JPH01271049A - Secondary cooling method in continuous casting - Google Patents

Abstract

PURPOSE:To save a cooling energy by deciding a spray pattern on each steel kind, setting the allowable limit value of an air-water mixture ratio as well and controlling the feeding amt. and pressure of a compressed gaseous body so as to maintain the allowable limit value mixture ratio while under continuous casting. CONSTITUTION:A molten steel 21 forms a solidified shell 22 from the surface layer with a primary cooling of a mold 1, is drawn continuously and cooled by a secondary cooling belt 40. The spray pattern formed by the mixture ratio of a gaseous body and cooling water is found in advance on each cast steel kind and the allowable limit value of an air-water mixture ratio is experimentally set. The valves 5, 7 for controlling the amt. of the cooling water and gaseous body in the air-water and valve opening control devices 6, 8 are set up and during continuous casting a secondary cooling control device 9 controls the amt. and pressure of the cooling water and gaseous body so as to maintain the allowable limit mixture ratio. The air-water secondary cooling in the necessary lowest limit is performed, so the cooling energy is drastically saved.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to secondary cooling in continuous casting, and provides a method that reduces energy loss in secondary cooling equipment and enables efficient cooling of slabs. It is something.

BACKGROUND OF THE INVENTION In recent years, efforts have been made to increase the speed of continuous casting and to directly connect the continuous casting and rolling processes (hereinafter referred to as direct rolling). In order to carry out this direct rolling efficiently, it is necessary to supply the hot slab to the rolling process in a defect-free state.

In continuous casting, molten steel is injected into a mold, solidified from the surface of the mold, and continuously drawn out below the mold. ) is injected onto the slab and cooled to cause the solidified shell to grow. Cooling within the mold is generally referred to as primary cooling, and cooling of the slab pulled out of the mold is referred to as secondary cooling. This secondary cooling has an extremely important effect on the production of the aforementioned high-temperature slabs without defects, and depending on the situation, it may be impossible to manufacture high-temperature slabs, or surface cracks or internal cracks may occur in the slabs. This defect will occur.

Therefore, in the past, for example, Japanese Patent Application Laid-Open No. 57-1914
4 and Japanese Patent Application Laid-Open No. 57-187150, cooling water necessary for efficient air-water cooling in response to changes in the amount of cooling water set from a cooling section required for cooling slabs, and A technique for determining an optimal control amount for gas pressure has been disclosed. That is, a technique is disclosed for controlling the amount of air-water cooling water to have the necessary cooling water capacity, the pressure of the cooling water necessary to ensure the optimum spray condition at that time, and the gas pressure.

Problems to be Solved by the Invention In the conventional method described above, a strand (hereinafter referred to as a strand is a slab that is drawn out of a mold and moves through continuous casting equipment before being cut into a predetermined size, and Regarding the cooling conditions for the strands (simply referred to as strands), only the technology for manufacturing high-temperature and defect-free slabs has been disclosed, and one of the cooling methods is to cool the strands with air and water as described above. A cooling method has been proposed. When such air-water cooling is performed, the cooling of the strand is made more uniform than when cooling only with cooling water, and it is a very effective means from the viewpoint of preventing defects in the manufactured slab.

For this reason, conventional methods have focused only on ensuring the best spray conditions, and the gas supply pressure has been extremely high. In other words, using the air-water cooling method is effective in improving the quality of the slab, especially the surface quality, but on the other hand, it is necessary to pressurize the gas to a high pressure using a well-known compressor in order to obtain a good spray condition. However, depending on the amount of gas required and the pressure of the gas after increasing the pressure, a large amount of energy is required to operate the compressor.

The present invention provides a method for efficiently cooling a strand by reducing the large energy consumption required for supplying gas for cooling the strand.

