NO961967L - Rolled metal product and process for its manufacture - Google Patents

Description

ROLLED METAL PRODUCT AND METHOD OF MANUFACTURE THEREOF

The present invention generally relates to sheet metal products for the construction of e.g. bodies for motor vehicles, using an increased percentage plate thickness reduction ratio that can be achieved with a work roll structure in a single, last stand in a rolling mill compared to what can be achieved with directionally ground rolls in several chairs. The provided sheet metal has improved functional properties for subsequent manufacturing processes, such as formability in presses, spot weldability, resistance to peeling or adhesive metal transfer from the sheet surface to the tool surface in presses that form the sheet into components, and the appearance of sheet-over songs before and after they are painted or coated.

Reference is hereby made to the applicant's former U.S. patent 5025547. This patent relates to micro-machined surface structures for both working and counter-holding rolls in each stand in a rolling mill, so that each stand is tailored to its particular rolling conditions including lubrication, wear and draft, the last stand of the rolling mill reducing the strip or plate's thickness in a way that characterizes the surface of the plate product with the surface structure of the work rolls. This is described at the bottom of column 3 of the patent, f.o.m. line 63. The surface structure of a work roll in the last stand in the rolling process is shown in the micrograph in Figure 5 of the patent drawings in the U.S. patent 5025547. It should be noted that structures formed in and on a strip surface in all racks except the last one are largely eliminated from the plate surface v.h.a. the effect of work rolls in later racks in the rolling mill.

Moreover, according to the applicant's above-mentioned patent, the work rollers in the last rack can be provided with a continuous screw groove, as described at the top of column 4 in the patent. Such a structure would produce the plate surface shown in Figure 28 in the applicant's present drawings. The continuous screw groove structure is anisotropic (directional) and the resulting structure on the plate is also directional, as shown in Figure 28 of the present application. With such a structure, it is difficult to make large reductions in plate thickness, i.e. reduction ratio at or above 55%. The purpose of the groove structure is to remove lubricant by pressing the lubricant to the channels in the roller structure. Consequently, the plate/work roll interface is not sufficiently lubricated to effect a large thickness reduction in the last stand in a cold rolling process. The helical groove structure was designed for rolling processes with low reduction where the purpose is to roll at high speeds to produce a very bright plate product, i.e. which has improved mirror-reflectivity.

With the crater structure in Figure 5 according to the above-mentioned patent, such large reductions are also not intended, as one purpose of the patent is to produce an isotropic and non-directional surface in the final plate product by embossing the plate surface with the surface structure of the work roll before the plate leaves the rolling mill. Any large reductions in thickness cause significant leveling of the plate surface, as described in detail below. In the above patent, the reductions are in the range of 15% to 54%, which range contains sheet thickness reductions that substantially exceed those made during "temper" rolling, which includes sheet thickness reductions of the order of three to ten percent.

Steel and aluminum plates are products produced in rolling mills that use work rolls to give the plate an engagement in a thickness-reduction process. The ground surfaces of the work rolls can be provided with different structures using surface treatment techniques such as sandblasting, electrical discharge structuring (EDT), C02 laser structuring and electron beam structuring. These methods can produce different micro-roughness morphologies in the area from very small craters, which are located in a discrete manner along the work roll surfaces (e.g. as shown in Figure 3 in the present drawings), to knotty irregularities that have a significant arbitrary element (eg as shown in Figure 13). Laser and electron beam techniques are capable of forming deterministic crater patterns in work roll surfaces, while the crater pattern formed by electrical discharge machining has a largely arbitrary element, as shown in Figure 13. These technologies are generally described in Iron and Steel Engineer, Vol. 68, No. 8, August 1991, and in Applicant's article "Focused Energy Beam Work Roll Surface Texturing Science and Technology", Journal of Materials Processing and Manufacturing Science, Vol. 2, July 1993.

The work roll surface structures provided by the above technologies are used to emboss the plate surface during rolling processes with a small percentage plate thickness reduction (e.g. typically equal to or less than 5% thickness reduction). A work roll surface with a crater structure similar to that shown in Figure 3 will indent the plate surface in the manner shown in Figure 4. This gives the plate surface an ordered group of annular depressions surrounding plateau areas, the surfaces of the latter exhibiting the ground termination of the work roller before structuring.

Today, such structured work roll surfaces are in common use in the production of car body panels. However, a number of significant rolling process-related problems remain for reasons described below. These problems have a tendency to worsen as the percentage plate thickness reductions made during a rolling process become larger and larger, thereby precluding the use of the structure in accompanying Figure 3 beyond a certain, small reduction.

The percentage plate thickness reduction ratios that can be achieved with rollers with directional grinding bottom e.g. into relatively low roller gap forces and roller torques, as described in detail below.

With a small percent plate thickness reduction ratio in a cold rolling process (eg, typically less than 3%) at a relatively low speed (eg, 500 ft/min (152 m/min)) without lubricant, it is also possible that the topography of a work roll surface is only partially pressed or impressed on the plate surface due to imperfect structure formation on the work roll surface. Such imperfect structure formation can e.g. is due to irregular absorption of radiant energy by different alloying agents in the work roll steel, which thereby leads to an imperfect work roll surface structure. Figure 5 of the accompanying drawings is an example of imperfect structuring, showing a stylus rendered topography of a representative area of a steel sheet surface showing imperfect embossing of an annular work roll crater structure (similar to that shown in

Figure 3) formed by electron beam technology.

The embossed plate roughness typically meets the performance requirements of automotive body steel plate and can have a better painted surface appearance than the appearance of a conventional plate surface rolled with a work roll with a largely anisotropic roughness in the form of a directional grind. However, the image clarity of the painted surface of a plate structured with a discrete roughness is not optimized due to the background roughness transferred from the ground parts of the roll surface (as seen in the accompanying Figure 4), as well as the structure itself which can be shown through a painted finish.

Formation of wear debris during sheet metal rolling is one of the most significant rolling process problems associated with roll surface structures produced with CO 2 , electron beam and electrical discharge structuring devices. In order to better understand the nature of this wear mechanism, it is appropriate to mention something about the structure-morphology produced with these devices. In the case of the CO 2 laser and electron beam devices, a single element of the roughness produced by these devices, known as a "crater", is produced by one or more pulses of sufficient average energy density (which is the product of the beam intensity and pulse activation time), thereby melting a microscopic amount of the work roll surface, the individual pulses consisting of electromagnetic radiation in the case of the CO 2 laser, or accelerated electrons in the case of the electron beam device. Each crater consists of an almost ring-shaped rim, which is raised in relation to the average roughness of the roller surface, and a depressed area, typically called a depression, formed in the roller surface, as shown in Figure 8. The crater rim is generally sloping relative to the average roll roughness, so that its angle of inclination or dip is quite steep (eg 15° to 50°). By adjusting the angle of a fast gas contributor relative to the position where the pulsed jet hits the roll surface, the CO2 device can produce an asymmetric crater morphology consisting of a single, raised "hump" and a single depression. Figure 10 is a stylus-rendered topography of such raised bumps in a representative area on a work roll surface. The drop of one of the bumps in relation to the average rolling roughness is also quite large.

The embossed plate surface structure as a result of small percentage plate thickness reduction rolling with a work roll structure formed by electrical discharge technology (EDT) typically consists of a series of plateaus and depressed areas, without any discernible order, which are formed (oppositely) on the work roll surface, i.a. multiple sparks from discharge electrodes through a dielectric (i.e. initially non-conductive) fluid medium. The plateau edges are also quite steep and rugged. This can be seen in Figure 13 in the accompanying drawings. Figure 13 is a stylus-rendered topography of a plate surface of aluminum alloy 2008, the steep and jagged surface being the indentation of a work roll structured with an electrical discharge device.

