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An investigation into the room temperature mechanical properties and microstructural evolution of thermomechanically processed TWIP steel
Materials Science & Engineering A 596 (2014) 200–206
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
An investigation into the room temperature mechanical properties and microstructural evolution of thermomechanically processed TWIP steel A.R. Khalesian, A. Zarei-Hanzaki n, H.R. Abedi, F. Pilehva Hot Deformation & Thermomechanical Processing of High Performance Engineering Materials Lab, School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 19 September 2013 Received in revised form 19 November 2013 Accepted 20 November 2013 Available online 28 November 2013
The present work aims to positively impact the utilization of high-Mn TWIP steels through gaining a fundamental understanding of their microstructural evolution during high temperature rolling procedure. Towards this end, full annealed TWIP steel was thermomechanically processed (TMP) through hot rolling the work-pieces at 800, 950, and 1100 1C by the reductions of 30 and 60%. The restoration processes (i.e., recovery and recrystallization) are found to be operative during hot rolling of the examined steel. The discontinuous and geometrical dynamic recrystallization mechanisms are considered to contribute generating the fine-equiaxed microstructures. In addition, the distorted annealing twins hold a great contribution to the formation of new recrystallized grains. The room temperature mechanical properties of the hot rolled products were examined by tensile testing method. The results show that a significant strengthening would be achieved by applying the predetermined TMP cycles without a dramatic decrement in elongation to fracture. The changes in the yield and ultimate strength as well as the ductility values have been properly addressed considering the corresponding microstructural evolutions and texture effects. & 2013 Elsevier B.V. All rights reserved.
Keywords: Twinning induced plasticity steel Hot rolling Restoration phenomena Mechanical characterization
1. Introduction The incremental rate of technological developments in automotive industries has dictated an unsaturated demand for new steels with desirable strength to weight ratio [1]. The extent of the conducted researches clearly indicates that the twinning induced plasticity (TWIP) steels provide a great potential owing to their excellent combination of strength and ductility [2]. The increase of strength enables car manufacturers to reduce the weight, whereas the increase of ductility allows for more sophisticated car design [1,3]. Generally, the cold rolling and subsequent annealing have been employed as the main route to process the strip products [4–6]. Accordingly the majority of previous researches have focused on the cold-rolled TWIP steels [7–9]. The evolution of the deformation mechanisms [10,11], the mechanical/annealing twinning characteristics [12,13], and the work hardening behavior [14,15] are some of the most important issues, which have been considered. In spite of the comprehensive involved researches, it is generally believed that the cold-rolling and annealing scheme has some shortcomings such as high energy consumption, complicated process and so on. In consequence, it is worth introducing and developing the high temperature deformation
n
Corresponding author. Tel.: þ 98 21 82084116; fax: þ 98 21 88006076. E-mail address: [email protected] (A. Zarei-Hanzaki).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.11.065
(consisting of the hot rolling and successive cooling) as an alternative or intermediate processing route [16–20]. Unfortunately, the low plasticity and high deformation resistance during hot rolling of TWIP steels due to high contents of Mn, Si and Al import some difficulties. For instance, severe edge cracks may be formed during hot strip rolling, leading to the edge cutting-off of the final products [18]. In addition, the thickness of the steel sheet cannot be exactly controlled and the microstructure cannot keep homogeneous through hot-rolling in comparison to cold rolling, and this may influence the final mechanical properties of the products [19,21]. Nevertheless, an acceptable edge and surface quality as well as excellent combinations of mechanical properties could be obtained through a proper control of the processing conditions. It has been shown that the uniform elongation of the hot rolled TWIP steel is not severely degraded although its strength is greatly enhanced, leading to superior tensile properties [20]. The occurrence of TWIP effect during subsequent deformation is thought to be responsible for the excellent mechanical properties of the hot-rolled steel [19]. In line with previous efforts, the aim of this research is to obtain a better insight in the microstructural evolution of a high Mn TWIP-steel upon hot rolling in order to assess their potential as strip products. The present specific objective is to understand the nature and magnitude of the dynamic restoration processes participate in strain accommodation during processing. The information gained
A.R. Khalesian et al. / Materials Science & Engineering A 596 (2014) 200–206
from this study would be used to link mechanical property improvements with microstructural changes during deformation thereby suggesting the cost-effective processing schedules for TWIP steels.
