Patent Application
20160311014
The invention relates to a casting mould for casting steel melt into a strand being continuously drawn, wherein on at least one of the inner surfaces of the casting mould facing towards the melt to be cast a surface texture is formed, which extends at least over the region of the mould, which, during operation, is wetted with slag floating on the melt that has been poured into the mould.
In continuous casting, steel melt is poured from a casting ladle into a distributor, also known as “tundish”, provided as a buffer and, where appropriate, to distribute the melt into several strands, and is transported from there into the respective casting mould by means of a dip tube. The pouring direction corresponds to the force of gravity here.
The strand is formed in the casting mould. Upon contact with the cool inner surfaces of the casting mould the melt begins to solidify such that the strand coming out of the casting mould in a vertical direction has a thin shell of solidified steel on its outer surfaces, the shell enclosing the still liquid melt inside the strand.
After emersion from the mould, the strand is redirected supported by rolls in a so-called ‘casting bow’ in a horizontal direction of flow. Systematically controlled cooling takes place in the region of the casting bow in order to effect a controlled solidification of the strand. From the strand being completely solidified and discharged in a horizontal direction slabs are then separated, which and conveyed for further processing.
Casting powder is scattered in the mould on the free surface of the melt to form slag. The slag covers the melt and prevents the melt from reacting with the surrounding atmosphere in the region of the so-called ‘meniscus’. At the same time the slag binds impurities ascending in the melt and acts as a lubricant between the solidifying shell of the steel strand and the mould. Alternatively, there are casting methods in which pre-molten casting powder is fed or in which the casting method uses so-called ‘casting oils’ i.e. liquid casting media, instead of casting powder. The latter technique is applied particularly in billet or circular continuous casting. The mould is generally moved in an oscillating manner in order to prevent the steel from sticking to the cooled walls of the mould and to support the discharge of the strand from the mould.
Continuous casting moulds can be composed of mould plates or designed as one individual piece. The internal sides of continuous casting moulds are generally made of copper. To improve their resistance to wear and tear the inner surfaces of said casting moulds that come into contact with the forming strand can be covered with a nickel coating (EP 0 125 509 B1). However, the nickel coating results in a significant reduction in heat flow. For this reason it is generally applied only at a certain distance from that upper edge of the casting mould, which is assigned to the distributor of the continuous casting plant.
Regardless of whether or not the inner surfaces of casting moulds are nickel coated, in the continuous casting method the steel melt cools particularly rapidly especially in the region of the meniscus. This can lead to surface defects in the case of sensitive steel grades due to the internal stresses that occur during the cooling process.
This issue was already addressed in EP 1 099 496 B1. Reference is made there to the publication ‘Über den Zusammenhang zwischen Anfangserstarrung and Beschaffenheit der Strangoberfräche bei peritektisch erstarrenden Stählen’ (Postdoctoral thesis by M. M. Wolf, Forch 2002, pages 61-64) according to which in particular the thermal flow through the mould wall in the region of the molten metal level thereof plays a crucial role in terms of the absence of cracks in the strand shell. If the heat flow is too great, this results in an increased risk of cracking. In order to increase the thermal flow between the forming strand shell and the inner surface of the mould, EP 1 099 496 B1 suggests reducing the thermal resistance in the region of the meniscus by roughening the mould surface. In this way, the strand shell forming in the mould should remain thinner for longer and be pressed evenly against the copper plate of the continuous casting mould by the ferrostatic pressure rising with increasing distance from the level of the molten metal. The surface of the mould is roughened in the process such that the machining depth of the roughness of the inner surface of the mould decreases in the casting direction such that a gradual transition is achieved from rough to smooth section of the mould and thus also a gradual transition from restricted to unrestricted thermal flow. One advantage considered there is that the macrostructure of the inner surface of the mould can be achieved by methods known per se such as shot blast texturing (SBT), electric discharge texturing (EDT), electron beam texturing (EBT), laser texturing (LT) or by a perforated texture (GLT) or using other methods.
In the light of the above-mentioned prior art, the object of the invention was to create a casting mould where with simple means an optimum solidification behaviour is guaranteed in the region of the casting mould that is critical in terms of the risk of crack formation.
This problem is solved according to the invention by a casting mould designed according to Claim 1.
Advantageous embodiments of the invention are stated in the dependent claims and explained in detail below along with the general inventive concept.
In a casting mould according to the invention for casting steel melt into a strand being continuously drawn, a surface texture is formed on at least one of the inner surfaces of the casting mould facing towards the melt to be cast in accordance with the prior art explained above. The surface texture extends at least over the region of the mould, which, during operation, is wetted with slag floating on the melt poured into the mould.
According to the invention, said surface texture is now designed as a closed structure with indentations which are completely bordered and randomly distributed indentations.
