JP2003290877A - Continuous casting method of molten metal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface property of a slab due to interference between a flow induced in a molten metal by an AC magnetic field and a discharge flow from an immersion nozzle in continuous casting while applying an AC electromagnetic field to the molten metal. Also eliminates fluctuations in the slab internals. SOLUTION: In a method of supplying molten metal into a mold from an immersion nozzle and continuously casting the molten metal in the mold while applying an alternating magnetic field, the immersion nozzle is configured to vertically lower the molten metal from a direction of a corresponding mold wall surface. And a slit portion for discharging from the vertically downward direction directly below the nozzle toward the opposite mold wall surface, and an area ratio between the slit portion and the two-hole portion, and a casting condition. And satisfy a particular relationship.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting molten metal, and more particularly, to applying an alternating-current electromagnetic field to a molten metal in a mold to raise the molten metal to a convex shape. The technology relates to continuous casting while casting. 2. Description of the Related Art In continuous casting, many methods have been proposed to improve the surface properties of a slab by controlling the oscillation mark and initial solidification. Japanese Patent Application Laid-Open No. 58-32824 discloses a continuous casting method in which a molten metal 2 is injected together with a lubricant 4 into a water-cooled mold 1 vibrating at a constant cycle and continuously drawn downward, as shown in FIG. Electromagnetic coil 5 provided in
A method for improving the surface properties of a cast slab by continuously supplying an alternating current to the molten metal 2 and using the electromagnetic force generated by the alternating magnetic field to build up the molten metal 2 in a convex shape. Japanese Patent Application Laid-Open No. 64-83348 discloses that when an electromagnetic force is applied to a molten metal in a mold by an electromagnetic coil, an alternating magnetic field is applied in a pulsed manner to intermittently apply the electromagnetic force. A method of further improving the surface properties in powder casting is disclosed. Further, in the re-publication publication (WO96 / 05926), an alternating current is applied to the solenoid-shaped electromagnetic coil 5 installed so as to surround the continuous casting mold wall 1 as shown in FIG.
A molten metal is raised to a convex shape while applying an electromagnetic force 22 acting on the solidifying molten metal 2 in a direction determined by the direction of the induced current 20 and the induced magnetic field 21 to separate the molten metal 2 from the mold wall. In the method of continuous casting, a method is disclosed in which the initial solidification is stabilized and the slab surface properties are improved by periodically changing the amplitude or waveform of an energized alternating current by a waveform generator 24. . As described above, many techniques for improving the surface properties of a slab by continuously casting a molten metal while applying an AC magnetic field to the molten metal have been developed. [0005] As described above, by applying an alternating magnetic field to the molten metal in the mold, an electromagnetic force in the direction of raising the molten metal in a convex shape can be applied. Due to this AC magnetic field, the molten metal in the mold is lowered along the mold wall above the vertical center position of the coil and along the mold wall below the vertical center position of the coil as shown in FIG. Then, a flow 11 in the upward direction is induced (hereinafter referred to as an induced flow). On the other hand, the flow of the molten metal and the discharge flow 7 discharged from the ordinary immersion nozzle 6 having two discharge ports are directed to the opposite mold wall, and the downward flow 9 flowing in the drawing direction.
Branching into an upward flow 10 that reverses upward. The upward flow 10 and the induced flow 11 generated by the AC magnetic field interfere with each other, and the flow of the molten steel is disturbed near the meniscus. This turbulence in the slab causes the heat flow in the vicinity of the meniscus to fluctuate, destabilizing the solidification interface, deteriorating the surface properties of the slab, or promoting entrainment of powder, etc., resulting in defects in the slab. This is one of the causes of defects. It is well known that the flow of molten metal in a continuous casting mold has a great effect on slab quality.
For example, inclusions in which the flow of the molten metal discharged from the injection nozzle is carried deep into the molten metal pool, are caught by the solidified shell, and cause internal defects in the slab. For this reason, many techniques for controlling the flow of the molten metal to prevent these defects have been proposed. For example, in Japanese Patent Application Laid-Open No. 2-284750, a DC magnetic field is applied to a molten metal by a coil provided below an injection nozzle,
A method is disclosed in which a brake is applied to the molten steel flow at that position to reduce the descending flow of the molten steel. Further, Japanese Unexamined Patent Publication No.
