JP4622172B2 - Continuous casting method for molten steel - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for continuously casting molten steel, and in particular, an upper nozzle and a submerged nozzle provided on the bottom of a tundish with molten steel held in a tundish and suspended in non-metallic inclusions (hereinafter simply referred to as inclusions). It is related with the technique which prevents that the inclusion which is suspended is caught in the molten steel inject | poured in a casting_mold | template through a series.
[0002]
[Prior art]
In order to manufacture a steel product, it is common to once cast molten steel that has been discharged from a refining vessel such as a converter into a ladle, and then rolling the steel slab. A continuous casting method is often used for casting the molten steel. As shown in FIG. 3, this continuous casting method is an intermediate container (referred to as a tundish 4) in which molten steel 3 with slag 2 held in a ladle 1 is made uniform in order to make the components of molten steel uniform before being poured into a mold. The semi-solid body 8 having a solidified shell formed on the surface thereof is continuously injected through the upper nozzle 5 and the immersion nozzle 6 sequentially into the water-cooled mold 7 disposed below the upper nozzle 5 and the immersion nozzle 6. Is a technique in which steel is cooled while being gradually extracted downward (in the direction indicated by →) to form a steel slab (not shown, but referred to as a slab, bloom, billet, etc. depending on the shape).
[0003]
By the way, when a steel slab obtained by this continuous casting method is rolled to produce a product such as a wire rod, it is rejected in ultrasonic inspection (hereinafter referred to as UT (Ultra Sonic Testing)) or the like. There is. This is because the inclusion 9 (which originates from the slag 2 and is suspended in the molten steel as particles having a particle diameter of 30 to 40 μm) is caught in the molten steel 3 injected into the mold 7 in a large amount by the continuous casting described above. In particular, it is remarkable when the steel type to be cast is high carbon steel such as S43C, S45C (JIS standard). Therefore, until now, while increasing the degree of superheat of the molten steel 3 held in the tundish 4, the gas that rises in the molten steel is blown from the upper nozzle 5, and the inclusions 9 that are entrained in the molten steel 3 rise to the slag 2. In order to reduce the number of defective products, it has been operated under conditions favorable to traps. Here, the superheat degree of the molten steel 3 is defined by the difference between the actual molten steel temperature and the melting point of the molten steel.
[0004]
However, when the tundish 4 does not include means such as a heater for heating the molten steel 3 held, it is difficult to adjust the superheat degree of the molten steel 3 held in the tundish 4 within a desired range. . In other words, the current situation is that casting is not always performed under the optimum conditions for reducing rejected products.
[0005]
[Problems to be solved by the invention]
In view of such circumstances, the present invention provides a continuous casting method of molten steel that can reduce the inclusion of inclusions in molten steel injected into a mold, even if a tundish that does not include molten steel heating means is used. The purpose is that.
[0006]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the inventor diligently studied the phenomenon of inclusion inclusion during casting and realized the result in the present invention.
[0007]
That is, the present invention provides a method for continuously casting a molten steel, which is held with a slag in a tundish and in which nonmetallic inclusions are suspended, by sequentially injecting the molten steel into a mold through an upper nozzle and an immersion nozzle. A rod-shaped body that can be moved up and down is provided above the upper nozzle, and the height of the molten steel head in the tundish is measured with an ultrasonic molten steel level meter, and the lower end of the rod-shaped body and the upper end surface of the upper nozzle It is a continuous casting method of molten steel, characterized in that the distance is adjusted to a range of 5 to 15% with respect to the measured value using a lifting device for a rod-shaped body , and molten steel is poured into a mold. In particular, 10 to 12% is preferable. In this case, it is preferable to use a stopper that closes the upper nozzle and stops pouring molten steel from the tundish into the mold.
[0008]
According to the present invention, the molten steel in the tundish does not generate a vortex immediately above the upper nozzle during casting, and inclusions can be prevented from being caught in the molten steel flowing into the upper nozzle. As a result, the quality of the steel slab is improved, and the acceptance rate of bar wire products and the like can be increased as compared with the prior art.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
In the following, embodiments of the present invention will be described in accordance with the circumstances leading to the invention.
[0010]
First, as a result of investigating the cause of the high defect rate in the UT of wire rod products, the inventor originated from the slag in the ladle based on an analysis photograph (EPMA) of the defective part by a micro analyzer. It was found that there was an inclusion. Subsequently, when pursuing a mechanism in which the inclusions are mixed into the molten steel injected into the water-cooled mold, a vortex 10 is generated immediately above the upper nozzle 5 provided at the bottom of the tundish 4 during casting as shown in FIG. And it became clear that the inclusion 9 suspended in the molten steel 3 was caught in the vortex 10. Accordingly, the present inventors have made intensive efforts to discover means for suppressing the generation of vortices, thought that it would be sufficient to obstruct the flow of molten steel by causing a rod-like body to exist at the position where the vortex was generated, and reached the present invention.
[0011]
