Grain Size Effect on Behaviour and Microstructure of a Warm Deformed Ti-IF Steel A. Oudin 1, M. R. Barnett 1 and P. D. Hodgson 1

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1 Grain Size Effect on Behaviour and Microstructure of a Warm Deformed Ti-IF Steel A. Oudin 1, M. R. Barnett 1 and P. D. Hodgson 1 1 School of Engineering and Technology - Deakin University - Geelong VIC Australia Keywords: IF steel, ferrite deformation, warm deformation, microstructure, torsion test. Abstract. The effect of grain size on the deformation behaviour in the ferrite region of a Titanium stabilized Interstitial Free steel was investigated by hot torsion. The initial work hardening regime is followed by a softening regime where a broad eak stress develos. The eak stress and the stress at final strain were relatively insensitive to grain size. However, at low values of the Zener-Hollomon arameter, the strain to the eak stress was strongly deendent on the grain size. A series of microstructural arameters were examined to exlain these observations. Introduction For about the last 15 years, "warm" or "ferritic" rolling of stri steel has gained in imortance, mainly because it can reduce the cost of stri steel rocessing and gives secific material roerties to the roducts [1,2]. However, desite the number of studies carried out on the hot deformation of ferrite [3-8], the effect of the initial grain size has not received much attention. Knowledge of the influence of grain size on the flow stress is imortant for the modelling of ferritic rolling. It is also exected that the effect of grain size should highlight the relative roles of the cometing microstructural rocesses of dynamic recovery and recrystallization. The resent work investigated the role of grain size on the warm deformation of a Titanium stabilized Interstitial Free steel using hot torsion. Exerimental Procedures Materials. A commercial interstitial free (IF) steel (.5%C,.3%N,.13% Mn,.84%Ti,.42% Al, by weight) was sulied as front end transfer bar cros by BHP Steel. Torsion samles with a gauge section of 2 mm and 6.7 mm in diameter were machined out of the as-received lates with the longitudinal axis arallel to the rolling direction. Two roughing rocedures and one heat treatment were used to achieve three different initial grain sizes (average linear grain intercet lengths) of 25µm, 75µm and 15µm (Figure 1). Torsion Test Procedure. Tests were erformed on a torsion rig described elsewhere [9]. Before testing, all secimens were soaked for 1 min at 8 o C to ensure comlete titanium carbide (TiC) reciitation [1]. Samles were then cooled or heated to the test temerature. After a 6s hold at the test temerature, the secimen was twisted to a strain of 2 and water quenched within.5s after the end of deformation. Torsion tests were erformed at temeratures between 765 o C and 85 o C at strain rates from.3s -1 to 1s -1 (Table 1). Exerimental Results Flow Curves. The flow curves (Figure 2) comrise a work hardening regime, which ends at a eak stress σ, followed by a work softening regime where a gradual stress decrease occurs (Figure 3). The onset of the work softening regime occurs at the strain to the eak stress (termed the eak strain, ε ). For the low strain rates and high temeratures (low Z values), a steady state stress is

2 attained equivalent to the stress at the final strain σ ε = 2, whereas for high strain rates and low temeratures (high Z values) a steady state regime is not achieved by the final strain (Figure 2). Overall, the influence of grain size on the flow curves was minor excet for high strain rates and low temerature (high Z values), where a slightly higher stress level was found for the finest grained samles. temerature 11 o C 1 o C 95 o C 15 µm 5 min 5 min & ε = 1s 1 ε = 2 w water quenched to RT 1 & ε =.1s ε = 1 w natural cooling to RT 15 min natural cooling to RT Fig.1 Exerimental schedules used to generate grain sizes of 25µm, 75µm and 15µm. time Table 1 Torsion tests schedule. T [ o C] ε& [s -1 ] grain size [µm] µm 5 12 T = 765 o C. ε = 1 s -1 4 σ σ ε=2 stress (MPa) 8 4 T = 8 o C. ε =.3 s -1 T = 85 o C. ε =.3 s -1 stress (MPa) work hardening ε work softening strain (mm/mm ) Fig. 2 Tyical flow curves for different test conditions and initial grain sizes strain (mm/mm) Fig. 3 Examle flow curve showing key arameters. Rate Equations. As roosed earlier by Sellars et al. [11], the values of σ and σ ε = 2, and also ε, were modeled over the full range of temeratures, strain rates and grain sizes using three different hyerbolic sine laws in the form of : [ sinh( α ] n Z = A ) (1) σ [ σ ] n Z = C sinh( α ε = 2) (2) [ sinh( δ ] l Z = E ) (3) ε where Z = ε&ex ( Q / RT ) is the Zener-Hollomon arameter, Q is an average aarent activation energy for deformation (372 kj/mol), R = J mol -1 K -1, T is the absolute deformation temerature and A, C, E, α, δ, n and l are material constants. The exerimental values are lotted according to Equations 1-3 in Figures 4-6.

