High strength cold rolled steel plate and method for production thereof

Abstract

The present invention provides a high strength cold-rolled steel sheet, consisting essentially of, by mass %, 0.04 to 0.10% C, 0.5 to 1.5% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol. Al, 0.005% or less N, and the balance being iron and inevitable impurities, and having a structure which substantially comprises ferrite phase and martensite phase. Since the high strength cold-rolled steel sheet of the invention has ductility in which an elongation is 18% or more, excellent stretch-flangeability in which a hole-expanding ratio is 60% or more and a tensile strength of 780 MPa or more, it is favorable for use in a structural member of automobile.

1 . A high strength cold-rolled steel sheet consisting essentially of, in terms of percentages by mass, 0.04 to 0.10% C, 0.5 to 1.5% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol. Al, 0.005% or less N, optionally at least one element selected from the group consisting of Cr, Mo, B, Ti, Nb, V and Zr, and the balance being iron and inevitable impurities, and having a structure substantially comprising a ferrite phase and a martensite phase.  
     
     
         2 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is in an amount of 0.04 to less than 0.070%, and said steel sheet has a tensile strength of from 780 MPa to less than 980 MPa.  
     
     
         3 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is in an amount of 0.070 to 0.10%, and said steel sheet has a tensile strength of from 980 MPa to less than 1180 MPa.  
     
     
         4 . The high strength cold-rolled steel sheet as set forth in  claim 1 , further containing at least one element selected from the group consisting of, in terms of percentages by mass, 0.01 to 1.0% Cr, 0.01 to 0.5% Mo, 0.0001 to 0.0020% B, 0.001 to 0.05% Ti, 0.001 to 0.05% Nb, 0.001 to 0.05% V and 0.001 to 0.05% Zr.  
     
     
         5 . The high strength cold-rolled steel sheet as set forth in  claim 2 , further containing at least one element selected from the group consisting of, in terms of percentages by mass, 0.01 to 1.0% Cr, 0.01 to 0.5% Mo, 0.0001 to 0.0020% a, 0.001 to 0.05% Ti, 0.001 to 0.05% Nb, 0.001 to 0.05% and 0.001 to 0.05% Zr.  
     
     
         6 . The high strength cold-rolled steel sheet as set forth in  claim 3 , further containing at least one element selected from the group consisting of, in terms of percentages by mass, 0.01 to 1.0% Cr, 0.01 to 0.5% Mo, 0.0001 to 0.0020% B, 0.001 to 0.05% Ti, 0.001 to 0.05% Nb, 0.001 to 0.05% V and 0.001 to 0.05% Zr.  
     
     
         7 . The high strength cold-rolled steel sheet as set forth in  claim 2 , wherein said steel sheet has a martensite phase in a volume fraction of 30% to 45%.  
     
     
         8 . The high strength cold-rolled steel sheet as set forth in  claim 5 , wherein said steel sheet has a martensite phase in a volume fraction of 30% to 45%.  
     
     
         9 . The high strength cold-rolled steel sheet as set forth in  claim 3 , wherein said steel sheet has a martensite phase in a volume fraction of 45% to 60%.  
     
     
         10 . The high strength cold-rolled steel sheet as set forth in  claim 6 , wherein said steel sheet has a martensite phase in a volume fraction of 45% to 60%.  
     
     
         11 . A method for producing a high strength cold-rolled steel sheet comprising the steps of: 
 (a) producing a steel sheet by hot rolling a steel slab comprising a composition as set forth in any one of  claims 1  to  6 , followed by cold rolling to provide a cold-rolled steel sheet;    (b) heating the cold-rolled steel sheet at from 750° C. to 870° C. for 10 seconds or more to provide a heated steel sheet;    (c) cooling the heated steel sheet down to from 550° C. to 750° C. to provide a cooled steel sheet; and    (d) cooling the cooled steel sheet down to 300° C. or less at a cooling rate of over 100° C./sec.    
     
     
         12 . The method for producing a high strength cold-rolled steel sheet as set forth in  claim 11 , wherein the steel sheet is cooled at a cooling rate of 20° C./sec. or less within a temperature range of from 550° C. to 750° C.  
     
     
         13 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, the Si is in an amount of 0.5 to 1.0% and the Mn is in an amount of 2.0 to 2.5%.  
     
     
         14 . The high strength cold rolled steel sheet as set forth in  claim 13 , further containing at least one element selected from the group consisting of, in terms of percentages by mass, 0.01 to 1.0% Cr, 0.01 to 0.5% Mo, 0.0001 to 0.0020% B, 0.001 to 0.05% Ti, 0.001 to 0.05% Nb, 0.001 to 0.05% V and 0.001 to 0.05% Zr.  
     
     
         15 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is 0.054%, Si is 1.0%, Mn is 2.3%, P is 0.015%, S is 0.002%, Sol.Al is 0.030%, and N is 0.003%.  
     
     
         16 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is 0.065%, Si is 1.4%, Mn is 2.1%, P is 0.010%, S is 0.003%, Sol.Al is 0.030%, and N is 0.003%.  
     
     
         17 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is 0.081%, Si is 0.8%, Mn is 2.0%, P is 0.006%, S is 0.001%, Sol.Al is 0.030%, and N is 0.003%.  
     
     
         18 . The high strength cold rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is 0.058%, Si is 0.9%, Mn is 1.9%, P is 0.010%, S is 0.001%, Sol.Al is 0.030%, N is 0.003%, B is 0.0010%, and Cr is 0.020%.  
     
     
         19 . The high strength cold rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is 0.045%, Si is 0.6%, Mn is 2.0%, P is 0.010%, S is 0.003%, Sol.Al is 0.030%, N is 0.003%, Ti is 0.02%, and Nb is 0.02%.  
     
     
         20 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is 0.065%, Si is 1.1%, Mn is 2.3%, P is 0.012%, S is 0.004%, Sol.Al is 0.030%, and N is 0.003%.  
     
     
         21 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is 0.073%, Si is 1.0%, Mn is 2.3%, P is 0.015%, S is 0.002%, Sol.Al is 0.030%, and N is 0.003%.  
     
     
         22 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is 0.095%, Si is 1.4%, Mn is 2.1%, P is 0.010%, S is 0.003%, Sol.Al is 0.030%, and N is 0.003%.  
     
     
         23 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is 0.083%, Si is 0.9%, Mn is 1.9%, P is 0.010%, S is 0.001%, Sol.Al is 0.030%, N is 0.003%, B is 0.0010%, and Cr is 0.020%.  
     
     
         24 . The high strength cold-rolled steel sheet as set forth in  claim 1 , wherein, in terms of percentages by mass, C is 0.088%, Si is 0.8%, Mn is 2.0%, P is 0.010%, S is 0.003%, Sol.Al is 0.030%, N is 0.003%, Ti is 0.02%, and Nb is 0.02%.  
     
     
         25 . The method for producing a high strength cold-rolled steel sheet as set forth in  claim 11 , wherein the steel sheet is cooled down to 100° C. or less at a cooling rate of 500° C./sec or more in step (d).  
     
     
         26 . The method for producing a high strength cold-rolled steel sheet as set forth in  claim 11 , further comprising a step of tempering the cooled steel sheet at from 150 to 390° C. for 5 to 20 minutes after step (d).