Means for Solving the Problems The present invention for solving the problems described above provides a secondary cooling method in continuous casting in which a hot slab pulled out of a mold is cooled by injecting a water mixture of cooling water and compressed gas. The air water at the time of casting is determined from the spray pattern determined from the mixture ratio (gas/water) of the compressed gas to the cooling water, and the correlation between the spray pattern and the casting defect occurrence rate determined in advance for each steel type. A permissible limit value of the mixture ratio is set, and the slab is cooled by controlling the supply amount of compressed gas or the supply pressure in addition to the supply amount so as to maintain the permissible limit mixture ratio during continuous casting. The present invention relates to a secondary cooling method in continuous casting.

Function As mentioned above, when using the strand cooling method using air water, which is a mixture of compressed gas and cooling water, it is necessary to increase the pressure of the gas with a compressor and supply it to the secondary cooling device, which reduces the amount of gas required. Depending on the gas pressure after pressurization, the pressure is 1iIl&
requires a large amount of energy. Naturally, the energy required by the compressor is determined by the amount of gas required. The required amount of gas can be determined from the amount of cooling water to the strands and the spray state of the cooling water. That is, the energy required to supply gas can be determined by the required amount of cooling water and the spray state of the cooling water.

The required amount of cooling water can be determined in a manner similar to the well-known technique described above. However, the spraying condition of the cooling water has conventionally been determined to be the best condition from the viewpoint of preventing surface defects, etc. of the slab. In other words, if the amount of cooling water is the same for all types of steel manufactured by continuous casting, the same amount of gas is mixed and controlled so that the spray state is the same.

In other words, the relationship between the amount of cooling water and the amount of gas, or the pressure of the gas, is set based on the following relational expression, for example, when the secondary cooling zone (described later) is the same, and the relationship between the amount of cooling water and the amount of gas, or the pressure of the gas is set to be a value proportional to the amount of cooling water. Even when the type of steel to be cast was changed, the control was performed according to the same relational expression as described above.

P=A-Q2 +B・Q+C P: pressure of gas, Q: amount of cooling water, A, B, C: coefficients Generally, the target materials cast in continuous casting equipment are multiple steel types. As a matter of course, different steel types have different susceptibilities such as occurrence of surface cracks on zero pieces. In other words, for steel types with high cracking susceptibility, compared to steel types with low cracking susceptibility, it is necessary to inject cooling water in a fine (good) spray state to the strand, which increases the amount of gas supplied, and Compressor energy also increases.

The present inventors conducted the research described below in order to efficiently cool the strands, that is, without causing defects such as surface cracks, and to minimize the energy required by the aforementioned compressor. I did it.

The present inventors first carried out air-water cooling of the gas and cooling water.
In the next cooling, a spray pattern of cooling water determined from the mixing ratio of gas and cooling water (ratio of gas/cooling water, hereinafter simply referred to as mixing ratio) was determined.

Figure 2 shows an example of the results, with the horizontal axis representing the amount of cooling water and the vertical axis representing the amount of gas. The atomization state of cooling water injected onto a flat plate separated by approximately 50 to 200 Ilm was investigated. The atomization state is
A good spray pattern in which the cooling water is in the form of fine particles and evenly distributed, a coarse spray pattern in which the cooling water is in the form of relatively coarse particles, and an intermittent spray in which the gas is intermittently supplied into the cooling water in a breath-like manner. This pattern was classified as a water spray pattern in which almost no gas was supplied, that is, only cooling water was sprayed irregularly.

As can be seen from Fig. 2, when the ratio of gas to cooling water, that is, the mixing ratio, is changed, there is a large difference in the spraying condition of cooling water; conversely, by controlling the mixing ratio, , any spray pattern can be obtained.

Next, casting tests were repeatedly conducted using the above-mentioned spray patterns determined by variously changing the steel type to be cast and the mixing ratio, and the relationship between defects in the slab and the spray patterns was investigated. FIG. 3 shows an example of the results, with the horizontal axis representing the spray pattern and the vertical axis representing the incidence of surface defects on the slab. In this example, we compared and investigated medium-grade aluminum guild steel, which has high cracking susceptibility, and low-grade aluminum guild steel, which has low cracking susceptibility. The number of surface defects divided by the number of slabs is expressed as an index.

As is clear from FIG. 3, even if the spray pattern is the same, the occurrence of defects changes greatly if the gas changes, indicating that it is not necessary to ensure a good spray pattern for all fIA types.

