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 - 22 . (cancelled)  
     
     
         23 . A high-ductility hot-dip galvanized steel sheet comprising a hot-dip galvanizing layer or an alloyed hot-dip galvanizing layer formed on the surface of a high-ductility 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 containin a primary phase containing a ferrite phase and a secondary phase containing a retained austenite phase in a volume ratio of not less than 1%.  
     
     
         24  A high-ductility hot-dip galvanized steel sheet comprising a hot-dip galvanizing layer or an alloyed hot-dip galvanizing layer formed on the surface of 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%.  
     
     
         25 . A high-ductility steel sheet to 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 phase containing a ferrite phase and a secondary phase containing a retained austenite phase in a volume ratio of not less than 1%, wherein the steel sheet is a hot-dip galvanized steel sheet having a hot-dip galvanizing layer or an alloyed hot-dip galvanizing layer formed on a surface of the steel sheet, and the primary phase containing a ferrite phase comprises a ferrite phase and a tempered martensite phase.  
     
     
         26 . A high-ductility steel sheet according to  claim 25 , wherein the 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.  
     
     
         27 . A high-ductility steel sheet according to  claim 26 , the composition further comprising, in weight percent, at least one of 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.    
     
     
         28 . A high-ductility steel sheet according to  claim 25 , wherein the 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%, 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%, and the balance Fe and incidental impurities.  
     
     
         29 . A high-ductility steel sheet according to  claim 28 , 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%.  
     
     
         30 . A method of manufacturing of a high-ductility hot-dip galvanized 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 primary heat-treating step of heating a steel sheet to a temperature of not less than the A C1  transformation point and rapidly cooling the steel sheet, the steel sheet 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%;    a secondary heat-treating step of heating the steel sheet to a temperature in the range of the A C1  transformation point to the A C3  transformation point; and    a hot-dip galvanizing step of forming a hot-dip galvanizing layer on the surface of the steel sheet.    
     
     
         31 . A method for manufacturing a high-ductility cold-rolled steel sheet according to  claim 30 , the composition further containing, in weight percent, at least one of 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.    
     
     
         32 . A method for manufacturing a high-ductility hot-dip galvanized steel according to  claim 30 , wherein the steel sheet is replaced with a steel sheet having 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%, 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%.  
     
     
         33 . A method for manufacturing a high-ductility hot-dip galvanized steel sheet according to  claim 32 , the composition further containing, in weight percent, at least one of Nb, Ti, and V in a total amount of not more than 2.0%.  
     
     
         34 . A method for manufacturing a high-ductility hot-dip galvanized steel sheet according to  claim 30 , further comprising a pickling treatment step of pickling the steel sheet between the primary heat treatment step and the secondary heat treatment step.  
     
     
         35 . A method for manufacturing a high-ductility hot-dip galvanized steel sheet according to  claim 30 , further comprising an alloying step of alloying the hot-dip galvanizing layer, subsequent to the hot-dip galvanizing step.  
     
     
         36 . A method for manufacturing a high-strength hot-dip galvanized steel sheet according to  claim 30 , wherein the steel sheet is a hot rolled steel sheet manufactured by hot-rolling a material under conditions including a heating temperature of not less than 900° C., a finish rolling end temperature of not less than 700° C. and a coiling temperature of not more than 800° C., or a cold-rolled steel sheet obtained by cold-rolling the hot-rolled steel sheet.  
     
     
         37 . A method for manufacturing a high-strength hot-dip galvanized steel sheet according to  claim 36 , wherein the cool-rolling is performed at a reduction ratio of not less than 40%.