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The annealing phenomena and thermal stability of severely deformed steel sheet
Materials Science and Engineering A 528 (2011) 5212–5218
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
The annealing phenomena and thermal stability of severely deformed steel sheet F. Khodabakhshi, M. Kazeminezhad ∗ Department of Materials Science and Engineering, Sharif University of Technology, Azadi Avenue, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 10 January 2011 Received in revised form 3 March 2011 Accepted 4 March 2011 Available online 11 March 2011 Keywords: Annealing behavior Constrained groove pressing Low carbon steel Nano-structure Thermal stability
a b s t r a c t However, there are many works on annealing process of SPDed non-ferrous metals, there are limit works on annealing process of SPDed low carbon steel. Therefore, in this study the annealing responses after constrained groove pressing (CGP) of low carbon steel sheets have been investigated. The sheets are subjected to severe plastic deformation at room temperature by CGP method up to three passes. Nanostructured low carbon steel sheets produced by severe plastic deformation are annealed at temperature range of 100–600 ◦ C for 20 min. The changes of their microstructures after deformation and annealing are studied by optical microscopy. The effects of large strain and annealing temperature on microstructure, strength and hardness evolutions of the nano-scale grained low carbon steel are examined. The results show that annealing phenomena can effectively improve the elongation of SPDed sheets with preserving the hardness and mechanical strength. Also, the thermal stability of microstructure and mechanical properties can be observed through annealing temperatures up to 400 ◦ C and temperature of 400 ◦ C is achieved as an optimum annealing temperature in which both strength and elongation are increased and hardness inhomogeneity of the sheet is minimum. Annealing at temperatures of higher than 400 ◦ C leads to abnormal grain growth. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Reduction in grain size leads to increase in the strength and toughness of material at ambient temperatures. If the small grain size is retained at elevated temperatures where diffusion is rapid, there is also a potential for achieving good formability and superplastic ductilities [1]. Ultra fine grained (UFG) materials can be synthesized by either bottom-up or top-down approaches. In the bottom-up approach, the nanostructure is produced through atom-by-atom and layer-by-layer arrangements that often leads to porous structure and is not applicable to industrial manufacturing [2]. In the top-down approach, the bulk materials microstructure is changed to nanostructure utilizing severe plastic deformation (SPD) in which materials are subjected to the impose of very large strains without concurrent changes in the cross-sectional dimensions of the samples [3]. The principle of this process includes increasing the dislocation density by heavily uniform deformation, forming of dense dislocation walls and acquiring ultrafine microstructure [4]. Several methods have been introduced for imposing the severe plastic deformation to the metals [5–14]. Some of these methods are: equal channel angular pressing (ECAP) [14], high pressure torsion (HPT) [6], accumulative roll bonding (ARB)
∗ Corresponding author. Tel.: +98 21 66165227; fax: +98 21 66005717. E-mail address: [email protected] (M. Kazeminezhad). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.03.024
[13], repetitive corrugation and straightening (RCS) [7], constrained groove rolling (CGR) [10], constrained groove pressing (CGP) [12], Friction stir processing [15], etc. Within these methods, the four methods of ARB, RCS, CGR and CGP are applicable for producing ultrafine grained structure in sheets. Accumulative roll bonding (ARB) and constrained groove pressing (CGP) are two major and useful processes for SPD of sheets. In ARB process a perfect bonding cannot be achieved and therefore, this is considered as a major defect of this method. Thus, ARB is less considered feasible for severe plastic deformation of sheet metals. But, constrained groove pressing that was originally proposed by Shin et al. [12], has the advantage of imposing more uniform severe plastic deformation on sheet metals. The principle of CGP is subjecting a material to a large amount of plastic shear deformation with asymmetrically grooved and flat dies, alternatively [4] (see Fig. 1). Recently, many researches have been carried out on mechanical properties and microstructure of some sheet metals such as copper, aluminum and low carbon steel, processed by CGP indicating that this process can effectively enhanced mechanical properties of sheet metals such as increase in yield strength and mean hardness [12,16–18]. Also, the results show that this process has the ability of producing submicrometer polygonized grain structure with well-defined grain boundary [12,17]. Refining the grain size of useful metals such as low carbon steel is of great interest as the yield stress is increased with grain refinement, significantly. This leads to an improvement in the strength to weight ratio, which is
F. Khodabakhshi, M. Kazeminezhad / Materials Science and Engineering A 528 (2011) 5212–5218
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Fig. 1. Schematic of constrained groove pressing (CGP) technique.
