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ISIJ International Vol. 60 (2020), No. 12

ISIJ International
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Grid List Abstracts
オンライン版ISSN: 1347-5460
冊子版ISSN: 0915-1559
発行機関: The Iron and Steel Institute of Japan

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キーワードランキング

21 Dec. (Last 30 Days)

ISIJ International Vol. 60 (2020), No. 12

Preface to the Diamond Jubilee Issue on “Selected Topics in Iron and Steel and Their Processing toward the New Steel Age”

Ryuji Uemori

pp. 2629-2629

DOI:

10.2355/isijinternational.60.2629

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Congratulations on the Diamond Jubilee of ISIJ International

Shuhei Onoyama

pp. 2630-2630

DOI:

10.2355/isijinternational.60.2630

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Congratulatory Messages from ISIJ Honorary Members

Harshad Kumar Dharamshi Hansraj Bhadeshia, Ruiyu Yin, Hae-Geon Lee, John William Morris

pp. 2631-2632

DOI:

10.2355/isijinternational.60.2631

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Review on the Phase Equilibria in Iron Ore Sinters

Eetu-Pekka Heikkinen, Mikko Iljana, Timo Fabritius

pp. 2633-2648

DOI:

10.2355/isijinternational.ISIJINT-2020-224

抄録

Sintering process is a commonly used pre-treatment process for iron containing burden materials with an aim to produce porous, agglomerated sinter material with suitable properties to be charged into the blast furnace. During the sintering process the material undergoes a series of reactions, during which the conditions vary considerably. These changes in temperature and state of oxidation cause changes in the mineralogical composition of the material and although the sintering process does not completely reach the chemical equilibrium, it is important to understand the phase equilibria of the sinter system in order to analyse and control the effect of various factors on the sintering process.The purpose of this paper is to give a review on the research related to phase equilibria in iron ore sinters. The main components of the sinter are FeO, Fe2O3, SiO2, CaO, Al2O3 and MgO and by studying the phase equilibria of this system, the behaviour of sinters can be evaluated. Based on the experimental data, oxide databases have been created to provide thermochemical data of all the necessary compounds within this system. Concerning the solutions, more research is required related to SFCA phases. These databases are commercially available with thermochemistry software and can be used to compute phase diagrams illustrating the effect of different factors on the phase equilibria within the FeO–Fe2O3–SiO2–CaO–Al2O3–MgO system. Phase diagrams provide a useful tool to study the behaviour of the material in both sintering process itself as well as in the following reduction processes such as the blast furnace.

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Dominant Factor Affecting Reducibility of Calcio-wüstite Originating from Silico-ferrite of Calcium and Aluminum

Miyuki Hayashi, Boyuan Cai, Masahiro Susa

pp. 2649-2658

DOI:

10.2355/isijinternational.ISIJINT-2020-140

抄録

The reducibility of calcio-wüstite (CW) originating from silico-ferrite of calcium and aluminum (SFCA) has been investigated from the perspectives of the morphology of SFCA and the concentration of FeO in CW. Two types of SFCA sample were prepared: columnar SFCA and acicular SFCA. The former was synthesized from reagent grade powders of Fe2O3, CaCO3, SiO2 and Al2O3, and contained columnar SFCA grains covered with slag. The latter was synthesized from iron ore and reagent grade CaCO3, and contained acicular SFCA grains, which were smaller than the columnar SFCA grains and had fine pores nearby. These samples were reduced in an XRD apparatus for high temperature use in a condition simulating a blast furnace where the oxygen partial pressure was controlled by CO–CO2 mixtures. The microstructures of the samples before and after reduction were observed by electron probe microanalysis (EPMA). XRD profiles indicated: (i) both SFCA samples were reduced to Fe via CW at 1000°C and (ii) acicular-SFCA-origin CW was reduced to Fe earlier than columnar-SFCA-origin CW, which suggests that the reducibility of acicular-SFCA-origin CW is higher than columnar-SFCA-origin CW. EMPA indicated: (i) most residual parts of acicular-SFCA-origin CW phase kept the morphologic feature of having fine pores as acicular SFCA during reduction as well and (ii) the FeO concentration in acicular-SFCA-origin CW was lower than that in columnar-SFCA-origin CW. Hence, it is concluded that the reducibility of SFCA-origin CW is dominated by the morphology of CW but not by the concentration/activity of FeO in CW.

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Mechanisms of Phase and Microstructure Formation during the Cooling of “Fe2O3”–CaO–SiO2–Al2O3 Melts in Air and Implications for Iron Ore Sintering

Stuart Nicol, Siyu Cheng, Evgueni Jak, Peter Charles Hayes

pp. 2659-2668

DOI:

10.2355/isijinternational.ISIJINT-2020-182

抄録

Experimental studies have been undertaken on the controlled solidification of iron oxide-rich melts in the system “Fe2O3”–CaO–SiO2–Al2O3 in air to determine the mechanisms of phase and microstructure formation during the cooling.Selected bulk compositions, containing approximately 2 wt% Al2O3 and CaO/SiO2 = 3.5, were cooled at a fixed rate of 2K/s from fully liquid melts. The samples were rapidly quenched from selected temperatures, and the microstructures and phases present were examined using scanning electron microscopy (SEM) and electron probe X-ray microanalysis (EPMA).It has been shown that, on non-equilibrium cooling in air, the magnetite and hematite phases are retained to sub-solidus temperatures despite the presence of the pseudo-ternary peritectic reaction H + L → SFCA + L that would occur under equilibrium cooling. The SFCA and SFC-I phases appear to nucleate preferentially at the interfaces between the magnetite and liquid phases; this phenomenon appears to be associated with common crystallographic features in the magnetite and the SFCA phases.It has also been shown that rapid formation of secondary hematite can take place through the liquid phase assisted oxidation of the primary magnetite grains. The mechanism of this reaction has not been previously reported.

