Hot-rolled steel sheet

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, in which, when a region of ⅛ to ⅜ of a sheet thickness in a sheet thickness direction from a surface is defined as a ¼ depth position, a microstructure at the ¼ depth position includes, by area fraction, retained austenite: less than 3.0%, ferrite: less than 30.0%, and pearlite: less than 5.0%, at the ¼ depth position, an average number density of Ti-based carbides having a longest diameter of 15 nm or more is 1.0×10 4 /mm 2 or more, and an average grain size dq is 15.0 μm 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: 0.050% to 0.200%;   Si: 0.005% to 2.000%;   Mn: 0.50% to 4.00%;   P: 0.100% or less;   S: 0.0100% or less;   sol. Al: 0.001% to 1.00%;   Ti: 0.150 to 0.400%;   N: 0.0010% to 0.0200%;   Nb: 0% to 0.200%;   V: 0% to 1.000%;   Mo: 0% to 1.000%;   Cu: 0% to 1.00%;   Ni: 0% to 1.00%;   Cr: 0% to 2.00%;   W: 0% to 1.00%;   B: 0% to 0.0040%;   Ca: 0% to 0.0100%;   Mg: 0% to 0.0100%;   REM: 0% to 0.0100%;   Bi: 0% to 0.0200%; and   a remainder: Fe and impurities,   wherein, when a region of ⅛ to ⅜ of a sheet thickness in a sheet thickness direction from a surface is defined as a ¼ depth position, a microstructure at the ¼ depth position includes, by area fraction,
 retained austenite: less than 3.0%, 
 ferrite: less than 30.0%, and 
 pearlite: less than 5.0%, 
   at the ¼ depth position,
 an average number density of Ti-based carbides having a longest diameter of 15 nm or more is 1.0×10 4 /mm 2  or more, and 
 an average grain size dq is 15.0 μm or less, and 
   a tensile strength is 980 MPa or more.   
     
     
         2 . The hot-rolled steel sheet according to  claim 1 ,
 wherein, when a region from the surface to 50 μm from the surface in the sheet thickness direction is defined as a surface layer portion, ds/dq, which is a ratio of an average grain size ds of the surface layer portion to the average grain size dq at the ¼ depth position, is 0.95 or less.   
     
     
         3 . The hot-rolled steel sheet according to  claim 1 , wherein the chemical composition contains, by mass %, one or more of
 Nb: 0.001% to 0.200%,   V: 0.005% to 1.000%,   Mo: 0.001% to 1.000%,   Cu: 0.02% to 1.00%,   Ni: 0.02% to 1.00%,   Cr: 0.02% to 2.00%,   W: 0.020% to 1.00%,   B: 0.0001% to 0.0040%,   Ca: 0.0002% to 0.0100%,   Mg: 0.0002% to 0.0100%,   REM: 0.0002% to 0.0100%, and   Bi: 0.0002% to 0.0200%.   
     
     
         4 . The hot-rolled steel sheet according to  claim 2 , wherein the chemical composition contains, by mass %, one or more of
 Nb: 0.001% to 0.200%,   V: 0.005% to 1.000%,   Mo: 0.001% to 1.000%,   Cu: 0.02% to 1.00%,   Ni: 0.02% to 1.00%,   Cr: 0.02% to 2.00%,   W: 0.020% to 1.00%,   B: 0.0001% to 0.0040%,   Ca: 0.0002% to 0.0100%,   Mg: 0.0002% to 0.0100%,   REM: 0.0002% to 0.0100%, and   Bi: 0.0002% to 0.0200%.