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Measurement of austenite-to-ferrite transformation temperature after multi-pass deformation of steels
MATERIALS SCIENCE & ENGINEERING ELSEVIER
Materials Science and Engineering, A194 (1995) L15-L18
A
Letter
Measurement of austenite-to-ferrite transformation temperature after multi-pass deformation of steels Xiaodong Liu, Jan Ketil Solberg, Ragnar Gjengedal Department of Metallurgy, NTH, N-7034 Trondheim, Norway Received 3 June 1994; in revised form 13 October 1994
Abstract The austenite-to-ferrite transformation temperature (Ar3) under multi-pass deformation has been studied using the continuous hot torsion testing method by monitoring the variation in the stress with temperature during the cooling period. The experimental results show that the effect of deformation on the At3 temperature decreases rapidly with decreasing cooling rate. This may be due to static recovery taking place within the deformed austenite during the cooling period, effectively reducing the nucleation of ferrite.
Keywords: Austenite; Ferrite; Deformation; Steel
1. Introduction It is well known that the mechanical properties of hot-rolled steel products can be improved by controlling the subsequent cooling rate after hot rolling. Hence, knowledge of the phase transformation of austenite is of great industrial importance. Continuous cooling transformation diagrams offer preliminary guidance for the selection of cooling rates for desired microstructures, and over the years such diagrams have been the subject of vigorous investigations applying various experimental techniques, such as thermal analysis, dilatometry, metallography etc. [1-3]. None of these techniques is capable of dealing with the continuous cooling transformation of steels as far as hot rolling is concerned. Recently, it has been reported [4] that a continuous cooling, continuous deformation test has been developed in a materials testing system to determine the transformation characteristics. However, until now the work has been limited to the simulation of continuous cooling transformation without multi-pass deformation. In this paper, a continuous hot torsion testing method, based on the continuous cooling,
continuous deformation test [4], is used to study the characteristics of continuous cooling transformation after simulated multi-pass deformation.
2. Experimental procedure The composition of the niobium-bearing steel studied is as follows (wt.%): C-~0.10, Mn~-1.32, Si ~ 0.30, Nb -~ 0.028, A1 = 0.042, N -- 0.008, P= 0.018, S-~ 0.015; Fe, balance. The tests were performed in a computerized hot torsion machine (see Ref. [5] for details). Solid specimens, 8 mm in gauge diameter and 5 mm in gauge length, were machined from the as-received steel plates. The specimens were first reheated to 1200 °C for 10 rain, and then cooled to the upper end of the selected temperature range (1150-910 °C) for 13-pass deformation. The processing parameters were chosen as: pass strain (axial strain in torsion test), 0.3; strain rate, 1 s- 1; interpass time, 20 s; pass interval, 20 °C. At the end of each pass, the stress was immediately decreased to a low, but nonzero, level. After multi-pass deformation, the speci-
0921-5093/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSD1 0921-5093(94)02717-X
LI6
Letter
/
Materials Science and Engineering A194 (1995) LI5-LI8
mens were continuously deformed at a strain rate of 5 x 10 -5 s-1 and simultaneously cooled at different cooling rates between 0.26 and 5 °C s 1. T h e stress was monitored during the cooling period and, as soon as the phase transformation took place, there was a decrease in the stress due to the softening from the ferrite transformation. T h e same test was also performed without deformation. A verification of the austenite-to-ferrite transformation temperature (Ar3) measured by this method was made by quenching the specimens just before and just after the stress decrease for metallographic examination.
To verify the measured A r 3 temperature, the cooling process of some specimens was interrupted by rapid quenching at a temperature just above or just below the measured A r 3 temperature for metallographic observation. T h e microstructures obtained are shown in Fig. 3.
