High-strength cold-rolled steel sheet excellent in coating film adhesion

Abstract

A cold-rolled steel sheet of DP (Dual Phase) type with a specific composition meets the requirements: (I) In the surface of the steel sheet, there exist Si—Mn complex oxides no larger than 5 μm in diameter of the equivalent circle as many as 10 or more per 100 μm 2 and the coverage of oxides composed mainly of Si on the surface of steel sheet is no more than 10% of surface area, and/or (II) The cross section near the surface of the steel sheet does not show cracks with a width no larger than 3 μm and a depth no smaller than 5 μm in arbitrary ten fields of observation under an SEM with a magnification of 2000. A high-strength cold-rolled steel sheet excellent in coating film adhesion and having a tensile strength no lower than 550 MPa is provided.

1 . A high-strength cold-rolled steel sheet excellent in coating film adhesion, which is a DP (Dual Phase) steel sheet of ferrite-tempered martensite type containing no more than 1 mass % of C (excluding 0 mass %), 0.05 to 2 mass % of Si, and 1 to 5 mass % of Mn, having a tensile strength no lower than 550 MPa, satisfying the equation (1) below, and being characterized by its surface in which there exist Si—Mn complex oxides no larger than 5 μm in diameter of the equivalent circle as many as 10 or more per 100 μm 2  and the coverage of oxides composed mainly of Si on the surface of steel sheet is no more than 10% of surface area.  
         [Si]/[Mn]≦0.4  (1)  
       where [Si] denotes an Si content (in mass %) and [Mn] denotes an Mn content (in mass %).  
     
     
         2 . A high-strength cold-rolled steel sheet excellent in coating film adhesion, which is a DP (Dual Phase) steel sheet of ferrite-tempered martensite type containing no more than 1 mass % of C (excluding 0 mass %), no more than 2 mass % of Si (excluding 0 mass %), and 1 to 5 mass % of Mn, having a tensile strength no lower than 550 MPa, and being characterized by its surface whose cross section does not show cracks with a width no larger than 3 μm and a depth no smaller than 5 μm in arbitrary ten fields of observation under an SEM with a magnification of 2000.  
     
     
         3 . A high-strength cold-rolled steel sheet excellent in coating film adhesion, which is a DP (Dual Phase) steel sheet of ferrite-tempered martensite type containing no more than 1 mass % of C (excluding 0 mass %), 0.05 to 2 mass % of Si, and 1 to 5 mass % of Mn, having a tensile strength no lower than 550 MPa, satisfying the equation (1) below, and meeting the following requirements (I) and (II). 
 (I) In the surface of the steel sheet, there exist Si—Mn complex oxides no larger than 5 μm in diameter of the equivalent circle as many as 10 or more per 100 μm 2  and the coverage of oxides composed mainly of Si on the surface of steel sheet is no more than 10% of surface area.    (II) The cross section near the surface of the steel sheet does not show cracks with a width no larger than 3 μm and a depth no smaller than 5 μm in arbitrary ten fields of observation under an SEM with a magnification of 2000.      [Si]/[Mn]≦0.4  (1)    where [Si] denotes an Si content (in mass %) and [Mn] denotes an Mn content (in mass %).    
     
     
         4 . The high-strength steel sheets as defined in  claim 1 , which satisfies the equations (2) and (3) below.  
         [P]+3[S]+1.54[C]<0.25  (2)  [C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34  (3)  
       where [C], [Si], [Mn], [P], and [S] denote the content (in mass %) of these elements.  
     
     
         5 . The high-strength steel sheets as defined in  claim 2 , which satisfies the equations (2) and (3) below.  
         [P]+3[S]+1.54[C]<0.25  (2)  [C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34  (3)  
       where [C], [Si], [Mn], [P], and [S] denote the content (in mass %) of these elements.  
     
     
         6 . The high-strength steel sheets as defined in  claim 3 , which satisfies the equations (2) and (3) below.  
         [P]+3[S]+1.54[C]<0.25  (2)  [C]+[Si]/30+[Mn]/20+2[P]+4[S]<0.34  (3)  
       where [C], [Si], [Mn], [P], and [S] denote the content (in mass %) of these elements.  
     
     
         7 . The high-strength steel sheets as defined in  claim 1 , wherein the coverage of oxides composed mainly of Si on the surface of steel sheet is no more than 5% of surface area.  
     
     
         8 . The high-strength steel sheets as defined in  claim 3 , wherein the coverage of oxides composed mainly of Si on the surface of steel sheet is no more than 5% of surface area.