High-strength composite steel sheet having excellent moldability and delayed fracture resistance

Abstract

The present invention provides a high-strength composite steel sheet which has a tensile strength of 980 MPa class as well as excellent and excellent anti-delayed fraction property, and also has excellent spot-weldability. The high-strength composite steel sheet comprises a steel satisfying: C: 0.10 to 0.25% (% by mass in case of a chemical component, the same shall apply hereinafter), Si: 1.0 to 3.0%, Mn: 1.5 to 3.0%, P: 0.15% or less, S: 0.02% or less, Al: 0.4% or less, and comprising the remnant made from iron and unavoidable impurities; the contents of Si, Al, Mn and Cr satisfy the relationship of “(Si+Al)/Mn or (Si+Al)/(Mn+Cr)=0.74 to 1.26”; and microstructure is specified.

1 . A high-strength composite steel sheet having excellent formability and anti-delayed fraction property, comprising a steel satisfying:
 C: 0.10 to 0.25% (% by mass in case of a chemical component, the same shall apply hereinafter),   Si: 1.0 to 3.0%,   Mn: 1.5 to 3.0%,   P: 0.15% or less,   S: 0.02% or less,   Al: 0.4% or less, and comprising the remnant made from iron and unavoidable impurities, wherein the contents of Si, Al and Mn satisfy the relationship of the following formula (I):
   (Si+Al)/Mn: 0.74 to 1.26  (I) 
   and a microstructure in a longitudinal section comprises, by an occupancy ratio based on the entire structure,   1) bainitic ferrite: 50% or more,   2) polygonal ferrite: 5 to 35%,   3) average grain size of polygonal ferrite: 10 μm or less], and   4) residual austenite: 5% or more.   
   
   
       2 . A high-strength composite steel sheet having excellent formability and anti-delayed fraction property, comprising a steel satisfying:
 C: 0.12 to 0.25%,   Si: 1.0 to 3.0%,   Mn: 1.5 to 3.0%,   Cr: 1.0% or less,   P: 0.15% or less,   S: 0.02% or less,   Al: 0.4% or less, and comprising the remnant made from iron and unavoidable impurities, wherein the contents of Si, Al, Mn and Cr satisfy the relationship of the following formula (II):
   (Si+Al)/(Mn+Cr): 0.74 to 1.26  (II) 
   and a microstructure in a longitudinal section comprises, by an occupancy ratio based on the entire structure,   1) bainitic ferrite: 50% or more,   2) polygonal ferrite: 5 to 35%,   3) average grain size of polygonal ferrite: 10 μm or less], and   4) residual austenite: 5% or more.   
   
   
       3 . The high-strength composite steel sheet according to  claim 1  or  2 , wherein the steel further contains, as other elements,
 Ti: 0.15% or less and/or Nb: 0.1% or less.   
   
   
       4 . The high-strength composite steel sheet according to  claim 1  or  2 , wherein the steel further contains,
 Ca: 30 ppm or less and/or REM: 30 ppm or less as other elements.   
   
   
       5 . The high-strength composite steel sheet according to  claim 1  or  2 , which has a tensile strength of 980 MPa or higher.