~l
~
Coolingrate(1*C/s)
301
2 0 ~ ~
( 740,0}
3. R e s u l t s and d i s c u s s i o n
T h e flow stress vs. temperature curve during and after 13-pass deformation is shown in Fig. 1. During the cooling period (from 910 to 6 5 0 ° C at a cooling rate of 1 °C s-J), the flow stress first decreased with decreasing temperature due to stress relaxation, and then with continuous deformation at a strain rate of 5 x 1 0 - 5 s- ], the flow stress increased almost linearly with a further decrease in temperature. At about 760 °C, there was an obvious decrease in the stress, and with a further decrease in temperature the stress decreased rapidly. T h e same test was also performed without deformation as illustrated in Fig. 2. In order to hold the specimen tightly in the beginning, a small amount of deformation ( e = 0.1 ) at a strain rate of 1 s- 1 was employed at a temperature of 1150°C. It is reasonable to assume that this strain did not affect the subsequent phase transformation. T h e decrease in stress observed in Fig. 1 is also present in Fig. 2, but is located at a lower temperature, i.e. at 740 °C. T h e temperature corresponding to the beginning of the stress decrease is defined as the austenite-to-ferrite transformation temperature (Ar3). T h e principle of the variation of the flow stress with temperature during the cooling period is the same as that reported in Ref. [4].
650
7o0
750
800
8so
900
95o
looo 1050 1100
Temperature,*C Fig. 2. Flow stress vs. temperature during cooling without previous deformation.
160 140 Coolingrate(1*C/s) 120
@. 100 80 ¢/I
60 40
Ar3 (760"C)
20
600 650 700 750 800 850 900 950 1000105011001150 Temperature,qC
Fig. 1. Flow stress vs. temperature during and after 13-pass deformation.
Fig. 3. Microstructures obtained by quenching just above and just below the Ar 3 temperature after 13-pass deformation: (a) above At3; (b) below Ar 3.
Letter
/
Muterials Science and Engineering A194 (1995) L1_5L18
Above the measured Ar, temperature, the microstructure is completely austenitic as can be seen from the completely martensitic microstructure in Fig. 3(a). A small amount of ferrite is observed to have nucleated mainly at the grain boundaries of the deformed austenite in Fig. 3(b), which shows the specimen quenched just below the measured Ar, temperature. Thus torsion testing was found to be a very effective method for measuring the continuous cooling transformation temperature under the conditions prevailing during the thermomechanical treatment of steels. By comparison of Figs. 1 and 2, it is also seen that, during the phase transformation, the stress decreases more rapidly when the austenite is deformed than when it is not. This means that deformation not only enhances the At-, temperature, but also speeds up the transformation process. This is mainly due to the fact that, compared with undeformed austenite, characterized by a large grain size of 156 pm [6], high-temperature 13-pass deformation not only greatly refines the austenite grain size (27 pm) [6] through complete recrystallization at temperatures above the non-recrystallization temperature T,,, (for this test measured to be 9.50 “C [7]), but also increases the accumulated strain within the unrecrystallized austenite at temperatures below T,,. All of these factors significantly accelerate the austenite-to-ferrite transformation, so that hightemperature deformation leads to an increase in both the Ar, temperature and the transformation rate. Fig. 4 shows the experimentally measured Ar, temperature as a function of the cooling rate under conditions of deformation and non-deformation. The relation between the Ar, temperature and the cooling rate was found to obey the following exponential function TAr,= TA,expl- (a l’
)“I
(1)
where a (with units of s “C-r) is 6.03 x 10 7 and 4.58 X lo-” and /3 is 0.162 and 0.210 for 13-pass
7001 0
’ 1
’ 2
I 3
4
’ 5
Fig. 4. Experimentally rate.
measured
No de$ymed
Ar,
temperature
deformation and non-deformation respectively, For this steel, TAI was calculated to be 848 “C using an empirical equation from ref. [S]. In both cases, the At-, temperature increases rapidly with decreasing cooling rate. It can also be seen that the influence of deformation on the Ar, temperature decreases with decreasing cooling rate. According to classical nucleation theory, the diffusion-dependent ferrite transformation usually starts preferentially at high-energy sites, such as grain boundaries, deformation bands, etc. Static recovery, which can reduce the level of accumulated strain within the heavily deformed austenite, is found to be the major physical event taking place during cooling, and the degree of static recovery is also dependent on the cooling rate. With a decrease in the cooling rate, the recovery of the dislocation structure is more likely to occur initially within the high-energy sites, such as heavily deformed grain boundaries, deformation bands, etc. These are the sites where the initial nucleation of ferrite is most likely to take place; therefore the potential for ferrite nucleation is strongly decreased. 4. Conclusions
(1) The
austenite-to-ferrite transformation (Ar3) temperature on multi-pass deformation can be measured using the continuous hot torsion testing method by monitoring the variation in the stress with temperature during the cooling period. The relation between the Ar, temperature and the (2) cooling rate for conditions of deformation and no deformation is found to obey the following exponential function
L, = TA,expl - (QI/ )"I
(2)
where a (with units of s”C-‘) is 6.03 X lo-’ and 4.58 X lo-’ and p is 0.162 and 0.210 for 13-pass deformation and non-deformation respectively. In both cases, the Ar, temperature increases rapidly with decreasing cooling rate, and the influence of deformation on the Ar, temperature decreases with decreasing cooling rate. These observations can be explained by static recovery taking place within the deformed austenite during the cooling period, effectively eliminating the high-energy sites for ferrite nucleation.
