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

ISIJ International Vol. 60 (2020), No. 1

  • A Review on Prevention of Sticking during Fluidized Bed Reduction of Fine Iron Ore

    pp. 1-17

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    DOI:10.2355/isijinternational.ISIJINT-2019-392

    The fluidized bed ironmaking technology has attracted the attention of many researchers for decades as a direct reduction ironmaking method with many advantages. This process has been applied as a pretreatment method in many non-blast furnace ironmaking processes. However, the sticking problem hindered its development greatly. Defining the essential cause of sticking, and fundamentally solving this problem are the key steps encountered by this process. The research works related to the prevention of sticking problem during fluidized bed reduction of fine iron ore are comprehensively summarized in this article. The causes of sticking, the influencing factors of sticking and the solution of sticking are firstly discussed, followed by the analysis on the possible development direction of future fluidized bed ironmaking technology.
  • Nanostructured Bainitic Bearing Steel

    pp. 18-30

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    DOI:10.2355/isijinternational.ISIJINT-2018-693

    Bearing is the most important component for nearly all mechanical equipment. Nanostructured bainitic steel, which is a new bearing steel, not only possesses necessary hardness and higher toughness, but exhibits excellent wear resistance and rolling contact fatigue performance, making it suitable for bearing application. In recent years, the research on nanostructured bainitic bearing steel has gained great progress and obtained attentions from bearing industry. To make a clear knowledge on nanostructured bainitic bearing steel, and reveal the further research direction on this filed, this paper reviews the development of nanostructured bainitic bearing steel, including the design of chemical composition, the heat treatment process, the feature of microstructure, the properties involving conventional mechanical properties, wear resistance and rolling contact fatigue performance, the effect of retained austenite, and the distribution of residual stress.
  • Coupled Experimental Study and Thermodynamic Modeling of the Al2O3–Ti2O3–TiO2 System

    pp. 31-41

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    DOI:10.2355/isijinternational.ISIJINT-2019-006

    A complete critical evaluation and re-optimization of phase diagrams and thermodynamic properties of the Al2O3–Ti2O3–TiO2 system at 1 atm pressure has been performed. Equilibration and quenching experiment in the Al2O3–TiO2 system in air was also performed to constrain the solubility limit of Al2O3 in TiO2 rutile solution at high temperatures. The molten oxide phase was described by the Modified Quasichemical Model considering the short-range ordering in molten oxide. While Al2TiO5 and Ti3O5 were treated as separate stoichiometric phases in the previous optimization, they were described in this study, using the Compound Energy Formalism, as part of pseudobrookite solid solution with a miscibility gap based on new experimental data. Corundum and rutile solutions were also described based on their crystal structures. New high temperature phase, Al6Ti2O13, was also considered for the first time. A set of optimized model parameters of all phases was obtained, which reproduces all available and reliable literature data within experimental error limits from 25°C to above the liquidus temperatures under oxygen partial pressures from metallic saturation to 1 atm. The newly optimized database was applied to calculate the inclusion diagram and reoxidation of Al-killed and Ti bearing steels.
  • Structural Evaluation of Molten Aluminosilicate by Combining Impedance Measurements and Cell Model Calculations

    pp. 42-50

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    DOI:10.2355/isijinternational.ISIJINT-2019-132

    Molten oxides have been used for a range of processes; however, their physical and rheological properties affect the quality of products manufactured from them. Although various reports of structural analysis exist, the methods employed are typically time consuming. Herein, a method is established for rapidly estimating the structures of melts by combining impedance measurements and thermodynamic calculations regarding the cell model. The melt structure is calculated using two thermodynamic parameters; however, these parameters have not yet been reported for systems containing alkali metal oxides. Thus, impedance measurements were carried out for SiO2–Al2O3–RO melt systems (R = Ca or Mg), and relationships between the equivalent circuit components and the thermodynamic parameters of the cell model were established. The structures of melts containing alkali metal oxides were then estimated by calculating the thermodynamic parameters of these systems by substituting the equivalent circuit components in the correlation equations. The structures estimated by the proposed method appear to correlate with those measured by NMR spectroscopy.
  • Structure and Viscosity of CaO–Al2O3–B2O3 Based Mould Fluxes with Varying CaO/Al2O3 Mass Ratios

    pp. 51-57

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    DOI:10.2355/isijinternational.ISIJINT-2019-234

