LAUSR

Reducing the weight of the automobile body while maintaining sufficient safety by using high strength steel sheet is an important concern of automakers from the viewpoint of both fuel efficiency and crash worthiness.1) When the high strength steel sheet is applied to automobile parts, high ductility is demanded to enable the press forming for complex shape of parts. The concept of advanced high strength steel sheets (AHSS) such as DP (Dual Phase)2) and TRIP (Transformation Induced Plasticity)3) steel sheets were developed and 980 and 1 180 TS grade steels have been progressively applied to body parts to satisfy these requirements recently.4–7) However since the combination of tensile strength (TS) and total elongation (T-El) of these steels is limited to about TSxT-El=20 000 MPa·%, the development of high strength steel sheet with higher El has been strongly demanded. Recently, steel sheet with much higher El has been focused on as the third generation AHSS. Q&P (Quench & Partitioning) steel8) and medium Mn steel9–18) are promising candidates for the third generation AHSS. In Effect of Microstructure before Intercritical Annealing on Mechanical Properties of 5% Mn Steel