High-strength galvanized steel sheet and method of producing the same

Abstract

A high-strength galvanized steel sheet includes a steel sheet and a particular galvanized layer. The steel sheet has a particular composition and a steel microstructure. The steel microstructure contains ferrite and carbide-free bainite constituting 25% or less (including 0%) by area fraction in total, tempered martensite and carbide-containing bainite constituting 70% to 97% by area fraction in total, martensite and retained austenite constituting 3% to 20% by area fraction in total, and retained austenite constituting 1% to 5% by area fraction. The ferrite, the carbide-free bainite, the martensite, and the retained austenite constitute 3% to 30% by area fraction in total. The retained austenite has a C content of 0.10% to 0.50%. The carbide-containing bainite and the tempered martensite have an average grain size of 5 to 20 μm.

1 .- 3 . (canceled) 
     
     
         4 . A high-strength galvanized steel sheet comprising:
 a steel sheet and a galvanized layer,   wherein the steel sheet has a composition and a steel microstructure, the composition including, on a mass percent basis,   C: 0.08% to 0.15%,   Si: 0.1% to 1.0%,   Mn: 2.0% to 3.0%,   P: 0.100% or less (excluding 0%),   S: 0.02% or less (excluding 0%),   Al: 0.01% to 1.0%, and   N: 0.010% or less, and   one or more selected from   Cr: 0.005% to 2.0%,   Ti: 0.005% to 0.20%,   Nb: 0.005% to 0.20%,   Mo: 0.005% to 2.0%, and   V: 0.005% to 2.0%,   Cr+Ti+Nb+Mo+V being greater than or equal to Si, the remainder being Fe and incidental impurities,   the steel microstructure containing ferrite and carbide-free bainite constituting 25% or less (including 0%) by area fraction in total, tempered martensite and carbide-containing bainite constituting 70% to 97% by area fraction in total, martensite and retained austenite constituting 3% to 20% by area fraction in total, and retained austenite constituting 1% to 5% by area fraction, the ferrite, the carbide-free bainite, the martensite, and the retained austenite constituting 3% to 30% by area fraction in total, the retained austenite having a C content of 0.10% to 0.50%, the carbide-containing bainite and the tempered martensite having an average grain size of 5 to 20 μm, and   the galvanized layer is disposed on the steel sheet and has an Fe content of 6% or more by mass.   
     
     
         5 . The high-strength galvanized steel sheet according to  claim 4 , wherein the composition further includes, on a mass percent basis, one or more selected from
 Ni: 0.005% to 2.0%,   Cu: 0.005% to 2.0%,   B: 0.0001% to 0.0050%,   Ca: 0.0001% to 0.0050%,   REM: 0.0001% to 0.0050%,   Sn: 0.01% to 0.50%, and   Sb: 0.0010% to 0.10%.   
     
     
         6 . A method of producing a high-strength galvanized steel sheet, comprising:
 an annealing step of hot-rolling and pickling a steel having the composition according to  claim 4  to form a hot-rolled sheet, optionally cold-rolling the hot-rolled sheet at a rolling reduction of 20% or more to form a cold-rolled sheet, heating the hot-rolled sheet or the cold-rolled sheet to a heating temperature of 780° C. to 950° C., a heating rate of 600° C. to 720° C. being 3.0° C./s or less, cooling the sheet to a temperature of 150° C. to 400° C., an average cooling rate of 450° C. to 720° C. being 10° C./s or more, a residence time at 780° C. to 950° C. in the heating and cooling being 30 seconds or more, and after the cooling, reheating the sheet at a heating temperature of more than 150° C. and 600° C. or less, a residence time at a temperature of more than 150° C. and 600° C. or less being 1000 seconds or less, or after the cooling, holding the sheet at a temperature of more than 150° C. and 400° C. or less for a residence time of 1000 seconds or less; and   a galvanizing step of galvanizing the annealed sheet after the annealing step, then heating the sheet to a temperature of 460° C. to 600° C. for plating and alloying treatment, and cooling the sheet to 50° C. or less.   
     
     
         7 . A method of producing a high-strength galvanized steel sheet, comprising:
 an annealing step of hot-rolling and pickling a steel having the composition according to  claim 5  to form a hot-rolled sheet, optionally cold-rolling the hot-rolled sheet at a rolling reduction of 20% or more to faun a cold-rolled sheet, heating the hot-rolled sheet or the cold-rolled sheet to a heating temperature of 780° C. to 950° C., a heating rate of 600° C. to 720° C. being 3.0° C./s or less, cooling the sheet to a temperature of 150° C. to 400° C., an average cooling rate of 450° C. to 720° C. being 10° C./s or more, a residence time at 780° C. to 950° C. in the heating and cooling being 30 seconds or more, and after the cooling, reheating the sheet at a heating temperature of more than 150° C. and 600° C. or less, a residence time at a temperature of more than 150° C. and 600° C. or less being 1000 seconds or less, or after the cooling, holding the sheet at a temperature of more than 150° C. and 400° C. or less for a residence time of 1000 seconds or less; and   a galvanizing step of galvanizing the annealed sheet after the annealing step, then heating the sheet to a temperature of 460° C. to 600° C. for plating and alloying treatment, and cooling the sheet to 50° C. or less.