JPH02133155A - Method for preventing longitudinal crack of continuously cast slab - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for preventing vertical cracking in continuously cast slabs.

[Conventional technology]

When continuously casting sea bream, vertical cracks may occur near the center of the long side of the slab depending on the WR111 conditions of the mold, the properties of the powder used, etc. When such vertical cracks occur, a slab maintenance process is required, and furthermore, hot charging or direct rolling, in which the slab is sent directly from the casting process to the hot rolling process, cannot be performed, which becomes a major obstacle to energy conservation.

In addition, the above-mentioned vertical cracking is particularly likely to occur in medium carbon steel with a carbon content of α09 to 015% by weight (hereinafter simply referred to as %), but this is because molten steel with a carbon content in the above range is in the mold. When solidifying, a non-uniform solidified shell is likely to be generated due to the involvement of peritectic reaction, which is thought to lead to cracking due to thermal stress, a phenomenon that is difficult to avoid in terms of composition.

Conventionally, the following methods have been known as means for preventing the above-mentioned vertical cracks. (2) A method of optimizing the physical properties of the powder so that the continuous casting powder flows between the mold and the solidified shell with a uniform thickness; (2) A method of bonding an alloy with low thermal conductivity to the inner surface of the mold. (3) A method of reducing the amount of heat removed from the slab when forming grooves and reducing the cooling rate of the slab to suppress uneven formation of a solidified shell. The method of flowing the molten metal between the immersion nozzle and the long side of the mold by blowing gas or arranging a screen plate near the discharge port of the immersion nozzle (Japanese Unexamined Patent Publication No. 172083/1983) and Kasa's method are used. ,
Or so it was proposed.

[Problem to be solved by the invention]

However, the above conventional method has the following problems. That is, in the method (1) for optimizing the physical properties of the powder, it is extremely difficult to determine values representative of the physical properties of the powder because the physical property values of the powder change every moment during casting.

Therefore, it is extremely difficult to prevent vertical cracks from occurring on the slab surface by optimizing the physical properties of the powder.

In the method of reducing the cooling rate of the slab in (2) above, when casting low carbon steel or other V4 types using the same mold, there is a risk that the growth of the solidified shell will change depending on the steel type, making it easier to cause breakouts. .

Furthermore, the method (3) of flowing the molten metal between the immersion nozzle and the long side of the mold by blowing gas into the molten metal or arranging a screen plate has the disadvantage that the cleanliness of the molten metal, such as molten steel, decreases.

It is an object of the present invention to solve the above problems of the prior art and to propose a method for preventing vertical cracks occurring near the center of the long side of a continuously cast slab.

(Means for Solving the Problems) In order to solve the above problems, the present inventors conducted repeated experiments and research focusing on elucidating the mechanism of longitudinal cracking in medium carbon steel.
1) Most of the vertical cracks occur in the part of the slab that is cast (hereinafter referred to as the unsteady part) during the period from the start of casting until the casting speed gradually increases and reaches a predetermined speed. It has been found that when this occurs, the molten steel temperature at the molten steel meniscus in the initial stage of casting is low, especially between the immersion nozzle and the long side of the mold, and the temperature difference between each part of the meniscus is large. From the above facts, if the drop in molten steel temperature at each part of the meniscus at the initial stage of casting, which corresponds to casting of an unsteady part, is prevented and made uniform, powder slag formation defects will not occur, and the powder will be uniform between the mold and the solidified shell. Based on the idea that a smooth inflow is ensured and no vertical cracks occur, the present invention was completed after further investigation. That is, in the present invention, when continuous casting is performed by supplying molten steel into a mold through an immersion nozzle, the depth of the immersion nozzle is made shallow in the initial stage of casting until the casting speed reaches a predetermined speed, thereby increasing the casting speed. Accordingly, the depth of the immersion nozzle is controlled so that the depth of the immersion nozzle is gradually deepened and reaches a predetermined depth when a predetermined casting speed is reached, and the degree of superheating of the molten steel at each part of the meniscus is maintained at 0°C or higher, and The present invention relates to a method for preventing vertical cracking in continuous casting slabs, which is characterized in that the difference in molten steel temperature at each part of the meniscus is kept at 5°C or less. Here, the predetermined casting speed refers to the target casting speed in a steady state, and @steel supermaturity refers to the difference between the molten steel temperature and the liquidus temperature.

[For production]

The depth of the immersion nozzle is shallow in the early stages of casting for the following reasons.In the early stages of casting, it is necessary to sufficiently develop the solidified shell thickness of the slab to prevent breakout.For this purpose, the casting speed must be At this time, if the depth of the immersion nozzle is increased, the flow velocity at the meniscus will be slow, the meniscus temperature will decrease, and the temperature difference between each part of the meniscus will increase. Therefore, the powder will not melt sufficiently, and the mold will not melt properly. When the casting speed is 0.5 m/min, the preferred nozzle depth is at most 100++n, and when the casting speed is 1 m/min, the nozzle depth is at most 100 ++ n. (But it's 200 dragons.

