EP0228335B1 - Continuous-casting mould with a hot top - Google Patents

Abstract

In their upper part the walls comprise an insert (3) consisting of a thin deposit of a refractory material of low heat conductivity, which has a good resistance to thermal shocks and intended to slow down the heat exchanges, at the meniscus, between the cast metal and the ingot mould. The deposit of refractory material is obtained by spraying with a detonation gun and is made of alumina, zirconia, boron nitride or chromium carbide. <IMAGE>

Description

The invention relates to a vertical or substantially vertical ingot mold for the continuous casting of molten metals such as steel. More particularly, the invention relates to an ingot mold made of copper or copper alloy, with a hot head, that is to say in which the heat exchanges, at the meniscus, between the cast metal and the ingot mold are slowed down so as to to reduce the oscillation wrinkles on the cast product.

We know, for example from documents FR-A-2 528 738 and FR-A-2 540 409, such an ingot mold, the walls of which are energetically cooled by circulation of a cooling liquid, have an insert at their upper part; metal of stainless steel, whose thermal conductivity is lower than that of the metal constituting the walls of the mold. In this way, the metal is less cooled in the vicinity of the meniscus, which makes it possible to reduce the depth of the oscillation wrinkles and the length of the solidified horn which accompanies their formation. It is known that such wrinkles constitute sensitive points where preferential surface cracks arise which are detrimental to the quality of the finished product.

The installation of this metal insert, the average thickness of which can be a few millimeters (the thickness can be uniform, or even decrease from top to bottom of the insert), is done using an expensive technique (generally plating by explosion) due to the need to obtain perfect contact between the metal insert and the wall of the mold. This necessity results in particular from the fact that any non-zero contact thermal resistance introduces a predetermined and non-reproducible datum into the overall thermal resistance of the mold. On the other hand, these ingot molds often include a surface wear coating of nickel intended to avoid the formation of star cracks in the cast product: the addition of a thermal contact resistance risks bringing the nickel to a close temperature. its limit around 600 ° C (temperature) of metal near the wall: around 1540 ° C).

The object of the invention is to propose a new type of insert for an ingot mold with a hot head, which does not have the aforementioned drawbacks.

According to the invention, the insert, about 10 to 20 cm long, is formed by a thin deposit of a refractory material with thermal conductivity less than 20 Wm ^-1 K ^-1 having good resistance to thermal shocks, such as that alumina, zirconia, boron nitride, chromium carbide, and the thickness of which, taken between approximately 200 and 500 μm, is chosen so that, at the contact temperature of the metal with said deposited material, said metal being solidified has a solid fraction of between 25 and 35% by weight.

The inventors have in fact found that if the solid fraction on the surface of the cast metal is greater than approximately 35%, a thick and rigid horn which cannot be obtained by cooling in the region of the meniscus near the wall of the mold. more deform, resulting in the formation of oscillation wrinkles.

Conversely, if the solid fraction is weak, the meniscus has no more consistency or rigidity; but maintaining a liquid fraction of less than 25% amounts to inhibiting a large part of the cooling in the mold.

In the field under consideration, the refractory materials chosen resist temperatures of around 1800 ° C. Their thermal conductivity is preferably less than 10W.m ^- ¹K ^- ¹, to be compared to conductivities of the order of 50, 100 and 300 Wm ^- ¹K ^- ¹ respectively for stainless steel, nickel and copper-silver alloy inserts.

The small thicknesses envisaged are advantageously between 2 and 5 / 10ths of a mm, to be compared with a few millimeters in thickness of the metal inserts.

The refractory materials are in particular alumina (melting temperature: 2040 ° C; thermal conductivity ranging between 2 Wm ^-1 K ^- ¹ and higher values depending on the treatment undergone), zirconia (melting temperature: 2700 ° C ; thermal conductivity: 1 Wm ^- ¹K ^- ¹), boron nitride, or, so preferred, chromium carbide (melting temperature: 1800 °; thermal conductivity: 7.5 Wm⁻¹K⁻¹).

The deposition of the material is obtained by spraying in a hot powder manner, for example by means of a plasma torch or preferably a detonation gun (according to the technique marketed by the company "Union Carbon Corporation"), followed polishing. This technique provides excellent adhesion between the support and the filler material, and a homogeneous coating free of roughness.

The deposit is made in a recess in the wall of the mold, of depth equal to the thickness of the deposit, made either in the copper of the wall, or in the surface coating of nickel or chromium if one is provided. .

Other characteristics and advantages will appear in the description which will be given of a particular embodiment of the invention. Reference is made to the appended drawings in which:

FIG. 1 represents a partial section of the wall of an ingot mold according to the invention,

FIG. 2 is a similar view when the mold is provided with a surface coating,

FIG. 3 is a graph showing the variation of the surface flux of heat dissipated by the ingot mold as a function of the distance from the surface of the meniscus (ordinate 0),

FIG. 4 is a representation of the profile of the oscillation wrinkles on slabs of continuous casting in a mold according to the invention,

- Figure 5 is a similar representation with an ingot mold without insert.

