JP3237177B2 - Continuous casting method - 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 for continuously solidifying a molten metal, in order to prevent segregation of trace elements in the center of the slab and obtain a homogeneous product. The present invention relates to a method for controlling the flow of molten steel remaining in steel.

[0002]

2. Description of the Related Art Generally, a slab obtained by a continuous casting method contains trace elements such as S, P and Mn, and these elements are concentrated at the center of the slab to form a region called center segregation. , Is a major cause of lowering product quality. It is said that this segregation phenomenon is caused by the residual molten steel in the slab at the end of solidification flowing due to solidification shrinkage of the slab or bulging (a phenomenon in which the shell between rolls expands due to the molten steel static pressure).

[0003] Therefore, in order to prevent this phenomenon, it is only necessary to reduce bulging between the rolls and further reduce the slab by an amount corresponding to the solidification shrinkage. JP-A-62-28056, JP-A-53-102225 and the like.

[0004]

However, reducing the unsolidified portion is extremely difficult for the following reasons. That is, as shown in FIG. 1, in a conventional slab type continuous casting machine, since a molten metal 13 is discharged into a water-cooled mold 12 from an immersion nozzle 11 usually called a two-hole nozzle, a large circulating downflow 14 15 are formed. Due to this downward flow, uniform cooling in the slab becomes impossible, and an unsolidified region having a fishtail-like tip (crater end) and a hairpin-like cross section as shown in FIG. 2 is formed.

FIG. 2 shows this unsolidified region at the same time as the slab sectional view. As is clear from this sectional view, since the unsolidified portion does not exist uniformly, it is impossible to perform uniform rolling. The cause of the formation of the non-uniform non-solidified region is caused by the flow of the molten metal, particularly the downward flow along both short sides of the mold.

[Means for Solving the Problems]

In order to solve the above-mentioned problem, it is only necessary to control the formation of an unsolidified region having a non-uniform shape as shown in FIG. If the downward flow of the discharge flow from the immersion nozzle can be controlled, the shape of the unsolidified region can be controlled.

The present invention is characterized in that an electromagnetic force is used as the downward flow control means. As means for applying the electromagnetic force, an electromagnetic brake using a DC magnetic field and a moving magnetic field using an AC are used.

[0008]

Next, the operation of the present invention will be described. A case where a DC magnetic field is used will be described with reference to FIG. The molten steel 2 supplied from the immersion nozzle 1 becomes a discharge flow 3 toward the short side of the mold. The eddy current 5 is induced by interaction with the static magnetic field 4 applied to this portion, and the current as shown here Form a loop. Due to the interaction between the induced current and the applied magnetic field, the Lorentz force 6
Acts to apply a braking force to the flow of molten steel.

As a result, as shown in FIG.
In the width direction and the thickness direction,
Control the flow with the flow rate distribution that is the fastest in the center
It becomes possible to do. And the solidified shell is this molten steel
Grows under the control of the flow. In other words, from the immersion nozzle
High temperature molten steel is supplied, and in the mainstream of the flow
It is difficult to cool compared to parts with a large amount of heat and little flow.
No. As a result, the solidified shell thickness near the main flow
It becomes thinner than the area. One using a conventional two-hole nozzle
In a general slab type continuous caster, as shown in FIG.
The main flow of the flow from the nozzle is a downward flow along the short side
Therefore, an unsolidified region as shown in the sectional view of FIG. 2 is formed.
It is. On the other hand, as shown in FIG.
Flow control is performed so that the flow goes down the center of the rub.
For example, reducing the thickness of the solidified shell in the center of the slab
Becomes possible. In addition, such control of the solidified shell thickness
Is the type of magnetic field, magnetic field strength and magnetic field for each casting condition.
Preliminarily study the effect of the applied area on the solidification shell thickness
By doing so, it can be easily implemented at the production site.

FIG. 5 shows an example of using a moving magnetic field by an alternating current. An electromagnetic force acts in the direction of the moving magnetic field, and it is possible to select between driving and braking for the flow. When a moving magnetic field is applied in the direction shown in FIG. 5, an unsolidified region similar to that shown in FIG. 4 can be formed. Hereinafter, the form of controlling the shape of the crater end will be further described with reference to FIGS.

In the slab type continuous caster, the flow pattern of the molten steel is as shown in FIG. The discharge flow from the nozzle forms a downward flow on the short side, and an upward flow at the center. An unsolidified region is formed according to this flow pattern. Therefore, the shape of the crater end can be controlled by changing the flow rate pattern of the molten steel.

Control method 1: Control of molten steel discharge flow from nozzle by electromagnetic brake As shown in FIG. 9, the flow pattern is controlled by applying a braking force to the molten steel flow discharged from the nozzle by a general electromagnetic brake. However, the shape of the crater end can be made convex downward.

Control Method 2: Uniform Flow by Electromagnetic Stirring As shown in FIG. 10, when general electromagnetic stirring is performed, the downward flow on the short side is made uniform, and the shape of the crater end is made uniform accordingly. You.