2. Experimental procedure The experimental steel was supplied in as-cast condition, the chemical composition of which is given in Table 1. In order to eliminate the undesired dendritic structure and to produce a uniform fine grained microstructure, the as-received material was forged at 1150 1C. This was followed by subsequent annealing at 1100 1C for 45 min to remove any segregation of the alloying elements in particular that of Mn. The typical microstructure of the solution heat-treated (1100 1C, 45 min, and water quenched) experimental steel is shown in Fig. 1. As is observed, the experimental steel contains a fully austenitic structure characterized by annealing twins. This is also confirmed through X-ray analysis (Fig. 2). The mean grain size, measured by the proper image analysis software is 80 mm. The as-annealed material was then rolled to 30% and 60% reduction at the temperatures of 800 1C, 950 1C, and 1100 1C with 10% thickness
Table 1 Chemical composition of experimental TWIP steel. TWIP
C
Mn
Al
Si
P
S
Fe
wt%
0.013
29.10
2.40
0.30
0.06
0.06
Bal.
γ (311)
800 400
γ (222)
1200
γ (220)
γ (111)
Intensity (arb.unit)
1600
γ (200)
Fig. 1. The initial microstructure of the experimental steel.
0 35
45
55
65
75
85
95
2θ (degree) Fig. 2. The XRD profiles of the initial microstructure.
201
reduction per pass. The rolling procedure was conducted through a Furouzandeh-GL-200 rolling machine furnished by two 10 cmdiameter rolls that rotate at 30 rpm. To investigate the final microstructures, the work-pieces were sectioned longitudinally (on the mid thickness planes parallel to the rolling direction), mounted using cold curing resin, ground and polished step by step up to the final polishing by 0.05 μm Al2O3 powders. The microstructures were optically characterized by etching the mechanically polished samples with 3% Nital. The X-ray diffraction analysis was also executed on the selected specimens to identify the constitutive phases in the related microstructures. Toward this end, a high resolution X-ray diffractometer (model Philips-X’pert, Netherlands) with a rotating copper anode (CuKα) radiation (wave length, λ¼1.5406 Å) was employed. The data were collected over a range of 10–110 in 2θ with an accelerating voltage of 40 kV and a scanning speed of 2 min 1. To assess the room temperature mechanical properties of the rolled products, the tensile tests were conducted according to ASTM E8M standard using cylindrical specimens with a reduced section diameter of 6 mm and a gauge length of 30 mm. The tension tests were carried out under the initial strain rate of 10 3 s 1 using an Instron-4208 universal testing machine, equipped with a contact extensometer. The elongation-to-failure was measured from the gauge length of the fractured specimens. The related fracture surfaces and any change in the subsurface regions were also examined using scanning electron microscopy to clarify the ductility behavior of the material.
3. Results and discussion 3.1. Microstructure evolution To explore the involved recrystallization mechanisms and to explain the effects of aforementioned processing parameters (rolling temperature and thickness reduction), the evolved microstructures through hot rolling were examined (Fig. 3). According to Fig. 3a and b, the obtained microstructures upon hot rolling at 800 1C are consisted of highly elongated grains associated with pancaked (non-recrystallized) structure. In contrast, the recrystallized grains are clearly evident in the microstructure of the specimens which have been rolled at 950 and 1100 1C. In fact, at the latter deformation conditions the thermal activity is high enough to cause the microstructure to be partially or fully refined due to the occurrence of dynamic recrystallization [22]. As is observed, the extensive recrystallization only occurs at higher temperatures and higher imposed strain (Fig. 3d and f) [23]. The obtained microstructures upon hot rolling at lower thickness reduction (Fig. 3c and e) show partially recrystallized grain structures where a combination of coarse and fine grains are identified in the microstructure. As a matter of course, under the specified deformation conditions the stored energy is not high enough to triger the dynamic recrystallization throughout the whole workpiece. Therfore, the nucleation, and in turn the formation of recrystallized grains would be confined to the specified regions [24]. On the other hand, the high temperature at which the material is held during straining would cause the initial unrecrystallized grains to coarsen just before quenching. These together would end in the appearance of such bimodal microstructure, as is seen in Fig. 3e. As is indicated by white arrows in Fig. 3c and d, most of the recrystallized grains were nucleated through grain boundary bulging mechanism which is the main feature of discontinous dynamic recrystallization. On the close inspection of the related microstructures (Fig. 4a and b), the occurrence of bulging is also clearly evident. It is worth to note that this result is in a good agreement with the previous results on the same material which
202
A.R. Khalesian et al. / Materials Science & Engineering A 596 (2014) 200–206
T= 950 °C
T= 1100 °C
R= 60%
R= 30%
T= 800 °C
Fig. 3. The typical microstructures obtained after hot rolling at different thermomechanical conditions.
Fig. 4. The typical microstructures obtained after hot rolling at (a) 950 1C, 30% (b) 950 1C, 60% and (c) 1100 1C, 60%.
