Improved method for the rolling of long products and rolling line which performs that method

Abstract: Improved method for the rolling of long products in a line comprising in sequence at least one roughing assembly (11), a possible reduction/sizing assembly (19) with three rolling passes, a fast pre-finishing block (25) and a fast finishing assembly (31), the method being suitable to roll products starting with a nominal size of section between 120 and 180 mm., the products of the range of dimensions between 20 mm. and 25 mm. being finished in a reduction/sizing assembly (19) with three rolling passes positioned upstream of the fast pre-finishing block (25), the relative changes of products being concentrated in the reduction/sizing assembly (19), the whole range of dimensions of the products between 4.0 mm. and 20 mm. being subjected to the same reduction sequence up to last rolling pass of the fast pre-finishing block (25), the changes of products being carried out only in the rolling passes of the fast finishing assembly (31), the temperature of a bar entering the fast finishing assembly (31) being about 700 to 750°C., in which method the bar upstream of the fast finishing assembly (31) is subjected to a cooling with water at a low pressure of about 6 bar such as will prevent the initiation of vibrations at the point of the bar travelling through the cooling line. Improved rolling line for long products which comprises at least one roughing assembly (11), a fast pre-finishing block (25) and a fast finishing assembly (31), the line being suitable to roll products starting with a nominal size of section between 120 and 180 mm., two pairs of cantilever rolling blocks (13) with rolling stands having alternate horizontal/vertical axes being included between the roughing assembly (11) and the fast pre-finishing block (25) and being followed by a reduction/sizing assembly (19) with three rolling passes having the function of finishing the products in the range between 20 mm. and 25 mm., the fast finishing assembly (31) having four rolling passes and comprising two separate modules, each having two rolling passes, in which line at least the fast finishing assembly (31) and/or the reduction/sizing assembly (19) with three rolling passes are associated with an assembly (218, 118) performing the fast change of equipment according to the required reduction.

Description: This invention concerns an improved method for the rolling of long products and also the rolling line which performs this method, as set forth in the respective main claims. The invention is applied to the iron and steel production field in the sector of the production of long products consisting of alloys containing iron such as, in particular but not only, bars, rods, wire rods, whether round or of any other required cross-section. The invention enables the steps of production and particularly the steps of finishing of the product to be improved and makes possible an increase in the output of the plant, an improvement of the surface and inner quality and a reduction of the times for corrective work and maintenance on the line. In the field of the production by rolling of long products such as bars, rods, wire rods, etc. the great competition between the producers entails more and more the need to meet very strict processing parameters in obtaining products of a high quality at ever lower costs and with ever shorter times. To be more exact, the requirements of the steelmakers as directed to the designers of plants concern the possibility of meeting the following specifications: dimensional tolerances in a range between ± 0.08 mm and ± 0.1 mm over the whole range of dimensions of the rolled product (this range of dimensions in this type of plant being generally between 4.0 and 25 mm); a speed of finishing of the product higher than 120 m/s and even as high as 140 m/s; a speed of production of the line at least equal to 120 tons/hr.; a percentage of discarded material not greater than 3%; a factor of plant usage at least equal to 90%; the ability to carry out a finishing process at a low temperature on the whole range of products so as to obtain a high surface and inner quality of the finished product. All these specifications are not normally met as a whole in the conventional rolling plants, or at least are not met over the whole ranges of products which can be produced by the line. This situation is due to a plurality of causes which arise mainly from the arrangement of the lay-out of the plant and from the rolling methods conventionally employed, particularly in the change from one dimensional range of products to another. The rolling method in the conventional plants, which includes a great number of specialised finishing passes for each type of section, entails the necessity of performing frequent and long processes for the change of equipment, and these processes involve long times and the use of labour, thus reducing the factor of exploitation and therefore the efficiency of the plant. Moreover, in the conventional rolling plants the necessity of performing in separate steps the cropping of the spirals of the leading and trailing ends of the coil entails the use of additional personnel with a resulting increase in costs and in working times for the completion of the process. Moreover, the conventional rolling method causes the necessity of discarding a great quantity of material, and this situation is now unacceptable in the present rolling plants. EP-A-0 606 966 discloses a rolling mill comprising a roughing section having a plurality of roughing stands, an intermediate section with intermediate stands, and a finishing section having first and second finishing blocks each comprising the a same number of stands, so that each finishing block is equal to the other one. The two finishing blocks are arranged respectively along a first and a second non-aligned paths and four switches are provided to selectively direct the rolling products coming from