TRIP型焼鈍マルテンサイト鋼の残留オーステナイト特性と引張特性に及ぼすCrとMoの影響

概要

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The effects of Cr and Mo on microstructure, retained austenite characteristics and tensile properties of a 0.2%C, 1.5%Si, 1.5%Mn, 0.05%Nb TRIP-aided steel with annealed martensite matrix, “TRIP-aided annealed martensitic steel; TAM steel”, were investigated to enhance the hardenability. Volume fraction and carbon concentration of retained austenite were decreased with increasing hardenability, although the volume fraction was increased in a 0.5% Cr bearing steel. Simultaneously, blocky second phase consisting of carbon-enriched martensite and retained austenite increased with increasing hardenability. These interesting characteristics were mainly caused by preferential nucleation of austenite on prior austenitic grain and packet boundaries on intercritical annealing and delayed bainitic transformation during austempering. Tensile strength of the steels was increased with increasing hardenability, accompanied with the decreased yield stress. Combination of tensile strength and total elongation decreased with increasing hardenability due to the increased blocky second phase.