Method and facility for producing high-strength galavanized steel sheets

A method for producing high-strength galvanized steel sheets, the method comprising:

• hot rolling a slab comprising, by mass %, C:
• 0.05 to 0.30%, Si:
• 0.1 to 2.0% and Mn:
• 1.0 to 4.0%, into a steel sheet, then coiling the steel sheet into a coil at a temperature Tc satisfying the relationship (1), and pickling the steel sheet;
• cold rolling the hot-rolled steel sheet resulting from the hot rolling;
• annealing the cold-rolled steel sheet resulting from the cold rolling, the annealing including (zone-A heating), (zone-B heating), and (zone-C heating);
• and
• galvanizing the annealed sheet resulting from the annealing in a galvanizing bath comprising 0.12 to 0.22 mass % Al, wherein:
• (zone-A heating):
• the cold-rolled steel sheet is heated in a direct-flame furnace at an air ratio α and an average heating rate at 200° C. and above is in a range of 10 to 50° C./sec to a target heating temperature T1 (° C.) satisfying the relationship (2),
• (zone-B heating):
• the cold-rolled steel sheet resulting from the zone-A heating is heated in a direct-flame furnace at an air ratio ≤0.9 and an average heating rate at above T1 is in a range of 5 to 30° C./sec to a target heating temperature T2 (OC) satisfying the relationship (3), and
• (zone-C heating):
• the cold-rolled steel sheet resulting from the zone-B heating is heated in an atmosphere containing H2 and H2O, the balance being N2 and inevitable impurities, at a log(PH2O/PH2) in a range of not less than −3.4 and not more than −1.1 and an average heating rate at above T2 is in a range of 0.1 to 10° C./sec to a prescribed target heating temperature T3 (° C.) in a range of 700 to 900° C., and is held at T3 for 10 to 500 seconds,
• TC≤−60([Si]+[Mn])+775 (1)
• T1≥28.2[Si]+7.95[Mn]−86.2α+666 (2)
• T2≥T1+30 (3)
• wherein [Si] and [Mn] are the contents of mass % Si and Mn present in the slab, α is not more than 1.5, and log(PH2O/PH2) is log(H2O partial pressure (PH2O)/H2 partial pressure (PH2)).