HOT-PRESSED MEMBER, COLD-ROLLED STEEL SHEET FOR HOT-PRESSED MEMBER, AND METHODS RESPECTIVELY FOR PRODUCING THESE PRODUCTS

摘要

Objects are to provide a hot-pressed member and a method for producing the hot-pressed member, the hot-pressed member having a tensile strength of 1780 MPa or greater and improved delayed fracture resistance, and to provide a cold-rolled steel sheet for a hot-pressed member and a method for producing the cold-rolled steel sheet.;A hot-pressed member has a predetermined chemical composition. In the hot-pressed member, a steel sheet has a microstructure in which a prior austenite average grain diameter is 8 µm or less, and martensite is present in a volume fraction of 95% or greater in a region within 30 µm of a surface; a Ni diffusion region having a thickness of 0.5 µm or greater exists in a surface layer; a standard deviation of Vickers hardness values is 35 or less; Mndif (mass%) in a sheet thickness direction ≤ 0.20, where Mndif (mass%) is a degree of Mn segregation; and a tensile strength is 1780 MPa or greater.