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

ISIJ International Advance Publication

  • Quantitative Correlation between Interfacial Heat Transfer Coefficient and Pressure for 19Cr-14Mn-0.9N High Nitrogen Steel Cylindrical Ingot

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    DOI:10.2355/isijinternational.ISIJINT-2020-010

    This research is aimed to correlate heat transfer coefficient to pressure at the interface between 19Cr-14Mn-0.9N high nitrogen steel cylindrical ingot and cast iron mould, during the pressurized solidification process of cylindrical ingot. The correlations were obtained by mathematical inverse model of heat conduction problem. Validation results indicate that this model is applicable to investigate the change in interfacial heat transfer coefficient during the pressurized solidification process of 19Cr-14Mn-0.9N high nitrogen steel, and guarantee the correlation accuracy. Combing with theoretical derivation for cylindrical steel ingot, the change in interfacial heat transfer coefficient with time can be described by hf,0.5 = 679.68t-0.12 W/(m3·K) for 0.5 MPa, hf,0.85 = 753.53t-0.12 W/(m3·K) for 0.85 MPa and hf,1.2 = 790.39t-0.12 W/(m3·K) for 1.2 MPa, quantitatively. Meanwhile, an empirical formula was presented to correlate interfacial heat transfer coefficient to pressure, which can be taken as heat transfer boundary to simulate the change in solidification state of 19Cr-14Mn-0.9N high nitrogen steel cylindrical ingot with pressure.
  • 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.
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    1. An Evaluation Method for Hydrogen Embrittlement of High Strength Steel Sheets Using U-bend Specimens ISIJ International Advance Publication
  • 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.
  • Control of Laser Focal Point by Using an Electrically Tunable Lens in Laser-induced Plasma Optical Emission Spectrometry

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    DOI:10.2355/isijinternational.ISIJINT-2020-170

    This paper suggests a method to control the focal point of laser on the on-focus position of a sample surface automatically in laser-induced breakdown spectrometry (LIBS). For this purpose, an electrically-tunable plano-convex lens was installed in a laser irradiation system, where it could vary the focal length of laser with a long working distance and a rapid response time, and the focal length could be periodically varied with a triangle waveform. Because the tunable lens was easily handled and inexpensive, the laser system could be modified with a low cost, as compared with commercial apparatuses having complicated optics to control the position of laser irradiation. A piece of scrapped stainless steel the surface of which was titled and had some roughness was investigated as a test specimen. A satisfactory result was obtained such that the plasma could be generated uniformly and firmly along a laser trace on the sample surface and thus could give the emission signal with a sufficient precision. The driving frequency of the tunable lens, which controlled a repetition period of the laser beam, was optimized to be 10 Hz when the scan rate of laser was fixed at 3.0 mm/s. As a result, it is expected that the LIBS system with the tunable lens can be applied to actual on-site/in-line analysis in material production.
  • Deoxidation Equilibria of Fe–Mn–Al Melt with Al2O3 or MnAl2O4 at 1873 and 1773 K

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    DOI:10.2355/isijinternational.ISIJINT-2020-177

    Deoxidation equilibria of Fe–Mn–Al melt with Al2O3 or MnAl2O4 were measured at 1773 K. Composition of melts doubly-saturated with Al2O3 and MnAl2O4 were also measured using a crucible comprising these two phases at 1873 or 1773 K. Equilibria with each solid oxide were analyzed using Wagner's Interaction Parameter Formalism (WIPF). In the case of Al2O3 saturation, Al deoxidation curve at 1773 K was similar in shape to that at 1873 K, and the equilibrium oxygen content was approximately 1/3 of that at 1873 K. The deoxidation equilibria were reproduced using WIPF at the composition range above 0.1 mass%Al by using -0.32 as and 10-13.4 as the equilibrium constant of Al2O3 dissolution reaction, both of which were determined through analysis of measured results for Fe–(20 to 30) mass% Mn melt. In the case of MnAl2O4 saturation, accurate values of equilibrium constant were not obtained because of the relatively significant influence of oxygen analysis error. On the contrary, using compositions doubly-saturated with Al2O3 and MnAl2O4, valid values of the equilibrium constant of MnAl2O4 dissolution reaction, 10-15.4 and 10-17.7 at 1873 and 1773 K, respectively, could be determined.
  • Surface Tension Calculation of Molten Slag in SiO2–Al2O3–CaO–MgO–‘FeO’–‘Fe2O3’ Systems Based on a Statistical Modelling Approach

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    DOI:10.2355/isijinternational.ISIJINT-2019-807

    A calculation model for surface tension of molten slags in SiO2–Al2O3–CaO–MgO systems, based on a statistical modelling approach, was further extended to SiO2–Al2O3–CaO–MgO–‘FeO'–‘Fe2O3' multi-component systems. A total number of 1493 surface tension data reported in literatures, including 661 iron-containing data in this study, have been collected and critically reviewed for optimizing model parameters. The model achieves an excellent agreement with literature values for iron -containing melts with an average error of 4.9% and overall absolute error of 29.75 mN/m. Moreover, the dependence of surface tension on composition and temperature the composition has been discussed using the present model. The results show that the surface tension always obviously decrease with the increment of SiO2 content and substitution of CaO or Al2O3 by MgO will cause a decrease in the surface tension. In case of silica-free ternary systems such as the Al2O3–CaO–‘Fe2O3' and CaO–MgO–‘Fe2O3', the surface tension decreases with increasing ‘Fe2O3' concentration with constant Al2O3 and CaO level respectively, indicating the network former role of ‘Fe2O3' in these studied systems. Temperature coefficient is closely related with the melt composition and positive values for high ‘FeO' and ‘Fe2O3' contents in the melts were found.
  • Enhanced Homogeneity of a Flat-rolled Wire in Twinning-induced Plasticity Steel Using the Pass Schedule Design

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    DOI:10.2355/isijinternational.ISIJINT-2020-029

    The effects of reduction in height per pass, roll diameter, and friction coefficient on the homogeneity of mechanical properties and shape change in flat-rolled twinning-induced plasticity steel wire were investigated. The goal was to improve the homogeneity of mechanical properties of a wire with area during flat rolling process using a numerical simulation, a hardness test, and electron backscatter diffraction techniques. Reduction in height per pass and roll diameter had large influences on both strain inhomogeneity and lateral spread of flat-rolled wire. Strain inhomogeneity and lateral spread increased with increasing the reduction in height per pass and roll diameter due to the higher length of the contact area. The underlying mechanism for the strain inhomogeneity and lateral spread of flat-rolled wire was highly related to the length of contact area. Hence, the length of the contact area needed reduction through controlling process conditions to improve the strain homogeneity of the flat-rolled wire. The effect of friction coefficient on lateral spread was negligible, whereas strain inhomogeneity slightly increased with friction coefficient. The combination of high and low reduction in height per pass with a smaller roll diameter improved the homogeneity of mechanical properties and microstructure over the area of the flat-rolled wire. Based on the results of numerical simulation and experimental test, a new practical strategy is proposed to achieve greater homogeneity of mechanical properties over the area of flat-rolled wire, which could be of great applicability in industrial fields.
  • Crack Propagation Behavior of Impact Fracture in Case Hardening Steel Subjected to Combined Heat Treatment with Excess Vacuum Carburizing and Subsequent Induction Hardening