In Japanese Patent Publication No. 285854, as shown in FIG. 7, the immersion nozzle 6 is provided with a slit-shaped discharge port 30 and the discharge flow from the immersion nozzle is directed from the vertical direction to the short side of the mold facing the downward discharge port. Continuous fan-shaped flow 2 spreading toward
9 is obtained, and the flow of molten steel is controlled by combining with the electromagnetic coil 27. However, the technique disclosed in Japanese Patent Application Laid-Open No. 9-285854 is to control the downward flow of the molten metal by a DC magnetic field, and does not relate to the induced flow and the inverted upward flow when an AC magnetic field is applied. According to the present invention, when a molten metal is continuously cast by applying an AC magnetic field, flow turbulence due to interference between an ascending flow due to a discharge flow from an immersion nozzle and a flow induced by an AC magnetic field is controlled, and An object of the present invention is to provide a continuous casting method capable of eliminating a change in properties. [0010] The present invention has been made to solve the above-mentioned problems, and has the following contents. A molten metal is injected from a dipping nozzle into a water-cooled mold having a pair of long sides and a pair of short sides vibrated at a constant cycle, and an alternating magnetic field generated by an energizing coil provided around the water-cooled mold. In the method of solidifying the molten metal by applying electromagnetic force in a direction to separate the molten metal from the mold wall, and continuously drawing the molten metal downward, the immersion nozzle is configured to vertically move the molten metal from the direction of the opposite mold wall surface. A two-hole portion to be discharged in the downward direction, and a slit portion to be discharged from the vertically downward direction directly below the nozzle toward the opposite mold wall surface are continuously provided, and the ratio of the discharge cross-sectional area of the slit portion to the two-hole portion is determined. A continuous casting method of molten metal, characterized by casting according to the relationship of formula (1). (2√2 · T · Vc COS θ / V ₀ √S ₁ ) −1 <β (1) where β: ratio of the discharge cross-sectional area between the slit portion and the two-hole portion T: thickness of the casting space (M) Vc: Casting speed (m / s) θ: Angle (degree) between the center line of the two holes and the horizontal plane S ₁ : Discharge cross-sectional area of the two holes (m ² ) V ₀ : Electromagnetic force of energizing coil In addition, when the left side is 0 or less, β = 0, that is, no slit portion is provided. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings of embodiments. FIG. 1 is a sectional view schematically showing a continuous casting apparatus for performing the method of the present invention. Around the water-cooled mold 1, an energizing coil 5 for supplying an alternating current is provided so as to be buried in the water-cooled mold such that the upper end thereof is substantially at the position of the meniscus 3. In casting, a molten metal is supplied from a tundish (not shown) above the water-cooled mold through the immersion nozzle 6 into the water-cooled mold. Note that the direction of discharge of the molten metal from the immersion nozzle, or the direction of the discharge port of the immersion nozzle, is usually on the short side when the shape of the horizontal cross section of the water-cooled mold is rectangular, and is substantially square. Is on either side. In the following, description will be made assuming that the horizontal cross-sectional shape of the water-cooled mold is a rectangular mold. Powder 4 is supplied as a lubricant to the surface of the molten metal 2 in the water-cooled mold 1, and the molten metal is cooled down while being oscillated by the water-cooled mold 1, solidified, and drawn down as a slab. At this time, several tens to several tens
When an alternating current of 0 Hz is applied, an induced magnetic field B is generated in the mold, and an induced current J is induced. By the action of the induced current J and the induced magnetic field B, the molten metal 2
The electromagnetic force F acts in the direction toward the center of the water-cooled mold, and the molten metal is raised in the vicinity of the meniscus 3 so that solidification proceeds while maintaining smooth lubrication with the water-cooled mold. As a result, the oscillation marks and the like of the slab can be reduced and the surface properties can be improved. However, as shown in FIG. In the metal, a flow 11 (induced flow) in a rising direction is induced below the vertical center position of the coil while falling below the vertical center position of the coil. On the other hand, the molten metal discharge flow 7 from the immersion nozzle 6 changes its flow direction downward near the wall surface on the short side of the mold, and mostly becomes a downward flow 9, but a part of the discharge flow Is
The flow is reversed upward and becomes an ascending flow 10, which interferes with the above-described induced flow 11 generated by the application of the alternating current to the coil, and as described above, the flow is disturbed near the meniscus. The present inventors have proposed a method for suppressing the fluctuation of the surface properties of a slab due to the disturbance of the flow near the meniscus due to the interference between the induced flow 11 and the ascending flow 10 to obtain a slab having excellent surface properties. The method was examined. That is, based on the knowledge that the rising flow on the short side determined by the casting conditions needs to be equal to or less than the induced flow due to the electromagnetic force, the shape of the nozzle, the casting conditions, etc. After examining the relationship with the surface properties in detail,
The present invention has been made. In the present invention, first, as shown in FIG. 2 (a), the discharge portion of the molten metal of the immersion nozzle 6 has a two-hole portion 12 for discharging the molten metal from the opposite mold wall surface direction to the lower wall surface direction. And a slit portion 13 for discharging from a vertical direction immediately below the nozzle to a wall direction opposite thereto, and the slit portion 13 is provided continuously to the two hole portions 12. In other words, by adopting a structure in which slits are continuously provided at two holes of the immersion nozzle, FIG.
