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

ISIJ International Advance Publication

  • Steam Reforming of Methane on Sponge Iron: Influence of Gas Composition on Reaction Rate

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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.
  • Interfacial Reactions and Inclusion Formations at an Early Stage of FeNb Alloy Additions to Molten Iron

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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.
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    1. Effects of Distributions of Constituent Phases on Mechanical Properties of C–Si–Mn Dual-phase Steel ISIJ International Advance Publication
  • Effects of Distributions of Constituent Phases on Mechanical Properties of C–Si–Mn Dual-phase Steel

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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 of the αa phase and 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 and 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 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.
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    1. Interfacial Reactions and Inclusion Formations at an Early Stage of FeNb Alloy Additions to Molten Iron ISIJ International Advance Publication
  • Effects of Grain Boundary Characteristics on Secondary Recrystallization Textures in Fe–Si Alloy

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    DOI:10.2355/isijinternational.ISIJINT-2020-437

    To understand the factors that determine the secondary recrystallization textures of Fe–Si alloy, the change in the secondary recrystallization textures of Fe–Si sheets was investigated by increasing the cold-rolled reduction rate (CR) from 70 up to 95%.The secondary recrystallization textures in the CRs=90–95% samples accumulated in specific orientations; the main components of the secondary recrystallization were {110}<001> in the CRs=90 and 93% samples, and {110}<001> with {110}<112> in the CR=95% sample. The experimental results were reproduced by calculations based on the idea that the secondary recrystallization texture is mainly determined by the frequency of the specific-orientation-grain-boundaries, for example, coincidence-site-lattice grain boundaries. In contrast, in the CR=70% sample, the secondary recrystallization texture was not accumulated but dispersed from {110}<001> to {110}<225> and was inaccurately reproduced by the above calculations.The study concludes that the secondary recrystallization textures in the CRs=90–95% samples are mainly determined by the grain boundary effect. It is also concluded that the secondary recrystallization textures of the CR=70% sample are determined by both grain boundary effect and nucleus effect. The difference in the mechanisms originates from the changes in the frequency of the specific-orientation-grain-boundaries in the matrix and of the nuclei at the surface in the primary recrystallization textures of various CRs.
  • Relationship between Creep Strength and Magnetic Properties of Cobalt-bearing High Chromium Ferritic Steel

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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.
  • Desulfurization Behavior of Low-sulfur Plastic Die Steel during ESR Process under Different Atmospheres

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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.
  • Ferrite Transformation from Fe-0.3N Austenite

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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.
  • Transformation from Ferrite to Austenite during/after Solidification in Peritectic Steel Systems: an X-ray Imaging Study

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    DOI:10.2355/isijinternational.ISIJINT-2020-486

    X-ray transmission imaging with X-ray diffractometry and time-resolved tomography with three-dimensional X-ray diffraction microscopy have been used to observe solidification and transformation in carbon steel and other Fe-based alloys. The imaging techniques showed a massive-like transformation, in which multiple austenite grains were produced in a single δ grain through a solid–solid transformation. The critical velocity from the diffusion-controlled growth of the γ phase to the massive-like transformation was as low as 5 µm/s. X-ray imaging indicated that the δ phase transforms massively to the γ phase in the conventional solidification processes, such as continuous casting. The massive-like transformation and multiple γ grains that were produced in the transformation were related to the subsequent microstructure evolution and casting defect formation. Solidification model including the massive-like transformation is expected to improve our understanding the solidification and the related phenomena in the peritectic steel systems.
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    1. Faster Generation of Nanoporous Hematite Ore through Dehydration of Goethite under Vacuum Conditions ISIJ International Advance Publication
    2. Numerical Modeling of the Inclusion Behavior during AC Flash Butt Welding ISIJ International Advance Publication
    3. Ferrite Transformation from Fe-0.3N Austenite ISIJ International Advance Publication
  • Formulation of a Generalized Flow Curve for 0.2% Carbon Steel under High-speed Hot Forming Conditions by a Regression Method

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    DOI:10.2355/isijinternational.ISIJINT-2020-175

    A precise flow curve for a wide range of forming conditions is important for accurately predicting forming force. Moreover, since the flow curve reflects microstructural changes, its accurate description must be obtained under various temperatures and strain rates up to 300 s-1. For practical forming processes such as hot strip rolling and wire rod rolling, the deformation behavior at high strain rates (50–200 s-1) must also be studied. However, a uniform axial high strain rate is difficult to achieve. Hence, a new deceleration method is developed. Also, the compression test at high strain rates is accompanied by marked internal heat generation, Therefore, temperature and deformation are highly inhomogeneous compared with those in tests at lower strain rates. In addition to this problem, heat conduction to the die and friction should be corrected using inverse analysis. By considering the internal temperature increase effect at high strain rates, the uniaxial flow curve obtained using inverse analysis is shown to be greater than the experimental apparent stress–axial strain curve. And then, a regression method is applied to obtain a generalized flow curve at high strain rates, which can cover wider ranges of strain rates and temperatures. Finally, they are compared with an extrapolated flow curve that is regressed using an intermediate strain rate in our previous research. By comparing those results, the extrapolated flow curve is greatly different from the flow curve obtained in the current research. To find the reason for the difference, a microstructure analysis using EBSD is implemented.
  • Faster Generation of Nanoporous Hematite Ore through Dehydration of Goethite under Vacuum Conditions

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

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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.
  • Microstructure Evolution and Tempering Transformation Kinetics in a Secondary Hardened M50 Steel Subjected to Cold Ring Rolling

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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.
  • Structure-Property Correlations of a Medium C Steel Following Quenching and Isothermal Holding above and below the Ms Temperature

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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.
  • Viscosity of Na–Si–O–N–F Melts: Mixing Effect of Oxygen, Nitrogen, and Fluorine

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    DOI:10.2355/isijinternational.ISIJINT-2020-326

