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Comparison of material flow and deformation resistance of HSLA steel deformed by hot rolling and by flat compression under simulated conditions
Journal of Mechanical Working Technology, 5 (1981) 267--280 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
267
COMPARISON OF MATERIAL FLOW AND DEFORMATION RESISTANCE OF HSLA STEEL DEFORMED BY HOT ROLLING AND BY FLAT COMPRESSION UNDER SIMULATED CONDITIONS
O. PAWELSKI
Max-Planck-Institut fiir Eisenforschung GmbH, Diisseldorf (West Germany) and V. GOPINATHAN*
Indian Institute of Technology, Madras-600036 (India) (Received April 21, 1980; accepted October 2, 1980)
Industrial Summary To arrive at an optimum rolling mill schedule several thousand experiments may have to be carried out; to accomplish this in industry under actual rolling conditions is not only time consuming but is also costly and measurements are difficult. Several laboratory simulation tests have thus been developed to tackle this problem, one c o m m o n example of which is the flat compression test. In an at t em p t to compare the results of rolling with those of the flat compression test the authors have carried out these tests under similar deformation conditions at different temperatures between 850 and 1100°C using Nb micro-alloyed steel NV4-4. The strain distribution, strain rate variation, temperature rise during deformation, deformation resistance variation and fibre flow have been analysed for both processes, and a comparison criterion has been proposed. Based on these results, it has been concluded that when the deformation resistance in rolling and that in flat compression are comparable, the two processes can be considered to be comparable. When this condition is satisfied all the mechanical properties and the microstructure are also comparable. It has also been concluded that the fiat compression test can be used as a powerful laboratory test for simulating the rolling process in order to evaluate the optimum rolling schedule.
1. Introduction The development of the optimum rolling schedule for the production of HSLA steel sheets calls for the carrying out of several thousand experiments in order to be able to determine the effects of variation of the major rolling parameters such as rolling temperature, number of passes, deformation in each pass, strain rate in each pass, cooling rate between passes and, at the end of rolling, austenitising temperature, etc. To do this in industry under actual *Present address: Central Metal Forming Institute, HMT Limited, Hyderabad-500 854, India. 0378-3804/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
268
rolling conditions is not only time consuming but is also costly and measurements are difficult.As a result, several laboratory simulation testshave been developed to tackle this problem, the four most commonly used of which are: cylindrical compression, tension, torsion and flat compression. Each test has its own relative merits and drawbacks, as describedelsewheze [1, 2], and the detailsof individual tests are also available in the literature [3--7]. From a study of these different tests,Pawelski [8] has shown that the flat compression test clearly meets most of the requirements of a rolling simulation test. However, whether or not the results obtained from the laboratory-simulated flat compression test are applicable in an actual rolling process, has not yet been clearly established. In an attempt to compare the results of rolling with those of the flat compression test, the authors carried out several experiments in rolling and in flat compression using a N b micro-alloyed steel. The hardness distribution, tensile properties, impact strength and austenite grain shape, size and distribution, all showed a very good agreement between the rolling and the flat compression test, under similar deformation conditions [9]. In the present paper the material flow and the deformation resistance of NV4-4 steel are reported for both the rolling and the flat compression test. A comparison criterion has been proposed and is supported by experimental results. 2. Criteria for comparison When two processes are to be compared, one must know clearly the basis of comparison. Figure 1 shows the different parameters which can be considered for comparison. When all the parameters mentioned in Fig. 1 are comparable for both o f the processes under consideration, t h a t will be the best conceivable criterion. However, this is very unlikety to occur in practice, which is why the question of a comparison criteria arises. Based on theoretical considerations Pawelski [10] has shown t h a t the deformation resistance of the material can be comparable for bar, sheet and wire rolling, forging and wire drawing. However, no report is available giving, explicitly, the criteria under which rolling and flat compression are comparable. Figure 2 illustrates the concepts behind the comparison of rolling and flat compression. The left half of the figure illustrates the material movement in the two processes under consideration, whilst the right half shows how the different passes in rolling can be considered to be the different consecutive deformation strokes in the flat compression test. In comparing these two processes, it must also be remembered t h a t the stress conditions existing in the two proceases are the ~ m e , viz. pure compression. It can also be seen how simple it would be to carry o u t a flat compression test in a laboratory, when compared with effecting rolling: herein lies the advantage of the flat compression test.