3 Q = J/mol α = 1/5 - n = Q = J/mol α = 1/5 - n = Z (s -1 ) 1 16 Z (s -1 ) µm sinh(α σ ) Fig. 4 Strain rate and temerature deendence of the eak stress sinh(α σ ε=2 ) 15 µm Fig. 5 Strain rate and temerature deendence of the stress at final strain. eak strain (mm/mm) 1. Q = J/mol δ = 1/.5 15 µm Z (s -1 ) Fig. 6 Strain rate and temerature deendence of the eak strain. Table 2 Model constants. arameter const. value Q 372 σ A n 3.77 α 1/5 ε E k 1 = k 2 = k 3 = 2.93 l k 4 = 5.92 k 5 = k 6 = 5. δ 2 σ ε=2 C n 3.77 α 1/5 Using individual coefficients for each stresses (eak stress and stress at final strain), the hyerbolic sine law gave an excellent fit with the data. However, for a more general solution, constant values of α and n were used for the two stresses. Consequently, A, C, α and n are material constants indeendent of the temerature, the strain rate and the grain size (Table 2). In contrast with the stresses, the eak strain was strongly deendent on the grain size (Figure 6). While the material constant δ was assumed to be grain size indeendent, the arameters E and l were significantly found deendent on the grain size. Two emirical functions were roosed to take into account the grain size effect: k3 ( k k d ) E = ex (4) l + k6 = k k d (5) 4 5 where k 1, k 2, k 3, k 4, k 5 and k 6 are material constants and d is the initial grain size (Table 2). Microstructures. EBSD analysis was carried out on samles deformed at low values of Z (85 o C -.3s -1 ) where a strong effect of grain size on the eak strain was observed. For these conditions, torsion tests were carried out at different strains from.1 to 4. Mas of the orientation data, showing boundaries with a misorientation greater than 3 and 15 and the contrast of the

4 Kikuchi attern (as grey scale) are shown in Figure 7. It can be seen that many of the subgrains are surrounded by boundaries with an angle of greater than 3. Isolated segments of high angle boundaries, aarently formed by a continuous rotation rocess, and bulged/serration high angle boundaries, aarently formed by migration, are evident at both grain sizes. o o o Fig. 7 EBSD mas showing low angle ( 15 > θ > 3, light lines) and high angle ( θ > 15, bold lines) boundaries in samles deformed at low Z conditions (T = 85 o 1 C, ε& =.3s, ε = 4 ) and with an initial grain size of 25µm (left) and 15µm (right). (The shear direction is vertical and the radial direction is normal to the age). To gain an imression of the evolution of the substructure, the average intercet length between boundaries with a misorientation greater than 1.5, l 1.5 (a value that reflects the effective limit of the technique and which aroximates the subgrain size) is resented in Figure 8. For the finest and coarsest grained samles, it can be seen that l 1.5 reaches a steady state value of ~14µm with increasing strain which aears indeendent on the grain size. This value is aroached more raidly in the finest grain sized samle and is reached in both cases at a strain close to the eak strain average intercet distance θ>1.5 o (µm) ε,25 ε,15 strain rate =.3 s -1 T = 85 o C 15 µm average intercet distance θ>15 o (µm) strain rate =.3 s -1 T = 85 o C 15 µm strain (mm/mm) Fig. 8 Influence of strain on the average linear intercet between boundaries greater than 1.5 in misorientation strain (mm/mm) Fig. 9 Influence of strain on the average linear intercet between boundaries greater than 15 in misorientation. In contrast, the linear intercet between high angle boundaries, l 15 (Figure 9) shows distinctly different deendencies on strain for the three grain sizes. For the coarsest grained samle, the value of l 15 dros raidly with increasing strain (as the grain boundary area is increased due to