A.R. Khalesian et al. / Materials Science & Engineering A 596 (2014) 200–206
203
3.2. Tensile properties The typical tensile engineering stress–strain curves of the hot rolled materials under different deformation conditions are shown in Fig. 6. As is clearly observed, the room temperature mechanical
Eng. Stress (MPa)
600 500 400 300 As-received
200
800 °C 30% 950 °C 30%
100
1100 °C 30%
0
Fig. 5. The developed microstructures after 30% reduction by rolling at 950 1C.
0
0.1
0.2
0.3
0.4
0.5
Eng. Strain 600
Eng. Stress (MPa)
have reported the occurrence of dynamic recrystallization during hot compression tests under the similar thermo-mechanical conditions [25]. The role of twinning in grain refinement is another issue which should be elaborated in terms of the changes in the frequencey and the appearance of twins. The hot rolling of the experimental alloy results in distortion and migration of the pre-existing twin boundaries. In this context, the presence of the distorted twins within the initial and recrystallized grains and along the grain boundaries have been pointed by red arrows in Figs. 3 and 4. As a matter of fact, the strain energy accumulation at twin boundaries providethe preferred sites for nucleation, to some extent that twin boundary bulge and form a new recrystallized grain. The formation of fine-equiaxed grains adjacent to the twin boundaries can be easily traced in the developed structures as is observed in Fig. 5. The role of twin boundaries in stimulating the DRX phenomenonmay also come from those that are newly formed in the microstructures, those which were not in the initial microstructure. These newly formed twins are generated simultaneously with the growth of DRX grains. In fact, once the misorientation between DRX grains and deformed lattice is inadequate, the growth will stop. In this condition, a series of twins form at DRX grain boundaries, which causes a distortion in the lattice and result in an enhancement of the misorientation in DRX grain boundaries. This would lead to a considerable decrease in the energy required for DRX grain growth [26]. This kind of twins is indicated with blue arrows in Fig. 3f and 4c. The repeated formation of twins during deformation causes an amplification of the number of DRX grains through the partitioning mechanism [27]. According to the microstructural features (Fig. 3b, dashed ovals), it is also speculated that the geometric dynamic recrystallization (GDRX) amy also contribute to the formation of fineequiaxed structures at lower temperature regime. This structural evolution is intensified where the dynamic recovery prevails and subgrains are well generated. The preferred dynamic recovery has occurred and has led to the formation of serrated-shaped grain boundaries in the the microstructures of the specimens which were rolled at 800 1C (Fig. 3b). The wavelength of these serrations is similar to the subgrain size. Interpenetration of the scalloped boundaries, then occur throughout the deformation process and this, in turn, results in a microstructure of small equiaxed grains. It is worth mentioning that, these findings are in accord with a number of previous investigations on Fe–Mn–Al–Si alloys which have reported the occurrence of GDRX during hot compression tests under similar thermomechanical conditions [25].
500 400 300 As-received
200
800 °C 60% 950 °C 60%
100
1100 °C 60%
0 0
0.1
0.2
0.3
0.4
0.5
Eng Strain Fig. 6. The typical engineering stress–strain curves of the hot rolled experimental material at room temperature.
Fig. 7. The room temperature tensile strength and ductility of the hot rolled material (a) 30% and (b) 60%.
204
A.R. Khalesian et al. / Materials Science & Engineering A 596 (2014) 200–206
properties of the examined alloy are substantially influenced by the rolling temperature and rolling thickness reduction. For further investigation, the variations of ductility (elongation-toUTS), yield and ultimate tensile strength with rolling temperature are plotted in Fig. 7. As is seen, a significant increase in strength
R= 60%
T= 1100 °C
T= 950 °C
T= 800 °C
R= 30%
has been achieved in comparison to the as-received material without a dramatic decrement in elongation to fracture values. In addition the higher room temperature ductility is achieved at higher rolling temperature and higher equivalent rolling strain. Moreover, an increasing trend is recognized for the strength
Fig. 8. The fracture surface of the tensile specimens elongated to the fracture at different hot rolling conditions.