269
[ Hot rolling
I
simulation
AVAILABLE TESTS 1 12 3 /.,
CYLINDER COMPRESSION FLAT COMPRESSION H b e s t suited TENSION TORSION
I Ro,,ing I
[Flat compression test I coMPAR,SO. POSSIBLE PARAMETERS
Strain Strain rate Temperature Friction
Yield or proof stress Tensile strength Frac~ure elongation Fracture strength
Chemical composition Thermal treatment Cooling condition
/
Impact strength Hardness
CrJteriafor comparison
II
Fig. 1. Diagram showing the possible parameters for comparison between hot rolling and the flat compression test.
(o) R o i l i n g
(b) F[Clt compression
test
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270
3. Experimental procedure The experiments were carried out using Nb micro-alloyed steel NV4-4, the composition of which is given in Table 1. All the rolling experiments were carried out using a 2-high rolling mill with a roll diameter of 180.3 mm and a barrel length of 200 ram. Specimens 200 X 100 X 20 mm were cut from hotrolled sheets of 500 mm width and 2150 mm length. Before rolling, the specimens were austenitised either at 1226 or 1250 °C for I h in an argon atmosphere, cooled in still air until the required rolling temperature was reached and then rolled to the required deformation. After rolling, the specimens were either cooled in still air or quenched in water or cooled in a furnace maintained at 600°C depending upon the requirements. The specimen temperature was controlled during and after rolling by reference to a chromel-alumel, 2-mm-outer
TABLE 1 Chemical composition of the steel tested. Steel type
NV 4-4
Chemical composition (weight %) C
Si
Mn
P
S
Nb
N
0.14
0.22
1.43
0.021
0.001
0.04
0.015
4. Results and discussions
4.1 Strain distribution Using the screw technique the strain distribution in rolling and in flat compression was determined. For this, 3-ram-diameter screws were screwed into position at different locations, as shown in Fig. 4. The protruding screw length was cut off and ground flush with the specimen surface. This particular *Abbreviation of the German word "Warmumformsimulator".
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272
specimen with screws was rolled at 950°C to 28.6% deformation. After rolling, the specimen was c u t across the screw locations and polished. The deformed screws at different locations are shown in Fig. 5, from which it is observed that the farther the screws are from the longitudinal axis of the specimen, the greater is the bending or buckling, indicating the edge effect. It can also be seen that for all locations there is little variation in the deformation characteristics
Fig. 4. Photograph showing the screws (for studying the strain distribution in rolling) in position prior to rolling.
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273
of the screws in the rolling direction, indicating that approximately steady-state rolling conditions have been achieved at these screw locations. Using a digital light microscope the distances between the successive threads of the screws were measured on both the sides of the screws. Knowing the distance between successive threads of the undeformed screw (0.5 mm in this case) the localised strains across the specimen thickness were calculated at different locations. From the results, presented in ref. 9, it was shown that within the experimental scatter the thickness strain distribution across the thickness is constant over the entire width of the specimen, indicating a uniform deformation throughout the volume, neglecting the edge effects. A similar measurement on a flat compression test specimen yielded an entirely different strain distribution, as can be seen from Fig. 6. This result shows that at the centre of the specimen the strain is almost double the nominal strain. The results o f Lee and Kobayashi [12] found by finite-element-method calculations also support this observation.
