When rolling elongated material, in particular metal strip material, the work rolls in a rolling stand have to exert considerable forces on the strip material so as to reduce the thickness thereof. To avoid bending of the work rolls, back-up rolls having a larger diameter support or back up the work rolls. For cold rolling steel strip, it is usual to use a four-high or six-high rolling stand; in a six-high rolling stand, intermediate rolls are placed between the work rolls and the back-up rolls. To provide a steel strip with an appropriate thickness, so without crown, the work rolls are usually not cylindrical, but are often more or less barrel shaped. Also, the back-up rolls (in a four-high rolling stand) or the intermediate and back-up rolls (in a six-high rolling stand) can have a non- cylindrical shape as well.

In practice the rolls wear during use. The work rolls are relatively often grinded so as to re-instate their original form, or new work rolls are used, but the back-up rolls should have a life span of more than 10 times that of the work rolls. However, sometimes the wear of the back-up rolls is such that the back-up rolls have to be grinded more often. This amounts to a considerable cost.

During use for instance a work roll contacts the back-up roll over a certain length, here indicated with a length from 0 to Y. Over this length of contact, the rolls are more or less flattened over a part of their circumference, and depending on the position of contact the pressure between the rolls is different. The rolls wear at the contact between the rolls.

To provide some background information, in a four-high rolling stand the work rolls usually have a diameter of at least 250 mm, and the back-up rolls have a diameter that is approximately 2,5 times as big. In a six-high rolling stand the diameter of the work rolls is the same, and the relation between the diameter of the work rolls, the intermediate rolls and the back-up rolls is usually approximately 4— 5— 12. The length of the rolls should of course be at least the same as the width of the steel strip that has to be rolled, which can be more than 2 meter.

It is an object of the invention to provide a rolling stand for rolling elongated material comprising four rolls, wherein the back-up rolls have a reduced wear during use in comparison to the wear of the back-up rolls in the presently used four-high roll stands.

It is another object of the invention to reduce the wear of the intermediate rolls and the back-up rolls in a six-high rolling stand.

It is a further object of the invention to provide a pair of cooperating rolls for use in a four-high rolling stand (so a pair consisting of a work roll and a back-up roll), wherein the back-up roll has a reduced wear.

It is furthermore an object of the invention to provide cooperating rolls for use in a six-high rolling stand (so cooperating rolls consisting of a work roll and an intermediate roll, or cooperating rolls consisting of an intermediate roll and a back-up roll) , wherein the intermediate roll and the back-up roll have a reduced wear.

It is also an object of the invention to provide a back-up roll for use in a rolling stand or in a pair of cooperating rolls, wherein the back-up roll has a reduced wear.

It is as well an object of the invention to provide an intermediate roll for use in a six-high rolling stand or in cooperating rolls for use in a six-high rolling stand, wherein the intermediate roll has a reduced wear.

According to a first aspect of the invention there is provided a rolling stand for rolling elongated material, comprising two rolls A and two rolls B, wherein the rolls A and/or the rolls B have a shape that is non-cylindrical, and wherein during rolling a roll A is in contact with a roll B over a length of the rolls from 0 to Y, wherein the radius R _B (X) of the roll B is a constant factor K times the radius R _A (X) of the roll A at all positions x of the contact between the roll A and the roll B from 0 to Y, such that R _B (X) = K-R _A (X), plus or minus a small difference in the radius of the roll A and roll B at all positions x between 0 and Y, this difference in radius being AR( _A ) and AR( _B ), respectively, which difference in radius is at most 0.01 % of the radius of the respective roll A and B. The inventors have found that wear of the rolls can be minimised at the roll-to-roll interfaces by reducing the circumferential speed differences between the contacting rolls. During rolling, wear of two rolls at their interface is governed by their line load and the circumferential speed differences between the rolls. When the ratio of the radii of the contacting rolls is constant at all width positions on the interface between the rolls, that is over the full length of contact between the rolls, then the circumferential speed difference of the rolls remains small for the line loads necessary for rolling in combination with the power transmitted between these rolls during rolling. For this reason, the circumferential speed difference should be very small and hence the difference in the radius AR has to be very small.

Moreover, as a general requirement the line load between the rolls has to stay below the maximum allowable line load that the roll can handle, which is specified by the manufacturer of the rolls.

According to the prior art it is claimed that a moderate line load minimises mechanical wear. This state of the art is described in Wang et al, Design and application of an optimum backup roll contour configured with CVC work roll in hot strip mill, ISIJ Int. 52 (9) (2012) 1637-1643. The inventors have found that this prior art solution does not work to reduce wear.

A rolling stand wherein the difference in the radius of the roll A AR _(A ) and/or the roll B AR(B) is at most 0.005% of the radius of the respective roll is preferred. Thus, the circumferential speed difference of the rolls is smaller, and thereby the wear of the rolls will be smaller.

