JPH06182416A - Cooling method for steel sheet - Google Patents

Abstract

PURPOSE:To uniformize the distribution of cooling stop temperatures in the longitudinal direction of a steel sheet by reducing the speed of the steel sheet which is being cooled according to the passing position of the sheet in a cooling device and passing the sheet through the passing position. CONSTITUTION:A thick steel sheet, etc., are passed and cooled by a dip type water cooling device. In that process, the speed of the steel sheet which is being cooled according to the passing position of the sheet is reduced and passed. In other words, the reduction of cooling capacity by the cooling device in the dipping condition is estimated by parameters of temperature for stopping cooling and of sheet thickness and the speed of the steel sheet which is passed and cooled is reduced. For example, when the length position of the steel sheet in which reduction of cooling capacity stops advances 11m from the tip into the cooling device, the passing speed is reduced from 120mpm to 100mpm. Consequently, the temperature for stopping cooling coincides with the temperature for stopping cooling at the tip part of the steel sheet over all the length and a uniform temperature for stopping cooling in the longitudinal direction is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for uniformly cooling a thick steel plate or the like in the longitudinal direction in water cooling.

[0002]

2. Description of the Related Art Generally, in a thick plate manufacturing process, a cooling process using water as a coolant is adopted for cooling for quenching and for controlled cooling immediately after rolling for the purpose of reducing alloy elements and heat treatment. Has been done. In control cooling, a technique of stopping this cooling at a temperature considerably higher than room temperature in the middle has been often used. In this process, in the method of sequentially cooling from the tip of the steel plate while passing the plate through the cooling device (hereinafter referred to as plate cooling), there is a difference in the cooling start time at each position in the steel plate longitudinal direction,
A difference occurs in the cooling start temperature at each position in the longitudinal direction of the steel plate, and when the plate is passed at a constant speed, the cooling stop temperature varies depending on the position in the longitudinal direction of the steel plate.

In addition, the temperature difference in the longitudinal direction at the end of rolling (or at the start of cooling) due to factors other than the above, such as the heating state of the slab before rolling and the effect of descaling during rolling, is called the cooling stop temperature deviation. Has occurred. Making the cooling stop temperature uniform in the longitudinal direction of the steel sheet is important for preventing the occurrence of residual stress in the steel sheet and for obtaining a uniform material in the longitudinal direction. For this problem, JP-A-61-213327,
In JP-A-61-253112, JP-A-62-199723 and JP-A-2-179826, the temperature at each position in the longitudinal direction of the steel sheet is detected on the inlet side of the cooling device, and the steel sheet is detected according to the result. Several proposals have been made to adjust the plate passing speed to make the cooling stop temperature uniform in the longitudinal direction.

However, almost all of these prior arts are methods for controlling cooling by spraying or columnar water from the top and bottom of the steel sheet, that is, a method of controlling the cooling in a state where the cooling capacity of the cooling water is always constant. The water is always replaced with fresh water, and the cooling capacity does not decrease during cooling due to the water staying around the steel plate as in the case of the drip pickling which is the object of the present invention.

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
This is a method of making the temperature in the longitudinal direction of the steel plate uniform when cooling is stopped with respect to the temperature difference in the longitudinal direction of the steel plate when cooling starts. This is a water-cooled facility with a maximum water density of about 100 l / m ² min, and the supplied water is replaced relatively quickly. That is, this method can be applied when fresh water is always ready to cool the surface of the steel sheet and the cooling capacity of the cooling device hardly changes during cooling of the steel sheet. However, when the water density is increased to obtain a higher cooling capacity, the steel sheet is cooled and the cooling mechanism in the cooling device is different from that when the water density is low.

[0006] In the cooling in the immersion state, the tip of the steel sheet is always cooled with fresh water that has been replaced, whereas the tail end of the steel sheet is cooled with water that has not been sufficiently replaced and has an increased temperature. Since there is a difference in cooling ability between the water cooled with low initial temperature and the water with increased temperature in the latter period, when the steel sheet is passed at a constant speed, the cooling stop temperature is low at the steel plate tip and stopped at the tail end. There is a problem that a phenomenon that the temperature becomes high occurs.

It is an object of the present invention to provide a uniform cooling technique in the longitudinal direction of a steel plate in immersion cooling which solves the above problems.

[0008]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention determines the passing speed of a steel sheet during cooling in consideration of a decrease in cooling capacity due to a rise in water temperature in a cooling device during cooling of a steel sheet in an immersed state. By decelerating, a uniform cooling stop temperature is obtained in the longitudinal direction of the steel sheet. That is, in the present invention, in a process of cooling a thick steel plate or the like while passing through a plate using an immersion type water cooling device, the steel plate speed during cooling is reduced according to the passage position of the plate in the cooling device, and the plate is passed. Is a method for cooling a steel sheet.

