The present invention relates to a process and to the relevant apparatus for the manufacture of thin and ultra-thin hot-rolled strips having improved dimensional and geometrical features, comparable with those of the cold-rolled strips having the same thickness values.

It is known, also from recent studies, that the market shares of the thin and ultra-thin hot-rolled strips (that is, in the thickness range of 0,6-1,5 mm) is at present increasing with respect to that of the cold-rolled ones, in the use in the various industry fields and particularly the motor-car field.

Particularly, the demand relates above all to steel strips having a low carbon content, microbound, and, more recently, also to those having a high carbon content. The development of these products is influenced by the limits of the traditional rolling processes which, because of the low thickness values, cannot guarantee the dimensional and geometrical features which are typical of the cold- rolled strips nor the surface quality, compromising the direct use thereof.

Particularly, a very negative aspect consist in the cross-profile of the strip because it is not completely parallel, being mainly of convex lenticular shape with notable thickness differences between the center and the edges thereof. As a matter of fact, these rolling processes generally use bearing or back-up cylinders which are wider than the strip to be rolled.

In order to obtain strips with thickness values of about 1 mm, as above indicated, it is necessary to increase the pressure over the rolling cylinders so as to compensate the opposite separating forces which react against the strip reduction, whereby an opposite thrust is created in the central zone of the working and back- up cylinders which causes them to bend, thus generating a convex-profiled strip.

This situation is shown, by way of an example, in figure 1 which represents the prior art. Two negative aspects of such a condition are shown, the former consisting in the convex or lenticular strip shape, whereas the latter is due to the

localized wear of the working cylinders at the strip edges which are indicated by the arrows F1 in figure 1. In figure 2, still relevant to the prior art, phase a again shows the situation of figure 1 as far as the convex profile of strip 1 is concerned and phase b evidences the deformations which may be generated in the working cylinders as a consequence of the wear localized at the strip edges, in the portions indicated by arrows F1 in phase a, and generated by the greater bending exerted by the cylinder in said zones with respect to the central zone for the"pincers effect"on the strip itself and for the greater hardness of the edges due to the lower temperature existing in said portion with respect to the rest of the strip. Also in the cold-rolling mills the back-up cylinders have a greater width than the strip which is to be rolled, but the above mentioned drawbacks do not occur because the strip has the same temperature from the edges to the center and the edges are trimmed, so that the strip has the same strength on the whole width.

On the other side, it is known that, in order to optimize the cylinder wear, most of the manufacturers of thin and ultra-thin hot-rolled strips use rolling techniques which involve strip sequences having variable width according to the so-called"coffin scheme"that is, with reference to figure 3, starting from narrow strips c, with intermediate phase d of broad strips again followed by decreasing widths e. This process improves on one side the geometrical features of the strips, but on the other side it worsens the productivity of the plants with high running costs and it is for this reason that the manufacturers of hot-rolled strips are reluctant to produce thin strips, especially from a slab of traditional thickness.

An attempt to the solution of this problem has been proposed in US 4.162.627 by making movable in the two opposite longitudinal directions the bearing cylinders, thus varying the contact zone thereof with the working cylinders. However, difficulties due to the contact of the back-up cylinders with differently and highly worn surfaces of the working cylinder occur, which may further negatively influence the strip profile.

It has also been proposed, for example by GB-A-2.202.174, to adopt working cylinders having one conical and one cylindrical end, from opposite sides of the strip to be rolled, said cylinders being axially movable one in the opposite

sense with respect to the other so that both the side edges of the strip to be rolled are positioned at a conical zone of the upper working cylinder and of the lower one respectively, moving of an amount corresponding to the wear of the same cylinders and anyway maintaining a shift with respect to the edges of the strip which is connected to the different profile which is desired for each length. This technology, as well as the one described in EP-A-899029 and WO 99/47283, which involve grooves at opposite sides respectively of the working and back-up cylinders which can be axially movable, is anyway always applicable together with the above mentioned"coffin"manufacturing system.

With patent application MI 98A000143 in the name of the same applicant a hot-rolling method for thin and ultra-thin strips, comprising subsequent steps of lateral shift in the axial direction of the working and bearing cylinders in opposite directions, has been proposed. Good results have been obtained by this method as far as the dimensional and geometrical features of the manufactured strip are concerned, but clearly this method requests that delicate devices for shifting the working and back-up cylinders are prepared, with relevant shifting synchronization organs.

Therefore, object of the present invention is providing a process and relevant apparatus for checking the profile and the tolerances of the hot-rolled strips, particularly those whose profile is within 0,6 and 1,5 mm, without pressing the edges thereof, on the contrary receiving from the edges of the strip an opposite thrust which tends to compensate the converging curvature, thus giving rise to a substantially parallel or"flat"profile, with geometrical features which are comparable with those of the cold-rolled strips. This, even leaving out of consideration axial shifting of the working and back-up cylinders and therefore avoiding the need for expensive and delicate control means therefor.

