JPH0218936B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for continuous casting of steel, and particularly to a method for continuous casting of steel capable of obtaining high-temperature slabs.

[Prior Art] An overview of the method of casting slabs in curved continuous casting equipment will be explained with reference to FIG.

Molten steel 3 injected from the ladle 1 into the tundish 2 is injected from the tundish 2 into the mold 4. The molten steel 3 injected into the mold 4 is guided from the lower part of the mold 4 through a secondary cooling zone 8 while being curved by a large number of slab guide rolls 5 . After this, slab 6
is gradually straightened horizontally, and the completely solidified slab 6 is cut into a predetermined length by a cutting device 7.
Transported to the next process.

[Problems to be Solved by the Invention] By the way, the cast slab cut into a predetermined length by the conventional cutting machine 7 is once cooled to room temperature and then subjected to flaw removal, etc. After that, it is heated in a heating furnace. Generally, it was reheated and transported to the rolling line for the next process. At that time, if the temperature of the slab before hot rolling falls below 900℃, precipitation of AlN (aluminum nitride) will occur.
Since it is no longer possible to obtain a good cold-rolled sheet material, it was necessary to reheat the slab before hot rolling and maintain it at a temperature of about 1050°C for a predetermined period of time to redissolve AlN into solid solution.

FIG. 2 shows the change in surface temperature of a slab when the slab is cast by a conventional continuous casting method.

When casting slabs using the above method, cooling equipment is required to cool the hot slabs of 900°C or higher to room temperature immediately after cutting, and a heating furnace is required to reheat the slabs after removing defects. This required a large amount of equipment costs and resulted in a large amount of energy loss.

In recent years, various improvements in casting methods have made it unnecessary to remove defects from cast slabs.
For the purpose of reducing the above equipment costs and saving energy, some attempts have been made to use so-called hot direct rolling (HDR), in which hot slabs immediately after being cut are transported directly to the rolling line without being cooled to room temperature. ing. An example of this is the Special Public Interest Publication No. 56-21330.
It is disclosed in the publication No.

[Means for Solving the Problems] The present invention provides a continuous casting method capable of obtaining high-temperature slabs necessary for carrying out the above-mentioned hot direct rolling, in which steel is cast using a mold. Then, the unsolidified slab in which the solidified shell has been formed is guided while being curved along the secondary cooling zone, and then, the solidification is completed while straightening the curved unsolidified slab in the horizontal direction, and the solidification is completed. In a continuous steel casting method, the unsolidified slab is cut into predetermined lengths in a horizontal state and transported, in which the unsolidified slab is placed in the upper part of the secondary cooling zone for 2 to 10 minutes.
The surface temperature of the slab is cooled to 600 to 850°C to increase the rigidity of the solidified shell, and then the surface temperature of the unsolidified slab that has passed through the upper secondary cooling zone after completion of cooling is reduced to 900°C. Reheat to above ℃, then
This method is characterized in that the surface temperature of the slab after reheating is maintained continuously for 10 minutes or more, thereby re-dissolving the precipitated AlN into solid solution.

The present invention will be further explained as follows. In order to efficiently produce cold-rolled deep-drawn steel sheets of good quality by performing hot direct rolling, the following two conditions must be satisfied at the same time.

(A) Ensure that the amount of AlN in solid solution is sufficient to guarantee cold-rolling deep drawability in the slab immediately before hot rolling, (B) Continuous casting and hot rolling can be carried out without any problems in terms of operation and quality. Ensure workable slab temperature conditions.

The following two measures are taken to deal with the condition in (A) above.
There are several possible points.

(i) There is no area in the slab where the temperature drops below 900℃ between the continuous casting mold and the start of hot rolling. (ii) If there is an area in the slab where the temperature drops below 900℃ during the continuous casting process. To do this, the area is reheated to over 900℃ using the latent heat and sensible heat possessed by the slab itself.
In addition, by holding this for 10 minutes or more, the re-solid solution of the AlN that has precipitated is promoted to a level that does not cause any practical problems.

Regarding the above condition (B), first of all, in order to carry out hot rolling without any problems in terms of operation and quality, it is important to secure the maximum amount of heat retained in the slab immediately before the start of hot rolling. To achieve this, we will perform high-speed casting by maximizing the length of the continuous casting machine, keep the temperature of the slab at the outlet of the continuous casting machine sufficiently high, and at the same time keep cutting and conveyance times as short as possible, and It is necessary to strengthen insulation through

In this case, in order to satisfy the conditions (A)-(i) above, the secondary cooling in the continuous casting machine must be directed to weak cooling, which lengthens the solidification time and reduces the machine length of the continuous casting machine. It has to be made longer. The situation is shown in Figures 3a and 4a.

