JP4211573B2 - Steel continuous casting method - Google Patents

Description

  The present invention uses a continuous casting immersion nozzle (hereinafter also referred to as a “swirl flow nozzle”) in which a twisted plate type swirl vane is installed in the inner pipe portion and a swirl flow is imparted to the molten steel flowing down the inner pipe portion. The present invention relates to a continuous casting method.

  When a swirl vane is installed in the inner pipe part and an immersion nozzle (two-hole nozzle) provided with two discharge holes facing the side wall near the bottom is used for transferring molten steel between the tundish and the mold, it flows down in the immersion nozzle. A swirl flow is applied to the molten steel, and the molten steel is forcibly distributed and discharged to the two discharge holes by centrifugal force, so fluctuations in the discharge flow rate and discharge speed from the two discharge holes are reduced and the discharge from the discharge holes is reduced. The flow state is stable, and uneven flow is less likely to occur in the mold. As its effect, it is possible to reduce slab surface flaws caused by trapping non-metallic inclusions, bubbles, mold powder, etc. in the solidified shell in the mold, especially when performing wide slab continuous casting with a large rectangular ratio It is known to be effective. For example, Patent Document 1 and Patent Document 2 disclose a swirl flow nozzle technique in which blades for swirling molten steel are installed in an immersion nozzle. Moreover, the effectiveness of the swirl flow nozzle is introduced in Non-Patent Document 1, for example.

  However, each disclosed technique has the following problems. With swirling nozzles, the flow passage cross section of the swirling blade installation site is small, so clogging due to adhesion of non-metallic inclusions such as alumina is likely to occur. Adhesion of non-metallic inclusions such as alumina proceeds on the inner surface of the nozzle, and the molten steel in the nozzle is not in a favorable swirl flow. Generally, a method of supplying Ar gas into the immersion nozzle is used as a countermeasure against the blockage of the immersion nozzle. When a large amount of Ar gas is blown into the molten steel, the amount of alumina or the like attached to the inner surface of the nozzle is reduced, and the casting time can be extended. However, boiling due to Ar gas bubbles occurs in the molten steel in the mold, mold powder is entrained, and powder defects occur in the surface of the slab, or Ar gas bubbles are trapped in the slab, resulting in a It is known to cause defects.

Japanese Patent Laid-Open No. 11-815291 JP-A-11-90593 CAMP-ISIJ Vol. 15 (2002), 165

Therefore, in order to suppress slab defects while preventing nozzle blockage, it is necessary to know an appropriate range of the Ar gas blowing amount (flow rate) in the swirling nozzle. Furthermore, in order to obtain the effectiveness of the swirl flow nozzle, it is necessary to form a sufficient swirl strength in the swirl flow nozzle.
However, the conditions required for putting a swirl flow nozzle having a twisted plate swirl vane in the inner pipe portion into practical use in a continuous casting method have not been sufficiently studied.
The present invention has been made in view of such circumstances, and an object of the present invention is to provide an optimum continuous casting method for practical use of a swirl flow nozzle in which a twist plate swirl blade is installed in an inner tube portion. .

The present inventor has conducted various experiments and simulations in order to find an optimum continuous casting method for practical use of a swirl flow nozzle in which a twisted plate swirl blade is installed in the inner pipe portion. As a result, it is clear that the swirl strength of the molten steel flow increases as the molten steel supply amount (molten steel passage amount) into the nozzle increases, but when the molten steel supply amount is low, a stable swirl flow is not formed. became.
Based on the above knowledge, in the present invention, the twist plate type swirl vane for swirling the molten steel flow is provided inside the tundish and the mold, which has two discharge holes opened facing the side wall near the bottom. In continuous casting in which the provided immersion nozzle is provided, the molten steel in the tundish is passed through the immersion nozzle and supplied into the mold, and the inert gas is supplied into the molten steel passing through the immersion nozzle. The flow rate q (N liters / min) of the inert gas supplied into the molten steel while controlling the supply amount Q (ton / min) of the molten steel passing through the nozzle in a range of 2.5 ≦ Q ≦ 5.2. Is controlled in the range of 2 ≦ q ≦ 15.
Moreover, in the method of the present invention, two layers are provided between the tundish and the immersion nozzle in order to limit the supply amount Q of the molten steel that passes from the tundish through the immersion nozzle and is supplied into the mold to the above range. Formula structure sliding gate is provided.

