Patent ID: 7919194
Filing Date: 2011-04-05
Classification: C21D,C22C,C23C,Y10T

Abstract:
1. A high strength steel sheet comprising: a component composition which includes 0.05 to 0.20 mass percent of C, 0.5 mass percent or less of Si, 1.5 to 3.0 mass percent of Mn, 0.06 mass percent or less of P, 0.01 mass percent or less of S, 0.3 to 1.5 mass percent of Al, 0.02 mass percent or less of N, 0.01 to 0.1 mass percent of Ti, and 0.0005 to 0.0030 mass percent of B; 0.4 to 1.5 mass percent of Cr; and the balance being Fe and inevitable impurities, wherein the high strength steel sheet is composed of a microstructure including 20% to 70% ferrite and 20% or more of martensite in volume fraction, has a tensile strength of 950 MPa or more, and has a strength-ductility balance of more than 18,000 (MPa. %); and wherein said high strength steel sheet is manufactured by a process comprising hot-rolling a slab comprising said component composition; cold-rolling the resulting hot-rolled sheet; annealing the resulting cold-rolled sheet at a temperature of 780 to 900° C for 300 seconds or less; and cooling the sheet to a temperature of 500° C or less at an average cooling rate of 7 to 30° C/second.