As shown in the figure, a flow of molten steel flowing from directly below the immersion nozzle to vertically downward, a descending flow 8 is generated, thereby limiting the ratio of the discharge flow 7 from the two holes, and the short side from the two holes. The upward flow 10 generated by the discharge flow toward the (wall surface) can be reduced, and interference between the upward flow and the above-described induced flow 11 can be suppressed. Further, in the immersion nozzle configured as described above, the casting speed Vc, the casting space thickness T, and the like are set in order to reduce the strength of the upward flow that interferes with the induced flow as described above. The distribution of the discharge flow at the two holes and the slit of the immersion nozzle may be determined according to the conditions. That is, the ratio β (= discharge cross-sectional area of the slit portion / discharge cross-sectional area of the two-hole portion) of the discharge cross-sectional area of the slit portion and the two-hole portion is determined by the discharge angle and the discharge cross-sectional area of the nozzle two-hole portion, The relationship is to satisfy Expression (1) in relation to the speed V ₀ of the induced flow. (2√2 · T · Vc COS θ / V ₀ √S ₁ ) −1 <β (1) In the equation (1), when the left side is 0 or less, β =
0, that is, no slit portion is provided. In the equation (1), β is a ratio (S2) between the discharge cross-sectional area S2 of the discharge part of the immersion nozzle 6, ie, the slit part and the discharge cross-sectional area S1 of the two hole part, as shown in FIG.
/ S1). The shape of the two holes is circular, elliptical, rectangular,
Although not particularly limited, such as a polygon, a circle, an ellipse, and the like are preferable from the viewpoint of reducing disturbance of the flow in the molten metal pool due to the discharge flow. The cross-sectional area S1 is
It can be obtained according to the shape of the discharge port (hole), and the discharge cross-sectional area S2 of the slit can be obtained as the sum of the respective rectangular cross-sectional areas. As shown in FIG. 2 (a), the slit is provided continuously at the two holes, and the center line in the thickness direction of the slit and the center line of the two holes of the immersion nozzle are: It shall be provided so that it may become substantially the same direction, and shall be installed so that it may become parallel to the long side of a casting mold at the time of casting. As shown in FIG. 2B, θ is 2
The angle (degree) between the center line of the hole and the horizontal plane, that is, the discharge angle, is usually -15 to 55 degrees, preferably 0 to 45 degrees.
Degrees. T is the thickness (m) of the casting space, in this case, the mold thickness (the length of the short side of the mold). In the case of a mold having a square horizontal cross section, the length is any side. Vc is a casting speed (m / s) and is determined in consideration of operating conditions such as the type of molten metal to be cast, the temperature of the molten metal, and the size of the slab. V ₀ is the velocity (m / s) of the flow (induced flow) 11 of the molten steel induced by the electromagnetic force of the energizing coil. In general, when the alloy has a relatively high solidification rate such as continuous casting, it solidifies in a dendritic manner (dendrites), and the dendrites incline to the upstream side of the flow during solidification growth, and the angle is determined by the alloy components and flow velocity. Are known. Therefore, the velocity V _{0 of the} induced flow can be obtained by this method. In addition, it can also be obtained by numerical calculation. As shown in FIG. 1, but the opposite direction of flow to each other in the upper and lower vertical center position of the coil, V ₀ is, upward, either induced flow downward in (1) Or the average speed of both. In the present invention, the velocity of this flow is preferably a velocity measured near the height of the coil. In other words, by measuring the velocity in the range from the lower end to the upper end of the coil, in the measurement of the velocity of the induced flow, it is possible to evaluate without being affected by the discharge flow from the immersion nozzle or the fluctuation of the molten metal pool surface. That's why. For example, Vo may be measured or calculated by the above-described method at a position equivalent to 3/4 height of the current-carrying coil, 1/2 of the width of the mold (long side), and 1 cm inside the mold wall. preferable. EXAMPLE A solenoid coil having a height of 150 mm was set on a meniscus portion of a mold having a width of 1500 mm, a height of 880 mm and a thickness of 250 mm so that the upper end thereof was located at the meniscus position. Casting was performed using an immersion nozzle having an outer diameter of 150 mm and an inner diameter of 90 mm, which had a different depth, with a different nozzle outlet.
The nozzle has the shape shown in FIG.
