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

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 5 μm or less;
• and
• 30% to 80% bainite and/or martensite each having an average grain size of 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 and has a galvanized zinc layer at a coating weight in a range of 20 to 150 g/m2 per side on a surface thereof.

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 5 μm or less,
• a tensile strength of at least 980 MPa, and a galvanized zinc layer at a coating weight in a range of 20 to 150 g/m2 per side on a surface thereof.

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.

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.