JP2012101237A - Method for controlling cooling of hot-rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for controlling the cooling of a hot-rolled steel sheet for cooling to a prescribed winding temperature with an irreducible minimum quantity of cooling water without deteriorating the grade of the surface of the hot-rolled steel sheet even when rolling speed varies on the midway of rolling.SOLUTION: In the method for controlling the cooling when performing the cooling by jetting cooling water onto the hot-rolled steel sheet which is hot finish-rolled by using cooling equipment from which cooling water is jetted, the cooling equipment comprises a plurality of cooling sections which are divided in the conveying direction of a run out table, each cooling section can control the on-off of jetting of the cooling water independently and a plurality of cooling means which are different in cooling property are parallelly provided in the cooling section. The cooling means in each cooling section is selected in accordance with the variation of the cooling rate so that the water consumption in the whole cooling unit becomes minimum and the surface temperature of the hot-rolled steel sheet during cooling is not lower than the quenching temperature.

Description

The present invention relates to a method for controlling the cooling of a hot-rolled steel sheet when the hot-rolled steel sheet that has been hot-finished and rolled is cooled by a cooling facility disposed on a hot run table. Specifically, the present invention relates to a cooling control method when cooling is performed by injecting cooling water onto a hot rolled steel sheet that has been hot-finished and rolled using a cooling facility that injects cooling water.

Generally, a hot-rolled steel sheet is manufactured by hot finish rolling, cooling to a predetermined temperature by a cooling facility disposed on a hot run table, and winding it with a winder. In order to increase the productivity of the hot-rolled steel sheet, the rolling speed is determined according to the maximum cooling capacity of the cooling equipment. On the other hand, until the end of the hot rolled steel sheet that has been hot finish rolled bites into the winder, the rolling speed is reduced in order to stably feed the sheet. In order to obtain a predetermined coiling temperature, it is necessary to change the cooling conditions in accordance with the reduction in rolling speed, and various cooling control methods have been conventionally proposed.

For example, Patent Document 1 discloses a cooling method including a cooling process for rapid cooling and a cooling process for slow cooling so that the amount of temperature drop is constant in the steel strip according to the rolling speed of the hot-rolled steel strip. Discloses a cooling method for controlling the cooling zone length of the rapid cooling process. In other words, when the rolling speed is low, the cooling zone length in the rapid cooling process is shortened so that the cooling time is the same for the entire steel strip.

Further, Patent Document 2 discloses a water cooling device having a water amount adjusting valve for adjusting the amount of cooling water to be injected, the opening degree being independently controllable, and a set value of the opening amount of the water amount adjusting valve depending on the rolling speed. A cooling method for correcting the above is disclosed.

JP 2001-246409 A JP 2009-56504 A

In the method disclosed in Patent Document 1, when the rolling speed is low, the hot-rolled steel sheet is cooled by being injected with cooling water in the previous stage of the rapid cooling process, and then passed through an air-cooling zone where no cooling water is injected. It will be cooled in the cooling process. Since the hot rolled steel sheet is reheated in the air cooling zone and the surface temperature of the steel sheet rises, in order to make the temperature at the entry side of the slow cooling process the same as when the rolling speed is high, the rapid cooling process must be performed at a lower temperature. It needs to be cooled. Moreover, since the cooling efficiency generally decreases as the cooling water flow rate increases, there is a problem that the power cost is high due to an increase in the amount of cooling water used. Furthermore, when cooling to a lower temperature, the steel sheet surface temperature may be lower than the quench point, and by shifting from the film boiling region to the transition boiling region, a locally overcooled part is generated and the cooling water is generated on the steel plate surface. The surface quality of the hot-rolled steel sheet deteriorates due to retention in the water, causing water patterns and scale peeling.

In the method disclosed in Patent Document 2, the amount of water is changed by adjusting the opening of the valve. However, since a time difference occurs until the flow rate actually changes by changing the valve opening, temperature control is difficult. In particular, when accelerating the rolling speed from a low state, there is a problem that it is very difficult to follow the change in flow rate with the change in the rolling speed.

Therefore, the present invention has been made in view of such problems of the prior art, and even when the rolling speed changes during rolling, the minimum necessary without deteriorating the surface quality of the hot-rolled steel sheet. It is an object of the present invention to provide a cooling control method for a hot-rolled steel sheet for cooling to a predetermined coiling temperature with a cooling water amount.

The method for controlling the cooling of a hot-rolled steel sheet according to the present invention for solving the above-described problems is as follows.
(1) A cooling control method for cooling a hot-rolled steel sheet hot-rolled by injecting cooling water using a cooling facility for injecting cooling water, wherein the cooling facility is in the direction in which the runout table is conveyed Each cooling section can independently control on / off of the cooling water jet, and the cooling section is provided with a plurality of cooling means having different cooling characteristics. It is characterized in that the cooling means of each cooling section is selected according to the change in rolling speed so that the amount of water used in the entire apparatus is minimized and the surface temperature of the hot-rolled steel sheet during cooling does not fall below the quench point. To control cooling of hot-rolled steel sheet.
(2) As the cooling means, a pipe laminar cooling means is disposed in each cooling section, a slit laminar cooling means is provided in the upstream cooling section, and a spray cooling means is provided in the downstream cooling section. The method for controlling cooling of a hot-rolled steel sheet according to (1), wherein:
(3) The method for controlling cooling of a hot-rolled steel sheet according to (1) or (2), wherein cooling means having a high cooling efficiency per flow rate is preferentially selected from the downstream cooling section.