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    DOI:10.2355/isijinternational.ISIJINT-2019-826

    The Charpy impact value of case hardening steel subjected to combined heat treatment with excess vacuum carburizing and subsequent induction hardening was evaluated. The purpose of this study is to clarify the relation between the crack propagation behavior and the microstructure in steels having different amounts of retained austenite and cementite. The vacuum carburizing treatment is performed at the hyper-eutectoid composition of 1.3 mass% C. Three different heating temperatures were chosen for induction hardening in the two-phase (austenite, cementite) region between Acm and A1 to obtain different amounts of retained austenite and cementite. Decreasing the induction heating temperature from 1143 K to 1043 K, increased crack propagation resistance by around 30% on average in both the quenched-only and the quenched-and-tempered specimens. The high crack propagation resistance of the samples with the low induction heating temperature was caused by the arrest effect of undissolved θ. By contrast, in the sub-zero treated specimens, crack propagation resistance showed an almost constant value irrespective of the induction heating temperature. That constant propagation resistance was attributed to the repeated bending and branching occurring during crack propagation.
  • Effect of Steel-refractory Reactions on Removal of Arsenic from Molten Steel with Lanthanum Additions

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    DOI:10.2355/isijinternational.ISIJINT-2020-052

    To formulate strategies to remove arsenic from molten steel by adding rare earth elements (REs), the evolution of inclusions in steel with different lanthanum additions was studied, and the effect of reactions between lanthanum and magnesium crucibles on the removal of arsenic was discussed. The results show that the addition of lanthanum can remove arsenic from molten steel, but steel-refractory reactions dramatically influenced the removal effect. The arsenic removal was determined by the generation of La–S–As. The reactions between lanthanum and magnesia crucibles partly consumed lanthanum and decreased its effective concentration acting on arsenic. Further, the reaction product dissolved magnesium consumed a part of sulfur that was disadvantageous for the formation of La–S–As. Besides, a sequence of reactions existed after the addition of lanthanum. The original Si–Mn–Al–O inclusions were changed to lanthanum-containing oxides first and then to MgO-rich oxides. The reaction to generate La–S–As mainly took place within 5 min. The consumption of REs by crucible refractories is an important issue that needs consideration. Alumina crucibles are more favored over magnesia crucibles when using REs to remove arsenic from molten steel.
  • Inclusion Characteristic in Tinplate Steel in RH Refining and Kinetics Limitation of Calcium Transfer by Refining Slag

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    DOI:10.2355/isijinternational.ISIJINT-2020-237

    The characteristics of inclusions including composition, morphology, number, and size in tinplate steel were studied by industrial experiments and thermodynamic calculations during the RH refining process. The results indicated that two types of Al2O3 inclusions including cluster and single-particle are generated at first after Al addition. With the slag-metal and refractory-metal reactions, Al2O3 inclusions, CaO·Al2O3 inclusions, MgO·Al2O3 spinel inclusions, and CaO–MgO–Al2O3 ternary system inclusions are found in the middle of RH refining. Only single-particle Al2O3, CaO·Al2O3 inclusions with high melting point, and CaO–MgO–Al2O3 ternary system inclusions are found at the end of RH refining. From Al addition to the end of RH refining, the total number of inclusions showed a decreasing trend and the proportion of the number density decreased by 70%. About 62% of inclusions are smaller than 10 µm at the end of RH refining, which are difficult to be removed from the liquid steel. The mass transfer of Ca from the refining slag to the liquid steel has a significant effect on the content of [Ca] in liquid steel. Al2O3 inclusions generated in liquid steel can only be modified to CaO·Al2O3 inclusions in the present RH refining time. Aiming to generate 12CaO·7Al2O3 inclusions quickly, moderate calcium treatment as a supplementary measure for refining slag is recommended to modify inclusions during the RH refining process.
  • Numerical Simulation of Fluid Flow and Solidification in a Vertical Round Bloom Caster Using a Four-port SEN with Mold and Strand Electromagnetic Stirring

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    DOI:10.2355/isijinternational.ISIJINT-2019-738

    A computational model coupling electromagnetic field with a macroscopic heat and fluid flow is developed to investigate the flow pattern and solidification in a vertical continuous caster using a four-port submerged entry nozzle (SEN) with mold and strand electromagnetic stirring (M-EMS and S-EMS). The flow pattern and solidification features of the bloom strand without and with EMS in the caster using the four-port SEN is analyzed and compared with that using a straight-port nozzle. The effects of the stirring parameters and the position of the strand stirrer on the flow and solidification are discussed. The approach to identify the optimum stirring parameters by the comparison of tangential velocity is suggested. The results show that the application of M-EMS in a four-port SEN can weaken the strength of the jet impingement from every port of the four-port SEN, and rapidly dissipate the superheat of the melt and reduce the liquid fraction in the mold. In spite of inhomogeneous shell growth in the mold, the swirl velocity obtained by a four-port SEN and M-EMS and the solidus fraction by S-EMS is above those of a single-port SEN with the same stirring strength, which is favorable for the formation of more equiaxed crystals. For the S-EMS, the solidified shell thickness is the main factor to determine the stirring position and the tangential velocity at the same stirring intensity. In terms of different ported SENs, it is necessary to perform specific optimization of the stirring parameters of M-EMS and S-EMS.
  • Numerical Analysis of Effect of Operation Conditions on Bubble Distribution in Steel Continuous Casting Mold with Advanced Bubble Break-up and Coalescence Models

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    DOI:10.2355/isijinternational.ISIJINT-2020-106