a desirable property for automobile industry. In the previous work of the present authors carried out on mechanical properties and microstructure evolution of low carbon steel sheet processed by CGP, it was shown that this process can effectively increase the mechanical properties of the sheets such as yield strength, ultimate tensile strength and mean hardness [16]. Also, this process can efficiently refine the ferritic coarse grained microstructure to nano-scale grained structure with well-defined grain boundary. By imposing the CGP method to the steel sheets, the strength is increased up to about 100% and the microstructure is approximately refined to 250 nm through third CGP pass. But elongation and formability of these sheets are decreased due to CGP process and it seems that annealing of these SPDed sheets for enhancing the formability properties is necessary. The annealing behavior of SPDed metals is very important, because some of these metals show thermal stability behavior through annealing. Due to this behavior, ductility of SPDed metal is improved with preserving strength and hardness; also the microstructure is modified and goes to a saturated condition. There are a high number of researches about annealing responses of SPDed ferrous and non-ferrous bulk metals, such as aluminum and its alloys, copper and low carbon steel [19–33]. Also, there are some researches about annealing of SPDed non-ferrous sheet metals such as aluminum and copper [34–36]. In most of these researches, it can be seen that 300 ◦ C is introduced as optimum post annealing temperature for achieving a good thermal stability in microstructure and mechanical properties of aluminum and copper. Thermal stability examinations on low carbon steel processed by ECAP and subsequent annealing show that via annealing at temperature range of 693–783 K, ultrafine ferrite grains were relatively stable with little grain growth. Therefore, as can be found, there are very limit works on annealing behavior of SPDed nonferrous sheet metals and no work on annealing of SPDed ferrous sheet metals. In the present study, the effects of annealing phenomena on the mechanical properties, hardness inhomogeneities and microstructures of low carbon steel sheets under CGP process are investigated. Also, the thermal stability of microstructure and mechanical properties of the CGPed steel sheets are studied and a critical annealing temperature leading to sudden drop in mechanical properties due to abnormal grain growth is introduced.
Fig. 2. Schematic of section for optical microscopy observations.
2. Experimental procedure A low carbon steel sheet was used to study the effectiveness of constrained groove pressing for strengthening and grain refining of low carbon steel at room temperature and in the following for investigating the effect of annealing on CGPed sheets. The specimens used for these studies were sheets of 84 mm × 45 mm × 3 mm. The chemical composition of studied sheet is shown in Table 1. The sheets were taken from industrial rolling production. One pass of CGP process contains four stages shown in Fig. 1. In the first step of groove pressing, the specimen was pressed between asymmetrically grooved dies (see Fig. 1). The deformed sheet was then removed from the dies and the second pressing was carried out using a set of flat dies. After the second pressing, the sample was rotated by 180◦ around an axis perpendicular to the plane of the sheet and pressed again using the grooved and flat dies. Therefore, the successive mentioned pressings with grooved and flat dies called one pass resulted in a homogeneous effective strain of 1.16 throughout the sample. Teflon layers were used as lubricant between low carbon steel sheet and dies. By using Teflon layers, it was possible to impose the strain magnitude of 3.48 to the low carbon steel sheets through three passes. After imposing the severe plastic deformation to steel sheets, the sheets were subjected to annealing phenomenon to enhance sheet elongations with controlling the hardness and strength. The samples during annealing were encapsulated at a controlled atmosphere in order to minimize the possible decarburization and oxidation. Annealing was conducted for 20 min on CGPed sheets at the temperature range of 100–600 ◦ C with 100 ◦ C interval. During annealing, temperature was controlled within ±2 ◦ C. To investigate the mechanical behavior of the sheets before and after annealing, tensile and hardness tests were carried out. Tensile specimens were cut along the longitudinal direction and machined according to the ASTM E8M standard. All tensile tests were conducted at room temperature using an Instron tensile test machine operating at a constant cross-head speed and with an initial strain rate of 5 × 10−4 s−1 . Vickers hardness measurements were car-
Fig. 3. Stress–strain curve of as-received low carbon steel sheet.
