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

ISIJ International Advance Publication

  • Effects of Substituting SiO2 with Oxidisers on the Reaction Performance and Physical Properties of Mould Flux for High Ti-bearing Steel

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    DOI:10.2355/isijinternational.ISIJINT-2020-446

    In order to eliminate the TiN inclusion emerging from the molten steel and maintain the stable performance of mould flux during continuous casting of high Ti-bearing steel, the influence of extra oxidisers, such as Mn2O3, Mn3O4, and Fe2O3 substituting SiO2, on the reaction performance and physical properties of CaO–SiO2 type mould flux was investigated in this study. A detailed study using thermodynamic calculation, X–ray diffraction, and thermogravimetric analysis shows that the oxidisability of oxidisers to TiN, ranging from strong to weak, is Fe2O3, Mn2O3, Mn3O4, and SiO2. The oxidisability of SiO2 in the mould flux is much weaker than that of Mn2O3 and Fe2O3. Results show that the precipitation of solid iron after the reaction between the CaO–SiO2–Al2O3 type or CaO–Al2O3 type mould fluxes with Fe2O3 and TiN worsens the liquidity of the mould flux and further deteriorate its physical properties. Mn2O3 is the best option, with a sustainable change in the physical properties at less than 10 wt.%. Moreover, following the reaction with TiN, the physical properties of the CaO–SiO2–Al2O3 type mould flux with 15 wt.% Mn2O3 show an acceptable change for the application of casting.
  • CO Desorption and Absorption in Molten Steel: A Review

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    DOI:10.2355/isijinternational.ISIJINT-2020-509

    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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    1. Improved Hydrogen Embrittlement Resistance of Steel by Shot Peening and Subsequent Low-temperature Annealing ISIJ International Advance Publication
  • On the CO Desorption and Absorption in Liquid Low-carbon Steel

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    DOI:10.2355/isijinternational.ISIJINT-2020-510

    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).
  • Effects of Ammonium Thiocyanate and pH of Aqueous Solutions on Hydrogen Absorption into Iron under Cathodic Polarization

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    DOI:10.2355/isijinternational.ISIJINT-2020-551

    To evaluate the susceptibility of steels to hydrogen embrittlement, it must be hydrogen charged and have its hydrogen content controlled before any mechanical testing. In this study, hydrogen permeation tests of an iron sheet were performed during potentiostatic hydrogen charging in various solutions containing ammonium thiocyanate to obtain a guideline for efficient hydrogen charging for a wide range of hydrogen contents. As the polarization potential shifted in the negative direction, the hydrogen permeation current increased before becoming almost constant. In all cases, besides when an aqueous sodium hydroxide solution was employed, the hydrogen permeation current increased due to the addition of ammonium thiocyanate. The effect of adding ammonium thiocyanate was enhanced as the aqueous solution pH was decreased. The hydrogen permeation current under various hydrogen charging conditions obtained in this study can be used as a reference for hydrogen charging of steels.
  • Influence of Micro-texture Distribution and Straining Direction on the Ridging of Ferritic Stainless Steels

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    DOI:10.2355/isijinternational.ISIJINT-2020-610

    Ridging means the appearance of surface profile undulations, i.e. peaks and valleys, as a result of plastic strain. The reasons for the different ridging behaviour of industrially produced, stabilized ferritic stainless steel sheets (EN 1.4509) have been investigated after straining in the rolling and transverse directions with low and high resistance towards ridging. The evolution of macro-texture has been measured by X-ray diffraction (XRD) both before and after ridging tests in the rolling and transverse directions. The macro texture results showed that straining along the rolling direction strengthened the α fibre whereas the γ fibre was strengthened by grain rotations after straining along transverse direction. Electron backscatter diffraction (EBSD) imaging was used to establish the micro-textural variations over the thickness of the sheets among the high and low ridging materials. Mean orientations of individual grains determined from the EBSD data were utilized to calculate plastic strain ratio r-values by considering all slip systems weighted according to their Schmid factors. The calculated r-values were used to predict the ridging surface profile after straining along the rolling and transverse directions. The results demonstrated the influence of local variations in micro-texture on the severity of ridging.
  • Evolution of Inclusions with Ce Addition and Ca Treatment in Al-killed Steel during RH Refining Process

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    DOI:10.2355/isijinternational.ISIJINT-2020-672

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

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    DOI:10.2355/isijinternational.ISIJINT-2020-677

    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.
  • Separation of Vanadium and Titanium Phase from the Vanadium Slag System Utilizing Supergravity

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    DOI:10.2355/isijinternational.ISIJINT-2020-333

    To effectively recycle and utilize vanadium and titanium resource from the vanadium slag, the crystallization and separation behaviors of V-spinel phase and Ti-spinel phase in the FeO–SiO2–V2O3–TiO2 system as the main components of vanadium slag were investigated. The results indicated that the suitable temperatures for precipitating V-spinel phase and Ti-spinel phase were chosen as 1723 K and 1623 K, respectively. With introduction supergravity at the parameter of G = 700, T = 1723 K and t = 10 minutes, the solid V-containing phase was intercepted by the filter to form the V-enriched slag, while the residual melt went through the filter into the lower crucible to form the Ti-containing slag. And the mass fraction of V2O3 in the V-enriched slag reached about 32.98 wt% and that of TiO2 in the Ti-containing slag reached about 19.41 wt%; the recovery ratios of V2O3 and TiO2 were about 86.50% and 76.80%, respectively. In addition, the Ti-spinel phase was further separated and concentrated in the Ti-enriched slag from the Ti-containing slag with the gravity coefficient G = 500 at 1623 K for 10 minutes. The mass fraction and the recovery ratio of TiO2 in the Ti-enriched slag could reach 30.83 wt% and 89.80%, respectively. In the whole process, the comprehensive recovery ratio of TiO2 could reach 68.97%.
  • Modelling of Hydrogen Diffusion in a Weld Cold Cracking Test: Part 1, Experimental Determinations of Apparent Diffusion Coefficient and Boundary Condition

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    DOI:10.2355/isijinternational.ISIJINT-2020-523

