JPH01299744A - Method for preventing longitudinal crack on surface of continuous cast slab - Google Patents

Abstract

PURPOSE:To prevent longitudinal crack developing on long side surface of a slab by applying refractory material on part within the specific size to casting direction from upper end of both long side walls in a mold and also within the specific size at both sides from center of the width direction and using the mold having the specific low quantity of heat conductivity. CONSTITUTION:At the time of continuously casting the medium carbon steel slab having 0.09-0.15wt.% carbon content, the continuous casting mold having heat conductivity in the part lower than the other part by 20-50%, is used by applying the refractory on the part within 200mm to the casting direction from the upper ends 10 of both long side walls 9 in the mold 3 and also within 100mm at both sides from the center 11 of the width direction. By this method, the sound cast slab can be stably produced without almost developing the surface crack even in the case of continuously casting the medium carbon steel slab.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is directed to the continuous casting of medium carbon steel slabs with a carbon content of 0.09 to 0.15% by weight, which are produced on the long sides of the slabs. This invention relates to a method for preventing vertical cracking.

<Prior art and its problems> When continuously casting a medium carbon steel slab with a carbon content of 0.09 to 0.15%, vertical cracks often occur in the center of the long side of the slab, especially It tends to occur frequently during the first or second charge in the initial stage of casting. When this vertical cracking occurs, a care process for the slab becomes necessary, making it impossible to carry out hot direct heating or hot direct rolling, which becomes a major hindrance to energy saving.

The reason why medium carbon steel tends to crack is thought to be that steel in this carbon content range has a peritectic composition. In other words, in steel with a peritectic composition, the solidification form is peritectic solidification, so it exhibits large shrinkage, and as a result, a local gap is created between the mold and the solidified shell, resulting in the generation of a non-uniform shell. By leveraging
When thermal stress is applied, cracking tends to occur.

Furthermore, during high-speed casting, the inflow of powder between the mold and the solidified shell tends to be uneven (the amount of powder inflow tends to be too large or too small in some parts), and the solidified shell that is thus produced also becomes uneven in the width direction of the slab. It is presumed that cracks due to thermal stress are more likely to occur.

For this reason, conventional methods for preventing the cracking include (A) a method of optimizing the viscosity and crystallization temperature of the powder to ensure uniform inflow of the powder;

(B) A method of reducing the amount of heat extracted from molten steel by bonding a metal with low thermal conductivity to the inner surface of the copper plate constituting the mold, or by providing minute grooves on the inner surface to maintain an air layer for heat insulation (so-called A method to prevent uneven solidification using a "slow cooling mold").

(C) A method of fluidizing the unsolidified molten metal between the immersion nozzle and the long side of the mold by blowing gas or adjusting the direction of molten metal injection (Japanese Patent Laid-Open No. 172663/1983).

Other methods have been proposed.

However, in the methods shown in (A) and (B) above, when the casting speed is relatively slow (especially 1.0 m/m
Although a considerable improvement effect was observed in cases where the casting speed was 1.1 m/min or higher, vertical cracking could not be completely prevented when casting was performed at a high speed of 1.1 m/min or higher.

In addition, with the method shown in item (C) above, it is difficult to sufficiently fluidize the molten steel and only local fluidity is ensured at best, so it is still difficult to reliably prevent vertical cracking. It was something to think about.

Therefore, in order to solve the above-mentioned problem, one of the inventors of the present invention first proposed the following method: We proposed a method for continuous casting of medium-carbon steel slabs by joining metals with low thermal conductivity and using a mold with a slow cooling section configured with minute grooves to maintain an air layer for insulation. (Patent Application No. 1981-1952). This proposal was based on the following findings (a) to (C1): (a) “Vertical cracks on the slab surface occur frequently at the center of the slab width at the early stage of casting. The reason for this is that in the center of the slab width, the flow velocity of the reversed flow that occurs when the discharge flow from the nozzle collides with the short side is small, and there is heat removal by the submerged nozzle.
The slag rim produced by the molten powder solidifying and adhering to the mold wall becomes large and non-uniform. In other words, due to the above-mentioned phenomenon related to the formation of slag rim, the inflow of powder between the mold wall and the solidified shell becomes uneven.
As a result, the solidified shell becomes thinner in areas where the slag rim thickness becomes too thick, which is the main cause of cracking.

Fig. 1 (a) shows a cross section of the mold part in the casting direction in continuous casting, and Fig. 1 (bl is A in Fig. 1 (al)
-A cross section, showing how molten steel 1 is poured into a mold 3 through an immersion nozzle 2, forms a solidified shell 4, and solidifies while increasing the thickness of the shell. Generally, a portion of the casting powder 5 solidifies and adheres to the mold wall to form slag rim 6. However, as shown in FIG. 1(b), in the vicinity of the immersion nozzle 2, the flow velocity of the reversed flow 8 generated when the molten steel discharge flow 7 from the nozzle collides with the short side of the mold is small, and the molten steel discharge flow 7 from the nozzle collides with the short side of the mold. Since the powder temperature and the molten steel temperature are lowered due to the thermal effect, the formation of slag rim 6 becomes more intense in this area, and the unevenness becomes more pronounced.

