Manufacturing apparatus of hot-rolled steel sheet and manufacturing method of steel sheet

1. A manufacturing apparatus of a hot-rolled steel sheet comprising:

a row of hot finish rolling mills; and

a cooling apparatus which is disposed on a lower process side of a final stand in the row of hot finish rolling mills in a manner capable of cooling a steel sheet being transported on transporting rolls,

wherein the final stand comprises a housing which holds a work roll;

the housing comprises a pair of standing side members;

the cooling apparatus comprises a plurality of rows of upper surface cooling nozzles which are provided with cooling nozzles capable of spraying cooling water over an upper surface of the transported steel sheet, and which are disposed along a transporting direction of the transported steel sheet;

the cooling apparatus comprises a plurality of rows of lower surface cooling nozzles which are provided with cooling nozzles capable of spraying cooling water over a lower surface of the transported steel sheet, and which are disposed along the transporting direction of the transported steel sheet;

the cooling apparatus comprises an upper surface guide which is disposed on the upper surface side of the transported steel sheet;

an end portion of the cooling apparatus on a side which is the closest to the final stand is disposed between the pair of the standing side members of the housing in the final stand; and

when defining a width of a uniformly cooled region as W [m]; defining an average gap distance between the end portion of the width of the uniformly cooled region and the standing side member of the housing as W sw [m]; defining a gravity acceleration rate as g [m/s 2 ]; defining an average water volume density of the width of the uniformly cooled region as Q q [m 3 /(m 2 ·s)]; defining a value determined by the average gap distance W sw and an average distance h [m] between the upper surface guide and the upper surface of the steel sheet as C; and satisfying a relation, Q q >0.08, a relation

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1

C

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2

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·

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Q

q

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W

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1

is satisfied.

2. The manufacturing apparatus of a hot-rolled steel sheet according to claim 1 , wherein the cooling nozzle provided to the row of cooling nozzles is a flat spray nozzle.

3. A manufacturing method of a hot-rolled steel sheet, comprising the steps of:

rolling a steel sheet in a row of hot finish rolling mills; and

cooling the steel sheet with water after the steel sheet has been rolled in the step of rolling;

wherein in the step of cooling with water, supplying water from both upper surface and lower surface sides of the steel sheet to cool the steel sheet, such that an average water volume density Q q [m 3 /(m 2 ·s)] in a width of a uniformly cooled region is larger than 0.08 [m 3 /(m 2 ·s)]; and

discharging the water used for cooling from a gap between an end portion of the width of the uniformly cooled region and a standing side member of a housing in a final stand of the row of hot finish rolling mills, the gap satisfying a relation

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1.7

+

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1

-

1

C

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2

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SW

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g

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1

when defining a width of a uniformly cooled region as W [m]; defining an average distance of the gap as W sw [m]; defining a gravity acceleration rate as g [m/s 2 ]; defining, as C, a value determined by the average gap distance W sw and an average distance h [m] between the upper surface guide used in the step of cooling with water and the upper surface of the steel sheet.

4. A manufacturing method of a hot-rolled steel sheet, according to claim 3 ; further comprising

rolling the steel sheet with the largest rolling reduction of the final stand of the row of hot finish rolling mills.

5. A manufacturing method of a hot-rolled steel sheet, according to claim 3 ; further comprising

starting cooling the steel sheet after pinching a top portion of the steel sheet by a pinch roll to apply tension to the steel sheet.