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

Tetsu-to-Hagané Vol. 106 (2020), No. 3

  • Dynamic Changes in Interfacial Tension between Liquid Fe Alloy and Molten Slag Induced by Chemical Reactions

    pp. 133-142

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    DOI:10.2355/tetsutohagane.TETSU-2019-057

    The authors investigated the change in the interfacial tension with time for various combinations of molten slag and liquid Fe to elucidate the mechanism of the change in interfacial tension between liquid Fe alloy and molten slag over time accompanying reduction/oxidation reactions. The behavior of the change in the interfacial tension over time can be explained by the adsorption of oxygen at the interface and the diffusion of oxygen from the interface into the bulk of the liquid Fe and molten slag. In addition to that, we found that the interfacial tension decreases slowly and greatly from its initial value to a minimum point and then increases slowly to the final equilibrium state when molten silicate slag with low viscosity is brought into contact with liquid Fe without Al content and some of its SiO2 decomposes and dissolves into the liquid Fe. From these results, we suggest that the detachment of oxygen adsorbed at the interface into the liquid Fe is very slow and may be the rate-limiting step.
  • Recrystallization Behavior and Formation of {411}<148> Grain from α-fiber Grains in Heavily Cold-rolled Fe-3%Si Alloy

    pp. 143-153

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    DOI:10.2355/tetsutohagane.TETSU-2019-086

    Recrystallization texture is essential to control the mechanical and magnetic properties of steels. Both γ-fiber (ND//<111>) and α-fiber (RD//<011>) textures are known to develop during the rolling process of bcc iron. Recrystallization behavior from γ-fiber has been extensively studied. On the other hand, recrystallization behavior from α-fiber, in particular after heavy cold rolling reduction, has not been sufficiently clarified. In this study, recrystallization behavior from α-fiber, focusing on the formation of {411}<148> recrystallized grain, was investigated by means of EBSD and TEM. {411}<148> region already existed in the vicinity of deformed grains having upper α-fiber orientation ({100}<011> ~ {211}<011>). TEM observation revealed the existence of the lamellar structure with {411}<148> relatively fine dislocation cells in the {211}<011> deformed grains. With the progress of the recovery, {411}<148> subgrains (dislocation cells) are postulated to easily form and are surrounded by the deformed matrix grains with high angle interface. Thus, it is easy to form the recrystallization nuclei having the potential to grow with the sake of both high driving force and high interface mobility. At the early stage of recrystallization, {411}<148> recrystallized grains developed in {211}<011> deformed grains. At the later stage, {411}<148> recrystallized grains from {211}<011> deformed grains encroach {100}<011> deformed grains and new {411}<148> recrystallized grains developed in {100}<011> deformed grains.
  • Mechanism of Improved Ductility of 1,500 MPa-class Ultra-high Strength Cold-rolled Steel Sheet Produced by Rolling and Partitioning Method

    pp. 154-164

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    DOI:10.2355/tetsutohagane.TETSU-2019-083

    By using a steel with standardized chemical composition and conventional manufacturing processes for flat-rolled steel strip, a 1,500 MPa class stainless steel sheet whose product of yield strength (YS) and total elongation (El) exceeds 30,000 MPa% was developed and its mass production has been 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, the name of which was called 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.
  • Influence of Iron Carbide on Mechanical Properties in High Silicon-added Medium-carbon Martensitic Steels

    pp. 165-173

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    DOI:10.2355/tetsutohagane.TETSU-2019-085

    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.
  • Hydrogen Embrittlement and Fracture Surface Morphologies of Tempered Martensitic Steels

    pp. 174-182

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    DOI:10.2355/tetsutohagane.TETSU-2019-087

    Hydrogen embrittlement behavior of tempered martensitic steels has been investigated using a conventional strain rate test (CSRT) for circumferential notched specimens with various notch tip radii. For smaller notch tip radii (below 0.8 mm), hydrogen-charged specimens initially fractured near the notch tip. In contrast, for larger notch tip radii (above 1 mm), hydrogen-charged specimens initially fractured near the center of the specimen; the crack then propagated and failure occurred. The cracks of hydrogen-charged specimens with smaller notch tip radii occurred on the load-displacement curves of uncharged specimens, and the load remained constant regardless of the increase in displacement. These specimens showed quasi-cleavage (QC) and/or intergranular (IG) fracture morphologies. The results indicated that QC and IG fracture modes were stable. Although fracture morphologies changed from dimple to QC to IG with increasing hydrogen content, the critical hydrogen content was identical regardless of the notch tip radii. Fractography of hydrogen-charged specimens with larger notch tip radii unloaded just after the maximum tensile load clearly indicated that QC fracture was stable since it originated at several points, propagated and then coalesced. The relationship between stress triaxiality and critical equivalent plastic strain used for dimple failure could also be considered applicable to QC fracture. The presence of hydrogen in specimens decreased markedly under the critical equivalent plastic strain in this relationship.
  • Dislocation Characterization by the Direct-fitting/modified Williamson-Hall (DF/mWH) Method in Ultra-low Carbon Martensitic Steel

    pp. 183-186

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    DOI:10.2355/tetsutohagane.TETSU-2019-053

    The Williamson-Hall (WH) plots are the basic approach for the dislocation characterization. However, the elastic anisotropy affects full width at half maximum in diffraction peaks and this makes the dislocation characterization difficult. In order to correct the effect of elastic anisotropy, Ungár developed a unique methodology using the contrast factor, so called the modified Williamson-Hall (mWH) method. On the other hand, authors developed a new methodology termed as “direct-fitting (DF) method” in which the elastic anisotropy is corrected directly applying the correction parameter; ωhkl. By the DF method, reliable values are obtained for the parameter α which contains an information of crystallite size. In this paper, the α-value obtained by the DF method was applied to the mWH method and the dislocation characterization was performed in an ultra-low carbon martensitic steel (Fe-18%Ni alloy) with cold rolling up to 20% thickness reduction. It was found that high dislocation density ρ of 2.1×1015/m2 is obtained in as-quenched specimen and the cold rolling does not give significant effect on dislocation density ρ. However, the parameter φ obtained by the mWH method changes markedly by charging small amount of cold rolling. As a result, the parameter A, that depends on the values of ρ and φ, changes markedly by charging small amount of cold rolling: A=0.77 in as-quenched specimen but A=0.60 in specimens with cold rolling. This result indicates that the dislocation arrangement has been changed from homogeneous to inhomogeneous distribution by cold rolling.

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