Method of producing galvannealed steel sheet

A method of producing a galvannealed steel sheet, the method comprising:

• an oxidizing process of heating a steel sheet having a chemical composition containing, in mass %, Si:
• 0.10% or more and 2.00% or less, and Mn:
• 1.0% or more and 5.0% or less,
• to 600° C. or more in an oxidizing atmosphere containing O2:
• 1000 volume ppm or more and 30,000 volume ppm or less;
• a reducing process of holding the steel sheet after the oxidizing process at 700° C. or more in a reducing atmosphere having a hydrogen concentration of more than 8 vol % and 30 vol % or less for 20 s or longer;
• a soaking process of holding the steel sheet after the reducing process at 750° C. or more in a soaking atmosphere having a hydrogen concentration of 0.2 vol % or more and 8 vol % or less for 20 s to 300 s;
• a cooling process of cooling the steel sheet after the soaking process;
• a coating process of immersing the steel sheet after the cooling in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet;
• an alloying process of carrying out an alloying treatment on the hot-dip galvanized steel sheet to obtain a galvannealed steel sheet;
• and
• a cooling-reheating process of cooling the galvannealed steel sheet to a cooling stop temperature that is the Ms temperature or less, then holding the galvannealed steel sheet at a temperature that is the cooling stop temperature or more, 100° C. or more, and 450° C. or less for 30 s or longer in a reheating atmosphere having a hydrogen concentration of 0.2 vol % or less.