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

ISIJ International Vol. 55 (2015), No. 10

  • Softening Behaviors of Acid and Olivine Fluxed Iron Ore Pellets in the Cohesive Zone of a Blast Furnace

    pp. 2039-2046

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    DOI:10.2355/isijinternational.ISIJINT-2015-023

    A ferrous burden loses its permeability in the cohesive zone of a Blast Furnace (BF) which has an effect on the flow of reducing gases. Iron ore pellets with various chemical compositions have different softening properties. Due to the occurrence of numerous simultaneous phenomena the clarification of different variables is difficult. In this study the effect of Reduction Degree (RD) on the softening behavior of individual acid and olivine fluxed iron ore pellets was experimentally tested under inert conditions. The acid pellet softened rapidly at 1150°C and reached about 40% contraction at 1200°C. The olivine fluxed pellet softened gradually in the range of 1150 and 1350°C and reached 30–35% contraction. The RDs of 50–70% for acid and 50–65% for olivine fluxed pellet had no significant effect on the softening behavior. However, the highest contraction-% was reached with the lowest RD. The results indicate that softening of the pellets is caused by the softening of the pellet core. The early softening of the acid pellet was attributed to high SiO2 content and formation of fayalite slag with high wüstite solubility. The superior properties of the olivine fluxed pellet were attributed to the low SiO2 content and favorable effects of fluxes to prevent wüstite dissolution. FactSage V6.4 -software and its FToxid-database was used to compute the phase equilibrium of a pre-reduced pellet with a quaternary FeO–SiO2–CaO–MgO system in the core region. The computed phase equilibrium provided additional information about the effects of different components in the phase equilibrium.
  • Operation Windows of the Oxygen Blast Furnace with Top Gas Recycling

    pp. 2047-2055

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    DOI:10.2355/isijinternational.ISIJINT-2015-083

    Due to global concern about climate change, efforts in the steel industry to decrease emissions must be introduced, as this industrial sector gives rise to 5–7% of the anthropogenic CO2 emissions. There are several possibilities to achieve this goal: Making the processes more energy efficient, changing to renewable energy sources, by introducing process modifications or completely new processes, etc. In this paper the concept of oxygen blast furnace, where pure oxygen is used as blast combined with recycling of CO2-stripped top gas, is studied numerically. In the analysis, special attention is paid to the amount of recycled top gas and how the gas should be divided between injection to the lower (hearth) and upper (shaft) tuyeres. The results shed light on the feasible operation window, under the given process constraints, for the oxygen blast furnace and also how different combinations of hearth and shaft gas injections are reflected in coke rate, oxygen rate and top gas composition, as well as in the emissions. The issue whether the recycled top gas must be heated in regenerative heat exchanges is also addressed by considering the alternative where the recycled gas to the lower tuyeres is injected cold, while the shaft gas is elevated to the reserve zone temperature by partial combustion. The findings of the study are expected to be useful for assessing the feasibility of operating the blast furnace under high top gas recycling rates.
  • Carburization, Melting and Dripping of Iron through Coke Bed

    pp. 2056-2063

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    DOI:10.2355/isijinternational.ISIJINT-2015-115

    The carburizing, melting, and dripping behavior of iron through graphite and coke beds was investigated by an in-situ observation technique with increasing temperature from room temperature to 1773 K. An iron sample placed on the graphite bed was fully melted at 1490 K, whereas that on the coke bed was not melted at 1773 K. From the microscopic analysis of the sample after experiments, it was revealed that solid ash formed at the metal-coke interface prevented the carburization and melting of the iron sample. In order to examine the role of gangue and flux materials in sinter, iron samples mixed with slag powders at different weight fractions (15, 30, and 50 wt%) were prepared and similar experiments were carried out. The sample containing 15 wt% slag powders was not melted, but the samples containing 30 and 50 wt% slag powders were carburized and fully melted at 1773 K after holding the sample at that temperature for 342 and 37 s, respectively. It was considered that the liquid slag absorbed solid ash formed at the coke surface, yielding carburization and melting of the iron.
  • Simultaneously Overdeveloped Central and Peripheral Gas Flows of a Blast Furnace

    pp. 2064-2073

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    DOI:10.2355/isijinternational.ISIJINT-2015-161

