METHOD FOR PRODUCING LARGE THICKNESS LOW YIELD RATIO HIGH-TENSILE STEEL PLATE

摘要

PROBLEM TO BE SOLVED: To provide a method for producing a low yield ratio high-tensile steel plate with which in the large thickness high tensile steel plate having ≥550 MPa tensile strength and exceeding 40 mm thickness, the low yield ratio and the high strength can be achieved in both of 1/4t and 1/2t positions with small characteristics difference in the plate thickness direction without needing Q' process.;SOLUTION: When the large thickness high tensile steel plate having ≥40 mm plate thickness containing, by mass, 0.01-0.18% C, 0.01-0.5% Si, 0.3-2.5% Mn and the balance Fe with inevitable impurities, is produced by heating a slab to 1,000-1,300°C, and accelerated-cooling with water-cooling to ≤500°C after hot-rolling, and thereafter, performing a tempering at the temperature of the Ac₁ transforming point or lower, the cooling from the surface of the steel plate is controlled so that the difference of the cooling speeds at the temperature zone of at least 700-500°C at the 1/4t position and the 1/2t position in the plate thickness direction at the accelerated-cooling time, becomes within 15%.;COPYRIGHT: (C)2011,JPO&INPIT