JPH09136145A - Method for working recessed parts on peripheral surface for continuously casting cast strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To extend the service life of a cooling roll by using shot grains having grain diameter distribution, in which the max. diameter of the shot grains is not larger by a prescribed value or larger than the average diameter. SOLUTION: On the peripheral surfaces of the cooling rolls 2a, 2b in twin roll type continuous casting apparatus for casting a cast strip 6 by supplying molten metal into a molten metal pool part 4 formed with one pair of cooling rolls 2a, 2b, recessed parts are worked with a shot blasting method. At this time, the shot grains having the grain diameter satisfying the max. diameter <=the average diameter +0.3mm. Therein, in order to obtain the recessed part having a prescribed average depth, the higher hardeness of the peripheral surface of the cooling roll is, the larger average diameter of the used shot grains is desired. The relation between the hardness of the cooling rolls 2a, 2b and the average grain diameter of the used shot grains, is decided according to a kind of the shot blast working device and the working condition.

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 depression on a peripheral surface of a cooling drum of a twin-drum type continuous casting apparatus for casting a strip cast.

[0002]

2. Description of the Related Art As a synchronous continuous casting process for producing a 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 faces of a pair of cooling drums 2a, 2b which are installed parallel to each other with their axes horizontal and close to each other and which rotate in opposite directions.
8 (the front side is not shown) is crimped and configured,
By injecting the molten metal from the tundish 1 through the nozzle 3 into the molten metal pool 4 formed by the cooling drums 2a, 2b and the side dams 8, 8, the molten metal is cooled by the cooling drums 2a,
A solidified shell is formed on the peripheral surface of 2b, and the solidified shell is pressed at a kissing point 5 to form a strip cast piece 6. The strip slab 6 is sent in the direction of the arrow by the pinch roll 7 and is rolled by the in-line rolling mill 9 if necessary. By the way, since the thickness of the thin strip cast piece 6 is as thin as about 1 to 7 mm, the surface condition of the solidified shell is significantly affected by the formation state of the solidified shell, and the unevenness of the solidified shell thickness causes defects such as cracks on the surface of the cast piece. May occur.

In order to solve such a problem, it is necessary to provide a large number of depressions on the peripheral surface of the cooling drum.
No. 449, etc. A gas gap, which serves as a heat insulating layer, is formed between the cooling drum and the solidification shell by this depression, and the amount of heat removed from the cooling drum is reduced to allow slow cooling of the molten metal, and at the same time, the convex transfer due to the depression is formed on the surface of the strip slab. Is formed and coagulation is started from the peripheral portion of the convex transfer, so that the coagulated shell thickness is made uniform in the plate width direction.

Further, in order to effectively prevent the surface cracking of the thin strip cast, there is a technique which defines the shape of the depressions, the size of the depressions, the distribution of the depressions, etc. on the peripheral surface of the cooling drum. It is known from JP-A No. 1-83342 and JP-A No. 3-110044.

In addition, the applicant of the present invention provides a method for producing a strip plate by in-line rolling a strip slab continuously cast by a twin-drum type continuous casting apparatus, in which a peripheral surface of the cooling drum is manufactured. Patent application for invention that prevents surface cracking of the slab by defining the average depth of the dents as the lower limit value and prevents scale biting flaws of the slab by defining the maximum depth of the dents as the upper limit value. The application was filed according to 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 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 ≧ 60 μm and the maximum depth ≦ 100 μm, and in the case of ordinary steel, the average depth ≧ 70 μm.
μm, maximum depth ≦ 200 μm.

By the way, the recesses formed on the peripheral surface of the cooling drum are slightly reduced due to wear during casting for a long time. Therefore, when the average depth of the recesses becomes smaller than the lower limit value, the life of the recesses is reached, and it is necessary to rework the recesses on the peripheral surface of the cooling drum. Therefore, in order to extend the life of the cooling drum, it is necessary that the average depth of the depressions is as large as possible within a range not exceeding the upper limit value of the maximum depth, and for that purpose, the variation in the depression depths may be small. is necessary.

Although it is relatively easy to process the depressions satisfying such conditions by the photoetching method or the laser processing method, these methods are more costly and more time-consuming than the shot blasting method. Is disadvantageous. On the other hand, the shot blast method is industrially advantageous because it can be processed in a shorter time and costs less than the photoetching method and the laser processing method, but when processed using ordinary shot grains, the processed dent There is a large variation in depth.

