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

Tetsu-to-Hagané Advance Publication

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

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    DOI:10.2355/tetsutohagane.TETSU-2022-049

    One of the objectives for the development of high-strength dual-phase (DP) steel is improving the stretch-flangeability. Large-strained sheared edges are deformed and frequently cracked during stretch-flange formation. Considering shearing as the first deformation, the stretch-flange deformation may be regarded as a secondary deformation. To improve the stretch-flangeability of the DP steels, many researchers have analyzed the microvoid formation. However, in these analyses, the shearing process was not considered. With this background, ex-situ mini-bending tests combined with scanning electron microscopy (SEM) monitoring of microvoid formation were conducted during the secondary deformation. Prior to the secondary deformation, several microvoids were observed on the sheared surface and fine subgrains formed in the ferrite. During secondary deformation, the preliminary microvoids present at the ferrite-martensite interface propagated into the ferrite phase. In contrast, this behavior was not observed for the reamed surface deformation, which was formed without preliminary deformation. Furthermore, microvoids were initiated on ferrite grains that were not present at the ferrite-martensite interface, and martensite islands were not cracked during secondary deformation. This result is noteworthy because martensite cracking was the main factor involved in microvoid initiation, in the absence of shearing. Electron backscattering diffraction analysis revealed that the work hardening of ferrite, prior to the secondary deformation, caused a deviation in the strain concentration sites from those found in the reamed surface deformation. Therefore, this study elucidated microvoid formation on preliminary deformed surfaces via shearing and provided insights for material development considering deformations on the sheared surfaces of materials.
  • Friction Stir Welding of 1.4 GPa-grade Tempered Martensitic Steel

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    DOI:10.2355/tetsutohagane.TETSU-2022-045

    The thermal hysteresis in fusion welding causes serious deterioration of welds of medium to high-carbon steels, so the development of an effective alternative welding process are expected. Friction Stir Welding (FSW) is considered to be an effective alternative. FSW is a solid-state joining process in atmosphere, which reduces the risks associated with melting and solidification of metals. Another advantage is the in-process flexible controllability of heat input by controlling welding parameters. From this perspective, the authors are engaged in a series of studies to elucidate the characteristics of friction stir welded joints for medium- to high-carbon steels, including high-strength tempered steel.This report describes the results of applying friction stir welding to 1.4 GPa-grade tempered JIS-S55C steel plates. Five types of joints with different welding parameters were obtained by varying the joining parameters e.g. tool rotation speed or welding speed. The temperature of the FSW tool and material interface during friction stir welding was measured using a thermal imaging camera. The microstructure of the friction stir welded butt joint was evaluated by optical microscopy and FE-SEM / EBSD. The mechanical properties of the welds were evaluated by Vickers hardness test and tensile test, and DIC analysis was applied to analyze the details of local deformation during the tensile test. The effects of joining parameters on microstructure, microstructure of welds and mechanical properties of welds were examined in detail by properly conducting FE-SEM micro-observations, EBSD measurements.
  • Effect of Welding Condition on Texture Evolution of Austenite in Stir Zone and Marternsitic Transformation Behavior during Cooling in Ni-C Steels

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    DOI:10.2355/tetsutohagane.TETSU-2022-011

