The present invention relates to a process and relevant plant for producing coils of steel strips, having characteristics of a cold-rolled product and directly obtained in a hot-rolling line from a continuous casting with arc-shaped path and hori¬ zontal outlet.

It is known that for obtaining hot-rolled steel strip coils, the following operations are provided, successively:

producing by casting a steel slab having a thickness of between 160 and 250 mm, and possibly storing the same;

heating such a slab, if coming from the store, or in any case bringing it again to a rolling temperature of at least 1050°C;

hot-rolling the slab for a first cogging and thereafter for obtaining hot-rolled strips having a minimum thickness of 2 mm;

taking again the hot-rolled strip and subjecting the same to annealing for a reconstruction of the grain which has been deformed and become dis- homogeneous during the preceding operations, in particular the hot-rolling step;

subjecting the product to pickling in order to eleminate from its surface the oxides previously formed, especially during annealing; and

causing the actual cold-rolling step to be performed, which comprises mounting the coil onto an unwinding reel to bring again the strip onto a plane, causing the strip to pass through at least one cold-rolling stand until obtaining thicknesses of less than 1 mm, down to 0.5-0.2 mm and finally winding the strip on a reel to obtain the final coil. It will be noted that the number of paths in the stands for cold-rolling depends on the desired final thickness and the reduction percentage which is to be obtained, in other words the ratio between thick¬ ness of the hot-rolled strip and thickness of the final product. For high values of such a percentage reduction it is not enough to increase the number of said paths, but it will be necessary to subject the strip to another annealing operation and the consequent pickling, otherwise the material hardens and final product results to be of low quality.

Although it is possible to obtain by hot-rolling strips having a thickness of less than 2 mm, it is usually avoided to reach these values, as this type of processing is considered anti-economical,

above all due to the reduced productivity that would be obtained in this case. The costs relating to the reduction of strip thickness are however ex¬ tremely high in any case. Assuming 100 the cost of hot-rolling, starting from liquid steel, the cost of cold-rolling step is of at least 80.

Attemps have been described as to making plants for obtaining thin strips by means of more compact operating cycles with respect to the above-mentioned conventional cycle, in order to reduce complexity and duration of the latter. For example EP-A-226446 describes a number of hot-rolling examples, all in line and at a very high speed (not less than 1500 mm/min) but the final product not only has a thick¬ ness of 2-6 mm, falling thereby in the range of hot-rolling, but also certainly does not show the structural features of a cold-rolled product. The main purpose of this application is in fact restricted to a high productivity while obtaining at the same time a product of good processability , but not of high quality.

In EP-370 575 there is described a method for the manufacture of a steel strip having a final thick¬ ness of between 0.5 and 1.5 mm, comprising the

steps of hot-rolling a steel slab of less than 100 mm thickness, at a temperature of between 300 C and a temperature at which at least 75% of the material is converted into ferrite, with a thickness reduction of over 30% in at least one reduction stage, and an exit speed after hot-rolling of less than 1000 m/min, with final coiling of the strip after recrystallization. This was an attempt to avoid the two successive cycles of hot- and cold- rolling, with intermediate stages of annealing and pickling, but also this attempt has been unsuccessful, and it could not find success, apart from the pro¬ posed solution, as in any case the inner structure of the material, when subjected to cold-rolling, is unsuitable to undergo this treatment for ob¬ taining a final product of acceptable quality. This occurs owing to the fact that the inner structure, is not recrystallized before cold-rolling, results to be dishomogenous under a dimensional aspect and with insufficiently fine grains, in comparision with the grain size which would be required by the conven- tonal cold-rolling technology according to the above-described cycle.

It is known on the other hand that an eccessive reduction of thickness with successive rolling stands on the same hot-rolling line gives rise to

such a temperature decrease to go below the re- crystallization point Ar„ , at which the steel is no longer austenitic, whereby a subsequent annealing above Ar„ restores the pre-existing structural situation without the benefits of grain reduction.

Instead it has been surprisingly found that the product obtained trough the hot-rolling process according to the published international patent application WO 89/11363 for the continuous production of steel strip or plates from flat products through continuous casting with arc-shaped path and hori¬ zontal outlets, shows an inner structure with fine grains, so uniformely distributed to have already the characteristics of a material suitable to be cold-rolled. Therefore it has been thought that rolling up to thicknesses of less than 1 mm can be obtained with no need of annealing and pickling, as it can be in practice performed in line with the hot-rolling carried out upstream.

In this way a technical prejudice can be overcome, which is extremely common and deep-rooted both among those skilled in hot-rolling, and those, normally distinct therefrom, who are skilled in cold-rolling, since the material obtained in the

SUBSTITUTE SHEET

hot-rolling line results to be suitable to cold- rolling, even if its temperature is caused to be lower than recrystallization point Ar„ .

Therefore it is an object of the present invention to provide a process and relevant plant for obtaining a cold-rolled product, of extremely thin thickness, directly starting from the hot-rolled product, being coupled thereto as to the speed and with no need of further treatment (such as annealing and pickling) on the material, thereby without any discontinuity in the manufacturing line.

