JP2005297030A - Manufacturing method of multilayer slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To cast a multilayer slab by using one type of molten steel, one injection nozzle per mold and a static magnetic field brake installed in the mold, and supplying an element added to the surface layer from above the molten steel in the mold. Even when a relatively short injection nozzle is used and the position of the nozzle outlet is above the position of the static magnetic field generating coil, the difference between the slab surface layer and the internal component can be obtained stably.
A multi-layer casting in which an injection nozzle having a downward discharge port is used per mold, and an element added to the surface layer is supplied from above the molten steel in the mold using a static magnetic field brake installed in the mold. In the piece casting method, the position of the nozzle outlet is above the coil position for generating the static magnetic field, and the average flow velocity at the nozzle outlet and the magnitude of the magnetic field of the static magnetic field brake satisfy the following relationship: . 0.08 ≦ B / Vq ≦ 0.12
B: Magnetic flux density (T) in the center of the coil height direction (casting direction) of the static magnetic field brake, Vq: Average flow velocity (m / s) of the injection nozzle outlet
[Selection] Figure 1

Description

  The present invention relates to a method for producing a multilayer slab having different components inside and outside the surface layer of the slab. Conventionally, multilayer steel materials have been manufactured by superimposing and rolling two or three kinds of steel materials so as to be surface layer-inside-surface layer. However, in recent years, there has been a casting method for directly manufacturing multilayer slabs. Has been developed. The present invention relates to a method for directly producing the multilayer slab by casting.

  As a conventional technique, there has been proposed a method of pouring different types of molten metal with a static magnetic field in a mold as a boundary and a tip of a nozzle for supplying a molten metal below in a trumpet shape (see Patent Document 1). This method has a problem that the manufacturing cost is high because two types of molten steel and two nozzles are used. Moreover, it is necessary to make the nozzle exit which supplies the molten metal below become under a static magnetic field.

  In addition, instead of using two types of molten steel, one type of molten steel and one injection nozzle per mold are used, and a solute element from the continuous casting powder is added to the molten steel pool in the upper part of the mold partitioned by a static magnetic field in the mold. Is added to enrich the element in the surface of the slab (see Patent Document 2). However, in this case, there is no description about the position of the nozzle, and if estimated from the figure, the position of the injection nozzle is at the upper end of the static magnetic field coil, so a long nozzle is required.

  Furthermore, molten steel is injected into the lower pool among the molten steel pools divided by the DC magnetic field, and a predetermined solute element is added to the molten steel in the upper pool from a wire or continuous casting powder to enrich the element on the surface of the slab. A method is shown (see Patent Document 3). Also in this case, it is necessary to make the position of the nozzle below the DC magnetic field, and a long nozzle is required.

  In addition, many methods for producing multi-layer slabs by continuous casting have been proposed, but for the injection nozzle position, a relatively short injection nozzle is used and the nozzle outlet is set above the position of the static magnetic field or DC magnetic field. In such a case, there is no invention that mentions a method for stably obtaining a clear slab surface layer and an internal component difference.

JP-A-1-83347 JP 7-284879 A JP 7-290195 A

  The present invention uses one type of molten steel, one injection nozzle per mold and a static magnetic field brake installed in the mold, and casts a multilayer slab that supplies elements to be added to the surface layer from above the molten steel in the mold. In the method, a relatively short injection nozzle is used to provide a method for stably obtaining a clear slab surface layer and an internal component difference even when the nozzle outlet is positioned above the coil position for generating a static magnetic field. To do.

  By obtaining a clear component difference stably, a slab having a targeted surface layer and internal functions can be obtained, and the manufacturing cost can be reduced.

  In order to achieve the above object, the present invention is characterized by the following configurations.

Using one injection nozzle with a downward discharge port per mold, the molten steel is injected into the mold, and the element to be applied to the portion corresponding to the slab surface layer is supplied from above the molten steel in the mold and installed in the mold. In the method of casting a multilayer slab while suppressing component mixing using a static magnetic field brake, the nozzle is disposed so that the nozzle discharge port is above the position of the static magnetic field generating coil, and from the nozzle discharge port A method for producing a multilayer slab, characterized in that casting is performed such that the average discharge flow rate and the magnitude of the magnetic field of the static magnetic field brake have the following relationship:
0.08 ≦ B / Vq ≦ 0.12
B: Magnetic flux density (T) at the center of the coil height direction (casting direction) of the static magnetic field brake
Vq: Average flow velocity at the injection nozzle outlet (m / s)

  According to the present invention, casting of a multilayer slab using one type of molten steel, one injection nozzle per mold and a static magnetic field brake installed in the mold, and supplying an element to be added to the surface layer from above the molten steel in the mold In the method, even when a relatively short injection nozzle is used and the position of the nozzle outlet is above the coil position for generating a static magnetic field, it is possible to stably obtain a clear slab surface layer and an internal component difference. It becomes.

