JP4058561B2 - Metal continuous casting method and ingot mold for carrying out the method - Google Patents

Abstract

An ingot mould has energetically cooled metal walls (1, 3) having means in their upper portion for decreasing the intensity of the heat flow extracted from the cast metal (2), such as a coating (6, 15) of a less conductive metal than that of the walls, or grooves (31) provided in said walls. On top of the walls is provided a heat insulating feeder bush (7) associated with gas injection means terminating in nozzles distributed on the inner periphery of said ingot mould. During the casting, the free surface of the liquid metal is maintained at the same level as the feeder bush, wherein the solid skin (21) only starts to solidify at the upper edge of the metal walls (3). The invention is particularly useful for the continuous casting of steel.

Description

The present invention relates to continuous casting of metals, particularly steel.
As is well known, the concept of continuous casting operation is to pour molten metal into an ingot mold that is basically composed of bottomless tubular elements that define the passage of the cast metal. The mold wall is made of copper, more commonly a copper alloy, and is forced to cool by circulating water. From the mold, a casting whose outside is solidified over several centimeters in thickness is continuously extracted. This solidification proceeds toward the center of the casting, and solidification is completed by the action of water spray in the so-called “secondary cooling” region while the casting descends downstream of the ingot mold. The resulting casting (bloom, billet or slab) is cut to a certain length and rolled before being transported to the customer or transformed into a bar, wire, profile, plate, sheet or the like.
Surface defects or subsurface defects in castings obtained by continuous casting of steel often cause defective products. This is because the defects cannot sufficiently withstand the rolling operation, or the defects are enlarged by rolling, and the metal properties of the rolled casting are lowered to an unacceptable level.
The molten metal is supplied to the ingot mold by a nozzle during casting, and forms a solid film in contact with the cooling wall of the ingot mold. This film is driven downward by the rhythmic movement of the ingot mold during vertical extraction, while the film thickness is increased by the extraction of heat that continues through the wall of the ingot mold.
Thus, a new film of solid metal occurs continuously at the level of the free surface of the metal in the ingot mold (called “meniscus”), which solidifies throughout the perimeter of the inner wall of the ingot mold, A solid ring is formed which tends to shrink due to the cooling it receives while descending in the ingot mold.
The degree of shrinkage of this ring increases as the amount of heat extraction increases, and further shrinks upon cooling, for example due to changes in the solid phase at the end of solidification, as in the case of 0.1% carbon steel or AISI 304 grade stainless steel. Increased by the inherent properties of the cast metal.
Due to the shrinkage of the peripheral edge, the solidified skin is easily separated from the wall of the ingot mold, and therefore, the contact between the skin and the cooling wall is reduced and the heat exchange efficiency is lowered. In general, this separation is not uniform around the periphery of the solidified skin and causes surface defects in the final casting.
In order to prevent or limit these defects, attempts have been made to reduce the heat flow extracted during skin formation and at the start of solidification.
That is, the heat flow can be reduced by using an ingot mold provided with an upper portion where the casting starts to solidify, and an insert that forms a heat shielding layer for preventing molten metal from directly contacting the cooling wall of the ingot mold. Tried. However, it has been found that the durability of such inserts is uncertain and the maintenance costs of such ingot molds are very high.
In addition, the ingot mold wall is given a surface finish, for example engraved on the wall to form embosses on the surface of the wall, or irregularly roughened by, for example, sand blasting, so that the cast metal directly contacts the cooling wall copper. Attempts have been made to reduce heat flow by reducing the area of the contact area. However, this method cannot greatly improve the quality of the casting surface. In fact, the turbulence caused by fluctuations in the free surface height of the cast metal is much more prevalent than the effect of mitigating the heat flow obtained by surface finishing the walls of the ingot mold, so that solidification heterogeneity remains. , Surface defects will not go away.
Attempts have also been made to form a vertical groove in the wall with the same purpose of reducing direct contact between the cast metal and the cold copper of the wall, but this groove is commonly used for coatings on the free surface of liquid metal. It was found that the slag quickly clogs and the expected thermal effect is weakened.