    Weld cold cracking is a kind of hydrogen embrittlement and is a serious problem when high strength steel is concerned. Cold cracking usually occurs at the area where hydrogen locally accumulates, and hence hydrogen diffusion modelling is important to understand the hydrogen distribution in a welded joint. In this paper, we conducted the permeation tests to measure the apparent diffusion coefficients of hydrogen with varying the hardness and the plastic strain of the steel. The experimental results were used to develop the empirical equation to predict the diffusion coefficient. The empirical equation is described as a function of temperature, hardness and plastic strain. Next, we measured the evolution curves of hydrogen released from the rectangular specimens. We considered the boundary condition of the surface exposed to the atmosphere, and the experimental results were used to determine the boundary condition. The empirical equation and the boundary condition obtained in this paper will be used for the numerical calculations of hydrogen diffusion in a weld cold cracking test in the companion paper.
  • Modelling of Hydrogen Diffusion in a Weld Cold Cracking Test: Part 2, Numerical Calculations

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    DOI:10.2355/isijinternational.ISIJINT-2020-524

    The purpose of this work is to compare the numerical calculation results of the hydrogen diffusion with the experimental results. We firstly conducted a y groove weld cracking test using 980 MPa grade steel and 780 MPa grade welding consumable to observe HAZ cracking. Next, we showed the present diffusion equation and the procedure to determine the physical properties for the calculations such as stress-strain curve, and using the apparent diffusion coefficient and the boundary condition obtained in the companion paper, we conducted the numerical calculations for the two cases, that is, the under-match case (980 MPa grade steel and 780 MPa grade welding consumable) and the even-match case (780 MPa grade steel and 780 MPa grade welding consumable). The calculation results of the under-match case show that hydrogen tends to accumulate in the stress concentration area of both the HAZ and the weld metal, which indicates crack may occur in both the HAZ and the weld metal. The results of the even-match case show not only the hydrogen accumulation around the stress concentration area but also high hydrogen concentration in the weld metal, which indicates crack may occur in the weld metal. Calculation results for both of the cases fairly agree with the experiments.
  • Numerical Simulation on the Effect of Gas Pressure on the Formation of Local Dry Underwater Welds

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    DOI:10.2355/isijinternational.ISIJINT-2020-367

    The obvious feature of local dry underwater welding is the local drainage of the welding area. The choice of drainage method is one of the important factors affecting the quality of underwater welds. Based on hydrodynamics and thermodynamics, the internal gas flow characteristics and arc combustion of the micro-drainage cover with dual-gas curtains is used to simulate which could not be directly observed in the micro-drainage cover with FLUENT, and then the influence of micro-drainage cover on the weld formation of local dry underwater welding is studied. The simulation results show that under the action of intake mode and channel structure, the drainage gas forms a spiral gas wall with a certain "stiffness" around the arc combustion area, which can both protect the stability of the internal dry environment, and cut down the interference of the low drainage pressure on the flow of the shielding gas. However, the low drainage pressure can not effectively achieve the internal drainage. That is, when the pressure of drainage gas exceeds 0.3 MPa, due to the Bernoulli effect, the turbulence situation is intensified owing to the influence of shielding gas, and the arc column fluctuates greatly, which is consistent with the results of underwater weld formation, and provide theoretical basis and basic data for further study of local dry underwater welding.
  • Simultaneous Measurements of Polarization Resistance and Hydrogen Permeation Current of Iron in an Aqueous NaCl Droplet

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    DOI:10.2355/isijinternational.ISIJINT-2020-583

    In this study, a Kelvin probe (KP) technique combined with the Devanathan-Stachurski electrochemical hydrogen permeation method was applied for simultaneous measurements of polarization resistance and hydrogen permeation current for iron to clarify the hydrogen uptake mechanism during drying of an NaCl droplet. The reciprocal of the polarization resistance, which is an index of the corrosion rate, the hydrogen permeation current, and the corrosion potential under the droplet were successfully measured. The corrosion potential decreased, and the hydrogen permeation current increased, after the NaCl droplet had been applied to the iron surface. The reciprocal of the corrosion resistance increased gradually during the drying of the droplet with increasing the corroded areas on the iron. The hydrogen permeation current decayed with the shift in the corrosion potential toward the noble side during the drying stage, before the droplet dried up completely. The hydrogen permeation current mainly followed the change in the corrosion potential. The hydrogen uptake mechanism of iron during corrosion is discussed in detail based on the corrosion potential, corrosion rate and hydrogen permeation behavior.
  • Tensile Properties and Stretch-Flangeability of TRIP Steels Produced by Quenching and Partitioning (Q&P) Process with Different Fractions of Constituent Phases

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    DOI:10.2355/isijinternational.ISIJINT-2020-388

    The influence of microstructure on tensile properties and stretch-flangeability of TRIP steels with tensile strengths higher than 1.2 GPa has been investigated under various Quenching and Partitioning (Q&P) conditions. As lowering the quenching stop temperature, QT, below Ms temperature in the range of 340°C to 280°C, volume fractions of tempered martensite and retained austenite increased and volume fractions of bainite and fresh martensite decreased in the final microstructure. The higher the QT temperature in the range of 280°C to 330°C, the more the relative proportion of untransformed austenite at the end of the partitioning step was transformed into fresh martensite. The microstructural characteristics of fresh martensite and retained austenite under different QT conditions were analyzed by EBSD. The fresh martensite phase was identified by a new method applying the threshold values of both Image Quality (IQ) and Kernel Average Misorientation (KAM). It is suggested that the decrease in the HER (Hole Expansion Ratio) value with increasing QT temperature is due to the increase in the size and the volume fraction of fresh martensite particle.The mechanical properties of Q&P steels were evaluated before and after tempering at 200°C for 1 hour. Under conditions where the initial volume fraction of fresh martensite before tempering was higher, tensile elongation and HER values were improved by tempering. Tensile elongation was increased with the volume fraction of retained austenite. Lower HER values were obtained with higher volume fractions of fresh martensite, regardless of tempering.
  • Numerical Study into Gravity Separation of Phosphorus from BOS Slag during Solidification

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    DOI:10.2355/isijinternational.ISIJINT-2020-450