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

[0010]

DISCLOSURE OF THE INVENTION The present invention is to accurately form as deep as possible a dent on the peripheral surface of the cooling drum of a twin-drum type continuous casting apparatus within a range in which the average depth of the dent does not exceed the upper limit of the maximum depth. The task is to work stably.

[0011]

According to the present invention, in order to solve the above-mentioned problems, when a recess is formed on the peripheral surface of a cooling drum by a shot blast method, maximum diameter ≤ average diameter +
It is characterized in that a recess is formed on the peripheral surface of the cooling drum by using shot particles satisfying the range of 0.30 mm.

At this time, the diameter of the recess 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 casting steel type is stainless steel, and ordinary steel , Average depth ≧ 70 μm, maximum depth ≦ 200 μm.

In order to prevent surface cracking of the strip cast,
(1) A gas gap is formed between the cooling drum and the solidification shell to allow the solidification shell to cool slowly, and (2) a convex transfer due to a depression is formed on the surface of the slab to solidify from the peripheral edge of the convex transfer. It is necessary to start the solidification and make the solidification uniform in the width direction of the slab. In order to obtain the effects of (1) and (2), it is advantageous that the average depth of the depression is large.

On the other hand, when the ingot is rolled in-line, a scale bite flaw is generated in the rolled ribbon slab, and the flaw remains even in the cold rolled thin plate product. In order to prevent the scale biting flaw, (3) the scale biting flaw is preferentially applied from a portion of the convex transfer portion having a high convex transfer, that is, a portion corresponding to a deep recess of the recess processed on the cooling drum peripheral surface. It is necessary to eliminate the deep depression because it occurs in the.

If rolling is not performed in-line after casting, no scale biting flaws occur, but after cold rolling, the convex transfer does not completely disappear and a trace remains. Since this convex transfer trace is also preferentially generated from a convex transfer portion, that is, a portion corresponding to a deep recess of the recess processed on the peripheral surface of the cooling drum, it is necessary to eliminate the deep recess. That is, it is necessary to limit the maximum depth of the dent in order to prevent scale biting flaws and convex transfer marks.

Further, the recess formed on the peripheral surface of the cooling drum is
With casting for a long time, it is slightly reduced due to wear. Therefore, it is advantageous for casting for a long time that the average depth of the depressions is as large as possible within the range not exceeding the maximum depth.

As described above, the depth of the recess formed on the peripheral surface of the cooling drum must satisfy the maximum depth and the average depth, but the average depth of the recess is extended to extend the life of the drum. Must be as large as possible without exceeding the upper limit of maximum depth.

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

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventor has carried out various researches on the processing of a recess around the cooling drum by the shot blast method, and as a result, processes the recess having a small difference between the maximum depth and the average depth by the shot blast method. Therefore, it was found that it is most effective to reduce the particle size distribution range (maximum diameter-average diameter) of shot particles.

That is, if the particle size distribution range of shot particles (maximum diameter-average diameter) is reduced, the distribution range of depression depth (maximum depth-average depth) is reduced. The type (air type, impeller type, etc.), processing conditions, and hardness of the peripheral surface of the cooling drum affect only the average depth of the depressions, and the distribution range of the depression depths (maximum depth-average depth) Has almost no effect.

FIG. 2 shows the relationship between the difference between the maximum diameter and the average diameter of the shot grains used when the depression was formed on the peripheral surface of the cooling drum by the shot blast method, and the difference between the maximum depth and the average depth of the depression. . In Fig. 2, the maximum diameter of the shot grain
In the range where the shot grain average diameter is larger than 0.30 mm, the value of the shot grain maximum diameter-the shot grain average diameter becomes smaller as the value of the shot grain maximum diameter-shot grain average diameter becomes smaller, but the shot grain maximum diameter-shot In the range where the average grain diameter is 0.30 mm or less, the value of the maximum depth of the recess-the average depth of the recess hardly changes.

On the other hand, narrowing the particle size distribution range of shot particles is disadvantageous in terms of cost because the yield in the step of screening shot particles during production of shot particles deteriorates.
Therefore, by setting the shot grains within the range of maximum diameter ≦ average diameter + 0.30 mm, it becomes possible to process the depression having a small value of maximum depth-average depth with a slight increase in cost. The effect of extending the life of the drum by this is far more cost-effective than the disadvantage of narrowing the shot grain size.