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Numerical Analysis of Raceway Formation in Isothermal and Non-reactive Packed Bed

Shinsuke Taya, Shungo Natsui, Jose Adilson de Castro, Hiroshi Nogami

pp. 2669-2677

DOI:

10.2355/isijinternational.ISIJINT-2020-167

抄録

Raceway zone is one of the most important regions in blast furnace, since it governs efficiency and stability of the blast furnace process through combustions of coke and auxiliary reducing agent, burden descent, fines generation, and so on. Therefore, quantitative understanding of the raceway behavior is indispensable to designing and realizing highly efficient operation of blast furnace. In the raceway, a cavity is formed and the coke particles circulate due to interaction between particles and high velocity blast gas, and consumption of coke. Thus, two-phase flow behavior is one of the key features of the raceway formation. In this study, the formation behavior of the raceway in the isothermal and non-reactive packed bed was numerically discussed from the viewpoint of two-phase flow. The mathematical model used in this study consisted of Lagrangean particle tracking method and Eulerian computational fluid dynamic technique, and these were combined through voidage distribution and momentum exchange. This model was applied to a small-scale cold-model condition. The simulation results revealed the raceway formation behavior such as variations of cavity size, particle motion, gas flow field, and contact force network. Furthermore, the effects of the initial packing structure on the raceway formation, the differences in the raceway formation behavior in the fixed bed and the moving bed and their mechanisms were clearly elucidated.

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Effect of Hydrogen Concentration in Reducing Gas on the Changes in Mineral Phases during Reduction of Iron Ore Sinter

Taichi Murakami, Hiroki Wakabayashi, Daisuke Maruoka, Eiki Kasai

pp. 2678-2685

DOI:

10.2355/isijinternational.ISIJINT-2020-180

抄録

In order to decrease CO2 emission from the ironmaking process, an increasing use of hydrogen in blast furnace (BF) ironmaking is a promising way. In this case, the properties of iron ore sinter such as reducibility and strength need to be optimized because hydrogen reduction of iron oxide is an endothermic reaction and temperature distribution in BF drastically changes. In this study, the effect of hydrogen concentration in the reducing gas on the changes in mineral phases during reduction of iron ore sinter is evaluated. Mineral composition of the ten types of sinter samples was analyzed by XRD and image analysis. Sinter sample was reduced under the simulated conditions such as Low-H2 (N2 − 48%(CO + CO2) − 5.8%(H2 + H2O)) and High-H2 (N2 − 48%(CO + CO2) − 13%(H2 + H2O)). After reduction, microstructure of the sample was observed. Iron ore sinters usually consist of mineral phases such as hematite, magnetite, calcium-ferrites and slag. Furthermore, calcium-ferrite phases are roughly divided into four types: 1) acicular texture coexisted with primary hematite (1H-ACF), 2) columnar texture coexisted with secondary hematite (2H-CF), 3) small and 4) large columnar textures coexisted with magnetite (M-FCF and M-CCF). An increase in hydrogen concentration of reducing gas accelerates the reduction of hematite, 1H-ACF, and 2H-CF in all sinter samples, while it does not affect the reduction of magnetite, and calcium-ferrite coexisted with magnetite.

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Numerical Investigation for the Temperature Dependency of Coke Degradation by CO2 Gasification Reaction in a Blast Furnace

Yui Numazawa, Yohsuke Matsushita, Hideyuki Aoki, Atsuki Komiya

pp. 2686-2694

DOI:

10.2355/isijinternational.ISIJINT-2020-198

抄録

To quantitatively evaluate the temperature dependency of coke degradation by CO2 gasification reaction in a blast furnace, kinetic analyses of gasification reaction with mass transfer for the coke model with approx. 200 million voxels developed from X-ray CT images at the reaction temperatures of 1373, 1573, 1773 and 1973 K were performed. At high reaction temperature, the gas concentration of CO2 was high in the external area of the coke model, and the coke matrix voxels vanished mainly around the external surface. Distinguishing surface area of interface between a carbon matrix voxel and a pore voxel with the gas concentration of CO2 at a neighbor pore voxel, although the surface area with the high gas concentration of CO2 accounted for the majority of the total surface area at 1373 K, the ratio was lower at over 1573 K than at 1373 K. In addition to this, from the effectiveness factor of catalyst, the initial rate-controlling step was chemical reaction at 1373 K but pore diffusion at over 1573 K. Also, although the frequency distribution of local porosity showed unimodal regardless of the progress of reaction, the standard deviation calculated from the distribution was changed by reaction. The change rate of the standard deviation by reaction seemed to be larger at high reaction temperature than at low reaction temperature. The logarithm of the change rate hardly depended on reaction temperature under 1573 K but was proportional to the inverse of the temperature over 1573 K. This study quantitatively showed that the rate-controlling step affects the coke structure after reaction largely.

