| 摘要 |
PROBLEM TO BE SOLVED: To provide an extra-low carbon cold rolled steel sheet excellent in stretch rigidity which is the rigidity of press parts having a relatively slow curved face and having an extremely large surface area to the sheet thickness such as an automotive outer board panel in the case external force is acted on the parts. SOLUTION: This is a cold rolled steel sheet contg., by mass, 0.0007 to 0.01% C and having 0.5 to 0.8 mm sheet thickness and 120 to 250 N/mm2 yield stress, and in the case it is applied with prestrain of 2% by equivalent strain, is subjected to heat treatment at 150 to 170 deg.C for 5 to 20 min and is again subjected to a tensile test, the ratio A (=X/Y) between the slope X of a stress- strain curve at 0.06% strain quantity and the Young's modulus Y satisfies A>0.8, and the increasing margin of the yield stress after the heat treatment is >=40 N/mm2.
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