    Phosphorus is known to partition to a dicalcium silicate – tricalcium phosphate solid solution (C2S–C3P) during the solidification of basic oxygen steelmaking (BOS) slag. Typically, C2S–C3P solidifies first and has a lower density than the remaining liquid slag, suggesting that gravity separation may be possible. This study simulated the cooling behaviour of BOS slag, and predicted the potential for spherical C2S–C3P particles to float. A lumped parameter heat transfer model based on ordinary differential equations was developed to predict the temporal variations of slag temperature in a 5 mm diameter Pt crucible. Hydrodynamic calculations were also carried out to study the floating behaviour of the spherical particles. The results showed reasonable agreement between predictions and experimental measurements for the slag's cooling rate. In the separation experiments, coarse C2S–C3P crystals were observed in the upper section of the crucible, while a glassy slag was observed in the lower section. This is consistent with the hydrodynamic calculations that showed the single particles floating up to the interface. Preliminary approximations were also performed for industrial slag pots which showed the higher possibility of separation for a shallow and insulated slag pot. Further studies are required to confirm the nucleation and growth behaviour in the experiments.
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    1. Volumetric Strain Dependence of Quantum Diffusion of Hydrogen in bcc Iron ISIJ International Advance Publication
  • Effect of Environmental Factors on Hydrogen Absorption into Steel Sheet under a Wet-dry Cyclic Corrosion Condition

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    DOI:10.2355/isijinternational.ISIJINT-2020-561

    The effects of temperature and chloride deposition on hydrogen absorption into steel were evaluated during wet/dry cyclic corrosion using a temperature-compensated hydrogen absorption monitoring system based on the electrochemical hydrogen permeation method. Hydrogen absorption into steel was detected through the measurement of hydrogen permeation currents during the wet periods under the wet/dry cyclic corrosion. The enhancement of hydrogen absorption was mainly caused by the changes in the solution chemistry during the wetting and drying periods, with a decrease in pH due to the hydrolysis reaction of Fe3+ at high Cl- concentration. Hydrogen absorption into steel increased with increasing temperature and chloride deposition. The reasons for the increment of hydrogen absorption are considered that enhancement of the hydrogen evolution reaction with temperature and that the corrosion potential shifted to less noble by increase in the electrolyte thickness with increasing chloride deposition. Based on these results, the amount of absorbed hydrogen map effected by these factors under atmospheric corrosion environment was described.
  • Relation between Intergranular Stress of Austenite and Martensitic Transformation in TRIP Steels Revealed by Neutron Diffraction

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    DOI:10.2355/isijinternational.ISIJINT-2020-503

    In situ neutron diffraction measurements of two low-alloy steels and a 304-type stainless steel during tensile and creep tests were performed at room temperature. Changes in the diffraction pattern, the integrated peak intensities of austenite (γ), and the peak positions of γ were analyzed and discussed to elucidate the relationship between intergranular stress in γ and the occurrence of martensitic transformation during deformation. Tensile loading experiments revealed that the susceptibility to martensitic transformation depended on the γ-(hkl) grains, where γ-(111) grains underwent martensitic transformation at the latest. The volume fractions of γ were found to decrease under an applied load but to remain almost unchanged under constant load in creep tests, where the lattice strains of γ-(hkl) grains were mostly unchanged. The γ-hkl dependence of the susceptibility to martensitic transformation was found to be controlled by the shear stress levels in γ-(hkl) grains, which were affected by the intergranular stress partitioning during deformation.
  • Influence of Aging Treatment on the Microstructure Evolution and Mechanical Properties of 316H Weld Metals with Different C Contents

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    DOI:10.2355/isijinternational.ISIJINT-2020-387

    The influence of 600°C aging treatment on the microstructure and mechanical properties of 316H stainless steel weld metals with different C contents have been studied. The results indicated that during the aging process, the rapid precipitation of M23C6 carbide occurred in δ-ferrite firstly owing to the high diffusion rate of C. After the C is depleted by precipitation of M23C6, the residual δ-ferrite transforms into σ phase through eutectoid transformation (δ-ferrite → σ phase + γ). Furthermore, after a long enough aging time, the transformation from M23C6 to σ phase occurred. The C content has a significant influence on the δ-ferrite transformation behavior, δ-ferrite in the low and medium C weld metals transforms into M23C6 and σ phase successively, while δ-ferrite in high C weld metal only transforms into M23C6 carbides. The variations of mechanical properties with aging conditions depended mainly on the microstructures at different aging conditions. For the low C weld metal, as the aging time increased, the increasing σ phase content improved the strength obviously. For the medium and high C weld metals, as the aging time increased, first the depletion of the solid solution C as a result the M23C6 precipitation deteriorated the strength, then the formation of σ phase improved the strength. Furthermore, with the increasing of the aging time, the precipitation of M23C6 and σ phase deteriorated the elongation and impact energy.
  • Effect of Iron Rust on Hydrogen Uptake during Steel Corrosion under an Aqueous NaCl Droplet

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    DOI:10.2355/isijinternational.ISIJINT-2020-528

    The effect of iron rust on hydrogen uptake into steel during corrosion under an aqueous NaCl droplet was investigated. Pre-rusted steel was obtained by exposing a steel coupon to natural environmental conditions for 1 month at the Choshi site of the Japan Weathering Test Center. The iron rust that formed on the coupon was partly removed, and model rust/steel samples differing in the area ratios of rusted and bare steel were prepared. The hydrogen permeation current and the corrosion potential were simultaneously measured by Devanathan-Stachurski (DS) method and the Kelvin probe technique, respectively. As the applied droplet of aqueous NaCl dried, the corrosion potential shifted in the negative direction and the hydrogen permeation current slightly increased in all model samples. However, the corrosion potential and hydrogen permeation current did not differ substantially among the model samples once the rusted area of the model sample exceeded 50%. These results indicate that iron rust cause a positive shift in the corrosion potential, and hydrogen uptake was significantly suppressed due to the inhibition of the hydrogen evolution reaction. The hydrogen uptake behavior of the model sample is discussed with consideration of the cathodic reduction reaction of iron rust.
  • Optimization of the Structure and Injection Position of Top Submerged Lance in Hot Metal Ladle

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    DOI:10.2355/isijinternational.ISIJINT-2019-680