Here, Figure 3 a shows a slab of 220 mm thick at 2.2 m/
This figure shows the change in temperature of the slab when the slab is pulled out at a speed of min. min and the slab surface temperature does not fall below 900°C. Here, the surface temperature of the slab is the surface temperature of the central portion of the slab in the width direction (the same applies hereinafter).
As a result, the crater end (solidification completion point) is 50 m from the meniscus. On the other hand, Figure 4a shows an example in which a slab with a thickness of 220 mm was initially strongly cooled at the same speed of 2.2 m/min, and the slab surface was once lowered to about 700°C. is 42 m from the meniscus, which is 8 m shorter than in the weak cooling case shown in Figure 3a. That is, in order to prevent the surface temperature of the slab from falling below 900°C, it is necessary to increase the length of the continuous casting machine, which leads to a rise in equipment costs.

Another method to maintain the surface temperature above 900℃ without extending the machine length is to increase the casting speed.
It would be fine to lower it to about 1.8m/min, but as a result,
The production capacity will be reduced by about 20%, and in the case of hot direct rolling, the production capacity of the hot rolling mill will be reduced.

Furthermore, considering the operational and quality aspects of continuous casting, in the case of FIG. 3a, a fatal problem occurs during high-speed weak cooling casting. In other words, as shown in Figure 3b, the surface temperature of the slab is 900°C from just below the mold to the outlet of the continuous casting machine.
Because the temperature is maintained above ℃, the solidified slab tends to swell due to the static pressure of the molten steel. This is called a bulging phenomenon. In particular, in the secondary cooling zone where the shell thickness is relatively thin, solidification interface strain occurs due to inter-roll bulging, and especially in the bending and straightening areas, the interface strain increases rapidly, reaching the critical cracking strain level of the shell of the steel type. As a result, many internal cracks occur in the cross section of the slab, and vertical cracks in the slab, especially in the upper secondary cooling zone where the shell thickness is thin, may cause breakout accidents, etc. cause fatal troubles in the operation.

The present invention was made in consideration of the extremely severe continuous casting operation and equipment conditions that satisfy the countermeasures in (A)-(i), and the gist thereof is the countermeasures in (A)-(ii). By realizing this, it is possible to perform high-speed casting with relatively ease and without any problems in terms of operation and quality.In addition, the heat insulator placed between the rolls in the rear half of the continuous casting machine to surround the slab allows for relatively low solidification. By making maximum use of the latent heat and sensible heat of the molten steel in the axial core of the slab, which has undergone rapid heating, the surface layer of the slab is rapidly recuperated by strong cooling in the first half of the continuous casting machine. Precipitated in a predetermined range
The goal is to re-dissolve AlN in solid solution to a level that does not pose a practical problem.

In other words, as shown in Figure 4a, the surface temperature of the slab is quickly reduced to 850°C in the upper secondary cooling zone directly below the mold.
℃ or less and 600℃ or more, and then
This is maintained continuously for 10 minutes to promote solidification while ensuring sufficient rigidity of the shell.Then, the surface layer of the slab is reheated in the heating zone in the latter half of the continuous casting machine, and the surface temperature of the slab is raised to over 900℃. By holding it for more than 10 minutes,
The AlN precipitated in the supercooled region, including the surface layer of the cross section of the slab, which initially dropped to below 850℃, is re-dissolved. The solidification interface strain profile of the slab cast under the above concept is shown in Figure 4b. Due to the strong initial cooling, the stiffness and thickness of the shell increase, and due to the synergistic effect of the two, the strain level reaches the 3rd level. This has been reduced by 50 to 70% compared to the case of weak-cooling high-speed casting shown in Figure b, and there is no longer any risk of occurrence of casting defects such as internal cracks or operational troubles. The lower limit of the surface temperature of the slab in the upper secondary cooling zone is limited to 600℃ because below this value,
This is because, at a temperature of , surface cracks occur at the corners of the slab due to bending and straightening, and it is no longer possible to guarantee that the slab will be free of defects when performing high-speed casting.

On the other hand, the upper limit was limited to 850°C because this is an essential condition for stably maintaining the strain level in the secondary cooling zone directly below the mold below its critical value.

Moreover, the reason why the holding time was limited to 2 to 10 minutes at 600 to 850°C is as follows. That is, if the holding time at 600 to 850° C. in the secondary cooling zone immediately below the mold is less than 2 minutes, the rigidity of the solidified shell will be insufficient and the risk of internal cracking will increase. On the other hand, if the holding time exceeds 10 minutes, solidification progresses too much, making it impossible to ensure sufficient and long-term heat recovery in the trailing zone (insulation zone).

