High-strength steel sheet for warm forming and process for producing same

Abstract

Provided is a high-strength steel sheet having good warm press formability and excellent strength and ductility after warm press forming, and a method for manufacturing such. The high-strength steel sheet has a tensile strength at room temperature not less than 780 MPa, a yield stress at a heating temperature range of 400° C. to 700° C. not more than 80% of the yield stress at room temperature, total elongation at the heating temperature range not less than 1.1 times the total elongation at room temperature, yield stress and total elongation after the steel sheet is heated to the heating temperature range, subjected to a strain of not more than 20%, and cooled from the heating temperature to room temperature, not less than 70% of the yield stress and total elongation, respectively, at room temperature before the heating.

1 . A high-strength steel sheet for warm press forming, wherein:
 the tensile strength at room temperature is not less than 780 MPa,   the yield stress at a heating temperature range of 400° C. to 700° C. is not more than 80% of the yield stress at room temperature,   the total elongation at the heating temperature range is not less than 1.1 times the total elongation at room temperature,   the yield stress of the steel sheet after the steel sheet is heated to the heating temperature range, subjected to a strain of not more than 20% and cooled from the heating temperature to room temperature is not less than 70% of the yield stress at room temperature before the heating, and   the total elongation of the steel sheet after the steel sheet is heated to the heating temperature range, subjected to a strain of not more than 20% and cooled from the heating temperature to room temperature is not less than 70% of the total elongation at room temperature before the heating.   
     
     
         2 . The high-strength steel sheet for warm press forming according to  claim 1 , wherein the steel sheet has a chemical composition containing, in mass %:
 C: not less than 0.03% and not more than 0.14%, Si: not more than 0.3%,   Mn: above 0.60% and not more than 1.8%, P: not more than 0.03%,   S: not more than 0.005%, Al: not more than 0.1%,   N: not more than 0.005%, and Ti: not more than 0.25%,   the balance comprising Fe and inevitable impurities, and satisfying Expressions (1) and (2) below, and wherein the steel sheet includes a microstructure which has a matrix having a ferrite grain diameter of not less than 1 μm and a ferrite phase area fraction of not less than 95% and in which a carbide having an average particle diameter of not more than 10 nm is precipitated in the matrix:
   ([Ti]/48+[V]/51+[Mo]/96+[W]/184)>0.0031  (1)
 
   0.8≦([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184)≧1.20  (2)
 
   
       wherein [C], [Ti], [V], [Mo] and [W] are contents (mass %) of respective elements. 
     
     
         3 . The high-strength steel sheet for warm press forming according to  claim 2 , wherein the chemical composition further contains, in mass %, one, or two or more of V: not more than 0.5%, Mo: not more than 0.5% and W: not more than 1.0%. 
     
     
         4 . The high-strength steel sheet for warm press forming according to  claim 1 , wherein the steel sheet has a coating layer on the surface. 
     
     
         5 . The high-strength steel sheet for warm press forming according to  claim 4 , wherein the coating layer is a hot-dip galvanized layer or a galvannealed layer. 
     
     
         6 . A method of working high-strength steel sheets for warm press forming, comprising heating the high-strength steel sheet for warm press forming described in  claim 1  to a heating temperature range of 400° C. to 700° C. and subjecting the steel sheet to a strain of not more than 20%. 
     
     
         7 . A method for manufacturing high-strength steel sheets for warm press forming, comprising:
 heating a steel slab to a temperature of not less than 1100° C. and not more than 1350° C.,   hot rolling the steel slab to a steel sheet at a finishing temperature of not less than 820° C.,   starting cooling within 2 seconds after the hot rolling,   cooling the steel sheet at an average cooling rate of not less than 30° C./s in the temperature range from a temperature of not less than 820° C. to a coiling temperature, and   coiling the steel sheet into a coil at a coiling temperature of not less than 550° C. and not more than 680° C.,   the steel slab having a chemical composition containing, in mass %:   C: not less than 0.03% and not more than 0.14%, Si: not more than 0.3%,   Mn: above 0.60% and not more than 1.8%, P: not more than 0.03%,   S: not more than 0.005%, Al: not more than 0.1%,   N: not more than 0.005%, and Ti: not more than 0.25%,   the balance comprising Fe and inevitable impurities, the chemical composition satisfying Expressions (1) and (2) below:
   ([Ti]/48+[V]/51+[Mo]/96+[W]/184)>0.0031  (1)
 
   0.8≦([C]/12)/([Ti]/48+[V]/51+[Mo]/96+[W]/184)≧1.20  (2)
 
   
       wherein [C], [Ti], [V], [Mo] and [W] are contents (mass %) of respective elements. 
     
     
         8 . The method for manufacturing high-strength steel sheets for warm press forming according to  claim 7 , wherein the chemical composition further contains, in mass %, one, or two or more of V: not more than 0.5%, Mo: not more than 0.5% and W: not more than 1.0%.