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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 106 (2020), No. 12

  • Mechanism of Mild Cooling by Crystallisation of Mould Flux for Continuous Casting of Steel

    pp. 871-882

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    DOI:10.2355/tetsutohagane.TETSU-2020-084

    Effects of crystallisation on heat transfer across solid mould fluxes have been examined on the basis of apparent thermal conductivities including radiative contribution. The apparent thermal conductivities were measured on glassy and crystallised mould flux samples under steep temperature gradients using a parallel plate method improved in the present work. Both surfaces of the samples were coated with silver paste to reduce contact thermal resistance. Thermal resistance except the sample itself was experimentally determined to be 2.27×10–4 m2KW–1 based upon measurements on Inconel 600. To confirm the reasonableness of this value, the method was applied to fused silica. Apparent thermal conductivities were in good agreement with reported values. Apparent thermal conductivities of mould fluxes were measured up to 900ºC at the high temperature side of the sample. The thermal conductivity of the glassy sample was 1.25 Wm–1K–1 below 300ºC in the central temperature (Tc) of the sample, and was lower than those of the crystallised samples. With increasing degree of crystallinity, the thermal conductivities increased around room temperature. Samples with higher degrees of crystallinity showed negative temperature dependence more remarkably and resultantly were close to that of the glassy sample where Tc ~ 350-500ºC. Where Tc > 500ºC, the thermal conductivity of the glassy sample was 1.54 Wm–1K–1 and was greater than that of a crystallised sample, 1.32 Wm–1K–1, which would be due to the radiation. Apparent thermal conductivity at a practical temperature has also been estimated, which suggests that crystallisation enables radiative thermal conductivity to be reduced.mould flux; continuous casting; mild cooling; thermal conductivity; temperature gradient; radiative heat transfer; crystallisation.
  • Effect of MC Type Carbides on Wear Resistance of High Wear Resistant Cast Iron Rolls Developed for Work Rolls of Hot Strip Mills

    pp. 883-891

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    DOI:10.2355/tetsutohagane.TETSU-2020-003

    High-speed steel type cast iron rolls were developed around 1990 and have been widely used in the former stands of hot strip mills. However, in the latter stands of the hot strip mills, the use of high-speed steel type rolls has been limited due to the insufficient crack resistance. Therefore, in order to improve the wear resistance of the latter stands, enhanced type high-nickel grain rolls in which MC type carbides of high hardness are crystallized in a conventional high-nickel grain roll has been developed. However, since the wear resistance of the enhanced type high-nickel grain roll is significantly inferior to that of the high-speed steel type roll. Therefore, The development of a new cast iron roll with superior wear resistance and is applicable to the latter stands of hot strip mills was studied. The present development roll has improved wear resistance by increasing the amount of addition of the high hardness MC type carbide forming elements. In addition, the reduction of the carbon equivalent for less amount of eutectic carbides resulted in the reduction of the residual stress down to the same level as the high-nickel grain roll, which improved the crack resistance. As a result, it was confirmed that the wear resistance was improved about three times compared with the conventional high-nickel grain rolls. In addition, the results suggest that the wear resistance of work rolls for the hot strip mill is largely controlled by the amount of MC carbides among the rolls with the same hardness.
  • Friction Stir Welding of High Phosphorus Weathering Steel– Weldabilities, Microstructural Evolution and Mechanical Properties

    pp. 892-901

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    DOI:10.2355/tetsutohagane.TETSU-2020-057

    Phosphorus (P) addition is expected to simultaneously increase the strength and corrosion resistance of weathering steels. However, P causes solidification cracking in the fusion welding process and reduces the toughness of steel. To avoid these problems, the P content of weldable SMA490AW weathering steel is currently limited to below 0.035 mass%. High P steels which are impossible to be joined by the fusion welding process, can be joined by a solid-state joining process, friction stir welding (FSW). Because the stir zone obtained by FSW contained very fine grains, its toughness was expected to improve. This study applies FSW to high-P weathering steels and examines the weldability of the product. The microstructural evolution and mechanical properties of the stir zone were investigated at different welding temperatures. The macroscopic cross-sectional observations of the FSW joints revealed crack-free structures even in steel containing 0.3 mass% P. Moreover, FSW significantly refined the grain structure in the stir zone. Consequently, the ductile-to-brittle transition temperature of the stir zone was approximately 150°C lower in the steel containing 0.3 mass% P and welded below A1 (average grain size = 2.5 µm) than in the base material (average grain size = 23 µm). It appears that the grain refinement by FSW overcomes the embrittlement caused by excessive P content.
  • Effect of Ammonium Thiocyanate on Hydrogen Entry Behavior of Low Alloy Steel under Galvanostatic Cathode Charging

    pp. 902-910

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    DOI:10.2355/tetsutohagane.TETSU-2020-045

