JP2001179630A - Depressing method of cooling drum peripheral surface for continuous casting of strip slab - Google Patents

Abstract

(57) [Summary] [Problem] When a pit is formed on the peripheral surface of a cooling drum of a twin-drum continuous casting apparatus by a shot blast method, a pit having a difference as small as possible between an average depth and a maximum depth of the pit is accurately and stably. And process. SOLUTION: The cooling drums 2a, 2b of a twin-drum continuous casting apparatus for casting a thin strip slab 6 by supplying a molten metal to a pool part 4 formed by a pair of cooling drums 2a, 2b.
When a depression is formed on the peripheral surface of 2b by the shot blast method, shot grains having one kind of particle size distribution satisfying a condition of maximum diameter ≦ average diameter + 0.30 mm are used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a recess in a peripheral surface of a cooling drum of a twin-drum continuous casting apparatus for casting a strip.

[0002]

2. Description of the Related Art As a synchronous continuous casting process for producing a thin strip slab having a thickness equal to or close to that of a hot strip by continuous casting, for example, a twin-drum type continuous casting apparatus is shown in FIG. Side dams 8 are provided on both end surfaces of a pair of cooling drums 2a and 2b that are horizontally parallel in axis, installed close to and parallel to each other, and rotated in opposite directions.
8 (the front side is not shown) is crimped,
By pouring the molten metal from the tundish 1 through the nozzle 3 into the pool 4 formed by the cooling drums 2a, 2b and the side dams 8, 8, the molten metal is cooled by the cooling drum 2a,
A solidified shell is formed at the peripheral surface of 2b, and the solidified shell is pressed at the kissing point 5 to form a strip slab 6. The strip slab 6 is fed in the direction of the arrow by a pinch roll 7 and is rolled by an in-line rolling mill 9 if necessary. However, since the thickness of the strip slab 6 is as thin as about 1 to 7 mm, the surface condition is significantly affected by the state of solidified shell formation, and defects such as cracks on the slab surface due to uneven thickness of the solidified shell. May occur.

In order to solve such a problem, it has been proposed to provide a large number of depressions on the peripheral surface of a cooling drum as disclosed in Japanese Patent Application Laid-Open No. 60-184.
No. 449, for example. The depression forms a gas gap that serves as a heat insulating layer between the cooling drum and the solidified shell, reduces the amount of heat removed from the cooling drum and slowly cools the molten metal. Is formed, and solidification is started from the peripheral portion of the convex transfer, so as to make the solidified shell thickness uniform in the plate width direction.

Further, in order to effectively prevent surface cracks of the strip slab, Japanese Patent Laid-Open No. 1-83340 discloses a technique in which the shape, size, or distribution of the depressions on the peripheral surface of the cooling drum is specified. Japanese Patent Application Laid-Open No. HEI 8-83342, Japanese Patent Application Laid-Open No. H3-1110044, and the like.

In addition, the present applicant relates to a method for producing a ribbon by in-line rolling a ribbon slab continuously cast by a twin-drum type continuous casting apparatus, wherein the cooling drum peripheral surface is formed. The invention, in which the average depth of the pit in the slab is defined as a lower limit to prevent surface cracks of the slab, and the maximum depth of the slab is defined as the upper limit to prevent scale scumming of the slab, The application was filed by Japanese Patent Application No. 7-093910 and Japanese Patent Application No. 7-205504.

As described above, the depth of the depression on the peripheral surface of the cooling drum needs to satisfy each value of the average depth and the maximum depth.
Although these values differ depending on the type of cast steel, for example, in the case of stainless steel, the average depth is ≧ 60 μm and the maximum depth is ≦ 100 μm. In the case of ordinary steel, the average depth is ≧ 70.
μm, maximum depth ≦ 200 μm.

The pits formed on the peripheral surface of the cooling drum are slightly reduced due to abrasion with long-time casting. For this reason, the time when the average depth of the depression becomes smaller than the lower limit is the life of the depression, and it is necessary to rework the depression on the peripheral surface of the cooling drum. Therefore, in order to extend the life of the cooling drum, the average depth of the depressions needs to be as large as possible within a range that does not exceed the upper limit of the maximum depth. It is necessary.

[0008] It is relatively easy to process a dent that satisfies such conditions by a photoetching method or a laser processing method. However, these methods are more expensive than the shot blast method in terms of cost and processing time. Is disadvantageous in terms of
On the other hand, in the case of the shot blasting method, processing can be performed in a shorter time as compared with the photoetching method or the laser processing method, and the cost is low, which is industrially advantageous. There is large variation in the depth of the depression.

As a result, if the average depth is increased to extend the life of the drum, the maximum depth may exceed the upper limit value and scale entanglement flaws may occur in the slab. Attempting to keep it below this value will decrease the average depth and shorten the life of the drum. In some cases, it may not be possible to machine a depression that satisfies both the conditions of the average depth and the maximum depth.

[0010]

SUMMARY OF THE INVENTION According to the present invention, the deepest possible dents are formed on the peripheral surface of the cooling drum of a twin-drum continuous casting apparatus within the range in which the average depth of the dents does not exceed the upper limit of the maximum depth. It is an object to process well and stably.

