EP0342082B1 - Method for cooling a metallic continuous casting product - Google Patents

Abstract

The method according to the invention is characterised in that an intense cooling of the product in the process of being continuously cast is carried out when the latter is, at its core, in the pasty solidification phase (8), as a result of which the differential thermal contraction between the pasty core and the outer shell which is already completely solidified produces a squeezing effect of the core by the shell (9). To this end, means for cooling the product are arranged on the casting machine in the region of the terminal portion of the metallurgical length. <??>The invention makes it possible to reduce, and even prevent the formation of internal cracks during the cooling of the cast product, which would lead to the presence of segregated areas in the axial region. It is applied advantageously to the casting of steels which are well known to be difficult to cast by continuous casting, such as steels with a wide solidification range, the carbon content of which is from 0.25 to 1.5%. <IMAGE>

Description

The present invention relates to a method of cooling a metal product during continuous casting intended to reduce, or even eliminate the presence of a large segregated zone in the central part of the product. This process is advantageously applicable to the continuous casting of steel products known to be difficult to cast according to this technique, such as steels having a wide solidification interval, that is to say for example those whose carbon content is between 0, 25 and 1.5% approximately.

For the good understanding of what follows, it will be advantageous to represent the product being solidified as the combination of three concentric bodies, namely: a ring formed by the outer crust, or skin, already solidified, enclosing another pasty ring which surrounds the liquid core of molten metal. By pasty state is meant a state where the metal is at a temperature between the liquidus and the solidus, and where coexist in variable proportions of the liquid metal and solid crystals. During the extraction of the product, it travels slowly along the machine while being cooled, so that solidification progresses from the periphery towards the center. The liquid core and the pasty ring thus have conical profiles whose points are oriented towards the bottom of the machine. The interfaces between these various concentric bodies constitute, respectively, as we are used to designating them, the fronts of ending and beginning solidification. At an advanced stage of solidification, the liquid core disappears (bottom of the beginning solidification well), and only a solidified crust and a pasty core remain. At a later stage, the pasty zone disappears in turn (closing of the finishing solidification well) and the product is completely solidified.

• la lingotière, où le métal liquide entre en contact avec des parois bonnes conductrices de la chaleur et énergiquement refroidies par circulation d'eau. C'est dans cette zone, dite de refroidissement primaire que débute la formation de la peau solidifiée qui enserre le coeur liquide du produit, et que le produit prend sa forme définitive ;
• la zone dite de "refroidissement secondaire", qui débute juste en-dessous de la lingotière et s'étend sur une longueur variable selon les conditions locales. Dans cette zone, la peau solidifiée du produit en défilement est arrosée par un fluide refroidissant (généralement de l'eau pulvérisée, ou un mélange-air eau), ce qui a pour effet d'accélérer la progression des fronts de solidification commençante et finissante vers l'intérieur du produit. Cependant, à l'endroit où cesse l'aspersion d'eau, la solidification complète du produit n'est pas réalisée, et le coeur du produit demeure à l'état liquide ;
• et la portion de la machine qui fait suite à la zone de refroidissement secondaire. Le produit en défilement n'y est plus arrosé et se refroidit de façon naturelle. C'est dans cette zone que s'achève la solidification du coeur du produit.

Solidification and cooling of the product during casting are normally ensured in three successive zones of the continuous casting machine, namely, in the direction of progression of the product during its extraction:

• the ingot mold, where the liquid metal comes into contact with walls that are good conductors of heat and energetically cooled by circulation of water. It is in this zone, called primary cooling that the formation of the solidified skin begins which encloses the liquid core of the product, and that the product takes its final form;
• the so-called "secondary cooling" zone, which begins just below the mold and extends over a variable length according to local conditions. In this zone, the solidified skin of the moving product is sprayed with a cooling fluid (generally water spray, or an air-water mixture), which has the effect of accelerating the progression of the beginning and ending solidification fronts. inward of the product. However, at the point where the spraying of water ceases, complete solidification of the product is not carried out, and the core of the product remains in the liquid state;
• and the portion of the machine that follows the secondary cooling zone. The scrolling product is no longer watered there and cools naturally. It is in this area that the solidification of the product core ends.

The forced cooling of the product in the mold and after its exit from the mold provides rapid growth in the thickness of solidified skin, in order to limit the risks of breakthrough and significantly increase the speed of extraction of the product, on which the productivity of the continuous casting machine.

