High-strength cold rolled steel sheet having excellent deep drawability and bake hardenability and method for manufacturing the same

Abstract

A high-strength cold rolled steel sheet having excellent deep drawability and bake hardenability, with a tensile strength ≧440 MPa, an average r-value ≧1.20 and a bake hardening value ≧40 MPa, is obtained by subjecting a steel raw material having a chemical composition including C: 0.010-0.06 mass %, Si: more than 0C:\Users\ejensen\Desktop\!TEMP\.5 mass % but not more than 1.5 mass %, Mn: 1.0-3.0 mass %, Nb: 0.010-0.090 mass %, Ti: 0.015-0.15 mass % and satisfying (Nb/93)/(C/12)<0.20 to hot rolling, cold rolling and then to annealing including steps of heating to a temperature of 800-900° C. while a temperature region of 700-800° C. is an average heating rate <3° C./s, and soaking and thereafter cooling at a rate ≧5° C./s from the soaking temperature to a cooling stop temperature ≦500° C. to thereby form a microstructure including ferrite phase with an area ratio ≧70% and martensite phase with an area ratio ≧3%.

1 . A high-strength cold rolled steel sheet having excellent deep drawability and bake hardenability, which has a chemical composition comprising C: 0.010-0.06 mass %, Si: more than 0.5 mass % but not more than 1.5 mass %, Mn: 1.0-3.0 mass %, P: 0.005-0.1 mass %, S: not more than 0.01 mass %, sol. Al: 0.005-0.5 mass %, N: not more than 0.01 mass %, Nb: 0.010-0.090 mass % and Ti: 0.015-0.15 mass % and the remainder being Fe and inevitable impurities, provided that Nb, Ti, N and S satisfy the following equations (1) and (2):
   (Nb/93)/(C/12)<0.20  (1)
     0.005≦C*≦0.025  (2)
   
       wherein C*=C−(12/93)Nb−(12/48){Ti−(48/14)N−(48/32)S} and each element symbol in the above equations represents a content of each element (mass %), and has a microstructure comprising a ferrite phase with an area ratio of not less than 70% and a martensite phase with an area ratio of not less than 3% and a tensile strength of not less than 440 MPa, an average r-value of not less than 1.20 and a BH quantity of not less than 40 MPa. 
     
     
         2 . A high-strength cold rolled steel sheet according to  claim 1 , which further contains not more than 0.5 mass % in total of one or more selected from Mo, Cr and V in addition to the chemical composition. 
     
     
         3 . A high-strength cold rolled steel sheet according to  claim 1 , which further contains one or two selected from Cu: not more than 0.3 mass % and Ni: not more than 0.3 mass % in addition to the chemical composition. 
     
     
         4 . A high-strength cold rolled steel sheet according to  claim 1 , which further contains one or two selected from Sn: not more than 0.2 mass % and Sb: not more than 0.2 mass % in addition to the chemical composition. 
     
     
         5 . A high-strength cold rolled steel sheet according to  claim 1 , which further contains Ta: 0.005-0.1 mass % in addition to the chemical composition, provided that C, Nb, Ta, Ti, N and S satisfy the following equation (3) instead of the equation (2):
   0.005≦C*≦0.025  (3)
   
       wherein C*=C−(12/93)Nb−(12/181)Ta−(12/48){Ti−(48/14)N−(48/32)S} and each element symbol in the above equations represents a content of each element (mass %). 
     
     
         6 . A method for manufacturing a high-strength cold rolled steel sheet having excellent deep drawability and bake hardenability by subjecting a steel raw material having a chemical composition comprising C: 0.010-0.06 mass %, Si: more than 0.5 mass % but not more than 1.5 mass %, Mn: 1.0-3.0 mass %, P: 0.005-0.1 mass %, S: not more than 0.01 mass %, sol. Al: 0.005-0.5 mass %, N: not more than 0.01 mass %, Nb: 0.010-0.090 mass % and Ti: 0.015-0.15 mass % and the remainder being Fe and inevitable impurities, provided that Nb, Ti, N and S satisfy the following equations (1) and (2):
   (Nb/93)/(C/12)<0.20  (1)
     0.005≦C*≦0.025  (2)
   
       wherein C*=C−(12/93)Nb−(12/48){Ti−(48/14)N−(48/32)S} and each element symbol in the above equations represents a content of each element (mass %), to hot rolling, cold rolling and annealing, characterized in that the said annealing is carried out by heating to an annealing temperature of 800-900° C. while a temperature region of 700-800° C. is an average heating rate of less than 3° C./s and thereafter cooling at an average cooling rate of not less than 5° C./s from said annealing temperature to a cooling stop temperature Tc of not higher than 500° C. 
     