    The gas flow distribution in the radial direction of the blast furnace is of vital importance for its stable and economical operation.
    In this study, an experimental apparatus, simulating bell-less top charging, was built on a 1/15 scale of 5800 m3 blast furnace in Shasteel. Experimental results shows that the charging mode designed for the simultaneously overdeveloped central and peripheral gas flow leads to a thin coke slit in the intermediate part and a large center coke column in the model furnace. The intermediate coke slit is more thinner under higher U/Umf, larger O/C and deeper stock line with the existence of coke collapse; and in the radial direction, the largest fluctuation of gas velocity exists at the center-adjacent point, the interface of center coke column and ore layer, where the small particle size coke gets fluidized, which coincides with the analysis of above-burden temperature fluctuation and observation from top camera during blast furnace operation. In combination with the actual online operation data, it is further revealed that the burden layer with locally-narrowed coke slit at the cohesive zone hinders the smooth gas flowing, resulting in a big fluctuation of gas flow at the bosh part and an unsmooth burden descending.
    Optimization of charging pattern was done in the cold model and applied to the 5800 m3 blast furnace. As a conclusion, optimization of coke charging mode to increase the intermediate thin part of coke layer is the most effective way to pursue the stable operation with high productivity.
  • Elimination Behaviors of NOx in the Sintering Process with Flue Gas Recirculation

    pp. 2074-2081

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    DOI:10.2355/isijinternational.ISIJINT-2015-180

    A novel sintering process is proposed based on part of flue gas is reused by reintroducing the flue gas into sintering bed, which is of great significance to cleaner production of sinter because it can reduce significantly the emission of exhaust gas. The reaction behaviors of NOx during flue gas recirculation (FGR) process are researched. The elimination behavior of NOx in sinter zone is NO–CO reaction on the catalysis of sinter, and the degradation rate of NOx reaches a maximum at 700°C. The NOx formation supervenes with coke combustion in combustion zone. The NOx formation decreases due to the significantly reduced O2, increased CO2, and increased NO of FGR gas. The combustion atmosphere is the key point on the NOx elimination since the fuel-N decomposition intermediate products could transform to NO or reduce NO. Moreover, the NO-carbon reaction conducting in combustion zone contributes to the NOx elimination further. Therefore, it can significantly reduce the NOx emission of sintering process when the FGR technology is taken in the sintering process.
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  • Reduction Behaviour of Self-reducing Blends of In-plant Fines in Inert Atmosphere

    pp. 2082-2089

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    DOI:10.2355/isijinternational.ISIJINT-2015-182

    Large amount of dust and sludge recovered during cleaning of iron and steel making process gases are annually put on landfill or intermediate storage. These by-products have high contents of iron (Fe) and carbon (C) that potentially could be utilized in the steel industry. However, due to the presence of impurity compounds as well as the unsuitable physical properties, these by-products cannot be recycled directly. The main objective of the present study is to investigate the possibilities to recover the valuable components Fe and C in these by-products and thereby decrease the need of landfills at the steel plants as well as reduce the consumption of virgin materials, including fossil coal, and reduce CO2 emissions. A recycling route has been investigated by means of laboratory trials and FactSage thermodynamic modeling. Four different blends of BF and BOF dusts and sludges are prepared in predetermined ratios. Reduction behavior of each blend is studied using TG/DTA/QMS and in-situ high temperature X-ray diffraction. High temperature physical properties like softening, swelling and melting are also investigated by means of heating microscope. The obtained results indicate the feasibility of both minimizing the impurity elements as well as recovering of valuable components.
  • Liquidus Lines in the Fe-rich Region of the FeOx–CaO–SiO2–Al2O3 System at 1573 K under Oxygen Partial Pressures between 10−6 and 10−2 atm

    pp. 2090-2097

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    DOI:10.2355/isijinternational.ISIJINT-2015-229

    The performance of iron ore sinter in a blast furnace such as strength and reducibility is strongly affected by the amount and the chemical composition of liquid phase in sinter during the sintering process. Thus, it is necessary to control the production of liquid phase during the sintering process. Iron ore sintering process consists of rapid heating of pulverized raw materials, partial reduction reaction of iron ores as well as liquid phase production, which are caused by combustion of cokes and suction of flaming gas. The heating time of raw materials is too short that the chemical reactions within the sinter never reach the thermodynamic equilibriums. Nevertheless, controlling the melt production could be attempted based on the thermodynamic data such as the phase diagrams. In this study, liquidus lines coexisting with iron oxides and/or 2CaOSiO2 in the FeOx–CaO–SiO2 system have been measured at 1573 K in the range of oxygen partial pressures between 10−6 atm and 10−2 atm. In addition, the effect of Al2O3 content on the liquidus has also been investigated as Al2O3 is a major gangue component affecting the quality of sinter. It has been found that the homogenous liquid region extends over a wide range of CaO/SiO2 (C/S) ratio between 0.6 and 2 or even more for all the measurement oxygen partial pressures. It has also been found that the FeOx content in liquid phase at C/S = 1.0 in equilibrium with solid FeOx is almost constant to be 40–45 mass% irrespective of oxygen partial pressures. However, the FeOx content decreases with an increase in Al2O3 content, indicating that the homogenous liquid region in the FeOx-rich side becomes narrower. As a consequence, it is considered that the amount of slag melt produced in sinter during the sintering process becomes smaller with increasing Al2O3 content.
  • Softening, Melting, and Permeation Phenomena of CaO–FeO–SiO2 Oxide on a Coke Bed

    pp. 2098-2104

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    DOI:10.2355/isijinternational.ISIJINT-2015-269

