High-strength galvanized steel sheet and method for producing the same

1. A high-strength galvanized steel sheet comprising:

a base steel sheet having a composition containing, by mass,

C: 0.05% to 0.30%,

Si: 3.0% less,

Mn: 1.5% to 4.0%,

P: 0.100% or less,

S: 0.02% or less, and

Al: 1.0% or less, with the balance being Fe and inevitable impurities,

wherein the composition further containing two or more elements selected from, by mass,

Cr: 0.005% to 2.0%,

Mo: 0.005% to 2.0%,

V: 0.005% to 2.0%,

Ni: 0.005% to 2.0%,

Cu: 0.005% to 2.0%,

Nb: 0.005% to 0.20%,

Ti: 0.005% to 0.20%,

B: 0.0001% to 0.0050%,

Ca: 0.0001% to 0.0050%,

REM: 0.0001% to 0.0050%,

Sb: 0.0010% to 0.10%, and

Sn: 0.0010% to 0.50%,

the base steel sheet having a microstructure including one or more of ferrite, carbide-free bainite, martensite, carbide-containing bainite, and retained austenite, a total area fraction of ferrite and carbide-free bainite being 0% to 65%, a total area fraction of martensite and carbide-containing bainite being 35% to 100%, and an area fraction of retained austenite being 0% to 15%,

a content of diffusible hydrogen in the base steel sheet being a 0.00008% by mass or less (including 0%); and

a galvanizing layer disposed on the base steel sheet,

wherein a density of gaps in the galvanizing layer, the gaps cutting across the entire thickness of the galvanizing layer in a cross section of the steel sheet, the cross section being taken in a thickness direction of the steel sheet perpendicular to a rolling direction of the steel sheet, is 10 gaps/mm or more, and

wherein the high-strength galvanized steel sheet has a tensile strength of 1000 MPa or more, and an average hole expansion of 25% or more to 70% or less.

2. The high-strength galvanized steel sheet according to claim 1 , wherein desorption of the diffusible hydrogen peaks at a temperature in the range of 80° C. to 200° C.

3. The high-strength galvanized steel sheet according to claim 1 , wherein the galvanizing layer is an alloyed galvanizing layer.

4. The high-strength galvanized steel sheet according to claim 2 , wherein the galvanizing layer is an alloyed galvanizing layer.

5. A method for producing a high-strength galvanized steel sheet according to claim 1 , the method comprising:

an annealing step in which a hot-rolled or cold-rolled steel sheet having the composition according to claim 1 is subjected to heating to an annealing temperature of 750° C. or more, then held as needed, and subsequently subjected to cooling such that an average cooling rate in a range of 550° C. to 700° C. is 3° C./s or more, the amount of retention time during which the steel sheet is held in a temperature range of 750° C. or more in the heating and the cooling being 30 seconds or more;

a galvanizing step in which, subsequent to the annealing step, the annealed steel sheet is galvanized and subsequently, as needed, subjected to an alloying treatment;

a bending-unbending step in which the galvanized steel sheet is bent and unbent in a direction perpendicular to a rolling direction of the steel sheet at a bend radius of 500 to 1000 mm in a temperature range of Ms to Ms−200° C. during cooling performed subsequent to the galvanizing step, each of the bending and the unbending being performed once or more;

a retention step in which the galvanized steel sheet is held for 3 s or more until the temperature reaches 100° C. after having been subjected to the bending-unbending step; and

a final cooling step in which the galvanized steel sheet is cooled to 50° C. or less after having been subjected to the retention step.

6. The method for producing a high-strength galvanized steel sheet according to claim 5 , wherein, in the annealing step, the H 2 concentration at the annealing temperature is 30% by volume or less.

7. The method for producing a high-strength galvanized steel sheet according to claim 5 , wherein, in the annealing step, the H 2 concentration during the cooling performed in the temperature range of 550° C. to 700° C. is 30% by volume or less.

8. The method for producing a high-strength galvanized steel sheet according to claim 6 , wherein, in the annealing step, the H 2 concentration during the cooling performed in the temperature range of 550° C. to 700° C. is 30% by volume or less.