    To optimize the structure and injection position of top submerged lance, a three-dimensional gas-liquid two phases flow model in hot metal ladle is established based on Euler-Euler approach. The effects of the orifices arrangement, immersion depth and eccentricity of lance on the mixing time and gas total volume in ladle are investigated. A water model experiment is conducted to investigate the bubbles distribution and mixing time. The results show that, the predicted bubbly plume and mixing time agree well with the experimental photos and measured data. When the separation angle decreases from 180 deg to 45 deg, the symmetrical liquid flow field is destroyed, and a large circulation is formed in ladle. Meanwhile, the liquid velocity at the bottom of ladle becomes intense. As the separation angle reduces, the mixing time initially decreases follow by an increase, while the gas total volume becomes smaller. At a deeper immersion depth, the liquid velocity at the bottom of ladle and gas total volume are bigger. The mixing time reaches its minimum when the immersion depth is 740 mm. With the increasing of eccentricity, the liquid velocity becomes more uniformity, and the mixing time and the gas total volume in hot metal ladle gradually decrease. It is recommended to use the submerged lance with separation angle of 90 deg, immersion depth of 740 mm and eccentricity of 0.2 in present system.
  • Improved Hydrogen Embrittlement Resistance of Steel by Shot Peening and Subsequent Low-temperature Annealing

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    DOI:10.2355/isijinternational.ISIJINT-2020-463

    The effect of shot peening and subsequent low-temperature annealing (SP treatment) on hydrogen embrittlement in tempered martensitic steel was investigated comparing the typical hydrogen charging methods, constant current controlled cathodic charging test and combined cyclic corrosion test (CCT). The hydrogen entry behavior was evaluated by hydrogen permeation technique and hydrogen visualization using secondary ion mass spectrometry. The SP treatment improved hydrogen embrittlement resistance in both hydrogen charging methods. On the other hand, the effect of SP treatment on the hydrogen entry was different depending on hydrogen charging method. The hydrogen entry was suppressed by SP treatment in the cathodic charging test because the compressive residual stress reduced hydrogen concentration in the surface layer and the potential increased by the increase of surface roughness and the formation of a surface film. In CCT, although the surface hydrogen concentration decreased due to compressive residual stress, the total hydrogen content did not decrease by SP treatment since the surface film disappeared by corrosion and an increase of surface roughness led to an increase in hydrogen entry sites. The improved hydrogen embrittlement resistance by the SP treatment in the cathodic charging test is a result of hydrogen entry suppression in addition to the effect of compressive residual stress. In CCT, the hydrogen embrittlement resistance was improved by SP treatment due to effect of compressive residual stress, i.e., the suppression of stress concentration as well as the stress-induced hydrogen diffusion in stress concentration area and the reduction of hydrogen concentration in the surface layer.
  • Composition Dependence of Normal Spectral Emissivity of Liquid Ni–Al Alloys

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    DOI:10.2355/isijinternational.ISIJINT-2020-233

    Ni–Al alloys are good candidates for the fabrication high-efficiency gas turbine blades. The Ni–Al system is also important as a solution for AlN crystal growth. To accurately model the casting process for turbine blade fabrication and design solution growth techniques for AlN, the thermophysical properties of the liquid alloy are required. In this study, the normal spectral emissivity of Ni–Al liquid alloys was measured using the electromagnetic levitation technique under a static magnetic field. Both the melting temperature of Cu and the eutectic temperature of the Ni–C system were used as fixed temperature points for spectrometer calibration to obtain the radiance of liquid Ni–Al alloys. The composition dependence of the normal spectral emissivity of liquid Ni–Al alloys had a maximum at ~40–50 mol%Al-Ni. The Ni–Al binary system had a stable intermetallic compound of NiAl with a melting temperature of 1911 K. The short range chemical ordering could be attributed to strong bonding between Ni and Al atoms, which affected the scattering cross section of the conduction electrons even in the liquid state; hence, the normal spectral emissivity had a maximum at ~40–50 mol%Al-Ni.
  • Hydrogen Absorption Behavior and Absorbed Hydrogen Trapping Sites in Rolling Contact Fatigue

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    DOI:10.2355/isijinternational.ISIJINT-2020-470

    Hydrogen absorption behavior and microstructural change of carburized JIS SCr420 steels containing different amounts of retained austenite in rolling contact fatigue were investigated. The thermal desorption analysis confirmed hydrogen desorption at the second-peak between 423 and 623 K after rolling contact fatigue. The hydrogen concentration at the second-peak increased with number of cycles in the rolling contact. This increment was larger when using the steel with higher amount of retained austenite before the fatigue test. It was still large even when the amount of martensitic transformation from retained austenite under cyclic stress to introduce dislocation with trapping capacity was small. The activation energies of desorption for the second-peak hydrogen were calculated to be 50.6 kJ·mol-1 for the steel with 10.4% retained austenite and 55.8 kJ·mol-1 for the steel with 4.9% retained austenite. The activation energies of cathodically charged 0.8%C steels with 10.9% and 6.0% retained austenite, simulating carburized layer before the test, were 36.2 and 42.2 kJ·mol-1, respectively. This means that the activation energy of hydrogen desorption increased during rolling contact. The absorbed hydrogen during the rolling contact fatigue was likely trapped in more stable trapping sites related to the retained austenite which were formed under cyclic stress.
  • Mathematical Modeling on Transient Multiphase Flow and Slag Entrainment in Continuously Casting Mold with Double-ruler EMBr through LES+VOF+DPM Method

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    DOI:10.2355/isijinternational.ISIJINT-2020-592

    To investigate the transient multiphase flow, slag entrainment and exposed slag eye in the mold with EMBr and argon injection, a transient multiphase LES model was constructed. In the model, the multiphase (steel-slag-air) flow was simulated through VOF method, argon bubbles were tracked using DPM method. The model was validated by previous experimental measurements through the particle image velocimetry. The results indicated that argon injection weakened the upper recirculation flow, and reduced the molten steel jet downward angle. Moreover, the turbulent flow in the mold was suppressed by EMBr. The slag entrainment mechanisms are different under different conditions. In the case without argon injection and EMBr, three slag entrainment mechanisms were found: shear flow, Von Kármán vortex, and level fluctuation. Nevertheless, when argon was injected, in addition to the above mechanisms, one extra mechanism was predicted that was bubble interaction. Additionally, bubble interaction was the unique slag entrainment mechanism in the case with argon injection and EMBr. The impact of argon bubbles on the steel-slag interface resulted in exposed slag eye. In the case without EMBr, the exposed slag eye phenomenon was intermittent. However, the phenomenon was persistent when EMBr was applied.
  • Effect of Humidity of Air on Hydrogen Absorption into Fe with Rust Layer Containing MgCl2 during Atmospheric Corrosion

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    DOI:10.2355/isijinternational.ISIJINT-2020-426

