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

摘要

The purpose of the present invention is to provide: a hot-pressed member having tensile strength of 1780 MPa or more and improved in delayed fracture resistance properties; a method for producing the hot-pressed member; a cold-rolled steel sheet for a hot-pressed member; and a method for producing the cold-rolled steel sheet. A hot-pressed member having a specified component composition, wherein the average crystal grain diameter of prior austenite is 8 μm or less and the volume fraction of martensite in a region extending from the surface to the depth of 30 μm or less is 95% or more in the micro structure of a steel sheet, there is an Ni-diffused region having a thickness of 0.5 μm or more in a surface layer, the standard deviation of Vickers hardness is 35 or less, the Mndif (mass%) value in the thickness direction is 0.20 or less wherein Mndif (mass%) represents an Mn segregation degree, and the tensile strength is 1780 MPa or more.