It is even more preferred when the difference in the radius of the roll A AR _(A ) and/or the roll B AR _(B ) is at most 0.003% of the radius of the respective roll. Wear of the rolls will be minimal for this difference in the radii of the rolls.

Preferably, the rolling stand is a four-high stand having two work rolls A and two back-up rolls B. This is the most used rolling stand having work rolls and back-up rolls.

According to another preferred embodiment the rolling stand is a six-high stand having two work rolls C, two intermediate rolls IM and two back-up rolls D, wherein the radius of a work roll C and the radius of an intermediate roll IM is the radius of roll A and roll B, respectively, as described above, and/or wherein the radius of an intermediate roll IM and a back-up roll D is the radius of the roll A and the roll B, respectively, as described above. In a six-high rolling stand it is thus possible to reduce the wear of the intermediate rolls IM and/or the wear of the back-up rolls D. Usually it is preferred to reduce the wear of both the intermediate rolls and the back-up rolls.

Preferably, the rolling stand is a rolling stand for rolling elongated metal material, in particular for rolling elongated aluminium or steel material, such as steel strip material. Rolling steel material causes high rolling forces exerted on the rolls, leading to high wear if the circumferential speed differences are too high.

The invention is particularly useful when the rolling stand is a rolling stand for cold rolling steel strip material or a rolling stand for hot rolling steel strip material. When steel strip material is cold or hot rolled, the rolling forces are often high.

According to a second aspect of the invention, a pair of cooperating rolls A and B is provided for use in a rolling stand according to the first aspect of the invention, wherein during use the roll A is in contact with the roll B over a length of the rolls from 0 to Y, characterised in that the radius R _B (X) of the roll B is a constant factor K times the radius R _A (X) of the roll A at all positions x of the contact between the roll A and the roll B from 0 to Y, such that R _B (X) = K R _A (x), plus or minus a small difference in the radius of the roll A and roll B at all positions x between 0 and Y, this difference in radius being AR( _A ) and AR( _B ), respectively, which difference in radius is at most 0.01 % of the radius of the respective roll A and B. This pair of rolls can be used in the rolling stand as described above.

According to a third aspect of the invention a back-up roll is provided for use in a rolling stand according to the first aspect of the invention or as part of the pair of cooperating rolls according to the second aspect of the invention, wherein the back-up roll has a radius R _B that is a constant factor K times the radius of the work roll or intermediate roll the back-up roll has to contact during use, at all positions of the contact between the back-up roll and the work roll or intermediate roll, plus or minus a small difference in the radius of the back-up roll at all positions of contact, this difference in radius being at most 0.01 % of the radius of the back-up roll. In this way, also the backup roll as such is covered, for instance when a back-up roll is ground after wear.

According to a fourth aspect of the invention an intermediate roll is provided for use in a six-high rolling stand according to the first aspect of the invention or as part of the pair of cooperating rolls according to the second aspect of the invention, wherein the intermediate roll has a radius R _M that is a constant factor K times the radius of the work roll the intermediate roll has to contact during use, at all positions of the contact between the intermediate roll and the work roll, plus or minus a small difference in the radius of the intermediate roll at all positions of contact, this difference in radius being at most 0.01 % of the radius of the intermediate roll. In this way, also the intermediate roll as such is covered, for instance when the intermediate roll is ground after wear.

The invention furthermore relates to a method for rolling elongated material using a four-high rolling stand, wherein two work rolls A and two back-up rolls B are present, wherein the rolls A and/or the rolls B have a shape that is non-cylindrical, wherein during use a work roll A is in contact with a back-up roll B over a length of the rolls from 0 to Y, and that at the start of a campaign of rolling the material the rolls have a radius such that the radius R _B (X) of the back-up roll B is a constant factor K times the radius R _A (X) of the roll A at all positions x of the contact between the roll A and the roll B from 0 to Y, such that R _B (X) = K R _A (x), plus or minus a small difference in the radius of the roll A and roll B at all positions x between 0 and Y, this difference in radius being AR _(A ) and AR _(B ), respectively, which difference in radius is at most 0.01 % of the radius of the respective roll A and B. This method thus relates to the use of work rolls and back-up rolls when rolling elongated material, and specifies that at the start of a campaign of rolling material the radii of the contacting rolls should be constant at all width positions on the interface between the rolls, apart from a small difference in the radii. During the campaign the radii cannot be measured, hence the prescription at the start of the campaign.