    For the low carbon operation of blast furnace, it is necessary to maintain the gas permeability in the furnace at low coke rate. The pressure drop in the cohesive zone of the blast furnace is significantly high, and the gas permeability of the cohesive zone has an influence on the productivity of the iron-making process. The gas permeability increases as the thickness of the cohesive zone decreases. The present study aimed to decrease the thickness of the cohesive zone by achieving a higher softening temperature and a lower temperature for the dripping of the sinter ore. The softening, melting, and permeation processes of CaO–FeO–SiO2 oxide on a coke bed was investigated. The change in the shape of the oxide tablet, the formation of the liquid droplet, and its permeation into the coke bed were observed with increasing temperature in several CO/CO2 atmospheres. Softening of the oxide was observed at a temperature that was slightly above the solidus temperature. The deformation of the oxide tablets was strongly affected by the liquid phase ratio. Furthermore, there was no relationship between the temperature of permeation and the melting of the oxide tablet. The permeation of the oxide melt into the coke bed is dependent on the wettability between them.
  • Predictcion of Next-Generation Ironmaking Process Based on Oxygen Blast Furnace Suitable for CO2 Mitigation and Energy Flexibility

    pp. 2105-2114

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    DOI:10.2355/isijinternational.ISIJINT-2015-264

    In the integrated steel works, huge quantities of coal and other fossil materials are consumed as reducing agents and energy resources. The steel industry must now deeply reexamine its utilization of carbon and energy from the viewpoints of global warming and energy security. Against this background, this paper focuses on the progressive design of an ambitious blast furnace for the future.Several blast furnace processes including the current blast furnace and the oxygen blast furnace were examined by using a material and energy balance model of the integrated steel works. First, the current blast furnace was evaluated considering expanded use of hydrogen-rich injectants. Next, the oxygen blast furnace with top gas recycling was examined, and the characteristics of the carbon and energy balances were clarified from the viewpoints of CO2 mitigation and the energy balance in the steel works as a whole. Although the results confirmed that CO2 emissions can be reduced by intensifying top gas recycling, the energy supply to the downstream processes became seriously insufficient.Then, the applicability of an oxygen blast furnace with a high injection rate of a hydrogen-rich gas such as natural gas instead of top gas injection was evaluated. This process, thanks to the intensified hydrogen reduction, enables the CO2 mitigation while maintaining the energy balance in the steel works. Based on the evaluation, the concept of the advanced oxygen blast furnace as a next-generation low carbon blast furnace with high energy flexibility was proposed.
  • Inclusion Evolution during Modification of Alumina Inclusions by Calcium in Liquid Steel and Deformation during Hot Rolling Process

    pp. 2115-2124

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    DOI:10.2355/isijinternational.ISIJINT-2015-064

    The formation mechanism of Al2O3–CaS based inclusions in Al-killed steel was studied by industrial trials and thermodynamic calculations of various calcium aluminates and the precipitated CaS. The chemical activities of CaO and Al2O3 of various calcium aluminates were expressed as a function of CaO and Al2O3 content of CaO–Al2O3 inclusions respectively by the third liner fitting equation obtained from different researchers. Massive Al2O3–CaS based inclusions were generated with a decrease of temperature of molten steel and subsequent solute segregation during casting. Based on the shapes of CaS layer, there are two formulations to consider: 1) the crescent-shaped CaS layer surrounded calcium aluminates was generated by reaction between dissolved Al, S and modified calcium aluminates or diffusion of S dissolved in inclusions; 2) the oval shape layer associated with calcium aluminates was generated by collision or adsorption of calcium aluminates and the precipitated CaS. In addition, the deformation mechanism of inclusions with different modification extents and sizes during hot rolling was proposed.
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  • A Comprehensive Static Model of an Iron Bath Smelting Reduction Process with Thick Slag for Alumina-Rich Iron Ore

    pp. 2125-2134

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    DOI:10.2355/isijinternational.ISIJINT-2015-255