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Fig. 4. Variations of tensile properties of low carbon steel sheets during CGP process.
ried out along the longitudinal direction on the surface of sheets. The mean hardness values were achieved from 10 measurements on each specimen. The microstructures of CGPed sheets were investigated after annealing by using an optical microscopy. Cross section that used for optical microscopy observations is shown in Fig. 2. Samples for these observations were ground on papers, polished and then etched in a solution of CH3 OH containing 3% HNO3 . 3. Results and discussion 3.1. Mechanical properties Stress–strain curve in Fig. 3 shows the tensile behavior of asreceived sample. It can be seen that as-received low carbon steel sheet has a tensile strength of 299 MPa, yield stress of 218 MPa and elongation of 47.3%. The tensile properties variations (tensile strength and elongation) of the deformed sheets versus pass number and magnitude of imposed strain are shown in Fig. 4. The general trend that can be found from Fig. 4 is that the tensile strength is increased and the elongation is decreased with increasing the strain. The tensile strength of the as-received sheet was 299 MPa and after one pass, it is increased to 418 MPa and then the rate of strength improvement is decreased. The decrease in the rate may be related to dislocation annihilation or appearance of microcracks [17,18]. After three passes of CGP, the strength reaches to 428 MPa. Increasing the tensile strength of materials during SPD is common and arises from the work hardening and grain refining mechanisms. Elongation is extremely decreased to 12% after first CGP pass, and
Fig. 5. Variation of tensile properties of CGPed low carbon steel sheets versus annealing temperature and CGP pass number.
Fig. 6. The effect of annealing temperature on elongation of CGPed steel sheets.
then the rate of its reduction is decreased. After third pass, it reaches to 10.4%. Fig. 5 shows the tensile strength of groove pressed low carbon steel sheets versus annealing temperature at different CGP pass numbers. As can be seen, for all passes the tensile strength is firstly increased with increasing temperature and then it is decreased. On the other hand, firstly with increasing the temperature, the tensile strength is preserved and may be increased up to 400 ◦ C and further annealing leads to strength drop that these variations are more obvious for specimens after one and two CGP passes. As an example for specimen after one CGP pass, the strength is increased from 418 MPa at room temperature (as-pressed state) to 445 MPa at 400 ◦ C annealing temperature. At this stage, increase of grain boundary density due to partial restoration phenomenon may compensate the decrease of dislocation density. Thus, the strength is not decreased sharply unless the temperatures above 400 ◦ C used for annealing of specimens. Through annealing at temperatures higher than 400 ◦ C, due to the existence of large amount of strain in CGPed specimens, recrystallization and grain growth phenomena occur extremely which lead to loss of main strength. For example, the strength of one pass CGPed sheet annealed at 600 ◦ C is decreased to 325 MPa. From the tensile strength variations of sheets with annealing temperature, it can be seen that there is a thermal stability up to annealing temperature of 400 ◦ C. Also, from this figure it can be seen that with increasing CGP pass number at a constant annealing temperature, strengths of CGPed steel sheets are decreased. This trend may be due to the fact that the driving force for recrystallization and subsequent grain growth is increased with increasing the stored strain energy at specimens. The effect of post annealing phenomena on microstructural evolution of SPDed steel sheets will be explained in the microstructure section (Section 3.3).
Fig. 7. The profile of Vickers hardness along the length of the as-received specimen.
F. Khodabakhshi, M. Kazeminezhad / Materials Science and Engineering A 528 (2011) 5212–5218
Fig. 8. The Vickers hardness profiles along the length of the first pass specimens with different annealing temperatures.
Fig. 9. The Vickers hardness profiles along the length of the second pass specimens with different annealing temperatures.
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Fig. 11. Mean hardness value versus CGP pass number at room temperature.
Fig. 12. Variation of mean hardness with annealing temperature in each pass.
Fig. 13. Variation of I.F. with annealing temperature and CGP pass number. Fig. 10. The Vickers hardness profiles along the length of the third pass specimens with different annealing temperatures.
Fig. 6 shows the effect of post annealing treatment on the elongation of CGPed steel sheets. As can be seen, the annealing phenomena can effectively improve the elongation of as-pressed sheets. From this figure, it can be seen that the elongation is continuously increased with increasing the annealing temperature for all passes. As an example for third pass specimen, after
annealing at 100 ◦ C elongation is 10.4%, and after annealing at 600 ◦ C it is improved to 21%. Elongation improvement of heavily deformed specimens versus post annealing temperature is common and it is due to decreasing of dislocation density and also occurring of recrystallization and grain growth phenomena with annealing.
Table 1 Chemical composition of the steel sheet (wt.%). Fe
C
Si
Mn
P
S
Cr
Ni
Mo
Base
0.0527
0.0229
0.203
0.006
0.0031
0.0088
0.0281
0.0024
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Fig. 14. Microstructure of as-received specimen.
