High-strength hot-dip galvanized steel sheet excellent in impact resistance property and manufacturing method thereof, and high-strength alloyed hot-dip galvanized steel sheet and manufacturing method thereof

A manufacturing method of a high-strength hot-dip galvanized steel sheet excellent in impact resistance property, comprising:

• a step of obtaining a base steel sheet, the step comprising:
• a hot-rolling step of performing hot rolling in which a slab containing, by mass %,
• C:
• 0.075 to 0.400%,
• Si:
• 0.01 to 2.00%,
• Mn:
• 0.80 to 3.50%,
• P:
• 0.0001 to 0.100%,
• S:
• 0.0001 to 0.0100%,
• Al:
• 0.001 to 2.00%,
• O:
• 0.0001 to 0.0100%,
• N:
• 0.0001 to 0.0100%, and
• a balance composed of Fe and inevitable impurities is heated to 1080° C. or more, the hot rolling is completed at a temperature of 850° C. to 950° C., and a rolling reduction in a temperature region of 1050° C. to a hot-rolling completion temperature satisfies the following Expression 1 to obtain a hot-rolled steel sheet;
• a cold-rolling step of performing cold rolling at a reduction ratio of 30% to 75% on the hot-rolled steel sheet to obtain a cold-rolled steel sheet;
• and
• an annealing step of performing annealing in which the cold-rolled steel sheet is made to pass through a preheating zone in which heating is performed by using mixed gas whose air ratio being a ratio between a volume of air contained in the mixed gas per unit volume and a volume of air which is theoretically required to cause complete combustion of fuel gas contained in the mixed gas per unit volume in the mixed gas of air and fuel gas used for preheating, is 0.7 to 1.2, to generate an oxide coating film on a surface layer portion, the steel sheet is made to pass through a reduction zone in an atmosphere in which a partial pressure ratio between H2O and H2 (P(H2O)/P(H2)) is 0.0001 to 2.00 at a maximum heating temperature of Ac3 point −50° C. or more to reduce the oxide coating film to form a decarburized layer, and bending with a bending radius of 800 mm or less is performed one time or more while applying a tension of 3 to 100 MPa, while performing cooling in a temperature region of 740° C. to 500° C. at an average cooling rate of 1.0° C./s or more;
• and
• a plating step of making the base steel sheet to be immersed in a plating bath in which an effective Al amount is 0.01 to 0.18 mass % to form a hot-dip galvanized layer on a surface of the base steel sheet to produce a hot-dip galvanized steel sheet:
• 
• Expression
• 
• 
• 1
• 0.10
• ≤
• {
• ∑
• i
• =
• 1
• N
• 
• (
• 5.20
• ×
• 10
• -
• 6
• ·
• T
• i
• 3
• -
• 1.06
• ×
• 10
• -
• 2
• 
• ·
• T
• i
• 2
• +
• 1.68
• ×
• 10
• ·
• T
• i
• -
• 5.67
• ×
• 10
• 3
• )
• 2
• ·
• (
• h
• i
• -
• 1
• h
• i
• )
• 2
• ·
• exp
• (
• -
• 2.20
• ×
• 10
• 4
• T
• i
• )
• ·
• t
• i
• }
• 0.5
• ≤
• 1.00
• (
• 1
• )
• wherein in Expression 1, N indicates a total number of passes from a start of the hot rolling to a completion of the hot rolling, i indicates an order of pass, Ti indicates a rolling temperature (° C.) at i-th pass, hi indicates a sheet thickness after processing (mm) at the i-th pass, and ti indicates an elapsed time from the i-th pass to a next pass;
• wherein when i equals to 1, h0 equals to a slab thickness;
• and wherein an elapsed time from a final pass to a next pass is set to an elapsed time from the final pass to a point of time at which cooling is started after the completion of hot rolling.