JPH04182045A - Method for continuously casting cast slab without any flaw below surface - Google Patents

Abstract

PURPOSE:To prevent a crack just below surface and to improve the product yield by covering the meniscus of a molten steel in a mold with non-melting heat insulating plate and also executing a casting while pouring a lubricating oil from between the non-melting heat insulating plate and the mold wall. CONSTITUTION:The molten steel 2 received into a tundish 1 from a ladle is poured into the mold 4 through an immersion nozzle 3 provided in the mold 4 together with the non-melting heat insulating plate 6 while penetrating through the plate 6, and the meniscus 5 of molten steel in the mold 4 is covered with the plate 6. Simultaneously, a continuous casting is executed while pouring the lubricating oil 7 from a gap (t) between the mold 4 and the plate 6. By this method, the crack just below the surface is prevented and the product yield is improved.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a continuous casting method for obtaining slabs free of subcutaneous defects, and in particular, low carbon steel (C, 0.18% or less ) Continuous casting method.

[Prior art and problems to be solved by the invention] Conventionally, as a continuous casting method for molten steel, for example, as shown in FIG.
The molten steel meniscus 5 in the mold 4 is poured through the pouring nozzle 8.
At the same time, a lubricating oil 7 is injected onto the molten steel meniscus 5 in the mold 4, and the lubricating oil 7 lubricates between the inner wall surface of the mold and the solidified shell, and then continuous casting is performed. As shown in FIG. 4, the molten steel 2 poured from a ladle (not shown) into the tundish plate L is poured into the molten steel in the mold 4 through the immersion nozzle 3, while the molten steel meniscus 5 in the mold 4 is poured into the molten steel in the mold 4. A method is used in which continuous casting is performed by coating with powder 9 and using this powder slug to lubricate the space between the inner wall surface of the mold and the solidified shell.

However, in the former continuous casting method, since the molten steel 2 is discharged from the pouring nozzle 8 onto the molten steel meniscus 5 in the mold, there is a problem that the discharged molten steel 2 is oxidized and the molten steel meniscus 5 in the mold is Since it is exposed to the atmosphere, there is a problem that the molten steel meniscus 5 oxidizes, the temperature of the molten steel meniscus 5 decreases due to heat radiation to the atmosphere, and the molten steel meniscus 5
There is also the problem of solidification, and its application is limited to small-section slabs with a small molten steel meniscus 5. In addition, in this continuous casting method, is it possible to use the immersion nozzle 3 instead of the pouring nozzle 8?If the immersion nozzle 3 is used, will the oxidation of the molten steel 2 poured by the immersion nozzle 3 be prevented? The heat dissipation from the meniscus 5 to the atmosphere becomes more intense, the temperature of the molten steel meniscus 5 drops significantly, and solidification and scum occur around the immersion nozzle 3, resulting in the inability to cast, or even if casting is possible, serious damage to the slab. This may result in serious quality defects, making it impractical.

On the other hand, in the latter continuous casting method, the immersion nozzle 3 is used to pour the molten steel 2 from the tandem tube 1 into the mold 4, and the molten steel meniscus 5 in the mold 4 is covered with powder 9. The problem of atmospheric oxidation and solidification of the molten steel meniscus 5, as in the continuous casting method of (0.18 wt% or less), subcutaneous defects occur in the slab due to the following mechanism.

First, in the early stage of solidification, streak-like depressions occur on the surface of the slab due to solidification contraction related to peritectic reaction. Powder slag then flows into this depression, thereby accelerating the cooling of the depression. This cooling acceleration increases the amount of peritectic reaction per hour, and with this increase, the amount of solidification shrinkage increases due to the increase in the amount of peritectic reaction, resulting in an increase in the depression and further powder slag flowing into this increased depression. By repeating these steps, the depth of the dent increases to some extent (for example, 0
．． 25 fights or more), a crack during solidification occurs just under the skin of the dent. Naturally, if the crack is filled with concentrated molten steel or if the solidified shell is weak, the surface of the slab and powder will be sucked in, resulting in a major defect in the product. Even if it does not appear on the surface, it appears as a product defect when it is scaled off during rolling because it is just under the skin.

Therefore, in view of the above-mentioned problems, the present invention provides a method for continuously casting molten steel without using powder that increases streak-like depressions due to solidification contraction associated with peritectic reactions that cause subcutaneous defects. The purpose is to provide

[Means to solve the problem]

In order to achieve the above object, the method of continuous casting of slabs without subcutaneous defects according to the present invention is such that molten steel is continuously poured into a mold through a submerged nozzle, while solidified slabs are poured from the bottom of the mold. In a continuous casting method for continuously drawing slabs, the molten steel meniscus in the mold is covered with a non-melting heat insulating plate, and lubricating oil is poured between the non-melting heat insulating plate and the mold wall during casting.

