JPH0649221B2 - Thin cast continuous casting machine - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a continuous casting machine for thin slabs using an endless metal belt.

(Prior Art) Conventionally, thin steel sheets are manufactured by continuously supplying molten steel into a water-cooled copper mold, cooling and solidifying to produce a slab with a thickness of 200 to 300 mm. It was rolled and manufactured by a rolling mill.

However, since the energy consumption in the hot rolling process is extremely large, it has been desired to realize an energy-saving steel plate manufacturing facility that replaces the hot rolling facility.

Under such circumstances, various methods or devices for directly producing thin cast pieces or sheets from molten steel have been proposed. For example, an endless track type continuous casting machine disclosed in Japanese Patent Application No. 61-180628 proposed by the present applicant is one of the inventions. As shown in FIG. 1, the thin cast continuous casting machine of this type has a drive roller 1, a pair of metal belts 2 circulated by the drive roller 1, and a metal sandwiched between both ends of these metal belts. The main configuration is that a dam block 3 that circulates together with the belt 2 is provided, and a casting space is formed by the pair of upper and lower metal belts and the pair of left and right dam blocks.

When casting a thin cast piece with this device, a large amount of cooling water 7 is jetted from the back surface of the metal belt, and molten steel 5 is injected into the casting space from the hot water supply trough 4 to the casting surface of the metal belt 2. The solidified shell 6 is generated and sequentially solidified to form a cast piece 8, and the cast piece 8 is continuously drawn backward to manufacture.

By the way, when a cast product is manufactured using the thin cast continuous casting machine described above, vertical cracks may occur on the surface of the cast product.

One is the problem of solidification structure, which occurs frequently in the case of medium carbon steel with a carbon content of 0.09 to 0.30% in the molten steel as in the case of ordinary thick slabs. In this regard, various studies have been conducted so far, and the molten steel with a carbon content of 0.09 to 0.30% is a peritectic reaction region, so the temperature at which the austenite single phase is formed is high, and the austenite grains become coarse and crack susceptibility is high. In addition to the above, the amount of shrinkage during solidification due to the peritectic reaction is large, so the solidification reaction does not proceed uniformly when rapidly cooled during solidification, and the thickness of the solidified shell becomes uneven, which causes It is said that cracking will occur.

In addition to this, not only medium carbon steel, but also vertical cracks occur due to problems peculiar to this apparatus. This is because when molten steel is injected between the belts and solidified shells are generated, the amount of heat removed becomes uneven in the vicinity of the meniscus due to rapid heat removal, and the thickness of the initial solidified shells becomes uneven. Then, it is considered that solidification shrinkage stress, thermal stress, or impact force due to belt vibration is applied to the thin portion of the solidified shell, and vertical cracking occurs. That is, in the device shown in FIG. 1, the metal belt 2
The casting surface of is in contact with the molten steel 5 of about 1600 ° C, and the back surface is cooled by the cooling water 7 so that it is operated under severe conditions, so that a steep temperature difference occurs in the thickness direction of the metal belt 2 and thermal expansion occurs. Distortion occurs due to. The belt caused by this distortion is shown in Fig. 2.
As shown in (a) and (b), it deforms in the width direction (a) and the casting direction (b). The corrugated deformation amount of the belt is greatly affected by the thermal expansion of the metal belt as shown in FIG.

As a result, the cooling rate of the surface of the slab that is in contact with the metal belt 2 varies, which causes the thickness of the solidified shell 6 to be uneven. That is, in the lower belt, FIG. 4 (a)
As shown in (b), when solidification progresses immediately after the molten steel 5 is poured (a), the growing solidified shell becomes a resistance against heat removal to the metal belt as the distance from the meniscus increases, and the temperature rise of the belt increases. In addition, since the metal belt is cooled, the amount of thermal deformation of the metal belt is reduced and the solidified shell 6 and the metal belt 2 are locally separated from each other, so that uniform cooling by the cooling water 7 is not performed, A solidified shell 6 having a non-uniform thickness having concave portions is formed. As a matter of course, the upper belt also forms a solidified shell having a non-uniform thickness like the lower belt. As described above, even in the case of a steel which is not a medium carbon steel, the formation of the solidified shell becomes non-uniform, and the thermal stress generated in the thin portion of the solidified shell causes cracks in the solid-liquid interface of the solidified shell or the surface of the solidified shell.

When vertical cracks are generated on the surface of the slab due to the reasons described above, it is necessary to remove the cracks before transferring to the subsequent rolling step. For this reason, the slab temperature decreases, and hot charging or direct rolling (direct rolling) cannot be performed, which is a serious problem in the production process.

(Problem to be Solved by the Invention) The present invention is a surface crack that occurs when casting a thin slab by a thin slab continuous casting machine, that is, a case that occurs in the case of medium carbon steel (C: 0.09 to 0.30%). A continuous casting machine for thin cast pieces capable of preventing cracks due to non-uniform solidification due to crystallization reaction, cracks due to non-uniform solidification due to thermal deformation of a metal belt, and the like.

(Means for Solving the Problems) The inventors of the present invention have conducted various studies on surface cracks that occur when casting a thin slab with a thin slab continuous casting machine, and as a result, a plurality of strips are formed on the back surface of the metal belt. If vertical grooves are formed and the amount of heat removed from the molten steel is made different in the belt width direction, a large number of fine recesses can be generated on the surface of the slab during the initial solidified shell formation. The initial solidified shell thickness of carbon steel can be generated uniformly. The contact area between the metal belt and the cast piece is reduced, so the heat removal amount is reduced and the thermal deformation due to the temperature rise of the metal belt can be suppressed. The present invention has been completed based on the knowledge that

That is, the gist of the present invention is to provide a "driving roller, a pair of metal belts circulated by the driving rollers, and a dam block sandwiched between both end portions of the both metal belts and circulating together with the metal belts. In a thin cast continuous casting machine surrounded by a dam block and forming a casting space, a plurality of vertical grooves are formed on the back surface of the metal belt.

