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

ISIJ International Vol. 61 (2021), No. 5

  • CO Desorption and Absorption in Molten Steel: A Review

    Zhuangzhuang Liu, Peter Tom Jones, Bart Blanpain, Muxing Guo

    pp. 1337-1347

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    DOI:10.2355/isijinternational.ISIJINT-2020-509

    Abstract

    Previous work on the mechanism of carbon monoxide absorption and desorption from liquid steel/iron is reviewed. The experimental set-up employed in these studies is summarized and the characteristics of each methodology are discussed and compared. The reaction kinetics, particularly the rate-limiting step of the CO gas-molten steel/iron reaction is analysed with respect to experimental parameters, comprising temperature, CO partial pressure in the gas mixture, gas flow rate, crucible materials, and carbon and oxygen content in the steel/iron. To further understand the CO absorption and desorption mechanisms in liquid steel, suggestions for future work are provided.

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    2. Effect of Na Ions on Melt Structure and Viscosity of CaO–SiO2–Na2O by Molecular Dynamics Simulations ISIJ International Vol.61(2021), No.5
    3. Phase Relationship and Activities of Components in CaO–SiO2–Cr2O3 Ternary System at 1573 K ISIJ International Vol.61(2021), No.5

  • Effect of Viscosity on Dynamic Evolution of Metallurgy Slag Foaming

    Bo Zhang, Ruifang Wang, Chao Hu, Chengjun Liu, Maofa Jiang

    pp. 1348-1356

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    DOI:10.2355/isijinternational.ISIJINT-2020-404

    Abstract

    The viscosity is of high significance for the dynamic evolution of slag foaming in the metallurgical processes. Through physical modelling, the dynamic evolutions of slag foaming with different viscosities were analyzed by characterizing the foam structure. Furthermore, the effects of viscosity on the characteristic parameters of dynamic evolution were investigated. The experimental results indicated that the slag foaming types evolved along Partial Foaming I → Entire Foaming → Partial Foaming II with increasing superficial gas velocity, while the bubble shape of the top layers gradually transformed from the spherical to non-spherical shape. Moreover, the rise in the slag viscosity decreased the critical superficial gas velocity between the various slag foaming types by hindering the bubble shape transformation. Despite constant injection of gas, the “foam pseudo-decaying” phenomenon was observed after the liquid level of the foamed slag reached its maximum, and this phenomenon exacerbated under the higher viscosity conditions. Under the various superficial gas velocity conditions, the effects of viscosity on the characteristic parameters of the dynamic evolution presented a considerable difference. Under a lower superficial gas velocity, the viscosity had no obvious effect on the foaming and the decaying rates, whereas, under a higher superficial gas velocity, the foaming and the decaying rates decreased significantly with increasing viscosity. The foaming height reached the peak value when the slag foaming developed into the Entire Foaming, and this foaming height is basically consistent with the theoretical peak value. The superficial gas velocity required to reach this peak value decreased with increasing viscosity.

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    2. Change of State of Lime Phase and Inhibition of Hydration Reaction by Coexisting Oxides in Steelmaking Slag ISIJ International Vol.61(2021), No.5
    3. Effect of Na Ions on Melt Structure and Viscosity of CaO–SiO2–Na2O by Molecular Dynamics Simulations ISIJ International Vol.61(2021), No.5

  • On the CO Desorption and Absorption in Liquid Low-carbon Steel

    Zhuangzhuang Liu, Peter Tom Jones, Martin Kendall, Bart Blanpain, Muxing Guo

    pp. 1357-1362

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    DOI:10.2355/isijinternational.ISIJINT-2020-510

    Abstract

    CO absorption and desorption in liquid steel comprise highly significant reaction mechanisms for steelmaking operations such as decarburization, ladle degassing, and the production of rimming steel ingots. However, until present the difference in the mechanism of CO absorption versus desorption has not been clarified. In this study, the CO desorption and absorption experiments were performed by blowing Ar + CO (0% and 5% in volume fraction) gas mixture bubbles into liquid steel with low carbon content (12–19 ppm). The experimental data show that the rate of CO desorption is much lower than that of absorption. The carbon mass transfer in liquid steel is found to be the rate-limiting step with respect to CO absorption. For CO desorption, in addition to the carbon mass transfer, the interfacial reaction at the gas-liquid interface is found to pose an additional kinetic barrier. The present finding improves the understanding of the basic C–O reaction kinetics involved in many steelmaking processes and contributes to accurate modeling and precise control of industrial practices such as basic oxygen furnace (BOF) and argon oxygen decarburization (AOD).

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    2. Accurate Viscosity Prediction for Molten Slags: A New Model and Database ISIJ International Vol.61(2021), No.5
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  • Evaporation Behavior of Al and Sn from Ti-6242 Alloy Melt

    Hideo Mizukami, Tomoyuki Kitaura, Yoshihisa Shirai

    pp. 1363-1369

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    DOI:10.2355/isijinternational.ISIJINT-2020-517

    Abstract

    For multi-component titanium alloy ingots that contain evaporable solute elements, the evaporation behavior of these elements is very important for optimizing the casting conditions. This study examined the evaporation of Al and Sn from molten Ti-6242, an alloy used for aircraft engines, after partial melting in a small electron beam furnace. The amount of Al and Sn evaporation depended on the melting time, and there equilibrium vapor pressure of the elements and activity in the molten alloy. The concentration of Al changed in proportion to that of Sn in the molten alloy during evaporation process and the amount of evaporation of Al was twice as Sn. The rate control step of evaporation of Al and Sn were limited by evaporation from the molten alloy surface to the vacuum phase. The liquidus temperature of the alloy melt also changed with the evaporation of Al and Sn. These findings will help predict the amount of solute evaporation from molten titanium alloys and maintain the surface and internal quality of the ingots.

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  • Surface Structures of CaFe2O4 (001), (100), (110) and (111): A Density Functional Theory Study

    Zhigang Que

    pp. 1370-1378

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    DOI:10.2355/isijinternational.ISIJINT-2020-541

    Abstract

    Calcium ferrite is the main binder phase in sinter, which affects the physical and metallurgical properties of sinter. It was also widely applied to photocatalysts, oxidation catalysts, photocathodes and gas sensors etc. The electronic, magnetic and chemical properties of it were investigated since its crystal structure was first reported. However, the right terminations, structures and relative stabilities of CaFe2O4 surface have not been systemically studied. Hence, the surface structures of CaFe2O4 (001), (100), (110) and (111) were calculated by using a generalized gradient approximation considering on-site Coulomb interaction of iron 3d electrons (GGA + U) in the paper. With U = 4.70 eV, the band gap for up-spin and down-spin energy of CaFe2O4 was calculated to be 1.91 and 1.81 eV, closed to the experimental value (1.90 eV). For the CaFe2O4 (001) surface, the O1 terminations were the most stable and the surface energy was 1.307 J·m−2. In the case of CaFe2O4 (100) surface, the surface energies at Ca and O1 terminations were 1.278 and 1.568 J·m−2, respectively. There were also two most stable CaFe2O4 (110) surfaces in close surface energies and terminated with the exposed Fe2 and O3 atoms. The surface energies of them were 1.489 and 1.570 J·m−2, respectively. Among the fifteen CaFe2O4 (111) terminations, the surface energies at O2 (l) and O4 (f) terminations were the lowest and they were 1.421 and 1.455 J·m−2. The calculated surface energies indicate that (100) was better than (001), (110) and (111) in thermodynamic, which agrees well with the experimental results.

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  • Accurate Viscosity Prediction for Molten Slags: A New Model and Database

    Rui Zhang, Samuel Hallström, Huahai Mao, Lina Kjellqvist, Qing Chen

    pp. 1379-1388

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    DOI:10.2355/isijinternational.ISIJINT-2020-586

    Abstract

    We present a new structurally-based viscosity model and database for accurately predicting the viscosity of multicomponent molten slags in the whole composition and large temperature ranges. The model is based on the CALPHAD (CALculation of PHAse Diagrams) approach and the thermodynamic two-sublattice ionic liquid formalism, which means that the underlying structure of oxide melts is taken into account and a full description of a multicomponent system is achieved by using the information of only binary and ternary systems. The model is implemented in the Thermo-Calc software package and is applied to optimize model parameters for the FeOx–CaO–MgO–Al2O3–SiO2–CaF2–CrOx–Na2O–MnOx–TiO2–ZrO2–P2O5–Gd2O3–La2O3–V2O5–NiO–CuOx system. Encompassing the obtained viscosity model parameters, the new thermodynamic database TCOX10 is shown to be able to give highly reliable calculation results for industrial and geological applications. Compared with previously reported modelling work by using the same large experimental datasets on the key subsystem CaO–MgO–Al2O3–SiO2 (CMAS), the present model and database are found to give the smallest deviations to the measurements, which is also proved true for the CMAS-based multicomponent glass that can significantly impact the performance of airplane turbine engines.