    Fluorine and nitrogen are important elements of metallurgical slags and fluxes. Studies on their viscosity have often focused on the additive effect of fluoride and nitride compounds (e.g., CaF2 and Si3N4), whereas the influence of anionic composition (i.e., oxygen, fluorine, and nitrogen concentrations) with a fixed cationic composition remains unclear. The present study reports the scarcely quantified viscosity variations due to changes in the anionic composition of a simple sodium silicate system by rotating crucible method under a controlled atmosphere. 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to characterize the structural changes against varying nitrogen and fluorine concentrations in the quenched glassy sample. The observed change in the local silicon structure was consistent with the expected variation from the conventional structural roles of nitrogen and fluorine in silicate glasses: nitrogen atoms tend to bond with silicon atoms, whereas fluorine atoms prefer to exist in surrounding sodium cations. Moreover, nitrogen tends to increase the viscosity, whereas fluorine strongly decreases the viscosity of the sodium silicate melts even with the enhancing effect of the latter on the polymerization of silicate anions. The viscosity of silicate melts has been commonly related to the overall polymerization degree of the liquid. However, the viscosity of fluorine-containing silicates cannot be explained by this conventional scenario. Fluorine ions tend to loosely bond with sodium cations. These sodium–fluorine complexes played a strong lubricant role in the network liquids.
  • Melting Characteristics of Multipiece Steel Scrap in Liquid Steel

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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.
  • Flow Stress of Duplex Stainless Steel by Inverse Analysis with Dynamic Recovery and Recrystallization Model

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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.
  • Texture Formation in a Polycrystalline Fe–Ni–Co–Al–Ti–B Shape Memory Alloy

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    DOI:10.2355/isijinternational.ISIJINT-2020-199

    In polycrystalline Fe-Ni-Co-Al-based shape memory alloys, control of the recrystallization texture is significantly important to improve the ductility by suppressing the brittle precipitates of the β-B2 phase at grain boundaries during aging treatment. In this paper, the texture evolution in the recrystallization process was systematically investigated by means of the electron backscatter diffraction (EBSD) method in an Fe–Ni–Co–Al–Ti–B polycrystalline alloy. The development of a {110}<112> texture was confirmed in the 98.5% cold-rolled sheet specimen. After primary recrystallization annealing, the γ matrix containing the β phase showed the continuous recrystallization remaining in the same orientation with a deformation texture after annealing at 1000°C. Grain growth of the γ phase was interrupted by the β phase. Then, abnormal grain growth of {210}<001> grains began occurring in concurrence with the dissolution of the β phase and the main recrystallization texture changed from {110}<112> to {210}<001> at temperatures higher than 1100°C.
  • Fuel Ratio Optimization of Blast Furnace Based on Data Mining

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    DOI:10.2355/isijinternational.ISIJINT-2020-238

    Despite the age of the process, the blast furnace (BF) ironmaking is still crucial to iron and steel industry. To improve the competitiveness of enterprises, fuel ratio (FR) in BF ironmaking process needs to be kept its lowest level possibly. In the work, a prediction model was established to predict FR of the BF by using feature selection and support vector regression (SVR). GA-SVR (genetic algorithm - SVR) method was employed to select the most informative five features from the candidate features.The experimental results indicated that the SVR model brought high learning precision and excellent prediction generalization ability. To explore and discover the laws of BF production, the influences of the five features on FR were discussed by simulation analysis of the model. All the calculations were performed on the computational platform of data mining developed by us. The work can provide guides for the operators on modulating input parameters in advance. The methods outlined here can provide valuable hints into revealing mechanisms of BF ironmaking process and realizing controlled production of BF with guidance of quantitative analysis methods.
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  • Optimization Analysis of Mechanical Performance of Copper Stave with Special-shaped Tubes in the Blast Furnace Bosh

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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.
  • Quantification of the Severity of Ridging in Ferritic Stainless Steel Sheets Using a Profilometric Technique

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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.
  • Accuracy Improvement of the XRD-Rietveld Method for the Quantification of Crystalline Phases in Iron Sintered Ores through the Correction of Micro-absorption Effects

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    DOI:10.2355/isijinternational.ISIJINT-2020-188

    The mass fraction of each crystalline phase in inorganic materials can be investigated using the Rietveld refinement of the X-ray diffraction (XRD) patterns. For quantitative analysis, differences in the values of the linear absorption coefficient, µ, among the crystalline phases must be considered when certain X-ray sources are used, because such differences often affect their mass fractions. Herein, we evaluate the effects of the differences between the Cu and Co Kα X-rays on the mass fractions of the crystalline phases in iron sintered ores using the XRD-Rietveld method by performing two types of XRD measurements. Type 1 samples modeled materials with two different particle size combinations of α-Fe2O3 and ZnO. Type 2 samples used powder mixtures to simulate iron sintered ores composed of α-Fe2O3, and synthesized SFCA and SFCA-I in various mass fractions. Moreover, a correction method was developed using the Taylor-Matulis (TM) correction that considers the µ of each phase and the average particle diameter R of each crystalline phase determined by scanning electron microscopy with energy dispersive spectroscopy. For type 1 samples, results that were in good agreement with the initially-charged mass fractions could be obtained using the TM correction, even in the presence of significant differences in R between α-Fe2O3 and ZnO. The results for type 2 samples confirmed that quantitatively accurate mass fractions could be obtained using the TM correction with an accuracy of approximately ±3 mass% for Cu and Co sources, whereas the error was greater than ±3 mass% for Cu source when the TM correction was not applied.
  • Extraction of Iron from Refractory Titanomagnetite by Reduction Roasting and Magnetic Separation

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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.
  • Melting Erosion Failure Mechanism of Tuyere in Blast Furnace

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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.
  • Potential Influences of Impurities on Properties of Recycled Carbon Steel

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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.
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    1. Melting Erosion Failure Mechanism of Tuyere in Blast Furnace ISIJ International Advance Publication
  • Mechanism Behind the Onset of Delamination in Wire-drawn Pearlitic Steels

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    DOI:10.2355/isijinternational.ISIJINT-2020-158

