EP0452294B1 - Method and device for the continuous casting of a 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

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 allowing continuous casting with a speed high.

Continuous casting is a well known casting method and widely used in technology, especially in industry steel. 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 metallic alloys.

We know that, according to its well-known principle, continuous casting consists of pour steel from a container, such as a ladle or distribution basket, in a bottomless ingot mold with side walls are energetically cooled. Steel begins to cool in this mold, and it is extracted from the bottom in the form of a continuous ingot which presents a solidified skin surrounding a liquid core. On leaving the ingot mold, the ingot undergoes cooling by splashing water, said secondary cooling, while being supported mechanically to avoid breakthroughs, i.e. steel spills liquid by tearing solidified skin. 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 outlet of the 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 transition point from the curved path to the path horizontal straight is usually 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 pouring vessel has a temperature higher than its temperature of liquidus, to avoid any risk of freezing due to cooling located. This temperature difference, called overheating, is typically on the order of 25 to 35 ° C.

We have already proposed, in particular by patent BE-A-08900414, a process secondary cooling of a continuously cast metal product, which includes an intense cooling step using a layer of water continues under pressure, followed by a moderate cooling step at by means of water flowing along the poured product.

This process makes it possible to obtain products which exhibit segregation very weak axial, as well as a surface practically free of cracks. In these products, however, there remain closed internal cracks which appear on the solidification front.

It is also known, from document EP-A-0 054 867, to apply bare secondary cooling in two stages of a continuously cast steel product. However, this process does not allow not prevent internal cracking of the product.

The object of the present invention is to provide a method for casting continuous of a metal, in particular steel, which does not have the disadvantage mentioned above and which therefore makes it possible to obtain free cast products cracks, both on the surface and inside the metal.

a) on abaisse la température du métal jusqu'à une valeur comprise entre T_f et (T_f + 20°C) avant et/ou pendant la coulée dudit métal dans la lingotière, T_f étant la température de liquidus dudit métal;

b) la première étape de refroidissement secondaire consiste en un refroidissement intense au moyen d'une couche continue de liquide de refroidissement sous pression formée à la surface dudit produit coulé; et

c) la seconde étape de refroidissement secondaire consiste en un refroidissement modéré au moyen d'un liquide de refroidissement assurant un coefficient d'échange de chaleur inférieur ou égal à 50 % du coefficient d'échange de chaleur réalisé par le liquide de refroidissement utilisé dans ladite première étape de refroidissement secondaire.

In accordance with the present invention, a method for the continuous casting of a metal, in which the said metal is poured into a bottomless ingot mold and the subdued product is then subjected to secondary cooling in two stages of different intensity 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 T _f and (T _f + 20 ° C) before and / or during the casting of said metal in the ingot mold, T _f being the liquidus temperature of said metal;

b) the first secondary cooling step consists of intense cooling by means of a continuous layer of pressurized coolant formed on the surface of said cast product; and

c) the second secondary cooling step consists 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 produced by the cooling liquid used in said first secondary cooling step.

According to a first implementation of the method of the invention, we lower the temperature of the metal in the casting container up to a value located in the aforementioned temperature range, and the temperature is maintained metal at this value during casting.

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

Another particular implementation of the process of the invention consists in lower the temperature of said metal by controlled cooling during the casting, that is to say between the casting container and the mold.

For this purpose, for example, a hollow jet of metal is formed which flows in a layer continues along a cooled wall before entering the ingot mold.

However, it has been found to be advantageous to lower the temperature. in two stages, namely a limited decrease in the container of casting and additional cooling during casting.

a) des premiers moyens pour abaisser la température du métal jusqu'à une valeur comprise entre T_f et (T_f + 20°C) avant et/ou pendant la coulée dudit métal dans la lingotière, T_f étant la température de liquidus dudit métal;

b) des deuxièmes moyens pour soumettre le produit coulé à une première étape de refroidissement secondaire consistant en un refroidissement intense au moyen d'une couche continue de liquide de refroidissement sous pression formée à la surface dudit produit coulé; et

c) des troisièmes moyens pour appliquer audit produit coulé une deuxième étape de refroidissement secondaire consistant en un refroidissement modéré au moyen d'un liquide de refroidissement.