In other words, compared to steel types with high crack susceptibility, steel types with low cracking susceptibility do not generate defects even at a small mixing ratio, and depending on the type of steel being cast, the minimum necessary amount is required to prevent surface defects. It was found that the spray patterns were different. By conducting such a survey in advance, it is possible to determine the mixing ratio that is acceptable for secondary cooling of high-temperature slabs that can be directly rolled without causing surface defects. It becomes possible to set the limit (hereinafter referred to as allowable limit mixture ratio) for each steel type.

Next, Figure 4 shows the relationship between the amount of gas supplied and the energy (power consumption) required for the compressor to supply it.
Figure 5 shows the relationship between the pressure of the gas boosted by the compressor and the energy (power consumption) required for it. requires more energy.

Once the above-mentioned allowable limit mixing ratio is determined, the amount of gas required to obtain the mixing ratio, or the pressure in addition to the amount of gas, can be set.

Now, FIG. 1 is an explanatory view showing an example of a well-known curved continuous casting equipment in which a secondary cooling device for carrying out the above-described invention is attached.

In the figure, 1 is a mold, and the molten steel 21 injected into the mold 1 undergoes primary cooling in the mold 1 to sequentially generate a solidified shell 22 from its surface layer, forming a strand 3.
The strand 3 is supported by a group of guide rolls 2 below the mold 1 and is continuously drawn out. A secondary cooling zone 40 is provided to cool the strand 3 after the mold 1, and divided cooling control is performed in each cooling zone 4 as necessary in the casting direction and the casting width direction.

In the secondary cooling of this example, immediately after exiting the mold 1, cooling was performed by spraying only the cooling water indicated by reference numeral 41, and after that, cooling was performed using air-water cooling water based on the present invention. In order to cool the strand 3 with air and water, a valve 5.7 for controlling the amount of cooling water and gas and a control device 6.8 for the opening degree of each valve are provided.
is installed.

In the present invention, information on the type of steel to be cast is input from the work instruction device 10 to the secondary cooling control device 9 which calculates and determines the amount of cooling water, the amount of gas, the pressure, etc. The secondary cooling control device 9 determines the amount of cooling water, gas, pressure, etc. based on the permissible limit mixture ratio determined in advance based on the information from the work instruction device 10, and then Emit each control signal. The control device 6.8 controls the cooling water, the amount of gas, the gas pressure, etc. during the casting process based on the control signal.

In this case, the pressure to be increased by the gas compressor 12 differs depending on the required amount of gas. Specifically, if the required amount of gas is small, it is possible to supply gas even if the pressure raised by the compressor is low, but if the required amount of gas is large, the pressure is the same as above. , even if the valve 7 is fully opened, the necessary amount of gas cannot be secured. Conversely, if the pressure is increased more than necessary when the required amount of gas is small, more energy than necessary will be given to the compressor, resulting in wasted energy.

This situation will be further explained in detail based on FIG. FIG. 6(a) is an enlarged view of a part of the gas supply system of the secondary cooling device in the continuous casting equipment shown in FIG. It is a figure showing the relationship between the gas pressure and the amount of cooling water at each device position in a).

In FIG. 6(b), the line P2 represents the case where the gas supply amount is large (the mixing ratio is large), and the line P21 represents the case where the gas supply amount is small (the mixing ratio is small). Naturally, the pressure P1 of the gas immediately before the valve 7 that controls the pressure needs to be higher than the pressure curve shown by P2, which is required in actual operation. Also compressor 1
2 and receiver tank 13 is controlled by valve 7.
It is necessary to set the pressure PC to be higher than the pressure loss ΔP1 caused by the inside of the piping. If the pressure at each device position is not considered in this way, the gas supply at the end will not be satisfactory.

