Rolling of the billets in four -high stands by scheme of the all-round compression is modern direction of the reconstruction continuously - billet and bar mills

Prof B N Poliakov

On the basis of the results of the experimental researches in production conditions of process rolling of the large billets in four-high calibre by the scheme of all-round compression, the high intensity of three-dimensional deformation, peculiar to this technology, as well as the high quality of metal (the equiaxial grain) and the high mechanical properties of received rolling the productions are shown. The design of the four-high stand collet of the type, realizing this technology, and promising the versions of its applications, at reconstruction of the continuous billet, medium- and small section mills, as well as in rolling conversion of continuously cast billets are offered.

Recently intensive technological and design developments are conducted in making the processes of the deformation, providing significant (even tenfold) single drawing (in one the pass or the groove), in prime particular, in continuous rolling mills are inculcated stands transverse-screw rolling, three-and four-high stands especially for roiling of difficult-deformable materials. The works on research of process of the rolling in four-high stands by scheme of the all-round compression of billets the small sections are known, but on study of the rolling in such stands of the billets the greater sections of the works are absent. For estimation of the possibility of using the four-high stands in lines continuously-billet mills (CBM) by their reconstructions, the problem to research experimental of the deformation and power-force the parameters by rolling of the billet the large square section in four-high groove by scheme of the all-round compression has been put [1].

With the object of checking in real production conditions of the principle technological possibility of rolling in four-high groove of the billets the large symmetrical section by size 100 x100 mm the device has been created, which can be installed in flow technological of acting CBM and uses under here existing drive one of the worker stand.

Particularity of rolling the billet square section in four-high groove, forms two horizontal and two vertical rolls, axis which are situated in one plane, is that, the billet is needed to give in rolls on edge, as calibration is built by scheme of the inserted square. Namely, such position occupies the billet in grooves of the clean group of the stands, where on existing technology rolling is conducted by system "rhomb-square" and draft occurs toward diagonals of the section of the billet alternately horizontal and vertical rolls, in this case from horizontal stands the square section billet leaves.

With the account of energy expenses, required for rolling in four-high groove, for carrying out experiments was chosen vertical stand No9 (penultimate stand of the finishing mill group) CBM 850/700/500 of West-Siberian Steel Corporation (WSSC), equipped by the most powerful drive.

The four-high cassettee was designed and manufactured for carrying out the experimental rolling. During carrying out the experienced rolling it was fixed instead of the existing two-high cassette.

The four-high cassette represents itself cylinder-conical reduction gear, allowing' to transfer rotation from two vertical shafts on all four worker rolls. The drive of the rolls was carried out from one of the main engine and multifunction reduction gear, through two flaps of the spindles, installed on out put slotted shafts of reducer, with specially constructing universal joints, with possibility of the movements the spindles on height, when installing the cassette.

The several stages of the experimental rolling were conducted in the four-high cassette. The purposes first of stage:

· to check possibility of the achievement in the real industrial conditions high meanings of the coefficient of drawing ( not less 2,0 ), when rolling with all drive rolls on different speeds with re provision of the quality surfaces, accuracy of the sizes and properties of the material on GOST;
· in substantiate necessities for designing the requirements to technology of the rolling, to design the parameters of the four-high stand and its of the hi~ conducted drive.

For rolling were applied worker rolls with the nominal diameter 420mm. In the character of the initial was taken the billet with nominal sizes of the section 100xl00mm, coming out of stand No 8 and rolled consecutively in stands No 7 and No8 from the billet 125x125mm by existing calibration.

The billets were rolled from steel BSt5ps (half-balanced steel) under initial temperature 1100-13000 C at speed 3.5m/s. The billets are received the correct geometric form with actual size 80±1.5mm (i.e. with deviations, admitted GOST 4693-77), with section, analogous existing technology, but with small fluctuations of the radius in corners of the profile, reaching 20mm. It is marked, that condition of the surface of the billets analogous condition of the surface of the billets, rolled by existing technology and answers the requirements GOST 535-79.Average value of the coefficient of drawing has formed 1.58, that corresponds to the general drawing in horizontal and the vertical stands (No 6 and No 7 or No 8 and No 9) the finishing mill group.