the intermediate section or towards both such two blocks, when products of small sizes are to be rolled, or alternatively towards one or the other of such two blocks, when products of larger sizes are to be rolled. In the latter case, since each finishing block is equal to the other, it is indifferent to direct the larger product sizes towards one or the other of such finishing blocks. In other words, this rolling mill provides for a single finishing section divided into two finishing blocks, wherein each one of them is indifferently used for rolling larger product sizes, while both the finishing blocks are used sequentially for rolling smaller product sizes. EP-A-0 358 917 discloses a bar steel rolling mill with a cooling segment for thermo-mechanical finishing rolling, comprising a roughing train, a first intermediate train, a lighter intermediate train, a first finishing train for bar steel and a second finishing train for wires, wherein the material to be rolled is alternatively worked by the first finishing train or by the second finishing train. A watercooling segment for adjusting the temperature of the material to be rolled to a value of approximately 750 °C is located downstream of the intermediate trains, before a shunting arrangement which can divert the material to one or another finishing train. In this rolling mill, therefore, the products to be rolled must in any case pass through the water-cooling segment, after which they are in any case directed towards one finishing train disposed downstream thereof. All these problems have induced the present applicants to design a rolling line suitable to meet the above specifications by providing an improved rolling method which improves the characteristics of exploitation and efficiency of the rolling plant, thus achieving a great financial advantage in the production of rolled steels together with quality levels and the ability to repeat those qualities hitherto unattainable in conventional plants. For this purpose the present applicants have designed, tested and embodied this invention. This invention is set forth and characterised in the respective main claims, while the dependent claims describe variants of the idea of the main embodiment. The purpose of the invention is to obtain an improved rolling method and also a rolling line which performs this improved method, the method and the line being such as will optimise the efficiency of a plant performing the rolling of long products, at the same time enabling a product to be made of a high surface and inner quality which is characterised by very small dimensional tolerances. Another purpose of the invention is to provide an optimised line suitable to work on the whole range of products with shorter and faster operations for the change of equipment, with less use of manual labour and therefore a saving of the labour force and with an improved logistical occupation of space. The rolling line according to the invention includes for all the ranges of products the following lay-out of the blocks: a roughing assembly normally comprising from 10 to 18 rolling mill stands, depending on the dimension of the starting billet, which is normally between 120 mm and 180 mm.; two pairs of cantilever rolling blocks, each of which comprises two rolling mill stands with alternate horizontal/vertical axes; a reduction/sizing assembly with three rolling stands; a pre-finishing block with a number of rolling passes between six and ten; a finishing assembly with four rolling passes, which comprises two modules, each with two rolling passes. According to the invention the whole range of dimensions of products between 4.0 mm and 20 mm follows the same processing line up to the last rolling pass of the pre-finishing block, the changes of product being obtained only in the four rolling passes of the finishing assembly. The products of the range between 20 mm and 25 mm are finished in the reduction/sizing assembly with three rolling passes, and thus the relative changes too are concentrated in this assembly. The rolling line according to the invention includes an assembly for the fast changing of equipment, this assembly being associated with the two modules of the finishing assembly in the step of the change of product. This situation enables the times for re-adaptation of the line to be reduced appreciably and reduces the downtimes and thus increases appreciably the factor of exploitation and therefore the output of the plant. According to a variant, an assembly for the fast changing of equipment is included in cooperation with the pairs of fast-rolling cantilever blocks positioned at the outlet of the roughing assembly. According to another variant, an assembly for the fast changing of equipment is included also in cooperation with the possible reduction/sizing assembly with three rolling passes located immediately downstream of the fast-rolling cantilever blocks. This assembly for the fast changing of equipment enables all the three rolling passes or only two rolling passes to be changed at the same time, or else only one pass, depending on the requirements of the rolling sequence. According to the invention an in-line system of cooling the rolled stock being fed is included at least between the pre-finishing block and the finishing assembly. This cooling system lowers the temperature of the rolled stock leaving the pre-finishing block and enables a finishing process to be carried out at a low temperature over the whole range of products. In particular, a bar can be finished at a temperature of about 750°C or even down to 700°C, and an excellent equalisation of the heat of the bar can be achieved just the same at the inlet of the finishing assembly, owing also to the ample distance provided between the last cooling box and the finishing assembly, this distance being enough to obtain the required degree of stabilisation of the bar in terms of temperature before the final deformation of the bar. Moreover, since the whole range