Steel sheet

Abstract

This steel sheet has a predetermined chemical composition, Ex. C that is obtained by Ex. C = (%C) - 12 ( (%Ti*)/48 + (%V)/51 + (%Nb)/93 + (%Mo)/96 + (%W)/184} is 0.020% or less, a microstructure at a ¼ depth position of a sheet thickness from a surface contains 60% or more of ferrite, 0% to 5% of MA and a total of 0% to 5% of pearlite and cementite with a remainder of bainite in terms of area fractions, in the microstructure, the average crystal grain diameter is 10.0 µm or less, the average aspect ratio of crystal grains is 0.30 or more, the standard deviation of a Mn concentration is 0.60 mass% or less, a Ti-based carbide having a Baker-Nutting orientation relationship in the ferrite is precipitated in a semi-coherent state, and a tensile strength is 980 MPa or more.

1 - 5 . (canceled) 
     
     
         6 . A steel sheet comprising, as a chemical composition, by mass%:
 C: 0.050% to 0.250%;   Si: 0.005% to 2.000%;   Mn: 0.10% to 3.00%;   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 0.0100%;   Nb: 0% to 0.100%;   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 1.000%;   B: 0% to 0.0020%;   Ca: 0% to 0.0100%;   Mg: 0% to 0.0100%;   REM: 0% to 0.0100%; and   Bi: 0% to 0.0200%   with a remainder of Fe and impurities,   wherein Ex. C obtained by the following formula (1) is 0.020% or less,   a microstructure at a ¼ depth position of a sheet thickness from a surface contains 60% or more of ferrite, 0% to 5% of MA and a total of 0% to 5% of pearlite and cementite with a remainder of bainite in terms of area fractions,   in the microstructure,   an average crystal grain diameter is 10.0 µm or less,   an average aspect ratio of crystal grains is 0.30 or more,   a standard deviation of a Mn concentration is 0.60 mass% or less,   a Ti-based carbide having a Baker-Nutting orientation relationship in the ferrite is precipitated in a semi-coherent state, and   a tensile strength is 980 MPa or more,   Ex. C = (%C) - 12{(%Ti*)/48 + (%V)/51 + (%Nb)/93 + (%Mo)/96 + (%W)/184} Formula (1)   here, “%Ti*” in the formula (1) is obtained from the following formula (2),   %Ti* = %Ti - 48 × {(%N)/14 + (%S)/32) Formula (2)   %C, %V, %Nb, %Mo, %W, %Ti, %N and %S in the formula (1) and the formula (2) are the amounts of C, V, Nb, Mo, W, Ti, N and S in the steel sheet by mass%.   
     
     
         7 . The steel sheet according to  claim 6 , comprising, as the chemical composition, by mass%, one or more selected from:
 Nb: 0.001% to 0.100%;   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: 0.02% to 1.000%;   B: 0.0001% to 0.0020%;   Ca: 0.0002% to 0.0100%;   Mg: 0.0002% to 0.0100%;   REM: 0.0002% to 0.0100%; and   Bi: 0.0001% to 0.0200%.   
     
     
         8 . The steel sheet according to  claim 6 ,
 wherein a plating layer is formed on a surface.   
     
     
         9 . The steel sheet according to  claim 7 ,
 wherein a plating layer is formed on a surface.   
     
     
         10 . The steel sheet according to  claim 8 ,
 wherein the plating layer is a hot-dip galvanized layer.   
     
     
         11 . The steel sheet according to  claim 9 ,
 wherein the plating layer is a hot-dip galvanized layer.   
     
     
         12 . The steel sheet according to  claim 10 ,
 wherein the hot-dip galvanized layer is a hot-dip galvannealed layer.   
     
     
         13 . The steel sheet according to  claim 11 ,
 wherein the hot-dip galvanized layer is a hot-dip galvannealed layer.