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4.2 Temperature rise during deformation From the temperature curves recorded during rolling and the fiat compression test the temperature rise during deformation was noted and the results plotted for different deformation temperatures (Fig. 7). In both rolling and flat compression, with an increase in the initial test temperature the temperature rise decreases; this is expected, because at higher forming temperatures the resistance o f the material to deformation is less and thus less work is re-
274
quired, which gives rise to a lower increase in temperature. Added to this, the radiation losses at high temperatures will be more than at low temperatures, which also reduces the temperature rise at high forming temperatures. However, at any given deformation temperature, the rise in temperature observed in the fiat compression test is almost double that in rolling. It should be recalled that in both cases the temperatures were measured at the specimen centre. At any given temperature, other ambient conditions remaining the same, the rise in temperature is mainly a function o f strain: the higher the strain the more will be the temperature rise and vice versa. It was noted (in Section 4.1) that in rolling there is no change in strain, either across the thickness or the width, whereas in the case of the fiat compression test the localised strain at the centre -- where the temperature is measured -- is almost double the nominal strain. This is the reason why there is almost double the temperature rise for the fiat compression test than there is for rolling. It will be shown subsequently that provided the nominal strains are constant for both processes, these variations in localised strains do not significantly alter the flow behaviour of the metal. Hence, it can be stated that strain and
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275
temperature rise during deformation cannot serve as a parameter for comparison of the two processes. 4.3 Strain rate variation At hot working temperature ranges, strain rate plays a very important role in deciding the f l o w behaviour of metals. Unfortunately, both in rolling as well as in flat compression, the strain rate does not remain constant during the deformation process. The variation of strain rate has been theoretically calculated for both processes and is plotted in Fig. 8 as a function of a dimensionless parameter $, which is defined as the ratio of the instantaneous strain e to the final strain e~. The value of ~ varies between 0 at the beginning of deformation to 1 at the end of deformation. These curves are presented for a mean strain rate of 60 s -~, for different end-strains of from 0.1 to 0.7. At l o w end-strains, the strain rate decreases continuously with ~ for both rolling and the flat compression test. At end-strains above 0.25 the strain rate increases initially with increase in strain ratio ~, reaches a maximum value, and thereafter decreases rapidly. In the case of the flat compression test a similar ~rend is also observed. In both cases the point at which the maximum strain rate is observed is shifted to higher ~ values with an increase in end-strain e~. Similar strain rate variation curves for a lower mean strain rate of 10 s -~ are shown in Fig. 9. In the case of rolling, the strain rate curves for different end,strains are similar to those described earlier for high mean strain rates. On the
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other hand, for the fiat compression test the curves at low mean strain rates even for end~trains as high as 0.6 -- exhibit a continuous increase in strain rate with ~ until the end of the stroke. In the case of the fiat compression test the strain rate is directly proportional to the contact velocity of the punch and indirectly proportional to the instantaneous height of the specimen. This means t h a t for the same initial thickness o f the specimen the contact velocity of the punch has to be increased in order to achieve higher strain rates. As the contact velocity of the punch is increased the elastic deformation of the press and the associated tools is also increased, particularly at high strains, and also the overshoot due to inertia of the punch is greater. In order to compensate for this effect, the ram m o v e m e n t has to be stopped a little before the theoretical end of the stroke and from this point onwards the strain rate starts to decrease. At large mean strain rates the contact velocity o f the punch is so large that the arrest of the ram has to be effected well in advance and hence a strain rate variation similar to the one shown in Fig. 8 is obtained. On the other hand, in the case of low mean strain rates the contact velocity of the punch is comparatively low and hence the ine~ial overshoot is also very low. Correspondingly, the ram has to be stopped only a short while before the theoretical end of the stroke and hence the strain rate variation as shown in Fig. 9 is obtained. These results show t h a t the strain rate variation during deformation in rolling and in the fiat compression test are comparable only at very high mean strain rates and high strains; at low mean strain rates there is poor agreement. However, it should be borne in mind t h a t these variations in strain rates between the two processes do n o t contribute significantly in altering the flow behaviour
277
of the material, as will be shown in Section 4.4. Hence, for comparison of the two processes the influence of strain rate variation need n o t be considered.
4.4 Resistance to deformation From the deformed specimens the actual contact areas were calculated as indicated in Fig. 10. From the force--travel diagrams recorded, the average force was calculated, and then divided by the contact area to give the deformation resistance. The latter was calculated for both rolling and the flat compression test at different temperatures and the results given in Fig. 11, from which it can be seen that for both processes an increase in test temperature causes a decrease in the deformation resistance, as would be expected. The scatter band is n o t very broad and the deformation resistance for rolling agrees very well with that for fiat compression. The above result suggests that the deformation resistance -- which is a function of strain, strain rate, temperature and microstructure -- can be conveniently employed as a parameter for comparing rolling with flat compression. In other words, it can be stated that when the deformation resistance in rolling and that in the flat compression test are comparable then the two processes can be considered to be comparable. The microstructures and mechanical properties reported in ref. 9 support the above statement.
