KR20040022871A - Method for Cooling Hot Rolled Steel Sheet by Spraying Water first on Under Face of the Sheet - Google Patents

Abstract

PURPOSE: A hot rolled strip cooling method is provided which is capable of solving problems of material defects due to detection error of coiling temperature by fundamentally preventing retention of cooling water on an upper part of hot rolled strip. CONSTITUTION: In a method for coiling the cooled hot rolled strip by cooling a hot rolled strip in a cooling facility comprising dynamic cooling part and feedback cooling part, the method for cooling the hot rolled strip by preferentially injecting water onto a lower part of the strip comprises the steps of setting the target coiling temperature range; injecting cooling water onto upper and lower parts of the hot rolled strip transferred by opening header of upper bank and lower bank of the dynamic cooling part; measuring coiling temperature of the hot rolled strip passing through the feedback cooling part following the dynamic cooling part; sequentially opening the header of the lower bank of the feedback cooling part and the header of the upper bank of the feedback cooling part in case that the measured coiling temperature values are higher than a target coiling temperature, thereby injecting cooling water onto the hot rolled strip so that measured coiling temperature values satisfy the target coiling temperature range; and sequentially closing the header of the upper bank of the feedback cooling part and the header of the lower bank of the feedback cooling part in case that the measured coiling temperature values are lower than the target coiling temperature so that measured coiling temperature values satisfy the target coiling temperature range.

Description

Method for Cooling Hot Rolled Steel Sheet by Spraying Water first on Under Face of the Sheet}

The present invention relates to a method for cooling a hot rolled steel sheet running on a run-out table of a hot rolling process, and more particularly, by cooling water remaining in a curved portion due to a temperature difference between upper and lower parts of a hot rolled steel sheet during a cooling process of a hot rolled steel sheet. It relates to a method for cooling a hot rolled steel sheet that can prevent material defects caused by winding temperature detection error.

In general, the hot rolled steel sheet is measured from the rear of the rolling mill when passing through the run-out table (hereinafter referred to as ROT) to obtain the desired material through the finish rolling mill 30, as shown in FIG. Cooling water is poured into the upper and lower parts of the hot-rolled steel sheet so that the finishing temperature of the steel sheet becomes the target winding temperature. The cooling water is injected into a cooling system consisting of a dynamic cooling unit and a feedback cooling unit to obtain a winding temperature in consideration of the rolling speed and the measured finish temperature of the hot rolled steel sheet.

In the dynamic cooling unit 10 and the feedback cooling unit 20 of the cooling facility, the banks 11, 12, 21, 22 provided with a plurality of headers are formed of a plurality of control units. The bank of the cooling section is a cooling water storage container and injects the cooling water to the hot-rolled steel sheet through the header. Both the dynamic cooling unit and the feedback cooling unit are provided with upper banks 11 and 21 provided on the upper portion of the hot rolled steel sheet and lower banks 12 and 22 provided on the lower portion of the hot rolled steel sheet. For example, the dynamic cooling unit 10 includes 16 upper and lower banks 11 and 12, one bank includes six headers 11a and 12a, and the feedback cooling unit 20 includes two upper and lower banks. It consists of (21, 22), and there are 12 headers 21a, 22a in one bank.

In the cooling control in the dynamic cooling unit and the feedback cooling unit, first, when the cooling water is poured to the target winding temperature in the dynamic cooling unit, the winding temperature is measured to correct the difference in the target winding temperature in the feedback cooling unit. Therefore, various cooling patterns may be implemented according to the opening and closing operation of the cooling water valve of the upper and lower banks. Typical cooling is to open the valve in the order from the front bank to the rear bank, and from the upper bank to the lower bank, and closing the valve in the reverse order. In other words, the hot-rolled steel sheet starts cooling water in the dynamic cooling unit immediately before the tip enters the cooling facility, and performs cooling while adjusting the opening amount at a predetermined length period. Then, the winding temperature is measured and compared with the target winding temperature, the header is opened in the order of the lower banks from the upper bank to the lower bank, and the closing is adjusted in the reverse order to adjust the winding temperature to the target winding temperature.

However, in some steel grades, bending occurs in the hot-rolled steel sheet as shown in FIG. 2 due to differences in upper and lower cooling capabilities of the hot-rolled steel sheet and stress differences. In this case, the upper cooling water stays on the upper surface of the hot-rolled steel sheet, and the curvature is accelerated by the staying water, and the water flows through the winding temperature measuring thermometer without being removed from the upper bank of the feedback cooling unit. Temperature detection errors 50 to 54 will occur. As a result, material defects occur regardless of the temperature of the actual hot rolled steel sheet. The bending of hot rolled steel sheet occurs a lot in high carbon steel and high tensile steel.

The present invention provides a method of cooling a hot rolled steel sheet that can prevent the problem of material defects due to a detection error of the winding temperature by preventing the retention of the cooling water in the upper portion of the hot rolled steel sheet.

1 is an exemplary view showing a cooling method of a hot rolled steel sheet in a winding cooling installation.