The same holds for a method for rolling elongated material using a six-high rolling stand, wherein two work rolls A, two intermediate rolls IM and two back-up rolls B are present, wherein the rolls A and/or the rolls B and/or the rolls IM have a shape that is non-cylindrical, and wherein during use a work roll A is in contact with an intermediate roll IM over a length of the rolls A and IM from 0 to X and an intermediate roll IM is in contact with a back-up roll B over a length of the rolls IM and B from 0 to Y, and that at the start of a campaign of rolling the material the rolls B and the rolls IM have a radius such that the radius R _B (Z) of the back-up roll B is a constant factor K times the radius R _M (z) of the intermediate roll IM at all positions z of the contact between the roll IM and the roll B from 0 to Y, such that R _B (Z) = K-R _M (z), plus or minus a small difference in the radius of the roll A and roll B at all positions z between 0 and Y, this difference in radius being AR( _B ) and respectively, which difference in radius is at most 0.01% of the radius of the respective roll B and IM, and/or that at the start of the campaign of rolling the material the rolls A and the rolls IM have a radius such that the radius R _M (z) of the intermediate roll IM is a constant factor L times the radius RA(Z) of the work roll A at all positions z of the contact between the intermediate roll IM and the work roll A from 0 to X, such that R _M (z) = L R _A (z), plus or minus a small difference in the radius of the roll IM and roll A at all positions z between 0 and X, this difference in radius being AR( _M) and AR( _A) , respectively, which difference in radius is at most 0.01% of the radius of the respective roll IM and A.

Preferably the elongated material to be rolled is metal material, in particular steel strip material.

In particular the rolling stand is a rolling stand in a cold rolling mill or in a hot rolling mill.

In the above further aspects of the invention and in the methods described above, it is preferred when the difference in radius is at most 0.005%, and even more preferred when the difference in radius is at most 0.003%.

The invention will be elucidated with reference to the figures.

Figure 1 shows a very schematic picture of two contacting rolls with their contact length.

Figure 2 shows a graph with the hollowing of the back-up roll versus the campaign length of original rolls and trial rolls.

Figure 3 shows a graph with the number of work roll changes because of wear in the original situation and during trials.

In Figure 1 a very schematic representation is given of two cooperating rolls, such as a work roll A and a back-up roll B. This figure shows what is meant by the length of the contact between the rolls A and B from 0 to Y, and the position x of the contact between the rolls A and B. It will be clear that the length of contact will depend on the rolls used and the rolling that is performed, and that the position x is always between the endpoints 0 and Y of the contact between the rolls.

Figure 2 shows the amount of wear of the back-up rolls in stand 5 of a cold rolling mill. In the original situation wear was high, leading to operational problems. In stand 5 rolls with a crown were used. The amount of wear was measured by measuring the hollowing of the back-up rolls used. With the term hollowing is indicated that the backup rolls wear out mostly in the middle of the roll, and the hollowing is measured where the wear is most severe. The hollowing of the back-up rolls could be more than 0.5 mm measured on the crown of the back-up rolls, and often a hollowing of 0.3 to 0.4 mm was measured. Of course the hollowing of the back-up rolls increases with increased campaign length. This is shown in Figure 2 with the dots. In this situation at the beginning of a campaign the back-up roll typically has a diameter of 1250 mm with a 75 μιη crown (being the difference between the centre and the mean of the edges of the roll), and the work roll has a diameter of 575 mm and a crown of 100 μιη or 200 μιη.

In November 2013 a trial was started, wherein a gradual move was made towards using work rolls and back-up rolls wherein the ratio of the crowns on the rolls matches the ratio of the diameters of the rolls, so the surface speeds matches along the whole length of the rolls. In this situation the rolls have a diameter as before, but the back-up roll has a 125 μιη crown and the work roll has a 70 μιη crown.

The result on the wear of the back-up rolls is shown in Figure 2 with the triangles. It is directly apparent that the wear of the back-up rolls is reduced, because the hollowing is diminished. The maximum hollowing measured during the trials is less than half of the hollowing in the original situation.

The general results of the original hollowing and the trial hollowing shown by the dotted lines in Figure 2 show that, by using the invention, the hollowing is reduced by a factor 3 approximately.

Not only the measured hollowing as indication of the wear of the back-up rolls shows the positive result of the use of the invention, but also the general improvement can be seen in the reduction of unplanned work roll changes, as shown in Figure 3.

Figure 3 clearly shows that after the start of the trials in November 2013 the number of unplanned work roll changes has reduced considerably. In 2014 and thereafter, the number of unplanned work roll changes has reduced by at least a factor 3. Thus not only a reduction in work roll and back-up roll cost is achieved, but also a considerable down time reduction. This is of course a huge advantage for the processing of a cold rolling line and is clearly the result of use of the invention in shaping the rolls.

In both Figure 2 and Figure 3 "a.u." means that the values on the respective axis are given in arbitrary units.

It will be clear to the person skilled in the art that the invention is particularly useful for the shaping of the rolls in the rolling stands of a cold rolling mill or a hot rolling mill for steel strip, but that the invention can be used for cooperating rolls for other roll stands as well.