The present invention relates to a method and a device for manufacturing cold rolled metal sheets or strips, through a cold rolling tandem mill.

The present invention relates also to metal sheets or strips obtained through cold rolling tandem mills by using the method and the device of the present invention. State of the art The cold rolling process consists essentially in pulling off the strip coming from the hot rolling mill from the uncoiler through a tandem mill comprising usually several stands of 2, 4 or 6 high rolls and to coil it up again. The rolled strip coil is then heated up in a furnace, this process is known as annealing process. Afterwards, the annealed coil passes again through cold roll mill called skin pass or temper mill.

It is a common practice in cold rolling steel sheet to apply a certain roughness to the work rolls of the last stand of the tandem mill or of the temper mill.

The roughness is usually obtained by engraving the tandem or temper mill rolls through shot blasting or Electron Discharge Technology (EDT) . The result of using such techniques is a stochastical roughness. It is also known to use the laser technology for texturing rolls intended to cold rolling mills (see Fachberichte Hϋttenpraxis Metallweiterverarbeitung Vol. 23,No. 10, 1985, pp 968-972). This technique creates isolated

craters with rims on the roll surface which are arranged in helicoidal pattern around the roll giving rise to periodic unidirectional phenomenon as far as the crater distances in the direction of the helix (roll circumference) are concerned.

The roughness of the last stand of the tandem mill causes three major consequences:

- avoiding cold welding (sticking) of the coil spires during batch annealing; - ascertaining adhesion of a coating on the steel substrate;

- giving rise to a final roughness after temper rolling, which is a mixture between the tandem mill roughness and the subsequent temper mill roughness.

Generally tandem mill rolls are roughed by shot blasting or EDT (Electron discharge technology) to a Ra value of about 3 to 4 μm, giving rise to a strip roughness of about 1.2 to 1.5 μm. The resulting strip roughness is stochastic and shows a more or less irregular profile with a number of peaks and valleys of irregular height and depth. The repartition of the peaks and valleys is also irregular. This rather high strip roughness is detrimental for the production of the final roughness after temper rolling, which is defined by the steel user. This final roughness has in many cases to be lower than the tandem mill roughness. For this reason, the present -tendency is to try to produce a lower tandem mill roughness, without jeopardizing the batch annealing and the coating adhesion. With classical roughing techniques, this effort is however limited due to the irregular character of the applied roughness.

Furthermore, full hard tandem rolled sheet is also used for coating e.g. by hot dip galvanizing or galvannealing. In the subsequent utilization of the coated strip, the adhesion of the coating layer is of prime importance, especially when deepdrawing is involved. When dealing with a zinc or zinc alloy coated strip, the loss of coating due to powdering and flaking during drawing is a well known detrimental effect.

Finally, the application of a high stochastical roughness to the full hard tandem rolled sheet has a major drawback. Indeed, after annealing, in the subsequent temper mill rolling, the temper mill rolls are also roughened, so that the final roughness of the temper rolled sheet is a mixture of the tandem mill roughness and the temper mill roughness. This mixed roughness is hard to control, and it is difficult to provide the steel user with the exact final surface roughness, needed for his applications. Furthermore, this mixed roughness can be detrimental to the forming (drawing) of the sheet as well as to the painting quality (gloss) of the finished part. Indeed, during drawing of the sheet, the underlaying tandem mill roughness can reappear, which can give rise to galling and tearing, and decreases the paint quality of the finished part. These negative effects are the more severe as the tandem mill roughness shows a higher Ra value. Aims of the present invention

A first aim of the present invention is to provide a method and a device for manufacturing cold rolled metal sheets or strips which for very little roughness performed on the metal sheets or strips, avoid cold welding of the coil spires during batch annealing.

A second aim of the present invention is to ascertain the adhesion of a coating on the metal substrate minimizing the powdering and flaking effect.

A third aim of the present invention is to provide a method and a device for manufacturing cold rolled metal sheets which are able to control the mixed roughness (mixture of tandem mill roughness and temper mill roughness) . Main characteristics of the present invention

The present invention relates to a method of producing metal sheets or strips by tandem rolling such sheet or strip with at least one pair of work rolls, at least one of which being a textured work roll in order to transfer the surface pattern of the textured work roll to the surface of said sheet or strip, characterized in that said pattern consists in regular deterministic bidimensional pattern in

the form of unit cells of spots wherein each spot has the form of a crater with a rim around it, said spot being obtained through an electron beam irradiation.

The roughness of the textured work roll in the tandem mill is comprised between 1-4 μm although the sheet roughness performed with the process according to the present invention is in the range of 0.3 - 1.5 μm.