    In order to make comprehensive utilization of alumina-rich iron ore, a two-step three-vessel smelting reduction process is proposed, including the Rotary Hearth Furnace (RHF) for the pre-reduction of alumina-rich iron oxide pellet, the Smelting Reduction Vessel (SRV) with a thick slag bath for the final reduction and melting of pellet, and the Gas Reforming Furnace (GRF) for reforming the exhaust gas from SRV. A three-level lances arrangement is designed to improve the energy utilization and production condition of the SRV. An overall process model is established for the whole process to calculate the mass and heat consumptions of all sections of the process, and to investigate the effect of operation conditions on the process performance. With 80% heat transfer efficiency of post combustion, the recommended post combustion ratio (PCR) of the SRV gas is 55% and the pellet metallization rate (PMR) is 80%. A zoned model of the SRV is developed to calculate the mass and heat balances of each reaction zones and especially to optimize the operation conditions. The recommended PCRs in the upper slag and lower slag are 15% and 0% respectively. The coal-only injection method is recommended in the lower slag.
  • Evaluation of Defect Distribution in Continuously-Cast Slabs by Using Ultrasonic Defect Detection System and Effect of Electromagnetic Brake on Decreasing Unbalanced Flow in Mold

    pp. 2135-2141

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    DOI:10.2355/isijinternational.ISIJINT-2015-149

    In order to clarify the suppression mechanism of an unbalanced flow in the continuous casting mold under high molten steel throughput conditions, the relationship between the unbalanced flow and the distribution of defects entrapped on the solidified shell was measured by ultrasonic defect detection. The relationship between the unbalanced flow and the defect distribution in the mold with an electromagnetic brake was also investigated. The main results are summarized as follows.
    (1) By using a new ultrasonic defect detection system, accurate measurement of defects was achieved for the defects’ diameter of more than 0.6 mm in the range 2–10 mm from surface.
    (2) The effect of the electromagnetic brake on decreasing molten steel momentum was measured. The measured value was consistent with the calculated value. Molten steel momentum could be reduced by more than 50% when the Stuart number (N) was more than 3.5 under molten steel throughput conditions of 4.7 and 5.3 ton/min.
    (3) The position of entrapped defects on the solidified shell in the mold was influenced by the molten steel throughput condition and the magnetic flux density of the electromagnetic brake.
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  • Development and Application of Final Permanent Magnet Stirring during Continuous Casting of High Carbon Rectangular Billet

    pp. 2142-2149

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    DOI:10.2355/isijinternational.ISIJINT-2015-183

    Production of continuously cast high carbon steel that very low center macrosegregation is an important object in meeting high quality requirements. Apart from the widely known methods of reducing macrosegregation during continuous casting, final permanent magnet stirring (FPMS) provides an alternative for producing steel with low macrosegregation. The permanent magnet stirring featuring with low power dissipation and high intensity of magnetic field, which is suitable for final stirring conditions. In the present research, a three-dimensional unsteady coupled mathematical model based on the electromagnetic field analysis software Opera-3D and the flow field software Fluent has been developed to analysis the magnetic flux density, the electromagnetic force and the molten steel flow under different stirring frequencies of FPMS. The calculated results in the residual liquid pool show that the center magnetic flux density is nearly invariable equals to 1335 Gs, the maximum electromagnetic force increases from 12 kN/m3 to 59 kN/m3 and the maximum tangential velocity in the width direction various form 0.09 m/s to 0.45 m/s as the stirring frequency increases from 1 Hz to 5 Hz. The industrial trials reveal that the FPMS with a stirring frequency of 5 Hz can effectively decrease the centre carbon segregation and improve the final product quality of high carbon steel.
  • Investigation on Thermo-mechanical Behavior of Mold Corner for Continuous Casting Slab

    pp. 2150-2157

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    DOI:10.2355/isijinternational.ISIJINT-2015-230

    Corner cracks in slab and billet have always been the important issues during continuous casting. For the purpose of investigating heat transfer, stress and distortion behaviors of corner regions for the slab continuous casting mold, a full-scale stress model of mold which integrated finite-element method with inverse heat transfer algorithm was developed. An inverse algorithm was applied to calculate the heat flux through the measured temperatures from thermocouples buried inside mold plates. Based on this, a full-scale finite-element stress model, including four copper plates, nickel layer and water slots of different depths, was built to reveal the complex thermo-mechanical behavior of mold corners. The results show that the corner regions generate large stress and obvious stress concentration. The mold bends toward the molten steel and it appears V-shaped distortion in the corner region. And the area of water slots decrease after distortion, but it would not apparently affect heat transfer. The cooling effect could be optimized by changing the position, depth and angle of sloped water slots, which helps to flexibly improve the solidification behavior of slab corner.
  • Characterization of Evolution of Microscopic Stress and Strain in High-Manganese Twinning-Induced Plasticity Steel

    pp. 2158-2165

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    DOI:10.2355/isijinternational.ISIJINT-2014-378

