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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575
Publisher: The Iron and Steel Institute of Japan

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Tetsu-to-Hagané Advance Publication

Flow Visualization and Heat Transfer Characteristics of Droplet Train Obliquely Impinging on a Moving Hot Solid

Katsutoshi Tatebe, Shunsuke Fujita, Hitoshi Fujimoto

Abstract

The spray cooling of moving hot solids is widely performed in the steel industry. Understanding flow and heat transfer when droplets impinge on moving hot solids is important. By simultaneous visualization with flash photography and temperature measurement using thermography, the flow and heat transfer of a droplet train obliquely impinging on a moving solid at high temperatures was experimentally investigated. A rectangular test piece (SUS303) was heated to 500 °C at a moving velocity of 0.25–1.5 m/s. The test liquid was water at approximately 25 °C. The pre-impact droplet diameter, impact velocity, and inter-spacing between two successive droplets were 0.69 mm, 2.2 m/s, and 2.23 mm, respectively. The tilt and torsional angles were 50° and -30–60°, respectively. No coalescence of droplets was observed; the droplets deformed independently on the moving solid, even though the torsional angle generated a velocity component along the width of the solid. The surface temperature of solid after droplet impingements depended on the experimental conditions. Wavy temperature profile was obtained when the moving distance of solid was large during two successive collisions. The temperature changed continuously for the small distances. In this regard, a simple model considering droplet movement, collisional deformation behavior, and solid migration can explain this phenomenon by the overlap of the cooling regions of the droplets. Furthermore, experimental and numerical analyses show that the heat removal rate of individual droplets is constant at approximately 12.5 MW/m2 and depends on the total contact time when multiple droplets collide.

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Flow Visualization and Heat Transfer Characteristics of Droplet Train Obliquely Impinging on a Moving Hot Solid

Effects of Cyclic Softening on Fatigue Crack Propagation Properties of Steel

Takayuki Yonezawa, Takashige Mori, Seiichiro Tsutsumi

Abstract

In this paper, the effect of cyclic softening properties on fatigue crack propagation behavior was investigated. Ferrite and ferrite-pearlite steels with different cyclic softening properties were produced by cold rolling process. The cold-rolled steels showed cyclic softening, and the cyclic softening rate increased as the cold reduction rate increased. As a result of fatigue crack propagation tests using CT specimens, the fatigue crack growth rate decreased with increasing the cold reduction rate. The crack growth rate and the cyclic softening rate showed a good correlation regardless of the microstructure. The cold-rolled steels showed crack closure/opening behavior, and the crack opening load increased with increasing cold reduction rate. In addition, the difference in fatigue crack propagation rates of cold-rolled steels was explained by effective stress intensity factor range. From these results, the decrease of fatigue crack growth by cold rolling was considered to be mainly due to the suppression of crack opening by cyclic softening near the fatigue crack tip.

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Effects of Cyclic Softening on Fatigue Crack Propagation Properties of Steel

Improvement of Fatigue Crack Propagation Property in Low Carbon Steel by Microstructural Control and an Investigation of its Practical Benefit

Yoshihiro Hyodo, Masao Yuga, Yasuyuki Kurihara, Thi-Huyen Doan, Takahiro Sakimoto, Yoshiaki Murakami, Koji Goto, Tetsuya Tagawa

Abstract

The fatigue crack propagation properties of newly-developed SM490 grade steels were investigated in comparison with a conventional steel of the same grade. The fatigue crack propagation rate of the developed steel in Region II of the da/dN-ΔK relationship was suppressed to about 1/2 that of the conventional steel, and its ΔKth value was more than twice as large as in the conventional steel. However, fatigue crack resistance for long crack propagation does not necessarily improve the fatigue life in a condition of increasing ΔK from a small defect, which is usually detected in practical fatigue damage in actual structures in service. The developed steels were subjected to surface crack propagation tests using specimens with artificial small defects to examine their potential under more practical conditions. The fatigue life of the developed steel was about three times longer than that of the conventional steel. A detailed analysis of the surface crack propagation revealed crack propagation below ΔKth only in the developed steels, which suggested the so-called “short crack regime” in a fatigue crack. The crack propagation from a surface defect that deviated from long crack behavior was convincingly explained by the corrected threshold using the R-curve model of a short crack proposed in the previous literature. Based on the experimental fatigue life improvement and its analytical estimation of the propagation resistance in the short crack regime, the effect of the ΔKth value for a long crack in the initial propagation stage just after fatigue crack initiation was discussed.

