LU87722A1 - PROCESS AND PLANT FOR CONTINUOUS CASTING OF METAL - Google Patents

Abstract

The present method for the continuous casting of a metal comprises the following successive operations: a) the temperature of the metal is lowered to a value between Tf and (Tf + 20 DEG C) before and/or during casting of the said metal in the ingot mould, Tf being the liquidus temperature of the said metal, for example by forming a hollow jet flowing along a cooled conduit; b) the cast product is subjected to an intense cooling by means of a continuous layer of cooling liquid under pressure formed at the surface of the said cast product; and c) a moderate cooling is then applied to the said cast product by means of a cooling liquid preferably coming from the preceding intense cooling. In this way, continuously cast products are obtained which have no internal or surface cracks. <IMAGE>

Description

DESCRIPTIVE MEMORY

filed in support of an INVENTION PATENT application

to the GRAND DUCHY OF LUXEMBOURG on behalf of: METALLURGICAL RESEARCH CENTER - CENTRUM VOOR RESEARCH IN DE METALLURGIE, Association sans but Lucratif -Vereniging zonder winstoogmerk à BruxeLLes, (Belgium) for a

PROCESS AND INSTALLATION FOR CONTINUOUS CASTING OF A METAL.

Method and installation for the continuous casting of a metal.

The present invention relates to the continuous casting of a metal, such as liquid steel; it relates on the one hand to a process and on the other hand to an installation making it possible to carry out continuous casting with a high speed.

Continuous casting is a casting method which is now well known and widely used in the art, and in particular in the steel industry. The description which follows is specially devoted to the continuous casting of steel, but it goes without saying that the present process could also find its application for the casting of other metals or metal alloys.

We know that, according to its well-known principle, continuous casting consists in casting steel from a container, such as a ladle or a distribution basket, in a bottomless ingot mold whose side walls are energetically cooled. . The steel undergoes a beginning of cooling in this ingot mold, and it is extracted therefrom by the bottom in the form of a continuous ingot which has a solidified skin surrounding a liquid core. At the exit of the ingot mold, the ingot undergoes cooling by spraying water, called secondary cooling, while being mechanically supported to avoid breakthroughs, that is to say flows of liquid steel by tearing the skin. solidified. The secondary cooling has the effect of increasing the thickness of the solidified skin to a value where there is no longer any risk of breakthrough. The length of the ingot between the exit of the ingot mold and the point where the solidification is complete is called metallurgical length.

Usually, the steel is poured vertically and the ingot then follows, during its solidification, a curved trajectory which brings it to the horizontal. The point of passage from the curved trajectory to the rectilinear horizontal trajectory is generally called the straightening point of the ingot, because its curvature becomes zero there.

In the current practice of continuous casting, the steel contained in the casting container has a temperature higher than its liquidus temperature, in order to avoid any risk of freezing due to localized cooling. This temperature difference, called overheating, is typically of the order of 25 to 35oC.

There has already been proposed, in particular by patent BE-A-08900414, a process for the secondary cooling of a continuously cast metal product, which comprises a step of intense cooling by means of a layer of continuous water under pressure, followed by a moderate cooling step by means of the water flowing along the cast product.

This process makes it possible to obtain products which have very low axial segregation, as well as a surface practically free from cracks. In these products, however, there remain internal, closed cracks, which appear at the solidification front. The object of the present invention is to provide a process for the continuous casting of a metal, in particular steel, which does not have the above-mentioned drawback and which therefore makes it possible to obtain cast products free from cracks. , both on the surface and inside the metal.

According to the present invention, a method for the continuous casting of a metal, in which said metal is poured into a bottomless ingot mold and then the cast product is subjected to secondary cooling by means of a coolant, is characterized in that it comprises the following operations, in combination: a) the temperature of the metal is lowered to a value between Tf and (Tf + 20eC) before and / or during the casting of said metal in the ingot mold, Tf being the liquidus temperature of said metal; b) subjecting the cast product to a first secondary cooling step consisting of intense cooling by means of a continuous layer of pressurized coolant formed on the surface of said cast product; and c) then applying to said cast product a second secondary cooling step consisting of moderate cooling by means of a cooling liquid ensuring a heat exchange coefficient less than or equal to 50% of the heat exchange coefficient achieved. by the coolant used in said first secondary cooling step.

According to a first implementation of the process of the invention, the temperature of the metal in the casting container is lowered to a value situated in the aforementioned temperature range, and the temperature of the metal is maintained at this value during casting .

This implementation may require heating of the metal to compensate for heat losses by radiation or convection.

Another particular implementation of the process of the invention consists in lowering the temperature of said metal by controlled cooling during casting, that is to say between the casting container and the mold. For this purpose, a hollow jet of metal is formed, for example, which flows in a continuous layer along a cooled wall before entering the ingot mold.

