JPS5937713B2 - Stirring method for unsolidified molten metal in continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for stirring unsolidified molten metal in continuous casting, in which the unsolidified molten metal is stirred and solidified using the magnetic field of a permanent magnet and the thrust generated by an electric current passing through the slab.

The center of the slab produced by continuous casting contains carbon, 2
Segregated areas enriched with phosphorus, phosphorus, etc. are likely to occur.

The macrostructure of this segregated area exhibits a color tone different from that of the normal area, and the mechanical properties and commercial value of products made from this slab are significantly reduced.

It is known that the above-mentioned center segregation can be reduced by producing a large number of equiaxed crystals in the center of the slab, and a proposed method for this is to stir the unsolidified molten metal inside the slab during solidification of the slab. has been done.

Conventional methods for stirring unsolidified molten metal include applying a rotating magnetic field or a moving magnetic field to a continuously cast slab during solidification and applying a thrust in the moving direction of the magnetic field to the unsolidified portion inside the slab. There is a method in which a stationary magnetic field is applied to the unsolidified portion inside the piece, and a direct current is passed through the unsolidified molten metal, and the interaction between this current and the magnetic field applies a thrust to the unsolidified molten metal to stir it.

An example of the latter method is the method shown in FIG.

That is, between the mold part and the part where the slab is completely solidified, there is a permanent magnet 2 with the N pole or S pole facing each other in proximity to one surface of the slab, and a permanent magnet 2 with the polarity as described above in proximity to the other surface. Permanent magnets 2 with S or N poles facing oppositely to the magnets are installed, and a static magnetic field 5 is applied to the inside of the slab so that the main direction of the magnetic lines of force is perpendicular to the drawing direction of the slab 4. At the same time, by passing a direct current in the part of the slab where this static magnetic field is acting so that the main direction of the current is parallel to the direction in which the slab is pulled out, an electromagnetic force is generated in the unsolidified part in the width direction of the slab. There is a method of stirring unsolidified molten metal by applying force F.

The slab 4 produced by normal continuous casting without electromagnetic stirring includes:
As shown in Fig. 2a, center segregation 6 is observed, but the slab 4 subjected to the electromagnetic stirring has (
No central segregation was observed.

However, due to this electromagnetic stirring, a new negative segregation band, the so-called white band T, is generated.

This white band 1 generally has a narrow width on the long side of the slab.
The short side appears wide, and the white band γ clearly remains in the rolled product as shown in Figure 3a, which not only significantly reduces the commercial value but also causes variations in mechanical strength and bath weldability. will occur.

On the other hand, even if the narrow white band on the long side remains, it is not noticeable as shown in Figure 3, so it does not affect the product value much.

Therefore, in the case of electromagnetic stirring, it is desirable to use a stirring method that narrows the wide white band on the short side.

Regarding this point, the inventor conducted various studies and found that in order to obtain a sufficient stirring effect by narrowing the wide white band on the short side and making it uniform and indistinct over the entire slab,
The direction of the current flowing through the slab can be changed for a short period of time, in the forward direction and in the reverse direction (
We have confirmed that it is effective to switch alternately.

Based on this knowledge, the present invention proposes an electromagnetic stirring method in which the unsolidified portion is stirred and solidified while passing a low-frequency alternating current. It is possible to obtain slabs that appear uniform and indistinct.

Next, embodiments of the invention will be described with reference to the drawings.

As shown in FIG. 1, the upper and lower rollers 3
Between the two sides of the slab (
Set this up.

In this case, each permanent magnet is provided so that its opposite magnetic poles (N and S poles) face each other on the mold surface, so that a static magnetic field 5 is generated inside the slab in a direction perpendicular to the direction in which the slab is pulled out.

Then, the upper and lower rollers close to the permanent magnet 2.2 are provided with energizing brushes 8, 8, which are connected to a low frequency AC power source of 30 Hz or less, and the current is applied to the slab 4 between the upper and lower rollers.
is configured to flow as part 9 of the circuit.

The alternating current is not desirable because the lower the frequency and the longer the current is applied in one direction, the more the stirring flow becomes stronger and the white band becomes thicker and clearer.

However, if the energization time in one direction is extremely short, a sufficient stirring effect cannot be obtained, so in reality, the frequency of energization is 5 to 30Hz.
It is desirable to use a low frequency alternating current.

The permanent magnet has a residual magnetic flux density Br of 5 to 10 kg.

At a coercive force Hc of 5 to 10 KOe, the maximum energy product [B
rHc]max is suitable, and YCo51
CeCa5*PrCo5tSmCo5. SmPrCo5
A rare earth cobalt magnet with the following components is optimal.