A.R. Khalesian et al. / Materials Science & Engineering A 596 (2014) 200–206
variation with increasing rolling strain and decreasing rolling temperature. The typical scanning electron microscopy images of the fracture surfaces are shown in Fig. 8. The presence of shallow and deep dimples with different size and depth (without any cleavage facet) predominantly indicates a ductile fracture mode [28]. This is in accord with the high ductility values obtained at the specified deformation conditions. It is worth mentioning that the initiation of the voids is from nucleating at the small discontinuity formed at the grain boundaries and develops by plane sliding [29]. By further deformation, new decohesion are developed alongside the original ones, allowing the voids growth as well as further plane sliding. This explains the elongated shape of the observed voids in Fig. 8. However, the size of the steps and the magnitude of the sliding are dependent on the deformation conditions. These steps can be readily traced in Fig. 9. In addition, as is seen in Fig. 8, the dimples are distributed more uniformly through the surface, the size and depth of which are decreased at higher rolling temperature and rolling equivalent strain. These changes in surface morphology well coincide with the observed changes in room temperature ductility values. The details of ductility and strength variation at different deformation conditions are described point by point are as follows. The mean grain sizes of the developed structures through hot rolling are presented in Table 2. As is observed, a significant grain refinement has been achieved through applying the hot rolling. The final mean grain size is reduced as the rolling temperature and equivalent strain are increased due to providing additional nucleation sites and higher dynamic recrystallization driving force. The grain refinement resulted from dynamic recrystallization has
205
a significant effect on increasing the yield stress of the specimens rolled in temperature range of 800–1100 1C (compared to the asreceived material). The lower yield stress of the material rolled at high temperature of 1100 1C is justified considering the higher recrystallization volume fraction at this deformation condition (refer to Table 2). Due to the extensive occurrence of dynamic recrystallization, the deformed grains are completely replaced by a new set of strain free grains with low density of dislocation. This leads to plastic deformation to start more readily and the yield stress value is decreased. In order to assess the post-yield flow behavior of the hot rolled materials, the variation of yield to tensile strength ratio at different rolling temperature and thickness reduction is presented in Fig. 11. The yield to tensile ratio is a measure for the safety margin against failure by plastic collapse, and indicates the ability of the material to experience plastic deformation before failure (is connected with the uniform elongation, n). As is observed, the obtained values lay above the range, which has been already reported for TWIP steels (Table 3) [30]. This represents the high work hardening capacity of rolled specimen at room temperature. It is worth to note that, the low yield to tensile ratio of o0.7 has been traditionally regarded as providing a safety margin against failure in the post-yield area. In addition, a decreasing trend of yield to tensile ratio is recognized with increasing rolling temperature and rolling strain. This is justified considering the work hardening behavior of the rolled specimens. In this regard, the variation of work hardening Table 3 Yield to tensile strength ratio of TWIP steels [30]. TWIP steels
YS/UTS
30%Mn–0.5%C 29%Mn–0.8%C 22%Mn–1.2%C 29.2%Mn–0.06%C Experimental steel
0.23 0.22 0.23 0.58 0.5–0.8
Fig. 9. Typical appearance of the ductile dimple developed on fracture surface of the examined specimens – representing the activation of the plane sliding mechanism.
Table 2 Average grain size (mm) and the DRX volume fraction (vol%) of the experimental steel upon different hot rolling conditions. R.S.
R.T. 800 1C
ε1 ¼ 30% ε2 ¼ 60%
950 1C
1100 1C
D (mm)
VDRX
D (mm)
VDRX (vol%)
D (mm)
VDRX (vol%)
72 67
– –
53 35
8 76
45 39
37 100
Initial grain size¼ 80 mm
Fig. 10. The variation of work hardening rate with true strain at different hot rolling conditions.
206
A.R. Khalesian et al. / Materials Science & Engineering A 596 (2014) 200–206
provides the preferred sites for nucleation of new recrystallized grains. The resulted grain refinement holds a significant effect on mechanical properties. The decreasing trend of the strength variation by decreasing the rolling strain and increasing the rolling temperature is justified considering the higher recrystallization volume fraction. In addition, the development of strain-free structures results in increasing the rate and the capacity of work hardening. This would result in increasing the ductility values with increasing rolling temperature and rolling strain. The texture hardening was also introduced as another factor which positively affect the formability of rolled products. Fig. 11. The yield to tensile strength ratio obtained from tensile tests of the materials rolled at different thermomechanical conditions.
References
Fig. 12. XRD profiles of the developed structures at different hot rolling conditions.
