JPH09174208A - Method for continuously casting steel and immersion nozzle therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method of a steel, capable of easily producing a cast slab reduced in blow hole and non-metallic inclusion on the surface layer part by forming the vertical component of molten steel flow in the range within a specified distance from interface between a solidified shell and the molten steel in a specified value or lower of the thickness of the solidified shell in a mold to downward stream. SOLUTION: A nozzle used in this method is formed to have an arc surface 3 of radius (γ) in the extending part of the wall 2 at the upper part of a spouting hole 1 so as to satisfy r>3h, where (h) is the height of the spouting hole 1. The molten steel spouted from the nozzle flows along the arc-state wall and flows slantingly upward. The molten steel flows along a meniscus, and after colliding to the mold, it flows along the mold downward. Therefore, the molten steel flows downward near the solidified shell in the inner part of the mold. Then, the vertical component of the molten steel flowing in the range within 20mm from the interface between the solidified shell and the molten steel in 0-10mm of the solidified shell thickness in the mold is made downward stream.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel continuous casting method and a dipping nozzle for continuous casting for producing a slab having a small amount of air bubbles and non-metallic inclusions in the surface layer portion.

[0002]

2. Description of the Related Art Thin steel sheets used for automobile bodies are required to have no flaws on their surfaces. It is generally known that air bubbles and non-metallic inclusions present in the surface layer of the cast slab are exposed on the surface during rolling and cause surface defects. Therefore, in order to reduce surface defects, it is necessary to reduce bubbles and non-metallic inclusions existing in the surface layer of the cast slab. Especially 100μ in the range within 10mm from the surface of the slab
It is important to sufficiently remove bubbles and non-metallic inclusions of m or more. In continuous casting, the slab is cooled in the mold and the secondary cooling zone below it, and the solidified shell develops. The length used in normal continuous casting is 800 to 1000, although it varies depending on the casting speed and cooling conditions in the mold.
With a mm mold, solidification within 10 mm from the surface of the slab is almost complete in the mold. Therefore, it is important to control the molten steel flow in the mold so that air bubbles and non-metallic inclusions are not captured by the solidified shell.

Generally, in continuous casting of steel, the molten steel supplied from the immersion nozzle 4 into the mold 5 collides with the solidified shell as shown in FIG. 2, and is divided into two upper and lower streams. Therefore, the upper part of the mold has an upward flow, and the lower part of the mold has a downward flow. In the past, it was thought that the downward flow in the lower part of the mold hindered the levitation of bubbles and inclusions, and tried to impart upward molten steel flow to promote the levitation of bubbles and non-metallic inclusions. For example, as shown in FIG. 3, a method of performing continuous casting by using an electromagnetic stirrer 6 while flowing molten steel upward on the inner wall surface of the mold is disclosed in JP-A-61-140356.

On the other hand, by the discharge flow from the immersion nozzle,
There is also a method of controlling the molten steel flow in the mold. In this case, the discharge angle from the immersion nozzle, the area of the discharge port, the immersion depth of the immersion nozzle, etc. are changed.

[0005]

However, as will be described later, when the flow of molten steel is directed upward, bubbles and non-metallic inclusions tend to adhere to the inner surface of the solidified shell, and therefore bubbles in the surface layer of the cast slab. And the number of non-metallic inclusions increases. Further, there is a problem that the variation of the meniscus position of the molten steel in the mold is increased and the entrainment of the mold powder is increased, and the entrained mold powder remains as a non-metallic inclusion in the slab. In order to obtain a downward molten steel flow in the vicinity of the solidification shell, there is a method in which the discharge angle of the nozzle is upward, the molten steel collides with the mold along the meniscus, and then descends, but the meniscus is disturbed by the discharge flow. Non-metallic inclusions due to mold powder increase.

The present invention has been made in order to solve the above-mentioned problems of the prior art, and is a continuous casting method for steel that enables production of a good quality slab with few bubbles and non-metallic inclusions in the surface layer. And an immersion nozzle for continuous casting suitable for use in this method.

