High strength galvanized steel sheet and method for producing the same

Abstract

A high-strength hot-dip galvanized steel sheet that even on the premise of ordinary CGL heat cycle, has a low yield stress and excels in resistance to natural aging and baking hardenability without reliance on the use of expensive Mo; and a process for producing the same. The constituent composition thereof comprises 0.01 to less than 0.08% C, 0.2% or less Si, more than 1.0 to 1.8% Mn, 0.10% or less P, 0.03% or less S, 0.1% or less Al, 0.008% or less N and more than 0.5% Cr so that the relationship 1.95≦Mn(mass %)+1.3Cr(mass %)≦2.8 is satisfied and comprising the balance iron and unavoidable impurities. The structure thereof has a ferrite phase and a martensite phase of 2 to 15% area ratio, and the cumulative area ratio of pearlite phase and/or bainite phase is 1.0% or less. In the production of this hot-dip galvanized steel sheet, the temperature and cooling rate are controlled during the annealing/plating operation subsequent to cold rolling.

1 . A high strength galvanized steel sheet having a composition which contains, on a mass percent basis, 0.01% to less than 0.08% of C, 0.2% or less of Si, more than 1.0% to 1.8% of Mn, 0.10% or less of P, 0.03% or less of S, 0.1% or less of Al, 0.008% or less of N, more than 0.5% of Cr, and the balance being iron and inevitable impurities, wherein 1.95≦Mn (mass percent)+1.3Cr (mass percent)≦2.8 and the microstructure includes a ferrite phase and 2% to 15% of martensite on an area ratio basis, and the total area ratio of perlite and/or bainite is 1.0% or less. 
     
     
         2 . The high strength galvanized steel sheet according to  claim 1 , wherein, on a mass percent basis, the Cr content is more than 0.65%, and the Mn content is more than 1.0% to 1.6%. 
     
     
         3 . The high strength galvanized steel sheet according to  claim 1 , wherein the composition further contains, on a mass percent basis, 0.01% or less of B. 
     
     
         4 . The high strength galvanized steel sheet according to  claim 1 , wherein the composition further comprises, on a mass percent basis, at least one selected from 0.15% or less of Mo, 0.5% or less of V, 0.1% or less of Ti, and 0.1% or less of Nb. 
     
     
         5 . A method for manufacturing a high strength galvanized steel sheet, comprising:
 performing hot rolling and cold rolling of a steel slab having the composition according to  claim 1 ;   performing annealing at an annealing temperature of more than 750° C. to less than 820° C.;   performing cooling at an average cooling rate of 3 to 15° C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed;   performing galvanizing; and   performing cooling at an average cooling rate of 5° C./s or more.   
     
     
         6 . The method according to  claim 5 , further comprising, after performing galvanizing, performing an alloying treatment of a galvanizing layer. 
     
     
         7 . The high strength galvanized steel sheet according to  claim 2 , wherein the composition further contains, on a mass percent basis, 0.01% or less of B. 
     
     
         8 . The high strength galvanized steel sheet according to  claim 2 , wherein the composition further comprises, on a mass percent basis, at least one selected from 0.15% or less of Mo, 0.5% or less of V, 0.1% or less of Ti, and 0.1% or less of Nb. 
     
     
         9 . The high strength galvanized steel sheet according to  claim 3 , wherein the composition further comprises, on a mass percent basis, at least one selected from 0.15% or less of Mo, 0.5% or less of V, 0.1% or less of Ti, and 0.1% or less of Nb. 
     
     
         10 . The high strength galvanized steel sheet according to  claim 7 , wherein the composition further comprises, on a mass percent basis, at least one selected from 0.15% or less of Mo, 0.5% or less of V, 0.1% or less of Ti, and 0.1% or less of Nb. 
     
     
         11 . A method for manufacturing a high strength galvanized steel sheet, comprising:
 performing hot rolling and cold rolling of a steel slab having the composition according to  claim 2 ;   performing annealing at an annealing temperature of more than 750° C. to less than 820° C.;   performing cooling at an average cooling rate of 3 to 15° C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed;   performing galvanizing; and   performing cooling at an average cooling rate of 5° C./s or more.   
     
     
         12 . A method for manufacturing a high strength galvanized steel sheet, comprising:
 performing hot rolling and cold rolling of a steel slab having the composition according to  claim 3 ;   performing annealing at an annealing temperature of more than 750° C. to less than 820° C.;   performing cooling at an average cooling rate of 3 to 15° C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed;   performing galvanizing; and   performing cooling at an average cooling rate of 5° C./s or more.   
     
     
         13 . A method for manufacturing a high strength galvanized steel sheet, comprising:
 performing hot rolling and cold rolling of a steel slab having the composition according to  claim 4 ;   performing annealing at an annealing temperature of more than 750° C. to less than 820° C.;   performing cooling at an average cooling rate of 3 to 15° C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed;   performing galvanizing; and   performing cooling at an average cooling rate of 5° C./s or more.   
     
     
         14 . A method for manufacturing a high strength galvanized steel sheet, comprising:
 performing hot rolling and cold rolling of a steel slab having the composition according to  claim 7 ;   performing annealing at an annealing temperature of more than 750° C. to less than 820° C.;   performing cooling at an average cooling rate of 3 to 15° C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed;   performing galvanizing; and   performing cooling at an average cooling rate of 5° C./s or more.   
     
     
         15 . A method for manufacturing a high strength galvanized steel sheet, comprising:
 performing hot rolling and cold rolling of a steel slab having the composition according to  claim 10 ;   performing annealing at an annealing temperature of more than 750° C. to less than 820° C.;   performing cooling at an average cooling rate of 3 to 15° C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed;   performing galvanizing; and   performing cooling at an average cooling rate of 5° C./s or more.