This is obtained by means of a process comprising the steps of casting in a mould at a thickness of less than 100 mm, preliminary reducing the thick¬ ness in a condition of liquid core of the casting product immediately under the mould, with further thickness reduction after solidification of casting in a first rolling stage until values of 10-30 mm at temperatures higher than 1100 C, induction re¬ heating up to a temperature as homogeneous as possible of about 1100°C and causing the flat product to pass through a least a further hot-rolling stage, above point Ar„, characterized by the fact of com¬ prising subsequently the steps of controlling the

temperature at the outlet of hot-rolling stage to o ' prefixable values in a range between 600 and 250 C, thus lower than point Ar„, and one or more stages in series of cold-rolling with the final coiling of the strip-shaped product obtained.

The plant for carrying out such a process will consist essentially of the plant described and claimed in the above-mentioned PCT patent application until exit of the last hot-rolling stand, being further characterized by a cooling and temperature controlling device, by one or more cold-rolling stands and a final strip coiling reel.

It should be appreciated that the expected tempe¬ rature at the outlet from the controlled cooling device is always less than that of recrystalli- zation point Ar„ , which varies according to the carbon content in the Steel, with a minimum of 690 C for 0.6% of carbon, up to a maximum of 900 C for lower or higher carbon contents. Therefore it is certain that the subsequent processing is actually a cold-rolling step, which is carried out on a material the inner structure of which has all the required characteristics in order that the cold- rolling operation is accomplished in the best way and the final product is provided, from a metallur-

gical point of view, with all the properties which are required to a cold-rolled product.

These and further objects, advantages and features of the process according to the present invention, as well as of the relevant plant, will be clear to those skilled in the art from the following detailed description of a preferred embodiment, given by way of a non-limiting example with reference to the annexed drawing showing a diagrammatic view of a plant according to the invention, useful to describe also the process of the invention.

From a continuous casting mould 10, the steel flat product 1, driven and guided by a known-type roller path being arc-shaped, from an initially vertical direction, passes through the arc-shaped path formed by the rollers 11, to a horizontal direction, the thickness of the casting product 1 is firstly reduced in a condition of liquid core, for example in two distinct sections of rollers 13 and thereafter, upon solidification, but still at a temperature of about 1100°C, in a first stage of rolling 15 at the end of the bent path 11 and at the beginning of the horizontal path. Subsequently, in an induction oven 21 the flat product 1 is reheated to bring it again to hot-rolling temperature, and then rolled in one or more rolling stands 27,

between which there may be possibly provided ad¬ ditional induction ovens (not shwon in the drawing) for maintaining the rolling temperature of at least 865 C at the outlet from the stand.

According to an embodiment of this first portion of the plant, substantially already known from WO 89/11363, immediately after the first rolling stage 15 there may be provided a shear 17 and before said stage 15 a discaling device 19 for eliminating scale from the surface of the product to be treated. Furthermore, between the induction oven 21 and the hot-rolling stands 27 there may be provided a winding and unwinding device 23 compri¬ sing a reel 22 for coiling the strip from oven 21, being coupled to a reel 24 for uncoiling the strip itself to be fed to stands 27, possibly after an additional discaling step in a suitable device 25 provided at the inlet of the first rolling stand.

According to the present invention, the hot-rolling strip 21, at the outlet of the last rolling stand 27 at a temperature certainly higher than the recrystallization point Ar ₃ , is caused to enter, still in the same production line, as cooling and temperature controlling apparatus 29, at the exit of which the strip has a temperature, controllable

at each time, comprised in a range of between 250 and 600 C. It substantially consists of a water- based cooling device, for example of the so-called "laminar rain" type, being provided with a tempe¬ rature detector with a feed-back controlling the valves for feeding water into the device. The value of temperature to be fixed for strip 1 at the inlet of the subsequent cold-rolling stage, with deviations of not more than 20 C, will depent on the type of steel (carbon content, etc.), the feeding speed of the strip and its thickness, but in any case it will be less than the temperature at the recrystal¬ lization point Ar„, which varies between 900 C and a minimum of 690 C for a carbon content of- 0.6%. As the maximum temperature provided a the outlet of apparatus 29, thereby at the inlet of subsequent cold-rolling stage 31, is of 600 C, the strip is surely under the point Ar„, and thus at the best conditions to undergo the cold-rolling step, due to the fine grain structure of the material from the upstream treatment, absolutely suitable to be subjected to cold-rolling.

Such a rolling occurs in at least one stand, for example of "six high" type, i.e. with six rolls mounted in vertical. The passes of cold-rolling may

however be more than one, but all in series when providing a multiplicity of stand side by side, contrary to the method of providing for a multi¬ plicity of subsequent passes in the same stands, as according to the concentional technology of cold- rolling.

Finally the cold-rolling strip, with a thickness of less than 1 mm, ready for use as it shows the tipical microcrystalline features of a cold- rolled product, such as a homogeneous distribution of grains, is coiled on a final reel 33. The lower limit of the thickness than can be obtained in this way will be only dictated by the nip of the cold- rolling stands 31, as well as their precision, not certainly by problems of material hardening or anyhow deriving from its metallurgical structure.