KR100447466B1 - Continuous casting method for metals and ingot mould for implementing same - 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

CONTINUOUS CASTING METHOD FOR METALS AND INGOT MOULD FOR IMPLEMENTING SAME

The present invention relates to continuous casting of metals, in particular steel.

Continuous casting operations, as is well known, consist schematically of pouring molten metal into an ingot mold that forms a passage of molten metal, which consists primarily of a bottomless tubular element, but here copper (usually made of a copper alloy) The wall of the ingot mold made of) is forcibly cooled by the circulating water, and the product which has already solidified outside is continuously drawn out of the ingot mold. The solidification then proceeds towards the center of the product and is completed in the so-called "second cooling" zone under the influence of the water spray line during the descent of the product downstream of the ingot mold. Next, the obtained product (bloom, billet or slab) is cut and rolled to a predetermined length or transformed into bars, wires, sections, plates, sheets or the like before being transported to the consumer.

Surface defects or subsurface defects in the product obtained by continuous casting of steel often make it difficult to withstand rolling operations, or are enlarged by rolling and dropping to an unacceptable level of metallurgical quality of the rolled product, often resulting in scrapping. Let's do it.

During casting, molten metal injected into the ingot mold forms a solid film at the free surface level of the metal when it comes into contact with the cold metal wall of the ingot mold. This film is moved downwards by a jerky movement caused by the vertical vibration of the ingot mold during withdrawal of the product. At the same time, its thickness increases due to continuous heat release through the walls of the ingot mold. Thus, a new film of solid metal is created continuously at the level of the free surface of the metal in the ingot mold (so-called “meniscus”), and this film solidifies over the entire boundary of the ingot mold inner wall, thus ingot mold Cooling during the descent within it forms a spherical ring that is susceptible to shrinkage.

The heat shrinkage of the solid skin is large when the heat release is increased, for example by the natural tendency of the molten metal to shrink during cooling by changes in the solid phase at the end of solidification, in which case low carbon steel or medium carbon steel or Particularly applicable to stainless steel grades.

This peripheral shrinkage separates the solidification angle from the wall of the ingot mold, thereby reducing heat exchange by poor contact between the solidification angle and the cooling wall. This separation is generally not uniform across the interface of the solidification angle and is the source of surface defects in the final product obtained.

In order to avoid or limit these defects, attempts have been made to reduce the heat flux emitted during the solidification angle formation at the start of solidification.

Thus, attempts to reduce this heat flux have been made by using an ingot mold with an insert formed thereon that forms a thermal barrier that prevents molten metal from making direct contact with the cooling wall of the ingot mold. However, such inserts have very irregular resistance over time, and the maintenance costs of such ingot molds are very expensive.

It is also possible to reduce the area of the area where the molten metal is in direct contact with the copper of the cooling wall, for example by uniformly carving the walls forming wafer patterns on these surfaces, or by irregular roughness obtained by sandblasting, for example. Attempts have been made to reduce the heat flux by performing a surface finish to the wall of the ingot mold. However, this method has not yet been found to significantly improve the surface quality of cast products. In practice, the variation caused by the fluctuation in the free surface level of the molten metal is dominant compared to the effect of ablation of the heat flux on the free surface of the molten metal obtained by surface finishing the wall of the ingot mold as described above, resulting in uneven solidification. With this present, surface defects are not eliminated.

It has also been attempted to make vertical grooves in the wall, with the aim of reducing the direct contact between the molten metal and the cold copper of the wall again. However, these grooves are quickly filled with slag which is commonly used as a layer covering the free surface of the liquid metal and it has been found that this reduces the thermal effect sought.

It is an object of the present invention to solve the above problems, in particular to ensure an effective reduction in the heat flux released during the formation of solidification angles and at the beginning of growth, while at the same time especially in the level of the free surface of the liquid metal in the ingot mold This ensures that casting products of very good surface quality are obtained while avoiding adverse effects.

