JPH0673727B2 - Continuous casting method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for continuously casting a billet or bloom by a mold equipped with an electromagnetic stirrer, and more particularly to a continuous method capable of improving center segregation of a slab. The present invention relates to a casting method.

[Prior Art] In continuous casting, an electromagnetic stirrer is installed in the mold for the purpose of improving the internal quality of the slab, and the molten steel in the mold is agitated so that the dendrite on the solidification front of the slab in the mold is removed. It is known and practiced to cut to increase the equiaxed crystal ratio and improve center segregation. In order to improve the center segregation due to this electromagnetic stirring, it is said that it is effective to strongly stir the molten steel in the mold as described below and to precisely control the superheat degree of the molten steel supplied into the mold. There is.

By the way, the conventional electromagnetic stirrer may be constrained by casting conditions because it is fixedly installed after setting the optimum production conditions at the stage of equipment design. Therefore, the present applicant proposes the following device in Japanese Patent Application No. 1-110682 for the purpose of adjusting the operating conditions in the continuous casting device.

FIG. 2 is a cross-sectional explanatory view showing an example of a continuous casting device equipped with an electromagnetic stirring device. Continuous casting machine 2 is tundish 1
And is configured to continuously supply the molten steel A ₀ in the tundish 1 into the mold 2 through the immersion nozzle 4. An electromagnetic stirrer 22 which is movable up and down in the directions of the arrows Y ₁ Y ₂ is arranged on the outer peripheral portion of the mold 21, and is vertically moved by a drive motor 24 via a speed reducer 23.

With this apparatus, the molten steel A ₀ in the tundish 1 is poured into the mold 21 through the discharge port 4a of the dipping nozzle 4, and the slab is formed while forming the solidified shell A ₁ in the mold 21.

[Problems to be Solved by the Invention] When carrying out continuous casting using the mold having the above-mentioned structure, the problem is how to impart the optimum casting conditions, and thereby the effect of improving center segregation Will make a difference. Conventionally, in order to improve the center segregation in the slab, it is necessary to increase the equiaxed crystal ratio of the slab,
It was considered necessary to satisfy the following conditions in order to increase the equiaxed crystal ratio.

Lower the temperature of molten steel overheating in the tundish.

Strongly stir the molten steel in the mold.

However, if the above conditions are attempted to be satisfied, the immersion nozzle may be clogged due to the low temperature, which may cause a significant deterioration in operation. Further, as shown in FIG. 3, when the superheating temperature is low, the equiaxed crystal ratio is high, but the inclusions (deoxidation products) B floating on the molten metal surface (hereinafter also referred to as meniscus) in the mold are reduced, and the inclusions are reduced. However, there is a problem in that it is easily rolled into the slab and a defect of the surface subcutaneous inclusion of the inclusion system is likely to occur.

On the other hand, when the strong agitation shown above is performed, the equiaxed crystal ratio increases as shown in FIG. 4, but the inclusions floating on the meniscus are not melted and are easily rolled into the slab as they are. It will result in many defects.

Therefore, the inventors of the present invention have carried out research for the purpose of providing a continuous casting method capable of ensuring a high equiaxed crystal ratio and capable of strongly agitating molten steel without causing powder entrapment defects, etc., and completed the present invention. did.

[Means for Solving the Problems] The present invention which has been able to achieve the above object is to supply molten steel into a mold through an immersion nozzle, and the vertical distance L ₁ from the outlet of the immersion nozzle to the molten steel stirring center is The purpose is to stir the molten steel while setting so that α ₁ · Vc · S / d ² <L ₁ <α ₂ · Vc · S / d ² is satisfied.

[Operation] The equiaxed crystal nuclei increase by breaking the dendrites generated from the inner surface of the solidified shell with a stirring flow, and this breaking is easily performed near the meniscus in the fine state of the dendrite structure. Therefore, in order to maintain a high equiaxed crystal ratio, it is desirable to set the stirring position near the meniscus.
However, as shown in FIG. 5, there is a limit between the stirring strength in the mold and the equiaxed crystal ratio, and as shown in the figure, the effect of increasing the equiaxed crystal ratio is above a certain stirring strength. It will be saturated. Further, as the stirring strength is increased, as shown in Fig. 4 mentioned above, the molten steel flow velocity in the meniscus in the mold increases, which may cause the occurrence of norokami defects due to the inclusion of the mold powder supplied on the molten steel surface of the mold. The existence of optimization conditions is found in the relationship between the two.