Specifically, as shown in FIG. 1, a rod-like body 11 is provided in the tundish 4 so as to be movable up and down, and is moved up and down, so that the lower end portion 13 of the rod-like body 11 and the upper end surface 14 of the upper nozzle 5 The distance 15 is adjusted to a range of 5 to 15% with respect to the molten steel head height 12 in the tundish 4. Further, an ultrasonic molten steel level meter and an electromagnetic level meter (not shown) are used for measuring the molten steel head height, and the position of the lower end portion 13 of the rod-shaped body is raised and lowered based on the measured value by the level meter. It sets with an apparatus (not shown). In this case, the central axis of the rod-shaped body 11 substantially coincides with the center of the nozzle. Further, the rod-like body 11 may use a stopper that closes the opening of the upper nozzle 5 and stops the outflow from the tundish 4 to the upper nozzle 5 of the molten steel 3. Therefore, it is desirable that the lower end portion of the rod-shaped body 11 has an R shape (forms a curved surface), and the rod-shaped body is formed of a heat-resistant material such as a refractory or ceramic.
[0012]
The distance 15 between the tip of the rod 11 and the upper end surface of the upper nozzle 5 is set to 5 to 15% with respect to the molten steel head height 12 in the tundish 4 for the following reason. That is, if it is less than 5%, the gap between the tip of the rod-like body 11 and the upper end surface of the upper nozzle 5 becomes too narrow, and the outflow path of the molten steel 3 cannot be ensured sufficiently, and the molten steel flow rate required for smooth casting. It is because it becomes impossible to satisfy. On the contrary, when the distance exceeds 15% with respect to the molten steel head height 12 in the tundish 4, the generation of the vortex 10 generated just above the nozzle cannot be suppressed. Preferably, this distance is 10 to 12% with respect to the molten steel head height 12 in the tundish 4. This method is effective for improving the quality of the cast slab even if the tundish heating means is not provided, but a heating means may be provided.
[0013]
【Example】
By the continuous casting method of the molten steel which concerns on this invention, it implemented with the high carbon steel whose steel type is S45C (JIS specification). The specific state is shown in FIG. As the rod-shaped body 11, the above-mentioned stopper was adopted. The lower end position of the rod-shaped body from the upper end surface of the upper nozzle 5 was adjusted according to the molten steel head height 12, and was continuously cast as shown in FIG. Here, the molten steel head was measured with an ultrasonic molten steel level meter, and the lower end position of the rod-shaped body was set with a stopper lifting device. Immediately thereafter, the steel slab was rolled to produce a steel bar product having a diameter of 50 mm, and the defects were examined by UT. In addition, as the tundish, one without any heating means was used.
[0014]
Defect rate in UT of a steel bar product obtained by continuously casting steel slabs with the distance from the upper end surface of the upper nozzle to the lower end of the rod-shaped body being 11% and 20% with respect to the molten steel head height FIG. 5 shows the number of accepted products / the total number of inspected products × 100). From FIG. 5, it is clear that when the rod-shaped body height is 20% (comparative example), there is no effect of suppressing the generation of vortex, and the defect rate is the same as when the conventional rod-shaped body is not used (conventional example). However, at a rod-like body height of 11% that satisfies the position adjustment range of the rod-like body according to the present invention, the occurrence of the UT defect rate is greatly reduced due to the effect of suppressing the generation of vortices. In all of the examples, the molten steel was cast at a superheat degree of 25 ° C.
[0015]
【The invention's effect】
As described above, according to the present invention, inclusion of inclusions in molten steel injected into a mold can be reduced as compared with the conventional technique even when a tundish not provided with a molten steel heating means is used. As a result, the quality of the continuously cast steel slab has been improved, and finally, the pass rate of the bar wire product can be increased as compared with the prior art.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram for explaining the effect of inhibiting inclusion inclusion by a continuous casting method according to the present invention.
FIG. 2 is a diagram showing specific implementation conditions of a continuous casting method according to the present invention.
FIG. 3 is a diagram for explaining a general method for continuous casting of molten steel.
FIG. 4 is a diagram showing a situation where inclusions are caught in molten steel in continuous casting.
FIG. 5 is a diagram showing an example of the effect of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Ladle 2 Slag 3 Molten steel 4 Tundish 5 Upper nozzle 6 Immersion nozzle 7 Water-cooled mold 8 Semi-solid body 9 Inclusion 10 Vortex 11 Rod body 12 Molten steel head height 13 Lower end 14 of rod body Upper end surface 15 of upper nozzle Distance between the lower end of the rod-shaped body and the upper end surface of the upper nozzle

Claims (2)

In the continuous casting by injecting molten steel held with slag in the tundish and suspending non-metallic inclusions into the mold through the upper nozzle and the immersion nozzle sequentially,
A rod-shaped body that can be moved up and down in the tundish is provided above the upper nozzle, and the height of the molten steel head in the tundish is measured with an ultrasonic molten steel level meter, and the upper end of the rod-shaped body and the upper nozzle are measured. A method for continuously casting molten steel, wherein the distance from the end surface is adjusted to a range of 5 to 15% with respect to the measured value using a rod-like body lifting device , and molten steel is poured into a mold.   2. The molten steel continuous casting method according to claim 1, wherein the rod-shaped body is a stopper that closes the upper nozzle and stops injection of molten steel from the tundish into the mold.

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