Improved hot-rolling process for sealess tubes with preliminary diameter reduction of the semifinished products

Abstract: In an improved process for hot-rolling of seamless tubes with preliminary diameter reduction of the semifinished products (axially pierced round blanks), the coreless reduction step (without mandrel) occurs immediately before the step of continuous rolling on a mandrel and practically at the same time of driving the mandrel into the pierced blank, without interferences therebetween. The apparatus carrying out this operation is positioned immediately upstream of the continuous mill with mandrel, so as to form with this a single unit on the same line, thus reducing the need for room, the operation time and consequently the blank cooling before rolling, whereby the last stand of the rolling mill is reached at a sufficient temperature for rolling without any need of intermediate heating steps.

Description: The present invention relates to an improved hot-rolling process for seamless tubes with preliminary diameter reduction of the pierced round blanks by means of a rolling mill for blanks reduction of the conventional type. It is known that in the manufacture of seamless tubes of metal or metallic alloys, particularly steel, having diameters defined as of small or medium-small size, i.e. not greater than about 170 mm, one of the most common processes is at present that of rolling on a mandrel. According to this process, the starting material in the form of a round billet obtained through one of the known technological processes, is heated in an oven up to a temperature suitable for rolling (about 1300°C for steel) and thereafter it undergoes a rolling process comprising various successive steps carried out by some main machines which very often are provided e.g. by a piercing mill with skew rolls to obtain from said billet a hollow body of big thickness, being called "pierced blank"; a continuous rolling mill formed of a series of multi-rolls stands which, by rolling the blank on a mandrel previously fed therein, progressively reduces the thickness thereof down to a size near to the finished tube size; and finally a drawing mill formed of a series of stands with two or more rolls each for coreless reducing the outer diameter until reaching the desired final size. Continuous rolling of seamless tubes on a mandrel is shown e.g. in JP-A56-30015 Obviously the number, arrangement and constructional characteristics of the machines upstream of the rolling mill may vary, according to the needs, to follow the requirements of the common technological knowledges. Furthermore the limits imposed to the various machines by technology and layout define the cross-section reductions which are possible, and consequently the diameter size at the various rolling steps. In particular the diameters of the initial round billets are chosen and determined according to the diameter size that is desired for the finished products. In fact from a specific diameter of starting billet a precise series of outer diameters of the finished tube can be obtained, which series is defined by the structural features of both the continuous rolling mill and the drawing mill. It is also known that the least expensive technology of manufacturing the raw round billet is that of the continuous casting process from the melt metal, then using directly the billet without preliminary processing before forming the pierced blank. However the continuous casting process is particularly advantageous, under the point of view of both the manufacturing costs and the quality of the product, only for diameter sizes greater than minimum size which may be defined, according to the present state of the art, as a minimum value of the billet diameter of 150 mm. By way of example a particular method of optimization of a plant designed to manufacture finished tubes with outer diameter of 20-90 mm would require to start from a round billet having a diameter of 120 mm, i.e. less than the minimum size that can be satisfactorily produced by the most common continuous casting plants. A billet of this size not only is of a quality that not always is reliable, but also is expensive and hardly available in the trade. In order to overcome this inconvenience a rolling mill for reducing the diameter of pierced blanks has been used for some time to reduce the outer diameter of the billet already pierced by a piercing mill with skew rolls, thus modifying the outer diameter from a size that is more convenient for the continuous casting plants to a size required to enter the rolling mill with mandrel. Furthermore with the provision of an intermediate machine the correspondence between the initial diameter of the billet and that of the finished tube is made less rigidly univocal, whereby the number of diameter sizes of the raw billets which are required for a given range of diameter sizes of the finished tubes may be reduced, thus simplifying the supplying operation through a less differentiated stored stock. At present, according to the existing state of the art, the apparatus for reducing the blank diameter is installed as a separate machine between the piercing mill with skew rolls and the rolling mill with mandrel according to a layout as shown in Fig. 1. This layout shows, with respect to a non-optimized plant, i.e. a process not providing for the step of preliminary diameter reduction of the blanks, some inconveniences mainly due to the considerable demand of room to be occupated by the feed facilities 5 relating to the blank reduction rolling mill 6 for feeding the latter with the blanks from the piercing mill with skew rolls 2, by the discharge facilities 3 for said blanks from the piercing mill 2 and by the handling means 4 between the discharging and feeding lines, respectively 3 and 5, as well as by the discharging and handling means, respectively 7 and 8, of the pierced blanks from the reducing rolling mill 6 to the station 9 for feeding mandrels before the continuous rolling mill 10 with mandrels. This results not only in additional high cost of the facilities and systems of feeding, discharging and handling, but also in a sensible increasing of the time between the outlet of the blank from piercing mill 2 and its inlet into the rolling mill 10 with mandrel, thus giving rise to