STUDY OF THE PROCESS OF REALIZATION OF HIGH-RATE PLASTIC DEFORMATION IN LENGTHWISE ROLLING

Transcription

1 STUDY OF THE PROCESS OF REALIZATION OF HIGH-RATE PLASTIC DEFORMATION IN LENGTHWISE ROLLING Abstract A.B. NAIZABEKOV, M.B. BYKHIN, K.A. NOGAEV, B.B. BYKHIN Karaganda state industrial university, Republic of Kazakhstan, Arselor Mittal Temirtau, Republic of Kazakhstan Study of the new technique of metal deformation during lengthwise rolling in cross shear providing high rate plastic deformation is considered in the work. The results of the study of the stressed and strained state of metal by mathematic modeling methods which show a possibility of achieving a sufficient high level of shear rate in rolling by the new technique in comparison with conventional rolling as a consequence of a more favourable scheme of the stressed state in the place of strain are given. Testing of the new rolling technology using rolls of a new design with rhomb-square pass system in laboratory conditions is described. Judging by the results of the study of the mechanical properties and the analysis of the metal structure it can be concluded that in industrial conditions it is possible to provide the required number of products when reducing the total number of passes in comparison with the existing rolling technology at metallurgical enterprises. Keywords: high - rate plastic deformation, new technique, stressed and strained state, rhomb-square pass system 1. PROCESS OF REALIZATION OF HIGH-RATE PLASTIC DEFORMATION Significant results have been achieved recently in the development and working out of new PMT processes realizing high-rate plastic deformation. It especially concerns the processes of extrusion and forging where the whole volume of the deformed billet is gradually subjected to high-rate plastic deformation due to the use of newly designed deforming tools providing controlled shear strain localization in definite place of deformation with sequential cyclic changing of the flow direction for the opposite one (sign-changing deformation) [1-5]. In the field of lengthwise rolling the development of technological processes of realization of high-rate plastic deformation is in the stage of search and individual tries [6]. It is known that a conventional rhomb-square pass system is used in rolling billets and sections, both in roughing and finishing passes of line, successive and continuous rolling mills, both rhomboid and square pass on the rolls are located diagonally with reference to the longtitudal axis of rolls [7]. But the nature of interaction of metal and rolls in the conventional rhomb-square grooving system doesn t sufficiently provide the required level of working of the stricture in individual passes though it solves the problem of making the required form and size of products. That is why in order to achive the necessary level of product quality it is necessary to increase the number of passes and consequently to increase power costs for rolling. In order to realize high-rate plastic deformation of metals during rolling there has been proposed the use of rhomb-square pass system with non-diagonal location of rhomboid pass in reference to the longtitudal roll

2 axis in such a way that the two opposite sides of the rhomb are located parallel to the roll axis and the other two sides of the rhomb at an angle to the roll axes (fig.1b). as a result a square or rectangular crosssection billet fed into this rhomb pass receives a cross scear along with a height reduction which provides high-rate plastic working of the structure. This result is possible due to the peculiar character of the interaction of every roll with the deformed metal in the place of deformation (fig.1,b). It is seen in the picture that the rolls in the rhomboid pass interact with the rolled metal in such a way that plastic deformation occurs as a result of height reduction and cross shear to the left on the side of the lower roll. This character of interaction of the upper and lower rolls causes unequally directed plastic flows in the pass. It is especially favourable for axial zones of the deformed billets where as a rule liquation places, axial macro and micropores (porosity) and gas blisters are located which are easily healed due to unequally directed shear displacement of metal particles on the side of the upper and lower rolls. а) b) Fig.1. Views of rhomb-square pass system a) conventional rhomb - square pass system; b) rhomboid pass with non-diagonal location in combination with conventional square pass 2. COMPARATIVE ANALYSIS OF THE STRESSED AND STRAINED METAL STATE. To make a comparative study of the stressed and strained metal state when rolling 40H steel whose hot plastic working is made at the temperature of 1100 C (the most often used temperature in the conventional rolling technology for this steel grade) in the conventional and proposed rhomb-square pass system there has been carried out modeling by finite elements (FEM) made in DEFORM 3D program complex (SFTC, USA) A mm square section prism was taken as a geometrical billet model. In modeling the inclination angle of the rhomboid pass walls with non-diagonal location is assumed as 33 which corresponds to the mean value of optimal shear angle in forging processes realizing high-rate shear strain [3]. The technological rolling parameters are: friction coefficient when biting metal 0.35, extension coefficient λ=1,2,3 which corresponds to the mean values of these parameter during rolling in roughing