rate with true strain at different rolling conditions is depicted in Fig. 10. As is expected, the rate and the capacity of work hardening are increased with increasing the rolling temperature and rolling strain. As a matter of fact, the development of strain-free structures results in increasing the austenite capacity to considerably work-harden to large imparted strain levels. In addition, the grain refinement resulted from recrystallization increases the rate of twining which causes a high value of the instantaneous hardening rate (n value). The present authors also believe that the occurrence of texture hardening during hot rolling may also influence the subsequent room temperature ductility of the products. As it is observed in Figs. 11 and 12, the (111) intensity is increased with increasing the rolling equivalent strain. Duggan et al. [31] also suggested that increasing the rolling reduction leads to the rotation of twin clusters to align with the rolling plane. Considering the fact that the mechanical twinning mainly occurs in {111} 〈uvw〉 orientations, the rate of twining is increased with increasing (111) intensity that leads to an increase in work hardening capacity and thus the elongation to fracture values. 4. Conclusion The experimental material is prone to be fully or partially recrystallized through discontinuous and geometrical mechanisms during high temperature rolling in the range of 800–1100 1C. In addition, the accumulated strain energy at twin boundaries
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
An investigation into the room temperature mechanical properties and microstructural evolution of thermomechanically processed TWIP steel A.R. Khalesian, A. Zarei-Hanzaki n, H.R. Abedi, F. Pilehva Hot Deformation & Thermomechanical Processing of High Performance Engineering Materials Lab, School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 19 September 2013 Received in revised form 19 November 2013 Accepted 20 November 2013 Available online 28 November 2013
The present work aims to positively impact the utilization of high-Mn TWIP steels through gaining a fundamental understanding of their microstructural evolution during high temperature rolling procedure. Towards this end, full annealed TWIP steel was thermomechanically processed (TMP) through hot rolling the work-pieces at 800, 950, and 1100 1C by the reductions of 30 and 60%. The restoration processes (i.e., recovery and recrystallization) are found to be operative during hot rolling of the examined steel. The discontinuous and geometrical dynamic recrystallization mechanisms are considered to contribute generating the fine-equiaxed microstructures. In addition, the distorted annealing twins hold a great contribution to the formation of new recrystallized grains. The room temperature mechanical properties of the hot rolled products were examined by tensile testing method. The results show that a significant strengthening would be achieved by applying the predetermined TMP cycles without a dramatic decrement in elongation to fracture. The changes in the yield and ultimate strength as well as the ductility values have been properly addressed considering the corresponding microstructural evolutions and texture effects. & 2013 Elsevier B.V. All rights reserved.
Keywords: Twinning induced plasticity steel Hot rolling Restoration phenomena Mechanical characterization
1. Introduction The incremental rate of technological developments in automotive industries has dictated an unsaturated demand for new steels with desirable strength to weight ratio [1]. The extent of the conducted researches clearly indicates that the twinning induced plasticity (TWIP) steels provide a great potential owing to their excellent combination of strength and ductility [2]. The increase of strength enables car manufacturers to reduce the weight, whereas the increase of ductility allows for more sophisticated car design [1,3]. Generally, the cold rolling and subsequent annealing have been employed as the main route to process the strip products [4–6]. Accordingly the majority of previous researches have focused on the cold-rolled TWIP steels [7–9]. The evolution of the deformation mechanisms [10,11], the mechanical/annealing twinning characteristics [12,13], and the work hardening behavior [14,15] are some of the most important issues, which have been considered. In spite of the comprehensive involved researches, it is generally believed that the cold-rolling and annealing scheme has some shortcomings such as high energy consumption, complicated process and so on. In consequence, it is worth introducing and developing the high temperature deformation
n
Corresponding author. Tel.: þ 98 21 82084116; fax: þ 98 21 88006076. E-mail address: [email protected] (A. Zarei-Hanzaki).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.11.065
(consisting of the hot rolling and successive cooling) as an alternative or intermediate processing route [16–20]. Unfortunately, the low plasticity and high deformation resistance during hot rolling of TWIP steels due to high contents of Mn, Si and Al import some difficulties. For instance, severe edge cracks may be formed during hot strip rolling, leading to the edge cutting-off of the final products [18]. In addition, the thickness of the steel sheet cannot be exactly controlled and the microstructure cannot keep homogeneous through hot-rolling in comparison to cold rolling, and this may influence the final mechanical properties of the products [19,21]. Nevertheless, an acceptable edge and surface quality as well as excellent combinations of mechanical properties could be obtained through a proper control of the processing conditions. It has been shown that the uniform elongation of the hot rolled TWIP steel is not severely degraded although its strength is greatly enhanced, leading to superior tensile properties [20]. The occurrence of TWIP effect during subsequent deformation is thought to be responsible for the excellent mechanical properties of the hot-rolled steel [19]. In line with previous efforts, the aim of this research is to obtain a better insight in the microstructural evolution of a high Mn TWIP-steel upon hot rolling in order to assess their potential as strip products. The present specific objective is to understand the nature and magnitude of the dynamic restoration processes participate in strain accommodation during processing. The information gained
A.R. Khalesian et al. / Materials Science & Engineering A 596 (2014) 200–206
from this study would be used to link mechanical property improvements with microstructural changes during deformation thereby suggesting the cost-effective processing schedules for TWIP steels.