According to the present invention, in continuous casting using a swirl flow nozzle, swirl vanes function effectively, and fluctuations in discharge flow rate and discharge speed from the discharge holes on the side surface of the nozzle bottom are reduced, so that stabilization is achieved.
In addition, according to the present invention, it becomes difficult for deposits on the swirl vane and the inner surface of the nozzle to be deposited due to the blowing of inert gas into the molten steel. Is prevented.
Therefore, it is difficult to cause uneven flow in the mold, and the effect of stably producing a slab having good surface quality can be expected by reducing slab surface flaws resulting from uneven flow in the mold.

In the continuous casting method of the present invention, when a swirl flow nozzle is installed between the tundish and the mold, and molten steel is supplied from the tundish into the mold, the molten steel in the tundish is fed from the entrance of the swirl flow nozzle to the nozzle inner tube. The swirl flow is generated by supplying and passing through the section, and the inert gas is blown into the flowing steel from the position upstream of the swirl flow nozzle or the inner wall of the swirl flow nozzle. The molten steel that has passed through the nozzle flow path is turned into a swirl flow and supplied into the mold from a discharge hole provided in the side wall near the bottom of the nozzle.
In the present invention, in order to stably produce a slab having a good surface quality by discharging a uniform and stable swirling flow into a mold for a long time, a molten steel supply amount Q passing through the swirling flow nozzle is set to 2.5 ton / min or more and 7.0 ton / min or less, preferably 3.5 ton / min or more and 7.0 ton / min or less, and the inert gas flow rate q supplied into the molten steel is 2 N liter / min or more and 15 N liter. / Min or less, preferably 2 N liter / min or more and 10 N liter / min or less.

In order to improve the surface quality of the slab, it is necessary to supply molten steel into the nozzle to some extent. When the molten steel supply rate is less than 2.5 ton / min, the energy required for swirling is insufficient and the stability of the swirling flow is deteriorated. As a result, it is difficult to obtain good slab surface quality.
On the other hand, when the molten steel supply rate exceeds 7.0 ton / min, the swivel force or the discharge pressure is too strong, and the mold surface level in the mold tends to fluctuate locally. As a result, powdery defects are likely to occur due to the entrainment of mold powder.
In addition, as an anti-clogging measure for the swirl flow nozzle, an inert gas is blown into the swirl flow nozzle. However, if the flow rate of the inert gas is less than 2 N liters / min, the clogging prevention effect is not sufficient. The deposits accumulate and the swirl flow becomes uneven or unstable, and it is difficult to obtain good slab surface quality, and the operation may be interrupted due to blockage.
On the other hand, when the flow rate of Ar gas blown into the swirling flow nozzle exceeds 15 N liters / min, the amount of blown air is too large, and therefore, a powdery defect and a bubble defect are likely to occur due to the mold powder being caught.