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Improvement of Fatigue Crack Propagation Property in Low Carbon Steel by Microstructural Control and an Investigation of its Practical Benefit

Hydrogen Trapping and Precipitation of Alloy Carbides in Molybdenum Added Steels and Vanadium Added Steels

Shunsuke Taniguchi, Miyuri Kameya, Yukiko Kobayashi, Kazuma Ito, Shingo Yamasaki

Abstract

Martensitic steels of Fe-0.1%C-2%Mn-1.6%Mo alloy and Fe-0.1%C-2%Mn-0.2%V alloy were subjected to tempering at 873 K to investigate hydrogen trapping of Mo carbides and V carbides. We carried out the detail analysis of the alloy carbides by atomic-resolution scanning transmission electron microscopy and atom probe tomography, and the evaluation of hydrogen trapping energy by ab initio calculation. The hydrogen content of the Mo added steel tempered for 1.8 ks increases from that of the quenched Mo added steel and the hydrogen content monotonically decreases as the tempering time increases. The hydrogen content of the V added steels increases during the tempering to 7.2 ks and then keeps almost constant. Plate-shaped B1-type Mo carbide with a chemical composition of MoC0.50 is precipitated in the Mo added steel tempered for 3.6 ks. Needle-shaped HCP Mo2C is precipitated and the B1-type Mo carbide decreases in the Mo added steel tempered for 14.4 ks. Plate-shaped B1-type V carbides with a chemical composition of VC0.75 is precipitated in the V added steel tempered for 14.4 ks. We found the positive correlation between the hydrogen content and the product of the interface area and the carbon vacancy fraction of B1-type alloy carbide. The hydrogen trapping energy of the carbon vacancy at the interface between BCC-Fe and B1-type Mo carbide is higher than that of the interstitial sites in BCC-Fe. These results suggest that the main trapping site in the tempered Mo added steel is the carbon vacancy at the interface of B1-type MoC0.50, not HCP Mo2C.

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Hydrogen Trapping and Precipitation of Alloy Carbides in Molybdenum Added Steels and Vanadium Added Steels

Challenges in Materials Integration

Masahiko Demura

Abstract

Materials Integration is the concept of accelerating materials development by linking processing, structure, property, and performance (PSPP) on a computer using any types of models such as theoretical, empirical, numerical-simulation, and machine learning models. In the first and second phases of Cross-ministerial Strategic Innovation Promotion Program in Cabinet Office, Japan, we have developed a system called MInt (Materials Integration for Network Technology), which links PSPP with computational workflows that combine modules implemented, in order to realize the concept of Materials Integration. MInt is equipped with an application programming interface (API) that can be called from various algorithms in the artificial intelligence (AI) field and one can use MInt-API together with the AI algorithms to inversely design materials and processes from desired performance. The target material systems have expanded to steel, aluminum alloys, nickel alloys, and titanium alloys, and the target processes have also expanded to welding, heat treatment, 3D additive manufacturing, and powder metallurgy. MInt is more than just software for materials design; it is designed to serve as a digital platform for industry-academia collaboration. The Materials Integration Consortium has been established with MInt as its core technology, based on the philosophy of sharing tools such as modules and workflows, while competing on how to use them. In materials research and development, which has traditionally been regarded as a competitive area, we hope that a digital collaborative area will be formed and that investment efficiency will be drastically improved.

