JP2002178109A - Method for producing continuously cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent variations of the thickness of surface layer in a cast slab and solute element concentration and to improve the yield of a product when double-layer cast slab is produced. SOLUTION: In an impressing state of a DC magnetic field over the whole width of the cast slab in the direction crossing the thickness of the cast slab with magnetic poles at a position in the lower part having a fixed distance in the casting direction from molten metal surface in a continuous casting mold, in the lower part of these magnetic poles, only the base molten steel is supplied and on the other hand, in the upper part of these magnetic poles, the base molten steel together with a specified element are supplied. In this way, when the double-layer cast slab concentrated with the specified element is produced, the supplying speed of the molten steel to the upper part of the magnetic poles, is made to 0.2<1.0 times of the speed consuming the molten steel with this solidification at the upper part from the upper end of the magnetic poles.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a continuous cast slab, and more particularly to a method for stably producing a multilayer slab having a high concentration of a specific element in a surface layer of the slab. is there.

[0002]

2. Description of the Related Art Conventionally, various methods have been proposed for producing a multilayer cast slab in which the component composition differs between the surface layer portion and the inside by continuous casting. For example, Japanese Patent Publication No. 3-20295 discloses that a direct magnetic flux is applied over the entire width of a slab in a direction perpendicular to the casting direction at a position below a fixed distance from a molten metal level in a continuous casting mold. A method is described in which two different types of molten metal are supplied above and below a static magnetic field zone formed by a magnetic flux as a boundary.

However, the method disclosed in Japanese Patent Publication No. 3-20295 described above separates the molten steel for the surface layer and the molten steel for the inside of the cast slab separately, and further molds them through separate tundishes and immersion nozzles. Because it requires an extremely complicated process of supplying the product inside, it is easy to cause production failure,
There is also a problem that the cost is increased.

In this respect, the method described in Japanese Patent Application Laid-Open No. Hei 8-257692 discloses a method in which a DC magnetic field is applied over the entire width of a mold below a certain distance from a meniscus to form a braking region, and nozzle discharge holes are formed at the upper and lower portions. By injecting molten steel of a constant composition using the immersion nozzle having, and further stirring the molten steel by the molten steel injection flow while continuously supplying the alloy element to the molten steel pool above the braking zone using a wire, This is a method for producing a slab having a uniform alloy element concentration, and the process is simple, so that the above-described problem does not occur.

[0005] However, in the above method, the distribution of molten steel to the upper and lower portions of the magnetic field zone must be performed in the immersion nozzle. In the continuous casting process of molten steel, non-metallic inclusions in the discharge hole of the immersion nozzle are required. Inevitably, phenomena such as adhesion and detachment of the steel or erosion of the nozzle discharge hole occur, so that there is a problem that the distribution ratio of the molten steel to the upper part and the lower part fluctuates during casting. Since such fluctuations cannot be detected during casting, if the supply ratio of molten steel to the upper part relative to the lower part of the magnetic field zone increases during casting, even if the fluctuation is slight, the boundary between the upper and lower molten steels will be lower. Not only does the slab surface move to increase the thickness of the slab surface layer, but also the product quality varies because the solute element concentration in the surface layer decreases. In particular, when this variation is large, the boundary deviates from the magnetic field zone and the upper molten steel flows downward, resulting in a problem that the product yield is significantly reduced.