Acknowledgements
J 6
Cooling rate, ‘C/s Defgmd
L17
vs. cooling
The authors wish to thank Fundia Norsk Jernverk AS for financial support. One of the authors (X.D. Liu) also thanks the Research Council of Norway for an award of a research fellowship.
L18
Letter
/
Materials Science and Engineering A194 (1995) L I 5 - L I 8
References [1] H. Yada, J. Tominaga, K.Y. Wakimoto and N. Matsuzu, in P.D. Southwiek (ed.), lnt. Conf. on Accelerated Cooling of Steel, TMS-AIME, Warrendale, PA, 1986, p. 55. [2] J.J.M. Too, in J.H. Beynon (ed.), Int. Conf. on Modelling of Metal Rolling Processes, The Institute of Metals, London, 1993, p. 343. [3] A. Hurkmans, G.A. Duit, T.M. Hoogendoorn, F. Hollander and H. Ijmuiden, in ED. Southwick (ed.), Int. Conf. on
[4] [51 [6] [7] [8]
Accelerated Cooling of Steel, TMS-AIME, Warrendale, PA, 1986, p. 481. A.Z. Hanzaki, R. Pandi, P.D. Hodgson and S. Yue, Metall. Trans. A, 24 (1993) 2657. J.K. Solberg and A.H. Sunde, Prakt. Met., 24 (1987) 468. X.D. Liu, J.K. Solberg, R. Gjengedal and A.O. Kluken, J. Mater. Process. Technol., 45 (1994) 497. X.D. Liu, J.K. Solberg, R. Gjengedal and A.O. Kluken, Mater. Sci. Technol., in press. K.W. Andrews, J. Iron Steellnst., 203 (1965) 721.
Materials Science and Engineering, A194 (1995) L15-L18
A
Letter
Measurement of austenite-to-ferrite transformation temperature after multi-pass deformation of steels Xiaodong Liu, Jan Ketil Solberg, Ragnar Gjengedal Department of Metallurgy, NTH, N-7034 Trondheim, Norway Received 3 June 1994; in revised form 13 October 1994
Abstract The austenite-to-ferrite transformation temperature (Ar3) under multi-pass deformation has been studied using the continuous hot torsion testing method by monitoring the variation in the stress with temperature during the cooling period. The experimental results show that the effect of deformation on the At3 temperature decreases rapidly with decreasing cooling rate. This may be due to static recovery taking place within the deformed austenite during the cooling period, effectively reducing the nucleation of ferrite.
Keywords: Austenite; Ferrite; Deformation; Steel
1. Introduction It is well known that the mechanical properties of hot-rolled steel products can be improved by controlling the subsequent cooling rate after hot rolling. Hence, knowledge of the phase transformation of austenite is of great industrial importance. Continuous cooling transformation diagrams offer preliminary guidance for the selection of cooling rates for desired microstructures, and over the years such diagrams have been the subject of vigorous investigations applying various experimental techniques, such as thermal analysis, dilatometry, metallography etc. [1-3]. None of these techniques is capable of dealing with the continuous cooling transformation of steels as far as hot rolling is concerned. Recently, it has been reported [4] that a continuous cooling, continuous deformation test has been developed in a materials testing system to determine the transformation characteristics. However, until now the work has been limited to the simulation of continuous cooling transformation without multi-pass deformation. In this paper, a continuous hot torsion testing method, based on the continuous cooling,
continuous deformation test [4], is used to study the characteristics of continuous cooling transformation after simulated multi-pass deformation.
