High-Strength Hot-Dip Galvanized Steel Sheet Excellent in Formability and Method for Producing Same

Abstract

There is provided a high-strength hot-dip galvanized steel sheet having excellent formability, the steel sheet eliminating special pre-structure control and capable of being produced by using a hot-dip galvanized steel-sheet production line that is not capable of sufficiently ensuring an austempering time after annealing. A high-strength hot-dip galvanized steel sheet includes, on the basis of mass percent, 0.05-0.3% C, 1.4% or less (including 0%) Si, 0.08%-3% Mn, 0.003-0.1% P, 0.07% or less S, 0.1-2.5% Al, 0.1-0.5% Cr, and 0.007% or less N, Si+Al≧0.5%, and the balance being Fe and incidental impurities, wherein the steel sheet has a retained austenite content of 3% or more by volume fraction, and wherein the average aspect ratio of retained austenite grains is 2.5 or less.

1 . A high-strength hot-dip galvanized steel sheet excellent in formability, comprising, on the basis of mass percent, 0.05-0.3% C, 1.4% or less (including 0%) Si, 0.08%-3% Mn, 0.003-0.1% P, 0.07% or less S, 0.1-2.5% Al, 0.1-0.5% Cr, and 0.007% or less N, Si+Al 3  0.5%, and the balance being Fe and incidental impurities,
 wherein the steel sheet has a retained austenite content of 3% or more by volume fraction, and wherein the average aspect ratio of retained austenite grains is 2.5 or less.   
   
   
       2 . The high-strength hot-dip galvanized steel sheet excellent in formability according to  claim 1 , further comprising, on the basis of mass percent, at least one element selected from 0.005-2% V and 0.005-2% Mo. 
   
   
       3 . The high-strength hot-dip galvanized steel sheet excellent in formability according to  claim 1 , further comprising, on the basis of mass percent, at least one element selected from 0.01-0.5% Ti, 0.01-0.1% Nb, 0.0003-0.005% B, 0.005-2.0% Ni, and 0.005-2.0% Cu. 
   
   
       4 . A method for producing a high-strength hot-dip galvanized steel sheet excellent in formability, the method comprising annealing a steel sheet in a first temperature region having a temperature of 700° C. to 900° C. for 15 to 600 seconds, the steel sheet containing, on the basis of mass percent, 0.05-0.3% C, 1.4% or less (including 0%) Si, 0.08%-3% Mn, 0.003-0.1% P, 0.07% or less S, 0.1-2.5% Al, 0.1-0.5% Cr, and 0.007% or less N, Si+Al 3  0.5%, and the balance being Fe and incidental impurities; and cooling the steel sheet to a second temperature region having a temperature of 360° C. to 490° C. at a cooling rate of 5° C./s or more, wherein a retention time in the second temperature region is controlled on the basis of Formula (1):
   5£E£200−0.003′(T−350) 2   (1)   
     where t represents the total retention time (second) in the temperature region having a temperature of 360° C. to 490° C., and T represents an average temperature (° C.) when the steel sheet is retained for the total retention time in the temperature region having a temperature of 360° C. to 490° C. 
   
   
       5 . The method for producing the high-strength hot-dip galvanized steel sheet excellent in formability according to  claim 4 , wherein the steel sheet further comprises, on the basis of mass percent, at least one element selected from 0.005-2% V and 0.005-2% Mo. 
   
   
       6 . The method for producing the high-strength hot-dip galvanized steel sheet excellent in formability according to  claim 4 , wherein the steel sheet further comprises, on the basis of mass percent, at least one element selected from 0.01-0.5% Ti, 0.01-0.1% Nb, 0.0003-0.005% B, 0.005-2.0% Ni, and 0.005-2.0% Cu. 
   
   
       7 . The high-strength hot-dip galvanized steel sheet excellent in formability according to  claim 2 , further comprising, on the basis of mass percent, at least one element selected from 0.01-0.5% Ti, 0.01-0.1% Nb, 0.0003-0.005% B, 0.005-2.0% Ni, and 0.005-2.0% Cu. 
   
   
       8 . The method for producing the high-strength hot-dip galvanized steel sheet excellent in formability according to  claim 5 , wherein the steel sheet further comprises, on the basis of mass percent, at least one element selected from 0.01-0.5% Ti, 0.01-0.1% Nb, 0.0003-0.005% B, 0.005-2.0% Ni, and 0.005-2.0% Cu.