| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot molded steel sheet member with a multiple phase structure having excellent ductility and a tensile strength of 980 MPa or higher, and in which surface decarburization is suppressed, and a production method capable of mass-producing the same.SOLUTION: Disclosed is a hot molded steel sheet having a chemical composition comprising 0.10 to 0.34% C, 0.5 to 2.0% Si, 1.0 to 3.0% Mn, 0.05% or less of P, 0.01% or less of S, 0.001 to 1.0% sol.Al and 0.01% or less of N, and the balance Fe with impurities, and in which, in the internal layer part satisfying (the area ratio of ferrite in the surface layer part)/(the area ratio of ferrite in the internal layer part)≤1.20), the content of the ferrite is 10 to 70%, the content of martensite is 30 to 90% or less and the total area ratio of the ferrite and martensite is controlled to 90% or more. Regarding its production, a slab is heated to 720°C to an Apoint to control the concentration of Mn in an austenitic phase to 1.20 times or higher of the concentration of Mn in a ferritic phase, and is subjected to hot forming in which the time when it is exposed to air-cooling from the completion of the heating to the start of the hot molding is controlled to below 15 s, and is cooled to a temperature range of an Ms point or less at the average cooling velocity of 10 to 500°C/s. |
