Secondary cooling method and secondary cooling device for casting product in continuous casting

1. A secondary cooling method for a casting product casted in a continuous casting machine,

the continuous casting machine including, in a secondary cooling zone below a mold, a plurality of pairs of support rolls that support the casting product from both sides of the casting product in a thickness direction,

a cooling device being disposed between support rolls adjacent to each other along a casting direction of the continuous casting machine,

the cooling device including

a coolant pipe that supplies a coolant, and

a coolant guide plate with a flat plate shape for spreading the coolant on the casting product, the coolant guide plate being disposed parallel to and spaced in a perpendicular direction from a surface of the casting product,

the secondary cooling method comprising:

a step of supplying the coolant from a coolant supply port provided in the coolant guide plate to a gap between the casting product surface and the coolant guide plate, and cooling the casting product using the coolant mainly in a transition boiling region, wherein using the coolant mainly in the transition boiling region comprises using the coolant under conditions such that 80% or more of a flow channel is in a transition boiling state,

wherein an interval between the casting product surface and the coolant guide plate is 5 mm or more, and time for the coolant to reach an upstream-side end or a downstream-side end of the coolant guide plate in the casting direction from the coolant supply port is 0.6 seconds or less.

2. The secondary cooling method for a casting product in continuous casting according to claim 1 , wherein the coolant supply port is a plurality of holes aligned in one row in a width direction of the casting product or a slit whose longitudinal direction is the width direction of the casting product.

3. The secondary cooling method for a casting product in continuous casting according to claim 1 , wherein the coolant is supplied from the coolant supply port in a liquid phase, and, in a flow channel between the casting product surface and the coolant guide plate, entirely enters a gas phase before reaching an upstream-side end or a downstream-side end of the coolant guide plate in the casting direction.

4. The secondary cooling method for a casting product in continuous casting according to claim 1 , wherein vapor of the coolant is discharged from at least one of an upstream-side end and a downstream-side end, in the casting direction, of the gap between the casting product surface and the coolant guide plate.

5. The secondary cooling method for a casting product in continuous casting according to claim 1 , wherein quantity of heat removal by cooling for the coolant to entirely enter a gas phase before reaching an upstream-side end or a downstream-side end of the coolant guide plate in the casting direction satisfies Formula (A) below:

Q/W≥ 59×10 6 [J/m 3 ]  (A),

where Q denotes quantity of heat removal by cooling, and W denotes water flow density.

6. A secondary cooling device for a casting product in continuous casting, the secondary cooling device being disposed between support rolls adjacent to each other along a casting direction, among a plurality of pairs of support rolls that support the casting product from both sides of the casting product in a thickness direction, in a secondary cooling zone below a mold of a continuous casting machine,

the secondary cooling device comprising:

a coolant pipe that supplies a coolant; and

a coolant guide plate with a flat plate shape for spreading the coolant on the casting product, the coolant guide plate being disposed parallel to and spaced in a perpendicular direction from a surface of the casting product,

wherein an interval between the casting product surface and the coolant guide plate is 5 mm or more, and is set in a manner that time for the coolant to reach an upstream-side end or a downstream-side end of the coolant guide plate in the casting direction from a coolant supply port provided in the coolant guide plate is 0.6 seconds or less, and

the coolant is supplied from the coolant supply port to a gap between the casting product surface and the coolant guide plate, and the casting product is cooled using the coolant mainly in a transition boiling region, wherein using the coolant mainly in the transition boiling region comprises using the coolant under conditions such that 80% or more of a flow channel is in a transition boiling state.

7. The secondary cooling device for a casting product in continuous casting according to claim 6 , further comprising

an interval control mechanism that controls the interval between the casting product surface and the coolant guide plate.

8. The secondary cooling device for a casting product in continuous casting according to claim 6 , wherein the coolant supply port s a plurality of holes aligned in one row in a width direction of the casting product or a slit whose longitudinal direction is the width direction of the casting product.

9. The secondary cooling device for a casting product in continuous casting according to claim 6 , further comprising

an exhaust part that discharges the coolant that has entered a gas phase from at least one of an upstream-side end and a downstream-side end, in the casting direction, of the gap between the casting product surface and the coolant guide plate.