Method for producing metal strips by continuous casting and rolling

Abstract

The invention relates to a method for producing metal strips by continuous casting and rolling, in which a slab (3) is first cast in a casting machine (2), wherein the slab (3) is cleaned in a cleaning device (4) placed after the casting machine (2) in the conveying direction, and the slab (3) then undergoes a heat treatment in a heat treatment device (11), wherein, between the cleaning device (4) and the heat treatment device (11), the slab (3) undergoes a smoothing process by means of two interacting smoothing rolls (13, 14). In order to reduce energy and production costs and to improve the surface of the slabs before the first forming process, according to the invention the smoothing process is carried out such that the slab (3) does not undergo a substantial reduction in thickness, and the roughness depth of the smoothed surface of the slab (3) is reduced.

Description

[0001] Process for producing metallic strips by casting and rolling

[0002] The invention relates to a method for producing metallic strips by casting rolls, in which a slab is first cast in a casting machine, wherein the slab is cleaned in a cleaning device downstream of the casting machine in the conveying direction and the slab is subsequently subjected to a heat treatment in a heat treatment device, wherein the slab is subjected to a smoothing process by means of two cooperating smoothing rolls between the cleaning device and the heat treatment device.

[0003] Such a process is disclosed in DE 10 2008 029 581 A1. The cast strand emerging from the casting machine is first descaled and, before entering a furnace, subjected to a roughing process using a roughing stand, during which a first relevant thickness reduction of the cast slab occurs (see Figure 6 of the document). After the slab emerges from the furnace, further rolling takes place in a rolling mill.

[0004] A similar method is described in WO 2008 / 113848 A1. Further similar and different solutions are shown in JP 2018 061999 A, US 2008 / 251232 A1, DE 10 2005 059 692 A1, CN 213495668 U, US 2010 / 116380 A1, DE 10 2007 022 931 A1, and DE 101 37 944 A1.

[0005] In the generically relevant previously known solutions, the slab is subjected to a relevant thickness reduction before entering the furnace, either in the area of ​​the casting machine, i.e. when the core of the cast strand is still liquid, or between the casting machine and the furnace in a roughing stand.

[0006] The previously known production method results in the following disadvantageous situation: If a forming process involving a significant reduction in the thickness of the solidified slab takes place before homogenization in a furnace, the temperature differences within the slab can lead to uneven forming, which has a detrimental effect on subsequent rolling steps. A further disadvantage of thickness reduction is the necessary structural extension of the continuous furnace, which is designed to be several times the length of the slab. Not only does the furnace length increase, but the longer contact of the slab with the furnace rollers also has a negative impact.

[0007] As far as the previously known solutions do not provide for a reduction in the thickness of the slab, the technological focus here is on improving the profile or geometry of a cast strip or a very thin slab.

[0008] The disadvantage of large thickness reductions before the furnace is the negative impact on the quality of the lateral slab and strip edge geometry, which leads to higher energy and cost consumption due to the necessary excess widths in the melting, casting and rolling processes and due to the wider trimming cut that is then required.

[0009] It has been observed that cast slabs, which may be thin, medium, or thick, often have an uneven surface, which can be caused by casting marks, among other things. During further processing, e.g., in a roller-hearth furnace, the slabs rest on these surface peaks, which can lead to rutting and unwanted scale rolling into the recesses, particularly when disc rollers are used in the downstream process. In addition, a long residence time of the slab in the furnace leads to increased scale buildup, which must first be removed before a subsequent rolling step. The scale is difficult to remove from the recesses if the surface is not smoothed.

[0010] The problems described lead to surface defects in the finished rolled product that cannot be repaired and, in surface-sensitive applications, may lead to the downgrading of the product. The invention is based on the object of developing a method of the type mentioned above in such a way that the aforementioned problem is eliminated. In particular, energy and production costs are to be reduced, which is to be achieved by specifically influencing the slab and strip edge geometry. In particular, the surface of the slabs is to be improved before the first forming step.

