JP2002263803A - Cooling drum for continuously casting thin cast slab and method for continuously casting twin-drum type thin cast slab using the drum - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cooling drum having dimples with which a thin cast slab having necessary shaped convex projections can be cast, and a long service life, and to provide a twin-drum type thin cast slab continuous casting method by using the cooling drum. SOLUTION: In the twin-drum type continuous casting, with which molten steel is poured into a molten steel pool part formed of a pair of cooling drums having parallel axes and rotated in the mutually reverse direction and the side weirs, and while cooling and solidifying the molten steel on the peripheral surfaces of the cooling drums, the thin cast slab is continuously cast, as the cooling drum for continuously casting the thin cast slab, the cooling drum forming a dimple having 200-2000 μm circle-equivalent average diameter and >200 to 1000 μm average depth on the peripheral surface at 30-70% area ratio, is used, and also, the molten steel pool part is covered with atmospheric gas composed of Ar and N2 . The mixing ratio of Ar is desirable to be 1-10%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cooling drum for continuous casting of a thin-walled slab, which is used in a twin-drum thin-walled slab continuous casting apparatus, and a twin-drum thin-walled slab continuous casting method using the cooling drum. is there.

[0002]

2. Description of the Related Art A pair of cooling drums for continuous casting of thin cast slabs (hereinafter sometimes referred to as "cooling drums") are arranged in parallel and rotated in opposite directions to form hot water formed on the peripheral surface of the cooling drum. A twin-drum type thin cast slab continuous casting technique for continuously casting thin cast slabs (hereinafter, sometimes referred to as “cast slabs”) by cooling and solidifying molten steel to be poured into the reservoir and solidifying the molten steel on the peripheral surface has been developed. ing.

[0003] Since this technique is to continuously cast a thin cast piece having a shape and a thickness close to that of a final product, it is difficult to obtain a final product having a required level of quality at a high yield. It is indispensable to cast a thin cast piece having no surface defects such as cracks, cracks, and unevenness. This surface defect is that when thin cast slab is cast, the form of solidified shell on the surface of the cooling drum is not uniform or uniform, that is, the form of rapid solidification of molten steel on the surface of the cooling drum is uniform or uniform. It is caused by not being. Therefore, in order to ensure the uniformity and uniformity of the rapid solidification of the molten steel, a large number of cooling drums having depressions (dimples) of a required diameter and depth formed on the drum peripheral surface have been proposed. I have.

[0004] For example, Japanese Patent Application Laid-Open No. Hei 4-238651 discloses that a depression having a depth of 50 to 200 μm is formed on a peripheral surface of a semiconductor device.
A cooling drum for continuous casting is disclosed which is formed at an area ratio of 0% and has a depression having a depth of 10 to 50 μm at an area ratio of 40 to 60%. Also, Japanese Unexamined Patent Publication No.
No. 8204 discloses that the peripheral surface has a diameter of 100 to 300 μm,
A depression having a depth of 100 to 500 μm is formed at an area ratio of 15 to 50%, and an angle between a line perpendicular to a tangent to a peripheral surface and a side surface of the depression is 45 to 400 μm and a depth of 10 to 100 μm. 40% to 60% depression of ~ 75 °
Discloses a cooling drum for continuous casting formed with an area ratio of 1.

[0005] These cooling drums suppress the occurrence of "cracks" and "cracks" on the slab surface, and have a remarkable effect in producing high quality stainless steel sheet products. By the way, in the rapid solidification of molten steel, when the molten steel comes into contact with a cooling drum having a depression having a required diameter and depth, the molten steel penetrates into the depression at a certain depth, rapidly solidifies in that state, and a solidified shell. Is formed. Therefore, on the surface of the thin cast slab sent out from the cooling drum, there is a convex projection (hereinafter, referred to as a “convex projection”) to which the above-mentioned dent has been transferred.

[0006] In the casting of thin cast slabs, it is necessary to form convex projections of a required height in order to suppress cracks in the slabs. It causes rolling flaws. Therefore, it is necessary to form a convex protrusion having an appropriate height on the surface of the thin cast slab, and this height forms not only the depth of the depression but also the atmosphere of the reservoir. It is known that the type of gas is also affected (JP-A-63-177945, JP-A-3-173945).
No. 77747).

