Galvanized steel sheet excellent in uniformity and method for producing the same

Abstract

A high-strength galvanized steel sheet has a steel composition which contains, by % by mass, 0.01 to 0.12% of C, 0.2% or less of Si, less than 2% of Mn, 0.04% or less of P, 0.02% or less of S, 0.3% or less of sol. Al, 0.01% or less of N, and over 0.3% to 2% of Cr, and which satisfies 2.1≦[Mneq]≦3 and 0.24≦[% Cr]/[% Mn], the balance being composed of iron and inevitable impurities, and has a steel microstructures containing ferrite and a second phase.

1 . A high-strength galvanized steel sheet comprising steel having a composition which contains, by % by mass, 0.01 to 0.12% of C, 0.2% or less of Si, less than 2% of Mn, 0.04% or less of P, 0.02% or less of S, 0.3% or less of sol. Al, 0.01% or less of N, and over 0.3% to 2% of Cr, and which satisfies 2.1≦[Mneq]≦3 and 0.24≦[% Cr]/[% Mn], the balance being composed of iron and inevitable impurities, the steel having a microstructure containing ferrite and a second phase, wherein the area ratio of the second phase is 2 to 25%, the area ratio of pearlite or bainite in the second phase is 0% to less than 20%, the average grain diameter of the second phase is 0.9 to 7 μm, and the area ratio of grains with a grain diameter of less than 0.8 μm in the second phase is less than 15%, wherein [Mneq] represents the Mn equivalent shown by [Mneq]=[% Mn]+1.3[% Cr] and [% Mn] and [% Cr] represent the contents of Mn and Cr, respectively. 
     
     
         2 . The high-strength galvanized steel sheet according to  claim 1 , wherein 2.2<[Mneq]<2.9 is satisfied. 
     
     
         3 . The high-strength galvanized steel sheet according to  claim 1 , wherein 0.34≦[% Cr]/[% Mn] is satisfied. 
     
     
         4 . The high-strength galvanized steel sheet according to  claim 1 , further comprising, by % by mass, 0.005% or less of B. 
     
     
         5 . The high-strength galvanized steel sheet according to  claim 1 , further comprising, by % by mass, at least one of 0.15% or less of Mo and 0.2% or less of V. 
     
     
         6 . The high-strength galvanized steel sheet according to  claim 1 , further comprising, by % by mass, at least one of less than 0.014% of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less of Cu. 
     
     
         7 . A method for producing a high-strength galvanized steel sheet comprising:
 hot-rolling and cold-rolling a steel slab having the composition according to  claim 1 ;   heating at an average heating rate of less than 3° C./sec in a temperature range of 680° C. to 740° C. in a continuous galvanizing line (CGL);   annealing at an annealing temperature of over 740° C. to less than 820° C.;   cooling from the annealing temperature at an average cooling rate of 3 to 20° C./sec;   dipping in a galvanization bath or dipping in the galvanization bath and further alloying the coating; and   cooling at an average cooling rate of 7 to 100° C./sec.   
     
     
         8 . The method according to  claim 7 , wherein heating is performed at an average heating rate of less than 2° C./sec in a temperature range of 680° C. to 740° C. in the CGL. 
     
     
         9 . The method according to  claim 7 , wherein hot rolling is performed by starting cooling within 3 seconds after hot rolling, cooling to 600° C. or less at an average cooling rate of 40° C./sec or more, and coiling at a coiling temperature of 400° C. to 600° C., and then cold rolled with a rolling reduction of 70 to 85% 
     
     
         10 . The high-strength galvanized steel sheet according to  claim 2 , wherein 0.34≦[% Cr]/[% Mn] is satisfied. 
     
     
         11 . The high-strength galvanized steel sheet according to  claim 2 , further comprising, by % by mass, 0.005% or less of B. 
     
     
         12 . The high-strength galvanized steel sheet according to  claim 3 , further comprising, by % by mass, 0.005% or less of B. 
     
     
         13 . The high-strength galvanized steel sheet according to  claim 10 , further comprising, by % by mass, 0.005% or less of B. 
     
     
         14 . The high-strength galvanized steel sheet according to  claim 2 , further comprising, by % by mass, at least one of 0.15% or less of Mo and 0.2% or less of V. 
     
     
         15 . The high-strength galvanized steel sheet according to  claim 3 , further comprising, by % by mass, at least one of 0.15% or less of Mo and 0.2% or less of V. 
     
     
         16 . The high-strength galvanized steel sheet according to  claim 4 , further comprising, by % by mass, at least one of 0.15% or less of Mo and 0.2% or less of V. 
     
     
         17 . The high-strength galvanized steel sheet according to  claim 2 , further comprising, by % by mass, at least one of less than 0.014% of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less of Cu. 
     
     
         18 . The high-strength galvanized steel sheet according to  claim 3 , further comprising, by % by mass, at least one of less than 0.014% of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less of Cu. 
     
     
         19 . The high-strength galvanized steel sheet according to  claim 4 , further comprising, by % by mass, at least one of less than 0.014% of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less of Cu. 
     
     
         20 . The method according to  claim 8 , wherein hot rolling is performed by starting cooling within 3 seconds after hot rolling, cooling to 600° C. or less at an average cooling rate of 40° C./sec or more, and coiling at a coiling temperature of 400° C. to 600° C., and then cold rolled with a rolling reduction of 70 to 85%