| 摘要 |
PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet which has good hole expandability while securing strength before and after hot stamping and is excellent in chemical conversion treatability and plating adhesion after hot stamping, an alloyed melt-zinc-plated cold rolling sheet, etc. and their production method.SOLUTION: A cold rolled steel sheet meets the relationship of an equation(A), and the metal structure before hot stamping comprises 40-95% of ferrite and 5-60% of martensite, in terms of the area ratio, with a sum of the area ratios of ferrite and martensite of 60% or higher. The hardness of the martensite, measured with a nanoindenter before hot stamping, meets equations (B) and (C), and the product TS×λ of the tensile strength TS by the hole expandability λ is 50,000 MPa% or greater. (A) (5×[Si]+[Mn])/[C]>10; (B) H2/H11<1.10; and (C) &sgr;HM<20. |
