1. Field of the Invention
The present invention relates to a continuous casting method and apparatus for effecting flow control of molten steel using a magnetic field during continuous casting of steel.
2. Description of Related Art
In continuous casting, an immersion nozzle is often used to pour a molten metal into a casting mold. If the flow speed of the surface molten metal is too high at that time, mold flux on the surface of the molten metal is entrained (or involved) into a body of the molten metal, and if the flow speed of the surface molten metal is too low, the molten metal stagnates and segregates there, thus finally giving rise to surface segregation. For reducing such surface defects, there is known a method of applying a static magnetic field and/or a moving magnetic field (AC moving magnetic field) to the molten metal in the mold for controlling the flow speed of the molten metal.
However, the known method has problems as follows. When a static magnetic field is applied to brake a flow of the molten metal (for electromagnetic braking), segregation tends to occur readily, particularly in a position where the molten metal stagnates. Also, when a moving magnetic field is applied to agitate the molten metal (for electromagnetic agitation), entrainment of the mold flux (flux entrainment) tends to occur readily in a position where the flow speed of the molten metal is high.
To cope with the above problems, several proposals have been made as to the manner of applying a magnetic field. For example, Japanese Unexamined Patent Application Publication No. 9-182941 discloses a method of periodically reversing the direction, in which a molten metal is agitated by a moving magnetic field, to prevent inclusions from diffusing downward from an agitation area. Japanese Unexamined Patent Application Publication No. 8-187563 discloses a method of preventing a breakout by changing the magnitude of a high-frequency electromagnetic force depending on vibration of a casting mold. Japanese Unexamined Patent Application Publication No. 8-267197 discloses a method of preventing inclusion defects by providing a gradient to a change rate of the magnetic flux density in the changeover process of an electromagnetic braking force so as to reduce changes of a molten metal flow. Furthermore, Japanese Unexamined Patent Application Publication No. 8-155605 discloses a method of applying a horizontally moving magnetic field at frequency of 10-1000 Hz through conductive layers, each of which has low electrical conductivity and is formed to extend continuously in the direction of transverse width of a casting mold, and imposing a pinching force on a molten metal so that a contact pressure between the casting mold and the molten metal is reduced.
However, none of these known methods has succeeded in satisfactorily preventing the occurrence of flux entrainment, because a macro flow of the molten metal is caused due to the moving magnetic field, or because the flow speed of the molten metal is increased in a position where the static magnetic field is small.
With the view of breaking through the limits of the related art set forth above, it is an object of the present invention to provide a continuous casting method and apparatus for metals which can produce a cast slab much less susceptible to flux entrainment, capture of bubbles and non-metal inclusions near the surface of a molten metal, and surface segregation.
As a result of conducting intensive studies, the inventors have made the following findings.
Aspect A of Invention: Application of Non-moving, vibrating AC Magnetic Field
1) Molten-metal flow control under application of a static magnetic field is very effective in preventing entrainment of mold flux 3 and occurence of inclusions. However, if the magnetic field is too strong, the flow speed of a molten metal is reduced and surface segregation 5 is caused due to semi-solidification at the surface of the molten metal. (See FIG. 1)
2) Molten-metal flow control under application of a moving magnetic field is able to prevent the surface segregation 5 and capture of foreign matters (bubbles and non-metal inclusions 4) at the solidification interface. With a resulting increase of the flow speed of the molten metal indicated by 2, however, the entrainment of the mold flux 3 is more likely to occur and an amount of the entrained mold flux 3 is apt to increase. (See FIG. 1)
3) A method of applying an electromagnetic force, which induces only vibration without inducing a macro flow, so as to act upon the molten metal is very effective in preventing the semi-solidification at the surface of the molten metal and the capture of foreign matters at the solidification interface while holding down the flux entrainment. Such an electromagnetic force can be produced by an AC magnetic field which is not moved but only vibrated (hereinafter referred to as a xe2x80x9cnon-moving, vibrating magnetic field)xe2x80x9d. Thus, the term xe2x80x9cnon-moving magnetic fieldxe2x80x9d as used herein connotes magnetic flux alternating in opposite directions, whereas a moving magnetic field connotes a magnetic flux continuing in a single direction.
The present invention according to this aspect A has been accomplished based on the above-mentioned findings.
More particularly, according to this aspect A of the present invention, there is provided a continuous casting method for metals, the method comprising the step of applying a non-moving, vibrating magnetic field to a molten metal in a casting mold to impose only vibration on the molten metal.
The non-moving, vibrating magnetic field is preferably produced by arranging electromagnets, each of which comprises an iron core and a coil wound over the iron core, in an opposing relation on both sides of the mold in the direction of transverse width thereof to lie side by side in the direction of longitudinal width of the mold, and supplying a single-phase AC current to each coil.
The iron core may comprise individual single iron cores separate from each other, or a comb-shaped iron core having a comb-teeth portion over which coils are wound.
The single-phase AC current preferably has frequency of 0.10 to 60 Hz.
Furthermore, a DC magnetic field and an AC magnetic field for producing the non-moving, vibrating magnetic field may be applied in superimposed fashion in the direction of transverse width of the mold.
Aspect B of Invention: Intermittent Application of Static Magnetic Field
1) Molten-metal flow control under application of a static magnetic field is very effective in preventing entrainment of mold flux and intrusion of inclusions. However, if the magnetic field is too strong, the flow speed of a molten metal is reduced and segregation is caused due to solidification at the surface of the molten metal, as shown on the left side of FIG. 6.
2) With molten-metal flow control under application of a moving magnetic field, the flow speed of the molten metal is increased and the flux entrainment is more likely to occur, as shown on the right side of FIG. 6.
In other words, when an area appears in which the molten metal slows down its flow speed and is semi-solidified, segregation occurs in that area and product defects are ultimately caused. Providing a macro flow to the molten metal to avoid the occurrence of segregation, however promotes the flux entrainment and gives rise to new defects.
3) A method of applying a static magnetic field intermittently is very effective in preventing the semi-solidification at the surface of the molten metal while holding down the flux entrainment.
According to this aspect B of the present invention, there is provided a continuous casting method for casting a metal while applying a static magnetic field in the direction of thickness of a cast slab, comprising the step of intermittently applying the static magnetic field. Herein, the term xe2x80x9cintermittent applicationxe2x80x9d means a process of alternately repeating application (on) of the static magnetic field and no application (off) of the static magnetic field.
Preferably, the static magnetic field is intermittently applied under setting of an on-time t1=0.10 to 30 seconds and an off-time t0=0.10 to 30 seconds. Also, the static magnetic field is preferably applied to a surface of a molten metal. It is more preferable to employ setting of an on-time t1=0.3 to 30 seconds and an off-time to t0=0.3 to 30 seconds.
According to another aspect of the present invention, when continuous casting is performed by applying a DC magnetic field and an AC magnetic field in superimposed fashion in the direction of transverse width of a casting mold at positions above and below an ejection port of an immersion nozzle immersed in a molten metal in the mold, the AC magnetic field may be moved in a longitudinally symmetrical relation from both ends to the center of the mold in the direction of longitudinal width thereof.
The above method can be implemented by a continuous casting apparatus for molten metals, the apparatus comprising a coil for producing an AC magnetic field moving in a longitudinally symmetrical relation from both ends to the center of the mold in the direction of longitudinal width thereof, and a coil for producing a DC magnetic field, both the coils being wound over each of common iron cores, the iron cores being arranged on both sides of the mold in the direction of transverse width thereof such that a direction of the magnetic fields produced by the coils is aligned with the direction of transverse width of the mold.