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278
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4.5 Fibre flow The theoretical slip-line solution for plane-strain deformation has been determined by many investigators. Typical solutions, along with experimental results, are reproduced in Fig. 12, from which it can be seen that provided that the ratio of the punch-width to the specimen-height in the flat compression test is equal to the ratio of the contact-length to the mean height of the specimen in rolling, the flow figures obtained by etching in both cases are comparable. The material flow in the flat compression test and in rolling for three different deformation temperatures is compared in Fig. 13. A study of these macrostructures indicates similarity in material flow between rolling and flat compression. Because the rolling process was interrupted, the actual lengths of contact seen in these photographs are larger than the theoretically calculated values. A n approximate slip-linenetwork constructed for these geometries for both rolling and the flat compression test are superimposed on the macrostructures of Fig. 13. Even though these slip-linenets are not exact, particularly for rolling, they strongly resemble the network proposed by Ford and Alexander [13]. Based on these results it can be seen that the material flow in rolling is similar to that in the flat compression test.
279
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Fig. 12. Comparison of the slip-line fields in rolling and in the flat compression test.
Fig. 13. Comparison of fibre flow in rolling and in the flat compression test, at three differ. ent forming temperatures.
280
5. Conclusions The following conclusions can be drawn: (1) Under steady-state conditions there is a uniform thickness strain distrib u t i o n over t h e entire volume o f the rolled specimen, neglecting edge effects, w h e m s s i n the ease of the fiat compression test the strain is not uniform and the m a x ~ u m strain - - o b s e r v e d at t h e centre of the specimen-- is almost double the nominal strain. (2) The strain rate variations in rolling and in the fiat compression test are comparable only at very high mean strain rates and high strains. (3) The temperature rise during deformation decreases with deformation t e m p e r a t u r e in a similar m a n n e r for rolling and the flat compression test. However, the absolute value o f the temperature rise is dependent u p o n the measurement p o i n t i n t h e fiat compression test b u t is independent of location in rolling. (4) The resistance t o deformation decreases with increase in deformation temperature in a similar manner for rolling a n d the flat compression test, ind i c a t i n g t h a t this c a n be used as a parameter for comparing the t w o processes, (5) The fibre lines in rolling and in the flat compression test seem to be comparable under similar deformation conditions. (6) Based on these results, it can be stated that when the deformation resistance in rolling and that in the flat compression test are comparable then the t w o processes can be considered comparable. When this condition is satisfied all the mechanical properties and the microstructure are also comparable [9]. (7) The flat compression test can be used as a powerful laboratory test for simulating the rolling process in order to evaluate the o p t i m u m rolling schedule.
References O. Pawelski, Z, Metallkd., 68 (2) (1977) 79--89, D. Bauer, M ~ , , 32 (8) (1978) 776--781.
8 9 10 11 12 13
S. Fulop, K.C. Cadien, M.J, Luton and H.J. M e ~ e e n , J. Testing and Evaluation, 5 (6) (1977) 419: W. V a n ~ and H. ~ n k l e r , Ber~. Hfittenwes,, (9) (1977~397, nt. ~ h : T o o t ~ Res~ Conf:, M~nchester, pp. 247--253. S ~ I Eis~m, 98 (1978) 181--189. O. Pawelski and V. Gopinathan, J. Mech. Work. Tech., 5 (1981) 45--68. O. Pawelski, Stahl Eisen, 83 (1963) 1440--1451. V. Gopinathan, Research report on "A comparative study of hot roiling with fiat compression test", Dept. Metal Forming, MPI, Diisseldorf, May 1980. C.H. Lee and Shiro Kobayashi, J. Mech. Sci., 12 (1970) 349--370. H. Ford and J. Alexander, Advanced mechanics of materials, Wiley, New York, 1963.