3 passes for the pass system under consideration [7]. The diameter value and rotational roll speed are assumed as 220 mm. and 3.6 rad/s accordingly proceeding from characteristics of semi finishing section DUO -220 rolling mill which later was used for laboratory experiments. A comparative analysis of the conventional and the new rolling processes was made according to the values of effective stresses and shear strain rate in the cross-section near to the exit from the strain place, and their distribution is illustrated in fig.2 and 3. а) b) Fig.2. Distribution of effective stress along billet cross-section a) Conventional rhomboid pass; b) Rhomboid pass with non-diagonal location. The character of distribution of effective stresses has symmetrical form in reference to the vertical and horizontal strip axes in conventional rhomboid pass rolling. In this case the zones of maximum values of effective stress are located at the side pass walls and change in the range of MPa and in the axial zone along the whole strip width the value of the effective stress is not more than 6 MPa (fig.2a). This low value of the effective stress in the axial zone is caused by insufficient working of metal structure in the above zone. Quite different picture can be seen in rolling cross shear in the rhomboid pass with non-diagonal location in reference to the roll axes. Along the whole width of the big diagonal of the rhomboid strip the efficient stress value is rather high in comparison with the conventional rolling scheme reaching Mpa (fig.2b). on the other places of the section the effective stress level is also rather high which provides highrate working of metal structure ( MPa). The character of distribution of logarithm shear strain rate in conventional rolling and in rolling with cross shear in rhomboid pass with non-diagonal is sufficiently different. In traditional rolling in axial zone along the strip length there are localized places with a low rate of logarithm shear strain in the centre of the billet 0,125 0,25, on the edges of the strip big diagonal less than 0,125 (Fig. 3a). In the rest parts of the axial zone Г changes from 0,25 to 0,375. The highest strain can be seen only on small places nearer to the pass top and are 0,75-1,0.

4 а) b) Fig.3 Distribution of shear strain rate along the billet cross-section a) Conventional rhomboid pass; b) rhomboid pass with non-diagonal location During rolling in rhomboid passes with non-diagonal location the strip is subjected to macroshear strain in opposite directions from the side of the upper and lower rolls due to the effect of side inclined pass walls, which causes a higher level of realization of high-rate shear deformation in comparison with conventional rolling. In this case the minimum level of the logarithm shear strain rate is not less than 0,375 (fig.3b). In the axial zone along the long rhomb diagonal line Г changes from 0,375 to 0,5. The places with maximum Г level are located on the inclined places of the rhomb and it is 0,75-1,0. Mean values of logarithm shear strain rate Г m in the surface, intermediate and axial zones were determined for comparative evaluation of the efficiency of working of the deformed metal in the zones of the section under consideration (fig.4). In conventional rolling Г m is: in the surface zone 0,455, in the intermediate 0,4, and in the axial zone 0,28, while in the new rolling process these values are 0,669, 0,547 and 0,528 accordingly. On the whole the average Г m through the whole section is 0,354 for conventional rolling and 0,528 for the new process which along with other equal conditions provides almost 1,5 fold increase of metal structure working efficiency. It is important that the non-uniformity of distribution of logarithm shear strain rate in conventional rolling is higher than in the new process. This is obvious from the ratio of Г m for surface and axial zones which comprises 0,455/0,28 = 1,625 in conventional rolling and 0,669/0,449 = 1,49 in the new process. Decreasing the non-uniformity degree in the new rolling process provides a higher quality of finished products. Fig.4 Comparative values of average rates of logarithm shear strain

5 3. CARRYING OUT THE EXPERIMENT In order to check the possibility of realizing high-rate plastic deformation in the proposed rolling technique and to reveal the effect of the new scheme of deformation of ingots and billets on metal quality an experimental research was carried out in a semi-industrial DUO-220 section rolling mill installed in the rolling laboratory of KSIU PMF chair. A new design of working rolls with the proposed rhomb-square system was worked out for these experiments (fig.5). Ingots measuring mm of 40H and 70H steel grades were used as initial pieces. The above steel grades are used for making sections of high quality employed for manufacturing critical parts in engineering industry. The choice of temperature rolling conditions for carrying out the experiments was made in accordance with the existing hot rolling practice for these steel grades in industrial conditions. The ingots heated to C in electric furnace were rolled in four roll passes of which the 1-st and the 3-d are rhomboid passes with non diagonal rhomb location, and the 2-nd and the 4-th square passes with diagonal square location on the rolls barrel (fig.5). This combination of passes provided high-rate sign changing plastic deformations with cross shear. Fig.5 Sketch of the new working rolls design (figures 1,2,3 and 4 in the passes show the sequence of passes) The initial piece a square measuring mm is fed into the first rhomboid pass according to fig. 1b. The metal suffers shear in the cross direction to the angle corresponding to the angle of the side pass walls and at the same time is subjected to reduction by the pass walls. Upon leaving this pass the produced rhomboid strip is fed to the 2-nd square pass with the long rhomboid diagonal in the vertical position being subjected to reduction along the vertical axis. The track of metal particles movement changes the direction and there