    The effect of CaO/Al2O3 ratio on the structure of CaO–Al2O3–B2O3 based glassy mould fluxes was investigated by employing 27Al and 11B Triple Quantum Magic-angle spinning nuclear magnetic resonance (3QMAS-NMR) and Raman spectroscopy. 27Al and 11B 3QMAS-NMR spectrums showed that Al3+ mainly forms [AlO4] as a network former and B3+ mainly forms [BO3] groups in CaO–Al2O3–B2O3 based glasses. Raman spectrum showed existences of different [AlO4] structure units and BO3 pyro-borate units. In addition, deconvolution results on Raman spectrums indicate that the degree of polymerization of aluminate network in CaO–Al2O3–B2O3 based glasses decreases with the increase of CaO/Al2O3 ratio. The effect of CaO/Al2O3 ratio on viscosity of CaO–Al2O3–B2O3 based glassy mould fluxes was investigated by employing the rotating-cylinder method. The viscosity decreases with increasing CaO/Al2O3 ratio in CaO–Al2O3–B2O3 based mould flux. Correlation between viscosity and structural information of investigated mould fluxes was explored.
  • Crystallisation of Silicate Glasses and Melts with Chemical Compositions in Primary Phase Region of Gehlenite

    pp. 58-64

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    DOI:10.2355/isijinternational.ISIJINT-2019-266

    Crystallisation behaviour has been investigated on samples with two compositions: (A) 42CaO-37SiO2-21Al2O3 and (B) 43CaO-27SiO2-19Al2O3-11CaF2 (all mass%). Composition (A) is within the primary phase region of gehlenite and composition (B) is just between gehlenite and cuspidine on a mass basis. Two types of sample were prepared from glassy and molten states for each composition. These samples were characterised by differential thermal analysis, X-ray diffraction analysis and electron probe microanalysis.- Crystallised samples A from glassy state:It contained gehlenite and wollastonite in the XRD profile, and in BE images gehlenite crystals grew more largely than wollastonite, suggesting that gehlenite precipitates in preference to wollastonite.- Crystallised samples B from glassy state:It contained cuspidine and gehlenite in the XRD profile. In BE images, cuspidine precipitated but gehlenite was not observed.- Crystallised samples A from molten state:It contained only gehlenite in the XRD profile, and in BE images there was dendritic gehlenite developed across the sample.- Crystallised samples B from molten state:It contained cuspidine and CaF2 as well as gehlenite in the XRD profile. In BE images, gehlenite comprised the major part, and it is likely that gehlenite crystallises in preference to cuspidine.It is also likely that the pseudo-binary system of gehlenite and cuspidine forms eutectics and that the eutectic composition is rather closer to cuspidine. Consequently, the increase in Al2O3 concentration would little affect crystallisation of the glassy portion in actual mould fluxes containing CaF2 but strongly affects crystallisation of the liquid portion.
  • Thermal Decomposition Reaction Kinetics of Hematite Ore

    pp. 65-72

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    DOI:10.2355/isijinternational.ISIJINT-2019-129

    In order to understand the thermal decomposition kinetics of hematite particles in inert atmosphere, thermogravimetriy was employed for isoconversional analysis. The kinetic triplet was estimated from the experimental data and the isothermal reaction kinetics was predicted. The results indicated that the thermal decomposition could be divided into two stages, of which the activation energies were 636 kJ/mol and 325 kJ/mol, respectively. The exponential form of pre-exponential factor, ln(A/s−1), for the two stages were estimated to be 42.9±6.6 and 14.1±3.08. At last, the kinetic mechanism of the first stage was suggested to match Sestak-Berggren model as f(α)=(1−α)1.38. The relatively slow reaction rate of the second stage was due to the slag formation during the reaction.
  • Sinter Strength and Pore Structure Development using Analogue Tests

    pp. 73-83

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    DOI:10.2355/isijinternational.ISIJINT-2019-247

