PROCESS AND PLANT FOR OBTAINING STEEL STRIP COILS HAVING COLD-ROLLED CHARACTERISTICS AND DIRECTLY OBTAINED IN A HOT-ROLLING LINE

Abstract: A process for obtaining steel strip coils with characteristics of a cold-rolled product, directly in a hot-rolling line, comprises subsequently to steps of casting and thickness reduction at a temperature of more than 1100 DEG C upon solidification, induction heating of the product and a further step of hot rolling, above point Ar3, a step of cooling and temperature control in a range of between 600 and 250 DEG C, thus lower than said point Ar3, as well as one or more passes of cold-rolling in series, with final coiling of the obtained product. Also a preferred plant is described for putting into practice such a process.

Description: "PROCESS AND PLANT FOR OBTAINING STEEL STRIP COILS HAVING COLD- ROLLED CHARACTERISTICS AND DIRECTLY OBTAINED IN A HOT-ROLLING LINE" 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 horizon¬ tal 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 fo 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 dishomogeneous during the preceding opera¬ tions, in particular hot-rolling step; - subjecting the product to pickling in order to eliminate from its surface the oxides previously formed, especiall during annealing;- and - causing the actual cold-rolling step to be performi ed, 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 ofpa_a_3.es 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 ratic 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 passes, but it will be necessary to subject the strip to another annealing operation and the consequent pickling, otherwise the material hardens and the 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 with a conventional rolling mill. The costs relating to the reduction of strip thickness are however extremely high in any case. Assuming 100 the cost of hot-rolling, starting from liquid steel, the cost of cold-rolling step alone 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-226 46 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 published application is in fact restrict¬ ed 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 thickness 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 - n - 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 recry- stallization. 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 coul d not find success indeed, apart from the proposed solution, as in any case the inner structure of the material, when subjected to cold-rolling, is unsui¬ table to undergo this treatment for obtaining a final product of acceptable quality. This occurs owing to the fact that the inner structure, if not recrystallized before cold-rolling, results to be dishomogenous under a dimensional aspect and with insufficiently fine grains, in comparison with the grain size which -would be required by the conventio- nal 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 recrystalliza- tion 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 through 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 horizon¬ tal outlet, shows an inner structure with fine grains, so uniformely distributed to have already the chara¬ cteristics 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 hot-rolling line results to be suitable to cold-rolling, even if its temperature is caused to be lower than recrystal- lization point Ar . Therefore it is an object of the present invention to provide a process and relevant plant for obtain¬ ing a cold-rolled product, of extremely thin thick- ness, 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 dis¬ continuity 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 thickness 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 tempera¬ tures 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 at least a further hot-rolling stage, above point Ar , characterized by the fact of comprising subse¬ quently the steps of controlling the temperature at the outlet of hot-rolling stage to 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 final coiling of the strip-shaped product obtained. The plant for carrying out such a process will consist essentially of the plant described and claim ed 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 coiler. It should be appreciated that the expected tempe¬ rature at the outlet from the controlled cooling device is always less than that of recrystallization 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 ope¬ ration is accomplished in the best way and the final product is provided, from a metallurgical 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 di¬ rection. 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 re¬ peated 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 shown 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. Fur¬ thermore, between the induction oven 21 and the hot-rolling stands 27 there may be provided a wind¬ ing and unwinding device 23 comprising a reel 22 for coiling the strip from oven 21 , being coupled to a reel 2-4 for uncoiling the strip itself to be fed to stands 27, possibly after an additional dis- caling step in a suitable device 25 provided at the inlet of the first rolling stand. According to the present invention, the hot-rol¬ led strip 1 , at the outlet of the last rolling stand 27 at a temperature certainly higher than the recrystallization point Ar , is caused to enter, still in the same production line, a 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 tempera¬ ture detector with a feed-back controlling the val¬ ves 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 depend 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 at 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 sub- jected 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 stands side by side, contrary to the method of providing for a multipli¬ city of subsequent passes in the same stands, as according to the conventional technology of cold- -rolling. Finally the cold-rolled 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 wound on a final coiler 33. The lower limit of the thickness that 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.

Claims: CLAIMS 1. A process for obtaining steel strip coils, having characteristics of cold-rolled product, being directly obtained in hot-rolling line, comprising the steps of: a) mould casting at a thickness of less than ΛOO mm; b) preliminary reduction of the thickness in a si ¬ tuation of liquid core of the casting product immediately under the mould; c) further reduction of thickness upon solidifica¬ tion of casting in a first stage of rolling at temperatures higher than 1100°C until values of 10-30 mm; d) induction re-heating up to a temperature as homo- geneous as possible of about 1100°C; e) subjecting the flat product a further stage of hot-rolling, characterized by the fact of comprising subsequently the steps of bringing the temperature of the product from the hot-rolling stage, still above point Ar , to prεfixable values lower than saϊd point Ar , pre¬ ferably in the range between 600 and 250°C; and one or more cold-rolling passes in series, with final coil- ing of the strip-shaped product obtained. 2. A process according to claim 1, further com¬ prising one or more of the following steps: i) coiling and subsequent uncoiling of the strip immediately after the induction heating, upon cutting the strip immediately after the first rolling stage; ii) at least one discaling step; iii) additional heating between two further stages of hot-rolling; 3- A plant for obtaining steel strip coils, hav¬ ing characteristics of cold-rolled product, directly obtained in a hot-rolling line, comprising: a) a mould (10) for continuous casting of flat pro¬ ducts (1) with a subsequent arc-shaped guide roller path (11)? b) a first reduction device (13, 15) for reducing thickness of the flat product in the curve path in a condition of liquid core and/or immediately thereafter upon solidification of product (1); c) device (21) for induction heating and homoge- neization of temperature along the cross-section of the flat product (1); d) at least one additional rolling stand (27), characterized by further comprising, immediately downstream of the last hot-rolling stand (27) an apparatus (29) for cooling and controlling the tem¬ perature of product (1) until under point Ar , one or more cold-rolling stands (3D and a final coiler (33) for winding the strip in a coil (1' ) . 4. A plant according to claim 3, characterized in that said cooling apparatus (29) is a water cool¬ ing device provided with a temperature detector with feed-back for automatic controlling of the cooling water feeding valves. 5. A plant according to claim 3 or 4, characteriz ed in that the range of variation of the temperatures at the exit of said device (29) is between 250 and 600°C, with a deviation of more or less 10°C from the prefixed value at the inside of said range ac- cording to the quality of steel, the feeding speed and the product (1) thickness. 6. A plant according to any of claims 3-5 further comprising a device (23) for winding and subsequently unwinding the strip immediately downstream of the induction oven (21), upstream of the latter there being provided a shear cutting device (17). 7. A plant according to any of claims 3-6, further comprising at least a discaling device (19, 25), respectively upstream of the first rolling stage (15) and downstream of said induction oven (21). 8. A plant according to any of claims 3-7, further comprising at least an additional induction oven interme d iate between two subsequent rolling stands (27).

Inventors: ARVEDI GIOVANNI, GOSIO GIOVANNI, ARVEDI, GIOVANNI, GOSIO, GIOVANNI
Applicants: ARVEDI GIOVANNI, ARVEDI, GIOVANNI
Application year: 1991
Main classification: B21B1
IPC codes: B21B1, B22D11, C21D8