[0007]

The present invention was devised to solve the above problems, and its gist is (1) when the thickness of the solidification shell in the mold is 0 mm or more and 10 mm or less,
A continuous casting method for steel, characterized in that the vertical component of the molten steel flow within 20 mm from the interface between the solidified shell and the molten steel is directed downward, (2) molten steel flow velocity at a position 20 mm from the interface between the solidified shell and the molten steel Vertical downward component of 5cm
/ s or more and 50 cm / s or less, (1) the continuous casting method for steel according to (1), which has an arc surface 3 of radius r in the extension of the upper wall 2 of the discharge port 1, A method for continuous casting of steel according to (1) and (2), characterized in that, when the height of the outlet 1 is h, an immersion nozzle having a discharge port portion where r> 3h is used, (4) discharge port 1 has an arc surface 3 with a radius r in the extension of the upper wall 2 and the height of the discharge port 1 is h,
An immersion nozzle for continuous casting of steel, characterized in that it has a discharge port with r> 3h.

[0008]

DETAILED DESCRIPTION OF THE INVENTION The contents and embodiments of the present invention will be described below. FIG. 4 shows the effect of the molten steel flow velocity in the mold on the number of bubbles in the surface layer of the cast slab. Here, the molten steel flow velocity is a vertical component at a position of 20 mm from the interface between the solidified shell and the molten steel, and was obtained from the inclination angle of the primary dendrite arm in the slab from the direction perpendicular to the surface of the slab. The number of bubbles is
It is the number of bubbles of 100 μm or more per unit area within a range of 10 mm from the surface of the slab. If the vertical component of the molten steel flow velocity is -5 cm / s or less, that is, if the vertical component of the molten steel flow velocity is 5 cm / s or more downward, the number of bubbles in the surface layer of the cast slab is small and surface defects are hardly generated even after rolling. .

FIG. 5 shows the effect of the flow rate of molten steel in the mold on the number of non-metallic inclusions in the surface layer of the cast slab. Here, the molten steel flow velocity is a vertical component at a position of 20 mm from the interface between the solidified shell and the molten steel, and was obtained from the inclination angle of the primary dendrite arm in the slab from the direction perpendicular to the surface of the slab. The number of non-metallic inclusions is 1 within the range of 10 mm from the surface of the slab.
It is the number per unit area of non-metallic inclusions of 00 μm or more. When the vertical component of the molten steel flow velocity is -5 cm / s or less, that is, when the vertical component of the molten steel flow velocity is 5 cm / s or more downward, the number of non-metallic inclusions in the surface layer of the cast slab becomes small. However, when the downward molten steel flow velocity exceeds 50 cm / s, the number of nonmetallic inclusions increases. This is because the downward flow increases the amount of non-metallic inclusions generated when the mold flux in the meniscus portion of the molten steel in the mold is caught in the molten steel. The amount of non-metallic inclusions entrained in the molten steel increases as the downward flow velocity increases.

Therefore, in order to reduce the number of air bubbles and non-metallic inclusions within 10 mm from the surface of the cast slab, especially the surface of the cast slab, the vertical position at a position 20 mm from the interface between the solidified shell and the molten steel is required. Limit the components downward to 5 cm / s or more and 50 cm / s or less.

The immersion nozzle shape for easily obtaining the above molten steel flow will be described with reference to the drawings. Normally, when the molten steel flow is controlled by the discharge flow from the immersion nozzle, the discharge angle and the area of the discharge port were the main control factors. However, even if these two factors are changed, the meniscus of the molten steel in the mold will change. It is difficult to form a downward flow near the solidified shell without disturbing the part.

In the present invention, attention has been paid to the shape of the ejection port including the upper wall of the ejection port, which has not been noticed in the past. FIG.
(A) Front view of the vicinity of the discharge port of the immersion nozzle of the present invention,
It is a side view and (c) sectional view. The upper wall 2 of the discharge port 1 has an arc surface 3 having a radius r in the extension thereof, and has a shape that satisfies r> 3h when the height of the discharge port 1 is h.
The molten steel discharged from the nozzle flows along the arcuate wall as shown in FIG. 6, and flows obliquely upward. The molten steel flows along the meniscus and, after colliding with the mold, flows downward along the mold. Therefore, the molten steel flows downward near the solidified shell inside the mold. Therefore, air bubbles and non-metallic inclusions are less likely to adhere to the solidified shell, and air bubbles and non-metal inclusions on the surface layer portion of the cast slab are reduced. In order to control the downward molten steel flow velocity, the height and width of the nozzle outlet may be adjusted. On the other hand, in the case of r <3h, the molten steel flow separates from the curved surface and thus does not flow obliquely upward and collides with the mold wall. Therefore, a part of the molten steel flow colliding with the mold wall becomes a downward flow, but most of the molten steel flow becomes an upward flow especially in the upper part of the mold.
Therefore, when viewed in the upper part of the mold, that is, in the slab, there are many bubbles and non-metallic inclusions in the surface layer part.