    Iron ore sinter quality is important for productive and efficient operation of the modern iron blast furnace. Understanding the driving mechanisms of industrial sinter quality is complicated by the variability in the product. Variability arises due to many factors including heterogeneous raw material distribution, spatial variation in temperature profile and gas atmosphere down the bed. To reduce product variability iron ore sinter analogues were created from the −1.0 mm fraction of an Australian iron ore. Samples were fired under tightly controlled thermal and atmospheric conditions in an infra-red rapid heating furnace. Maximum temperature was found to have a strong influence on the porosity of the fired tablets and hence their strength. The more commonly used integrated time at temperature above 1100°C (EA, enclosed area) was found to be not as useful a predictor of sinter strength for these analogue tests. Analogues fired to 1320°C showed a minimum in porosity and maximum strength after holding for 1 min. With longer firing, over-sintering occurred caused by pore swelling, leading to a decrease in strength. The proposed mechanisms of pore swelling are bubble coalescence and gas generation from hematite decomposition to magnetite. Extrapolating these results suggests over-sintering may occur in the lower part of the industrial sinter bed if the temperature is too high for too long, leading to lower strength product.
  • Morphology and Composition of Inclusions in Si–Mn Deoxidized Steel at the Solid-Liquid Equilibrium Temperature

    pp. 84-91

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    DOI:10.2355/isijinternational.ISIJINT-2019-313

    Morphology and composition of inclusions change with temperature. However, besides the temperature conditions during steelmaking or continuous casting, other factors contributing to changes in the morphology and composition of inclusions during solidification are still unknown. In this study, the formation of complex inclusions in Si–Mn deoxidized steel after isothermal holding at the solid-liquid equilibrium temperature (TS) was investigated.The typical inclusions found in the alloy were MnO–SiO2 based, spherically shaped and homogeneously distributed. With isothermal holding at the solid-liquid equilibrium temperature, formation of a secondary SiO2-rich inclusion phase occurred. The changes in the composition of the inclusions depended on the manganese and silicon contents in the metal.The general mechanism of inclusion formation observed in this study can be divided into three steps: 1) the formation of primary MnO–SiO2 inclusions above the liquidus temperature when the steel is in a completely molten state as a result of the deoxidation process; 2) the nucleation of secondary inclusions as the molten steel becomes supersaturated with the solute elements while holding at the solid-liquid equilibrium temperature; and 3) the growth and coalescence of inclusions due to natural convection in the molten alloy. From this, the inclusions formed in Si–Mn deoxidized alloys held isothermally at the solid-liquid equilibrium temperature were of three types: primary MnO–SiO2 inclusions, secondary SiO2 inclusions and complex inclusions with both primary MnO–SiO2 inclusions and precipitated secondary SiO2 inclusions.
  • Effect of Solute Elements on Boron Segregation in Boron-Containing Steels

    pp. 92-98

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    DOI:10.2355/isijinternational.ISIJINT-2019-258

    The addition of boron to steel alloys results in an increase in both hardenability and casting defects. The casting difficulties are predicted to stem from a metatectic reaction, δ + γ → L + γ, where a fully solidified material begins to locally remelt as the temperature decreases. Another possible source of casting defects is a boride-rich phase that is predicted to remain liquid at low temperatures. To experimentally determine which reaction is the likely source of the casting defects, the predicted reactions and the effect of solute elements on those reactions are investigated. Levitation zone melting is used to control segregation in a ternary Fe–C–B alloy and a commercial 22MnB5 alloy. Carbon segregation and a peritectic reaction result in a peritectic jump during directional solidification where the first directionally solidified (DS) zone undergoes δ-bcc solidification followed by a peritectic jump to steady state planar solidification of γ-fcc in the second DS zone. The presence of other solute elements in the zone melted 22MnB5 alloy lead to a breakdown in the planar solidification front before steady state solidification could be achieved in the second DS zone. With a cellular solid/liquid interface, boron-rich intercellular liquid formed low melting iron boro-carbide particles. The controlled solidification conditions in a levitation zone melter were unable to prevent ~0.003 wt% boron from segregating to high enough levels to form boride particles. Therefore, it is likely that during commercial casting, the formation of the low melting boride phase from interdendritic segregation is a key source of the casting issues.
  • Introduction of Dendrite Fragmentation in Microstructure Calculation by Cellular Automaton Method

    pp. 99-105

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    DOI:10.2355/isijinternational.ISIJINT-2019-304