The invention also relates to an installation for the continuous casting of a metal, in particular steel, which, in addition to a casting container and a bottomless ingot mold, comprises in combination:

a) first means for lowering the temperature of the metal to a value between T _f and (T _f + 20 ° C) before and / or during the casting of said metal in the ingot mold, T _f 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 cooling liquid.

The first ways to lower the temperature of the metal may be in a device limiting the cooling of the metal caused by heat losses in the pouring vessel, ladle or basket distributor; such a device can for example include a cover thermal insulation and / or burners.

Another embodiment of said first means consists of a vertical duct arranged between the pouring container and the ingot mold, having 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, by 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 leaving of the scope of the invention.

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

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

It will be readily understood that said second means could also be made up, in whole or in part, by any other secondary cooling capable of providing the desired cooling. By example, this device may consist of a conventional set of water sprinklers alternating with rollers to support the cast product.

Another interesting way to ensure the desired rapid cooling consists in using an enclosure surrounding the product poured over a length appropriate, the inner wall of this enclosure being pierced with orifices directed to the product to be cooled. This enclosure is supplied with water under pressure; this escapes through the orifices in the form of jets which strike the surface of the cast product to cool and ensure its rapid cooling.

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

Figure 1

représente schématiquement une installation combinée conforme à la présente invention; et la

Figure 2

illustre graphiquement l'amélioration obtenue par le procédé de l'invention en ce qui concerne la fissuration interne des produits coulés; la

Figure 3

illustre graphiquement l'amélioration obtenue par le procédé de l'invention en ce qui concerne la ségrégation axiale dans les produits coulés.

We will now describe a currently preferred embodiment of the method and installation of the invention, with reference to the accompanying drawings, in which the

Figure 1

schematically represents a combined installation in accordance with the present invention; and the

Figure 2

graphically illustrates the improvement obtained by the process of the invention with regard to the internal cracking of the cast products; the

Figure 3

graphically illustrates the improvement obtained by the process of the invention with regard to axial segregation in the cast products.

The continuous casting installation, shown schematically in the Figure 1, is arranged essentially under a pouring container, by example a distribution basket symbolized by its pouring nozzle 1.

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

The first device encountered by steel is a distribution device 2, domed, arranged in the inlet portion of a vertical duct 3, which is provided with a water cooling circuit 4.5. This conduit vertical 3 is extended downwards by a pouring tube 6.

The steel, deflected by dome 2, flows in a thin layer along the inner surface of the vertical duct 3, and therefore undergoes a first cooling. The path of the liquid steel is indicated in dotted lines in Figure 1. This first cooling eliminates part of the overheating steel.

The pouring tube 6 is introduced into a casting mold 7, of the type conventional, 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 beginning of solidification in the mold 7, and it leaves this in the form of a continuous ingot 9 having a solidified skin relatively thin.

The partially solidified ingot 9 then enters a device for intense cooling 10; this one basically consists of a box surrounding the ingot 9 and whose wall facing the ingot is pierced with slots 11 inclined in the direction of advancement of the ingot 9. These slots allow passage to pressurized coolant, usually water, which forms a continuous layer between 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 a enclosure 12 which surrounds the ingot under the casing 10. The lower end of this enclosure 12 is constricted so as to form an outlet slot 13 calibrated around ingot 9. Due to this constriction, the water collected in enclosure 12 forms a relatively calm bath where the ingot undergoes a moderate cooling. A pyrometer 14 measures the surface temperature of the ingot 9 at the outlet of enclosure 12 and optionally controls an adjustment 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 T _R is also measured at the straightening 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 (ΔT). This was between 31 ° C and 15 ° C; the temperature of the blooms at the rectification point (T _R ) was between 870 ° C and 910 ° C.