A micro-cutting wear mechanism therefore tends to be the dominant mode of formation of wear debris in the roll gap of structured work rolls. With work rollers such as used in vehicle plate rolling, the average crater-edge height or average height of an asymmetric "hump" is typically greater than the average background roughness of the work roll as a result of the roll's grinding operation. Thus, microcutting occurs when the crater edges or the asymmetric "bumps" plow the plate-over-laten to thereby detach small particles from the plate-over-laten. The total volume of wear debris formed in the roll gap during rolling with a structured roll is proportional to the sliding distance between the work roll and the plate-over tracks. The sliding distance is a function of the plate's gauge thickness, thickness-reduction ratio and the diameter of the work roll. Another factor that contributes to the formation of wear debris in the structured roll gap is the average drop of the crater edge or the asymmetric "hump" in relation to the average background roughness of the roll. If this dip generally exceeds 20° or so, high levels of wear debris can be expected in rolling processes where plate thickness reductions in excess of 15% are made.

Significant wear debris thereby leads to an aesthetic problem with the final plate product because the debris can either

is rolled into the plate, or it can be transferred to the work roll surface where it can act as sharp cutting edges that further damage the plate surface. For this reason, the use of structured rollers has generally been limited to temper rolling with a small percentage plate thickness reduction ratio, because there is relatively little slip between the grinding roller surface and the plate surface. Such a low reduction has a tendency not to form a significant amount of debris.

Temper rolling is sometimes carried out in a rolling mill that has a four-high design (eng.: "four-high"), i.e. two counter-holding rollers and two working rollers. In this case, the contact stresses between the work roll and the counter-holding roll are much greater than the stresses at the interface between the strip and the work roll. The reason for this is the fact that the width of the area over which the surfaces of the counter-holding roller and the working roller come into contact is significantly smaller than the width of the contact area between the surfaces of the working roller and the plate. Also, the plate is typically subjected to a tensile stress which acts to reduce the normal-contact stress required to achieve a desired reduction. Consequently, counter-roll surface wear and ultimately severe damage to the counter-roll surface can result from crater edges (in the cases of the laser or electron beam technologies) or sharp cutting edges (in the case of electrical discharge technology) repeatedly cutting into the counter-roll. - the surface during the rolling process.

Work roll steel is generally harder than the counter roll steel and consequently the highest parts of the structured work roll surface can cut into the counter roll surface, which leads to the formation of small steel particles and depressions in the counter roll surface. With some simple engineering estimates, it is possible to demonstrate, at least in the case of the ring crater morphology in Figure 8 or the asymmetric "hump11 morphology in Figure 10, that the work roll structure will cut into the counter roll surface under rolling process conditions that typically found in the aluminum industry. This first requires an estimate of the lubricant film thickness between the work roll and counter roll surface to determine whether the lubricant film separates the two surfaces or whether the structure height exceeds the average film thickness. Under these conditions where the work roll surface structure actually comes into contact with the backing roll surface, it is then necessary to determine whether the associated contact stresses along the work roll/backing roll contact are large enough to promote damage to the backing roll surface.Although the following developments are limited to the case of the ring crater morphology , eg as shown in Figure 3, they can also be extended to the case me d the hump morphology according to Figure 10.

The lubricant film thickness between the work roll and the counter roll can be estimated using the well-known Dowson and Higginson formula for isothermal line contacts using the following expression, found in "Elastohydrodynamic Lubrication" by D. Dowson and G.R. Higginson, Pergamon Press, London, 1996:

where:

!—-— ; width of plate/work roller-aBU=Jy$Rv

contact (along the flow direction)

Ub • Uw; respectively counter-holding roll and work roll surface speeds in relation to contact area

w<=>ay(aBWLaw) ; load on the work roller due to plate deformation per length unit (along the roll axis and consequently across the roll direction)

<y>^; original plate thickness before

rolling

Eb; Young's modulus for the counter-holding roller E^; Young's modulus for the working roller E'; effective Young's modulus

1 1 1- vl 1- we t ...

—; = — (—-— + —-—) ; reciprocal value of effective

E12 Eb Eb

Young's modulus

G = yE' ; dimensionless material parameter

Law ; length unit of rectangular contact piece (along the roll axis and consequently across the roll direction) in the plate and work roll interface hvlh ; length unit of rectangular contact piece (along the roll axis and consequently across the roll direction) in the working roll and counter-holding roll interface Rb; radius of the counter-holding roller

R w ; radius of the work roller

r,/ _R*F-b.

~ ~R—+~R^ ' effective roller radius

_V0(uw+ub)

~2E'r' ' dimens3ons10s speed

w-w

e'r'l b' dimensionless load

/? ; plate thickness-reduction ratio y ; pressure viscosity coefficient HQ ; the base viscosity of the lubricant v ; Poisson's ratio ay ; the plate's tensile yield strength

The following material and process parameters, which are representative of many four-high roll designs in the aluminum industry, are inserted into Equation (l):

<u>^<=>ub<=>7.64m/s

<y>L<=>0.001 m

F^= Eb=207 GPa

L8W= 0.0254 m

<I>^b=0.0254 m

P^= 0.27 m

Rb= 0.64 m

0 = 0.03

Y 14.5GPa"1

HQ= 2.7 x IO"<3>Pa-sec

vw = vb= 0.33

ay = 1.4 x IO<8>Pa (for aluminum alloy 2008)

where the values for the contact lengths L8W and L^ are chosen only to show the concept. It should be noted that the percentage plate thickness reduction ratio is simple /? x 100, or in the present situation, 3%.

The calculated sizes are thus:

<a>sw=0.0028 m

w = 1.0 x 10<4>,N

E' = 232 GPa

G = 3364

R' = 0.19 ro

U = 4.7 X 10"<13>

W = 8.9x10"<6>

where the appropriate unit designation is "Pascal" for "Newton/meter<2>", the prefix "G" representing giga, which entails multiplication by IO<9>.

Using the above values in Equation (1), the lubricant thickness between the counter roll and the work roll is given by Equation (1):

hmin=°'43 Mm

It is obvious that the crater rims carry most of the load because the film thickness is much less than a typical crater rim height (eg 3.0/xm to 10.0/xm) produced either by laser or electron beam - the technology. Consequently, the majority of the craters on the work roll will come into direct contact with the counter-holding roll surface. Because Equation (1) fits isothermal line contacts, it is likely that the estimate for hmin will be even smaller due to thermal effects (from plastic heating of the plate, interfacial friction, etc.) on the lubricant viscosity.