The object of the present invention is to solve the above-mentioned problems, effectively reduce the heat flow extracted during the formation of the solidified shell and at the start of growth, and to reduce the adverse effect of fluctuations in the height of the free surface of the liquid metal in the ingot mold. An object of the present invention is to provide a method capable of obtaining a casting which is prevented and has an extremely excellent surface quality.
The subject of the invention has a substantially vertically extending forced cooled metal wall defining a passage for the cast metal, extracting the heat flow of the cast metal over the entire height of the metal wall to cool the metal. Cooled in a method of continuously casting molten metal, especially steel in an ingot mold, which gradually solidifies and reduces the strength of the heat flow extracted at a height where the metal begins to solidify in contact with the wall. An insulating frame is placed on the upper side of the metal wall, and during casting, the height of the free surface of the cast metal is maintained inside the insulating frame, and the ingot is at least at the height of this lower edge at the height of the insulating frame. A method is characterized in that gas is injected into the mold and the injected gas is distributed over the inner peripheral edge of the passage in the form of a jet stream.
In the present invention, the free surface (meniscus) of the liquid metal bath is located in the frame. Since the frame is made of an insulating material, the solidified metal film begins to form uniformly only at the upper edge of the metal wall. For this purpose, a purge gas is sent to the bottom of the frame to accurately separate the desired uniform solidification in the cooled metal part from all the undesired local solidification that can occur in the part made of thermal insulation material. The solidification of the metal is started only at a certain distance from the meniscus. This solidification initiation region is almost completely horizontal and is not affected by the inevitable fluctuations and turbulence that agitate the free surface of the bath. The solid ring composed of the first solidified hull is geometrically perfectly uniform, the new ring that follows is also almost perfectly uniform, as is the solidified hull that grows gradually as the casting descends. It is.
Furthermore, since the solidification of the cast metal does not begin in the frame, the cast metal does not shrink at this height. The cast metal maintains contact with the wall of the frame and prevents slag from entering between the cast metal and the wall. As a result, the intrusion of slag between the outer shell that solidifies in contact with the wall and the metal wall of the ingot mold cannot be the case where the solidified outer shell tends to be separated from the wall due to the shrinkage of the solidification. In addition, the static pressure of the liquid metal iron produced by the liquid metal contained in the frame prevents this separation, thus maintaining contact with the metal wall surface of the skin, and the thickness and solidification of the skin is ingot. Since it is uniform at the inner periphery of the mold, it is maintained uniformly throughout the inner periphery of the ingot mold.
Therefore, the heat flow extraction performed at the upper part of the metal wall is also performed uniformly throughout the inner periphery of the casting due to the regularity at this inner periphery, and the local separation of the solid skin and the resulting thickness. Insufficiency is prevented and the strength of the heat flow extracted in the solidification initiation region is very uniform across the inner periphery of the passage defined by the metal wall.
The strength of the extracted heat flow is preferably lowered in a predetermined height region from the lower edge of the frame without greatly changing the overall characteristics of the heat extracted by the ingot mold. By limiting this height, the heat flow extracted in the region where the solid metal skin is generated can be reduced, and the shrinking action and separation of the metal skin observed in the conventional casting process can be prevented.
In the first embodiment of the present invention, a substantially constant heat flow is extracted at the entire height of the region. In this case, the reduction rate of the extracted heat flow can be high, but it is a narrow height range, for example, about 10 mm.
In the second embodiment of the present invention, the heat flow extraction capability increases from top to bottom in the above region. In this case, the decreasing rate of the extracted heat flow gradually decreases toward the bottom of the ingot mold in a region having a large height range. As a result, the reduction rate of the extracted heat flow can be increased compared to the above case in a large height range of this region, and the ingot mold that requires a frame with less heat flow to be extracted and the maximum heat flow extraction. It is possible to give a certain degree of gradual change to the change in the heat flow extracted between the cooled metal parts.
Another object of the present invention is a metal wall extending in a substantially vertical direction defining a passage of cast metal, a cooling means disposed inside the wall for forcibly cooling the wall over substantially the entire height of the metal wall, and a metal wall A continuous cast ingot mold having a means for reducing the strength of the heat flow through the inner surface of the wall produced by the cooling means provided on the upper side of the metal plate, the thermal insulation disposed on the upper extension of the metal wall on the upper side of the metal wall A frame made of material and an injection means for injecting the pressurized gas into the ingot mold in the form of a jet stream, the pressurized gas being at the height of the lower edge at least at the height of the frame. The ingot mold is distributed over the entire inner peripheral edge of the passage.