For process of the rolling in four-high cassette is typical of full absence of the dynamic phenomenons and fluctuations of power-force and speed parameters, in particular, of the moment of the rolling ( current of the rotor) as in transitional stages of the seizing and the issuing of metal from rolls, so and in establishment of stage, which are organic characteristic of traditional method of the rolling in all stands CBM. The maximum values of the moment of the rolling (current of the rotor) at the seizing and the issuing substantially do not differ from value of the moment in establishment of stage.

The billets of the greater section were rolled on second stage also in square 80 from steel BSt3kp (acid Bessemer rimmed steel) with the further rolling in the reinforcement twisted ribbed bar on small section tech mill 250. Average value of the coefficient of drawing equal 1. 77 was achieved, that approximately corresponds to general drawing in three consecutive stands of the finishing mill group, for example, in stands C, D and No 7 CBM WSSC.

It is marked, that process of the rolling in four-high groove with all drive rolls by scheme of the all-round compression (by calibration of the inserted square) creates high intensity of the deformation in comparison with traditional scheme of the rolling on CBM. This promotes penetration of deformation in deep layers of the section and forming significant speeds of the current of the different layers of metal (particularly of central layers), providing penetration of plastic deformation on the whole section (i.e. its quality) and ultimately greater of drawing to achieve. The metal not only does not flow in lateral clearance between rolls, as it was expected, does not form supposed hangnails, but on the contrary, all directs in drawing, that can even somewhat distort regulated form of section: to increase the radius of the comers and even to form the drawing-in of the section. Thus, technology of the rolling by scheme of the all-round compression is characteristically more high, in comparison with rolling in two-high stands, efficiency of the forming: absence of the spread and increased coefficient of drawing. The face the parts of the received billets has a correct spherical form, that promotes the stable seizing, when by rolling in the subsequent stands or rolling conversions.

The significant intensity of the are, deformation at rolling by scheme of the all-round compression brings to notice increase of the temperature of the metal in the centre of deformation, that the load capacity technological of stand lowers, but the thermal crack of the surface and the wear of rolls can promote

With the purpose revelation the quality metal of the billets, rolled in four-high cassette, met allographic of the study on cross templets were executed. The macrostructures of metal appeared dense, fine-grained, defects of the metallurgical character is not discovered. The liquationed "square" is localized clearly in the a m narrow central zone, with sizes of more than 20mm; this is explained by symmetrical character of the current intensive of the metal in three directions in process of the deformation. The structure of the metal is characterized by density and homogeneity, small impurity sulphide of the inclusions.

The microstructures of the metal of the billets is uniform, small-grained, ferrite-pearlite with ferrite predominance. The grain size close by the surface in the comers and in average part verges to the billets corresponding to mark 7-8. The grain is homogeneous approaches are equiaxial. Thus, undoubted f dignity of new technology is revealed as means of management and achievements ( in combination with the following thermal processing with rolling heating) of high mechanical, plastic, and the official properties of the final products. As a whole, the results of metallographic researches are evidence of good quality of the n metal and the microstructure, exceeding the quality of the rolled products, received by the existing technology.

The tests of the mechanical properties and following statistical analysis have shown, that regardless of the place of the selection of the trials on section of the billet (the comer, the medium of the surface, in the central zone) all the descriptions he exceed to the requirements of GOST.

The high values of the mechanical properties are fixed by the set reinforcement twisted ribbed bar, rolled from the received billets on the small section mill 250 (without of the thermal treatment).

Particularly the high value of the of impact strength, equal 2.0-2.1MJ/m 2, is discovered, exceeding in 2.5-3.0 times the level standardized of GOST. Besides, the increased stability of the mechanical properties on the section and the length of the billet is noted: the values of coefficients of variation their functions of distributions are insignificant and enough narrow interval of 2.6-12.8% are changed.

