High-stiffness high-strength thin steel sheet and method for producing the same

Abstract

There is provided a high-stiffness high-strength thin steel sheet having a tensile strength of not less than 590 MPa and a Young's modulus of not less than 230 GPa, which comprises C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.0-3.5%, P: not more than 0.05%, S: not more than 0.01%, Al: not more than 1.5%, N: not more than 0.01% and Ti: 0.02-0.50% as mass %, provided that C, N, S and Ti contents satisfy Ti*=Ti−(47.9/14)×N−(47.9/32.1)×S≧0.01 and 0.01≦C−(12/47.9)×Ti*≧0.05 and the remainder being substantially iron and inevitable impurities, and has a texture comprising a ferrite phase as a main phase and having a martensite phase at an area ratio of not less than 1%.

1 . A high-stiffness high-strength thin steel sheet comprising C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.0-3.5%, P: not more than 0.05%, S: not more than 0.01%, Al: not more than 1.5%, N: not more than 0.01% and Ti: 0.02-0.50% as mass %, provided that C, N, S and Ti contents satisfy the relationships of the following equations (1) and (2):  
         Ti*=Ti−(47.9/14)×N−(47.9/32.1)×S≧0.01  (1)  0.01≦C−(12/47.9)×Ti*≦0.05  (2)  
       and the remainder being substantially iron and inevitable impurities, and having a texture comprising a ferrite phase as a main phase and having a martensite phase at an area ratio of not less than 1%, and having a tensile strength of not less than 590 MPa and a Young's modulus of not less than 230 GPa.  
     
     
         2 . A high-stiffness high-strength thin steel sheet according to  claim 1 , which further contains one or two of Nb: 0.005-0.04% and V: 0.01-0.20% as mass % in addition to the above composition and satisfies the relationships of the above equation (1) and the following equation (3) instead of the equation (2):  
         0.01≦C−(12/47.9)×Ti*−(12/92.9)×Nb−(12/50.9)×V≦0.05  (3)  
     
     
         3 . A high-stiffness high-strength thin steel sheet according to  claim 1  or  2 , which further contains one or more of Cr: 0.1-1.0%, Ni: 0.1-1.0%, Mo: 0.1-1.0%, Cu: 0.1-2.0% and B: 0.0005-0.0030% as mass % in addition to the above composition.  
     
     
         4 . A method for producing a high-stiffness high-strength thin steel sheet comprising subjecting a starting material of steel comprising C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.0-3.5%, P: not more than 0.05%, S: not more than 0.01%, Al: not more than 1.5%, N: not more than 0.01% and Ti: 0.02-0.50% as mass %, provided that C, N, S and Ti contents satisfy the relationships of the following equations (1) and (2):  
         Ti*=Ti−(47.9/14)×N−(47.9/32.1)×S≧0.01  (1)  0.01≦C−(12/47.9)×Ti*≦0.05  (2)  
       to a hot rolling step under conditions that a total rolling reduction below 950° C. is not less than 30% and a finish rolling is terminated at 800-900° C., coiling the hot rolled sheet below 650° C., pickling, subjecting to a cold rolling at a rolling reduction of not less than 50%, raising a temperature to 780-900° C. at a temperature rising rate from 500° C. of 1-30° C./s to conduct soaking, and then cooling at a cooling rate up to 500° C. of not less than 5° C./s to conduct annealing.  
     
     
         5 . A method for producing a high-stiffness high-strength thin steel sheet according to  claim 4 , wherein the starting material of steel further contains one or two of Nb: 0.005-0.04% and V: 0.01-0.20% as mass % in addition to the above composition and satisfies the relationships of the above equation (1) and the following equation (3) instead of the equation (2):  
         0.01≦C−(12/47.9)×Ti*−(12/92.9)×Nb−(12/50.9)×V≦0.05  (3)  
     
     
         6 . A method for producing a high-stiffness high-strength thin steel sheet according to  claim 4  or  5 , wherein the staring material of steel further contains one or more of Cr: 0.1-1.0%, Ni: 0.1-1.0%, Mo: 0.1-1.0%, Cu: 0.1-2.0% and B: 0.0005-0.0030% as mass % in addition to the above composition.