Cooling water temperature control method and cooling water temperature control device for steel sheet

A cooling water temperature control device for a steel sheet that controls temperature of cooling water in a line including cooling equipment that cools an annealed steel sheet using the cooling water, a draining roll that removes a water film on the steel sheet cooled by the cooling equipment, drying equipment that is disposed on an exit side of the draining roll and dries the steel sheet, and coating equipment that is disposed on an exit side of the drying equipment and coats the steel sheet, the cooling water temperature control device comprising a processor comprising hardware, the processor being configured to:

• (a) calculate thickness of the water film remaining on the steel sheet on the exit side of the draining roll;
• (b) calculate a change in the thickness of the water film from the exit side of the draining roll to the exit side of the drying equipment considering line speed;
• (c) calculate a change in temperature of the steel sheet from the exit side of the draining roll to an entrance side of the coating equipment considering the line speed;
• (d) calculate, using calculation results of (a) to (c), a steel sheet temperature on the exit side of the draining roll at which a position where the thickness of the water film on the steel sheet becomes zero coincides with an exit side position of the drying equipment, and set the calculated steel sheet temperature to a lower limit value of cooling water temperature;
• (e) calculate, using the calculation results of (a) to (c), a steel sheet temperature on the exit side of the draining roll at which the steel sheet temperature on the entrance side of the coating equipment coincides with a predetermined temperature and set the calculated steel sheet temperature to an upper limit value of the cooling water temperature;
• and
• (f) control the temperature of the cooling water within a range of the lower limit value and the upper limit value set in (d) and (e).