[0006]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and not only can effectively prevent fluctuations in the thickness of the slab surface layer and the concentration of solute elements, but also significantly increase the product yield. It is an object of the present invention to propose a method for stably producing a multilayer slab which can be improved.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and have obtained the following findings. 1) Distribution of solute concentration in the mold when the supply ratio of molten steel to the upper part of the magnetic field zone is reduced The inventors investigated the vertical flow rate ratio by recovering the immersion nozzle during casting, and examined the slab cast just before the recovery. The following findings were obtained by conducting the survey. If the molten steel supply rate to the upper part of the magnetic field zone is higher than the consumption rate of molten steel consumed by solidification above the upper end of the magnetic pole and smaller than the consumption rate of molten steel consumed by solidification above the lower end of the magnetic pole, Boundary exists between the upper and lower ends of the magnetic pole. Between the upper and lower ends of this magnetic pole, the flow of the molten steel is significantly suppressed by the braking by the magnetic field, so that the upper and lower molten steels do not mix, and the boundary moves up and down according to a slight change in the molten steel feed rate ratio . On the other hand, when the molten steel supply rate to the upper part of the magnetic field zone is smaller than the consumption rate of the molten steel consumed by solidification above the upper end of the magnetic pole, in the region above the upper end of the magnetic pole between the upper end and the lower end of the magnetic pole. To compensate for the supply shortage of molten steel, a gentle upward flow rectified by the magnetic field is generated. Further, in a region above the upper end of the magnetic pole, the magnetic field sharply decreases and the mixing of the molten steel proceeds, so that the boundary between the upper and lower molten steels becomes constant at the upper end of the magnetic pole. In other words, by lowering the molten steel supply rate to the upper part of the magnetic field zone from its consumption rate, the magnetic field zone can be made into a rectification region having an upward flow over the whole, and as a result, the boundary between the upper and lower molten steels becomes the upper end of the magnetic pole. It becomes possible to fix to the part.

2) Fluctuation of molten steel distribution ratio As described above, the distribution ratio of molten steel to the upper and lower portions of the magnetic field zone changes due to the attachment or detachment of nonmetallic inclusions to the nozzle discharge holes or the erosion of the nozzle discharge holes. At this time, the change in the molten steel supply rate to the upper part of the magnetic field zone is approximately 20% of the designed supply ratio.
While repeating the change of about%, the average value gradually increases or decreases. The increase or decrease in the average value varies depending on the type of steel to be cast and the casting time. Note that when distributing and supplying molten steel using an immersion nozzle having a plurality of discharge holes, the supply ratio is determined by the cross-sectional area ratio of the discharge holes, the shape of the discharge holes, and the like. It can be determined by actual measurement of the flow rate ratio by the following method.

The present invention has been developed based on the above findings, and the gist configuration thereof is as follows. That is, in the present invention, at the time of continuous casting of molten metal, a DC magnetic field over the entire width of the slab in a direction crossing the thickness of the slab by the magnetic pole, at a position below a certain distance in the casting direction from the molten metal level in the continuous casting mold. In the applied state, the specific element is concentrated on the surface layer of the slab by supplying only the mother molten steel to the lower part of the magnetic pole and supplying the mother molten steel and the specific element together to the upper part of the magnetic pole. In a continuous casting method for producing a multilayered cast slab, the feed rate of molten steel to the upper part of the magnetic pole is 0.2 times or more and less than 1.0 times the rate at which molten steel is consumed by solidification above the upper end of the magnetic pole. This is a method for producing a continuous cast slab.

In the present invention, the supply rate of molten steel into the continuous casting mold is 0.3 to 0.8 times the consumption rate of molten steel consumed by solidification above the top of the magnetic pole. It is preferable to use an immersion nozzle having two upper and lower discharge holes designed to be as follows.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings. FIG. 1 schematically shows an example of an injection procedure of molten steel according to the present invention. In the figure, reference numeral 1 denotes a mold, 2 denotes an immersion nozzle, and 3 denotes a magnetic pole (for example, an electromagnet). The magnetic pole 3 enables a DC magnetic field to be applied in the thickness direction of the slab over the entire width of the slab. Reference numeral 4 denotes a boundary between the upper molten steel (the molten steel in which the solute elements are concentrated) and the lower molten steel (the mother molten steel). The boundary 4 coincides with the upper end of the magnetic pole 3. further,
Reference numeral 5 denotes a lower discharge hole of the immersion nozzle 2, and 6 denotes an upper discharge hole. Numerals 7 and 8 denote injection flows from the lower discharge hole and the upper discharge hole. 9 is a solute element (wire), 10 is a solidified shell, and 11 shows the upward flow of molten steel from the lower part to the upper part of the DC magnetic field zone.