[0011] The solution to this problem by the invention is characterized in that the smoothing process is carried out in such a way that the slab does not experience a significant reduction in thickness and the roughness depth of the smoothed surfaces of the slab is reduced.

[0012] The average roughness of the rolled surfaces of the slab after smoothing is preferably no more than 10 pm, particularly preferably no more than 8 pm. This reduces the average roughness compared to the usual value for casting from a mold (approximately 11 pm to 14 pm).

[0013] The slab is preferably subjected to the smoothing process in a fully solidified state.

[0014] The reduction in thickness of the slab during the smoothing process is preferably less than 10%, particularly preferably less than 5% and most particularly preferably less than 3%.

[0015] The smoothing process is carried out using two cooperating smoothing rollers, which preferably exert a force acting normally on the slab between 3,000 kN and 5,000 kN, and particularly preferably between 3,500 kN and 4,500 kN.

[0016] The smoothing process with the two interacting smoothing rollers is preferably carried out with smoothing roller diameters between 500 mm and 700 mm, preferably between 550 mm and 650 mm. The cleaning process in the cleaning device is preferably a descaling process, in particular by fluid descaling or scarfing.

[0017] The smoothing rollers can be cooled during the smoothing process.

[0018] The smoothing process is preferably carried out at a speed between 1.0 and 6.0 m / min.

[0019] The proposed method is therefore particularly suitable for casting and rolling hot slabs to prepare the surfaces of the slab before entering the furnace. The surfaces of the fully solidified slab emerging from the casting machine are smoothed.

[0020] The central aspect of the invention is the smoothing of the surface of the cast strand or slab, whereby cleaning, in particular descaling (e.g., by fluid descaling, scarfing, or gas cleaning), takes place beforehand, and the cleaned slab surface is smoothed by a pair of rollers. No relevant thickness reduction takes place, i.e., no penetrative deformation and thus no associated microstructural transformation from a cast structure to a rolled structure (which only occurs with a corresponding thickness reduction). Rather, the intended smoothing process (using the pair of smoothing rollers) merely smooths the surface peaks of the slab, thus reducing the surface roughness to create the flattest possible surface.

[0021] The material from the "surface peaks" thus simply flows from them into the depressions or "valleys." This reduces the potential surface area for scale formation, and the flatter surface scales more evenly during the subsequent process, without forming further surface-damaging peaks and depressions.

[0022] The thickness reduction of the slab resulting from the application of force is less than 10%, preferably less than 5%, and particularly preferably less than 3%. Specifically, a thickness reduction of less than 1% can be provided, whereby only elastic deformation occurs in the slab itself during the smoothing process. However, due to the surface peaks, this deformation causes the desired slight deformation (as explained above) and is limited to the layer of the slab closest to the edge.

[0023] The pair of smoothing rolls used for the described smoothing process can be adjusted to the slab thickness. The applied force can be set using a process model. The parameters for adjusting the smoothing rolls are transmitted via a control device. The pair of smoothing rolls is therefore not a fully-fledged rolling stand with corresponding actuators.

[0024] Depending on one possible design, the pair of smoothing rollers can be moved into and out of the line. However, it can also remain stationary in the system.

[0025] Preferably, the smoothing rollers are cooled. The pair of smoothing rollers can be housed together with a shear in a common frame. Furthermore, the scale washer can also be integrated into this common frame.

[0026] The invention is preferably used for a slab thickness between 30 mm and 180 mm, particularly preferably for a slab thickness between 50 mm and 150 mm.

[0027] The preferred contact force of the smoothing rolls, approximately 4,000 kN, differs significantly from the forces otherwise specified. This force is too low for the desired slab thickness reduction, but it is also significantly higher than the force specified for the contact force of the pinch rolls (which is only approximately 20% to 50% of the contact force of the smoothing rolls). The design of the operating parameters is naturally also dependent on the strip width.

[0028] The preferred diameter of the smoothing rollers (approx. 600 mm) is also significantly larger than that of the typical drive rollers used in casting machines.

[0029] The design of the diameter and optionally also the bale shape depends on the belt width.