This object is obtained by a process and an apparatus having the features specified in the claims.

Further objects, advantages and features of the present invention will appear more clearly from the following description of process and relevant apparatus with reference to the accompanying drawings, wherein:

figures 1 and 2 schematically show assemblies of cylinders of a rolling stand according to the prior art, for the manufacture of thin or ultra-thin strips; figure 3 schematically shows the width trend of the strip rolled by the so- called''coffin''technology ; figure 4 and figure 5 schematically show the cylinders of a rolling stand according to the present invention, showing more in detail in the latter the forces distribution.

With reference to the drawings and particularly to figures 4 and 5 which are representative of the present invention, whereas figures 1-3 have been already considered in the description of the prior art, 1 is again the strip which is to be rolled, clearly not shown in scale, but with a greater thickness, related to the width, than that which would result for a thin or ultra-thin strip, and this for the sake of a better illustrative clarity. The two bearing or back-up cylinders have been indicated with 2, whereas 3 are the two working cylinders contacting strip 1, which are indicated as perfectly parallel with respect to each other. The process according to the present invention and the apparatus which carries it out do not imply any help of special systems such as organs for shifting the cylinders in the axial direction and for adjusting and synchronizing said shifting also without excluding axial shiftings of the cylinders, but back-up cylinders suitably profiled and sized so that the length 1 of the contact zone with the working cylinders 3 is smaller than the length L of strip 1. Preferably, the back-up cylinders 2 are beveled at their ends so that they have an axial length greater than 1, forming an angle 4 with the working cylinders.

As it is better shown in figure 5, pressure F exerted by the back-up cylinders, transmitted by means of the working cylinders 3, spreads over all the width of strip 1 which is being rolled so that also the separating force G or reaction of the strip has the same trend against the cylinders, thus preventing them from bending or anyway reducing the bending thereof to the minimum.

Anyway, this will be proportional to the acting forces and to the resistance G opposed by the material, as well as to pre-loading forces F2 applied to the working cylinders. In this way the preferential wear at the edges, that is, in the

zones Fl indicated in figure 1, is avoided, because the specific pressure exerted by cylinders 3 is evenly spread over all the strip width, unlike what happens with the traditional systems wherein it concentrates at the edges, thus causing the cylinders wear in those portions (see also figure 2). It is worth noting that the side beveling of cylinders 2, which determines the angle zone 4 by reducing the zone of contact with the working cylinders to a section smaller than the width L of the strip which is to be rolled, enables an even distribution of the forces over all the width of the strip thus eliminating the negative effect of the cylinder bending and anyway compensating it so that wear zones localized between the two pairs of cylinders, at the edges of working cylinders 2, are avoided. It has been found that preferably the ratio between the back-up cylinders contact zone, indicated with 1 in figure 5, and the width of strip L, that is 1/L, is within 0,5 and 1, preferably 0,9. The beveling depth, that is the maximum distance between the back-up and working cylinders at angles 4 can vary between 0 and 10 mm, with a preferential value around 2,25 mm.

As stated above, because of the forces distribution, the working cylinders will not be subjected to sensible concavities, such as shown in figures 1 and 2, but it is more probable that they take a convex shape which anyway will have no negative effect, particularly on the wear due to the contact with the back-up cylinders, by virtue of the beveled shape of the last mentioned ones. Anyway, the deviations due to the concave or convex shape thereof will be not higher than 0, 30 mm and preferably 0, 20 mm.

In a practical example, a hot-rolling mill for thin and ultra-thin strips has five stands whose working and back-up cylinders have a length of 1900 and 1500 mm respectively. The last mentioned ones have a maximum diameter of 1450 mm and a minimum diameter of 1300 mm, with a total beveling of 150 mm in the axial direction. The maximum diameter of the working cylinders is 700 mm for the first two stands and 600 mm for the other three, whereas the minimum diameter is 630 mm for the first two stands and 540 mm for the others, preferably having a concave shape.

The working cylinders are provided with a certain concavity, substantially

null in the first stand up to a value of-0,2 mm in the last stands. The bearing section 1 between back-up cylinders and working cylinders is constant, equal to 1200 mm, lower than width L of the strip which is 1330 mm.

From the preceding description it is evident also the advantage of the present invention of enabling the manufacture of thin and ultra-thin hot-rolled strips at the maximum width without having to continuously change the strip width, as it is on the contrary necessary by following the working program of figure 3, whereas it is possible to reduce the frequency of the changes of the cylinders up to about 100 km of rolled strip, as it appears from the experimental tests which have been carried out. Further, the thin and ultra-thin hot-rolled strips having a maximum width obtained by the process and the apparatus according to the present invention have geometrical and dimensional features comparable with those of the cold-rolled strips having the same thickness and width.