    An Fe plate, whose one side was electro-polished and the other covered with the rust layer containing 25.7 g·m-2 MgCl2, was used as the specimen to investigate the effect of humidity on the hydrogen absorption of the plate. The specimen was subjected to an electrochemical hydrogen-absorption test during which the rusted surface was exposed to the air with controlled relative humidity (RH) and atmospheric corrosion occurred on it. When the rusted surface was subjected to dry (RH 0%)–wet (RH 27%) repeated cycle tests for 10.8 ks each, the anodic current density corresponding to the hydrogen-absorption rate was measured on the hydrogen detection surface. The maximum current density was almost independent of the cycle during the first 10 cycles, after which it decreased with an increase in the cycle, reaching almost a steady-state after about 40 cycles. After 55 cycles of the dry–wet repeated cycle test, the specimen was subjected to an electrochemical hydrogen-absorption test to obtain the relationship between the steady-state hydrogen-absorption rate and RH. Hydrogen absorption was observed at RH of about 15%, and the absorption rate increased rapidly with an increase in RH, reached a maximum at RH of about 30%, and then decreased rapidly. When RH increased beyond 40%, the absorption rate increased again, reached a maximum value at RH of 80%, and then decreased gradually. The specimen with the rust layer containing 39.8 g·m-2 MgCl2 also showed two peaks in the hydrogen-absorption rate versus RH plot.
  • Hydrogen Absorption into Fe Plates with Rust Layers Containing Various MgCl2 Amounts during Atmospheric Corrosion with Controlled Humidity

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    DOI:10.2355/isijinternational.ISIJINT-2020-502

    Fe plates with rust layers containing various MgCl2 amount were prepared as specimens. Each specimen was subjected to dry/wet repeat test beyond 50 cycles, and then subjected to electrochemical hydrogen absorption test under atmospheric corrosion in the air with controlled relative humidity (RH). For an MgCl2 amount of 39.8 g·m-2, a hydrogen absorption rate (iH) started to increase from an RH around 15%, steeply increased with an increase in RH up to about 30%, steeply decreased up to about 35%, gradually increased up to 65% and gradually decreased up to about 92%. A decrease in MgCl2 amount in the range between 0.514 and 39.8 g·m-2 induced a decrease in iH in wide RH range. The maximum iH at an RH around 30% increased with an increase in MgCl2 amount in the rust layer. Besides, the RH where the maximum iH was obtained beyond an RH of 40% increased with a decrease in MgCl2 amount. From theoretical relationship between RH and thickness of MgCl2 solution film on the Fe plate without rust layer, it is found that the solution film thicknesses at the RHs were about 0.18 mm, almost independent of MgCl2 amount. In addition, thicknesses of the rust layers containing 25.7 and 39.8 g·m-2 MgCl2 were measured to be almost 0.18 mm each other. The trends of iH depending on MgCl2 amount were tried to be explained using nature of deliquescence for MgCl2.
  • Graphitization Behaviors of Creep-ruptured 0.3% Carbon Steel at 673 to 773 K

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    DOI:10.2355/isijinternational.ISIJINT-2020-575

    Graphitization in carbon steels must be prevented because it reduces the amount of carbon in the matrix, which degrades the material performance due to loss in strength. In addition, when graphite particles are aligned, they can cause fracture by their linkage. The safety management standards for carbon steels in high-temperature applications state that graphitization should be considered at 698 K and above. The number of reported cases on graphitization in steels below 698 K is limited, and the mechanism has not yet been well investigated. This paper reports the finding of unprecedented graphitization at 673 K in creep-ruptured carbon steel and an elongated form of graphite that appears after a much shorter time at 673–773 K than other previously reported times. Furthermore, the formation mechanism of this elongated graphite is discussed. Dislocations and inclusions in the vicinity of grain boundaries may facilitate graphitization kinetics at these temperatures.
  • Improved Responsivity and Sensitivity of Hydrogen Mapping Technique in Pure Iron Using WO3 Thin Film by Control of Pd Intermediate Layer

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    DOI:10.2355/isijinternational.ISIJINT-2020-563

    The objective in this study is to improve the responsivity and the sensitivity of the hydrogen mapping technique using the WO3 thin film by the optimization of the Pd intermediate layer. Especially, the effect of the thickness of Pd on the responsivity and the sensitivity of the hydrogen detection during hydrogen charging was investigated. Pd and WO3 thin films were coated on the detection side of the pure iron sheet by a magnetron sputtering system. No color change was observed on the hydrogen detection side of the specimens with the Pd intermediate layer more than 25 nm in thickness during the hydrogen detection test for 7.2 ks. The responsivity for the hydrogen mapping technique using WO3 was essentially improved by decreasing the Pd thickness. In terms of the onset time of the average color change, the earliest response was obtained when the Pd thickness was 4 nm because of the uneven distribution of the color change in the case of the Pd thickness of 2.5 nm. The sensitivity for the hydrogen mapping technique using WO3 was improved by decreasing the Pd thickness. Taking into account the responsivity, the sensitivity, and the spatial resolution comprehensively, the best thickness of the Pd intermediate layer seems to be 4 nm in this study.
  • In-Situ Observation Experimental Study on the Agglomeration and Dispersion of Particles at the Interface of High-temperature Melts

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    DOI:10.2355/isijinternational.ISIJINT-2020-585

    Understanding the mechanism of particle aggregation and dispersion at a liquid surface is important for the design and fabrication of novel materials. The behaviors of particles with various compositions at the interface of high-temperature melts, including the rapid growth of particles, the linear aggregation or curvilinear motion, and the wetting and separation processes, were observed in situ by high-temperature confocal laser scanning microscopy (HT-CLSM). We experimentally investigated the interaction force on particle pairs as a function of the interparticle distance and the size, composition and shape of particles. The experimental results indicate the attractive force between particle pairs decreases with increasing distance, and it increases as the size of the guest particle increases. The acting length of particles increases in the order of the alumina-magnesia particle (30 µm)< calcium aluminate particle with low CaO contents (80 µm)<alumina particle (110 µm). The estimated attractive interaction between particle pairs based on in situ observation increases in the order of Al2O3·35%CaO< Al2O3·19%MgO < Al2O3·38%MgO < Al2O3·15%CaO < Al2O3. The complex selective attraction has been observed in the in situ experiments which is attributed to the polydirectional attractive force and anisotropy in the particle morphology. The relationship between the roundness with acceleration rate of particles indicates that the attraction between spherical particles tends to be less than the attraction between irregular particles with edges.
  • Roles of Hydrogen Content and Pre-strain on Damage Evolution of TRIP-aided Bainitic Ferrite Steel

