LAUSR

The continuous annealing process, first put into practical operation in the 1970s, made it possible to manufacture dual-phase (DP) steel sheets1) in which the martensite (α’) phase was dispersed in the ferrite (α) matrix. Triggered by this innovation, the research and development on DP steel sheet as a new high-strength steel (HSS) with an excellent balance between tensile strength (TS) and total elongation (El)2,3) was carried out worldwide. Regarding the essential mechanism behind the excellent mechanical properties of DP steel, research articles providing useful guidelines for the recent development of HSSs were published.4–6) Subsequently, transformation-induced plasticity (TRIP) steel with TS > 1 000 MPa and TS×El > 30 000 MPa%7) was developed through a continuous annealing process that Mechanism of Improved Ductility of 1 500 MPa-class Ultra-high Strength Cold-rolled Steel Sheet Produced by Rolling and Partitioning Method