JPH06218505A - Method for continuously casting molten metal - Google Patents

Abstract

PURPOSE:To stably produce a cast slab having good surface characteristic in the industrial scale by energizing AC current having a specific frequency on molten metal surface in a mold and heating. CONSTITUTION:The molten steel 3 is poured into the mold 1 for continuous casting through an immersion nozzle 2. Two electrodes 8 are dipped into this molten steel (molten metal) 3 and an AC source 9 is connected with these electrodes and the AC current having >=200Hz frequency is energized between the electrodes 8. By this method, the surface of the molten steel 3 is heated. The energization is executed in the range, which the impressed current value I (A) satisfies the inequality. In the inequality, t: thickness of the mold (m), l: length between the electrodes (m), f: AC frequency (s<->). By this method, the surface part of the molten steel in the mold can be heated with a little input energy without contaminating the molten steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for molten metal, especially molten steel, and particularly to improve the surface quality of the slab by suppressing the capture of mold powder and other oxides on the surface layer of the slab. To do.

[0002]

2. Description of the Related Art In continuous casting of molten metal, particularly molten steel (the examples of molten steel will be described below), in order to omit the maintenance work of the slab by directly connecting with the hot rolling which is the next step, It is important to reduce defects in the slab, especially surface defects, as much as possible.

As shown in FIG. 1, the surface defects of the slab are obtained by casting a mold powder 4, in a molten steel 3 injected through a dipping nozzle 2 into a continuous casting mold (hereinafter simply referred to as "mold") 1. The alumina inclusions 5 and the air bubbles 6 are entrained, and they are trapped by the initially solidified shell 7 of the slab and are generated.

Therefore, in order to improve the surface quality of the slab,
(A) It is necessary to reduce the amount of mold powder, alumina inclusions, and bubbles entrained in the molten steel, and (b) to reduce the growth rate of the solidified shell in the meniscus portion.

[0005]

With respect to the above (a), measures such as optimization of the shape of the dipping nozzle, control of molten steel flow by electromagnetic force, and reduction of oxygen concentration in steel have been taken. Haven't come to satisfactory results.

With regard to the above (b), measures have been taken such as heating molten steel in the meniscus portion and utilizing exothermic mold powder. However, the use of exothermic mold powder is due to the presence of metallic Al in the mold flux.
Since oxidization heat generation is utilized by mixing and ferrosilicon and the like, an oxidizing agent (iron or Mn oxide) must be added. Therefore, it is difficult to control the reaction between the exothermic agent and the oxidizing agent, and a sufficient calorific value cannot be obtained, and the oxidizing agent reacts with Al in the bath to contaminate the molten steel.

On the other hand, the heating of molten steel in the meniscus portion is described in
As described in Japanese Patent Laid-Open No. 57-109552, this is a method of immersing a carbon electrode in a mold powder to conduct electricity and utilizing Joule heat in the mold powder. However, the voltage may become unstable due to the carburization of molten steel by the carbon electrode or the fluctuation of the molten metal surface level, and there were many problems to be solved before applying it to the actual operation of continuous casting. .

Further, both of the above two measures (b) are for heating the mold powder to indirectly heat the molten steel, and the amount of heat radiated from the mold powder to the atmosphere above the bath surface is Since it is large, there is a disadvantage that the amount of input energy must be increased.

Therefore, an object of the present invention is to propose an advantageous heating method for a molten steel meniscus portion in a mold by solving the above problems.

[0010]

The present invention is a continuous casting method for molten steel, characterized in that the molten metal surface charged in a continuous casting mold is heated by applying an alternating current having a frequency of 200 Hz or more. Is. The energization is performed via a pair of electrodes immersed in molten steel, electrically insulated from each other,
What is done between the opposing inner walls of the mold, and the applied current value I
It is advantageous in practice that (A) is performed within a range that satisfies the following formula (1). Note I ≧ 3.26 × 10 ⁴ {t / (l · f ^1/2 )} ^1/ 2-(1) where, t: mold thickness (m) l: distance between electrodes (m) f: alternating current Frequency (s ^-1 )

Next, the method of the present invention will be described in detail with reference to FIG. That is, two electrode rods 8 are immersed in the molten steel 3 injected into the mold 1 through the immersion nozzle 2 and these electrode rods 8 are connected to an AC power source 9 respectively, and the frequency between the electrode rods 8 is increased. Apply an alternating current of 200 Hz or more. This electric current flows through the molten steel 3 to the other electrode rod 8 via the one electrode rod 8, and as a result, the molten metal surface of the molten steel 3 between these electrodes is heated. When the fluctuation of the molten metal level is small, it is not necessary to insert the electrode rod 8 into the molten steel as shown in the figure, and the tip of the electrode rod 8 may be immersed in a molten layer of mold powder. The material of the electrode rod is C-containing refractory such as graphite, ZrB ₂ , alumina graphite or zirconia graphite, or a composite material thereof,
Furthermore, water-cooled metal is suitable. However, since graphite may be carburized, it is applied only to steel grades that are not harmful.

