Manufacturing method of hot-rolled steel sheet

1. A manufacturing method of a hot-rolled steel sheet, wherein cooling water is sprayed at an inside of a housing of a final stand on a lower process side of the final stand, which is disposed on an end on the lower process side in a row of hot finishing mills, to rapidly cool a rolled material;

a surface temperature of the rolled material is measured on an entry side of the final stand, to obtain an entry-side surface temperature; and

an entry-side target surface temperature, which is a target surface temperature of the rolled material at the measurement position of the entry-side surface temperature, is calculated from an exit-side target surface temperature, which is a target surface temperature of the rolled material on an exit side of the final stand, based on: a temperature increase by a heat generated by rolling processing in the final stand; a temperature reduction by contact of a work roll in the final stand with the rolled material; and a temperature reduction by air cooling in transportation from the measurement position of the entry-side surface temperature to the work roll of the final stand.

2. The manufacturing method of a hot-rolled steel sheet according to claim 1 , wherein in calculating the entry-side target surface temperature, changes in the surface temperature of the rolled material in a rolling direction are included as an element.

3. The manufacturing method of a hot-rolled steel sheet according to claim 1 , wherein in a case when rolling by the row of hot finishing mills is carried out by a rolling method to change a rolling reduction of the rolling mills during the rolling, sheet thickness changes and changes in a friction coefficient between the rolled material and the work roll due to the rolling reduction changes are also included as elements in calculating the entry-side target surface temperature.

4. The manufacturing method of a hot-rolled steel sheet according to claim 1 , wherein the rolled material is cooled by cooling water also in the entry side of the final stand; and

in calculating the entry-side target surface temperature, the temperature reduction by water cooling at a time of transportation from the measurement position of the entry-side surface temperature to the work roll of the final stand is also included as an element.

5. The manufacturing method of a hot rolled steel sheet according claim 1 , wherein a measuring device of the entry-side surface temperature is a thermometer disposed at a position opposing to the surface of the rolled material; and

as to the thermometer, a plurality of the thermometers are disposed in a sheet width direction of the rolled material, or one width direction thermometer is disposed.

6. The manufacturing method of a hot-rolled steel sheet according to claim 1 , wherein the measuring device of the entry-side surface temperature is a water column thermometer comprising: a radiation thermometer disposed at the position opposing to the surface of the rolled material; and a water column forming device for forming a watercourse as a light waveguide between the rolled material and the radiation thermometer.

7. The manufacturing method of a hot-rolled steel sheet according to claim 1 , wherein the entry-side target surface temperature and the entry-side surface temperature are compared based on the calculated entry-side target surface temperature and the measured entry-side surface temperature; and

a command is sent to at least one of a coilbox, rough bar heater, descaler, and between-stand cooling device such that the entry-side surface temperature becomes the entry-side target surface temperature.

8. The manufacturing method of a hot-rolled steel sheet according to claim 1 , wherein when rolling is carried out without performing the rapid cooling, a surface temperature of the rolled material is measured in a zone supposed to be rapidly coolable or immediately after the zone to obtain a post-rolling surface temperature,

wherein a surface temperature of the rolled material on the exit side of the final stand is calculated based on the measured entry-side surface temperature and the post-rolling surface temperature; the calculated surface temperature is compared with a surface temperature of the rolled material on the exit side of the final stand obtained by a calculation process of the entry-side target surface temperature; and the calculation of the entry-side target surface temperature when performing the rapid cooling is modified based on an error.

9. The manufacturing method of a hot rolled steel sheet according to claim 1 , wherein water flow density of the cooling water is 10 m 3 /(m 2 ·min) or more.

10. A manufacturing method of a hot-rolled steel sheet, wherein cooling water is sprayed at an inside of a housing of a final stand on a lower process side of the final stand, which is disposed on an end on the lower process side in a row of hot finishing mills, to rapidly cool a rolled material;

a surface temperature of the rolled material is measured on an entry side of the final stand, to obtain an entry-side surface temperature;

a prescribed entry-side target surface temperature and the measured entry-side surface temperature are compared; and

a command is sent to at least one of a coilbox, rough bar heater, descaler, and between-stand cooling device such that the measured entry-side surface temperature becomes the entry-side target surface temperature.

11. The manufacturing method of a hot rolled steel sheet according to claim 10 , wherein water flow density of the cooling water is 10 m 3 /(m 2 ·min) or more.