High-tensile steel sheet and a method of manufacturing the same excellent bending workability and low-temperature toughness

摘要

PROBLEM TO BE SOLVED: To impart superior bending property and low temperature toughness to a high-tensile strength steel sheet having a tensile strength of ≥1,150 MPa and a sheet thickness of about 7-50 mm.;SOLUTION: The high-tensile strength steel sheet has a steel composition comprising, by mass, 0.10-0.25% C, 0.05-1.5% Si, 0.5-2.0% Mn, 0.3-2.2% Cr, 0.2-1.4% Mo, 0.03-0.1% V, 0.005-0.1% Al, 0.0005-0.006% N, ≤0.02% P, ≤0.005% S, and 0.0003% B, wherein formula (1) 0.8≤0.5[% Cr]+1.2[% Mo]+5[% V]≤2.1 is satisfied, and the balance Fe with inevitable impurities. The high-tensile strength steel sheet has a steel structure in which martensite structure has a volume fraction of ≥95%, and the average grain diameter of old austenite grains is ≤20 μm in terms of the diameter of the equivalent circle, in the martensite structure, and has a tensile strength of ≥1,150 MPa.;COPYRIGHT: (C)2012,JPO&INPIT