Patent ID: 9708697
Date: 2017-07-18
CPC Classifications: C21D,C22C

Claim:
1. A cold-rolled steel sheet, comprising: C: 0.05-0.19 mass %; Si: 3.0 mass % or less, exclusive of 0 mass %; Mn: 0.1-5.0 mass %; P: 0.1 mass % or less, exclusive of 0 mass %; S: 0.02 mass % or less, exclusive of 0 mass %; Al: 0.01-1.0 mass %; and N: 0.01 mass% or less, exclusive of 0 mass % respectively; iron and inevitable impurities, wherein a microstructure comprises ferrite that is a soft first phase by 20-50% in terms of area ratio, and tempered martensite, tempered bainite, or both, that is a hard second phase; a difference between area ratio Vαs of ferrite of a steel sheet surface layer section from a steel sheet surface to a depth of 100 μm and area ratio Vαc of ferrite of a center section of t/4-3t/4 ΔVα=Vαs−Vαc is less than 10%, where t is a sheet thickness; and a ratio of hardness Hvs of the steel sheet surface layer section and hardness Hvc of the center section RHv=Hvs/Hvc is 0.75-1.0.