Method for producing metal strips by continuous casting and rolling

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a national stage application, filed under 35 U.S. C. § 371, of International Patent Application PCT/EP2023/070506, filed on Jul. 25, 2023, which claims the benefit of German Patent Application DE 10 2022 208 498.5, filed on Aug. 16, 2022.

BACKGROUND

DE 10 2008 029 581 A1 discloses a method for producing metal strips by continuous casting and rolling. The casting strand exiting the casting machine is first descaled and undergoes a pre-rolling process by means of a pre-rolling mill stand prior to entering a furnace, wherein a first relevant reduction in thickness of the cast slab is carried out (see FIG. 6 of the document). After the exit of the slab from the furnace, it is rolled again in a rolling mill.

A similar method is described in WO 2008/113848 A1. Other 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.

With the previously known solutions relevant to the type, the slab undergoes a relevant reduction in thickness prior to being fed into the furnace either in the region of the casting machine, i.e. when the core of the casting strand is still liquid, or between the casting machine and the furnace in a pre-rolling mill stand.

The previously known manufacturing method results in the following disadvantageous situation: If a forming process with a significant reduction in thickness of the solidified slab is carried out prior to homogenization in a furnace, the temperature differences in the slab can lead to an uneven forming process, which has a detrimental effect in subsequent rolling steps. A further disadvantage of reduction in thickness is the necessary structural extension of the continuous furnace, which is designed according to a multiple of a slab length. The longer the furnace, the higher the energy consumption; the longer the contact of the slab with the rolls of the furnace also has a negative impact.

To the extent that no reduction in thickness of the slab is provided for in the previously known solutions, the technological focus here is on improving the profile / geometry of a cast strip or a very thin slab.

The disadvantage of a large reduction in thickness prior to the furnace is the negative influence 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 process and the wider trimming cut that is then required.

It has been found that cast slabs, which can be thin, medium or thick slabs, often have an uneven surface, which can be attributed to casting marks, among other things. During further treatment, e.g. in a roller hearth furnace, the slabs rest on these surface peaks, which can lead to track grooves and unwanted scale indentations in the recesses, in particular when using disk rolls in the region of the subsequent process. In addition, a long dwell time of the slab in the furnace leads to an increased scale build-up, which must first be removed prior to a subsequent rolling step. The scale is difficult to remove from the recesses if the surface is not smoothed.

The problems described lead to surface impairments in the finished rolled product, which cannot be rectified and may result in the product being “downgraded” in the case of surface-sensitive applications.

SUMMARY

The disclosure relates to a method for producing metallic strips by casting rolling, in which a slab is first cast in a casting machine. The slab is cleaned in a cleaning device placed after the casting machine in the conveying direction. The slab then undergoes a heat treatment in a heat treatment device. Between the cleaning device and the heat treatment device, the slab undergoes a smoothing process by means of two interacting smoothing rolls.

The disclosure is based on the object of further developing a method of the type mentioned above such that the problem mentioned is eliminated. In particular, energy and production costs are to be reduced, which is to be carried out by influencing the slab and strip edge geometry on a targeted basis. In particular, the surface of the slabs is to be improved prior to the first forming process.

The solution to this object is characterized by the fact that the smoothing process is carried out such that the slab does not undergo a substantial reduction in thickness, and the roughness of the smoothed surfaces of the slab is reduced.

The mean roughness value of the rolled surfaces of the slab after smoothing is preferably at most 10 μm, particularly preferably at most 8 μm. This means that the mean roughness value is reduced compared to the usual value when casting from a mold (approximately 11 μm to 14 μm).

The slab preferably undergoes the smoothing process in a fully solidified state.

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

The smoothing process is carried out with two interacting smoothing rolls, which preferably exert a force of between 3,000 kN and 5,000 kN, and particularly preferably between 3,500 kN and 4,500 kN, acting normally on the slab.

