HIGH TENSILE STRENGTH STEEL HAVING EXCELLENT LOW TEMPERATURE TOUGHNESS AND REDUCED STRENGTH ANISOTROPY, AND METHOD FOR PRODUCING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a high strength steel having excellent low temperature toughness satisfying the tensile strength of ≥600 MPa and reduced strength anisotropy, and to provide a method for producing the same.;SOLUTION: The steel has a composition, preferably, containing, by mass, 0.02 to 0.25% C, 0.01 to 0.8% Si, 1.0 to 2.0% Mn, 0.005 to 0.1% Al, 0.0005 to 0.008% N, ≤0.02% P and ≤0.004% S, and, if required comprising one or more kinds of elements selected from Mo, Nb, V, Ti, Cu, Ni, Cr and W, and in which the X-ray random intensity ratio in the {110} face at the 1/4 position of the sheet thickness is 1.2 to 4.0 and also the X-ray random intensity ratio in the {211} face at the 1/2 position of the steel sheet is 1.2 to 4.0, and the balance Fe with inevitable impurities. After casting, without cooling to an Ar₃ transformation point or below or after reheating to an Ac₃ transformation point or above, by hot rolling at a draft of 10 to 60% in an unrecrystallized region, its sheet thickness is controlled to the prescribed one, successively, cooling is performed from an Ar₃ transformation point or above to ≤350°C at the average cooling rate of ≥2°C/s, and thereafter, tempering is performed in such a manner that the maximum arrival temperature in the central part of the sheet thickness is controlled to an Ac₁ transformation point or below.;COPYRIGHT: (C)2009,JPO&INPIT