An estimate of the total pressure or normal contact stress carried by the crater edges is therefore required to investigate the likelihood of the work roll surface structure cutting into a counter roll surface. One must first calculate the width awbav the rectangular contact area between the work roll and counter roll surface, as shown in Figure 6. This is carried out using Hertz's classical contact theory, for which there is a description in "Contact Mechanics" by K. L. Johnson, Cambridge University Press, New York, 1985. For two elastic cylinders in contact under a normal load w pr. axial unit length, the resulting planar contact zone has a width of aw along the rolling direction. Consequently,

where the numerical values are taken from the above list of process parameters and material properties. In the hexagonal crater pattern shown in Figure 3 (produced by the electron beam structuring technology), and schematically in Figure 7, the unit cell is a parallelogram. Within each unit cell lies one entire crater. The area of a parallelogram Ap is given by where Ca is the distance between the centers of adjacent craters in a single cell in the hexagonal pattern as indicated in Figure 7. Thereby the percentage of area covered by the crater rim in a single unit cell, %Acr, can be expressed as where r0 and rL are the outer radius and inner radius of the crater, respectively. A typical crater produced with the electron beam roll structuring device has an outer radius rQ of 76.2 µm, an edge width of 25.4 nm, and consequently an inner radius rL of 50.8 µm. A typical distance Ca between the centers is 203 µm. The area of the crater rim is thus 10.1x IO"<9>m<2>, the area of a unit cell parallelogram Ap is 35.6 xIO"<9>m<2> and the percentage of area of a parallelogram cell covered of crater rim %Acrer 28%. Number of craters "N" in a small rectangular area of length 0.025 m (along the roll axis and therefore across the roll direction) and width awb= 204.2/im (as the previously calculated contact width in the roll direction) along the work roll/counter roll contact area is given by

where N has been rounded to the nearest integer value.

The total area AT, which is covered by all crater edges along a unit length, is therefore

The average pressure pA distributed over the crater edges along the plate/work roll interface is therefore

It is appropriate to assume that the pressure pA is transferred to the work roll/counter roll interface and thereby onto these crater edges temporarily at this interface. The yield strength of a resist roll can be as high as 1.0 GPa. Accordingly, an estimate of the counter-roll indentation pressure is 2.6 GPa because the indentation pressure is approximately 2.6 times the yield strength of the counter-roll material. A pressure of 7.4 GPa on the crater edges obviously exceeds the indentation pressure of the counter-roll surface. It is therefore likely that a crater edge will cut into the counter-holding roll surface, and the actual contact area will extend beyond the total area covered by the craters even in situations where the percentage plate thickness reduction ratio is 3%. Over time, this will cause serious damage to the counter-holding roll surface. Such damage can only be stopped by changing the counter-holding roller and this leads to production downtime, increased process intensity, more labor and increased cost.

Significant surface wear on a textured work roll occurs when the protruding edges or cutting edges of the work roll surface become detached from the work roll surface during rolling. In the case of the ring crater morphology e.g. formed by the impact of a CO2 laser beam, the crater rim is formed by the rapid acceleration of a microscopic bath of molten metal onto the portion of the roller's ground surface surrounding the central depression. The metal that solidifies on the roll surface (i.e. the crater edge) attaches itself to the local roll roughness which is typically a ground finish. The firmness of this bond depends on the adhesion of the solidified material to the roll surface. This bond may actually be quite weak because the molten metal may not sufficiently "fill" the ground roll roughness. Consequently, the crater rim can detach from the work roll surface when exposed to the alternating contact stresses in the roll gap. This causes a breakdown of the structured work roll surface, which ultimately requires improvement of the roll. The problems associated with bonding the molten material that solidifies and forms the annular crater rim during CO 2 laser structuring of work rolls is described in U.S. Pat. patent 4806731.

It is not possible to form a crater depression without a rim because molten material below the impinging jet flows radially out of a growing depression, outward fluid velocities to molten material being primarily induced by a spatial surface tension gradient extending along the surface of a melt bath formed by the beam. This gradient is the product of a variation with temperature of surface tension and a spatial surface temperature gradient. Variation of the surface tension gradient produces a shear stress that accelerates the surface of the molten material. The shear stress a is related to the spatial surface stress gradient through the following relationship:

where C is the variation of surface tension with temperature. The distribution of energy in a beam pulse typically decreases radially outward from the pulse center, and consequently the local work roll surface decreases radially outward from the point of impact of the beam. According to this equation, the surface tension thereby increases radially outwards from the point of impact. Thereby, shear stress on a layer of molten metal is significant enough to displace the melt to the slopes of the depression, and the rapidity of heating pro-

the session overrides fluid movement below the surface, and consequently more material is transported radially outward than can be replaced by any recirculating current below the outward flowing layer of molten work roll surface material. The result is a build-up of material, followed by rapid freezing of material, which thereby leads to the formation of an annular rim or lip around the recess. A description of the surface melting and shearing effect during laser and electron beam processing of materials can be found in "Thermocapillary Convection in Materials Processing", by M. M. Chen in Interdisciplinary Issues in Materials Processing and Manufacturing, S. K. Samanta, R. Komanduri et al. . eds ASME Publications, (1987), pages 541-558.

The above melting and surface cutting process takes place both with and without a contributing gas, although the contributing gas will modify the maximum height of the molten fluid which will eventually form the solidified lip. If the velocity of the contributing gas is made very high, a crater lip is still formed because the above-mentioned surface shear occurs as soon as a melt bath is formed under the jet. In addition, a contributing gas at high velocity will seek to expel molten material from the developing crater beneath the jet. A fraction of the ejected molten material can be re-deposited onto the work roll surface and then solidify into many sharp cutting edges. The cutting edges from the redeposited material will produce wear debris in rolling processes with large percentage plate thickness reduction ratios.

Another problem arises from the relative crater spacing along the work roll surface. In the rolling process, the work roll surface is moved at a higher speed than the plate surface before the neutral plane, at which the roll and plate surface have the same speed. After a plate surface element passes the neutral plane, the surface element is moved at a higher speed than the work roll surface.

(This phenomenon is described in detail in the applicant's previous patent, which is hereby referred to.) If the reduction gene-

reit is less than approx. 5%, there is minimal relative leveling effect between the plate and roll surface and the result is an almost perfect pressing of the roll structure onto the plate surface. At larger reductions, the craters in the work roll surface cut into the plate surface and level the plate surface against the rolling direction before the time when a plate surface element reaches the neutral plane; thereby forming smoothed grooves on the plate surface (i.e. forward smoothing). After the plate surface element passes through the neutral plane, the same action is repeated, but in the opposite direction because the plate surface speed exceeds the roll speed due to volume conservation by plastic

deformation (backward smoothing). The net effect of the backward and forward smoothing effect is the formation of short and narrow "grooves" on the plate surface, and the plate structure is thereby significantly distorted.

Figures 9a and 9b show the surface morphologies of two plate surfaces of aluminum alloy 2008 which were rolled with a structure similar to the asymmetric hump structure in Figure 10 at 35% and 60% respectively, under otherwise identical rolling process conditions. In each case, the structured plate surface will not correctly reproduce the indentation of the work roll surface structure. This is particularly clear in Figure 9b, where the plate structure exhibits a significant directional component due to the fact that the bumps in the work roll cut into and plow the plate instead of just cutting into the plate as was the case with the small reduction rolling process that produced the plate surface shown in Figure 4. Consequently, a greater reduction in plate thickness has a tendency to extend the crater indentations on the plate surface, thereby forming long, smooth grooves because the craters generally lie along a helical path on the work roll surface. As the percentage plate thickness reduction ratio is increased, the grooves can, due to nearby craters or bumps connect to each other because the average length of a single groove due to a single structural element increases with increasing reduction and there is little control over crater location or total crater pattern on the roll surface during C02-Iaser structuring where external chopping of a continuous wave beam is included. This leads to the formation of grooves in the plate surface which, taken together, form an anisotropic roughness. It is therefore possible to start with a largely non-directional structure (such as that shown in Figure 3) on the work roll surface, and on the rolled plate surface to end up with a roughness that has a significant directional component.

