bending excellent in high-strength cold-rolled steel sheet, high-strength galvanized steel plate and high-strength alloy of molten zinc plated steel sheet and method of manufacturing the same

摘要

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet having excellent bendability and 900 MPa or higher tensile maximum strength, and to provide a method for manufacturing the high-strength steel sheet.;SOLUTION: A steel sheet is composed of steel containing 0.075-0.300% C, 0.30-2.50% Si, 1.30-3.50% Mn, 0.001-0.050% P, 0.0001-0.0100 S, 0.001-1.500% Al, 0.0001-0.0100% N, 0.0001-0.0100% O, and having the structure containing ≥50% of bainite and 3-30% of retained austenite, and a ratio of the hardness Hvs on the steel sheet surface layer to the hardness Hvb at 1/4 thickness of the steel sheet being 0.35-0.90. The steel sheet is manufactured in such a manner that after cold rolling, an annealing is applied in the temperature range near the Ac3 transformation point under atmosphere of -3.0 to 0.0 log (water partial pressure/hydrogen partial pressure), and retention at 300-500°C for 10-1000 sec is performed on the way of cooling after annealing.;COPYRIGHT: (C)2013,JPO&INPIT