FIG. 1 shows an example of the configuration of an apparatus that can carry out the continuous casting method of the present invention. In the continuous casting apparatus 1 of FIG. 1, an upper nozzle 3, a two-layer sliding gate 4, and a swirling flow nozzle 5 are sequentially connected to an outlet provided at the bottom of a continuous casting tundush 2. The bottom part (lower end) of the swirl flow nozzle 5 is immersed in the molten steel in the mold 6, and the two opposing discharge holes (5a) provided in the side wall near the bottom part are immersed in the molten steel in the mold.
FIG. 2 is an enlarged sectional view showing an outline of the two-layer structure sliding gate 4 and the swirl flow nozzle 5 installed in the apparatus of FIG. In the two-layer structure sliding gate, the upper plate 4a and the lower plate 4b are overlapped so that either one or both of them can slide. The upper plate is provided with an inert gas blowing nozzle 4c and is connected to a flow path inner wall 4d made of porous refractory. The inert gas passes through the blowing nozzle 4c and is blown into the molten steel from the porous refractory flow passage inner wall 4d of the upper plate.
A torsion plate type swirl vane 7 is installed in the inner tube portion of the swirl flow nozzle 5, and swirl flow is generated when the molten steel passes through the position of the swirl vane. The dimensions of the swirl nozzle are usually about 50 to 300 mm in inner diameter and about 500 to 1000 mm in length. The swirl flow nozzle body is made of, for example, alumina-carbon, alumina-silica-carbon, or the like.
FIGS. 3A, 3B and 3C show a twisted plate type swirl blade. Regarding the reference numerals in the figure, L means the torsion plate length, D means the torsion plate width, and T means the torsion plate thickness. The torsion plate type swirl blade is usually formed of a refractory material such as a boron nitride sintered product. The swirl vanes usually have a length L / width D ratio (L / D) of 0.4 to 2.0, a twist angle θ of 90 ° to 180 °, and a twist pitch of 0.8 to 2.0 rotations. Within range. Here, the torsion pitch is an index representing the torsional strength, which is how many times the width D is required for the torsion plate to be twisted 180 °.

The configuration of the apparatus can be changed as appropriate. For example, the tundush and the swirl flow nozzle can be connected via other members such as an intermediate nozzle as necessary. Further, a gate for adjusting the molten steel supply amount may be provided between the tundush and the swirl nozzle. As the gate, for example, a sliding gate such as a two-layer structure or a three-layer structure, a stopper, or the like can be used. When a two-layer structure sliding gate is used, a twisted plate-type swirl vane in a swirl flow nozzle is used. Since the molten steel flows down uniformly and easily forms a uniform swirling flow with respect to the nozzle cross-section at a position immediately above, it is preferable.
A plurality of discharge holes of the swirl flow nozzle may be opened in the side wall near the bottom or may be opened in the bottom. When the discharge holes are provided in the side wall near the bottom of the swirl flow nozzle, the number of discharge holes is not particularly limited, but usually two or three discharge holes are provided at equal intervals. In particular, a two-hole nozzle having two discharge holes facing the side wall in the vicinity of the bottom portion discharges molten steel accompanied by a swirl flow to the two discharge holes forcibly by centrifugal force and discharges them. Variations in flow rate and discharge speed are reduced. As a result, the state of the discharge flow is stabilized, and uneven flow is less likely to occur in the mold.

As the inert gas, Ar (argon) gas is preferably used, but is not particularly limited. The means for supplying the inert gas may be provided at any position on the flow path from the outlet of the bottom of the tundish to the discharge hole of the submerged nozzle. For example, the upper nozzle in the tundish, the plate of the sliding gate, the submerged nozzle, etc. Blowing can be performed at the position. As the structure and material of the blowing portion, a steel thin tube, a porous refractory, or the like that is usually used may be used.
In order to prevent clogging at the position of the torsional plate-like swirl vane, an inert gas is supplied into the flow channel upstream of the swirl vane and the twisted plate-like swirl is performed with the molten steel sufficiently containing the inert gas. It is preferable to pass the position of the blade. From this point of view, the inert gas supply means is preferably blown from an apparatus around the sliding gate such as an upper nozzle in the tundish or a sliding gate plate.
That is, in the present invention, supplying the inert gas into the molten steel passing through the immersion nozzle means supplying the inert gas so that the molten steel can pass through the immersion nozzle in a state containing the inert gas. However, the present invention is not limited to the case where the inert gas is supplied to the molten steel passing through the immersion nozzle.

  The following is a mold having the basic configuration shown in FIG. 1, a 1860 mm wide × 210 mm thick mold, a two-layered sliding gate and a two-hole swirl nozzle (immersion nozzle shape: inner diameter φ70 mm; discharge hole shape: two holes, 55 mm high) Length × 90 mm width, upward 10 °; swirl blade shape (see FIG. 3): continuous casting apparatus having diameter D = 100 mm, length L = 100 mm, twist angle θ = 120 °, twist plate thickness T = 15 mm) The results of the continuous casting of the molten steel of ultra-low carbon steel with [C] ≦ 30 ppm until the total molten steel amount reaches 460 tons while performing Ar gas blowing, and the slab surface flaw occurrence rate will be described. For comparison with the above test, a normal two-hole nozzle in which the swirling blades were removed from the swirling flow nozzle was used, and continuous casting was performed in the same manner under other conditions.