With this objective in mind, the present invention extends vertically to form a passage of molten metal, and also forcibly cools to release the heat flux of the molten metal to cool and gradually solidify the molten metal over the entire height of the wall. In an ingot mold consisting of a metallic metal wall, a method of continuous casting of molten metal, in particular steel, according to the method, the strength of the heat flux released at the level at which the metal is in contact with the wall to start solidification is reduced. A feature of this method is that an insulating frame is disposed on the cooling metal wall, during casting, a free surface level of molten metal is maintained inside the frame and around the entire boundary of the passageway at the level of the lower edge of the frame. Gas is injected into the ingot mold in the form of jet gas distributed.

According to the invention, the free surface (meniscus) of the liquid metal bath is located in the frame. The frame is made of a refractory insulating material so that the solidified metal film starts to form uniformly only from the upper edge of the metal wall, which is such a uniform uniform solidification on the cooling metal part from the undesirable localized solidification part which may occur in the fire resistant part. Enables proper separation of wealth Accordingly, solidification of the molten metal is only initiated at certain intervals in the meniscus. As a result, this solidification initiation region is completely flat, horizontal and free of fluctuations or disturbances that inevitably disturb the free surface of the bath. The solid ring comprising the first solidification angle is geometrically completely uniform and the continuous regeneration of the ring upon lowering of the cast product is almost completely uniform in the same way as the growth of this solidification angle.

In addition, solidification of the molten metal is not initiated in the frame so that the metal does not shrink at this level. The metal is in contact with the wall of the frame and prevents infiltrations of slag between the metal and the wall. As a result, even when the solidification angle is separated from the wall due to the solidification shrinkage, there is no inclusion of slag between the metal wall of the ingot mold and the solidification angle at their contact portion. In addition, the ferro-static pressure of the molten metal due to the height of the metal in the frame inhibits this separation, thus maintaining the solidification angle with respect to the surface of the metal wall and the solidification angle is the entire interior of the ingot mold. The circumference is uniform and thus the thickness and solidification state of the solidification angle are also uniform circumference.

Due to this uniformity of the circumference, therefore, heat dissipation performed at the top of the metal wall is also achieved in the same way around the entire interface of the product, avoiding local separation and thickness underestimation that can occur at the solidification angle and starting to solidify The intensity of heat flux emitted in the region can be ensured in much the same way around the entire boundary of the passageway formed by the metal wall.

Preferably, the intensity of the heat flux released does not significantly change the overall amount of heat released by the ingot mold but is reduced over a region of a predetermined height from the lower edge of the frame. This limited height thus ensures a reduction in the heat flux emitted in the region where the solidification angle forms and also avoids the effects of separation and shrinkage of the metal solidification angle observed in the casting process according to the prior art.

According to a first variant, the region has a rather constant heat flux release capacity over its entire height. In this case, the reduction in the released heat flux can be large, for example only for small heights of 10 mm.

According to a second variant, the region has a heat flux release capability which increases from top to bottom. In this case, the reduction of the released heat flux gradually decreases towards the bottom of the ingot mold for areas with long heights. This allows an even greater reduction in the heat flux emitted than in the previous case in areas where this decrease is due to the long height of this region, and also allows the released flow rate to be as low as possible and the ingot mold with the maximum heat flux release. It guarantees a kind of graduality in the fluctuations of the heat flux emitted between the cooling metal parts of.

The object of the present invention also includes a metal wall extending vertically to form a passage of molten metal and cooling means disposed in the interior of the wall such that the wall is forcibly cooled relative to the height of the entire wall. Means for reducing the intensity of the heat flux caused by the cooling means and passing through the inner surface of the wall, the ingot mold being disposed above the metal wall and extending upwardly, and the level of the lower edge of the frame And injection means for injecting pressurized gas into the ingot mold in the form of jet gas distributed around the entire boundary surface of the passage.