3.2. Hardness The hardness distribution across the length of as-received specimen on the surface plane is shown in Fig. 7. It can be seen from this figure that the hardness distribution along the length of as-received sheet is more uniform with an average hardness of 98 VHN.
The hardness distributions across the length of the CGPed specimens annealed at 25 (as-pressed), 100, 200, 300, 400, 500 and 600 ◦ C are shown in Figs. 8–10, respectively. As can be seen in these figures, in each pass the hardness is firstly increased with increasing the post annealing temperature. But in the following, with increasing the annealing temperature more than 400 ◦ C, the hardness is decreased. After annealing at 600 ◦ C, a sharp decrease in hardness distribution occurs for all passes. Also, it can be seen in these figures that with increasing the CGP pass number, the hardness values are decreased at constant post annealing temperature. Using the data presented in Figs. 8–10, the uniformity of hardness is quantitatively discussed later. Fig. 11 shows the mean values of hardness as a function of strain and CGP pass number at room temperature. As can be seen, the hardness values of sheets are rapidly increased from 98 VHN to 163 VHN through first pass. However in the following passes, the rate of increase in hardness is reduced and after three passes reaches to 180 VHN. Fig. 12 illustrates the mean hardness values of CGPed sheets after annealing at 25 (as-pressed) to 600 ◦ C for different passes. As can be seen, mean hardness values for all passes are preserved or slightly increased up to 400 ◦ C and therefore samples with higher post-annealing temperatures show higher hardness values. For example, hardness of the sheet after three CGP passes is increased from 158.3 VHN at annealing temperature of 100 ◦ C to 170.5 VHN at that of 400 ◦ C. Annealing at temperature higher than 500 ◦ C shows a sharp decrease in hardness value and tensile strength due
Fig. 15. The microstructures of CGPed steel sheets after post annealing at 400 ◦ C: (a) one pass, (b) two passes and (c) three passes.
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Fig. 16. The microstructures of CGPed steel sheets after post annealing at 600 ◦ C: (a) one pass, (b) two passes and (c) three passes.
to rapid grain growth (that is described in next section). Consequently, the temperatures higher than 400 ◦ C are not appropriate for post-annealing of CGPed low carbon steel sheets. The variations of hardness versus annealing temperature show a thermal stability behavior for annealing up to 400 ◦ C temperature. As mentioned, the trends for hardness variations are similar to that for strength and the descriptions are the same. In order to investigate the uniformity of hardness distributions, the inhomogeneity factor (I.F.) is used. This can be calculated by [36]:
n I.F. =
i=1
¯ (Hi − H) ¯ H
2
0.5
/n − 1
(1)
where n is the number of hardness measurements on each spec¯ is the imen, Hi is the hardness value of ith measurement, and H mean hardness value. Less I.F. value indicates higher homogeneity of mechanical properties. In conventional metal forming processes, as the annealing temperature is increased, the inhomogeneity factor is decreased [37]. In contrast to the expectation, the results of this study show that there is an optimum post-annealing temperature for acquiring the least I.F. Fig. 13 shows the variation of I.F. value with post annealing temperature in different CGP passes. As reported in this figure, the I.F. value of as-received specimen is least, but after imposing of one CGP pass, it is increased. Also among CGPed sheets, the I.F. is decreased with increasing the CGP pass number indicating increase of strain homogeneity.
With increasing the annealing temperature, the I.F. value is rapidly increased at first and then it is decreased up to annealing at 400 ◦ C. In annealing at temperatures higher than 400 ◦ C, the I.F. value is increased again. As it is shown in Fig. 13, the least I.F. can be obtained by post annealing of CGPed specimens at 400 ◦ C. The increase in I.F. after annealing at temperatures higher than 400 ◦ C can be interpreted by abnormal grain growth presented in next section.