    Argon bubbles are usually injected into steel continuous casting mold to prevent the clogging of submerged entry nozzle (SEN), but some bubbles may be entrapped to form defects in the final slab. In order to provide a reference for improving the quality of steel, a mathematical model based on the Eulerian-Lagrangian approach with advanced bubble break-up and coalescence models was established to study the effect of operation conditions on bubble distribution in a steel continuous casting mold. A bubble break-up model based on a daughter bubble fraction, which is suitable for the continuous casting system, was considered. The mathematic model was validated by comparing of the size and number of captured bubbles with the plant measurements of previous work. The result shows that argon gas injection has obvious effect on the flow pattern in the upper recirculation zone of the mold. In the upper recirculation zone, the bubbles mean diameter decreases and the bubble number increases with increasing casting speed, and both of the bubble size and number increase with the increase of gas flow rate. From the result, it can be found that the number and diameter of bubbles arriving at the advancing solidified shell region increase with increasing casting speed. In addition, the increase of gas flow rate causes more bubbles arriving at the advancing solidified shell region, but has little effect on the size of bubbles.
  • Solid-liquid Interfacial Energy for Fe–Cr Alloy under Temperature Gradient from Molecular Dynamics Simulation

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    DOI:10.2355/isijinternational.ISIJINT-2019-769

    The solid-liquid interfacial energy of Fe–Cr alloy under temperature gradient is investigated by molecular dynamics (MD) simulations in conjunction with a capillary fluctuation method including the effect of temperature gradient. It is revealed from the MD simulation that fluctuation of the solid-liquid interface decreases with increasing temperature gradient. This results in a large value of the solid-liquid interfacial energy under large temperature gradient. On the other hand, there is a competing effect reducing the solid-liquid interfacial energy with increasing temperature gradient in the formulation of the capillary fluctuation method including the effect of temperature gradient. As a result, the solid-liquid interfacial energy doesn't change significantly at small temperature gradient. Moreover, it is confirmed that the solid-liquid interfacial energy of Fe–Cr alloy decreases with increasing Cr composition at Fe-rich composition regardless of the temperature gradient.
  • Availability of Opal Photonic Crystal Films for Visualizing Heterogeneous Strain Evolution in Steels: Example of Lüders Deformation

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    DOI:10.2355/isijinternational.ISIJINT-2020-223

    An opal photonic crystal film was applied to characterize local strain evolution associated with Lürders band propagation in an annealed low carbon steel. A local change in color of the opal film was observed, which corresponded to the propagation of the Lürders band. In particular, we carried out two tensile experiments for line and area analyses of RGB (Red-Green-Blue) values of the opal films pasted on the specimens. Both of the experiments clearly exhibited a quantitative correspondence between color variation and local strain evolution, namely, the present study demonstrated the potential of the opal films to analyze heterogeneous strain evolution in steels.
  • Phase Composition and Formation Mechanism of Slag Crust in Blast Furnace

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    DOI:10.2355/isijinternational.ISIJINT-2020-113

    Copper stave damage is common problem in blast furnace operations, and the formation of slag crust is beneficial to reduce the damage of copper stave. Therefore, an in-depth understanding of phase composition and formation mechanism of slag crust is helpful to clarify the protection mechanism of copper stave, so as to control the growth of the slag crust and to increase the service life of copper staves. In this study, the slag crust from a copper stave blast furnace was sampled, and the phase composition and structure of the slag crust were characterized in detail through XRD analysis and SEM-EDS. The results indicated that the slag crust presented apparent layer structure as the solid slag layer and viscous layer, which primarily consisted of gehlenite (Ca2Al2SiO7), calcium aluminate (CaAl4O7), magnesia-alumina spinel (MgAl2O4), pleonaste (Mg0.7Fe0.23Al1.97O4), kaliophilite (KAlSiO4) and metallic iron. In addition, the ternary phase diagram analysis of CaO–SiO2–Al2O3 showed that the primary crystal phase of the slag is in the gehlenite region, and that the primary crystal region migrates to the calcium aluminate region with the increasing of Al2O3 content, which are beneficial to the slag crust formation. Finally, the formation mechanism of slag crust was proposed.
  • Effects of Concentrations of Micro-alloying Elements and Hot-forging Temperature on Austenite Grain Structure Formed during Carburization of Case-hardening Steel

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    DOI:10.2355/isijinternational.ISIJINT-2020-123

    Effects of fine precipitates on the austenite (γ) grain structures were investigated in JIS SCM420-based case-hardening steels with several different concentrations of the micro-alloying elements and hot-forging temperatures. Micro-alloyed steels of 18Al (0.018 mass% Al) and 35Al–32Nb (0.035 mass% Al, 0.032 mass% Nb) were forging-simulated at 1150°C or 1250°C, normalized at 1070°C, and carburized at 1050°C. When the as-received 18Al steel was normalized and carburized without forging-simulated heating, a uniform γ grain structure was observed with the distribution of fine AlN precipitates. However, coarsening of AlN occurred when the forging-simulated temperature was 1150°C and it caused abnormal grain growth during carburization. In 35Al–32Nb steel, the same heating did not induce the abnormal grain growth owing to the AlN–Nb(C,N) combined particles. The size of these particles increase with an increase in the forging-simulated temperature. The high forging-simulated temperature caused the dissolution of the fine precipitates, followed by reformation and coarsening of the precipitates during the subsequent cooling and the normalization heating, which resulted in a decreased pinning force and γ grain coarsening. Furthermore, TEM observations revealed that a considerable amount of Nb(C,N) particles exist near large eutectic MnS particles. Thermodynamic calculations based on the Scheil's condition showed that the formation of these Nb(C,N) particles was due to segregation during solidification. It was suggested that such local concentration of the precipitate particles in the last solidifying region leads to ununiform distribution of the pinning force that may induce the abnormal grain growth.
  • Influence and Mechanism of CaO on the Oxidation Induration of Hongge Vanadium Titanomagnetite Pellets

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    DOI:10.2355/isijinternational.ISIJINT-2020-091

    The oxidation induration process of Hongge vanadium titanomagnetite pellets (HVTMP) with different amounts of CaO was investigated. This research forms part of an ongoing study on a novel and clean smelting process for the comprehensive utilization of Hongge vanadium titanomagnetite. The results revealed that the compressive strength of HVTMP could be enhanced by adding only 1% CaO because of the formation of fine grains and homogenous microstructures. However, a further increase in CaO decreased the compressive strength. Excessive amounts of CaO decreased the grain size but increased the porosity, impeding the oxidation induration process of HVTMP. An increase in CaO slightly influenced the phase compositions but largely reduced their peak intensities. An innovative index, namely induration degree, was proposed to directly estimate the induration behavior of HVTMP. A schematic diagram was presented to clearly describe the induration mechanism of HVTMP with different amounts of added CaO.
  • Sinter Pot for Temperature Measurement of the Top Layer during and After the Ignition

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    DOI:10.2355/isijinternational.ISIJINT-2020-090

