EP4640327A1 - Work roll for use in a cold rolling mill and method for cold rolling a flat material, and a cold rolling mill - Google Patents

Abstract

The invention relates to a work roll (1) for use in a cold rolling mill and a method for cold rolling a flat material, as well as a cold rolling mill.

Description

The invention relates to work rolls for use in a cold rolling mill and a method for cold rolling a flat material, as well as a cold rolling mill.

Work rolls in cold rolling mills producing flat steel are typically given a specific surface roughness, usually by grinding. The roughness of the roll surfaces alters the friction in the roll gap. It prevents slippage of the flat material from causing surface defects. For the flat material to be gripped by the work roll at the entrance of the roll gap and driven between the roll surfaces, the friction between the flat material and the roll surface must be sufficiently high to ensure proper gripping.

The surface roughness of the work rolls tends to decrease with use. A higher grinding roughness thus offers the possibility of a longer rolling service life. However, the production sequence must be adapted to the changing friction conditions with regard to the flat material, its thickness, and width. Furthermore, excessive grinding roughness can lead to significant contamination of the work rolls and the flat material.

Textured work rolls offer the possibility of introducing a defined surface structure into the surfaces of the flat product. Random, so-called stochastic structures are created by blasting the roll surfaces with grains (shot blasting textureding), by electrical discharge texturing (EDT), or by electroplating (electrochemical deposition).

Using laser or electron beams, valleys and plateaus can be selectively created in a regular shape and arrangement, so-called deterministic structures (laser texturing, electron beam texturing), as exemplified in the WO 1995/026836 A1 explained.

The EP 1 368 140 B1 reveals a cold rolling process with a sequence of several textured work rolls to produce a surface roughness similar to electropolishing on an aluminium flat product.

The SU 1 733 159 A1 This reveals a cold rolling roll with a micro-relief on the working surface, designed to enhance technological capabilities. The micro-relief consists of periodically arranged holes with a depth of 0.4 mm to 0.5 mm and a diameter of 1.5 mm to 2.0 mm, spaced 2 mm to 6 mm apart. These structures are intended to make the working rolls more resistant and improve the quality of the bent profile.

The rationale behind conventional surface finishing of work rolls is that applying a metallic protective layer, such as hard chrome plating, provides improved wear protection with regard to service life and has a positive impact on the required strip cleanliness. EP 0 184 568 B1 The processing of such a layer using laser beams for alloy formation and microstructuring is revealed.

The object of the invention is to provide the surface of the work roll with a suitable texture so that it can better meet the requirements of cold rolling. In particular, the rolling force must be reduced to conserve resources and when rolling high-strength materials. Further objectives include increasing service life, while ensuring proper gripping conditions, especially in conjunction with sufficient flatness and cleanliness of the flat material. An additional wear-resistant coating may be unnecessary.

The problem relating to a work roll for use in a cold rolling mill is solved by the features of claim 1. The problem relating to a method for cold rolling a flat material is solved by the features of claim 6. The problem relating to a cold rolling mill is solved by the features of claim 9.

The teaching of the invention is that a reduction in rolling force can be achieved without impairing the cold rolling conditions by suitable texturing of the surface of the work roll.

The existing literature does not reveal a solution for how the surface topography on work rolls should be designed to reduce the rolling force required for a cold rolling process.

The surface topography of work rolls according to the invention preferably includes continuous valleys transverse and/or oblique to the rolling direction, which function as lubricant channels. This structuring allows for the targeted adjustment of rolling forces to a lower level. Reducing the rolling force is advantageous, among other things, for processing high-strength materials, preferably steels, for increasing flat material widths and/or pass thicknesses, for extending tool life, for increasing the energy efficiency of the forming process, and/or for reducing contamination.

These structures, along with the parameters subsequently stressed, also have a positive influence on wear resistance. This allows the gripping condition to be maintained for a longer period during the rolling process, and also prevents slippage of the flat material in the roll gap for a longer time. Consequently, the rolling forces can remain more constant during operation than with the prior art.