    Friction stir welding (FSW) was performed under the two welding conditions (rotation speed - traveling speed) of 150 rpm - 100 mm/min and 200 rpm - 400 mm/min using 24%Ni - 0.1%C (mass%) steel. The rapid cooling utilizing liquid CO2 was exploited to strictly evaluate the microstructural evolution of austenite in stir zone after FSW. In addition, the effect of the microstructural characteristics of austenite on martensitic transformation behavior during cooling were compared with that of 6%Ni - 0.63%C steel. Irrespective of the welding conditions, fine equiaxial grains with simple shear texture of {111}<110> orientation formed. At 200 rpm - 400 mm/min, finer grain formed without grain growth during cooling. On the other hand, at 150 rpm - 100 mm/min, recovery and grain growth occurred during relatively slow cooling after FSW. The grain growth preferentially proceeded presumably due to the strain-induced grain boundary migration of grain with lower dislocation density having {110}<110> orientation. Larger amount of retained austenite remained in the stir zone of 6%Ni - 0.63%C steel FSWed at 200 rpm - 400 mm/min due to both finer austenite grain size and higher dislocation density. Moreover, the influence of austenite grain orientation on the thermally induced martensitic transformation during cooling was revealed to be negligible, which is different from that of deformation induced martensitic transformation.
  • Numerical Analysis for Interaction of Fluid and Sphere Penetrating into Liquid Bath

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    DOI:10.2355/tetsutohagane.TETSU-2022-027

    In the steelmaking process, it is necessary to decrease impurities in steel to meet the increasing demand for high-grade products. Top blowing and blasting of powder reagent are desirable for the purpose and the deeper penetration of particles into the bath is important for efficient refining. In the present work, CFD calculation with VOF (Volume of Fluid) method and dynamic mesh was executed to study the reported penetration and residual bubble behavior of polypropylene sphere (diameter of 9.6 mm) with a static contact angle of 87° and 143° and an entry solid sphere velocity of 0.63, 0.89, and 1.53 m/s in the water model experiments. Calculated results showed the numerical analysis could evaluate the formation and breakup of air column behind the sphere and the generation of consequent residual bubble on the sphere. Good wettability and high entry speed promoted the deeper penetration of the sphere. Calculated dynamic contact angle on the basis of Kistler’s model indicated that the difference between static and dynamic contact angles was within 13.7° in the present conditions and the discrepancy could not wield a substantial influence on the result of CFD calculation. The adoption of base and refined mesh without parallel zone around the sphere could not give a good agreement with the experimental results. On the other hand, the use of layer mesh was appropriate for reproducing the penetration depth and residual bubble volume observed in the experiments.
  • Sulfide Stress Cracking (SSC) of Low Alloy Linepipe Steels in Low H2S Content Sour Environment

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    DOI:10.2355/tetsutohagane.TETSU-2022-013

    Resistance to Sulfide Stress Cracking (SSC) caused by local hard zones of pipe inner surface has been required in low alloy linepipe steel. In this study, using two samples with different surface hardness, the detailed SSC initiation behavior was clarified by four-point bend (4PB) SSC tests in which immersion time and applied stress were changed in a sour environment containing 0.15 bar hydrogen sulfide (H2S) gas. SSC cracks occurred when the applied stress was higher than 90% actual yield strength (AYS) in higher surface hardness samples over 270 HV0.1. From the fracture surface observation of SSC crack sample, it was found that the mechanism gradually shifted from active path corrosion (APC) to hydrogen embrittlement (HE), and that the influence of APC mechanism remained partially in the process of SSC initiation at the tip of corrosion pit or groove. The polarization measurement in the 4PB SSC test showed that the anodic and cathodic reactions (especially cathodic reactions) were activated when the applied stress was 90% AYS or higher. The FEM coupled analysis simulating the stress and strain concentration at the bottom tip of the corrosion groove and the hydrogen diffusion and accumulation was carried out. The principal stress in the tensile direction showed the maximum value at 0.04-0.06 mm away from the tip of the corrosion groove, and the hydrogen accumulation became the maximum. It was analytically found that the SSC crack initiated and propagated with HE mechanism dominated type when the threshold value of about 0.82 ppm is exceeded.
  • Evaluation of Interfacial Energy between Molten Fe and Fe-18%Cr-9%Ni Alloy and Non-Metallic Inclusion-Type Oxides

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    DOI:10.2355/tetsutohagane.TETSU-2021-107

