EP2054178B1

EP2054178B1 - Crystalliser

Info

Publication number: EP2054178B1
Application number: EP07802561.6A
Authority: EP
Inventors: Nuredin Kapaj; Gianluca Bazzaro; Alfredo Poloni
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 2006-08-11
Filing date: 2007-08-10
Publication date: 2014-03-05
Anticipated expiration: 2027-08-10
Also published as: EP2054178A1; WO2008017711A1; ITMI20061622A1

Description

Field of the invention

The present invention relates to a crystalliser, in particular to a tubular crystalliser for continuous casting of blooms, billets or rods.

State of the art

Various types of crystallisers are known and used in continuous casting of liquid steel in an ingot mould. The main tasks which must be performed by a crystalliser are to guarantee good heat transfer from the steel outwards, so as to favour solidification, and to lubricate its internal surface which comes into contact with the steel to prevent problems of sticking and breakage of the skin of the billet which is forming with the consequent break-out of molten material.
In the past, tubular crystallisers having a thin thickness, ranging from 5 to 8 mm, were used for continuous casting of billets and blooms. In this way the attempt was to intensify steel cooling in order to guarantee a rapidly solidified skin. However, with these thickness values, the crystalliser was not sufficiently stiff and consequently was highly deformed causing also the deformation of the cast product which presented geometrical shape defects, such as rhomboid errors, and off-corner cracks.
Subsequently, in order to reduce this excessive deformation, the thickness of the crystalliser was increased to values equal to approximately 10% of the side of the cast product. The problem of excessive deformation of the cast product was partially solved in this way, however to the detriment of heat transfer and the creation of other problems, such as:

a) increased temperature on the internal wall of the crystalliser in contact with the liquid steel and consequently excessive wear of the crystalliser itself;
b) high temperature gradients along the thickness and along the cross section and, consequently, the generation of surface cracks on the chromium coating in the meniscus area.

These problems considerably reduce the life of the crystalliser, thus increasing processing costs.
In particular, the temperature on the internal wall of the crystalliser drastically increases when the casting speed increases. In these cases, in addition to the problems described above, the plastic deformation of the crystalliser also increases. The high temperature on the internal wall of the crystalliser, especially in the meniscus area, is a severe problem especially for free jet casting because the lubrication oil flowing on the internal wall is burnt and consequently there is no more lubricant between the billet, or other cast product, and the crystalliser. In this case, the solidified skin breaks causing the break-out of liquid steel, interrupting the casting process and at the same time soiling the casting line with considerably increase of production times and costs.
A partial solution to these problems is given by the crystalliser described in GB2177331 . Such crystalliser is provided on its external surface with longitudinal cooling grooves of a predetermined length. These grooves, essentially parallel to each other, each define a direction essentially parallel to the longitudinal axis of the crystalliser or die. They are arranged along a single horizontal row at the molten material meniscus which fluctuates during casting. The most intense thermal deformation occurs, indeed, on the portion of internal wall of the die corresponding to this region.
Since, however, during the casting operation, thermal deformation is oriented on the walls of the die in a non-uniform manner along the longitudinal direction, said crystalliser still presents the following disadvantages:

high temperatures on the wall in contact with the liquid steel;
still high temperature gradients along the thickness and along the length or longitudinal extension of the crystalliser;
non-uniform cooling along the transversal section;
presence of deformations and generation of cracks on the chromium coating in the meniscus zone;
not controllable internal taper;
stiffness not sufficiently high to avoid cast product quality problems, such as rhomboid error and off-corner cracks.

The need for a crystalliser capable of solving the aforesaid drawbacks is therefore felt.

Summary of the invention

The primary object of the present invention is to construct a crystalliser for continuous casting of blooms and billets, provided on the external surface with grooves or channels, made for example by milling and in which cooling water flows, said grooves being appropriately dimensioned and being distributed so as to obtain an optimal cooling of the internal wall of the crystalliser thus making its temperature nearly uniform.
A further object of the present invention is to make a crystalliser of considerable stiffness despite of being not very thick.
The present invention, therefore, aims to reach the objects discussed above by making a crystalliser for continuous casting of blooms, billets or rods which, according to claim 1, comprises a tubular structure defining a longitudinal axis and whose section on a plane orthogonal to said axis defines the shape of said blooms, billets or rods, longitudinal grooves made on an external surface of said tubular structure, characterised in that said grooves are arranged on at least two rows essentially orthogonal to said axis and the grooves on one row are staggered with respect to the grooves of another adjacent row.
Advantageously, in order to improve the performance of conventional crystallisers in terms of lowering the temperature of the copper of the tubular structure in contact with the liquid steel, of uniformity of temperature on the internal wall of the crystalliser and of improved stiffness of the crystalliser, cooling channels were made on the external wall of the crystalliser, by milling or electron discharge machining or chemical evaporation.
These cooling channels are advantageously arranged on horizontal rows, mutually staggered one row with respect to the other, and have different depths. The maximum depth is obtained at the meniscus zone where the highest temperatures occur in the conventional tubular crystalliser.
The advantages obtained with the crystalliser of the invention with respect to the conventional solution include:

lower temperatures on the wall in contact with the liquid steel;
low temperature gradients along the thickness and along the casting direction;
a very limited deformation;
an uniform cooling along the transversal section;
a more controllable internal taper;
long life because no cracks are generated on the chromium coating in the meniscus zone;
a very high stiffness allowing to eliminate or minimise rhomboid errors, off-corner cracks and other defects.