The valleys divide the surface of the work roll into separate plateaus or plateau areas. To ensure a long roll service life, it is desirable for the transition from the plateau segment to the valley to be as abrupt as possible. The depressions, or the resulting valleys, in the roll surface are preferably created by laser irradiation. An alternative method for producing these valleys could, for example, involve cross-grinding.

The valleys serve primarily to collect the abrasion particles inevitably generated during the cold rolling process from the roll gap. The valleys are essentially continuous, allowing, for example, a liquid rolling fluid to escape and carry the abrasion particles with it. A key characteristic of the work roll surface is that it incorporates open valleys and thus lubricant channels, which preferably run obliquely to the rolling direction.

A closed texture on the surface of a work roll, such as could be created by isolated laser shots, would quickly become clogged by abrasion. Any lubricant trapped within would thus reduce friction to such an extent that the flat material being rolled would slip and, in the worst case, become unrollable.

Furthermore, the particularly open design of the valleys improves gripping conditions by increasing friction at the entrance to the roll gap. Excess rolling medium can escape from the plateaus and spread out in the valleys.

Roughness loss occurs predominantly on the plateau surfaces. As long as the introduced depressions or valleys form a continuous network or open structure, the gripping condition remains fulfilled, even if the roughness on the plateau surfaces has completely disappeared. The gripping condition is thus largely decoupled from the loss of roughness.

A first teaching of the invention relates to a work roll for use in a cold rolling mill with a surface wherein the topography defines valleys and plateaus or plateau areas and has the following topography properties according to DIN EN ISO 25178-2:2023-09: a mean arithmetic height Sa between 0.30 µm and 4.0 µm, a core height Sk between 0.40 µm and 8.0 µm and an area of the valleys Sak2 between 0.15 ml/m ² and 5.0 ml/m ² .

The topography on the surface of the work roll according to the invention is designed in such a way as to ensure rollability. For this purpose, the valleys of the work roll are designed such that the area of the valleys Sak2 is in particular at least 0.18 ml/ ^m² , preferably at least 0.20 ml/ ^m² , more preferably at least 0.25 ml/ ^m² , and most preferably at least 0.30 ml/ ^m² . The area of the valleys Sak2 can be up to 3.0 ml/ ^m² , in particular up to 2.6 ml/ ^m² , more preferably up to 2.3 ml/ ^m² , and more preferably up to 2.0 ml/ ^m² .

The mean arithmetic height Sa of the described topography has an influence on the entirety of the cold rolling process, the lubrication and adhesion conditions, the friction, etc.

Friction acting at the contact surfaces of two touching bodies makes it more difficult for them to move relative to each other. The frictional force acts in the opposite direction to the relative motion of the bodies. Friction is both an advantage and a disadvantage for the rolling process: When the material enters the roll gap, and the material moves slower than the work roll without external forces, friction is essential so that the work roll can grip the material and drive it forward in the strip direction. Gripping is possible when the angle of engagement is smaller than the angle of friction. From the so-called At the flow gap, the material being rolled moves faster than the work roll. To facilitate strip movement, friction should be as low as possible from this point onward. The integration of lubrication pockets and/or channels into the work roll ensures a sufficient supply of lubricant, which reduces friction at the roll gap exit. Insufficient lubrication would cause friction to increase again, hindering the rolling process and potentially leading to strip warping.

The mean arithmetic height Sa of the topography can, in particular, be at least 0.40 µm, 0.50 µm, or 0.60 µm. The mean arithmetic height S _a of the topography can, in particular, be limited to a maximum of 4.90 µm, 4.70 µm, or 4.50 µm, preferably to a maximum of 4.10 µm, 3.90 µm, or 3.70 µm, and more preferably to a maximum of 3.50 µm, 3.30 µm, or 3.0 µm. If the mean arithmetic height Sa of the topography falls below 0.30 µm, the required rolling conditions cannot be guaranteed, and therefore sufficient rolling or the desired deformation in the rolling stand cannot be ensured. A mean arithmetic height Sa of the topography above 5.0 µm can also have a negative effect on rolling.

The topography of the work roll features plateaus, which are preferably smooth or have grinding structures or a microstructure with low roughness. Smoothly polished plateaus are also possible. The microstructuring can be carried out, for example, using a laser. A core height Sk of particularly less than 7.0 µm, preferably less than 6.0 µm, more preferably less than 5.0 µm, most preferably less than 4.0 µm, and more preferably less than 3.0 µm, results in less dirt accumulating in the roll gap.