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  • Effect of Na Ions on Melt Structure and Viscosity of CaO–SiO2–Na2O by Molecular Dynamics Simulations

    Xiaobo Zhang, Chengjun Liu, Maofa Jiang

    pp. 1389-1395

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    DOI:10.2355/isijinternational.ISIJINT-2020-635

    Abstract

    Molecular dynamics (MD) simulations have been used to study the effect of Na ions on the structure properties of CaO–SiO2–Na2O melts. The short-range structure and medium-range structure of CaO–SiO2–Na2O in this study are consistent with existing data. The replacement of Ca2+ with Na+ in CaO–SiO2–Na2O melts has almost no effect on the degree of polymerization and distribution of bond angles of Si–O tetrahedron. From micro perspective, Na ions enhance the mobility of CaO–SiO2–Na2O melts by multiple ways. Firstly, the modification effect of Na+ on the melt network structure is weaker than that of Ca2+, the Si–O tetrahedron around Na+ is sparser than Ca2+, which is more conducive to ions movement. Secondly, the diffusion capacity of Na+ is much greater than other ions in CaO–SiO2–Na2O system, which the overall diffusion capacity of the system can be improved by adding more Na+. Thirdly, since Na+ has only one charge, there is no electrostatic restraint on the depolymerized tetrahedron which happened in multivalent charges such as Ca2+, so that the mobility of CaO–SiO2–Na2O is stronger than that of CaO–SiO2. The micro changes provide an explanation for the improvement of macro liquidity.

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  • Agglomeration Characteristics of Various Oxide Inclusions in Molten Steel Containing Rare Earth Element under Different Deoxidation Conditions

    Yeguang Wang, Chengjun Liu

    pp. 1396-1403

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    DOI:10.2355/isijinternational.ISIJINT-2020-684

    Abstract

    Comprehensive investigation of agglomeration behavior of oxide inclusions in steel-containing rare earth (RE) under different deoxidation conditions is essential for improving the nozzle-clogging problem during continuous casting. The present study first optimized the thermodynamic model by supplementing thermodynamic data of the RE-compound to study the formation of various inclusions in the Ce–Si–Al–O system. On this basis, laboratory-scale experiments with different deoxidation conditions were designed to obtain samples containing one single type of inclusions, viz., SiO2, SiO2–Ce2O3 or SiO2–Al2O3–Ce2O3, respectively. Subsequently, the agglomeration behavior of inclusions was observed in situ by confocal laser scanning microscopy (CLSM), and the corresponding attractive forces were calculated to compare the agglomeration tendency of various inclusions. The results indicate that, in the Si deoxidized samples, SiO2–Ce2O3 inclusions agglomerate to form clusters with the attractive force of 4.3 × 10−17N to 4.2 × 10−15N, which is weaker than that of Al2O3 and Al2O3–Ce2O3 inclusions after Al deoxidation. Although SiO2 inclusions can agglomerate, their attractive force is only 6.3 × 10−19N to 1.2 × 10−16N, which is the weakest of the above inclusions. In the case of Si/Al complex deoxidization, spherical SiO2–Al2O3–Ce2O3 liquid inclusions are formed after Ce treatment, and no agglomeration of such inclusions was observed in situ. The study could provide theoretical guidance for improving the nozzle-clogging problem of RE-containing steel from inclusion-controlling perspective under different deoxidation conditions.

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  • Phase Relationship and Activities of Components in CaO–SiO2–Cr2O3 Ternary System at 1573 K

    Kodai Iwahashi, Shuji Hashimoto, Ryohei Yamauchi, Keijiro Saito, Masakatsu Hasegawa

    pp. 1404-1411

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    DOI:10.2355/isijinternational.ISIJINT-2020-752

    Abstract

    The knowledge of Cr2O3 activity in slag is necessary to decarburize chromium-containing special steel while minimizing the oxidation loss of chromium. In the present study, firstly, the phase relationship in the CaO–SiO2–Cr2O3 system was determined at 1573 K. Subsequently, the Cr2O3 activities in two- and three-phase coexisting slags at 1573 K were measured by equilibrating molten copper with oxide phases under a stream of Ar + H2 + CO2 gas mixtures, and the Gibbs energy changes of the formations of CaCr2O4 and Ca3Cr2Si3O12 were derived. It was found that the present results were compatible with the phase diagram and the literature data reported at higher temperature than 1573 K.

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  • Effects of Iron Ores on the Combustion Behavior of Coke and NOx Emission during Sintering Process

    Zhigang Que, Xianbin Ai

    pp. 1412-1422

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    DOI:10.2355/isijinternational.ISIJINT-2020-222

    Abstract

    Calcium ferrite could promote the CO–NO reduction reaction, and its formation is affected by iron ores during sintering process. In present study, effects of iron ore coating layers on coke combustion rate and NOx emission were investigated in a visualize combustion equipment, and an optimized ore blending structure was proposed by sinter pot test. Due to the melting of iron ore coating layers at high temperature, coke transformed from coated to naked. With increasing of the binary basicity of the iron ore coating layers, the formation of calcium ferrite increased, resulting in increasing of the melt fluidity. The lower the formation temperature of the melts, the sooner coke was exposed, and the peak combustion rate linearly increased with the melt fluidity of iron ore coating layers. Meanwhile, compared to the high-silicon ores, the maximum NOx emission concentration and conversion rate of N element were lower with the low-silicon ores. NOx emission concentration showed an inverted W-shape trend and had an 8-shape relation with coke combustion rate. Due to the difference of the capability of calcium ferrite formation in coating layers, the conversion rate of N element was linearly negative and positive correlated with the basicity of iron ore coating layers and mass of CO emission, respectively. In addition, with the proportions of the low-silicon limonite and hematite increased in sinter mixture, NOx emission gradually decreased. As a consequence, with exclusively using low-silicon lignite and hematite in sinter mixture, NOx emission decreased by about 20% and sinter indexes significantly improved.

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  • Fracture Behavior in Blending Coke Using the Model Compound of Low-quality Coal

    Yuumi Miyashita, Yasuhiro Saito, Yohsuke Matsushita, Hideyuki Aoki, Takashi Matsui, Issui Akishika, Daisuke Igawa

    pp. 1423-1430

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    DOI:10.2355/isijinternational.ISIJINT-2020-255

    Abstract

    The adhesion failure of coke with low-quality coal affects its strength. However, it would be difficult to specify the interface between caking coal and low-quality coal where the adhesion failure arises. In this study, the interface was specified using alumina beads that can be identified by the X-ray CT (computed tomography) as a model compound of low-quality coal, and thus fracture behavior of coke assuming that low-quality coal was blended was investigated experimentally and numerically. To quantitatively evaluate the adhesiveness between coke matrices and alumina beads, the three-dimensional structure of coke was evaluated by the X-ray CT. As a result, the adhesiveness decreased with an increase in the volume ratio of alumina beads, while the adhesiveness was not related to the particle diameter. Also, the fracture strength of coke decreased with an increase in the volume ratio and the particle diameter of alumina beads. For the fracture analysis using RBSM (Rigid Bodies-Spring Model), the fracture strength was also negatively correlated with the volume ratio and particle diameter of alumina beads. Focusing on fracture behavior of coke model, the interface between coke matrices and alumina beads fractured at the non-uniform part where alumina beads existed closely. Therefore, this study suggested that the distribution of low-quality coal within the coke as well as the adhesion failure has an effect on strength of coke with low-quality coal.

