Cooling device of hot-rolled steel strip

1. A cooling device that cools a hot-rolled steel strip which has passed through a finishing rolling, the cooling device comprising:

a rapid cooling device that cools a target surface of the hot-rolled steel strip from a first temperature of not lower than 600° C. and not higher than 650° C. to a second temperature of not higher than 450° C., with cooling water in a water amount density of not lower than 4 m 3 /m 2 /min and not higher than 10 m 3 /m 2 /min, with respect to an area of the target surface, an area of a portion where a plurality of spray jets of the cooling water directly strike on the target surface is at least 80%.

2. The cooling device that cools the hot-rolled steel strip according to claim 1 , wherein the rapid cooling device comprises a plurality of spray nozzles that eject the cooling water, the plurality of the spray nozzles ejecting the cooling water such that the cooling water strikes on the target surface with a velocity of not lower than 20 m/sec.

3. The cooling device that cools the hot-rolled steel strip according to claim 1 or 2 , wherein the rapid cooling device comprises a plurality of spray nozzles that eject the cooling water, the plurality of the spray nozzles ejecting the cooling water such that the cooling water strikes on the target surface with a pressure of not lower than 2 kPa.

4. The cooling device that cools the hot-rolled steel strip according to claim 1 or 2 , wherein the rapid cooling device comprises a plurality of the spray nozzles eject the cooling water in a conical shape, and an impact angle of the cooling water to the target surface is not smaller than 75 degrees and not larger than 90 degrees when viewed from a steel strip rolling directions.

5. The cooling device that cools the hot-rolled steel strip according to claim 1 or 2 , further comprising:

a first water-blocking mechanism that blocks the cooling water which flows on an upper surface of the hot-rolled steel strip at an upstream side from a position where a supply of the cooling water starts; and

a second water-blocking mechanism that blocks the cooling water which flows on the upper surface of the hot-rolled steel strip at a downstream side from a position where the supply of the cooling water finishes.

6. The cooling device that cools the hot-rolled steel strip according to claim 5 , wherein:

the first water-blocking mechanism comprises a first water-blocking nozzle that ejects blocking water to an upstream side from the target surface; and

the second water-blocking mechanism comprises a second water-blocking nozzle that ejects blocking water to a downstream side from the target surface.

7. The cooling device that cools the hot-rolled steel strip according to claim 6 , wherein:

the first water-blocking mechanism comprises a first water-blocking roll provided at a downstream side from the first water-blocking nozzle; and

the second water-blocking mechanism comprises a second water-blocking roll provided at an upstream side from the second water-blocking nozzle.

8. The cooling device that cools the hot-rolled steel strip according to claim 1 or 2 , wherein the rapid cooling device cools only an upper surface of the hot-rolled steel strip.

9. The cooling device that cools the hot-rolled steel strip according to claim 1 or 2 , wherein:

the rapid cooling device cools an upper surface and a lower surface of the hot-rolled steel strip; and

a cooling performance for the upper surface of the hot-rolled steel strip is not less than 0.8 times and not more than 1.2 times of a cooling performance for the lower surface of the hot-rolled steel strip.