Patent ID: 9139885
Filing Date: 2015-09-22
Classification: C21D,C22C,C23C,C25D

Abstract:
1. A high-strength steel sheet comprising 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% of N; 0.0001 to 0.008% of 0; and a remainder composed of iron and inevitable impurities, wherein a steel sheet structure contains by volume fraction 10 to 45% of a ferrite phase, 10 to 50% of a tempered martensite phase, and a remaining hard phase which is selected from the group consisting of a bainitic ferrite phase, a bainite phase, a fresh martensite phase, and a mixture thereof, wherein when a plurality of measurement regions with diameters of 1 μm or less are set in a range from ⅛ to ⅜ of a thickness of the steel sheet, hardness measurement values in the plurality of measurement regions are arranged in ascending order to obtain a hardness distribution, an integer N0.02 which is a number obtained by multiplying a total number of the hardness measurement values by 0.02 and, if present, by rounding up a decimal number, is obtained, a hardness of a measurement value which is an N0.02-th largest value from a smallest hardness measurement value is regarded as a 2% hardness, an integer N0.98 which is a number obtained by multiplying the total number of the hardness measurement values by 0.98 and, if present, by rounding down the decimal number is obtained, and a hardness of a measurement value which is an N0.98-th largest value from the smallest hardness measurement value is regarded as a 98% hardness, the 98% hardness is 1.5 or more times as high as the 2% hardness, wherein a kurtosis K* of the hardness distribution between the 2% hardness and the 98% hardness is equal to or more than −1.2 and equal to or less than −0.4, wherein an average crystal grain size in the steel sheet structure is 10 μm or less, wherein a tensile strength is 900 MPa or more, wherein a difference between a maximum value and a minimum value of Mn concentration in a base iron in a thickness range from ⅛ to ⅜ of the steel sheet is equal to or more than 0.4% and equal to or less than 3.5% when converted into the mass percentage, and wherein the hard phase includes a fresh martensite phase of 10% or less.