Figure 2 is a photograph showing the C curve by the retention of cooling water on the top of the hot rolled steel sheet

3 is a graph showing a winding temperature detection error due to retention of cooling water on top of a hot rolled steel wave.

4 is an example of a cooling control flowchart according to the present invention.

Figure 5 is a picture of first pouring the cooling water in the lower portion of the hot rolled steel sheet according to the present invention.

6 is a graph showing the coiling temperature detected when cooling the hot rolled steel sheet according to the present invention.

* Description of the main parts of the drawings *

10 ...... Dynamic Cooling Unit

20 ...... Feedback Cooling Unit

11, 21 ...... Upper Bank

12, 22 ...... Bottom Bank

11a, 11b, 12a, 12b ...... header

30 ...... finish rolling roll

40 ...... winding temperature measuring instrument

Hot rolled steel sheet cooling method of the present invention for achieving the above object,

In a method for cooling and winding a hot rolled steel sheet in a cooling facility comprising a dynamic cooling unit and a feedback cooling unit, setting a target winding temperature range,

Injecting coolant into the upper and lower portions of the hot rolled steel sheet which opens and transfers the headers of the upper and lower banks of the dynamic cooling unit;

Measuring a winding temperature of the hot rolled steel sheet passing through the dynamic cooling unit and passing through a feedback cooling unit;

When the measurement winding temperature is higher than the target winding temperature, the header is opened in the order of the header of the feedback cooling unit upper bank from the header of the feedback cooling unit lower bank, and the cooling water is injected into the hot rolled steel sheet so that the measurement winding temperature is within the target winding temperature range. Satisfying,

If the measurement winding temperature is lower than the target winding temperature, closing the header in the order of the header of the feedback cooling unit lower bank from the header of the upper bank of the feedback cooling unit so that the measurement winding temperature satisfies the target winding temperature range. do.

Hereinafter, the present invention will be described in detail.

In the present invention, the header is opened in the order of cooling water feed of the hot-rolled steel sheet in the feedback cooling unit in the order of the upper bank to the lower bank of the feedback cooling unit, and the closing order is reversed to minimize the retention of the coolant on the top of the hot-rolled steel sheet. have.

Hot rolled steel sheet to be applied to the present invention is not particularly limited to the steel grade or thickness, but when applied to the steel species that a lot of bending occurs, the application effect is large. Usually, high carbon steel (C: 0.05% or more) or high tensile steel (strength: 50kg / mm ² or more), especially when the thickness of the hot rolled steel sheet is 3 to 5mm, a lot of bending occurs.

In the present invention, when the tip of the hot-rolled steel sheet enters the run-out table, the cooling in the dynamic cooling unit is performed according to a conventional method, and the cooling in the feedback cooling unit adopts the lower priority casting method.

In general, the cooling of the hot rolled steel sheet is cooled in the dynamic cooling unit, and the remaining cooling amount ΔT _Cool , that is, the feedback cooling unit cooling amount is obtained by the following equation and feedback cooling.

[Relationship 1]

ΔT _Cool = FDT- ΔT _air -ΔT _water

ΔT _Cool : Cooling residual amount

FDT: Finished rolling back temperature

ΔT _air : air cooling drop

ΔT _water : water cooling drop

In the relation (1) in the condition of ΔT <0, the initial cooling amount of the feedback cooling unit is decreased, and vice versa, the target winding temperature is increased by increasing the feedback cooling unit water supply amount. That is, when the tip of the hot rolled steel sheet passes through the coiling temperature detecting thermometer 40 and the coiling temperature is measured, the amount of cooling of the feedback cooling unit is calculated by comparing the measured coiling temperature with the target coiling temperature, and the amount of cooling per header of the bank. In order to determine the opening and closing order of the header.

In this case, in the present invention, the header of the lower bank of the feedback cooling unit is first opened and closed, and the cooling water is inferior to the lower part of the hot-rolled steel sheet, and if only the header of the lower bank of the feedback cooling unit is insufficient, the header of the upper bank of the feedback cooling unit is opened to open the hot-rolled steel sheet. Pour the coolant at the top of the To reduce the amount of chilled water in reverse, do the reverse. In general, the amount of water cooling per header of the bank is calculated as in Equation 2.

[Relationship 2]

Vx: Mail speed

C: Specific heat

ρ: material density

H _F : Thickness

l _BNK : Cooling Zone length

Q _x : Heat flux

In Equation 2, the number of openings of the bank header is increased until the condition of ΣT> ΔT _Cool is satisfied.

Figure 4 is a flow chart of the lower cooling first cooling method in the present invention, with reference to this one example of the cooling method of the present invention. FIG. 4 illustrates control diagrams to which the present invention is applied to steel grades in which bending occurs by dividing steel grades in which bending occurs frequently and steel grades in which bending does not occur. In this case, it is also possible to apply the present invention to all steel grades, regardless of the steel grade.

First, it is determined whether the bending occurs according to the material and thickness of the hot rolled steel sheet.