    Fully pearlitic steel was wire-drawn up to a strain of 2.2. Torsion tests were performed using two types of specimens—one was an as-drawn specimen, and the other was aged at 423 K for 3.6 ks. A delamination crack propagated along the longitudinal direction of the wire in the aged specimen, whereas normal fracture was exhibited perpendicular to the longitudinal direction in the as-drawn specimen during torsion tests. Backscattered electron images indicated that the cementite lamellae beneath the delamination crack had vanished, whereas, in the as-drawn specimen, the cementite lamellae beneath the normal fracture surface had rotated until the fracture. Torsion tests with different strain rates indicated an inverse strain-rate dependence of the onset of the delamination, suggesting that the plastic deformability of ferrite and existence of the thermally activated process that controls the cementite dissolution indicate the onset of the delamination. In the present study, the effect of aging and deformability of ferrite on delamination is discussed, suggesting that the delamination crack propagates as a result of the local plastic instability on the scale of several microns.
  • Characterization of Non-metallic Inclusions and Clusters during Production of Low-carbon IF Steel

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    DOI:10.2355/isijinternational.ISIJINT-2020-202

    One of the quality criteria for Interstitial Fee (IF) steels is the metal purity with respect to non-metallic inclusions (NMI), which are harmful for the plastic properties of the material. Furthermore, they cause a formation of surface defects in flat rolled products and reduce the rate of steel casting due to nozzle clogging. This article presents the results of a study of the content, composition, size and morphology of non-metallic inclusions and clusters in steel samples taken during ladle treatment, casting as well as from slabs and steel sheets after rolling of IF steel. The characteristics of NMI and clusters were determined by using conventional two-dimensional quantitative metallographic investigations of polished sections of steel samples (2D method), electrolytic extraction (EE method) of samples followed by investigations of inclusions and clusters by using scanning electron microscopy and energy dispersive spectroscopy and fractional gas analysis (FGA method). By using EE method, different types of inclusions and clusters, their formation, growth and behavior during different stages of IF steel production were studied. The results obtained by the EE method agreed well with the results of the quantitative determination of oxide NMI by using the FGA method. The method of fractional gas analysis shows the dynamics of changes in the content of various types of oxide non-metallic inclusions during ladle treatment and casting of steel. The obtained results can be used to analyze the causes of the formation of harmful NMI in the metal and to optimize ladle treatment of IF steel grades.
  • Torque Model in Plate Rolling Process with Biting Impact Considered

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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

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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.
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    1. Removal of Non-Metallic Inclusions from Aluminum by Electroslag Refining MATERIALS TRANSACTIONS Vol.52(2011), No.12
  • Evaluation and Prediction of Blast Furnace Status Based on Big Data Platform of Ironmaking and Data Mining

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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.
  • Influence of Soil Particle Size, Covering Thickness, and pH on Soil Corrosion of Carbon Steel

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    DOI:10.2355/isijinternational.ISIJINT-2020-261

    The effects of environmental factors, such as particle size, covering depth, and pH, on the corrosion behavior of carbon steel in silica sand filled with a 3% NaCl solution were investigated by electrochemical impedance spectroscopy (EIS) and polarization measurements.The corrosion rate initially decreased with the decrease in the oxygen concentration near the steel surface, which was a result of the consumption of the dissolved oxygen during the corrosion reaction. The corrosion rate became constant at approximately 10 μm y-1 regardless of the particle size and covering thickness. At the steady state, both the anodic reaction (iron dissolution) and cathodic reaction (oxygen reduction) appeared to be suppressed by the formation of an oxide film on the steel surface. In the silica sand filled with non-buffer and buffer solutions of pH 3–6, the corrosion rate was initially significantly enhanced by the hydrogen ions (H+). The period of the enhancement depended on the buffering capacity. However, the corrosion rate was subsequently independent of the pH due to the neutralization of the solution in the vicinity of the steel surface.
  • Microstructure Evolution and Mechanical Properties Improvement in Magnetic-controlled Electroslag Remelted Bearing Steel

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    DOI:10.2355/isijinternational.ISIJINT-2020-173

    The transverse static magnetic field (TSMF) was introduced into the electroslag remelting (ESR) process to produce GCr15 steel ingots and the microstructure, non-metallic inclusions, chemical composition and mechanical properties of the ingots were analyzed to investigate the effect of TSMF during the ESR process. The transverse section of the ingots indicated that the application of a 130 mT static magnetic field resulted in a refined dendritic structure. The coverage ratio of the homogeneous crystallites area in the center of the transverse section increased to 52%. The metallic solid-liquid interface with different magnetic flux density (MFD) was recorded during ESR process. The depth of the metallic molten pool was 44.2 mm without the TSMF. When a 130 mT TSMF was applied, the molten pool became noticeably shallower (14.2 mm). And the oxide inclusions count in the scan area of 5.117 mm2 decreased to 239 from 1212. When the TSMF implemented, the tensile, friction and wear and Rockwell hardness properties of ingots showed a significant improvement. These results showed that the application of TSMF during the ESR process of GCr15 steel not only refine the dendritic structure, but also improve the efficiency of inclusion removal and mechanical properties.
  • Development of Fe–P–C–Cu Immiscible Amorphous Alloys with Liquid Phase Separation

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    DOI:10.2355/isijinternational.ISIJINT-2020-211

    The rapid solidification microstructure and magnetic properties of melt-spun ribbons in the (Fe0.75P0.125C0.125)100-xCux (at%) alloys were investigated, focusing on the occurrence of liquid-phase separation and simultaneous amorphous-phase formation. The (Fe0.75P0.125C0.125)100-xCux alloys were designed as a combination of Fe–P–C alloy with high glass-forming ability and Cu. Amorphous-phase formation was observed in the melt-spun ribbons of the (Fe0.75P0.125C0.125)100-xCux (x = 10, 20) alloys. For the melt-spun ribbons of the (Fe0.75P0.125C0.125)100-xCux (x = 10) alloy, a composite of Fe–P–C amorphous matrix and FCC–Cu globules was obtained, whereas in the melt-spun ribbons of the (Fe0.75P0.125C0.125)100-xCux (x = 20) alloy, multistep liquid-phase separation resulted in a particular solidification microstructure.
  • In situ Phase Analysis during Self-sintering of BOS Filter Cake for Improved Recycling

