High strength galvanized steel sheet and production method therefor

A method for producing a high strength galvanized steel sheet that has a tensile strength of 780 MPa or higher and includes, at a position ½ of a thickness of the steel sheet, 5% or more and 80% or less of a ferrite phase in terms of area fraction, 20% or more and 70% or less of a martensite phase in terms of area fraction, and 0% or more and 25% or less of a bainite phase in terms of area fraction, in which a hardness at a position 5 μm from a surface layer of the steel sheet in a thickness direction is 80% or less of a hardness at the position ½ of the thickness and a hardness at a position 15 μm from the surface layer of the steel sheet in the thickness direction is 90% or more of the hardness at the position ½ of the thickness, the method comprising:

• hot rolling a steel slab having a composition comprising, in terms of % by mass, C:
• 0.05% or more and 0.15% or less, Si:
• 0.01% or more and 1.00% or less, Mn:
• 2.2% or more and 3.5% or less, P:
• 0.001% or more and 0.050% or less, 0.010% or less, sol. Al:
• 0.005% or more and 0.100% or less, N:
• 0.0001% or more and 0.0060% or less, at east one element selected from the group consisting of Nb:
• 0.01% or more and 0.10% or less and Ti:
• 0.01% or more and 0.10% or less, and the balance being Fe and unavoidable impurities;
• cold rolling the resulting hot rolled sheet;
• performing primary annealing, soft reduction rolling, pickling, and secondary annealing;
• and
• galvanizing the resulting steel sheet,
• wherein in the primary annealing, heating is carried out at an average heating rate of 0.1° C./second or more and less than 3° C./second in a temperature range of 600° C. to 750° C., an annealing temperature in the range of 750° C. to 850° C. is held for 10 to 500 seconds, and cooling is carried out at an average cooling rate in the range of 1 to 15° C./second from the annealing temperature to a cooling stop temperature not higher than 600° C.,
• in the soft reduction rolling, soft reduction is carried out at a rolling reduction of 0.3% or more and 2.0% or less,
• in the pickling, a pickling weight loss of the steel sheet is adjusted to be in the range of 0.05 to 5 g/m2 on an Fe basis,
• in the secondary annealing, an annealing temperature in the range of 750° C. to 850° C. is held for 10 to 500 seconds and cooling is then carried out at an average cooling rate in the range of 1 to 15° C./second from the annealing temperature, and the resulting steel sheet is dipped in a zinc coating bath to galvanize the steel sheet, and
• cooling is carried out to a temperature of 150° C. or lower at an average cooling rate in the range of 5 to 100° C./second after the galvanizing.

A method for producing a high strength galvanized steel sheet that has a tensile strength of 780 MPa or higher and includes, at a position ½ of a thickness of the steel sheet, 5% or more and 80% or less of a ferrite phase in terms of area fraction, 20% or more and 70% or less of a martensite phase in terms of area fraction, and 0% or more and 25% or less of a bainite phase in terms of area fraction, in which a hardness at a position 5 μm from a surface layer of the steel sheet in a thickness direction is 80% or less of a hardness at the position ½ of the thickness and a hardness at a position 15 μm from the surface layer of the steel sheet in the thickness direction is 90% or more of the hardness at the position ½ of the thickness, the method comprising:

• hot rolling a steel slab having a composition comprising, in terms of % by mass, C:
• 0.05% or more and 0.15% or less, Si:
• 0.01% or more and 1.00% or less, Mn:
• 2.2% or more and 3.5% or less, P:
• 0.001% or more and 0.050% or less, S:
• 0.010% or less, sol. Al:
• 0.005% or more and 0.100% or less, N:
• 0.0001% or more and 0.0060% or less, at least one element selected from the group consisting of Nb:
• 0.01% or more and 0.10% or less and Ti:
• 0.01% or more and 0.10% or less, at least one element selected from the group consisting of Mo:
• 0.05% or more and 1.00% or less, V:
• 0.02% or more and 0.50% or less, Cr:
• 0.05% or more and 1.00% or less, and B:
• 0.0001% or more and 0.0030% or less, and the balance being Fe and unavoidable impurities;
• cold rolling the resulting hot rolled sheet;
• performing primary annealing, soft reduction rolling, pickling, and secondary annealing;
• and
• galvanizing the resulting steel sheet,
• wherein in the primary annealing, heating is carried out at an average heating rate of 0.1° C./second or more and less than 3° C./second in a temperature range of 600° C. to 750° C., an annealing temperature in the range of 750° C. to 850° C. is held for 10 to 500 seconds, and cooling is carried out at an average cooling rate in the range of 1 to 15° C./second from the annealing temperature to a cooling stop temperature not higher than 600° C.,
• in the soft reduction rolling, soft reduction is carried out at a rolling reduction of 0.3% or more and 2.0% or less,
• in the pickling, a pickling weight loss of the steel sheet is adjusted to be in the range of 0.05 to 5 g/m2 on an Fe basis,
• in the secondary annealing, an annealing temperature in the range of 750° C. to 850° C. is held for 10 to 500 seconds and cooling is then carried out at an average cooling rate in the range of 1 to 15° C./second from the annealing temperature, and the resulting steel sheet is dipped in a zinc coating bath to galvanize the steel sheet, and
• cooling is carried out to a temperature of 150° C. or lower at an average cooling rate in the range of 5 to 100° C./second after the galvanizing.