[0011]

According to the present invention, in order to solve the above-mentioned problems, when a depression is formed on a peripheral surface of a cooling drum by a shot blast method, a maximum diameter ≦ an average diameter +
The method is characterized in that a depression is formed on the peripheral surface of the cooling drum using shot grains having one kind of particle size distribution satisfying 0.30 mm.

At this time, the diameter of the recess to be processed is 0.5 mm
2.0 mm, area ratio is 30-70%, average depth and maximum depth are average depth ≧ 60 μm, maximum depth ≦ 100 μm when stainless steel is cast, and ordinary steel , Average depth ≧ 70 μm and maximum depth ≦ 200 μm.

[0013] In order to prevent surface cracks of the ribbon slab,
(1) Forming a gas gap between the cooling drum and the solidified shell to slowly cool the solidified shell; and (2) Forming a convex transfer by a depression on the surface of the slab to solidify from the peripheral edge of the convex transfer. And solidification must be uniform in the slab width direction. In order to obtain the effects (1) and (2), it is advantageous that the average depth of the depression is larger.

On the other hand, when the strip slab after casting is rolled in-line, scale entanglement flaws occur in the strip slab after rolling, and these flaws remain even in a cold-rolled sheet product. In order to prevent the scale biting flaws, (3) the scale biting flaws are formed from the high convex transfer portion of the convex transfer portion, that is, the portion corresponding to the deep concave portion among the concave portions formed on the cooling drum peripheral surface. Since it occurs preferentially, it is necessary to eliminate deep depressions.

When rolling is not performed in-line after casting, scale biting flaws do not occur, but even after cold rolling, the convex transfer does not completely disappear and traces remain. Since this convex transfer mark is also preferentially generated from a high convex transfer portion, that is, a portion corresponding to a deep dent among pits processed on the peripheral surface of the cooling drum, it is necessary to eliminate the deep dent. In other words, in order to prevent scale biting flaws and convex transfer marks,
It is necessary to limit the maximum depth of the depression.

Further, the depression formed on the peripheral surface of the cooling drum is
Along with casting for a long time, the amount slightly decreases due to wear. For this reason, it is advantageous for casting for a long time to make the average depth of the depression as large as possible without exceeding the maximum depth.

As described above, it is necessary that the depth of the depression formed on the peripheral surface of the cooling drum satisfy the values of the maximum depth and the average depth. The height must be as large as possible without exceeding the upper limit of the maximum depth.

As a method of processing the dent, a photo-etching method, a laser processing method, a shot blast method, and the like are known, but a shot blast method which can be processed in a short time and at low cost is most advantageous industrially. . However, with the shot blast method using shot grains that are commonly used, it is difficult to precisely control the depth of the pit, and it is difficult to stably process the pit that satisfies the necessary conditions depending on the type of casting steel. .

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The inventor of the present invention has conducted various studies on the processing of a depression around a cooling drum by the shot blast method. To this end, it has been found that it is most effective to reduce the particle size distribution range (maximum diameter-average diameter) of shot particles having one type of particle size distribution.

That is, in a shot grain having one kind of grain size distribution, the grain size distribution range (maximum diameter-average diameter)
Is smaller, the distribution range of the depression depth (maximum depth-average depth) becomes smaller. In this case, the type of shot blasting device (air type, impeller type, etc.), processing conditions, cooling drum peripheral surface Hardness or the like affects only the average depth of the depressions, and hardly affects the distribution range of the depression depth (maximum depth-average depth).

FIG. 2 shows the difference between the maximum diameter and the average diameter and the maximum depth of the depression in the “shot particles having one kind of particle size distribution” used when the depression was formed on the peripheral surface of the cooling drum by the shot blast method. The relation of the difference of average depth is shown. In FIG. 2, in the range where the shot grain maximum diameter−shot grain average diameter is larger than 0.30 mm, as the value of shot grain maximum diameter−shot grain average diameter becomes smaller, the value of dent maximum depth−dent average depth also increases. Although the value becomes smaller, the value of the maximum depth of the depression-the average depth of the depression hardly changes when the maximum diameter of the shot particle-the average diameter of the shot particle is 0.30 mm or less.

On the other hand, in the case of shot grains having one kind of particle size distribution, narrowing the range of the particle size distribution reduces the yield in the step of sieving the shot grains during the production of shot grains, so that cost is reduced. Be disadvantaged. Therefore, by setting the particle diameter of the shot grains in the range of maximum diameter ≦ average diameter + 0.30 mm, it is possible to process a dent having a small value of maximum depth−average depth with a small increase in cost. The effect of extending the drum life resulting from this has a far greater cost advantage than the disadvantage of narrowing down the shot grain size.