If continuous casting is carried out by this device, the slab 4 that is in the middle of solidification at the roller apron part after being drawn from the mold 1 (as shown in Figure 1), the static magnetic field 5 perpendicular to the drawing direction and the same Due to the action of the direct current in the direction, an electromagnetic force F in the width direction of the slab acts on the unsolidified molten metal inside the slab.

By switching the direct current in the forward and reverse directions at short intervals, the direction of the electromagnetic force F changes alternately.

Therefore, as mentioned above, due to the electromagnetic force F whose direction changes for a short time, the unsolidified bath molten metal is stirred while flowing alternately in the left and right directions of the width of the slab, and the segregation in the center of the slab is eliminated. In addition, the white band becomes uniform throughout and becomes unclear.

Example 1601 3-charge low carbon aluminum-silicon killed steel continuously melted in a converter (components: 0.16% carbon, 0.3% silicon, 1.45% manganese, 0.018% phosphorus) , 0.013% sulfur, iron remaining) was continuously cast using a two-strand curved slab continuous casting machine under conditions of a casting temperature of 1540°C and a drawing speed of 0.8 n1/min, with cross-sectional dimensions of 190 m x 1600 mm. Tani strand 2
40 were manufactured.

At this time, the valley strand f is equipped with an electromagnetic stirring device shown in Fig. 1, and two permanent magnets (S
mCo 5 ) were installed in the width direction of the slab (at a position Q 400 Crrl below the upper surface of the mold so that the N and S poles faced each other, and energizing brushes were provided for each of the 10th and 11th rollers under the mold. In the case of the first strand, a DC current with a voltage of 20 V and a current of 5500 A is passed in the direction of pulling out the slab, and in the case of the second strand, a DC current of 20 V and a current of 5,500 A is applied in the direction of pulling out the slab.
A low frequency alternating current with a voltage of 5500 V and a frequency of 5" and 30 Hz was applied.

Then, test pieces were cut out from 20 m, 50 m, and 80 m after the start of casting, and the cross sections were macro-etched to examine the uniformity and shading of the white band.

The results are not shown in FIGS. 4 and 5.

Figure 4 shows a schematic diagram of cross-sectional macro etching, and a
The figure shows a slab obtained by a comparative method, and figure b shows a slab obtained by implementing the present invention using a 10 Hz low frequency alternating current.

In the case of figure a, a white band 12 of an unrestricted thickness appears clearly between the dendrites 10 on the outer periphery and the equiaxed crystals 11 inside, while in the case of figure b, the white band 12'
is unclear (I can see that it is happening).

FIG. 5 shows a 9r made from a slab according to the present invention.
Ten samples were taken from each plate with a thickness of rrm, and the center segregation of the microstructure and the presence or absence of a white band, l
The results of the hard judgment are shown as a ratio, but 5
It can be seen that a sufficient stirring effect is obtained when a low frequency alternating current of ~30 Hz is used.

[Brief explanation of drawings]

Fig. 1 is a perspective view (Fig. a) showing the main parts of a curved continuous casting machine according to the present invention, and a cross-sectional view of the magnet installation part (Fig. B). Fig. 2 shows a cast slab without electromagnetic stirring. Cross-sectional view (a
Figure 3 is a cross-sectional view of a slab made from the part including the wide white band of the slab (Figure b), and Figure 3 is a cross-sectional view of a finished product made from the part of the slab that was subjected to the conventional electromagnetic stirring method (Figure b). Figure 4 is a macrostructure diagram in the cross section of a cast slab when conventional electromagnetic stirring is performed (Figure a) and when the electromagnetic stirring of this invention is implemented (Figure b). Figure 5 is a 9rrtyt It is a chart showing the results of determining the center segregation of the macrostructure and the presence or absence of a white band in a thick plate. In the figure, 1...Mold, 2...Permanent magnet,
3...roller, 4...slab, 5...
...static magnetic field, 6...center segregation, T...
・・White band, ・・・Electrifying brush, 9・
... Part of the circuit, 10 ... Dendrite, 11 ... Equiaxed crystal, 12 ... White band, F ... Electromagnetic force.

Claims (1)

[Claims] 1 In the secondary cooling zone where there is an unsolidified part inside the slab,
Continuous casting involves creating a magnetic field in the unsolidified area using a permanent magnet, passing an electric current through the slab, and using the resulting thrust to stir and solidify the unsolidified area. There is also a method of stirring unsolidified molten metal in continuous casting, which is characterized by stirring the unsolidified portion while passing a low-frequency alternating current.