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    DOI:10.2355/isijinternational.ISIJINT-2020-514

    From the aspect of crack/void initiation and growth characteristics, the effects of pre-strain on the hydrogen embrittlement resistance of a transformation-induced plasticity-aided bainitic ferrite steel were examined. The hydrogen uptake in the specimens without pre-strain caused degradation of crack growth resistance, but the crack initiation probability did not change significantly. It is noteworthy that the degree of degradation was independent of the hydrogen content in the present hydrogen charging condition. Pre-straining to 3% and 6% improved the crack growth resistance of the hydrogen-charged specimens because of a reduction in the probability of austenite presence at the crack tip. Furthermore, a high level of pre-strain provided high hydrogen concentration and resulted in strain-age-hardening, which caused an acceleration of quasi-cleavage fracture, an increase in yield strength, and a stress/strain concentration associated with Lüders deformation. These factors diminished the crack initiation resistance and crack growth resistance.
  • Effects of Residual Stress on Hydrogen Embrittlement of a Stretch-Formed Tempered Martensitic Steel Sheet

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    DOI:10.2355/isijinternational.ISIJINT-2020-492

    The effects of residual stress on the hydrogen embrittlement behavior of a tempered martensitic steel sheet with 1-GPa-class tensile strength stretch-formed by a hemisphere punch simulating press-formed automotive structural parts were investigated. Cracking on the stretch-formed specimen induced by potentiostatic hydrogen charging was initiated in the foot of the impression of the specimen and propagated to the radial direction both toward the hillside and the plain. The mixture of quasi cleavage and intergranular fractures were observed whole through the fracture surface. Residual stress in the stretch-formed specimens was analyzed by using energy-dispersive X-ray diffraction method utilizing the synchrotron X-ray radiation at SPring-8. In addition, stress and plastic strain distributions in the specimen were analyzed by using Finite Element Method (FEM). These analyses depicted that the high tensile stress in the circumferential direction was in the foot of the impression, corresponding to the direction of the crack growth. The FEM analysis revealed that the high triaxial stress was in the foot suggesting accumulation of hydrogen. It was considered that the preferential crack initiation at the foot was promoted by the high residual stress in the circumferential direction and the hydrogen accumulation due to stress-induced diffusion.
  • Effect of Rare Earth Ce on the Morphology and Distribution of Al2O3 Inclusions in High Strength IF Steel Containing Phosphorus during Continuous Casting and Rolling Process

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    DOI:10.2355/isijinternational.ISIJINT-2020-053

    This paper analyzes the control of Al2O3 inclusions in high strength IF steel containing phosphorus. The inclusion statistics and two-dimensional morphology of samples with slab thickness of 1/8, 1/2 and 7/8, and the samples with hot rolling, cold rolling and continuous annealing processes were observed and compared by ASPEX SEM. In addition, the three-dimensional morphology of the inclusions extracted from the electrolysis of slab samples and the original morphology of inclusions from the rolling process samples were observed and compared. The results show that: When rare earth Ce is added to the steel, the combination of Ce with activity O and S in the steel has lower Gibbs free energy, and it is easy to generate CeAlO3, Ce2O2S, Ce2O3, composite rare earth inclusions combined with other inclusions. The concentration and supersaturation of aluminum and oxygen are reduced, and the ability of single particle Al2O3 to aggregate into large-scale cluster inclusions is reduced. The average size of Al2O3 inclusions decreased from 5–7 µm to 2–5 µm in each thickness direction. The morphology of inclusions changed from long strip, sharp angle and cluster to spherical, spindle and round surface. Meanwhile, the number density of Al2O3 inclusions increase, but the area density decreases during continuous casting and rolling process. The size of Al2O3 inclusions in the steel without rare earth addition is mostly large-scale strip shape, and it is crushed during rolling,resulting quality defects on the exposed surface and causing the stamping crack problem. When rare earth is added to the steel, the inclusions in the steel change into small-sized circular inclusions with dispersion distribution. The modulus of elasticity is close to that of the steel matrix, which will not affect the continuity of the strip structure, and is beneficial to the relevant properties of the product.
  • Visualization of Hydrogen in Stress and Strain Fields Using SIMS

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    DOI:10.2355/isijinternational.ISIJINT-2020-191

    In order to clarify the mechanism of hydrogen embrittlement in high strength steel, it is necessary to evaluate local hydrogen concentration in stress and strain fields where hydrogen embrittlement is considered to occur. There are several ways to visualize hydrogen, but SIMS is advantageous in that it can detect hydrogen directly without reaction. In this study, visualization of hydrogen accumulated in stress and strain field was tried using SIMS. Hydrogen flux intensity in the bent portion of the U-bend specimen, where the stress gradient exists, changed according to the stress gradient, and the hydrogen flux intensity was higher on the outside of the bending than on the inside, with the center of the plate thickness at the boundary. On the shearing surface with strain field, the hydrogen flux intensity had a positive correlation with the stress. On the other hand, it remained constant with respect to the strain. From these observations, the hydrogen partitioning behavior in steel could be visualized semi-quantitatively by using the isotope labeling method with SIMS.
  • Crystal Structure Analysis of Top Dross in Molten Zinc Bath by First Principles Calculation and Synchrotron X-ray Diffraction

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    DOI:10.2355/isijinternational.ISIJINT-2020-428

    In a molten zinc bath in a continuous galvanizing line, the top dross particles crystallize as a Zn-containing intermetallic Fe2Al5 compound, which generates surface defects in the final product by easily adhering to the steel sheets. The present study focused on analysis of the crystal structure of a top dross by first principles calculation and synchrotron X-ray diffraction of the top dross prepared in a laboratory. The following results were obtained: first principles calculation on the top dross suggested that two Al atoms at four of the partially occupied Al sites in the Fe2Al5 structure proposed by Mihalkovič et al., were replaced by two Zn atoms. In addition, the Al atoms at both the types of partially occupied Al sites in Fe2Al5 proposed by Burkhardt et al., were equally replaced by Zn atoms. The proposed crystal structure of the top dross was verified by conducting X-ray diffraction profile analysis using RIETAN-FP simulations as well as the experimentally determined lattice constant of the Zn-containing top dross.
  • Numerical Simulation of Flow, Heat, Solidification, Solute Transfer and Electromagnetic Field for Vertical Mold and Curved Mold of Billet