Control Method 3: Control of Molten Steel Discharge Flow from Nozzle Using Moving Magnetic Field As shown in FIGS. 11 and 12, a moving flow magnetic field is used to positively generate a center downflow. This method is most excellent in flexibility of flow control. For the above flow control
The present invention which performs reduction by controlling the crater end shape more
The method can be easily implemented. That is, as shown in the sectional view of FIG.
In the unsolidified region of the shape, the solidified shell formed in the center
Prevents the reduction of the unsolidified portion, whereas in FIG.
In a slab having an unsolidified region as shown in the cross-sectional view,
Because it can be performed smoothly, it is possible to reduce the amount according to the amount of coagulation shrinkage.
You. The supplementary explanation of the reduction according to coagulation shrinkage is given below.
I do. The residual molten steel in the unsolidified region at the end of solidification solidifies the slab
Unevenness due to flow due to shrinkage or bulging between rolls
The analysis occurs as described above. For example,
As disclosed in Japanese Unexamined Patent Publication No. 53-102225,
So that the slab becomes progressively thinner from the
According to the method of reducing the roll gap by reducing the roll gap,
As long as the lower volume corresponds to the coagulation shrinkage,
Can prevent the flow of residual molten steel and suppress segregation.
You. This technology uses a bloom that has a cross-sectional shape
It has been used in continuous casting machines for molds. In bloom
Is a fishtail like that found in continuous slab casting.
Crater end
I can do it. But in slabs, usually fish tail
Crater end
It is necessary to control the shape of the data end.

[0015]

FIG. 7 shows the results of an example in which continuous casting was carried out under the conditions shown in Table 1 in the apparatus shown in FIG. 6, the long sides L ₁ m, short L ₂ m flow of molten steel in a mold having a cross-section of Q ton
In the case of continuous casting at / min, a slab after solidification using a mold (a) not applying an electromagnet, a mold (b) applying a static magnetic field, and a mold (c) applying a moving magnetic field to molten steel. (A band at an equal distance from both short sides and shown in the upper right figure in FIG. 7) was measured for the amount of C segregation.

In FIGS. 6 (b) and 6 (c), a,
b is the effective (cut) area of the magnet for generating a magnetic field, and x and y represent the distance from the center line of the magnet to the center line of the immersion nozzle and the upper surface of the mold, respectively. The maximum magnetic flux density in the static magnetic field was 0.35 Tesla, and the maximum magnetic flux density in the moving magnetic field was 0.17 Tesla. These conditions are as shown in Table 1. Here is the slab
Slab cutting from the position where the average cross-sectional temperature becomes the solidification start temperature
Pressure until the surface average temperature reaches the solidification completion temperature
The lower roll applied the reduction in two stages. Setting of reduction amount
The values were 2.5 mm at the front and 2.0 mm at the rear.

[0017]

[Table 1]

As is apparent from the results shown in FIG. 7, the flow can be controlled by a DC magnetic field or an AC magnetic field to improve the shape of the unsolidified region, so that the rolling can be easily performed and the center segregation is greatly improved. It was possible to do. Incidentally, in case (a), the allowable reduction was 3.5 mm, but in cases (b) and (c), the reduction was 4.5 mm. In case (a),
As shown in Fig. 2, the solidified shell at the center of the slab
It was a hindrance, but it could only be reduced by 3.5 mm.
However, in cases (b) and (c), there is no hindrance of reduction, and
Thus, a 4.5 mm reduction was possible. This is,
Flow control reduces the thickness of the solidified shell at the center of the slab.
This is the effect of such control. It was also observed that the degree of segregation of the C content at the center was reduced from about 1.4 to 1 or less. Here, the degree of segregation is represented by the equation (1).

[0019]

## EQU1 ## (local density-average density) / average density

[0020]

As described above, in continuous casting of molten metal, the shape of the unsolidified region is controlled by electromagnetic force to enable uniform reduction of the unsolidified region, so that most segregation occurs at the center. It has become possible to obtain a high quality and homogeneous cast slab.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing a flow of molten steel in a conventional continuous casting machine.

FIG. 2 is a view showing a shape of an unsolidified region in a conventional continuous casting machine.

FIG. 3 is a diagram showing an interaction between a static magnetic field and molten steel metal.

FIG. 4 is a diagram showing the flow of molten metal controlled by electromagnetic force and the shape of an unsolidified region.

FIG. 5 is a diagram showing a situation of flow control of molten metal using a moving magnetic field by alternating current.

FIG. 6 is a diagram showing an apparatus used for the method of the present invention and the method of the comparative example.

FIG. 7 is a graph showing a relationship between a position in a thickness direction and a degree of segregation of carbon in a cast billet.

FIG. 8 is a diagram showing a flow pattern of molten steel in a continuous slab casting apparatus.

FIG. 9 is a diagram showing an aspect of control of molten steel discharge flow from a nozzle by an electromagnetic brake.

FIG. 10 is a diagram showing a mode of uniform flow by electromagnetic stirring.

FIG. 11 is a diagram showing a mode of controlling molten steel discharge flow from a nozzle by one moving magnetic field.

FIG. 12 is a diagram showing a mode of controlling molten steel discharge flow from a nozzle by a plurality of moving magnetic fields.

[Explanation of symbols]

 Reference Signs List 1 immersion nozzle 2 molten metal 3 discharge flow 4 static magnetic field 5 eddy current 6 Lorentz force

Continuation of front page (56) References JP-A-59-4943 (JP, A) JP-A-62-203648 (JP, A) JP-A-62-72458 (JP, A) JP-A-3-243260 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B22D 11/115 B22D 11/128 350 B22D 11/16 B22D 11/20

Claims (1)

(57) [Claims] In a continuous casting method of molten metal for continuously drawing a slab, a shape of a solidification line in the slab is controlled by controlling a flow of a molten metal continuously supplied into a mold by an electromagnetic force. A continuous casting method characterized by continuously reducing the unsolidified slab to prevent center segregation while controlling the thickness of the shell at the center of the slab to be small.