If a core height Sk of 8.0 µm is exceeded, it cannot be ensured that the flat material can be drawn through and that the gripping condition is met; in particular, fluctuations in the rolling force or excessive rolling force cannot be avoided.

The area of the valleys Sak2 is in particular at least 0.60 ml/m ² , preferably at least 0.70 ml/m ² . With this size of the area of the valleys Sak2, the rolling force required for the rolling process can be reduced.

It may also be advantageous if the reduced valley floor depth Svk is at least 2.50 µm, in particular at least 3.50 µm, preferably at least 4.50 µm to 12.0 µm, in particular up to 11.0 µm, preferably up to 10.0 µm.

The topography or surface of the work roll can preferably be determined using an areal measuring system, for example, with a measuring area of at least 1 mm × 1 mm. Depending on the structure size of the topography to be measured, or depending on the structure itself, the measuring area can be, for example, up to 10 mm × 10 mm, preferably up to 5 mm × 5 mm. A square measuring area is not strictly necessary; formats such as 2 mm × 5 mm are also acceptable. The surfaces to be evaluated can, for example, be scale-limited according to DIN EN ISO 25178-2:2023-09 in section 3.1.9. The refinement index of the L-filter can be, for example, 1.0 mm and that of the S-filter, for example, 0.010 mm. Preferably, an areal Gaussian filter according to ISO 16610-21:2011 is used for filtering.

The parameter Sa is the "mean arithmetic height" according to DIN EN ISO 25178-2:2023-09 (Chapter 4.2.8). It describes the mean of the absolute ordinate values of the scale-bounded surface.

The parameter Sk is the "core height" according to DIN EN ISO 25178-2:2023-09 (section 4.5.4.2 in conjunction with Figure 14). It describes the distance between the highest and lowest levels of the "core surface".

The parameter Smrk1 is the "material fraction of the hills" in percent according to DIN EN ISO 25178-2:2023-09 (Chapter 4.5.4.7 in conjunction with Figure 14).

The parameter Sak1 ("area of the hills") according to DIN EN ISO 25178-2:2023-09 (Chapter 4.5.4.9) corresponds to the material volume per unit area Sak1 = Vm (Smrk1), expressed in the unit [ml/ ^m² ]. A material volume per area of 1 ml/ ^m² is equivalent to an average height of 1 µm.

The reduced peak height according to DIN EN ISO 25178-2:2023-09 (Chapter 4.5.4.3) results from Spk = 2* Sak1 * 100% / Smrk1.

The parameter Smrk2 is the "material content of the valleys" in percent according to DIN EN ISO 25178-2:2023-09 (Chapter 4.5.4.2 and Chapter 4.5.4.8).

The parameter Sak2 ("area of valleys") according to DIN EN ISO 25178-2:2023-09 (Chapter 4.5.4.10) corresponds to the void volume of the valleys per unit area: Sak2 = Vv (Smrk2), expressed in the unit [ml/ ^m² ]. A material volume per area of 1 ml/ ^m² is equivalent to an average height of 1 µm.

The reduced valley floor depth according to DIN EN ISO 25178-2:2023-09 (Chapter 4.5.4.5) results from Svk = 2* Sak2 * 100% / (100% - Smrk2), expressed in the unit [µm].

Among other things, Figure B1 in DIN EN ISO 25178-2:2023-09 serves to illustrate and calculate the aforementioned parameters.

Topography can be recorded using a confocal light microscope. The data acquired is then subjected to filtering operations, which can be performed using evaluation software such as MountainsMap from digital surf. The aforementioned parameters are standard parameters in MountainsMap.

At the same time, it was shown that a topography according to the invention on a work roll exhibited a comparable service life to standard work rolls with a surface coated with a metallic protective layer. Thus, the work roll according to the invention can be considered an "uncoated" work roll with a bare surface.

Work rolls for cold rolling processes are usually made of metal, for example, they are forged or cast. It goes without saying that the topography is designed axially on the surface of the work roll to such an extent that the flat material to be rolled can be gripped by the topography across its entire width in the rolling direction.