According to another preferred embodiment of the method of the present invention, the metal sheet or strip after tandem rolling is temper rolling with at least one pair of work rolls, at least one of which being a textured work roll in order to transfer the surface pattern of the textured work roll to the surface of said sheet or strip, said pattern consisting in a regular deterministric bidimensional pattern in the form of unit cells of spots wherein each spot has the form of a crater with a rim around it, said spot being obtained through' an electron beam irradiation.

Preferably the metal sheet or strip which was textured on one side through the tandem mill should be textured on the same side by the textured work roll in the temper mill.

If the metal sheet or strip is not reversed between the tandem and the temper rolling phases, this means that the upper and/or lower work roll of the pair of work rolls in the temper mill should correspond to the textured upper and/or lower work roll of the pair of work rolls in the tandem mill.

If the metal sheet or strip is reversed between the tandem and the temper rolling phases, this means that the upper end/or lower work rolls of the pair of work rolls in the temper mill should correspond to the textured lower and/or upper work roll of the pair of work rolls in the tandem mill.

The roughness of the textured work roll in the temper mill is comprised between 1-8 μm. According to another preferred embodiment of the method of the present invention, both rolls of the pair of work rolls in the tandem mill and/or the temper mill are subjected to a texturing of surface pattern which is a

deterministic bidimensional pattern.

Actually, the tandem or temper mill rolls are engraved in the "EBT process" by an electron beam under vacuum. By this technique, over a few square microns, an electron beam brings the roll surface to melting temperature and, under plasma pressure, the molten metal is ejected and forms around the crater a circular rim integrated within the base metal. The texture is reproduced on metal sheet during the rolling process. Preferably the unit cell of pattern configuration is a regular centered hexagon.

The distance between two next craters is comprised between 90 and 500 μm with a depth comprised between 2 and 35 μm. The inside diameter of the crater is comprised between 50 and 300 μm, although the width and the height of the rim are in the range of 10-35 μm and 3-20 μm respectively.

The present invention relates also to a device for producing metal sheets or strips comprising at least a tandem roll mill with several stands of work rolls characterized in that at least one roll of the pair of work rolls of the last stand has a texture of surface pattern which consists in regular bidimensional deterministic pattern of spots wherein each spot has the form of a crater with a rim around it, said spot being obtained through an electron beam irradiation.

According to another preferred embodiment of the device of the present invention, it comprises a temper roll mill with at least one pair of work rolls wherein at least one roll of the pair of the work rolls in the temper mill has a texture of surface pattern which consist in a regular deterministic bidimensional pattern in the form of unit cell of spots wherein each spot has a form of a crater with a rim around it, said spot being obtained through an electron beam irradiation.

According to another preferred embodiment of the device of the present invention, both work rolls of the last stand of rolls in the tandem mill and/or the temper mill have

a texture of surface pattern which is a regular bidimensional deterministic pattern.

The present invention also relates to metal sheets or strips having a surface pattern which consists in regular bidimensional deterministic pattern in the form of unit cell of spots, each spot having the form of a circular indentation surrounding a protuberance.

Preferably, the unit cell of the pattern configuration is a regular centered hexagon. The distance between two next protuberances is comprised between 90 μm and 500 μm with a height of the protuberance comprised between 1 μm and 10 μm.

The inside diameter of the circular indentation is comprised between 15 μm and 300 μm, although the width and the depth of the circular indentation are in the range of 10- 35 μm and 3-20 μm respectively.

As already mentioned, the roughness of the metal sheet is comprised between 0.3 - 1.5 μm.

Preferably, the metal sheet or strip is characterized by the fact that high wavelengths are practically absent from the power spectrum. Brief description of the figures

- Figure 1 is a schematic view of the EBT machine which is intended to texture cold rolls used in the method and the device of the present invention.

- Figure 2 is a schematic view of the electron beam gun of the machine of Figure 1.

- Figure 3 represents a view of a typical deterministic EBT texture on cold rolls.

- Figures 4 & 5 represent a view wherein the behavior of the strip in the tandem (Fig. 4) and temper mill (Fig. 5) is shown.

- Figure 6 represent three-dimensional measurements of a tandem mill rolled strip obtained with a method according to the state of the art. -Figures 7 & 8 represent three dimensional measurements

of a textured work roll in a tandem mill and a tandem mill rolled strip according to the present invention.

- Figure 9 is a diagram wherein Ra evolution of the EBT textured roll in the tandem mill is represented during a single rolling schedule.

