Cooling apparatus and cooling method for hot rolling

1. A cooling apparatus for hot rolling which is installed on a downstream side of a finishing mill of a continuous hot rolling mill, and cools a steel sheet rolled by the finishing mill while being conveyed, the apparatus comprising:

first pinch rolls which, during an interval when the steel sheet fed out from a final stand of the finishing mill moves from a position of the final stand to a position where a surface temperature of the steel sheet reaches 850° C. or less, pinch the steel sheet while applying tension of 3.9 MPa or greater, the first pinch rolls being installed at a position immediately after where the surface temperature of the steel sheet is cooled to 850° C.,

second pinch rolls being installed at a position where a surface temperature of the steel sheet is from 650° C. to 550° C.,

third pinch rolls being installed at a position where a surface temperature of the steel sheet is from 450° C. to 350° C., and

a rapid cooling unit configured to rapidly cool the steel sheet to a temperature of 850° C. or lower at an immediate rapid cooling zone provided between the final stand and the first pinch roll,

wherein the second pinch rolls and the third pinch rolls apply tension of 3.9 MPa or greater between the position of the steel sheet where the second pinch rolls are installed and the position of the steel sheet where the third pinch rolls are installed.

2. The cooling apparatus for hot rolling according to claim 1 , wherein a lower limit of the surface temperature of the steel sheet at a position where the tension is applied exceeds 650° C.

3. A cooling method for hot rolling, that, on a downstream side of a finishing mill of a continuous hot rolling mill, cools a steel sheet rolled by the finishing mill while being conveyed,

the method comprising:

a first step of controlling a surface temperature of the steel sheet fed out from a final stand of the finishing mill to 850° C. or lower;

a second step of pinching the steel sheet while applying tension of 3.9 MPa or greater, from a position of the final stand to a position where a surface temperature of the steel sheet reaches 850° C. or less;

a third step in which, between a position where the surface temperature of the steel sheet reaches 650° C. to 550° C., and a position where the surface temperature of the steel sheet reaches 450° C. to 350° C., tension of 3.9 MPa or greater is applied, and

a rapid cooling step wherein the steel sheet is rapidly cooled in a region between the final stand and a position at which the surface temperature of the steel sheet becomes 850° C.,

wherein in the second step of pinching the steel sheet, the tension is applied at a position immediately after where the surface temperature of the steel sheet cools to 850° C.

4. The cooling method for hot rolling according to claim 3 , wherein, in the first step, a lower limit of the surface temperature at a position where the tension is applied in the subsequent second step is a temperature exceeding 650° C.