In the case of steel grades which do not bend, it is cooled according to the usual upper priority casting method.

And, in the case of the steel type in which the curvature occurs, the feedback cooling unit is cooled by the lower priority casting method. That is, if the measured coiling temperature of the hot rolled steel sheet is higher than the target coiling temperature, the feedback cooling unit opens the header in order from the header of the lower bank to the header of the upper bank to inject the coolant so that the measuring coil temperature satisfies the target coiling temperature range Do it. If the measured winding temperature of the hot rolled steel sheet is lower than the target winding temperature, the feedback cooling unit closes the header in the order of the header of the upper bank to the header of the lower bank, thereby reducing the amount of cooling water to be measured. To be satisfied.

In the present invention, when the tip of the hot-rolled steel sheet passes through the dynamic cooling unit, water may not start in the feedback cooling unit. Alternatively, when the bank consists of two banks, a part of the lower header of the first bank may be opened, and the lower header of the second bank and the lower header of the second bank may be opened instead of opening all of the lower header of the first bank. The upper header may not be open. The selection may be determined according to the cooling capacity according to the target winding temperature. Most preferably, when the tip of the hot-rolled steel sheet enters the feedback cooling unit, the remaining header is opened while a part of the lower header of the first bank of the feedback cooling unit is opened. By adjusting the opening of the cooling water to adjust the amount of water.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

C: 0.07 ~ 0.09%, Mn: 0.8 ~ 1.0%, Si: ~ 0.02%, P: 0.03 ~ 1.10%, B: 0.0005 ~ 0.0015 The finish rolling temperature of 4mm thick hot rolled steel sheet is 880 ^o C Rolled. Before the end of the hot rolled steel sheet proceeds to the dynamic cooling section, the header of the upper and lower banks of the dynamic cooling section was completely opened, and the coolant was injected to meet the target winding temperature range (630 to 670 ^° C.). The cooling residual amount (cooling amount of the feedback cooling unit) for the target winding temperature was calculated as (40 ^° C). Therefore, as shown in Table 1, the operation was started by opening the header of the dynamic cooling unit and the feedback cooling unit.

Bank Dynamic Cooling Unit Feedback Cooling Unit One 2 3 4 5 6 7-14 Bank 15 16 Number of top headers open 6 6 6 6 6 6 0 5 0 Lower header opening count 6 6 6 6 6 4 0 12 0 The dynamic cooling unit consists of 14 banks, one bank consists of 6 headers up and down, the feedback cooling unit consists of 2 banks, and one bank consists of 12 headers up and down.

As shown in Table 1, the winding temperature was measured while filling the coolant with the upper and lower headers of the second bank closed, while the lower headers of the first bank were all opened and only the upper headers were opened. When the measured winding temperature is higher than the target winding temperature range, the upper bank of the first bank of the feedback cooling unit was opened, and if necessary, the header was opened from the lower header of the second bank to the upper header in order to increase the amount of cooling water. . If the measured winding temperature is lower than the target winding temperature range, the cooling pattern is set to satisfy the target winding temperature range by closing the header from the header of the upper bank of the feedback cooling unit to the header of the lower bank. Quantity was added or subtracted. 5 is a photo showing the header first opening method of the lower bank in the feedback cooling unit.

The results are shown in FIG.

As shown in FIG. 6, it can be seen that the measured winding temperature satisfies the target winding temperature range, and no winding temperature detection error occurs.

As described above, according to the present invention, the feedback cooling unit controls the target winding temperature by ensuring the winding temperature detection reliability of the hot rolled steel sheet by the lower priority cooling method of the hot rolled steel sheet while simultaneously performing the lower portion having a lower cooling capacity than the upper portion. It can be finely adjusted to solve the problem of material defects due to the winding temperature detection error, in particular, there is an effect that can reduce the winding temperature variation in the winding coil of the hot rolled steel sheet.

Claims (2)

In a method of cooling and winding a hot rolled steel sheet in a cooling facility comprising a dynamic cooling unit and a feedback cooling unit, Setting a target winding temperature range, Injecting cooling water into the upper and lower portions of the hot rolled steel sheet which opens and transfers the headers of the upper and lower banks of the dynamic cooling unit, Measuring a winding temperature of the hot rolled steel sheet passing through the dynamic cooling unit and passing through a feedback cooling unit; When the measurement winding temperature is higher than the target winding temperature, the header is opened in the order of the header of the feedback cooling unit upper bank from the header of the feedback cooling unit lower bank, and the cooling water is injected into the hot rolled steel sheet so that the measurement winding temperature is within the target winding temperature range. Satisfying, If the measurement winding temperature is lower than the target winding temperature, the lower step comprising the step of closing the header in the order of the feedback cooling unit lower bank in the header of the upper bank of the feedback cooling unit to satisfy the target winding temperature range First, a method of cooling hot rolled steel sheets by pouring water. The method of claim 1, wherein when the front end of the hot-rolled steel sheet passes through the feedback cooling unit, part of the header of the lower bank of the feedback cooling unit is opened to pour the coolant.