3.3. Microstructure In previous sections, it was discussed that abnormal grain growth leads to sharp strength and hardness drops in specimens annealed at elevated temperatures. The microstructure of the asreceived low carbon steel sheet from the thickness plane (Fig. 2) is shown in Fig. 14. As can be seen in this figure, the as-received steel consists of approximately 6 vol% pearlite (dark contrast) and the remainder is ferrite (bright contrast). As can be observed in Fig. 14, as-received low carbon steel sheet has an average grain size of 30 m. The microstructural evolutions of low carbon steel sheets during constrained groove pressing were studied in the work of present authors [16]. It was revealed that an ultrafine grain structure with cell size of 200–300 nm can be achieved in low carbon steel sheets by imposing the CGP method up to three passes. For example, the microstructures after post-annealing of specimens at two different annealing temperatures (400 ◦ C and 600 ◦ C)
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are shown in Figs. 15 and 16. Fig. 15a, b and c shows the microstructures of annealed sheets at 400 ◦ C after one, two and three CGP passes, respectively. The normal grain growth can be observed. As shown in Fig. 16, with increasing the postannealing temperature up to 600 ◦ C, the abnormal grain growth can be observed. Regarding the presence of strain inhomogeneity in CGPed sheets, driving force for restoration is not uniform, which leads to dissimilar grain sizes. Consequently, in the grain growth phenomenon occurs at elevated temperatures, the kinetic of growth results in abnormal grain growth. The same results for abnormal grain growth have been reported for CGPed copper [36]. As mentioned in previous sections, hardness and tensile strength variations show the thermal stability behavior at annealing temperatures up to 400 ◦ C temperature. From the microstructural investigations, a reliable thermal stability up to that temperature can be observed. In all of previous discussions, it was shown that with post annealing of CGPed sheets at 400 ◦ C, in addition to preserving the hardness and strength, elongation is significantly increased that this subject can improve the formability of the severely deformed steel sheets. The I.F. examinations show that hardness distributions of CGPed specimens are more uniform after annealing at 400 ◦ C. Also microstructural investigations show that no abnormal grain growth occurs in this temperature. Therefore, temperature of 400 ◦ C can be introduced as an optimum post annealing temperature for CGPed low carbon steel sheets. Also, it can be considered that severely deformed steel sheets annealed at this temperature have a thermal stability condition. This result is consistent with that reported in Section 1. 4. Conclusions In this research, method of constrained groove pressing (CGP) is carried out on the sheets of low carbon steel at the room temperature and the strain magnitudes up to 3.48 is imposed to these sheets. The effects of post-annealing phenomena on mechanical properties and microstructures of these CGPed sheets have been studied. The main results of this study can be presented as follows: 1. Tensile strength of the steel sheet is increased from 299 MPa to 428 MPa through imposing three passes of CGP at room temperature, and elongation is decreased from 47.3% to 10.4%. Also, the hardness is continuously increased from 98 VHN to 180 VHN with increasing the pass number. 2. Tensile strength and hardness of CGPed sheets are increased with increasing annealing temperature up to 400 ◦ C and higher annealing temperatures cause to strength and hardness drops. 3. Post annealing phenomena can effectively increase the elongation of severely deformed sheets. 4. The I.F. of CGPed and annealed sheets at 400 ◦ C is the lowest. In the other words, specimens in this temperature have the most homogenous mechanical properties and microstructure.
