JPH067907A - Production of continuously cast slab excellent in surface characteristic - Google Patents

Abstract

PURPOSE:To improve the surface characteristic of a cast slab in an unsteady condition at the time of starting a casting. CONSTITUTION:At the time of pouring a molten steel 8 into a mold 4 from a tundish 7, mold powder 1 added into the mold 4 is beforehand melted by using a melting furnace 2. After adding the molten mold powder 1 into the mold 4 simultaneously with the of start of the casting, electrodes 3 are dipped in the molten layer and energized, and the lowering of temp. is prevented by Joule heat generated in the powder layer, and the molten thickness of the powder is secured. Therefore, since the powder layer melted at the starting time is secured, the surface characteristic of the cast slab can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a continuous cast slab of steel, and more particularly to a method for producing a continuous cast slab having excellent surface properties at the non-steady portion of the casting start.

[0002]

2. Description of the Related Art In continuous casting of steel, for the purpose of preventing reoxidation of molten steel in a mold, adsorption of inclusions in molten steel, lubrication of mold and slab, and prevention of temperature drop of molten steel surface, CaO, SiO ₂ etc. It is common to use mold powders composed of oxides. For the slab corresponding to the start of continuous casting, the melting of the mold powder is unstable, the temperature of the molten steel to be cast drops, and the casting speed fluctuates and is in an unsteady state, causing a sluggish defect. Or, in the case of ultra-low carbon steel applied to outer panels for automobiles, the phenomenon that molten steel is carburized by the contact between unmelted powder containing a few percent of carbon and molten steel is induced.

As a conventional technique for preventing the above problems, continuous casting as shown in JP-A 1-202439, JP-A 63-230259, JP-A 61-086055, etc. There is a method of preventing the defects caused by the mold powder by adding a previously melted mold powder into the mold during non-steady state. In this method, since the powder added in the mold is in a molten state, heat is largely removed from the surface of the powder, the surface of the powder is solidified, and the temperature of the molten steel meniscus is lowered.

[0004]

DISCLOSURE OF THE INVENTION The object of the present invention is to eliminate problems such as cracking of a slab and carburization from unmelted powder, which are caused by insufficient melt thickness of the mold powder at the start of casting. It is a thing.

[0005]

The present invention aims to form a molten layer of a mold powder at an early stage by adding a mold powder that has been melted in advance at the start of continuous casting into the continuous casting mold, and thereafter, the molten mold powder. In order to prevent the temperature from decreasing due to the radiation of the powder, one or more electrodes are immersed in the molten layer of the mold powder and resistance heating is used to prevent heat removal from the surface of the powder and to produce a slab with excellent surface quality. It is provided.

[0006]

[Operation] At the start of casting in continuous casting of molten steel,
Molten steel in the ladle that was transported to the continuous casting factory through the converter and secondary refining was poured into an empty tundish that was preheated with a burner,
When the molten steel level in the tundish reaches a certain level, it is injected into the mold through a refractory material nozzle.
The casting at that time is performed at a low speed for several minutes in order to avoid a breakout at the start of casting, and then is accelerated to a target casting speed.

Therefore, at the start of casting, the molten steel surface becomes unstable due to the lowering of the molten steel temperature and the speed increase (increase of the pouring amount) compared to the steady state of casting. The mold powder added to the mold at the start of casting is less likely to melt because the molten steel temperature in the mold is low, which promotes a decrease in the thickness of the molten layer and causes cracks on the slab surface in steel types with high crack susceptibility. ,
In ultra-low carbon steel, the frequency of contact between the unmelted mold powder and molten steel increases, so the carbon in the unmelted powder enters the molten steel. It may trigger out.

In order to solve these problems, it is necessary to secure a sufficient molten thickness of the mold powder, and the present invention solves these problems. The feature of the present invention is that, as shown in FIG. 1, when the molten steel 8 in the tundish 7 is injected into the mold (4) in which the dummy bar (7) is set through the immersion nozzle 9, it is added to the mold 4. The mold powder (1) to be melted is previously melted by using a melting furnace (2), etc., added at the same time as the start of casting through a supply gutter (10), and then the electrode (3) immersed in the molten layer is energized, This is to prevent the temperature of the powder from lowering due to the Joule heat generated in the powder layer and to secure the melted thickness of the powder. If a sufficient melt thickness can be secured at the start of casting, cracking defects on the surface of the slab can be avoided, and the problem of carburization is solved because the melted powder does not contain carbon.

As described above, simply adding the melted mold powder (1) does not serve the purpose of the mold powder (1) because the powder (1) is solidified immediately after the addition due to radiation to the atmosphere. The phenomenon that molten steel abnormally solidifies in the meniscus portion called Dickel and the insufficient lubrication between the mold (4) and the solidification shell (5) result in inducing breakout.