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  • Effect of Basicity on Metallurgical Properties of Fluxed Pellets with High MgO Content

    Zichao Yang, Zhenggen Liu, Mansheng Chu, Lihua Gao, Cong Feng, Jue Tang

    pp. 1431-1438

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    DOI:10.2355/isijinternational.ISIJINT-2020-445

    Abstract

    The basicity (CaO/SiO2) has an important influence on the metallurgical properties of fluxed pellet with high MgO content. This paper studied the effect and function of basicity on metallurgical properties of fluxed pellet, such as compressive strength, reduction disintegration, reduction swelling and the softening-melting behaviour systematically with simulating BF conditions. From the results, with increasing basicity from 0.6 to 1.3, the compressive strength increases from 2842 N to 3532 N, the pore structure become more compact, and the following phase transformation is observed from Fe2SiO4 → Ca2SiO4, MgXFe2−XSiO4 → MgFe2O4. Additionally, the reduction properties of high magnesium fluxed pellets are improved, the low temperature disintegration index increases from 89.8% to 97.78%, and the reduction swelling index decreases from 19.02% to 8.45%. With the increase of basicity in the pellets, the softening interval and melting interval of the pellets decrease slightly, and the position of the cohesive zone shift down and gradually become thinner, which improve the permeability of the cohesive zone. In the basicity range of 0.6–1.3, the optimum metallurgical properties of pellets are observed for the basicity of 1.1.

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  • Effect of Varied Oxygen Levels on the Oxidation of a Magnetite Pellet Bed during Pot Furnace Induration

    Anna Eriksson, Charlotte Andersson, Hesham Ahmed, Anders Dahlin, Telkicherla Kamesh Sandeep Kumar, Pär Semberg

    pp. 1439-1449

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    DOI:10.2355/isijinternational.ISIJINT-2020-469

    Abstract

    An excess amount of oxygen originating from hydrogen production is likely to be available as part of the HYBRIT (Hydrogen Breakthrough Ironmaking Technology) initiative, aimed at producing fossil-free steel by replacing coking coal with hydrogen. Oxygen enrichment during magnetite pellet induration can lead to reduced fuel amounts and increased productivity. Induration of magnetite iron ore pellets liberates considerable amounts of heat when magnetite is oxidised to hematite. Elevated oxygen levels in the process gas are expected to promote the oxidation reaction, resulting in increased process efficiency. However, more information is required to enable the transition towards a higher oxygen level process and improved production rate, while maintaining the metallurgical properties of the pellet bed. In this study, interrupted pot furnace experiments were conducted on a magnetite pellet bed (approximately 100 kg) at Luossavaara-Kiirunavaara Aktiebolag to investigate the effect of oxygen levels at approximately 6%, 13%, and 30% O2. Temperature profiles are measured and pellet properties (compression strength, porosity, oxidation degree, microstructures) are analysed at different bed heights. The higher oxygen level (approximately 30% O2) intensifies the oxidation reaction, resulting in increased temperature, oxidation rate and compression strength across the vertical bed height. Three different pellet oxidation profiles are identified, namely, homogenous oxidation across the pellet, complete oxidation of the pellet shell and an unreacted core with a sharp/distinct interface, and partial oxidation of the pellet shell and an unreacted core. A higher oxygen level results in an increased oxidation rate, while the temperature controls the pellet oxidation profile.

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  • Numerical Investigation of Hematite Flash Reduction-biomass Steam Gasification Coupling Process

    Xingnan Wang, Guiqin Fu, Wei Li, Miaoyong Zhu

    pp. 1450-1458

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    DOI:10.2355/isijinternational.ISIJINT-2020-566

    Abstract

    The flash ironmaking process is a novel ironmaking technology; the direct use of biomass as the reductant and fuel in this process can take full advantage of the heat and syngas produced during the biomass gasification. This study establishes a three-dimensional computational fluid dynamics model that incorporates turbulent flow, mass transfer, and heat transfer to describe the complex gas-particle reaction behavior of the hematite flash reduction-biomass steam gasification (FR-BSG) coupling process in an entrained flow reactor to explore its feasibility. The temperature and species distributions in the FR-BSG coupling process are analyzed, and the effects of steam/carbon molar ratio (S/C) and ore/biomass mass ratio (O/B) are investigated. The results show that the reduction degree of hematite particles reaches 76.67% in the residence time of 1.65 s under the conditions of S/C=0.1, O/B=1.0 and T=1673 K. The increase of S/C can enhance the production of H2 but reduce the molar fractions of H2 and CO in biomass syngas, which leads to the decrease of hematite reduction degree. A higher reduction degree of hematite and lower carbon conversion of biomass can be obtained at lower O/B values. These results provide a theoretical basis for the use of biomass as energy in flash ironmaking technology.

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    3. Quantitative Analysis of Solidification of Compacted Graphite Irons – A Modelling Approach ISIJ International Vol.61(2021), No.5

  • Gasification Behavior of Phosphorus during Pre-reduction Sintering of Medium-high Phosphorus Iron Ore

    Yanbiao Chen, Haibin Zuo

    pp. 1459-1468

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    DOI:10.2355/isijinternational.ISIJINT-2020-564

    Abstract

    The effects of dephosphorization agents (SiO2, Na2CO3 and Al2O3) and reduction time on phosphorus gasification during pre-reduction sintering gasification dephosphorization were studied. The results indicate that the dephosphorization rate of apatite by carbothermal reduction is 23%, the dephosphorization rate of adding SiO2 and Al2O3 is 25%, and the optimal dephosphorization rate is 36% by adding SiO2 and Na2CO3. The dephosphorization rate increased from 13% to 31% with the reduction time increased from 20 min to 60 min. Thermodynamic analysis shows that the promotion effect of Na2CO3 on apatite reduction is stronger than that of Al2O3, and the thermodynamic conditions of apatite reduction are optimized to the greatest extent by adding SiO2 and Na2CO3, with the starting temperature of carbothermal reduction of apatite decreased from 1464°C to 746°C. P and C have the same variation and change trend, and the migration of phosphorus ito metallic iron is related to the carburizing reaction. The good heat storage of the sinter layer in pre-reduction sintering increases the time for maintaining the apatite reaction temperature and promotes the occurrence of dephosphorization reaction, however part of the phosphorus gas is absorbed by liquid metallic iron to form stable iron phosphorus compounds,which leads to the decrease of dephosphorization rate. Besides addition of dephosphorization agent, restricting migration of phosphorus into iron phase is the key point to increase gasification dephosphorization efficiency in sintering process.

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    3. Quantitative Analysis of Solidification of Compacted Graphite Irons – A Modelling Approach ISIJ International Vol.61(2021), No.5

  • Iron Ore Sintering in Milli-Pot: Comparison to Pilot Scale and Identification of Maximum Resistance to Air Flow

    Tejbir Singh, Huibin Li, Guangqing Zhang, Subhasish Mitra, Geoffrey Evans, Damien O’dea, Tom Honeyands

    pp. 1469-1478

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    DOI:10.2355/isijinternational.ISIJINT-2020-574

    Abstract

    In the iron ore sintering process, the resistance to air flow is a major factor in deciding the flame front speed, which influences the sinter productivity and quality. In this work, pressure drop during sintering and the resistance to air flow was investigated in milli-pot sintering for different coke rates. The sintering experiments were conducted in a milli-pot (diameter 53 mm, height 400 mm) and pressure and temperature were measured at the same locations in the bed by four taps located equidistant to each other. The yield of sinter product was measured following a modified drop test and the mineralogy of the sinter product was analysed. The results from milli-pot sintering were then compared to the reported results from standard pilot-scale sintering, and it was found that the lower half of the milli-pot bed gave a reasonable representation of the pilot-scale sintering process. The results of sinter mineralogy, yield and productivity of the lower half of milli-pot at 5.5–8.0% coke rate were found to be similar to pilot-scale sintering tests at a corresponding coke rate from 3.5 to 5.5%. The maximum resistance to air flow in the bed was found to be in the region between the leading edge of the flame front at ~100°C and the trailing edge of the flame front at ~1200°C. This suggests that the maximum resistance to air flow includes the effect of de-humidification and combustion in addition to the high temperature “flame front” region usually defined at temperatures above 1100°C or 1200°C.