According to the present invention, the amount of cooling water used in the entire cooling device can be reduced by selecting a cooling unit according to a change in rolling speed using a cooling facility provided with cooling units having different cooling characteristics. . Moreover, since the cooling which avoided the quench point is attained, a hot-rolled steel plate with a favorable surface quality can be manufactured.

It is equipment layout which shows one example of the cooling equipment arrangement which implements this invention. It is a conceptual diagram of the model which calculates the plate thickness direction temperature distribution in this invention. It is a figure explaining the Example of the cooling control which reduces the use cooling water flow rate in this invention. It is a figure explaining the Example of the cooling control which avoids the quench point in this invention and improves the surface quality of a steel plate. It is a figure which shows the surface temperature of a steel plate, a heat transfer rate, and a quench point. When cooling with the conventional cooling method, it is a figure explaining a mode that a steel plate surface temperature becomes below a quench point and a water pattern can be made locally. It is a figure which shows the control logic at the time of implementing this invention. It is a figure showing to the cooling means determination of S105 of FIG. It is a figure showing to the cooling means determination of S110 of FIG. It is a figure showing to the cooling means determination of S115 of FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

An example of the cooling equipment according to the embodiment of the present invention is shown in FIG. On the runout table 1 between the hot finish rolling mill and the winder, a plurality of cooling sections divided in the longitudinal direction are provided, the left side of FIG. 1 is the upstream side, and the right side is the downstream side . The upper cooling means of each cooling section includes a pipe laminar cooling means 2 and a slit laminar cooling means 4 in the upstream cooling section, and a pipe laminar cooling means 2 and a spray cooling means 5 in the downstream cooling section. Yes. The lower cooling means of each cooling section is a pipe laminar cooling means 3. Table 1 shows the specifications of each cooling means. Table 1 is a table showing the cooling characteristics of each cooling means.

In addition, a finishing delivery thermometer (FT7) 6 is provided on the exit side of the finishing mill and a No. 1 intermediate thermometer (MT1) 7 is provided in the middle of the cooling equipment. 2. An intermediate thermometer (MT2) 8 and a winding thermometer (CT) 9 are installed immediately before the winder, and the temperature at the center of the width of the upper surface of the steel sheet is measured over the entire length. No. 1 intermediate thermometer (MT1) 7, No. 1 The temperature data measured by the two intermediate thermometers (MT2) 8 and the winding thermometer (CT) 9 are reflected in the feedforward / feedback control function of the cooling calculation.

  Selection of the cooling means of each cooling section according to the rolling speed (steel plate speed) is performed according to the flow procedure shown in FIG.

  The cooling curve in each cooling control means is calculated using a cooling calculation model (FIG. 2) that can predict the thickness direction temperature distribution. Each time the steel sheet advances by a certain length, the cooling means is selected so as to achieve the target winding temperature according to the procedure of FIG. 7 based on the conveying speed, the steel sheet temperature, the water temperature, and the steel sheet temperature. Hereinafter, the selection means of the cooling means shown in FIG. 7 will be described.

S101: The thickness, the steel plate speed, the steel plate temperature, and the water temperature of the finish-rolled steel plate are measured by each measuring device (not shown) arranged in the hot rolling line.

S102: As shown in FIG. 8, a temperature calculation is performed by cooling with spray cooling means in each cooling section from the CT side toward the upstream side to obtain a cooling curve, and each from the FT side toward the downstream side In the cooling section, a temperature calculation is performed when the pipe laminator cooling means is used to obtain a cooling curve, and an intersection point a between the two is obtained.

S103: It is determined whether or not the intersection point a shown in FIG. 8 is within the cooling section in which the spray cooling means is disposed.

S104: As shown in FIG. 8, when S103 is Yes, it is determined whether or not the temperature at the intersection a is equal to or higher than the quench point of the pipe laminator cooling means.

S105: As shown in FIG. 8, when S104 is Yes, the cooling section from the FT side to the intersection point a is determined as the pipe laminator cooling means, and the cooling section downstream from the next cooling section at the intersection point a is determined as the spray cooling means. To do.

S106: As shown in FIG. 9, when S104 is No, the intersection point a is moved to the high temperature side on the cooling curve of the spray cooling means until it becomes the quench point of the pipe laminar cooling, and this point is set as the intersection point a '.

S107: It is determined whether or not the intersection point a 'shown in FIG. 9 is within the cooling section in which the spray cooling means is disposed.

S108: As shown in FIG. 9, when S107 is Yes, the temperature calculation is performed when cooling is performed by the slit laminator cooling means in each cooling section from the FT side toward the downstream side, a cooling curve is obtained, and the intersection point a ′ is determined. An intersection point with the cooling curve of the pipe laminator cooling means moved so as to pass is defined as an intersection point b.