Furthermore, the solubility in iron of alloying elements, such as carbon, is lower when the iron is in the solid state than in the liquid state. In the pasty ring, there are therefore locally in the liquid differences in concentration, for example in carbon.
If, within the pasty ring, there is movement of the liquid enriched in carbon, this results in the presence, in the center of the completely solidified product, of so-called "segregated" zones, where the carbon concentration (and / or other segregating elements) is significantly higher than in other regions. The other alloying elements have a behavior similar to that of carbon, and the location of the segregated zones can be deduced from the tests commonly called "Baumann imprints" which make it possible to identify the distribution of the sulfur on a polished section of the product. These segregated zones, also identifiable on metallographic attacks, have a detrimental influence on the homogeneity of the mechanical properties of the product. Thus, the relatively higher concentration of carbon in the center leads to higher hardness in these areas than in the rest of the product after rolling.

This phenomenon is particularly marked in the case of steels heavily loaded with alloying elements, such as those containing 0.5 to 1.5% of carbon and commonly called steels with a large solidification interval, such as the grade of bearing steel. 100 C6 for example. A "Baumann imprint" made on a sample of the product taken along its longitudinal axis would show that the segregations are distributed around the axis of the product according to "ves" whose formation mechanisms are not, moreover, still completely elucidated.

An attempt has been made to resolve this problem by applying an electromagnetic stirring of the metal in the pasty solidification zone so as to force the segregated liquid to be distributed over a more extensive zone. But in doing so, we actually correct the effects without really addressing the causes of the phenomenon. In addition, this technique involves the acquisition of at least one stirring inductor, as well as significant operating costs.

The object of the present invention is to propose a simple and economical solution for reducing, or even eliminating the highly segregated zones in the core of the continuously cast products, by attacking the very cause responsible for their formation. It can be added to or a substitute for electromagnetic stirring in the end of pasty solidification zone.

To this end, the subject of the invention is a method of cooling a metal product, in particular steel, during continuous casting, characterized in that a forced cooling of the product is carried out, while the product is in pasty solidification phase, this cooling being carried out so that the differential thermal contraction between the pasty core and the already completely solidified crust which envelops it, permanently causes a tightening effect of the heart by the crust. This cooling is carried out in an area which extends at least between the place where, in the absence of such cooling, the rate of cooling of the pasty core of the product would exceed that of the surface of the product, and a place where the thermomechanical behavior of the pasty core during cooling is identical to that of the solidified outer crust.

As will be understood, the invention consists in fact in using the solidified outer crust as a vice which accompanies the contraction of the heart during cooling. In other words, the inside diameter of the ring formed by the solidified crust must decrease faster than the diameter of the pasty core would decrease if the crust had no effect on the heart. This vice is put into action, by thermal, simply by means of an accelerated cooling of the surface of the product in the lower part of the machine, where usually the product was allowed to cool naturally.

It was indicated above that the causes of formation of segregated "ves" in the central part of the cast product have not yet been fully identified and explained.

However, the hypothesis made by the inventors as being the most probable and which underlies the present invention, can be schematically explained as follows.

When passing through the secondary cooling zone, the skin of the product cools rapidly, while the liquid core remains at an almost constant temperature. As the product passes through the natural cooling zone, the cooling of the skin, which is no longer watered, becomes much slower. On the other hand, given the length usual in the secondary cooling zone, it is only when the product is already largely engaged in the natural cooling zone that the temperature of the core (which is then in the pasty state), tends to drop appreciably.

The pasty internal part of the product then cools more quickly than the solid layer which envelops it and undergoes a greater thermal contraction. The mechanical stresses thus created are released by the formation of cracks in the central block previously "pasty", cracks in which can penetrate by suction, highly segregated liquid.

Thus, in the completely solidified product, the locations of these cracks will be identified by their high concentration of alloying elements, leading to the defects mentioned above.

In the case of steels heavily loaded with alloying elements, such as carbon, such as 100 C6 for example, the difference between the temperatures at the start and end of solidification is relatively large, and pasty solidification is therefore likely to be carried out over a wider area than in the case of low alloy grades. This, together with a greater sensitivity to the segregation of the elements between the liquid and solid phases, explains why the alloyed grades are so subject to the formation of segregated zones in the axial region of the continuously cast products. In certain extreme cases, such defects make it impossible to obtain finished products of sufficient quality, and impose the necessity of giving up producing them by continuous casting.