    Control of the solidification microstructure in continuous casting of steel is necessary because the microstructure affects material properties. In order to predict the solidification microstructure, the effect of dendrite fragmentation was indirectly introduced in the cellular automaton method by using Vcrit, which is the velocity threshold of the molten steel flow. Calculations were carried out with various Vcrit (200 µs−1 ≦ Vcrit ≦ 1 mms−1), and the results were compared with the results of a casting experiment using high carbon steel. The observed specimens were extracted from the upper part and lower part of casting. Equiaxed grains and branched columnar grains were observed in the microstructure of the upper specimen, whereas only columnar grains were observed in the microstructure of the lower specimen. The calculation results with Vcrit greater than 400 µm showed good qualitative agreement with the microstructures of both observed specimens. The microstructures were calculated because the flow velocity of the molten steel around the upper specimen was much greater than that around the lower specimen. There is a possibility that solute transportation, which induces fragmentation, occurs even if the velocity of the molten steel flow is on the order of 10−4 ms−1.
  • Optimization of Thermal Soft Reduction on Continuous-Casting Billet

    pp. 106-113

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    DOI:10.2355/isijinternational.ISIJINT-2019-409

    Thermal soft reduction (TSR) is an effective technique to improve the inner quality of continuous-casting billet, but it may lead to undesired internal and surface cracks. In this work, the technologic parameters of TSR were optimized to ensure its effect and control the cracks of 82A tire cord steel billet. A heat transfer model with comprehensive thermo-physical parameters was established to simulate the thermal behavior of continuous-casting billet. The model was verified by comparing the measured surface temperatures and the calculated ones. According to the mechanism of TSR on billet, both the location and water flow rate were comparatively optimized. TSR was determined to locate at 6.96 m–8.46 m from meniscus, where the temperature of billet center dropped rapidly to liquid impenetrable temperature. The water flow rate of TSR was set to 2.2 m3/h, which allowed the reheating rate and surface temperature in a reasonable range and prevented the formation of the cracks. Plant trials were conducted to verify the effect of the optimized TSR. The results showed that the central porosity, V segregation and central segregation of the billet were obviously improved by applying TSR. Meanwhile, the internal and surface cracks were well controlled in the billet.
  • Observation of Chemical State for Interstitial Solid Solution of Carbon in Low-carbon Steel by Soft X-ray Absorption Spectroscopy

    pp. 114-119

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    DOI:10.2355/isijinternational.ISIJINT-2019-206

    The near-edge X-ray absorption fine structure at the carbon K edge was measured for determining the chemical state of interstitial carbon in a low-carbon steel. In addition, the wavelength dependence of the photoelectron spectrum of the surface of the steel was evaluated, and a contamination and oxidation layer of 3 nm thickness was found. As a result, it was possible to observe a change in the chemical state of carbon existing in bulk iron located deeper than the oxidation and contamination layers, by evaluating the difference spectra between the sample and a reference. Furthermore, by evaluating the shape change of the difference spectra based on the heat treatment time, it was found that the chemical state of carbon in bulk iron changes with heat treatment.
  • Development of Analysis Method for Sulfide in Steel with Chelating Agent of Copper

    pp. 120-127

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    DOI:10.2355/isijinternational.ISIJINT-2019-327

    Copper sulfide (CuxS) has been frequently observed in steel samples, prepared using selective potentiostatic etching by electrolytic dissolution (SPEED). It is often the case that CuxS is detected unexpectedly from the precipitates extracted from steel samples by selective potentiostatic etching, although such CuxS formation during the heat treatment conducted is not anticipated by the thermodynamic equilibrium calculations. In this study, we observed such artificial CuxS along with manganese sulfide (MnS) precipitates, which were extracted from steel materials by SPEED, using secondary electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy. These CuxS–MnS sulfide complex would be formed by the following mechanism: as the solubility of CuxS is far bigger (i.e. 10 times or more) than that of MnS, Cu2+ ion dissolved from steel matrix would be exchanged with Mn2+ ion on the MnS surface during the etching process, leading to a formation of CuxS–MnS sulfide complex.In order to suppress the formation of such CuxS, we propose the use of following electrolyte: a non-aqueous solution of 4% methyl salicylate + 1% salicylic acid + 1% tetramethylammonium chloride (TMAC) + 5% Triethylenetetramine (TET) in volume fraction, in methyl alcohol (Cu ion selective hold etching by electrolytic dissolution, abridged as CUSH electrolyte). Then, this electrolyte was applied to precipitates in steel samples. It was effective to prevent the formation of sulfides in electrolyte, with the effect of metallic (Cu2+, Ag+, Pb+, etc.) chelating ability of TET.
  • Influence of Injection Distance on Water Droplet Behavior in High Pressure Descaling