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Numerical Approach to Comprehend for Effect of Melts Physical Properties on Iron-slag Separation Behaviour in Self-reducing Pellet

Ko-ichiro Ohno, Shungo Natsui, Sohei Sukenaga, Kazui Tonya, Takayuki Maeda, Kazuya Kunitomo

pp. 2695-2704

DOI:

10.2355/isijinternational.ISIJINT-2020-218

抄録

A smooth iron-slag separation during ironmaking process is necessary for the steel refining process, even in case of “Zero Carbon Ironmaking”. For a fundamental comprehension of the effect of the physical properties of the melts on the iron-slag separation behaviour, a numerical approach with a practical multi-interfacial smoothed particle hydrodynamics (SPH) simulation for the tracking of the iron-slag separation behaviour is undertaken in this study.Experimental values for iron-slag separation conditions from a previous work and estimated physical properties from literature were used for the numerical analysis. The CLS-SPH method was able to reproduce the iron-slag separation behaviour where iron aggregated in a unitary sphere and the slag discharged onto the iron surface. A less viscous slag may reduce the negative impact on the separation. A slag with a high surface tension enables the slags to agglomerate and decreases the number of elements that may disturb the iron agglomeration. A highly dense slag has a strong influence on the variation of the iron-slag interface due to a larger momentum. The interfacial tension showed no obvious effect on the separation behaviour in the range of experimental values considered in this study.

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Review on the High-Temperature Thermophysical Properties of Continuous Casting Mold Fluxes for Highly Alloyed Steels

Zhanjun Wang, Il Sohn

pp. 2705-2716

DOI:

10.2355/isijinternational.ISIJINT-2019-522

抄録

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

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Thermodynamic Modeling of Liquid Steel

Youn-Bae Kang

pp. 2717-2730

DOI:

10.2355/isijinternational.ISIJINT-2020-101

抄録

Thermodynamic property of liquid steel is often described by activity coefficient of solute in the steel (the other form of partial excess Gibbs energy of the solute). Reliable description of the activity coefficient is required in order to predict equilibrium content of the solute as accurately as possible. In the present article, a number of such approaches are reviewed, with emphases on basic assumption and inherent character of each formalism/model, and on its applicability at high alloyed liquid steel. Chemical interaction between elements was categorized as weak interaction (i.e., between metal and metal) and strong interaction (i.e., between metal and non-metal). Each formalism/model was analyzed in the view of thermodynamic consistency (Gibbs-Duhem equation and Maxwell’s relation). It is concluded that two issues should be explicitly and simultaneously considered: obeying thermodynamic consistency and treating strong chemical interaction. The former ensures its applicability at higher solute content, and the latter is necessary to properly handle the strong interaction between metallic elements and non-metallic elements, contrary to conventional random mixing assumption.

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Development and Prospects of Refining Techniques in Steelmaking Process

Naoki Kikuchi

pp. 2731-2744

DOI:

10.2355/isijinternational.ISIJINT-2020-186

抄録

The development of refining techniques in the steelmaking process over the last 60 years and the prospects for the future were reviewed. In Japan, hot metal pretreatment started in the 1960s with the aim of reducing refining costs and improving quality, and its purposes have now transitioned to meeting new requirements for reduced treatment time, reuse of steelmaking slag and use of diverse iron sources. In converter refining, in addition to high speed decarburization, visualization of phenomena and sensing modeling techniques are becoming more important for combined use with data science techniques. In ladle metallurgy (secondary refining), techniques to realize high speed treatment and heating of the molten steel are key issues. The necessity of process revolution to contribute to a sustainable social environment is also discussed briefly.

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Quantitative Phase-field Modeling and Simulations of Solidification Microstructures

Munekazu Ohno

pp. 2745-2754

DOI:

10.2355/isijinternational.ISIJINT-2020-174

抄録

This review presents the development of quantitative phase-field models for simulating the formation processes of solidification microstructures, with particular attention to the theoretical foundation and progress in modeling. The symmetry of interpolating functions required to reproduce the free-boundary problem in the thin-interface limit and the necessity of antitrapping current in the diffusion equation are discussed. In addition, new cross-coupling in the phase-field equation for two-sided asymmetric diffusion is briefly described. Recent achievements of large-scale simulations using high-performance computing techniques are explained. Furthermore, some important applications of quantitative phase-field simulations such as investigations of cellular and dendritic growth, microsegregation, and peritectic reaction in carbon steel are discussed.

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

Hideyuki Yasuda, Kohei Morishita, Masato Yoshiya, Taka Narumi

pp. 2755-2764

DOI:

10.2355/isijinternational.ISIJINT-2020-486

抄録

X-ray transmission imaging with X-ray diffractometry and time-resolved tomography with three-dimensional X-ray diffraction microscopy have been used to observe solidification and transformation in carbon steel and other Fe-based alloys. The imaging techniques showed a massive-like transformation, in which multiple austenite grains were produced in a single δ grain through a solid–solid transformation. The critical velocity from the diffusion-controlled growth of the γ phase to the massive-like transformation was as low as 5 µm/s. X-ray imaging indicated that the δ phase transforms massively to the γ phase in the conventional solidification processes, such as continuous casting. The massive-like transformation and multiple γ grains that were produced in the transformation were related to the subsequent microstructure evolution and casting defect formation. Solidification model including the massive-like transformation is expected to improve our understanding the solidification and the related phenomena in the peritectic steel systems.

