The present invention relates to a continuous casting method of molten metal, more particularly relates to an improvement of a flow of molten metal in a casting mold.
In a continuous casting method of molten metal, a casting mold having a casting space for forming a cast slab surrounded at four sides by water-cooled copper plates is used, molten metal is injected into the casting mold, the part of the molten metal contacting the casting mold solidifies to form a shell, the shell is pulled out from the bottom of the casting mold while growing, and the metal finally finishes solidifying whereupon a continuously cast slab is formed.
In continuous casting of a cast slab of a flat shape, the casting space in the casting mold also has a rectangular cross-section. The surfaces of the casting mold facing the long sides of the cross-sectional rectangular shape are called the “long side surfaces” while the surfaces of the casting mold facing the short sides of the rectangular shape are called the “short side surfaces”. The molten metal is supplied through a submerged entry nozzle into the casting mold. The submerged entry nozzle is a closed bottom cylindrical shape. Near the bottom end of the submerged entry nozzle, discharge ports are formed oriented in two directions in the longitudinal direction of the casting space. The discharge ports discharge molten metal inside the casting mold. The discharge flow from the discharge ports of the submerged entry nozzle penetrates in the molten metal pool in the casting mold and strikes the casting mold short sides whereupon it is divided in an upward oriented flow and a downward oriented flow.
At the surface of the molten metal pool formed in the casting mold, continuous casting mold flux is supplied forming a layer. This is melted by the heat of the molten metal and flows into the gap between the casting mold and the shell to form a mold flux film there. This functions as a lubricant between the casting mold and shell. The casting mold constantly vibrates in the vertical direction (called “oscillation”) to promote the inflow of the mold flux film and facilitate withdrawal of the cast slab. On the other hand, the cast slab surface is formed with relief shapes called “oscillation marks” due to the casting mold oscillation.
If arranging an electromagnetic coil around the casting mold having a current path surrounding the casting space and running an alternating current through this electromagnetic coil, a pinch force acts on the molten metal in the casting mold. Japanese Patent Publication (A) No. 52-32824 describes an invention making this electromagnetic force act near the meniscus of the molten metal and thereby causing the molten metal near the meniscus in the casting mold to receive force in a direction separating it from the casting mold wall and making the meniscus strongly bend and simultaneously enlarging the gap between the casting mold and the shell to thereby promote the inflow of powder, reduce oscillation marks, and improve the shape of the cast slab surface.
On the other hand, the thus acting electromagnetic force simultaneously forms an electromagnetic ally driven flow at the molten metal pool in the casting mold. The electromagnetic ally driven flow is formed at the center of the electromagnetic coil in the height direction heading from the shell to the center of the molten metal pool and is divided into the upward oriented flow and the downward oriented flow at the pool center. At a location corresponding to the top half of the electromagnetic coil, a circulating flow is formed comprised of an upward oriented flow at the pool center, an outwardly oriented flow at the meniscus part, and a downward oriented flow near the shell. At a location corresponding to the bottom half of the electromagnetic coil, a rotary flow is formed comprised of a downward oriented flow at the pool center, an outwardly oriented flow near the bottom end of the electromagnetic coil, and an upward oriented flow near the shell.
Japanese Patent Publication (A) No. 11-188460 describes, in an example of casting a billet having a circular or rectangular casting cross-section, a method of continuous casting arranging a molten metal injection nozzle having discharge ports opening in a downward oriented direction so that the discharge ports are positioned below the center of the electromagnetic coil and injecting molten metal into the casting mold from the discharge ports of the molten metal injection nozzle. In what is described in Japanese Patent Publication (A) No. 11-188460, due to this, the rotary flow flowing upward oriented at the center of the molten metal pool is not affected by the discharge flow from the molten metal injection nozzle, so it is considered that a cast slab superior in surface properties is cast.
The molten metal refined by oxygen for decarburization at a refining furnace contains free oxygen, so when transferring molten metal from the refining furnace to a ladle, a deoxidizing agent with a strong deoxidizing power is added into the molten metal to convert the free oxygen to oxides. The nonmetallic oxides formed mostly float up in the molten metal to be separated, but part remains floating in the molten metal and are transferred as is to the tundish. For this reason, the molten metal supplied from the tundish through the immersion nozzle to the inside of the casting mold includes nonmetallic inclusions. Further, to prevent the nonmetallic inclusions in the molten metal from sticking to the inside walls of the submerged entry nozzle, nonoxidizing gas is blown in the submerged entry nozzle. The blown nonoxidizing gas is entrained in the molten metal to become bubbles which move together with the molten metal. These nonmetallic inclusions and bubbles in the molten metal are supplied from the discharge ports of the submerged entry nozzle together with the discharge flow to the inside of the casting mold. If the nonmetallic inclusions and bubbles are entrained in the cast slab, they form quality defects, so it is preferable as much as possible to make them float up in the molten metal in the casting mold and have them absorbed by the continuous casting mold flux covering the meniscus for separation.