In the present invention, the supply speed of molten steel to the upper part of the magnetic pole is set to be less than 1.0 times the consumption rate of molten steel consumed by solidification above the upper end of the magnetic pole. Therefore, the thickness of the surface layer of the slab having a high concentration of the added solute element is also constant. Also,
Since the molten steel that is insufficient for the solidification amount is supplied from the lower part to the upper part through the magnetic pole part, the ratio of the base molten steel supplied to the upper part and the specific element is always constant, and therefore the solute element concentration in the surface layer of the slab also It will be constant. However, if the supply of molten steel to the upper part becomes too small, the inflow from the lower part, which flows in slowly, on the contrary, increases, the stirring effect in the upper part decreases, and the concentration of the surface layer of the slab becomes non-uniform Therefore, the supply speed of molten steel to the upper part of the magnetic pole needs to be 0.2 times or more the consumption rate of molten steel consumed by solidification above the upper end of the magnetic pole. It is preferably at least 0.3 times.

When the supply of molten steel to the upper and lower portions of the mold as described above is performed using a single immersion nozzle, the supply speed of the molten steel to the upper portion of the magnetic pole is increased by solidification above the upper end of the magnetic pole. It is preferable to use an immersion nozzle having two upper and lower discharge holes designed so that the consumption speed of the molten steel to be used is 0.3 times or more and 0.8 times or less. This is based on the finding that the variation of the molten steel supply ratio to the upper and lower portions is about 20%, as described above. When such a nozzle is used, the variation of the molten steel supply ratio occurs. In addition, the thickness and the solute element concentration distribution of the surface layer of the slab can be made constant.

For a long-term increase or decrease in the supply ratio, a nozzle having a small supply ratio to the upper part is used in the case of casting of an increasing steel type, while a nozzle to the upper part is used in casting of a decreasing steel type. By using a nozzle having a relatively high ratio in advance, continuous casting can be performed for a long period of time.

For comparison, FIG. 2 shows a procedure for supplying molten steel according to the method described in Japanese Patent Application Laid-Open No. 8-257692. In this method, it is possible to fix the boundary position between the upper and lower molten steels in the magnetic field zone by making the supply speed of the molten steel to the upper part of the magnetic pole equal to and constant to the speed consumed by solidification at the upper part of the magnetic pole. . However, as described above, the ratio of the supply rates of molten steel to the upper and lower sides inevitably fluctuates during casting, so even if the supply rate to the upper part is slightly increased, the boundary between the upper and lower molten steels moves downward. This causes an increase in the surface layer thickness and a decrease in the solute element concentration. In particular, when this variation is large, the boundary deviates from the magnetic field zone and the solute element of the upper molten steel flows downward, which significantly lowers the product yield.

[0016]

EXAMPLE A continuous cast slab was produced using the continuous casting mold shown in FIG. 1 under the following conditions (an example of adaptation of the present invention).・ Inner diameter of the mold Long side: 1.2 m Short side: 0.26 m Height: 1.0 m ・ DC magnetic field application position (distance from the level of the mold surface to the center of the magnetic pole height): 0.6 m Magnetic pole height: 0.2 m Distance from mold level in mold to top of magnetic pole: 0.5 m Strength of applied magnetic field: 0.3 T ・ Immersion nozzle Upper discharge hole: 2 holes, discharge angle θ = 0 ° (horizontal direction), hole size 16 × 31 mm □ Lower discharge hole: 2 holes, discharge angle θ = 60 ° (downward), hole size 77 mmφ ・ Distance h from lower discharge hole to center of height of magnetic pole: h: -0.3
m ・ Distance h 'from the upper discharge hole to the center of height of the magnetic pole: 0.2
m ・ Solute element (pure Ni wire) Supply position of pure Ni wire: hot surface ・ Casting speed Vc: 1.6 m / min

It is known that the growth thickness d (m) of the solidified shell in the above continuous caster is given by the following equation. d = 0.022 × (L / Vc) ^0.5 where L is the distance (m) from the level of the molten metal to the top of the magnetic pole, and Vc is the casting speed (m / min). Therefore,
From the above formula, the solidified shell thickness at the upper and lower pool boundary is 12
It turns out that it is about mm. As a result, the consumption rate Q of the molten steel solidified above the upper end of the magnetic pole is 0.39 t / min.
On the other hand, the molten steel supply rate Q ′ from the upper discharge port is known to be 6.7% of the total throughput from a water model or the like, and Q ′ = 0.234 t / min. Therefore, the supply speed Q 'of molten steel from the upper discharge hole is Q' = 0.6 Q (0.6 times the consumption speed of molten steel solidified above the upper end of the magnetic pole).