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Phase Identification of Crystal Precipitated from Molten CaO–SiO2–FeOx–P2O5 Slag by High Temperature In-situ X-ray Diffraction

Masanori Suzuki, Honami Serizawa, Norimasa Umesaki

pp. 2765-2772

DOI:

10.2355/isijinternational.ISIJINT-2020-148

抄録

For the first time, we have succeeded in directly identifying the crystalline phase precipitated from the fully liquid slag of the CaO–SiO2–FeOx–P2O5 system by high-temperature in-situ X-ray diffraction analysis. Dephosphorization from molten iron can be promoted by 2CaO·SiO2 precipitates in molten P2O5-containing slag because they form a solid solution with 3CaO·P2O5. Knowledge of the crystal structure of the 2CaO·SiO2 precipitate is important because it strongly affects the phosphorus solubility. Although it is believed that the α phase of the 2CaO·SiO2–3CaO·P2O5 solid solution precipitates from the molten slag, the crystal structure of the precipitate has not been identified because the crystal structure of the 2CaO·SiO2 compound rapidly changes by phase transition when cooled from high temperature. In this study, slag samples were aerodynamically levitated and completely melted by laser heating under an Ar atmosphere, and then the diffraction patterns were obtained by transmitting a high-energy and high-intensity X-ray beam into the molten slag. We verified that the α-2CaO·SiO2 phase precipitated as the primary crystal phase from molten slag containing 10–30 mass% FeOx and 5 mass% P2O5, whereas nagelschmidtite precipitated for the molten slag with high P2O5 content. The α-2CaO·SiO2 precipitates contained much higher FeOx content than the reported solubility limit, which was supported by the diffraction angles positively deviated from those of the FeOx-free α phase in the CaO–SiO2–P2O5 system and chemical analysis of the quenched slag sample. This excess FeOx solute may influence the phosphorus distribution in the α-2CaO·SiO2 precipitates.

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Determination of Thermal Diffusivity/conductivity of Oxide Scale Formed on Steel Plate by Laser Flash Method through Thermal Effusivity Measurement by Transient Hot-strip Method

Rie Endo, Hiroki Hayashi, Mu Li, Megumi Akoshima, Hikaru Okada, Hiroshi Tanei, Miyuki Hayashi, Masahiro Susa

pp. 2773-2779

DOI:

10.2355/isijinternational.ISIJINT-2020-163

抄録

Thermal diffusivity and conductivity were determined for oxide scale formed on steel plate by the laser flash method in combination with thermal effusivity measurement by the transient hot-strip method. The thermal effusivity measurement technique was confirmed by measurement of silica glass, and the value was determined to be 2.52 kJ m−2 s−1/2 K−1 for the oxide scale formed on an ultra-low-carbon steel plate by oxidation in air at 900°C for 3600 s. Thermal diffusivity measurements were also conducted for 1 mm-thick steel plates oxidized in air at 900°C for 770–3600 s by the laser flash method. The apparent thermal diffusivity of samples provided the thermal diffusivity of the oxide scale based on three-layered analysis by inputting the measured value of the thermal effusivity. The measured values suggested that no significant boundary resistance exists between the oxide scale and the steel plate. The thermal conductivity and diffusivity of the oxide scale were calculated to be 1.6 W m−1 K−1 and 4.0 × 10−7 m2 s−1, respectively.

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Activities of P2O5 in Solid Solutions between Di-calcium Silicate and Tri-calcium Phosphate at 1573 K

Taihei Saito, Toru Nishimura, Keijiro Saito, Masakatsu Hasegawa

pp. 2780-2786

DOI:

10.2355/isijinternational.ISIJINT-2020-181

抄録

The knowledge of P2O5 activities in solid solutions between Ca2SiO4 and Ca3P2O8 is required for more effective dephosphorization. In the present study, firstly, the P2O5 activities in these solid solutions coexisting with CaO or CaSiO3 at 1573 K were measured by equilibrating molten copper with oxide phases under a stream of Ar + H2 + H2O gas mixtures. Subsequently, the sub-regular solution model was applied to Ca2SiO4–Ca3P2O8 solid solutions. The P2O5 activities calculated with the solution model agreed well with the present experimental results and the literature data. It was found that the P2O5 activity in Ca2SiO4–Ca3P2O8 solid solution coexisting with CaO was about 7 digits lower than that coexisting with CaSiO3.

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Deoxidation Equilibria of Fe–Mn–Al Melt with Al2O3 or MnAl2O4 at 1873 and 1773 K

Ryosuke Nishigaki, Hiroyuki Matsuura

pp. 2787-2793

DOI:

10.2355/isijinternational.ISIJINT-2020-177

抄録

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

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

Sohei Sukenaga, Masayuki Ogawa, Yutaka Yanaba, Mariko Ando, Hiroyuki Shibata

pp. 2794-2806

DOI:

10.2355/isijinternational.ISIJINT-2020-326

抄録

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

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Viscosity of Slag Suspensions with a Polar Liquid Matrix

Noritaka Saito, Daigo Hara, Seiyu Teruya, Kunihiko Nakashima

pp. 2807-2818

DOI:

10.2355/isijinternational.ISIJINT-2020-396

抄録

Under the operating temperatures employed in steelmaking, most slags and fluxes often contain solids, such as undissolved CaO and its reaction products; thus, they are more viscous than their fully liquid states. However, few studies have considered the dielectric interactions of solid particles with the liquid matrix in such systems. In the present study, the viscosity of suspensions of dispersed particles consisting of polyethylene beads in a matrix of silicone oil or aqueous glycerol at room temperature was measured. Then, empirical models for estimating the viscosity based on the Einstein–Roscoe equation were proposed. Furthermore, the viscosity of suspensions of CaO and MgO particles dispersed in a matrix of CaO–Al2O3–SiO2–MgO slag at 1773 K was measured, and the feasibility of the proposed viscosity equations was investigated. As expected, the viscosities of the suspensions of polyethylene beads dispersed in silicone oil and glycerol increased with an increasing bead volume fraction. Under comparable measurement conditions, the viscosities of the glycerol suspensions were higher than those of the silicone oil suspensions. The proposed viscosity models based on the Einstein–Roscoe equation and the capillary number reproduced the viscosity of the silicone oil suspensions but underestimated that of the glycerol suspensions. The trend of increasing viscosity of the molten slag suspensions with dispersed CaO and MgO particles was similar to that of the room-temperature suspensions, exhibiting Bingham non-Newtonian behavior. The viscosity model composed with the results from the glycerol aqueous suspensions underestimated the slag viscosity, which can be attributed to the repulsive forces in the high-polarity liquid matrix.