2. Experimental procedure The experimental steel was supplied in as-cast condition, the chemical composition of which is given in Table 1. In order to eliminate the undesired dendritic structure and to produce a uniform fine grained microstructure, the as-received material was forged at 1150 1C. This was followed by subsequent annealing at 1100 1C for 45 min to remove any segregation of the alloying elements in particular that of Mn. The typical microstructure of the solution heat-treated (1100 1C, 45 min, and water quenched) experimental steel is shown in Fig. 1. As is observed, the experimental steel contains a fully austenitic structure characterized by annealing twins. This is also confirmed through X-ray analysis (Fig. 2). The mean grain size, measured by the proper image analysis software is 80 mm. The as-annealed material was then rolled to 30% and 60% reduction at the temperatures of 800 1C, 950 1C, and 1100 1C with 10% thickness
Table 1 Chemical composition of experimental TWIP steel. TWIP
C
Mn
Al
Si
P
S
Fe
wt%
0.013
29.10
2.40
0.30
0.06
0.06
Bal.
γ (311)
800 400
γ (222)
1200
γ (220)
γ (111)
Intensity (arb.unit)
1600
γ (200)
Fig. 1. The initial microstructure of the experimental steel.
0 35
45
55
65
75
85
95
2θ (degree) Fig. 2. The XRD profiles of the initial microstructure.
201
reduction per pass. The rolling procedure was conducted through a Furouzandeh-GL-200 rolling machine furnished by two 10 cmdiameter rolls that rotate at 30 rpm. To investigate the final microstructures, the work-pieces were sectioned longitudinally (on the mid thickness planes parallel to the rolling direction), mounted using cold curing resin, ground and polished step by step up to the final polishing by 0.05 μm Al2O3 powders. The microstructures were optically characterized by etching the mechanically polished samples with 3% Nital. The X-ray diffraction analysis was also executed on the selected specimens to identify the constitutive phases in the related microstructures. Toward this end, a high resolution X-ray diffractometer (model Philips-X’pert, Netherlands) with a rotating copper anode (CuKα) radiation (wave length, λ¼1.5406 Å) was employed. The data were collected over a range of 10–110 in 2θ with an accelerating voltage of 40 kV and a scanning speed of 2 min 1. To assess the room temperature mechanical properties of the rolled products, the tensile tests were conducted according to ASTM E8M standard using cylindrical specimens with a reduced section diameter of 6 mm and a gauge length of 30 mm. The tension tests were carried out under the initial strain rate of 10 3 s 1 using an Instron-4208 universal testing machine, equipped with a contact extensometer. The elongation-to-failure was measured from the gauge length of the fractured specimens. The related fracture surfaces and any change in the subsurface regions were also examined using scanning electron microscopy to clarify the ductility behavior of the material.
3. Results and discussion 3.1. Microstructure evolution To explore the involved recrystallization mechanisms and to explain the effects of aforementioned processing parameters (rolling temperature and thickness reduction), the evolved microstructures through hot rolling were examined (Fig. 3). According to Fig. 3a and b, the obtained microstructures upon hot rolling at 800 1C are consisted of highly elongated grains associated with pancaked (non-recrystallized) structure. In contrast, the recrystallized grains are clearly evident in the microstructure of the specimens which have been rolled at 950 and 1100 1C. In fact, at the latter deformation conditions the thermal activity is high enough to cause the microstructure to be partially or fully refined due to the occurrence of dynamic recrystallization [22]. As is observed, the extensive recrystallization only occurs at higher temperatures and higher imposed strain (Fig. 3d and f) [23]. The obtained microstructures upon hot rolling at lower thickness reduction (Fig. 3c and e) show partially recrystallized grain structures where a combination of coarse and fine grains are identified in the microstructure. As a matter of course, under the specified deformation conditions the stored energy is not high enough to triger the dynamic recrystallization throughout the whole workpiece. Therfore, the nucleation, and in turn the formation of recrystallized grains would be confined to the specified regions [24]. On the other hand, the high temperature at which the material is held during straining would cause the initial unrecrystallized grains to coarsen just before quenching. These together would end in the appearance of such bimodal microstructure, as is seen in Fig. 3e. As is indicated by white arrows in Fig. 3c and d, most of the recrystallized grains were nucleated through grain boundary bulging mechanism which is the main feature of discontinous dynamic recrystallization. On the close inspection of the related microstructures (Fig. 4a and b), the occurrence of bulging is also clearly evident. It is worth to note that this result is in a good agreement with the previous results on the same material which
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T= 950 °C
T= 1100 °C
R= 60%
R= 30%
T= 800 °C
Fig. 3. The typical microstructures obtained after hot rolling at different thermomechanical conditions.
Fig. 4. The typical microstructures obtained after hot rolling at (a) 950 1C, 30% (b) 950 1C, 60% and (c) 1100 1C, 60%.