When the swirl flow nozzle is used, the relationship between the molten steel supply amount and the slab surface flaw occurrence rate is the result shown in the graph of FIG. When the molten steel supply rate is less than 2.5 ton / min, the energy required for swirling is insufficient and the stability of the swirling flow is deteriorated. As a result, good slab surface quality cannot be obtained.
On the other hand, if the molten steel supply rate exceeds 7.0 ton / min, the swirl force or discharge pressure is too strong, and the mold surface level tends to fluctuate locally. Defects are likely to occur.

The relationship between the Ar gas flow rate and the slab surface flaw occurrence rate when the swirl flow nozzle was used was the result shown in the graph of FIG. The results of a comparative test using a normal two-hole nozzle are also shown on the same graph.
When Ar gas is blown into the nozzle as a countermeasure against clogging of the swirling flow nozzle, if the flow rate of Ar gas into the nozzle exceeds 15 N liters / min, powder defects and bubble defects due to entrainment of mold powder As a result, the surface quality of the slab deteriorated.

  The relationship between the Ar gas flow rate when the swirl flow nozzle is used and the inclusion adhesion thickness of the nozzle inner tube portion is the result shown in the graph of FIG. The results of a comparative test using a normal two-hole nozzle are also shown on the same graph. When Ar gas was blown into the nozzle as a countermeasure against clogging of the swirling flow nozzle, nozzle clogging occurred when the Ar gas flow rate was less than 2 N liters / min.

FIG. 5 and FIG. 6 show a comparison of the proper Ar gas blowing amount between the normal two-hole nozzle and the swirl flow nozzle used in the present invention.
When a normal two-hole nozzle is used, clogging of the immersion nozzle can be prevented by flowing a small amount of Ar gas. However, at a flow rate of 2 to 3 N liters / min or more, powder defects or bubble defects are present on the surface of the slab. It can be seen that slab surface defects are likely to occur. When a normal two-hole nozzle is used, the flow of molten steel in the mold may be uneven, and by blowing Ar gas in that state, the flow of molten steel becomes more uneven, resulting in slab surface defects. This is likely to occur.

On the other hand, in the case of the swirling flow nozzle used in the method of the present invention, by setting the Ar gas flow rate to 2 N liter / min or more, it is possible to effectively prevent the clogging of the immersion nozzle and to make it 15 N liter / min or less. It turns out that said slab surface defect can be prevented.
Thus, in the case of the swirl flow nozzle used in the method of the present invention, the appropriate flow rate range of the Ar gas flow rate becomes wide. By using the swirl flow nozzle, the flow of the molten steel in the mold is made uniform, so that the Ar gas blowing flow rate can be increased.

  On the other hand, the molten steel flowing down in the swirling nozzle is branched into two flows by a twisted plate swirling blade installed in the nozzle inner pipe, and after twisting the flow, it joins at the lower end of the swirling blade. A swirling flow in which two high flow velocity portions are distributed in a double spiral shape is formed. In this swirl flow forming process, the molten steel flowing down the nozzle inner pipe portion is evenly distributed on both sides of the twisted plate swirl blade, whereby a more uniform swirl flow is formed and the slab quality is improved. Therefore, it is desirable that the molten steel flow that passes through the nozzle inner pipe portion spreads over the entire cross section of the nozzle inner pipe at the position directly above the swirl vane and is less biased.

The deviation of the molten steel flow relative to the nozzle inner tube cross-section is affected by the structure of the sliding gate.
In the case of a two-layer sliding gate, the flow rate is adjusted by sliding the lower plate, so the molten steel spreads from the sliding gate outlet to the entire upper inner pipe of the immersion nozzle in the shape of a fan. Reach directly above.
On the other hand, in the case of the three-layer type, the upper and lower plates are fixed, and the flow rate of the molten steel is adjusted by an intermediate plate. Therefore, the molten steel flows down from the sliding gate outlet directly below the inner pipe of the immersion nozzle. To do.
Therefore, since the sliding gate of the two-layer structure rather than the three-layer structure has reached a position directly above the twisted plate-type swirl blade, the molten steel spreads over the entire nozzle cross section, resulting in a uniform molten steel flow. A more stable swirl flow is formed, and the effect of the swirl flow nozzle is increased.