    Electron backscatter diffraction was used to observe the microstructure of an austenitic high-manganese twinning-induced plasticity steel and investigate the crystal orientation of grains in this steel. The results showed that mechanical twins are formed in a grain with a high Schmid factor during the tensile test. The orientation data obtained were used to estimate the anisotropic elasticity of the grains in the steel. The microscopic stress and strain evolved in the microstructure of the steel unloaded after plastic deformation were estimated using finite element method simulation in which the elastic anisotropy of the steel was taken into account. The simulation indicated that the evolution of microscopic stress and strain in the microstructure is considerably influenced by the crystal orientation of the grains. Furthermore, white X-ray diffraction with microbeam synchrotron radiation was used to characterize the evolution of microscopic stress and strain in the grains of the steel. The stress analysis during white X-ray diffraction indicated the formation of residual microscopic stress after tensile deformation, which was found to be distributed heterogeneously in the steel. It was also shown that the direction of the maximum principal stresses at different points in the microstructure under loading were mostly oriented along the tensile direction. These results are fairly consistent with the results obtained by the simulation, although absolute values of the real principal stresses may be influenced by the heterogeneously evolved strain and the several assumptions used in the simulation.
  • New Post-combustion Ultraviolet Fluorescence Spectroscopy for Precise Determination of Trace Sulfur in Steel

    pp. 2166-2172

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    DOI:10.2355/isijinternational.ISIJINT-2015-108

    We have developed an analytical method which enables highly precise and rapid quantitative analysis of ultra low sulfur content in steel samples. This development has been termed the “post-combustion ultraviolet fluorescence method”, which combines a high frequency induction furnace with a continuous UV fluorescence analyzer. Despite the same ease of operation as the conventional IR method, the newly developed method showed high sensitivity and good precision. The quantitation limit of sulfur in steel was 0.5 mass ppm. In addition, it was shown that this equipment has sufficient stability for use as a process control analysis apparatus in continuous operation.
  • Evaluation of Inclusion Characteristics in Low-Alloyed Steels by Mainly Using PDA/OES Method

    pp. 2173-2181

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    DOI:10.2355/isijinternational.ISIJINT-2015-172

    The characteristics of non-metallic inclusions (such as number, size and volume fraction) in liquid steel samples taken during ladle treatment and casting of industrial heats of two low-alloyed Ca-treated steel grades were evaluated by using the Pulse Distribution Analysis with Optical Emission Spectroscopy (PDA/OES) method. These results were compared to data obtained by Scanning Electron Microscope observations of inclusions after electrolytic extraction from steel samples (the EE method). It was found that the PDA/OES method can be used for a relative estimation of the homogeneity of the distribution of non-metallic inclusions in steel samples. Bottom and middle parts of the steel samples showed more homogeneous results with respect to the characteristics of the investigated Al2O3, CaO–Al2O3 and CaO–Al2O3–CaS inclusions. The numbers of inclusions in the size ranges 2.0–5.7 µm and 1.4–5.7 µm in samples taken before and after a Ca addition, respectively, showed a relatively good agreement between both methods. Furthermore, the calculated volume fractions for inclusions in the size range 2–13 µm obtained by the PDA/OES method agreed satisfactorily well with those obtained from the EE method. Finally, the minimum sizes of inclusions in steel samples, which can reliably be detected by the PDA/OES method, were estimated for steels with different concentrations of Al in steel and Al2O3 in inclusions.
  • Delayed Cracking of Metastable Austenitic Stainless Steels after Deep Drawing

    pp. 2182-2188

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    DOI:10.2355/isijinternational.ISIJINT-2015-078

    Certain metastable austenitic stainless steels, especially low-nickel grades, can be susceptible to delayed cracking after forming operations. Small amount of hydrogen is always present in stainless steels as an inevitable impurity. Delayed cracking results from a time-dependent hydrogen redistribution process driven by stress, either applied or residual. In this study, delayed cracking susceptibility of three metastable austenitic stainless steels 301, 301LN and 304 after deep drawing was examined by Swift cup tests. The objective was to clarify the role of alloy composition, strain-induced α’-martensite and residual stresses in delayed cracking. Hydrogen content of the stainless steels, tested in as-produced condition, was < 3 wppm. Detailed characterization of the Swift cups was performed using X-ray diffraction, magnetic permeability measurement, nanoindentation, FEG-SEM and EBSD. Stainless steel 304 with the highest Ni content was not susceptible to delayed cracking regardless of the presence of α’-martensite. Stainless steel 301LN with lower Ni content and lower austenite stability showed minor cracking at the highest drawing ratio. Stainless steel 301 was the most susceptible material, and had the highest residual stresses after deep drawing. Residual stress level in the stainless steels was found to be affected by the amount of α’-martensite and also by the hardness of the phases, both of which depend on chemical composition. Fracture mechanism in delayed cracking was predominantly transgranular quasi-cleavage and crack propagation occurred through α’-martensite. High residual stresses and presence of α’-martensite are essential in delayed cracking of austenitic stainless steels.
  • Creep-Rupture Life Prediction for 9Cr-1Mo-Nb-V Weld Metal

    pp. 2189-2197

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    DOI:10.2355/isijinternational.ISIJINT-2014-783

