High-ductility steel sheet excellent in press formability and strain age hardenability, and method for manufacturing the same

Abstract

A steel sheet composition contains appropriate amounts of C, Si, Mn, P, S, Al and N and 0.5 to 3.0% Cu. A composite structure of the steel sheet has a ferrite phase or a ferrite phase and a tempered martensite phase as a primary phase, and a secondary phase containing retained austenite in a volume ratio of not less than 1%. In place of the Cu, at least one of Mo, Cr, and W may be contained in a total amount of not more than 2.0%. This composition is useful in production of a high-ductility hot-rolled steel sheet, a high-ductility cold-rolled steel sheet and a high-ductility hot-dip galvanized steel sheet having excellent press formability and excellent stain age hardenability as represented by a ΔTS of not less than 80 MPa, in which the tensile strength increases remarkably through a heat treatment at a relatively low temperature after press forming.

1 - 11 . (cancelled)  
     
     
         12 . A high-ductility cold-rolled steel sheet excellent in press formability and in strain age hardenability as represented by a ΔTS of not less than 80 mpa, comprising a composite structure containing a primary ferrite phase and a secondary phase containing a retained austenite phase in a volume ratio of not less than 1%,.  
     
     
         13 . A high-ductility steel sheet according to  claim 12 , wherein the cold-rolled steel sheet has a composition comprising, in weight percent, C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.1%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, Cu: 0.5 to 3.0%, and the balance Fe and incidental impurities.  
     
     
         14 . (Original) A high-ductility steel sheet according to  claim 13 , the composition further comprising, in weight percent, at least one of the following Groups A to C, in addition to the above-mentioned composition: 
 Group A: Ni: not more than 2.0%;    Group B: at least one of Cr and Mo: not more than 2.0% in total; and    Group C: at least one of Nb, Ti, and V: not more than 0.2% in total.    
     
     
         15 . A high-ductility steel sheet according to  claim 12 , wherein the cold-rolled steel sheet has a composition comprising, in weight percent: C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0% Mn, P: not more than 0.1%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, at least one selected from the group consisting of Mo: 05 to 2.0%, Cr: 0.05 to 2.0% and W: 0.05 to 2.0%, not more than 2.0% in total, and the balance Fe and incidental impurities.  
     
     
         16 . A high-ductility steel sheet according to  claim 15 , the composition further comprising, in weight percent, at least one of Nb, Ti, and V, in a total amount of not more than 2.0%.  
     
     
         17 . A method for manufacturing a high-ductility cold-rolled steel sheet excellent in press formability and in strain age hardenability as typically represented by a ΔTS of not less than 80 MPa, comprising: 
 a hot rolling step of hot-rolling a steel slab having a composition containing, in weight percent, C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.1%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, and Cu: 0.5 to 3.0% as a material to form a hot-rolled steel sheet;    a cold rolling step of cold-rolling the hot-rolled steel sheet into a cold-rolled steel sheet; and    a recrystallization annealing step of applying recrystallization annealing to the cold-rolled steel sheet into a cold-rolled annealed steel sheet, the recrystallization annealing step including a heat treatment of heating and soaking the steel sheet in a ferrite/austenite dual phase region within a temperature range of the A C1  transformation point to the A C3  transformation point, cooling the sheet, and retaining the sheet in the temperature region of 300 to 500° C. for 30 to 1,200 seconds.    
     
     
         18 . A method for manufacturing a high-ductility cold-rolled steel sheet according to  claim 17 , the composition further comprising, in weight percent, at least one selected from the following Groups A to C: 
 Group A: Ni: not more than 2.0%;    Group B: at least one of Cr and Mo: not more than 2.0% in total; and    Group C: at least one of Nb, Ti, and V: not more than 0.2% in total.    
     
     
         19 . A method for manufacturing a high-ductility cold-rolled steel sheet according to  claim 17 , wherein the steel slab is replaced with a steel slab having a composition containing, in weight percent, C: not more than 0.20%, Si: not more than 2.0%, Mn: not more than 3.0%, P: not more than 0.10%, S: not more than 0.02%, Al: not more than 0.3%, N: not more than 0.02%, and at least one selected from the group consisting of Mo: 0.05 to 2.0%, Cr: 0.05 to 2.0% and W: 0.05 to 2.0% in a total amount of not more than 2.0%.  
     
     
         20 . A method for manufacturing a high-ductility cold-rolled steel sheet according to  claim 19 , the composition further comprising, in weight percent, at least one of Nb, Ti, and V in a total amount of not more than 2.0%.  
     
     
         21 . A method for manufacturing a high-ductility cold-rolled steel sheet according to  claim 17 , wherein the hot-rolling step includes heating the steel slab at a temperature of not less than 900° C., rolling the slab at a finish rolling end temperature of not less than 700° C., and coiling the hot-rolled steel sheet at a coiling temperature of not more than 800° C.  
     
     
         22 . A method for manufacturing a cold-rolled steel sheet according to  claim 17 , wherein all or part of the hot rolling is lubrication rolling.  
     
     
         23 - 37 . (Cancelled)