MANUFACTURING METHOD OF LARGE THICKNESS LOW YIELD RATIO HIGH-TENSILE STEEL PLATE

摘要

PROBLEM TO BE SOLVED: To provide a method for manufacturing a large thickness low yield ratio high-tensile steel plate having a thickness exceeding 40 mm and a tensile strength of ≥ 550 MPa in which the low yield ratio and the high strength can be achieved at both 1/4 t position and the 1/2 t position without any Q' process and with small characteristic difference in the thickness direction.;SOLUTION: When manufacturing a large thickness high-tensile steel plate of a thickness of ≥ 40 mm by heating a slab to 1,000-1,300°C, to be hot-rolled, and then being subjected to accelerated cooling with water cooling to a temperature of ≤ 500°C, and then being tempered at a temperature equal to or lower than the Ac₁ transformation point, the cooling from the surface of the steel plate is suppressed so that the difference in the cooling rate in the temperature zone of at least 700-500°C at the 1/4 t position and the 1/2 t position in the thickness direction during the accelerated cooling is within 15%.;COPYRIGHT: (C)2006,JPO&NCIPI