The smoothing process with the two interacting smoothing rolls is preferably carried out such that smoothing roll diameters of between 500 mm and 700 mm, preferably between 550 mm and 650 mm, are provided.

The cleaning process in the cleaning apparatus is preferably a descaling process, in particular by fluid descaling or scarfing.

The smoothing rolls can be cooled during the smoothing process.

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

Accordingly, the proposed method is used in particular for casting-rolling hot slabs in order to prepare the surfaces of the slab prior to being fed into the furnace. The smoothing of the surfaces of the solidified slab exiting the casting machine is carried out.

The central aspect of the disclosure is the smoothing of the surface of the casting strand/slab, wherein cleaning, in particular descaling (e. g, by fluid descaling, scarfing or gas cleaning), is carried out beforehand and the cleaned slab surface is smoothed by a pair of rolls. There is no relevant reduction in thickness, i.e. no penetrating forming process and thus no associated structural transformation from a cast structure to a rolled structure (which only occurs with a corresponding reduction in thickness).

Rather, the provided smoothing process (by using the pair of smoothing rolls) merely smooths the surface peaks of the slab and thus reduces the surface roughness, such that the surface is as even as possible.

The material of the “surface peaks” thus only flows from these into the recesses/“valleys.” This reduces the potential attack surface for the formation of scale and the flatter surface scales more evenly in the further process, without forming further surface-damaging peaks and recesses.

The reduction in thickness of the slab resulting from the application of force is less than 10%, preferably less than 5% and particularly preferably less than 3%. In particular, a reduction in thickness of less than 1% can be provided, wherein only elastic deformation takes place in the slab itself during the smoothing process, which, however, causes the desired slight forming process (as explained above) due to the surface peaks there and is limited to the layer of the slab close to the edge.

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. Thus, the pair of smoothing rolls is not a fully-fledged rolling mill stand with corresponding actuators.

The pair of smoothing rolls can be moved in and out of the line according to a possible configuration. However, it can also remain stationary in the system.

The smoothing rolls are preferably cooled. The pair of smoothing rolls can be accommodated together with a shear in a common frame. Furthermore, it can be provided that the descaler is also integrated into this common frame.

The method is preferably used with a slab thickness of between 30 mm and 180 mm, particularly preferably with a slab thickness of between 50 mm and 150 mm.

The preferred pressure force of the smoothing rolls of approximately 4,000 kN differs substantially from the forces otherwise provided. This force is too low for the desired reduction in thickness of the slab, on the other hand it is significantly higher than the force that is required for the contact of drive rolls (this is only approximately 20% to 50% of the contact force of the provided roll contact force of the smoothing rolls).

Of course, the design of the operating parameters is also carried out as a function of the strip width.

The preferably provided diameter of the smoothing rolls (approximately 600 mm) is also significantly larger than the typical drive rolls used in the casting machine sector.

The design of the diameter and optionally also the barrel shape is also carried out as a function of the strip width.

The arrangement of the smoothing rolls is provided close to the outlet of the casting machine and can also be partially located beneath the casting platform.

Due to the proposed procedure, substantial advantages arise, which first 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.

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

Advantageously, energy can also be saved, since less scale has to be removed/can be removed more easily due to the lower and simpler scale build-up. Thus, the required amount of water is also reduced, such that the cooling of the slab/the pre-strip, which has to be compensated for later, requires less energy. Finally, production costs are reduced as a result of the fact that unnecessary excess widths can be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing shows an exemplary embodiment.

FIG. 1 shows a system for producing metallic strips by casting rolling,

FIG. 2 shows an embodiment of a smoothing device,

FIG. 3 shows a possible sequence of method steps, and

FIG. 4a and FIG. 4b schematically show, in a highly magnified view, the surface structure of a slab treated in accordance with the disclosed method.

