HIGH-STRENGTH STEEL SHEET HAVING SUPERIOR IMPACT RESISTANCE, METHOD FOR PRODUCING SAME, HIGH-STRENGTH GALVANIZED STEEL SHEET, AND METHOD FOR PRODUCING SAME

摘要

This high-strength steel sheet, which has superior impact resistance and a maximum tensile strength of at least 900 MPa has a steel sheet structure containing C, Si, Mn, P, S, Al, Ti, N, and O in predetermined quantities, the remainder comprising iron and unavoidable impurities; 1-8% by volume fraction of residual austenite is contained in the 1/8-3/8 thickness range centered on 1/4 of the sheet thickness; the average aspect ratio of the residual austenite is no greater than 2.0; the amount of solid-solution Mn in the residual austenite is at least 1.1 times the average amount of Mn; TiN grains having an average grain size of no greater than 0.5 µm are contained; and the density of AlN grains having an average grain size of at least 1 µm is no greater than 1.0 grains/mm2