HIGH-STRENGTH COLD-ROLLED STEEL SHEET WITH EXCELLENT DUCTILITY AND STRETCH FLANGEABILITY, HIGH-STRENGTH GALVANIZED STEEL SHEET, AND METHOD FOR PRODUCING BOTH

摘要

This high-strength steel sheet includes by mass percentage: 0.05 to 0.4% of C; 0.1 to 2.5% of Si; 1.0 to 3.5% of Mn; 0.001 to 0.03% of P; 0.0001 to 0.01% of S; 0.001 to 2.5% of Al; 0.0001 to 0.01% ofN; 0.0001 to 0.008% of O; and a remainder composed of iron and inevitable impurities, wherein a steel sheet structure contains by volume fraction 10 to 50% of a ferrite phase, 10 to 50% of a tempered martensite phase, and a remaining hard phase, wherein a 98% hardness is 1.5 or more times as high as a 2% hardness in a range from 1/8 to 3/8 of a thickness of the steel sheet, wherein a kurtosis K* of the hardness distribution between the 2% hardness and the 98% hardness is -1.2 to -0.4, and wherein an average crystal grain size in the steel sheet structure is 10µm or less.