JPH0390249A - Method for continuously casting cast steel strip with thin thickness - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention is applicable to continuous casting of thin-walled cast steel slabs from molten steel such as 18-8 stainless steel using a continuous casting machine that uses a rotating cooling drum as a mold. This invention relates to a continuous casting method for thin-walled cast steel slabs in which the composition of molten steel and the atmosphere of the casting equipment have been improved in order to prevent the occurrence of defects such as raw horizontal wrinkles on the slabs and improve the surface quality.

[Conventional technology]

Recently, T! 1mm to 10m from molten metal such as 1lWA
A method of directly manufacturing thin slabs with a wall thickness of approximately 1.0 m using a continuous casting machine is attracting attention. This continuous casting method eliminates the need to manufacture thin plates from thick slabs through a multi-step hot rolling process as in the past, and requires only a light cold rolling process to shape the slab to its final thickness. Therefore, a large economic effect can be obtained by simplifying the process and equipment, and it is often preferable from the viewpoint of metallographic structure.

As shown in FIG. 2, this type of continuous casting method includes a twin drum method in which a pool 3 is formed between a pair of cooling drums 1 and 2 that rotate in opposite directions, and a cooling drum method (not shown). There is a drum-belt method in which a water reservoir is formed between the cooling drum and the belt, and a single drum method in which a water reservoir is formed in a part of one cooling drum.

In these methods, the molten metal in the pool 3 is cooled at the part where it contacts the surfaces of the cooling drums 1 and 2, begins to solidify from the surface, and gradually solidifies toward the center of the thickness while forming a solidified shell. As the molten metal progresses, oxides of the molten metal (called scum) are present on the surface layer of the molten metal in the pool 3.
is floating, and a part of it gets caught up in the cooling drum 1,
If it partially adheres to the surface of the thin slab 2 (referred to as uneven dirt), not only will it leave a dent (transfer) on the surface of the thin slab 4 depending on the thickness of the adhered material (also referred to as dirt), but it will also cause the adhesion to become uneven. Since the heat conductivity is different between the part with the kimono and the part without it, the progress of cooling on the surface of the slab 4 becomes uneven, which causes the thickness of the slab 4 to be uneven. In some cases, wrinkles or cracks may occur.

Therefore, it was considered to scrape off the deposits on the surfaces of the cooling drums 1 and 2 with a brush roll 5, and
Inventions such as those described in Japanese Patent Application Laid-open No. 184449 and Japanese Patent Application Laid-Open No. 176650/1983 have been proposed.

[Problem to be solved by the invention]

The brush roll 5 according to the prior art is provided for the purpose of scraping off deposits on the surface of the cooling drum 1.2, but it is not always possible to reliably prevent cracking of the slab 4. Rather, vertical cracks in the cast slab due to streak-like deposits that are not completely scraped off by the brush roll 5, or uneven bands in the axial direction on the surface of the cooling drum due to eccentricity of the brush roll 5, etc. The remaining deposits 6.7 may cause defects such as periodic wrinkles and horizontal cracks that appear on the slab.

An object of the present invention is to focus on the cause of uneven dirt generation and provide a measure to directly deal with the cause, without relying solely on mechanical means such as scraping off deposits (dirt) with the brush roll 5. That is.

[Means to solve the problem]

The cause of uneven staining due to adhesion of scum on the surface of the cooling drum 1.2 is that manganese contained in the molten steel in the sump 3 is deposited unevenly on the surface of the cooling drum 1.2. This can be explained by considering that . Since manganese is a metal that evaporates relatively easily, it is thought that it evaporates easily from the molten steel, deposits unevenly on the surface of the cooling drum 1.2, oxidizes, and remains in the form of a MnO film. It will be done. In fact, when the deposits on the cooling drum 1.2 are removed and analyzed, a relatively high concentration of MnO is detected.

If MnO adheres non-uniformly to the surface of the cooling drum 1.2, the molten metal will simply float on the surface layer of the molten steel in the sump 3 because it is not compatible with the material of the cooling drum 1.2.
Oxides, etc. (scum) of the bottom of the bottom are deposited on the surface of the cooling drum 1.2, so MnO, which is a similar metal oxide and is familiar, is deposited on the MnO deposited part. It is thought that the slag is dragged in and cooled, and as a result, it adheres non-uniformly to the surface of the cooling drum 1.2, resulting in uneven staining, and as a result, defects are generated in the slab 4.