a harmful lowering of temperature of the blank to be rolled (Fig. 8) and an additional secondary oxidation of the inner and outer surface of the pierced blank. The temperature decrease may compromise the rolling itself if additional intermediate heating step is not provided, which obviously involves additional costs. In any case cooling and secondary oxidation give rise to negative consequences to the forces required for rolling, the ductility of the material to be rolled, as well as the surface quality of the finished tube, as it is known to those skilled in the art. JP-A60-102211 discloses a method for the manufacture of thick steel tubes according to which the coreless reduction step is distinct from a final rolling step on mandrel, the latter being carried out by means of caliber rolls to obtain finished products. This method provides for merely making longer the continuous rolling mill by increasing the number of stands, thus having this final operation in line, but only because no previous preparation of the pierced blank is foreseen. A first consequence is that not actually round shapes are obtained, in particular of the inner tubular walls, as shown by the drawings of this prior patent. Furthermore, during the first stage of rolling, wherein mandrel and tubular blank are not in contact therebetween, a reduction of the outer and inner diameter together with an elongation is experienced, whereas at a subsequent stage, with mandrel and tube in contact, the outer diameter only is reduced, with a general reduction of the thickness, but without problems, as in this case the thickness is still high enough, according to the particular product (thick tubes) in demand. It is an object of the present invention to provide an improved hot-rolling process for seamless tubes of outer diameter not greater than 170 mm and obtained through piercing by skew rolls a continuously cast and re-heated round billet, which does not show any of the above-mentioned drawbacks, although the advantages given by an optimized process with intermediate step of diameter reduction are kept unchanged. The process according to the invention, comprising a step of coreless reduction of the pierced blank diameter before its continuous rolling on a mandrel, is characterized by the fact that said reduction step of the blank without mandrel is carried out immediately before and on the same line of the continuous rolling step on mandrel, without intermediate transferring of the blank and at the same time as the mandrel is driven into the blank, without touching it, at the location of said reducing mill. It is also an object of the present invention to provide a rolling plant for manufacturing by hot-rolling seamless tubes with outer diameter not greater than 170 mm from a continuously cast round billet, comprising a heating oven, a piercing mill with skew rolls, a coreless diameter reducing mill for pierced blanks and a continuous rolling mill on mandrel, with a feeding station for inserting mandrels into the blanks, characterized by the fact that said coreless reducing mill is positioned immediately upstream of the continuous rolling mill with mandrel, so as to form therewith a single unit, in correspondence with and on the same line of the mandrel feeding station. These and additional objects, advantages and features of the process according to the invention, and of the plant carrying out the process will result more clearly from the following detailed description of an embodiment thereof, given by way of a non limiting example with reference to the annexed drawings, in which: Figure 1 shows a diagrammatic view of a rolling plant for seamless tubes with preliminary reduction of the pierced blank diameter according to the prior art; Figure 2 shows a diagrammatic view of an example of a coreless (without mandrel) reducing mill, in line with a continuous rolling mill with mandrel according to the invention; Figure 3 shows a diagrammatic view of an example of a coreless (without mandrel) reducing mill, in line with a continuous rolling mill with mandrel according to the invention; Figure 4,5 show cross-section views, respectively taken along lines IV-IV and V-V in correspondence with the first and the last roll stand of the reducing mill; Figure 6,7 show cross-section views, respectively taken along lines VI-VI and VII-VII in correspondence with the first and the last roll stand of the continuous rolling mill with mandrel; Figure 8 shows a graph of the temperature variation, respectively at the skin and the core of the bar or billet, starting from the oven until the exit of tube from the continuous rolling mill, before the last step of reduction by drawing (not considered in the present description), for a process according to the prior art; and Figure 9 shows the same diagram of temperatures of Fig. 8 for a process according to the invention. With reference to the drawings, as already stated above, Fig. 1 shows a diagrammatic representation or a layout of a known plant carrying out the conventional process of manufacturing seamless tubes, being optimized by the fact of comprising a coreless reducing mill 6 of the pierced blanks between the piercing mill 2 and the continuous rolling mill 10 with mandrel. In the foregoing it has been already observed which are the inconveniences due to the space being occupied, the increasing of costs and the longer time of processing that results in a considerable cooling until reaching temperature values which, at the continuous rolling mill on mandrel, are near to or even lower than the values generally admitted for rolling, of about 1180°C (for the steel), as can be seen in Fig. 8. Referring to Fig. 2 the process according to the invention provides for causing a bar or billet to exit from heating oven, carrying the same to a piercing mill 2 and feeding the pierced blank therefrom, through transfer lines 3, 4, 5 to a pierced blank reducing mill 6 which is designed and made according to the prior art, and according to the invention is installed directly upstream of, adjacent to a rolling mill 10 with mandrel, whereby these machines form a single unit. It should be noted that the pierced blank feed 5 to the reducing mill 6 in this case is concident with the mandrel feeding station for the mandrels to be used in the mill 10. Therefore