JP2953857B2 - Continuous casting method using static magnetic field - Google Patents

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 using a static magnetic field, which greatly improves the surface and internal quality of a steel slab obtained by continuous casting and enables high-speed casting.

[0002]

2. Description of the Related Art Conventionally, in continuous casting of steel slabs such as slabs used for manufacturing wide steel sheets, a flow path of molten steel between a tundish containing molten steel and a continuous casting mold is usually made of refractory. An immersion nozzle is used. In this immersion nozzle, alumina tends to adhere to the inner surface of the nozzle particularly during continuous casting of aluminum killed steel, so that the flow path of the molten steel is narrowed with the elapse of the casting time, and there has been a problem that a desired flow rate of the molten steel cannot be obtained. For this reason, an inert gas such as argon is usually supplied into the nozzle during the supply of molten steel to cope with this.However, when the throughput becomes high and the discharge flow rate of the molten steel is large, the gas is used. As shown in Fig. 1 (a), it was caught by the molten steel flow rate and could not float on the molten metal surface in the mold.
(B), the gas was trapped in the solidified shell a, so that the gas-induced blistering defect sometimes occurred after cold rolling.

Further, simply blowing an inert gas into an immersion nozzle is not enough to prevent nozzle clogging.
Needs frequent replacement work of nozzle replacement, especially
As shown in FIGS. 7 (a) and 7 (b), in the two-hole nozzle type immersion nozzle 2 provided with the symmetrical discharge port 7 at the tip of the immersion nozzle 2, the quality is deteriorated due to the left and right asymmetric blockage of the discharge port. Had a problem. As a problem of quality deterioration caused by nozzle blockage, there is not only a problem of a gas trap but also entrapment of inclusions and mold powder due to drift caused by blockage of a nozzle discharge port.

Attempts to solve such problems include:
Attempts have been made to use nozzles containing alumina and CaO, which produces a compound with a low melting point, but this has not been effective enough. In addition, Japanese Patent Application Laid-Open No. 60-92064 discloses a method of injecting molten metal that suppresses nozzle blockage by applying a direct current magnetic field to a molten metal flow in an immersion nozzle to make the molten metal flow laminar. However, since the molten metal flows down deep into the molten metal crater in the mold, accompanying inclusions may not be able to float and may be trapped in the solidified shell. Japanese Patent Application Laid-Open No. Sho 62-3857 discloses the use of the same straight nozzle as that of the present invention. However, there is still room for improvement only by obtaining a slab having a quality equivalent to that of a conventional multi-hole nozzle. A combination of a two-hole nozzle and a two-stage static magnetic field is disclosed in Japanese Patent Laid-Open No. 2-2847.
Although disclosed in Japanese Patent Publication No. 50, there is still room for improvement in quality.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems in continuous casting and to propose a continuous casting method using a static magnetic field capable of obtaining a steel slab having good surface and internal quality. Is the purpose. In particular, in order to achieve the above object, it is necessary for the molten steel discharged from the nozzle to have a uniform downward flow early in the mold, and to control the flow of molten steel in the vicinity of the discharge port and the uniform flow of molten steel in the mold. Are made possible by the present invention.

[0006]

[Means to solve the problem] As a result of various investigations and examinations on nozzle clogging during continuous casting using aluminum killed steel mainly deoxidized with Al, which has a carbon concentration of 500 ppm or less, the oxygen content in molten steel was found. When the concentration was adjusted to 30 ppm or less, more preferably 20 ppm or less, and the tip of the nozzle body of the immersion nozzle was opened and this was used as a discharge port of molten steel, it became clear that there was almost no nozzle clogging. Further, it was also clarified that the turbulence of the molten metal surface was extremely reduced by all the discharge flows going downstream. However, in such a straight nozzle, since the discharge flow velocity of the molten steel goes toward the exit side of the mold, that is, downward, there is a possibility that inclusions or bubbles in the molten steel may penetrate deep into the crater formed inside the slab. Exists.

