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|1. (WO2013118679) HIGH-STRENGTH COLD-ROLLED STEEL SHEET AND PROCESS FOR MANUFACTURING SAME|
|Applicants:||NIPPON STEEL & SUMITOMO METAL CORPORATION
|Title:||HIGH-STRENGTH COLD-ROLLED STEEL SHEET AND PROCESS FOR MANUFACTURING SAME|
This high-strength cold-rolled steel sheet has: a composition which contains, in mass%, 0.075 to 0.300% of C, 0.30 to 2.50% of Si, 1.30 to 3.50% of Mn, 0.001 to 0.050% of P, 0.0001 to 0.0100% of S, 0.001 to 1.500% of Al and 0.0001 to 0.0100% of N; a surface microstructure which comprises, in volume fraction, 3 to 10% of retained austenite and at most 90% of ferrite; and an internal microstructure which contains 3 to 20% of retained austenite in volume fraction. The internal microstructure is such observed at a depth of t/4 from the surface [wherein t is the thickness of the steel sheet]. Further, the Hvs/Hvb ratio exceeds 0.75 and is up to 0.90, Hvs being the hardness in the surface layer of the steel sheet and Hvb is the hardness at a depth of t/4 from the surface. The maximum tensile strength of the high-strength cold-rolled steel sheet is 700MPa or higher.