In recent continuous casting, the mold is made a vertical bending type provided with a vertical part directly under the meniscus to promote the flotation and separation of the nonmetallic inclusions and bubbles at the vertical part. Further, if the discharge flow from the discharge ports of the submerged entry nozzle strikes the casting mold short sides, then flows downward along the casting mold short sides too strongly, the nonmetallic inclusions and bubbles riding this flow will reach the deep parts of the cast slab and be entrained in the solidified cast slab.
By running an alternating current to an electromagnetic coil arranged around the casting mold so as to surround the casting space, it is possible to control the meniscus shape to improve the cast slab surface properties. However, as described in the above-mentioned Japanese Patent Publication (A) No. 11-188460, if arranging the molten metal injection nozzle having discharge ports opening in the downward oriented direction so that the discharge ports are positioned below the center of the electromagnetic coil for casting, the cast slab surface properties are improved, but the nonmetallic inclusions and bubbles trapped inside the cast slab cannot be sufficiently reduced.
The present invention has as its object the provision of a continuous casting method of molten metal using electromagnetic force to improve the cast slab surface properties and reduce the nonmetallic inclusions and bubbles trapped inside the cast slab.
In the case of using a submerged entry nozzle 5 having discharge ports 6 opening in the downward oriented direction described in Japanese Patent Publication (A) No. 11-188460 (
As opposed to this, as shown in
The present invention was made based on this discovery and has as its gist the following:
(1) A continuous casting method of molten metal injecting molten metal into a casting mold having a casting space of a rectangular cross-sectional shape through a submerged entry nozzle, arranging an electromagnetic coil having an electric current path surrounding the casting space around the casting mold, running an alternating current through this electromagnetic coil, and using said alternating current so that the molten metal near the meniscus in the casting mold receives force in a direction separating it from the casting mold wall while continuously casting the molten metal,
said continuous casting method of molten metal characterized by forming a discharge flow discharged from discharge ports of molten metal provided at a front end of the submerged entry nozzle oriented upward from the horizontal toward the short sides of the casting mold and in that a direction of a center line of said discharge flow is oriented upward from an intersection of the casting mold short sides and meniscus.
(2) A continuous casting method of molten metal injecting molten metal into a casting mold having a casting space of a rectangular cross-sectional shape through a submerged entry nozzle, arranging an electromagnetic coil having an electric current path surrounding the casting space around the casting mold, running an alternating current through this electromagnetic coil, and using said alternating current so that the molten metal near the meniscus in the casting mold receives force in a direction separating it from the casting mold wall while continuously casting the molten metal,
said continuous casting method of molten metal characterized by providing discharge ports of molten metal provided at a front end of the submerged entry nozzle oriented upward from the horizontal toward the short sides of the casting mold and in that a direction of a center line of said discharge ports is oriented upward from an intersection of the casting mold short sides and meniscus.
(3) A continuous casting method of molten metal as set forth in (1) or (2) characterized in that 0.8 of an angle between an opening direction X of said discharge ports and the horizontal direction is larger than an angle between a direction from the discharge port center C to the intersection A of the casting mold short side and meniscus and the horizontal direction.
(4) A continuous casting method of molten metal as set forth in (1) or (2) characterized in that a casting direction length of the electromagnetic coil 4 is made L and the center C of the discharge ports 6 is positioned above the bottom end of the electromagnetic coil 4 by more than ¼·L.
(5) A continuous casting method of molten metal as set forth in (1) or (2) characterized in that two or more discharge ports are arranged aligned in a vertical direction.
The present invention relates to a continuous casting method of molten metal. As shown in
The present invention, further, as shown in
By running an alternating current to the electromagnetic coil 4, the pinch force acts and simultaneously an electromagnetic induction flow is formed in the molten metal pool in the casting mold. The electromagnetic induction flow, as shown in
In the present invention, as shown in
In the present invention, as shown in
By defining the direction X of the opening of the discharge ports in the above way, it is possible to obtain the effect of the present invention. On the other hand, in actual continuous casting, the direction X of the opening of the discharge ports and the discharge direction of the discharge flow 14 sometimes do not match. Therefore, the inventors changed the discharge angle of the discharge ports of the submerged entry nozzle during continuous casting of steel given electromagnetic force in an actual machine to various angles and investigated the relationship between the direction X of the openings of the discharge ports and the direction of the actual discharge flow 14. Specifically, the inventors confirmed using sulfur as a tracer whether the discharge flow directly strikes the meniscus or strikes the shell of the casting mold short sides in the range of a linear speed of the discharge flow from the discharge ports of 0.5 to 2 m/sec. When sulfur is detected in the cast slab after casting, it can be judged that the discharge flow strikes the shell at the casting mold short sides, while when sulfur is not detected at the cast slab after casting, it can be judged that the discharge flow directly strikes the meniscus. As a result, it is learned that when having upward oriented discharge ports, the angle between the direction of the actual discharge flow and the horizontal direction becomes about 80% of the angle between the direction X of the opening of the discharge ports and the horizontal direction.