6 takes place a sign changing deformation as compared to the movement track of the same metal particles in the first pass. The produced square strip is fed to the third pass (rhomboid) in such a way that metal cross shear takes place in the direction opposite to the shear sign in the first pass. The rhomboid strip produced in the third pass is fed to the fourth pass (square) and is deformed as in the case of the second pass. Thus the deformation process from pass to pass takes place with sign changing strain with summation of the store strain providing high-rate plastic deformation. 4. STUDY OF MECHANICAL PROPERTIES In order to study mechanical properties of 40H and 70H steel of the rods rolled according to the new technology standard samples for tension were made. In the course of mechanical tests there were determined the following mechanical properties for 40H and 70H: steels ultimate strength σ B, yield strength σ T, elongation δ, reduction of area ψ. Average value of mechanical properties obtained after statistical processing of the tests results are given in table 1. Figure 6 illustrates changing of mechanical properties depending on the metal rolling temperature. As it is seen from the graph (fig.6a) rising of rolling temperature reduces strength characteristics. For instance, rising rolling temperature from 950 to 1150 reduces ultimate strength σ 02 from 630 MPa to 530 MPa for 40H steel, and from 710 MPa to 630 MPa for 70H steel, and ultimate strength σ b from 955 MPa to 910 MPa for 40H steel, and from 1070 MPa to 980 MPa for 70H steel accordingly Table 1 Mechanical properties of 40H and 70H steel samples rolled according to the new technology Steel 40H 70H Rolling temperature, ºС σ 02, МПа σ В, МПа δ % ψ % , , ,0 54 Note: rolling rate is 0.4 m/s which provides rolling time in individual passes less than 1s. Ductility characteristics on the contrary tend to grow alongside rolling temperature rising (fig.6b). Elongation δ for 40H steel grows from 8% at rolling temperature 950 C to 14% at 1150 C, and for 70H steel from 5,6% at 950 C to 8% at 1150 C accordingly. Reduction of area Ψ grows for 40H and 70H steels from 35% and 30% accordingly at rolling temperature 950 C to 52% and 54% at 1150 C. The indexes of mechanical properties obtained in the laboratory experiments show that there are sufficient reserves of raising these qualities due to the increase of the number of passes at the enterprises having continuous multi-stand mills e.g at the small section mill of Arselor Mittal Temirtau JSC the total number of stands is 16 which provides 16 passes depending on the required final cross-section. However it should be mentioned that the use of the new rolling process realizing high-rate shear deformation the total number of passes will be less than in conventional rolling. Besedes, high rigidity of the stands of modern rolling mills allows to reduce the temperature of the end of rolling which will suppress the post deformation recrystallization processes causing grains to grow during phase transformation of the initial austenite which gives rise to the formation of finer grains and a high complex of mechanical properties. An extra reserve of raising the levels of final properties is the use of thermomechanical processing at the cost of the heat in the metal at the moment of completing plastic

7 deformation without subsequent thermal processing σ, МПа 70H 1000 σ в H H σ H а) 500 Т, ºС δ, ψ, % ψ 40H 40 70H δ 40H b) 70H 0 Т, ºС Fig.6. Mechanical metal properties depending on rolling temperature. a) Strength characteristics: σ 02 conventional yield strength, σ B ultimate strength; b ductility characteristics; δ elongation; ψ reduction of arca 5. METALLOGRAPHIC STRUCTURAL ANALYSES. In order to evaluate the effect of the technological rolling mode on the peculiarities of structure formation there were carried out metallographic investigations of samples. These investigations were made in accordance with STST and STST requirements using a metallographic microscope DM IRM of Leica company (Germany) and an electronic microscope JEOL (Japan). As a result of the metallographic investigation, there were found the microstructure and the fine electronic substructure of the rolled metal.