The structure provided as surface texture according to the invention and formed from entirely defined indentations reduces the transfer of heat between mould and liquid melt. Some of the solidifying slag covers the indentations on the random surface structure and sticks there unlike in the case of open surface structures. Thus, the slag adhering to the inner surface of the mould acts as heat insulation which prevents direct contact of the melt with the inner surface. Said insulating effect of the slag layer leads to a lower, and over the breadth of the mould, to a more uniform heat supply in the region of the meniscus. As a consequence of the overall reduced and more uniform heat supply, fewer internal stresses occur in the strand shell during the cooling process when using a structured mould surface according to the invention compared with a conventional mould surface. Consequently, the risk of surface defects forming is reduced. If casting oils are used, the surface texture described here is wetted. The oil layer then adhering in the indentations also acts as thermal insulation.
An open surface and roughness structure created using the method referred to in EP 1 099 496 B1, for example, or by shot blasting or similar methods, in which the respective indentations overlap and accordingly are not defined from each other, but merge together, obtains its roughness from elevations in the material, which occur due to a shifting of the mould material. The closed surface structure provided according to the invention is, however, characterised by indentations and cavities that are not connected. It turns out that said closed, and according to the invention randomly distributed, indentations ensure better slag adhesion and prevent slag run off.
In addition to the topographical appearance that occurs in this manner, the mean roughness index Ra and the mean roughness depth Rz are important in terms of designating said surface structure. Both the mean roughness index Ra and the mean roughness depth Rz must be determined in accordance with DIN EN ISO 4287. In the case of a surface structure according to the invention, the ideal mean roughness index Ra is between 10 m and 50 μm and the mean roughness depth Rz between 80 μm and 250 μm. Mean roughness values and mean roughness depths in said value ranges result in a maximum reduction of surface defects and stable process reliability. This applies particularly if the mean roughness index Ra is between 10 μm and 50 μm, in particular between 15 μm and 50 μm.
Optimum adhesion of the slag on the surface texture is produced if the maximum depth of indentations of the surface texture is 500 μm. The indentations should be at least 5 μm deep in order to reliably achieve the pursued roughness.
Casting moulds of the type referred to here are normally made of a non-ferrous metal alloy, which is generally cooled on the side facing away from the melt. The mould cross-section can be designed as square or rounded. In order to produce strands of varying widths when using rectangular or square moulds, at least one of the plates defining the narrow sides of the mould opening can be adjustable in the width direction (EP 0 985 471 A1 ).
The surface structure provided according to the invention is provided on at least one of the inner surfaces defining the casting mould openings. Naturally, this includes the option to form a corresponding surface structure on all or at least opposite inner surfaces of the casting mould. Also in the case of width-adjustable casting moulds, the surface texture structured according to the invention should be present on at least one of the inner surfaces. The region, which is covered during adjustment of the side of the mould moved relative to said inner surface, can remain free of the surface structure according to the invention if this is advantageous in terms of sealing the corner regions in which the surfaces defining the mould opening touch. Accordingly, in the case of a casting mould, the thickness or width of which can be adjusted by displacing at least one of the sides thereof, the surface texture extends over the width of the inner surface provided with said surface texture, by means of which the inner surface comes into contact with the melt to be cast if the smallest thickness or width of the casting mould is configured.
The surface texture structured according to the invention should extend at least over the region of the respective inner surface of the casting mould, which is wetted during casting operations by the slag covering the meniscus. It has proven useful in the case of casting moulds used today if the surface texture extends over an area which, measured in the casting direction, begins at a distance of at least 10 mm below the upper mould edge and ends at a maximum distance of 600 mm.
In the event that the inner surface provided with the surface structure is covered with a layer of nickel over a section beginning at a distance from the upper edge of the mould, it has proven particularly advantageous in terms of reducing surface defects in the casting strand if the surface texture designed according to the invention overlaps the edge region assigned to the upper edge of the mould. In practice, overlapping areas have proven useful here, which, measured in the casting direction, are at least 50 mm. The overlapping of the surface texture according to the invention with the nickel coating prevents an abrupt break in thermal conductivity in the transition zone between the non-coated to the nickel-coated section of the respective inner surface.
The structure according to the invention of the surface texture provided on the respective mould surface can be introduced into the surface by embossing (pressure) or by strike or impact momentum, using needles, for example. The structure is introduced by deforming the mould surface without removing material in the process. The cold work hardening effected as a result of the striking or pressing strain on the respective mould inner surface can contribute towards a longer useful life of the mould.
