Dual-phase steel sheet excellent in stretch flange formability and production method thereof

Abstract

Disclosed is a dual-phase steel sheet having low yield ratio, excellent in the balance for strength-elongation and for strength-stretch flange formability, and also excellent in bake hardening property containing (on the mass% basis). C: 0.01-0.20%, Si: 0.5% or less, Mn: 0.5-3%, sol. Al: 0.06% or less (inclusive 0%), P: 0.15% or less (exclusive 0%), and S: 0.02% or less (inclusive 0″), and in which the matrix phase contains tempered martensite; tempered martensite and ferrite; tempered bainite; or tempered bainite and ferrite, and the second phase comprises 1 to 30% of martensite at an area ratio based on the entire structure.

What is claimed is:  
     
         1 . A dual-phase steel sheet of excellent bake hardening property and stretch flange formability containing, on the mass % basis (here and hereinafter), 
 C: 0.01-0.20%,    Si: 0.5% or less,    Mn: 0.5-3%,    sol. Al: 0.06% or less (inclusive 0%),    P: 0.15% or less (exclusive 0%), and    S: 0.02% or less (inclusive 0%), wherein 
 the matrix phase contains tempered martensite; tempered martensite and ferrite; tempered bainite; or tempered bainite and ferrite, and  
 the second phase comprises from 1 to 30% of martensite as an area ratio based on the entire structure.  
   
     
     
         2 . The dual-phase steel sheet as defined in  claim 1 , wherein the bake hardening property is improved by controlling sol.Al to 0.025% or less.  
     
     
         3 . The dual-phase steel sheet as defined in  claim 2 , which further containing, 
 N: 0.0050% or more and satisfying the following relation (1):   0.001%≦[N]−(14/27)×]sol.Al]≦0.001%  (1)   (where [ ] represents the content for each element).    
     
     
         4 . The dual-phase steel sheet as defined in  claim 1 , further containing 0.003% or less of B (exclusive 0%).  
     
     
         5 . The dual-phase steel sheet as defined in  claim 1 , further containing 1% or less of at least one of Cr and Mo in total (exclusive 0%).  
     
     
         6 . The dual-phase steel sheet as defined in  claim 1 , further containing at least one of 
 Ni: 0.5% or less (exclusive 0%), and    Cu: 0.5% or less (exclusive 0%).    
     
     
         7 . The dual-phase steel sheet as defined in  claim 1 , further containing at least one of 
 Ti: 0.1% or less (exclusive 0%),    Nb: 0.1% or less (exclusive 0%), and    V: 0.1% or less (exclusive 0%).    
     
     
         8 . The dual-phase steel sheet as defined in  claim 1 , further containing at least of 
 Ca: 0.003% less (exclusive 0%), and    REM: 0.003% (exclusive 0%).    
     
     
         9 . A method of producing a dual-phase steel sheet in which the matrix phase is tempered martensite or tempered bainite as defined in  claim 1  by applying an hot rolling step and a continuous annealing step or galvanization step, wherein 
 the hot rolling step includes a step of completing finish rolling at a temperature of (A γ3 −50)° C. or higher; and a step of cooling and at an average cooling rate of 20° C./s or more down to Ms point or lower, or Ms point or higher and Bs point or lower, followed by coiling and  
 the continuous annealing step or galvanization step includes a step of heating to a temperature of A 1  point or higher and A 3  point or lower; and a step of cooling at an average cooling rate of 3° C./s or more and cooling down to Ms point or lower; and, optionally, a step of further applying averaging at a temperature from 100 to 600° C.  
 
     
     
         10 . A method of producing a dual-phase steel sheet in which the matrix phase is tempered martensite or tempered bainite as defined in  claim 1  by applying a hot rolling step, a cold rolling step, a first continuous annealing step and a second continuous annealing step or a galvanization step, wherein 
 the first continuous annealing step include a step of heating to and retaining at a temperature of A 3  point or higher; and a step of cooling at an average cooling rate of 20° C./s or more down to a temperature of Ms point or lower, or Ms point or higher and Bs point or lower, and  
 the second continuous annealing step or galvanization step includes a step of heating at a temperature of A 3  point or higher and A 3  point or lower; a step of cooling at an average cooling rate of 3° C./s or more down to a temperature of Ms point or lower; and, optionally, a step of further applying overaging at a temperature from 100 to 600° C.  
 
     
     
         11 . A method of producing a dual-phase steel sheet, in which the matrix phase is tempered martensite and ferrite or tempered bainite and ferrite as defined in  claim 1 , by applying a hot rolling step, and a continuous annealing step or a galvanization step, wherein 
 the hot rolling step includes a step of completing finish rolling at a temperature of (A γ3 −50)° C. or higher; and a step of cooling and at an average cooling rate of 10° C./s or more down to Ms point or lower, or Ms point or higher and Bs point or lower, followed by coiling, and    the continuous annealing step or galvanization step includes a step of heating to a temperature of A 1  point or higher and A 3  point or lower; and a step of cooling at an average cooling rate of 3° C./s or more down to Ms point or lower; and, optionally, a step of further applying overaging at a temperature from 100 to 600° C.    
     
     
         12 . The production method as defined in  claim 11 , wherein the hot rolling step includes a step of completing the finish rolling at a temperature of (A γ3 −50° C.) or higher; a step of cooling at an average cool rate of 30° C./s or more down to a temperature region in a range of 700±100° C.; a step of conducting air cooling for 1 to 30 sec in the temperature region; and a step of cooling at an average cooling rate of 30° C./s or more down to a temperature of Ms point or lower or Ms point or higher and Bs point or lower after air cooling, followed by coiling.  
     
     
         13 . The method of producing a dual-phase steel sheet in which the matrix phase is tempered martensite and ferrite or tempered bainite and ferrite as defined in  claim 1 , by applying a hot rolling step, a cold rolling step, a first continuous annealing step and a second continuous annealing step or a galvanization step, wherein 
 the first continuous annealing step includes a step of heating to and retaining at a temperature of A 1  point or higher and A 3  point or lower; and a step of cooling at an average cooling rate of 10° C./s or more down to a temperature of Ms point or lower, or Ms point or higher and Bs point or lower and    the second continuous annealing step or galvanization step includes a step of heating at a temperature of A 1  point or higher and A 3  point or lower; and a step of cooling at an average cooling rate of 3° C./s or more down to a temperature of Ms point or lower and, optionally, a step of further applying overaging at a temperature from 100 to 600° C.