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 225 GPa, which comprises C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.5-4.0%, 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 Nb: 0.02-0.40% as mass %, provided that C, N and Nb contents satisfy 0.01≦C+(12/14)×N−(12/92.9)×Nb≦0.06 and N≦(14/92.9)×(Nb−0.01) 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: 1.5-4.0%, 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 Nb: 0.02-0.40% as mass %, provided that C, N and Nb contents satisfy the relationships of the following equations (1) and (2):
   0.01≦C+(12/14)×N−(12/92.9)×Nb≦0.06   (1)     N≦(14/92.9)×(Nb−0.01)   (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 225 GPa. 
     
     
         2 . A high-stiffness high-strength thin steel sheet according to  claim 1 , which further contains one or two of Ti: 0.01-0.50% and V: 0.01-0.50% as mass % in addition to the above composition and satisfy the relationships of the following equations (3) and (4) instead of the equations (1) and (2):
   0.01≦C+(12/14)×N*−(12/92.9)×Nb−(12/47.9)×Ti*−(12/50.9)×V≦0.06   (3)     N*≦(14/92.9)×(Nb−0.01)   (4)   
       provided that N* in the equations (3) and (4) is N*=N−(14/47.9)×Ti at N−(14/47.9)×Ti>0 and N*=0 at N−(14/47.9)×Ti≦0, and Ti* in the equation (3) is Ti*=Ti−(47.9/14)×N−(47.9/32.1)×S at Ti−(47.9/14×N−(47.9/32.1)×S>0 and Ti*=0 at Ti−(47.9/14)×N−(47.9/32.1)×S≦0. 
     
     
         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: 1.5-4.0%, 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 Nb: 0.02-0.40% as mass %, provided that C, N and Nb contents satisfy the relationships of the following equations (1) and (2):
   0.01≦C+(12/14)×N−(12/92.9)×Nb≦0.06   (1)     N≦(14/92.9)×(Nb−0.01)   (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 Ar 3 -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 1-40° 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 Ti: 0.01-0.50% and V: 0.01-0.50% as mass % in addition to the above composition and satisfies the relationships of the following equations (3) and (4) instead of the equations (1) and (2):
   0.01≦C+(12/14)×N*−(12/92.9)×Nb−(12/47.9)×Ti*−(12/50.9)×V≦0.06   (3)     N*≦(14/92.9)×(Nb−0.01)   (4)   
       provided that N* in the equations (3) and (4) is N*=N−(14/47.9)×Ti at N−(14/47.9)×Ti>0 and N*=0 at N−(14/47.9)×Ti≦0, and Ti* in the equation (3) is Ti*=Ti−(47.9/14)×N−(47.9/32.1)×S at Ti−(47.9/14)×N−(47.9/32.1)×S>0 and Ti*=0 at Ti−(47.9/14)×N−(47.9/32.1)×S≦0. 
     
     
         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.