of dimensions between 4 mm and 20 mm is finished in the last four rolling passes downstream of this cooling system, the product being fed passes through the cooling line with a minimum diameter greater than that normally envisaged in conventional rolling trains and at a relatively low temperature. This condition makes it possible to have a more rigid and stable product travelling through the cooling line and also to keep always functioning the delivery of water in the cooling line, thus enabling the whole product from the leading end to the trailing end of the bar to be cooled in an even and controlled manner. This situation represents a considerable advantage over the conventional plants in which the diameters of product below 8 mm are not stable during their travel through the cooling line owing to their modest section, to the high speed and to the high temperature, and therefore the products of conventional plants do not always permit the delivery of water to be kept always working in the cooling line, for such delivery has to be shut off when the leading part of the bar passes through. This has the effect that the leading part of the bar leaves the cooling line with water at a temperature different from that of the rest of the bar and thus has to be discarded from the coil with a resulting loss of material. This loss of material is avoided in the system disclosed by our invention, with the resulting advantage of reducing the length of the end segments of the rolled stock not subjected to the cooling treatment and therefore having to be discarded from the coil. Furthermore, this situation also enables the working life of the valves delivering the cooling water to be prolonged owing to the lesser number of cycles of opening/closing to be carried out over a period of time. According to the invention a high-speed cropping shears is included upstream of the finishing assembly with the four rolling passes for the purpose of removing automatically the leading and trailing end segments before sending the rolled product to the finishing step. Thus the high-speed in-line shears makes possible the elimination of the cropping operations conventionally carried out and thus achieves a considerable saving of the labour force employed. Besides, the wound coil thus produced is ready to be pressed, tied and despatched without undergoing any further manual processing. According to the invention electronic-control drawing systems are positioned along the line so as to keep the bar always under tension by self-adaptation to any change of section and therefore to any change of the relative speed and tension. The attached figure is given as a non-restrictive example and shows a preferred lay-out of the rolling line which performs the improved rolling method according to the invention. A rolling line 10 shown in the attached figure is arranged, as an example, for a billet having an initial square cross-section and sides of about 160 mm or else a rectangular cross-section which can be likened thereto. This rolling line 10 comprises in this case a roughing train 11 including twelve rolling mill stands 12 with alternate horizontal/ vertical axes, which is followed by two cantilever rolling blocks 13 performing variable reduction, each of which defines two rolling passes 14 having alternate horizontal/vertical axes. Between the first fast cantilever rolling block 13a and the second fast cantilever rolling block 13b is positioned a first loop-forming machine 15a, which can possibly be disconnected from the line, whereas a second loop-forming machine 15b, which too can possibly be disconnected from the line, is included at the outlet of the second fast cantilever rolling block 13b. Each of the cantilever rolling blocks 13 is associated with its own drive motor 16 through a double-ratio gear transmission 17. The cantilever rolling blocks 13 are associated with a device 18 for the quick change of rolling equipment, by means of which device the whole changing operation can be carried out automatically in a very short time, less than five minutes and probably less than four minutes. The cantilever rolling blocks 13a and 13b are perfectly equal to each other and can be interchanged to increase the working flexibility of the rolling line 10. In this example a reduction/sizing assembly 19 with three rolling passes is included in-line downstream of the second loop-forming machine 15b and can possibly be removed from the rolling line 10. This reduction/sizing assembly 19 with three rolling passes too is associated with its own device 118 for the fast changing of the rolling passes, the device 118 being analogous to the device 18 and having also the purpose of the possible removal of the reduction/sizing assembly 19 from the rolling line 10. The drive of the reduction/sizing assembly 19 is provided by motors 20. An assembly 21 to measure dimensions of the product being fed and also to monitor surface faults is included at the outlet of the reduction/sizing assembly 19 and is followed by a first fast cooling assembly 22 employing water. The product leaving this fast cooling assembly 22 passes through a shears 23 performing cropping of the leading and trailing ends of the product and is then fed into a third loop-forming machine 24, which can possibly be disconnected from the rolling line 10. The product is then sent to the pre-finishing block 25, which in this case comprises eight reduction rolling passes. In the method according to the invention the pre-finishing block 25 carries out an average percentage reduction of the cross-section of the rolled product amounting to between about 14% and 20%. According to the invention, for each range of dimensions of the finished product, for instance between 4.5 mm and 20 mm, each product follows the same reduction process from its entry into the roughing train 11 up to the last rolling pass of the pre-finishing block 25. The specific differentiation of each single rolled product is obtained downstream of