According to a first embodiment, said heat flux intensity reducing means comprises a metal layer having a lower thermal conductivity than a metal forming a wall, said metal layer comprising eg nickel deposited on copper or copper alloy of the ingot mold wall by an electrolytic process. .

According to a first variant, the layer is located above the wall, ie between the refractory material constituting the frame and the wall, the thickness of which may be for example about 1 millimeter.

According to another variant, the layer also extends above a height of about several centimeters with respect to the inner surface of the cooling metal wall. The poor conductive metal layer then forms a thermal barrier between the very good thermal conductive metal of the ingot mold wall and the solidification angle of the molten metal. Over the entire height at which this failing layer extends, the heat flux released is greatly reduced (reduction may reach or even exceed 50%) when compared to the shape where molten metal is in direct contact with the conductive metal of the wall. have).

The formation of a conductive metal layer on the copper or copper alloy wall and on the inner side simultaneously results in an average of the heat flux emitted from the top of the cooling wall and the spread of the flow rate according to the distance from the top edge of the metal wall to the molten metal film solidification start level. It becomes deformable and can also precondition and facilitate this spread by gradually decreasing the thickness of the layer from top to bottom.

According to a second embodiment, said heat flux intensity reducing means comprises a groove made on the inner surface of said wall to extend somewhat vertically. Thus, in the region where the solidification angle is formed and with respect to the circumferential wall of the passage formed by the wall of the ingot mold, the portion where the molten metal is in direct contact with the copper or copper alloy of the cooling wall and the heat dissipation is reduced. The parts corresponding to will alternately exist. Unlike the prior art using the groove system mentioned at the beginning of this specification, the start of solidification is, according to the invention, made below a certain distance of the free surface of the metal bath, ie away from the slag, so that the slag is inserted into the slot during casting. It will prevent infiltration and blockage of the slot.

According to a variant of this configuration, the grooves are at least partly filled with a material having a lower thermal conductivity than the metal forming the wall.

Other features and advantages will be apparent from the description of the continuous steel casting ingot mold according to the present invention.

Referring to the accompanying drawings:

1 is a schematic view of a first variant of the first embodiment , showing a longitudinal cross-sectional view of an upper portion of an ingot mold;

FIG. 2 shows the change in distance from the upper edge of the metal wall versus the heat flux released during casting in such an ingot mold.

3 and 4 show another modification of the first embodiment of the ingot mold and correspond to FIGS. 1 and 2, respectively.

FIG. 5 is a schematic illustration of the top of an ingot mold wall for the second embodiment using the grooves of the top section of the metal wall.

FIG. 6 is an enlarged horizontal sectional view taken from the top of the metal wall along the line IV-IV of FIG.

FIG. 7 is a horizontal cross-sectional view as shown in FIG. 6, illustrating a case where the groove is filled with a weakly conductive metal.

FIG. 8 shows a particularly effective frame configuration with thermal resistors, showing only the vertical cross section of the upper part of one of the four walls of the ingot mold.

9 is a reduced schematic cross-sectional view of the ingot mold taken from the top along the line VIII-VIII in FIG. 8.

The ingot mold shown in FIG. 1 has a metal wall 1 which is cooled in an originally known manner by the internal circulation of water, which forms a tubular body and forms a vertical passage for the cast steel 2. . The upper part of this metal wall is preferably made of copper or copper alloy, for example in the form of a ring-shaped part 3, and with the lower part, which is provided with a self-cooling circulation device schematically shown by the water circulation channel 4. It is made of separate components.

This ring-shaped portion 3 can be easily replaced at a lower cost than the metal wall is formed in a single part with respect to its entire height.

For example, an electrolytic nickel layer 6 having a thickness of about 1.5 mm is added to the upper surface 5 of the ring portion 3.

Insulation frame comprising a top 8 made of a high insulation refractory material, for example 200 mm in height, and a lower insulation but lower strength, for example a bottom 9 made of a fire resistant material known as SiAlON, for example 20 mm thick. 7 is arranged above this ring-shaped portion 3, for example having a height of 40 mm.