Regarding the upper limit of the radius r of the arc surface 3 of the discharge port 1, if the radius r is too large, the material cost of the nozzle becomes high and the cost becomes high. Therefore, it is desirable that r is approximately 150 mm or less. Further, the immersion nozzle of the present invention is a method usually used for manufacturing an immersion nozzle for continuous casting of steel,
That is, it can be manufactured by the rubber pressing method.

[0014]

【Example】

(Example 1) C: 0.001-0.006 by weight%,
Si: 0.005-0.02, Mn: 0.05-0.
2, P: 0.01 to 0.02, S: 0.002 to 0.0
2, Al: 0.02 to 0.1, Ti: 0.001 to 0.
05, the balance Fe and the steel consisting of unavoidable impurity elements were continuously cast. The width of the slab is 1500 mm and the thickness is 240 mm
And the casting speed is 1.5 m / min. At this time, the immersion nozzle has a wall 2 on the upper portion of the discharge port 1 as shown in FIG.
2 has an arc surface 3 with a radius of 100 mm and a discharge port 1 having a height of 30 mm. Comparative Example 1 is a case where a linear motor type electromagnetic stirrer is used so that the molten steel flow in the vicinity of the mold wall in the mold is directed upward.

In the method of the present invention, the solidified shell thickness is 10 mm.
In the range up to, the molten steel flow velocity at a position of 20 mm from the solidified shell was 5 to 50 cm / s vertically downward. On the other hand, in the method of Comparative Example 1, the molten steel flow velocity at a position of 20 mm from the solidified shell was 5 to 50 cm / s vertically upward in the range of the solidified shell thickness up to 10 mm.

10 existing within 10 mm from the surface of the slab
As a result of investigating the number of bubbles and non-metallic inclusions of 0 μm or more, as shown in Table 1, in the method of the present invention, both the number of bubbles and the number of non-metallic inclusions are 1/10 or less as compared with Comparative Example 1. Diminished.

[Table 1]

(Example 2) C: 0.001% by weight
0.006, Si: 0.005 to 0.02, Mn: 0.
05-0.2, P: 0.01-0.02, S: 0.00
2 to 0.02, Al: 0.02 to 0.1, Ti: 0.0
Steel consisting of 01 to 0.05 and the balance Fe and unavoidable impurity elements was continuously cast. The width of the slab is 1500 mm, the thickness is 240 mm, and the casting speed is 1.5 m / min. At this time, as shown in FIG. 1, the immersion nozzle has an arc surface 3 having a radius of 100 mm in an extension of the upper wall 2 of the discharge port 1 and a discharge port portion having a height of the discharge port 1 of 30 mm. I used one. In the comparative example 2, the radius of the extension of the upper wall 2 of the discharge port 1 is 8
This is the case of using the one having the arc surface 3 of 0 mm and the ejection port portion in which the height of the ejection port 1 is 30 mm. That is, r
This is the case of <3 h.

In the method of the present invention, the solidified shell thickness is 10 mm.
In the range up to, the molten steel flow velocity at a position of 20 mm from the solidified shell was 5 to 50 cm / s vertically downward. On the other hand, in the method of Comparative Example 2, the molten steel flow velocity at the position of 20 mm from the solidified shell was from 50 cm / s vertically upward to 10 cm / s vertically downward in the range where the thickness of the solidified shell was up to 10 mm.

10 existing within 10 mm from the surface of the slab
As a result of investigating the number of bubbles and non-metallic inclusions of 0 μm or more, as shown in Table 2, in the method of the present invention, both the number of bubbles and the number of non-metallic inclusions are 1/5 or less as compared with Comparative Example 2. Diminished.