the pre-finishing block 25. The rolled product downstream of the pre-finishing block 25 undergoes a fast cooling process, which in this case includes two units 26a and 26b spraying water. A device 27 to make the tension of the product uniform is included in this case between the two water spraying units 26a, 26b. These water spraying units 26a, 26b lower the temperature of the product for the purpose of obtaining a finishing process at a low temperature on the whole range of dimensions of the products; this condition enables a better surface and inner quality of the finished product to be achieved and also enables the parameters of tolerance in relation to the established nominal dimension to be improved. In the water-spraying units 26a, 26b both the cooling nozzles and the elements which shut-off the water sprayed by those cooling nozzles work at a pressure below 0,6 MPa. This low pressure enables the initiation of vibrations to be prevented on the point of the bar travelling through the cooling line, and as a result the whole bar from its leading end to its trailing end is evenly cooled. Downstream of the cooling units 26a, 26b is positioned an assembly 28 performing drawing with two rolls, at the outlet of which is located in-line a high-speed flying shears 29 which crops the leading and trailing ends of the product and is associated downstream with a scrap shears 30, which breaks up the cropped ends. In this case, since the product entering the cooling units 26a, 26b possesses, for each dimension of the finished product, a minimum diameter less than that normally envisaged in the conventional rolling trains and travels at a reasonably low speed, the end segments to be cropped are shorter than those normally envisaged. The quantities of product discarded are therefore reduced to a value appreciably lower than 3%, which represents a value improved as compared to that which can be achieved in the conventional rolling trains. Lastly, the product is delivered into the finishing assembly 31. In this case, the finishing assembly 31 comprises two blocks 32, each of which includes two reduction rolling passes. In the method according to the invention the two finishing blocks 32 perform a reduction which will vary according to the dimension of the finished product to be achieved and are associated with a third quick-change device 218, which is suitable to perform the changing of the rolling equipment in very short times. The reduction of section achieved by this finishing assembly 31 is, for all the ranges of dimensions obtainable, at least about 12% to 16%, and this situation enables dimensional tolerances to be achieved which are even smaller than the ± 0.1 mm required by the specifications of the producers. The inclusion of the quick-change device 218 in association with the finishing assembly 31 enables a quick adaptation of the rolling line 10 to the various ranges of diameters to be achieved and an exploitation of the rolling line 10 to be obtained which is up to at least 94% or even more as compared to values of about 85% to 87% with the conventional rolling lines. This enables the output of the plant to be increased appreciably and, at the same time, a smaller labour force to be employed in operations on the line 10. The two finishing blocks 32 are driven individually by respective motors 33, through respective double-ratio gear transmissions 34. The two motors 33 are coupled to each other by an electric control group governed by the automation system which governs the whole the rolling line 10, the purpose being to be able to adjust extremely accurately the relationship of speed between the two modules having two rolling passes according to the required reduction. According to a variant, the motor of the pre-finishing block 25 is coupled to the two motors 33 of the finishing assembly 31 by means of an electric control group governed by the automation system of the line 10 so as to be able to adjust accurately the relationship of speed between the pre-finishing block 25 and finishing assembly 31 according to the reduction required. A drawing assembly 35 with two rolls is included at the outlet of the finishing assembly 31 and sends the product next to the third loop-forming machine 36. All the drawing assemblies, including at least the drawing assembly 28 and the drawing assembly 35, of the rolling line 10 are equipped with automated adjustment systems governed by the automation system of the line 10, so as to keep the bar under tension between the various assemblies and blocks during every step. In particular, the drawing assemblies provide a self-adaptation to any variation in the cross-section of the bar during the rolling.

Claims: Method of rolling long products such as bars, rods, wires or similar products having a starting nominal transverse dimension between 120 and 180 mm and a final nominal transverse dimension between 4 and 25 mm, in a rolling line having in sequence: at least one roughing assembly (11); a reduction and sizing assembly (19) with three rolling passes; a pre-finishing assembly (25) with a plurality of rolling passes; a water cooling assembly (26); and a finishing assembly (31); comprising the steps of: feeding long products with a starting nominal transverse dimension between 120 and 180 mm to said roughing assembly (11) to produce respective roughing product bars; optionally feeding said roughing product bars to said reduction and sizing assembly (19) only when the final nominal transverse dimension of said long products is comprised in a first range between 20 and 25 mm, to reduce said optionally fed roughing product bars to the desired final nominal transverse dimension without passing through any further rolling assembly, whereby the final sizing of said long products comprised in said first range is obtained in said optional reduction and sizing assembly (19) by passing through said three rolling passes; feeding said roughing product bars directly to said pre-finishing assembly (25) without passing through said reduction and sizing assembly (19), only when the final nominal transverse dimension of said long products is comprised in a second range between 4 and 20 mm to produce respective pre-finishing product bars, wherein each of said roughing product bars in said pre-finishing assembly (25) is subjected to a same reduction sequence up to the last rolling pass of said plurality of rolling passes; passing said pre-finishing product bars through said water cooling assembly (26) to reduce the temperature of said pre-finishing product bars to about 700 to 750° C, wherein the cooling water in said water cooling assembly (26) is ejected towards said pre-finishing product bars at a low pressure of about 0.6 MPa ; and feeding said pre-finishing product bars in said finishing assembly (31) to produce respective finishing long products, whereby the final sizing of said long products comprised in said second range is obtained solely in said finishing assembly (31). Method as in Claim 1, in which the distance between the outlet of said water cooling assembly (26) and the first rolling mill stand of the finishing assembly (31) is at least such as to make possible a controlled equalisation of heat of the whole pre-finishing product bar at the inlet of the finishing assembly (31). Method as in Claim 1 or 2, in which the speed of finishing of the long products in the finishing assembly (31) is about 140 m/sec. Method as in any claim hereinbefore, in which the finishing assembly (31) cooperates with a first quick-change device (218) to change equipment. Method as in any claim hereinbefore, in which the reduction/sizing assembly (19) cooperates with a second quick-change device (118) to change equipment, the second quick-change device (118) being suitable to change any or all the three rolling passes at the same time, according to the requirements of the rolling sequence. Method as in any claim hereinbefore, in which the finishing assembly (31) comprises two modules connected to two corresponding motors (33) coupled therebetween by an electric control group so as to obtain an accurate adjustment of the relative speeds of the two motors (33) according to the required reduction of the transverse dimension of the long products. Method as in Claim 6, in which the pre-finishing assembly (25) is fed by a third motor coupled to the two motors of the finishing assembly (31) by said electric control group so as to obtain an accurate adjustment of the relative speeds of said three motors according to the required reduction of the transverse dimension of the long products. Method as in any claim hereinbefore, in which the delivery of cooling water in the water cooling assembly (26a, 26b) is always kept in operation due to the high transverse dimension of said pre-finishing product bars and the low corresponding speed thereof through said water cooling assembly. Method as in any claim hereinbefore, in which upstream of the finishing assembly (31) both leading and trailing ends of the pre-finishing product bars are sheared by flying shears (29). A rolling line for long products such as bars, rods, wires or similar products, having a starting nominal transverse dimension between 120 and 180 mm and a final nominal transverse dimension between 4 and 25 mm, comprising in sequence: at least one roughing assembly (11); two pairs of cantilever rolling blocks (13) with rolling stands having alternate horizontal-vertical axes; an reduction and sizing assembly (19) with three rolling passes able to finish said long products when their final nominal transverse dimension is in a range between 20 mm and 25 mm; a pre-finishing assembly (25) having a plurality of rolling passes; a water cooling assembly (26) able to reduce the temperature of the long products to about 700 to 750°C; and a finishing assembly (31) for finishing said long products when their final nominal transverse dimension is lower than 20 mm, the finishing assembly (31) having four rolling passes and comprising two separate modules, each having two rolling passes, wherein at least the finishing assembly (31) and/or the reduction and sizing assembly (19) is associated with an equipment changing assembly (218, 118) able to perform the fast change of equipment. A rolling line as in Claim 10, in which between the pre-finishing assembly (25) and the finishing assembly (31) water-spraying cooling units (26a, 26b) are disposed, equipped with cooling elements and with elements to shut-off the water thus sprayed, the cooling elements and water-shutoff elements working at a pressure of about 0.6 MPa. A rolling line as in Claim 11, in which a device (27) to make uniform the tension of any long products passing through is provided between the two water-spraying cooling units (26a, 26b). A rolling line as in any of Claims 10 to 12 inclusive, in which a high-speed flying shears (29) to crop the leading and trailing ends of long products is provided upstream of the finishing assembly (31). A rolling line as in any of Claims 10 to 13 inclusive, in which an assembly (21) to measure the dimensions of any long products is provided at the outlet of the reduction/sizing assembly (19). A rolling line as in any of Claims 10 to 14 inclusive, in which each block (32) of the finishing assembly (31) is associated with its own motor (33). A rolling line as in any of Claims 10 to 15 inclusive, in which the motors (33) are coupled by an electric control group. A rolling line as in any of Claims 10 to 16 inclusive, further comprising drawing assemblies (28, 35) equipped with automated adjustment systems to keep any long products under tension between the assemblies, with self-adaptation to every variation of transverse dimension of any long products during the rolling.

Inventors: LESTANI MASSIMO, NINI ROBERTO, LESTANI, MASSIMO, NINI, ROBERTO
Applicants: DANIELI OFF MECC, DANIELI & C. OFFICINE MECCANICHE S.P.A.
Application year: 1997
Main classification: B21B1
IPC codes: B21B45, B21B31, B21B1