5 deformation). Whereas, for the fine grain samle, l 15 is relatively insensitive to strain. As would be exected, l 15 is greater in the coarse grained samles. Discussion The values of aarent activation energy, Q and strain rate exonent, n obtained in the resent study are similar to those reorted in revious studies on similar materials [2]. The fact that the aarent activation energy is high (372 kj/mol) comared to that for self diffusion in ure iron (239 kj/mol) has reviously been attributed to the resence of Ti. Though the recise reasons for this are not clear it can be imagined that the effect is more due to the interaction between dislocations and reciitates than it is to a role of the solute atoms; Ti is exected to be largely resent in reciitate form [1]. The degree of softening is relatively minor comared to that seen in conventional dynamic recrystallization (DRX) of austenite and the microstructural observations and measurements made in the resent work shed no extra light on why softening should be manifest. It is quite ossible that a texture softening effect, similar to that reorted in aluminium, is resonsible [12]. However, a reduction in load due to the migration of boundaries, similar to that seen in conventional DRX, but at a lesser scale is also ossible. If this were the case then one might exect that the softening would be more evident at lower values of Z (where DRX occurs more raidly) and at finer grain sizes (because there is more high angle boundary to migrate). In the resent work neither of these can be clearly demonstrated, but this may be due to the subtle nature of the effect (the values for σ ε=2 fell ~5-1% lower than those for σ ) and the scatter inherent in the tests. Grain size and Z were seen to exert a noticeable influence on ε. This arameter was sensitive to the initial grain size at low values of Z and was not at higher levels of Z. The high temerature and low rate tests carried out in the resent work fall within the DRX region roosed by Matsubara et al. (σ<~4mpa) [7]. It is, therefore, quite reasonable to exect that the acceleration of DRX nucleation in the fine grained samles would lead to lower values of ε at low values of Z. The resent microstructural observations show that if this is true then it is only the beginnings of conventional DRX, e.g. boundary bulging, that is occurring even at a strain of 4. In addition, even for the coarsest grained samle, there is no evidence in the microstructures for the traditional necklace tye formation of DRX grains. In concetual suort of this idea is the relative insensitivity of l 15 to strain in the fine grained samle (Figure 9), which suggests that some rearrangement of high angle boundaries is occurring during deformation in this samle. If this was not the case, the value of l 15 would continue to dro with increasing strain due to the change in grain shae dictated by the deformation. The role this lays in determining the stress is currently unclear but it is quite conceivable that high angle boundary migration lays a art in this rearrangement. The data in Figure 8 suggest another ossible exlanation for the low values of ε seen in the fine grained samles. It is evident that, for the conditions examined, ε correlates with the strain at which the subgrain size assumes, more or less, a steady state. It can be imagined that a steady state subgrain size of ~14µm is far more readily established in a samle starting with a grain size of 25µm than it is in a samle with a grain size of 15µm. The amount of dislocations required to make u the subgrain network is far less due to the amount of re-existing boundary. In contrast, at higher values of Z, the subgrain size is smaller and the head start towards a steady state subgrain size made ossible by the fine initial grain size would not be so great. Assuming that the steady state stress is achieved at the same oint as the steady state subgrain size, one could exect that the influence of grain size on ε would reduce with increasing Z, as observed.

6 Summary The effect of the initial grain size on the warm deformation behaviour of a commercial IF steel was studied by hot torsion. It has been shown that there is work hardening followed by a slight degree of softening over a wide range of deformation conditions. The initial grain size does not have a significant effect on the eak stress or on the stress at the final strain. Furthermore there is little effect of grain size on the strain to the eak stress at high values of the Zener Hollomon arameter, although there is a ronounced effect at lower values. The strain to eak stress, the eak stress and the stress at final strain were modeled using three distinctive hyerbolic sine laws. No grain size effect was assumed for the stresses whereas the grain size effect was taken into account for the eak strain. The limited studies of the structural evolution using EBSD show a comlicated behaviour in the generation of new boundaries during deformation that varies for the fine and coarse initial grains. The exact mechanism for flow softening has not been deduced in the current work, although some otential exlanations are roosed based on analysis of the emerging microstructure. Acknowledgements The authors acknowledge the Australian Research Council (ARC) for financial suort. They also thank BHP Steel for the rovision of material for this study. References [1] J.C. Herman and V. Leroy: The Future of Flat Rolled Products (Chicago, June 11-14, 1995). [2] M. R. Barnett and J. J. Jonas: ISIJ Int., 39 (1999), [3] C. M. Sellars: Annealing rocesses - Recovery, Recrystallization and Grain Growth, N. Hansen, D. Juuljensen, T. Leffers and B. Ralh Editors, (1986), [4] C. M. Sellars: Metals Forum, (1981),. 75. [5] A. Najafi-Zadeh, J. J. Jonas and S. Yue: Met. Trans. A, 23A Setember (1992), [6] J. Baczynski and J. J. Jonas: ReX'96 Third International Conference on Recrystallization and Related Phenomena, (1996),. 3. [7] Y. Matsubara, N. Tsuji and Y. Saito: Thermec'97 International Conference on Thermomechanical Processing of Steels & Other Materials, T. Chandra and T. Sakai Editors, (1997), [8] C. Huang, E. B. Hawbolt, X. Chen, T. R. Meadowcroft and D. K. Matlock: Acta Mater., 49 (21), [9] P. D. Hodgson, D. C. Collinson and B. Perett: Proceedings of the 7th International Symosium on Physical Simulation, Tsukuba, Jaan, (1997), [1] I. Ochiai, H. Ohba and A. Kawana: Wire Journal Int., December (1994),. 74. [11] C.M. Sellars and W.J. McTegart: Acta Metall., 14 (1966), [12] M.E. Kassner, M.Z. Wang, M.-T. Perez-Prado and S. Alhajeri: Met. Trans. A, 33A October (22),