These groove effects can give the plate surface a directional appearance such as that found on a ground surface finish or the appearance of the rasp structure shown in Figure 28. Such a surface may in some cases be undesirable for a customer, particularly in situations where the customer wants a quasi-isotropic plate surface roughness, because the optical properties of such a surface are less dependent on the direction from which the plate surface is viewed by the human eye, and such a surface will seek to retain lubricant rather than freely channel lubricant during a forming cycle .

It can generally be concluded that the structures that are formed by the above C02 laser structuring process cannot be used in aluminum rolling processes with large reduction (i.e. greater than or equal to 15% thickness reduction). The reasons are as follows: (1) a large plate thickness reduction ratio leads to a large roller separation force. This increases the propensity of the crater edges (or bumps in the case of the asymmetric morphology shown in Figure 10) to damage the counter-holding roll surface due to the previously described contact stress mechanism, thereby leading to premature wear of the crater edges; (2) if the annular craters are too close together (i.e. along the circumferential direction of the work roll surface), it is estimated that a plate thickness reduction of 15% may result in the formation of separate leveling grooves which, in situations involving significantly larger thickness reductions than the above-mentioned 15%, can be interconnected so that continuous grooves are formed in the plate surface. The interconnected grooves form "rough" bands, while on the surrounding (unstructured) plate surface, which is smoothed v.h.a. relatively flat areas of the roller, smoother and narrower bands appear from the grinding process. Figure 11 shows a plate surface of aluminum alloy 2008 rolled at 40% reduction with the ring crater morphology of the roll surface formed by C02-Iaser structuring. The distance between the centers of the craters is sufficiently short to cause the grooves to form on the plate surface; (3) because the CO2 laser beam is mechanically chopped with a serrated disk, it is not possible to accurately control the position of one crater relative to its neighbors, i.e. to produce hexagonal or square cells. This is today only possible either with the electron beam technology, because this technology has been adapted from rotogravure printing with an internally pulsed C02, where the active elements in the laser resonator are modulated with a radio frequency device. Further details on the application of electron beam technology in rotogravure printing can be found in "A Rapid Electron Beam Engraving Process for Engraving Metal Cylinders", by W. Boppel, Optik, Vol. 77, No. 2, (1987), pages 83- 92; (4) the crater lips or bumps produced by the CO 2 laser structuring process will lead to unacceptably high levels of wear debris formation during rolling processes with large plate thickness reductions. Too much wear debris puts a further burden on the rolling mill's oil filtration housing and will eventually lead to the cessation of the rolling process. Similar conditions can be treated regarding the electron beam structuring.

The painted appearance of sheet material rolled with laser-structured rolls is often inappropriate for sheet customers in the automotive industry. The roll structure with an annular crater leads to an annular depression in the plate surface, and the hump structure leads to an almost circular depression in the plate surface. In each case, the plate surface depressions are surrounded by flat areas that act as bearing areas through which the load from a forming tool is transferred to the plate. It is possible that the deformations during pressing operations are not sufficiently large to cause the indentations on the plate to completely disappear from the plate surface. The embossed plate structure can therefore be shown through the painted finish so that the painted finish is given a background structure. This is often a basis for rejecting the formed plate component, especially for cars in the luxury class. Description of a remaining structure after pressing work can be found in chapter 5 of "Optimierung der Oberflachenmikrogeometrie von Aluminiumfeinblech fur das Karosserieziehen" (Springer-Verlag, 1988). In Norwegian, the title is "Optimization of the surface micro-geometry of aluminum plate for pulling car body panels", by R. Balbach.

The present invention overcomes the disadvantages of temper rolling processes, where only a small percentage plate thickness reduction ratio, less than five percent, is allowed at low rolling speeds, by allowing large percentage plate thickness reduction ratios at high speeds, i.e. percentage plate thickness reduction ratio at or above 55%, in a single and last rack in a rolling mill where plates are advanced at the order of 2000 feet per minute (610 m/min) in the production of car body sheet and 5,000 feet per minute (1525 m/min) in the production of can board. And this is carried out without the incessant formation of wear debris, while damage to the counter-roller in roll designs with counter-rollers has largely been eliminated, and with the maintenance of the necessary draft levels at the plate/work roll interface and the work roll/counter roll interface, thereby avoiding slippage along these boundaries. This is accomplished by superimposing, upon a substantially smooth work roll surface whose roughness is in the range of 0.007 lim to 0.25 µm, a deterministic pattern of micron-sized craters whose solid edges are removed from the roll surface, as that bowl-shaped depressions are left with mainly smooth entrance areas or mouths. This work roller structure is best seen in the stylus-rendered topography in Figure 12 among the accompanying drawings.

The craters are preferably formed in the work roll surface by a pulsed laser or electron beam device through the melting and cutting action previously described. The jets in such devices create indentations in the work roll surface and a lip or rim around each indentation which, in the present invention, is largely removed by e.g. one or more of known polishing techniques, such as honing, fine grinding, belt polishing, grinding, and/or chemical polishing. In this way, work roll material, which could otherwise create wear debris in the roll gap, is removed before the work roll is used. Otherwise, wear debris becomes embedded in the plate surface during rolling, so that a very dirty rolled product is formed which is unwanted by the customers of the product manufacturer. It should be noted that it is not possible to machine away the crater edges with a standard diamond tool because the diamond will chemically break down during the machining of steel.

After the edges have been removed, the roll surface can be coated with a hard, high-density material such as e.g. a chrome.

By making the large plate thickness reductions according to the present invention, in contrast to temper rolling or other rolling processes with small reduction, the plate material is forced to partially extrude into the bowl-shaped depressions formed on the work roll surface. As the plate material is extruded, it is leveled due to the difference in plate and work roll surface speeds. The simultaneous effects of extrusion and smoothing in the roll nip lead to the transient and very short-lived formation of microscopic, raised structures on the plate surface, which are formally termed "protrusions". During the process of forming protrusions, any excess lubricant trapped by the recesses on the roll surface is forced by the extruding plate material to flow out of the recesses. The excess lubricant therefore flows into the plate/work roll interface and improves the tribological properties of the interface. As described in detail in what follows, there is again a very small rear protrusion on the plate surface after the above-mentioned short-term protrusions disappear from the plate surface. When the plate is then used in a secondary forming operation, such as deep drawing, the remaining protrusions act as lubricant carriers as well as lubricant barriers along the plate/sink interface. Thereby, the protrusions improve the tribological properties of secondary sheet forming processes by minimizing the undesirable effects of tearing or adhesive metal transfer because metallic contact is significantly minimized.

The plate's background surface between the remaining protrusions is also leveled in the above-mentioned process with a large reduction, i.e. the smooth work roll surface existing between the indentations on the work roll surface, so that a substantially smooth, bright plate surface is formed between the protrusions.

The number and spacing of the roll craters depends on the hardness and alloy of the material to be rolled, the amount of thickness reduction to be made, the speed at which the material will be rolled, the amount and type of coolant and lubricant provided, and the nature of the secondary forming process (such as pulling or pushing).

Traction between the work roll and the counter-holding roll surface is ensured by cutting the lubricant film and a temporary, partial filling of the softer counter-holding roll material into recesses on the work roll surface. The filling process is due to elastic deformation of the counter-holding roller surface. The areas of the counter-holding roll surface that partially fill the work roll surface's cups act as lips that carry the tensile forces between the work roll and counter holding roll surface.

The protrusions left on the surface of the rolled sheet are leveled and eventually flattened in the surface during press work operations because they cannot support the normal forces transferred to the sheet surface from a forming tool or die during all stages of a forming process. Consequently, it is much less likely that any remaining structure on the plate surface after a pressing operation will be visible through a paint finish than the structures due to the crater or hump morphologies.