[0030] The arrangement of the smoothing rollers provided according to the invention is close to the outlet of the casting machine and can also be partially located below the casting platform.

[0031] The proposed procedure results in significant advantages, which initially lie in the reduction of scale formation, which results from the reduction of surface roughness and thus the avoidance of running marks on the slab.

[0032] This results in quality improvements, especially for surface-sensitive alloys and applications.

[0033] Energy savings are also advantageous, as the smaller and simpler scale buildup means less scale needs to be removed, or it can be removed more easily. This also reduces the amount of water required, so the subsequent cooling of the slab or transfer strip requires less energy. Finally, production costs are reduced by avoiding unnecessary overwidths.

[0034] The drawing shows an embodiment of the invention.

[0035] Figure 1 shows a plant according to the invention for producing metallic strips by casting rolls, Figure 2 shows an embodiment of a smoothing device,

[0036] Figure 3 shows a possible sequence of process steps in the process according to the invention and

[0037] Figure 4a and

[0038] Figure 4b shows schematically, greatly enlarged, the surface structure of a slab treated according to the invention.

[0039] Figure 1 shows a schematic representation of a front part of the casting-rolling plant 1 according to the invention for producing metallic strips, in particular steel strips. The plant 1 initially comprises a casting device 2, over which slabs 3 with a thickness of 30 to 150 mm are cast. These slabs are typically formed from a vertical to a horizontal position via guide rollers (not shown). The slabs 3 leaving the casting device 2 are so hot that an oxide layer in the form of scale immediately forms upon contact with atmospheric oxygen. This oxide layer must be removed from the surface of the slabs 3 for the subsequent heat treatment.

[0040] For this purpose, the system 1 comprises a cleaning unit 4 arranged directly behind the casting device 2 in the strip travel direction, which has a first upper and a first lower scale washer 5, 6. The scale washer 5, 6 arranged above and below the slab 3 can remove the majority of the scale already formed from the slab surface.

[0041] Downstream of the cleaning unit 4 in the strip travel direction, the system 1 further comprises a cutting device 7 with two shears 8, 9, by which the slab 3 is divided, if necessary, before entering a heat treatment device 11. The heat treatment device 11 is designed as a roller-hearth furnace 11. The roller-hearth furnace 11 serves both for reheating and for equalizing the slab temperature.

[0042] In order to improve the transport conditions in the roller hearth furnace 11 on the one hand and to remove the portion of the scale that cannot be removed by the scale washers 5, 6 on the other, the system 1 according to the invention comprises a smoothing device 12 arranged between the cleaning device 4 and the separating device 7, said device having an upper and a lower drivable smoothing roller 13, 14. Both smoothing rollers 13, 14 are spaced from one another in such a way that a smoothing pass can be carried out on the cleaned slabs 3. The smoothing pass, on the one hand, smooths the surface of the slab 3 and, on the other hand, effectively breaks up the scale that cannot be removed by the upstream scale washers 5, 6. This is then removed via a second cleaning unit 15, which is arranged downstream of the separating device 7 and has a second upper and a second lower scale washer 16, 17, so that a virtually scale-free split slab 10 can be fed to the roller hearth furnace 11.

[0043] The distance between the two smoothing rolls 13, 14 is set in this case such that, according to one possible embodiment of the invention, the slabs 3 experience a thickness reduction of at least 3.0% and a maximum of 5.0%, whereby this reduction is kept constant between the two smoothing rolls 13, 14 throughout the entire process. Furthermore, the system 1 can comprise a force and / or position control device (not shown) via which a hydraulic and / or mechanical device (not shown) of the casting device 2 can be controlled.

[0044] Figure 2 shows a sectional view of an embodiment of the smoothing device 12. The smoothing device 12 comprises the two smoothing rollers 13, 14 arranged in a stand 19, as well as a force measuring sensor 20 arranged on the operator and drive sides, by means of which the forces across the slab width can be determined. This allows useful information on the shape of the slabs, such as in particular the thickness and / or wedge shape, to be obtained at an early stage of the process. This information can be transmitted via appropriate signaling systems to the casting device 2 and / or to the downstream rolling mill in order to optimize the rolling process as a whole.