The flat material to be cold-rolled is preferably generally a steel flat product in strip form (steel strip). Alternatively, for example, an aluminum flat product (aluminum strip) can also be cold-rolled with the work roll according to the invention.

Due to its function, the work roll in a cold rolling process must meet different requirements compared to rolls in tempering processes, so-called temper rolls. In addition to the aforementioned tasks of removing abrasion and extending service life, a virtually slip-free cold rolling process must also be ensured. Therefore, the surface topography of work rolls for the cold rolling process must be specifically designed and is not comparable to the surface topography of temper rolls.

The plateaus are preferably arranged in a regular shape and at regular intervals on the surface of the work roll. This design can be considered deterministically textured, meaning that a surface structure with recurring patterns is created, with plateaus and valleys that have a defined shape and/or configuration, cf. EP 2 892 663 B1 .

A thermal process can be used for texturing, for example, laser texturing. Devices for carrying out the laser texturing process are state of the art; see also EP 2 892 663 B1 also the EP 3 172 006 B1 and the EP 3 877 112 A1 In this way, it is possible to create a topography on the surface of the work roll, which, when laser-blasted, causes ablation, i.e., material removal in the area of impact. Through precise adjustment, the laser beams can overlap or be spaced so far apart that a near-original state remains between them. The areas not hit by the laser beam thus form the plateaus on the surface of the work roll.

Alternatively, a mechanical texturing method can be used, for example, engraving. Devices for carrying out engraving are also state of the art.

Alternatively, a chemical texturing process can also be used, for example etching. Devices for carrying out etching are also state of the art.

According to one embodiment of the work roll, the valleys have a depth between 2 and 40 µm. The depth can be at least 3 µm, 4 µm, 5 µm, preferably at least 7 µm, 8 µm, 9 µm, in order to counteract unavoidable wear during cold rolling. The desired topography is maintained to ensure a certain service life for the work roll. The depth can be limited, in particular, to a maximum of 35 µm, 30 µm, 25 µm, preferably to a maximum of 20 µm, 18 µm, 15 µm.

According to one embodiment of the work roll, the valleys can have a width between 5 and 100 µm. The width can be at least 10 µm, 15 µm, 20 µm, and preferably at least 25 µm, in order to ensure, in conjunction with the depth, a sufficient void volume for receiving abrasion and thus for the corresponding removal during cold rolling. The width can be limited, in particular, to a maximum of 85 µm or 60 µm to prevent the formation of unwanted protrusions and thus near-surface profiling on the flat material to be rolled, especially in the rolling direction.

According to a second teaching, the invention relates to a method for cold rolling a flat material or a metal flat material in a cold rolling mill with at least one, in particular at least two, preferably at least three, preferably at least four and at most nine, in particular at most seven, preferably at most five rolling stands, in particular of a steel or aluminum flat product, wherein in at least one rolling stand two work rolls according to the invention are used, which act on the flat material and cause a thickness reduction of at least 5 and at most 90%, wherein the thickness reduction corresponds to the ratio of the thicknesses of the flat material before entering the rolling stand and after exiting the same rolling stand.

Depending on the number of rolling stands in the cold rolling mill, the number of the rolling stand in a series of several rolling stands (cold rolling line), whether it is the first, second, third rolling stand, etc., through which the flat material is successively cold-rolled and its thickness reduced, and especially depending on the target thickness to be achieved, the thickness reduction in a rolling stand can be at least 10%, preferably at least 15%, and preferably at least 20%. The thickness reduction in a rolling stand can be at most 80%, preferably at most 70%, more preferably at most 60%, further preferably at most 50%, and most preferably at most 40%, for example, per pass.

To avoid repetition, reference is made to the aforementioned designs of the work roll.

Materials requiring high rolling forces, especially steel materials with tensile strengths greater than 600 MPa, particularly greater than 780 MPa, preferably greater than 900 MPa, preferably greater than 1050 MPa, can also be cold-rolled more effectively and with greater process stability, so that, among other things, limit forces of the rolling stand may not be reached, which also makes it possible to roll larger dimensional widths and/or thicknesses.