Therefore, the line Y is defined as shown in
Regarding the electromagnetic coil 4 having an electric current path around the casting mold surrounding the casting space 8, the casting direction length of the electromagnetic coil 4 is made “L”. It is necessary that the alternating current flowing through the electromagnetic coil 4 cause the molten metal near the meniscus in the casting mold to receive a force in a direction separating it from the casting mold wall, so the top end position of the electromagnetic coil 4 becomes a position near the meniscus 11 in the casting mold.
The discharge ports 6 of the submerged entry nozzle 5 of the present invention are positioned are preferably positioned so that until the discharge flow 14 discharged from the discharge ports 6 reaches the meniscus 11, the discharge flow 14 continuously receives a pinch force from the electromagnetic coil 4 and the spread of the discharge flow 14 in the cast slab thickness direction is suppressed. Therefore, the casting direction position of the center of the discharge ports 6 is preferably above the bottom end position of the electromagnetic coil 4.
On the other hand, near the bottom end of the electromagnetic coil 4, a pinch force acts on the molten metal toward the center direction of the cast slab thickness, but, as shown in
In the present invention, as shown in
The present invention was applied in a continuous casting machine for casting a cast slab of a width of 1200 mm and a thickness of 250 mm in cross-sectional shape. The casting mold had a height of 900 mm, had a vertical part of 2.5 m right below the casting mold, and further had a bent part of a radius of curvature of 7.5 m and bent back horizontal part.
As shown in
The submerged entry nozzle 5 has an outside diameter of 150 mm and an inside diameter of 90 mm. As shown in
The inventors changed the conditions by the four types of opening directions X of the discharge ports 6, changed them further by presence or absence of electromagnetic force of the electromagnetic coil 4, cast low carbon Al-killed steel by a casting speed of 1.5 m/min, and evaluated the quality of the cast slabs. Conditions of no electromagnetic force and discharge ports of a downward oriented 30 degrees were used as reference conditions.
For discharge ports of an upward oriented 30 degrees, the opening directions X of the discharge ports, the direction of the line Y, and the direction of the actual discharge flow 14 all reach the meniscus 11 before striking the short side shell 13. For an upward oriented 20 degrees, the opening directions X of the discharge ports directly reached the meniscus 11 and the direction of the line Y was a direction reaching just slightly above the intersection A of the casting mold short side and meniscus right near it, but the direction of the actual discharge flow 14 directly reached the meniscus 11 in the invention examples with electromagnetic force and struck the short side shell 13 in the comparative examples without electromagnetic force. On the other hand, for discharge ports of an upward oriented 10 degrees and a downward oriented 30 degrees, the opening directions X of the discharge ports, the direction of the line Y, and the direction of the actual discharge flow 14 all directly struck the short side shell 13.
For the cast slab surface properties, the roughness of the surface was measured by a laser displacement meter. A total of five lines were selected: at 50 mm positions from the two short sides with respect to the width of the cast slab and at ¼ width, ½ width, and ¾ width. The surface relief of the cast slab surface was measured over a 200 mm length in the casting direction while moving the laser displacement meter with a spot diameter of 0.2 mm at a 0.2 mm pitch. The difference between the maximum displacement and minimum displacement for each 10 mm length on each line was obtained. This was compared over the total length. The maximum value was defined as the roughness degree. Further, the relative roughness degree indexed to the roughness degree of a sample of the reference production conditions as “1” was made the final definition.
Regarding the internal quality due to the nonmetallic inclusions and bubbles, the states of formation of surface layer inclusion and bubble defects and internal inclusion and bubble defects were evaluated. The “surface layer” is a depth of 20 mm from the cast slab surface and substantially corresponds to the thickness of solidification within the casting mold. The “internal” is the depth up to 20 mm to 50 mm depth of the casting surface layer and is a region including the part of the bent part forming a defective zone in a vertical bending continuous casting machine. For the surface layer, the entire width of the cast slab was milled over a 200 mm length in the casting direction at a 1 mm pitch in the thickness direction and the numbers of inclusions and bubbles were visually counted. For the inside, the entire width was milled over a 1 mm length in the casting direction at a 5 mm pitch in the thickness direction and the numbers of inclusions and bubbles were visually counted. For both, relative number indexes indexed to the number index of the sample of the reference production conditions as “1” was made the final definition.
The results are shown in Table 1. The invention examples with discharge ports upward oriented 30 degrees and electromagnetic force gave the best results in all of the cast slab surface roughness degree, surface layer bubble defects, and internal bubble defects compared with all of the comparative examples. The invention examples with discharge ports upward oriented 20 degrees and electromagnetic force also gave good results compared with the comparative examples.
The present invention makes the discharge flow from the submerged entry nozzle discharge ports reach the meniscus without striking the short side shell and without contacting the long side shell either, so nonmetallic inclusions and bubbles can be kept from being trapped at the short side shell and the long side shell and the internal quality of the cast slab can be improved. Along with this, by running an alternating current through an electromagnetic coil arranged around the casting mold to surround the casting space to control the meniscus shape, the cast slab surface properties can be improved.
Number | Date | Country | Kind |
---|---|---|---|
2006-328273 | Dec 2006 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/JP2007/073731 | 12/3/2007 | WO | 00 | 5/22/2009 |