Steel sheet and method for manufacturing steel sheet

Abstract

This steel sheet includes, in mass % C: 0.15% to 0.50%, Si: 0.01% to 1.00%, Mn: 1.00% to 3.00%, P: 0% to 0.0200%, S: 0.0001% to 0.0200%, Al: 0.001% to 0.100%, and N: 0% to 0.0200%, with the remainder being Fe and impurities, in which a metallorgraphic structure has an area fraction of 0% to 10.0% of retained austenite and 0% to 5.0% of pearlite, ferrite, and bainite in total, with the remaining structure being martensite and tempered martensite, a maximum diameter of MnS predicted by extreme value statistics is 30 μm or less, a surface roughness Ra is 5 μm or less, and a surface layer has a Vickers hardness of greater than or equal to a tensile strength TS (MPa) of the steel sheet×0.25.

1 . A steel sheet having a component composition including, in mass %,
 C: 0.15% to 0.50%,   Si: 0.01% to 1.00%,   Mn: 1.00% to 3.00%,   P: 0% to 0.0200%,   S: 0.0001% to 0.0200%,   Al: 0.001% to 0.100%,   N: 0% to 0.0200%,   Co: 0% to 0.500%,   Ni: 0% to 1.000%,   Mo: 0% to 1.000%,   Cr: 0% to 2.000%,   O: 0% to 0.020%,   Ti: 0% to 0.5000%,   B: 0% to 0.0100%,   Nb: 0% to 0.500%,   V: 0% to 0.500%,   Cu: 0% to 0.500%,   W: 0% to 0.1000%,   Ta: 0% to 0.1000%,   Sn: 0% to 0.0500%,   Sb: 0% to 0.0500%,   As: 0% to 0.0500%,   Mg: 0% to 0.0500%,   Ca: 0% to 0.0500%,   Zr: 0% to 0.0500%, and   REM: 0% to 0.1000%,   with the remainder being Fe and impurities,   wherein a metallographic structure has an area fraction of   0% to 10.0% of retained austenite and   0% to 5.0% of pearlite, ferrite, and bainite in total, with the remaining structure being martensite and tempered martensite,   wherein a maximum diameter of MnS predicted by extreme value statistics is 30.0 μm or less,   wherein a surface roughness Ra is 5.0 μm or less, and   wherein a surface layer has a Vickers hardness of greater than or equal to a tensile strength TS (MPa) of the steel sheet×0.25.   
     
     
         2 . The steel sheet according to  claim 1 ,
 wherein the component composition includes, in mass %, one or more of   Co: 0.010% to 0.500%,   Ni: 0.010% to 1.000%,   Mo: 0.010% to 1.000%,   Cr: 0.001% to 2.000%,   O: 0.0001% to 0.020%,   Ti: 0.0010% to 0.5000%,   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: 0.0010% to 0.1000%,   Ta: 0.0010% to 0.1000%,   Sn: 0.0010% to 0.0500%,   Sb: 0.0010% to 0.0500%,   As: 0.0010% to 0.0500%,   Mg: 0.0001% to 0.0500%,   Ca: 0.0010% to 0.0500%,   Zr: 0.0010% to 0.0500%, and   REM: 0.0010% to 0.1000%.   
     
     
         3 . The steel sheet according to  claim 1 ,
 wherein the component composition includes, in mass %,   Mn: 1.00% to 2.00% and   Si: 0.30% to 1.00%.   
     
     
         4 . The steel sheet according to  claim 1 , wherein a tensile strength is 1,470 MPa or more. 
     
     
         5 . The steel sheet according to  claim 1 ,
 wherein a coating film layer containing at least one of zinc, aluminum, magnesium, and their alloys is provided on a single surface or both surfaces of the steel sheet.   
     
     
         6 . A method for manufacturing the steel sheet according to  claim 1 , comprising:
 a refining process in which molten steel subjected to a vacuum degassing treatment to adjust a component composition of the molten steel to have an Al concentration of 0.0500 mass % or less and to include, in mass %,   C: 0.15% to 0.50%,   Si: 0.01% to 1.00%,   Mn: 1.00% to 3.00%,   P: 0% to 0.0200%,   S: 0.0001% to 0.0200%,   N: 0% to 0.0200%,   Co: 0% to 0.500%,   Ni: 0% to 1.000%,   Mo: 0% to 1.000%,   Cr: 0% to 2.000%,   O: 0% to 0.020%,   Ti: 0% to 0.5000%,   B: 0% to 0.0100%,   Nb: 0% to 0.500%,   V: 0% to 0.500%,   Cu: 0% to 0.500%,   W: 0% to 0.1000%,   Ta: 0% to 0.1000%,   Sn: 0% to 0.0500%,   Sb: 0% to 0.0500%,   As: 0% to 0.0500%,   Mg: 0% to 0.0500%,   Ca: 0% to 0.0500%,   Zr: 0% to 0.0500%, and   REM: 0% to 0.1000%,   with the remainder being Fe and impurities;   a casting process in which a slab is manufactured using the molten steel after the refining process;   a hot rolling process in which slab is heated directly or after temporary cooling and hot-rolled to obtain a hot-rolled steel sheet;   a coiling process in which hot-rolled steel sheet is coiled in a temperature range of 700° C. or lower;   a pickling process in which the hot-rolled steel sheet after the coiling process is pickled;   a cold rolling process in which the hot-rolled steel sheet after the pickling process is cold-rolled at a rolling reduction of 30 to 90% to obtain a cold-rolled steel sheet; and   an annealing process in which the cold-rolled steel sheet is annealed in an atmosphere with a dew point of −80° C. and −15° C. in a temperature range of 820° C. to 900° C.   
     
     
         7 . The method for manufacturing a steel sheet according to  claim 6 ,
 wherein, in the refining process, a deoxidation time is shorter than 5 minutes.   
     
     
         8 . The method for manufacturing a steel sheet according to  claim 6 ,
 wherein the component composition includes, in mass %,   
       Mn: 1.00% to 2.00% and 
       Si: 0.30% to 1.00%. 
     
     
         9 . The method for manufacturing a steel sheet according to  claim 6 , comprising:
 a coating film layer forming process in which a coating film layer containing at least one of zinc, aluminum, magnesium, and their alloys is formed on a single surface or both surfaces of the cold-rolled steel sheet in the annealing process.   
     
     
         10 . The steel sheet according to  claim 2 , wherein a tensile strength is 1,470 MPa or more. 
     
     
         11 . The steel sheet according to  claim 2 ,
 wherein a coating film layer containing at least one of zinc, aluminum, magnesium, and their alloys is provided on a single surface or both surfaces of the steel sheet.   
     
     
         12 . The method for manufacturing a steel sheet according to  claim 7 , comprising:
 a coating film layer forming process in which a coating film layer containing at least one of zinc, aluminum, magnesium, and their alloys is formed on a single surface or both surfaces of the cold-rolled steel sheet in the annealing process.