JPS5944944B2 - Stirring method for unsolidified molten metal in continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to electromagnetic stirring of unsolidified molten metal in continuous casting, in which the flow form of the molten metal is controlled to eliminate the non-uniformity of white bands occurring in slabs. This invention relates to a method for stirring molten metal.

Segregation areas enriched with carbon, sulfur, phosphorus, etc. are likely to occur in the center of slabs manufactured by continuous casting.

The macrostructure of this segregated area exhibits a color tone different from that of the normal area, and products made from this slab exhibit variations in mechanical properties and weldability, as well as defects that significantly reduce commercial value.

In order to improve the above drawbacks, 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 inventor found that We have confirmed that it is effective to alternately switch the direction of the direct current flowing between the forward and reverse directions in a short period of time.

Based on this knowledge, the present invention proposes an electromagnetic stirring method in which a white band appears narrowly, uniformly, and indistinctly over the entire slab.

That is, the present invention provides a secondary cooling zone in which an unsolidified portion exists inside a continuously cast slab, in which a magnetic field is formed by a permanent magnet in the unsolidified portion, and a thrust is generated by passing an electric current through the slab. During continuous casting, where the unsolidified portion is solidified while stirring, the direction of the direct current applied to the slab is set parallel to the direction in which the slab is pulled out, and at short intervals of 1 to 2 A.
A method of stirring unsolidified molten metal in continuous casting, the gist of which is to alternately switch the current value within the range of /lrA.
The feature is that the macro grade overall judgment (A), which is expressed as the ratio of the number of white band obscured samples to the total number of samples subjected to the white band presence test, is 40% or more, eliminating segregation in the center of the slab. This is due to making the white band uniform throughout and making it unclear.

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

As shown in FIG. 1, one magnetic pole is placed close to the surface of the slab 4 and facing the surface of the slab 4 over the entire length of the slab between the upper and lower rollers 3 at an arbitrary point on the roller apron in the secondary cooling zone below the mold 1. Permanent magnets 2, 2 are placed oppositely on both sides of the slab.

In this case, the permanent magnets are provided so that the opposite magnetic poles of N and S poles face the surface of the slab, so that static magnetic fields 5 are generated inside the slab in each direction perpendicular to the direction in which the slab is pulled out.

Then, energizing brushes 8, 8 are provided on the upper and lower rollers close to the permanent magnets 2, 2, and these are connected to a DC power supply circuit 9.
The current is configured to flow between the upper and lower rollers through the slab as part of a circuit, and this circuit is equipped with a current switching device 10.
Insert.

The current switching device 10 uses a generally known DC current exchanger or the like, and alternately switches a current in a range of DC current values from 1 to 2 A/4 in the forward and reverse directions for 60 seconds or less.

The DC current value was set in the range of 1 to 2 A/Jut because the range was selected to provide a sufficient effect to eliminate segregation, and a sufficient stirring effect was not obtained below IA/cfL. Moreover, if it exceeds 2 A/1rrt, the agitation flow will be too strong and negative segregation will occur, and the negative effects cannot be eliminated even by current conversion.

Further, it is not desirable that the current is applied in one direction for a long time because the longer the current is applied, the stronger the stirring flow becomes and the white band becomes thicker and clearer.

However, if the energization time is extremely short and is less than a few seconds, a sufficient stirring effect cannot be obtained, so it is actually desirable to switch the energization time within the range of 5 seconds to 60 seconds.

In addition, a permanent magnet has a residual magnetic flux density Br5~10KG, a coercive force Hc5~10KOe, and a maximum energy product (B r H
c) A large max is suitable, and YCO
5, CeC05y PrCO5* SmCo5. SmP
A rare earth cobalt magnet having a component such as rCo5 is most suitable.

When continuous casting is performed using this device, the slab 4 that has been pulled out of the mold 1 and is in the process of solidifying at the roller apron portion is exposed to a static magnetic field 5 perpendicular to the drawing direction and a static magnetic field 5 in the same direction as the drawing direction, as shown in Figure 1. Due to the interaction with the direct current, 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 every short time, the unsolidified molten metal is stirred while flowing alternately in the left and right directions in the width direction of the slab, and the segregation in the center of the slab is eliminated and the white The bands become uniform and indistinct throughout.