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    2. Effect of Silica in Ash of Coke on Carburization and Melting of Iron ISIJ International Vol.61(2021), No.5
    3. Effect of Basicity on Metallurgical Properties of Fluxed Pellets with High MgO Content ISIJ International Vol.61(2021), No.5

  • Effect of Silica in Ash of Coke on Carburization and Melting of Iron

    Taichi Murakami, Daisuke Maruoka, Eiki Kasai

    pp. 1479-1487

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    DOI:10.2355/isijinternational.ISIJINT-2020-612

    Abstract

    Reducing energy consumption in ironmaking process is essential for decreasing the amount of carbon dioxide emission from the blast furnace process. To effectively reduce carbon dioxide emission, a faster reduction of iron ore, as well as the acceleration of carburization and melting of the reduced iron is important. Understanding the ash behavior of coke in blast furnace is necessary for controlling carburization because ash prevents carburization. In this study, the following were investigated:1) Condensation behavior of the ash components on the surface of coke by gasification under the simulated blast furnace condition2) Melting behavior of iron tablet by the direct contact with coke plate3) In-situ observation of the melting behavior of coke-iron composite and effect of deashing treatment4) In-situ observation of the melting behavior of Fe–Si alloy using graphiteThe results revealed that ash condenses on the coke surface via gasification, and the ash coverage ratio of the coke surface increases with increasing in the coke reactivity. In addition, the melting of iron by carburization with coke proceeds at lower temperature when the coke reactivity is high. However, the effect of ash coverage on this behavior is significantly higher than that of coke reactivity, and an increase in the ash coverage ratio suppresses the carburization and melting of iron. In addition, during holding at high temperature, SiO2 in ash is reduced to Si, and it alloys with metallic iron. Furthermore, an increase in Si concentration in metallic iron increases the melting temperature of Fe–Si system. Consequently, carburization and melting iron are suppressed.

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    3. Quantitative Analysis of Solidification of Compacted Graphite Irons – A Modelling Approach ISIJ International Vol.61(2021), No.5

  • Modeling of Coke Particle Breakage in Blast Furnace Considering Pore Structure by Discrete Element Method

    Koichi Takahashi, Aya Yoshino, Taihei Nouchi, Junya Kano, Shingo Ishihara, Tatsuro Ariyama

    pp. 1488-1497

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    DOI:10.2355/isijinternational.ISIJINT-2020-630

    Abstract

    Cokes play an important role in the blast furnace as a spacer for maintaining gas permeability. Since blast furnaces with large inner volumes exceeding 5000 m3 are now common in Japan, use of high strength coke has become a crucial issue for modern blast furnaces. However, general methods for evaluating coke strength, for example, the drum test, are insufficient for understanding the breakage behavior of coke in detail. In order to evaluate the coke breakage behavior in blast furnaces, a coke breakage model based on the discrete element method (DEM) with cluster particles and parallel bonds was developed.According to experiments using the indirect tensile test, the tensile strength of cokes shows a wide distribution because of the randomness of the pore arrangement. Therefore, a DEM simulation model for coke breakage was developed considering pores with random positions. DEM simulations of the indirect tensile test were conducted for 10 cases of random pore arrangements for each of Coke A (small porosity) and Coke B (large porosity). The tensile strength obtained from the experiments and DEM simulations was compared by a Weibull analysis. The simulation results were in agreement with the experimental results including the distribution of coke strength. Finally, the probability distributions of coke breakage obtained from the Weibull analysis were applied to the DEM simulation result of the material flow in a 5000 m3 blast furnace, and the percentage of coke breakage in the deadman region of the blast furnace was evaluated for Coke A and Coke B.

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    3. Quantitative Analysis of Solidification of Compacted Graphite Irons – A Modelling Approach ISIJ International Vol.61(2021), No.5

  • Nitrogen Solubility in Liquid Fe–Nb, Fe–Cr–Nb, Fe–Ni–Nb and Fe–Cr–Ni–Nb Alloys

    Shouxing Yang, Huabing Li, Hao Feng, Xuze Li, Zhouhua Jiang, Tong He

    pp. 1498-1505

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    DOI:10.2355/isijinternational.ISIJINT-2020-627

    Abstract

    The N-containing Fe–Cr–Ni–Nb austenitic heat-resistant steels have become the research focus of high-temperature material. Nitrogen plays an important role on the strength and structural stability of the steels, and thus the accurate control of nitrogen content is of great significance to the smelting process. In this paper, the nitrogen solubility in liquid Fe, Fe–Nb, Fe–Cr–Nb, Fe–Ni–Nb and Fe–Cr–Ni–Nb systems from 1823 to 1873 K were investigated by gas-liquid metal equilibrium experiments. In liquid Fe–Nb system with a niobium content of 5 to 20%, the solubility of nitrogen increased with niobium content. The first-order interaction parameter of niobium on nitrogen at 1873 K and its relationship with temperature were determined as follows: [ This part of text is omitted ], [ This part of text is omitted ]. In the liquid Fe–Cr–Nb and Fe–Ni–Nb systems, the second-order cross-interaction parameters of chromium or nickel with niobium on nitrogen were determined as follows: [ This part of text is omitted ], [ This part of text is omitted ]. Furthermore, a more accurate nitrogen solubility prediction model for the liquid Fe–Cr–Ni–Nb system was established based on the existing thermodynamic parameters and the interaction parameters obtained in this study.

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  • Evolution of Inclusions with Ce Addition and Ca Treatment in Al-killed Steel during RH Refining Process

    Ruming Geng, Jing Li, Chengbin Shi

    pp. 1506-1513

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    DOI:10.2355/isijinternational.ISIJINT-2020-672

    Abstract

    The evolution of inclusions with Ce addition and Ca treatment in Al-killed steel during RH refining process was investigated through experimental observations and thermodynamic calculations. The results indicated that the typical inclusions before Ce addition are CaO–Al2O3 inclusions, which were a liquid state during RH refining. After Ce addition, the typical inclusions was transformed from calcium aluminate inclusion to (Ca–Ce–S–O)+(Ce–Al–Ca–O) complex inclusion. After Ca treatment, the types and morphologies of typical inclusions in steel did not change. Experimental observation and thermodynamic calculations shown that a certain amount of Ca addition can’t affect the formation of Ce-containing inclusion, which may indicate that Ca treatment should not be carried out for rare earth treated steel.

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    2. On the CO Desorption and Absorption in Liquid Low-carbon Steel ISIJ International Vol.61(2021), No.5
    3. Agglomeration Characteristics of Various Oxide Inclusions in Molten Steel Containing Rare Earth Element under Different Deoxidation Conditions ISIJ International Vol.61(2021), No.5

  • A Novel Model to Design the Equilibrium Slag Compositions for Bearing Steel: Verification and Application

    Yunpeng Wang, Guoguang Cheng, Shijian Li, Weixing Dai, Dajun Shang, Chunguang Lu

    pp. 1514-1523

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    DOI:10.2355/isijinternational.ISIJINT-2020-671

    Abstract

    A new equilibrium calculation model for the slag composition design of bearing steel is established through the combination between IMCT and FactSage software. With this model, the equilibrium slag could be obtained quantitatively when the initial compositions of steel were given. It is validated by the comparison of predictive results with experimental data with different slag system, reflecting the reliability of the model. After that, the effects of Al in steel as well as CaO, MgO and Al2O3 in slag on the dissolved oxygen, magnesium and calcium were discussed in detail. Furthermore, Industrial trials with five different slags was carried out in a special steel plant in China. Results show a good correlation between the calculated contents in molten steel after LF refining and number density of oxide inclusions in final steel, which means this model have a good guidance for the production of high clean bearing steel.

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    3. Interaction between M(C, N) and Ferrite in Electropulsing Microalloyed Steel ISIJ International Vol.61(2021), No.5

  • Reaction Behaviour between Cerium Ferroalloy and Molten Steel during Rare Earth Treatment in the Ultra-low Carbon Al-killed Steel

    Min Wang, Shuai Gao, Xin Li, Gui-xuan Wu, Li-dong Xing, Hao Wang, Jian-guo Zhi, Yan-ping Bao

    pp. 1524-1531

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    DOI:10.2355/isijinternational.ISIJINT-2020-678

    Abstract

    In this work, the melting process and reaction behaviour of cerium ferroalloy in the liquid ultra-low carbon interstitial free steel was investigated through a special designed hot crucible experiment using standard metallographic techniques including SEM, EDS, XRF, and XRD in combination with the thermodynamic software FactSage 7.2. The evolution mechanism of cerium-containing phases in the cerium ferroalloy during the melting process was proposed. The cerium-containing phases stretched and migrated along the direction of heat flow, in further the arms of the network structures became thinner and finally dissolved into the molten steel. It was concluded that the thinner of network structures’ arms, the higher of the Ce content in cerium-containing phases. During the melting process, the cerium content in the cerium-enriched phase was increased from 43.9 wt% in the matrix of the cerium ferroalloy to 90.44 wt% in the reaction zone, and the average width of the network arms was decreased from 18 µm to 2 µm accordingly. The main reaction products in the interface among cerium ferroalloy, molten steel, and slag were Ce2O3 and CeAlO3.