8 The microstructure of the initial ingot of 40H steel in the annealed state is given in figure 7a, b. Annealing of low alloyed steel is usually aimed at diminishing the effect of coarse casting defects on the results of structure formation during hot rolling. In complete annealing with phase recrystallisation there can usually be made the initial equilibrium structure with the transformation of cast needle structure into equiaxial one along with substantial reduction of the level of chemical liquation of steel elements and healing small inner discontinuities. Annealing was carried out during 3 hours and one hour soaking at 1150 c with subsequent air cooling. а) b) Fig.7 Microstructure of the initial ingot of 40H steel in the annealed state a) in optic DM IRM microscope (200x) b) in electronic JEOL microscope (1000x) In fig.7a the microstructure of 40H steel (200x) is shown. After annealing the microstructure consists of ferrite and pearlite with the mean grain size d m 35 mcm and round equiaxial form. In fig. 7b a fine electronic structure (1000x) is shown which was obtained in JEOL electronic microscope. The microstructure clearly shows the structures of pearlite colonies and the possibility of determining the sizes of the pearlite plates. The measurements showed that the ferrite and cementite plates in the initial pearlite has the mean size 0,5 mcm along the plate thickness pl and the plate length l pl 20 35mcm. Thus the sizes of initial pearlite grains are determined by the value not exceeding the maximum length l pl of pearlite, plates in the grain and is 35mcm. In this case the size of the initial pearlite grains corresponds to the maximum length of pearlite plates and is d m 35mcm. During heating before rolling there takes place the transformation of pearlite and ferrite into austenite, and the austenite grains are subjected to plastic deformation. The sizes of austenite grains formed during heating before rolling depend on the heating temperature which maximum value in this case is 1150 C, and also on the duration and soaking at this temperature. It is known that at steel heating temperature higher than 1200 C and extended soaking in this temperature range there takes place sudden grain coarsening because of overheating and the so called secondary recrystallization. As the assumed highest heating temperature before rolling is 1150 C which is lower critical overheating temperature, and soaking at this temperature does not exceed 30 min., the sizes of the initial obtained austenite will not exceed the sizes of the initial pearlite and ferrite grains. In the course of high-rate plastic deformation the form changing and austenite grain reduction take place as a result of the joint action of the main plastic deformation mechanisms: sliding, simple shear, turnings, rotation of fragments, which result in formation of new small grains and sub-grains with new boundaries of deformation origin. In order to judge the true sizes of the new formed grains at the moment of completing

9 deformation it is necessary to study the peculiarities of the structure of these new grains at the moment of completion of deformation in the last pass at the real end rolling temperature which is physically impossible to do in reality. Subsequent post deformation cooling accompanied by opposite phase transformation of the deformed austenite into pearlite ferrite phases reflect on the whole the peculiarities of the deformed austenite structure provided the final thickness of the bar leaving the last pass is not more than 30 mm. In this case collective statistic recrystallization does not have time to start and the newly formed pearlite and ferrite grains do not have time to grow to big sizes and on the whole inherit, though with a little tendency to growth, the sizes of the the deformed austenite formed at the moment of the plastic deformation completion. On fig. 8a a fine electronic structure (8000x) formed after rolling and subsequent cooling of the pearlite component of 40H steel microstructure rolled at C is shown. As it is seen from fig. 8a the pearlite plates formed after rolling and subsequent cooling have the following dimensions: plate thickness pl mcm, plate length l pl 1.4 mcm which testifies to the fact that high-rate plastic deformation took place in austenite during rolling. A similar fine electronic structure of the pearlite component of 40H steel after rolling at C and subsequent cooling is shown in fig. 8b. Here the form changing picture repeat in kind the peculiarities of the change which took place at C. The difference is in the dimensions of the deformed plates of the cementite component of pearlite plate thickness pl mcm and l pl 0.7 mcm. а) b) c) Fig. 8 Fine electronic structure of 40H steel depending on deformation temperature (8000x) a at C; b - at C; c - at C