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3.2. Tensile properties The typical tensile engineering stress–strain curves of the hot rolled materials under different deformation conditions are shown in Fig. 6. As is clearly observed, the room temperature mechanical
Eng. Stress (MPa)
600 500 400 300 As-received
200
800 °C 30% 950 °C 30%
100
1100 °C 30%
0
Fig. 5. The developed microstructures after 30% reduction by rolling at 950 1C.
0
0.1
0.2
0.3
0.4
0.5
Eng. Strain 600
Eng. Stress (MPa)
have reported the occurrence of dynamic recrystallization during hot compression tests under the similar thermo-mechanical conditions [25]. The role of twinning in grain refinement is another issue which should be elaborated in terms of the changes in the frequencey and the appearance of twins. The hot rolling of the experimental alloy results in distortion and migration of the pre-existing twin boundaries. In this context, the presence of the distorted twins within the initial and recrystallized grains and along the grain boundaries have been pointed by red arrows in Figs. 3 and 4. As a matter of fact, the strain energy accumulation at twin boundaries providethe preferred sites for nucleation, to some extent that twin boundary bulge and form a new recrystallized grain. The formation of fine-equiaxed grains adjacent to the twin boundaries can be easily traced in the developed structures as is observed in Fig. 5. The role of twin boundaries in stimulating the DRX phenomenonmay also come from those that are newly formed in the microstructures, those which were not in the initial microstructure. These newly formed twins are generated simultaneously with the growth of DRX grains. In fact, once the misorientation between DRX grains and deformed lattice is inadequate, the growth will stop. In this condition, a series of twins form at DRX grain boundaries, which causes a distortion in the lattice and result in an enhancement of the misorientation in DRX grain boundaries. This would lead to a considerable decrease in the energy required for DRX grain growth [26]. This kind of twins is indicated with blue arrows in Fig. 3f and 4c. The repeated formation of twins during deformation causes an amplification of the number of DRX grains through the partitioning mechanism [27]. According to the microstructural features (Fig. 3b, dashed ovals), it is also speculated that the geometric dynamic recrystallization (GDRX) amy also contribute to the formation of fineequiaxed structures at lower temperature regime. This structural evolution is intensified where the dynamic recovery prevails and subgrains are well generated. The preferred dynamic recovery has occurred and has led to the formation of serrated-shaped grain boundaries in the the microstructures of the specimens which were rolled at 800 1C (Fig. 3b). The wavelength of these serrations is similar to the subgrain size. Interpenetration of the scalloped boundaries, then occur throughout the deformation process and this, in turn, results in a microstructure of small equiaxed grains. It is worth mentioning that, these findings are in accord with a number of previous investigations on Fe–Mn–Al–Si alloys which have reported the occurrence of GDRX during hot compression tests under similar thermomechanical conditions [25].
500 400 300 As-received
200
800 °C 60% 950 °C 60%
100
1100 °C 60%
0 0
0.1
0.2
0.3
0.4
0.5
Eng Strain Fig. 6. The typical engineering stress–strain curves of the hot rolled experimental material at room temperature.
Fig. 7. The room temperature tensile strength and ductility of the hot rolled material (a) 30% and (b) 60%.
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properties of the examined alloy are substantially influenced by the rolling temperature and rolling thickness reduction. For further investigation, the variations of ductility (elongation-toUTS), yield and ultimate tensile strength with rolling temperature are plotted in Fig. 7. As is seen, a significant increase in strength
R= 60%
T= 1100 °C
T= 950 °C
T= 800 °C
R= 30%
has been achieved in comparison to the as-received material without a dramatic decrement in elongation to fracture values. In addition the higher room temperature ductility is achieved at higher rolling temperature and higher equivalent rolling strain. Moreover, an increasing trend is recognized for the strength
Fig. 8. The fracture surface of the tensile specimens elongated to the fracture at different hot rolling conditions.