2. Experimental procedure The composition of the niobium-bearing steel studied is as follows (wt.%): C-~0.10, Mn~-1.32, Si ~ 0.30, Nb -~ 0.028, A1 = 0.042, N -- 0.008, P= 0.018, S-~ 0.015; Fe, balance. The tests were performed in a computerized hot torsion machine (see Ref. [5] for details). Solid specimens, 8 mm in gauge diameter and 5 mm in gauge length, were machined from the as-received steel plates. The specimens were first reheated to 1200 °C for 10 rain, and then cooled to the upper end of the selected temperature range (1150-910 °C) for 13-pass deformation. The processing parameters were chosen as: pass strain (axial strain in torsion test), 0.3; strain rate, 1 s- 1; interpass time, 20 s; pass interval, 20 °C. At the end of each pass, the stress was immediately decreased to a low, but nonzero, level. After multi-pass deformation, the speci-
0921-5093/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSD1 0921-5093(94)02717-X
LI6
Letter
/
Materials Science and Engineering A194 (1995) LI5-LI8
mens were continuously deformed at a strain rate of 5 x 10 -5 s-1 and simultaneously cooled at different cooling rates between 0.26 and 5 °C s 1. T h e stress was monitored during the cooling period and, as soon as the phase transformation took place, there was a decrease in the stress due to the softening from the ferrite transformation. T h e same test was also performed without deformation. A verification of the austenite-to-ferrite transformation temperature (Ar3) measured by this method was made by quenching the specimens just before and just after the stress decrease for metallographic examination.
To verify the measured A r 3 temperature, the cooling process of some specimens was interrupted by rapid quenching at a temperature just above or just below the measured A r 3 temperature for metallographic observation. T h e microstructures obtained are shown in Fig. 3.
~l
~
Coolingrate(1*C/s)
301
2 0 ~ ~
( 740,0}
3. R e s u l t s and d i s c u s s i o n
T h e flow stress vs. temperature curve during and after 13-pass deformation is shown in Fig. 1. During the cooling period (from 910 to 6 5 0 ° C at a cooling rate of 1 °C s-J), the flow stress first decreased with decreasing temperature due to stress relaxation, and then with continuous deformation at a strain rate of 5 x 1 0 - 5 s- ], the flow stress increased almost linearly with a further decrease in temperature. At about 760 °C, there was an obvious decrease in the stress, and with a further decrease in temperature the stress decreased rapidly. T h e same test was also performed without deformation as illustrated in Fig. 2. In order to hold the specimen tightly in the beginning, a small amount of deformation ( e = 0.1 ) at a strain rate of 1 s- 1 was employed at a temperature of 1150°C. It is reasonable to assume that this strain did not affect the subsequent phase transformation. T h e decrease in stress observed in Fig. 1 is also present in Fig. 2, but is located at a lower temperature, i.e. at 740 °C. T h e temperature corresponding to the beginning of the stress decrease is defined as the austenite-to-ferrite transformation temperature (Ar3). T h e principle of the variation of the flow stress with temperature during the cooling period is the same as that reported in Ref. [4].
650
7o0
750
800
8so
900
95o
looo 1050 1100
Temperature,*C Fig. 2. Flow stress vs. temperature during cooling without previous deformation.
160 140 Coolingrate(1*C/s) 120
@. 100 80 ¢/I
60 40
Ar3 (760"C)
20
600 650 700 750 800 850 900 950 1000105011001150 Temperature,qC
Fig. 1. Flow stress vs. temperature during and after 13-pass deformation.
Fig. 3. Microstructures obtained by quenching just above and just below the Ar 3 temperature after 13-pass deformation: (a) above At3; (b) below Ar 3.
Letter
/
Muterials Science and Engineering A194 (1995) L1_5L18
Above the measured Ar, temperature, the microstructure is completely austenitic as can be seen from the completely martensitic microstructure in Fig. 3(a). A small amount of ferrite is observed to have nucleated mainly at the grain boundaries of the deformed austenite in Fig. 3(b), which shows the specimen quenched just below the measured Ar, temperature. Thus torsion testing was found to be a very effective method for measuring the continuous cooling transformation temperature under the conditions prevailing during the thermomechanical treatment of steels. By comparison of Figs. 1 and 2, it is also seen that, during the phase transformation, the stress decreases more rapidly when the austenite is deformed than when it is not. This means that deformation not only enhances the At-, temperature, but also speeds up the transformation process. This is mainly due to the fact that, compared with undeformed austenite, characterized by a large grain size of 156 pm [6], high-temperature 13-pass deformation not only greatly refines the austenite grain size (27 pm) [6] through complete recrystallization at temperatures above the non-recrystallization temperature T,,, (for this test measured to be 9.50 “C [7]), but also increases the accumulated strain within the unrecrystallized austenite at temperatures below T,,. All of these factors significantly accelerate the austenite-to-ferrite transformation, so that hightemperature deformation leads to an increase in both the Ar, temperature and the transformation rate. Fig. 4 shows the experimentally measured Ar, temperature as a function of the cooling rate under conditions of deformation and non-deformation. The relation between the Ar, temperature and the cooling rate was found to obey the following exponential function TAr,= TA,expl- (a l’
)“I
(1)
where a (with units of s “C-r) is 6.03 x 10 7 and 4.58 X lo-” and /3 is 0.162 and 0.210 for 13-pass
7001 0
’ 1
’ 2
I 3
4
’ 5
Fig. 4. Experimentally rate.