For comparison, continuous cast slabs were also produced by the conventional molten steel supply procedure shown in FIG. 2 (the method disclosed in Japanese Patent Application Laid-Open No. Hei 8-257692). At that time, the casting conditions were as follows: ・ Immersion nozzle Upper discharge hole: 2 holes, discharge angle θ = 0 ° (horizontal direction), hole size 26 × 32 mm □ Lower discharge hole: 2 holes, discharge angle θ = 60 ° ( (Downward), the hole size was changed to 75 mmφ. As a result, the molten steel supply speed Q ′ from the upper discharge hole Q ′: Q ′ = Q, and the distance h from the lower discharge hole to the center of the height of the magnetic pole h: −0.1
m (0.1 m below the center of the magnetic pole). Other conditions were the same as those of the applicable example of the present invention.

The slabs of the conforming example and the comparative example of the present invention are compared, and the thickness and concentration of the slab surface layer are varied (standard deviation).
FIGS. 3 and 4 show the results of an investigation on the above, and FIG. 5 shows the results of an investigation on the incidence of product defects due to an increase in the concentration inside the slab. As shown in FIGS. 3 to 5, it can be understood that the variation in the surface layer thickness and the surface layer concentration is remarkably reduced and the occurrence rate of defective products is significantly reduced in the conforming example as compared with the comparative example. In addition, it can be seen that the incidence of defects inside the slab due to the inclusion of inclusions is reduced by half.

[0020]

As described above, according to the present invention, the boundary between the upper and lower molten steels in the mold does not change, and the supply speed of the mother molten steel supplied to the upper region becomes constant. Multilayer cast slabs with extremely small solute element variations can be stably manufactured, and furthermore, the outflow of solute elements below the mold can be suppressed, so that the product yield can be significantly improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of a procedure for pouring molten steel according to the present invention.

FIG. 2 is a schematic diagram showing an example of a procedure for injecting molten steel according to a comparative example.

FIG. 3 is a diagram showing a comparison of a variation (standard deviation) in the thickness of a slab surface layer in an example of the present invention and a comparative example.

FIG. 4 is a view showing a comparison (variation (standard deviation)) of the Ni concentration in the surface layer of the cast slab in an example of the present invention and a comparative example.

FIG. 5 is a diagram showing a comparison between the examples of the present invention and a comparative example in terms of the rate of occurrence of product defects due to an increase in the Ni concentration in the slab.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 mold 2 immersion nozzle 3 magnetic pole 4, boundary between upper molten steel and lower molten steel 5 lower discharge hole 6 upper discharge hole 7 injection flow from lower discharge hole 8 injection flow from upper discharge hole 9 solute element (wire) 10 Solidification shell 11 Upflow of molten steel

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B22D 11/108 B22D 11/108 D 11/11 11/11 D (72) Inventor Hiromitsu Shibata Chiba, Chiba 1F Kawasaki-cho, Chuo-ku F-term in Kawasaki Steel Engineering Laboratory (reference) 4E004 AA09 FB06 FB10 MB11 MB14

Claims (2)

[Claims] In a continuous casting of a molten metal, a DC magnetic field is applied across the entire width of a slab by a magnetic pole at a position below a level in a casting direction by a predetermined distance from a molten metal level in a continuous casting mold. In this state, the specific element was concentrated on the surface layer of the slab by supplying only the mother molten steel to the lower part of the magnetic pole and supplying the mother molten steel and the specific element together to the upper part of the magnetic pole. A continuous casting method for producing a multilayer slab, wherein a supply speed of molten steel to an upper portion of the magnetic pole is set to be 0.2 times or more and less than 1.0 times a speed at which molten steel is consumed by solidification above an upper end of the magnetic pole. A method for producing continuous cast slabs. 2. The method according to claim 1, wherein the supply rate of the molten steel into the continuous casting mold is set at a rate of 0.3% of a rate at which the molten steel is consumed by solidification above the upper end of the magnetic pole.
A method for producing a continuous cast slab, wherein the method is performed using an immersion nozzle having two upper and lower discharge holes, which is designed to be at least twice and at most 0.8 times.