    Creep tests were carried out with shielded-metal-arc weld metal for 9Cr-1Mo-Nb-V steel. The testing temperature and stress levels were chosen so that rupture would occur around 1000 hours. The three creep-rupture life prediction methodologies, namely the Monkman-Grant law, the modified θ projection, and the Ω method, were applied to each experimental creep curve, and the constant numbers for individual equations were obtained. All these three were proven to be practical for weld metals similar to the base metal. However, the Monkman-Grant law has been proven to be the most effective when predicting creep rupture life at an early stage of creep test. The equation, obtained in this study, was relevant also when the Mn+Ni content varied in 9Cr-1Mo-Nb-V weld metal.
  • Crystallographic Analysis of Lath Martensite in Ferrite-Martensite Dual Phase Steel Sheet Annealed after Cold-Rolling

    pp. 2198-2205

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    DOI:10.2355/isijinternational.ISIJINT-2014-641

    Cold-rolled and annealed ferrite-martensite dual phase (DP) steel sheets are useful for automotive applications because of their excellent balance of strength and elongation. Although martensite in ferrite-martensite DP steels has important roll in the mechanical properties, the crystallography and microstructure of lath martensite in DP steels have not been investigated in detail. In this paper, the crystallography and microstructure of lath martensite in a ferrite-martensite DP steel were studied using electron microscopy and electron diffraction analysis, and the crystallographic orientation relationship between the ferrite and martensite was analyzed. The main results are as follows: (1) The lath martensite in DP steels consists of some packets, each of which contains several blocks. This structure is the same as that of lath martensite in fully martensitic steels. (2) The common parallel close-packed plane of the martensite laths in a packet tend to be parallel to the close-packed plane of the adjacent ferrite grain, whose fraction is about 30%.
  • Effect of the Cr Content and Coiling Temperature on the Properties of Hot Rolled High Strength Lower Bainitic Steel

    pp. 2206-2211

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    DOI:10.2355/isijinternational.ISIJINT-2015-086

    The effect of the Cr content and the coiling temperature on the tensile properties and the micostructure of an ultra high strength hot rolled lower bainitic steel has been evaluated by lab simulations. It was shown that an increase in Cr reduces the yield strength, but increases the tensile strength and the elongation. This observation was made for both coiling temperatures (425°C and 500°C) tested. For coiling at 500°C, the effect was directly attributed to the presence of an increasing fraction of secondary phases, mainly martensite-austenite islands. At low coiling temperature (425°C), a banded structure of granular and lower bainite occured. The granular bainite bands contained more martensite-austenite islands for the high Cr level, explaining the low yield strength, high tensile strength and high elongation. It was shown that the lower bainite forms prior to coiling, whereas the granular bainite forms during slow coil cooling. The yield and the tensile strength is higher for coiling at 425°C compared to 500°C as a consequence of a higher lower bainite fraction.
  • Correlation between Recrystallization Texture and Heterogeneities in Deformed Structure of an Electrical Steel by Electron Back-scatter Diffraction

    pp. 2212-2216

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    DOI:10.2355/isijinternational.ISIJINT-2015-159

    The correlation of recrystallization nucleation and the deformation microstructure in an electrical steel was studied by examining micro-indenter marked specimens with electron back-scatter diffraction (EBSD). The deformed grains were classified according to the density of high-angle boundaries in the grain interiors, and the local misorientation of substructure was characterized by using both the kernel average misorientation (KAM) and the high misorientation region (HMR) analyses. The EBSD observation confirmed that recrystallization was preferentially evolved from areas having high local misorientation revealed by HMRs. The recrystallization nuclei in partially recrystallized specimen were observed to be directly related to the high-KAM areas in the deformed grains. The orientation distribution of the areas having high local misorientation in the cold-rolled structure, which fulfill both criteria of high-KAM and HMR, was found to be associated with the texture of recrystallized grains.
  • Effect of Nb Microalloying on Reversion and Grain Growth in a High-Mn 204Cu Austenitic Stainless Steel

    pp. 2217-2224

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    DOI:10.2355/isijinternational.ISIJINT-2015-156