5. Annealing of CGPed sheets at temperatures higher than 400 ◦ C leads to abnormal grain growth. 6. The microstructural and mechanical properties investigations show acceptable thermal stability up to 400 ◦ C. 7. The temperature of 400 ◦ C is introduced as an optimum post annealing temperature to achieve a high strength low carbon steel sheet with reliable elongation. Acknowledgements The authors wish to thank the research board of Sharif University of Technology for the financial support and the provision of the research facilities used in this work. References [1] M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A 324 (2002) 82–89. [2] P.G. Sanders, J.A. Eastman, J.R. Weertman, Acta Mater. 46 (1998) 4195–4202. [3] Y.W. Tham, M.W. Fu, H.H. Hng, M.S. Yong, K.B. Lim, J. Mater. Process. Technol. 192–193 (2007) 575–581. [4] E. Hosseini, M. Kazeminezhad, A. Mani, E. Rafizadeh, Comput. Mater. Sci. 45 (2009) 855–859. [5] I.V. Alexandrov, R.Z. Valiev, Nanostruct. Mater. 12 (1999) 709–712. [6] Z. Horita, T.G. Langdon, Scripta Mater. 58 (2008) 1029–1032. [7] J. Huang, Y.T. Zhu, D.J. Alexander, X. Liao, T.C. Lowe, R.J. Asaro, Mater. Sci. Eng. A 371 (2004) 35–39. [8] J.Y. Huang, Y.T. Zhu, H. Jiang, T.C. Lowe, Acta Mater. 49 (2001) 1497–1505. [9] J. Kratochvíl, M. Kruzík, R. Sedlácek, Acta Mater. 57 (2009) 739–748. [10] J.W. Lee, J.J. Park, J. Mater. Process. Technol. 130–131 (2002) 208–213. [11] Y. Saito, N. Tsuji, H. Utsunomiya, T. Sakai, R.G. Hong, Scripta Mater. 39 (1998) 1221–1227. [12] D.H. Shin, J.J. Park, Y.S. Kim, K.T. Park, Mater. Sci. Eng. A 328 (2002) 98–103. [13] N. Tsuji, Y. Saito, H. Utsunomiya, S. Tanigawa, Scripta Mater. 40 (1999) 795–800. [14] R.Z. Valiev, T.G. Langdon, Prog. Mater. Sci. 51 (2006) 881–981. [15] R.S. Mishra, Z.Y. Ma, Mater. Sci. Eng. R 50 (2005) 1–78. [16] F. Khodabakhshi, M. Kazeminezhad, A.H. Kokabi, Mater. Sci. Eng. A 527 (2010) 4043–4049. [17] A. Krishnaiah, U. Chakkingal, P. Venugopal, Mater. Sci. Eng. A 410–411 (2005) 337–340. [18] A. Krishnaiah, U. Chakkingal, P. Venugopal, Scripta Mater. 52 (2005) 1229–1233. [19] W.Q. Cao, A. Godfrey, W. Liu, Q. Liu, Mater. Sci. Eng. A 360 (2003) 420–425. [20] W.Q. Cao, A. Godfrey, W. Liu, Q. Liu, Mater. Lett. 57 (2003) 3767–3774. [21] R. Daly, S. Zghal, N. Njeh, Phys. Proc. 2 (2009) 677–684. [22] S. Ferrasse, V.M. Segal, F. Alford, Mater. Sci. Eng. A 372 (2004) 235–244. [23] W.Z. Han, S.D. Wu, S.X. Li, Y.D. Wang, Mater. Sci. Eng. A 483–484 (2008) 430–432. [24] M.V. Markushev, M.Y. Murashkin, Mater. Sci. Eng. A 367 (2004) 234–242. ˜ [25] M.A. Munoz-Morris, C. Garcia Oca, G. Gonzalez-Doncel, D.G. Morris, Mater. Sci. Eng. A 375–377 (2004) 853–856. [26] K.T. Park, S.Y. Han, B.D. Ahn, D.H. Shin, Y.K. Lee, K.K. Um, Scripta Mater. 51 (2004) 909–913. [27] D.H. Shin, B.C. Kim, K.T. Park, W.Y. Choo, Acta Mater. 48 (2000) 3245–3252. [28] D.H. Shin, Y.S. Kim, E.J. Lavernia, Acta Mater. 49 (2001) 2387–2393. [29] P. Szczygiel, H.J. Roven, O. Reiso, Mater. Sci. Eng. A 493 (2008) 202–206. [30] D.M. Jiang, J.L. Ning, J.F. Sun, Z.M. Hu, Y. Hou, Trans. Nonferrous Met. Soc. China 18 (2008) 248–254. [31] M. Kazeminezhad, Comput. Mater. Sci. 43 (2008) 309–312. [32] Y.H. Zhao, X.Z. Liao, Z. Jin, R.Z. Valiev, Y.T. Zhu, Acta Mater. 52 (2004) 4589–4599. [33] A. Zi, I. Stulikova, B. Smola, Mater. Sci. Eng. A 527 (2010) 1469–1472. [34] C. Kwan, Z. Wang, S.B. Kang, Mater. Sci. Eng. A 480 (2008) 148–159. [35] N. Tsuji, Y. Ito, Y. Saito, Y. Minamino, Scripta Mater. 47 (2002) 893–899. [36] E. Rafizadeh, A. Mani, M. Kazeminezhad, Mater. Sci. Eng. A 515 (2009) 162–168. [37] M. Kazeminezhad, A. Karimi Taheri, A. Kiet Tieu, Comput. Mater. Sci. 38 (2007) 765–773.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
The annealing phenomena and thermal stability of severely deformed steel sheet F. Khodabakhshi, M. Kazeminezhad ∗ Department of Materials Science and Engineering, Sharif University of Technology, Azadi Avenue, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 10 January 2011 Received in revised form 3 March 2011 Accepted 4 March 2011 Available online 11 March 2011 Keywords: Annealing behavior Constrained groove pressing Low carbon steel Nano-structure Thermal stability