It is therefore an object of the invention to provide a bright plate product both before and after it has been painted by reducing the product very much in the last stand in a rolling mill.

Another purpose of the invention is to provide a work roll surface which will significantly reduce the formation of wear debris, so that very large thickness reductions in a rolled plate can be made by the roll in a single stand in a rolling mill without significant formation of debris material and embedding of this in the surface of the plate being rolled.

Yet another object of the invention is to provide a work roll surface structure which allows cold rolling to be carried out without damaging counter-holding rolls used with work rolls having the structures according to the present invention and which extends the surface life of both the work roll surface and the counter-holding roll surface.

It is yet another object of the present invention to provide a work roll surface structure which will allow very large percentage plate thickness reduction ratios in a single stand in a rolling process, which is otherwise not possible either with a ground roll surface finish, ring crater structures or the asymmetric hump structures, and which will not lead to failure in the rolling process.

Another object of the invention is to provide a structure which not only transports lubricant into a plate/tool interface, such as occurs during a press operation, but also acts to cause a lubricant film formed at this interface to persist until later stages in the process due to the blocking effect, as described in detail below.

Yet another object of the invention is to provide car body panel products with great image clarity formed by large specular reflection of light from a mainly smooth background surface of the panel.

Another object of the invention is to provide a largely non-directional plate surface structure which, after the plate is formed into a component including a number of secondary forming processes, will not be visible through or otherwise clearly prominent under a paint finish applied to the component's surface by an automotive manufacturer.

The invention, together with its objects and advantages, will be best understood by considering the following detailed description and accompanying drawings in which: Figure 1 schematically shows two rotating work rolls in a rolling mill, which reduce the thickness of an intermediate metal material; Figure 2 shows on a larger scale a partial section of the interface between the work roll and the metal material according to Figure 1, the surface of the upper roll having very small, shallow, bowl-shaped depressions in which very small amounts of plate surface material are extruded; Figure 3 is a stylus rendered topography of a portion of a work roll surface showing the annular crater structure in a hexagonal array caused by an electron beam patterning device; Figure 4 is a stylus rendered topography of an aluminum plate surface embossed with a crater structure on a work roll surface during a small reduction rolling process; Figure 5 is a stylus rendered topography of a portion of a steel plate surface imperfectly embossed with a crater structure on a work roll surface; Figure 6 shows Hertz's line contact width between a counter-holding roll and a working roll in a rolling mill; Figure 7 schematically shows a hexagonal crater pattern (such as the one shown in Figure 3) formed in the surface of a work roll v.h.a. an electron beam device, whose basic unit is a parallelogram; Figure 8 is a stylus-rendered topography of a single, ring-shaped crater formed in the surface of a work roll v.h.a. double pulses from an electron beam patterning device; Figures 9a and 9b show the surface leveling of two aluminum plates which have been rolled at 35% and 60% thickness reductions respectively, i.e. the asymmetric "hump" CO 2 laser structure shown in Figure 10; Figure 10 is a stylus rendered topography of a portion of a work roll surface with the asymmetric "hump" structure produced with a mechanically chopped CO 2 laser and a suitable propellant gas according to the method of U.S. Pat. patent 4806731; Figure 11 is a photomicrograph of a plate of aluminum alloy 2008 (magnified 200 times) reduced in thickness by 40% using a work roll surface with the ring crater morphology of Figure 8; Figure 12 is a stylus-rendered topography of smooth, bowl-shaped depressions in the surface of a steel work roll, with the crater edges removed to form smooth entry areas in the bowls; Figure 13 is a stylus-rendered topography of a plate surface of aluminum alloy 2008 rolled in a rolling process with a small thickness reduction with a work roll structured v.h.a. electrical discharge; Figure 14 shows on a larger scale a partial section of the interface between the work roll and plate surface according to Figures 1 and 2, where further details are shown in the forward formation of protrusions on the plate surface using the method according to the invention (before the time when the partial cuts reach the neutral plane in a roll gap, after which a backward projection is formed); Figure 15 is a photomicrograph of the forward protrusions in Figure 14 enlarged 425 times; Figure 16 shows on a larger scale a partial section of the interface between the work roll and the plate surface after the original two work roll surface depressions in Figure 14 have been moved to the vicinity of the neutral plane where the forward protrusions according to Figures 14 and 15 have been leveled and flattened, the plate surface material undergoing further plastic deformation in the process of partially filling the two depressions in the work roll surface so that there are elevations; Figure 17 is a photomicrograph of metal elevations formed as the plate passes through the neutral plane, magnified 425 times; Figure 18 shows the plate surface in a position past the neutral plane in the roll gap after the original two work roll surface depressions in Figure 14 have been moved to the vicinity of the exit plane in the roll gap, as the elevations according to Figures 16 and 17 have been flattened and the plate surface material undergoes further plastic deformation in the process of partially filling the two depressions in the work roll surface to become rearward protrusions; Figure 19 is a photomicrograph of the rearward projection material shown in Figure 18; Figure 20 is a stylus rendered topography of a representative plate surface area containing the forward protrusions shown in Figures 14 and 15; Figure 21 is a stylus rendered topography of a representative plate surface area containing elevations similar to those shown in Figures 16 and 17; Figure 22 is a stylus-rendered topography of a representative plate surface area containing rearward projections similar to those shown in Figures 18 and 19; Figure 23 shows the relative size of elevations formed on a section of a plate surface which is in the region of the neutral plane of the roll nip and rearward protrusions formed on a nearby section of the same plate surface which has crossed the neutral plane but which has not yet reached the exit plane; Figure 24 is a graph showing rolling force variation in relation to percentage plate thickness reduction ratio that can be achieved with ground rolls in a four-high rolling process; Figure 25 is a graph showing roll force variation in relation to the percentage plate thickness reduction ratio that can be achieved with the work roll structure according to the present invention in a four-high rolling process; Figure 26 is a graph showing variation of roll torque versus percent plate thickness reduction ratio that can be achieved with ground rolls in a four-high rolling process; Figure 27 is a graph showing variation of roller torque in relation to percentage plate thickness reduction ratio that can be achieved with the work roll structure according to the present invention in a four-high rolling process; and Figure 28 is a stylus rendered topography of an aluminum alloy 5182 plate surface rolled with the work roll structure disclosed in applicant's U.S. Pat. patent 4996113.

Referring to Figure 1, upper and lower rolls 10 and 11 in a rolling mill (otherwise not shown) are shown during the process of reducing the thickness of a metal plate 12. The upper roll has a working surface 14 which is provided with several, spaced apart , cup-shaped depressions 16 with micron size, three of which are shown in Figure 2, and which are also shown on a very large scale for reasons of clarity. Before the depressions are formed in the surface 14 of one or more work rolls 10 and/or 11, the surface(s) are prepared in a way that gives a very smooth finish, e.g. a finish in the order of 0.007/xm to 0.2/xm Ra (arithmetic mean roughness). Measurement with the ISO roughness standard is appropriate. Surface roughness standards are described in "Surface Texture Analysis, The Handbook", by L. Mummery, Hommelwerke, Germany, 1990. Such a surface forms a mainly smooth surface on the metal material which is reduced in thickness in the roll gap according to Figure 1. Aesthetically, the result is a rolled product with a light surface, as any smoothing of the surface during rolling increases the product's overall lightness as seen by the human eye.