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Characterization of Non-metallic Inclusions and Clusters during Production of Low-carbon IF Steel

Dmitry Gorkusha, Andrey Vladimirovich Karasev, Olga Komolova, Konstantin Vsevolodovich Grigorovich, Pär Göran Jönsson

pp. 2819-2828

DOI:

10.2355/isijinternational.ISIJINT-2020-202

抄録

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

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Effect of Deoxidizing Element on the Hot Ductility of Boron-Containing Steel

Kenji Taguchi, Shin Takaya, Mitsuhiro Numata, Toru Kato

pp. 2829-2837

DOI:

10.2355/isijinternational.ISIJINT-2020-192

抄録

Improving the surface quality of the casting slab increases the productivity of steel. The slab surface sometimes has transverse cracks along a grain boundary when the slab is bent and/or unbent around the transformation temperature from the austenite to ferrite phases. In particular, for boron-containing steel, the defect is strongly influenced by the precipitation of BN on the grain boundary. In this study, the effect of deoxidizing elements, such as aluminum, calcium, and zirconium, on the hot ductility of boron-containing steel have been investigated fundamentally. The addition of zirconium or calcium improves the hot ductility of boron-containing steel by comparing with that of aluminum. An oxide containing calcium or zirconium acts as a more effective nucleus for BN precipitation than alumina, and the excess precipitation of BN on the grain boundary is suppressed. This improves the hot ductility of boron-containing steel in the region of single-phase austenite. Moreover, the hot ductility of Al+Zr-added steel is the best, even in the region of coexisting austenite and ferrite phases, because the precipitation of ferrite (α-Fe) on the grain boundary is suppressed compared to that of Al+Ca-added steel.

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Optimization of the Interfacial Properties between Mold Flux and TiN Substrate Through the Regulation of B2O3

Lejun Zhou, Zihang Pan, Wanlin Wang, Junyu Chen

pp. 2838-2844

DOI:

10.2355/isijinternational.ISIJINT-2020-184

抄録

Titanium nitride (TiN) inclusions are easy to precipitate in the high temperature processing of titanium alloying steels, which tends to introduce numerous surface defects on the final continuous casting slabs. This study utilizes B2O3 to regulate the interfacial properties between the designed mold fluxes and TiN, with the aim to resolve above problems. The results show that the spreading behavior of the mold flux on the TiN substrate is enhanced, and the interfacial contact angle starts to drop at a lower temperature (from 1473 K for Sample 1 to 1343 K for Sample 4) with the addition of 0–9 wt.% B2O3, as the melting behavior of the designed mold fluxes has been improved. The interfacial reactions between the TiN substrate and molten fluxes are also promoted with the addition of B2O3, where more bubbles are observed in the tested mold fluxes samples. For Sample 1 without B2O3, quite a few TiN particles couldn’t be dissolved and remains in the matrix phase, where the major formed phase is perovskite (CaTiO3) that would deteriorate the high temperature properties of mold flux severely. However, most TiN particles have been dissolved in the optimized mold fluxes, as major of them have reacted with mold fluxes, resulted in the more generation of titanium oxides phase in the samples. In addition, the calculated phase diagram of CaO–SiO2–TiO2 slag system under different B2O3 contents indicates that the formation and precipitation of CaTiO3 can be effectively inhibited by the addition of B2O3.

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Control of Laser Focal Point by Using an Electrically Tunable Lens in Laser-induced Plasma Optical Emission Spectrometry

Yusuke Fugane, Shunsuke Kashiwakura, Kazuaki Wagatsuma

pp. 2845-2850

DOI:

10.2355/isijinternational.ISIJINT-2020-170

抄録

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

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

Takayuki Harano, Yu Nemoto, Reiko Murao, Masao Kimura

pp. 2851-2858

DOI:

10.2355/isijinternational.ISIJINT-2020-188

抄録

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

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Kinetic Analysis Considering Particle Size Distribution on Ca Elution from Slags in CaO–SiO2–MgO–Al2O3–Fe2O3 System

Yoshiaki Kashiwaya, Seitarou Tauchi, Takanori Nomura, Tomohiro Akiyama

pp. 2859-2869

DOI:

10.2355/isijinternational.ISIJINT-2020-265

抄録

The understanding of the behavior of alkali elution from slags is important for their effective recycling and utilization. In a previous study, it was reported that the addition of iron oxide to steel slags significantly inhibited alkali elution. A lower modified basicity, i.e., CaO/(SiO2 + Fe2O3) ratio, indicates a lower alkali elution from the slags. In addition to the effect of Fe2O3 content, particle size distribution is an important parameter to determine the elution of Ca quantitatively.In this study, a kinetic model considering particle size distribution is developed and applied to the results of a dissolution experiment using samples of a slag, which is designated as SlagF4, with different particle size distributions. In the new kinetic model, an effective surface area and the effectiveness of total surface area α are introduced, and a kinetic analysis is performed.The rate constant k obtained is a fixed value for one sample type; k decreased with the increase in Fe2O3 content.The values of α increased significantly with the increase in the total surface area ST (i.e., a decrease in particle diameter) in different particle size distributions. It is discovered that α typically represents the Ca elution tendency. Additionally, the change in α is small with the change in the Fe2O3 content. It is demonstrated that the developed kinetic model is valid for the analysis of Ca elution for samples with different particle size distributions.