• la figure 1 représente schématiquement une installation de coulée continue courbe de demi-produits en acier, de conception classique ;
• la figure 2 représente l'installation de la figure 1, modifiée selon l'invention par adjonction d'une rampe de refroidissement dans la zone de fin de solidification, du produit ;
• la figure 3 montre un cas d'évolution des vitesses de refroidissement de la surface et du coeur du produit au cours de son défilement dans la partie inférieure de la machine. Sont figurés les deux cas de l'absence et de la présence d'un dispositif de refroidissement dans la zone de fin de solidification du produit.

We have just quickly seen how the invention, by causing a thermal contraction of the peripheral solid crust, counteracts the velocity of the product to form these cracks interns responsible for highly segregated central areas. However, the invention will be well understood and other characteristics and advantages will emerge from the following detailed description given with reference to the accompanying drawing plates, in which:

• Figure 1 schematically shows a continuous casting installation of semi-finished steel semi-finished products, of conventional design;
• 2 shows the installation of Figure 1, modified according to the invention by adding a cooling ramp in the end of solidification zone, of the product;
• FIG. 3 shows a case of evolution of the cooling speeds of the surface and of the core of the product during its movement in the lower part of the machine. The two cases of the absence and the presence of a cooling device in the zone of end of solidification of the product are shown.

Figure 1 is a schematic longitudinal section of a conventional continuous casting installation, and in particular it shows the product being solidified. A pocket, not shown, feeds liquid steel 1 to a distribution basket 2. The liquid steel 1 then flows into one or more ingot molds 3 with copper or copper alloy walls energetically cooled by water. It is in each of these ingot molds or primary cooling zones Ⓧ that the solidification of a product 4 begins at its periphery, which thus takes its final section. The mold shown in Figure 1 has a curvature, and it is found on the product. The case of the right ingot mold giving birth to a right product is also encountered in industrial practice. Just below the ingot mold 3 begins the secondary cooling zone Ⓨ in which the product 4 is sprayed over a variable length according to the machines by a ramp of injectors 5. These project all around the product a cooling fluid , usually sprayed or atomized water. Next comes the natural cooling zone Ⓩ, where a conventional machine such as that shown schematically does not have means for cooling the product. In the lower part of the machine are the means (not shown) for decentring the product, responsible for giving it a straight shape, and means (not shown) for cutting up the product for cutting it to length.

FIG. 1 makes it possible to distinguish several concentric regions inside the product during casting, corresponding to the physical state of the material which they contain. In a product section located in the upper part of the machine (for example in zone Y), there are successively three regions. At the core (region 6) the metal is entirely in the liquid state; the section of this zone decreases as the product solidifies, and after the point of closure of the liquid well 7, there is no longer any liquid metal alone. Around the liquid core 6, a pasty region 8, corresponding to the metal being solidified, contains both liquid and solid. The proportion of the latter increases as the temperature decreases. Around the pasty region, the crust 9 consists only of solidified metal. Beyond the closing point of the ending solidification well 10, this region 9 covers the entire product, the solidification of which is then completed. The zone of the installation which extends between the meniscus and the level corresponding to the point of closure of the finishing solidification well 10 is called "metallurgical length".

Figure 2 shows the continuous casting machine of Figure 1 modified according to the invention. The elements common with Figure 1 are identified by the same numbers. The difference between the two configurations lies in the addition to the original machine of a second ramp of injectors 11, located in the zone Ⓩ of the machine where the product completes its solidification.

• format du produit : billettes de section carrée, de 105 mm de côté,
• composition du produit : acier à 0,7 % de carbone,
• vitesse d'extraction du produit : 3,3 m/min.

FIG. 3 shows examples of changes in the speed V of cooling of the metal at the surface and to the core as the product advances in the zone Ⓩ of the machine where it completes its solidification. This advancement is expressed by the distance D to the meniscus, that is to say the surface of the liquid metal in an ingot mold. The curves were plotted using mathematical models similar to those available to users of continuous casting machines. They are valid for the following casting conditions:

• product format: square section billets, 105 mm side,
• product composition: 0.7% carbon steel,
• product extraction speed: 3.3 m / min.

Under these conditions, the complete solidification of the product is carried out at a distance of 11.20 m from the meniscus, marked in the figure by the line S.