    For achieving high sinter yield and quality, various technologies are being implemented and developed to control the heat pattern during the sintering reaction. Further improvements in these technologies necessitate detailed time-course profiles of temperature at all sinter-bed heights; however, no technique has yet been reported for determining the temperature distribution in the top layers of the sinter bed at high spatial and time resolutions. Herein, detailed heat patterns in these layers were visualized by a newly developed pot test apparatus having ~300-mm sinter-bed height. The developed apparatus demonstrated the effect of ignition time on heat patterns during combustion and immediately after ignition. Ignition times of 30, 60, and 90 s demonstrated that the high-temperature holding time increased with an increase in ignition time, and this effect is more evident in the top layer. All parameters, including high-temperature holding time, flue gas composition, and sinter yield, suggest that a longer ignition time intensified coke combustion in the top half layer. The developed technique to measure the temperature in the top layer will quantitatively clarify the effect of segregation or ignition condition on the heat pattern in the top layer.
  • Effect of Titanomagnetite Ironsand Coal Composite Hot Briquette on Softening-melting Performance of Mixed Burden under Simulated Blast Furnace Conditions

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    DOI:10.2355/isijinternational.ISIJINT-2020-098

    Titanomagnetite ironsand coal composite hot briquette (ICHB) was proposed as a novel type of burden to enhance the incremental and high-efficiency utilization of ironsand in blast furnace. ICHB was prepared firstly under laboratory conditions, with a compressive strength higher than 3000 N. Then the national charging ratio of ICHB in the mixed burdens was explored to conduct softening-melting experiments with the simulated BF conditions. Finally the softening-melting-dripping mechanism of mixed burdens was discussed by thermodynamic calculations, SEM-EDS, and XRD detections in this work. It was showed that the softening-melting-dripping behavior and the permeability of mixed burdens could be improved obviously by an appropriate ICHB charging. With the increasing of ICHB charging ratio, the location of cohesive zone was shifted down gradually and its thickness was the narrowest at the ICHB charging ratio of 10%, which was beneficial to BF smelting. Meanwhile, the dripping ratio of mixed burden also achieved the maximum value of 66.71% when the ICHB charging ratio was 10%. However, the excessive ICHB charging would promote the precipitation of Ti(C,N) with a high melting point at the interface between metal and slag, which resulted in the deterioration of dripping and further worsening the gas permeability of mixed burdens. Comprehensively considering the softening-melting-dripping behavior and the permeability of mixed burden, and the precipitation of Ti(C,N), the recommended ICHB charging ratio was 10%.
  • Influence of H2–H2O Content on the Reduction of Acid Iron Ore Pellets in a CO–CO2–N2 Reducing Atmosphere

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    DOI:10.2355/isijinternational.ISIJINT-2019-734

    Using hydrogen as a reducing agent for iron production has been the focus of several studies due to its environmental potential. The aim of this work is to study the influence of H2–H2O content in the gas phase on the reduction of acid iron ore pellets under simulated blast furnace conditions. Temperature and gas compositions for the experiments were determined with multi-point vertical probes in an industrial blast furnace. The results of the reduction tests show that higher temperatures and H2 content increase the rate and extent of reduction. For all the gas and temperature combinations, morphological, mineralogical, and microstructure changes were observed using different characterization techniques. Microscopy images reveal that H2–H2O, in the gas phase, has a positive influence on reduction, with metallic iron forming at the pellet's periphery and core at lower temperatures compared to CO–CO2–N2 reducing gas. Porosity and surface area changes were determined using a gas pycnometer and the BET method. The results indicate that increasing the reduction temperatures and H2 content results in greater porosity and a larger surface area. Moreover, carbon deposition did not take place, even at lower temperatures. A rate minimum was detected for pellets reduced at 800°C, probably due to metallic iron formation, hindering the diffusion of reducing gases through the product iron layer.
  • Improvement in Reduction Behavior of Sintered Ores in a Blast Furnace through Injection of Reformed Coke Oven Gas

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    DOI:10.2355/isijinternational.ISIJINT-2020-063

    As an innovative route to mitigating CO2 emissions in ironmaking, increasing the hydrogen reduction in a blast furnace is promising. One possible method is the shaft injection or blast tuyere injection of coke oven gas (COG) with its hydrogen concentration enhanced by steam-reforming methane and tar. Therefore, the reduction behavior of sintered ores in a blast furnace by injecting reformed COG was investigated using a softening-melting tester and counter-current reaction simulator (BIS). The shaft injection of reformed COG promoted the reduction and improved the permeability of the ore layer, particularly in the wall area of the blast furnace. An injection rate larger than 200 Nm3/t-HM was required for reformed COG for a limiting intermediate distribution ratio of injection gas lower than 20% in a large blast furnace. Unchanged shaft temperature and increased hydrogen reduction were observed during the shaft injection of hot reformed COG in the BIS test. The water-gas shift reaction below the temperature of the thermal reserve zone was insignificant even for the shaft injection of reformed COG. As for tuyere injection, direct reduction was decreased by increasing the injection rate of reformed COG from tuyere. The injection of COG with or without reforming from tuyere reduced the carbon consumption of the blast furnace by 10 kg/t-HM. The influence of the composition of COG on carbon consumption was insignificant. Direct observation of hydrogen reduction revealed a decrease in flooding molten slag in the upper coke layer during reduction, thus explaining the improved permeability of the ore layers.
  • Fundamentals of the Reduction of SFC in CO/CO2 Gas at 1000°C

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    DOI:10.2355/isijinternational.ISIJINT-2020-064

    SFC (Sillico-Ferrite of Calcium) is a kind of simplified SFCA (Sillico-Ferrite of Calcium and Aluminum) with no Al2O3. SFC and SFCA are believed to be the most desirable bonding phase in sinter. In order to better understand the fundamentals of the reduction of SFC, a series of experiments on the SFC reduction were carried out in the present work, including phase equilibria tested by XRD, morphology tested by SEM-EDS, and reduction pathway under the different CO/CO2 mixture gas at 1000°C. The experimental results indicated, (1) in the case of CO = 20% and 40%, most of Fe2O3 in SFC was reduced to FeO. The equilibrium phases were FeO, CaO·Fe2O3, and CaO·SiO2. (2) In the case of CO = 60%, CaO·Fe2O3 was reduced to generate FeO and 2CaO·Fe2O3. The equilibrium phases were FeO, 2CaO·Fe2O3, and CaO·SiO2. (3) In the case of CO = 80% and 90%, FeO was reduced to Fe, and 2CaO·Fe2O3 was reduced to generate Fe and CaO. The equilibrium phases were Fe, CaO, and CaO·SiO2. The findings from this work may provide guidelines for the improvement of sintering production and blast furnace performances.
  • 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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    DOI:10.2355/isijinternational.ISIJINT-2020-027