Thus, when cold rolling flat materials, an acceptable roll service life can be achieved through the use of work rolls according to the invention, due to reduced rolling force in conjunction with reduced friction. A reduction in contamination, particularly through the reduction of iron abrasion during the cold rolling of flat steel materials, is also possible, which in turn can have a beneficial effect on strip contamination and the cleanliness of the cooling lubricant emulsion.

According to a third teaching, the invention relates to a cold rolling mill with at least one, in particular at least two, preferably at least three, more preferably at least four and at most nine, in particular at most seven, more preferably at most five rolling stands, wherein at least one rolling stand has two work rolls according to the invention, which are manufactured in particular according to the second teaching of the invention. A cold rolling mill with one rolling stand can, for example, be a reversing stand. In the case of two or more rolling stands of a cold rolling mill, two work rolls according to the invention can be provided, particularly when viewed in the process direction, at least in the first and second or in the second and third rolling stand. Preferably, the cold rolling mill is a tandem cold rolling mill or a part thereof. Other equipment can also be connected upstream and/or downstream of the cold rolling mill in the process direction, such as a continuous pickling line in the upstream section of the cold rolling mill. The arrangement of work rolls in a rolling stand of a cold rolling mill, as well as the structure and operation of a cold rolling mill or a tandem cold rolling mill, are state of the art.

An exemplary embodiment of a work roll (1) according to the invention and the corresponding topography (3) is shown in Figure 1 shown.

In the upper right is a sketch of a work roll (1) with a surface (2) for use in a cold rolling mill. It is preferably open and deterministically textured. From the In the small window, there is a magnification on the left. Figure 1 shown, which schematically represents a partial top view of the surface (2) of the work roll (1) with a topography (3). The illustration at the bottom right in Figure 1 Figure 1 shows a partial cross-section of the work roll (1), which defines the topography (3) on the surface (2) with plateaus (3.1) and valleys (3.2). The depth (T), which can range from 2 to 40 µm, and the width (B), which can range from 5 to 100 µm, of the valley (3.2) are illustrated in the partial cross-sectional view. The depth (T) is determined from the highest point of the plateau (3.1) surrounding the valley (3.2) on the surface (2) of the work roll (1), and the lowest point is measured within the valley (3.2) formed in the work roll (1). The width (B) is determined on the surface (2) between two opposing points where the curvature or radius, and thus the indentation of the valley (3.2), begins.

Investigations were carried out in a four-stand tandem cold rolling mill, wherein the second rolling stand in the process direction was equipped with two work rolls (1) according to the invention, as upper and lower work rolls in contact with the strip. Hot-rolled strips with a tensile strength greater than 780 MPa were processed, and essentially the same parameters were set, in particular with an identical thickness reduction of approximately 33% at the second rolling stand. A total of 14 different work roll pairs were investigated, with work roll pairs 1 to 14 exhibiting the characteristic parameters listed in Table 1 .

Work roll pairs 1 and 2 had a double-I structure in different orientations, work roll pair 3 a herringbone structure. Work roll pairs 4 to 11 had a square configuration, cf. Figure 1 bottom left, with different sizes ranging from 2 µm × 2 µm to 35 µm × 35 µm.

In the work roll pairs 4 to 11, which were work rolls (1) according to the invention, the average depth (T) of the valleys (3.2) was approximately 7 µm, and the average width (B) of the valleys (3.2) was approximately 35 µm. Similarly, in the work roll pairs 1 to 3, the average depth (T) of the valleys (3.2) was approximately 7 µm, and the average width (B) of the valleys (3.2) in pairs 1 and 3 was also approximately 35 µm, while in pair 2 it was approximately 70 µm.

The work roll pairs 12 to 14 were machined (only) with a grinding stone and exhibited different grinding roughness. The higher the grinding roughness, the greater the rolling force. All work roll pairs were uncoated.