(b) Therefore, in order to stably produce continuously cast slabs with a sound surface without surface cracks even during continuous casting of medium carbon steel slabs, it is necessary to suppress the formation of slag rim that solidifies and adheres to the inner surface of the mold as much as possible. It is important to make the thickness uniform in the width direction.

fc) However, the amount of heat removed at the "widthwise center of the long side of a slab mold" where the influence of the heat removal effect by the immersion nozzle is large and the flow rate of the molten steel discharged from the immersion nozzle is slow is calculated from If the thickness of the slag rim is reduced by bonding a metal with low thermal conductivity or by providing a microgroove to maintain an air layer for heat insulation, the thickness of the slag rim in this area will be reduced to about A, and the unevenness will also be reduced, resulting in a gain. The frequency of vertical cracks occurring on the surface of the slab is drastically reduced. Moreover, if the heat removal reduction range of the mold and the degree of heat removal reduction in the area are adjusted to a specific range, there will be no particular problem in casting efficiency.

However, according to the previously proposed "continuous casting mold in which specific parts are slowly cooled by providing micro grooves for joining low thermal conductivity metals and maintaining an insulating air layer", it is said that it has excellent vertical crack prevention for medium carbon steel. Although the effect is obtained, subsequent examination through actual operation revealed that the production cost of the slab increases and that breakout occurs when trying to cast low carbon steel with a carbon content of about 0.05% by weight using the same mold. It has become clear that there are points that need to be further improved, as they tend to cause operational troubles such as:

Means for Solving the Problems> From the above-mentioned viewpoints, the present inventors have developed a method that can stably and reliably prevent vertical cracking in medium carbon steel slabs, and that can also be used when continuously casting steel other than medium carbon steel. As a result of further research to find a more practical and less costly continuous casting method that does not require mold replacement, we were able to obtain the knowledge shown in a) to step) below.

a) Since vertical cracks in medium carbon steel tend to occur frequently at the early stage of casting, the above-mentioned "partial slow cooling of the mold", which is effective in preventing vertical cracks in slabs, is sufficient only at the early stage of casting.

b) On the other hand, when casting steels other than medium carbon steel, there is no need to replace the mold (in order to use the above-mentioned "mold designed for partial slow cooling", it is necessary to easily and easily change the slow cooling means for the mold). It would be better if the configuration was such that it could be attached and detached.

1) A simple and inexpensive method for partially slow cooling of a continuous casting mold is to apply a refractory material and dry it.
With this method, as the casting progresses, the slow cooling effect becomes smaller and eventually disappears. Because it exhibits a cooling effect, it is possible to stably manufacture sound medium carbon steel slabs without vertical cracks, and since the refractory coating layer can be attached and removed extremely easily, it is possible to manufacture steels other than medium carbon steel using the same mold. There is no particular problem with casting (
(without problems such as breakouts due to delayed growth of the solidified shell).

Furthermore, in the case of a slow-cooling mold made by forming microgrooves, joining metal plates with low thermal conductivity, or providing a thick plating layer or thermal spraying layer according to the previous proposal (Japanese Patent Application No. 195281/1981), it is possible to In addition, since slow cooling continues until the end of casting, it was found that the middle and final stages of casting, where vertical cracking is less likely to occur, can cause operational problems such as breakouts due to delayed growth of the solidified shell. However, with slow cooling molds coated with refractory material, the slow cooling effect disappears as the casting progresses, as described above, so there is no need to worry about the above trouble.

This invention was made based on the above knowledge, and as shown in FIG.
00 mm or less, and from the center 11 in the width direction to both sides 10
By applying refractory material to the area within 0.00 meters (the area marked with a mesh pattern in Figure 2), the amount of heat removed from that area can be reduced by 20 to 50.
By performing continuous casting of steel using a continuous casting mold with a lower %, it is possible to stably produce sound slabs with almost no surface cracks even during continuous casting of medium carbon steel slabs. It is characterized by the fact that it is made in such a way that

Here, the material and coating thickness of the refractory material to be applied are not particularly limited, and may be appropriately selected and adjusted so that the amount of heat removed from the portion of the mold is reduced by 20 to 50%. The refractory material is preferably applied before the casting operation and then sufficiently dried using a mold preheating process, and the coating thickness is preferably 10 mm or less.

In addition, in the method of the present invention, the reason why the slow cooling area of the long side wall of the mold is numerically limited as described above is as follows.

That is, the reason why the cracks were first limited to within 100 mm on both sides from the center in the width direction of the mold was because, as shown in FIG. The area was limited to within 200 fins from the top of the mold.
This is because the influence of slag rim on slab cracking is especially large only near the meniscus, and if even this area is slowly cooled, vertical cracking of the slab can be suppressed. In addition, the above-mentioned FIG. 3 shows a medium carbon steel slab (1200
It is a graph showing the relationship between the "distance from the center of the slab width" and the "vertical crack length" on the long side surface when continuous casting was performed at a casting speed of 1.5 m/min.