Therefore, the present invention is based on the investigation of this cause and attempts to prevent the formation of uneven stains by suppressing the evaporation of manganese, which is the cause of uneven stains. That is, in the continuous casting method for thin-walled cast steel, a continuous casting machine equipped with a cooling drum that cools molten steel to form thin-walled steel slabs is used to suppress the evaporation of manganese contained in the molten steel in the sump. In order to prevent vapor deposition on the cooling drum, by adjusting the respective contents of oxygen O and sulfur S in the molten steel and determining the concentration of nitrogen atmosphere N, the portion near the surface of the molten steel 0 and S in
The sum of each content [○] and (S) (unit: ppm) and N2 concentration [:N (unit: VO1%)] is 0.0334 [S1 + 0.0223 (OF + 0.06)
7 [N! According to the continuous casting method of the present invention, by adjusting the composition of the molten steel and selecting the casting atmosphere, the above-described amount or more can be produced. Oxygen, sulfur, and nitrogen contained in the molten steel are concentrated near the surface of the molten steel in the molten metal pool, and act to trap the manganese in the molten steel and prevent it from evaporating. Therefore, the manganese in the molten steel does not evaporate and nonuniformly deposit on the surface of the cooling drum to become MnO. As a result, the scum floating on the upper layer of the molten steel gets caught up and adheres to the MnO, and the scum is deposited on the surface of the cooling drum. This also eliminates the formation of uneven stains on the surface.

Oxygen, sulfur, and nitrogen present in the molten steel in amounts greater than the specified amount also act as surfactants, lowering the surface tension of the molten steel and increasing its wettability with respect to the cooling drum. Therefore, the cooling drum and the molten steel become compatible, and the molten steel is cooled while being in close contact with the cooling drum.

Both of the above two effects help improve the surface properties of the thin cast steel piece, and defects such as horizontal wrinkles and vertical cracks do not occur in the thin cast steel piece.

〔Example〕

Using a twin-drum continuous casting machine as shown in Fig. 2, 18-8 stainless steel molten metal is supplied to the sump 3, and a slab 4 with a plate thickness of 2.4 mm is cast at a casting speed. 65m/l
Figure 1 shows the results of continuous casting at 1 inch.
, 60ppm, 210ppm or 4ooppm
) was used to measure how the thickness of uneven MnO staining, that is, the difference in thickness between thick and thin parts of unevenly deposited MnO, changes with respect to the total amount of oxygen contained in the molten metal. It is.

When the thickness of MnO contamination unevenness becomes 10 - or more, scum adhesion to it becomes uneven and the amount of horizontal wrinkles on the slab 4 increases. This is a suitable range.

Although it is clear that a large amount of S is desirable, it is not possible to increase the amount of S to a large extent due to the properties required for the product.
It is necessary to adjust the amount of S while considering the allowable amount within the range of 5 to 4 ooppm.

Regarding the amount of ○ (total oxygen amount), the higher the amount, the more desirable results can be obtained in terms of preventing uneven staining even if the amount of S is small, and it can be seen that S and O have complementary effects. If the amount of ◯ exceeds 4ooppm, inclusions in the steel will increase significantly and cause surface scratches, so 4ooppm is the upper limit. Therefore, the amount of ◯ is adjusted in consideration of the S content and the atmospheric concentration of N2.

As for the casting atmosphere, Nt is better than Ar in terms of preventing uneven staining, and when the amount of N dissolved on the surface of the molten metal increases due to the N3 atmosphere, it acts as a surfactant along with the aforementioned S and ○. This reduces the surface tension of the molten metal and improves its adhesion to the cooling drum 1.2 (increases wettability).

At the same time, when S, 0, and N become concentrated on the surface of the molten metal, Mn is trapped and cannot evaporate from the molten metal, and Mn vapor is no longer deposited unevenly on the cooling drum 1.2.

As a result, scum is less likely to adhere to the cooling drum,
It is possible to prevent defects such as horizontal wrinkles and vertical cracks from occurring in the slab.

〔Effect of the invention〕

According to the present invention, vapor deposition of MnO and uneven adhesion of scum due to the deposition of MnO do not occur on the surface of the cooling drum, and defects in thin-walled cast steel pieces caused by this can be prevented.

Therefore, it becomes possible to continuously cast thin-walled cast steel slabs of good quality close to the final thickness at low cost, and the cold rolling process is also light, reducing time loss.

[Brief explanation of drawings]

FIG. 1 is a diagram showing test results in an example of the present invention;
FIG. 2 is a perspective view conceptually showing a twin-drum type continuous casting machine. 1.2... Cooling drum, 3... Hot water reservoir, 4...
Thin cast steel billet, 5... brush roll, 6.7...
- Dark and light deposits. Procedural amendment (voluntary) Hei 7i! 2 years 71 month date

Claims (1)

[Claims] A continuous casting machine equipped with a cooling drum that cools molten steel to form a thin slab is used to suppress the evaporation of manganese contained in the molten steel in a sump and deposit it on the cooling drum. In order to prevent this, the respective contents of oxygen O and sulfur S in the molten steel are adjusted, and the concentration of the nitrogen atmosphere N_2 is determined to prevent each of O and S in the portion near the surface of the molten steel. Content [O] and [S] (unit: pp
m) and N_2 concentration [N_2] (unit: vol%) is 0.0334[S]+0.0223[O]+0.06
7. A continuous casting method for thin-walled cast steel pieces, characterized in that the condition [N_2]>10 is satisfied.