Tadashi Furuhara

The mechanical properties of TRIP steels depend on heterogeneities of chemical composition and grain size in the retained γ structure, although these heterogeneities have not been characterized in detail. Therefore, in this study, we quantitatively investigate the inhomogeneous carbon concentration and grain size distribution, and its effects on the thermal stability of the retained γ in Fe-2Mn-1.5Si-0.4C (mass%) TRIP steel using FE-EPMA, EBSD, Mössbauer spectroscopy, and in-situ neutron diffraction during bainitic transformation at 673 K. In-situ neutron diffraction experiments detects high-carbon γ evolving during bainite transformation, in addition to the original γ, and the time variation of the volume fraction of high-carbon γ agrees well with the fraction of γ retained at room temperature. Williamson-Hall analysis based on peak width suggests that heterogeneity of carbon content exists even within the high-carbon γ. Compositional analysis using FE-EPMA and three-dimensional atom probe directly revealed that fine filmy γ was highly enriched with carbon compared to larger blocky γ, and the carbon content in blocky γ decreases with increasing blocky γ size. DICTRA simulation qualitatively reproduces the size dependency of carbon enrichment into γ. It was also found that γ tends to be retained at higher carbon content and smaller γ grain size since the smaller grain size directly improves thermal stability and the smaller γ size further contributes to the thermal stability via enhanced carbon enrichment.

Effects of transformation temperature and carbon content on the effective grain bounded by high‐angle boundary, i.e., “Bain size”, in bainite structure are studied in Fe–2Mn–(0.05–0.75)C (mass%) alloys transformed isothermally between 773 and 673 K. Bainitic ferrite (BF) nucleates at austenite (γ) grain boundary with strong variant selection for carbon content less than 0.2 mass%, resulting in formation of coarse BF regions consisting of the Kurdjumov–Sachs (K–S) variants belonging to the same Bain variant. As temperature is lowered, K–S variant pairs with large misorientation are frequently formed. Variant selection at γ grain boundary becomes weaker for higher carbon content due to frequent intragranular nucleation particularly at 673 K. The Bain size decreases generally as transformation temperature is lowered, which is ≈15 μm at 773 K but down to ≈5 μm at 673 K, and also slightly decreases at 773 K with increasing carbon content. At lower temperatures, it decreases with the increase of carbon content in low carbon range less than 0.2 mass%C but inversely increases in higher carbon range. The observed changes in the variant paring and the Bain size are well understood in terms of the driving force of BF formation and self‐accommodation of transformation strain.