[0045] Figure 2 also indicates that corresponding control or regulating devices 21 are provided for the position of the smoothing rollers 13, 14 and the force they exert on the slab. To ensure the possibility of retrofitting existing systems, the cleaning units 4, 15, the separating device 7, and the smoothing device 12 arranged therebetween can be designed as a single unit 18 (see Fig. 1).

[0046] Figure 3 shows the sequence of the individual process steps as they can be provided in the process according to the invention. The cast strand or slab is produced in the casting machine in step A. Induction heating of the slab can be provided in step B. Subsequently, in step C, the slab enters the cleaning device or scale washer. In step D, the slab can then be separated into individual pieces. In step E, the central smoothing of the surface of the slab according to the invention then takes place by means of the smoothing rollers 13 and 14. In step F, the pretreated slabs are then further transported or heated in the furnace.

[0047] A possible alternative to this sequence is that step E (smoothing) is performed before step D (separating).

[0048] The effect intended according to the invention, which the smoothing pass is intended to produce by means of the smoothing rollers 13 and 14, is schematically illustrated in Figures 4a and 4b: Figure 4a schematically shows the slab with its thickness D, with the "surface mountain" resulting as usual after casting in the mold. It should be noted that with normal care, the mean roughness value R a the surface is typically between 11 pm and 14 pm. After smoothing by means of the smoothing rollers 13 and 14, as outlined in Figure 4b, the slab 10 (split thin slab) has essentially retained its thickness D, ie, no relevant thickness reduction has occurred. However, the "surface mountain range" has been significantly leveled, ie, the mean roughness R a is significantly reduced. This is preferably done to a value below 10 pm, particularly preferably below 8 pm. Lower values ​​below 6 pm are even more favorable and are targeted.

[0049] The first cleaning unit 4, the separating device 7, and the smoothing device 12 arranged therebetween are preferably designed as a unit 18. At least one of the two smoothing rollers 13, 14 of the smoothing device 12 can comprise a force measuring sensor 20 on the operating and / or drive side, by means of which the force across the slab width can be determined.

[0050] Preferably, no rolling mill is provided between the casting device 2 and the heat treatment device 11. The heat treatment device 11 can be designed as a roller hearth furnace and / or as an induction heater.

[0051] The slab surface is thus evened out before the cutting device or the possible cutting step, but after the cleaning unit or after the cleaning step, by smoothing both surfaces, i.e., the top and bottom of the slab, using the smoothing pass. This reduces local roughness peaks to such an extent that the formation of unwanted ruts is effectively prevented, thus improving the transport processes in the heat treatment facility. Surprisingly, it has also been shown that the smoothing pass leads to reduced scale formation on both surfaces of the slab as soon as it leaves the heat treatment facility and is fed into the rolling mill. It can therefore be assumed that this effect is based on the reduction of the active surface on which scale preferentially forms.This reduction in scale formation has a positive effect on the plant's output by reducing material losses due to scale formation, which then has to be removed later.

[0052] By means of the cleaning device, which is designed as a scale washer, a large portion of the already formed scale is removed from the slab surfaces before the cutting device. The portion of the scale that cannot be removed by the scale washer can be effectively broken up by the smoothing pass using the proposed process, resulting in higher heat transfer and thus a more effective heat treatment process.

[0053] The separating device can be in the form of shears and / or a laser-based cutting device. Depending on the operating mode, the continuous cast slab is divided into individual slabs or passes through the separating device as a continuous slab for further processing in subsequent process steps. Both operating modes can be provided individually or jointly by the system and method and are equally suitable for the implementation of the invention without restrictions.

[0054] For the purposes of the present invention, the term "smoothing device" refers to a device with two driven smoothing rollers arranged at a defined distance from each other. Therefore, it is preferred that the system between the casting device and the heat treatment device not include a rolling mill.

