JPS6111704B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a continuous slab casting method using a vertical bending type continuous casting machine, and particularly to a slab casting method for steel types containing additive elements with high cracking susceptibility such as A and Nb. In recent years, curved continuous casting machines have become mainstream due to the height of the equipment. There is a problem that inclusion defects increase due to being collected on the solidified surface of the curved inner side.
Therefore, in recent years, there has been a tendency to install vertical bending type continuous casting machines in which the mold and the part immediately below it (usually about 1 to 3 m from the bottom end of the mold) are vertical, and the unsolidified slab is then bent. In other words, although the equipment height of vertical bending continuous casting machines (excluding those that bend the slab after it solidifies) is slightly higher than that of the curved type, it is easier to compare the vertical type and the vertical solidification curved type. This is because the height of the equipment can be significantly reduced and, like the vertical type, inclusions float to the surface of the molten metal, resulting in fewer inclusion defects. Incidentally, the surface defects of slabs produced by continuous casting machines include vertical cracks, flux entrainment, corner cracks, and horizontal cracks, but vertical bending continuous casting machines do not suffer from the above defects compared to other types of continuous casting machines. Of the various surface defects, corner cracks and lateral cracks are particularly likely to occur. In other words, in a vertical bending type continuous casting machine for slab casting, as shown in FIG. Tensile stress acts on the curved inner side 2b of the slab 2 near the unbending part 4 where the slab 2 transitions from a curved state to a horizontal state, and therefore corner cracks and Horizontal cracks are likely to occur. FIG. 2 shows an example of how transverse cracks 5 and corner cracks 6 occur in a slab. Although such cracking is not so noticeable in general steel types, it is known that it is likely to occur in steel types containing additive elements such as A, Nb, and V that have a high so-called cracking susceptibility. This invention was made in view of the above-mentioned circumstances.
The purpose is to prevent the occurrence of corner cracks, especially at the bending part, when casting a steel slab containing one or more additive elements of Nb, V, Ti, and Cu using a vertical bending continuous casting machine. That is. The present inventors investigated the occurrence of cracks in steel containing one or more of the additive elements A, Nb, V, Ti, and Cu, and found that the occurrence of cracks is closely related to the surface temperature of the slab. We conducted detailed experiments and studies on the relationship between secondary cooling conditions and corner cracks in slabs in a vertical bending continuous casting machine. ) was closely related to the amount of spray water received, leading to this invention. That is, the present inventors used general 40 kg steel that does not substantially contain A, specifically C0.16% (weight%, the same hereinafter), Si0.20%, Mn0.60%, P0.018
%, S0.012%, A0.006%, and A, which is considered to be particularly susceptible to cracking.
-Nb-V steel, specifically C0.10%, Si0.25
%, Mn0.150%, P0.015%, S0.003%, A0.032
%, V 0.040%, Nb 0.035%, Cu 0.20%. Tensile tests were conducted at various temperatures and the relationship between the tensile test temperature and the aperture value was investigated. The results shown in Figure 3 were obtained. From this result, the reduction of area of ordinary steel that does not substantially contain A hardly changes even if the temperature changes, whereas the A-Nb-V-containing steel has a temperature of 700 to 900℃.
The aperture value decreased significantly. Therefore,
It is clear that A-Nb-V-containing steel is susceptible to cracking if tensile stress is applied in the temperature range of 700 to 900°C, and this phenomenon also occurs in Ti,
The same was true for steel containing Cu. The cause of this is AN and
This seems to be because NbC, TiN, VC, Cu, etc. are continuously precipitated at the austenite grain boundaries, leading to embrittlement. From the above experimental results, in order to avoid the occurrence of corner cracks and lateral cracks in vertical bending continuous casting machines, it is necessary to keep the slab surface temperature below 700℃ or above 900℃ where tensile stress is applied. It is presumed to be good. Therefore, the inventors first
We thought about lowering the temperature to 700°C or less, but since the bending position in Figure 1 where tensile stress acts is close to the mold surface, it would be difficult to reduce the temperature suddenly to 700°C or lower. It is difficult to prevent corner cracks and horizontal cracks by any method. Therefore, we investigated ways to increase the slab surface temperature at the bending position to 900℃ or higher, and found that transverse cracks occur on the wide side of the slab, and since the bending position is close to the mold, the slab surface temperature from just below the mold to the bending position By appropriately controlling the amount of spray water received by the wide side, the surface temperature of the wide side of the slab at the bending position can be easily maintained at 900℃ or higher, making it possible to prevent the occurrence of horizontal cracking relatively easily and effectively. It was confirmed that On the other hand, regarding corner cracks, the heat dissipation at the corners of the slab is greater than that at the wide sides, and the temperature drops rapidly. It is thought that it would be extremely difficult to prevent corner cracks by maintaining the temperature above 900°C.