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    DOI:10.2355/isijinternational.ISIJINT-2020-142

    The self-sintering of basic oxygen steelmaking (BOS) filter cake has been studied using in situ high temperature X-ray diffraction (XRD) during heating in air from room temperature to 1273 K. The aim of the study was to improve the understanding of the self-sintering process, using this in situ method to identify what reactions occur at different temperature ranges.The in situ phase analysis measurements correspond well, and are consistent with, the previously reported characteristics of BOS filter cake oxidation. However, the in situ measurements have allowed a more detailed analysis of the reactions taking place over the temperature ranges studied. Wüstite was the first component of the BOS filter cake to react with air, reacting at temperatures of approximately 373 K to 773 K to form a magnetite-zinc ferrite spinel solid solution. The reaction of metallic iron began at higher temperatures than wüstite, beginning at ~633 K and finishing at ~873 K. The decomposition of fluxes (mainly CaCO3) occurred at temperatures above 873 K. At temperatures higher still, at approximately 1073 K, hematite reacted with zinc oxide to form zinc ferrite.The knowledge that wüstite reacts at the lowest temperatures is an improvement in the understanding of the self-sintering of BOS filter cake, and may give insights into the initiation of the self-sintering process. This improved understanding will aid analysis and approaches focused on process optimisation to increase the amount of filter cake that can be recycled back to the BOS.
  • An Empirical Comparative Study of Renewable Biochar and Fossil Carbon as Carburizer in Steelmaking

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    DOI:10.2355/isijinternational.ISIJINT-2020-135

    Approximately 60–70% of the direct greenhouse gas emissions in electric arc furnace (EAF) steelmaking originate from the use of fossil carbon charge during melting of steel scrap. Regarding short-term solutions to mitigate the climate impact of steelmaking, there is greater potential to replace fossil carbon charge with renewable carbon in the EAF than in integrated blast furnace steelmaking where mechanical strength requirements on carbon charge are too demanding. Therefore, the present study aims to provide an experimental and practical foundation for using renewable biochar in the EAF as a relatively simple step to decrease the climate impact of steelmaking.In order to evaluate the inherent performance of biochar as a carburizing agent, lab-scale tests where completed using four different types of carbonaceous materials: synthetic graphite, anthracite coal and two types of biochar from woody biomass (BC1 and BC2). The first order dissolution rate constants from experiments ranged between 0.7 to 1.9 × 10-4 m/s, which agrees well with previously reported results. Furthermore, lab-scale results show that biochar properties commonly seen as detrimental, such as low carbon crystallinity and high porosity, do not necessarily constitute a disadvantage for biochar utilization as carburizer in steelmaking.In order to further assess the results from lab-scale tests, an industrial trial including six consecutive heats was performed in a 50 t EAF at the Höganäs Halmstad Plant. Results show that 33% substitution of standard Anthracite carbon charge with biochar BC2 gave no deviation from normal operating conditions in the EAF.
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    1. Dissection Investigation of Forming Process of Titanium Compounds Layer in the Blast Furnace Hearth ISIJ International Advance Publication
  • Dissection Investigation of Forming Process of Titanium Compounds Layer in the Blast Furnace Hearth

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    DOI:10.2355/isijinternational.ISIJINT-2020-146

    In this paper, titanium compounds samples were obtained by dissecting the blast furnace for which protected the hearth by adding titanium ores for a long time, and the formation process of the titanium protective layer in the blast furnace was deduced. It was found that Ti(C, N) precipitated in the molten iron initially, and then grew up in the slag by grain boundary migration between grains. Finally, Ti(C, N) grains consolidated with slag and iron to form a protective layer. Furthermore, the 3D morphology of the precipitated phase of Ti(C, N) was also observed by the acid attack method, found that the Ti(C, N) grains mainly in the form of the layered structure and stepped rectangular structure. Besides, the internal of the Ti(C, N) grains was not dense packing. The above results can provide an in-depth understanding of furnace protection with titanium ores.
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    1. An Empirical Comparative Study of Renewable Biochar and Fossil Carbon as Carburizer in Steelmaking ISIJ International Advance Publication
  • Numerical Modeling of the Inclusion Behavior during AC Flash Butt Welding

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    DOI:10.2355/isijinternational.ISIJINT-2020-143

    The entrapment of inclusions in the solidified weld zone is detrimental to its mechanical properties. In an AC flash welding process, the upsetting rate, the initial temperature of the weld pool, and the size of inclusions may affect the final distribution of the inclusions. Additionally, the concentration of sulfur may induce Marangoni convection in the weld pool, which possibly affects the pushing and engulfment of inclusions by the solid at the solid-liquid interface. In the present work, a two-dimensional numerical model based on Computational Fluid Dynamics (CFD) has been developed to investigate the behavior of alumina inclusions during the AC flash welding of a thin SPFH590 steel plate. The Volume of Fluid (VOF) numerical model was coupled with the dynamic mesh model for the motion of plates, discrete phase for inclusion particles and solidification model. The simulation results show that the upsetting parameters significantly affect the overall inclusion motion. A high upsetting rate pushes the inclusions away from the welded joint. The high initial flash temperature does not affect the removal of inclusions from the weld zone. A similar outcome has been noted with respect to the increase in the diameter of the inclusions. Furthermore, the predicted results show that inclusions are prone to engulfment by the solidification front under the influence of higher interfacial tension between the inclusions and melt. Nevertheless, the inclusion displacement under the influence of an interfacial tension gradient is diminutive because of the rapid solidification rate of the weld pool.
  • Influence of Basicity on the Viscosity and Crystallization Characteristics of Chromium-containing High-titanium Slag

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    DOI:10.2355/isijinternational.ISIJINT-2020-244