[0010]

EXAMPLES The present invention will be described below with reference to examples using the apparatus having the configuration shown in FIG. Example 1 0.09 to 0.12% C, 0.9 to 1.2% Mn, 0.006 to 0.012%
Molten steel containing P, 0.06-0.010% S, 0.025-0.040% Al is poured into a pre-heated tundish, and then
When casting into a mold of 1000 to 1500 mm width x 220 mm thickness and starting casting, as an example of the present invention, the mold powder (2.9%
C, 34.4% SiO ₂ , 33.0% CaO, 6.1% Al ₂ O ₃ , 13.4% Na
₂ O, 8.1% F) was melted in a silicon knit furnace, and immediately after it was added to the mold, an iron electrode with a cross section of 100 mmφ was
The mold was set at 1/4 and 3/4 of the width of the mold so that the immersion depth was 15 mm, and electricity was applied at a current value of 500 to 700A. The amount of the melt mold powder added was kept at a melt thickness of 20 to 30 mm up to a casting length of 5 m regardless of the mold size.

Taking the effect of the present invention as a comparative example, when powder powder having the same composition as that of the present invention was added at the start of casting with the same steel type and mold size (Comparative Example 1).
The case where the mold powder of the same component was melted and added without heating using the electrodes (Comparative Example 2) was performed. The number of surface cracks was measured for a length of 3 m of the cast slab at the starting portion thus cast. The results are shown in Table 1. As shown in Table 1, in Comparative Example 1, the number of cracks was small because the powder had a thin melt thickness.
In the comparative example, the number of cracks was 4.5 / m because the molten powder added in the comparative example was solidified by heat radiation to the atmosphere and consequently the molten thickness became thin. On the other hand, according to the present invention, the amount of powder flowing between the mold wall and the slab is sufficient, and the slab can be uniformly cooled in the mold. Therefore, the slab cracks corresponding to the casting start portion are extremely small at 0.3 pieces / m. There were few.

Therefore, it was confirmed that the present invention is extremely effective for the cracking of the surface caused by the insufficient powder melt thickness at the start portion.

[0013]

[Table 1]

Example 2 0.0015 to 0.0025% C, 0.09 to 0.12% Mn, 0.006 to 0.012
% P, 0.06-0.010% S, 0.035-0.045% Al containing ultra-low carbon molten steel was cast in the same manner as in Example 1 described above, and a slab corresponding to the starting portion was cooled at a position corresponding to a length of 3 m. Carbon analysis of the rolled steel sheet was performed. On the other hand, also in the comparative example, casting was carried out by the same method as the above-mentioned conventional method, and carbon analysis of the cold rolled steel sheet corresponding to the starting portion 3 m was also conducted. The results are shown in Table 2.

In the example of the present invention, all the carbon in the mold powder in the melting furnace is burned, and no carbon is contained at the stage of adding into the mold, so that the molten steel in the tundish (T / D) and the cold rolled steel sheet are The carbon concentration of the cold-rolled steel sheet is about 6 ppm higher than the carbon content of the molten steel in the tundish in the comparative example, while the carbon concentration is almost unchanged. In addition to this experiment, an experiment was conducted under the same casting conditions in which an unheated molten mold powder using an electrode was added, but the added powder solidified in the molten steel meniscus, and a deckle defect occurred inside the slab.

From this example, it was confirmed that the present invention is effective for carburizing.

[0017]

[Table 2]

In the above embodiments, the silicon knit furnace was used to melt the mold powder, but the type of furnace is not limited as long as it is for melting. Also, the electrode diameter,
The number of electrodes and the amount of electricity supplied are not particularly limited as long as the molten powder to be added has a heat amount that does not solidify by cooling to the atmosphere.

[0019]

According to the present invention, the mold powder melted at the start of continuous casting is added into the mold, and the electrodes immersed in the added molten powder layer are energized for resistance heating. It is possible to avoid cracking and carburization and improve the yield of slabs.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing the configuration of the present invention.

[Explanation of symbols]

 1 Melt mold powder 2 Molten steel furnace 3 Electrode 4 Mold 5 Solidification shell 6 Dummy bar 7 Tundish 8 Molten steel 9 Immersion nozzle 10 Supply gutter

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Seiji Taguchi, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Headquarters (72) Inventor Hiroshi Sekiguchi 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama ( No address) Mizusaki Works, Kawasaki Steel Co., Ltd.

Claims (1)

[Claims] 1. At the start of continuous casting of steel,
A method for producing a continuous cast slab having excellent surface properties, which comprises adding a melted mold powder into a continuous casting mold and then immersing one or more electrodes in the molten powder for resistance heating.