3-charge low carbon aluminum-silicon killed steel manufactured by continuously melting Example 160 in a converter (components: 0.16% carbon, 0.3% silicon, 1.45% manganese, 0.018% phosphorus) 0.013% sulfur, remaining iron) was continuously cast in a two-strand curved slab continuous casting machine at a casting temperature of 1540°C and a drawing speed of 0.8 m/mvt, with a cross-sectional dimension of 190 m/mvt. Each strand, 240 pieces, was manufactured from slabs of rnm/1600 mm.

At this time, each strand is provided with an electromagnetic stirring device shown in Figure 1, and the magnetic field strength is 1 at the center of the slab in the thickness direction.
．． Two 0KG permanent magnets have north and south poles in the width direction of the slab.
They are placed 400cm below the top of the mold so that the poles are facing each other, and energizing brushes are installed on each of the 10th and 11th rollers below the mold, and in the case of the first strand, a voltage of 20V and a current are applied in the direction of pulling out the slab. A direct current of 5500 A is passed through the second strand, and the current direction can be switched every 5 seconds, 10 seconds, 15 seconds, 20 seconds, and 30 seconds by inserting a current converter into the DC circuit according to the present invention. did.

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 shown in FIGS. 4 to 6.

FIG. 4 shows a cross-sectional macro-etch, in which figure a is a slab obtained by the comparative method, and figure b is a slab obtained by implementing the present invention in which the direct current is switched every 20 seconds.

In the case of figure a, a white band 13 of an unrestricted thickness appears clearly between the dendrites 11 on the outer periphery and the equiaxed crystals 12 inside, while in the case of figure b, a white band 13' appears clearly.
It can be seen that it is unclear.

Moreover, as a result, it has excellent commercial value and has less variation in mechanical strength and weldability.

In addition, Figure 5 shows the phosphorus content in the thickness direction of the slab expressed as the amount of segregation with respect to the surface analysis value, but in the case of Figure a obtained by the comparative method, negative segregation occurs near the surface and a white band appears. It can be seen that it appears clearly.

On the other hand, it can be seen that in the case of Figure b according to the present invention, there is less segregation in the center and less negative segregation near the surface, and the white band is unclear.

Figure 6 shows the macrostructure of a 9mm thick rolled plate made from the slab of the second strand. Ten samples were taken at each energization time. There was no center segregation and the white band was unclear, and it was judged to be good. It shows the ratio of the sample to all samples, that is, the overall macrograde judgment (%) expressed by the formula below, and the judgment value is 40 when no energization switching is performed.
Regarding this, it can be seen that all the devices in which energization was switched in a switching time of 5 to 30 seconds according to the present invention had a judgment value of 60% or more, indicating that good results were obtained.

Macro grade overall judgment (salary)

[Brief explanation of drawings]

Figure 1 is a perspective view (Figure 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 (Figure B), and Figure 2 is a continuous cast slab without electromagnetic stirring. (Fig. a) and a cross-sectional view of a slab subjected to conventional electromagnetic stirring (Fig.
Fig. b), Fig. 3 is a cross-sectional view of a finished product made from the part including the wide white band part of the slab (Fig. a), and a cross-sectional view of the finished product made from the part including the narrow white band part of the slab. (
Figure b), Figure 4 shows the case of conventional electromagnetic stirring (Figure a).
A macrostructure diagram of the cross section of the slab when the electromagnetic stirring of this invention is applied (Figure b), Figure 5 is a diagram showing the state of phosphorus segregation in the cross section of the central part of the slab, and Figure 6 is a diagram showing the state of phosphorus segregation in the cross section of the central part of the slab. Create a diagram showing the relationship between time and the state of the macro organization. In the figure, 1... Mold, 2... Permanent magnet, 3... Roller, 4... Slab, 5... Stationary magnetic field, 6... Center segregation, 7... White band, 8...Electrifying brush,
9... DC power supply circuit, 10... Current switching device, 11
... Dendrite, 12... Equiaxed crystal, 13... White band, F... Electromagnetic force.

Claims (1)

[Claims] 1 In the secondary cooling zone where an unsolidified portion exists inside the continuously cast slab, a permanent magnet forms a magnetic field in the unsolidified portion, and an electric current is passed through the slab, and the resulting thrust stirs the unsolidified portion. During continuous casting, the direction of the direct current applied to the slab is parallel to the slab drawing direction, and is alternately switched within a current value range of 1 to 2 A/la at short intervals. Features: Stirring method for unsolidified molten metal in continuous casting.