    In this paper, the influence of basicity (CaO/SiO2) on the viscosity and crystallization characteristics of chromium-containing high-titanium slag was studied. The melting temperature and viscosity were measured by employing the hemisphere and rotating cylinder methods, and the composition and morphology of the crystallized phase were analyzed by X-ray diffraction and scanning electron microscopy-energy dispersive spectroscopy. The results showed that the crystallization sequence during the cooling process was spinel, followed by anosovite. The increased basicity led to enhanced softening, hemispherical, and flowing temperatures of chromium-containing high-titanium slag; however, the viscosity significantly decreased. The basicity obviously affected the crystallized phase composition. Furthermore, anosovite was the dominant crystallized phase and a proper level of basicity was beneficial for titanium enrichment.
  • Method for Evaluating Hydrogen Embrittlement of High-Strength Steel Sheets Considering Press Formation and Hydrogen Existence State in Steel

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    DOI:10.2355/isijinternational.ISIJINT-2019-822

    High strength steel sheets are increasingly being used due to the growing need to improve fuel efficiency by reducing vehicle weight. Steel sheets are usually used as automotive parts formed by pressing, etc., and complicated strain is generated in the steel sheet after forming. This strain is thought to affect the occurrence of hydrogen embrittlement. In this study, a new hydrogen embrittlement evaluation method using a forming limit diagram was devised. A forming limit diagram of a steel sheet with a strength level adjusted to 1470 MPa was prepared for uniaxial, plane strain and biaxial modes, and stress and hydrogen were applied to the specimens formed with respective strain modes to evaluate the occurrence of fracture. In order to examine the relationship between the hydrogen content and the cracking, evaluation of the hydrogen content by Thermal Desorption Analysis, visualization of hydrogen by Secondary Ion Mass Spectrometry and microstructure analysis by Electron BackScatter Diffraction were carried out. It was found that cracks are generated in the strain mode in which hydrogen accumulates locally in the steel even when the apparent hydrogen content is small, and it was clarified experimentally that evaluation of local hydrogen concentration is important for the evaluation of hydrogen embrittlement.
  • An Evaluation Method for Hydrogen Embrittlement of High Strength Steel Sheets Using U-bend Specimens

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    DOI:10.2355/isijinternational.ISIJINT-2020-260

    An evaluate method for hydrogen embrittlement property of high strength steel sheet has been proposed in this study. To take into consideration of the effect of plastic strain in addition to the effects of applied/residual stress and diffusible hydrogen, U-bend specimens have been adopted because steel sheets for automobiles are usually used after press forming into various parts. After U-shape bending, the specimen was loaded using a bolt. The proposed evaluation method is based on the measurement of critical hydrogen content or critical hydrogen charging condition for hydrogen embrittlement fracture at given stress and strain conditions. The hydrogen charging current density was increased in step-wise manner until cracking was observed, and cracking was detected by optical observation and by monitoring voltage between the sample and a counter electrode. The critical hydrogen contents for specimens with varied applied stress were obtained by means of thermal desorption spectroscopy. For the critical hydrogen content, both the hydrogen contents in strained portion of the specimen and no-strained portion were measured. The former is affected by introduced dislocations caused by straining and the latter is thought to be proportional to the hydrogen fugacity. Both critical hydrogen contents tended to be decreased slightly when the applied stress was relatively high.
  • Control of Laser Focal Point by Using an Electrically Tunable Lens in Laser-induced Plasma Optical Emission Spectrometry

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    DOI:10.2355/isijinternational.ISIJINT-2020-170

    This paper suggests a method to control the focal point of laser on the on-focus position of a sample surface automatically in laser-induced breakdown spectrometry (LIBS). For this purpose, an electrically-tunable plano-convex lens was installed in a laser irradiation system, where it could vary the focal length of laser with a long working distance and a rapid response time, and the focal length could be periodically varied with a triangle waveform. Because the tunable lens was easily handled and inexpensive, the laser system could be modified with a low cost, as compared with commercial apparatuses having complicated optics to control the position of laser irradiation. A piece of scrapped stainless steel the surface of which was titled and had some roughness was investigated as a test specimen. A satisfactory result was obtained such that the plasma could be generated uniformly and firmly along a laser trace on the sample surface and thus could give the emission signal with a sufficient precision. The driving frequency of the tunable lens, which controlled a repetition period of the laser beam, was optimized to be 10 Hz when the scan rate of laser was fixed at 3.0 mm/s. As a result, it is expected that the LIBS system with the tunable lens can be applied to actual on-site/in-line analysis in material production.
  • Deoxidation Equilibria of Fe–Mn–Al Melt with Al2O3 or MnAl2O4 at 1873 and 1773 K

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    DOI:10.2355/isijinternational.ISIJINT-2020-177

    Deoxidation equilibria of Fe–Mn–Al melt with Al2O3 or MnAl2O4 were measured at 1773 K. Composition of melts doubly-saturated with Al2O3 and MnAl2O4 were also measured using a crucible comprising these two phases at 1873 or 1773 K. Equilibria with each solid oxide were analyzed using Wagner's Interaction Parameter Formalism (WIPF). In the case of Al2O3 saturation, Al deoxidation curve at 1773 K was similar in shape to that at 1873 K, and the equilibrium oxygen content was approximately 1/3 of that at 1873 K. The deoxidation equilibria were reproduced using WIPF at the composition range above 0.1 mass%Al by using -0.32 as and 10-13.4 as the equilibrium constant of Al2O3 dissolution reaction, both of which were determined through analysis of measured results for Fe–(20 to 30) mass% Mn melt. In the case of MnAl2O4 saturation, accurate values of equilibrium constant were not obtained because of the relatively significant influence of oxygen analysis error. On the contrary, using compositions doubly-saturated with Al2O3 and MnAl2O4, valid values of the equilibrium constant of MnAl2O4 dissolution reaction, 10-15.4 and 10-17.7 at 1873 and 1773 K, respectively, could be determined.
  • Surface Tension Calculation of Molten Slag in SiO2–Al2O3–CaO–MgO–‘FeO’–‘Fe2O3’ Systems Based on a Statistical Modelling Approach