    The effect of ammonium thiocyanate (NH4SCN) on the behavior of hydrogen entry into low alloy steel under cathode charging was investigated using electrochemical hydrogen permeation technique. In this study, hydrogen entry sides were charged galvanostatically to control the rate of hydrogen evolution reaction. The potential, hydrogen charging current density and hydrogen permeation current density were measured at pH of 3.0 in acetic buffer solution with and without 3 g·L–1 NH4SCN. From the Tafel slope of the cathode reaction and the dependence of hydrogen concentration on hydrogen charging current density, it was confirmed that the hydrogen evolution reaction proceeded under Volmer-Tafel mechanism in this study. NH4SCN drastically increased hydrogen entry into steel. To analyze the results of this study, the efficiency of hydrogen entry was calculated from the relationships among hydrogen charging current density, hydrogen permeation current density and hydrogen overpotential. It was found that the hydrogen entry efficiency was drastically higher in NH4SCN environment than that in NH4SCN free environment. However, the coverage of adsorbed hydrogen atoms on hydrogen entry side was decreased in NH4SCN environment. To discuss the mechanism that NH4SCN increases hydrogen entry efficiency, the activation energies of hydrogen adsorption and hydrogen absorption were estimated by temperature dependence of the hydrogen charging current density and the hydrogen permeation current density. It is suggested that NH4SCN increased the activation energy of hydrogen adsorption although it decreased that of hydrogen absorption.
  • Thermodynamic Study on Solubility Products of Ti4C2S2 in Fe Using First-Principles Calculations

    pp. 911-923

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    DOI:10.2355/tetsutohagane.TETSU-2020-033

    This study elucidates the solubility product of Ti4C2S2 in steels by means of first-principles calculations and thermodynamic analysis. For this purpose, the Gibbs formation energy of Ti4C2S2 was calculated theoretically by considering the effect of lattice vibration and thermal expansion. In addition, the Gibbs energies of the bcc and fcc phases in the Fe–Ti–C ternary system were also obtained using the cluster expansion and cluster variation method. Although some experimental data were considered as required, those results were evaluated as the calculation of phase diagrams (CALPHAD)-type thermodynamic parameters through fitting to the sublattice model. By using those thermodynamic functions, an approximate expression of the solubility product for Ti4C2S2 was derived. The result agrees with an experimental result measured in a relatively large temperature range. Furthermore, the formation behavior of precipitates in typical interstitial-free steels was discussed, incorporating an earlier thermodynamic analysis on the Fe–Ti–S ternary system. The results show that NiAs-type TiS was the main precipitate at higher temperatures and that Ti4C2S2 was the main precipitate at lower temperatures.
  • In situ Observation of Abnormal Grain Growth of Austenite in Case Hardening Steel

    pp. 924-933

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    DOI:10.2355/tetsutohagane.TETSU-2020-042

    In order to clarify the mechanism of abnormal grain growth of austenite in case hardening steel, in-situ observation by high temperature EBSD was performed using JIS SCM420 containing Nb (0.2C-0.2Si-0.8Mn-1.1Cr-0.2Mo-0.024Nb steel in mass%). After heating at 1143 K, two growing grains several times larger than the surrounding grains were observed. These growing grains grew abnormally by holding at 1193 K and were adjacent to each other. Since the boundary between the two abnormal grains is a twin boundary, the abnormal grains were observed as if they were one larger abnormal grain. The growth rate of abnormal grain is as high as the initial stage of growth and negligibly small at the latter stage of growth. That is, the grain size of abnormal grain growth of austenite is mainly decided by rapid grain growth for a short time after the start of grain growth. The generation of growing grain of abnormal grain growth of austenite is not affected by orientation and strain distribution. When distant grains which have a twin relationship are adjacent to each other in the grain growth process, grain connection occurs. It doubles the area surrounded by high angle grain boundaries without twin boundary. In addition to the encroachment of surrounding grains by larger grains, austenite grain connection through twin boundary also affects abnormal grain growth of austenite.
  • Effect of Carbon Content on V-bending in High-Strength TRIP-aided Dual-Phase Steel Sheets with Polygonal Ferrite Matrix

    pp. 934-943

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    DOI:10.2355/tetsutohagane.TETSU-2020-012

    Effect of carbon content on V-bending in high-strength TRIP-aided dual-phase (TDP) steel sheets with polygonal ferrite matrix was investigated for automobile applications. V-bending test was performed on a hydraulic testing machine at a processing speed of 1 mm/min, using a rectangular specimen (50 mm in length, 5 mm in width, 1.2 mm in thickness). The main results are as follows.(1) The (0.1-0.4)C-1.5Si-1.5Mn, mass% TDP steel sheets were able to perform V-bending by strain-induced martensitic transformation of TRIP effect. On the other hand, ferrite-martensite dual-phase (MDP0) steel sheet of 900 MPa grade was not able to perform 90-degree V-bending because of initiation of crack in tension region.(2) The 0.4C-1.5Si-1.5Mn,mass% TDP4 steel sheet of 1100 MPa grade was able to enable the 90-degree V-bending that considered an amount of springback (Δθ=θ2θ1), in which the θ1 and the θ2 are a bending angle on loading and a bending angle after unloading respectively, of more than 2-degree by controlling a displacement of punch bottom dead center.
  • Analysis of Nano-hardness Distribution Near the Ferrite-martensite Interface in a Dual Phase Steel with Factorization of Its Scattering Behavior

    pp. 944-952

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    DOI:10.2355/tetsutohagane.TETSU-2020-037