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Challenges in Materials Integration

Effects of Cooling Conditions and γ Grain Size on each Behavior of Transformations, 3-Dimensional Thermal Deformation and Stress Generation during Immersion Cooling of Steel Bloom

Kohichi Isobe, Yuga Kumagai, Takumi Satou

Abstract

To optimize reverse transformation treatment, which is effective in preventing hot rolling cracking in the CC-HCR (Continuous Casting - Hot Charge Rolling) process, three-dimensional metallo-thermo-mechanical analyses were performed. The metallo-thermo-mechanics and a CCT (Continuous Cooling Transformation) diagram estimation method using JMatPro were used to obtain the required cooling time for reverse transformation in immersion cooling with water jet and to elucidate the effects of nonuniform cooling and γ grain size on thermal deformation and stress generation behaviors during cooling and the mechanisms of these events.These analyses clarified the following. When diffusion-controlled transformation occurs during cooling, the required cooling time increases as Dγ (Diameter of γ grain) increases, and becomes substantially constant when only martensite transformation occurs. In addition, the difference of Dγ causes a difference in the type of transformation that occurs during cooling and the temperature range where transformation occurs during cooling, and these differences produce differences in the amount of transformation expansion, which greatly affects the bloom deformation behavior during cooling. Furthermore, there is a difference in the level of the maximum generated stress during cooling depending on whether the transformation that occurs near the bloom surface layer during cooling is diffusion-controlled transformation or martensitic transformation. In addition, this difference in transformation behavior also causes a difference in the mechanism of maximum stress generation.

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Effects of Cooling Conditions and γ Grain Size on each Behavior of Transformations, 3-Dimensional Thermal Deformation and Stress Generation during Immersion Cooling of Steel Bloom

Current Trends on Deep Learning Techniques Applied in Iron and Steel Making Field: A Review

Kazumasa Tsutsui, Tokinaga Namba, Kengo Kihara, Junichi Hirata, Shohei Matsuo, Kazuma Ito

Abstract

In recent years, remarkable advances have been made in statistical analyses based on deep learning techniques. Applied studies of deep learning have been reported in various industrial fields, and those of the iron and steel industry are no exception. The production of iron and steel requires a variety of processes, such as processing of ingredients, iron making, casting, and rolling. As a result, the data acquired from them are diverse, and various tasks exist for which deep learning algorithms can assist. Hence, providing a summary of the application is helpful not only for researchers specializing in information science to grasp the current trend of applied studies on deep learning techniques, but also for researchers specializing in each field of the iron and steel making industry to understand what types of deep learning techniques are being utilized in other specialized fields. Therefore, in this paper, we summarize current studies on the application of deep learning in the iron and steel making field by organizing them into several categories of processes and analytical methodologies. Furthermore, based on the results, we discuss future perspectives on the development of deep learning techniques in this field.

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Current Trends on Deep Learning Techniques Applied in Iron and Steel Making Field: A Review

Comparison of Crack Initiation Sites and Main Factors Causing Hydrogen Embrittlement of Tempered Martensitic Steels with Different Carbide Precipitation States

Naoki Uemura, Takahiro Chiba, Kenichi Takai

Abstract

The dependence of crack initiation sites and main factors causing hydrogen embrittlement fracture on carbide precipitation states has been investigated for tempered martensitic steels with the same tensile strength of 1450 MPa. Notched specimens charged with hydrogen were stressed until just before fracture and subsequently unloaded. The crack initiation site exhibited intergranular (IG) fracture at 21 μm ahead of the notch tip as observed by scanning electron microscopy (SEM) for 0.28% Si specimens with plate-like carbide precipitates on prior austenite (γ) grain boundaries. This crack initiation site corresponded to the vicinity of the maximum principal stress position as analyzed by a finite element method (FEM). The initiation site corresponded to the triple junction of prior γ grain boundaries as analyzed by electron backscattered diffraction (EBSD). In contrast, the crack initiation site exhibited quasi-cleavage (QC) fracture at the notch tip for 1.88% Si specimens with fine and thin carbide particles in the grains. This crack initiation site corresponded to the maximum equivalent plastic strain site obtained by FEM. Additionally, the crack initiated on the inside of prior γ grain boundaries and propagated along the {011} slip plane with higher kernel average misorientation (KAM) values as analyzed by EBSD. These findings indicate that differences in carbide precipitation states changed the crack initiation sites and fracture morphologies involved in hydrogen embrittlement depending on mechanical factors such as stress and strain and microstructural factors.