In the first embodiment of the present invention, the means for reducing the heat flow strength consists of a layer of metal having a lower thermal conductivity than the metal constituting the wall, for example nickel, which is applied to the copper or copper alloy of the wall of the ingot mold. It can be formed by an electrolysis method.
In a first embodiment of the invention, the layer is located on the metal wall, and thus between the metal wall and the insulating material constituting the frame, and its thickness can be, for example, about 1 mm.
In another embodiment of the invention, the layer extends along the inside surface of the cooled metal wall, which can be about a few centimeters high. The poorly conductive metal layer forms a thermal barrier layer between the solidified outer shell of the cast metal and the highly conductive metal of the ingot mold. The heat flow extracted at the entire height where this poorly conductive layer extends is greatly reduced compared with the case where the cast metal is in direct contact with the highly conductive metal on the metal wall (the reduction rate is 50% or less). Can be reached).
By forming a poorly conductive metal layer on both the upper side of the copper or copper alloy wall and the inner surface of the wall, the average value of the heat flow extracted in the upper part of the cooling wall is changed, and the upper end of the metal wall The heat flow can be distributed at the same time depending on the distance from the edge to the height at which the cast metal film begins to solidify, and this distribution is easily preconditioned by gradually decreasing the layer thickness from top to bottom. be able to.
In a second embodiment of the invention, the means for reducing the heat flow strength comprises a substantially vertically extending groove formed on the inner surface of the metal wall. The point where the cast metal is in direct contact with the copper or copper alloy of the cooling wall along the inner peripheral edge of the passage formed by the wall of the ingot mold in the region where the solidified film is formed by this groove, and the extraction amount of the heat flow corresponding to this groove is The decreasing points can be arranged alternately. Unlike the method described at the beginning of this specification, this grooved device can initiate solidification at a distance below the free surface of the metal bath, and thus in the absence of slag. It can be understood that slag cannot enter the slit and clog the slit during casting. In one embodiment of this configuration, at least a portion of the groove is filled with a material that is less conductive than the metal forming the metal wall.
Other features and advantages of the present invention will be better understood from the following description of the steel continuous casting ingot mold of the present invention and its implementation.
Refer to the attached drawing:
FIG. 1 is a longitudinal sectional view of an upper portion of an ingot mold conceptually showing a first embodiment of the present invention.
FIG. 2 is a graph showing the change in heat flow extracted during casting in an ingot mold as a function of the distance from the upper edge of the metal wall.
FIG. 3 is a view corresponding to FIG. 1 of the second embodiment of the ingot mold of the present invention.
FIG. 4 is a graph corresponding to FIG. 2 of the ingot mold of the second embodiment.
FIG. 5 is a conceptual diagram of the upper portion of the wall of the ingot mold of the second embodiment in which grooves are formed in the upper portion of the metal wall.
6 is an enlarged horizontal sectional view of an upper portion of the metal wall of FIG.
FIG. 7 is a view similar to FIG. 6 in which the groove is filled with a metal having poor conductivity.
FIG. 8 is a particularly advantageous design drawing of a frame having a resistance heating element.
9 is a reduced cross-sectional view of the ingot mold taken along line IX-IX in FIG.
The ingot mold shown in FIG. 1 forms a tubular body, has a metal wall 1 cooled with internal circulating water as is well known, and has a metal wall 1 that defines a vertical passage of cast steel 2. The upper part of the metal wall 1 is preferably made of an independent member from its lower part. This independent member is, for example, in the form of an annular part 3, made of the same copper or copper alloy and with its own cooling circuit, which is conceptually indicated by a water circulation conduit 4.
The annular part 3 can be replaced more easily and cheaply when the metal wall is formed of a single part over the entire height of the wall. An electrolytic nickel layer 6 having a thickness of, for example, 1.5 mm is applied to the upper side surface 5 of the annular portion 3.
The heat insulating frame 7 is made of a heat insulating material having a high insulating property, for example, an upper portion 8 having a height of, for example, 200 mm, and a heat insulating material that is less insulative than the upper portion 8 but has a superior strength, for example, It has a lower part 9 made of a material called SiAlON with a thickness of 20 mm, for example. The heat insulating frame 7 has a height of, for example, 40 mm and is disposed on the upper side of the annular portion 3.