    pp. 128-135

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    DOI:10.2355/isijinternational.ISIJINT-2019-070

    Hydraulic descaling is used in hot rolling mills in order to remove scale and prevent surface defects. Because the impact pressure of the descaling jet is one important factor from the viewpoint of mechanical breaking and applying thermal shock to scale layers, the water jet structure and the droplet velocity should have large effects on scale breaking properties. However, the influence of the injection distance on the jet structure and the droplet velocity has not been clearly understood. In this work, the behavior of changes in the descaling jet structure and attenuation of the water droplet velocity along the injection distance were investigated experimentally. High pressure descaling nozzles with pressures up to 25 MPa were used, and the injection distance was varied in the range from 30 to 400 mm. The jet structure was observed with a high speed camera, and the water droplet velocity and diameter were measured with a phase Doppler analyzer. The results confirmed that the jet structure changes continuously through a process of continuous flow, break-up, water lumps, and water droplets. It was found that a continuous flow can be maintained for a long distance by using a low injection pressure and large flow rate, and the water droplet diameter also becomes larger, which reduces velocity attenuation. These deformation properties of the jet structure are related to the Weber number expressed by the relative velocity between a water droplet and the surrounding air. A smaller Weber number is effective for reducing velocity attenuation over a long injection distance.
  • Research and Application of Model and Control Strategies for Hot Rolled Strip Cooling Process Based on Ultra-Fast Cooling System

    pp. 136-142

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    DOI:10.2355/isijinternational.ISIJINT-2019-104

    Ultra-fast cooling technology as an effective method for control microstructure and property, is widely used in hot rolled strips. For precise control of strip temperature in cooling process, a mathematical model based on UFC is established to calculate UFC-T and CT in high pressure mode, or only CT in low pressure mode. Temperature calculation compensation strategy is obtained to solve the situation that re-reddening after UFC process affects CT calculation. Furthermore, for existing self-learning strategy care less about evolution of strip temperature and has no ability to eliminate errors quickly, a multi-dimensional self-learning control strategy is proposed including dynamic self-learning gain, distributed temperature self-learning strategy and velocity coefficient for heat transfer self-learning. With help of proposed control strategies, strip temperature in cooling process is precise calculated and controlled. The model and strategies have been applied successfully in a 2050 HSM for development of low cost and feature strip products.
  • Modeling of Surface Crack Defects Developed on Shear Edge in High-strength Automotive Steel Sheets

    pp. 143-152

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    DOI:10.2355/isijinternational.ISIJINT-2019-326

    Surface crack defects developed on the shear edge cause a problem in shearing of high-strength steels. The surface crack formation mechanism was clarified by microstructural examinations and numerical simulation. Two types of 780 MPa grade hot-rolled steel sheets with a thickness of 2.6 mm were chosen for the evaluations because the materials show different surface crack susceptibilities. Cleavage fracture was responsible for the surface cracks, and micro-ductile cracks with a length of 30 µm to 40 µm were detected in the interrupted punching samples. A numerical simulation demonstrated that a tensile stress was developed in the direction of the micro-ductile cracks opening during punching process. The critical length of the micro-ductile crack for cleavage fracture as a crack initiation site was given by linear fracture mechanics; for example, the critical length is 23 µm or longer under the applied tensile stress of 910 MPa. The tensile stress causing cleavage fracture decreased by reducing the tool clearances, and it was shown experimentally that surface crack defects can be prevented by controlling the clearance appropriately.
  • Friction Stir Welding of Medium Carbon Steel with Laser-Preheating

    pp. 153-159

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    DOI:10.2355/isijinternational.ISIJINT-2019-394