In other words, the wide side and side surfaces (short side surfaces) of the slab directly under the mold are forcibly cooled by spraying, but the amount of spray water on the wide side and the side surface is both reduced and the bending area is reduced. If an attempt is made to maintain the surface temperature of the corner section above 900°C, there is a risk of breakout accidents or bulging occurring. Therefore, the present inventors investigated the effect on corner cracking when reducing only the amount of spray water on the side surface without reducing the amount of water sprayed on the wide side of the slab, and found that We obtained new knowledge that the amount of spray water is closely related to the occurrence of corner cracks, and that corner cracks can be effectively prevented by controlling the amount of water received on the side surfaces to 150/m ² or less. . That is, the present inventors et al.
Si0.15-0.30%, Mn1.30-1.60%, P0.022% or less, S0.005% or less, A0.025-0.050%, V0.020
~0.050%, Nb0.015~0.050%, Cu0.10~0.30%
For high-strength steel with a composition of
Casting speed 1.0 ~ using vertical bending type continuous casting machine
A slab of 200 x 1900 mm was continuously cast at 1.3 m/min, and the total amount of spray water received per unit area of the side of the slab up to the bending start position was 25~25 m/min.
When we investigated the occurrence of corner cracks on the outside of the curve of the slab by varying it within a range of 400/ ^m2 , we found that
The results shown in the figure were obtained. However, this experiment was conducted with the amount of spray water received on the wide side of the slab set to ^about 250/m2, as in the past. Furthermore, in FIG. 3, the corner crack occurrence index represents the average number of cracks occurring at the curved outer corner portion per unit length of the slab and the index change of this number. From Figure 4, the total amount of spray water received on the side of the slab up to the bending start position is calculated.
It is clear that if it is 200/m ² or less, the occurrence of corner cracks will decrease rapidly, and if it is 150/m ² or less, corner cracks can be completely prevented from occurring. Therefore, the slab casting method of this invention includes A,
When casting a steel slab containing one or more additive elements among Nb, V, Ti, and Cu using a vertical bending continuous casting machine, the support on the side of the slab from just below the mold to the bending start position is It is characterized by controlling the amount of water to 150/m ² or less, and by controlling it in this way, breakout accidents do not occur and corner cracks can be effectively prevented. Furthermore, the casting method of this invention will be explained in detail as follows.
The steel types targeted by this invention are steel types with high cracking susceptibility as mentioned above, namely A, Nb, V,
This is a type of steel that contains one or more of Ti and Cu. Among these, especially A and
Steels to which Nb, V, etc. are added in combination tend to crack easily, and therefore the method of the present invention is most effective for such steels. The amount of these additive elements is not particularly limited, but steel is usually made with intentionally added elements in excess of the amount contained as unavoidable impurities.
That is, A0.020~0.060%, Nb0.010~
About 0.100%, V0.010~0.100%, Ti0.010~
Steels containing one or more of about 0.100% Cu and about 0.10 to 0.60% Cu are targeted. On the other hand, the continuous casting machine used in the casting method of the present invention has a vertical mold, and transitions from the vertical state to the curved state at a position relatively close to the lower end of the mold, that is, at a position where the inside of the slab is not solidified. This is a vertical bending type continuous casting machine. The bending position in this type of continuous casting machine is usually about 1 to 3 meters from the bottom end of the mold. Therefore, in this invention, the secondary cooling water in this range, especially the side surface of the slab (short side surface), is received. The secondary cooling spray water will be controlled. The amount of spray water received on the side of the slab is normally at least 250/m ² or more in conventional vertical bending continuous casting machines.