Therefore, by the shot blast method,
In order to efficiently process a dent with a small difference between the maximum depth and the average depth on the cooling drum peripheral surface, the maximum diameter ≤ the average diameter +
It is necessary to use shot grains having one kind of particle size distribution satisfying 0.30 mm. Here, in order to obtain a depression having a desired average depth, it is necessary to increase the average diameter of shot particles used as the hardness of the peripheral surface of the cooling drum increases. The relationship between the hardness of the cooling drum and the average particle size of the shot particles used is determined according to the type of shot blasting device (air type, impeller type, etc.), processing conditions, and the like.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the twin-drum continuous casting apparatus shown in FIG.
A SUS304 stainless steel strip cast having a thickness of 3.5 mm was cast. This strip slab is subjected to in-line rolling following casting to produce a 2.7 mm-thick strip after rolling.
Next, cold rolling was performed to produce a thin sheet product having a sheet thickness of 0.5 mm.

When manufacturing the above slab, a depression was formed on the peripheral surface of a cooling drum having a width of 800 mm and a diameter of 1200 mm under the conditions shown in Table 1. In the present embodiment, an air-type shot blasting apparatus was used for processing the depression, and N ₂ was used as an atmosphere during casting.

FIGS. 3 and 4 show typical grain size distributions of shot particles, and FIGS. 5 and 6 show typical recess depth distributions on the peripheral surface of the cooling drum. 3 and FIG.
4 (Comparative Example), FIG. 4 and FIG. 3 (Example of the present invention). The results of the indentation process on the cooling drum peripheral surface and the surface quality of the thin plate product were as follows.

[Table 1]

No. Cases 1 and 5: Since the particle size distribution range (maximum diameter-average diameter) of the shot grains exceeds 0.30 mm, the distribution range of the pit depth becomes large, and pits satisfying the average pit depth are provided. And the maximum depth of the depression is 100
μm was exceeded. As a result, this cooling drum was not used.

No. Cases 2 and 6: Since the particle size distribution range (maximum diameter-average diameter) of the shot grains exceeds 0.30 mm, the distribution range of the pit depth is large, and the upper limit of the maximum pit depth is satisfied. When the recess was formed, the average depth of the recess was close to the lower limit of 60 μm. As a result, cracks did not occur in the ribbon slab at the beginning of casting, but due to wear of the dents due to long-time casting, cracks occurred in the ribbon slab, and the life of the drum was short. .

No. In the case of 3, 4, 7, and 8: Since the particle size distribution range (maximum diameter-average diameter) of the shot grains is 0.30 mm or less, the dents satisfy the predetermined values for both the average depth and the maximum depth. . As a result, neither cracks nor scale entrapment flaws occurred in the strip slab, and the difference between the average depth of the depressions and its lower limit was large, so that the drum life was extended.

[0030]

According to the present invention, when a depression is formed by a shot blast method on the peripheral surface of a cooling drum of a twin-drum continuous casting apparatus, the average depth of the depression does not exceed the upper limit of the maximum depth. Thus, the largest possible dent can be stably processed with high accuracy. Thus, by performing the processing of the depression on the peripheral surface of the cooling drum by the shot blast method, the processing cost and processing time can be reduced to about 1/100 or less as compared with, for example, the photo etching method. Also, since the difference between the average depth and the maximum depth of the dents can be reduced, the cost of shot particles is increased by about 20% compared to the case where conventional shot particles are used.
The life of the cooling drum can be extended several times or more. As described above, according to the present invention, a dent having a long life can be stably processed at low cost industrially, so that the industrial effect is extremely large.

[Brief description of the drawings]

FIG. 1 is a partially sectional side view showing a twin-drum continuous casting apparatus for carrying out the present invention.

FIG. 2 is a diagram showing a relationship between a difference between a maximum diameter and an average diameter of shot grains used when processing a depression on the cooling drum peripheral surface and a difference between a maximum depth and an average depth of the depression on the cooling drum peripheral surface. .

FIG. 3 is an example of a comparative example of the particle size distribution of shot grains used when processing a depression on the peripheral surface of a cooling drum (No. 1 in Table 1).
It is a figure which shows 1).

FIG. 4 shows an example of the present invention of the particle size distribution of shot particles used when processing a depression on the peripheral surface of a cooling drum (No. 1 in Table 1).
It is a figure which shows 3).

FIG. 5 is a view showing an example (No. 1 in Table 1) of a comparative example of a depth distribution of a pit processed on a cooling drum peripheral surface.

FIG. 6 is a view showing an example (No. 3 in Table 1) of the present invention of a distribution of a depth of a pit processed on a peripheral surface of a cooling drum.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Tundish 2a, 2b ... Cooling drum 3 ... Nozzle 4 ... Pool hole 5 ... Kissing point 6 ... Thin strip slab 7 ... Pinch roll 8, 8 ... Side weir (The front side is not shown) 9 ... In-line rolling Machine

Claims (1)

[Claims] When a hollow is formed by a shot blast method on a peripheral surface of the cooling drum of an apparatus for supplying a molten metal to a pool portion formed by a pair of cooling drums and continuously casting it into a thin strip, A method for forming a depression on the peripheral surface of a cooling drum for continuous casting of a strip slab, wherein shot particles having one kind of particle size distribution satisfying a maximum diameter ≦ the average diameter + 0.30 mm are used.