    Friction Stir Welding (FSW) has expanded to many metallic materials with higher melting points or much higher strength than the aluminum alloys. If the tool travels too quickly along the welding seam during the welding process or if the melting point of the workpiece is high, the frictional heat generated between the tool and the workpiece may not be sufficient to cause material flow. Insufficient heat input results in the formation of groove or tunnel-shaped defects in the stir zone and also severe wear or breaking of the FSW tool. To solve these problems, a higher heat input is required to soften the materials. Therefore, several preheating methods have been adopted to increase the heat input. In this study, a fiber laser was used as the preheating source during the FSW. In this experiment, the effect of the laser-preheating on the defect formation and tool rotational torque during the FSW was investigated. Additionally, a difference in the material flow during the conventional FSW and laser-preheating FSW was observed by two pairs of X-ray transmission real-time imaging systems. As a result, it was found that the laser preheating reduced the defect formation and the tool rotational torque during the FSW. Furthermore, laser beam irradiation on the retreating side (RS) was the most effective in reducing the defect formation. On the other hand, the irradiation on the advancing side (AS) was the most effective in reducing the tool rotational torque.
  • Accuracy Evaluation of Phase-field Models for Grain Growth Simulation with Anisotropic Grain Boundary Properties

    pp. 160-167

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    DOI:10.2355/isijinternational.ISIJINT-2019-305

    The phase-field method has been widely employed recently for simulating grain growth. Phase-field grain growth models are classified into two types according to their conservation constraints for phase-field variables: the multi-phase-field model and the continuum-field model. In addition, within the multi-phase-field model framework, three models with different formulations exist. These models are reported to accurately simulate grain growth under conditions of isotropic or weakly anisotropic grain boundary energy and mobility. However, for cases of strongly anisotropic grain boundary properties, the accuracy of these models has not yet been examined in detail. In this study, using the continuum-field model and three different multi-phase-field models, systematic grain growth simulations with anisotropic grain boundary energies and mobilities are performed. Through the detailed investigation of the accuracy of the simulated results, the suitability of each model for anisotropic grain growth simulations is revealed. Furthermore, based on the higher-order terms, accuracy improvement of the phase-field models is attempted.
  • Precipitation Process of TiC in Low Alloy Martensitic Steel and Its Effect on Wear Resistance

    pp. 168-174

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    DOI:10.2355/isijinternational.ISIJINT-2019-151

    Traditional low-alloy martensitic steel is widely used as wear-resistant steel parts.But the contradiction between hardness and processing as well as welding performance is prominent,which limits the development of wear resistance. Titanium carbide is often used in industrial production as a metal-based wear-resistant phase, which is an inexpensive reinforcement that contributes to excellent overall performance. In this paper, a series of low-alloy martensitic steel sheets with same hardness grade and different Ti content of 0 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt% and 0.7 wt% are designed, respectively. The morphology and distribution of TiC precipitates in steel were characterized by OM, SEM, TEM and EPMA. The wear resistance was studied by wet sand semi-free abrasive wear test, and three modes of interaction between TiC particles and abrasives during wear process were summarized. The thermodynamic calculation was used to help explain the solidification precipitation process of titanium carbide particles.
  • Growth Behavior of a Mechanically Long Fatigue Crack in an FeCrNiMnCo High Entropy Alloy: A Comparison with an Austenitic Stainless Steel

    pp. 175-181

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    DOI:10.2355/isijinternational.ISIJINT-2019-230

    The fatigue crack growth characteristics of an Fe20Cr20Ni20Mn20Co high-entropy alloy (HEA) were investigated by ΔK increasing compact tension test in comparison with SUS316L. The fatigue crack growth rate of the HEA was lower than that of the SUS316L. The predominant crack growth path was the grain interior for both alloys. A difference was observed in the crack roughness; i.e., the fatigue crack growth path of the HEA was more distinctly deflected than that of the SUS316L. This indicates that roughness-induced crack closure is a key factor reducing the crack growth rate of the HEA. Another key factor is the noncrystallographic transgranular crack growth mechanism. The SUS316L exhibited crack growth via crack blunting/re-sharpening, while the HEA exhibited transgranular crack growth associated with dislocation substructure alignment.
  • Influence of Iron Carbide on Mechanical Properties in High Silicon-added Medium-carbon Martensitic Steels

    pp. 182-189

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    DOI:10.2355/isijinternational.ISIJINT-2019-331