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Development of Metal Supported SOEC for Carbon Recycling Iron Making System

Hiroki Takasu, Yuko Maruyama, Yukitaka Kato

pp. 2870-2875

DOI:

10.2355/isijinternational.ISIJINT-2020-506

抄録

A new solid oxide electrolysis cell (SOEC)—a metal-supported SOEC (MS-SOEC) where the SOEC is structured on a metal support and is capable of extending the cell surface area to a wider extent than a conventional ceramics base SOEC—was designed for carbon dioxide (CO2) recycling in the ironmaking process. The MS-SOEC demonstrated CO2 reduction and CO and oxygen production properties. The possibility of carbon cycling was examined with the CO2 resource utilization technology using MS-SOEC and its application to the iron-making process. The required cell area of MS-SOEC for an iACRES, which combines an active carbon recycling energy system (ACRES) and a steelmaking process, was estimated using experimental results. By improving the performance of the cell, MS-SOEC was expected to be applied to a carbon recycling ironmaking system that could contribute to the establishment of a zero-emission CO2 iron making system.

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Development of a Shape Meter Employing the LED Dot Pattern Projection Method for a Hot Strip Finishing Mill

Yoshito Isei, Tomoya Kato, Masahiro Osugi, Takeshi Ohta

pp. 2876-2885

DOI:

10.2355/isijinternational.ISIJINT-2019-718

抄録

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

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Dynamic Control of Flatness and Elongation of the Strip in a Skin Pass Mill

Tomoyoshi Ogasahara, Takuya Kitamura, Shin-ichiro Aoe, Jun-ichi Tateno, Kazuya Asano

pp. 2886-2895

DOI:

10.2355/isijinternational.ISIJINT-2020-147

抄録

This paper proposes a dynamic control method for strip flatness and elongation in a skin pass mill. In conventional feedback control, the target values of flatness and elongation are fixed. However, elongation control to a fixed target value is often insufficient to achieve strip flatness when rolling force manipulation for elongation control causes work roll deflection. To improve flatness control performance, we propose an elongation control method which considers flatness. In the proposed method, an optimization problem is solved periodically. The objective function, including flatness control error, is minimized subject to constraints such as the range of elongation and strip thickness and the control outputs. A feedforward control method which suppresses elongation deviations during mill speed changes is also proposed. Accurate online prediction of rolling force using a physical model is difficult due to the heavy computational load. The proposed method utilizes the relationship between the strip strain rate and deformation resistance, which is measured offline, and the designated rolling force change from low to top rolling speed, and does not require a physical model or heavy computation load. An evaluation by simulation and experiments showed that the proposed method improves flatness and elongation control performance.

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Formulation of a Generalized Flow Curve for 0.2% Carbon Steel under High-speed Hot Forming Conditions by a Regression Method

Hyeon-Woo Park, Kyunghyun Kim, Hyung-Won Park, Sheng Ding, Jun Yanagimoto

pp. 2896-2904

DOI:

10.2355/isijinternational.ISIJINT-2020-175

抄録

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

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Uniform Hot Compression of Nickel-based Superalloy 720Li under Isothermal and Low Friction Conditions

Satoko Horikoshi, Akira Yanagida, Jun Yanagimoto

pp. 2905-2916

DOI:

10.2355/isijinternational.ISIJINT-2020-214

抄録

Isothermal compression tests at 1000°C and 0.1 s−1 strain rate, in which mica or glass sheets were used as a lubricant, were conducted. Isothermal condition was achieved by placing high-heat-resistant (HHR) alloys between a workpiece and ceramic tools in the induction-heating configuration to prevent heat from escaping to the ceramic tools. To perform high compression tests, it was necessary to increase the diameter of the HHR alloy, for which a new single-turn coil was designed using FEM calculation coupled with deformation-temperature electromagnetic fields. In order to obtain the correct flow stress, inverse analysis was conducted using the FEM calculation, in which temperature and strain rate fluctuations were compensated. However, the compensation was insufficient when the distribution of temperature and strain rate was large. The use of glass sheets as a lubricant considerably reduced friction and uniform deformation was achieved. Thus, flow stress obtained using the inverse analysis became extremely reliable. The flow stress of mica obtained using inverse analysis with a constant friction coefficient was different from that of glass. Introducing new friction model that the friction coefficient changed from 0.02 to 0.3, the flow stress of mica was consistent with that of glass. Therefore, the flow stress obtained using the inverse analysis for the new configuration proposed in this study proved to be reliable.