[0055] The smoothing pass is characterized by a particularly small thickness reduction and is therefore different from a conventional rolling pass. Therefore, it is preferably provided that the distance between the two smoothing rollers or the gap between them is designed such that the smoothing pass causes a thickness reduction of a maximum of 5.0% relative to the thickness of the incoming slab, more preferably a thickness reduction of 3.0 to 5.0% relative to the thickness of the incoming slab. In other words, the gap between the two smoothing rollers is set such that it is slightly smaller than the incoming slab.

[0056] It is therefore particularly preferred that the distance between the two smoothing rollers be constant and remain constant even during the smoothing pass. For this purpose, the system advantageously comprises a force and / or position control device, via which a hydraulic and / or mechanical device of the casting device can be controlled.

[0057] In the embodiment of the invention described above, a second cleaning device is arranged between the separating device and the heat treatment device, which is particularly preferably designed in the form of scale washers. The scale washers, located downstream of the separating device in the strip travel direction, can remove any remaining loose scale from the surface of the slabs before they are subsequently subjected to heat treatment.

[0058] In order to ensure the possibility of retrofitting old systems, it is particularly preferred that the first cleaning unit, the separating device and the smoothing device arranged therebetween are designed in the form of a unit.

[0059] List of reference symbols:

[0060] 1 system

[0061] 2 Casting machine / casting device

[0062] 3 slab / thin slab

[0063] 4 first cleaning device / cleaning unit

[0064] 5 first upper scale washer

[0065] 6 first lower scale washer

[0066] 7 Separator

[0067] 8 upper scissors

[0068] 9 lower scissors

[0069] 10 split thin slabs

[0070] 11 Heat treatment facility (roller hearth furnace)

[0071] 12 Smoothing device

[0072] 13 upper smoothing roller

[0073] 14 lower smoothing roller

[0074] 15 second cleaning device / cleaning unit

[0075] 16 second upper scale washer

[0076] 17 second lower scale washer

[0077] 18 units

[0078] 19 Scaffolding

[0079] 20 force measuring sensor

[0080] 21 Control / regulating device for position or force

Claims

Patent claims:

1. A method for producing metallic strips by casting rolls, in which a slab (3) is first cast in a casting machine (2), the slab (3) being cleaned in a cleaning device (4) downstream of the casting machine (2) in the conveying direction, and the slab (3) is then subjected to a heat treatment in a heat treatment device (11), the slab (3) being subjected to a smoothing process by means of two cooperating smoothing rolls (13, 14) between the cleaning device (4) and the heat treatment device (11), characterized in that the smoothing process is carried out in such a way that the slab (3) does not experience any significant reduction in thickness and the roughness depth of the smoothed surfaces of the slab (3) is reduced.

2. Method according to claim 1, characterized in that the mean roughness of the smoothed surfaces of the slab (3) after smoothing is at most 10 pm, preferably at most 8 pm.

3. Method according to claim 1 or 2, characterized in that the slab (3) is subjected to the smoothing process in a fully solidified state.

4. Method according to one of claims 1 to 3, characterized in that the reduction in thickness of the slab (3) during the smoothing process is less than 10%, preferably less than 5%.

5. Method according to claim 4, characterized in that the thickness reduction of the slab (3) during the smoothing process is less than 3%.

6. Method according to one of claims 1 to 5, characterized in that the smoothing process is carried out with two cooperating smoothing rollers (13, 14) which exert on the slab a force acting normally on the slab of between 3,000 kN and 5,000 kN, preferably between 3,500 kN and 4,500 kN.

7. Method according to one of claims 1 to 6, characterized in that the smoothing process is carried out with two cooperating smoothing rollers (13, 14), each having a roller diameter between 500 mm and 700 mm, preferably between 550 mm and 650 mm.

8. Method according to one of claims 1 to 7, characterized in that the cleaning process in the cleaning device is a descaling process, in particular by fluid descaling or flame cutting.

9. Method according to one of claims 1 to 8, characterized in that the smoothing rollers (13, 14) are cooled during the smoothing process.

10. Method according to one of claims 1 to 9, characterized in that the smoothing process is carried out at a speed between 1.0 and 6.0 m / min.

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