measured
No de$ymed
Ar,
temperature
deformation and non-deformation respectively, For this steel, TAI was calculated to be 848 “C using an empirical equation from ref. [S]. In both cases, the At-, temperature increases rapidly with decreasing cooling rate. It can also be seen that the influence of deformation on the Ar, temperature decreases with decreasing cooling rate. According to classical nucleation theory, the diffusion-dependent ferrite transformation usually starts preferentially at high-energy sites, such as grain boundaries, deformation bands, etc. Static recovery, which can reduce the level of accumulated strain within the heavily deformed austenite, is found to be the major physical event taking place during cooling, and the degree of static recovery is also dependent on the cooling rate. With a decrease in the cooling rate, the recovery of the dislocation structure is more likely to occur initially within the high-energy sites, such as heavily deformed grain boundaries, deformation bands, etc. These are the sites where the initial nucleation of ferrite is most likely to take place; therefore the potential for ferrite nucleation is strongly decreased. 4. Conclusions
(1) The
austenite-to-ferrite transformation (Ar3) temperature on multi-pass deformation can be measured using the continuous hot torsion testing method by monitoring the variation in the stress with temperature during the cooling period. The relation between the Ar, temperature and the (2) cooling rate for conditions of deformation and no deformation is found to obey the following exponential function
L, = TA,expl - (QI/ )"I
(2)
where a (with units of s”C-‘) is 6.03 X lo-’ and 4.58 X lo-’ and p is 0.162 and 0.210 for 13-pass deformation and non-deformation respectively. In both cases, the Ar, temperature increases rapidly with decreasing cooling rate, and the influence of deformation on the Ar, temperature decreases with decreasing cooling rate. These observations can be explained by static recovery taking place within the deformed austenite during the cooling period, effectively eliminating the high-energy sites for ferrite nucleation.
Acknowledgements
J 6
Cooling rate, ‘C/s Defgmd
L17
vs. cooling
The authors wish to thank Fundia Norsk Jernverk AS for financial support. One of the authors (X.D. Liu) also thanks the Research Council of Norway for an award of a research fellowship.
L18
Letter
/
Materials Science and Engineering A194 (1995) L I 5 - L I 8
References [1] H. Yada, J. Tominaga, K.Y. Wakimoto and N. Matsuzu, in P.D. Southwiek (ed.), lnt. Conf. on Accelerated Cooling of Steel, TMS-AIME, Warrendale, PA, 1986, p. 55. [2] J.J.M. Too, in J.H. Beynon (ed.), Int. Conf. on Modelling of Metal Rolling Processes, The Institute of Metals, London, 1993, p. 343. [3] A. Hurkmans, G.A. Duit, T.M. Hoogendoorn, F. Hollander and H. Ijmuiden, in ED. Southwick (ed.), Int. Conf. on
[4] [51 [6] [7] [8]
Accelerated Cooling of Steel, TMS-AIME, Warrendale, PA, 1986, p. 481. A.Z. Hanzaki, R. Pandi, P.D. Hodgson and S. Yue, Metall. Trans. A, 24 (1993) 2657. J.K. Solberg and A.H. Sunde, Prakt. Met., 24 (1987) 468. X.D. Liu, J.K. Solberg, R. Gjengedal and A.O. Kluken, J. Mater. Process. Technol., 45 (1994) 497. X.D. Liu, J.K. Solberg, R. Gjengedal and A.O. Kluken, Mater. Sci. Technol., in press. K.W. Andrews, J. Iron Steellnst., 203 (1965) 721.