JPH0847758A - Method for cotinuously casting wide and thin cast slab - Google Patents

Abstract

PURPOSE:To eliminate the solidified segregation during, what is called, transition stage from the time of starting casting to the stationary condition at the time of starting the operation of a continuous casting method by executing rolling reduction to the slab in the unsolidified condition successively from the operation of rolling reduction rolls positioned on the downstream side in the casting direction to that of rolling reduction rolls positioned on the upstream side after a dummy bar passes these rolling reduction rolls. CONSTITUTION:The unsolidified rolling is executed to a wide cast slab having an unsolidified layer 7 with a continuous casting apparatus having the rolling reduction roll groups 1-5 in a roller apron zone 6. At the time of executing the rolling reduction in such apparatus, the rolling reduction is started from the roll group 5 positioned on the downstream side in the casting direction and the rolling reduction is executed in order of the roll group 4 to the roll group 3 to ... to roll group 1 after, preferably immediately after the dummy bar passes the roll group 1. At the time of starting the rolling reduction with the roll group 5, since the rolling reduction is not executed to the unsolidified layer 7 existing at the part corresponding to the roll groups 4-1 on the upstream side in the casting direction, the flowing passage and the fluidity for discharging the concd. molten steel toward the upstream part are secured and in consequence, the center segregation is eliminated and the stable operation can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for wide and thin slabs, and more particularly to a continuous casting method for improving the internal quality of wide and thin slabs at the start of casting.

[0002]

2. Description of the Related Art In recent years, due to remarkable progress in refining technology and forging technology, it has become possible to easily manufacture slabs with good quality properties, and the idea of labor and energy saving has increased. Attempts to directly and continuously manufacture thin sheet materials from molten metal without significantly omitting the hot rolling process or hot rolling have been conducted not only for non-ferrous metals such as aluminum and copper with relatively low melting points but also for iron-based materials. Even metal has come to be used.

The following methods have been proposed so far as means for continuously casting the thin cast pieces. (1) Continuous casting method using a belt type wall surface moving mold (belt type continuous casting method). (2) Continuous casting method using a modified cross-section mold (SMS method, modified cross-section mold casting method). (3) Twin roll continuous casting method.

However, (1) the belt type continuous casting method has a problem that the maintenance cost and the running cost are high because the belt is difficult to cool, and the immersion nozzle, which is apt to be difficult to dispose in this type of mold. There is a problem that it is very difficult to maintain the surface quality without degassing casting by (2). (2) The modified cross-section mold casting method gradually reduces the cross-sectional area in the mold. Therefore, it has been pointed out that a large frictional force is generated between the inner surface of the mold and the surface of the slab, and this frictional resistance causes severe wear of the inner surface of the mold and shortens the life of the mold. (3) Twin rolls The continuous casting method has the problems that the molten metal flows strongly during roll pressure in the unsolidified portion, it is difficult to float and separate inclusions, and segregation easily occurs.

Thus, the conventional thin cast continuous casting method is
All of them are thin slabs of satisfactory quality with good workability,
From the viewpoint of stable production, there are many unsolved problems, and in the industrial production of iron-based thin metal sheet materials, the current situation is that the conventional method involving hot rolling has not yet been reached. It was

Therefore, even in the conventional method, as an alternative method to the above method, a method of producing a thin cast piece by reducing the cast piece having the same mold thickness as the conventional one in a continuous casting apparatus in the solidification stage Is proposed. This is the so-called unsolidified rolling method or unsolidified rolled casting method.

[0007] That is, this method is a technique of applying reduction at the stage where the unsolidified layer remains in the cast slab. By performing the reduction, the concentrated molten steel in the central portion is discharged in the upstream direction of the casting direction, that is, the casting mold direction to achieve thinning and complete solidification, so that improvement of center segregation of the slab can be expected.

Several techniques have already been proposed as techniques for achieving such an object. For example, JP-A-59-202145.
The inventions disclosed in Japanese Patent Publication No. 60-162563, Japanese Patent Publication No. 60-162564, Japanese Patent Publication No. 60-162564, Japanese Unexamined Patent Publication No. 1-220350, Japanese Patent Publication No. 3-124352 and the like can be mentioned, but sufficient results are still obtained I can't say that

[0009]

Conventionally, in such a non-solidification reduction method, a slab whose thickness gradually changes in the length direction from the start of reduction after passing through the dummy bar to the target thickness is obtained. Since the cast piece from the position of the dummy bar to the target thickness is difficult to process thereafter and is usually cut off, it is necessary to shorten the length of this portion from the viewpoint of improving the yield.