However, it has proved advantageous to effect this lowering of the temperature in two stages, namely a limited decrease in the pouring vessel and additional cooling during the casting. The invention also relates to an installation for the continuous casting of a metal, in particular steel, which, in addition to a pouring vessel and a bottomless ingot mold, comprises in combination: a) first means for lowering the temperature of the metal to a value between Tf and (Tf + 20eC) before and / or during the casting of said metal in the ingot mold, Tf being the liquidus temperature of said metal; b) second means for subjecting the cast product to a first secondary cooling step consisting of intense cooling by means of a continuous layer of pressurized coolant formed on the surface of said cast product; and c) third means for applying to said cast product a second secondary cooling step consisting of moderate cooling by means of a coolant.

The first means for lowering the temperature of the metal can consist of a device limiting the cooling of the metal caused by thermal losses in the pouring vessel, ladle or distributor basket; such a device can for example comprise a heat-insulating cover and / or burners.

Another embodiment of said first means consists of a vertical conduit disposed between the casting container and the ingot mold, comprising at its inlet end a metal distribution member which extends transversely to near the interior surface of said drove. This vertical duct is preferably cooled externally, for example by circulation of water, over at least part of its length.

The two aforementioned embodiments of these first means, as well as other equivalent forms, can obviously be combined without departing from the scope of the invention.

The second means, which provide intense secondary cooling of the cast product, advantageously comprise a box surrounding at least partially and at a certain distance the cast product, and at least one wall of which faces towards said product is pierced with orifices for passage of the liquid. cooling.

Such cooling means make it possible to establish a continuous layer of pressurized coolant on the surface of the cast product and thus not only to cool it quickly but also to support it uniformly.

It will easily be understood that said second means could also be constituted, totally or partially, by any other secondary cooling device capable of ensuring the desired cooling, for example by a conventional set of water jets alternating with rollers for supporting the cast product. .

According to a particular embodiment of the installation of the invention, said third means for the moderate cooling of the cast product, comprise an enclosure situated under the aforementioned second means, which surrounds the cast product by defining at its lower end a slot of calibrated width with the poured product. Means can also be provided for measuring the temperature of the product poured at the outlet of said enclosure and for adjusting the width of said slot as a function of the measured temperature.

We will now describe a currently preferred embodiment of the method and installation of the invention, with reference to the accompanying drawings, in which Figure 1 schematically shows a combined installation according to the present invention; and FIG. 2 graphically illustrates the improvement obtained by the process of the invention as regards the internal cracking of the cast products. The continuous casting installation, represented diagrammatically in FIG. 1, is arranged essentially under a casting container, for example a distribution basket symbolized by its casting nozzle 1.

The various devices constituting said installation will be described in the order in which they are traversed by the cast metal, from the liquid state to the solid state. In the present case, the illustrated installation is intended for the casting of steel.

The first device encountered by steel is a distribution member 2, in the form of a dome, disposed in the inlet portion of a vertical duct 3, which is provided with a water cooling circuit 4.5. . This vertical duct 3 is extended downwards by a pouring tube 6. The steel, deflected by the dome 2, flows in a thin layer along the inner surface of the vertical duct 3, and it is thereby subjected a first cooling. The path of the liquid steel is indicated in dotted lines in Figure 1. This first cooling eliminates part of the steel overheating.

The pouring tube 6 is introduced into a casting mold 7, of conventional type, and the lower end of this tube is immersed in the liquid steel 8 present in the ingot mold 7. The steel undergoes cooling and a start of solidification in the ingot mold 7, and it leaves it in the form of a continuous ingot 9 having a solidified skin of a relatively small thickness.

The partially solidified ingot 9 then enters an intense cooling device 10; this essentially consists of a box surrounding the ingot 9 and whose wall facing the ingot is pierced with slots 11 inclined in the direction of progression of the ingot 9. These slots provide passage to a pressurized coolant, usually water, which forms a continuous layer between the ingot 9 and said wall and which cools and uniformly supports said ingot 9.

This water then flows along the ingot 9 and is collected in an enclosure 12 which surrounds the ingot under the box 10. The lower end of this enclosure 12 is constricted so as to form an outlet slot 13 calibrated around the ingot 9. Due to this constriction, the water collected in the enclosure 12 forms a relatively calm bath where the ingot undergoes moderate cooling. A pyrometer 14 measures the surface temperature of the ingot 9 at the outlet of the enclosure 12 and optionally controls the width of the slot 13.