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Comparison of Crack Initiation Sites and Main Factors Causing Hydrogen Embrittlement of Tempered Martensitic Steels with Different Carbide Precipitation States

Crystal Plasticity Finite-element Simulation of Non-uniform Deformation Behavior at Grain Level of Ultralow Carbon Steel

Takayuki Hama, Masashi Oka, Takuna Nishi, Takashi Matsuno, Seiji Hayashi, Kenji Takada, Yoshitaka Okitsu

Abstract

In this study, deformation behaviors at the grain level of coarse-grained ultralow carbon steel subjected to uniaxial tension and simple shear were simulated by using a crystal plasticity finite-element method. Heterogeneity of strain distributions appeared at the early stage and remained almost unchanged in the following deformation. Localized strain bands occurred at the grain level, but the directions of the bands depended on the deformation mode. These trends agreed well with experimental results reported in a previous paper [Hama et al., ISIJ Int., 61 (2021), 1971]. The mechanisms that the direction of the localized strain bands depended on the deformation mode were studied on the basis of the slip activities. The activities of slip systems roughly followed the Schmid factor, and the slip directions of the most active slip systems were consistent with the directions of localized strain bands, suggesting that the direction of localized strain bands were determined primarily by the Schmid factor.

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Crystal Plasticity Finite-element Simulation of Non-uniform Deformation Behavior at Grain Level of Ultralow Carbon Steel

Carbide Precipitation and Hydrogen Trapping Behavior in Mo and V Added Tempered Martensitic Steel

Miyuri Kameya, Syunsuke Taniguchi, Yukiko Kobayashi, Naoki Matsui, Shingo Yamasaki

Abstract

To investigate the effect of the combined addition of V to Mo-added steel on the hydrogen trapping behavior, 0.1C-2Mn-1.6Mo mass% steel (Steel A) and 0.1C-2Mn-1.6Mo-0.2V mass% steel (Steel B) were prepared, quenched, and tempered at 873 K for 0.5 to 10 hours. The thermal desorption analysis of hydrogen-charged specimens confirmed that Steel B showed a higher hydrogen trapping capacity than Steel A. According to thermodynamic equilibrium calculations, it was predicted that only M2C precipitated in Steel A and B after tempering at 873 K. However, according to TEM observation of specimens tempered for 4 hours, coarse M2C and fine MC precipitated in Steel A, whereas only fine MC precipitated in Steel B. Based on the three-dimensional atom probe analysis, MC of both Steel A and B show a composition close to MC0.5, in which Mo is the primary element in metal sites. It was found that the carbon-site vacancy (C vacancy) ratio of MC in the present work is higher than that of V4C3 (VC0.75). The hydrogen trapping capacity showed a good correlation with the product of the area of interface of MC and the C vacancy ratio in MC. The reason of the higher hydrogen trapping capacity of Steel B than that of Steel A is considered to be below; 1) The combined addition of V suppressed the precipitation of M2C and increased the amount of MC. 2) C vacancies in MC which act as hydrogen trapping sites increased by the partitioning of Mo into MC.