    Highly refined grain size can be obtained in metastable austenitic stainless steels by martensitic reversion treatment, but the annealing must be controlled due to the high coarsening tendency of ultrafine grains. The effect of Nb microalloying on reversion and retardation of grain growth has been investigated in an austenitic 204Cu stainless steel containing Nb from 0 to 0.45 wt.%. Hot-rolled samples were cold rolled to a 60% reduction, leading to the formation of 47–60% strain-induced martensite, and subsequently annealed at various temperatures between 973 and 1373 K up to 1000 s. Grain size and precipitate structures were examined with optical, scanning and transmission electron microscopes. In the Nb-bearing steels, precipitate particles were found to retard the reversion. The reversion could be completed even at the low temperature of 973 K, but at 973–1073 K the structure remained non-uniform consisting of fine reversed austenite grains and coarser grains with different degree of recrystallization. More uniform grain size in the range of 2–9 µm was obtained by reversion treatment at 1173–1373 K for 1 s, the grain size decreasing with increasing Nb content. Grain growth was effectively retarded by 0.28 wt.% Nb alloying even at 1373 K and by 0.11 wt.% Nb at 1273 K. The activation energy for grain growth increases with increasing Nb content from 363 to 458 kJ/mol. Zener’s model and the flexible boundary model for the driving and pinning forces, respectively, can explain the observed retardation of grain growth.
  • Effect of Ni, Mn, V, and Al on Toughness of Blade Steels

    pp. 2225-2228

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    DOI:10.2355/isijinternational.ISIJINT-2014-606

    In modern high carbon steels, Mn alloying binds S, and usually Al or V precipitates prevent grain growth. Primitive steels do not contain these alloying elements, but they may contain Ni, obtained from iron meteorites. Totally unalloyed carbon steel (0.7C–0.02Mn–0.01S–0.01P) and Ni steel (0.7C–4Ni–0.03Mn–0.01S–0.01P) were made in a laboratory foundry and their properties were compared with modern high carbon steels in a bladesmith forge. The unalloyed carbon steel suffered grain growth. Therefore, after low temperature tempering it had exceptional poor toughness, but after higher 250°C tempering it was as tough as equally hard (57 HRC) modern fine grained carbon steel. The Ni alloying increased toughness in the case of low temperature tempering, but it also reduced attainable hardness. The studied Ni steel was equal to modern carbon steels up to 62 HRC. The study showed that Mn, Al, or V alloying is not mandatory for the best quality blade steels.
  • Characterization of Microstructures and Mechanical Properties of Cold-rolled Medium-Mn Steels with Different Annealing Processes

    pp. 2229-2236

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    DOI:10.2355/isijinternational.ISIJINT-2015-187

    The cold rolled medium-Mn steel was processed by two different heat treatments (intercritical annealing and quenching+intercrictical annealing). The microstructure evolution was characterized by scanning electron microscopy, electron back scattered diffraction and transmission electron microscope. It was found that austenite reverted transformation took place during annealing processes. Blocky ultrafine ferrite/austenite duplex structure was formed after intercritical annealing, while lath ultrafine duplex structure was developed after quenching+intercritical annealing. Such ultrafine duplex structure has both high strength and high plasticity. The excellent properties and tensile behavior were related to the phase transformation from retained austenite to martensite, which was associated with the carbon content, the morphology of the retained austenite and the matrix microstructure in the steel with different processes.
  • Relationship between Microstructure, Mechanical Properties and Damage Mechanisms in High Martensite Fraction Dual Phase Steels

    pp. 2237-2246

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    DOI:10.2355/isijinternational.ISIJINT-2015-186

    The relationships between microstructure, mechanical properties and damage mechanisms in as-quenched and in quenched-and-tempered high martensite fraction (>60%) dual phase (DP) steels were investigated. The mechanical behaviour was determined by tensile and hole expansion (HE) tests. In the as-quenched condition, the HE ratio decreased with increasing ferrite content and showed a non-linear inverse relation to the uniform elongation. Tempering significantly improved the HE ratio for all studied martensite fractions; the increase in HE was found to be monotonic for tempering temperatures between 230°C and 460°C, even though the yield stress dependence was complex in this range. Tempering studies showed that, at constant martensite fractions, there was a linear dependence between the HE ratio and the ductile fracture strain, εf. However, the parameters of the linear relation changed significantly when the martensite fraction was varied. The dominant damage mechanism in simple tensile tests evolved from ferrite/martensite or martensite/martensite interface decohesion in the as-quenched state to martensite/carbide interface decohesion after tempering. The damage mechanisms were qualitatively described using the Beremin local criteria.
  • Influence of Enthalpy Changes on the Temperature Dependency of the Viscosity of Pure Liquid Metals

    pp. 2247-2251

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    DOI:10.2355/isijinternational.ISIJINT-2015-214