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Impact of the Different Friction Coefficients on the Tools on the Mechanics of the Mannesmann 2-roll Tube Piercing

Meriane Fernandes, Nabil Marouf, Pierre Montmitonnet, Katia Mocellin

pp. 2917-2926

DOI:

10.2355/isijinternational.ISIJINT-2020-290

抄録

A numerical parametric study on friction in cross-roll tube piercing is reported in this paper, in order to assess the role of the different friction coefficients on the different parts of the complex tooling of this process (friction on cross-rolls, Diescher disks, piercing plug). Their effects on entrainment speed, state of strain and stress are quantitatively evaluated using the 3D Finite Element Method (FEM). This knowledge allows measures to be taken in case of friction-dependent defects occurring on the piercing mill. Simple regression formulae are proposed which highlight which friction coefficient (s) most impact feed efficiency, twist angle, piercing plug force and torques on the different tools. Based on these relations, a strategy is developed, involving measurements to be performed and equations to be used for an unambiguous friction coefficients identification procedure.

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Calibration of Distortional Plasticity Framework and Application to U-draw Bending Simulations

Shin-Yeong Lee, Seong-Yong Yoon, Jin-Hwan Kim, Frédéric Barlat

pp. 2927-2941

DOI:

10.2355/isijinternational.ISIJINT-2020-391

抄録

A new version of a distortional plasticity framework, the so-called homogeneous anisotropic hardening (HAH), was investigated regarding its calibration and its application to the numerical analysis of the U-draw bend test. First, the mechanical properties of a dual-phase steel sheet sample, DP780, were characterized using uniaxial tension, bulge and elastic loading-unloading tests to provide the data for the calibration of a conventional constitutive description assuming isotropic hardening. Then, tension-compression with different load reversal numbers and two-step tension tests were performed to produce strain path changes that lead to anisotropic hardening effects. The coefficients of this new model, called HAH20, were determined with an optimization procedure. Several sets of coefficients were identified depending on the number of reversals considered in tension-compression. U-draw bend test simulations were also carried out to validate the HAH20 model and its implementation in a finite element code, to compare the results with an older version of the HAH model family, and to assess the influence of the input data used in the coefficient calibration. In addition, the influence of the cross-loading effect on the U-draw bending predictions was examined. The main conclusion of this work is that the HAH20 coefficients calibrated for the description of reverse loading depend on the number of reversals during the tension-compression tests. The results of the U-draw bending simulations indicate that this dependence may lead to significant differences in the predicted amounts of springback.

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Interaction of Alloying Elements with Migrating Ferrite/Austenite Interface

Goro Miyamoto, Tadashi Furuhara

pp. 2942-2953

DOI:

10.2355/isijinternational.ISIJINT-2020-216

抄録

Alloying elements greatly influence phase transformation kinetics in steel due to local partitioning and interfacial segregation to migrating interface and quantitative understanding of the alloying effects is a key for tailoring mechanical properties of modern high strength steels. Energy dissipation for interface migration is an important concept to understand the alloying effects and also to control carbon enrichment in untransformed austenite in multiphase high strength TRIP or DP steels. In this review, possible sources causing energy dissipation for interface migration are introduced and the energy dissipation for interface migration in formations of grain boundary ferrite (α), Widmanstatten α and bainitic α investigated in various systems are summarized.

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Control of Core-shell Type Second Phase Formed via Interrupted Quenching and Intercritical Annealing in a Medium Manganese Steel

Toshihiro Tsuchiyama, Takayuki Sakamoto, Shohei Tanaka, Takuro Masumura

pp. 2954-2962

DOI:

10.2355/isijinternational.ISIJINT-2020-164

抄録

Medium manganese steel (Fe-5.0%Mn-1.2%Si-0.10%C alloy) was subjected to interrupted quenching from the austenite single-phase region to a temperature between Ms and Mf followed by intercritical annealing in the ferrite and austenite dual-phase region at 923 K. As a result, a core-shell type second phase, which consisted of a fresh martensite core surrounded by a film-like retained austenite shell, was formed. The mechanism and kinetics of reversion for the interrupted-quenched specimens were analyzed with DICTRA simulation and TEM observation. With regard to the effect of the core-shell type second phase on mechanical properties, it was inferred that the fresh martensite core functioned as a hard second phase and enhanced work hardening by stress partitioning similar to DP steel, while the film-like retained austenite contributed to improved ductility due to the TRIP effect. As the interrupted quenching temperature decreased, the volume fraction of the fresh martensite core decreased, while the stability of the retained austenite shell increased. This showed potential for controlling the strength and ductility balance of medium manganese steel. A possible beneficial effect of the core-shell type second phase on the ductile fracture behavior was also discussed in terms of stress/strain relaxation at the interfaces between hard martensite and ferrite matrix.

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Electron Theory Calculation of Thermodynamic Properties of Steels and Its Application to Theoretical Phase Diagram of the Fe–Mo–B Ternary System

Masanori Enoki, Kota Takahashi, Soei Mitomi, Hiroshi Ohtani

pp. 2963-2972

DOI:

10.2355/isijinternational.ISIJINT-2020-189

抄録

In this study, the ground structures of the Fe–Mo–B ternary systems were estimated by first-principles calculations based on genetic algorithm, and the free energies of their structures are evaluated by electronic calculations and statistical thermodynamic techniques. In addition, the phase diagram at finite temperature was theoretically constructed using the calculated free energies, and the result was compared with the experimental knowledge. The space groups and compositions of many ground structures obtained by the calculations correspond well with the experimental findings, but the agreement is not perfect. However, by including metastable structures by only a few kJ/mol than the ground state, it becomes clear that the appearance of almost all structures can be predicted based on this technique. The new calculation technique of such theoretical phase diagrams suggested in the present study is expected to open up the possibility of estimation of unknown phase diagram, reexamination of experimental phase diagrams and discovery of new phases. On the other hand, examining the calculation conditions for improving the accuracy of energy calculation, consideration of the anharmonicity of atomic vibration, magnetic entropy effect, handling of solid solution, etc. are mentioned as problems requiring further consideration.