a b s t r a c t However, there are many works on annealing process of SPDed non-ferrous metals, there are limit works on annealing process of SPDed low carbon steel. Therefore, in this study the annealing responses after constrained groove pressing (CGP) of low carbon steel sheets have been investigated. The sheets are subjected to severe plastic deformation at room temperature by CGP method up to three passes. Nanostructured low carbon steel sheets produced by severe plastic deformation are annealed at temperature range of 100–600 ◦ C for 20 min. The changes of their microstructures after deformation and annealing are studied by optical microscopy. The effects of large strain and annealing temperature on microstructure, strength and hardness evolutions of the nano-scale grained low carbon steel are examined. The results show that annealing phenomena can effectively improve the elongation of SPDed sheets with preserving the hardness and mechanical strength. Also, the thermal stability of microstructure and mechanical properties can be observed through annealing temperatures up to 400 ◦ C and temperature of 400 ◦ C is achieved as an optimum annealing temperature in which both strength and elongation are increased and hardness inhomogeneity of the sheet is minimum. Annealing at temperatures of higher than 400 ◦ C leads to abnormal grain growth. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Reduction in grain size leads to increase in the strength and toughness of material at ambient temperatures. If the small grain size is retained at elevated temperatures where diffusion is rapid, there is also a potential for achieving good formability and superplastic ductilities [1]. Ultra fine grained (UFG) materials can be synthesized by either bottom-up or top-down approaches. In the bottom-up approach, the nanostructure is produced through atom-by-atom and layer-by-layer arrangements that often leads to porous structure and is not applicable to industrial manufacturing [2]. In the top-down approach, the bulk materials microstructure is changed to nanostructure utilizing severe plastic deformation (SPD) in which materials are subjected to the impose of very large strains without concurrent changes in the cross-sectional dimensions of the samples [3]. The principle of this process includes increasing the dislocation density by heavily uniform deformation, forming of dense dislocation walls and acquiring ultrafine microstructure [4]. Several methods have been introduced for imposing the severe plastic deformation to the metals [5–14]. Some of these methods are: equal channel angular pressing (ECAP) [14], high pressure torsion (HPT) [6], accumulative roll bonding (ARB)
∗ Corresponding author. Tel.: +98 21 66165227; fax: +98 21 66005717. E-mail address: [email protected] (M. Kazeminezhad). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.03.024
[13], repetitive corrugation and straightening (RCS) [7], constrained groove rolling (CGR) [10], constrained groove pressing (CGP) [12], Friction stir processing [15], etc. Within these methods, the four methods of ARB, RCS, CGR and CGP are applicable for producing ultrafine grained structure in sheets. Accumulative roll bonding (ARB) and constrained groove pressing (CGP) are two major and useful processes for SPD of sheets. In ARB process a perfect bonding cannot be achieved and therefore, this is considered as a major defect of this method. Thus, ARB is less considered feasible for severe plastic deformation of sheet metals. But, constrained groove pressing that was originally proposed by Shin et al. [12], has the advantage of imposing more uniform severe plastic deformation on sheet metals. The principle of CGP is subjecting a material to a large amount of plastic shear deformation with asymmetrically grooved and flat dies, alternatively [4] (see Fig. 1). Recently, many researches have been carried out on mechanical properties and microstructure of some sheet metals such as copper, aluminum and low carbon steel, processed by CGP indicating that this process can effectively enhanced mechanical properties of sheet metals such as increase in yield strength and mean hardness [12,16–18]. Also, the results show that this process has the ability of producing submicrometer polygonized grain structure with well-defined grain boundary [12,17]. Refining the grain size of useful metals such as low carbon steel is of great interest as the yield stress is increased with grain refinement, significantly. This leads to an improvement in the strength to weight ratio, which is
F. Khodabakhshi, M. Kazeminezhad / Materials Science and Engineering A 528 (2011) 5212–5218
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Fig. 1. Schematic of constrained groove pressing (CGP) technique.