And so, the technology of the rolling by scheme of the all- round compression with the following thermal treatment may be considered as the instrument of improvement of the structure and of increase of the mechanical the properties of the metal of the ordinary marks of steels, as well as the manner in which to approach them to the values corresponding to the properties of the low-alloyed and alloyed steels.

Thus, in real industrial conditions of CBM of WSMC, in the result series of the experiment's rolling in specially manufactured the four-high cassette with all the drive rolls, realizing the deformation by scheme of the all-round compression, principle technological possibility of the stable rolling of the square billet 80x80mm is shown. At this the large value of the coefficient of single drawing by the order 1.8 (equivalent general drawing in three stands of CBM) is reached, at the high quality of the metal and of the mechanical properties exceeding requirements of GOST. And so, at reconstruction of CBM, particularly at high density of mechanical equipment, expeditious application of technology of the rolling in the four-high stands, providing vastly greater the single drawing for one pass in comparison with traditional scheme of the interchanges of the horizontal and the vertical two-high stands, besides, combined with the technological line of the reconstructed blooming mills with the following line of the thermal treatment.

* ±37 - the width of 95% confidence interval for the arithmetic average value.

Application the four-high stands in line of CBM minimum at least twice reduces the amount of the stands (its is 8-14 now), but consequently, and the length of the technological line, reduces the volume of equipment and of capital expenditures, lowers the consumption of the electric power, raises the accuracy and the consumers' properties of the rolled products, as well as it can to increase the productivity of CBM at the expense of the rolling the more large billet ( at condition of the reconstruction of following small section mills). Besides, at simultaneous transition to rolling the more large final billet (l00xl00mm and more) instead of square 80x80mm the equipment unloads essentially, increases and is created the determined reserve of productivity of the blooming - billet mill complex.

The application of the four-high Co stands at reconstructions of CBM causes necessity of the reconstruction following of the small section mills. At this potential possibility of the increase of their productivity is created by the estimate preliminary minimum on 7-8%. The installation of the four-high stand on the small section mills as last ( in exchange of three stands) with following thermal treatment in stream of the mill vastly raise the mechanical and the official properties of the rolled products ( at the expense of the forming and fastening of the equiaxial grain, i.e. improvements of the structure), guaranteeing the raised accuracy of its sizes in the field of the negative tolerances.

Follows to pay attention to, that rolling by scheme all-round compression in four ( or more)-high calibre, creating the high intensity of the three-dimensional deformation, can be effectively applied also at designing of the equipment for conversion of the continuously cast billets.

In spite of the advantages, various variants and significant possibilities of the applications of the technology of rolling in four- high calibre by scheme of the all-round compression, this method, unfortunately, has not yet received broad application, on highly efficient the billets mills particularly. The reason consists in that, are not yet developed such designs of the stands, which would have provided their stable work in the course of prolonged time.

The main constructive difficulty of the development of such multiple-high stands is concluded in that, in extremely limited space necessary to place, for example, the four rolls, the axis which are located in one plane, the bearing supports of the rolls, the mechanisms of their tuning and the details of the drive.

It is represented to us, that the most efficient will be a creation of the stand such as the cassette, which I would borrow a minimum time for installing it in the working position, for example, on type of the cassette, applied at guiding the experiments. Introduction is also necessary to have a possibility of the regulation of the between rolls distance on small range, that evidently, is reasonable to realise on the testing ground for assembly of the stands, before installing of the cassette in working position. Besides, for compensation of a wears is necessary to have adjusted the regulation in process of the rolling. The given technological and operational requirements are realised completely in the offered design of the four-high stand collet the of the type [2].

Thus, the experimental researches, the design developments, technological and economic the calculations have confirmed the efficiency and the perspectivity in applications of the technology of rolling of the bar billets in four-high calibre by scheme all-round compression, particularly jointly with the thermal treatment at reconstructions of the CBM, of the medium-and small section mills.