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    DOI:10.2355/isijinternational.ISIJINT-2019-807

    A calculation model for surface tension of molten slags in SiO2–Al2O3–CaO–MgO systems, based on a statistical modelling approach, was further extended to SiO2–Al2O3–CaO–MgO–‘FeO'–‘Fe2O3' multi-component systems. A total number of 1493 surface tension data reported in literatures, including 661 iron-containing data in this study, have been collected and critically reviewed for optimizing model parameters. The model achieves an excellent agreement with literature values for iron -containing melts with an average error of 4.9% and overall absolute error of 29.75 mN/m. Moreover, the dependence of surface tension on composition and temperature the composition has been discussed using the present model. The results show that the surface tension always obviously decrease with the increment of SiO2 content and substitution of CaO or Al2O3 by MgO will cause a decrease in the surface tension. In case of silica-free ternary systems such as the Al2O3–CaO–‘Fe2O3' and CaO–MgO–‘Fe2O3', the surface tension decreases with increasing ‘Fe2O3' concentration with constant Al2O3 and CaO level respectively, indicating the network former role of ‘Fe2O3' in these studied systems. Temperature coefficient is closely related with the melt composition and positive values for high ‘FeO' and ‘Fe2O3' contents in the melts were found.
  • Enhanced Homogeneity of a Flat-rolled Wire in Twinning-induced Plasticity Steel Using the Pass Schedule Design

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    DOI:10.2355/isijinternational.ISIJINT-2020-029

    The effects of reduction in height per pass, roll diameter, and friction coefficient on the homogeneity of mechanical properties and shape change in flat-rolled twinning-induced plasticity steel wire were investigated. The goal was to improve the homogeneity of mechanical properties of a wire with area during flat rolling process using a numerical simulation, a hardness test, and electron backscatter diffraction techniques. Reduction in height per pass and roll diameter had large influences on both strain inhomogeneity and lateral spread of flat-rolled wire. Strain inhomogeneity and lateral spread increased with increasing the reduction in height per pass and roll diameter due to the higher length of the contact area. The underlying mechanism for the strain inhomogeneity and lateral spread of flat-rolled wire was highly related to the length of contact area. Hence, the length of the contact area needed reduction through controlling process conditions to improve the strain homogeneity of the flat-rolled wire. The effect of friction coefficient on lateral spread was negligible, whereas strain inhomogeneity slightly increased with friction coefficient. The combination of high and low reduction in height per pass with a smaller roll diameter improved the homogeneity of mechanical properties and microstructure over the area of the flat-rolled wire. Based on the results of numerical simulation and experimental test, a new practical strategy is proposed to achieve greater homogeneity of mechanical properties over the area of flat-rolled wire, which could be of great applicability in industrial fields.
  • Crack Propagation Behavior of Impact Fracture in Case Hardening Steel Subjected to Combined Heat Treatment with Excess Vacuum Carburizing and Subsequent Induction Hardening

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    DOI:10.2355/isijinternational.ISIJINT-2019-826

    The Charpy impact value of case hardening steel subjected to combined heat treatment with excess vacuum carburizing and subsequent induction hardening was evaluated. The purpose of this study is to clarify the relation between the crack propagation behavior and the microstructure in steels having different amounts of retained austenite and cementite. The vacuum carburizing treatment is performed at the hyper-eutectoid composition of 1.3 mass% C. Three different heating temperatures were chosen for induction hardening in the two-phase (austenite, cementite) region between Acm and A1 to obtain different amounts of retained austenite and cementite. Decreasing the induction heating temperature from 1143 K to 1043 K, increased crack propagation resistance by around 30% on average in both the quenched-only and the quenched-and-tempered specimens. The high crack propagation resistance of the samples with the low induction heating temperature was caused by the arrest effect of undissolved θ. By contrast, in the sub-zero treated specimens, crack propagation resistance showed an almost constant value irrespective of the induction heating temperature. That constant propagation resistance was attributed to the repeated bending and branching occurring during crack propagation.
  • Effect of Steel-refractory Reactions on Removal of Arsenic from Molten Steel with Lanthanum Additions

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    DOI:10.2355/isijinternational.ISIJINT-2020-052

    To formulate strategies to remove arsenic from molten steel by adding rare earth elements (REs), the evolution of inclusions in steel with different lanthanum additions was studied, and the effect of reactions between lanthanum and magnesium crucibles on the removal of arsenic was discussed. The results show that the addition of lanthanum can remove arsenic from molten steel, but steel-refractory reactions dramatically influenced the removal effect. The arsenic removal was determined by the generation of La–S–As. The reactions between lanthanum and magnesia crucibles partly consumed lanthanum and decreased its effective concentration acting on arsenic. Further, the reaction product dissolved magnesium consumed a part of sulfur that was disadvantageous for the formation of La–S–As. Besides, a sequence of reactions existed after the addition of lanthanum. The original Si–Mn–Al–O inclusions were changed to lanthanum-containing oxides first and then to MgO-rich oxides. The reaction to generate La–S–As mainly took place within 5 min. The consumption of REs by crucible refractories is an important issue that needs consideration. Alumina crucibles are more favored over magnesia crucibles when using REs to remove arsenic from molten steel.
  • Inclusion Characteristic in Tinplate Steel in RH Refining and Kinetics Limitation of Calcium Transfer by Refining Slag

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    DOI:10.2355/isijinternational.ISIJINT-2020-237