A space formed between the nickel layer 6 and the SiAlON 9 forms a slit 10 having a low height, for example, a height of several 1/10 mm. The slit 10 opens to the inner surface of the ingot mold along the entire inner peripheral edge of the ingot mold and communicates with a pressurized inert gas source such as an argon source 110 conceptually shown in FIG.
The supply of the liquid metal to the ingot mold is achieved by a nozzle 11 opened at the height of the frame 7 made of a heat insulating material, for example, a lateral opening opened at a position approximately half the height of the upper portion 8 of the frame 7. This can be done with a well-known nozzle 11 having a part 12.
At the time of casting, molten steel accommodated in a tundish (not shown) to which the nozzle 11 is attached fills the ingot mold through the nozzle and its opening 12. The height of the liquid metal free surface 13 is maintained between the upper edge of the frame 7 and the opening 12, which is immersed in the liquid steel bath 2, and generally the free surface is a slag layer 13. Covered.
As can be seen from FIG. 1, the solid steel skin 21 begins to form at the height of the upper edge of the nickel layer 6 and gradually thickens towards the bottom due to the cooling caused by the metal walls of the ingot mold. Naturally, the outer shell moves, and in fact, continuously moves downward as the casting is extracted, and is continuously updated by solidification of the liquid metal in contact with the nickel layer 6.
Supplying pressurized argon from the slit 10 produces a gas jet flow that is substantially perpendicular to the inner surface of the wall of the ingot mold. Since this gas jet stream shears any initial solidification that may occur in contact with the lower part 9 of the frame, the outer skin 21 is located at the height of the upper edge 14 of the nickel layer 6 at the inner periphery of the same horizontal plane. Start to solidify reliably along the whole.
FIG. 2 shows the change in the extracted heat flow Φ as a function of the vertical distance d from the edge 14. The solid curve 22 shows the heat flow extracted when the ingot mold of the present invention shown in FIG. 1 is used, and the broken curve 23 shows the case where the nickel layer 6 is not present, ie, the outer skin 21 is the upper portion 3 for comparison. It shows the heat flow extracted when it begins to form in direct contact with copper.
In the vertical region corresponding to the thickness of the nickel layer, the extracted heat flow is reduced and this decrease in heat flow continues further several millimeters downward from the nickel layer, but in the entire heat flow extracted by the entire annular part 3. It can be understood that there is no significant effect.
FIG. 3 shows a second embodiment of the ingot mold. Members corresponding to those in FIG. 1 are given the same reference numerals. In this embodiment, an additional nickel layer 15 is applied on the inner lateral surface 16 of the annular part 3 and this annular part is pre-cut for a nickel adhesion layer and after this layer 15 is formed. The inner surface 17 of this layer is substantially flush with the extension of the inner surface of the lower portion of the ingot mold. The thickness of the nickel layer 15 is preferably gradually decreased from the top to the bottom in the height direction of the annular portion 3.
FIG. 4 is a view similar to FIG. 2 of this second embodiment, showing that the heat flow extracted by the entire annular portion (curve 24) is greatly reduced compared to the case where no nickel coating is present (curve 23). Show.
FIG. 5 is a conceptual projection of a portion of the upper portion of an ingot mold according to another embodiment of the present invention. As can be seen from the enlarged view of FIG. Is formed. For example, the depth and width of the groove 31 can be 0.2 mm, and the interval can be 1.5 mm.
As shown in FIG. 7, in this embodiment, the groove 31 can be filled with a metal having low conductivity, such as a nickel deposit 33. In this case, for example, the groove can have a width of 1 mm, a depth of 0.5 mm, and an interval of 2 mm. The nickel deposit formed in the groove is for preventing cast steel from entering the bottom of the groove. The amount of heat flow reduction extracted in this example is the same as the heat flow obtained by the reduction of the exchange surface proportional to the surface occupied by the groove filled with poorly conductive metal.
To simplify the design, an inert gas injection slit is not shown in FIG. 5, but it is clear that such an injection device can be used preferentially for this ingot mold.