DETAILED DESCRIPTION

FIG. 1 shows a schematic representation of a front part of the casting-rolling system 1 for producing metallic strips, in particular steel strips. The system 1 first comprises a casting device 2, which is used to cast slabs 3 with a thickness of 30 to 150 mm, which are typically formed from the vertical to the horizontal using guide rolls not shown. In doing so, the slabs 3 leaving the casting machine 2 are so hot that an oxide layer in the form of scale forms immediately on contact with atmospheric oxygen. This must be removed from the surface of the slabs 3 for the subsequent heat treatment.

For this purpose, the system 1 comprises a cleaning unit 4 arranged directly after the casting device 2 in the direction of strip travel, which has a first upper and a first lower descaler 5, 6. The descalers 5, 6 arranged above and below the slab 3 can be used to remove most of the scale that has already formed from the slab surface.

In the direction of strip travel after the cleaning unit 4, the system 1 also comprises a separating device 7 with two shears 8, 9 via which the slab 3 can be divided prior to entering a heat treatment device 11. In the present case, 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.

In order to improve the transport conditions in the roller hearth furnace 11, on the one hand, and to remove the part of the scale that cannot be removed by means of the descalers 5, 6, on the other hand, the system 1 comprises a smoothing device 12 arranged between the cleaning device 4 and the separating device 7 with an upper and a lower drivable smoothing roll 13, 14. Both smoothing rolls 13, 14 are spaced apart such that a smoothing pass can be carried out on the cleaned slabs 3. Due to the smoothing pass, on the one hand, the surface of the slab 3 is smoothed, and on the other hand, the scale that cannot be removed by the upstream descalers 5, 6 is effectively broken up. This is then removed by a second cleaning unit 15 downstream of the separating device 7, which has a second upper and a second lower descaler 16, 17, such that an almost scale-free divided slab 10 can be fed to the roller hearth furnace 11.

In the present case, the distance between the two smoothing rolls 13, 14 is set such that the slabs 3 undergo a reduction in thickness of at least 3.0% and a maximum of 5.0% in accordance with a possible embodiment, wherein this 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.

FIG. 2 shows a sectional view of an embodiment of the smoothing device 12. The smoothing device 12 comprises the two smoothing rolls 13, 14 arranged in a stand 19 along with a force measuring sensor 20 arranged on the operator and drive side in each case, by means of which the forces across the slab width can be determined. As a result, useful information about the shape of the slabs, such as their thickness and/or wedge shape, can be obtained at an early stage of the process and transmitted via appropriate signaling systems to the casting device 2 and/or the subsequent rolling mill, in order to optimize the rolling process as a whole.

FIG. 2 also indicates that corresponding control/regulating devices 21 are provided for the position of the smoothing rolls 13, 14/the force that they exert on the slab.

In order to ensure the option of retrofitting old systems, the cleaning units 4, 15, the separating device 7 along with the smoothing device 12 arranged between them can be designed in the form of a unit 18 (see FIG. 1).

FIG. 3 shows the sequence of the individual method steps as they may be provided for in the method. The casting strand/slab is produced in step A in the casting machine. In accordance with step B, inductive heating of the slab can be provided. The slab then enters the cleaning device/descaler in step C. In accordance with step D, a separation of the slab into individual pieces can then be provided. In accordance with step E, the central smoothing of the surface of the slab is then carried out by means of the smoothing rolls 13 and 14. In accordance with step F, the further transport/heating of the pretreated slabs in the furnace is then carried out.

A possible alternative to this sequence is that step E (smoothing) is carried out prior to step D (separating).

The intended effect, which is to be caused by the smoothing pass by means of the smoothing rolls 13 and 14, is schematically illustrated in FIGS. 4a and 4b: In FIG. 4a, the slab with its thickness D is schematically sketched, wherein the “surface mountain range” results as usual after casting in the mold. It should be noted that with normal care, the mean roughness value Ra of the surface is typically between 11 μm and 14 μm. After smoothing by means of the smoothing rolls 13 and 14, as sketched in FIG. 4b, the slab 10 (divided thin slab) has substantially retained its thickness D, i.e., a relevant reduction in thickness has not been carried out. However, the “surface mountain range” is significantly leveled; i.e., the mean roughness value R_a is significantly reduced.