Now, if the amount of gas supplied is small, for example, as mentioned above, the sixth
As shown by line P21 in Figure (b), it is below the curve P2. The pressure loss in the piping mentioned above is the flow rate,
Although it varies depending on the pressure, when the amount of gas is small, the gas supply can be satisfied if the pressure PCI of the compressor 12 and receiver tank 13 is set to the pressure added to at least the above-mentioned pressure loss ΔP1. In other words, by reducing the amount of gas, in this example, Δ
The original pressure of the gas can be reduced by P1 minute. In other words, if the original pressure of the gas remains unchanged even if the amount of gas is reduced, the compressor energy required to increase the pressure of ΔPi mentioned above will be wasted. Become.

Therefore, based on the determined required amount of gas, a set pressure to be increased is set from the bubble control device 9 to the boost control device 11 that controls the pressure increase of the compressor 12.
Based on this, the pressure increase of the compressor 12 is controlled and gas is supplied to the secondary cooling device via the receiver tank 13, thereby making it possible to efficiently cool the strands with less energy loss.

EXAMPLE The present invention was carried out in a continuous casting machine with a continuous casting function, a monthly production of 180,000 tons, and a machine length of 37 m.

During actual casting, the correlation between the spray pattern and the casting defect occurrence rate was determined in advance for all steel types to be cast, and a mixing ratio within the permissible limit at which the above-mentioned defects would not occur was determined.

The casting conditions are as shown in Table 1 and below.

Casting speed: 1.8m/win, casting width: 1000m
m Casting thickness: 250 m1 Steel type A: Medium carbon aluminum guild steel Steel type B: Low carbon aluminum guild steel Table 1 For steel type A, which is prone to defects, the amount of gas is 130 m
However, for steel type B, it has been determined in advance that the limit mixing ratio may be low, and the amount of gas determined based on the limit mixing ratio is 55m''/s.
In the method of the present invention, 130 m''/sin, which is the same as steel type A, was used in the conventional method.When casting under the above casting conditions, there were no defects in the cast slabs in both the conventional method and the method of the present invention. Good quality slabs were produced without any problems.

On the other hand, the energy required to supply gas under both casting conditions is shown in Fig. 7. As is clear from Fig. 7, by applying the method of the present invention, the strands can be made with approximately 50% of the energy consumption of the conventional method. It was found that cooling can be achieved.

Effects of the Invention As is clear from the above explanation, the present invention involves determining in advance the limit cooling conditions in which defects do not occur in manufactured slabs when cooling the strands, and cooling the strands based on that. This enables an efficient cooling method with little energy loss, and its practical effects are extremely large.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the basic configuration of the present invention,
FIG. 2 is a diagram showing the relationship between the mixture ratio of gas and cooling water and the spray pattern, FIG. 3 is a diagram showing the relationship between the occurrence of defects in manufactured slabs and the mixture ratio of air and water, and FIG. Figure 5 shows the relationship between the amount of gas and the energy required by the compressor, and Figure 6 shows the relationship between the pressure of gas after compression and the energy required by the compressor, Figures 6 (a) and (b). FIG. 7 is a diagram showing the relationship between the amount of gas and the pressure after increasing the pressure, and FIG. 7 is a diagram showing the effect when the method of the present invention is applied. 1... Mold, 2... Guide roll, 3. Fist/strand, 4.40.41,... 112nd cooling zone, 5... Cooling water control valve, 6 Fist... Cooling water control device, 7... fist gas control valve, 8... gas control device, 9... secondary cooling control device, 10... Φ work instruction device, lle fist pressure boost control device, 12... compressor, 13-...・Receiver tank, 21・molten t14.22... solidified shell.

Claims (1)

[Claims] In a secondary cooling method in continuous casting, in which a mixture of cooling water and compressed gas is injected into a high-temperature slab pulled from a mold to cool it, it is determined from the mixture ratio (gas/water) of compressed gas to the cooling water. The permissible limit value of the air-water mixture ratio during the casting is set based on the correlation between the spray pattern determined in advance for each steel type and the casting defect occurrence rate, and the permissible limit mixture ratio during continuous casting is set. A secondary cooling method in continuous casting, characterized in that the slab is cooled by controlling the supply amount of compressed gas, or the supply pressure in addition to the supply amount, so as to maintain the following.