267
COMPARISON OF MATERIAL FLOW AND DEFORMATION RESISTANCE OF HSLA STEEL DEFORMED BY HOT ROLLING AND BY FLAT COMPRESSION UNDER SIMULATED CONDITIONS
O. PAWELSKI
Max-Planck-Institut fiir Eisenforschung GmbH, Diisseldorf (West Germany) and V. GOPINATHAN*
Indian Institute of Technology, Madras-600036 (India) (Received April 21, 1980; accepted October 2, 1980)
Industrial Summary To arrive at an optimum rolling mill schedule several thousand experiments may have to be carried out; to accomplish this in industry under actual rolling conditions is not only time consuming but is also costly and measurements are difficult. Several laboratory simulation tests have thus been developed to tackle this problem, one c o m m o n example of which is the flat compression test. In an at t em p t to compare the results of rolling with those of the flat compression test the authors have carried out these tests under similar deformation conditions at different temperatures between 850 and 1100°C using Nb micro-alloyed steel NV4-4. The strain distribution, strain rate variation, temperature rise during deformation, deformation resistance variation and fibre flow have been analysed for both processes, and a comparison criterion has been proposed. Based on these results, it has been concluded that when the deformation resistance in rolling and that in flat compression are comparable, the two processes can be considered to be comparable. When this condition is satisfied all the mechanical properties and the microstructure are also comparable. It has also been concluded that the fiat compression test can be used as a powerful laboratory test for simulating the rolling process in order to evaluate the optimum rolling schedule.
1. Introduction The development of the optimum rolling schedule for the production of HSLA steel sheets calls for the carrying out of several thousand experiments in order to be able to determine the effects of variation of the major rolling parameters such as rolling temperature, number of passes, deformation in each pass, strain rate in each pass, cooling rate between passes and, at the end of rolling, austenitising temperature, etc. To do this in industry under actual *Present address: Central Metal Forming Institute, HMT Limited, Hyderabad-500 854, India. 0378-3804/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
268
rolling conditions is not only time consuming but is also costly and measurements are difficult.As a result, several laboratory simulation testshave been developed to tackle this problem, the four most commonly used of which are: cylindrical compression, tension, torsion and flat compression. Each test has its own relative merits and drawbacks, as describedelsewheze [1, 2], and the detailsof individual tests are also available in the literature [3--7]. From a study of these different tests,Pawelski [8] has shown that the flat compression test clearly meets most of the requirements of a rolling simulation test. However, whether or not the results obtained from the laboratory-simulated flat compression test are applicable in an actual rolling process, has not yet been clearly established. In an attempt to compare the results of rolling with those of the flat compression test, the authors carried out several experiments in rolling and in flat compression using a N b micro-alloyed steel. The hardness distribution, tensile properties, impact strength and austenite grain shape, size and distribution, all showed a very good agreement between the rolling and the flat compression test, under similar deformation conditions [9]. In the present paper the material flow and the deformation resistance of NV4-4 steel are reported for both the rolling and the flat compression test. A comparison criterion has been proposed and is supported by experimental results. 2. Criteria for comparison When two processes are to be compared, one must know clearly the basis of comparison. Figure 1 shows the different parameters which can be considered for comparison. When all the parameters mentioned in Fig. 1 are comparable for both o f the processes under consideration, t h a t will be the best conceivable criterion. However, this is very unlikety to occur in practice, which is why the question of a comparison criteria arises. Based on theoretical considerations Pawelski [10] has shown t h a t the deformation resistance of the material can be comparable for bar, sheet and wire rolling, forging and wire drawing. However, no report is available giving, explicitly, the criteria under which rolling and flat compression are comparable. Figure 2 illustrates the concepts behind the comparison of rolling and flat compression. The left half of the figure illustrates the material movement in the two processes under consideration, whilst the right half shows how the different passes in rolling can be considered to be the different consecutive deformation strokes in the flat compression test. In comparing these two processes, it must also be remembered t h a t the stress conditions existing in the two proceases are the ~ m e , viz. pure compression. It can also be seen how simple it would be to carry o u t a flat compression test in a laboratory, when compared with effecting rolling: herein lies the advantage of the flat compression test.
