HOT-STEELED STEEL SHEET FOR ROLLED PREPARATION OF VARIABLE THICKNESS, ROLLED PREPARATION OF VARIABLE THICKNESS AND METHODS FOR THEIR PRODUCTION

摘要

A heat-rolled steel plate for a tailored rolled blank is provided that has high tensile strength and is excellent in cold formability. The heat-rolled steel plate has: a chemical composition that contains, in mass%, C, Si, Mn, P, S, Al, N and Ti, with the balance being Fe and impurities, and that satisfies Formula (1); and a microstructure containing, in terms of area ratio, 20% or more of bainite, wherein 50% or more in terms of area ratio of the balance is ferrite. In the interior of the heat-rolled steel plate an average value of pole densities of an orientation group {100} 011 to {223} 110 is 4 or less, and a pole density of a {332} 113 crystal orientation is 4.8 or less. In an outer layer of the heat-rolled steel plate, a pole density of a {110} 001 crystal orientation is 2.5 or more. Furthermore, among Ti carbo-nitrides in the heat-rolled steel plate, the number density of fine Ti carbo-nitrides having a particle diameter of 10 nm or less is 1.0×10 17 per cm 3 or less, and a bake hardening amount is 15 MPa or more. Ti ˆ’ 48 / 14 × N ˆ’ 48 / 32 × S ‰¥ 0