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Carbide Precipitation and Hydrogen Trapping Behavior in Mo and V Added Tempered Martensitic Steel

Effect of Al2O3 Addition on the Reaction between 2CaO·SiO2 and Iron Ore

Chihiro Shimizu, Yuji Iwami, Takahide Higuchi, Takashi Watanabe, Rie Endo, Masahiro Susa, Miyuki Hayashi

Abstract

The effect of Al2O3 addition on the reaction between 2CaO·SiO2 (C2S) and iron ore has been elucidated from the perspective of silico-ferrite of calcium and aluminum (SFCA) formation. Hematite iron ore, synthesized C2S, and reagent grade CaO, Al2O3, and SiO2 were mixed so that the Fe2O3/SiO2 (mass%) ratio was 14.43. The mixed powders were uniaxially pressed into the shape of a tablet, which was sintered in air at 1250oC for 5 min. The constituent phases were investigated using XRD and EPMA for the samples where CaO/SiO2 (mass%) ratio was varied from 0.22 to 2.60 and for the samples where Al2O3 concentration was varied from 1.44 mass% (T1) to 3.98 mass% (T2). (i) The constituent phases are mainly Fe2O3 and SiO2 for the samples with a CaO/SiO2 (mass%) ratio of 0.22, and Fe2O3, SiO2, and SFCA for the samples with ratios above 1.97. (ii) The compositions of the SFCA phases are all on the plane connecting CaFe6O10, CaAl6O10 and Ca4Si3O10. The Al2O3 concentrations of SFCA in the T1 and T2 samples are in the range of 1-5 mass% and 2-8 mass%, respectively, and the Fe2O3 concentration tends to decrease with increasing Al2O3 concentration. (iii) The Rietveld analyses of XRD profiles have revealed that adding CaO and Al2O3 increases the fraction of the SFCA phase in the sintered sample. Hence, it is suggested that the placement of CaO and Al2O3 near C2S would promote SFCA formation, which leads to mitigating the decrease in strength relevant to C2S-added iron ore sinters.

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Effect of Al2O3 Addition on the Reaction between 2CaO·SiO2 and Iron Ore

Microstructural Ductile Fracture Analysis of 1180-MPa Class Martensite-Matrix Dual-phase Steel via in-situ Tensile Test

Yuto Watanabe, Takashi Matsuno, Takayuki Hama, Tomoko Matsuda, Yoshitaka Okitsu, Seiji Hayashi, Kenji Takada, Tadashi Naito

Abstract

Martensite-matrix dual-phase (DP) steel is increasingly used for high-strength automobile parts owing to its excellent compatibility, ductility, and tensile strength. However, its higher fracture strain, reflected by the hole expansion ratio, remains an issue hindering further adoption of this material. Therefore, this study conducted a microscale investigation of the ductile fracture behavior of 1180-MPa class martensite-matrix DP steel to obtain a guideline for microstructural design realizing improved fracture strain. In this investigation, in-situ tensile testing was conducted simultaneously with scanning electron microscope observations and crystal plasticity finite-element analysis (CP-FEA). The in-situ tensile test results indicated that microcracks initiated at particular martensite packets and did not propagate into other packets; the CP-FEA results revealed that the martensite crystal orientation caused this behavior to induce remarkable stress and strain localization at interfaces in the vicinity of ferrite islands, relaxing the stress and strain localization at distant martensite packets. Although the cracks observed around the ferrite–martensite interfaces were similar to those observed in conventional ferrite-matrix DP steel, such matrix-phase cracks have rarely been reported except immediately prior to final fracture. Thus, the optimization of ferrite island distribution to suppress the formation of stress and strain localization sites was identified as the key aspect of martensite-matrix DP steel microstructure design. This design aspect can be achieved using a combination of data science and CP-FEA.