Curves A and B correspond to the case of FIG. 1, where the product, in the terminal part of the machine, is not subjected to any forced cooling. Curve A represents the rate of surface cooling of the product. It shows that this speed remains substantially constant (a loss of 0.5 ° C / s) over the entire length of the zone considered. Curve B represents the rate of cooling of the pasty core of the product. It shows that, at the start of the zone considered, the temperature of the pasty core remains practically constant, as the cooling rate appears close to 0 ° C / s. It is only from a distance to the meniscus of approximately 8 m that the cooling of the pasty heart accelerates significantly. At a distance from the meniscus of 9.5 m, curve B intersects curve A. This means that beyond this point, the pasty heart begins to lose more than 0.5 ° C / s, and therefore that the cooling rate of the pasty core begins to exceed the cooling rate of the product surface. This results in a stronger thermal contraction of the heart than that of the surface, a phenomenon which we have seen that, according to the hypothesis made by the inventors, it was at the origin of the defects in the product which the invention aims to 'to avoid.

Curves C and D correspond to the case of FIG. 2, where the product, in accordance with the invention, is subjected to forced cooling in the zone Ⓩ at the end of solidification by means of the injector ramp 11. These curves have been traced in the hypothesis where the product is watered, between the distances to the meniscus 8.40 m and 11.20 m, with water at a flow rate of 12 m³ per hour and per m² of product sprayed, this flow rate being evenly distributed over the entire watering area. The distance to the meniscus of 8.40 m was chosen from curves A and B in Figure 3, that is to say a distance which is less than the distance of 9.50 m at which, in the absence of such a watering zone (case of FIG. 1), the rate of cooling of the pasty core begins to exceed the rate of cooling of the surface of the product. Curve C represents, when the product is watered according to the invention, the rate of cooling of the surface of the product, and curve D represents, under the same conditions, the rate of cooling of the pasty core. Upstream of the cooling zone, these curves merge with curves A and B respectively. From the start of the forced cooling zone, the cooling of the surface accelerates suddenly, reaching 9 ° C / s at a distance at the meniscus 9 m. Then, the cooling becomes more and more slow, because of the progressive deterioration of the quality of the heat exchanges between the cooling water (whose flow and temperature are constant) and the product (whose temperature decreases as and as it progresses through the cooling zone). At the same time, forced cooling has the effect of accelerating the cooling of the pasty heart, but this effect is only felt late (from the distance to the meniscus 10 m), and gradually. In the end, it is only at a distance from the meniscus of 11 m, that curve D intersects curve C. This means that at this distance, the cooling of the pasty core becomes faster than that of the surface of the product. At this level, the pasty heart has practically solidified, and its thermomechanical behavior is sufficiently close to that of the fully solidified crust so that the phenomenon of differential thermal contraction is negligible, and that segregated "ves" cannot be formed.

The example described above is, of course, not limiting. A figure similar to Figure 3 can be drawn for any continuous casting machine, on which a given product would be cast under defined conditions.

It is considered that, beyond the point where the solid fraction of the pasty core of the product reaches 90%, it is useless to continue watering. In some cases, it is even sufficient to water only up to a solid fraction of 60%.

It is advisable to continue forced cooling of the product to approximately 1 m beyond the end of solidification point determined by the calculation. With this in mind, in FIG. 3, the cooling ramp 11 is represented as extending beyond point 10. Likewise, the uncertainty of calculation on the determination of the point of intersection between the curves A and B in Figure 3 is approximately ± 1 m. The choice of the point where forced cooling begins must take this uncertainty into account. It is therefore advisable to place the first injectors of the ramp 11 at least 1 m upstream of said point of intersection, which was done in the digital example of FIG. 3, as explained previously. But it is also necessary to ensure that this advancement of the start of cooling does not cause a premature crossing of the curves C and D of FIG. 3, that is to say which would take place at a point where the solid fraction of the pasty core would be less than 60% at least.

The recommended cooling water flows are of the order of 8 to 15 m³ / h and per m² of sprinkled metal. Preferably, a flow rate of 12 m³ / m².h is chosen.

This process is easily adaptable to all continuous casting machines intended for the manufacture of steel products. It is more specifically designed for the casting of steel grades containing approximately from 0.25 to 1.5% of carbon.

A variant of this method would consist in designing the cooling ramp 11 so that the flow of cooling fluid varies between the start and the end of the cooling zone. The value of the average overall flow over the entire area would be unchanged compared to the configuration described above. In this way, it would be possible to better control the flow of heat extracted from the product along the cooling zone, in order to attenuate the reduction, visible in FIG. 3, of the speed of cooling at the surface of the product. Thus, one would increase the probability of having until the extreme end of solidification a cooling at heart less rapid than in skin.