As described below and as shown in Figures 3, 7 and 8, the recesses 16 are formed when a focused energy beam strikes the roll surface 14 so that craters 18 are formed, which include raised edge material 20 surrounding each recess. The beam hits the roll surface at a right angle because the beam path is aligned with the roll axis. Figure 2 reproduces the interface between one work roll according to the invention and one surface of a plate that is rolled by the work roll during a rolling process with a large reduction in plate thickness, and is a section on a larger scale of the small framed area in Figure 1. The presence of the bowl-shaped depressions in the work roll surface, together with the normal load exerted by the work roll surface on the plate surface (denoted pA in Figure 2) during rolling with a large reduction in plate thickness, as well as the kinematics of the roll gap, causes the topography of the plate surface to undergo a series of changes in the roller gap before it comes out of the roller gap. Figure 2 shows forward protrusions 22 which are microscopic, semi-circular, raised portions of plate surface material, and are formed in the area of the interface which lies in front of the vicinity of the neutral plane of the roll gap, which neutral plane is given the reference number 24 in the figures. The elevations 26 are formed on the plate surface near the neutral plane. In the exit region, rearward projections 28 are formed on the plate surface and remain on the plate as the final plate surface structure. The transient nature of the plate surface shown in Figure 2 is not due to the work roll surface structure marking the plate surface, which is the case with the plate topography shown in Figure 4 and in the applicant's previously described patent. Instead, microscopic amounts of plate material are subjected to a combination of backward extrusion into the bowls (due to the normal load exerted by the work roll surface on the plate surface) and leveling of the backward extruded material due to the significant relative movement between the work roll surface and the plate surface that takes place during rolling with a large reduction in plate thickness.

A laser or electron beam device (not shown) is used to create craters 18 (see Figures 3 and 8) in the work roll surface, each crater comprising a central depression 16 and a raised, ring-shaped edge 20 surrounding the depression. The jets from such devices place each crater precisely on the roll surface and at a size and frequency determined by the extent of thickness reduction to be effected, the alloy and temper of the plate 12 being reduced in thickness, the type and temperature of coolant and lubricant used. in the reduction process, and the rate at which the reduction is carried out. In general, a typical indentation depth in a work roll surface is in the range of 0.4 µm - 10.0 µm for generally circular indentations with outer diameters in the range of 50.0 µm - 255 µm.

When a pulsed laser beam with a Gaussian distribution of intensity about the beam center, or a beam with a Gaussian current density emitted from an electron beam device, strikes the roll surface normally so as to form craters 18, the edges 20 of roll material are formed around recesses 16 (below the beam) because the energy density of the beam is significant enough to melt the roll material. With such melting, there is subsequent, rapid displacement of a proportion of the molten material to the circumference of the depression due to surface cutting which is due to the melting bath's surface tension varying with temperature, as previously described. In order to prevent such edge material from breaking away from the roll surface during rolling, which contributes to the wear debris problem described above, the edge material is removed, so that the result is the work roll surface topography in Figure 12, and the roll surface is preferably covered with a hard high density material, such as chrome, to provide a durable wear surface that lasts for a long time. Removal of the edge material, e.g. v.h.a. polishing, forms a substantially smooth surface finish 30 (Figure 12) around recesses 16 in the work roll surface. Of particular importance is the fact that the entrance areas to the depressions shown in Figure 12 are also smooth and will not thereby act as cutting edges that create wear debris as plate surface material is simultaneously extruded and leveled against the inner areas of the depression during large reductions in plate thickness. Because the surface finish 30 of the background work roll is also substantially smooth, as previously described, the result of rolling with such a structured work roll is a bright strip or plate product. With large sheet thickness reduction ratios, the resulting smoothing process, as previously described, will increase the lightness of the rolled product. Without removal of the material of the edges 20, the result can be the formation of wear debris and damage to the counter-holding rollers, as described above. Furthermore, under large reductions in sheet thickness, the crater design of the work roll surface (Figure 3) produces elongated parallel grooves 32 in the sheet surface, which largely follow the rolling direction, as shown in Figures 9a, 9b and 11. Such a surface is typically undesirable in car body sheet- the customer, because it is not aesthetically pleasing as a surface and product and does not possess tribological properties that improve shaping of the plate during secondary forming processes, such as drawing and pressing.

In contrast, with the removal of edge material 20, and the creation of curved protrusions or raised elevations on the plate surface during rolling processes involving large percentage plate thickness reduction ratios, as described below, a plate product is produced that is substantially free of wear debris, with significant elongation of the lifetime of the counter-holding roll and the work roll, while producing a plate or a plate product that the customer wants.

Figures 14, 16, 18 and 20 to 22 show a microscopic part of a plate surface rolled with a work roll surface structure in Figure 12 during a rolling operation with a large plate thickness reduction in a rack at the end. These figures show the most significant changes that take place in the plate surface at three selected times caused by the same two indentations (now designated 16a and b) in the work roll surface as the indentations move from the entrance area to the exit area in the roll gap, in connection with the effects of the work roll's normal load on the plate, and the kinematics in the roll gap, as the plate passes through the roll gap. Figures 15, 17 and 19 are photomicrographs of representative sections of a plate surface illustrating plate surface structural changes occurring at the stages of the rolling process shown in

Figures 14, 16 and 18. Figures 20, 21 and 22 are stylus-rendered topographies of plate surface structural elements similar to those shown in Figures 15, 17 and 19 respectively. Figures 14 to 22 capture the very short-lived nature of the plate's surface roughness as it passes through the roll gap where at least one work roll surface has been structured with microscopic indentations (Figure 12) and finished using the method according to the invention.

In Figure 14, the two roller surface depressions 16a and b, formed according to the invention, are loaded (at an initial time t1) against the surface of the plate 12 v.h.a. a normal load pA, before they reach the neutral plane 24 in the roller gap. The neutral plane is also indicated by a vertical broken line 24 on the far right in Figure 14. The front edge of the recess 16b slope, which in Figure 14 is denoted LB2, is at a distance dA from the neutral plane. The depression's slope is defined as a surface revolution ring of small width, just below the mouth or entrance part of, and consequently in, the depression. This is the area along which contact with an extruded surface material is likely to occur. The contact area between the recess slope and the extruded plate surface material can be considered as an approximate sector area or a continuous triangle for which the sum of the angles exceeds 180°. The diameter of each recess 16 is denoted "D" in Figure 14 although in practice there may be a small distribution of the diameters within an ordered group of roller surface recesses due to a number of different conditions that exist during the time when the roll is being structured. These include changes in the pulse energy density of the forming beam, focusing of the beam, machine tool vibration, irregular absorption of beam energy by alloying agents in the roll surface, associated control electronics used for structuring the work roll, etc. The normal load and speed difference between work roll and plate surfaces causes a small amount of plate surface material to partially flow or be extruded into recesses 16a and b, as previously indicated. As a result of this normal pressure and the fact that the plate surface velocity is less than the roller surface velocity, two forward projections 22a and b are formed along the rear areas of the slopes of the recesses 16a and b as in

Figure 14 is denoted TB-1 and TB2.