In FIG. 1, the copper wall 1 has a recess 2 of 3/10 ^th mm in thickness and about fifteen centimeters in height covering the region of the meniscus taking account of the oscillations on the one hand and possible variations in average level. The recess 2 is filled with a deposit 3 of Cr₃C₂ obtained by projection with detonation cannon, followed by polishing.

In Figure 2, the recess 2ʹ is made from the chrome coating 4, 0.3 mm thick, covering the wall 1ʹ of the mold. The material 3ʹ is flush with the internal surface of the mold.

The graph in FIG. 3 makes it possible to compare the values of the local densities of heat flow Φ _c extracted by the ingot mold equipped with an insert according to the invention, made of chromium carbide (curve B), with the values obtained using a conventional ingot mold. without insert (curve A). The straight line C in broken lines located at d ≃ 70 mm, corresponds to the lower limit of the insert, which, in this particular case, and for an insert of approximately 150 mm high, indicates that the meniscus is substantially mid-height of said insert. The two curves A and B correspond to the continuous casting of slabs with an extraction speed of 1.3 m / min.

Under the heat exchange conditions adopted for this example, the inventors were able to observe, by studying a mathematical model established for cooling in an ingot mold without insert, that the surface temperature of the steel in contact with the ingot mold is close to the solidus of the steel grade (1493 ° C), which indicates a significant solidification of the meniscus horn. To bring the solidified fraction in contact with the ingot mold to about 30%, the inventors have found that it was necessary to divide by four the heat extraction provided by the copper walls in the vicinity of the meniscus.

It can be seen in FIG. 3 that, in accordance with the aim sought, in the area of the insert the density of thermal flux extracted by the ingot mold is substantially divided by a factor 4. In fact in a conventional ingot mold, the density of flux extracted at the meniscus is around 2 MW / m² (curve A), and the use of a thin chromium carbide insert brings this value to around 500 kW / m².

In addition, as soon as it leaves the insert (dashed line), the locally measured heat extraction is close values obtained on a conventional ingot mold, which shows that the overall cooling in the ingot mold below the insert is little disturbed by the presence of the latter.

Figures 4 and 5 compare the longitudinal profiles of the surface of a low carbon steel slab cast at the speed of 1.3 m / min with an oscillation frequency of 117 strokes per minute. Figure 4 corresponds to a profile obtained by solidification against the wall of an ingot mold equipped with an insert according to the invention, and Figure 5 corresponds to an equivalent profile obtained with a conventional ingot mold without insert.

It is clear that the use of an insert significantly reduces the depth of oscillation wrinkles on the surface of the product, and improves their regularity. During tests carried out under similar casting conditions, a systematic reduction of 25 to 40% in the depth of the oscillation wrinkles has been observed.

Other tests carried out with medium-carbon steel slabs (that is to say with a carbon content close to 0.1%) cast at a speed of 0.8 m / min with an oscillation frequency of 80 strokes per minute made it possible to highlight the difference in the solidification structure of the first skin of a slab according to whether it is solidified in a mold without insert in accordance with the invention.

The observation of samples taken longitudinally in said slabs and attacked by the Bechet-Beaugeard reagent, reveals that, in the case of the conventional ingot mold without insert, the wrinkles are deep and are prolonged by a heterogeneity of solidification resulting from the formation solidified horn at the meniscus. The depth of the solidified horn is then about 1.7 mm. In the case of the ingot mold with insert according to the invention, the wrinkles appear much less marked and the depth of penetration of the solidified horn is only 0.9 mm or about half only the value observed in the absence of an insert.

Of course, the invention is not limited to the embodiments and to the applications described above by way of example. In particular, the dimensions of the insert can be adapted to the molds and cast products. The casting parameters (extraction speed, frequency of oscillations) can be adapted and the mold according to the invention can be used both for continuous casting of slabs and other products (blooms).

Claims (5)

1. Vertical or substantially vertical ingot mould for continuous casting of molten metals, such as steel, made from copper or copper alloy, the walls of which, energetically cooled by contact with a circulating cooling liquid, comprise at their upper part an insert made from a material with a thermal conductivity which is less than that of copper or of copper alloy, intended to slow down thermal exchanges at the level of the meniscus between the cast metal and the ingot mould, characterized in that the said insert, from 10 to 20 cm in length approximately, consists of a deposition of a refractory material (3, 3') with a thermal conductivity which is less than 20 w.m^-¹K^-¹, having a good resistance to thermal shocks, such as alumina, zirconia, boron nitride or chromium carbide, and whose thickness, taken between approximately 200 and 500 µm, is chosen such that, at the temperature of contact of the metal with the said deposited material, the said metal which is in the process of solidification has a solid fraction of between 25 and 35% approximately by weight.
2. Ingot mould according to Claim 1, characterized in that the refractory material (3,3') has a melting temperature greater than 1800°C.
3. Ingot mould according to Claim 1 or 2, characterized in that the refractory material (3, 3') has a thermal conductivity of less than 10 W.m^-¹K^-¹.
4. Ingot mould according to any one of the preceding claims, characterized in that the deposition of refractory material (3, 3') is obtained by hot spraying of pulverulent material.
5. Ingot mould according to Claim 4, characterized in that the deposition is performed by means of a detonation gun.

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