  Table 1 shows an example of the flow analysis simulation result of the molten steel flow in the submerged nozzle when two-layer and three-layer sliding gates are used. Mold shape: 1800 mm width x 250 mm thickness, casting speed: 1.2 m / min, immersion nozzle shape: inner diameter φ: 70 mm, discharge hole: 90 mm height x 55 mm width, discharge angle: upward flow 10 ° went.

In Table 1, “deviation” is the ratio of the molten steel discharge flow rate from the left and right discharge holes, and means that the uniformity of the cross section of the molten steel flowing down the nozzle cross section is impaired as the deviation increases. Defined by
(Definition formula for deviation)
Deviation (%) = (Molten steel discharge amount of left discharge hole-Molten steel discharge amount of right discharge hole) / [(Molten steel discharge amount of left discharge hole + Molten steel discharge amount of right discharge hole) / 2] × 100
As shown in Table 1, it can be seen from the flow analysis results that the deviation, which is the difference between the flow rates of the molten steel flowing out from the left and right discharge holes, is smaller in the two-layer type than in the three-layer type.

From the above knowledge, the present inventors have come up with an appropriate continuous casting method when applying a swirl flow nozzle. In order to further confirm the effect of the present invention, the present inventors conducted slab continuous casting tests (Examples and Comparative Examples) of ultra-low carbon steel ([C] ≦ 30 ppm) under various conditions as shown below. The discharge flow deviation (%), nozzle clogging evaluation, surface flaw occurrence rate (%), and surface flaw evaluation were investigated.
Table 2 shows the conditions and results of Examples and Comparative Examples of the present invention. The slab size is as shown in Table 2. In the case of a normal two-hole nozzle, the immersion nozzle shape was such that the nozzle inner diameter was 80 mm, the discharge hole width was 55 mm, the discharge hole height was 90 mm, and the discharge hole upward angle was 10 °. In the case of a swirling flow nozzle, a nozzle having an inner tube diameter of 100 mm at a position where a swirl vane made of refractory material is provided among the sizes of the two-hole nozzles was used. The shape of the swirl vane was a diameter D = 100 mm, a length L = 100 mm, a twist angle θ = 120 °, and a twist plate thickness T = 15 mm.

  Among the investigation items, the discharge flow deviation (%) was calculated by measuring the discharge amount of each of the left and right discharge holes and calculating the deviation as described above. In the nozzle clogging evaluation, the total amount of molten steel passing through the immersion nozzle of ultra-low carbon steel ([C] ≦ 30 ppm) was set to 460 tons, the inside of the nozzle after casting was observed, and the inclusion adhesion thickness of the nozzle inner tube was measured. : Maximum adhesion thickness ≦ 5 mm, Δ: Maximum adhesion thickness = 6 to 10 mm, ×: Maximum adhesion thickness ≧ 11 mm In addition, the surface flaw occurrence rate is the weight ratio of the coil being downgraded (downgraded) to the alumina sliver defect generated on the coil surface obtained by cold rolling the slab to 3.0 mm after hot rolling. is there. In the evaluation of surface flaws, ◎: surface flaw occurrence rate <0.5%, ○: 0.5% ≦ surface flaw occurrence rate <2.0%, Δ: 2.0% ≦ surface flaw Occurrence rate <5.0%, x: 5.0% ≦ surface flaw occurrence rate.

  In Examples F to J and L to N, the molten steel supply amount in the immersion nozzle is in the range of 2.5 to 7.0 ton / min, and the Ar gas flow rate into the nozzle is 2 to 15 N liters / min. As a result of adopting a sliding gate having a two-layer structure within the range, a uniform and necessary swirl flow was formed, and excellent slab surface quality was obtained. In Examples K and O, a sliding gate having a three-layer structure was adopted instead of the two-layer structure, but satisfactory slab surface quality was obtained.