10 A more sufficient change in the thin electronic structure took place after rolling at C and subsequent cooling (fig.8 c) Here one can see a sharper disorientation and breaking of pearlite plates in the space with some retardation of the change of the plate thickness and length ( pl 0.25 mcm, l pl 0.7 mcm). In this case on the finished bar, due to insufficient post-deformation static collective recrystallization, the plates manage to grow and there appear coarse pearlite grains of round form and one can clearly see the direction of macroshear strain which caused breaking and relative displacement of the fragments, forming new sub-boundaries and surfaces of fragments and blocks separation. For 70H steel similar pictures of the initial microstructure and fine electronic structure after rolling in various temperature conditions are given in figures 9 and 10. Steel 70H - structural low alloyed containing 0.7% carbon and % chromium in balance structure (in the annealed state) - is close to eutectoid one, i.e. it mainly consists of pearlite with insufficient quantity of independent ferrite phase (fig.9). In pearlite composition ferrite is present as an equally independent component like a mechanical mixture in the form of plates alternating with cementite plates with approximetely the same plate thickness pl 0.5 mcm and l pl 35 mcm with the morphology similar to 40H steel (fig.7a,b). Fine electronic structure of 70H steel after rolling at C, C and C and subsequent air cooling is shown in fig.10 a, b, c, accorolingly. Characteristic changes of this steel fine structure in the above said temperature intervals on the whole repeat in kind the peculiarities of such structures for 40H steel (fig. 8a,b,c). Insufficient difference consists in the sizes of pearlite plates and in almost complete absence of a separate ferrite structure phase, i.e. the microsection metallographic specimen is filled with the pearlite phase consisting of ferrite cementite mixture of various dispersion and orientation degree. Thus at deformation temperature C (fig.10a) there can be seen in the structure both small discontinuous, almost round forms of cementite plates broken into separate fine dispersion components, and more oblong but very thin scraps of these plates of various orientation which is the result of shears, turnings and breaking of the initial structure. In this case the plate thickness range is pl mcm and the plate length l pl mcm. Fig.9 Microstructure of the 70H steel ingot in the annealed state (200x)

11 а) b) c) Fig.10 Fine electronic structure of 70H steel depending on deformation temperature (8000x): a at C; b - at C; c - at C At the deformation temperatures C and C the plate dimensions are bigger, i.e. at C pl mcm, l pl mcm; and at C pl mcm, l pl 1 5 mcm. A higher level of ultimate strength and a low level of ductility of 70H steel in comparison with 40H steel are predetermined by a higher content of carbon in it. As it can be seen from the comparison of the dimensions of pearlite plates at various rolling temperatures smaller dimensions are formed at lower temperatures. 6. CONCLUSIONS In the course of the studies there were obtained the following results: 1) A new metal deforming process in lengthwise rolling with cross shear providing high-rate plastic deformation has been developed. 2) Investigation of the stressed and strained state of metal by methods of mathematic modeling of the new process shows the possibility of achieving sufficiently high level of the shear deformation rate in comparison with the conventional rolling technology which is the result of a more favourable scheme of the stressed state in the place of deformation, particularly a higher level of compession stress in the axial zone provides the removal of inner defects characteristic of this zone, and a more uniform strain distribution along the metal volume results in the increase of the product quality. 3) A new rolling technology using rolls of a new design with rhomb-square pass system has been tested.

12 4) The results of the study of mechanical properties and the analysis of the metal structure, rising the rate of plastic deformation, as a result of using the new technology provides the required quality of the product alongside cutting the total number of passes in comparison with the existing rolling technology at metallurgical enterprises. In its turn reduction of the number of passes results in reduction of the total number of operating rolls, in cutting total operating time which in the final analysis provides the increase of the mill efficiency and cuts the cost of finished products. LITERATURE [1] Segal V.M. Development of material processing by high-rate shear deformation. Metals, p [2] Segal V.M. Processes of metal forming by high-rate plastic deformation. - Metals, p [3] Naizabekov A.B. Scientific and technological bases of increasing the efficiency of forging processes in sign changing deformations. Almaty: Publishing house RPC on educational and methodical literature, p. [4] Akhmadeev N.A., Valiyev R.Z., Kopylov V.I., Mulukov R.R. Forming sub-micrograined structure in copper and nickel using highrate shear deformation // Metals, p [5] Naizabekov A.B., Ashkeev Zh.A., Toleuova A.R., Vorobiova N.N. Extrusion of billets in equichanneled step die // Technology of producing metals and by-products.- Temirtau, p [6] Pashinskaya E.G., Tolpa A.A. Possibilities of high-rate rolling with shear for forming ultra-fine-grained structure after the example of carbon eutectoid steel. // Metals, p [7] Smirnov V.K., Shilov A.V., Ignatovich Yu.V. Grooving of rolls. M.: Metallurgy, 1987,368 p. [8] Steel and alloys reference book/ V.G.Sorokin, A.V. Volosnikova, S.A.Vyatkin, et al. Edited by V.G.Sorokin. - M.: Machinebuilding, 1989, 640 p.