    The temperature dependency of the viscosity of molten pure metals has been thermodynamically discussed as a function of the liquid–state enthalpy. The following equation has been derived:
    [ This part of text is omitted ]
    where η and H are the viscosity (mPa·s) and liquid–state enthalpy (J/mol) at a given temperature T (K), η0 and H0 are the viscosity (mPa·s) and liquid–state enthalpy (J/mol) at the melting point or at any other temperature T0 (K), higher than the melting point, C is a proportionality constant with a value of 0.986 (mPa·s), K is the constant coefficient, which is 1/4 (-) for pure metals, and R is the gas constant (J/mol/K). It has been found that the temperature dependency of various pure metals can be predicted using this equation.
  • Alkali Elution Behavior of Steelmaking Slag into Seawater by Continuous Stirred Tank Reactor

    pp. 2252-2257

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    DOI:10.2355/isijinternational.ISIJINT-2015-137

    In order to understand the alkali elution behavior of steelmaking slag under continuous flow, a kinetic study was carried out by continuous stirred tank reactor (CSTR). Tap water and seawater were used for measurements of circumferential velocity and temporal change in pH, respectively. Liquid was fed into the vessel and flowed constantly during the impeller rotation operation in the vessel. The effect of the various operating factors such as vessel inner diameter, slag size, slag composition, ratio of liquid volume to mass of slag, seawater feed rate, impeller height from the slag, impeller rotation speed etc. were investigated.
    The circumferential velocity in the vessel was not affected by seawater feed rate but by the impeller rotation speed. The alkali elution rate increased with the increase in the circumferential velocity. The alkali elution rate was not influenced by the ratio of liquid volume to mass of slag and impeller height from the slag surface. The alkali elution rate decreased with the decrease in diameter of slag particle below 0.01 m due to the insufficient penetration of seawater between slag particles. The alkali elution rate increased with the increase in seawater feed rate at the same circumferential velocity. The non-dimensional correlation equation of alkali elution rate was shown by Sh = 8.26×10−4Rep0.62 (Nτ)−0.37 where Sh: Sherwood number, Rep: Particle Reynolds number and (Nτ): the product of impeller rotation speed and residence time.
  • Heat Recovery Process from Packed Bed of Hot Slag Plates

    pp. 2258-2265

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    DOI:10.2355/isijinternational.ISIJINT-2015-169

    Reduction of CO2 emissions from the steelmaking process is strongly required for prevention of global warming. One promising heat resource that is estimated to have great potential for energy saving is the waste heat of steelmaking slag, which has a temperature of above 1673 K in the molten state. This molten slag can be solidified in a plate-like shape by feeding it on the surface of water-cooled rolls, and the heat of the plate-like slag can be recovered easily in spite of its low heat conductivity. When these hot slag plates are packed in a slag chamber, the heat of the slag can be recovered by heat exchange with a counter current gas flow. Because the efficiency of gas-slag heat transfer changes depending on the shape of the packed slag, it is difficult to estimate the efficiency of slag heat recovery without evaluating the accuracy of the heat transfer coefficient in the bed. In this work, the effect of the slag shape on the accuracy of the heat transfer equation was evaluated by conducting both laboratory-scale and pilot-scale slag heat recovery experiments and performing a fitting analysis by using a slag packed bed heat transfer simulation model. A comparison of the experimental results and calculation results confirmed that the heat transfer coefficient can be estimated by using Johnson-Rubesin’s equation modified by a correction factor in case the packed materials are plate-like. The effect of the correction factor on the efficiency of slag heat recovery at the industrial scale was also estimated.
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  • Wettability Effect on Terminal Velocity of a Single Solid Sphere Falling in a Vertical Pipe

    pp. 2266-2268

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    DOI:10.2355/isijinternational.ISIJINT-2015-010

    Water model experiments were carried out to understand the dynamic behavior of refining agents in a reactor pipe of a continuous refining process. The terminal velocity of a poorly-wetted single solid sphere falling in a vertical pipe was measured as a function of the diameter ratio, λ(=dp/D), where dp is the sphere diameter and D is the pipe diameter. The particle Reynolds number, Rep, was chosen to be greater than 1000. The results were compared with those for a wetted single sphere to make clear the wettability effect on the terminal velocity.
  • εγ Reverse Transformation-induced Hydrogen Desorption and Mn Effect on Hydrogen Uptake in Fe–Mn Binary Alloys

    pp. 2269-2271

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    DOI:10.2355/isijinternational.ISIJINT-2015-215

    The effects of ε-martensite and Mn on hydrogen uptake and desorption were investigated through cryogenic thermal desorption analysis. Increasing Mn content promoted hydrogen uptake, and reverse transformation from ε to γ phases induced hydrogen desorption.

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