US-12624414-B2 - Cooling water temperature control method and cooling water temperature control device for steel sheet

US12624414B2US 12624414 B2US12624414 B2US 12624414B2US-12624414-B2

Abstract

A cooling water temperature control method, includes: calculating thickness of a water film remaining on a steel sheet; calculating a change in the thickness of the water film; calculating a change in temperature of the steel sheet; calculating a steel sheet temperature on an exit side of a 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 a drying equipment, and setting the calculated temperature to a lower limit value; calculating a steel sheet temperature on the exit side of the draining roll at which the steel sheet temperature on the entrance side of coating equipment coincides with a predetermined temperature and setting the calculated temperature to an upper limit value; and controlling the temperature of cooling water within a range of the lower limit value and the upper limit value.

Inventors

Masaki Hirai
Yusuke Ota
Ichiro Tanokuchi

Assignees

JFE STEEL CORPORATION

Dates

Publication Date: 20260512
Application Date: 20211227
Priority Date: 20210405

Claims (2)

1 . A cooling water temperature control method for a steel sheet for controlling 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 method comprising: (a) calculating thickness of the water film remaining on the steel sheet on the exit side of the draining roll; (b) calculating 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) calculating 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) calculating, 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 setting the calculated steel sheet temperature to a lower limit value of cooling water temperature; (e) calculating, 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 setting the calculated steel sheet temperature to an upper limit value of the cooling water temperature; and (f) controlling the temperature of the cooling water within a range of the lower limit value and the upper limit value set in (d) and (e).
2 . 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).

Description

FIELD The present invention relates to a cooling water temperature control method and a cooling water temperature control device for a steel sheet. BACKGROUND In an annealing process line for a thin steel sheet, coating is performed on the surface of the annealed steel sheet using coating equipment. In general, from the viewpoint of characteristics of coating liquid used for coating and coating quality, it is necessary to set the temperature of the steel sheet to a predetermined temperature (for example, 30° C.) or lower on an entrance side of the coating equipment. Therefore, the temperature of the annealed steel sheet is controlled to a predetermined temperature or lower by immersing the steel sheet in cooling water having a predetermined temperature or lower or spraying the cooling water having the predetermined temperature or lower on the steel sheet using water cooling equipment (see Patent Literature 1). If the cooling water used in the water cooling equipment remains as a water film on the steel sheet, since a problem occurs in the coating quality, the water film is removed by providing drying equipment on an exit side of the water cooling equipment and blowing dehumidified air from the drying equipment (see Patent Literature 2). Here, a method of cooling and drying the annealed steel sheet on the entrance side of the coating equipment is specifically explained with reference to FIG. 4. FIG. 4 is a schematic diagram for explaining a method of cooling and drying the annealed steel sheet on the entrance side of the coating equipment. As illustrated in FIG. 4, in this method, an annealed steel sheet S is cooled to a cooling water temperature by immersing the annealed steel sheet S in cooling water W in the f water cooling tank 1 and, thereafter, a water film on the steel sheet S is removed (drained) by a draining roll 2 on an exit side of the water cooling tank 1. However, the water film on the steel sheet S cannot be completely removed only by the draining roll 2. In general, a water film having thickness of 3 μm or less remains on the steel sheet S after the water film is removed by the draining roll 2. When the steel sheet S is coated in a state in which the water film remains, a problem in coating quality occurs. Therefore, dehumidified air is blown against the steel sheet S using drying equipment 4 after the draining, the water film remaining on the steel sheet S is completely removed, and, thereafter, the steel sheet S is coated by coating equipment 5. Note that, in a cooling process for the steel sheet S, the temperature of the cooling water W rises with heat quantity taken from the steel sheet S having temperature of 100° C. or lower after annealing. Therefore, the cooling water W in the water cooling tank 1 is sent to a chiller 7 by a circulation pump 6 and the cooling water W is cooled to predetermined temperature using the chiller 7 and thereafter returned to the water cooling tank 1 to make it possible to continuously cool the steel sheet S. In a drying process for the steel sheet S, if the dehumidified air is at a high temperature, the steel sheet temperature exceeds a predetermined temperature, which causes a problem in coating quality. Therefore, in general, hot air (approximately 80 to 95° C.) subjected to heat exchange by steam is used as the dehumidified air. However, for the above reason, the steel sheet S is dried using the dehumidified air having low temperature from a dehumidifier 8. Subsequently, a relation between a steel sheet temperature and a water film thickness is explained with reference to FIG. 5. FIG. 5 is a schematic diagram for explaining the relation between the steel sheet temperature and the water film thickness. In FIG. 5, the vertical axis indicates the steel sheet temperature and the water film thickness and the horizontal axis indicates the distance from the draining roll 2. AS illustrated in FIG. 5, first, in the position of the draining roll 2, the steel sheet temperature coincides with a cooling water temperature. Subsequently, in a path to an entrance side of the drying equipment 4, the steel sheet temperature is slightly lowered by heat extraction (heat of vaporization) due to evaporation (vaporization) of the water film generated from the difference between water vapor concentration in the atmosphere and water vapor concentration of the water film. Subsequently, in the drying equipment 4, the steel sheet temperature is changed by addition and subtraction (heat input during drying) of heat quantity received from the dehumidified air 9 and heat quantity removed when the water film is vaporized. In a bath to the coating equipment 5, a temperature change of the steel sheet S due to heat transfer caused by the temperature difference between the atmosphere and the steel sheet S is taken into account to set the steel sheet temperature on an entrance side of the coating equipment 5 be equal to or lower than a predetermined temperature necessary for coating quality.