  As a method of stably obtaining a clear slab surface layer and an internal component difference even when a relatively short injection nozzle is used, the inventors adjusted the nozzle discharge port downward and simultaneously weakened the braking force. Therefore, it was devised to actively supply molten steel from the injection nozzle to the lower pool.

  Details of the present invention will be described below. The conventional idea when producing a multilayer slab by continuous casting is to divide the molten steel pool up and down by static magnetic field and direct current magnetic field in the mold so that the molten steel supplied from the injection nozzle enters the lower pool as much as possible. The solute elements supplied from the wire and continuous casting powder to the upper pool were prevented from entering the lower pool as much as possible.

  In this case, if the injection nozzle outlet for supplying the molten steel is placed above the static magnetic field or DC magnetic field, the effect of braking the molten steel flow velocity will be reduced, and the upper pool was stirred more than necessary and supplied from wire or continuous casting powder. As a result, the solute element may penetrate into the lower pool, and as a result, the concentration difference between the slab surface layer and the solute element in the slab may be obscured.

  However, the use of a long injection nozzle has some limitations and drawbacks. For example, the nozzle refractory cost increases, and depending on the continuous casting machine, a long nozzle cannot be inserted into the mold.

  Contrary to the conventional idea, the present inventors set the injection flow even if the injection nozzle outlet is above the brake band by reducing the braking force and installing the nozzle injection port so as to face downward. Inspired to pass the brake belt to reach the lower pool.

  Therefore, the present inventor produced a casting apparatus shown in FIG. 2 based on the above idea. An electromagnetic stirring device 3 is provided above the mold 4, and a static magnetic field brake 5 is provided below (the center part of the mold). The injection nozzle 1 for injecting molten steel is provided so that the outlet thereof is located above the static magnetic field brake 5.

  In the mold 4 configured as described above, when the braking force of the static magnetic field brake is weakened and molten steel is injected from the injection nozzle 1, the injection flow passes through the brake zone and reaches the lower pool. A multi-layer slab can be cast by forming a solidified shell (inside) 7 on a solidified shell (surface layer) 6 which is formed of a broken upper pool and increases in thickness as it moves downward.

  However, even in this case, if the braking force is too weak, the solute element supplied from the wire or continuous casting powder to the upper pool passes through the brake zone and mixes with the lower pool, or the flow of molten steel injected into the lower pool. May reverse and disturb the molten steel in the upper pool.

  Therefore, an appropriate condition for the braking force was obtained through experiments. Casting was performed using the steel shown in Table 1, and the molten steel flow rate from the nozzle discharge port and the braking force were changed to perform casting, and the state of the difference between the surface layer and the internal component of the obtained multilayer slab was investigated. The element enriched in the surface layer was P, and was supplied from continuous casting powder. It mixed with the continuous casting powder in the form of FeP, and the concentration was obtained from several test results.

With respect to the target value of the surface layer P concentration this time, the analysis result of the component whose FeP concentration in the continuous casting powder is 20% is shown in FIG. Here, the component separation degree is defined as follows, and a separation degree of 60% or more indicates that there is a clear component difference.
Component separation degree (%) = (5-10 mm inside and 30-40 mm inside from the surface layer of the element)
Concentration difference) x 100 / (Difference between the surface layer of the element and the internal target concentration)

From the figure, when the relationship between the average flow velocity Vq (m / s) at the injection nozzle outlet and the magnetic flux density (T) at the center of the coil height direction of the static magnetic field brake satisfies the following formula (1), It was found that the internal P concentration was clearly different. That is, it was found that a multilayer slab with a clear component separation can be obtained under these conditions.
0.08 ≦ B / Vq ≦ 0.12 (1)

  The reason why such a result was obtained is considered as follows. That is, when B / Vq is larger than 0.12, since the braking force is too strong, a part of the molten steel supplied from the injection nozzle rises because it cannot go to the static magnetic field brake lower pool, and the static magnetic field This is because the brake upper pool is disturbed to generate a non-uniform flow, and the concentration of the solute element supplied from the upper pool heavy wire or continuous casting powder is made non-uniform for each place in the mold.