[Example] Next, an example of the present invention will be described. Curved continuous casting equipment with a water-cooled mold with long side dimensions of 1250 mm and short side dimensions of 220 mm allows a casting speed of 2.2 m/min.
A mild steel slab was cast at min. The continuous casting equipment used has a machine length of 40 m and is equipped with a secondary cooling zone consisting of a mist cooling nozzle with extremely high cooling capacity within a 10 m range directly below the mold, and a heat insulating device installed in the remaining 30 m. .

The temperature of the unsolidified slab that passed through the secondary cooling zone for about 5 minutes was 800°C. After that, in about 5 minutes, the surface temperature of the unsolidified slab rose to about 950℃ due to reheating.
The temperature rose to . After this, the surface temperature of the slab is maintained at the temperature after reheating for about 10 minutes by a heat insulating device.
Once solidified, the slab leaves the continuous casting facility. Next, the slab is cut into a predetermined length using a cutting device,
This was conveyed to a rolling facility by a roller table. In order to prevent the temperature of the slab from decreasing even when cutting the slab, a heat insulating device was installed around the slab, and even when cutting was completed, the temperature of the slab could be maintained almost at the temperature at the time of reheating. The temperature of the slab decreased slightly during the process of conveying the cut slab to the rolling equipment using the roller table, but the surface temperature of the slab immediately before rolling started was approximately 900°C, so there was no problem with rolling. It never came. Note that even if the temperature of the slab immediately before the start of rolling falls below 900°C, no problem will arise in rolling as long as this time is 1 minute or less. FIG. 3 shows the temperature change on the slab surface at this time.

When we examined the AlN of the slab just before rolling started, we found that
It was confirmed that the solid solution had been re-dissolved to the extent that there was no practical problem. Further, when the rolled material was rolled into a thin plate and the structure after annealing was examined, it was confirmed that the material had uniformly expanded grains.

[Effects of the Invention] As explained above, according to the present invention, AlN can be solid-dissolved again without reheating the slab.
It is now possible to perform highly efficient hot direct rolling while maintaining stable slab quality and operating conditions, and the heat energy consumption is also lower than that of conventional methods.
A very useful effect is brought about in that the amount of 270,000 to 30,000 Kcal/T is almost no longer needed.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing a conventional continuous casting method;
Figure 2 shows the temperature change on the surface of the slab cast by the conventional continuous casting method, and Figure 3 shows the temperature change on the surface of the slab cast by the conventional continuous casting method.
Fig. 3a is a graph showing the temperature change on the surface of a slab when a slab of mm in diameter is drawn at a casting speed of 2.2 m/min and weakly cooled so that the surface temperature does not fall below 900°C. Figure 3b is a diagram showing the relationship between the distance from the meniscus, slab temperature, and crater end (solidification completion point), and Figure 3b is a diagram showing the relationship between the distance from the meniscus, solidification interface strain, and critical strain for internal crack generation. Figure 4 is a diagram showing an example of calculation when the upper part of the secondary cooling zone is strongly cooled for a slab with a thickness of 220 mm at a casting speed of 2.2 m/min, and Figure 4a shows the distance from the meniscus, slab temperature, and crater. Figure 4b, which is a diagram showing the relationship with the end (solidification completion point), is a diagram showing the relationship between the distance from the meniscus, the solidification interface strain, and the critical strain for internal crack generation. In the drawings, 1... ladle, 2... tundish, 3... molten steel, 4... mold, 5... slab guide roll, 6...
Slab, 7... Cutting device, 8... Secondary cooling zone.

Claims (1)

[Claims] 1. Casting steel using a mold, guiding the unsolidified slab with a solidified shell while curving it along a secondary cooling zone, and then straightening the curved unsolidified slab in the horizontal direction. In a continuous steel casting method, the solidification is completed, and the solidified slab is cut into a predetermined length in a horizontal state and transported.
In the upper part of the next cooling zone, the unsolidified slab is
Cool the slab for 10 minutes until the surface temperature reaches 600 to 850°C to increase the rigidity of the solidified shell, and then cool the surface of the unsolidified slab after passing through the upper secondary cooling zone. The temperature is reheated to 900°C or higher, and then the surface temperature of the slab after reheating is maintained continuously for 10 minutes or more, thereby re-dissolving the precipitated AlN into solid solution. Continuous casting method for steel.