[0007] Japanese Patent Application Laid-Open No. 11-10288 discloses that the diameter of a cooling drum is 400 to 2000 μm.
A depression having a depth of 10 to 200 μm is formed at an area ratio of 30 to 70%, an atmosphere on the upper surface of the molten steel in the reservoir is an atmosphere of nitrogen gas or a mixture of nitrogen gas and argon gas, and an inclined surface of the depression and a cooling drum And a volume ratio y (%) of the argon gas in the atmosphere satisfying a predetermined relational expression. Continuous cast thin slabs are disclosed. In the thin cast slab continuous casting method, the atmosphere (nitrogen gas or a mixture of nitrogen gas and argon gas) on the molten steel upper surface of the basin prevents oxidation of the molten steel surface in the basin, and the surface of the thin slab , For forming convex protrusions (transfer of dents) having a required shape and height.

As described above, the most important aspect of the twin-drum type thin cast slab continuous casting technique, that is, the “rapid solidification of molten steel on the peripheral surface of the cooling drum”, is the shape of the depression formed on the peripheral surface of the cooling drum. -Since the structure and the type of atmosphere gas covering the basin affect each other, it is necessary to continuously cast high-quality thin-walled slabs with less surface defects than before.
It is necessary to further clarify the effects of these factors.

Further, in recent years, twin-drum type thin cast slab continuous casting tends to be performed for a long time. In particular, in a cooling drum that operates continuously for a long time under severe atmosphere and operating conditions, extending the life of the cooling drum is not only an improvement in productivity but also a continuous casting for a long time. This is also the biggest issue from the viewpoint of maintaining the quality that a high quality thin cast slab can be manufactured with high quality.

[0010]

DISCLOSURE OF THE INVENTION In view of the above technical background, the present invention is intended to cast a thin cast slab having no surface defects and having a convex projection of a required shape on the surface. A cooling drum for continuous casting of thin cast slabs, which has an area ratio "dent" on the drum peripheral surface and has a long service life, and a twin-drum thin cast that continuously casts high-quality thin cast slabs using the cooling drum. It is an object of the present invention to provide a single continuous casting method.

[0011]

Means for Solving the Problems The present inventors have, by adjusting the N ₂ and the mixing ratio of the Ar constituting the atmosphere over the molten steel top surface of the basin section of the convex protrusions formed on the surface of the thin cast strip The shape can be adjusted (see JP-A-11-10288).
That is, the size (volume) of the "gas gap" formed between the depression formed in the peripheral surface of the cooling drum and the molten steel can be adjusted. By adjusting the "gas gap", the cooling drum and the By reducing the heat flux between the molten steels, the mode of rapid solidification of the molten steel can be optimized, and as a result, a thin cast slab without surface defects can be cast while forming convex projections of a desired height on the surface. And the possibility of extending the life of the cooling drum.

Based on the above idea, the present inventor has found that the mixing ratio of Ar and N ₂ constituting the atmosphere covering the molten steel basin, the shape of the depression formed on the peripheral surface of the cooling drum,
The size (form) and the relationship between the heat flux generated between the cooling drum and the molten steel were investigated diligently. One example of the result is shown in FIG.
Shown in FIG. 1 shows the relationship between the shape of the depression and the heat flux when continuous casting is performed by covering the molten steel pool in an “Ar + N ₂ ” atmosphere (mixing ratio of Ar: 0 to 12%).

From the relationship between the shape of the depression and the heat flux shown in FIG. 1, it is understood that the heat flux between the cooling drum and the molten steel can be controlled by appropriately changing the shape of the depression formed on the peripheral surface of the cooling drum. The present invention has been made based on the knowledge obtained as a result of the above investigation (optimal relationship among the above three factors), and the gist thereof is as follows.

(1) A cooling drum for continuously casting thin slabs, wherein an average diameter equivalent to a circle is 2
00 to 2000 μm, average depth more than 200 to 1000 μ
A cooling drum for continuous casting of thin cast slabs, wherein m is formed. (2) In the cooling drum for continuous casting of thin cast slab according to (1), the average diameter of a circle is 300 to 1000 μm.
m, an average depth of 300 to 700 μm, a cooling drum for continuous casting of thin cast slabs.

(3) In the cooling drum for continuous casting of thin cast slab according to (1) or (2), the depression is formed with an area ratio of 30 to 70%. Cooling drum for casting. (4) A pair of thin cast slab cooling drums are arranged in parallel and rotated in opposite directions to cool molten steel to be poured into a basin formed on the peripheral surface of the cooling drum on the peripheral surface. ,
In the twin-drum type thin cast slab continuous casting method of solidifying and continuously casting thin cast slabs, the cooling drum for continuous casting of thin cast slabs has an average diameter equivalent to a circle of 20 throughout the peripheral surface.
0-2000 μm, average depth over 200-1000 μm
With use of the cooling drum formed depressions, twin drum type thin slab method continuous casting, characterized in that covering the basin section in an atmosphere gas consisting of Ar and N _2.