As the casting speed increases, the nozzle depth increases to obtain a suitable molten steel flow rate. That is, if the nozzle depth is too shallow, the flow velocity of molten steel in the meniscus becomes too high, causing ripples in the meniscus, which deteriorates the surface of the slab. The preferred nozzle depth is 25 m/min for a casting speed of 1.5 m/min.
0■1 or more.

By increasing the degree of superheating of the molten steel, sufficient molten teeth of the powder can be obtained. If the degree of superheating of the molten steel is 0°C, the meniscus will partially begin to solidify and the powder will not melt sufficiently. The upper limit is not particularly specified, but is a temperature that is naturally determined from the casting temperature.

If the temperature difference at each part of the meniscus is equal, the thickness of the solidified shell will develop evenly, and the melting rate of the powder will also be uniform. If the temperature difference between the various parts of the meniscus exceeds 5°C, the thickness of the solidified shell will be greatly uneven, and vertical cracks will occur in the slab. The temperature can be measured at any location, but when measuring at two points, convenient locations are the vicinity of the immersion nozzle and one width of the long side of the mold. The temperature of each part of the meniscus may be measured by a pattern measurement method.

The measurement interval may be continuous or intermittent.

〔Example〕

The following will explain based on examples.

In a two-strand continuous casting machine with a bending radius of 10 m,
Using molten steel having the composition shown in Table 1, the first strand was formed using the conventional method with a constant immersion nozzle depth of 300 mm, and the second strand was formed using the method of the present invention, with a slab cross-sectional size of 20 mm. ■Casting speed of X1600■ slab 1.7rn
The steel was cast at a speed of 1/min, and the degree of overmaturity of the steel and the occurrence of vertical cracks were investigated. The measurement position of the meniscus temperature in the method of the present invention is between the immersion nozzle and the center of the long side of the mold. The casting speed is shown in Figure 1 (
As shown in Figure 1 (0), the comb becomes larger as the casting length increases, and the immersion nozzle depth increases as the casting speed increases as shown in Figure 1 (0). ) deepened.

Table 1 (Unit: %) The degree of superheating of the molten steel at this time is shown in Figure 2. Further, the temperature difference between the temperature measurement position and one width part of the long side of the mold is shown in FIG. The solid line in FIG. 2 shows the case when the method of the present invention is applied, and the broken line shows the case when the conventional method is applied. From the same figure, it can be seen that the degree of superheating of molten steel is 0°C or less in the unsteady part with a casting length of 0 to 20 m in the conventional method, but it is 0°C or more when the method of the present invention is applied. The solid line in FIG. 3 shows the case when the method of the present invention is applied, and the broken line shows the case when the conventional method is applied. From the same figure, it can be seen that in the conventional method, the temperature difference in the meniscus portion is large at the initial stage of casting, but when the method of the present invention is applied, the temperature difference is 5° C. or less.

Figure 4 shows the results of an investigation into the occurrence of vertical cracks. The solid line in FIG. 4 shows the case when the method of the present invention is applied, and the broken line shows the case when the conventional method is applied. The horizontal axis is the casting length, and the vertical axis is the vertical crack code, which indicates the occurrence of vertical cracks. From the same figure, it can be seen that vertical cracks occur at the early stage of casting in the conventional method, whereas
It can be seen that when the method of the present invention is applied, there are almost no vertical cracks from the initial stage of casting.

〔Effect of the invention〕

As explained above, when manufacturing slabs by continuous casting, by applying the method of the present invention that controls the depth of the immersion nozzle, the degree of superheating of molten steel in the meniscus part, and the temperature difference of molten steel in the meniscus part according to the casting speed, the longitudinal direction of the slab can be improved. Cracks can be prevented. As a result, hot charging and direct rolling are possible after the casting process, resulting in significant energy savings.
It is possible to reduce costs, and the industrial value is extremely large.

[Brief explanation of drawings]

FIGS. 1 to 4 are explanatory diagrams of embodiments of the present invention,
Figure 1 (a) shows the casting speed versus casting length.
b) is a diagram showing the immersion nozzle depth against the casting length, Figure 2 is a diagram showing the relationship between the casting length and degree of superheating of molten steel, and Figure 3 is a diagram showing the relationship between the casting length and the temperature difference at the meniscus part. A line diagram showing
FIG. 4 is a diagram showing the occurrence of vertical cracks. Fig. and Tee J Fig. gun loading table S ('WL) Fig. Fig. / D 20 30 hook Δ k
(%p

Claims (3)

[Claims] (1) When continuously casting molten steel by supplying it into a mold through an immersion nozzle, the depth of the immersion nozzle is made shallow in the early stages of casting until the casting speed reaches a predetermined speed, and gradually increases as the casting speed increases. A method for preventing vertical cracking in a continuously cast slab, characterized by controlling the depth of an immersion nozzle so that it reaches a predetermined depth when a predetermined casting speed is reached. (2) The method for preventing vertical cracking in a continuously cast slab according to claim 1, characterized in that the depth of the immersion nozzle is controlled so as to maintain the degree of superheating of the molten steel at each part of the meniscus to 0° C. or higher. (3) The method for preventing vertical cracking in a continuously cast slab according to claim 1, characterized in that the depth of the immersion nozzle is controlled so that the difference in temperature of the molten steel at each part of the meniscus is 5° C. or less.