    Herein, we investigated the local preliminary hardening of ferrite near the ferrite–martensite interfaces in a dual-phase (DP) steel. Geometrically necessary dislocations (GNDs), generated due to interfacial misfit between different phases, may cause preliminary hardening of ferrite around such interfaces. However, for nano-hardness distribution, the hardened zone was not evidently detected by scattering measurement. Thus, we factorized nano-hardness scattering to estimate the actual ferrite hardness near ferrite–martensite interfaces.First, nano-hardness was measured around a martensite island using a conical nano-indenter in the DP steel containing 10% martensite by volume. Taking into account the scattering, the nano-hardness measurement converged to the hardness of ferrite, exceeding the distance corresponding to the nano-indenter radius. Thus, a preliminary hardening zone was not detected. Subsequently, the surface of the nano-indented microstructure was polished and observed using scanning electron microscopy (SEM) by analyzing electron back scattering diffraction (EBSD). This analysis confirmed the presence of the nano-indented microstructure under ferrite. Moreover, it established that the majority of the irregularly higher nano-hardness was caused by the buried martensite under ferrite. The value of the kernel average misorientation (KAM), which is proportional to the GND density for other irregularly higher nano-hardness points, was higher for the nano-indented microstructure as compared to that of the buried martensite. On the other hand, the ferrite was expanded under the nano-indented points for the majority of the irregularly lower nano-hardness, with some exceptions. Further, soft martensite was observed to induce irregularly lower nano-hardness locally around the interface.
  • Deformation-Induced Martensitic Transformation Behavior of Retained Austenite during Rolling Contact in Carburized SAE4320 Steel

    pp. 953-960

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    DOI:10.2355/tetsutohagane.TETSU-2020-048

    The deformation-induced martensitic transformation is a very effective phenomenon that improves mechanical properties of steels, and well known to be beneficial also in rolling contact fatigue (RCF) of bearings. In the present study, the characteristics of the deformation-induced martensitic transformation in the case of RCF of carburized, quenched and tempered SAE4320 steel were investigated in detail using a scanning electron microscopy – electron backscattering diffraction and an automated crystal orientation mapping - transmission electron microscopy. These analyses clarified that the extremely fine deformation-induced martensites as small as several tens of nm were formed with different variants within an austenite grain in the case of RCF, and the martensites were speculated to have the Kurdumov-Sachs or the Nishiyama-Wasserman relationship with retained austenite. Furthermore, the deformation-induced martensites were formed preferentially within the retained austenite grains, not from the interface between tempered martensite and retained austenite. Therefore, it was suggested that the deformation-induced martensites in RCF were formed from some localized regions that were plastically introduced within the retained austenite grains.
  • Biochemical Analysis of Microbial Adsorption Behavior on Iron and Steel Slag Using DNA-specific Fluorescent Reagent, and the Effect of Microbial Biofilm Attached to Slag on pH Buffering Action

    pp. 961-972

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    DOI:10.2355/tetsutohagane.TETSU-2020-026

    To evaluate microbial potentials for the material development of iron and steel slag, this study particularly investigated the chemical effect of slag, which was artificially coated with a microbial biofilm, on buffer action. Prior to evaluating the slag, this study also developed a method to determine the amount of microbes adhering to slag. To encourage the growth of Bacillus bacteria on slag, the slag was mixed with the bacteria in LB medium for 24 hours. After extracting microbial DNA using the hot-alkaline DNA extraction method, the microbial quantity attaching tightly to slag was determined from the concentration of the microbial DNA using Pico Green-based fluorometry. The adsorption isotherm between the microbial quantity attached to the slag and the corresponding reacting microbial amount was analyzed using the Langmuir and Freundlich adsorption models. To examine the buffering action of slag coated with and without microbes, each slag was immersed in distilled water for seven days. Next, both pH levels of each slag-containing solution and each amount of microbes attached to slag were determined. The pH increased in both solutions containing slag coated without biofilm and with partially desquamated one; in contrast, the slag coated with well-preserved biofilm showed a buffering action, resulting in an inhibited increase in pH. These results show that slag coated with biofilm is distinctively different from an original slag coated without biofilm in terms of buffer action. This processing technique using microbes could contribute to the development of a novel application of slag as a recycled material.
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  • Editorial Board and Copyright Page

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