On the other hand, the reason why the amount of heat removed from the mold slow cooling section is reduced by 20 to 50% is that if the degree of reduction in the amount of heat removed from this section is less than 20%, a sufficient effect of suppressing vertical cracks cannot be obtained, and the amount of heat removed If the degree of reduction exceeds 50%, the effect of reducing slag rim thickness and suppressing unevenness will be saturated, and the growth delay of the solidified shell will become significant, which may cause operational problems such as breakouts. This is because it becomes expensive.

When slabs are continuously cast according to the conditions stipulated in the present invention, the thickness of the slag rim that solidifies and adheres to the upper part of the long side of the mold near the center in the width direction is almost the same as that of other parts, and is uneven. As the thickness of the solidified shell of the slab becomes uniform, the frequency of cracking due to shrinkage during solidification is minimized, and the degree of cracking is slight, resulting in a slab with a sound surface. can be obtained stably. Furthermore, when casting steels other than medium carbon steel, it is possible to avoid applying refractory material or remove the refractory coating layer (actually, the refractory coating layer disappears during the previous casting process). It is possible to perform stable operations using the same mold without replacing the mold.

Next, the present invention will be specifically explained with reference to Examples.

<Example> The cast slab dimensions are long side 1200 mm x short side 270 mm,
100 meters on both sides (200 ffiffl in total width) from the widthwise center of the inner surface of both long sides, and 200 meters from the upper end in the casting direction.
A copper continuous casting mold was prepared in which the refractory material having the composition shown in Table 1 was coated to a thickness of 5 mm, and 0.11%C-0.5%Mn was cast using a vertical-curved continuous casting machine. Steel slabs were cast at a casting speed of 1.6 m/min.

In the above casting test, the thickness of the slag rim near the meniscus was investigated, and the results were summarized in terms of the degree of heat removal reduction measured by a thermocouple embedded in the copper plate constituting the mold, as shown in Figure 4.

In addition, the crack occurrence rate of the obtained slabs was investigated, and the results are also shown in Fig. 5, which are also organized by the degree of heat removal reduction.

As is clear from the results shown in Figures 4 and 5, the degree of gradual cooling (heat removal reduction It can be seen that when the slag rim thickness becomes 20% or more, the slag rim thickness near the meniscus becomes approximately 20%, and at the same time, the unevenness of the slag rim becomes smaller and the incidence of vertical cracks is halved.

Furthermore, Figure 6 is a graph that organizes the vertical crack occurrence rate and heat removal reduction degree by casting length. The results for the slab (comparative slab) are also shown, but from this Figure 6, the effect of reducing the amount of heat removed is lost in the first or second charge after casting, and the applied refractory material is not applied. However, the occurrence rate of vertical cracks did not increase after that, indicating that the uniformity of the slag rim shape at the initial stage of casting greatly contributed to the prevention of vertical cracks.
In this case, no adverse effects on the steel, such as an increase in inclusions due to elution of the refractory material, were observed.

Of course, when casting low carbon steel, it is possible to use the same mold without applying refractory material, and it is possible to cast low carbon steel, medium carbon steel, etc. without changing the mold. was also confirmed.

Summary of Effects> As explained above, according to the present invention, vertical cracking on the slab surface can be prevented at low cost even when medium carbon steel is continuously cast at high speed, and furthermore, the present invention can prevent vertical cracks on the slab surface at low cost. This brings about extremely useful effects in practice, such as the ability to stably cast other types of carbon steel with different carbon contents using the same mold.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram illustrating the casting situation of the mold part in continuous slab casting. Fig. 2 is a conceptual diagram illustrating the slow cooling part of the mold used in the method of the present invention. Fig. 3 is a diagram showing a slab of a continuous cast slab FIG. 4 is a graph showing the relationship between the distance from the width center and the length of vertical cracks that occur. Figure 4 shows the measurement results of the slag rim thickness near the meniscus where it solidifies and adheres in the example. This is a graph organized by the degree of reduction in the amount of heat removed by the cooling part. Figure 5 shows the measurement results of the vertical cracking incidence of continuous cast slabs obtained in Examples, with some of the results of the slow cooling of the slow cooling mold. Fig. 6 is a graph showing the vertical cracking incidence of continuously cast slab slabs and the degree of slow cooling partial heat removal reduction, organized by casting length. In the drawing, ■... Molten steel, 2... Immersion nozzle. 3... Mold, 4... Solidified shell. 5... Casting powder, 6... Slag rim. 7... From nozzle. Molten steel discharge flow. 8... Reverse flow of molten steel, 9... Long side wall of mold, 1
0...Top end of long wall of mold. 11... Center of the long wall of the mold in the width direction.

Claims (1)

[Claims] When continuously casting medium carbon steel slabs with a carbon content of 0.09 to 0.15% by weight, cast iron is applied within 200 mm from the upper ends of both long walls of the mold in the casting direction, and within 100 mm on both sides from the center in the width direction. A method for preventing vertical surface cracks in a continuously cast slab, the method comprising using a continuous casting mold in which the amount of heat removed from the part is reduced by 20 to 50% by coating with a refractory material.