269
[ Hot rolling
I
simulation
AVAILABLE TESTS 1 12 3 /.,
CYLINDER COMPRESSION FLAT COMPRESSION H b e s t suited TENSION TORSION
I Ro,,ing I
[Flat compression test I coMPAR,SO. POSSIBLE PARAMETERS
Strain Strain rate Temperature Friction
Yield or proof stress Tensile strength Frac~ure elongation Fracture strength
Chemical composition Thermal treatment Cooling condition
/
Impact strength Hardness
CrJteriafor comparison
II
Fig. 1. Diagram showing the possible parameters for comparison between hot rolling and the flat compression test.
(o) R o i l i n g
(b) F[Clt compression
test
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Fig. 2. Simulation of rolling deformation by the flat compression test.
3Pass
etc
270
3. Experimental procedure The experiments were carried out using Nb micro-alloyed steel NV4-4, the composition of which is given in Table 1. All the rolling experiments were carried out using a 2-high rolling mill with a roll diameter of 180.3 mm and a barrel length of 200 ram. Specimens 200 X 100 X 20 mm were cut from hotrolled sheets of 500 mm width and 2150 mm length. Before rolling, the specimens were austenitised either at 1226 or 1250 °C for I h in an argon atmosphere, cooled in still air until the required rolling temperature was reached and then rolled to the required deformation. After rolling, the specimens were either cooled in still air or quenched in water or cooled in a furnace maintained at 600°C depending upon the requirements. The specimen temperature was controlled during and after rolling by reference to a chromel-alumel, 2-mm-outer
TABLE 1 Chemical composition of the steel tested. Steel type
NV 4-4
Chemical composition (weight %) C
Si
Mn
P
S
Nb
N
0.14
0.22
1.43
0.021
0.001
0.04
0.015
4. Results and discussions
4.1 Strain distribution Using the screw technique the strain distribution in rolling and in flat compression was determined. For this, 3-ram-diameter screws were screwed into position at different locations, as shown in Fig. 4. The protruding screw length was cut off and ground flush with the specimen surface. This particular *Abbreviation of the German word "Warmumformsimulator".
271
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Fig. 3. Geometry of rollingand fiatcompression test specimens.
in
272
specimen with screws was rolled at 950°C to 28.6% deformation. After rolling, the specimen was c u t across the screw locations and polished. The deformed screws at different locations are shown in Fig. 5, from which it is observed that the farther the screws are from the longitudinal axis of the specimen, the greater is the bending or buckling, indicating the edge effect. It can also be seen that for all locations there is little variation in the deformation characteristics
Fig. 4. Photograph showing the screws (for studying the strain distribution in rolling) in position prior to rolling.
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Fig. 5. Photographs showing the deformed screws and their location in the rolling specimen.
273
of the screws in the rolling direction, indicating that approximately steady-state rolling conditions have been achieved at these screw locations. Using a digital light microscope the distances between the successive threads of the screws were measured on both the sides of the screws. Knowing the distance between successive threads of the undeformed screw (0.5 mm in this case) the localised strains across the specimen thickness were calculated at different locations. From the results, presented in ref. 9, it was shown that within the experimental scatter the thickness strain distribution across the thickness is constant over the entire width of the specimen, indicating a uniform deformation throughout the volume, neglecting the edge effects. A similar measurement on a flat compression test specimen yielded an entirely different strain distribution, as can be seen from Fig. 6. This result shows that at the centre of the specimen the strain is almost double the nominal strain. The results o f Lee and Kobayashi [12] found by finite-element-method calculations also support this observation.