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  • Effect of Cooling Rate on the Structure of CaO–SiO2–CaF2-based Glassy Mold Flux

    Jiangling Li, Feifei Lai, Mei Leng, Yangfan Chen, Qingcai Liu

    pp. 1532-1538

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    DOI:10.2355/isijinternational.ISIJINT-2020-423

    Abstract

    To evaluate the effect of cooling rate on the structure of CaO–SiO2–CaF2 based mold flux, the structure of glassy mold fluxes prepared at different cooling rates ranging from 20°C/s to 100°C/s were investigated by Raman spectroscopy, 29Si MAS-NMR and XPS techniques. The results shown that the structural species Q2 and Q3 slightly increased with the increase of cooling rate, while the ratio of Q0 slightly decreased. The change of Q1 was negligible. The average number of bridging oxygen atoms increased with the increase of cooling rate. The polymerization degree of the silicate structural network of glassy CaO–SiO2–CaF2-based mold fluxes was found to increase with the increase of cooling rate. The change of the cooling rate obviously caused the structural change of the CaO–SiO2–CaF2 mold flux. The faster the cooling rate, the higher temperature structural feature was founded. The effect of the cooling rate on F-bonds was negligible, and F was found to be mainly in the form of F–Ca bonds.

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  • Quantitative Analysis of Solidification of Compacted Graphite Irons – A Modelling Approach

    Jacques Lacaze, Anna Regordosa, Jon Sertucha, Urko de la Torre

    pp. 1539-1549

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    DOI:10.2355/isijinternational.ISIJINT-2020-476

    Abstract

    Several X-ray topography studies which have appeared recently in the literature show compacted graphite in cast iron consisting of coarse interconnected graphite lamellas. This suggested that solidification of these alloys could be described as done for irregular eutectics by accounting for limited branching of graphite lamellas. The corresponding growth law has been inserted in appropriate mass balances for describing the successive solidification stages. Predictions of the model thus obtained have been compared to quantitative experimental information previously gained on solidification of a series of hyper-eutectic alloys. The deep undercooling and marked recalescence which are so characteristic of the solidification of compacted graphite cast irons in the stable system are reproduced and appear to be closely related to the limited branching of graphite lamellas in the compacted graphite cells. The competition between stable and metastable solidification could be described and leads to a decrease of the recalescence amplitude that was properly reproduced.

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    3. Interaction between M(C, N) and Ferrite in Electropulsing Microalloyed Steel ISIJ International Vol.61(2021), No.5

  • Interaction between M(C, N) and Ferrite in Electropulsing Microalloyed Steel

    Zhenghai Zhu, Hongbiao Dong, Ke Zhang, Tao Jia, Lizhong Chang, Xiaofang Shi, Fei He

    pp. 1550-1555

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    DOI:10.2355/isijinternational.ISIJINT-2020-538

    Abstract

    Microstructure and M(C, N) significantly affect the quality of Nb microalloyed steel, and the control microstructure evolution and M(C, N) precipitation behavior is the key. In this study, the interaction between ferrite and M(C, N) with electropulsing is quantitatively analyzed. Results reveal that electropulsing promotes the precipitation of M(C, N) and ferrite phase from austenite phase. The precipitated M(C, N) affects the position of ferrite precipitation, and the precipitation of ferrite can then conversely affect the distribution of M(C, N). The influence of austenite to ferrite transformation on M(C, N) precipitation is much more significant than that of electropulsing. This observation can be applied to control of microstructure and M(C, N) in continuous casting.

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  • Development Technology for Prevention of Macro-segregation in Casting of Steel Ingot by Insert Casting in Vacuum Atmosphere

    Kohichi Isobe

    pp. 1556-1566

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    DOI:10.2355/isijinternational.ISIJINT-2020-603

    Abstract

    Important large components, such as rotors for power generation steam turbines, pressure vessels and reaction vessels, are manufactured through ingot casting. However, it is quite tasking to manufacture the materials with sufficient properties because of the macro-segregation of ingots.To develop effective and versatile macro-segregation countermeasures in the casting of large steel ingots for manufacturing large parts for power plants, the insert casting in vacuum atmosphere, in which a core material similar in composition as the base steel, is placed at the center of the mold, was studied. The effectiveness of the proposed insert casting as a macro-segregation countermeasure was evaluated in the casting experiments with 0.5 mass% carbon steel using cast iron mold with a 150 mm square inner cross section, insulated to reduce solidification rate. In addition, it is now confirmed that good bonding between the core material and base material can be achieved even under conditions where bonding by normal insert casting in air atmosphere is hard to achieve.The behavior of the core material melting and the solidification of the molten steel in the experiments of macro-segregation reproduction casting and insert casting were investigated using the direct finite difference method. The mechanism by which this method suppresses the macro-segregation formation and solidification conditions for the suppression, the reasons, and conditions for good bonding in this insert casting are clarified by the analyses. Furthermore, the cause of internal crack formation in the insert casting was investigated and guidelines for preventing the crack formation were presented.

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  • Time-resolved and in-situ Observation of Semisolid Deformation in Al–Cu Alloys with Equiaxed and Columnar Grain Structures by Using a Combination Technique of 4D-CT and 3DXRD

    Taka Narumi, Takumi Nakata, Ryuya Kobayashi, Hideyuki Yasuda

    pp. 1567-1578

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    DOI:10.2355/isijinternational.ISIJINT-2020-650

    Abstract

    Time-resolved combined absorption tomography and three-dimensional X-ray diffraction was developed to study semisolid deformation of metallic alloys, which combined time-resolved tomography and three-dimensional X-ray diffraction microscopy (3DXRD). The combined technique allowed observation of configuration and crystallographic orientation of solid grains, whereby translation and rotation of solid grains induced by interaction between solid grains during semisolid deformation was analyzed. During the compression tests of a semisolid Al–10mass%Cu alloy with the equiaxed grain structure, translation and rotation of solid grains rather than the plastic deformation played a dominant role in the deformation until strain reached −0.04. In a portion of the specimen, a gap between solid grains expanded owing to the interaction between solid grains, and consequently apparent volume expansion (dilatancy) occurred. The solid grains around the expanded region possessed similar rotational axis directions, which were perpendicular to the normal direction of the expanded region. As the compression proceeded, both translation/rotation and the plastic deformation of solid grains occurred owing to an increase in physical contact between solid grains. For a semisolid Al–10mass%Cu alloy with the columnar grain structure, solid grain comprised a single crystallographic domain. Inflection of the dendrite arms was confirmed by 3DXRD, where the compression of strain at −0.25 caused the grain to be inflected up to 12 degrees. The strain distribution in the grain was estimated using the inflection angle. In-situ observation revealed that the behaviors of solid grains and the liquid phase in semisolid alloys during the compression tests differed according to the structure.

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  • Surface Defects Classification of Hot Rolled Strip Based on Improved Convolutional Neural Network

    Wenyan Wang, Kun Lu, Ziheng Wu, Hongming Long, Jun Zhang, Peng Chen, Bing Wang

    pp. 1579-1583

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    DOI:10.2355/isijinternational.ISIJINT-2020-451

    Abstract

    Surface defect classification of hot-rolled strip based on machine vision is a challenge task caused by the diversity of defect morphology, high inter-class similarity, and the real-time requirements in actual production. In this work, VGG16-ADB, an improved VGG16 convolution neural network, is proposed to address the problem of defect identification of hot-rolled strip. The improved network takes VGG16 as the benchmark model, reduces the system consumption and memory occupation by reducing the depth and width of network structure, and adds the batch normalization layer to accelerate the convergence speed of the model. Based on a standard dataset NEU, the proposed method can achieve the classification accuracy of 99.63% and the recognition speed of 333 FPS, which fully meets the requirements of detection accuracy and speed in the actual production line. The experimental results also show the superiority of VGG16-ADB over existing classification models for surface defect classification of hot-rolled strip.

