JPH0413471A - Method for preventing surface cracking of slab - Google Patents

Abstract

PURPOSE:To decrease the heat extraction quantity of a molten steel and to prevent the generation of the surface longitudinal cracking of a slab by providing a heat insulating layer of a ceramics sheet around the molten steel flow hole of an immersion nozzle for continuous casting and pouring the melt of a middle-carbon steel contg. carbon at a specific ratio at a high velocity to a casting mold. CONSTITUTION:The immersion nozzle body 3 having a straight cylinder shape is provided with the heat insulating layer 6 embedded with the ceramics sheet concentrically with the molten steel flow hole 4 from the upper part thereof to the lower part. The slab is continuously cast by immersing the immersion nozzle 1 into the molten steel in the casting mold and discharging the molten steel toward the short side faces of the casting mold from discharge holes 5 on both sides. The slab is continuously cast by passing the melt of the middle-carbon steel contg. 0.09 to 0.15wt.% carbon through the immersion nozzle 1 at the high velocity. The heat extraction quantity of the molten steel from the immersion nozzle is prevented as far as possible, by which the temp. drop is hindered and the generation of the surface longitudinal cracking of the slab is surely prevented. The slab of the middle-carbon steel having good quality is thus cast with high productivity.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for preventing surface cracking of slabs in continuous casting.

Prior Art When continuously casting a slab of medium carbon steel having a carbon content of 0.09 to 0.15% by weight, the higher the casting speed, the more likely vertical cracks will occur on the long side surface of the slab. In particular, it tends to occur frequently during the first or second charge at the initial stage of casting.

When the above-mentioned vertical cracks occur in the slab, a care process for the slab becomes necessary, and direct hot heating and direct hot rolling become impossible, which becomes a major hindrance to energy saving.

The reason why medium carbon steel tends to crack is that the carbon content is 0°09~
If it is 0.15% by weight, peritectic solidification occurs, and the large amount of contraction during solidification creates local gaps between the mold and the solidified shell, which tends to generate uneven solidified shells and lead to cracking due to thermal stress. it is conceivable that.

In addition, the reason why cracks are more likely to occur due to high-speed casting is that, especially when high-speed casting is performed at 1.1 m/min or higher, the amount of molten powder injected between the mold and the solidified shell is partially excessive or insufficient. Therefore, it is thought that the solidified shell becomes non-uniform in the width direction of the slab, causing thermal stress and cracking.

Problems to be Solved by the Invention Therefore, in order to prevent the above-mentioned vertical cracking of the slab, conventional methods have been used to (1) optimize the viscosity of the molten powder and ensure uniform inflow of the molten powder, (2) improve the inner surface of the mold steel plate. (3) A method of fluidizing the molten metal between the immersion nozzle and the long side of the mold (a special method) JP-A-61-172663) and the like have been proposed.

Means to Solve the Problem However, methods (1) and (2) above have the effect of improving the scale when the casting speed is relatively slow, especially below 1.0 m/min, but when the casting speed is 1.1+/min or more, When it comes to high-speed casting, it was not possible to reliably prevent vertical cracks on the surface of the slab.

In addition, method (3) above aims to prevent vertical cracks on the surface of the slab and to increase the casting speed, but even with this method, it is difficult to sufficiently fluidize the molten metal, and the flow remains only locally. , it is difficult to reliably prevent vertical cracking.

Therefore, the present inventors conducted research from various aspects such as the temperature of the molten steel in the mold, the heating of the immersion nozzle, and the properties of the molten powder. As a result, the following was found.

There is a high probability that vertical cracks on the surface of the slab will occur near the immersion nozzle, as shown in FIG. As shown in FIG. 7, a temperature difference occurs in the molten steel between the immersed nozzle portion and a portion away from the immersed nozzle, and the molten steel temperature around the immersed nozzle becomes lower than that at the center. In addition, as shown in Figure 8, the temperature of the immersion nozzle changes from the end of preheating to the start of casting by approximately 1
The temperature drops rapidly from 200°C to about 800°C. Furthermore, as shown in Figure 9, the Arcs concentration in the molten powder is high in the vicinity of the immersion nozzle, resulting in poor fluidity. As a result, the slub hair becomes large and becomes unevenly large.

From these facts, vertical cracks on the surface of slabs in continuous casting occur because the thickness of the SR in the center of the long side of the mold becomes excessive and the solidification well becomes thin, as shown in Figure 10. It can be determined that vertical cracking occurs due to force.

Means for Solving the Problems The present invention was developed in view of the above-mentioned problems, and in order to solve the above-mentioned problems, a heat insulating layer of ceramic sheets is provided around the molten steel flow hole of the immersion nozzle for continuous casting. The immersion nozzle is installed from the top to the bottom of the main body, and the immersion nozzle is attached to a tundish of a continuous casting machine, and medium carbon steel molten steel with a carbon content of 0.09 to 0.15% by weight is poured from the tundish at high speed. It is an object of the present invention to provide a method for preventing surface cracking of a slab, which is characterized by flowing the above-mentioned immersion nozzle and pouring into a mold for slab molding.

According to the present invention, the immersion nozzle is circulated through the immersion nozzle by means of the heat insulating layer of the ceramic sheet provided on the immersion nozzle body.
It is possible to prevent the temperature of the trt 14 from decreasing due to heat removal through the immersion nozzle.