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    DOI:10.2355/isijinternational.ISIJINT-2020-019

    A three-dimensional model, which coupled flow, heat transfer, solidification, solute transport and electromagnetic field, was separately developed for vertical mold and curved mold of billet continuous casting. Therefore, the characteristics and disciplinarians of macroscopic transport behavior for different types of mold were revealed. The influence of mold electromagnetic stirring (M-EMS) and mold curvature on the flow, heat transfer, solidification, solute transport in the mold were investigated in detail. The results indicate that the M-EMS can cause obvious rotating flow in the mold and enhance the superheat dissipation of the molten steel to promote the growth of solidification shell. The mold curvature has a profound effect on the flow field distribution of the mold, and further affects the temperature field and solute distribution in the mold. Therefore, the mold curvature is necessary to consider for predicting the macroscopic transmission phenomenon in the mold. Moreover, the predicted and experimental results for distributions of magnetic flux density and near surface element C content compare agreeably, which indicates the validity of the coupled model in current work.
  • Positive Catalytic Effect and Mechanism of Iron on the Gasification Reactivity of Coke using Thermogravimetry and Density Functional Theory

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    DOI:10.2355/isijinternational.ISIJINT-2020-348

    The catalytic gasification characteristics and kinetics of metallurgical coke by iron were investigated by non-isothermal thermogravimetry using volumetric (VM), unreacted core (URCM), and random pore (RPM) models. Density functional theory (DFT) calculations were used to analyse the interaction mechanism of CO2 on the iron catalyst surface. Carbon conversion curves were shifted to a lower-temperature zone upon iron addition, indicating the strong catalytic effect of iron on carbon gasification. Kinetic analysis showed that RPM described coke gasification better than VM and URCM, with an RPM activation energy of 197.1–218.1 kJ/mol. DFT calculations indicated that CO2 molecules parallel to the crystal surface can easily interact with the iron surface. Three stable adsorption configurations with energies of -0.59, -0.62, and -0.78 eV were obtained. In the Löwdin population analysis, the C atom acts as a major electron acceptor from Fe. The C and O orbitals overlap with Fe 3d, 4s, and 4p, indicating stronger hybridisation and demonstrating that Fe (001) can activate CO2.
  • Identification of Cracking Issues and Process Improvements through Plant Monitoring and Numerical Modelling of Secondary Cooling during Continuous Casting of HSLA Steels

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    DOI:10.2355/isijinternational.ISIJINT-2020-442

    A holistic approach to diagnose the occurrence of cracking on HSLA steel slabs and propose countermeasures to prevent them is presented. The approach consisted on plant monitoring, including direct temperature measurements in the strand with pyrometers. Extensive characterization was performed via thermo-mechanical tests and microscopy techniques which revealed combination of Widmanstätten ferrite, acicular ferrite and secondary phases that promote embrittlement during casting with a minimum ductility between 700°C–800°C (± 50°C) which is responsible for cracking in this steel. Finally, 1D and 3D numerical models were developed to test possible cooling strategies which proved that reductions in water flowrate can have a positive effect in slab quality by avoiding the low ductility zone. Corrective actions included decreasing cooling to increase the overall temperature of the strand before the straightener to increase the overall temperature. Yet, some slabs still observed the presence of cracks which points at secondary factors such as high tundish temperatures >1530°C producing cracking. Other secondary factors include strong temperature variations up to ± 250°C during measurements which would send the strand corners into the low ductility range producing cracking despite having a hot slab centre. Although these optimization strategies are particular to each caster and steel grade, a similar approach could be applied to address secondary cooling issues during continuous casting. The models presented are an ideal toolkit to analyse the influence of product size and operation parameters in combination with plant monitoring and extensive microstructure characterization to improve the quality and productivity of the process.
  • Structural and Mechanical Characterizations of Top Dross in a Molten Zinc Bath

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    DOI:10.2355/isijinternational.ISIJINT-2020-487

    In a molten zinc bath in a continuous galvanizing line (CGL), top dross particles crystallize as Fe–Al–Zn intermetallic compounds. These particles easily adhere to the steel sheets causing surface defects. Therefore, controlling the top dross particles is a key issue. Our study focused on the structural and mechanical characterizations of the top dross particles using an electron probe micro analyzer, X-ray diffraction, electron back scattering diffraction, Vickers hardness measurements, and nano-indentation measurements. The following results were obtained: (1) The crystal structure of the top dross particles Fe2Al5Znx having 37–38 wt% Fe, 44–45 wt% Al, and 18–19 wt% Zn belongs to the orthorhombic system with lattice constants of a = 7.61 Å, b = 6.48 Å, and c = 4.23 Å. The a-axis of Fe2Al5Znx becomes shorter, while its b- and c-axes become longer compared to those of the binary Fe2Al5. (2) The coarsening of the top dross particles with the faceted interface was postulated to occur as a result of the driving force provided by the anisotropic interface energy between the top dross particles and molten Zn, rather than via the aggregation mechanism. (3) The hardness and the elastic modulus of the top dross particles are the lowest in the [001] orientation, similar to Fe2Al5, and are lower than those of Fe2Al5. (4) The fracture toughness of the top dross particles is approximately 1.1 MPa·m1/2, which is slightly lower than that of Fe2Al5.
  • Volumetric Strain Dependence of Quantum Diffusion of Hydrogen in bcc Iron

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    DOI:10.2355/isijinternational.ISIJINT-2020-340

    To predict hydrogen embrittlement in steels and clarify its mechanism, it is necessary to understand the time variation of hydrogen distribution. Since the quantum mechanical effect is remarkably observed in hydrogen diffusion, even at room temperature, there is a need to take into account it for analyses. In this study, we evaluated the diffusion coefficients of hydrogen in body-centered cubic (bcc) iron via density functional theory and small-polaron theory calculations. The analyses were carried out under various magnitudes of volumetric strains to investigate its effect on the diffusion coefficient. The temperature dependence of the diffusion coefficient was found to change at about 400 K. This is attributed to the fact that the tunneling between ground states dominantly contributes to the diffusion at lower temperatures, whereas at high temperatures, that between low excited states contributes dominantly. The diffusion coefficient was also found to increase with compressive volumetric strain and decrease with tensile volumetric strain. The volumetric strain dependence of the diffusion coefficient was clarified by the volumetric strain dependence of the tunneling matrix elements and that of the activation enthalpy.
  • Evaluation of Hydrogen-induced Cracking Behavior in Duplex Stainless Steel by Numerical Simulation of Stress and Diffusible Hydrogen Distribution at the Microstructural Scale