[0009]

According to the research conducted by the inventors, when the steel sheet is cooled in the immersion state, if the steel sheet is passed at a constant speed, the water temperature in the cooling device gradually rises, and the cooling capacity gradually increases from the tip to the tail of the steel sheet. Gradually decreases. However, from the front end of the steel plate to the end of the cooling device, the cooling capacity stops decreasing and the cooling capacity reaches a steady state.

According to the present invention, when the leading end of the steel sheet enters the cooling device and reaches the cooling device exit side, the stripping speed of the steel sheet is reduced, and then the steel sheet is stripped at a constant speed. Since the cooling time in the longitudinal direction is changed, it is possible to obtain uniform cooling in which the cooling stop temperature is constant in the longitudinal direction of the steel sheet, regardless of the cooling capacity of the cooling device.

[0011]

[Example] Fig. 2 is a steel plate obtained by rolling a steel plate having a plate thickness of 22 mm and a length of 30 m and then cooling the steel plate at a steel plate temperature of 760 ° C by a submerged water cooling device having a facility length of 11 m at a plate passing speed of 120 mpm. It is a cooling stop temperature distribution in the longitudinal direction. FIG. 3 shows changes in water temperature on the inlet side, the center, and the outlet side of the cooling device during cooling of the steel sheet. The cooling stop temperature in the longitudinal direction of the steel plate rises for about 11 m from the tip of the steel plate to the length position of the cooling equipment, and thereafter, the cooling stop temperature becomes almost constant. From this, the inventors have found that the cooling capacity of the cooling device gradually decreases as the cooling of the steel sheet progresses,
It was clarified that the cause was the rise of water temperature in the cooling device.

FIG. 4 shows changes in the cooling stop temperature and the cooling speed depending on the position in the longitudinal direction of the steel plate when the passing speed is constant. FIG. 1 shows changes in the cooling stop temperature and the cooling rate depending on the position in the longitudinal direction of the steel plate when the speed is reduced during cooling by the method of the present invention. When the steel plate length position where the cooling capacity of the cooling equipment stops decreasing about 11 m from the tip to the inside of the cooling device, the stripping speed is reduced from 120 mpm to 100 mpm, so that the cooling stop temperature is The cooling stop temperature coincides with the cooling stop temperature, and the cooling stop temperature becomes uniform in the longitudinal direction.

The reduction rate of the cooling capacity of the immersion type water cooling device depends on the thickness of the steel sheet and the strip passing speed. FIG. 5 shows the cooling stop temperature and the cooling capacity decrease rate. Here, the cooling rate decrease rate is η _CR ,
If the cooling rate at the steel plate tip is C _R0 , the cooling rate at the steel plate tail C _R1 is C _R1 = C _R0 × (1-η _CR ) The tip for obtaining a uniform cooling stop temperature in the steel plate longitudinal direction,
The cooling time at the tail end is as follows.

Δ T _L = (T ₀ −T ₁ ) / C _R0 Δ T _T = (T ₀ −T ₁ ) / C _R1 where T ₀ : cooling start temperature T ₁ : cooling stop temperature deceleration amount [delta] V in the strip passing during cooling When L is ΔV = {(L / Δ T L) - (L / Δ T T)} × 60 = L {C R0 / (T 0 - T 1) - C R0 ( 1-η _CR ) / (T ₀ − T ₁ )} × 60 = {L ・ C _R0 / (T ₀ − T ₁ )} ・ η _CR × 60 (mpm) Furthermore, the circulation speed of the cooling water in the cooling device If the cooling capacity is changed by changing it, decrease the strip running speed by 2 to
By using multiple stages such as dividing into three stages, more detailed cooling control is possible.

[0015]

EFFECT OF THE INVENTION The present invention employs a method in which the cooling capacity deterioration due to the cooling device in the immersed state is evaluated by the parameters of the cooling stop temperature and the plate thickness, and the steel plate speed during the passage cooling is reduced halfway. It became possible to make the cooling stop temperature distribution in the direction uniform.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing a relationship between a cooling speed at a position in a longitudinal direction of a steel plate and a cooling stop temperature due to deceleration during the passage of the steel plate.

FIG. 2 is a characteristic diagram showing changes in water temperature on the inlet side, the center, and the outlet side of a cooling device when cooling a strip.

FIG. 3 is a characteristic diagram showing a relationship between a cooling rate at a position in the longitudinal direction of a steel plate and a cooling stop temperature when the plate passing speed is constant.

FIG. 4 is a characteristic diagram showing a cooling stop temperature distribution in the longitudinal direction of the steel plate when the stripping speed is constant.

FIG. 5 is a characteristic diagram showing changes in cooling capacity lowering rate depending on cooling stop temperature and plate thickness.

Claims (1)

[Claims] 1. In a process of cooling a thick steel plate or the like while passing the plate by using an immersion type water cooling device, the speed of the steel plate during cooling is reduced according to the position of the plate passing through the cooling device to pass the plate. A method for cooling a steel sheet, characterized by: