JP2011212716A - Continuous casting method of steel cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method of a steel cast slab, by which a high quality cast slab with very little internal inclusions such as alumina and slag can be stably manufactured without requiring complicated molten steel flow control.SOLUTION: A DC magnetic field application apparatus is arranged on both short side ends of a long arm of a casting mold to apply static magnetic field. Meantime, in the center of the long arm of the casting mold, there is installed a non-magnetic field application region which is at least 200 mm wide, with no static magnetic field applied thereon. Also, in a region from the lower end of the casting mold to 500 mm in the casting direction, an AC moving magnetic field apparatus is arranged to apply an AC moving magnetic field to the molten steel, thereby producing a swirl flow with a flow speed of 10-40 cm/s in the horizontal direction of the molten steel.

Description

  The present invention relates to a continuous casting method for steel slabs, and intends to advantageously reduce internal inclusions at the center of the steel slab during vertical bending die continuous casting using a two-hole nozzle.

In vertical bending die continuous casting using a two-hole nozzle, when molten steel is supplied from a tundish into a mold, non-metallic inclusions such as alumina and slag flow together with the molten steel. Usually, these non-metallic inclusions are floated and separated in a mold. However, when the non-metallic inclusions are submerged in the strand portion along with the discharge flow from the two-hole nozzle, it is extremely difficult to lift the non-metallic inclusions in the mold and separate them.
In this way, the non-metallic inclusions that have penetrated deep into the strand part become internal inclusions and remain on the slab, and when the slab is processed into tinplate or automobile materials, it becomes the origin of press cracks. . Therefore, it is necessary to reduce the internal inclusions in the slab as much as possible.

  Conventionally, a vertical bending type continuous casting machine using a two-hole nozzle is provided with a vertical portion in order to promote the floating separation of the non-metallic inclusions as described above. Although the vertical separation and floating action has a certain effect on the floating separation of coarse nonmetallic inclusions, it has been difficult to float and separate minute nonmetallic inclusions (about 300 μm or less). However, even such a small internal inclusion may be a source of press cracks, and therefore it has been desired to reduce it.

In response to the above problems, by applying a static magnetic field or the like to the strand part under the mold, the speed of the molten steel flow discharged from the immersion nozzle is intentionally reduced, and the amount of inclusions entering the strand part is reduced. Many methods have been proposed (see, for example, Patent Document 1).
However, reducing the speed of the discharge flow with a static magnetic field electromagnetic device also reduces the speed of the reverse flow generated by the discharge flow. Therefore, the floating effect of the non-metallic inclusions due to the reverse flow is reduced, and the reduction effect of the internal inclusions is offset. Therefore, this is not an effective solution.

  In addition, by installing an electromagnetic stirrer in the strand part under the mold and stirring in the horizontal direction and in the casting direction upward, the downward flow velocity is reduced, and the maximum immersion depth of the jet from the immersion nozzle is reduced, There has been proposed a method for preventing the inclusion of internal inclusions (see, for example, Patent Document 2).

  However, although the above-described method has a reduction effect on the internal inclusions of a certain size or more, the problem still remains in view of the above-described reduction of the fine internal inclusions.

JP-A-1-150450 JP-A-57-97849

Iron and steel (61 (1975) p.69)

In slabs for manufacturing tinplate, automotive steel plates, etc., it is difficult to reduce the fine internal inclusions with the conventional technology, as mentioned above, despite the need for reduction of the internal inclusions. In addition, it was not possible to prevent the inclusion of minute impurity inclusions in the thin plate as a product.
For this reason, when a considerable amount of impurities is detected by an internal inclusion sensor such as an ultrasonic flaw detector in the product inspection process of the slab, the slab must be disposed of.

  The present invention has been made in view of the above circumstances. For non-metallic inclusions such as alumina and slag, not only coarse internal inclusions, but also high-quality cast slabs with extremely few fine internal inclusions are obtained. An object of the present invention is to provide a continuous casting method of a steel slab that can be obtained stably.

  The inventors diligently studied and studied to solve the above-described problems. As a result, in order to cast a high-quality slab with few minute internal inclusions, it has been found that it is effective to use an electromagnetic brake by a static magnetic field and molten steel flow in the mold. That is, the reverse flow formed by the discharge flow is caused by flowing the molten steel directly under the mold while decelerating the downward flow formed by the molten steel flow discharged from the two-hole nozzle by the electromagnetic brake colliding with the short side of the mold. This is a method of ensuring the flow rate of

This invention is made | formed based on the said knowledge, and the summary structure of this invention is as follows.
1. When performing continuous casting using a two-hole nozzle in a vertical bending type continuous casting machine, a DC magnetic field applying device is placed on both short sides of the long side of the mold to apply a static magnetic field, while the center of the long side of the mold is The part is provided with a non-magnetic field application region that does not apply a static magnetic field with a width of at least 200 mm, and an AC moving magnetic field device is disposed between the lower end of the mold and the casting direction of 500 mm to apply an AC moving magnetic field to the molten steel. And the continuous casting method of the steel slab characterized by producing the swirling flow whose flow speed is 10-40 cm / s in the horizontal direction of this molten steel.

2. 2. The steel casting according to 1, wherein a width of the mold: W (mm) and a width of the non-magnetic field application region: L ₁ (mm) satisfy the relationship of the following formula (1): Method for continuous casting of pieces.

W / 5 ≦ L ₁ ≦ W / 2 (1)

3. The outer diameter of the two-hole nozzle is N (mm), the width of the DC magnetic field application device is L ₂ (mm), and the distance from the hole center of the two-hole nozzle to the upper end of the DC magnetic field application device is L ₃ (mm ), And when the discharge hole angle of the two-hole nozzle is α (°), the relationship of the following expression (2) is satisfied for these, and the continuous steel slab according to 1 or 2 above Casting method.

(W-N) / 2- L 3 / tanα ≦ L 2/2 ··· (2)

  According to the present invention, a static magnetic field is applied to both short sides of the long side of the mold, while a region where no static magnetic field is applied is provided at the center of the long side of the mold, and further, in the mold and in the vertical part of the strand by electromagnetic stirring. By flowing the molten steel, it is possible to effectively reduce minute internal inclusions in the molten steel and stably cast a high quality slab having very few internal inclusions.

It is the figure which showed the casting_mold | template part of the vertical bending type | mold continuous casting machine used for this invention. It is the figure which showed typically the flow state of the molten steel in the conventional general casting_mold | template and a strand perpendicular | vertical part. It is the figure which showed typically the flow state of the molten steel in a casting_mold | template and a strand perpendicular | vertical part at the time of applying a static magnetic field with respect to discharge molten steel within a casting_mold | template. It is the figure which showed typically the flow state of the molten steel in a casting_mold | template and a strand perpendicular | vertical part at the time of applying a static magnetic field with respect to discharge molten steel within a casting_mold | template, and applying a horizontal swirl flow just under a casting_mold | template. When a static magnetic field is applied to the discharge molten steel only on the short side of the long side in the mold, and the horizontal swirl flow is applied directly under the mold, the flow state of the molten steel in the mold and in the vertical part of the strand It is the figure shown typically.

Hereinafter, the present invention will be specifically described.
In general, molten steel injected into a mold is mixed with alumina and slag, which are products during deoxidation, and non-metallic inclusions generated by melting a tundish refractory.
When molten steel mixed with these non-metallic inclusions is injected into the mold, if it gets inside the strand and is captured by the solidified shell, it becomes an internal inclusion in the steel slab. Cannot be used as a thin plate product.

Therefore, the inventors investigated in detail the phenomenon of non-metallic inclusions entering the strands, and conducted research on the relationship between the flow state of molten steel and the phenomenon of non-metallic inclusions entering the strands.
As a result, the method described below succeeded in greatly reducing the intrusion of nonmetallic inclusions into the strand, in particular, the infiltration of minute nonmetallic inclusions of about 50 to 200 μm, thereby completing the present invention. It reached.

That is, the present invention applies a static magnetic field using a DC magnetic field application device on both short sides of the long side of the mold when performing continuous casting of a steel slab using a vertical bending die continuous casting machine, About the center part of the long side of a casting_mold | template, the nonmagnetic field application area | region which does not apply the static magnetic field of a width of at least 200 mm is provided.
Furthermore, the AC moving magnetic field device is arranged so that its center is located between the lower end of the mold and the casting direction of 500 mm, and an AC moving magnetic field is applied to the molten steel, so that the molten steel is horizontal (perpendicular to the casting direction). ), And the flow velocity of the swirling flow is 10 to 40 cm / s.

FIG. 1 conceptually shows a mold part of a vertical bending type continuous casting machine used in the present invention. In the figure, 1 is a mold, 2 two-hole nozzle, 3 is the mold bottom, the width of the mold at is W, the outer diameter of the two-hole nozzle N, the width of the non-magnetic field application region L _1, DC magnetic field the width of the application device L _2, showing the distance from the hole center of the two-hole nozzle to the upper side of the DC magnetic field application device L _3. The discharge hole angle of the two-hole nozzle is indicated by α.
In addition, the arrow in a figure has shown the casting direction.

FIG. 2 schematically shows a flow state of molten steel in a conventional general mold and in a strand vertical part. As shown in the figure, even when there is no molten steel flow due to electromagnetic stirring, not all non-metallic inclusions in the molten steel poured into the mold become internal inclusions. If it is a coarse non-metallic inclusion, it floats with its own buoyancy, and even a small inclusion is partly associated with the reverse flow from the discharge hole of the two-hole nozzle as shown in FIG. It can rise and separate.
This means that the floating of the non-metallic inclusions poured into the mold can be promoted by increasing the speed of the reverse flow from the discharge hole.

  Moreover, when a static magnetic field (electromagnetic brake) using a DC magnetic field application device is applied to the downward flow formed after the discharge flow from the two-hole nozzle shown in FIG. 2 collides with the short side of the mold, the molten steel is lowered. The flow velocity is reduced, and the amount of inclusions inside the molten steel is reduced. On the other hand, as shown in FIG. 3, when the downward flow is decelerated by the electromagnetic brake, the flow velocity of the reverse flow is also reduced, and when the reverse flow passes from the bottom to the top of the electromagnetic brake, the flow velocity of the reverse flow is further reduced. Therefore, the effect of reducing internal inclusions due to the reverse flow is lost.

Therefore, the inventors conducted an experiment using a water model experimental apparatus. As a result, it has become clear that in order to increase the speed of the reverse flow described above, water should flow in the horizontal direction at a position between the mold lower end 3 and 500 mm in the casting direction.
Furthermore, the inventors conducted a water model experiment using a tracer simulating internal inclusions. However, even if the electromagnetic brake and the flow immediately below the mold are applied in combination, the reverse flow is canceled out by the electromagnetic brake as shown in FIG. The amount of the object did not decrease, and in the case of FIG. 4, it became clear that the effect of reducing the internal inclusions due to the flow was not observed.
That is, in order to further reduce the internal inclusions, it is necessary to find a condition where the effects of both the electromagnetic brake and the molten steel flow by electromagnetic stirring are compatible.

Therefore, the inventors conducted water model experiments under various conditions, and investigated conditions under which both the electromagnetic brake in the mold and the effect of molten steel flow by electromagnetic stirring directly under the mold could be achieved.
As a result, at the center of the long side of the mold, a region having at least 200 mm of width without braking is provided, and further, swirling at a flow rate of 10 to 40 cm / s in the horizontal direction from the lower end of the mold to the casting direction of 500 mm. It has become clear that by generating the flow, the speed of the water reversal flow can be efficiently increased, and the penetration of non-metallic inclusions can be greatly reduced. In the present invention, the flow rate of water was measured using an anemometer.

  That is, as shown in FIG. 5, in the central part of the long side of the mold, by applying no electromagnetic brake, the reverse flow that has flowed by electromagnetic stirring reaches the two-hole nozzle and the meniscus part without decelerating. The effect that internal inclusions can be greatly reduced is achieved.

The above configuration will be described more specifically.
As a result of conducting experiments with various swirl flows that generate a swirl flow at a flow rate of 10 to 40 cm / s in the horizontal direction, internal inclusions were greatly reduced at a flow rate of 10 cm / s or more. On the other hand, when the flow rate exceeded 40 cm / s, the molten steel surface level fluctuated significantly, causing the phenomenon of powder entrainment and affecting the surface quality of the steel slab. Therefore, the flow velocity of the swirling flow in the present invention is set to 10 to 40 cm / s.

The installation position of the AC moving magnetic field device is from the lower end of the mold to 500 mm in the casting direction. When the AC moving magnetic field device is installed in the mold and the molten steel flows horizontally, the discharge flow that passes through this flow region as it is. The ratio of the amount increases, and the effect of reducing internal inclusions becomes small.
On the other hand, when the AC moving magnetic field device is installed below the position of 500 mm at the lower end of the mold, the flow velocity increases only in a part of the lower part of the reverse flow, so the effect of reducing internal inclusions is limited. End up.
For these reasons, the installation position of the AC moving magnetic field device is between 500 mm in the casting direction from the lower end of the mold.
In addition, the reference | standard of installation of an alternating current magnetic field apparatus shall be the center of an alternating current magnetic field apparatus. That is, in the present invention, the installation position of the AC moving magnetic field device is between the center of the AC moving magnetic field device and the casting direction from the lower end of the mold to 500 mm.

Non-magnetic field application region to which a static magnetic field having a width of at least 200 mm is not applied The width: L ₁ of the non-magnetic field application region in the present invention is at least 200 mm. This is because if the width of the non-magnetic field application region where no static magnetic field is applied is less than 200 mm, the width of the reverse flow increased by the swirling flow of the AC moving magnetic field is reduced, so that the reverse flow rises. This is because, as a result, the effect of reducing internal inclusions is not sufficient.
On the other hand, the upper limit of _{L 1} is preferably set to about 1000 mm. This is because the effect of the non-magnetic field application region is saturated when L ₁ exceeds 1000 mm. Further, if L ₁ is too wide, the discharge flow from the immersion nozzle avoids the static magnetic field region and passes through the non-magnetic field application region (L ₁ ).

Next, expressions (1) and (2) in the present invention will be described.
As shown in FIG. 1, the width of the mold is W (mm), the outer diameter of the two-hole nozzle is N (mm), the width of the region where no static magnetic field is applied at the center of the long side is L ₁ (mm), The width of the DC magnetic field device installed on the short side is L ₂ (mm), the distance from the center of the 2-hole nozzle to the upper side of the DC magnetic field device is L ₃ (mm), and the discharge hole angle of the 2-hole nozzle is α (° ), It is advantageous to satisfy the following relational expression.

That is, in the vertical bending die continuous casting, in order to efficiently lift and separate inclusions, the width L ₁ of the non-magnetic field application region is set to 200 mm or more, and L ₁ is constant with respect to the width W of the mold. It is preferable that the reversal flow path be secured.
The condition is shown in Formula (3).
L ₁ ≧ W / 5 (3)
As a result of the water model experiment, when the width L ₂ of the DC magnetic field device is too narrow, flows as discharge stream avoid brake, for thereby lowered from the brake no longer side center, the L ₂ to W It is desirable to make it more than a certain ratio. The condition is shown in Formula (4).
L ₂ ≧ W / 4 (4)
Here, since W = L ₁ + 2L ₂ , the following formula (1) can be obtained by combining the formula (3) and the formula (4).
W / 5 ≦ L ₁ ≦ W / 2 (1)

In addition, an appropriate electromagnetic brake is applied to the discharge flow from the two-hole nozzle, and in order to effectively generate a reverse flow in the molten steel center below the electromagnetic brake application position, the nozzle discharge hole angle The relationship between α and L ₂ and L ₃ is important.
In other words, two-hole nozzle of a line drawn along the nozzle discharge hole angle α from the center of the hole, as it is shown in Figure 1, than L _2/2 is half the length of the electromagnetic brake, mold length It is desirable to intersect on the short side of the side, and this is shown by the following formula (2).
(W-N) / 2- L 3 / tanα ≦ L 2/2 ··· (2)
Therefore, by satisfying the conditions of Formula (1) and Formula (2) together, the DC magnetic field application device located on the short side of the long side of the mold can more effectively prevent the electromagnetic brake against the discharge flow. Application and good reversal flow formation is achieved.

As described above, by controlling the flow of molten steel according to the present invention, it is possible to produce a high-quality slab with very few internal inclusions, particularly minute internal inclusions due to non-metallic inclusions such as alumina and slag. Is possible.
If surface quality is required at the same time, the effect of the present invention can be maintained even if an AC moving magnetic field device capable of applying a swirling flow in the mold near the discharge hole of the two-hole nozzle is maintained. At the same time, a steel slab having good internal quality can be produced.
This is because when a swirl magnetic field is further applied in the mold in addition to the static magnetic field and the strand swirl magnetic field in the mold, a cleaning effect occurs at the solidification interface of the molten steel.

Here, any conventionally known apparatus can be used as the DC magnetic field applying apparatus used in the present invention. Moreover, the use conditions are a magnetic flux density of 0.1-0.5 Tesla.
Incidentally, from the two-hole nozzle center (the center of the discharge hole), the distance L ₃ to the upper DC magnetic field application apparatus is preferably in the range of about 200 to 500 mm.

Furthermore, as the AC moving magnetic field device used in the present invention, any of the conventionally known devices can be used. In particular, an AC moving magnetic field applying device is installed on both long sides, and the same direction in the width direction in each device. A magnetic field device that can generate a swirling flow by applying a moving magnetic field in the opposite direction (so-called swirl magnetic field device) can be preferably used.
The magnetic field device described above is a linear coil type that applies an alternating current with a maximum current of 1000 A and a frequency of 2 to 3 Hz, and the center of the magnetic field device is 1.0 to 1.5 m from the meniscus as the installation position. There are some examples.
Moreover, the use conditions are especially a magnetic flux density of 0.01-0.15 Tesla.

Here, the width and thickness of the mold used in the present invention, the outer diameter of the two-hole nozzle, the discharge hole angle, and the casting speed can be those conventionally known when using a vertical bending type continuous casting machine. , The range shown below, ie
Mold width: W is 1000-1800 mm, mold thickness is 200-300 mm, outer diameter of 2-hole nozzle: N is 100-200 mm, discharge hole angle: α is 0-50 °, and casting speed is , About 1.0 to 3.0 m / min is preferable.

Hereinafter, the test casting result of 14 charges performed by the slab continuous casting machine will be described.
Low charge tin molten steel of about 200 tons per charge was cast into 14 charges (test Nos. 1 to 14). In the inventive examples (Test Nos. 1 to 6, 8, 9, 13, and 14), a DC magnetic field applying device is installed so that a static magnetic field is applied only to the vicinity of both short sides of the mold long side of these molten steels. Then, at the center of the long side of the mold, a non-magnetic field application region that does not apply a static magnetic field of at least 200 mm is provided, and an AC moving magnetic field device is installed at a position of 200 mm in the casting direction from the lower end of the mold. Continuous casting was performed using a vertical bending continuous casting machine under the condition of applying a swirling flow having a flow rate of s. Table 1 shows the main casting conditions.

Further, in the inventive examples (Test Nos. 1 to 6), the mold width: W, the width of the DC magnetic field application device: L ₂ that satisfies both the expressions (1) and (2), and the position of the DC magnetic field application device: Casting was performed at L ₃ , the outer diameter N of the two-hole nozzle, and the discharge hole angle α.

Furthermore, as a comparative example, continuous casting was performed with a vertical bending continuous casting machine. Table 1 shows the main casting conditions.
In Comparative Example (Test No.7) were adjusted to non magnetic field application region L ₁ so as to not satisfy the equation (1). In the comparative examples (test Nos. 10 to 11), casting was performed under the condition that the flow velocity by the AC moving magnetic field apparatus exceeded 40 cm / s. In the comparative example (Test No. 12), casting was performed under the condition that the speed of the molten steel flow was less than 10 cm / s.

When a swirling flow of 30 cm / s was applied to the molten steel with an AC moving magnetic field, a swirling magnetic field having a magnetic flux density of 0.09 Tesla was applied. Further, when a swirl flow was applied to the molten steel at a rate exceeding 40 cm / s, a swirl magnetic field having a magnetic flux density of 0.15 Tesla was applied.
Furthermore, the electromagnetic brake uniformly applied a static magnetic field with a magnetic flux density of 0.3 Tesla.

The molten steel flow velocity was confirmed by collecting a sample from the cast slab and solidifying the sample. That is, a specimen for speculum is cut out from a cast slab, mirror-finished and then corroded with acid to reveal a solidified structure. From the inclination angle of dendritic dendrites in the solidified structure, Okano et al. The molten steel flow velocity was obtained using Reference 1).
The flow velocity when a moving magnetic field of 0.15 Tesla was applied with an AC moving magnetic field was 50 cm / s. The flow rate when a moving magnetic field of 0.05 Tesla was applied was 3 cm / s.

In addition, a surface perpendicular to the casting direction of the cast slab after casting was cut out by 30 mm in the casting direction, and the number of inclusions in the full width and thickness was measured with an ultrasonic flaw detector. The survey results are shown in Table 2. Furthermore, the number of internal inclusions by inclusion sensors in the steel plates after rolling of these slabs was also investigated. The results are also shown in Table 2.
The above rolling was cold rolling, and a continuously cast slab was hot-rolled and cold-rolled into a steel plate, and this steel plate was subjected to tin plating.

  As shown in Table 2, all of the inventive examples (Test Nos. 1 to 6, 8, 9, 13, 14) have fewer internal inclusions at the slab stage than the comparative examples, and The number of internal inclusions in the rolled steel sheet was also significantly reduced. In particular, the invention examples (Test Nos. 1 to 6) satisfying both the formula (1) and the formula (2) have a small number of internal inclusions at the slab stage and a small number of internal inclusions in the steel sheet. As a result.

Further, when the particle size of the internal inclusions in the steel plate was investigated, the particle size distribution of impurities in the steel plate according to the present invention was shifted to a smaller side, and the number of internal inclusions themselves was greatly reduced. I understood that.
It was also confirmed that the surface defects due to the powder were very few.

  According to the present invention, it is possible to obtain a high-quality steel slab having very few internal inclusions such as alumina and slag in the vertical bending continuous casting.

DESCRIPTION OF SYMBOLS 1 Mold 2 2-hole nozzle 3 Mold lower end L ₁ Width of area where static magnetic field is not applied L ₂ Width of DC magnetic field device installed on both short sides L ₃ Distance from center of 2-hole nozzle to upper side of DC magnetic field device W Mold width N Outer diameter of 2 hole nozzle α Discharge hole angle of 2 hole nozzle

Claims (3)

  When performing continuous casting using a two-hole nozzle in a vertical bending type continuous casting machine, a DC magnetic field applying device is placed on both short sides of the long side of the mold to apply a static magnetic field, while the center of the long side of the mold is The part is provided with a non-magnetic field application region that does not apply a static magnetic field with a width of at least 200 mm, and an AC moving magnetic field device is disposed between the lower end of the mold and the casting direction of 500 mm to apply an AC moving magnetic field to the molten steel. And the continuous casting method of the steel slab characterized by producing the swirling flow whose flow speed is 10-40 cm / s in the horizontal direction of this molten steel. 2. The steel according to claim 1, wherein the width of the mold: W (mm) and the width of the non-magnetic field application region: L ₁ (mm) satisfy the relationship of the following formula (1): A continuous casting method for slabs.

W / 5 ≦ L ₁ ≦ W / 2 (1)
The outer diameter of the two-hole nozzle is N (mm), the width of the DC magnetic field application device is L ₂ (mm), and the distance from the hole center of the two-hole nozzle to the upper end of the DC magnetic field application device is L ₃ (mm ), And when the discharge hole angle of the two-hole nozzle is α (°), the relationship of the following expression (2) is satisfied for these, and the steel slab according to claim 1 or 2 Continuous casting method.

(W-N) / 2- L 3 / tanα ≦ L 2/2 ··· (2)

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