Therefore, according to the shot blast method,
In order to efficiently form a dent with a small difference between the maximum depth and the average depth on the peripheral surface of the cooling drum, maximum diameter ≤ average diameter +
It is necessary to use shot grains satisfying 0.30 mm. Here, in order to obtain the depressions having the desired average depth, it is necessary to increase the average diameter of the 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 shot particles to be used is determined according to the type of shot blasting apparatus (air type, impeller type, etc.) and processing conditions.

[0024]

EXAMPLE A recess was formed on the peripheral surface of a cooling drum of the twin-drum type continuous casting apparatus shown in FIG.
A SUS304 stainless steel thin strip slab having a plate thickness of 3.5 mm was cast. This thin strip slab is subjected to in-line rolling subsequent to casting to produce a thin strip plate having a plate thickness of 2.7 mm after rolling,
Next, cold rolling was performed to manufacture a thin plate product having a plate thickness of 0.5 mm.

When manufacturing the above-mentioned cast piece, a recess was formed under the conditions shown in Table 1 on the peripheral surface of a cooling drum having a width of 800 mm and a diameter of 1200 mm. In the present example, an air type shot blasting machine was used to process the depression, and N ₂ was used as the atmosphere during casting.

3 and 4 show typical grain size distributions of shot grains, and FIGS. 5 and 6 show representative pit depth distributions on the peripheral surface of the cooling drum. 3 and 5 show No. 1 in Table 1.
1 (comparative example), FIG. 4 and FIG. 3 (Example of the present invention). The results of denting the surface of the cooling drum and the surface quality of the thin plate product were as follows.

[Table 1]

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

No. In the case of Nos. 2 and 6, since the particle size distribution range (maximum diameter-average diameter) of shot particles exceeds 0.30 mm, the distribution range of the pit depth becomes large and the upper limit value of the pit maximum depth is satisfied. When the depressions were provided, the average depth of the depressions became close to the lower limit value of 60 μm. As a result, cracks did not occur in the strip slabs at the beginning of casting, but due to wear of the dents caused by casting for a long time, the strip slabs eventually cracked, resulting in a shorter drum life. .

No. In the case of 3, 4, 7 and 8, since the particle size distribution range (maximum diameter-average diameter) of shot particles is 0.30 mm or less, the average depth and the maximum depth of the dents both satisfy the predetermined values. . As a result, neither the crack nor the scale biting flaw occurred in the thin strip cast, and the difference between the average depth of the depression and the lower limit thereof was large, so that the drum life was extended.

[0030]

EFFECTS OF THE INVENTION According to the present invention, when the recess is machined on the peripheral surface of the cooling drum of the twin-drum type continuous casting apparatus by the shot blast method, the average depth of the recess does not exceed the upper limit of the maximum depth. Thus, it is possible to accurately and stably process as large a depression as possible. As a result, the processing cost and the processing time can be reduced to about 1/100 or less as compared with, for example, the photoetching method by processing the depression on the peripheral surface of the cooling drum by the shot blast method. Further, since the difference between the average depth and the maximum depth of the depressions can be made small, the cost of shot grains is increased by about 20% and the life of the cooling drum is several times that of the conventional shot grains. It can be extended above.
As described above, according to the present invention, a recess having a long life can be manufactured industrially at low cost and stably, 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 the relationship between the difference between the maximum diameter and the average diameter of shot grains used when processing the depression on the peripheral surface of the cooling drum, and the difference between the maximum depth and the average depth of the depression on the peripheral surface of the cooling drum. .

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

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

FIG. 5 is a diagram showing an example (No. 1 in Table 1) of a comparative example of the depth distribution of depressions formed on the peripheral surface of the cooling drum.

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

[Explanation of symbols]

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

Claims (1)

[Claims] 1. When processing a recess by a shot blast method on the peripheral surface of the cooling drum of an apparatus for continuously casting a strip cast by supplying molten metal to a molten metal pool portion formed by a pair of cooling drums, A method for forming a depression on the circumferential surface of a cooling drum for continuous casting of thin strip casting, using shot particles having a particle size distribution satisfying maximum diameter ≤ average diameter + 0.30 mm.