  On the other hand, when B / Vq is smaller than 0.08, as described above, the braking force is too weak, and the solute element supplied from the wire or the continuous casting powder to the upper pool passes through the brake zone and passes through the lower pool. This is because the flow of the molten steel mixed into the lower pool or the flow of the molten steel injected into the lower pool is reversed to disturb the molten steel in the upper pool, and similarly, the concentration of the solute element is made nonuniform in each place in the mold.

  Next, the reason for defining the conditions of the invention and the specific application method of the invention will be described. The subject process is continuous casting and the purpose is the production of multilayer slabs.

  Among several multi-layer slab manufacturing methods, the present invention uses one injection nozzle per mold, uses a static magnetic field brake installed in the mold, and adds elements to the surface layer in the mold. It is limited to the casting method of the multilayer slab supplied from above the molten steel. This is because the method using two or more injection nozzles is expensive in cost.

  In the method using one injection nozzle per mold, the addition of a solute element enriched in the slab surface layer requires addition by a wire or continuous casting powder, so both methods were adopted. The static magnetic field installed in the mold is used because the static magnetic field type among electromagnetic brakes has a relatively uniform magnetic field distribution in the slab width direction.

  The reason for having a downward discharge port for the type of nozzle used is to allow the molten steel injected to be supplied to the lower pool without stirring the upper pool as much as possible. The reason why the position of the nozzle outlet is set above the position of the coil that generates the static magnetic field is to avoid the increase in the length of the nozzle as much as possible.

Regarding the average flow velocity at the nozzle outlet and the magnitude of the magnetic field of the static magnetic field brake, the following relationship was defined based on the results obtained in the actual machine test. That is, a multilayer slab with a clear component separation can be obtained only when the following conditions are satisfied.
0.08 ≦ B / Vq ≦ 0.12
B: Magnetic flux density (T) at the center of the coil height direction (casting direction) of the static magnetic field brake
Vq: Average flow velocity at the outlet of the injection nozzle (m / s)

  In the present invention, when the molten steel surface is stirred not only by the brake but also by electromagnetic stirring at the upper part of the mold, the solute element added by the continuous casting powder or the wire tends to become more uniform in the upper molten steel pool.

  Multi-layer slabs were produced using a continuous casting machine. The components of the slab are shown in Table 2. Table 3 shows the experimental conditions and evaluation methods common to each level. The experimental levels and results are shown in Table 4.

  From Table 4, when the conditions in the case of the present invention were satisfied, a multilayer slab having a clear separation of components between the slab surface layer and the inside could be obtained.

  On the other hand, in any of the comparative examples of comparative 1, comparative 2 and comparative 3, it was not possible to obtain a multilayer slab with a clear separation of the components on the slab surface layer and inside.

  That is, in Comparison 1, the brake was too strong to obtain an appropriate B / Vq, and the degree of separation decreased. Moreover, in Comparative 2, the brake was too weak to obtain an appropriate B / Vq, and the degree of separation decreased. Furthermore, in Comparative 3, since the casting speed was low and the discharge flow rate from the injection nozzle was small, an appropriate magnetic flux density was not obtained, so that an appropriate B / Vq could not be obtained and the degree of separation deteriorated.

  As described above, according to the present invention, it is possible to provide a multi-layer slab with a clear separation of components between the slab surface layer and the inside. Since the multilayer slab is used for various applications, the present invention has great industrial applicability.

It is a figure showing the relationship between parameter | index B / Vq and a component separation degree. It is a figure explaining the outline of a casting apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Injection nozzle 2 ... Continuous casting powder 3 ... Electromagnetic stirring apparatus 4 ... Mold 5 ... Static magnetic field brake 6 ... Solidification shell (surface layer)
7 ... Solidified shell (inside)

Claims (1)

Using one injection nozzle with a downward discharge port per mold, the molten steel is injected into the mold, and the element to be applied to the portion corresponding to the slab surface layer is supplied from above the molten steel in the mold and installed in the mold. In the method of casting a multilayer slab while suppressing component mixing using a static magnetic field brake, the nozzle is disposed so that the nozzle discharge port is above the position of the static magnetic field generating coil, and from the nozzle discharge port A method for producing a multilayer slab, characterized in that casting is performed such that the average discharge flow rate and the magnitude of the magnetic field of the static magnetic field brake have the following relationship:
0.08 ≦ B / Vq ≦ 0.12
B: Magnetic flux density (T) at the center of the coil height direction (casting direction) of the static magnetic field brake
Vq: Average flow velocity at the injection nozzle outlet (m / s)

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