Patent ID: 10329636

Abstract:
A high-strength cold-rolled steel sheet having good ductility, hole expandability, and delayed fracture resistance and being excellent in material homogeneity. The steel sheet has a chemical composition containing, in mass %, C: 0.15 to 0.25%, Si: 1.2 to 2.2%, Mn: 1.7 to 2.5%, P: 0.05% or less, S: 0.005% or less, Al: 0.01 to 0.10%, N: 0.006% or less, Ti: 0.003 to 0.030%, B: 0.0002 to 0.0050%, and Fe and inevitable impurities. The steel sheet has a microstructure that includes ferrite having an average crystal grain diameter of 4 μm or less at a volume fraction of 5 to 20%, retained austenite at a volume fraction of 5% or less, including 0%, and tempered martensite at a volume fraction of 80 to 95%. Additionally, the mean free path of the ferrite is 3.0 to 7.5 μm.