    The characteristics of inclusions including composition, morphology, number, and size in tinplate steel were studied by industrial experiments and thermodynamic calculations during the RH refining process. The results indicated that two types of Al2O3 inclusions including cluster and single-particle are generated at first after Al addition. With the slag-metal and refractory-metal reactions, Al2O3 inclusions, CaO·Al2O3 inclusions, MgO·Al2O3 spinel inclusions, and CaO–MgO–Al2O3 ternary system inclusions are found in the middle of RH refining. Only single-particle Al2O3, CaO·Al2O3 inclusions with high melting point, and CaO–MgO–Al2O3 ternary system inclusions are found at the end of RH refining. From Al addition to the end of RH refining, the total number of inclusions showed a decreasing trend and the proportion of the number density decreased by 70%. About 62% of inclusions are smaller than 10 µm at the end of RH refining, which are difficult to be removed from the liquid steel. The mass transfer of Ca from the refining slag to the liquid steel has a significant effect on the content of [Ca] in liquid steel. Al2O3 inclusions generated in liquid steel can only be modified to CaO·Al2O3 inclusions in the present RH refining time. Aiming to generate 12CaO·7Al2O3 inclusions quickly, moderate calcium treatment as a supplementary measure for refining slag is recommended to modify inclusions during the RH refining process.
  • Solid-liquid Interfacial Energy for Fe–Cr Alloy under Temperature Gradient from Molecular Dynamics Simulation

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    DOI:10.2355/isijinternational.ISIJINT-2019-769

    The solid-liquid interfacial energy of Fe–Cr alloy under temperature gradient is investigated by molecular dynamics (MD) simulations in conjunction with a capillary fluctuation method including the effect of temperature gradient. It is revealed from the MD simulation that fluctuation of the solid-liquid interface decreases with increasing temperature gradient. This results in a large value of the solid-liquid interfacial energy under large temperature gradient. On the other hand, there is a competing effect reducing the solid-liquid interfacial energy with increasing temperature gradient in the formulation of the capillary fluctuation method including the effect of temperature gradient. As a result, the solid-liquid interfacial energy doesn't change significantly at small temperature gradient. Moreover, it is confirmed that the solid-liquid interfacial energy of Fe–Cr alloy decreases with increasing Cr composition at Fe-rich composition regardless of the temperature gradient.
  • Availability of Opal Photonic Crystal Films for Visualizing Heterogeneous Strain Evolution in Steels: Example of Lüders Deformation

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    DOI:10.2355/isijinternational.ISIJINT-2020-223

    An opal photonic crystal film was applied to characterize local strain evolution associated with Lürders band propagation in an annealed low carbon steel. A local change in color of the opal film was observed, which corresponded to the propagation of the Lürders band. In particular, we carried out two tensile experiments for line and area analyses of RGB (Red-Green-Blue) values of the opal films pasted on the specimens. Both of the experiments clearly exhibited a quantitative correspondence between color variation and local strain evolution, namely, the present study demonstrated the potential of the opal films to analyze heterogeneous strain evolution in steels.
  • Phase Composition and Formation Mechanism of Slag Crust in Blast Furnace

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    DOI:10.2355/isijinternational.ISIJINT-2020-113

    Copper stave damage is common problem in blast furnace operations, and the formation of slag crust is beneficial to reduce the damage of copper stave. Therefore, an in-depth understanding of phase composition and formation mechanism of slag crust is helpful to clarify the protection mechanism of copper stave, so as to control the growth of the slag crust and to increase the service life of copper staves. In this study, the slag crust from a copper stave blast furnace was sampled, and the phase composition and structure of the slag crust were characterized in detail through XRD analysis and SEM-EDS. The results indicated that the slag crust presented apparent layer structure as the solid slag layer and viscous layer, which primarily consisted of gehlenite (Ca2Al2SiO7), calcium aluminate (CaAl4O7), magnesia-alumina spinel (MgAl2O4), pleonaste (Mg0.7Fe0.23Al1.97O4), kaliophilite (KAlSiO4) and metallic iron. In addition, the ternary phase diagram analysis of CaO–SiO2–Al2O3 showed that the primary crystal phase of the slag is in the gehlenite region, and that the primary crystal region migrates to the calcium aluminate region with the increasing of Al2O3 content, which are beneficial to the slag crust formation. Finally, the formation mechanism of slag crust was proposed.
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  • Effects of Concentrations of Micro-alloying Elements and Hot-forging Temperature on Austenite Grain Structure Formed during Carburization of Case-hardening Steel

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    DOI:10.2355/isijinternational.ISIJINT-2020-123

    Effects of fine precipitates on the austenite (γ) grain structures were investigated in JIS SCM420-based case-hardening steels with several different concentrations of the micro-alloying elements and hot-forging temperatures. Micro-alloyed steels of 18Al (0.018 mass% Al) and 35Al–32Nb (0.035 mass% Al, 0.032 mass% Nb) were forging-simulated at 1150°C or 1250°C, normalized at 1070°C, and carburized at 1050°C. When the as-received 18Al steel was normalized and carburized without forging-simulated heating, a uniform γ grain structure was observed with the distribution of fine AlN precipitates. However, coarsening of AlN occurred when the forging-simulated temperature was 1150°C and it caused abnormal grain growth during carburization. In 35Al–32Nb steel, the same heating did not induce the abnormal grain growth owing to the AlN–Nb(C,N) combined particles. The size of these particles increase with an increase in the forging-simulated temperature. The high forging-simulated temperature caused the dissolution of the fine precipitates, followed by reformation and coarsening of the precipitates during the subsequent cooling and the normalization heating, which resulted in a decreased pinning force and γ grain coarsening. Furthermore, TEM observations revealed that a considerable amount of Nb(C,N) particles exist near large eutectic MnS particles. Thermodynamic calculations based on the Scheil's condition showed that the formation of these Nb(C,N) particles was due to segregation during solidification. It was suggested that such local concentration of the precipitate particles in the last solidifying region leads to ununiform distribution of the pinning force that may induce the abnormal grain growth.
  • Effect of Titanomagnetite Ironsand Coal Composite Hot Briquette on Softening-melting Performance of Mixed Burden under Simulated Blast Furnace Conditions

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    DOI:10.2355/isijinternational.ISIJINT-2020-098

