High tensile-strength galvanized steel sheet and process for manufacturing high tensile-strength galvanized steel sheet

1. A high tensile-strength galvanized steel sheet, comprising:

as a percentage of mass,

C: at least 0.05% but less than 0.12%, Si: at least 0.01% but less than 0.35%,

Mn: 2.0% to 3.5%, P: 0.001% to 0.020%,

S: 0.0001% to 0.0030%, Al: 0.005% to 0.1%,

N: 0.0001% to 0.0060%, Cr: more than 0.5% but not more than 2.0%,

Mo: 0.01% to 0.50%, Ti: 0.010% to 0.080%,

Nb: 0.010% to 0.080%, and B: 0.0001% to 0.0030%,

the remainder being Fe and unavoidable impurities,

having a microstructure that contains 20% to 70% by volume ferrite having an average grain size of 3.5 μm or less,

a tensile strength of at least 980 MPa, tensile strength (TS) x elongation (El) of at least 15000 MPa·%, tensile strength (TS)×hole expansion ratio (λ) of at least 43000 MPa·% and a critical bending radius of not more than 1.5×sheet thickness (t) of the steel sheet, and a galvanized zinc layer at a coating weight in a range of 20 to 150 g/m 2 per side on a surface thereof.

2. A high tensile-strength galvanized steel sheet, comprising:

as a percentage of mass,

C: at least 0.05% but less than 0.12%, Si: at least 0.01% but less than 0.35%

Mn: 2.0% to 3.5%, P: 0.001% to 0.020%,

S: 0.0001% to 0.0030%, Al: 0.005% to 0.1%,

N: 0.0001% to 0.0060%, Cr: more than 0.5% but not more than 2.0%,

Mo: 0.01% to 0.50%, Ti: 0.010% to 0.080%,

Nb: 0.010% to 0.080%, and B: 0.0001% to 0.0030%,

the remainder being Fe and unavoidable impurities, and containing,

as a percentage by volume,

20% to 70% ferrite having an average grain size of 3.5 μm or less; and

30% to 80% bainite and/or martensite each having an average grain size of 3.5 μm or less,

wherein the amount of remaining microstructure is 5% or less (including zero),

and the high tensile-strength galvanized steel sheet has a tensile strength of at least 980 MPa, tensile strength (TS)×elongation (El) of at least 15000 MPa·%, tensile strength (TS)×hole expansion ratio (λ) of at least 43000 MPa·% and a critical bending radius of not more than 1.5×sheet thickness (t) of the steel sheet, and has a galvanized zinc layer at a coating weight in a range of 20 to 150 g/m 2 per side on a surface thereof.

3. A high tensile-strength galvanized steel sheet, comprising:

as a percentage of mass,

C: at least 0.05% but less than 0.10%, Si: at least 0.01% but less than 0.35%,

Mn: 2.0% to 3.5%, P: 0.001% to 0.020%,

S: 0.0001% to 0.0020%, Al: 0.005% to 0.1%,

N: 0.0001% to 0.0050%, Cr: more than 0.5% but not more than 2.0%,

Mo: 0.01% to 0.50%, Ti: 0.010% to 0.080%,

Nb: 0.010% to 0.080%, and B: 0.0001% to 0.0030%,

the remainder being Fe and unavoidable impurities,

having a microstructure that contains 20% to 60% by volume ferrite having an average grain size of 3.5 μm or less,

a tensile strength of at least 980 MPa, tensile strength (TS)×elongation (El) of at least 15000 MPa·%, tensile strength (TS)×hole expansion ratio (X) of at least 43000 MPa·% and a critical bending radius of not more than 1.5×sheet thickness (t) of the steel sheet, and a galvanized zinc layer at a coating weight in a range of 20 to 150 g/m 2 per side on a surface thereof.

4. A process for manufacturing a high tensile-strength galvanized steel sheet comprising:

subjecting a steel slab to a hot-rolling process, coiling, cold-rolling, and galvanizing to manufacture a galvanized steel sheet,