- Figure 10 represents the zinc loss (due to powdering effect) during drawing of galvanized matter.

-Figures 11 & 12 represent the resulting final texture of a metal sheet or strip after having been successively tandem and temper rolling and wherein a controlled mixture of shallow and deep circular valleys appears for coarse roughness (Fig. 11) and f i n e roughness (Fig. 12) . -Figures 13 & 14 represent the wavelength spectrum for a sheet or strip having been successively tandem and temper rolling in the case of using shot blasting (Fig. 13) and EBT (Fig. 14) textures respectively.

- Figure 15 & 16 represent two views of three-dimensional texture measurements of a temper rolled strip with a fine (Fig. 15) and a coarse

(Fig. 16) deterministic EBT texture. Detailed description of the present invention

Figure 1 describes an EBT machine which is intended to produce specific textures on cold rolls to be used in the method and the device of the present invention.

In general, one can compare the EBT machine to a highly energetic TV set, wherefrom the screen has been replaced by the roll surface to be textured. From this, the main advantages are: - flexibility

- reproducibility

- predictability

- productivity

- reliability

- full automation.

The EBT machine is essentially composed of the following parts: - the texturing chamber (1) ;

- the electron gun (8) ;

- the vacuum pump (13) ;

- the closed circuit heat exchanger (not represented)

- the electrical control cabinets (not represented) . The texturing chamber (1) consists of a cast steel base and aluminum cover, giving rise to an airtight unit. The cover has a movable lid on the top, in order to enable the loading and unloading of the rolls (2) . During texturing the vacuum in the chamber (1) is kept constant at 10 ^"1 mbar. The roll is rotated by means (3, 4, 5) of a continuously variable speed drive motor (6) between 0 and 1000 rpm, whereas a shifting mechanism (7) takes care of the translation of the roll (2) in front of the fixed position of the electron gun (8) . From the moment a texture is chosen and the roll (2) is introduced into the texturing chamber (1), the machine set-up and the texturing process is performed automatically. The EBT machine is controlled by means of five microprocessors which are linked to each other and to the central control PC via a LAN (Local Area Networks) system, which is executed with fiber optics in order to avoid unwanted noise.

The principal part of the EBT machine is the electron gun (8) which is rigidly attached to the backside of the texturing chamber (1). As represented in figure 2, the electron beam gun (8) is composed of three parts: - the accelerator unit (10) with the cathode (9)

- the zoom lens unit (11)

- the focusing unit (11)

The electron gun can be described as a classical triode, equipped however with fast pulsing and zoom lens optics, which make it unique. The crater and rim forming process is schematically represented in Figure 2. The gun operates under a vacuum of 10 ^"3 to 10 ^' mbar and uses an accelerating voltage of 200 kV at a maximum current of 75 mA.

A direct heated cathode produces the electrons. The pulse frequency of the gun is continuously variable, with a maximum of 200 kHz. The shot cycles for the formation of a single crater, which can be performed in single or double shot, is represented as follows:

preheating - avoids metal sputtering welds the rim to the base metal single shot first shot postheating - controls width and height of the crater rim double shot I preheating second shot postheating

The total shot cycle time per crater (first and possible second shot) is in the range of 2 to 15 μsec.

The electron beam is deflected in order to follow the translation and rotation of the roll during a crater formation. In this way, the whole roll surface is textured with perfectly circular craters. The shift speed is continuously variable from 0.03 to 0.36 m/min. The shift speed is controlled by the shift and rotation speed of the roll, monitored by decoders, which in turn control the timing of the impact of the electron beam.

Although any pattern configuration can be produced

(square, rectangle, etc.) and serves the purpose of the invention, normally use is made of a centered regular hexagon. Indeed, this configuration allows the maximum of craters on the minimum of surface (Fig. 3) . The range of the crater and pattern parameters is defined as follows.

- pattern: regular centered hexagon or any other configuration. - crater: distance (Sc) : 90 to 500 μm, controlled by roll rotation and translation; depth (K) : 2 to 35 μm, controlled by

shot t ime inside diameter (d) : 50 to 300 μm, controlled by the dynamic zoom lenses (energy density of the electron beam) - rim: width (B) : 10 to 35 μm height (H) : 3 to 20 μm

"B" and "H" are controlled by postheating. The choice of the combination of the parameters depends on the application of the tandem rolled sheet. It is indeed possible to obtain the same Ra value for different sets of pattern parameters. However, once a set of parameters has been fixed, the created pattern is unique and entirely defined by it.

