JPH08168850A - Method for starting continuous casting of duplex layer cast slab - Google Patents

Abstract

PURPOSE: To provide a starting method of a continuous casting, by which a duplex layer continuously cast slab having different component in the surface layer and inner layer is stably produced directly from molten steel in high yield. CONSTITUTION: A dummy bar head 12 having a partition plate 13 at the upper part is set in a mold 1 of a continuous caster for producing the duplex layer cast slab and the spouting hole of a nozzle 3 for inner layer is positioned at lower part than the partition plate to start the pouring of the molten steel. After the molten steel 5 for inner layer reaches the partition plate level, successively the pouring of the molten steel is started from an immersion nozzle 2 for surface layer at upper part than a DC magnetic field zone 6 while throttling the molten steel for inner layer. When the molten steel surface reaches a prescribed level position, the molten steels 4, 5 for surface/inner layers are made to be set flow rates, and the drawing-out of the dummy bar head 12 is started. By this method, the mixing length of the molten steels for surface/inner layers can be shortened, and therefore, the duplex layer cast slab having different components in the surface layer and the inner layer can stably be produced in the high yield.

Description

Detailed Description of the Invention [Industrial applications]

[0001] The present invention relates to a starting method for directly producing a multi-layered continuous cast slab having different surface layer and inner layer components from molten steel.

[0002]

2. Description of the Related Art The inventors of the present invention previously applied a DC magnetic field across a thickness of a slab at a position below a certain distance in the casting direction from a molten metal meniscus in a continuous casting mold, and the DC magnetic field band thereof was applied. The process of continuous casting of multi-layer slabs of which the surface layer and the inner layer are made of different kinds of metal by solidifying and drawing while supplying different kinds of molten metal to the upper pool and the lower pool The method and apparatus were invented and filed as Japanese Patent Application No. 61-252898.

This technique has made it possible to obtain a multi-layer cast product in which the metal of the upper pool is separated into the surface layer and the metal of the lower pool is separated into the inner layer and solidified.

[0004]

Problems to be Solved by the Invention This Japanese Patent Application No. 61-25
According to the technique proposed in No. 2898, it has become possible to obtain a multi-layer cast product composed of two kinds of molten steel compositions in which the surface layer and the inner layer are respectively injected. However, if proper casting is not started, molten steel on the surface / inner layer will be mixed in the early stage of casting, casting will become steady, and even if the quality of the slab is stabilized, the separation of the surface / inner layer will not be stable, resulting in a longer cut-off portion. However, there is a problem in that the yield of cast slabs deteriorates.

For example, as shown in the process diagram of FIG. 1, the nozzle 2 for injecting the surface molten steel and the nozzle 3 for injecting the inner layer molten steel into the mold 1 are set at substantially the same height, and the surface molten steel 4 is set.
While starting pouring first, while pouring the inner layer molten steel 5 immediately before the molten metal surface reaches the discharge hole of the inner layer molten steel injecting dipping nozzle 3, the dummy bar head 12 then starts drawing and at the same time dipping for injecting the inner layer molten steel. While lowering the nozzle 3 at a constant speed, the injection flow rate of the surface / inner layer molten steel is controlled to a predetermined value.

According to this method, the inner layer molten steel 5 flowing out when the discharge hole of the immersion nozzle 3 for injecting the inner layer molten steel is inside the surface layer pool 4 is mixed with the surface layer molten steel as shown in FIG. 1 (c). I will end up.

Further, since this mixed molten steel is only consumed by the solidified shell, it is inevitable that a certain time, that is, a certain casting length is required to completely separate the surface / inner layer molten steel.

Further, as in Japanese Patent Application No. 63-100551, there is disclosed a method of completely suppressing mixing in the initial stage of casting by using a box type dummy bar head 12 (FIG. 2), but the shape of the dummy bar head is devised. Was required, and a casting start method that was as simple as possible was required from the viewpoint of operability.

In view of the above problems, the present invention provides a method for starting continuous casting, which is capable of stably producing a multi-layer slab with a high yield when producing a slab having different surface layer and inner layer components.