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    DOI:10.2355/isijinternational.ISIJINT-2020-399

    Duplex stainless steels possess ferrite and austenite microstructures, which exhibit different mechanical properties. The strength level and hydrogen diffusion constant of the phases are different; therefore, it is expected that the microscopic stress and hydrogen concentration distribution are inhomogeneous. Assuming that hydrogen-induced cracking occurs at locally stress-concentrated and hydrogen-accumulated locations, it is important to consider the influence of the microstructure in the evaluation of hydrogen-induced cracking. In order to observe crack locations at the microstructural scale, a slow strain rate test of the hydrogen-charged specimen was performed and the cross-section of the specimen was observed following the test. Hydrogen-induced cracks were mainly observed in the ferrite phase. A numerical simulation was performed to determine the contribution of the stress and hydrogen concentration distribution to the initiation of hydrogen-induced cracks. A microstructure-based finite element model consisting of ferrite and austenite phases was designed based on the micrograph of the duplex stainless steel used. The stress–strain curves of the ferrite and austenite phases were used and macroscopic tension was applied to calculate the microscopic stress distribution. The microscopic distribution of hydrogen concentration was calculated by incorporating the stress distribution into the hydrogen diffusion simulation as one of the driving forces. From the simulation results, the stress concentration and hydrogen accumulation occurred at the ferrite phase or at the ferrite/austenite boundary. This tendency corresponds closely to the experimentally observed results; therefore, the above approach can be applied to the evaluation of hydrogen-induced cracking at the microstructural scale.
  • Numerical Simulation on Effect of Microstructure on Hydrogen-induced Cracking Behavior in Duplex Stainless Steel Weld Metal

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    DOI:10.2355/isijinternational.ISIJINT-2020-400

    Duplex stainless steels and their deposited weld metal have ferrite and austenite microstructures with different material properties. In addition, the microstructure of the base metal and weld metal clearly differs, affecting hydrogen diffusion and accumulation, and hydrogen-induced cracking behavior at the microstructural scale. In this study, the influence of microstructure on hydrogen-induced cracking behavior of the duplex stainless-steel weld metal was investigated. Duplex stainless-steel weld metal specimens were prepared and slow strain rate tensile test was performed after hydrogen charging. Cracks were observed at the ferrite/austenite boundaries. A microstructure-based finite element simulation was performed to clarify the concentration distribution at the microstructural scale. A finite element model based on the cross-section of the microstructure was designed to calculate the stress and hydrogen concentration distribution. The simulation result showed that hydrogen accumulation occurred at the ferrite/austenite boundaries, which corresponded to the locations where cracks were observed. On the other hand, the hydrogen concentration at the accumulation site in the weld metal was lower than that in the base metal. Therefore, the influence of the phase fraction and stress–strain curves of the ferrite and austenite phases on the hydrogen concentration was investigated by the proposed numerical simulation. Both phase fraction and stress–strain curves significantly influenced the microscopic distribution of hydrogen concentration.
  • Effects of Grain Boundary Characteristics on Secondary Recrystallization Textures in Fe–Si Alloy

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    DOI:10.2355/isijinternational.ISIJINT-2020-437

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

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    DOI:10.2355/isijinternational.ISIJINT-2020-135

    Approximately 60–70% of the direct greenhouse gas emissions in electric arc furnace (EAF) steelmaking originate from the use of fossil carbon charge during melting of steel scrap. Regarding short-term solutions to mitigate the climate impact of steelmaking, there is greater potential to replace fossil carbon charge with renewable carbon in the EAF than in integrated blast furnace steelmaking where mechanical strength requirements on carbon charge are too demanding. Therefore, the present study aims to provide an experimental and practical foundation for using renewable biochar in the EAF as a relatively simple step to decrease the climate impact of steelmaking.In order to evaluate the inherent performance of biochar as a carburizing agent, lab-scale tests where completed using four different types of carbonaceous materials: synthetic graphite, anthracite coal and two types of biochar from woody biomass (BC1 and BC2). The first order dissolution rate constants from experiments ranged between 0.7 to 1.9 × 10-4 m/s, which agrees well with previously reported results. Furthermore, lab-scale results show that biochar properties commonly seen as detrimental, such as low carbon crystallinity and high porosity, do not necessarily constitute a disadvantage for biochar utilization as carburizer in steelmaking.In order to further assess the results from lab-scale tests, an industrial trial including six consecutive heats was performed in a 50 t EAF at the Höganäs Halmstad Plant. Results show that 33% substitution of standard Anthracite carbon charge with biochar BC2 gave no deviation from normal operating conditions in the EAF.
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    1. Dissection Investigation of Forming Process of Titanium Compounds Layer in the Blast Furnace Hearth ISIJ International Vol.60(2020), No.11
  • Method for Evaluating Hydrogen Embrittlement of High-Strength Steel Sheets Considering Press Formation and Hydrogen Existence State in Steel

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    DOI:10.2355/isijinternational.ISIJINT-2019-822

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

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    DOI:10.2355/isijinternational.ISIJINT-2020-260

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

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    DOI:10.2355/isijinternational.ISIJINT-2020-018

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

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    DOI:10.2355/isijinternational.ISIJINT-2019-510

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

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  7. Preface to the Diamond Jubilee Issue on “Selected Topics in Iron and Steel and Their Processing toward the New Steel Age” ISIJ International Vol.60(2020), No.12
  8. In situ Observation of Reduction Behavior of Multicomponent Calcium Ferrites by XRD and XAFS Tetsu-to-Hagané Advance Publication
  9. Improved Hydrogen Embrittlement Resistance of Steel by Shot Peening and Subsequent Low-temperature Annealing ISIJ International Advance Publication
  10. Intraparticle Temperature of Iron-Oxide Pellet during the Reduction Tetsu-to-Hagané Vol.60(1974), No.9

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