The thin cast continuous casting machine of the present invention will be described below with reference to the drawings. FIG. 5 is a sectional view in the width direction of the metal belt, which is a feature of the present invention, in which a plurality of vertical grooves 9 are formed in the belt width direction on the back side of the metal belt 2. At the time of casting, a large amount of cooling water is caused to flow on the back surface side of the belt 2 to remove heat from the molten steel present on the casting surface side (flat surface in FIG. 5). Since the heat removal amounts from the thin portion D _A and the thick portion D _B are different from each other, a minute concave portion is generated on the surface of the treated solidified shell, and the concave portion allows gentle cooling. Needless to say, the smaller the spacing between the vertical grooves, the finer the recesses and the better the cooling effect.

FIG. 6 shows the results of examining the heat removal amount ratio between the thin wall portion and the thick wall portion at a position 20 mm below the meniscus when the depths of the vertical grooves formed in the belt are variously changed. From this figure, it can be seen that the heat removal ratio Q _A / Q _B increases in direct proportion as the wall thickness ratio D _A / D _B increases. Note that D _A is the thickness of the thin portion, D _B is the thickness of the thick portion, Q _A is the heat removal amount from the thin portion, and Q _B is the heat removal amount from the thick portion.

FIG. 7 shows the results of examining the relationship between the heat removal amount ratio of the adjacent thin and thick portions and the nonuniformity of the solidified shell thickness at a position 20 mm below the meniscus.

Here, the nonuniformity of the solidified shell thickness is determined by the standard deviation indicating the variation amount of the solidified shell thickness. The larger this value, the larger the nonuniformity Y of the solidified shell thickness. .

In addition, n is the number of measurement points of the thickness of the solidified shell, Xi is the thickness of the solidified shell measured at regular intervals, and is the average value of the thickness of the solidified shell.

From this figure, it can be seen that when the heat removal amount ratio approaches 1, the generation of minute recesses decreases, the heat removal amount from the molten steel increases, and the belt thermally deforms, resulting in an increased degree of non-uniform solidification.
On the other hand, it can be seen that when the heat removal amount approaches 0, the nonuniformity of the solidified shell thickness increases due to local heat removal. Further, from this figure, when the heat removal amount ratio is in the range of 0.4 to 0.6, the solidification non-uniformity is small, and therefore it is preferable to manufacture in this range in order to make a slab without surface cracks.

FIG. 8 shows the relationship between the nonuniformity of the thickness of the solidified shell and the total length of the surface crack of the slab. From this figure, it can be seen that the smaller the nonuniformity of the solidified shell thickness, the smaller the total crack length.

(Example) A slab was manufactured by using a thin slab continuous casting machine shown in FIG. 1 with metal belts having different vertical groove shapes on the back surface. The groove spacing of the metal belts used was 10 mm, but the wall thickness ratio was 0.
It was changed to 4, 0.5, and 0.6 (indicated by ● in Fig. 6).
The molten steel used at that time was two kinds of steel, medium carbon steel and low carbon steel shown in Table 1, and the casting conditions were as follows:
The thickness was 50 mm and the casting speed was 3.0 to 6.0 m / min.

As a result of casting, in the case of medium carbon steel, since the heat removal amount ratio is at the position indicated by the ● mark in FIG. 7, a uniform solidified shell is formed,
A good thin slab with no surface cracks could be manufactured. Also,
In the case of the low carbon steel as well, the belt was not thermally deformed and the waving amplitude of the belt could be suppressed. Therefore, as can be seen from FIG. 9, a thin slab having an extremely good surface quality was obtained.

(Effect of the invention) As described above, when the thin cast continuous casting machine of the present invention is used, surface cracks due to uneven solidification at the initial stage of casting during the production of thin casts can be prevented and the surface properties can be improved. It is possible to manufacture thin cast pieces. Moreover, the thin cast continuous casting machine of the present invention has an excellent effect that it can be applied to casting of any kind of rope.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a thin cast continuous casting machine, FIGS. 2 (a) and 2 (b) are views showing thermal deformation of a metal belt, and FIG. 3 is a relationship between a coefficient of thermal expansion of a metal belt and a wavy amplitude. Fig. 4 (a) and (b) are views showing a state in which a solidified shell having a non-uniform thickness is formed, and Fig. 5 is a cross section in the width direction of the metal belt of the thin cast continuous casting machine according to the present invention. Fig. 6 is a diagram showing the relationship between the wall thickness ratio of the metal belt and the heat removal amount ratio, Fig. 7 is a view showing the relationship between the heat removal amount ratio and the nonuniformity of the solidified shell thickness, and Fig. 8 is the solidification. FIG. 9 is a diagram showing the relationship between shell nonuniformity and the total length of surface cracks, and FIG. 9 is a diagram showing the relationship between the standard deviation of the corrugation amplitude of a metal belt and surface cracks. 1 is a drive roller, 2 is a metal belt, 3 is a dam block, 4 is a hot water supply gutter, 5 is molten steel, 6 is a solidified shell, 7 is cooling water,
8 is a slab, 9 is a vertical groove

Claims (1)

[Claims] 1. A drive roller, a pair of metal belts circulated by the drive rollers, and a dam block sandwiched between both ends of the both metal belts and circulated together with the metal belts. A continuous casting machine for thin cast pieces, characterized in that a plurality of vertical grooves are formed on the back surface of the metal belt in a continuous casting machine for thin cast pieces which forms a casting space surrounded by.