The ingot is finally guided along a curved trajectory, in a manner known per se, to the straightening point where it takes a horizontal trajectory. Its temperature Tr is also measured at the recovery point, because it is this which conditions certain subsequent operations, in particular the hot charging of the cast products.

In an installation of the type shown in Figure 1, a 0.8% C steel was poured into square blooms of 220 mm side, with different values of overheating (AT). This was between 31 ° C and 15 * C; the temperature of the blooms at the rectification point (Tr) was between 870 ° C and 910eC.

For the aforementioned product type and steel grade, the casting speed is normally 0.85 m / min with a temperature Tr * 930 * C. Under the conditions of the invention, the casting speed could be brought to values from 1.2 m / min to 1.6 m / min without the occurrence of surface cracks, whatever the overheating.

However, it appeared that overheating above 20 ° C led to internal cracking at the solidification front, while these internal cracks no longer occurred for overheating below 20 ° C.

This result is clearly illustrated in Figure 2, where the black dots represent the flows which gave rise to internal cracks, while the white points represent the flows without cracking.

The method of the invention makes it possible to obtain products having optimal properties, in particular a very fine solidification structure, practically zero central segregation, a total absence of cracks both internal and surface and a very low porosity. An additional advantage is that these properties are obtained with a substantially increased casting speed compared to conventional practice; the productivity of the installation and certain subsequent operations such as hot charging are thereby favored.

It should be emphasized that the improvements brought about by the process of the invention are unexpected; there is in fact no predictable relationship between the reduction in the overheating of the metal before casting and the behavior of this same metal during its solidification.

Although the invention has been described and illustrated with reference to a particular product and steel, it goes without saying that its object is not limited to this single application. On the contrary, many variants can be envisaged by a person skilled in the art, not only with regard to the type of metal but also in the means used to provide the various coolings required by the claimed process.

Claims (11)

1. A method for the continuous casting of a metal, in which said metal is poured into a bottomless ingot mold and the subdued product is then subjected to secondary cooling by means of a cooling liquid, characterized in that it comprises the following operations, in combination: a) the temperature of the metal is lowered to a value between Tf and (Tf + 20 ° C) before and / or during the casting of said metal in the ingot mold, Tf being the liquidus temperature of said metal; b) subjecting the cast product to a first secondary cooling step consisting of intense cooling by means of a continuous layer of pressurized coolant formed on the surface of said cast product; and c) then applying to said cast product a second secondary cooling step consisting of moderate cooling by means of a cooling liquid ensuring a heat exchange coefficient less than or equal to 50% of the heat exchange coefficient achieved. by the coolant used in said first secondary cooling step. 2. Method according to claim 1, characterized in that the temperature of the metal is lowered before casting to a value situated in the aforementioned temperature range, and the temperature of said metal is maintained at this value during casting. 3. Method according to claim 1, characterized in that the temperature of said metal is lowered during casting by forming a hollow jet of said metal and causing said hollow jet to flow in a continuous layer along a cooled wall before entering the mold. 4. Installation for the continuous casting of a metal according to the process which is the subject of the preceding claims, characterized in that in addition to a casting container (1) and a bottomless molding machine (7), it comprises in combination: a) first means (3) for lowering the temperature of the metal to a value between Tf and (Tf + 20eC) before and / or during the casting of said metal in the ingot mold, Tf being the temperature liquidus of said metal; b) second means (10) for subjecting the cast product to a first secondary cooling step consisting of intense cooling by means of a continuous layer of pressurized coolant formed on the surface of said cast product (9); and c) third means (12) for applying to said cast product (9) a second secondary cooling step consisting of moderate cooling by means of a coolant. 5. Installation according to claim 4, wherein said first means (3) for lowering the temperature of the metal consist of a vertical duct (3) disposed between the casting container (1) and the bottomless ingot mold (7) and comprising its inlet end a metal distribution member (2) which extends transversely to near the interior surface of said duct (3), said vertical duct (3) preferably being cooled externally. 6. Installation according to either of claims 4 and 5, in which said second means (10) comprise a box (10) at least partially and at a certain distance surrounding the cast product, and of which at least one wall facing the said product is pierced with holes (11) for the passage of the coolant. 7. Installation according to either of claims 4 to 6, in which said third means (12) comprise an enclosure (12) surrounding the cast product (9) and located under said second cooling means (10). 8. Installation according to claim 7, wherein said enclosure (12) has a lower end which defines a slot (13) of adjustable width with the cast product (9). 9. Installation according to either of claims 4 to 8, in which the vertical duct (3) is provided with a pouring tube (6) which extends it vertically downwards and which penetrates into the mold without bottom (7). 10. Installation according to either of Claims 6 to 9, in which the said orifices are slots (11) of adjustable width, inclined in the direction of progression of the said cast product (9).