High tensile strength galvanized steel sheet excellent in formability and method for manufacturing the same

Abstract

A high tensile strength galvanized steel sheet with excellent formability and anti-crush properties contains, in terms of % by mass, 0.05 to 0.3% of C, 0.01 to 2.5% of Si, 0.5 to 3.5% of Mn, 0.003 to 0.100% of P, 0.02% or less of S, 0.010 to 1.5% of Al, 0.007% or less of N, in addition, 0.01 to 0.2% in total of at least one element selected from Ti, Nb, and V, the remainder being Fe and unavoidable impurities, the steel sheet having a microstructure composed of, in terms of area fraction, 20 to 87% of ferrite, 3 to 10% in total of martensite and residual austenite, and 10 to 60% of tempered martensite, and a second phase composed of the martensite, residual austenite, and tempered martensite having an average crystal grain diameter of 3 μm or less.

1 . A high tensile strength galvanized steel sheet with excellent formability, comprising, in terms of % by mass, 0.05 to 0.3% of C, 0.01 to 2.5% of Si, 0.5 to 3.5% of Mn, 0.003 to 0.100% of P, 0.02% or less of S, 0.010 to 1.5% of Al, and 0.007% or less of N, the remainder being Fe and unavoidable impurities, and having a microstructure composed of, in terms of area fraction, 20 to 87% of ferrite, 3 to 10% in total of martensite and residual austenite, and 10 to 60% of tempered martensite. 
     
     
         2 . The high tensile strength galvanized steel sheet of  claim 1 , further comprising at least one element selected from the group consisting of, in terms of % by mass, 0.005 to 2.00% of Cr, 0.005 to 2.00% of Mo, 0.005 to 2.00% of V, 0.005 to 2.00% of Ni, and 0.005 to 2.00% of Cu. 
     
     
         3 . The high tensile strength galvanized steel sheet of  claim 1 , further comprising at least one element selected from the group consisting of, in terms of % by mass, 0.01 to 0.20% of Ti and 0.01 to 0.20% of Nb. 
     
     
         4 . The high tensile strength galvanized steel sheet of  claim 1 , further comprising, in terms of % by mass, 0.0002 to 0.005% of B. 
     
     
         5 . The high tensile strength galvanized steel sheet of  claim 1 , further comprising at least one element selected from the group consisting of, in terms of % by mass, 0.001 to 0.005% of Ca and 0.001 to 0.005% of REM. 
     
     
         6 . The high tensile strength galvanized steel sheet of  claim 1 , wherein the galvanized steel sheet is a galvannealed steel sheet. 
     
     
         7 . A method for making a high tensile strength galvanized steel sheet with excellent formability, comprising:
 subjecting a slab having an elemental composition of  claim 1  to hot rolling and cold rolling thereby making a cold rolled steel sheet;   subjecting the cold rolled steel sheet to annealing including heating and maintaining the steel sheet in a temperature range from 750 to 950° C. for 10 seconds or more;   cooling the steel sheet from 750° C. to a temperature range from (Ms point −100° C.) to (Ms point −200° C.) at an average cooling rate of 10° C./s or more;   reheating and maintaining the steel sheet in a temperature range from 350 to 600° C. for 1 to 600 seconds; and   subjecting the annealed steel sheet to galvanizing treatment.   
     
     
         8 . The method of  claim 7 , wherein the galvanizing treatment is followed by galvannealing treatment. 
     
     
         9 . A high tensile strength galvanized steel sheet with excellent formability and anti-crush properties, comprising, in terms of % by mass, 0.05 to 0.3% of C, 0.01 to 2.5% of Si, 0.5 to 3.5% of Mn, 0.003 to 0.100% of P, 0.02% or less of S, 0.010 to 1.5% of Al, and 0.01 to 0.2% in total of at least one element selected from the group consisting of Ti, Nb, and V, the remainder being Fe and unavoidable impurities, and having a microstructure composed of, in terms of area fraction, 20 to 87% of ferrite, 3 to 10% in total of martensite and residual austenite, and 10 to 60% of tempered martensite, and a second phase composed of the martensite, residual austenite, and tempered martensite having an average crystal grain diameter of 3 μm or less. 
     
     
         10 . The high tensile strength galvanized steel sheet of  claim 9 , further comprising at least one element selected from the group consisting of, in terms of % by mass, 0.005 to 2.00% of Cr, 0.005 to 2.00% of Mo, 0.005 to 2.00% of Ni, and 0.005 to 2.00% of Cu. 
     
     
         11 . The high tensile strength galvanized steel sheet of  claim 9 , further comprising, in terms of % by mass, 0.0002 to 0.005% of B. 
     
     
         12 . The high tensile strength galvanized steel sheet of  claim 9 , further comprising at least one element selected from the group consisting of, in terms of % by mass, 0.001 to 0.005% of Ca and 0.001 to 0.005% of REM. 
     
     
         13 . The high tensile strength galvanized steel sheet of  claim 9 , wherein the galvanized steel sheet is a galvannealed steel sheet. 
     
     
         14 . A method for making a high tensile strength galvanized steel sheet with excellent formability and anti-crush properties, comprising:
 subjecting a slab having an elemental composition of  claim 9  to hot rolling and cold rolling thereby making a cold rolled steel sheet;   subjecting the cold rolled steel sheet to annealing including heating the steel sheet in a temperature range from 500° C. to the Ac 1  transformation point at an average temperature rising rate of 10° C./s or more;   heating and maintaining the steel sheet in a temperature range from the Ac 1  transformation point to (Ac 3  transformation point +30° C.) for 10 seconds or more;   cooling the steel sheet to a temperature range from (Ms point −100° C.) to (Ms point −200° C.) at an average cooling rate of 10° C./s or more;   reheating and maintaining the steel sheet in a temperature range from 350 to 600° C. for 1 to 600 seconds; and   subjecting the annealed steel sheet to galvanizing treatment.   
     
     
         15 . The method of  claim 14 , wherein the galvanizing treatment is followed by galvannealing treatment. 
     
     
         16 . The high tensile strength galvanized steel sheet of  claim 2 , further comprising at least one element selected from the group consisting of, in terms of % by mass, 0.01 to 0.20% of Ti and 0.01 to 0.20% of Nb. 
     
     
         17 . The high tensile strength galvanized steel sheet of  claim 2 , further comprising, in terms of % by mass, 0.0002 to 0.005% of B. 
     
     
         18 . The high tensile strength galvanized steel sheet of  claim 3 , further comprising, in terms of % by mass, 0.0002 to 0.005% of B. 
     
     
         19 . The high tensile strength galvanized steel sheet of  claim 2 , further comprising at least one element selected from the group consisting of, in terms of % by mass, 0.001 to 0.005% of Ca and 0.001 to 0.005% of REM. 
     
     
         20 . The high tensile strength galvanized steel sheet of  claim 3 , further comprising at least one element selected from the group consisting of, in terms of % by mass, 0.001 to 0.005% of Ca and 0.001 to 0.005% of REM.