Patent ID: 7648597
Filing Date: 2010-01-19
Classification: C21D,C22C

Abstract:
1. A method for manufacturing high tensile strength steel plate comprising the steps of: casting a steel consisting essentially of 0.02 to 0.18% C, 0.05 to 0.5% Si, 0.5 to 2.0% Mn, 0.005 to 0.1% Al, 0.0005 to 0.008% N, 0.03% or less P, 0.03% or less S, by mass, and balance of Fe and inevitable impurities; hot-rolling the cast steel without cooling the steel to the Ar 3 transformation point or lower temperature, or after reheating the steel to the Ac 3 transformation point or higher temperature, to a specified plate thickness; cooling the steel by direct quenching from the Ar 3 transformation point or higher temperature, or by accelerated cooling, to 400° C. or lower temperature; and then tempering the steel, using a heating apparatus being installed directly connecting the manufacturing line containing a rolling mill and a direct-quenching apparatus or an accelerated cooling apparatus, to 520° C. or above of the maximum ultimate temperature at the plate thickness center portion at an average temperature-rising rate of smaller than 1° C./s at the plate thickness center portion between the tempering-start temperature and 460° C., and at an average temperature-rising rate of 1° C./s or larger at the plate thickness center portion up to a specified tempering temperature between 460° C. and the Ac 1 transformation point.