Depending on Sak2, Svk, and Sk, determined according to DIN EN ISO 25178-3:2023-09, the rolling forces were recorded and compared with the rolling force of work roll pair 12 as the standard work roll pair. The average relative rolling force of several test series/campaigns is shown as a mean value in Table 1 for work roll pairs 1 to 14. Work roll pairs 1 and 3, and 12 to 14, exhibited, among other things, a small Sak2 and thus did not lead to the desired reduction in rolling force; on the contrary, the rolling force was even higher compared to work roll pair 12. The underlined values, and therefore also the corresponding work roll pairs 1 to 3 and 12 to 14, are not in accordance with the invention. <b>Table 1</b> Roller pair Sa [µm] Sk [µm] Spk [µm] Svk [µm] Smrk1 [%] Smrk2 [%] Sak1 [ml/ ^m² ] Sak2 [ml/m ² ] relative rolling force [%] 1 3.54 11.54 3.39 2.07 12.4 96.4 0.21 0.04 +7 2 3.97 13.63 2.70 1.66 5.7 94.6 0.08 0.06 +5 3 3.96 12.77 3.30 1.51 11.3 97.1 0.19 0.02 +4 4 3.45 7.68 3.17 4.85 6.6 67.7 0.10 0.78 -5 5 2.72 1.56 0.34 9.36 4.4 63.4 0.01 1.71 -19 6 2.41 1.88 3.00 9.39 13.9 73.5 0.21 1.24 -20 7 1.96 2.08 2.73 8.89 12.2 79.3 0.17 0.92 -20 8 1.22 0.97 2.03 7.17 10.6 82.8 0.11 0.62 -17 9 1.01 0.96 1.39 6.64 11.4 85.1 0.08 0.50 -10 10 0.71 0.79 1.21 5.11 11.4 86.8 0.07 0.34 -8 11 0.63 0.92 1.25 3.99 9.1 86.3 0.06 0.27 -5 12 0.24 0.78 0.28 0.36 9.1 89.6 0.01 0.02 0 13 0.74 2.34 0.76 1.20 8.3 88.4 0.03 0.07 +4 14 0.95 2.98 1.18 1.28 9.4 88.3 0.06 0.07 +8

Claims (12)

Work roll (1) for use in a cold rolling mill with a surface (2) wherein the topography (3) defines valleys (3.2) and plateaus (3.1) and has the following topography properties according to DIN EN ISO 25178-2:2023-09: a mean arithmetic height Sa between 0.30 µm and 5.0 µm, a core height Sk between 0.40 µm and 8.0 µm and a valley area Sak2 between 0.15 ml/m ² and 3.0 ml/m ² . Working roller according to claim 1, wherein the reduced valley depth Svk according to DIN EN ISO 25178-2:2023-09 is at least 2.50 µm to 12.0 µm. Working roll according to one of the preceding claims, wherein the plateaus (3.1) are arranged in a regular shape and at regular intervals on the surface (2) of the working roll (1). Working roller according to one of the preceding claims, wherein the valleys (3.2) have a depth (T) between 2 µm and 40 µm. Working roller according to one of the preceding claims, wherein the valleys (3.2) have a width (B) between 5 µm and 100 µm. Method for cold rolling a flat material in a cold rolling mill with at least one and at most nine rolling stands, in particular a flat steel or aluminum material, characterized in that in at least one rolling stand two work rolls (1) according to one of the preceding claims are used, which act on the flat material and cause a thickness reduction of at least 5% and at most 90%, wherein the thickness reduction corresponds to the ratio of the thicknesses of the flat material before entering the rolling stand and after exiting the same rolling stand. Method according to claim 6, wherein the flat material is a steel material with a tensile strength greater than 600 MPa, which is cold-rolled. Method according to claim 6 or 7, wherein the flat material is a steel material with a tensile strength greater than 780 MPa, preferably greater than 900 MPa, which is cold-rolled. Cold rolling mill with at least one and at most nine rolling stands, characterized in that at least one rolling stand has two work rolls (1) according to one of claims 1 to 5. Cold rolling mill according to claim 9, wherein, viewed in the process direction, at least two work rolls (1) are provided in the first and second rolling stand or in the second and third rolling stand. Cold rolling mill according to claim 9 or 10, wherein the cold rolling mill is a tandem cold rolling mill. Cold rolling mill according to claim 9, wherein the cold rolling mill comprises a reversing stand.