High strength steel sheet

Abstract

A high strength steel sheet according to the present invention contains a predetermined chemical composition, a metallographic structure includes, by an area ratio, ferrite: 20% to 70%, residual austenite: 5% to 40%, fresh martensite: 0% to 30%, tempered martensite and bainite: 20% to 75% in total, and pearlite and cementite: 0% to 10% in total, in a range of a ⅛ thickness to a ⅜ thickness from a surface, a number proportion of residual austenite having an aspect ratio of 2.0 or more with respect to the number of all residual austenite is 50% or more, at a sheet thickness ¼ position of a cross section parallel to a rolling direction and perpendicular to the surface, a standard deviation of area ratios of ferrite measured at 10 points every 50 mm along a width direction is less than 10%, and a tensile strength is 780 MPa or more.

1 . A high strength steel sheet comprising, as a chemical composition, by mass %:
 C: 0.030% to 0.280%;   Si: 0.50% to 2.50%;   Mn: 1.00% to 4.00%;   sol. Al: 0.001% to 2.000%;   P: 0.100% or less;   S: 0.0200% or less;   N: 0.01000% or less;   O: 0.0100% or less;   B: 0% to 0.010%;   Ti: 0% to 0.20%;   Nb: 0% to 0.20%;   V: 0% to 1.000%;   Cr: 0% to 1.000%;   Mo: 0% to 1.000%;   Cu: 0% to 1.000%;   Co: 0% to 1.000%;   W: 0% to 1.000%;   Ni: 0% to 1.000%;   Ca: 0% to 0.0100%;   Mg: 0% to 0.0100%;   REM: 0% to 0.0100%;   Zr: 0% to 0.0100%; and   a remainder including Fe and impurities,   wherein a metallographic structure includes, by an area ratio,
 ferrite: 20% to 70%, 
 residual austenite: 5% to 40%, 
 fresh martensite: 0% to 30%, 
 tempered martensite and bainite: 20% to 75% in total, and 
 pearlite and cementite: 0% to 10% in total, 
   in a range of a ⅛ thickness to a ⅜ thickness from a surface, a number proportion of residual austenite having an aspect ratio of 2.0 or more with respect to the number of all residual austenite is 50% or more,   at a sheet thickness ¼ position of a cross section parallel to a rolling direction and perpendicular to the surface, a standard deviation of area ratios of ferrite measured at 10 points every 50 mm along a width direction is less than 10%, and   a tensile strength is 780 MPa or more.   
     
     
         2 . The high strength steel sheet according to  claim 1 ,
 wherein, at 10 positions at intervals of 50 mm in the width direction, a standard deviation of surface roughness Ra is 0.5 m or less.   
     
     
         3 . The high strength steel sheet according to  claim 1 , comprising, as the chemical composition, by mass %, at least one from the group of:
 B: 0.001% to 0.010%;   Ti: 0.01% to 0.20%;   Nb: 0.01% to 0.20%;   V: 0.005% to 1.000%;   Cr: 0.005% to 1.000%;   Mo: 0.005% to 1.000%;   Cu: 0.005% to 1.000%;   Co: 0.005% to 1.000%;   W: 0.005% to 1.000%;   Ni: 0.005% to 1.000%;   Ca: 0.0003% to 0.0100%;   Mg: 0.0003% to 0.0100%;   REM: 0.0003% to 0.0100%; and   Zr: 0.0003% to 0.0100%.   
     
     
         4 . The high strength steel sheet according to  claim 2 , comprising, as the chemical composition, by mass %, at least one from the group of:
 B: 0.001% to 0.010%;   Ti: 0.01% to 0.20%;   Nb: 0.01% to 0.20%;   V: 0.005% to 1.000%;   Cr: 0.005% to 1.000%;   Mo: 0.005% to 1.000%;   Cu: 0.005% to 1.000%;   Co: 0.005% to 1.000%;   W: 0.005% to 1.000%;   Ni: 0.005% to 1.000%;   Ca: 0.0003% to 0.0100%;   Mg: 0.0003% to 0.0100%;   REM: 0.0003% to 0.0100%; and   Zr: 0.0003% to 0.0100%.