[Table 2]

(Example 3) C: 0.001-wt%
0.006, Si: 0.005 to 0.02, Mn: 0.
05-0.2, P: 0.01-0.02, S: 0.00
2 to 0.02, Al: 0.02 to 0.1, Ti: 0.0
Steel consisting of 01 to 0.05 and the balance Fe and unavoidable impurity elements was continuously cast. The width of the slab is 1500 mm, the thickness is 240 mm, and the casting speed is 1.5 m / min. At this time, as shown in FIG. 1, the immersion nozzle has an arc surface 3 having a radius of 100 mm in an extension of the upper wall 2 of the discharge port 1 and a discharge port portion having a height of the discharge port 1 of 30 mm. I used one. In Comparative Example 3, the radius of the extension of the upper wall 2 of the discharge port 1 is 8
This is the case where the one having the arc surface 3 of 0 mm and the ejection port portion in which the height of the ejection port 1 is 20 mm is used. In this case, the discharge flow velocity becomes high.

Comparative Example 3 is a case where a molten steel flow in the vicinity of the mold wall is directed downward by using a linear motor type electromagnetic stirrer. At this time, in the range where the thickness of the solidified shell was up to 10 mm, the molten steel flow velocity at a position of 20 mm from the solidified shell was set to 50 to 70 cm / s vertically downward. On the other hand, in the method of the present invention, in the range where the thickness of the solidified shell is up to 10 mm, the molten steel flow velocity at the position of 20 mm from the solidified shell is 5 downward in the vertical direction.
It was ~ 50 cm / s.

10 within 10 mm from the surface of the slab
As a result of investigating the number of bubbles having a size of 0 μm or more and the non-metallic inclusions, as shown in Table 3, the number of bubbles was equal to that of the method of the present invention as compared with Comparative Example 3, but the number of non-metallic inclusions was It decreased to less than 1/5.

[Table 3]

[0023]

According to the method of the present invention, it is possible to easily manufacture a slab having few bubbles and non-metallic inclusions in the surface layer portion. Therefore, hot rolling of the slab produced by the method of the present invention, the surface quality of the thin steel sheet produced by cold rolling is improved, the occurrence of defective products due to surface defects is reduced, yield Can be improved. Further, according to the casting dipping nozzle of the present invention, the above-described method can be carried out in a suitable state, and the practical effect is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing a nozzle of the present invention.

FIG. 2 is a diagram showing molten steel flow in a mold in normal continuous casting.

FIG. 3 is a diagram showing molten steel flow in a mold when a downward flow is applied to the molten steel near the mold wall using an electromagnetic stirrer.

FIG. 4 is a diagram showing the relationship between the molten steel flow rate in the mold and the number of bubbles in the surface layer of the cast slab.

FIG. 5 is a diagram showing the flow rate of molten steel in the mold and the number of non-metallic inclusions on the surface layer of the slab.

FIG. 6 is a diagram showing molten steel flow in a mold when the method of the present invention is applied.

[Explanation of symbols]

 1 Discharge Port 2 Wall of Upper Discharge Port 3 Curved Wall 4 Immersion Nozzle 5 Mold 6 Electromagnetic Stirrer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ikuo Sawada 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Nippon Steel Corporation Technology Development Division

Claims (4)

[Claims] 1. The thickness of the solidified shell in the mold is 0 mm or more 1
20 mm or less from the interface between the solidified shell and molten steel within the range of 0 mm or less
A continuous casting method for steel, wherein the vertical component of the molten steel flow in the range of mm or less is directed downward. 2. A vertically downward component of the molten steel flow velocity at a position 20 mm from the interface between the solidified shell and the molten steel is 5 cm / s or more.
The continuous casting method for steel according to claim 1, characterized in that it is 0 cm / s or less. 3. An extension of the upper wall (2) of the discharge port (1) has an arc surface (3) of radius r, and the height of the discharge port (1) is h.
In this case, a continuous casting method for steel according to claim 1 or 2, characterized in that an immersion nozzle having a discharge port portion with r> 3h is used. 4. An extension of the upper wall (2) of the discharge port (1) has an arc surface (3) of radius r, and the height of the discharge port (1) is h.
The immersion nozzle for continuous casting of steel, characterized in that it has a discharge port with r> 3h.