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4.2 Temperature rise during deformation From the temperature curves recorded during rolling and the fiat compression test the temperature rise during deformation was noted and the results plotted for different deformation temperatures (Fig. 7). In both rolling and flat compression, with an increase in the initial test temperature the temperature rise decreases; this is expected, because at higher forming temperatures the resistance o f the material to deformation is less and thus less work is re-
274
quired, which gives rise to a lower increase in temperature. Added to this, the radiation losses at high temperatures will be more than at low temperatures, which also reduces the temperature rise at high forming temperatures. However, at any given deformation temperature, the rise in temperature observed in the fiat compression test is almost double that in rolling. It should be recalled that in both cases the temperatures were measured at the specimen centre. At any given temperature, other ambient conditions remaining the same, the rise in temperature is mainly a function o f strain: the higher the strain the more will be the temperature rise and vice versa. It was noted (in Section 4.1) that in rolling there is no change in strain, either across the thickness or the width, whereas in the case of the fiat compression test the localised strain at the centre -- where the temperature is measured -- is almost double the nominal strain. This is the reason why there is almost double the temperature rise for the fiat compression test than there is for rolling. It will be shown subsequently that provided the nominal strains are constant for both processes, these variations in localised strains do not significantly alter the flow behaviour of the metal. Hence, it can be stated that strain and
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Fig. 7. Comparison of temperature rise during deformation in rolling and in the flat compression test, at dlfferent defom~ation temperatures.
275
temperature rise during deformation cannot serve as a parameter for comparison of the two processes. 4.3 Strain rate variation At hot working temperature ranges, strain rate plays a very important role in deciding the f l o w behaviour of metals. Unfortunately, both in rolling as well as in flat compression, the strain rate does not remain constant during the deformation process. The variation of strain rate has been theoretically calculated for both processes and is plotted in Fig. 8 as a function of a dimensionless parameter $, which is defined as the ratio of the instantaneous strain e to the final strain e~. The value of ~ varies between 0 at the beginning of deformation to 1 at the end of deformation. These curves are presented for a mean strain rate of 60 s -~, for different end-strains of from 0.1 to 0.7. At l o w end-strains, the strain rate decreases continuously with ~ for both rolling and the flat compression test. At end-strains above 0.25 the strain rate increases initially with increase in strain ratio ~, reaches a maximum value, and thereafter decreases rapidly. In the case of the flat compression test a similar ~rend is also observed. In both cases the point at which the maximum strain rate is observed is shifted to higher ~ values with an increase in end-strain e~. Similar strain rate variation curves for a lower mean strain rate of 10 s -~ are shown in Fig. 9. In the case of rolling, the strain rate curves for different end,strains are similar to those described earlier for high mean strain rates. On the
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other hand, for the fiat compression test the curves at low mean strain rates even for end~trains as high as 0.6 -- exhibit a continuous increase in strain rate with ~ until the end of the stroke. In the case of the fiat compression test the strain rate is directly proportional to the contact velocity of the punch and indirectly proportional to the instantaneous height of the specimen. This means t h a t for the same initial thickness o f the specimen the contact velocity of the punch has to be increased in order to achieve higher strain rates. As the contact velocity of the punch is increased the elastic deformation of the press and the associated tools is also increased, particularly at high strains, and also the overshoot due to inertia of the punch is greater. In order to compensate for this effect, the ram m o v e m e n t has to be stopped a little before the theoretical end of the stroke and from this point onwards the strain rate starts to decrease. At large mean strain rates the contact velocity o f the punch is so large that the arrest of the ram has to be effected well in advance and hence a strain rate variation similar to the one shown in Fig. 8 is obtained. On the other hand, in the case of low mean strain rates the contact velocity of the punch is comparatively low and hence the ine~ial overshoot is also very low. Correspondingly, the ram has to be stopped only a short while before the theoretical end of the stroke and hence the strain rate variation as shown in Fig. 9 is obtained. These results show t h a t the strain rate variation during deformation in rolling and in the fiat compression test are comparable only at very high mean strain rates and high strains; at low mean strain rates there is poor agreement. However, it should be borne in mind t h a t these variations in strain rates between the two processes do n o t contribute significantly in altering the flow behaviour
277
of the material, as will be shown in Section 4.4. Hence, for comparison of the two processes the influence of strain rate variation need n o t be considered.
4.4 Resistance to deformation From the deformed specimens the actual contact areas were calculated as indicated in Fig. 10. From the force--travel diagrams recorded, the average force was calculated, and then divided by the contact area to give the deformation resistance. The latter was calculated for both rolling and the flat compression test at different temperatures and the results given in Fig. 11, from which it can be seen that for both processes an increase in test temperature causes a decrease in the deformation resistance, as would be expected. The scatter band is n o t very broad and the deformation resistance for rolling agrees very well with that for fiat compression. The above result suggests that the deformation resistance -- which is a function of strain, strain rate, temperature and microstructure -- can be conveniently employed as a parameter for comparing rolling with flat compression. In other words, it can be stated that when the deformation resistance in rolling and that in the flat compression test are comparable then the two processes can be considered to be comparable. The microstructures and mechanical properties reported in ref. 9 support the above statement.