Therefore, when this immersion nozzle is attached to a tundish and molten steel of medium carbon steel with a carbon content of 0.09 to 0.15% by weight is continuously cast at high speed, even if the flow rate of the molten steel is 1.1
Even at high speeds of m/min or higher, the amount of heat removed from the molten steel can be reduced and the occurrence of vertical cracks on the surface of the slab can be prevented.

Example Hereinafter, the present invention will be explained based on examples.

1 to 4 show an embodiment of the present invention.

As shown in Fig. 1, the immersion nozzle l for continuous casting is formed into a cylindrical shape made of a prescribed refractory material, and its upper end is formed into a slightly wide stepped part 2 so as to be attached to a tundish (not shown). A melt flow hole 4 of a predetermined diameter is formed in the center of the cylindrical immersion nozzle main body 3, and discharge holes 5 are formed on both sides of the lower end thereof. As shown in the figure, there are molten steel flow holes 4 extending from the top to the bottom of the cylindrical immersion nozzle body 3.
A heat insulating layer 6 in which ceramic sheets are embedded concentrically is provided.

It is preferable that this heat insulating layer 6 is continuously long from the top to the bottom as long as the strength of the submerged nozzle body 3 allows, and is disposed as close to the molten steel flow hole 4 as possible. If necessary, it can also be arranged on the inner circumferential surface of the immersion nozzle main body 3. In addition, as the ceramic sort for the heat insulating layer 6,
The thickness can be set to about 0.5 to 5 cm, and the insulation effect is large, and the strength of the immersed nozzle is not reduced.

The immersion nozzle 1 thus formed is immersed in the molten steel of the mold 7 as shown in FIG. 2, and the molten steel is discharged from the discharge holes 5 on both sides in the direction of the short sides of the mold 7, as shown in FIG. 3. The slab 8 is continuously cast.

Example of Use The above-mentioned immersion nozzle of the present invention was applied to an actual continuous casting machine, and compared with a conventional immersion nozzle shown in FIG. 5 made of the same material. The molten steel to be cast is C0, 10%, SiO, 80%.
, Mn 1.0%, PO,009%, SO,008
%, 5ol It is a medium carbon steel containing 0.30% of Al, the casting speed of the molten steel is 1.5 m/min, and the mold has a thickness of 25 m/min.
0m, width 1600m.

As a result, the incidence of vertical cracks on the surface of slabs was 33
%, but there was none in the present invention, and the intended purpose was fully achieved.

Regarding the products of the present invention and the conventional products, as shown in Figs. 1 and 5, the temperature of the nozzle inner walls A and C and the nozzle outer walls B and D, which correspond to the inside and outside of the heat insulating layer at the meniscus part of the submerged nozzle, is The measured results were as shown in FIG.

As shown in FIG. 4, in the submerged nozzle of the present invention, the temperature of the nozzle outer wall is lower and the temperature of the nozzle inner wall can be maintained higher than that of the conventional submerged nozzle. This is because a hollow part for heat insulation is provided in the immersion nozzle, which is the intended purpose, and it can be clearly demonstrated that the amount of heat removed from the molten steel through the immersion nozzle is reduced.

In the example, the heat insulating layer of the ceramic sheet was disposed continuously above and below the immersion nozzle, but within the scope of the spirit of the present invention, it is also possible to implement the heat insulating layer in an appropriate modification such as discontinuously. Further, the ceramic sheet is not limited to a flat plate-shaped one, but a cylindrical one can also be used.

Effects of the invention As described above, in the present invention, the carbon content is 0.09
Even if molten steel of ~0.15% by weight of medium carbon steel is passed through the immersion nozzle at high speed to continuously cast a slab, the amount of heat removed from the molten steel through the immersion nozzle can be prevented as much as possible, preventing a temperature drop, and the vertical surface of the slab. It is possible to reliably prevent the occurrence of cracks and to cast high quality medium carbon steel slabs with high productivity.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view of a submerged nozzle according to an embodiment of the present invention, FIGS. 2 and 3 are explanatory views of the same usage state, respectively, and FIG. 4 is a nozzle inner and outer wall of a product of the present invention and a conventional product. Figure 5 is a longitudinal cross-sectional view of a conventional immersion nozzle, Figures 6 to 9 are explanatory diagrams of experimental data for the immersion nozzle, and Figure 10 is an explanation of the occurrence of vertical cracks on the surface of the same slab. It is a diagram. 1... Immersion nozzle, 3... Immersion nozzle body, 4...
- Molten steel distribution hole, 5... Discharge hole, 6... Heat insulation layer, 7.
··template. Applicant Kawasaki Rozai Co., Ltd. Agent Patent Attorney Kuni Morimoto Figure 1 Figure 5 [Z≧5 Figure 2

Claims (1)

[Claims] (1) A heat insulating layer of ceramic sheet is provided around the molten steel flow hole of the immersion nozzle for continuous casting from the top to the bottom of the immersion nozzle body, and this immersion nozzle is attached to the tundish of the continuous casting equipment. is 0.09~
A method for preventing surface cracking of a slab, comprising flowing 0.15% by weight of medium carbon steel molten steel from the tundish at high speed through the immersion nozzle and into a mold for slab forming.