By applying EBT regular texturing of the tandem mill rolls, the problem of cold welding of coil spires during batch annealing is easily solved, even for a roll having a texture with a very low Ra value of about 1 μm.

Rolling was performed with such rolls, yielding a sheet roughness of 0.3 to 0.5 μm Ra on sheet gauges ranging from 0.6 to 0.9 mm. Although those gauges are very prone to cold welding, surprisingly they did not show any sticking after batch annealing, and hence produced a lot less noise during uncoiling at the temper mill. Research yielded the reason of this exceptional behavior. The EBT texture produces on the rolls craters (100) with rims (101) , arranges in a bidimensional deterministic pattern (see Fig. 7) . One normally would expect the rims to be imprinted on the metal sheet as a ring, whereas the metal would not enter the craters, as was observed during temper mill rolling (Fig. 4) . However, due to the high pressure and sharing forces in the tandem mill roll bite, the rolled material enters the craters, forming spikes or protuberances (103) on the metal sheet surface (Fig. 5 & 8) . Those spikes (103) are surrounded by a circular indentation (102) and are all about of the same height and show a regular repartition in accordance to the applied bidimensional EBT pattern (see Fig. 8 compared with Fig. 6 which shows a strip obtained with a conventional method) .

From this it appears that, although the crater rims are somewhat flattened by the sharing forces during tandem mill rolling, the important crater depths however remain untouched. Hence, the initial Ra value will decrease during the first few kilometers rolled, because of some flattening of the crater rims. Thereafter, the Ra value will remain stable, as can be observed from Fig. 9. This last phenomenon is most important as it ensures a long working time of a textured pair of work rolls and a constant roughness on the metal strip.

According to the method of the present invention, it is also observed that the adhesion of the coating (e.g. zinc or zinc alloy) to the metal support is advantageously superior with EBT texturing than with other roughing techniques. Indeed, the EBT technique creates a texture with a homogeneous peak and valley profile, attaching the coating layer and the metal support to each other like a zipper. This superior adhesion also minimizes powdering and flaking during deformation of the coated metal strip. As an example, tandem rolled steel sheet with shot blast texture, imitation shot blast by EBT (stochastic) and deterministic EBT texture were galvannealed. After galvannealing a classical shot blast texture was applied to the zinc layer by temper rolling. Powdering tests were performed by the cup drawing method, whereby the zinc loss during drawing was measured. From the results, represented in Fig. 10, clearly appears the advantage of the deterministic EBT tandem mill texture, reducing the zinc loss by a factor of about 5 versus stochastical texturing. This conclusion could be reached, as the only parameter, different from the normal procedure, was the tandem mill texture. In addition, the application of a deterministic EBT texture on the zinc layer during the temper mill rolling can only enhance those positive results.

Advantageously, the interference of the tandem mill roughness with the temper mill roughness is minimized with the method of the present invention. The homogeneous pattern of EBT texture with regular peaks of the same height (5 to 10 μm) is completely ironed out by the subsequent temper

rolling.

However, the imprinted corresponding circular shallow valleys or circular indentations around the peaks or protuberances remain in the texture after temper rolling. As described above, during the temper rolling process only circular valleys and no peaks are produced (Fig. 4) . Those temper mill valleys can be made deeper than the tandem mill valleys so that the resulting final texture is composed of a controlled mixture of shallow and deep circular valleys as shown in Fig. 11 and 12. From this phenomenon three unique and unexpected advantages arise:

- the final mixed sheet texture is absolutely predictable and reproducible;

- as far as deep drawing is concerned, the mixture of shallow and deep valleys allows the lubrication oil to be released gradually, when needed. Indeed, the shallow valleys readily release the oil content (e.g. at small drawing ratios), whereas the deeper valleys retain the oil much longer, favoring high drawing ratios; - the paint appearance of the finished part is enhanced because high wavelengths are practically absent from the power spectrum as shown in Fig. 13. However, when a stochastic component from the tandem or temper mill roughness is present, long wavelengths (> 0.6 mm) always appear strongly in the power spectrum, which is detrimental to the paint quality (see Fig. 14) .

Moreover, when EBT deterministic texturing is also applied on the temper mill rolls, an extra advantage is obtained. The temper mill load with EBT textured rolls is about 10 kN higher than with stochastical textures. This fact, although not expected, was observed during temper mill tests and is due to the lasting roughness on the work rolls. This higher load gives rise to a better imprint of the roll texture on the metal sheet and to a higher stability of the temper mill.

In Fig. 15 and 16, three-dimensional texture measurements are represented of a high and a low temper mill roughness pattern, having the same EBT tandem pattern.