the steel slab containing,

as a percentage of mass,

C: at least 0.05% but less than 0.12%, Si: at least 0.01% but less than 0.35%,

Mn: 2.0% to 3.5%, P: 0.001% to 0.020%,

S: 0.0001% to 0.0030%, Al: 0.005% to 0.1%,

N: 0.0001% to 0.0060%, Cr: more than 0.5% but not more than 2.0%,

Mo: 0.01% to 0.50%, Ti: 0.010% to 0.080%,

Nb: 0.010% to 0.080%, and B: 0.0001% to 0.0030%,

the remainder being Fe and unavoidable impurities,

wherein, in the hot-rolling process, the slab is hot-rolled at a reheating temperature in a range of 1150° C. to 1300° C. and a finishing temperature in a range of 850° C. to 950° C., is then cooled from the finishing temperature to (finishing temperature - 100° C.) at an average cooling rate in a range of 5° C. to 200° C./s, and is coiled at a temperature in a range of 400° C. to 650° C., and

after cold rolling, the hot-rolled steel sheet is heated from 200° C. to an intermediate temperature at a first average heating rate in a range of 5° C. to 50° C./s, the intermediate temperature being in a range of 500° C. to 800° C., heated from the intermediate temperature to an annealing temperature at a second average heating rate in a range of 0.1° C. to 10° C./s, the annealing temperature being in a range of 750° C. to 900° C., held in the annealing temperature range for 10 to 500 seconds, cooled to a temperature in a range of 450° C. to 550° C. at an average cooling rate in a range of 1° C. to 30° C./s, and then subjected to hot-dip galvanizing and, optionally, alloying and wherein galvanized steel sheet is characterized by having a microstructure that contains 20% to 70% by volume ferrite with an average grain size of 3.5 μm or less, a tensile strength of at least 980 MPa, tensile strength (TS)×elongation (El) of at least 15000 MPa·%, tensile strength (TS)×hole expansion ratio (λ) of at least 43000 MPa·% and a critical bending radius of not more than 1.5×sheet thickness (t) of the steel sheet, and a galvanized zinc layer at a coating weight in a range of 20 to 150 g/m 2 per side on a surface thereof.

5. A process for manufacturing a high tensile-strength galvanized steel sheet comprising:

subjecting a steel slab to a hot-rolling process, coiling, pickling, cold-rolling, and galvanizing to manufacture a galvanized steel sheet,

the steel slab containing,

as a percentage of mass,

C: at least 0.05% but less than 0.10%, Si: at least 0.01% but less than 0.35%,

Mn: 2.0% to 3.5%, P: 0.001% to 0.020%,

S: 0.0001% to 0.0020%, Al: 0.005% to 0.1%,

N: 0.0001% to 0.0050%, Cr: more than 0.5% but not more than 2.0%,

Mo: 0.01% to 0.50%, Ti: 0.010% to 0.080%,

Nb: 0.010% to 0.080%, and B: 0.0001% to 0.0030%,

the remainder being Fe and unavoidable impurities,

wherein, in the hot-rolling process, the slab is hot-rolled at a reheating temperature in a range of 1150° C. to 1300° C. and a finishing temperature in a range of 850° C. to 950° C., then cooled from the finishing temperature to (finishing temperature - 100° C.) at an average cooling rate in a range of 5° C. to 200° C./s, and coiled at a temperature in a range of 400° C. to 600° C., and

after pickling, a hot-rolled steel sheet is cold-rolled, heated from 200° C. to an intermediate temperature at a first average heating rate in a range of 10° C. to 50° C./s, the intermediate temperature being in the range of 500° C. to 800° C., heated from the intermediate temperature to an annealing temperature at a second average heating rate in a range of 0.1° C. to 10° C./s, the annealing temperature being in the range of 750° C. to 900° C., held in the annealing temperature range for 10 to 500 seconds, cooled to a temperature in a range of 450° C. to 550° C. at an average cooling rate in a range of 1° C. to 30° C./s, and then subjected to hot-dip galvanizing and, optionally, alloying and wherein galvanized steel sheet is characterized by having a microstructure that contains 20% to 70% by volume ferrite with an average grain size of 3.5 μm or less, a tensile strength of at least 980 MPa, tensile strength (TS)×elongation (El) of at least 15000 MPa·%, tensile strength (TS)×hole expansion ratio (λ) of at least 43000 MPa·% and a critical bending radius of not more than 1.5×sheet thickness (t) of the steel sheet, and a galvanized zinc layer at a coating weight in a range of 20 to 150 g/m 2 per side on a surface thereof.

6. The high tensile-strength galvanized steel sheet, according to claim 1 , wherein:

the C content is 0.05% or more but less than 0.10%,

the S content is 0.0001% to 0.0020%, and

the N content is 0.0001% to 0.0050%.

7. The high tensile-strength galvanized steel sheet, according to claim 1 , wherein a microstructure contains 20% to 60% by volume ferrite.