     
         7 . The method for manufacturing a high-strength cold rolled steel sheet according to  claim 6 , wherein the steel raw material further contains not more than 0.5 mass % in total of one or more selected from Mo, Cr and V in addition to the chemical composition. 
     
     
         8 . The method for manufacturing a high-strength cold rolled steel sheet according to  claim 6 , wherein the steel raw material further contains one or two selected from Cu: not more than 0.3 mass % and Ni: not more than 0.3 mass % in addition to the chemical composition. 
     
     
         9 . The method for manufacturing a high-strength cold rolled steel sheet according to  claim 6 , wherein the steel raw material further contains one or two selected from Sn: not more than 0.2 mass % and Sb: not more than 0.2 mass % in addition to the above chemical composition. 
     
     
         10 . The method for manufacturing a high-strength cold rolled steel sheet according to  claim 6 , wherein the steel raw material further contains Ta: 0.005-0.1 mass % in addition to the above chemical composition, provided that C, Nb, Ta, Ti, N and S satisfy the following equation (3) instead of the equation (2):
   0.005≦C*≦0.025  (3)
   
       wherein C*=C−(12/93)Nb−(12/181)Ta−(12/48){Ti−(48/14)N−(48/32)S} and each element symbol in the above equations represents a content of each element (mass %). 
     
     
         11 . The method for manufacturing a high-strength cold rolled steel sheet according to  claim 6 , wherein a rolling reduction of a final pass in a finish rolling of the hot rolling is not less than 10% and a rolling reduction of a pass before the final pass is not less than 15%. 
     
     
         12 . The method for manufacturing a high-strength cold rolled steel sheet according to  claim 6 , wherein the cooling is started within 3 seconds after the finish rolling of the hot rolling and carried out up to a temperature zone of not higher than 720° C. at an average cooling rate of not less than 40° C./s and coiling is conducted at a temperature of 500-700° C. and thereafter the cold rolling is carried out at a rolling reduction of not less than 50%. 
     
     
         13 . A high-strength cold rolled steel sheet according to  claim 2 , which further contains one or two selected from Cu: not more than 0.3 mass % and Ni: not more than 0.3 mass % in addition to the chemical composition. 
     
     
         14 . A high-strength cold rolled steel sheet according to  claim 2 , which further contains one or two selected from Sn: not more than 0.2 mass % and Sb: not more than 0.2 mass % in addition to the chemical composition. 
     
     
         15 . A high-strength cold rolled steel sheet according to  claim 3 , which further contains one or two selected from Sn: not more than 0.2 mass % and Sb: not more than 0.2 mass % in addition to the chemical composition. 
     
     
         16 . A high-strength cold rolled steel sheet according to  claim 2 , which further contains Ta: 0.005-0.1 mass % in addition to the chemical composition, provided that C, Nb, Ta, Ti, N and S satisfy the following equation (3) instead of the equation (2):
   0.005≦C*≦0.025  (3)
   
       wherein C*=C−(12/93)Nb−(12/181)Ta−(12/48){Ti−(48/14)N−(48/32)S} and each element symbol in the above equations represents a content of each element (mass %). 
     
     
         17 . A high-strength cold rolled steel sheet according to  claim 3 , which further contains Ta: 0.005-0.1 mass % in addition to the chemical composition, provided that C, Nb, Ta, Ti, N and S satisfy the following equation (3) instead of the equation (2):
   0.005≦C*≦0.025  (3)
   
       wherein C*=C−(12/93)Nb−(12/181)Ta−(12/48){Ti−(48/14)N−(48/32)S} and each element symbol in the above equations represents a content of each element (mass %). 
     
     
         18 . A high-strength cold rolled steel sheet according to  claim 4 , which further contains Ta: 0.005-0.1 mass % in addition to the chemical composition, provided that C, Nb, Ta, Ti, N and S satisfy the following equation (3) instead of the equation (2):
   0.005≦C*≦0.025  (3)
   
       wherein C*=C−(12/93)Nb−(12/181)Ta−(12/48){Ti−(48/14)N−(48/32)S} and each element symbol in the above equations represents a content of each element (mass %). 
     
     
         19 . The method for manufacturing a high-strength cold rolled steel sheet according to  claim 7 , wherein the steel raw material further contains one or two selected from Cu: not more than 0.3 mass % and Ni: not more than 0.3 mass % in addition to the chemical composition. 
     
     
         20 . The method for manufacturing a high-strength cold rolled steel sheet according to  claim 7 , wherein the steel raw material further contains one or two selected from Sn: not more than 0.2 mass % and Sb: not more than 0.2 mass % in addition to the above chemical composition.