As a result of research and development for solving this problem, as a method of preventing the discharge flow from the straight nozzle, it is very important to use a static magnetic field including the vicinity of the discharge port and parallel to the short side of the mold. It turned out to be effective. However, if the static magnetic field is simply continuous in the casting direction, the diffusion force of the molten steel is weak, and it has been difficult to obtain a uniform downward flow to the lower end of the mold. Therefore, the discharge flow velocity is reduced by the static magnetic field including the meniscus portion and the vicinity of the nozzle discharge port, and a gap is provided below the static magnetic field, and the downward discharge flow is more effectively uniformed by applying the static magnetic field again. It was confirmed that it was possible to control the flow downward. If the range of the applied magnetic field including all the magnetic pole lengths is equal, the downflow becomes more uniform due to the existence of the gap. Further, it is possible to completely prevent the fluctuation and the flow of the molten metal surface at the meniscus portion from being adversely affected by the static magnetic field.

The present invention is based on the above findings, and the gist thereof is as follows. That is, the present invention is to supply molten steel from a tundish into a continuous casting mold composed of a combination of a pair of short side walls and a long side wall through a straight immersion nozzle in which the tip of a nozzle body connected to the tundish is opened. For continuous casting of steel slabs, an upper static magnetic field generator was placed on the back side of the long side wall of the mold corresponding to the area including the outlet of the straight immersion nozzle, and a gap was provided below the upper static magnetic field generator. A continuous casting method using a static magnetic field, characterized in that the static magnetic field generator is arranged and casting is performed while constantly generating a static magnetic field from one long side wall to the other long side wall.

In the present invention, it is preferable that the static magnetic field generator is structured so as to cover the entire widthwise direction of the long side wall of the mold and that the casting is performed while always generating a static magnetic field. It is preferable to include the surface of the molten metal. In the present invention, casting can be performed while always generating a static magnetic field without blowing an inert gas into the immersion nozzle.

[0010]

The configuration and operation of the present invention will be described with reference to the drawings. 1 (a) and 1 (b) show a configuration of a main part of a continuous casting apparatus suitable for use in the embodiment of the present invention.
Is a continuous casting mold comprising a pair of short side walls 1a and a long side wall 1b;
Is a straight immersion nozzle connected to a tundish, and the straight immersion nozzle 2 has a structure in which a tip end portion of the nozzle body is released to form a straight discharge port 4 of molten steel. Numeral 3 is disposed on the back of the long side wall 1b of the continuous casting mold 1 and includes the discharge port of the straight immersion nozzle 2 and the meniscus, and is parallel to the short side wall from the long side wall 1b to another long side wall 1b. It is a device that generates a magnetic field. Thereby, the molten steel discharged from the straight immersion nozzle is decelerated, and at the same time, the fluctuation of the molten metal level is suppressed, and the entrainment of the powder is prevented.

Reference numeral 5 denotes a gap near a non-magnetic field for further uniformizing the flow of the slowed molten steel. The static magnetic field generator 3 decelerated the static magnetic field generated by the gap 5 and the static magnetic field generator 6 below and running from the long side wall 1b toward the other long side wall 1b in parallel with the short side wall 1a. The molten steel descends while advancing in the direction of the short side wall 1a. As a result, the flow is sufficiently decelerated and a uniform downflow of molten steel can be obtained.

FIG. 7 shows that the discharge port 7 of the molten steel is symmetrical.
In the two-hole immersion nozzle 2 as shown in (a) and (b), as described above, alumina and the like are likely to adhere particularly in the vicinity of the discharge port, and the nozzle is easily clogged. In the present invention, as shown in FIGS. 1 (a) and 1 (b), a continuous casting mold 1 is used as a dipping nozzle using a straight dipping nozzle 2 having a straight discharge port 4 in which the tip of a nozzle body is opened.
The molten steel supplied into the casting mold 1 is braked in the magnetic pole region of the upper static magnetic field generator 3 disposed in the continuous casting mold 1 so that the molten steel surface is settled by the static magnetic field generator 3 and the gap 5 is formed. Uniform downflow of molten steel. Also, by performing casting while applying braking to the molten steel also by the lower static magnetic field generator 6, there is no problem that nozzle clogging due to alumina adhesion occurs, and therefore the molten steel is cast at a desired speed.
Even if it is injected into the inside, inclusions may penetrate deep into the molten steel,
The upward flow of molten steel does not involve the powder on the bath surface.

[0013]

Next, the present invention will be described based on embodiments. Example 1 Using a two-strand continuous caster, a low-charge aluminum-killed steel having an oxygen concentration of 20 to 30 ppm was subjected to a three-charge continuous casting experiment using the immersion nozzle of the present invention. The casting conditions at this time are shown below.