    The contact angles between three non-metallic inclusion-type oxide substrates, viz. Al2O3, MgO, and MgO·Al2O3, and molten Fe and molten Fe-based stainless steel (Fe-Cr-Ni alloy) were measured using the sessile drop method in Ar atmosphere at 1873 K. The contact angles between molten Fe and oxide substrates ranged between 111° and 117°, while that between molten Fe-Cr-Ni alloy and substrates ranged between 103° and 105°. The angles between the alloy and each of the substrates were smaller than the corresponding values for Fe, which was attributed to the superior wettability of molten Fe-Cr-Ni alloy on the substrates. The wettability of the molten materials is related to the interfacial tension between the molten metals and each substrate. Thus, the interfacial tension between the molten metals and the non-metallic substrates was quantitatively evaluated using Young’s equation and the measured contact angles; the interfacial tension for molten Fe ranged from 1.862 to 2.781 N·m−1, while that for molten Fe-Cr-Ni alloy ranged from 1.513 to 2.286 N·m−1. Owing to the higher reactivity between molten Fe-Cr-Ni alloy and the substrates, the interfacial tension and energy between them were lower than those between molten Fe and the substrates.
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  • Wettability of Molten Fe-Al Alloys Against Oxide Substrates with Various SiO2 Activity

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    DOI:10.2355/tetsutohagane.TETSU-2021-110

    The contact angle between molten Fe-Al alloy with 0.03, 0.3, and 3 mass% Al composition, and Y2O3 matrix oxide substrate with 0.002, 0.32, and 1 SiO2 activity was measured using sessile drop method in Ar atmosphere at 1873 K, and the interfacial tension was evaluated. The contact angle and interfacial tension between the molten Fe-0.3 Al alloy and the Y2Si2O7 + SiO2 (aSiO2 = 1) substrate decreased over time during 60 s after the molten alloy was dropped onto the substrate. The decrease of the contact angle was 20°, and that of the interfacial tension was 628 mN・m1 Conversely, the other contact angles and the other interfacial energies were almost stable during the same period. The decrease of the contact angles ranged between 0° and 7°, and that of the interfacial tensions ranged 4 and 195 mN・m1. By observing the wetting behavior for 60 min, it was recognized that the interfacial reaction between the Fe-Al alloy and the oxide substrate was the redox reaction between Al composition in the alloy and SiO2 composition in the substrate, composed of SiO2 decomposition reaction and Al2O3 formation reaction between oxygen absorbed at the interface and Al composition in the alloy. In addition, it was indicated from the interfacial tension dependence on SiO2 activity that the medium SiO2 volume slag for the molten low-Al steel and the low SiO2 volume slag for the molten high-Al steel were effective in preventing the small droplets of molten slag into the molten steel.
  • Formation and Evolution of Inclusions in High Chromium Steel

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    DOI:10.2355/tetsutohagane.TETSU-2021-121

    Controlling inclusion content in high chromium steel is very important to prevent submerged entry nozzle from clogging in continuous casting and avoid the negative impacts of inclusions on steel properties. Therefore, effects of temperature and content of elements on phase stability diagram should be clarified in chromium bearing steel. However, the effect of chromium content on boundaries of MgO, MgO∙Al2O3 and Al2O3 in phase stability diagram are much different among the researchers. The direction of boundaries shift is affected by chromium content differently. Temperature dependencies of deoxidation equilibrium constants below 1873 K are also scattered. Calcium, which is used to avoid the negative effect of MgO∙Al2O3 inclusion, enlarges liquid region in phase stability diagram. However, the region replaced by liquid oxide is understood differently in low alloyed steel and high chromium steel. In TiOx-Al2O3-MgO system inclusion, commercial thermochemical software predicts that boundaries of Ti2O3, Ti3O5, Al2O3 and TiOx-Al2O3 shift toward lower titanium content in high chromium steel. However, the calculated phase stability diagrams vary among studies even in liquid iron or low alloyed steel. Therefore, equilibrium experiments under various conditions and reliable technique of thermodynamic calculation with high accuracy are desired.

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