[0010]

DISCLOSURE OF THE INVENTION The present invention provides a substantially uniform slab across the entire width of the slab so as to traverse the thickness of the slab at a position a certain distance below the molten steel meniscus in the continuous casting mold in the casting direction. By applying a DC magnetic field of magnetic flux density and continuously casting molten steel with different components in the upper and lower molten steel pools divided by this DC magnetic field band, the surface layer and the inner layer are composed of molten steel with different components In the continuous casting start method for producing a multi-layer cast product, a dummy bar head having a partition plate at the upper portion is set in the mold so that the partition plate is located below the DC magnetic field band and the partition plate is in the DC magnetic field band, First, start pouring from the inner layer immersion nozzle installed so that the discharge holes are located below the partition plate, and if the inner layer molten steel level reaches the partition plate level,
After that, while squeezing the molten steel in the inner layer, pouring was started from the surface layer dipping nozzle installed so that the discharge hole was located above the DC magnetic field band, and when the molten metal surface reached the prescribed level position, the dummy bar head This is a continuous casting start method for a multi-layer cast product, characterized in that, at the same time when drawing is started, injection is continued with a set flow rate of the surface / inner layer molten steel.

The inventors of the present invention provided a dummy bar head having a partition plate on the upper side below the DC magnetic field band, and set the dummy bar head in the mold so that the partition plate was in the DC magnetic field band, and then the partition plate. Start pouring from the immersion nozzle that injects the molten steel with the inner layer composition set lower than the above, pour only the inner layer molten steel to the partition plate, and then pour the inner layer molten steel by squeezing It was found that the mixing part at the initial stage of casting can be minimized by starting the molten metal, and after the level of the molten metal reaches the position of the drawing start, by starting the drawing and at the same time injecting the surface / inner layer molten steel at the set flow rate. It was

[0012]

Function: The DC magnetic flux that crosses the slab in the thickness direction over the width direction of the slab is divided by the magnetic field band extending over the entire width direction of the slab. When molten steel with a different composition is supplied to each position in a predetermined ratio, the molten steel stays in the part where a DC magnetic field with a certain magnetic flux density or more acts, and in the other upper and lower pools due to the molten steel flow. A region where each molten steel component is uniform is formed.

When continuous casting is carried out while maintaining this state, a multi-layer cast product in which the surface layer and the inner layer have respective molten steel compositions is produced. To describe this situation in more detail, the state in the pool is as shown in FIG.

That is, when a DC magnetic field having a magnetic flux density B having a magnetic flux density distribution 11 of a certain value or more is applied substantially uniformly over the entire width of the slab, the upper and lower portions of the region retained by the DC magnetic field respectively Although the components of the molten steels 4 and 5 injected into the region are maintained, the region sandwiched between these uniform concentration regions, that is, the retention region, is a region where two types of molten steels 4 and 5 diffuse and mix with each other. Exists and a concentration gradient is formed.

The component concentration distribution in the casting direction in this pool is as shown in FIG. 3 (b). When continuous casting is performed while maintaining such a pool structure, the resulting cast piece is as shown in FIG. 3 (c). That is, the surface layer 7
There is a boundary layer 8 having a concentration gradient between the inner layer 9 and the inner layer 9, but since the thickness of this boundary layer is generally sufficiently thinner than the surface layer, a multilayer cast product in which the surface layer and the inner layer are apparently separated is produced. You can do it.

However, in order to stably cast such a multi-layer slab from the early stage of casting, such a strand pool, that is, a state in which the surface layer molten steel 4 and the inner layer molten steel 5 are vertically separated by a DC magnetic field is required. It must be formed from the beginning of casting. For that purpose, a partition plate may be provided between the surface molten steel and the inner molten steel. As a casting start method provided with a partition plate, a method as shown in FIG. 4 may be used.

That is, as shown in FIG. 4, the discharge hole of the immersion nozzle 3 for injecting the inner layer molten steel 5 is provided below the partition plate 13, and only the inner layer molten steel 5 is injected first. The injection is continued for a while as it is, and when the inner layer molten steel level reaches the height of the partition plate 13, the injection of the surface layer molten steel 4 is started.

At this time, since the discharge hole of the inner-layer immersion nozzle 3 is in the form of being immersed in the molten steel, the injection is continued without stopping the injection once. After that, when the molten metal level reaches a predetermined drawing start position, drawing is started by the dummy bar head 12 and, at the same time, the front and inner molten steels 4, 5 are melted.
Constantly control the pouring amount of. According to this method, it is possible to create a state in which the surface layer molten steel and the inner layer molten steel are vertically separated by the DC magnetic field from the initial stage of casting.

[0019]

EXAMPLE In a continuous casting process in which a DC magnetic field having a uniform magnetic flux density in the width direction can be applied 0.45 m below the meniscus, a width of 1.5 m and a thickness of 0.
Using a 25 m mold, a molten steel of the type shown in Table 1 was injected into the upper pool and the lower pool, which were sectioned by the DC magnetic field zone, at a casting speed of 0.6 m / min, while casting a multi-layer cast piece.