    Using a medium-carbon steel containing 2 mass% Si, we investigated the effect of its tempered martensite microstructure on its mechanical properties. We found that the tensile strength of tempered martensite continuously decreases with increasing tempering temperature and that its yield strength markedly decreases in a tempering temperature range of 673 K to 723 K. To investigate the correlation with the microstructure, we examined the effect of tempering temperature on the microstructure by SEM and TEM and identified Fe carbide phases by TEM nanobeam diffraction pattern analysis and X-ray diffractometry. In the tempering temperature range where the yield strength significantly decreases, the morphology of the ε carbide precipitated in martensite blocks changed from platelike to granular and the χ carbide was precipitated in a small amount in the samples tempered at 723 K. SAXS quantitative evaluation of the ε carbide revealed that the decrease in the size and volume fraction of the ε carbide with the increase in the tempering temperature was far greater than with the samples tempered at 673 K and below. The sharp decrease in the yield strength was suggested to be correlated with the increase in the mobility of dislocations with the decrease in the precipitate volume fraction resulting from the dissolution of ε carbide in the transformation process of the Fe carbides.
  • Effect of Aluminum on the Solubility of Calcium in Liquid Iron at Low Calcium and Aluminum Contents

    pp. 190-192

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    DOI:10.2355/isijinternational.ISIJINT-2019-283

    The solubility of calcium in liquid iron as a function of aluminum content and calcium potential, at compositions relevant to production of aluminum killed steels, was studied experimentally at 1873 K. The measurements were made using a closed molybdenum chamber in which iron-aluminum alloys were held. The calcium potential was fixed using pure liquid calcium held at different temperatures. The calcium contents in the iron varied between 6 and 22 ppm by weight and the aluminum contents varied between 70 and 1900 ppm by weight. The results indicate that the effect of aluminum on the solubility of calcium in iron is very low in the composition ranges studied.
  • Application of Quaternary Acid Mixture to Microwave Digestion Effective for Various Kinds of Steel Samples

    pp. 193-195

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    DOI:10.2355/isijinternational.ISIJINT-2019-393

    This paper suggests a procedure of an improved digestion method, which can be applied to a variety of steel samples, for preparing the sample solution to quantify alloyed elements and sulfur simultaneously in inductively coupled plasma atomic emission spectrometry. A conventional digestion method using a mixture of hydrochloric and nitric acid has a poor ability to decompose tool steel completely. Alternatively, a microwave digestion method, in which an acid mixture of hydrochloric acid, hydrofluoric acid, nitric acid, and phosphoric acid was prepared with 1:1:1:1 volume ratio, enabled various steel samples including tool steel and stainless steel to be fully decomposed. Due to no addition of sulfuric acid, the sulfur content in the samples could be determined. The suggested procedure was applicable to determine sulfur, vanadium, chromium, manganese, cobalt, nickel, molybdenum, and tungsten in a variety of steel alloys using the same dissolution procedure.
  • Artificial MnS Inclusions in Stainless Steel: Fabrication by Spark Plasma Sintering and Corrosion Evaluation by Microelectrochemical Measurements

    pp. 196-198

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    DOI:10.2355/isijinternational.ISIJINT-2019-408

    Spark plasma sintering was used to fabricate type 304L stainless steel specimens containing artificial manganese sulfide (MnS) inclusions, and a microelectrochemical technique was used to characterize the pit initiation behavior at the MnS. A 200 μm square electrode area that included an artificial MnS particle was potentiodynamically polarized in 0.1 M NaCl, and the electrode surface was observed in situ by optical microscopy. The anodic dissolution of the MnS particle was observed in the passive region of the stainless steel. The pit occurred at the boundary between the particle and the steel matrix after the particle dissolved slightly. The dissolution potential and pit initiation behavior at the artificial MnS particles in the sintered stainless steel were confirmed to be similar to those at MnS inclusions in commercial stainless steels.

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