Tests conducted by the inventor have yielded satisfactory results from the metallurgical point of view of the casting. That is, a decrease in heat flow extracted in the solidification initiation region of 50% or more is observed, and in particular, a casting having no depression or crack type surface defects is a grade of 0.1% carbon steel that is particularly vulnerable to such defects. Was obtained.
The above embodiment is merely an example, and the present invention is not limited to this. In particular, a metal with poor conductivity other than nickel can be used. In order to reduce maintenance costs, it is preferable to reduce the heat flow extracted at the height of the upper part independent of the lower part forming the wall of the ingot mold, but only a certain height from the upper edge of the metal wall. Each of the above embodiments can also be used directly on a metal wall.
In addition, if the purge gas at the bottom of the frame is a “treatment” means to prevent unwanted local solidification of the walls made of the frame's insulating material during the casting solidification process, it heats the frame to this means. Additional “prevent” measures can be added to complete its action.
That is, in the present invention, as shown in the conceptual diagrams of FIGS. 8 and 9, an electrical resistance heating element, for example, an electrical resistance heating element in the form of a graphite ribbon 71 (PAPYER (registered trademark) type or SIGRAFLER (registered trademark)) is provided. It is advantageous to incorporate it into the frame 7. Since this electric resistance heating element is bent without breaking, it can be wound around the passage of the cast metal 2 (see FIG. 9). The heat generating ribbon 21 can be molded inside the heat insulating material of the frame, or preferably placed inside an annular groove (72) formed in the frame. In this case, the frame is made up of, for example, two parts 73, 74 that are stacked one above the other. If an inert gas such as argon is selected as the purge gas, the problem of oxidation of the graphite heating resistor does not occur.

Claims (12)

A forcedly cooled metal wall (1) extending in a substantially vertical direction defining a passage for the cast metal, extracting a heat flow of the cast metal over the entire height of the metal wall, and cooling the metal (2) gradually solidified Te, metal reduces the strength of the heat flow to be extracted at the height begins to solidify in contact with the wall, in the method for continuously casting a molten metals in ingot mold,
An insulating frame (7) is placed above the cooled metal wall, and the height of the free surface of the cast metal (2) is maintained inside the insulating frame (7) during casting, and the height of the insulating frame (7) is maintained. A gas is injected into the ingot mold at least at the height of the lower edge, and the injected gas is distributed over the inner peripheral edge of the passage in the form of a jet stream. The method according to claim 1, wherein the strength of the extracted heat flow is reduced in a region from the lower edge of the frame to a predetermined height. The method of claim 2, wherein the region has a substantially constant heat flow extraction capability over its entire height. The method according to claim 2, wherein the heat flow extraction capability in the region increases from top to bottom. A substantially vertically extending metal wall (1, 3) defining a passage for the cast metal (2), a cooling means disposed inside the wall for forcibly cooling the wall over substantially the entire height of the metal wall, and a metal wall A continuous cast ingot mold having means (6, 15, 31) for reducing the strength of the heat flow through the inner surface of the wall produced by the cooling means provided on the upper side of
There is a frame (7) made of a heat insulating material disposed on the upper extension of the metal wall on the upper side of the metal wall, and an injection means (10) for injecting the pressurized gas into the ingot mold in the form of a jet flow. The ingot mold is characterized in that the pressurized gas is distributed over the entire inner peripheral edge of the passage at least at the lower edge of the frame. 6. The ingot mold according to claim 5, wherein the means for reducing the heat flow strength comprises a metal layer (6, 15) having a lower thermal conductivity than the metal constituting the metal wall (1, 3). The ingot mold according to claim 6, wherein the layer (6) is located above the metal wall (3). The ingot mold according to claim 6 or 7, wherein the layer (15) extends on the inner surface (16) of the metal wall (3). 9. Ingot mold according to claim 8, wherein the thickness of the layer (15) decreases from top to bottom. 6. Ingot mold according to claim 5, wherein the means for reducing the heat flow strength comprises a substantially vertically extending groove (31) formed in the inner surface (32) of the metal wall (3). The ingot mold according to claim 10, wherein at least a part of the groove (31) is filled with a material (33) having a lower thermal conductivity than the metal forming the metal wall. The ingot mold according to claim 5, wherein the electric resistance heating means (71) is incorporated in the frame (7).

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