[0020]

[Table 1]

The magnetic flux density of the added DC magnetic field is 0.53.
T. Solidified shell thickness d (m) in this continuous casting machine
It has been found that the growth rate of No. 1 is given by the following formula (1), and it can be seen from this that the thickness of the surface layer is 20 mm.

[0022]

## EQU1 ## d = 0.0237 × (L / Vc) ^0.637 (1) where Vc is the casting speed (m / min) L is the distance (m) from the meniscus.

FIG. 5 shows the Cr concentration distribution from the surface of the cast slab obtained by the method of the present invention. A layer 20 mm from the surface is a component of stainless steel, and the inside thereof is It can be seen that the composition is that of medium carbon steel.

Further, as a result of starting the casting by the casting method described in detail in the section of the above-mentioned action, as shown in FIG.
What was there could be shortened to less than 2m.

[0025]

As described above, according to the present invention, a dummy bar head having a partition plate on the upper side is set in a mold, and the discharge holes of the inner layer nozzles are positioned below the partition plate for pouring. Once the inner layer molten steel reaches the partition plate level, pouring is started from the surface layer dipping nozzle above the DC magnetic field band while squeezing the inner layer molten steel, and the molten metal surface reaches the predetermined level position. In that case, the dummy bar head is pulled out and started with the set flow rate of the surface / inner layer molten steel, and it is possible to create a state where the surface / inner layer molten steel is vertically separated by the partition plate and the DC magnetic field. It is possible to shorten the mixing length of the surface / inner layer molten steel in the initial stage, and thus it is possible to stably produce a multi-layer cast product having different components in the surface layer and the inner layer with a high yield.

[Brief description of drawings]

FIG. 1A to FIG. 1D are schematic side views showing an example of a conventional procedure for starting continuous casting.

FIG. 2 is a schematic side view showing an example of a conventional box-type dummy bar head.

FIG. 3 is a drawing showing formation of surface layers, inner layers and boundary layers (a), component distribution in a pool (b) and component distribution (c) of a cast multi-layer cast product when the present invention is carried out.

4 (a) to (d) are schematic side views showing an example of a procedure for starting continuous casting according to the present invention.

FIG. 5 is a drawing showing an example of the Cr concentration distribution from the surface of the cast slab obtained by the method of the present invention.

FIG. 6 is a drawing comparing the mixing length of the surface / inner layer molten steel in the early stage of casting between the conventional example and the present invention example.

[Explanation of symbols] 1 mold 2 surface layer immersion nozzle 3 inner layer immersion nozzle 4 surface molten steel 5 inner layer molten steel 6 direct current magnetic field band 7 serving as boundary layer 8 surface layer 8 boundary layer 9 inner layer 10 direct current magnetic field generator 11 magnetic flux density distribution 12 dummy bar head 13 Partition plate B Magnetic flux density B _C Minimum magnetic flux density required to retain molten steel C Solute concentration C _A Surface layer solute concentration C _B Inner layer solute concentration Z Casting direction Z ₀ Molten steel meniscus level Z ₁ Upper limit of molten steel retention area Z ₂ Molten steel retention Lower limit of area d Cast thickness direction d ₀ Cast surface d ₁ Surface layer / boundary layer interface position d ₂ Boundary layer / inner layer interface position d ₃ Inner layer / boundary layer interface position d ₄ Boundary layer / surface layer interface position d ₅ Slab surface

Claims (1)

[Claims] 1. A DC magnetic field having a substantially uniform magnetic flux density is applied across the entire width of the slab so as to traverse the thickness of the slab at a position below a certain distance in the casting direction from the molten steel meniscus in the continuous casting mold. , Continuous production by holding molten steel with different components in the upper and lower molten steel pools divided by this DC magnetic field zone and continuously casting, to produce a multi-layer slab composed of molten steel with different surface and inner layers In the casting start method, a dummy bar head having a partition plate on the upper side is set in the mold at a position lower than the DC magnetic field band and the partition plate is in the DC magnetic field band, and first discharged below the partition plate. When pouring is started from the inner layer immersion nozzle installed so that the holes are located, and when the inner layer molten steel level reaches the partition plate level, the inner layer molten steel is then squeezed and then the DC magnetic field band is applied. Pouring is started from the surface layer immersion nozzle installed so that the discharge hole is located above it, and when the molten metal surface reaches a predetermined level position, the dummy bar head is pulled out, and at the same time the surface / inner layer molten steel is melted. A continuous casting start method for a multi-layer cast slab, characterized in that the injection is continued at a set flow rate.