As schematically shown in the drawing, a space is formed between the nickel layer 6 and SiAlON 9, which forms an injection hole 10 having a small height of, for example, several tens of millimeters, It is led to its inner surface around the entirety of the ingot mold and is also connected to a pressurized inert gas source 110, such as argon, schematically shown in the figure.

The supply of the liquid metal to the ingot mold comprises a nozzle 11 of known type, comprising a side opening 12 located at the level of the insulating frame 7, for example near the middle height of its upper part 8. Is achieved using.

During casting, molten steel in an unshown tundish to which the nozzle 11 is attached fills the ingot mold through the nozzle and its opening 12. The level of the free surface 13 of the liquid steel is maintained between the upper edge of the frame 7 and the top of the opening 12, so that the opening is submerged in the bath of the liquid steel 2. Typically, the free surface 13 is covered by a slag layer.

As can be seen in FIG. 1, the solidification angle 21 begins to solidify at the upper edge level of the nickel layer 6 and becomes thicker towards the bottom due to cooling by the metal walls of the ingot mold, of course. This solidification angle is in fact continuously updated by the solidification of the liquid steel which continues to move downward with the withdrawal of the cast product and which comes into contact with the nickel layer 6.

The supply of pressurized argon through the injection hole 10 produces a jet gas which is somewhat orthogonal to the inner surface of the ingot mold wall, which jet gas is located exactly at the level of the upper edge 14 of the nickel layer 6. It is used to remove the initial solidification which may occur in contact with the lower part 9 of the frame to ensure that the solidification angle 21 starts to solidify around the entire horizontal plane.

FIG. 2 shows the change in the vertical distance d from this edge 14 to the released heat flux Φ. The solid line curve 22 shows the discharged flow rate when the ingot mold of the present invention shown in FIG. 1 is used, while the dotted line curve 23 for comparison has no nickel layer 6, that is, the solidification angle 21 ) Shows the heat flux released when it starts to form in direct contact with the copper of the upper part 8. It can be seen that in the vertical region corresponding to the thickness of the nickel layer, the discharged flow rate decreases, and this change in the flow rate can also continue several mm below the nickel layer, but by the entire ring-shaped portion 3 It does not significantly affect the total flow rate released.

3 shows a configuration variant of the ingot mold, wherein the same reference numerals as in FIG. 1 are used to indicate corresponding components. In this variant, an additional nickel layer 15 is deposited on the inner side 16 of the ring portion 3, which ring portion is pre-processed for nickel deposition so that after such layer 15 is formed, The inner surface 17 is kept coplanar with the inner surface of the lower part of the ingot mold. The thickness of the nickel layer 15 gradually decreases from the top to the bottom with respect to the height of the ring portion 3.

4 for the second configuration variant, similarly to FIG. 2, shows that the heat flux (curve 24) totally released by the ring portion is greatly reduced compared to the case without the nickel coating (curve 23). see.

FIG. 5 is a perspective view schematically showing a part of an upper portion of an ingot mold wall according to another configuration, in which a vertical groove 31 on the inner surface 32 of the ring portion 3 as shown in the enlarged view of FIG. 6. ) Is made. The grooves 31 have a depth and a width of, for example, 0.2 mm and are spaced at 1.5 mm intervals.

In this variant, as shown in Fig. 7, the grooves can be filled with a deposit 33 of a less conductive metal, such as nickel. In this case, the grooves can be spaced 2 mm apart, for example with a width of 1 mm and a depth of 0.5 mm. Nickel deposits formed in the grooves prevent the cast steel from penetrating into the bottom of the grooves. Therefore, the reduction in the heat flux released in this variant is the same as that obtained by the reduction in the exchange surface which is proportional to the surface occupied by the grooves filled with the less conductive metal.

In order to simplify the configuration, although an inert gas injection hole is not shown in FIG. 5, it is clear that such an injection system is also preferably used for such an ingot mold.

