High-strength electrogalvannealed steel sheet and method for manufacturing the same

A method for manufacturing a high-strength electrogalvannealed steel sheet having a tensile strength of 780 MPa or higher, the method comprising:

• a heat treatment step of holding a steel sheet in a temperature range of 600° C. or higher and 950° C. or lower in an atmosphere having a H2 concentration of 1.0 vol % or more and 20.0 vol % or less and a dew point of 0° C. or lower for 20 s or longer and 900 s or shorter,
• the steel sheet having a composition comprising, in terms of % by mass,
• C:
• 0.030% or more and 0.250% or less,
• Si:
• 0.01% or more and 3.00% or less,
• Mn:
• 1.00% or more and 10.00% or less,
• P:
• 0.001% or more and 0.100% or less, and
• S:
• 0.0001% or more and 0.0200% or less, with the balance being Fe and unavoidable impurities;
• a holding step of cooling the steel sheet to 200° C. to 600° C. after the heat treatment step and then holding the cooled steel sheet in a temperature range of 200° C. to 600° C. in an atmosphere having a H2 concentration of less than 1.0 vol % for 30 s or longer;
• an electrogalvanizing step of subjecting the steel sheet after the holding step to electrogalvanization;
• and
• an alloying step of subjecting the electrogalvanized steel sheet to an alloying treatment, wherein the high-strength electrogalvannealed steel sheet has a diffusible hydrogen content of 0.2 wt ppm or less.

A method for manufacturing a high-strength electrogalvannealed steel sheet having a tensile strength of 780 MPa or higher, the method comprising:

• a heat treatment step of holding a steel sheet in a temperature range of 600° C. or higher and 950° C. or lower in an atmosphere having a H2 concentration of 1.0 vol % or more and 20.0 vol % or less and a dew point of 0° C. or lower for 20 s or longer and 900 s or shorter, wherein the heat treatment step suppresses decarburization that lowers the strength of a surface layer of the steel sheet,
• the steel sheet having a composition comprising, in terms of % by mass,
• C:
• 0.030% or more and 0.250% or less,
• Si:
• 0.01% or more and 3.00% or less,
• Mn:
• 1.00% or more and 10.00% or less,
• P:
• 0.001% or more and 0.100% or less, and
• S:
• 0.0001% or more and 0.0200% or less, with the balance being Fe and unavoidable impurities;
• a holding step of cooling the steel sheet to 200° C. to 600° C. after the heat treatment step and then holding the cooled steel sheet in a temperature range of 200° C. to 600° C. in an atmosphere having a H2 concentration of less than 1.0 vol % for 30 s or longer;
• an electrogalvanizing step of subjecting the steel sheet after the holding step to electrogalvanization;
• and
• an alloying step of subjecting the electrogalvanized steel sheet to an alloying treatment.