Continuous casting method, and device therefor

Details Continuous casting method, and device therefor Continuous casting method, and device therefor

2000-03-31

2001-11-13

Dunn, Tom (Department: 1722)

Metal founding

Process

Shaping liquid metal against a forming surface

C164S499000, C164S504000

Reexamination Certificate

active

06315029

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a continuous casting method, and a device for use in the casting method. More specifically, the present invention relates to a continuous casting method, and a device for use in the casting method, in which the flow state of the discharged molten metal is properly controlled, and thus, the amounts of residual non-metallic inclusions and gas bubbles within the molten metal are decreased, so that continuously cast slabs of a good quality can be produced.
BACKGROUND OF THE INVENTION
The molten metal continuous casting method has been adopted over the whole world since 1960s. This method has various advantages compared with the general ingot making method, and therefore, it is utilized for a considerable part of the manufactured steel.
The quality of a continuously cast metal is classified into a surface quality and an internal quality, and these qualities are closely related to the flow of molten metal within the mould.
FIGS. 1
a
and
1
b
illustrate a mould used in the general continuous casting method. Referring to these drawings, a molten metal is supplied into a mould
10
through a submerged nozzle
11
which has two discharge holes
11
a
. The molten metal which is discharged from the two discharge holes forms jet flows toward a narrow face
13
, and the jet flow collides with the narrow face
13
to be divided into an ascending flow U and a descending flow D. That is, the jet flow is divided into four recirculating streams U
1
, U
2
, D
1
and D
2
. In
FIG. 1
b
, reference code S indicates a turning point of the recirculating streams.
The molten metal which is introduced into the mold contains non-metallic inclusions (also called “inclusions” below) such as Al
2
O
3
, MnO, SiO
2
and the like which have been formed in the pre-treating stage or have come from the refractory materials. The molten metal further includes inert gas bubbles (also called “gas bubbles” below) which have been injected into the submerged nozzle
11
, for preventing the clogging of the nozzle
11
. The gas bubbles have sizes of several scores of microns to several millimeters. The inclusions and gas bubbles which are contained in the upper recirculating streams have a density lower than that of the molten metal. Therefore, they are subjected to a floating force in a direction opposite from gravity, and therefore, they move in the combined vector direction of the molten metal flow and the floating force. Then they gradually move toward the meniscus of the molten metal, to be captured by the mold flux
14
.
However, the inclusions and gas bubbles which are contained in the lower recirculating streams D pass through the jet flow region near the nozzle discharge holes
11
a
before moving toward the upper recirculating streams U. The velocity of the jet flow is faster than the ascending velocity due to the floating force, and therefore, the inclusions and the gas bubbles rarely pass through the jet flow. Accordingly, the inclusions and the gas bubbles which are contained in the lower recirculating streams cannot reach the meniscus of the molten metal, but continuously circulate along with the lower recirculating streams. Therefore, they are likely to remain within the cast metal. Particularly, in the case of the continuous curved caster, the particles contained in the lower recirculating streams spirally move due to the influence of the floating force to be ultimately adhered on the solidified layer, i.e., on the upper layer of the cast piece, thereby forming an inclusion/gas bubble accumulated region in the upper layer of the cast piece.
When the cast piece is subjected to a rolling, the residual inclusions and gas bubbles are exposed to the surface, thus causing surface defects. Or they remain within the cast piece, and when an annealing is carried out, the gas bubbles expand to cause internal defects.
In order to solve this problem and to improve the quality of the cast piece, conventionally the discharge angle &THgr; of the submerged nozzle is properly adjusted, so as to improve the quality of the cast piece. The discharge angle &THgr; of the submerged nozzle gives a great influence to the flow of the molten metal.
If the discharge angle &THgr; is increased, the amount of the descending flow increases, while that of the ascending flow decreases. As a result, the velocity of the molten metal on the meniscus of the melt is slowed, so that a stable surface of the melt is maintained. Therefore, the workability is improved, and the initial solidification is stably carried out, thereby upgrading the surface quality of the cast piece. However, if the discharge angle &THgr; is increased, large amounts of inclusions and gas bubbles are buried deeply into the cast piece, because they lose the opportunity of floating to the meniscus of the melt. Thus the internal quality of the cast piece is aggravated.
On the other hand, if the discharge angle &THgr; is decreased, the amount of the descending flow decreases, and therefore, the defects due to the inclusions and the gas bubbles may decrease. However, if the discharge angle is decreased, the amount of the ascending flow increases, and the velocity of the molten metal at the meniscus of the melt steeply increases. Therefore, the surface quality of the cast piece is decreased due to the entrainment of the mould flux at the melt surface, and due to the formation of vortex. These problems become much more serious as the casting speed becomes faster.
Thus, if only the submerged nozzle is employed, a limit in controlling the flow of the molten is confronted. Therefore, as shown in
FIG. 2
a
, an electromagnetic brake ruler (EMBR)
20
is installed immediately below the discharge hole
11
a
of the submerged nozzle. Thus the Lorentz force based on a magnetic field and a flow is utilized to decrease the flow velocity. (This is proposed in Swedish Patent SE 8,003,695, and U.S. Pat. No. 4,495,984.)
The method of
FIG. 2
a
has been put to the practical use, but it is not used at present because flow distortions occur in the direction of evading the flow resistance of the magnetic field, rather than decreasing the flow velocity by the magnetic field.
In order to overcome this problem, the magnetic field is horizontally distributed over the entire width of the mould as shown in
FIGS. 2
b
and
2
c
. (Swedish Patent SE 9,100,184, U.S. Pat. No. 5,404,933, and Japanese Patent Application Laid-open No. Hei-2-284750). However, the distortion phenomenon has been observed in these methods all the same.
When a dc magnetic field is not applied, the molten metal which has been discharged from the discharge holes
11
a
of the submerged nozzle
11
forms flow fields as shown in
FIG. 3
a
. However, if the magnetic field is applied over the entire width of the mould, the flow steams are formed as shown in
FIG. 3
b
. That is, compared with the case where there is magnetic field, the jet flow is markedly spread in the thickness direction of the mould. Therefore, the average velocity of the jet flow directed toward the mould narrow face is slowed.
As the velocity of the jet flow is slowed, the inclusions and the gas bubbles of several scores to several hundreds of microns have a long way to travel from the descending flow region to the ascending flow region, compared with the case where a magnetic field is not applied.
Meanwhile, most of the inert gas which has been injected through the nozzle into the molten metal has of several millimeters, and floats from between the narrow faces to the meniscus of the melt (the floating distance depends on the molten metal injection speed and on the amount of the injected gas, and this distance corresponds from near the discharge hole to the narrow face in the case where the minimum gas amount is injected, while it corresponds from immediately above the discharge hole to the narrow face in the case where the maximum gas amount is injected). If the velocity of the main flow is light, the direction of the main flow is not greatly affected by the floating of the inert gas bubbles. However, if th

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