If a stamping method is used, a negative of the structure to be produced is applied to a matrix, a sphere or a roller. Said negative is then used to apply the surface structure to the mould, depending on pressure and tool surface. If the structure is produced using a method based on strike or impact momentum, the structure defined according to the invention is produced by a tool striking with high momentum. So-called ‘needles’ with which specific surface roughnesses can be generated are suitable for this, such as in DE 199 07 827 A1, for example.
The invention is explained in greater detail below using drawings relating to an embodiment. Each of the figures represents a schematic view.
To cast a steel melt M into a strand S in the strand casting plant 1 shown in
When the dip tube 5 is open, the steel melt M flows into a casting mould 7, which is composed of cooled plates 8, 9, 10, 11, which are made of a non-ferrous metal or a non-ferrous metal alloy. Preferably, copper or copper alloys are used. The casting mould 7 has an opening cross-section that is substantially rectangular when viewed from above. The long sides of said cross-section are respectively delimited by a wide mould plate 8, 9 and the short sides respectively delimited by a narrow mould plate 10, 11.
On their inner surfaces 13 respectively assigned to the casting mould opening 12, the mould plates 8-11 can often be covered with a nickel layer 14, which, measured in the vertically aligned direction of flow F of the steel melt M, begins at a variable distance from the upper edge 15 of the casting mould 7 assigned to the distributor 3. The distance A1 is 300 mm in this case, but can be configured as generally variable. A rectangular mould with a nickel layer is used as an example here. However, other mould shapes with different coatings are also possible.
The strand S forming in the casting mould 7 from the steel melt M comes out of the casting mould 7 in a vertical direction of flow F and is guided in a horizontal direction Fh by means of a casting bow 16. In the region of the casting bow 16 the strand is guided by rollers 19, 20. Intensive cooling takes place at the same time such that the strand S has completely solidified to the greatest possible extent by the time it reaches the end of the casting bow 16 and can be conveyed for further processing.
A surface texture 22 is configured on the inner surfaces 13 of the mould plates 8-11 defining the mould opening 12 in a section 21 assigned to the upper edge of the mould 15. In this embodiment, the surface texture 22 begins in the direction of flow F at a distance A2 of 10 mm and ends at a distance A3 of 400 mm from the upper edge of the mould 15. Accordingly, the surface texture 22 overlaps the nickel layer 14 in an overlapping region U over a length measured in the direction of flow F of 100 mm. The surface texture can generally also be introduced up to a distance A3 of 600 mm as seen from the upper edge of the mould 15. In the section covered by the surface texture 22, the slag K floating at the meniscus of the melt M to be cast during casting operations wets the inner surface 13 of the copper plates 8-11.
The surface texture 22 is formed by a plurality of indentations 23, which are each completely enclosed by a partition wall 24. Each partition wall 24 defines two adjacently arranged indentations 23. The indentations 23 can be formed as individual hole-like impressions with a substantially round opening cross-section or from several such impressions merging together, which are then in turn bordered by a self-contained partition wall 24 encircling the respective indentation 23. Material ridging, which is produced when using the shot-blasting method, for example, is undesirable in this structure as said ridging is worn down by the strand shell. This would result in a degeneration of the structure reducing the roughness properties. Indentations are rather introduced into the mould material in order to achieve cold work hardening and maintain the surface structure. In the mould plates 8, 9 defining the mould opening 12 on the long sides thereof, the width B of the surface texture 22 is restricted to the narrowest region, which, if the mould plates 10, 11 defining the mould opening 12 on the short sides thereof are moved, is not covered by the copper plates 10, 11.
The indentations 23 being up to 500 μm deep have been produced by needles, using a standard needle device, that is not shown here. The needles in the needle device have been driven into the inner surface using high force and have compacted the material with which they have come into contact thus forming the respective indentation 23. No material abrasion occurred. In order to maintain the structure comprising indentations 23 and partition walls 24 shown in
The mean roughness depth Rz and mean roughness index Ra for two surface textures produced in this way inside and outside the overlapping region of surface texture 22 and nickel layer 14 of the inner surfaces 13 are shown in Table 1.
1 Continuous casting plant
2 Ladle
3 Distributor (Tundish)
4 Ladle shroud
5 Dip tube
6 Stopper
7 Casting mould
8-11 Copper plates
12 Casting mould opening
13 Inner surfaces of casting mould 7
14 Nickel layer
15 Upper edge of mould
16 Casting bow
19,20 Rollers
21 Section of the inner surfaces 13
22 Surface texture
23 Indentations
24 Partition wall
A1-A3 Distances, measured in direction of flow F
B Width of section of the inner surface 13 provided with the surface texture
F Direction of flow of steel melt M in casting mould 7
Fh Horizontal direction of flow
K Slag
M Melt
S Strand
Ü Overlapping region
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2013 114 326.1 | Dec 2013 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/003374 | 12/16/2014 | WO | 00 |
