained results allow one to simultaneously solve the problems of slag utilization and Se removal from wastewater or natural water, contributing to the establishment of a greener society.
  • Effect of MgO on Phase Development in Hematite-ilmenite Ore Sinter

    pp. 513-515

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    DOI:10.2355/isijinternational.ISIJINT-2020-490

    The effect of MgO on phase development in a hematite-ilmenite ore sinter was investigated employing a vertical tube furnace by raising the temperature at 30 K/min to 1553 K. The phases developed after sintering were identified in terms of XRD and EDS analyses. The distribution of metallic elements after sintering were figured out through EPMA mappings. The results showed that the formation of MgO·Fe2O3(s) and Mg-rich SFCA increased in the sinter blend, and that more Ti was retained in the Fe-rich phase with increasing MgO. It is believed that the formation of CaO·TiO2·SiO2 phase decreased with increasing the addition of MgO.

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