[For production]

In the present invention, since the immersion nozzle is used to pour the metal from the tundish into the mold, oxidation of the molten steel discharged from the immersion nozzle is prevented. In addition, since a non-melting heat insulating plate is provided above the molten sea bream meniscus in the mold, the molten steel meniscus is reliably kept warm together with the molten steel flow below the meniscus of the molten steel discharged from the immersion nozzle, and solidification of the molten steel meniscus is prevented. At the same time, oxidation of the molten steel meniscus surface is prevented. In addition, this heat insulating plate is made of a material that does not melt and burn with molten steel, such as carbon fiber or flame-retardant flaky graphite powder, and is made of a material that is not melted and burned by molten steel. Therefore, even if streak-like depressions occur due to coagulation contraction related to peritectic reaction, there is no effect that would increase them. Furthermore, since lubricating oil is used as lubrication between the mold and the solidified shell, it is almost always the case that the lubricating oil carbonizes at or just below the self-melting steel meniscus of the mold and flows between the mold and the solidified shell as a liquid. Therefore, like the above-mentioned heat-retaining plate, even if streak-like depressions occur due to solidification contraction related to peritectic reaction, there will be no effect to increase this.

In order to obtain the above effect, the size of the non-melting heat insulating plate is preferably such that there is a gap of 1 to 5 mm between it and the mold. The reason for this is that if the gap is less than 1 mm, the inflow of lubricating oil will be insufficient, and problems such as seizure with the mold will occur, and problems such as breakout of the slab will occur, making casting impossible. Moreover, if the gap exceeds 5 degrees, the lubricating oil will flow in more and the heat retention effect of the molten steel meniscus will decrease.

〔Example〕

FIG. 1 is a conceptual diagram of pouring metal from the tundish to the mold when the continuous casting method for slabs without subcutaneous defects according to the present invention is applied. In the drawings, components that are the same as those of the prior art are designated by the same reference numerals.

In the figure, 1 is a tandem shell, 2 is a molten steel, 3 is an immersion nozzle, 4 is a mold, 5 is a molten steel meniscus, 6 is a non-melting heat insulating plate, and 7 is a lubricating oil.

With this pouring configuration, the molten steel 2 received from the ladle (not shown) into the tundish plate 1 passes through the non-melting heat insulating plate 6 and passes through the immersion nozzle 3 provided in the mold 4 together with the non-melting heat insulating plate 6. The molten steel meniscus 5 in the mold 4 is covered with a non-melting heat insulating plate 6, and continuous casting is carried out while lubricating oil 7 is allowed to flow in from the gap t between the mold 4 and the non-melting heat insulating plate 6. Ru.

In the above manner, molten steel with the carbon content shown in Fig. 2 was prepared by arranging carbon fibers with flame-retardant flaky graphite powder to a thickness of 50 mm.
Using a non-melting heat insulating plate solidified so that the gap t = 3 mm, continuous casting was performed into a slab with a square cross section of 155 mm on the - side, and a depth of about 0.25 mm was generated on the surface of the resulting slab.
The incidence of streak-like depressions due to coagulation contraction associated with the above peritectic reaction was investigated. The survey results are also shown in Figure 2.

As is clear from Figure 2, the carbon content is 0.05 to 0.
．． It can be seen that in the range of 15 wt%, the incidence of dent defects is generally lower in the method of the present invention than in the conventional method using powder. Furthermore, the decrease in the occurrence rate of dent defects, which was previously particularly high, is noticeable in the range of 0.07 to 0.12 wt%.

〔Effect of the invention〕

As described above, according to the method for continuous casting of slabs without subcutaneous defects according to the present invention, continuous casting can be performed without increasing streak-like depressions due to solidification contraction associated with the peritectic reaction that causes subcutaneous defects. By doing so, it is possible to prevent cracks directly under the skin that occur due to the increase in dents, and improve product yield.

[Brief explanation of drawings]

Figure 1 is a conceptual diagram of the process from the tundish to the mold when the method of the present invention is applied, and Figure 2 shows the relationship between the carbon content of molten steel and the incidence of dent defects on the slab surface. The diagrams illustrating the relationship, FIGS. 3 and 4, are explanatory diagrams of the prior art. l Tandate 2 Molten steel 3 Immersion nozzle 4 Mold 5 Molten steel meniscus 6 Non-melting heat insulating plate 7 Lubricating oil patent applicant Kobe Steel Co., Ltd. agent Patent attorney Akira Kanemaru - Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] Molten steel is continuously poured into the mold through an immersion nozzle, while solidified slabs are continuously drawn out from the bottom of the mold.
In a continuous casting method for slabs, the molten steel meniscus in the mold is covered with a non-melting heat insulating plate, and casting is performed while lubricating oil is applied from between the non-melting heat insulating plate and the mold wall. Continuous casting method for pieces.