A simple understanding of outgrowth formation can be achieved with the following analogy. Point M on the recess 16a in Figure 14 is located on the rear section of the slope of the recess 16a; this sloping slope resembles the more familiar sloping surface of a water ski, although the water ski surface lacks the lateral curvature of the depression slope. When a person water skis, the skis are typically tilted at a slight angle to the surface of the water in front of the person, and this tilt along with their forward speed creates an accumulation or ridge of water under the skis. If one follows this water crest over time while following the person's movement, one will find that it is simply a traveling wave. The wave's intensity can increase if the person hits an area on the water's surface where the water's speed is strongly opposed to his direction of movement, which e.g. can occur when a boat passes parallel to the person's direction. On the other hand, the wave's intensity can decrease if the person hits an area on the water surface where the water is moving strongly in his direction. Each projection can therefore be regarded as a solid or plastic wave, formed by partially rearward extrusion of a microscopic portion of plate surface material into a roll surface depression, such extrusion taking place primarily along the trailing edge of the slope of the depression which has not yet reached the neutral plane 24, see Figure 14. The plate surface material, which partially fills the recess, is leveled in the rolling direction as it is extruded. This is due to the relative surface movement between the roller and plate surface before the time when the partially filled depression passes through the neutral plane. Figure 15 is a photomicrograph at 425X magnification showing the protrusion structure on a plate surface of aluminum alloy 2008, such structure being a result of the partially backward extrusion mechanism shown in Figure 14. In Figures 14 and 15, two forward protrusions are shown 22a and b. Each protrusion has a "semi-round" morphology, as can be seen from Figure 15. The distance between the protrusion edges corresponds to the separation between the centers of the two depressions 16a and b in the roll surface that formed the protrusions (Figure 14). The convex edges of the protrusions face the rolling direction. The inner or concave edges of the protrusions therefore face the entry point of the roll gap. There is a sloping end portion of material that rises and forms the convex edge of an outcrop. If one were to climb from left to right along the middle of this slope, i.e. from left to right along a forward projection in Figure 15, one would consequently climb a slope that swings in each side direction, i.e. in the two directions which are normally on the rolling direction, and which quickly fall off along the convex edge of the projection. This is better illustrated in Figure 20, which is a stylus-rendered topography of a representative area of the surface shown in Figure 15. The outcrops resemble water waves on the surface of the sea. The lateral curvature of the outcrops is also visible. The height of the protrusions is in the range 0.2 5/xm - 3.0 ixm. The accumulation of metal that leads to the formation of protrusions is due to the leveling of extruded plate material, i.a. a sector area of the recess slope rear edge. The semi-circular geometry of the protrusions in Figure 20, which appears to be in contrast to that of the depressions, is due to two factors: (a) metal is only partially extruded into a depression and forms

a plastic wave which is stationary relative to the indentation, and (b) the greatest "thrust" felt by the metal as it flows into the indentation comes from the trailing edge of the indentation slope.

At a later time t2>tx, i.e. in Figure 16, the plate and work roll surface which formed the interface shown in Figure 14 will leave the entry plane and move towards the vicinity of the neutral plane 24, although they reach the vicinity of the neutral plane at a slightly different time due to the above-mentioned smoothing effect which causes the plate surface to be displaced over the roller surface. Thereby, the projections 22a and b are then leveled against the output area v.h.a. unstructured areas 30 on the roller surface and finally flattened in the plate surface prior to the time when the plate surface reaches the vicinity of the neutral plane. There is a small zone surrounding the neutral plane where the plate and roll surface velocities are almost identical, i.e. near the neutral plane 24. An observer moving with the roller surface in the vicinity of the recesses 16a and b would therefore see the plate surface transiently at, or very close to, his own speed. Under these conditions, the relative leveling of the plate surface is negligible, so that the plate surface is thereby only affected by the normal pressure applied through the work roll surface. This normal pressure then forces the sheet metal to be mainly extruded into the recesses 16a and b in the form of elevations 2 6a and b much in the same way as the seal of a notary public stamps a legal document. A theoretical calculation of the normal load during rolling predicts that the normal load on the plate surface is highest near the neutral plane. This is the reason for the more uniform "partial filling" of the depressions with the plate material near the neutral plane and subsequent formation of the elevations. The calculation of normal loads during rolling is described in "Theory of Plasticity" by J. Chakrabarty, Chapter 7, McGraw-Hill,