[0012]

Since an alternating current having a frequency of 200 Hz or higher has a skin effect, as shown in FIG. 3, energization is carried out only in the region of the current permeation thickness δ of a certain constant thickness. Joule heating is possible on the surface of the molten metal. Here, the reason why the alternating current is 200 Hz or more is that the current frequency is
This is because if it is less than 200 Hz, the current penetration thickness δ becomes thick, the resistance value of the molten steel in the electric circuit becomes low, and the amount of heat generation decreases.

By the above operation, the temperature of the molten steel surface in the mold rises, the growth rate of the initially solidified shell at the meniscus portion is suppressed, and the surface quality of the cast piece is improved. Further, since the thickness of the molten layer of the mold powder is increased, it is possible to prevent the unmelted mold powder from carburizing the molten steel in casting the low carbon steel or the ultra low carbon steel.

[0014]

【Example】

Example 1 Using a vertical bending type continuous casting machine (vertical part: 3 m), Table 1
Continuous casting was performed under the conditions shown in. This continuous casting machine is of a two-strand type, in which one strand is subjected to the molten metal surface heating by passing the alternating current shown in FIG. 2, and the other strand is cast without the molten metal surface heating.

[0015]

[Table 1]

AC energization conditions were a frequency f: 50 kHz, a current I: 2550 A, an electrode rod made of ZrB ₂ and having a diameter of 100 mmφ was used, and an electrode rod interval (distance between shaft centers): 1300 mm and bath Depth from the surface: 20 mm was placed by immersion. By this electrical heating, the molten steel surface temperature can be increased by an average of 18 ° C. in the molten steel superheat degree ΔT (molten steel temperature-liquidus temperature).
An average temperature rise of 14 ° C could be achieved compared to the case without heating.

The above operation was carried out for 18 heats, and for each heat, a 1.2mm-thick cold rolled sheet was manufactured through hot rolling and then cold rolling, and the product quality of each strand was investigated.
The results are shown in Table 2.

[0018]

[Table 2]

From Table 2, by heating the molten steel surface according to the present invention, the surface properties of the obtained slab are improved, the quality of the cold-rolled sheet is remarkably improved, and the defect occurrence rate is
Compared with the case of not heating the molten steel surface (conventional example),
You can see that it became 1/10 or less.

Under the same conditions as above, when casting was performed while changing the current frequency f variously, when the current frequency f was less than 200 Hz, the current I had to be 10000 A or more. However, it was impossible to apply it to the actual machine because the capacity of was extremely large.

Even if the short side and the long side of the mold are electrically insulated and either the short side or the long side of the mold is used as an electrode, the same result as in the above embodiment can be obtained. I was able to. That is, the electrodes may be used in any arrangement or in any combination as long as an alternating current flows through the molten steel surface due to the structure of the continuous casting machine facility.

Example 2 Under the same conditions as in Example 1, the current frequency f: 200-
At 100 × 10 ³ (Hz), mold thickness t: 0.180 to 0.34 (m) and inter-electrode distance l: 0.200 to 1.400 (m), casting was performed by changing the energizing current value I variously, and the same as Example 1. Cold-rolled sheet was manufactured. Then, in this production, the molten steel superheat degree ΔT of the molten steel surface in the mold and the surface defect index of the cold rolled sheet were investigated.

As shown in FIGS. 4 and 5, the results of the investigation show that when the energization current value I is in the range satisfying the above-mentioned formula (1), the molten steel superheat degree ΔT on the molten metal surface rises sharply and It can be seen that the defect index is also significantly reduced.

[0024]

As described above, according to the present invention, the surface layer portion of the molten steel in the mold can be heated without contaminating the molten steel and with a small input energy, and a slab having a good surface property can be manufactured. It is possible to manufacture stably on a dynamic scale.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating a generation mechanism of a surface defect of a slab in continuous casting.

FIG. 2 is a schematic diagram illustrating a method for heating a surface layer portion of molten steel in a mold according to the present invention.

FIG. 3 is a schematic diagram illustrating a heating mechanism of a molten steel surface layer portion.

FIG. 4 is a graph showing a relationship between an energization current value I and a molten steel superheat degree ΔT on a molten metal surface.

FIG. 5 is a graph showing a relationship between an energization current value I and a surface defect index.

[Explanation of symbols]

 1 Mold 2 Nozzle 3 Molten Steel 4 Mold Powder 5 Alumina Inclusion 6 Bubble 7 Initial Solidification Shell 8 Electrode Rod 9 AC Power Supply

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kenichi Sorimachi, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Prefecture Technical Research Division, Kawasaki Steel Co., Ltd. (72) Tetsuya Fujii 1, Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Corporation Technical Research Division

Claims (4)

[Claims] 1. A continuous casting method for molten metal, which comprises heating an molten metal surface in a continuous casting mold by applying an alternating current having a frequency of 200 Hz or more. 2. The method according to claim 1, wherein the current is applied through a pair of electrodes immersed in the molten metal. 3. The method according to claim 1, wherein electric current is applied between opposing inner wall surfaces of the mold which are electrically insulated from each other. 4. The method according to claim 1, wherein the applied current value I (A) is energized in a range satisfying the following formula. Note I ≧ 3.26 × 10 ⁴ {t / (l · f ^1/2 )} ^1/2 where t: mold thickness (m) l: distance between electrodes (m) f: alternating frequency (s ⁻¹ ).