The equiaxed crystal ratio in the cast slab is determined by the difference between the amount of produced equiaxed crystals and the amount of equiaxed nuclei that are remelted by the molten steel discharge flow from the immersion nozzle. On the other hand, when a horizontal swirling stirring force is applied to the molten steel that is stored in the mold and has an upper free surface, this swirling flow forms an upward reversal flow in the mold, and the discharge flow from the immersion nozzle If the reverse flow is properly combined with the above, the flow velocity of the discharge flow can be weakened, and the above-mentioned re-dissolution of the nucleus of the equiaxed crystal can be suppressed to the minimum. That is, if the spread portion of the discharge flow, which is determined by the flow velocity of the discharge flow, is electromagnetically stirred, a large amount of equiaxed crystal is produced, and the remelting of nuclei is minimized to increase the equiaxed crystal ratio of the cast slab. You can Therefore, as a result of examining the relationship between the molten steel flow rate and the stirring strength in the meniscus in the mold with the installation position of the electromagnetic stirring device in the mold based on the above findings, the result as shown in FIG. 6 can be obtained. did it. The vertical axis is the stirring strength, and the horizontal axis is the vertical distance L from the meniscus to the center of stirring to determine the molten steel flow velocity in the meniscus in the mold. The stirring position for the proper region in FIG. 7 (from the molten steel supply point The distance to the electromagnetic stirring coil center) can be obtained.
In other words, it is essential that the stirring device has a considerable distance from the meniscus / molten steel feed point and that strong stirring is performed at the stirring position. Was examined and the results shown in FIG. 8 were obtained. From this result, particularly at the preferred superheating temperature of the present invention of 30 to 40 ° C., L ₁ is 170 to 700 mm,
It can be seen that it is more preferably 170 to 500 mm.

Further, the casting speed Vc (m / mi) that is proportionally related to the supply rate of molten steel to the mold, in other words, the flow rate of the discharge flow.
Focusing on n), it was found that L ₁ can be determined by the following relational expression when setting the electromagnetic stirring position. That is, when the vertical distance L ₁ (unit: m) from the discharge port of the immersion nozzle to the center of electromagnetic stirring is set to [L-1] as shown in Fig. 2, α ₁ · Vc · S / d ² <L ₁ <Set by α ₂ · Vc · S / d ² .

However, S is the cross-sectional area of the mold (m ² ), d is the inner diameter of the immersion nozzle (m), α ₁ and α ₂ are coefficients based on the shape of the immersion nozzle, dimensions, etc., and the experiment like the example shown in FIG. The result is obtained. In this case, α ₁ is 0.0037 and α ₂ is 0.011 to 0.0155.

On the other hand, the superheated temperature of molten steel in the tundish is closely related to the levitation effect of inclusions.The higher the temperature, the greater the effect, but generally the equiaxed crystal ratio deteriorates. Although it is conventionally considered to be up to about 30 ° C., it is preferable to keep it constant at 30 ° C. or higher because it can be expected to increase the cooling effect of molten steel due to the above-mentioned upward reversal flow in the mold described above. By keeping the temperature high, the floating and elution of inclusions can be promoted, and the billet or bloom having excellent inclusion results and surface quality and high equiaxed crystal ratio can be cast. More preferably, it is recommended to keep the constant superheating temperature within the range of 35 to 45 ° C.

And down the discharge port 4a of EXAMPLES immersion nozzle 4 in Figure 2, the vertical distance L ₁ between the stirring center C ₁ to finish the magnetic stirring device 22 0.02
The equiaxed crystal ratio at each superheating temperature when changing to 0, 0.220, 0.310 m is shown in the graph of FIG. The distance L ₁ is a value obtained by subtracting (subtracting) the immersion depth 1 of the immersion nozzle 4 from the distance L between the meniscus 5 and the stirring center C ₁ , where l is 0.18 m,
The inner diameter d of the immersion nozzle 4 is 0.03 m, and the cross-sectional area S of the mold 21 is
0.0256m ² , casting speed Vc is 1.6m / min, electromagnetic stirring strength is 200A
And The coefficients α ₁ and α ₂ are set to α ₁ = 3.7 × 10 ⁻³ and α ₂ = 1.55 × 10 ⁻² , respectively, based on the experimental data shown in FIG.

Substituting the above values for α ₁ · Vc · S / d ² <L ₁ <α ₂ · Vc · S / d ² yields 0.186 <L ₁ <0.705 (m), and L ₁ = 0.220 and 0.310 (m It is understood that the one set to () is an example that satisfies the requirements of the present invention.