On the other hand, it was noted that good homogeneity of the core of the product on which the process was to be applied was favorable to the reproducibility of the good metallurgical results sought. It has been observed that this homogeneity could advantageously be obtained by setting the liquid core in motion in the secondary cooling zone, or even in an ingot mold. This setting in motion can be favorably obtained using electromagnetic stirring means, now widely known in the field of continuous casting. These means may be constituted by annular polyphase inductors arranged around the cast product and producing a magnetic field rotating around the casting axis, or by polyphase inductors of planar structure producing a sliding field, parallel to the casting axis or perpendicular to the latter. Literature now abounds about this type of brewing. For more details, reference may be made, if desired, to the following documents: French patent 2,315,344 for mixing by rotating field in an ingot mold, French patent 2,211,304 relating to mixing by rotating field in the secondary cooling zone. , the Luxembourg patent 67 753 concerning the mixing using inductors producing a sliding field perpendicular to the pouring axis in the secondary cooling zone. The lessons of these various documents are included by reference in the present description.

It goes without saying that the invention is not limited to the examples described, but extends to multiple variants or equivalents insofar as the characteristics mentioned in the appended claims are respected. In particular, the method according to the invention can be applied to vertical, straight or curved continuous casting machines, as well as to horizontal continuous casting machines, as well as to existing or future installations for casting. direct continuous of thin products.

On the other hand, the invention does not apply restrictively to steel semi-finished products, but extends its field of application to any metallurgical product which is poured continuously, or which may be.

Likewise also, the invention applies equally to any metallurgical product cast continuously whatever its format: blooms, billets or slabs, in particular those intended for slitting to form blooms.

Claims (13)

1. Process for cooling a metal product (4), in particular made of steel, during continuous casting, characterised in that forced cooling of the product (4) is performed when, at the core, the product is in a phase of pasty solidification, the said cooling being conducted so that the differential thermal contraction between the pasty core (8) and the already completely solidified shell (9) surrounding it permanently gives rise to a squeezing effect of the core (8) by the shell (9) and being implemented in a zone extending along the casting machine at least between the point where, in the absence of such cooling, the speed of cooling of the pasty core (8) of the product (4) would exceed that of the surface of the product (4) and a point where the thermomechanical behaviour of the pasty core (8) during cooling is identical to that of the solidified outer shell (9).
2. Process according to Claim 1, characterised in that the cooling is maintained so that the effect of squeezing the pasty core (8) by the solidified shell (9) is continued up to a point where the proportion of solid material within the pasty core (8) is at least 60%.
3. Process according to Claims 1 or 2, characterised in that the forced cooling is performed by spraying a cooling fluid, such as water, onto the surface of the cast product (4).
4. Process according to Claim 3, characterised in that the cooling is performed with water at a mean flow rate of between 8 and 15 m³ per hour and per m² of sprayed product.
5. Process according to Claim 4, characterised in that a value of approximately 12 m³ per hour and per m² of sprayed product is chosen for the said mean flow rate.
6. Process according to Claim 3, characterised in that the flow rate of cooling fluid varies between the start and the finish of the cooling zone.
7. Process according to Claims 1, 2, or 3, characterised in that it is applied to the casting of products made of steel whose content of carbon by weight is of the order of 0.25 to 1.5%.
8. Process according to Claims 1, 2, or 3, characterised in that, simultaneously, the liquid core (6) of the product (4)is stirred.with the aid of agitation means.
9. Process according to Claim 8, characterised in that the said agitation means consist of at least one inductor with a movable electromagnetic field.
10. Process according to Claim 9, characterised in that use is made of an inductor surrounding the cast product (4) and generating a magnetic field rotating about the casting axis.
11. Process according to Claim 9, characterised in that use is made of an inductor of plane structure producing a sliding field within the cast product.
12. Installation for the continuous casting of metallic products (4), particularly made of steel, such that means (11) for cooling the product are arranged in the end portion of the metallurgical length, in a zone extending along the casting machine at least between the location where, in the absence of such cooling means, the speed of cooling of the pasty core (8) of the product (4) would exceed that of the surface of the product (4), and a location where the thermomechanical behaviour of the pasty core (8) during cooling is identical to that of the solidified shell (9).
13. Installation for continuous casting according to Claim 12, characterised in that the said cooling means (11) consist of spraying ramps spraying a cooling fluid onto the surface of the cast product (4).

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