However, in order to solve such a drawback, after the casting is performed in a steady state, all the reduction rolls are simultaneously reduced until the target plate thickness is reached, so that the reduction is rapidly performed. If it is advanced to, the molten steel reverse flow rate due to the volume change due to the reduction and the throughput change amount due to the change in the slab cross-section greatly disturb the balance with the hot water supply amount, which makes it difficult to control the molten metal level and impairs operational stability. Recent improvements in rolling technology are making it possible to roll slabs with a change in thickness in the casting direction, and it is becoming unnecessary to perform the rapid reduction shown here, which impairs the operational stability. However, the following problems regarding quality still remain large, and the current situation is unavoidable.

That is, when the series of reduction rolls arranged in the roller apron zone start the reduction, when the dummy bar passes through the last reduction roll and starts the reductions all at once, as shown in FIG. As shown in (b), due to uneven solidification in the width direction and the casting direction, the concentrated molten steel is discharged to the solidification delay portion. This concentrated molten steel can be completely discharged and a slab without center segregation can be obtained if fluidity to the upstream portion in the casting direction can be secured.

However, when the above series of rolling rolls are rolled all at once, the solid fraction of the central portion in the thickness direction increases and the fluidity also decreases in all the rolled rolls. Therefore, as shown in FIGS. 2 (c) and 2 (d), the fluidity of the concentrated molten steel discharged to the solidification delay part should be lowered in the upstream part in the casting direction where it should be discharged, and trapped at that position. There is a problem that a significant segregation occurs due to the concentration of chemical liquid steel.

Further, if the molten steel is passed through the dummy bar and then rolled down all at once, the flow of the concentrated molten steel is complicated and flows not only to the upstream portion in the casting direction but also to the downstream portion. There is a possibility that they will remain, which is a problem.

In order to prevent the segregation due to the concentrated molten steel trapped in the solidification-delayed portion, firstly, the solidification-delayed portion in which the concentrated molten steel is trapped is eliminated, that is, in the width direction and the casting direction. Means for eliminating the uneven solidification are conceivable.

However, for that purpose, it is first necessary to obtain knowledge as to in which position in the width direction and in the casting direction the nonuniform solidification occurs and to what degree the nonuniform solidification occurs. Further, since the heterogeneous solidification phenomenon varies depending on the steel type, a huge number of experiments are required to take countermeasures and substantially improve it, which is difficult. It can hardly be considered practical.

Thus, an object of the present invention is to provide a continuous casting method by a non-solidification reduction method which eliminates solidification segregation in the so-called transition process from the start of casting to a steady state in the starting operation of the continuous casting method. Is.

[0017]

Means for Solving the Problems The inventors of the present invention have conducted various studies on means for solving the above problems, and as a result, each roll has a target reduction amount immediately after the start of casting, that is, it is sequentially reduced from the upstream side. As a result of the initiation of the above, it was found that the uncoagulated portions tend to be gathered on the downstream side, and the present invention was achieved.

Here, the gist of the present invention is that in a continuous casting method for wide and thin slabs, thin slabs are manufactured by rolling down a slab having an unsolidified layer during casting with a roller apron band. In the unsolidified rolling casting method, after passing through the dummy bar, the unrolled rolling is started sequentially from the rolling roll located on the downstream side in the casting direction to the rolling roll located on the upstream side. This is a continuous casting method for pieces.

Therefore, according to the casting method of the present invention, the concentrated molten steel can be discharged to the upstream portion in the casting direction without the need to eliminate the uneven solidification, and a cast piece without center segregation can be easily obtained. be able to.

[0020]

Next, the function and effect of the present invention will be described more specifically. FIG. 1 is a conceptual diagram of a continuous casting facility having rolling roll groups 1 to 5 for performing unsolidification rolling on a roller apron strip 6 in which an unsolidified layer 7 exists for a wide slab.

According to the present invention, when performing the reduction in such equipment, the reduction is started from the roll group 5 located on the downstream side in the casting direction, preferably as soon as possible after passing through the dummy bar, and then the rolls. Group 4 → roll group 3 → ...
→ Roll down in the order of roll group 1. When the rolling of the roll group 5 is started, the unsolidified layer 7 in the portion corresponding to the roll groups 4-1 in the upstream portion in the casting direction is not rolled down, and thus the flow path for discharging the concentrated molten steel to the upstream portion and Liquidity is secured.

Since the roll reduction is usually controlled for each roll group, the reduction of each roll group may be sequentially performed in the upstream direction. The roll reduction may be performed simultaneously for each roll group. It is needless to say that it is preferable that the roll located on the downstream side in the casting direction is rolled down before the roll rolled on the upstream side.

First, according to the method of the present invention, as shown in FIG. 2, the slab portion where the unsolidified layer remains (see FIGS. 2 (a) and 2 (b)).
In the casting direction from the downstream portion, that is, the crater end side, to completely solidify and to discharge the concentrated molten steel only in the casting upstream portion, that is, the mold direction, so that the concentrated molten steel is not trapped in the solidification delay portion. It is possible to obtain a cast piece without center segregation. FIG. 2 (e) schematically shows this relationship, and it can be seen that the concentrated molten steel is easily discharged to the upper part as shown by the arrow in the figure because the reduction starts from the downstream side.

At this time, in order not to leave the concentrated molten steel,
The roller apron band for rolling down should be installed in the range downstream of the crater end in the casting direction.
Further, when referring to the reduction rate, preferably, the space where the concentrated molten steel is discharged is narrow, that is, the downstream side of the casting direction where the unsolidified layer is small, the reduction rate is kept small to ensure that the concentrated molten steel is discharged, and the casting direction It is preferable to increase the rolling speed toward the upstream portion.

When the reduction in the upstream portion in the casting direction is started, the reduction in the downstream portion has already progressed, so the change in throughput is small, and the factor affecting the fluctuation of the molten metal level is only the volume change due to the reduction, so the reduction speed is reduced. Can be big. Further, according to the method of the present invention, the timing at which the volume change due to the reduction flows backward is gradually shifted, so that the fluctuation of the molten metal level can be minimized.

When the reduction of each roll is completed, the casting reaches a steady state and thereafter stable continuous casting becomes possible. Next, the operation of the present invention will be described more specifically based on Examples.

[0027]

EXAMPLES Table 1 was used in the curved continuous casting machine shown in FIG.
Casting speed of 3.0 m / mi of medium carbon aluminum killed steel with the composition shown in
cast in n.

The target slab size is a slab with a thickness of 50 mm and a width of 1500 mm, but a mold size of 100 mm and a width of 1500 mm is applied, and a slab having an unsolidified portion is pressed down during casting in a continuous casting device to obtain a thickness of 50 mm. It was cast down.

In the present example, the rolling-down zone in the continuous casting apparatus was set at a length of 3 m from 1 seg (roll group) to 5 seg (roll group) in the roller apron zone. The reduction pattern was a uniform reduction of 20 mm for each segment. On the roll of the rolling segment, conical protrusions having a height of 5 mm were arranged in a zigzag pattern at a pitch of 15 mm in the circumferential direction in a range of 100 mm on both ends.

As the dipping nozzle, a flat type nozzle having an outer shape of 60 mm × 150 mm was used, and the surface of the molten metal in the mold was covered with powder. By doing so, stable casting was possible without the occurrence of operational troubles such as bridging between the immersion nozzle and the solidified shell, which is a problem when casting with a narrow mold.

On the other hand, when the thickness of the mold is 50 mm, the immersion nozzle cannot be inserted into the mold, and the conventional immersion nozzle hot water supply is impossible. In that sense, it was found that the non-solidification reduction technology in the continuous casting machine is very effective in the production of slabs with a slab size of 50 mm.

Next, in order to confirm the effect of the present invention in the same manner, casting was carried out by the rolling-down method as shown in Table 2 and the center segregation degrees were compared. The results are shown graphically in FIG. The relative evaluation of the segregation grade indicates that the closer the center segregation degree shown in Equation 1 is to 1.0, the better.

[0033]

[Equation 1]

As can be seen from these results, the slabs obtained under the conditions A and B in which the order of reduction was other than that of the present invention were found to have local segregation of strength, and the average Even with such a segregation grade, the grade was lower than that of the slab obtained under the conditions of the present invention, indicating the effectiveness of the present invention.
Further, under the condition C of the present invention, the fluctuation of the molten metal level was small and stable operation was possible.

[0035]

[Table 1]

[0036]

According to the method of the present invention, the center segregation of the wide and thin slab in the unsolidified reduction casting method can be improved, and stable operation with less fluctuation of the molten metal level can be achieved.

[Brief description of drawings]

FIG. 1 is a schematic view of a continuous casting apparatus for explaining the method of the present invention.

2 (a) to 2 (e) are schematic diagrams showing a process in which concentrated molten steel is discharged to a solidification delay portion.

FIG. 3 is a graph showing the effect of the method of the present invention on center segregation.

Claims (1)

[Claims] 1. A continuous casting method for wide and thin slabs, wherein a slab having a non-solidified layer being cast is pressed by a roller apron band to produce a thin slab, which is produced after passing through a dummy bar. A continuous casting method for a wide thin slab, which comprises sequentially starting unsolidified rolling from a rolling roll located on the downstream side in the casting direction toward a rolling roll located on the upstream side.