| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for producing a high-tensile cold-rolled steel sheet with a tensile strength of 340-490 MPa class, which shows a good surface property after press molding and is excellent in bake hardenability, resistance to natural aging and press moldability. SOLUTION: Steel comprising, by mass,≥0.0025% but <0.04% C,≤0.5% Si, 0.5-2.5% Mn,≤0.05% P, 0.01% S,≤0.15% sol.Al, <0.008% N, 0.05-2.0% Cr and the balance Fe with inevitable impurities is subjected to hot-rolling and cold-rolling and subsequently to continuous annealing, wherein the steel is soaked at a temperature equal to or above the Ac<SB>1</SB>transformation point but below the Ac<SB>3</SB>transformation point or a temperature equal to or above the Ac<SB>3</SB>transformation point but below (Ac<SB>3</SB>transformation point+100°C), is subsequently cooled at a rate of 15-200°C/s from 650°C to 450°C and is cooled at a rate of <10°C/s within a temperature range defined by the formulas given in the text. The steel has such a structure that the main phase is a ferrite phase and the second phase contains a phase generated through low-temperature transformation. COPYRIGHT: (C)2006,JPO&NCIPI |
