In recent years, enhancement of fuel efficiency of automobiles has become an important issue from the viewpoint of global environment protection. Consequently, there is an active movement to reduce the thickness of vehicle body components through increases in strength of vehicle body materials, and thereby reduce the weight of vehicle body itself.
In general, however, strengthening of steel sheets leads to deterioration in formability, causing the problem of cracking during forming. It is thus not simple to reduce the thickness of steel sheets. Therefore, it is desirable to develop materials with increased strength and good formability. In addition, in an automobile manufacturing process, steel sheets with a tensile strength (TS) of 980 MPa or more are subjected to press working, assembled using arc welding, spot welding, and the like, and combined into modules. Such steel sheets are thus required to have high dimensional accuracy on assembly.
Accordingly, such steel sheets should have good formability and be resistant to defects that could occur after working, such as springback. To this end, it is important for the steel sheets to have low yield ratio (YR) before subjection to working.
For example, JPS61157625A (PTL 1) proposes a high-strength steel sheet with extremely high ductility having a tensile strength of 1000 MPa or higher and a total elongation (EL) of 30% or more, utilizing deformation induced transformation of retained austenite.
In addition, JPH1259120A (PTL 2) proposes a steel sheet that is obtained from high-Mn steel, aiming to achieve well-balanced strength and ductility through heat treatment in a ferrite-austenite dual phase region.
Moreover, JP2003138345A (PTL 3) proposes a steel sheet that is obtained from high-Mn steel, aiming to achieve improved local ductility by being subjected to hot rolling to have a microstructure containing bainite and martensite, followed by annealing and tempering to cause fine retained austenite, and subsequently tempered bainite or tempered martensite in the microstructure.