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  • Improvement of Bragg-edge Neutron Transmission Imaging for Evaluating the Crystalline Phase Volume Fraction in Steel Composed of Ferrite and Austenite

    Hirotaka Sato, Miyuki Sato, Yuhua Su, Takenao Shinohara, Takashi Kamiyama

    pp. 1584-1593

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    DOI:10.2355/isijinternational.ISIJINT-2020-257

    Abstract

    The Bragg-edge neutron transmission imaging method can quantitatively visualize various types of crystalline microstructural information inside a bulk material over a large visualization field area. In this study, we investigated and improved both the experimental method and the data analysis method for the evaluation of crystalline phase volume fraction in steel composed of ferrite/martensite and austenite. For wavelength-resolved neutron transmission imaging experiments, we confirmed that accurate measurement of neutron transmission intensities was crucial. Therefore, the background neutrons scattered from a sample must be reduced. Simultaneously, we confirmed that a neutron wavelength resolution of approximately 1% was required. For the data analysis of the measured Bragg-edge neutron transmission spectrum, we used double March-Dollase orientation distribution functions for each crystalline phase to achieve effective spectrum correction of the crystallographic texture effect. As a result, this data analysis method allows improved evaluation of the crystalline phase volume fraction, compared with the use of a single March-Dollase function for each phase.

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  • Change of State of Lime Phase and Inhibition of Hydration Reaction by Coexisting Oxides in Steelmaking Slag

    Hiromi Eba, Kousuke Yonemoto, Yuuki Suzuki

    pp. 1594-1602

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    DOI:10.2355/isijinternational.ISIJINT-2020-602

    Abstract

    To promote the utilization of steelmaking slag, the hydration reactivity and electronic state of free lime (CaO) were observed in this study. It was confirmed by X-ray diffraction (XRD) and thermal analysis that the hydration reactivity decreases when CaO forms a lime-phase solid solution with FeO, and it was revealed by X-ray photoelectron spectroscopy (XPS) observations that the decrease in reactivity occurs due to deepening of the energy levels of the electron orbitals. The morphology and hydration reactivity of the free lime contained in a synthesized slag were observed by XRD and electron probe microanalysis. Fe-containing phases and double oxides were found to be adjacent to free lime; the former is considered to be related to the formation of a solid solution with FeO, and the latter closely surrounds the free lime and shields it from the outside; thus, both contribute to suppressing the hydration reaction. Furthermore, it was found that simply mixing lime with double oxides suppressed the hydration reaction. XPS and thermal analysis confirmed that a chemical change occurred on the surface of the free lime; that is, the energy levels of the electron orbitals are deepened and the hydration reaction is suppressed due to chemical interactions with the double oxides to stabilize the lime surface. It was clarified that in order to effectively promote the hydration of free lime, it must be exposed by breaking the structure of the slag that protects the free lime and suppresses hydration.

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    2. Effect of Na Ions on Melt Structure and Viscosity of CaO–SiO2–Na2O by Molecular Dynamics Simulations ISIJ International Vol.61(2021), No.5
    3. Effect of Cooling Rate on the Structure of CaO–SiO2–CaF2-based Glassy Mold Flux ISIJ International Vol.61(2021), No.5

  • Ensemble Learning Based Methods for Crown Prediction of Hot-Rolled Strip

    Guangtao Li, Dianyao Gong, Xing Lu, Dianhua Zhang

    pp. 1603-1613

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    DOI:10.2355/isijinternational.ISIJINT-2020-639

    Abstract

    The strip crown or profile generated by the cooperation of finishing mills is affected by many factors, so obtaining an accurate crown has always been a challenge in hot strip rolling. As a kind of solution to ensure the crown accuracy of hot-rolled strips, this study develops three novel strip crown prediction models using the well-performing and efficient tree-based ensemble learning algorithms, including Random Forest (RF), eXtreme Gradient Boosting (XGBoost), and Light Gradient Boosting Machine (LightGBM), respectively. The comparison results of measured and predicted strip crown show that all developed strip crown prediction models perform well based on the accurate extraction of key features as model inputs, the collection and pre-processing of a large amount of modeling data, and the use of the Bayesian optimization technique. By comparison, the LightGBM model with both high efficiency and high accuracy is considered the most recommended method for hot-rolled strip crown prediction. Besides, according to the feature importance scores of the input variables calculated based on the LightGBM model, the impact levels of each input variable on the strip crown are measured, and the calculation results fit well with the classical hot rolling theory indicating the modeling route as a reliable one.

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  • Effects of Mn and Al on Acicular Ferrite Formation in SAW Weld Metal

    Naoto Fujiyama, Genichi Shigesato

    pp. 1614-1622

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    DOI:10.2355/isijinternational.ISIJINT-2020-407

    Abstract

    The acicular ferrite (AF) formation behaviors of submerged arc weld metals with various Al/O ratios and Mn contents were investigated. The nucleation sites of AF consisted of inclusions surrounded by a TiO layer. The Al/O ratio affected the number of inclusions that acted as AF nucleation sites. At high Al/O ratios, the number of inclusions was low and the microstructure was coarse.We considered that the formation of AF was promoted by the presence of the TiO layer via two mechanisms: good lattice matching with ferrite and formation of a Mn-depleted zone owing to the absorption of Mn. The Baker–Nutting orientation relationship between TiO and AF was observed. Moreover, we determined that Mn-depleted zones were formed around the inclusions surrounded by TiO. The Mn content of weld metals affected the formation of AF via the formation of Mn-depleted zones. When the Mn content of the weld metals was high, Mn-depleted zones were formed.

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    3. Size Effect on Flow Field and Dynamic Deposition of Bottom Dross in a Molten Zinc Pot ISIJ International Vol.61(2021), No.5

  • Adhesive Scale Formation on Low Silicon Steel Surface; Characterization and Mechanism

    Manindra Manna, Kedar Bhave, Abhishek Subhash Pathak

    pp. 1623-1632

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    DOI:10.2355/isijinternational.ISIJINT-2020-134

    Abstract

    A systematic study has been conducted to characterize adhesive/non-adhesive scale on low silicon containing hot rolled steel surface and mechanism for formation of such scale. Different characterisation tools like XRD, SEM-EDS, GDOES, Scanning Kelvin probe and XPS were used to identify the chemical composition and characteristics of adhesive and non-adhesive scales. The characterization studies confirmed that the adhesive scale consists of CaFeSi2O6 compound along with magnetite and haematite with red in colour whereas non-adhesive scale consists of grey colour hematite-magnetite phase. Humidity in environment promotes oxidation of steel and depending on relative humidity different oxides are formed on the steel surface. Low humidity persists in India during the winter season which lead to lower oxidation potential and steel surface is oxidised accordingly. The wüstite phase is formed under lower oxygen potential at high temperature when slab is released from the mould. Wüstite is reactive to mould powder (tricalcium silicate) at high temperature and forms CaFeSi2O6 compound on the steel surface which is very adhesive in nature. Lots of cracks are inheritance attributes of non-adhesive scale whereas eventually no crack exists in adhesive scale. The compound present in the adhesive scale remains on the steel surface even after subsequent hot rolling and acid pickling operation as chloride ions of acid may not able to dissolve or penetrate through such stable compound. The quality of the finished product is deteriorated due to the presence of such adhesive scale as it leads to poor product performance during subsequent processing like cathodic paint deposition.

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    3. Size Effect on Flow Field and Dynamic Deposition of Bottom Dross in a Molten Zinc Pot ISIJ International Vol.61(2021), No.5

  • Size Effect on Flow Field and Dynamic Deposition of Bottom Dross in a Molten Zinc Pot

    Hongzhu Fei, Jian Li, Hongying Li

    pp. 1633-1640

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    DOI:10.2355/isijinternational.ISIJINT-2020-686

    Abstract

    During the continuous hot-dip galvanizing process, the effect and mechanisms of production parameters on flow field and dynamic deposition behaviors of bottom dross are critical issues for solving problems on harmless accumulation and control technologies of bottom dross. In this paper, a combined method of numerical simulation and water modeling experiment is presented to investigate the effects of steel strip width and velocity on flow field and dynamic deposition of bottom dross in a zinc pot. The molten zinc flow in an industrial-sized model were numerically simulated by the computational fluid-dynamics method. A 1/5 reduced-scale physical water model is established to study deposition behaviors of bottom dross based on the theoretical framework of similarity. NaCl aqueous solutions and black particles of acrylonitrile butadiene styrene are employed as model fluid and bottom dross, respectively. The results show that the width of strip is a key factor for the intensity of two impact flows in the bath. The condition of interaction between two impact flows determines the accumulation morphology of bottom dross. The influence of the strip width on the deposition directon of bottom dross exhibits an obvious size effect. With the decreasing of strip width, the deposition direction of the scattered bottom dross under sink roll is gradually changed from backward to backward as well as forward. The velocity of strip has no significant impact on the flow characteristics, but shows a strong positive correlation with the intensity of flow field and the number of scatted bottom drosses.