Size of continuous casting mold: 200 mm in thickness direction,
1500mm width direction, 800mm height direction Superheat of molten steel in a tundish: Approx. 30 ° C Casting speed: 2.0m / min A casting experiment was conducted by applying a vertical static magnetic field to one strand using a straight immersion nozzle. For the strand, a casting experiment was performed using a conventional two-hole immersion nozzle for comparison. Gas for preventing nozzle clogging is applied to both the strand to which a static magnetic field is applied and the strand using the conventional immersion nozzle.
Casting was performed while blowing 10 Nl / min.

FIG. 1 schematically shows the strength of a static magnetic field and its generator.
It was shown to. The conditions at that time are as follows. Upper static magnetic field generator: Width 1700mm Height 300mm
(L ₁ ) Maximum magnetic flux density 0.4 T Lower static magnetic field generator: 1700mm in width direction 300mm in height direction
(L ₂ ) Maximum magnetic flux density 0.4 T Interval between upper and lower static magnetic fields: 300 mm from the lower end of the upper static magnetic field generator to the upper end of the lower static magnetic field generator All magnetic pole ranges: L ₁ + L ₂ = 600 mm As a result, the conventional two holes In the continuous casting using a mold immersion nozzle, a layer of alumina deposits having a maximum thickness of 12 mm was observed in the vicinity of the nozzle discharge port. However, in the continuous casting according to the present invention, the thickness of the alumina deposit was reduced by the discharge. The average was 1.0 mm at the opening of the outlet, and it was clear that nozzle clogging was extremely small.

Example 2 A continuous casting experiment was performed under the same conditions as in Example 1 and without blowing gas on both strands. The casting speed at this time was 2.0 m / min, which was the same as in Example-1.
Ladle refining also reduces the oxygen concentration in the molten steel.
The experiment was performed with the concentration reduced to 15 to 20 ppm. As a result, in the two-hole immersion nozzle, the opening of the sliding nozzle starts to open from the second charge, and the original flow rate control becomes difficult, and a predetermined injection speed cannot be achieved due to nozzle clogging near the end of the third charge pouring. , The casting speed decreased. However, in a casting experiment in which a static magnetic field was applied using the straight immersion nozzle of the present invention, nozzle clogging did not occur, the injection speed did not decrease, and thus the casting speed did not decrease.

Both nozzles were recovered after the experiment was completed, and their clogging conditions were compared. As a result, the nozzles cast using the method of the present invention had only an average of 1.0 mm or less of alumina adhered. . On the other hand, when the conventional two-hole immersion nozzle is used, the adhesion of the alumina to the discharge port portion occurs at the same time, and the two holes of the immersion nozzle are not uniformly clogged.
As a result, it became clear that the left and right discharge flows were unbalanced.

Further, the results of the examples are shown in FIG. The results shown in FIG. 2 show the average of the defects per unit area measured by a magnetic flaw detector when a cast slab was subjected to hot rolling and then cold rolling to form a cold rolled sheet.
Further, after the magnetic flaw detection measurement, when the cause of the defect was investigated, it was found that the problem was caused by gas, inclusions, and powder. Assuming that the incidence of surface defects of the cold-rolled sheet in the case of Example-1 was 1, the incidence of other surface defects was shown by comparison.

FIG. 2 shows Examples 1 and 2 which are the results of a comparative experiment between the conventional casting method and the casting method of the present invention. From these results, it is clear that the present invention has significantly reduced the internal defects of the slab as compared with the prior art. As shown in Example-2 of FIG. 2, especially when the cleanliness of the molten steel is high, not only is there no nozzle clogging, but also since no gas is blown, there is no blowhole defect, and very good results are obtained. It has gained.

Example 3 Using the straight nozzle of the present invention, a comparative experiment was conducted between a casting method in which a static magnetic field was applied in two stages including a gap and a casting method in which only one stage was used. The experimental conditions at that time are shown below. The experiment was conducted with the gas for preventing nozzle clogging limited to 15 Nl / min in total from the upper nozzle and the sliding nozzle.

Size of casting mold: 200 mm in thickness direction,
1500mm width direction, 800mm height direction Superheat of molten steel in a tundish: approx. 30 ° C Casting speed: 1.9m / min At this time, low-carbon aluminum killed steel with an oxygen concentration of 28ppm was used, and a continuous 3-charge experiment was performed for each. went.