    By using a steel with standardized chemical composition and conventional manufacturing processes for flat-rolled steel strip, a 1500 MPa class stainless steel sheet, whose product of yield strength (YS) and total elongation (El) exceeds 30000 MPa%, was developed and its mass production was established. Besides the excellent YS–El balance, the developed steel sheet has excellent performance for not only an anti-secondary work embrittlement but also high cycle fatigue endurance.Core technology of the developed method is composed of a combination of high precision cold-rolling and isothermal partitioning treatment in a batch furnace, named as a rolling and partitioning (R&P) method. By the R&P method, the microstructure of steel is controlled to the mixture of a strain-induced martensite as the matrix phase, and an optimum amount of retained austenite as the second phase which is dispersed in isolation and surrounded by the transformed martensite.In this paper, the microstructure formation during the R&P process and the deformation mechanism that would bring about the excellent strength–ductility balance are discussed based on the results obtained from the in situ neutron diffraction measurement. The results have revealed that the typical Lüders-like stress–strain curve of R&P steel is caused by competitive plastic flow between austenite and martensite, and an effective transformation induced plasticity phenomenon.
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    DOI:10.2355/isijinternational.ISIJINT-2019-497

    Digital image correlation was applied to analyze the strain distribution and deformation-induced martensitic transformation of retained austenite in a low alloy transformation-induced plasticity (TRIP) steel plate under tension. The distribution of strain instilled by tensile deformation was inhomogeneous at a microscopic scale. Strain generated by deformation-induced martensitic transformation was successfully visualized and it led to a homogeneous strain distribution. The retained austenite in the high strain region transformed to martensite preferentially, which demonstrates that inhomogeneous strain distribution affects the stability of retained austenite. The high resolution strain distribution exhibited that a certain amount of strain instilled into retained austenites and there are a lot of strain concentration sites at ferrite/austenite interfacial boundaries in high strain region. Therefore, the stress concentration at the ferrite/retained austenite interfacial boundary occurs due to the difference of strain between the ferrite matrix and retained austenite. These strain accumulation in a retained austenite and/or stress concentration at ferrite/retained austenite interfacial boundary may induce martensitic transformation in high strain regions.
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    DOI:10.2355/isijinternational.ISIJINT-2019-823

    We have constructed an automatic in situ observation system for monitoring the behavior of small fatigue cracks at the microstructural level that, when used in conjunction with a digital-image correlation (DIC) technique, permits the continuous and automatic tracking and recording of microscopic deformation behavior. To verify the effectiveness of this system, we applied it to the evaluation of small fatigue cracks in heat-treated low-carbon steel. The results confirmed that our system can be used in the automatic tracking and recording of the initiation and early growth behavior of microstructurally small fatigue cracks. By the use of DIC analysis, we also succeeded in visualizing the opening-and-closing behavior of small fatigue cracks as well as the behavior of microscopic microstructural deformations, such as inhomogeneous strain concentrations, that caused the fatigue cracks. Although the early-stage growth of fatigue cracks propagates faster than that of long cracks, it is consistent with long-crack data if the effective stress intensity factor range ΔKeff which calculated by crack opening stress measured by DIC is used.
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    DOI:10.2355/isijinternational.ISIJINT-2019-718

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

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    DOI:10.2355/isijinternational.ISIJINT-2019-776

    The industrial application of nanofluids had been explored by many researchers since nanofluids were proposed. However, there were different opinions on the effect in jet cooling. In this paper, 0.4 vol%, 0.8 vol%, 1.2 vol%, 1.8 vol%, 2.4 vol% Al2O3-water, TiO2-water, SiO2-water nanofluids and pure water were used as quenching coolants to complete single jet cooling experiments on the free surface of 50 mm high-temperature steel plate. The results showed that using low concentration (0.4–1.2 vol%) nanofluids could significantly improve the maximum heat fluxes, cooling speed peaks, and moving velocities of peaks along the thickness direction compared with pure water. However, the cooling uniformity in the horizontal direction was reduced, especially with high concentration nanofluids (≥1.8 vol%). Through comprehensive comparison, when 1.2 vol% Al2O3 + water was used as coolant, the optimal cooling efficiency could be achieved, and cooling speed peaks along the thickness were 8.14%–19.70%, 2.16%–3.48% and 0.74%–1.44% higher than that of pure water respectively.
  • Fluid Flow Characteristic of EAF Molten Steel with Different Bottom-Blowing Gas Flow Rate Distributions

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    DOI:10.2355/isijinternational.ISIJINT-2019-794

    As an efficient stirring method, bottom-blowing technology was applied in the present electric arc furnace (EAF) steelmaking process to improve the dynamic conditions of the molten steel. This article describes the development of a numerical model to simulate the 3D multiphase flows (gas, steel, and slag). Comparisons among uniform and non-uniform (linear and triangle distributions) bottom-blowing gas rate arrangements were performed with both metallurgic and dynamic parameters obtained from the numerical simulation process and separated liquid steel analysis. The numerical simulation results indicated that the bottom-blowing scheme with the gas rate in a linear change distribution had the best stirring effects in the molten bath. In addition, the dynamic conditions in the molten bath were worse when the bottom-blowing gas rate change was focused on the nozzle near the eccentric bottom tapping area. Furthermore, water model and industrial experiments were performed. For this purpose, 120 sets of heat industry data were collected in the 100 t EAF steelmaking process. The results showed that the non-uniform bottom-blowing scheme is more able to improve the dynamic conditions of the molten bath compared with the conventional uniform gas rate distribution, which further validated the reliability of the present numerical simulation results.
  • Industrial Ladle Furnace Slag Composition Analysis with Optical Emissions from the Arc

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    DOI:10.2355/isijinternational.ISIJINT-2019-676

    With the strict standards for steel quality and high production rates, the demand for faster and more convenient slag composition analysis for both electric arc and ladle furnaces has become a major issue in industrial steel plants. To overcome the time-delay between slag sampling and results of the slag composition analysis, an on-line slag composition analysis is required. Such a method that can be used in on-line analysis and is also chemically sensitive to the slag composition is optical emission spectroscopy. In this work, the optical emissions from the arc have been measured in an industrial ladle furnace and used for slag composition analysis. This article focuses on CaF2 and MgO, since the CaF2 is a common additive material in the ladle treatment and high MgO content means that the ladle refractory lining is dissolving into the slag. The analysis has been carried out by comparing emission line ratios to the XRF analyzed ratios of CaF2/MgO and MnO/MgO, respectively. The results show that several atomic emissions lines of calcium, magnesium, and manganese can be used to evaluate the CaF2/MgO and MnO/MgO ratios in the slag. It was found out that the plasma temperature derived from Ca I emission lines has a non-linear relation with the CaF2 content of the slag. Additionally, the dissociation pathways of molecular slag components were determined and studied in different plasma temperatures with equilibrium composition computation in order to determine the relations between the slag and plasma compositions.
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    1. Softening–melting Properties and Slag Evolution Behavior of High Titanium Sinter ISIJ International Vol.60(2020), No.7
  • Effect of Direct Powder Additions on the Solidification Structure and Microsegregation of 42CrMo4 Steel

