Method for manufacturing high-strength galvanized steel sheet

A method for manufacturing a high-strength galvanized steel sheet, the method comprising performing an oxidizing treatment, reduction annealing, and a galvanizing treatment in this order on a steel sheet having a chemical composition containing, by mass %,

• C:
• 0.3% or less,
• Si:
• 0.1% to 2.5%,
• Mn:
• 0.5% to 3.0%,
• P:
• 0.100% or less,
• S:
• 0.0100% or less, and the balance being Fe and inevitable impurities,
• wherein heating in a first half of the oxidizing treatment is performed at a temperature of 400° C. or higher and 750° C. or lower in an atmosphere having an O2 concentration of 1000 vol.ppm or more and an H2O concentration of 1000 vol.ppm or more,
• wherein heating in a second half of the oxidizing treatment is performed at a temperature of 600° C. or higher and 850° C. or lower in an atmosphere having an O2 concentration of less than 1000 vol.ppm and an H2O concentration of 1000 vol.ppm or more,
• wherein heating in a heating zone having an upper part and a lower part for the reduction annealing is performed prior to soaking to raise a temperature of the steel sheet to a temperature of 650° C. or higher and 900° C. or lower at a heating rate of 0.1° C./sec or more in an atmosphere having an H2 concentration of 5 vol. % or more and 30 vol. % or less and an H2O concentration of 500 vol.ppm or more and 5000 vol.ppm or less with the balance being N2 and inevitable impurities,
• wherein soaking in a soaking zone for the reduction annealing is performed after the steel sheet exits the heating zone at the temperature of 650° C. or higher and 900° C. or lower to maintain the temperature of the steel sheet with a temperature variation of within ±20° C. for 10 seconds to 300 seconds in an atmosphere having an H2 concentration of 5 vol. % or more and 30 vol. % or less and an H2O concentration of 10 vol.ppm or more and 1000 vol.ppm or less with the balance being N2 and inevitable impurities, and
• wherein the H2O concentration in the heating zone is more than 1000 vol.ppm and 5000 vol.ppm or less, and the H2O concentration in the soaking zone is 10 vol.ppm or more and less than 500 vol.ppm.