As shown in FIG. 1, the experimental results in the case where the nozzle discharge port exists in the upper magnetic field with the two-stage static magnetic field as shown in FIG. 1, and the comparison of the experimental results with the one-stage static magnetic field shown in FIG. 6 as a comparative example FIG. 3 shows the results of the above. The specifications of each static magnetic field generator are also shown. Two-stage static magnetic field generator Upper static magnetic field generator: 1700mm in width direction 300mm in height direction
(L ₁ ) Maximum magnetic flux density 0.4 T Lower static magnetic field generator: 1700mm in width direction 300mm in height direction
(L ₁ ) Maximum magnetic flux density 0.4 T Interval between upper and lower static magnetic fields: 300 mm from the lower end of the upper static magnetic field generator to the upper end of the lower static magnetic field generator All magnetic pole ranges: L ₁ + L ₂ = 600 mm Single-stage static magnetic field generator: width direction 1700mm Height 600mm
(L) Maximum magnetic flux density 0.4 T FIG. 3 shows the occurrence rate of defects by the magnetic flaw detector as in Examples 1 and 2. Similarly, assuming that the conventional defect occurrence rate is 1, the defect occurrence rates of the respective comparative examples are shown. As a result, it is understood that the defect occurrence rate of the present invention is clearly low.

The reason why the defect generation rate of the comparative example is higher than that of the present invention is that there is no gap in the applied magnetic field, so that the molten steel flow is harder to diffuse than the present invention, and the discharge flow is less likely to be a uniform downward flow. Therefore, inclusions, bubbles, and the like are not good because they are caused to flow in the discharge flow and are trapped in the shell immediately below the nozzle. However, these are comparisons in an applied magnetic field, and it is apparent that the comparison is much better than in the conventional case where no magnetic field is applied. This is because the fluctuation of the molten metal level is suppressed by the applied static magnetic field in both the present invention and the comparative example.

Furthermore, in the present invention, the discharge flow is not only decelerated, but the discharge flow is diffused by providing a gap between the upper and lower stages and the static magnetic field. It is clear that it will flow. Example-4 A comparative experiment was performed for the case where the region to which the static magnetic field was applied was full width and the case where the region was partially applied. In the experiment, a low-strength aluminum killed steel having an oxygen concentration of 20 to 24 ppm was used using a two-strand continuous caster. In both cases, gas blowing for preventing nozzle clogging was performed at 10 Nl / min. The casting conditions at this time are shown below.

The size of the casting mold: 200 mm in the thickness direction,
Superheat of molten steel in a tundish with a width of 1500mm and a height of 800mm: about 30 ° C Casting speed: 2.2m / min Figure 5 shows a static magnetic field generator when a static magnetic field is partially applied. At this time, the specifications of the static magnetic field generator are as follows.

Upper static magnetic field generator: 800 mm in width direction, 300 mm in height direction, maximum magnetic flux density 0.4 T Interval of gap: 300 mm from lower end of upper static magnetic field to upper end of lower static magnetic field generator Lower static magnetic field generator: 800 mm in width direction , Height direction 300mm,
Maximum magnetic flux density 0.4T An experiment was conducted by arranging the above equipment on one strand. Further, another strand experiment was performed for comparison with the experimental method of the present invention, and the experiment was performed under the same conditions as in Example-1. The result is shown in FIG. From the result of FIG. 4, it was clear that it is better to apply a voltage of 1700 mm. However, it is clear that even when a static magnetic field is partially applied at this time, it is better than the conventional casting method.

Thus, the following can be said from the results of these experiments. By using a straight nozzle and a multi-stage static magnetic field, continuous casting can be achieved without nozzle clogging, thereby improving productivity, and more importantly, suppressing the drift of molten steel flow by eliminating nozzle clogging. And a clean slab can be manufactured. Also, by applying a static magnetic field to a position including the molten metal surface in the continuous casting mold, the fluctuation of the molten metal surface can be suppressed, and a static magnetic field is applied near the nozzle discharge port, and a gap is provided to reduce the static magnetic field downward. With this addition, a uniform downward flow of molten steel can be obtained, so that a cleaner steel slab without entrainment of mold powder can be manufactured. Particularly in the vicinity of the meniscus, it is important to generate a static magnetic field so as to cover the entire surface of the molten metal. For example, when a magnetic field is generated only at the lower part of the molten steel surface without applying a static magnetic field to the molten steel surface, it is possible to brake the flow under the molten steel surface but cannot suppress the vibration of the molten steel surface. Therefore, mold powder entrainment of the molten metal surface due to the vibration of the molten metal surface occurs.