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Microstructural Ductile Fracture Analysis of 1180-MPa Class Martensite-Matrix Dual-phase Steel via in-situ Tensile Test

Effect of Oxygen Enrichment on Melting Behavior in Sintering Process

Kenta Takehara, Takahide Higuchi, Tetsuya Yamamoto

Abstract

In the sintering process, productivity and strength are essential indicators, and it is known that they are closely related to the sintering time and temperature. Since the sintering time and high temperature holding time vary greatly depending on the combustion behavior of coke breeze, various studies have been conducted on the influence of oxygen which is critical element in the sintering phenomena. However, the influence of oxygen enrichment on sinter strength and yield has not yet been unified. In this study, the sintering productivity, strength, and yield were investigated when the oxygen concentration of the inlet gas was increased to 40 vol%. In addition, the heat profile on pot tests and the effect of oxygen partial pressure on the melting property were analyzed from the viewpoint of thermodynamics. As a result, it was found that the yield showed the maximum value at 30 vol% oxygen concentration, and the strength increased with oxygen enrichment. As for the yield, it was found that the effect of the heat profile was significant, and the deviation between the heat transfer speed and the combustion speed started when the oxygen concentration exceeded 30 vol%. On the other hand, it was suggested that the strength improvement was caused by the increase in the amount of melt produced by oxygen enrichment in addition to the effect of high temperature holding time.

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Effect of Oxygen Enrichment on Melting Behavior in Sintering Process

Effect of Alloy Elements on Eutectic Carbide Morphology for High-speed Tool Steel

Shiho Fukumoto, Hirofumi Miyahara

Abstract

Effect of alloying elements on the composition distribution, morphology and volume fraction of eutectic carbides were investigated for high-speed tool steels with uniform hardness from 68 to 70HRC after hot working and quenching and tempering. The stability of eutectic carbides at high temperature was also evaluated. M2C, M6C and MC eutectic carbide are observed in as-cast samples similar to the general high-speed tool steels. M2C type carbide increases in volume with increasing Si and V contents, and M6C and MC carbides appear with increasing W and V contents. The amount of M2C eutectic carbide varies with the composition in liquid phase just prior to eutectic solidification. The morphology of the eutectic carbide changes from fine fiber or lamellar type to coarse lamellar or feather type, and the interlayer spacing of eutectic carbide tends to increase with increasing the area fraction of M2C eutectic carbide. Moreover, after heat treatment at 1140℃ for 16 hours, some M2C carbides remain stabled but MC and M6C carbide appears.

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Effect of Alloy Elements on Eutectic Carbide Morphology for High-speed Tool Steel

Microvoid Formation of Ferrite-martensite Dual-phase Steel via Tensile Deformation after Severe Plastic Shear-deformation

Takashi Matsuno, Nanami Kinoshita, Tomoko Matsuda, Yoshiaki Honda, Takashi Yasutomi

Abstract

One of the objectives for the development of high-strength dual-phase (DP) steel is improving the stretch-flangeability. Large-strained sheared edges are deformed and frequently cracked during stretch-flange formation. Considering shearing as the first deformation, the stretch-flange deformation may be regarded as a secondary deformation. To improve the stretch-flangeability of the DP steels, many researchers have analyzed the microvoid formation. However, in these analyses, the shearing process was not considered. With this background, ex-situ mini-bending tests combined with scanning electron microscopy (SEM) monitoring of microvoid formation were conducted during the secondary deformation. Prior to the secondary deformation, several microvoids were observed on the sheared surface and fine subgrains formed in the ferrite. During secondary deformation, the preliminary microvoids present at the ferrite-martensite interface propagated into the ferrite phase. In contrast, this behavior was not observed for the reamed surface deformation, which was formed without preliminary deformation. Furthermore, microvoids were initiated on ferrite grains that were not present at the ferrite-martensite interface, and martensite islands were not cracked during secondary deformation. This result is noteworthy because martensite cracking was the main factor involved in microvoid initiation, in the absence of shearing. Electron backscattering diffraction analysis revealed that the work hardening of ferrite, prior to the secondary deformation, caused a deviation in the strain concentration sites from those found in the reamed surface deformation. Therefore, this study elucidated microvoid formation on preliminary deformed surfaces via shearing and provided insights for material development considering deformations on the sheared surfaces of materials.

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Microvoid Formation of Ferrite-martensite Dual-phase Steel via Tensile Deformation after Severe Plastic Shear-deformation

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