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    DOI:10.2355/isijinternational.ISIJINT-2019-787

    Inoculation and its link with the solidification structure is a relatively new field for low alloy steels. In this study, a cold crucible setup is used to realize direct particle inoculation of 50 g steel ingots. Eight different inoculants powders (oxides, nitrides and carbides) were tried with a 0.3 mass% level addition. Solidification structure sizes and morphologies, presence of inoculant particles and microsegregation have been characterized for all the samples. The best grain refinements were obtained for Si3N4, TiN and CeO2 additions whereas the lowest microsegregation intensities are achieved for Si3N4, HfC and W2C additions. The properties of the inoculants – misfits, solubility products and terminal velocities – are used to discuss those changes. The grain refinement could be linked to the misfit in good agreement with the literature. Other morphological changes (secondary arm spacing and dendricity) were attributed to the presence of inert particles staying inside the liquid during the solidification. Last but not the least, the flattest microsegregation profiles were possibly due to inoculant dissolution leading to a change in the MnS precipitation sequence.
  • Measurement and Thermodynamics of Carbon Solubilities in Molten Si–Fe, Si–Ni, and Si–Cr–Fe Alloys at 2073 K

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    DOI:10.2355/isijinternational.ISIJINT-2019-511

    The equilibrium phase relations of molten Si–Fe, Si–Ni, and Si–Fe–Cr alloys saturated with either silicon carbide (SiC) or graphite, which are candidates for the solvent for rapid solution growth of SiC, have been investigated. The measured carbon solubilities at 2073 K were 0.19–6.6 mol% for the Si–(24.1–70.1) mol% Fe, 0.061–5.2 mol% for Si–(30.0–85.0) mol% Ni, and 1.1–3.9 mol% for Si–(50-x) mol% Fe–x mol% Cr (x = 10.4–40.1) alloys. A quasi-chemical model that assumes that the carbon atoms are introduced into the interstitial sites of the Si–Fe, Si–Ni, and Si–Fe–Cr solvents and obstruct the bonding between solvent atoms was used to evaluate the activity coefficient of carbon in each alloy. The estimation reproduced the trends of the measured carbon solubilities fairly well. However, the estimation using the sub-regular solution model often overestimated the carbon solubilities. Thus, the carbon behavior in molten silicon–transition metal alloys is well described by the quasi-chemical model.
  • Partitioning of Solute Elements and Microstructural Changes during Heat-treatment of Cold-rolled High Strength Steel with Composite Microstructure

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    DOI:10.2355/isijinternational.ISIJINT-2019-723

    The partitioning of solute elements during intercritical annealing and the effects of partitioning on ferrite transformation during slow cooling after intercritical annealing in a 0.17% C–1.5% Si–1.7% Mn (mass%) steel were investigated by a new FE-EPMA (field emission electron probe microanalysis) technique. This new technique enables highly accurate measurement of the C distribution. During the intercritical annealing, C and Mn concentrated into austenite, while Si concentrated into ferrite. The distribution of Mn in austenite was inhomogeneous, and austenite with small Mn content was transformed into ferrite during slow cooling. This ferrite transformation proceeded in the NPLE (negligible partitioning local equilibrium) mode. Two kinds of ferrite were produced due to slow cooling, one being intercritically-annealed ferrite, and the other transformed ferrite. The transformed ferrite had larger Mn content than the intercritically-annealed ferrite. Furthermore, the transformed ferrite was classified into the ferrite grown epitaxially from the intercritically-annealed ferrite and that nucleated in the austenite with relatively small Mn content. Prior microstructure and distribution of solute elements before cooling are determined by the intercritical annealing conditions, and then control the ferrite transformation. Precise control of the ferrite transformation is effective for stable production of cold-rolled high strength steel with composite microstructure.
  • Crystallographic Orientation Control of 316L Austenitic Stainless Steel via Selective Laser Melting

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    DOI:10.2355/isijinternational.ISIJINT-2019-744

    In recent years, additive manufacturing has attracted attention as a technology that enables control of the crystallographic texture of metallic materials. We achieved successful control of the crystallographic texture of 316L austenitic stainless steel using selective laser melting (SLM). Three distinguished textures were achieved by changing the laser scan speed, namely: the single crystalline-like texture with {001} orientation in the build direction, the crystallographic lamellar texture in which two kinds of grains with {011} and {001} orientations in the build direction are alternately stacked, and polycrystalline with relatively random orientation. The melt pool shape and the solidification behavior (thermal gradient and migration velocity of solid/liquid interface) in a melt pool could be important controlling factors for the evolution of the crystallographic texture under the SLM process.
  • Evolution of TiN and Oxide Inclusions in Ti-containing Fe-25Ni-15Cr Alloy during Electroslag Remelting

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    DOI:10.2355/isijinternational.ISIJINT-2019-603

    The present study was undertaken to investigate the evolution of inclusions in a Ti-containing Fe-25mass%Ni-15mass%Cr alloy during electroslag remelting (ESR). The effect of slag composition on the inclusions in alloy was studied. The inclusions in both consumable electrode and remelted ingots are mainly 1 to 3 µm in size. The inclusions in consumable electrode are TiN, Al2O3–Ti2O3, Al2O3–Ti2O3 with a surrounded TiN layer. The inclusions in liquid metal pool and remelted ingots are TiN, MgO–Al2O3–Ti2O3 inclusion surrounded by TiN, MgO·Al2O3 inclusions, MgO·Al2O3 inclusions with an outer Ti2O3-rich layer. Increasing TiO2 content in slag has no influence on the types of inclusions in remelted ingots. The original TiN inclusions in consumable electrode cannot be dissociated at the electrode tip during the ESR process. TiN inclusions in remelted ingots mainly generated in liquid metal pool during ESR, and the TiN inclusions formed during the solidification of liquid alloy takes up a small amount fraction. Part of Al2O3–Ti2O3 inclusions in consumable electrode were removed through absorbing them into molten slag, and the remaining Al2O3–Ti2O3 inclusions in the liquid alloy reacted with Mg dissolved from ESR slag to form MgO–Al2O3–Ti2O3 inclusions which served as the nucleation sites for TiN inclusion formation. MgO·Al2O3 inclusions in the remelted ingots precipitated in the liquid metal pool during ESR process. The generation of MgO·Al2O3 inclusions with an outer Ti2O3-rich layer originated from the reaction between soluble titanium in liquid alloy and MgO·Al2O3 inclusion to form an outer Ti2O3-rich layer on unreacted MgO·Al2O3 inclusion core.
  • Effects of Nitrogen Gas Pressure on the Solidification Parameters and As-cast Microstructure Revolution during Pressurized Electroslag Remelting AISI 304 Stainless Steel