Although the magnetic field plays an important role in the present invention, it is important to perform the following in this magnetic field region. First, regarding the static magnetic field, it includes the tip of the nozzle and applies below it.
Especially when the discharge port at the tip of the nozzle exists in the magnetic field,
The molten steel discharge flow is divided into a gentle horizontal flow sufficiently decelerated by the magnetic field and a downward flow. As a result, the discharge flow becomes a more uniform downward flow over the entire area of the mold due to the downward magnetic field, and it becomes possible to cast a slab having good internal and surface quality.

Further, it can be seen that generating a static magnetic field so as to cover the entire continuous casting mold at the lower portion where molten steel is jetted from the nozzle discharge port is better than casting by partially generating a static magnetic field. Further, in this method, the effect of applying the static magnetic field can be further improved by providing a gap and providing multiple stages, as shown in Example-3, by changing the magnetic field from one stage to two upper and lower stages. It is clear from the results.

[0030]

As described above, according to the present invention, continuous casting can be stably performed, and quality and productivity can be improved. In particular, by forming the static magnetic field in multiple stages including the gaps, it is possible to obtain a high-quality continuous cast slab which has not been obtained conventionally. Also, when the oxygen concentration of the molten steel is low, it can be confirmed that continuous casting can be performed without blowing gas for preventing nozzle clogging, and at the same time, defects due to gas can be eliminated.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a continuous casting apparatus in which a static magnetic field generator of the present invention is installed in two stages, upper and lower.

FIG. 2 is a graph showing a comparison between a surface defect occurrence rate (index) of the present invention and a conventional example.

FIG. 3 is a graph showing the relationship between the defect occurrence rate (index) and the comparative example in the present invention.

FIG. 4 is a graph showing a comparison of a defect generation rate (index) between a case where the static magnetic field generator is installed over the entire width and a case where the static magnetic field generator is partially installed.

FIG. 5 is a cross-sectional view of a continuous casting apparatus according to a comparative example in which a static magnetic field generator is provided in two stages, upper and lower, and partially in the width direction.

FIG. 6 is a sectional view of a continuous casting apparatus according to a comparative example in which only one stage of a static magnetic field generator is installed.

FIG. 7 is an explanatory view showing a conventional two-hole type immersion nozzle.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Continuous casting mold 1a Short side wall 1b Long side wall 2 Immersion nozzle 3 Static magnetic field generator 4 Straight nozzle discharge port 5 Gap 6 Static magnetic field generator 7 Two hole nozzle discharge port

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hisao Yamazaki 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corp. Technical Research Division (56) References JP-A-5-104218 (JP, A) JP-A Heisei 4-361858 (JP, A) JP-A-5-96351 (JP, A) JP-A-5-84550 (JP, A) JP-A-5-77009 (JP, A) JP-A-5-77007 (JP, A) A) JP-A-5-77006 (JP, A) JP-A-4-344858 (JP, A) JP-A-4-339545 (JP, A) JP-A-3-142049 (JP, A) JP-A-2 -284750 (JP, A) JP-A-1-127158 (JP, A) JP-A-62-130752 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 11/10 350 B22D 11/04 311 B22D 27/02 B22D 11/10 330

Claims (4)

(57) [Claims] 1. A steel slab is supplied from a tundish into a continuous casting mold comprising a combination of a pair of short side walls and a long side wall through a straight immersion nozzle having a nozzle tip open to the tundish. In continuous casting, an upper static magnetic field generator is arranged on the back side of the long side wall of the mold corresponding to the molten steel area including the straight immersion nozzle discharge port, and a gap is provided below the upper static magnetic field generator to form one or more stages. A continuous casting method using a static magnetic field, wherein a static magnetic field generator is arranged and casting is performed while constantly generating a static magnetic field from one long side wall to the other long side wall. 2. The continuous casting method using a static magnetic field according to claim 1, wherein the static magnetic field generator is structured so as to cover the whole width direction of the long side wall of the mold, and the casting is performed while constantly generating a static magnetic field. 3. The continuous casting method using a static magnetic field according to claim 1, wherein the static magnetic field generating area of the upper static magnetic field generator includes a molten metal surface. 4. The continuous casting using a static magnetic field according to claim 1, wherein the casting is performed while generating a static magnetic field without blowing an inert gas into the immersion nozzle. Method.