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    3. Resistance to Temper Softening of Low Carbon Martensitic Steels by Microalloying of V, Nb and Ti ISIJ International Vol.61(2021), No.5

  • Resistance to Temper Softening of Low Carbon Martensitic Steels by Microalloying of V, Nb and Ti

    Yongjie Zhang, Chao Zhao, Mitsutaka Sato, Goro Miyamoto, Tadashi Furuhara

    pp. 1641-1649

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    DOI:10.2355/isijinternational.ISIJINT-2020-412

    Abstract

    The resistance to temper softening in low carbon martensite with its underlying origin, by microalloying of strong carbide-forming alloying elements (V, Nb and Ti) to an Fe-0.1C-1.5Mn-0.05Si (mass%) alloy, was investigated in this study. With similar hardness in as-quenched condition in all the alloys used, the hardness of tempered martensite is increased by V, Nb and Ti additions, particularly after treatment at higher temperature with longer time. The increment in hardness becomes larger by more amount of V addition, while with almost the same amount of microalloying additions, Nb and Ti provide larger strengthening than that of V. Atom probe measurements have revealed that a high density of nano-sized alloy carbides are formed in those alloys with V, Nb and Ti additions at 923 K, where large secondary hardening was observed. At 723 K, where there is some resistance to temper softening, however, almost no precipitation of V, Nb and Ti can be detected. The X-ray line profile analysis of the tempered alloys implies that the reduction in dislocation density during tempering is strongly retarded by V, Nb and Ti additions. This should be the major reason for their resistance to temper softening at relatively lower temperature, even without nano-precipitation of alloy carbides.

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    3. Size Effect on Flow Field and Dynamic Deposition of Bottom Dross in a Molten Zinc Pot ISIJ International Vol.61(2021), No.5

  • Effect of the Initial Microstructure and Thermal Path on the Final Microstructure and Bendability of a High Strength Ferrite-martensite Dual Phase Steel

    Huifang Lan, Shuai Tang, Linxiu Du, Jianping Li, Sybrand van der Zwaag

    pp. 1650-1659

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    DOI:10.2355/isijinternational.ISIJINT-2020-613

    Abstract

    The combined effects of the initial microstructure and heating rate on ferrite recrystallization, austenite formation and transformation kinetics of a regular ferrite-martensite dual phase steel are investigated. Special attention is given to the effect of the martensite distribution on mechanical properties, particularly the bendability. Ferrite recrystallization in the cold rolled bainitic grade proceeds faster than in the ferrite-pearlite grade. The reason for this is the higher defect density leading to a lower activation energy. The junctions between the cementite and the ferrite grain boundaries are the preferred austenite nucleation sites. Those ferrite boundaries far away from the cementite can only be invaded by austenite through carbon supplied by cementite dissolution and carbon segregation. When using a high heating rate, nucleation of austenite can occur in a massive-like manner, and the austenite transformation kinetics is remarkably accelerated. The bendability of DP steel is determined by a combination of structural parameters, such as grain size, volume fraction as well as spatial distribution of martensite. A short in-line holding-step during heating can alleviate the banding severity and improve the bendability of this ferrite-martensite DP steel.

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    3. Resistance to Temper Softening of Low Carbon Martensitic Steels by Microalloying of V, Nb and Ti ISIJ International Vol.61(2021), No.5

  • Effects of the Destabilisation Heat Treatments on the Precipitation of Secondary Carbides and Their Effect on the Corrosion of 27 wt.% Chromium White Cast Iron

    Hue Dang Thi Hong, Hai Nguyen Hong, Minh Nguyen Ngoc, Quyen Hoang Thi Ngoc

    pp. 1660-1668

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    DOI:10.2355/isijinternational.ISIJINT-2020-581

    Abstract

    A 27 wt.% Cr white cast iron has been subjected to various destabilization heat treatments. The transformation of the matrix phase as well as the precipitatiopn of secondary carbides by destabilization heat treatments, which have been clearly determined in this paper. The results revealed that in the destabilization temperature (about 880°C), the matrix phase is enriched with elements such as C and Cr, due to dissolution of eutectic carbides at high temperature. The secondary carbides were precipitated along grain boundary of C, Cr- rich matrix, they grew up within the matrix phase. When the destabilization temperature increases up to 1000°C the number, volume and size of secondary carbides also increase, respectively (secondary carbide size up to 2,22 µm). At 1050°C/3 h, the size of secondary carbides reduce significantly with a high distribution density in the matrix phase (grain size reduce to below 0,8 µm). At 1100°C and holding time for 3 hours, secondary carbides were dissolved into the matrix, and therefore, reduce the number and grain size of secondary carbides. Effects of secondary carbides on corrosion of alloy were determined by polarization test of alloys in H2SO4 5 vol.% solution at room temperature, by the depth of corrosion layer and by microstructure analyzing of corroded surfaces of alloys. The 1050°C/3 h alloy is the best corrosion resistance between the tested alloys with a large amount of fine secondary carbides and uniformly distributed within the matrix.

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  • Transformation Delay and Texture Memory Effect of Columnar Grained Cast Slab in Low Grades Non-oriented Electrical Steels

    Xiaolong Wu, Chen Gu, Ping Yang, Xinfu Gu, Shufang Pang

    pp. 1669-1678

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    DOI:10.2355/isijinternational.ISIJINT-2020-623

    Abstract

    The cast slab of low grades non-oriented electrical steels experiences twice diffusional solid phase transformation and demonstrates the features of strong morphological memory referring to columnar structure and texture memory referring to the preferred <100> texture at room temperature. In this paper, electron backscatter diffraction (EBSD), quasi-insitu observation of heating samples, and dilatometry are used to study and analyze the two kinds of memory phenomena. The results show that the cast slab consists of about 70% coarse columnar grains and 30% small equiaxed grains. Many small equiaxed grains show Σ3 misorientations with columnar grains indicating K-S orientation relationship obeyed during phase transformation. Coarse columnar grains show a typical <100>||growing axis orientation which is the same as solidified columnar grains. The quasi-insitu observation shows that transformation of columnar grained ferrite to austenite is very sluggish and columnar grained ferrite can still be seen even at a superheating degree of 176°C for 1 hour. Dilatometry measurement indicates that the starting transformation temperatures for a columnar-grain-dominant sample and a small-equiaxed-grain-dominant sample are similar, whereas their transformation extents are quite different with columnar grained sample showing a low dilatational amount due to insufficient transformation. It is most likely that the coarse columnar grains in cast slab are retained and untransformed high temperature δ-ferrite are not subjected to twice complete transformations. These retaining columnar grains in low grades of electrical steels can be used to improve magnetic properties through optimizing processing parameters.

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  • Effect of Isothermal Transformation Treatment and Tempering on the Microstructure and Hardness of a Medium C and High Si Steels

    Gülten Kafadar, Ali Kalkanli, Abbas Tamer Özdemir, Bilgehan Ögel

    pp. 1679-1687

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    DOI:10.2355/isijinternational.ISIJINT-2020-419

    Abstract

    This research work aims to study the effect of 2wt%Mo on bainitic transformation kinetics and bainite morphology in 0.6wt%C-1.2wt%Si-1.0wt%Mn-0.2wt%Cr steel. Specimens are austenitized at 950°C and rapidly cooled in salt bath and isothermally treated between 200°C–300°C for different time intervals. Another set of specimens are rapidly cooled in oil after an austenitization treatment and then tempered in the temperature range of 200°C–550°C. In C–Mn–Si specimens, the amount of retained austenite increases with increasing the amount of bainite but no retained austenite is observed in bainitic C–Mn–Si–Mo specimens. The tempering behavior of C–Mn–Si–Mo alloy is considerably different than that of the Mo free alloy. Mo in a martensitic microstructure of C–Mn–Si–Mo alloy shows the secondary hardening effect peak upon tempering at 500°C. The examination of the secondary hardening would also contribute to the behavior of Mo in tempered bainitic steels. The bainitic specimens do not show a peak hardness but softening is retarded upon tempering at the same temperature range.

