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

Tetsu-to-Hagané Advance Publication

  • Effects of Alloy Elements on Carbon Partitioning at Early Stages of Proeutectoid Ferrite Transformation in Low Carbon Mn-Si Steels

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    DOI:10.2355/tetsutohagane.TETSU-2019-020

    Controlling carbon concentration and its distribution among constituent phases is one of the most important issues to achieve high strength and ductility in the design of steel. The carbon distribution near the α/γ interface at the early stage of isothermal holding at 750ºC was measured and visualized in Fe-C-Mn-Si alloys, containing 2 mass%Si and 1.5 or 2 mass%Mn, using recently developed high precision FE-EPMA, and results were compared with the theory of ferrite growth in multi-component low alloy steel. The carbon concentrations at α/γ interfaces in austenite were generally between the NPLE/PLE and paraequilbrium γ/(γ+α) boundary concentrations. In alloys of carbon content smaller than the NPLE/PLE boundary, it seems that α/γ interfaces migrated under the condition close to paraequilibrium or with partially developed spikes of alloy elements at early stages. On the other hand, in alloys of bulk composition on the boundary and its higher carbon concentration side, Mn enrichment was observed at the interfaces, and the carbon concentration tended to be higher than those in alloys of lesser carbon content, albeit there were variations at individual interfaces.
  • Lowest Fatigue Limit Estimation of Ductile Cast Iron Joints by Considering Maximum Defect Size to Replace Welded Joints

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    DOI:10.2355/tetsutohagane.TETSU-2019-017

    In our earlier study, the authors revealed that the fatigue limit of ductile cast iron (DCI) specimens whose shapes are similar to the welded joint shapes is about three times larger than that of the welded joint specimens. However, since many defects are usually included in the DCI specimens, the fatigue limit of DCI joints decreases with increasing the maximum defect size. In this paper, therefore, the maximum defect size is estimated by using statistics of extremes. Then, the lowest fatigue limit corresponding to the maximum defect size is estimated from the 4 parameter model and compared with the lowest fatigue limit of the welded joint. As a result, it was confirmed that the lowest fatigue limit of the DCI specimens is about twice as large as the welded joint.
  • Impact Property of Case Hardening Steel Subjected to Combined Heat Treatment with Excess Vacuum Carburizing and Subsequent Induction Hardening

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    DOI:10.2355/tetsutohagane.TETSU-2018-166

    Charpy impact value of the 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 effect of retained austenite and cementite on the impact property. The characteristic of combined heat treatment is that the initial microstructure can be designed easily. The initial microstructure is designed by carburizing and annealing at the hyper-eutectoid composition of 1.3 mass% C and subsequent induction heating temperature is chosen between Acm and A1 to obtain different amounts of retained austenite and cementite. The impact value improves by the induction hardening with increasing heating temperature and the tempering. The steel treated at the low heating temperature shows intra-granular fracture irrespective of the presence of pro-eutectoid cementite. As the heating temperature increases, the formation of C solid solution progresses by the decomposition of cementite and increased retained austenite transforms into the deformation-induced martensite by the impact energy, thereby increases intra-granular strength. Hence critical fracture strength transits to grain boundary strength and showed inter-granular fracture at the interface of cementite and matrix in this study. The impact value showed the correlation with the amounts of retained austenite before the test and the decrement in retained austenite before and after the test. The effect of retained austenite is due to the plastic deformation of austenite, the increase of the compressive residual stress generated by deformation-induced martensite transformation, and the consumption of the impact energy as the driving force for deformation-induced martensite transformation.
  • Relationship between the Morphology of Mn-oxides Simulated by Ion Plating and Phosphatability for Mn-added High Strength Cold-Rolled Steel Sheets

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    DOI:10.2355/tetsutohagane.TETSU-2019-007

    It is well known that Si, Mn and B, the alloying elements for high strength steel sheets, easily form oxides on the steel surface during annealing in a reducing atmosphere, and those oxides have a large influence on the surface performance of steel sheets, such as phosphatability. In this work, we discovered that the oxidation behavior of Mn-added high strength cold-rolled steel sheets could be simulated on mild steel sheets by using an ion plating method and investigated the relationship between the morphology of Mn oxides and phosphatability under the condition that both the amount and kind of Mn oxides were fixed. In a simulated Mn-O layer, fine surface oxides, which covered most of the steel surface, were observed after annealing. On the other hand, in a Mn-B-O layer, large globular surface oxides were observed on the steel surface, and the Fe surface was partially bare. The B-Mn compound oxide is considered to be in a molten phase during annealing because the melting point of the compound oxide is lower than the annealing temperature, and as a result, it is thought that large B-Mn compound oxides coagulate and grow during annealing. In addition, it was found that the large B-Mn compound oxides (about 500 nm) interfere with steel dissolution in the phosphate solution. These results demonstrate the importance of controlling the morphology as well as the amount and kind of surface oxides for obtaining good phosphatability of Mn-added high strength cold-rolled steel sheets.
  • Texture Evolution during Recrystallization and Grain Growth in Heavily Cold-rolled Fe-3%Si Alloy

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    DOI:10.2355/tetsutohagane.TETSU-2019-013

    Recrystallization and grain growth are important phenomena for controlling the mechanical and magnetic properties of steels through texture. Only a limited number of studies have been carried out on texture evolution during recrystallization and grain growth in heavily cold-rolled Si steel. The present study first focuses on clarifying the texture evolution during normal grain growth, followed by an investigation into the development of the {411}<148> component during recrystallization. The {411}<148> component is remarkably developed during normal grain growth after the completion of recrystallization. At just fully recrystallized stage, the diameters of the {411}<148> grains were larger than that of the grains with other orientations. Therefore, the {411}<148> grains significantly grew owing to the size advantage.Just at the commencement of recrystallization, differences in grain diameter of recrystallized grains in terms of crystal orientation were not detected. However, it is worthwhile to mention that the nucleation of {411}<148> recrystallized grains is unexpectedly fast in heavily cold-rolled Si steel. Recrystallized {411}<148> grains were observed to nucleate in the deformed α-fiber grains, especially near the grain boundaries. Nuclei with {411}<148> orientation grow easily due to the high mobility of the interface between the recrystallized/non-recrystallized grains and the high driving force. Consequently, the diameter of a {411}<148> recrystallized grain becomes relatively large upon the completion of recrystallization. This contributes to the selective grain growth during the normal grain growth stage because of the size effect.
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  • Structural Change in Hot-Stamped GA Coating and Influence on Corrosion Resistance by Tempering

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    DOI:10.2355/tetsutohagane.TETSU-2018-155

    Hot-stamped galvannealed steel sheets (GA) are widely applied to produce automobile components that require high strength. The hot-stamped components tempered for the control of strength improve the collision property. This study investigated the structural changes in the hot-stamped GA coating and the influence on corrosion resistance by tempering. It has been revealed that by tempering between 500-600 degrees Celsius, the Fe-Zn solid-solution phase in the hot-stamped GA coating changed into two phases: the Γ phase and the Fe-Zn solid-solution phase with less Zn, and these changes in the coating structure improved the corrosion resistance. The phase decomposition is considered to result from discontinuous precipitation of the Fe-Zn supersaturated-solid-solution.

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