As a result, as is clear from the graph of FIG. 1, in this Example, even if the heating temperature was 30 ° C. or higher, the equiaxed crystal ratio did not significantly decrease, and the central segregation could be stably improved. did it. At this time, the inclusions and the surface quality of the slab were good. Molten steel in the tundish above 35-45 ℃
As a means for controlling within the optimum range, a plasma heating device, an induction heating device or a resistance heating device is used as the heating device 3 (see FIG. 2).

On the other hand, when manufacturing a billet or bloom with a side of 300 mm or less on one side, it is recommended to use a long mold with a length of 900 mm or more, which reduces the influence on the meniscus surface even if strong stirring is performed with an electromagnetic stirrer. It is possible to suppress the occurrence of powder entrapment defects.

On the other hand, as shown in FIG. 7, in order to set the meniscus surface flow velocity to the optimum range (the flow velocity range in which inclusions are levitated by an appropriate amount and melted by an appropriate amount) without disturbing the meniscus surface, the electromagnetic stirring device center C from the nozzle outlet. distance L ₁ to _1, is preferably in the proper position range determined by the above equations.

[Effects of the Invention] Since the present invention is configured as described above, while maintaining a high equiaxed crystal ratio, it is possible to maintain high inclusion inclusions in the slab as shown in Fig. 9 and improve center segregation. It is now possible to continuously cast ingots of excellent quality. That is, molten steel having a superheating temperature of 30 ° C. or higher is strongly stirred by an electromagnetic stirrer without causing powder entrainment, and high quality slabs can be efficiently manufactured.

[Brief description of drawings]

FIG. 1 is a graph illustrating the difference in the equiaxed crystal ratio between the inventive example and the comparative example, FIG. 2 is a cross-sectional explanatory view showing an example of a continuous casting apparatus, and FIG. 3 is a conventional superheating temperature and equiaxed crystal. Ratio and the formation of inclusions, FIG. 4 is a graph showing the relationship between the stirring temperature, the equiaxed crystal ratio and the powder entrainment defect in the prior art, and FIG. 5 is the relationship between the stirring strength and the equiaxed crystal ratio. Graph,
FIG. 6 is a graph showing the relationship between the stirring strength and the meniscus-centering distance of the stirring, FIG. 7 is a graph showing the relationship between the meniscus flow velocity and the quality of the slab, and FIG. 8 is the vertical distance L ₁ and the equiaxed crystal ratio of the slab. 9 is a graph showing the relationship between the molten steel heating temperature and the inclusion defect rate. 1 ...... tundish, 2 ...... continuous casting apparatus 4 ...... immersion nozzle, 4a ...... discharge port 21 ...... template, 2a ...... electromagnetic stirring apparatus C ₁ ...... stirring center, A ₀ ...... molten steel A ₁ ...... Solidification shell

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masamichi Takeuchi Inventor 2 Nadahama Higashi-cho, Nada-ku, Kobe-shi, Hyogo Prefecture Kondo Steel Works, Ltd. Kobe Steel Works (72) Yoshinori Onoue 2 Nada-hama Higashi-cho, Nada-ku, Kobe, Hyogo Kido Steel Works Kobe Steel Works (72) Inventor Takahiko Sato 2 Nadahama Higashi-cho, Nada-ku, Kobe-shi, Hyogo Prefecture Kado Works Steel Works Kobe Steel Works (72) Inventor Kenzo Ayada 2 Nada-hama-higashi, Nada-ku, Kobe City Hyogo Prefecture Company Kado Steel Works Kobe Steel Works (56) References Japanese Patent Publication Sho 64-10305 (JP, B2)

Claims (3)

[Claims] 1. A method of continuously casting while stirring a molten steel in the mold using a mold equipped with an electromagnetic stirrer in which a horizontal swirling stirring force is applied to the molten steel, wherein A range in which the vertical distance L ₁ from the outlet of the immersion nozzle to the center of molten steel stirring satisfies α ₁ · Vc · S / d ² <L ₁ <α ₂ · Vc · S / d ² while being supplied into the mold The continuous casting method is characterized by the following. Where Vc is the casting speed; m / min, S is the cross-sectional area of the mold; m ² , d
Is the inner diameter of the immersion nozzle; m, α ₁ and α ₂ are coefficients. 2. The continuous casting method according to claim 1, wherein the molten steel superheated in the tundish to which the immersion nozzle is connected is cast at a constant superheat of 30 ° C. or higher. 3. The continuous casting method according to claim 1, wherein the molten steel superheat temperature in the tundish to which the immersion nozzle is connected is cast at a constant superheat degree of 35 to 45 ° C.