| 摘要 |
PROBLEM TO BE SOLVED: To provide a superhigh strength cold-rolled steel sheet having a superior bendability and formability for an extension flange and a manufacturing method therefor. SOLUTION: The super high strength cold-rolled steel sheet contains 0.03 to 0.2% C, 0.05 to 2% Si, 0.5 to 3.0% Mn,≤0.1% P,≤0.01% S, 0.01 to 0.1% SolAl,≤0.005 % N and is composed of the balance Fe and inevitable impurities, has a soft layer of 10 to 100μm in thickness at≥90% in ferrite volumetric rate in the surface layer of the steel sheet, in which the structure in the central part is≥30% in the tempered martensite volumetric rate and the balance is a ferrite phase. The steel sheet is manufactured by subjecting the steel sheet to recrystallization annealing at a temperature above Ac1 point and below 950°C after cold rolling, then to two-stage cooling of (1) cooling for a cooling time of 0.05 to 5 seconds at a cooling rate of 5 to 50°C/second from a cooling initiation temperature≥550°C, then (2) cooling at a cooling rate of 100 to 2,000°C/s down to a cooling stop temperature of≤100°C and tempering the steel sheet at 150 to 500°C. COPYRIGHT: (C)2006,JPO&NCIPI
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