Experimental tests performed by the inventors show particularly satisfactory results from the metallurgical point of view of the cast product, where a more than 50% reduction in the heat flux released at the beginning of the solidification zone has been observed and in the form of depressions or cracks. Products without surface defects have been obtained, in particular for 0.1% carbon steel grades, which are particularly sensitive to such defects.

The invention is not limited to only the above variations described as examples. In particular, a coating metal having a low conductivity other than nickel may be used. In order to reduce the maintenance costs, it is desirable to achieve a reduction in the heat flux emitted at the level of the upper part independent of the lower part constituting the wall of the ingot mold, but the various designs described above only allow the metal to be above a limited height above its upper edge. Can be used directly against the wall.

In addition, if the sweeping gas at the base of the frame is a "treatment" means that counteracts the local undesired coagulation effect on the fire wall of the frame during the casting product solidification process, such means is a "prevention" means of heating the frame. Can be completed by Thus, according to the present invention, for example, a graphite ribbon 71 that can be bent without the risk of being broken so that it can be wound around the passage of molten metal 2 (see FIG. 9), as schematically shown in FIGS. 8 and 9. It may be effective to embed an electrical heating resistor in the frame 7 in the form of PAPYER® or SIGRAFLER®. This heating ribbon 21 may be formed in the refractory material of the frame or preferably arranged in a ring-shaped groove 72 in the frame for this purpose, for example made of two overlaps 73, 74. If care is taken to select an inert gas such as argon as the sweeping gas, the oxidation problem of the graphite heating resistor will not arise.

Claims (12)

Forced-cooled metal walls 1, 3 extending vertically to form passages of molten metal and releasing the heat flux of molten metal to cool and gradually solidify the molten metal 2 over the entire height of the wall. In an ingot mold consisting of: a method of continuously casting molten metal, in particular steel, while reducing the intensity of the heat flux released at the level at which the metal contacts the wall to start solidification, An insulating frame 7 is arranged above the cooling metal wall, and during casting, the free surface level of molten metal 2 is maintained inside the frame and the entirety of the passageway at the level of the lower edge of the frame 7 Injecting gas into the ingot mold in the form of jet gases distributed around the interface. The method of claim 1, And the intensity of the heat flux released is reduced for an area extending from the lower edge of the frame. The method of claim 2, And said region has a constant heat flux release capability over its entire height. The method of claim 2, And said region has a heat flux release capability that increases from top to bottom. Metal walls (1, 3) extending vertically to form passages of molten metal (2) and cooling means arranged inside said wall such that said walls are forced to cool with respect to the height of the entire wall, In the upper part, there is provided a continuous casting ingot mold provided with means for reducing the strength of the heat flux caused by the cooling means and passing through the inner surface of the wall, Injection means for injecting pressurized gas into the ingot mold in the form of an insulating frame 7 disposed above the metal wall and extending upwards and jet gas distributed around the entire boundary of the passageway at the level of the lower edge of the frame ( 10) comprising an ingot mold. The method of claim 5, wherein The ingot mold, characterized in that the heat flux intensity reducing means consists of a metal layer (6, 15) having a lower thermal conductivity than the metal constituting the metal wall (1, 3). The method of claim 6, Ingot mold, characterized in that the metal layer (6) is arranged on the metal wall (3). The method according to claim 6 or 7, Ingot mold, characterized in that the metal layer (15) extends with respect to the inner surface (16) of the metal wall (3). The method of claim 8, Ingot mold, characterized in that the thickness of the metal layer (15) is reduced from the top to the bottom. The method of claim 5, wherein The heat flux intensity reducing means is an ingot mold, characterized in that it consists of a groove (31) formed in the inner surface (32) of the metal wall (3) while extending vertically. The method of claim 10, Ingot mold, characterized in that the groove (31) is partially filled with a material (33) having a lower thermal conductivity than the metal forming the wall. The method of claim 5, wherein Ingot mold, characterized in that the electrical resistor heating means (71) is embedded in the insulating frame (7).