(5) In the twin-drum thin cast slab continuous casting method according to the above (4), an average diameter equivalent to a circle of a depression formed in the cooling drum for continuous casting of the thin cast slab is 300.
A twin-drum type thin-walled slab continuous casting method, characterized in that the thickness is from 1000 to 1000 µm and the average depth is from 300 to 700 µm. (6) In the twin-drum thin cast slab continuous casting method according to the above (4) or (5), the depression formed in the cooling drum for continuous casting of the thin cast slab is formed at an area ratio of 30 to 70%. A twin-drum thin cast slab continuous casting method.

(7) In the twin drum type thin cast slab continuous casting method according to the above (4), (5) or (6), the mixing ratio of Ar in the atmosphere gas (Ar / Ar + N ₂ )
Is a twin-drum type thin cast slab continuous casting method, wherein 1 to 10% is used.

[0018]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in detail. First, the cooling drum for continuous casting of thin cast slabs of the present invention (hereinafter referred to as “the cooling drum of the present invention”) will be described.
The cooling drum of the present invention has a large number of circular or elliptical depressions formed by shot blasting, electron beam processing, plasma processing, laser processing, steel ball pressing, etc., over the entire peripheral surface thereof, The depression has an average diameter equivalent to a circle (hereinafter sometimes referred to as “diameter”) of 200 to 200.
2,000 μm and an average depth (hereinafter sometimes referred to as “depth”) of more than 200 to 1000 μm.

The diameter equivalent to a circle is the diameter itself when the depression is a perfect circle, and is the average value of the maximum diameter and the minimum diameter when the depression is an ellipse. The ratio of the maximum diameter to the minimum diameter indicating the degree of the ellipse is preferably 0.6 or more. As described above, since the drum of the present invention has the depression of the form defined above, it has a convex projection of a desired height on its surface, and can produce a thin cast slab having no surface defects by continuous operation for a long time. It can be manufactured.

Here, the form of the depression in the cooling drum of the present invention will be described in detail. The average diameter of the depression is 20
If it is less than 0 μm, the penetration of the molten steel into the recess becomes insufficient, so that a convex projection having a desired height cannot be obtained on the surface of the thin cast slab and the slab is cracked. And On the other hand, the average diameter of the depression is 2000μ
If m exceeds m, the amount of stress and strain accumulated in units of depressions increases, and cracks easily occur on the slab surface.
000 μm.

The lower limit of the average diameter of the dents is 3 from the viewpoint of ensuring the formation of appropriately shaped projections on the slab surface.
00 μm is preferred, while the upper limit is preferably 1000 μm from the viewpoint of reducing the surface cracks of the slab. In the cooling drum of the present invention, when the depth of the depression is 200 μm or less,
The molten steel penetrates to the vicinity of the bottom of the pit, and the effect of reducing the heat flux by forming the “gas gap” cannot be obtained sufficiently, shortening the life of the cooling drum and causing surface defects in the thin cast slab. , With the lower limit exceeding 200 μm.

On the other hand, if the average depth of the depressions exceeds 1000 μm, the above-mentioned heat flux reducing effect can be obtained, but the effect is too large, and the solidification ability of the cooling drum, that is, the productivity is excessively reduced. , And the upper limit is 1000 μm. The average depth of the depression is preferably 300 to 700 μm from the viewpoint of ensuring an appropriate heat flux reduction effect and productivity.

The average diameter and the average depth of the depression are measured, for example, from a chart using a laser displacement meter. In the cooling drum of the present invention, the depression is formed over the entire peripheral surface of the drum, but when the depression is formed at an area ratio of 30 to 70%,
Appropriate heat flux reduction effect and productivity can be ensured more reliably.

Here, the area ratio is a ratio of the total area of the depression formed on the peripheral surface of the cooling drum to the area of the entire peripheral surface of the cooling drum. If the depression is formed with an area ratio of less than 30%, a sufficient heat flux reduction effect cannot be obtained even if the shape of the depression is within the range specified in the present invention. Therefore, the lower limit is preferably 30%. On the other hand, the area ratio of the depression is 7
If it exceeds 0%, there is no problem in securing the heat flux reduction effect, but the contact area between the slab and the cooling drum is too small, and the non-dented portion of the cooling drum surface (portion where no dent is formed) ), The stress is excessively concentrated, and as a result, the non-dented portion is crushed.
Is preferred.