S109: It is determined whether or not the intersection point b shown in FIG. 9 is greater than or equal to the quench point in the slit laminator cooling means.

S110: As shown in FIG. 9, when S109 is Yes, the cooling section from the FT side to the intersection b is the slit laminator cooling means, and the cooling section next to the intersection b to the cooling section at the intersection a 'is the pipe laminar cooling. As a means, the cooling section downstream from the next cooling section at the intersection a ′ is determined as the spray cooling means.

S111: When S109 is No, the calculation from S102 is executed again under the condition that the steel plate speed is reduced by a certain amount.

S112: When S107 is No, the calculation from S102 is executed again under the condition that the steel plate speed is reduced by a certain amount.

S113: As shown in FIG. 10, when S103 is No, the intersection point a is moved to the inlet side of the cooling section on the most upstream side where the spray cooling means is arranged, and the point is designated as the intersection point a ″. An intersection point between the curve and the cooling curve of the pipe laminator cooling means is defined as an intersection point c.

S114: It is determined whether or not the temperature at the intersection c shown in FIG. 10 is equal to or higher than the quench point of the pipe laminator cooling means.

S115: As shown in FIG. 10, when S114 is Yes, the cooling section from the FT side to the intersection c is the pipe laminator cooling means, the cooling section next to the intersection c to the cooling section at the intersection a ″ is air-cooled, The cooling section downstream from the cooling section next to the intersection a ″ is determined as the spray cooling means.

S116: When S114 is No, the calculation from S102 is executed again under the condition that the steel plate speed is reduced by a certain amount.
The above calculation is repeated every time the steel sheet advances by a certain length, and cooling is performed by sequentially setting the cooling means of each cooling section.

According to the above procedure, the amount of cooling water used in the entire cooling device can be reduced by preferentially selecting cooling means having a high cooling efficiency per flow rate from the downstream cooling section.

Using the hot-rolled steel sheet under the following conditions, the cooling water flow rates used in the conventional cooling method and the cooling method of the present invention were compared.
Steel type: 440 MPa class high strength hot-rolled steel sheet Thickness: 4.0 mm
Target winding temperature: 500 ° C
Conveyance speed: 630 mpm → 1000 mpm (accelerated rolling)

FIG. 3 shows a comparison of the cooling water flow rate between the conventional cooling method and the cooling method of the present invention at a conveyance speed of 630 mpm at the beginning of rolling. In the conventional cooling method, a cooling method at a maximum rolling speed of 1000 mpm is first set, and when the rolling speed at the initial stage of rolling is slow, the length of pipe laminar cooling is shortened. On the other hand, in the cooling method of the present invention, since the spray cooling means with high cooling efficiency is poured through the entire length of the cooling equipment, the use range of the pipe laminar cooling can be shortened. The water flow rate was reduced by 16.2%. As the conveyance speed is accelerated, the usage range of pipe laminar cooling water is expanded to reduce the amount of cooling water used. The amount of cooling water used at the maximum plate passing speed is the same, but the total amount is 2.6%. Cooling water reduction was possible.

The cooling histories of the conventional cooling method and the cooling method of the present invention were compared for the hot-rolled steel sheet having the above conditions.

FIG. 4 shows the cooling history of the steel sheet surface temperature in the cooling facility. In the conventional cooling method, laminar cooling is used up to the quench point (FIG. 5), and a portion that is locally supercooled as shown in FIG. On the other hand, in the cooling method of the present invention, since it is switched to weak cooling immediately before the quench point is lowered, a steel sheet with excellent surface quality could be manufactured.

  The present invention can be used in the field of cooling hot-rolled steel sheets as a cooling method that contributes to reducing the manufacturing cost of the steel sheet by reducing the flow rate of the cooling water used and improves the surface quality, which is the basic quality of the steel sheet. .

1: Run-out table 2: Upper pipe laminar cooling means 3: Lower pipe laminar cooling means 4: Upper slit laminar cooling means 5: Upper spray cooling means 6: Finishing side thermometer (FT7)
7: No. 1 Intermediate thermometer (MT1)
8: No. 2 Intermediate thermometer (MT2)
9: Winding thermometer (CT)

Claims (3)

  A cooling control method for cooling a hot rolled steel sheet hot-rolled by injecting cooling water using a cooling facility for injecting cooling water, wherein the cooling facility is divided in a conveyance direction of a runout table. Each cooling section can independently control on / off of the cooling water jet, and the cooling section is provided with a plurality of cooling means having different cooling characteristics. The cooling method of each cooling section is selected according to the change in rolling speed so that the amount of water used is minimized and the surface temperature of the hot-rolled steel sheet during cooling does not fall below the quench point. Steel sheet cooling control method.   As the cooling means, a pipe laminar cooling means is disposed in each cooling section, a slit laminar cooling means is provided in the upstream cooling section, and a spray cooling means is provided in the downstream cooling section. The method for controlling cooling of a hot-rolled steel sheet according to claim 1. The method for controlling cooling of a hot-rolled steel sheet according to claim 1 or 2, wherein a cooling means having a high cooling efficiency per flow rate is preferentially selected from the cooling section on the downstream side.

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