For the aforementioned product type and steel grade, the casting speed is normally 0.85 m / min with a temperature T _R = 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 cracks internals 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 that the white dots represent the flows without cracking.

Figure 3 illustrates the evolution of axial segregation as a function of overheating. Axial segregation is expressed by the ratio (C _max / C ₀ ) between the maximum carbon content, essentially in the axis, and the average carbon content in the cross section of the cast product. Overheating (ΔT) is expressed in degrees centigrade. The products considered here are square billets of 220 mm per side, with a nominal carbon content of 0.8%. The Figure shows a curve (a) representing the variation of the aforementioned ratio in the case of products cast with conventional cooling and a casting speed of 0.85 m / min. The set of points (b) corresponds to products obtained in accordance with the invention, cast with a speed of 1.5 m / min. It is clearly seen that the ratio C _max / C ₀ is much closer to 1 for these latter products, which translates into a marked reduction in axial segregation by virtue of the method of low overheating casting and two-stage cooling proposed by the present invention.

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

The unexpected nature of the improvements should be emphasized. by the method of the invention; there is indeed no predictable relationship between the reduction of the metal overheating before casting and the behavior of this same metal during its solidification.

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

Claims (10)

1. Process for the continuous casting of a metal, in which the said metal is cast into a bottomless mould and the cast product is then subjected to secondary cooling in two steps of different intensity by means of a liquid coolant, characterized in that it comprises, in combination, the following operations:

   a) the temperature of the metal is lowered to a value of between T_f and (T_f + 20°C) before and/or while casting the said metal into the mould (7), T_f being the liquidus temperature of the said metal;

   b) the first secondary cooling step consists of intense cooling by means of a continuous layer of pressurized liquid coolant formed on the surface of the said cast product; and

   c) the second secondary cooling step consists of moderate cooling by means of a liquid coolant ensuring a heat-exchange coefficient of less than or equal to 50% of the heat-exchange coefficient produced by the liquid coolant used in the said first secondary cooling step.
2. Process according to Claim 1, characterized in that the temperature of the metal is lowered before casting to a value lying within the aforementioned temperature range and the temperature of the said metal is maintained at this value during casting.
3. Process according to Claim 1, characterized in that the temperature of the said metal is lowered during casting, by forming a hollow stream of the said metal and making the said hollow stream flow as a continuous layer along a cooled wall before entering the mould.
4. Plant for the continuous casting of a metal according to the process which forms the subject of the preceding claims, characterized in that, in addition to a pouring vessel (1) and a bottomless mould (7), it includes, in combination:

   a) first means (3) for lowering the temperature of the metal to a value of between T_f anf (T_f + 20°C) before and/or while casting the said metal into the mould, T_f being the liquidus temperature of the 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 liquid coolant formed on the surface of the said cast product (9); and

   c) third means (12) for applying a second secondary cooling step to the said cast product (9), said step consisting of moderate cooling by means of a liquid coolant.
5. Plant according to Claim 4, in which the said first means (3) for lowering the temperature of the metal consist of a vertical duct (3) placed between the pouring vessel (1) and the bottomless mould (7), and having at its inlet end a device (2) for distributing the metal which extends transversely right up close to the inside surface of the said duct (3), the said vertical duct (3) preferably being cooled on the outside.
6. Plant according to either of Claims 4 and 5, in which the said second means (10) comprise a box (10) which surrounds, at least partially and at a certain distance, the cast product and at least one wall of which, facing the said product, is pierced by holes (11) for passage of the liquid coolant.
7. Plant according to any one of Claims 4 to 6, in which the said third means (12) comprise an enclosure (12) which surrounds the cast product (9) and lies under the said second cooling means (10).
8. Plant according to Claim 7, in which the said enclosure (12) has a lower end which defines a slot (13) whose width can be adjusted with the cast product (9).
9. Plant according to any one 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 enters the bottomless mould (7).
10. Plant according to any one of Claims 6 to 9, in which the said holes are slots (11) whose width can be adjusted, the said slots (11) being inclined in the direction of advance of the said cast product (9).

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