Cooling device for hot-dip plated steel sheet

1. A cooling device for a hot-dip plated steel sheet which is provided on an upper side of a plating thickness control device, including a pair of wiping nozzles, in a conveyance route of the hot-dip plated steel sheet that is conveyed from a plating bath in a vertically upward direction, the cooling device comprising:

a main cooling device, including a plurality of main cooling gas spraying nozzles, that vertically sprays a main cooling gas to the hot-dip plated steel sheet; and

a preliminary cooling device, including a plurality of preliminary cooling nozzles, that is provided in a preliminary cooling section between the main cooling device and the plating thickness control device in the conveyance route, and sprays a preliminary cooling gas to a plurality of gas collision positions which are set along the preliminary cooling section,

wherein the preliminary cooling device sprays the preliminary cooling gas to each of the gas collision position in an obliquely upward direction, and

wherein the closer the gas collision position is to a lower stage of the preliminary cooling section, the smaller an angle, which is made by a spraying direction of the preliminary cooling gas and the conveyance direction of the hot-dip plated steel sheet, becomes.

2. The cooling device for a hot-dip plated steel sheet according to claim 1 ,

wherein the preliminary cooling device comprises:

a temperature sensor that detects a surface temperature of the hot-dip plated steel sheet at the gas collision position of at least a lowest stage,

a first flow velocity sensor that detects a flow velocity of a gas stream that downwardly flows from the gas collision position of at least the lowest stage along a surface of the hot-dip plated steel sheet, and

a first controller that is configured to control an ejection flow velocity of the preliminary cooling gas that is sprayed to the gas collision position of at least the lowest stage on the basis of a temperature detection result obtained from the temperature sensor and a flow velocity detection result that is obtained from the first flow velocity sensor, and

wherein when the temperature detection result obtained from the temperature sensor is defined as T (° C.),

the flow velocity detection result obtained from the first flow velocity sensor is defined as Vd (m/s), and

a limit descending flow velocity, at which a wrinkle occurs on the surface of the hot-dip plated steel sheet, is defined as a wrinkle occurrence limit descending flow velocity VL 1 (m/s),

the first controller is configured to control the ejection flow velocity of the preliminary cooling gas that is sprayed to the gas collision position of the lowest stage in order for the following Expression (3) and Expression (4) to be satisfied with respect to the gas collision position of at least the lowest stage,

VL 1 =A ·( T−C ) 2 +B ·( T−C )− D   (3)

|Vd|≤|VL1|  (4)

(in Expression (3), A, B, C, and D represent integers).

3. The cooling device for a hot-dip plated steel sheet according to claim 2 ,

wherein when a solidification initiation temperature of the hot-dip plated steel sheet is defined as Ts (° C.),

the first controller performs a control of the ejection flow velocity in a case where the temperature detection result T (° C.) obtained from the temperature sensor satisfies the following Conditional Expression (5):

Ts− 49≤ T≤Ts +9  (5).

4. The cooling device for a hot-dip plated steel sheet according to claim 1 ,

wherein the preliminary cooling device comprises:

a second flow velocity sensor that detects a flow velocity of a gas stream that flows from the gas collision position of at least a lowest stage to an upward direction along a surface of the hot-dip plated steel sheet, and

a second controller that is configured to control an ejection flow velocity of the preliminary cooling gas that is sprayed to the gas collision position of at least the lowest stage on the basis of a flow velocity detection result obtained from the second flow velocity sensor, and

wherein when the flow velocity detection result obtained from the second flow velocity sensor is defined as Vu (m/s), and

a limit ascending flow velocity, at which a wrinkle occurs on a surface of the hot-dip plated steel sheet, is defined as a wrinkle occurrence limit ascending flow velocity VL2 (m/s),

the second controller is configured to control the ejection flow velocity of the preliminary cooling gas that is sprayed to the gas collision position of the lowest stage in order for the following Expression (6) to be satisfied with respect to the gas collision position of at least the lowest stage,

|Vu|≤|VL2|  (6).

5. The cooling device for a hot-dip plated steel sheet according to claim 1 ,

wherein the preliminary cooling device comprises a plurality of preliminary cooling nozzles which are individually independent.

6. The cooling device for a hot-dip plated steel sheet according to claim 5 ,

wherein the preliminary cooling device is provided with a gap, through which the preliminary cooling gas that is used in cooling of the hot-dip plated steel sheet is discharged, between the preliminary cooling nozzles adjacent to each other.

7. The cooling device for a hot-dip plated steel sheet according to claim 1 ,

wherein the main cooling device and the preliminary cooling device are configured integrally with each other.