The area ratio of the depression is more preferably 40 to 60% from the viewpoint of obtaining an appropriate heat flux reduction effect and maintaining the depression form reliably for a long time. next,
The twin drum type thin cast slab continuous casting method of the present invention (hereinafter referred to as “the present invention continuous casting method”) will be described. According to the continuous casting method of the present invention, a pair of thin-walled cast slab cooling drums are arranged in parallel, rotated in opposite directions to each other, and molten steel poured into a pool formed on the peripheral surface of the cooling drum is cooled. cooled on the surface, solidifying, in twin drum type thin slab continuous casting method for continuously casting a thin slab, as the thin cast slab cooling drum for continuous casting, with use of the present invention the cooling drum, the the basin portion Is covered with an atmosphere gas composed of Ar and N ₂ .

When performing continuous casting using the cooling drum of the present invention, the surface of the molten steel in the basin is covered with an atmosphere gas composed of Ar and N ₂ because (a) the surface of the molten steel is covered with an inert gas. In addition to preventing oxidation of the molten steel surface and suppressing scum generation as much as possible, (b) an atmosphere gas is introduced between the surface of the molten steel penetrating into the inside of the recess and the bottom of the recess. The depth of molten steel that penetrates into the recess is adjusted to an appropriate depth to uniformly disperse the strain caused by the solidification shrinkage of the shell, prevent slab cracks, and furthermore, the cooling drum And to reduce the heat flux generated between the steel and the molten steel.

When N ₂ is used as the atmospheric gas, it is possible to prevent the surface of the molten steel in the basin from being oxidized. However, as the cooling drum rotates, the N ₂ is caught in the depression on the peripheral surface of the cooling drum. However, since N ₂ is absorbed by the molten steel, as shown in FIG.
Between the bottom inside of the molten steel 2 penetrated the surface and depressions observed 3 "gas gap" is less likely to be formed.

In the rapid solidification mode of the molten steel 2 shown in FIG. 2, the rate of transfer of the pits to the thin cast slab is high without depending on the form of the pits 3 on the peripheral surface of the cooling drum 1. Since the displacement between the slab and the cooling drum is unlikely to occur, the strain accompanying solidification shrinkage can be evenly dispersed, which is extremely effective in preventing slab cracking.However, since the heat flux reduction effect cannot be expected at all, the cooling drum is Short-lived.

On the other hand, when the surface of the molten steel in the basin is covered with an atmosphere gas composed of “Ar and N ₂ ”, FIG.
As shown in, the atmospheric gas, and enters the inside of the circumferential surface recesses 3 of the cooling drum 1, and the surface of molten steel 2 forming a convex protrusion 4, caught between the bottom of the depressions viewed 3 "gas A gap 5 "is formed. In this case, N ₂ in the atmosphere gas is absorbed by the molten steel, but Ar remains, and “gas gap 5” is formed in the depression 3.

Therefore, the mixing ratio of Ar (Ar / Ar) in the “Ar and N ₂ ” atmosphere gas covering the molten steel pool portion
+ N ₂ ) can be adjusted appropriately to obtain the “gas gap”.
Can be appropriately adjusted. This is a feature of the present invention. As a result, the height (x in FIG. 3) of the convex protrusion formed on the surface of the thin cast slab and the degree of the heat flux generated between the cooling drum and the molten steel can be adjusted to required values. A thin cast piece having no surface defects can be cast, and the life of the cooling drum can be extended.

A in the "Ar and N ₂ " atmosphere gas
The mixing ratio of r (Ar / Ar + N ₂ ) can be appropriately selected according to the casting conditions and the desired slab, but the projections having a height (x in FIG. 3) of 100 μm or more on the surface of the thin slab are obtained. In order to form, 1 to 10% is preferable. If the height of the projections formed on the surface of the thin cast slab is less than 100 μm, cracks occur on the surface of the thin cast slab. From this point, the height of the convex projections may be 100 μm. preferable.

If the mixing ratio is less than 1%, N ₂ in the atmosphere gas becomes too large, and the size (volume) of the “gas gap” remains extremely small. Although a projection having a height of 100 μm or more can be formed, the required heat flux reduction effect cannot be obtained. Therefore, the lower limit of the Ar mixing ratio is set to 1%. On the other hand, if the mixing ratio exceeds 10%, the amount of Ar in the atmosphere gas becomes too large, and the size (volume) of the “gas gap” cannot be adjusted sufficiently while keeping the size (volume) large.
On the surface of the thin cast slab, a convex protrusion with a height of 100 μm or more,
The upper limit of the Ar mixing ratio is set to 10% because it is not difficult to form a uniform layer and an appropriate heat flux reduction effect cannot be obtained.

Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the combination of conditions used in the embodiments, and various conditions are set within a range in which the object of the present invention is achieved. It can be set and adopted as appropriate.

[0034]

EXAMPLE A depression was formed on the peripheral surface of a cooling drum having a diameter of 1200 mm and a width of 1300 mm under the casting conditions shown in Table 1. Using this cooling drum, the amount of Ar in the “Ar + N ₂ ” atmosphere covering the molten steel pool was also changed according to the casting conditions shown in Table 1, and 60 tons of SUS304 molten steel was cast into a 3.5 mm thick thin cast metal. The pieces were continuously cast.

The casting results are also shown in Table 1. From the conditions and results shown in Table 1, it can be seen that in continuous casting using the cooling drum of the present invention, the heat flux between the cooling drum and the molten steel was appropriately controlled, and a sound thin slab was produced.

[0036]

[Table 1]

[0037]

According to the present invention, it is possible to cast a thin cast piece having no surface defects while forming a convex protrusion having a desired height without rolling defects on the surface. Further, according to the present invention, since the diameter and depth of the depression are larger than the conventional depression, the wear allowance of the depression is larger than before, and since the heat flux is small, the life of the cooling drum can be extended, High quality thin cast slabs without surface defects can be manufactured continuously for a long time while maintaining high quality.

Therefore, the present invention greatly contributes to the development of the twin-drum continuous casting technique.

[Brief description of the drawings]

FIG. 1 is a view showing a relationship between a heat flux between a cooling drum and molten steel and a form of a depression when a molten steel pool is covered with an “Ar + N ₂ ” atmosphere.

FIG. 2 is a diagram showing a state of rapid solidification of molten steel in a case where an atmosphere gas covering a basin of molten steel is “N ₂ ”.

FIG. 3 is a diagram illustrating a rapid solidification state of molten steel in a case where an atmosphere gas covering a basin portion of molten steel is composed of “Ar and N ₂ ”.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Cooling drum 2 ... Molten steel 3 ... Depression 4 ... Convex projection 5 ... Gas gap x ... Convex projection height y ... Depth of depression

Continued on front page (72) Inventor Hideaki Kobayashi 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation Hikari Works (72) Inventor Isao Mizuchi 3434 Shimada, Hikari-shi, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation F term (reference) in Hikari Works 4E004 DA13 HA10 NB07 QA01 SB07

Claims (7)

[Claims] 1. A cooling drum for continuously casting thin cast slabs, wherein an average diameter of a circle is 200 to
A cooling drum for continuous casting of thin cast slabs, wherein a dent having a depth of 2000 μm and an average depth of more than 200 to 1000 μm is formed. 2. The cooling drum for continuous casting of a thin-walled slab according to claim 1, wherein an average diameter of a circle is 300 to 1000.
A cooling drum for continuous casting of thin cast slabs, characterized in that the cooling drum has a thickness of 300 µm and an average depth of 300 to 700 µm. 3. The cooling drum for continuous casting of a thin cast slab according to claim 1, wherein the depression has an area ratio of 30 to 70.
%, A cooling drum for continuous casting of thin cast slabs. 4. A pair of thin-walled cast slab cooling drums are arranged in parallel, rotated in opposite directions to each other, and molten steel injected into a pool formed on the peripheral surface of the cooling drum is placed on the peripheral surface. In the twin-drum type thin cast slab continuous casting method of continuously casting thin cast slabs by cooling and solidifying, the cooling drum for continuous casting of thin cast slabs has an average circular equivalent diameter of 200 to 200 mm over the entire peripheral surface. 2000 μm, average depth over 200 to 100
Using a cooling drum with a 0 μm depression,
Twin drum type thin slab method continuous casting, characterized in that covering the basin section in an atmosphere gas consisting of Ar and N _2. 5. The twin-drum thin cast slab continuous casting method according to claim 4, wherein an average diameter corresponding to a circle of the depression formed in the cooling drum for continuous casting of the thin cast slab is 300 to 100.
A twin-drum thin cast slab continuous casting method, characterized in that the thickness is 0 μm and the average depth is 300 to 700 μm. 6. The twin-drum thin cast slab continuous casting method according to claim 4, wherein the recess formed in the cooling drum for continuous casting of the thin cast slab is formed with an area ratio of 30 to 70%. A twin drum type thin cast slab continuous casting method characterized by the above-mentioned. 7. The twin-drum type thin cast slab continuous casting method according to claim 4, wherein a mixing ratio of Ar (Ar / Ar + N ₂ ) in the atmosphere gas is 1 to 10%. Twin-drum type thin cast slab continuous casting method.