Hot-rolled steel sheet

Abstract

A hot-rolled steel sheet has a predetermined chemical composition, a microstructure of an inner region includes 40% to 80% of one or two of martensite and bainite in total and 20% to 60% of ferrite by area ratio, an area ratio of a remainder in microstructure is less than 5%, αs/αc, which is a ratio of a ferrite area ratio αs of a surface layer region to a ferrite area ratio αc of the inner region, is in a range of 1.15 to 2.50, (1−Hvs/Hvc), which is a hardness difference ratio between a Vickers hardness Hvs of the surface layer region and a Vickers hardness Hvc of the inner region, is 0.20 or less, and a tensile strength is 980 MPa or more.

1 . A hot-rolled steel sheet comprising, as a chemical composition, by mass %:
 C: 0.02% to 0.30%;   Si: 0.10% to 2.00%;   Mn: 0.5% to 3.0%;   sol. Al: 0.10% to 1.00%;   Ti: 0.06% to 0.20%;   P: 0.1000% or less;   S: 0.0100% or less;   N: 0.0100% or less;   Nb: 0% to 0.100%;   Ca: 0% to 0.0060%;   Mo: 0% to 0.50%;   Cr: 0% to 1.00%;   V: 0% to 0.40%;   Ni: 0% to 0.40%;   Cu: 0% to 0.40%;   B: 0% to 0.0020%; and   Sn: 0% to 0.20%,   with a remainder of Fe and impurities,   a microstructure of an inner region includes 40% to 80% of one or two of martensite and bainite in total and 20% to 60% of ferrite by area ratio, and an area ratio of a remainder in microstructure is less than 5%,   αs/αc, which is a ratio of a ferrite area ratio αs of a surface layer region to a ferrite area ratio αc of the inner region, is in a range of 1.15 to 2.50,   (1−Hvs/Hvc), which is a hardness difference ratio between a Vickers hardness Hvs of the surface layer region and a Vickers hardness Hvc of the inner region, is 0.20 or less, and   a tensile strength is 980 MPa or more.   
     
     
         2 . The hot-rolled steel sheet according to  claim 1 ,
 wherein the chemical composition comprises, by mass %, one or more of   Nb: 0.010% to 0.100%,   Ca: 0.0005% to 0.0060%,   Mo: 0.02% to 0.50%,   Cr: 0.02% to 1.00%,   V: 0.01% to 0.40%,   Ni: 0.01% to 0.40%,   Cu: 0.01% to 0.40%,   B: 0.0001% to 0.0020%, and   Sn: 0.01% to 0.20%.