    Titanomagnetite ironsand coal composite hot briquette (ICHB) was proposed as a novel type of burden to enhance the incremental and high-efficiency utilization of ironsand in blast furnace. ICHB was prepared firstly under laboratory conditions, with a compressive strength higher than 3000 N. Then the national charging ratio of ICHB in the mixed burdens was explored to conduct softening-melting experiments with the simulated BF conditions. Finally the softening-melting-dripping mechanism of mixed burdens was discussed by thermodynamic calculations, SEM-EDS, and XRD detections in this work. It was showed that the softening-melting-dripping behavior and the permeability of mixed burdens could be improved obviously by an appropriate ICHB charging. With the increasing of ICHB charging ratio, the location of cohesive zone was shifted down gradually and its thickness was the narrowest at the ICHB charging ratio of 10%, which was beneficial to BF smelting. Meanwhile, the dripping ratio of mixed burden also achieved the maximum value of 66.71% when the ICHB charging ratio was 10%. However, the excessive ICHB charging would promote the precipitation of Ti(C,N) with a high melting point at the interface between metal and slag, which resulted in the deterioration of dripping and further worsening the gas permeability of mixed burdens. Comprehensively considering the softening-melting-dripping behavior and the permeability of mixed burden, and the precipitation of Ti(C,N), the recommended ICHB charging ratio was 10%.
  • Hydrogen Absorption Rate into Fe with Rust Layer Containing NaCl during Atmospheric Corrosion in Humidity-controlled Air

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    DOI:10.2355/isijinternational.ISIJINT-2020-018

    The research aimed to detect the rate of hydrogen absorption into Fe with rust layer during atmospheric corrosion in humidity-controlled air, and to realize the effect of relative humidity (RH) on hydrogen absorption rate. One side of an Fe plate specimen was covered by electrochemical Ni plate and the other side was covered with rust layer containing NaCl. The specimen was set between the double cells for electrochemical hydrogen permeation test. The cell for hydrogen detection was filled with 1 kmol·m-3 NaOH solution and the Ni side of the specimen was subjected to 0 VAg/AgCl in the solution. The cell for hydrogen absorption was filled with the air with a controlled RH to make the rust layer side corrode. During the corrosion, a hydrogen absorption current and an RH were continuously monitored. In the tests, the following results were obtained. In the region of RH between 42 and 74%, a hydrogen absorption rate increased with an increase in an RH. At an RH of 80%, a hydrogen absorption rate suddenly decreased. In the region of RH between 80 to 95%, a hydrogen absorption rate again increased with an increase in an RH. The pH in the rust layers during the corrosion under the tested RH range was estimated to be 4.2 and 4.3, slightly acidic.
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  • Hydrogen-assisted Crack Propagation in Pre-strained Twinning-induced Plasticity Steel: from Initiation at a Small Defect to Failure

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    DOI:10.2355/isijinternational.ISIJINT-2019-510

    Hydrogen-assisted crack growth of pre-strained twinning-induced plasticity (TWIP) steel was investigated using artificial defects (micro-drilled holes), which acted as artificial crack initiation sites. Hydrogen was introduced into the specimens by electrochemical hydrogen charging during slow strain rate tensile test. The quasi-cleavage crack propagation observed was due to repeated crack initiation near the crack tip and subsequent coalescence. Crack initiation near the crack tip occurred after plastic deformation of the crack tip, and pre-straining facilitated plasticity-driven crack initiation. The early stage of plasticity-driven crack growth was sensitive to the crack length and remote stress level. Accordingly, the crack growth rate in the early stage increased with the increase in the initial defect size. In the following stage of the crack growth, the crack growth rate exhibited a complicated trend with respect to the crack length, which is possibly due to the plastic-wake-altered stress field around the crack tip, which depends on the initial defect size.
  • Development of a Shape Meter Employing the LED Dot Pattern Projection Method for a Hot Strip Finishing Mill

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    DOI:10.2355/isijinternational.ISIJINT-2019-718

    In recent years, to improve the fuel efficiency of automobiles by reducing their weight while maintaining their strength, smaller-thickness and higher-strength steel sheets tends to be used as automobiles' construction materials. For stable and accurate production of these sheets, it is crucial for them to be flattened through the hot strip rolling process. Therefore, to realize accurate automatic flatness control (AFC), a new shape meter that employed the light-emitting diode (LED) dot pattern projection method was developed. This consists of an LED dot pattern projector that can project the staggered periodic dot pattern, made of 1200 power LED chips, on the rolled strip and area camera that captures the image of the projected pattern. Then, instantaneous strip flatness is measured to analyze the pattern pitch correlative with inclination angle. The shape meter was installed at the hot strip finishing mill's exit, and its measurement accuracy and stability were evaluated. As a result, its inclination angle measurement error was within 0.45 degrees (two sigma) when compared to the set angle of the standard target, and the measured flatness of the rolling strip was consistent with the visually observed one. Its measurement success rate per entire coil was above 98.5%. These results indicated that the developed shape meter could be applied to the AFC. In addition, applying the measured flatness to the AFC of the work roll bender and leveling, it was confirmed that the strip flatness was improved in a short time.
  • Review on the High-Temperature Thermophysical Properties of Continuous Casting Mold Fluxes for Highly Alloyed Steels

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    DOI:10.2355/isijinternational.ISIJINT-2019-522

    Several recently developed highly alloyed steel grades have shown unsurpassed performance in terms of physical, chemical, and electromagnetic properties. However, broader commercialization of these steels has been hampered by limitations in mold flux performance. Newly developed steels containing considerable amounts of dissolved Al, Mn, and Ti actively react with typical CaO-SiO2-based mold fluxes, which severely changes the composition and subsequently the thermophysical properties of the mold flux that determine the external and internal quality of the as-cast steels. These dynamic changes result in nonuniform heat transfer, lubrication issues, surface defects, and caster breakouts. This work critically assesses the current status of the high-temperature thermophysical properties of CaO-SiO2-based and CaO-Al2O3-based mold fluxes intended for use in casting highly alloyed steel grades. Thermophysical properties, including viscosity, crystallization, thermal conductivity, and heat flux, have been evaluated. The effect of compositional variables including CaO/SiO2, CaO/Al2O3, and Al2O3/SiO2 mass ratios and the additions of CaF2, B2O3, Li2O, K2O, Na2O, TiO2, and BaO on these high-temperature thermophysical properties are discussed.

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