In this invention, the amount is less, i.e. 150
Control below / ^m2 . By controlling in this manner, the surface temperature of the slab corner portion in the bending portion is maintained at a sufficiently high temperature (estimated to be 900° C. or higher), and corner cracking is effectively prevented. Note that the amount of spray water received up to the bending start position on the wide side of the slab may be 200/m ² to 300/m ² as in the conventional case. In this way, by reducing only the spray water on the side surfaces without reducing the spray water on the wide side, it is possible to prevent breakout accidents and bulging as much as possible while preventing corner cracks. is an important feature of this invention. Furthermore, after the bending start position, most ordinary slab casting machines do not spray water on the sides of the slab.
The same thing can be done when implementing this invention. Further, the amount of spray water received on the wide side of the slab after the bending start position may be the same as in the conventional case. The amount of spray water received on the side of the slab from the bottom end of the mold to the bending start position is 150/m ² or less as mentioned above, but if the amount of water received is extremely small, especially if the slab is thicker than a certain level. When casting slabs, there is a risk of breakout accidents occurring, so the lower limit is usually set at around 25/m ² to prevent breakouts, and the range is 150/m ² to 25/m ²
It is desirable to control within the range of . Examples of this invention are described below. Example High tensile strength steel containing A, Nb, V, i.e.
C0.09~0.12%, Si0.20~0.25%, Mn1.40~1.50
%, P0.020% or less, S0.004% or less, A0.030~
For steel with chemical compositions of 0.040%, Nb 0.025%, and V 0.030%, a vertical bending continuous casting machine with a distance of 1.6 m from the bottom of the mold to the bending start position was used, and the casting speed was 1.1 to 1.3 m/min. 200mm at min
A slab of ×1900mm was continuously cast. The amount of spray water received by the wide side of the slab from directly below the mold to the bending start position is set to about 230/ ^m2 as before, while the amount of spray water received by the side of the slab from directly below the mold to the bending start position is set to 50~
300/m ² , and the occurrence of cracks in the curved outer corner of the obtained slab was investigated, and the results shown in Table 1 were obtained. However, as in the case of FIG. 3, the corner crack occurrence index here is an index obtained by averaging the number of cracks occurring per unit length of the slab. From the results in Table 1, the amount of spray water received by the side of the slab can be determined.
It is clear that corner cracks can be completely prevented from occurring if the thickness is 150/m ² or less.

[Table] As is clear from the above description, the casting method of the present invention is suitable for casting slabs of steel with added elements such as A and Nb, which are highly susceptible to cracking, using a vertical bending continuous casting machine. It can effectively prevent the occurrence of corner cracks that tend to occur due to tensile stress, and the risk of breakout accidents and bulging does not increase significantly, so it is suitable for continuous casting machines for this type of steel. This can greatly contribute to improving slab quality.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the outline of a vertical bending continuous casting machine, Fig. 2 is a schematic perspective view showing the occurrence of cracks in slabs, and Fig. 3 is a general steel that does not substantially contain A and A- Diagram showing the relationship between test temperature and reduction of area in the tensile test of Nb-V containing steel, Part 4
The figure shows the relationship between the total amount of spray water received on the side surface of the slab from just below the mold to the bending start position of A--Nb--V containing steel and the corner crack occurrence index. 1...Mold, 2...Slab, 3...Bending part.

Claims (1)

[Claims] 1. When continuously casting a steel slab containing at least one additive element among A, Nb, V, Cu, and Ti using a vertical bending continuous casting machine, The amount of spray water that the wide side of the slab receives up to the slab bending start position is 200 to 300/ ^m2 , and the amount of spray water that the short side of the slab receives.
A slab casting method using a vertical bending continuous casting machine, which is characterized by controlling the thickness to 150/m ² or less. 2. A slab casting method in a vertical bending continuous casting machine according to claim 1, wherein the amount of water received and sprayed on the short side of the slab is controlled to 150/m ² or less and 25/m ² or more.