In recent years, in order to improve fuel efficiency by reducing the weight of automobile bodies and to improve collision safety, application of the steel sheets having a tensile strength (TS) of 980 MPa or more to automobile framework parts has been positively promoted. Recently, the application of even stronger steel sheets has been studied.
High strength steel sheets with TS: 1180 MPa or more used to be commonly applied to members subjected to general working, such as bumper reinforcements and door impact beams. The application of such steel sheets to automobile framework parts having various complicated shapes due to press forming has recently been studied to ensure further collision safety and to improve fuel efficiency by reducing the weight of vehicle bodies. Therefore, steel sheets having excellent formability are highly demanded.
However, increase in the strength of the steel sheets is in general likely to be accompanied by reduction in their formability. Accordingly, prevention of fractures caused during press forming has been a major challenge in promoting the application of high strength steel sheets. Further, in cases where the strength of the steel is increased to TS: 1180 MPa or more in particular, extremely expensive rare elements such as Nb, V, Cu, Ni, Cr, and Mo are often required to be intentionally added in addition to C and Mn in order to ensure sufficient strength.
Examples of the conventional techniques regarding a high-strength cold-rolled steel sheet having excellent formability include such techniques of obtaining a high-strength cold-rolled steel sheet having martensite phase or retained austenite phase as a constituent phase of the steel composition through restriction of the steel components and microstructure and optimization of hot rolling and annealing conditions for the production of the steel sheets as disclosed in PTL 1 (JP 2004-308002 A), PTL 2 (JP 2005-179703 A), PTL 3 (JP 2006-283130 A), PTL 4 (JP 2004-359974 A), PTL 5 (JP 2010-285657 A), PTL 6 (JP 2010-059452 A), and PTL 7 (JP 2004-068050 A).