In Examples F, G, and L, the mold size was the same, the Ar gas flow rate was the same at 2 N liter / min, and the molten steel supply amount Q was changed. The nozzle clogging evaluation, the slab surface flaw occurrence rate, and the evaluation thereof are improved in the order of Examples F, G, and L. The immersion Nords blockage situation is improved by reducing the casting time as the molten steel supply amount increases, and the surface flaw state also improves the cleaning effect of the solidified shell in the mold and the molten steel as the molten steel supply amount increases. The improvement effect of the meniscus temperature was improved.
In Examples H, I, and J, the mold size was the same, the molten steel supply amount Q was also the same at 4.0 ton / min, and the Ar gas flow rate was changed. Since the molten steel supply amount was the same, the clogging situation of the immersion nozzle was the same, and the smaller the Ar gas flow rate, the better the slab surface flaw.

In Examples G and K, only the difference between the two-layer type and the three-layer type sliding gate is different, and other conditions are the same. In Example G using the two-layer system, the slab surface flaw occurrence rate and the evaluation thereof were better.
In Example M, the Ar flow rate is the same as in Example F, the mold size is small, and the molten steel supply amount Q is small. Since the molten steel supply amount is small, the slab surface flaw occurrence rate is slightly worse.
Similarly, compared with Example H, Example N has the same Ar flow rate, a small mold size, and a small amount of molten steel supply Q. Since the molten steel supply amount is small, the slab surface flaw occurrence rate is slightly worse.

  On the other hand, Comparative Examples A to C are tests in the case of using a normal two-hole nozzle having two discharge holes facing the side wall near the bottom, and surface flaws occurred on the slab. In Comparative Example D, when the swirl flow nozzle was applied, the flow rate of the molten steel flowing down the nozzle was small, sufficient swirl strength could not be obtained, and nozzle blockage and slab surface flaws occurred. In Comparative Example E, the Ar gas blowing flow rate in the nozzle was excessive, and many surface defects were generated.

  Of course, the present invention is not limited to the above-described embodiments, and the effects of the configuration added to or deleted from the above-described embodiments can be added to or reduced from other embodiments. It doesn't matter.

It is a structure schematic diagram of the continuous casting apparatus containing a swirl flow nozzle. It is an expanded sectional view showing the outline of a two-layer structure sliding gate and a swirl flow nozzle. 3A is a perspective view of a torsion plate type swirl blade, FIG. 3B is a plan view of the torsion plate type swirl blade, and FIG. 3C is a side view of the torsion plate type swirl blade. . It is the graph which showed the influence of the molten steel supply amount which has on the slab surface defect. It is the graph which showed the influence of the Ar flow volume which has on the slab surface flaw. It is the graph which showed the influence of the Ar flow volume which acts on the inclusion adhesion thickness of the pipe | tube in a nozzle.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Continuous casting apparatus 2 ... Tandush 3 ... Upper nozzle 4 ... Two-layer structure sliding gate 4a ... Upper plate 4b ... Lower plate 4c ... Inert gas blowing nozzle 4d ... Channel inner wall 5 ... Swirling flow nozzle 5a ... Discharge hole 6 ... Mold 7 ... Twist plate type swirl blade

Claims (1)

Between the tundish and the mold, there is provided an immersion nozzle having two discharge holes opening facing the side wall near the bottom and having a twisted plate type swirl blade for swirling the molten steel flow inside. A continuous casting method in which molten steel in a dish is passed through the immersion nozzle and supplied into a mold, and an inert gas is supplied into the molten steel that passes through the immersion nozzle, and passes through the immersion nozzle. The molten steel supply amount Q to be set is 2.5 ton / min to 5.2 ton / min, the inert gas flow q supplied into the molten steel is set to 2 N liter / min to 15 N liter / min, and tundish In order to limit the supply amount Q of molten steel that passes through the immersion nozzle and is supplied into the mold to the above range, a two-layer structure sliding between the tundish and the immersion nozzle Continuous casting method of steel, characterized in that provided over bets.

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