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    DOI:10.2355/isijinternational.ISIJINT-2019-203

    Three AISI 304 stainless steel electrodes were remelted using the lab-scale pressurized electroslag remelting furnace under different nitrogen gas pressure conditions. The solidification parameters and microstructure evolution have been investigated with the sulfur print method method, color metallography and EPMA. The results showed that the pool depth, SDAS and mushy zone width firstly increased and then decreased with the increase of gas pressure from 0.1 to 1.2 MPa. With an approximately equal melting rate, the variation of solidification parameters is dependent on the competition between the heat transfer rate at the slag/pool interface and the ingot/mould interface, because increasing the nitrogen gas pressure could simultaneously increase the two heat transfer rates. Under the current pressure range, the solidification mode and microsegregation during solidification are not affected by the variation of gas pressure. In addition, the variation of nitrogen gas pressure could simultaneously change the nitrogen content and cooling rate in ingots. Both the nitrogen content and cooling rate could affect the content and composition of residual ferrite. However, under the current experiment conditions, the variation of nitrogen content plays a more important role in the content of residual ferrite than the cooling rate, because nitrogen is a strong austenite former element and the cooling rate has no wide variation.
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    1. Evolution of TiN and Oxide Inclusions in Ti-containing Fe-25Ni-15Cr Alloy during Electroslag Remelting ISIJ International Advance Publication
    2. Deoxidation of Electroslag Remelting (ESR) – A Review ISIJ International Vol.60(2020), No.6
  • Iron Powder-based Metal Matrix for Diamond Cutting Tools: A Review

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    DOI:10.2355/isijinternational.ISIJINT-2019-662

    New material technology, advancement in manufacturing technology and development of alternate materials are key enablers for today's process and product development. Although Diamond cutting tools has a long history started back in 1862, this industry has gone through a tremendous change in last 50 years with the development of synthetic diamond and continues exploration of alternate metal matrix materials (alternate to cobalt). For each specific application, powder for metal matrix needs careful selection based on its chemical composition, size, shape and thermal stability. Cobalt powder was the most commonly used metal matrix powder. However, fluctuating price and health concerns associated with cobalt powder has compelled the industry to search for more economical and sustainable alternative. In this context Iron and Iron based alloy powders turned out to be promising metal matrix powder alternatives for use in diamond cutting tools catering cutting, drilling, grinding, etc applications.This paper gives an overview on the diamond cutting tools history, evolution, trends and developments with special focus on the usages of iron powder as an alternate matrix material for diamond cutting tools.
  • Sample Temperature Effect on Steel Measurement Using SP-LIBS and Collinear Long-short DP-LIBS

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    DOI:10.2355/isijinternational.ISIJINT-2019-740

    Laser induced breakdown spectroscopy (LIBS) has been investigated as a potential multi-element quantitative analysis tool for the quality control of on-line steel production. This research investigated influence of sample temperature on steel sample measurement using collinear long-short dual-pulse LIBS (long-short DP-LIBS) and single-pulse LIBS (SP-LIBS). The standard steel sample has been uniformly heated in a muffle furnace from 20°C to 700°C. The experimental results show that sample temperature has significantly effect on measurement result using SP-LIBS. However, long-short DP-LIBS can effectively reduce the sample temperature effect on measurement result. The detection characteristics of long-short DP-LIBS and SP-LIBS were compared using the intensity ratio of I Mn 404.136 nm/I Fe 400.524 nm and I Fe 402.187 nm/I Fe 400.524 nm under different delay time and different sample temperature conditions. The signal intensity and plasma temperature can be maintained higher and more stable for a period of time and at different sample temperature by long-short DP-LIBS with smaller error bar compared with that of SP-LIBS, which indicated long-short DP-LIBS has better measurement repeatability than SP-LIBS. The plasma temperature correction method was applied to compare the detection features of long-short DP-LIBS and SP-LIBS. The signal stability of long-short DP-LIBS measurement was improved significantly at different sample temperature with plasma temperature correction. These results demonstrated that the effect of sample temperature can be reduced using long-short DP-LIBS method to improve the on-line detection capability for steel measurement in complex environment.
  • Multiscale Analysis of MnS Inclusion Distributions in High Strength Steel

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    DOI:10.2355/isijinternational.ISIJINT-2019-739

    In the present study, manganese sulfide (MnS) inclusions in the high-strength steel were observed by mainly three observation methods (optical microscope, ultrasonic test and serial sectioning) to characterize the size, location and shape distributions across multiple length scales. For the inclusion size, ultrasonic C-scan imaging and three-dimensional internal structure observation with serial sectioning were used to measure the distributions of the square root of the projected area of the inclusion. The obtained size distributions were combined by setting the threshold of ultrasonic amplitude. The validity of the amplitude threshold was verified by observing several inclusions with X-ray CT. The spatial distributions of inclusions were also obtained by the three observation methods, and analyzed on the basis of the coefficient of variation of the mean near-neighbor distance of inclusions (COVd). The results of analyzing COVd in both 2D and 3D spaces revealed that the inclusions in this material were arranged in clusters. For the inclusion shape, the three-dimensional geometries of inclusions were reconstructed from the images obtained by the serial sectioning method, and simplified to ellipsoid by principal component analysis. From the above results, the distributions of inclusion size, aspect ratio and direction (angle between rolling direction and major axis) were successfully obtained. The inclusion distributions were applied to fatigue prediction model, and the fatigue crack initiation life and total fatigue life of the high-strength steel were calculated. The calculation results showed that the multiscale analysis of inclusions would be useful for fatigue life prediction.
  • Modelling and Crystal Plasticity Analysis for the Mechanical Response of Alloys with Non-uniformly Distributed Secondary Particles

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    DOI:10.2355/isijinternational.ISIJINT-2019-754