The diameter 60 mm toward the short side of the mold space, the discharge angle θ is
Three having two holes having angles of 15 degrees, 30 degrees, and 45 degrees, and a thickness a continuously provided at the lower end of each of the two holes so as to be parallel to the long side direction of the mold. Nine slits having different slits were used. Table 1 shows the discharge angles θ of the two holes of the nozzle and the thickness a of the slit provided in the nozzle. [Table 1] A low-carbon aluminum-killed steel was cast at a casting speed of 1.5 m / mim by applying a sine wave alternating current at a frequency of 150 Hz to the electromagnetic coil so that the rise of the molten metal became 20 mm. At this time, the flow velocity Vo measured from the slab dendrite inclination angle corresponding to the vicinity of the 通電 height of the energizing coil was 0.2 m / s. The surface properties of the slab cast using these nozzles were measured, and the measurement results are shown in FIG. 3 as the surface roughness standardized to an allowable level of roughness. As can be seen from FIG. 3, none of the slits 1 indicated by black circles satisfy the relationship of the equation (1), and the surface roughness greatly exceeds the surface roughness standardized at an allowable level. Many and large variations. On the other hand, the slits 2 indicated by black squares and the slits 3 indicated by black diamonds, which satisfy the relationship of the present invention, are provided.
Are below the acceptable level,
It can be seen that the variation is small. Further, among the relations satisfying the relation of the present invention, those provided with the slits 3 indicated by black diamonds which largely satisfy the relation of the equation (1) also satisfy the relation of the equation (1). It can be seen that the surface has more stable surface properties as compared with those provided with the slits 2 indicated by black squares. According to the present invention, during continuous casting while applying an alternating current electromagnetic field to the molten metal, interference between the flow induced in the molten metal by the alternating magnetic field and the discharge flow from the immersion nozzle. Thus, it is possible to obtain a slab excellent in surface properties and slab quality.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing an embodiment of a continuous casting method according to the present invention, wherein (a) is a top view and (b) is a cross-sectional view. FIGS. 2A and 2B are schematic diagrams showing the shape of a nozzle according to an embodiment of the present invention, wherein FIGS. 2A and 2B are cross-sectional views, and FIG. FIG. 3 is a diagram showing a relationship between a slit thickness a and a discharge angle θ of a nozzle and a slab surface roughness standardized at an allowable level in an embodiment of the present invention. FIG. 4 is a schematic cross-sectional view showing an example of a conventional method for performing continuous casting by applying an electromagnetic force. FIG. 5 is a schematic sectional view showing another conventional example of continuous casting by applying an electromagnetic force. FIGS. 6A and 6B are schematic views of another conventional example of continuous casting by applying an electromagnetic force, wherein FIG. 6A is a top view and FIG. 6B is a cross-sectional view. FIGS. 7A and 7B are schematic views showing a continuous casting technique using an electromagnetic brake, wherein FIG. 7A is a cross-sectional view and FIG. 7B is a top view. [Description of Signs] 1 ... water-cooled mold 2 ... molten metal 3 ... meniscus 4 ... powder 5 ... energizing coil 6 ... immersion nozzle 7 ... discharge flow from two holes 8 ... discharge flow 9 from slit part 9 ... descending flow 10 ... Ascending flow 11 ... Induced flow 12 ... Nozzle 2 hole 13 ... Nozzle slit 20 ... Induced current 21 ... Induced magnetic field 22 ... Electromagnetic force 23 ... Electromagnetic force induced flow 24 ... Waveform generator 25 ... Power supply device 26 ... Exciting current 27 ... Electromagnetic coil 28 ... Electromagnetic coil upper end 29 ... Dispersed flow pattern of molten steel descending flow 30 ... Slit-shaped discharge port S1 ... Discharge cross-sectional area of two holes S2 ... Discharge cross-sectional area of slit part θ ... Discharge angle a ... Slit Width T: short side length of mold B: induction magnetic field F: electromagnetic force J: induction current

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Hiroshi Harada             20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation             Company Technology Development Division F-term (reference) 4E004 AA09 AD01 FB04 FB06 MB12                       MC05 NA02 NB01 NC01

Claims (1)

Claims: 1. A molten metal is injected from an immersion nozzle into a water-cooled mold having a pair of long sides and a pair of short sides vibrated at a constant cycle and provided around the water-cooled mold. In the method of solidifying the molten metal by applying an electromagnetic force in a direction to separate the molten metal from the mold wall by an alternating magnetic field generated by the energized coil, and continuously drawing the molten metal downward, the immersion nozzle includes: A two-hole portion that discharges from the opposite mold wall surface direction to the vertically downward direction, and a slit portion that discharges from the vertically downward direction directly below the nozzle to the opposite mold wall direction, and the two-hole portion and the slit portion Wherein the ratio of the cross-sectional area of the molten metal to the molten metal is cast as a relation of the formula (1). (2√2 · T · Vc COS θ / V ₀ √S ₁ ) -1 <β (1) where β: ratio of the discharge cross-sectional area between the slit portion and the two holes T: thickness of the casting space (m) Vc: Casting speed (m / s) θ: Angle (degree) between the center line of the two holes and the horizontal plane S ₁ : Discharge cross-sectional area of the two holes (m ² ) V ₀ : Induced by the electromagnetic force of the conducting coil Note that when the left side is 0 or less, β = 0, that is, no slit portion is provided.