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  • Comparative Study of Microstructure-sensitive Fatigue Crack Propagation in Coarse- and Fine-grained Microstructures between Stable and Metastable Austenitic Stainless Steels Using Miniature Specimen

    Aya Matsushita, Shohei Ueki, Yoji Mine, Kazuki Takashima

    pp. 1688-1697

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    DOI:10.2355/isijinternational.ISIJINT-2020-659

    Abstract

    Microstructure-sensitive fatigue crack propagation was studied on coarse- and fine-grained stainless steels with different austenite stabilities using miniature compact-tension specimens. For coarse-grained 310S stable austenitic steel, the crack growth rate was increased by shear-localised bands formed ahead of the crack tip. For fine-grained 310S with an average grain size of ~0.25 µm, the crack-tip plastic strain was concentrated on the grains favourable to dislocation multiplication, rather than being dependent on the distance from the crack surface, which led to discontinuous crack propagation. Consequently, the fatigue crack growth rate was lower in the fine-grained 310S steel than in the coarse-grained one. In 304 metastable austenitic steel, the fatigue crack propagated within the martensite that formed ahead of the crack tip, and the crack growth rate was lower than that in the 310S steel. The grain refinement of 304 steel to a ~0.99 µm average grain size enhanced the crack growth resistance. Electron back-scatter diffraction analysis of the fracture surface revealed microstructural fragmentation due to single-variant transformation for each grain in the fine-grained 304 steel. These findings indicate that the microstructural evolution ahead of the crack tip dominates the rate of mechanically short fatigue crack propagation in austenitic stainless steels.

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  • Approximation and Characteristic Times in Precipitation Modelling

    Rutger J. Slooter, Marcel H. F. Sluiter, Winfried G. T. Kranendonk, Cornelis Bos

    pp. 1698-1707

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    DOI:10.2355/isijinternational.ISIJINT-2020-431

    Abstract

    A new approximation to the equations describing Classical Nucleation and Growth Theories, is proposed providing quick, and intuitive insight. It gives a prediction of the mean precipitate radius and number density development under quasi-isothermal conditions. Current “mean-radius”, and “multi-class” approaches to modelling classical nucleation and growth theory for precipitation, require considerable computation times. An analytical approximation is proposed to solve the equations, and its results are compared to numerical simulations for quasi-isothermal precipitation. From the approximation a start and end time for the nucleation stage is predicted, as well as a time at which growth occurs and when the coarsening stage starts. Ultimately, these times, outline the numerical solution to the precipitation trajectory, providing key insight before performing numerical simulations. This insight can be used to more efficiently simulate precipitate development, as time scales at which the various stages in precipitate development occur can be predicted for individual precipitates. When these time scales are known a numerical simulation can be used for a specific goal, for instance to only simulate nucleation and growth, thus saving computational time. Moreover, for a first indication of the precipitate development in a composition under a particular heat treatment a numerical simulation is no longer necessary. This is also useful for process control as consequences of changes in treatment can be assessed on-line. Using these approximate analytical results an estimate can be made for the matrix concentration of precipitate forming elements. Additionally some dimensionless parameters are established to provide intuitive details to the precipitation trajectory.

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  • Estimation of Solute Carbon Concentration by Electrical Resistivity Measurement in Martensitic Steel

    Takuro Masumura, Taiga Taniguchi, Shohei Uranaka, Issei Hirashima, Toshihiro Tsuchiyama, Naoki Maruyama, Hiroyuki Shirahata, Ryuji Uemori

    pp. 1708-1715

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    DOI:10.2355/isijinternational.ISIJINT-2020-669

    Abstract

    The electrical resistivity of low-carbon martensitic steels was measured to estimate the carbon concentration in the solid solution. Since electrical resistivity is influenced not only by solute carbon but also by substitutional elements, lattice defects, and second phase, the effects of these factors need to be subtracted from the total electrical resistivity to obtain an accurate solute carbon concentration via this method. Consequently, the effects of dislocations and grain boundaries were much smaller than those of solute elements, representing approximately 1–2% of the total electrical resistivity in martensitic steel. However, substitutional elements and retained austenite were found to be significantly effective. By subtracting these effects from the measured value, the change in electrical resistivity owing to solute carbon (Δρsol.C) could be formulated as a function of the carbon concentration in the solid solution of martensite (Csol) as follows:Δρsol.C[mΩmm] = 0.25 × Csol[mass%]The estimated solute carbon concentration was confirmed to correspond to the directly measured value by atom probe tomography.

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  • Behavior of Crack Generation of Slag in Continuous Solidification Process of Blast Furnace Slag

    Yasutaka Ta, Takeru Hoshino, Hiroyuki Tobo, Keiji Watanabe, Katsunori Takahashi

    pp. 1716-1724

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    DOI:10.2355/isijinternational.ISIJINT-2020-632

    Abstract

    A continuous solidification process of blast furnace slag was developed to promote the use of air-cooled slag coarse aggregate for concrete. In this process, the molten slag can solidify in only 120 s and the slag thickness is about 25 mm. This process suppresses gas generation and greatly reduces water absorption. Most of the slag is crystalline, and part of the slag has a glass layer on its surface. Slag with a glass layer is brittle because it contains several cracks. Therefore, microscopic observation and thermal stress analysis of the solidified slag were carried out to clarify the mechanism of crack generation in the plate-like slag. In the microscopic observation, several cracks with a length of about 8 mm were found in the slag with the glass layer. From the analysis, in the cooling pattern of the slag on the piled slag a temperature difference of about 200 K exists between the center and the mold side in the slag pit, and keeping this difference results in tensile stress of more than 50 MPa. However, in the cooling pattern of the crystalline slag in the piled slag, the temperature gradient in the slag in the slag pit was very small because the slag was retained in the piled slag, and as a result, the thermal stress was almost 0 MPa.

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  • Selective Separation of Metallic Fe Remaining in Slags Using Electrical Pulse Disintegration

    Hironari Kubo, Masahiro Nakanishi

    pp. 1725-1735

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    DOI:10.2355/isijinternational.ISIJINT-2020-683

    Abstract

    Metallic Fe (hereinafter abbreviated as M.Fe) is suspended in steelmaking slags due to the stirring action during blowing and is mainly recovered via pulverization, classification, and magnetic separation. However, steelmaking slags are hard, and it is difficult to transform irregular-shaped and fine M.Fe in slags into free particles through the conventional pulverization method, which requires a large energy consumption. In this study, pulverization and separation experiments of steelmaking slags were performed using electrical pulse disintegration, which is completely different from the conventional pulverization method and capable of causing preferential fracture at the heterophase interface. As a result, several free particles of M.Fe with almost no slag attached were obtained from the coarse and fine pulverized particles. In addition, the electric field analysis results of a system where spherical M.Fe exists in a slag show that electric field concentration occurs in the front and back directions of the external magnetic field. The findings also show that a fracture can occur at the interface between the M.Fe and slag due to the combination of increased discharge probability, concentration of thermal energy, and generation of the Maxwell stress. Furthermore, the larger the pulverized mass, the higher the pulverization efficiency. In sum, electrical pulse disintegration may be advantageous for actual operations, where large quantities of oxides employed in the steel industry, such as steelmaking slag, spent refractories, and raw materials, should be treated in a short time with low energy consumption.

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  • Quantitative Evaluation of Solute Hydrogen Effect on Dislocation Density in a Low-carbon Stable Austenitic Stainless Steel

    Kengo Kamei, Yuuki Koumura, Arnaud Macadre, Koichi Goda

    pp. 1736-1738

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    DOI:10.2355/isijinternational.ISIJINT-2020-677

    Abstract

    The effects of hydrogen on dislocations are generally understood through Transmission Electron Microscope studies. Novel methods of X-Ray Diffraction analysis provide the means of quantitative measurements of dislocation densities and the evolution of cross-slip in austenitic stainless steels. In a low-carbon austenitic stainless steel (SUS316L) with and without solute hydrogen, and strained by cold-rolling, the maximum dislocation densities were measured, with hydrogen clearly increasing the maximum dislocation density, and the ratio of screw dislocations was shown to be similar regardless of hydrogen content.

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    3. Quantitative Analysis of Solidification of Compacted Graphite Irons – A Modelling Approach ISIJ International Vol.61(2021), No.5
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