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1. (WO2013118679) HIGH-STRENGTH COLD-ROLLED STEEL SHEET AND PROCESS FOR MANUFACTURING SAME

Pub. No.:    WO/2013/118679    International Application No.:    PCT/JP2013/052468
Publication Date: Fri Aug 16 01:59:59 CEST 2013 International Filing Date: Tue Feb 05 00:59:59 CET 2013
IPC: C22C 38/00
C21D 9/46
C22C 38/58
C23C 2/02
C23C 2/06
C23C 2/28
C23C 22/07
C23C 28/00
C25D 5/26
Applicants: NIPPON STEEL & SUMITOMO METAL CORPORATION
新日鐵住金株式会社
Inventors: AZUMA Masafumi
東 昌史
NOZAKI Takayuki
野崎 貴行
WAKABAYASHI Chisato
若林 千智
SATO Koichi
佐藤 浩一
KAWATA Hiroyuki
川田 裕之
FUJITA Nobuhiro
藤田 展弘
Title: HIGH-STRENGTH COLD-ROLLED STEEL SHEET AND PROCESS FOR MANUFACTURING SAME
Abstract:
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.