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278
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4.5 Fibre flow The theoretical slip-line solution for plane-strain deformation has been determined by many investigators. Typical solutions, along with experimental results, are reproduced in Fig. 12, from which it can be seen that provided that the ratio of the punch-width to the specimen-height in the flat compression test is equal to the ratio of the contact-length to the mean height of the specimen in rolling, the flow figures obtained by etching in both cases are comparable. The material flow in the flat compression test and in rolling for three different deformation temperatures is compared in Fig. 13. A study of these macrostructures indicates similarity in material flow between rolling and flat compression. Because the rolling process was interrupted, the actual lengths of contact seen in these photographs are larger than the theoretically calculated values. A n approximate slip-linenetwork constructed for these geometries for both rolling and the flat compression test are superimposed on the macrostructures of Fig. 13. Even though these slip-linenets are not exact, particularly for rolling, they strongly resemble the network proposed by Ford and Alexander [13]. Based on these results it can be seen that the material flow in rolling is similar to that in the flat compression test.
279
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Fig. 12. Comparison of the slip-line fields in rolling and in the flat compression test.
Fig. 13. Comparison of fibre flow in rolling and in the flat compression test, at three differ. ent forming temperatures.
280
5. Conclusions The following conclusions can be drawn: (1) Under steady-state conditions there is a uniform thickness strain distrib u t i o n over t h e entire volume o f the rolled specimen, neglecting edge effects, w h e m s s i n the ease of the fiat compression test the strain is not uniform and the m a x ~ u m strain - - o b s e r v e d at t h e centre of the specimen-- is almost double the nominal strain. (2) The strain rate variations in rolling and in the fiat compression test are comparable only at very high mean strain rates and high strains. (3) The temperature rise during deformation decreases with deformation t e m p e r a t u r e in a similar m a n n e r for rolling and the flat compression test. However, the absolute value o f the temperature rise is dependent u p o n the measurement p o i n t i n t h e fiat compression test b u t is independent of location in rolling. (4) The resistance t o deformation decreases with increase in deformation temperature in a similar manner for rolling a n d the flat compression test, ind i c a t i n g t h a t this c a n be used as a parameter for comparing the t w o processes, (5) The fibre lines in rolling and in the flat compression test seem to be comparable under similar deformation conditions. (6) Based on these results, it can be stated that when the deformation resistance in rolling and that in the flat compression test are comparable then the t w o processes can be considered comparable. When this condition is satisfied all the mechanical properties and the microstructure are also comparable [9]. (7) The flat compression test can be used as a powerful laboratory test for simulating the rolling process in order to evaluate the o p t i m u m rolling schedule.
References O. Pawelski, Z, Metallkd., 68 (2) (1977) 79--89, D. Bauer, M ~ , , 32 (8) (1978) 776--781.
8 9 10 11 12 13
S. Fulop, K.C. Cadien, M.J, Luton and H.J. M e ~ e e n , J. Testing and Evaluation, 5 (6) (1977) 419: W. V a n ~ and H. ~ n k l e r , Ber~. Hfittenwes,, (9) (1977~397, nt. ~ h : T o o t ~ Res~ Conf:, M~nchester, pp. 247--253. S ~ I Eis~m, 98 (1978) 181--189. O. Pawelski and V. Gopinathan, J. Mech. Work. Tech., 5 (1981) 45--68. O. Pawelski, Stahl Eisen, 83 (1963) 1440--1451. V. Gopinathan, Research report on "A comparative study of hot roiling with fiat compression test", Dept. Metal Forming, MPI, Diisseldorf, May 1980. C.H. Lee and Shiro Kobayashi, J. Mech. Sci., 12 (1970) 349--370. H. Ford and J. Alexander, Advanced mechanics of materials, Wiley, New York, 1963.