the operating steps of the process according to the invention comprise, subsequent to discharging the bar from the heating oven 1 and its carrying to the piercing mill 2, the handling of pierced blank to the mandrel feeding station, where the mandrel is driven into the pierced blank, then passing together with the latter through the blank reducing mill and the rolling mill on mandrel 10 in an immediate succession, as is better seen in Fig. 3, in which 11 designates the pierced blank and 12 the mandrel. With particular reference to Figs. 4-7 it is noted that the diameter reduction in 6 is actually performed "coreless", i.e. without contact with mandrel 12, as it were not present at the inside of blank 11, in the same way as the conventional method of Fig. 1, when the mandrel was to be fed only at a later time. Therefore it will be appreciated that with the process and related plant of this invention the mandrel 12, in spite of having also to pass longitudinally throughout the reducing mill 6, in addition to rolling mill 10, shall have the same length as required with the prior systems of Fig. 1, since during the reduction step the mandrel moves forward coaxially and at the same time with blank 11 while the diameter of the latter is reduced, without any inteference therewith (Fig. 4, 5). Mandrel and pierced blank come into contact each other only starting from the first stand of rolling mill 10, as it occurred in the prior art. In fact it is should be considered that the speed and mutual spacing between rolls of the stands of the blank reducing mill 6 are adjusted automatically for this purpose, e.g. by means of a programmable logic suitable to control at the same time two machines at different speeds, so that at each moment the mandrel position is the prefixed one. With interlocking arrangements of this type depending on the regulation of rolling mill 10, the synchronization of the various parts of the plant and the exact feeding of pierced blank to the first stand of rolling mill are ensured automatically and under whichever condition. When comparing Figs. 1 and 2 is clearly seen that with the layout of machines as provided by the present invention there is no need of feeding, discharging and handling facilities, in particular those referred to as 7-9 in addition to those being strictly necessary for carrying out the non optimized operational cycle, i.e. such that no preliminary reduction of the blank diameter is provided. The advantages of the solution proposed by the present invention are thereby evident under the aspects of room required and of installation and operating costs. In particular the conveying time for moving from piercing roll 2 to the continuous rolling mill does not change in consequence of installing the blank reducing mill and practically the diagram of temperatures given at Fig. 9 is the same as it would be for a non-optimized plant, showing no coreless reducing mill. Another advantage obtained with the process and plant of the invention is that the mandrel 12 being present at the inside of the pierced blank 11 when passing through the reducing stands 6 puts a limit to oxidation of the blank inner surface, owing to the decreased cross-section for passage of air caused by mandrel 12 and possible presence of antioxidizer agents provided on the mandrel surface. This effect is of course the more important, the greater is the affinity with air shown by the blank metal. With reference to the example referred to in the foregoing, relating to a modification of an existing plant in order to use round billets with a diameter of 150 mm instead of 120 mm, the advantages which can be obtained with the proposed solution may be summarized as from the following table. Further operating steps such as descaling etc. could be provided, in the process according to the invention, possibly associated to the pierced blank rolling, without substantially modifying the above-mentioned advantageous results.

Claims: A process for hot-rolling of seamless tubes of outer diameter not greater than 170 mm and obtained through piercing by skew rolls a continuously cast and re-heated round billet, comprising a coreless (without mandrel) reduction (6) of the pierced blank (11) diameter before its continuous rolling (10) on a mandrel (12), characterized by the fact that said step (6) for coreless reduction is accomplished on the same line (5) and immediately upstream of the continuous rolling step (10) which is carried out by using a mandrel, without intermediate transferring of the blank (11) and at the same time with feeding the mandrel (12) into the said blank without touching it, at the location of said reducing mill (10). A process according to claim 1, characterized by the fact that said pierced blanks (11) and mandrels (12) pass throughout said coreless reducing mill (6) coaxially and without any interference therebetween until entering the first stand of said rolling mill (10) where blank (11) and mandrel (12) come into contact. A process according to claim 1 or 2, characterized by the additional step of coating said mandrel (12) with antioxidizer substances along its whole lateral surface. A rolling plant for the manufacture by hot-rolling of seamless tubes of outer diameter not greater than 170 mm from a continuously cast round billet, comprising a heating oven (1), a piercing mill (2) with skew rolls, a coreless (without mandrel) diameter reducing mill (6) for pierced blanks (11) and a continuous rolling mill (10) on mandrel (12) with a feeding station (5) for inserting mandrels (12) into the blanks (11), characterized by the fact that said reducing mill (6) is installed immediately upstream of the continous rolling mill (10) and on the same line of feeding station (5),whereby a single unit is formed therewith. A plant according to claim 4, characterized by further comprising automatic control means for rolls and stands of the reducing mill (6), being interlocked to the control means of rolling mill (10), whereby at every moment and in any condition the synchronization of the various parts of the plant with a correct positioning of the mandrel is provided.

Inventors: D AVANZO RAIMONDO, D'AVANZO, RAIMONDO
Applicants: DALMINE SPA, DALMINE S.P.A.
Application year: 1990
Main classification: B21B17
IPC codes: B21B17, B21B25, B21B23