    The relationship between yield stress and the distribution of microscopic plastic deformation was numerically investigated by using a crystal plasticity finite element method (CP-FEM) in the model where particles were randomly distributed. It was in order to reveal which particle spacing. i.e., the maximum, minimum or average particle spacing, can be taken as the representative length which controls yielding. The critical resolved shear stress for the onset of the slip deformation in any element was defined under the extended equation in the Bailey-Hirsch type model. The model includes the term of the Orowan stress obtained from the local values of the representative length. Each particle spacing was distributed with a standard deviation of approximately 2 to 3 times larger than the average particle spacing. The macroscopic mechanical properties obtained with CP-FEM were in good agreement with those experimentally obtained. The onset of microscopic slip deformation depended on the particle distribution. Plastic deformations started first in the area where the particle size is larger, then the plastic region grows in the areas where the particle spacing is smaller. Slip deformation had occurred in 90% of the matrix phase by the macroscopic yield point. The length factor in the Orowan equation was the average spacing of the particles in the model, which is in good agreement with Foreman and Makin. The CP-FEM indicated that in dispersed hardened alloys, microscopic load transfer occurred between the areas where the large particles spacing and the small one at the yielding.
  • Effect of B2O3 on Structure of CaO–Al2O3–SiO2–TiO2–B2O3 Glassy Systems

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    DOI:10.2355/isijinternational.ISIJINT-2019-679

    Titanium-bearing blast furnace slag is an important metallurgical waste, but presently, it is difficult for it to be effectively utilized. B2O3 is an important modifier to greatly promote the mass diffusion of crystallization of Ti-enriched phases in molten slags. To furtherly understand the effect of B2O3 on the structure of Ti-bearing slag, CaO–Al2O3–SiO2–TiO2–B2O3 glasses of various B2O3 were investigated by combining Raman, FT-IR, and X-ray photoelectron spectroscopy. The results showed that BO3 was the dominant structure, which decreased slightly as the B2O3 content increased, while BO4 increased. Three coordination forms (TiO4, TiO5, and TiO6) of the Ti-structure were discovered in the prepared Ti-bearing glasses. The percentage of TiO4 gradually increased and became the main structural unit as the increased B2O3. The increase of BO4 and TiO4 leaded to an increased amount of network connection units, such as SiO4, to increase the degree of polymerization of the prepared Ti-bearing glasses.
  • Phase Composition and Properties Distribution of Residual Iron in a Dissected Blast Furnace Hearth

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    DOI:10.2355/isijinternational.ISIJINT-2019-712

    Dissection research is the most meaningful way to understand the internal conditions of blast furnace. Because it can obtain samples from hearth after a blast furnace was shut down. In this paper, a 2200 m3 commercial blast furnace was shut down with residual iron remained inside. And then core drilling method was used to obtain a horizontal sample of residual iron, which was located 1610 mm below the centerline of taphole and had a length of 1840 mm. A variety of techniques, such as scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Optical Microscope, X-ray diffraction (XRD) and Raman spectroscopy were applied to analysis the microscopic morphology, phase composition and properties distribution of residual iron samples at different position. The results show that complex phases appear near the cold side, including Fe, C, Ti3N2, KAlSi2O6 and Al2O3. And distribution of properties along the core sample from inside to cold side like density, degree of graphitization and thermal diffusion also have a big change. These results are considered to be related to the function of cooling system.
  • Sulfide Capacities of Solid Oxides in Calcium-Aluminate Systems

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    DOI:10.2355/isijinternational.ISIJINT-2019-793

    To clarify the mechanism of CaS formation on the oxide inclusion of the CaO–Al2O3 system, the sulfide capacities of solid oxides was measured in the present study. The results show that the sulfide capacity of (12CaO·7Al2O3; C12A7) was much larger than of other compounds, and increased with temperature. The value for C12A7 was larger than that measured for the liquid oxide of the same composition. Furthermore, the diffusion behavior of sulfur in solid steel to the inclusion of the CaO–Al2O3 system, was investigated using a diffusion couple. After heating at 1473 K for 72 h, in the case of C12A7, the intensity of sulfur in the oxide was high, but the formation of CaS was not detected. This suggests that the formation of CaS was suppressed around the C12A7 particles by the diffusion of sulfur in the solid oxide.
  • Physico-chemical Properties of Mill Scale Iron Powders

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    DOI:10.2355/isijinternational.ISIJINT-2019-737

    Powdered form of iron typically < 250 microns is phrased as iron powder. Commercial manufacturing of iron powder is majorly restricted to atomization, carbonyl, electrolytic and reduction routes. Powders from the later three techniques generally cater to special iron powders applicable to non-powder metallurgy segments such as food fortification, chemical reagents, water purification, etc. In this work, mill scale iron powders are synthesized by thermo-chemical reduction of mill scale, a steel industry by product, and their properties are analysed and compared with commercial iron powders such as carbonyl, electrolytic and reduced. Chemical and physical characterizations such as Optical Microscopy, Scanning Electron Microscopy and X-ray Diffraction of the powders are performed. Obtained results reveal that, mill scale iron powders MIP and MIP45 possess good product properties. Especially, MIP45 grade exhibited finer particle size of D50< 30 microns and BET surface area of 0.63 m2/g along with Fe (T) > 98%, true density-7.55 g/cc, apparent density 2.67–2.83 g/cc, packed bulk density of 3.44 g/cc and good flowability. This product of mill scale with highest apparent density and good surface area is expected to qualify to new segment of applications along with other commercial iron powders.
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    3. Determination of Free Lime Contents in Slags by Solution Calorimetry ISIJ International Vol.36(1996), No.Suppl
  • Segregation Mechanism of Al-based Oxides on Surface of Zn-0.2mass%Al Hot-dip Galvanized Steel Sheets

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    DOI:10.2355/isijinternational.ISIJINT-2019-749

    It is known that the Al added to the Zn coating layer of hot-dip galvanized steel sheets (HDG) segregates on the surface of temper-rolled HDG as Al-based oxides with increasing aging time in air at room temperature. In this study, the surfaces of Zn-0.2mass%Al HDG with and without temper rolling were investigated to clarify the segregation mechanism. Specimens with a Zn coating weight of 55–57 g/m2 including 0.19–0.20 mass% of Al were used. The specimens were aged in air at 20°C or held in liquid nitrogen, and the surface and cross sections of the specimens were then observed and analyzed by XRF, SEM-EDX and EBSD. As a result, it was found that the velocity of Al-based oxide segregation on the surface of the temper-rolled HDG was much higher than that of the HDG without temper rolling. This was attributed to the difference in the area where formation of Al-based oxides was possible. It was also found that the Zn crystal grains in the coating layer were refined by recrystallization due to contact with the temper roll, resulting in an increased number of grain boundaries that can serve as Al diffusion paths. Some unrecrystallized grains also remained after temper rolling and could increase the number of formation sites for Al-based oxides, as they contain numerous dislocations that can serve as Al diffusion paths. These two different formation sites could lead to difference in the segregation rates observed in this study.
  • Review on the High-Temperature Thermophysical Properties of Continuous Casting Mold Fluxes for Highly Alloyed Steels

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    DOI:10.2355/isijinternational.ISIJINT-2019-522

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

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