Patent ID: 8840834

Claim:
A high strength steel sheet comprising a composition including, on a mass basis: C: 0.12% or more and 0.50% or less; Si: 2.0% or less; Mn: 1.0% or more and 5.0% or less; P: 0.1% or less; S: 0.07% or less; Al: 1.0% or less; and N: 0.008% or less, with the balance Fe and incidental impurities, with a steel microstructure including on an area ratio basis, 80% or more of autotempered martensite, less than 5% of ferrite, 10% or less of bainite, and 5% or less of retained austenite; and the mean number of precipitated iron-based carbide grains each having a size of 5 nm or more and 0.5 μm or less and included in the autotempered martensite is 7 ×10 4 or more per 1mm 2 and a TS of 1400 MPa or more and a maximum hole-expanding ratio greater than or equal to 15%, wherein autotempered martensite microstructure includes a portion that contains a larger number of iron-based carbide grains having a size of 0.1 μm to 0.5 μm and a portion that contains a smaller number of iron-based carbide grains having a size of 0.1 μm 0.5 μm in a mixed manner such that the area ratio of autotempered martensite in which the number of precipitated iron-based carbide grains each having a size of 0.1 μm or more and 0.5 μm or less is 5 ×10 2 or less per 1 mm 2 to the entire autotempered martensite is 3% or more.