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variation with increasing rolling strain and decreasing rolling temperature. The typical scanning electron microscopy images of the fracture surfaces are shown in Fig. 8. The presence of shallow and deep dimples with different size and depth (without any cleavage facet) predominantly indicates a ductile fracture mode [28]. This is in accord with the high ductility values obtained at the specified deformation conditions. It is worth mentioning that the initiation of the voids is from nucleating at the small discontinuity formed at the grain boundaries and develops by plane sliding [29]. By further deformation, new decohesion are developed alongside the original ones, allowing the voids growth as well as further plane sliding. This explains the elongated shape of the observed voids in Fig. 8. However, the size of the steps and the magnitude of the sliding are dependent on the deformation conditions. These steps can be readily traced in Fig. 9. In addition, as is seen in Fig. 8, the dimples are distributed more uniformly through the surface, the size and depth of which are decreased at higher rolling temperature and rolling equivalent strain. These changes in surface morphology well coincide with the observed changes in room temperature ductility values. The details of ductility and strength variation at different deformation conditions are described point by point are as follows. The mean grain sizes of the developed structures through hot rolling are presented in Table 2. As is observed, a significant grain refinement has been achieved through applying the hot rolling. The final mean grain size is reduced as the rolling temperature and equivalent strain are increased due to providing additional nucleation sites and higher dynamic recrystallization driving force. The grain refinement resulted from dynamic recrystallization has
205
a significant effect on increasing the yield stress of the specimens rolled in temperature range of 800–1100 1C (compared to the asreceived material). The lower yield stress of the material rolled at high temperature of 1100 1C is justified considering the higher recrystallization volume fraction at this deformation condition (refer to Table 2). Due to the extensive occurrence of dynamic recrystallization, the deformed grains are completely replaced by a new set of strain free grains with low density of dislocation. This leads to plastic deformation to start more readily and the yield stress value is decreased. In order to assess the post-yield flow behavior of the hot rolled materials, the variation of yield to tensile strength ratio at different rolling temperature and thickness reduction is presented in Fig. 11. The yield to tensile ratio is a measure for the safety margin against failure by plastic collapse, and indicates the ability of the material to experience plastic deformation before failure (is connected with the uniform elongation, n). As is observed, the obtained values lay above the range, which has been already reported for TWIP steels (Table 3) [30]. This represents the high work hardening capacity of rolled specimen at room temperature. It is worth to note that, the low yield to tensile ratio of o0.7 has been traditionally regarded as providing a safety margin against failure in the post-yield area. In addition, a decreasing trend of yield to tensile ratio is recognized with increasing rolling temperature and rolling strain. This is justified considering the work hardening behavior of the rolled specimens. In this regard, the variation of work hardening Table 3 Yield to tensile strength ratio of TWIP steels [30]. TWIP steels
YS/UTS
30%Mn–0.5%C 29%Mn–0.8%C 22%Mn–1.2%C 29.2%Mn–0.06%C Experimental steel
0.23 0.22 0.23 0.58 0.5–0.8
Fig. 9. Typical appearance of the ductile dimple developed on fracture surface of the examined specimens – representing the activation of the plane sliding mechanism.
Table 2 Average grain size (mm) and the DRX volume fraction (vol%) of the experimental steel upon different hot rolling conditions. R.S.
R.T. 800 1C
ε1 ¼ 30% ε2 ¼ 60%
950 1C
1100 1C
D (mm)
VDRX
D (mm)
VDRX (vol%)
D (mm)
VDRX (vol%)
72 67
– –
53 35
8 76
45 39
37 100
Initial grain size¼ 80 mm
Fig. 10. The variation of work hardening rate with true strain at different hot rolling conditions.
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provides the preferred sites for nucleation of new recrystallized grains. The resulted grain refinement holds a significant effect on mechanical properties. The decreasing trend of the strength variation by decreasing the rolling strain and increasing the rolling temperature is justified considering the higher recrystallization volume fraction. In addition, the development of strain-free structures results in increasing the rate and the capacity of work hardening. This would result in increasing the ductility values with increasing rolling temperature and rolling strain. The texture hardening was also introduced as another factor which positively affect the formability of rolled products. Fig. 11. The yield to tensile strength ratio obtained from tensile tests of the materials rolled at different thermomechanical conditions.
References
Fig. 12. XRD profiles of the developed structures at different hot rolling conditions.
rate with true strain at different rolling conditions is depicted in Fig. 10. As is expected, the rate and the capacity of work hardening are increased with increasing the rolling temperature and rolling strain. As a matter of fact, the development of strain-free structures results in increasing the austenite capacity to considerably work-harden to large imparted strain levels. In addition, the grain refinement resulted from recrystallization increases the rate of twining which causes a high value of the instantaneous hardening rate (n value). The present authors also believe that the occurrence of texture hardening during hot rolling may also influence the subsequent room temperature ductility of the products. As it is observed in Figs. 11 and 12, the (111) intensity is increased with increasing the rolling equivalent strain. Duggan et al. [31] also suggested that increasing the rolling reduction leads to the rotation of twin clusters to align with the rolling plane. Considering the fact that the mechanical twinning mainly occurs in {111} 〈uvw〉 orientations, the rate of twining is increased with increasing (111) intensity that leads to an increase in work hardening capacity and thus the elongation to fracture values. 4. Conclusion The experimental material is prone to be fully or partially recrystallized through discontinuous and geometrical mechanisms during high temperature rolling in the range of 800–1100 1C. In addition, the accumulated strain energy at twin boundaries
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