(1987), pages 551-574. Figure 17 is a photomicrograph at 425 times magnification showing a set of microscopic elevations on the plate-over-laten formed w.h.a. the method described in Figure 16. Figure 21 is a stylus rendered topography of a representative area of the surface shown in Figure 17. After the work roll surface depressions 16a and b pass through the vicinity of the neutral plane 24 in Figure 16, the plate surface velocity begins to exceed the roll -exceeding the loading speed due to volume constancy during plastic deformation and the elevations 2 6a and b are consequently flattened back to the plate surface. The plate surface elevations, formed as the plate passes through the vicinity of the neutral plane and into the vicinity of the exit region, are therefore then leveled by the leading edges of the slopes LB^ and LB2 to the recesses 16a and b respectively, as seen from Figure 16, eventually as the plate 12 moves faster than the roller 10, as the elevations are then flattened. As the work roll surface depressions 16a and b move to the exit area and arrive in the vicinity of the exit area shown in Figure 18 just after the time when the plate section shown in Figure 18 arrives in this same area, rearward protrusions 28a and b are formed on the plate -the surface as shown in Figure 18. Figure 19 is a 425X photomicrograph of an aluminum plate surface with rearward projections 28a and b formed by the mechanism according to Figure 18. The convex edge of each forward projection faces the neutral plane, and consequently the concave edge of each forward projection faces the roll nip exit plane at the far right in Figure 19. Figure 22 is a stylus-rendered topography of a representative area of the surface shown in Figure 19, which contains forward protrusions 22a and b formed v.h.a. the mechanism shown in Figure 18. Figure 23 shows a stylus-rendered topography of a plate surface rolled with the method according to the present invention where the rolling process has been interrupted. Elevations formed on the plate surface near the neutral plane are shown along with the forward protrusions formed on the plate surface as the plate enters the vicinity of the exit region. Figure 23 also demonstrates the short-term nature of the plate surface topography during rolling at large plate thickness reductions, with at least one work roll structured with the topography in Figure 12. Therefore, the plate surface will have the forward protrusion structure according to Figures 22 and 23 when starting from the roller gap. Figure 24 presents the variation of rolling force with percentage plate thickness reduction ratio recorded during four-high rolling of aluminum alloy 2008-F using ground work rolls in a single rolling mill stand. As the percentage thickness reduction ratio increases, the rolling force increases non-linearly until the mill's force capacity (i.e. 100 kilopounds (445 kN)) is reached at 45% reduction. At this point the rolling process had to be terminated, because reductions beyond 45% could not be carried out. Figure 25 shows the variation of rolling force with percentage plate thickness reduction ratio recorded under conditions that were identical to the conditions for Figure 24, but with work rolls that had the bowl structure according to the present invention shown in Figure 12. The rolling force increases somewhat more slowly than the rolling force variation shown in Figure 24, which indicates more desirable tribological conditions when the structure according to the present invention is used. The mill's rolling power capacity will not be reached until percentage plate thickness reductions of the order of 70% are carried out. Excessive friction in the roller gap is minimized due to the absence of a significant roll grinding on the structured work roll surface together with the fact that into the roll gap v.h.a. the bowl structure is fed lubricant which is wholly or partially expelled to the plate/work roll interface as microscopic amounts of plate coating material are extruded into the bowls 16 in the manner shown in Figures 2, 14, 16 and 18. Figures 26 and 27 show a comparison of the variation of roll torque with percentage plate thickness reduction ratio during rolling with ground work rolls and work rolls structured with the invention under the conditions listed for Figures 24 and 25. Again, the structured work rolls extend the rolling process capacity of the simple roll stand far beyond the which can be achieved with ground work rolls in that the stand's torque capacity (3000 ft.-lb (4061 Nm)) during rolling with the structured work rolls is not reached until 70% reductions are made (Figure 27), while the rolling process using ground work rolls (Figure 26) had to be terminated at close to 2700 ft.-lb (3659 Nm) when a reduction ratio of 45% was achieved. Figures 24 to 27 clearly demonstrated that when rolling with work rolls structured by the method according to the present invention in a single stand, the rolling mill capacities can be stretched far beyond the artificial limitations imposed on the work due to ground work roll surface finishes, and consequently the work can be made more productive. In addition to the above-mentioned rolling process improvements due to work roll structure according to the present invention, the backward projection structure 28a and b left on the sheet surface when it emerges from the roll gap will improve lubrication and metal flow during subsequent metal forming operations, such as press forming of an automobile body screen, through two important mechanisms. In the first mechanism, the backward projections 28a and b can draw lubricant into the plate/tool interface thanks to their "semi-circular" morphology. In the second mechanism, the backward projections act as "barriers" that obstruct lubricant flow along the plate/tool interface. Because the lubricant cannot freely flow unhindered into and out of the interface, the lubricant film will persist until later stages of the forming process. This will reduce tearing, minimize premature failure of the plate due to wear, as well as protect the tool surface. A description of the lubricant obstruction effect at lubricated interfaces can be found in "An Average Flow Model for Determining Effect of Three-Dimensional Roughness on Partial Hydrodynamic Lubricant", by N. Patir and H. S. Cheng, ASME Journal of Lubrication Tribology, vol. 100 (1978), pages 12-17. It should also be noted that slippage between the plate and roll surface will not occur even in the extreme case of a "mirror-ended" work roll structured using the method of the present invention. This is due to the fact that the projection/elevation formation is the main pulling mechanism in the roll gap and consequently "slipping" does not take place during rolling in a single stand in the work where the large percentage plate thickness reduction ratios previously stated are made. With the method according to the present invention, sliding is independent of the background roughness of the roll, even if a background roughness in the form of a directional grinding can contribute to the overall pull between the plate and roll surface. It should further be noted that the method according to the present invention produces plate surface structures which are completely different from those produced by the usual roller structuring technologies such as laser and electron beam structuring. The crater edges 20 (Figure 8) or the asymmetric humps (Figure 10) produced during the above-mentioned smel tea and surface cutting process due to energy pulses in these technologies are typically left in place along the roll surface without any alteration of them under any circumstances. As such, these edges will impress or cut into microscopic depressions in the plate surface (Figures 4 and 5), as such depressions lie below the plate surface. One can think of the embossed ring-crater structure in Figure 4 as a design like a castle's plateau and moat, where the ring-shaped edges on the work roll surface form the moat on the plate surface (which is then filled with lubricant before a pressing operation), and the castle's plateau is simply a flat area which is essentially at the nominal roller surface roughness, and which does not perform any significant function in terms of improvement of lubrication in subsequent plate-forming processes. However, the method according to the invention produces precisely the opposite effect in that it leads to microscopic protrusions as the final plate surface structure, as the plate emerges from the last stand in a rolling mill, which stands up in relation to the average plate roughness. The springs also perform two functions in terms of improvement of lubrication at plate/workpiece interfaces, instead of the one lubricant confinement function performed by the ring crater morphology. The protrusions not only bring lubricant to a plate/tool interface, such as formed during a press work operation, but they also act as obstacles in the path of lubricant flow along such an interface and consequently cause the lubricant to persist along the interface in later stages of forming processes. The lubricant that is brought into the interface v.h.a. moreover, the protrusions are easily distributed to the plate/tool interface in a press work operation because it is not fully enclosed by an individual protrusion. This is often not the case with the annular crater morphology impressed into a plate surface during rolling with a small thickness reduction because there may well be a small amount of lubricant remaining in the remainder of the annular "moat" areas of the plate after the forming process is complete. The protrusions according to the invention have a significantly smaller overall size than the ring crater morphology and the hump morphology described above with reference to Figures 3 and 10. In general, car manufacturers do not want the plate structure to "show through" a paint layer, and this problem has been linked to the two latter structures -morphologies. As such, the protrusions will not show noticeably through a paint finish on a car part.

Claims (7)

1. Bright, rolled metal product, at least one surface of which contains several microscopic, raised protrusions formed by partial back-extrusion of strip material into cup-shaped depressions with smooth entrance formed in a roll surface, and by leveling of the strip surface material due to the relative speed of the strip and the roll which occurs when large reductions are made in the thickness of a strip in a rolling mill. 2. Product according to claim 1, characterized in that the strip material is aluminum and the light product with the microscopic protrusions is used for the production of vehicle bodywork components, the protrusions acting as lubricant carriers and blocking the regulation of lubrication of tools used to manufacture the bodywork components. 3. Structured sheet product with surface protrusions that are easily flattened in the surface of the sheet product, including tools used in pressing operations that produce metal parts of the product, which resulting parts have a bright surface and largely no directional pattern, so that the possibility of any residual plate surface structure showing through a final paint applied to the bright surface of the part is significantly reduced. 4. Rolled aluminum product with a mirror-reflective surface provided by an isotropic surface and structure consisting of a reflective surface area provided with raised protrusions deterministically formed on the product's mirror-reflective surface v.h.a. a work roll in a rolling mill deterministically designed with discrete, cup-shaped depressions with a smooth entrance into which material from a sheet of aluminum is extruded and leveled v.h.a. the effect of relative velocities of the work roll and sheet of metal when a large reduction in thickness takes place in a final stand in a rolling mill producing the rolled aluminum product. 5. Rolled aluminum product with at least one isotropically structured surface of micron-sized protrusions located on reflective surface areas of the rolled product, characterized in that it is produced by the following steps: advance of aluminum material between two lubricated rotating work rolls in a rolling mill, at least one of the rolls having a structured surface consisting of deterministically located, very small, bowl-shaped depressions with a smooth entrance, compression of the aluminum material between the rotating work rollers which causes a reduction ratio in the order of 70%, and applying the compacting step to form short-term protrusions on at least one surface of the aluminum material and protrusions that are maintained on said one surface of the aluminum material after the material exits the work rolls. 6. Rolled aluminum product with at least one isotropically structured surface of micron-sized protrusions located on reflective surface areas of the rolled product, characterized in that it is produced by the following steps: advancing aluminum material between lubricated rotating work rolls in a stand in a rolling mill at a stand torque capacity of the order of 3000 foot-pounds (4061 Nm), at least one of the rolls having a textured surface consisting of deterministically located, very small, bowl-shaped depressions with smooth entrance, compression of the aluminum material between the two lubricated rotating work rollers, and applying the compacting step to form short-term protrusions on at least one surface of the aluminum material and protrusions that are maintained on said one surface after the aluminum material exits the work rolls. 7. Aluminum sheet product having a mirror-reflective surface provided by a generally isotropic structure consisting of reflective surface areas and very small raised protrusions located on the reflective surface, which protrusions form the isotropic structure on the surface, and having predetermined, controlled, uniform dimensions in the micron size ranges, as formed by a work roll arranged in a rolling mill with a mirror surface and discrete, bowl-shaped depressions with a smooth entrance into which the sheet product material enters when large reductions are made in the thickness of a sheet of aluminum material forming the sheet product, the entry of aluminum material into the bowl-shaped depressions forms the protrusions and the isotropic protrusion structure.