Steel sheet

Abstract

What is provided is a steel sheet having a predetermined chemical composition, in which a microstructure includes, by area ratio, ferrite: 5.0% or less, martensite and tempered martensite: more than 90.0% in total, and a remainder: one or two or more of bainite, pearlite, and residual austenite, in a cross section in a sheet thickness direction, regions having an Mn content of 1.1×[Mnave] or more, where the [Mnave] is an average Mn content throughout the sheet thickness direction, have a number density of 5.0×10 −4 pieces/μm 2 or more and are present so that the average interval between closest regions having an Mn content of 1.1×[Mnave] or more is 10.0 μm or less, and a tensile strength is 1,500 MPa or more.

1 . A steel sheet comprising, as a chemical composition, by mass %:
 C: 0.150% to 0.400%;   Si: 0.01% to 2.00%;   Mn: 0.8% to 2.0%;   P: 0.0001% to 0.0200%;   S: 0.0001% to 0.0200%;   Al: 0.001% to 1.000%;   N: 0.0001% to 0.0200%;   O: 0.0001% to 0.0200%;   Co: 0% to 0.500%;   Ni: 0% to 1.000%;   Mo: 0% to 1.000%;   Cr: 0% to 2.000%;   Ti: 0% to 0.500%;   B: 0% to 0.0100%;   Nb: 0% to 0.500%;   V: 0% to 0.500%;   Cu: 0% to 0.500%;   W: 0% to 0.100%;   Ta: 0% to 0.100%;   Mg: 0% to 0.050%;   Ca: 0% to 0.050%;   Y: 0% to 0.050%;   Zr: 0% to 0.050%;   La: 0% to 0.050%;   Ce: 0% to 0.050%;   Sn: 0% to 0.050%;   Sb: 0% to 0.050%;   As: 0% to 0.050%; and   a remainder: Fe and impurities,   wherein a microstructure includes, by area ratio,
 ferrite: 5.0% or less, 
 martensite and tempered martensite: more than 90.0% in total, and 
 a remainder: one or two or more of bainite, pearlite, and residual austenite, 
   in a cross section in a sheet thickness direction, regions having an Mn content of 1.1×[Mnave] or more, where the [Mnave] is an average Mn content throughout the sheet thickness direction,
 have a number density of 5.0×10 −4  pieces/μm 2  or more and 
 are present so that an average interval between closest regions having an Mn content of 1.1×[Mnave] or more is 10.0 μm or less, and 
   a tensile strength is 1,500 MPa or more.   
     
     
         2 . The steel sheet according to  claim 1 ,
 wherein the chemical composition comprises one or more of   Co: 0.01% to 0.500%,   Ni: 0.01% to 1.000%,   Mo: 0.01% to 1.000%,   Cr: 0.001% to 2.000%,   Ti: 0.001% to 0.500%,   B: 0.0001% to 0.0100%,   Nb: 0.001% to 0.500%,   V: 0.001% to 0.500%,   Cu: 0.001% to 0.500%,   W: 0.001% to 0.100%,   Ta: 0.001% to 0.100%,   Mg: 0.0001% to 0.050%,   Ca: 0.001% to 0.050%,   Y: 0.001% to 0.050%,   Zr: 0.001% to 0.050%,   La: 0.001% to 0.050%,   Ce: 0.001% to 0.050%,   Sn: 0.001% to 0.050%,   Sb: 0.001% to 0.050%, and   As: 0.001% to 0.050%.   
     
     
         3 . The steel sheet according to  claim 1 ,
 wherein a coating layer containing zinc, aluminum, magnesium, or an alloy of these metals is provided on a surface.   
     
     
         4 . The steel sheet according to  claim 2 ,
 wherein a coating layer containing zinc, aluminum, magnesium, or an alloy of these metals is provided on a surface.