With the crystalliser of the invention, the risk of sticking of the skin on the wall of the crystalliser, and consequent breakage of the skin itself with break-out of the liquid metal, is also reduced. Indeed, the low temperatures on the internal wall of the crystalliser allow the lubricant to flow along the wall itself without being burnt, also in the case of free jet casting.
The dependent claims describe preferred embodiments of the invention.

Brief description of the drawings

Further features and advantages of the present invention will be more apparent in the light of the detailed description of preferred, but not exclusive, embodiments of a crystalliser illustrated by way of non-limitative example, with reference to the accompanying drawings, in which:

Fig. 1 shows the distribution of grooves on one part of the external surface of the crystalliser according to the invention;
Fig. 2 shows a distribution of the temperature on an internal wall of the crystalliser;
Fig. 3 shows respectively a side view of a variant of the crystalliser according to the invention and its top view;
Fig. 4 shows a partial side view of the variant in Fig. 3 and its longitudinal and transversal sections, respectively.

Detailed description of preferred embodiments of the invention

With reference to Fig. 1, it is shown a first embodiment of the crystalliser of the invention comprising a monolithic tube 1, preferably made of copper, defining a longitudinal axis X and having either a square or a rectangular or a round cross section, according to the shape of the product to be cast.
Cooling grooves or channels 2, 2' arranged along a plurality of rows essentially orthogonal to said axis X are made by milling, electron discharge machining or chemical evaporation on the external wall of the crystalliser. In the case of Fig. 1, there are preferably two rows.
Said grooves 2, 2' advantageously present an appropriate distribution on said external wall and appropriate dimensions so as to obtain an optimal heat exchange and therefore an optimal cooling of the internal wall of the crystalliser, making the temperature on it nearly uniform, with low gradients along the thickness and along the longitudinal extension of the crystalliser.
It has indeed been found that the isothermal lines on the internal surface of the crystalliser are of the half elliptical or parabolic type, as lines 3 and 4 shown in Fig. 2. In particular, the region indicated with reference 5 is the one presenting the maximum temperatures because it corresponds to the region of the meniscus; the adjacent region indicated with reference 6, comprised between two isothermal lines 3, 4, presents instead lower temperatures.
Furthermore, each zone between the two adjacent isothermal lines 3, 4 has a different temperature range. In order to reduce this difference of temperature between the various zones of region 6, corresponding grooves 2' with differentiated depth between each zone and with the distribution shown in Fig. 1 were made on the external surface. Similarly, grooves 2 were made at region 5. Advantageously, grooves 2, 2' are arranged along horizontal rows, one for each region, according to a distribution with tends to cover regions 5 and 6 respectively following the profile of isothermal lines 3 and 4. The longitudinal extension of the different grooves is therefore different.
Advantageously, grooves 2 are deeper because it is in region 5 corresponding to the meniscus that, in the conventional tubular crystalliser, the highest temperatures occur. In region 6, instead, grooves 2' are less deep so as to uniform the temperature on the internal wall of the crystalliser. In this way, temperature uniformity is obtained between the various zones thus reducing the thermal gradients on the crystalliser wall and consequently its non-uniform deformation.
A further advantage of the crystalliser of the invention is represented by the fact that in region 6 grooves 2' are staggered with respect to grooves 2 of region 5 in order to guarantee a higher stiffness of the crystalliser itself and, at the same time, to increase the turbulence of the cooling water, and ultimately, the heat exchange coefficient.
Fig. 3 shows a second embodiment of the crystalliser of the invention in which three horizontal rows 11, 12, 13 of grooves or channels are provided on the external surface.
Each row is advantageously staggered with respect to the row above and/or to the row underneath to increase the stiffness of the tubular structure and the heat exchange.
In this case, the longitudinal extension is the same for all grooves, but their depth varies. The depth is advantageously decreasing from the first row 11 of grooves towards the third row 13. The differentiated depth guarantees a uniform temperature between the various zones and consequently less deformation of the crystalliser.
All the embodiments described above advantageously allow:

a) a lowering of the copper temperature in contact with liquid steel,
b) a uniform temperature on the internal wall of the crystalliser,
c) an improvement of the stiffness of the crystalliser as the staggering of groove rows gives it a greater structural continuity,
d) the guaranteed introduction of lubricant under the skin being formed, for a determined length, from the first solidification point.

Advantageously, the crystalliser is cooled by means of the water, circulating on its external surface, which therefore slides within the longitudinal grooves.
The crystalliser of the invention may be used in particular for the production of billets with section from 50x50 mm to 200x200 mm, for the production of blooms having longer side from 200 to 600 mm and for the production of rods of any diameter.

Claims

A crystalliser for continuous casting of blooms, billets and rods, comprising:
- a tubular structure (1) defining a longitudinal axis (X) and whose section on a plane orthogonal to said axis (X) defines the shape of said blooms, billets or rods,

- longitudinal grooves (2, 2') obtained on an external surface of said tubular structure (1),
characterised in that said grooves (2, 2') are arranged on at least two rows (11, 12) essentially orthogonal to said axis (X) and the grooves (2) of one row (11) are staggered with respect to the grooves (2') of another adjacent row (12).
A crystalliser according to claim 1, wherein the grooves of one row (11) have a different depth from that of another adjacent row (12).
A crystalliser according to claim 1 or 2, wherein a first row (11) of grooves (2) is arranged at the region in which a meniscus is formed during a molten metal casting and there is provided at least one second row (12) of grooves (2') underneath said first row (11).
A crystalliser according to claim 3, wherein the grooves of the first row (11) are deeper than the grooves of the second row (12).
A crystalliser according to claim 4, wherein there is provided a third row (13) of grooves underneath the second row (12).
A crystalliser according to claim 5, wherein the grooves of the third row (13) are less deep than the grooves of the second row (12).