Preferably this is carried out to a value below 10 μm, particularly preferably below 8 μm. Smaller values below 6 μm are even more favorable and are aimed for.

The first cleaning unit 4, the separating device 7 and the smoothing device 12 arranged between them are preferably formed as a unit 18. At least one of the two smoothing rolls 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 over the slab width can be determined.

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 an induction heater.

The slab surface is thus evened out prior to the separating device/the possible separating step but after the cleaning unit/after the cleaning step by smoothing the two surfaces, i.e. the top side and bottom side of the slab, with the smoothing pass. As a result, the local roughness peaks are reduced to such an extent that the formation of undesirable track grooves is effectively avoided, thus improving the transport processes in the heat treatment device. 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 device and is fed into the rolling mill. In this respect, it can be assumed that this effect is based on the reduction of the active surface on which the scale preferably forms. Thus, this reduction in scale formation has a positive effect on the output of the system by reducing material losses due to scale formation, which in turn has to be removed later.

By means of the cleaning device, which is designed in the form of descalers, a large part of the scale already formed is removed from the surfaces of the slab prior to the separating device. The part of the scale that cannot be removed by means of the descaler can be effectively broken up by the smoothing pass by means of the proposed method, which leads to a higher heat transfer and thus a more effective heat treatment process.

The separating device can take the form of a shear and/or a laser-based cutting device. Depending on the mode of operation, 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 stages. Both modes of operation can be provided individually or jointly by the system and the method and are equally suitable for the implementation of the disclosure, without any limitation.

Within the meaning of the present disclosure, the term “smoothing device” refers to a device with two driven smoothing rolls that are arranged at a defined distance from one another. Therefore, it is preferably provided that the system does not comprise a rolling mill between the casting device and the heat treatment device.

The smoothing pass is characterized by a particularly small reduction in thickness and can therefore be distinguished from a classic rolling pass. Therefore, it is preferably provided that the distance between the two smoothing rolls/the gap between them is formed such that the smoothing pass causes a maximum reduction in thickness of 5.0% in relation to the thickness of the incoming slab, more preferably a reduction in thickness of 3.0 to 5.0% in relation to the thickness of the incoming slab. In other words, the gap between the two smoothing rolls is set such that it is slightly smaller than the incoming slab.

Therefore, it is particularly preferably provided that the distance between the two smoothing rolls is constant and also remains constant 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.

In the embodiment described above, it is provided that a second cleaning device, which is particularly preferably designed in the form of descalers, is arranged between the separating device and the heat treatment device. The remaining loose scale can be removed from the surface of the slabs via the descalers arranged after the separating device in the direction of strip travel before the slabs are then fed for heat treatment.

In order to ensure the option of retrofitting old systems, it is particularly preferably provided that the first cleaning unit, the separating device and the smoothing device arranged between them are formed as a single unit.

LIST OF REFERENCE SIGNS

• • 1 System
  • 2 Casting machine/Casting device
  • 3 Slab/thin slab
  • 4 First cleaning device/cleaning unit
  • 5 First upper descaler
  • 6 First lower descaler
  • 7 Separating device
  • 8 Upper shear
  • 9 Lower shear
  • 10 Divided thin slabs
  • 11 Heat treatment device (roller hearth furnace)
  • 12 Smoothing device
  • 13 Upper smoothing roll
  • 14 Lower smoothing roll
  • 15 Second cleaning device/cleaning unit
  • 16 Second upper descaler
  • 17 Second lower descaler
  • 18 Unit
  • 19 Stand
  • 20 Force measuring sensor
  • 21 Control/regulating device for position/force