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

Doyup Lee, Toshihiro Omori, Kwangsik Han, Yasuyuki Hayakawa, Ryosuke Kainuma

pp. 2973-2982

DOI:

10.2355/isijinternational.ISIJINT-2020-199

抄録

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

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Reduction of Thermal Expansion of Ferritic/martensitic Heat Resistant Steels -Alloying Effects on Thermal Expansion of α-Fe Phase-

Satoru Kobayashi, Hayato Fukunishi

pp. 2983-2989

DOI:

10.2355/isijinternational.ISIJINT-2020-205

抄録

Alloying effects on the thermal expansion of the α-Fe phase in connection with the magnetic states were investigated with a final goal to design ferritic/martensitic heat resistant steels with a reduced thermal expansion coefficient at high temperatures. The thermal expansion coefficient decreases with increasing the content of the alloying elements M (M = Co, Cr and V) regardless of the types of the elements in the ferromagnetic state well below the curie temperature in each alloy. The thermal expansion coefficient increases with temperature more significantly in the paramagnetic states compared to the ferromagnetic states. The temperature dependence of the thermal expansion coefficient in the paramagnetic states is not much influenced by the type of alloying element. As a result, the lower the Tc is, the higher the thermal expansion coefficient tends to become in the paramagnetic states at high temperatures. Based on the results obtained, a way is proposed to design heat resistant ferritic steels with reduced thermal expansion coefficients; to add both Cr and Co as major alloying elements such that the Tc and the A3 temperatures are placed above their service temperature.

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Limiting Retained Austenite Decomposition in Quenched and Tempered Steels: Influences of Rapid Tempering and Silicon

Virginia Katherine Euser, Don Lee Williamson, Amy Jean Clarke, John Gordon Speer

pp. 2990-3000

DOI:

10.2355/isijinternational.ISIJINT-2020-263

抄録

Tempering reactions are critical to microstructure and property control in martensitic steels. Here, retained austenite decomposition and cementite precipitation are monitored using Mössbauer spectroscopy in 4340 and 300-M steel under conventional and rapid tempering conditions. Tempering times are compared at a constant tempered hardness by increasing tempering temperatures associated with short time conditions to achieve equivalent matrix softening to that of longer tempering times. Time-temperature combinations that provide equivalent tempered hardness generated microstructures with similar dislocation densities and cementite precipitation fractions; these mechanisms are controlled by self-diffusion. However, systematic differences in retained austenite content were observed at a given degree of softening, where shorter tempering times exhibited higher levels of retained austenite compared to more conventional conditions. At low temperatures, the differences in retained austenite preservation between explored time-temperature conditions are attributed to corresponding differences in carbon diffusion distance (in austenite), the controlling diffusional process of retained austenite decomposition. At higher temperatures, retained austenite decomposition exhibits C-curve kinetic behavior in 4340. Thus, reduced thermodynamic driving force for cementite and ferrite formation at higher temperature is believed to play a role in restricting retained austenite decomposition within some short-time, high temperature tempering regimes. The addition of silicon pushes cementite precipitation and retained austenite decomposition to higher temperatures, although retained austenite decomposition is suppressed to a greater extent than cementite precipitation. Potential is illustrated for coupling rapid tempering with silicon alloying to produce appreciably tempered martensite (~490 HV) with relatively less retained austenite decomposition compared to conventional tempering conditions.

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Editors’ Postscript

Masahiro Susa, Jun Yanagimoto

pp. 3001-3001

DOI:

10.2355/isijinternational.60.3001

抄録

We present to you ISIJ International Vol. 60 No. 12 (2020), “The Diamond Jubilee Issue: Selected Topics in Iron and Steel and Their Processing toward the New Steel Age.”The Iron and Steel Institute of Japan (ISIJ) celebrated its 100th anniversary in 2015, and in 2014, we published the Centennial Anniversary volume of Tetsu-to-Hagané throughout the year. The ISIJ has also been in service of publication of ISIJ International for 60 years until 2020, which the members of the editorial board noticed in 2018. Since then, to celebrate this event, we have organized the publication of ISIJ International Vol. 60 No. 12 (2020) as a memorial issue titled The Diamond Jubilee Issue. We are very happy to publish this issue at the end of 2020, on schedule. The year 2020 should have been a special year for Japan and its people because the Olympic Games were scheduled to be held in Tokyo after an absence of 56 years. Unfortunately, the Olympic Games have been postponed to 2021 because of the spread of the coronavirus disease (COVID-19).The Diamond Jubilee Issue includes original and review papers on various research fields relevant to iron and steel and their processing. All the papers were invited by the editorial board, taking into consideration the balance between academic and industrial points of view. All the papers are based on research projects commonly applied around 2020 in the world. We expect that these papers will be of great help to research and development of the readers, and hope our readers and the authors will look back on these research projects in 10 or 20 years by the Diamond Jubilee Issue.We also hope that readers will submit their original papers to ISIJ International in the near future, too, and also hope to celebrate the Platinum Jubilee in 10 years, and the Centennial Anniversary in 40 years. Finally, we are extremely grateful to all the members of the editorial board, including the staff members (Ms. M. Sato, Ms. Y. Matsumoto, and Ms. M. Sugiyama) as well as the authors for their great contributions to the publication of the Diamond Jubilee Issue.

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