a desirable property for automobile industry. In the previous work of the present authors carried out on mechanical properties and microstructure evolution of low carbon steel sheet processed by CGP, it was shown that this process can effectively increase the mechanical properties of the sheets such as yield strength, ultimate tensile strength and mean hardness [16]. Also, this process can efficiently refine the ferritic coarse grained microstructure to nano-scale grained structure with well-defined grain boundary. By imposing the CGP method to the steel sheets, the strength is increased up to about 100% and the microstructure is approximately refined to 250 nm through third CGP pass. But elongation and formability of these sheets are decreased due to CGP process and it seems that annealing of these SPDed sheets for enhancing the formability properties is necessary. The annealing behavior of SPDed metals is very important, because some of these metals show thermal stability behavior through annealing. Due to this behavior, ductility of SPDed metal is improved with preserving strength and hardness; also the microstructure is modified and goes to a saturated condition. There are a high number of researches about annealing responses of SPDed ferrous and non-ferrous bulk metals, such as aluminum and its alloys, copper and low carbon steel [19–33]. Also, there are some researches about annealing of SPDed non-ferrous sheet metals such as aluminum and copper [34–36]. In most of these researches, it can be seen that 300 ◦ C is introduced as optimum post annealing temperature for achieving a good thermal stability in microstructure and mechanical properties of aluminum and copper. Thermal stability examinations on low carbon steel processed by ECAP and subsequent annealing show that via annealing at temperature range of 693–783 K, ultrafine ferrite grains were relatively stable with little grain growth. Therefore, as can be found, there are very limit works on annealing behavior of SPDed nonferrous sheet metals and no work on annealing of SPDed ferrous sheet metals. In the present study, the effects of annealing phenomena on the mechanical properties, hardness inhomogeneities and microstructures of low carbon steel sheets under CGP process are investigated. Also, the thermal stability of microstructure and mechanical properties of the CGPed steel sheets are studied and a critical annealing temperature leading to sudden drop in mechanical properties due to abnormal grain growth is introduced.
Fig. 2. Schematic of section for optical microscopy observations.
2. Experimental procedure A low carbon steel sheet was used to study the effectiveness of constrained groove pressing for strengthening and grain refining of low carbon steel at room temperature and in the following for investigating the effect of annealing on CGPed sheets. The specimens used for these studies were sheets of 84 mm × 45 mm × 3 mm. The chemical composition of studied sheet is shown in Table 1. The sheets were taken from industrial rolling production. One pass of CGP process contains four stages shown in Fig. 1. In the first step of groove pressing, the specimen was pressed between asymmetrically grooved dies (see Fig. 1). The deformed sheet was then removed from the dies and the second pressing was carried out using a set of flat dies. After the second pressing, the sample was rotated by 180◦ around an axis perpendicular to the plane of the sheet and pressed again using the grooved and flat dies. Therefore, the successive mentioned pressings with grooved and flat dies called one pass resulted in a homogeneous effective strain of 1.16 throughout the sample. Teflon layers were used as lubricant between low carbon steel sheet and dies. By using Teflon layers, it was possible to impose the strain magnitude of 3.48 to the low carbon steel sheets through three passes. After imposing the severe plastic deformation to steel sheets, the sheets were subjected to annealing phenomenon to enhance sheet elongations with controlling the hardness and strength. The samples during annealing were encapsulated at a controlled atmosphere in order to minimize the possible decarburization and oxidation. Annealing was conducted for 20 min on CGPed sheets at the temperature range of 100–600 ◦ C with 100 ◦ C interval. During annealing, temperature was controlled within ±2 ◦ C. To investigate the mechanical behavior of the sheets before and after annealing, tensile and hardness tests were carried out. Tensile specimens were cut along the longitudinal direction and machined according to the ASTM E8M standard. All tensile tests were conducted at room temperature using an Instron tensile test machine operating at a constant cross-head speed and with an initial strain rate of 5 × 10−4 s−1 . Vickers hardness measurements were car-
Fig. 3. Stress–strain curve of as-received low carbon steel sheet.
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Fig. 4. Variations of tensile properties of low carbon steel sheets during CGP process.
ried out along the longitudinal direction on the surface of sheets. The mean hardness values were achieved from 10 measurements on each specimen. The microstructures of CGPed sheets were investigated after annealing by using an optical microscopy. Cross section that used for optical microscopy observations is shown in Fig. 2. Samples for these observations were ground on papers, polished and then etched in a solution of CH3 OH containing 3% HNO3 . 3. Results and discussion 3.1. Mechanical properties Stress–strain curve in Fig. 3 shows the tensile behavior of asreceived sample. It can be seen that as-received low carbon steel sheet has a tensile strength of 299 MPa, yield stress of 218 MPa and elongation of 47.3%. The tensile properties variations (tensile strength and elongation) of the deformed sheets versus pass number and magnitude of imposed strain are shown in Fig. 4. The general trend that can be found from Fig. 4 is that the tensile strength is increased and the elongation is decreased with increasing the strain. The tensile strength of the as-received sheet was 299 MPa and after one pass, it is increased to 418 MPa and then the rate of strength improvement is decreased. The decrease in the rate may be related to dislocation annihilation or appearance of microcracks [17,18]. After three passes of CGP, the strength reaches to 428 MPa. Increasing the tensile strength of materials during SPD is common and arises from the work hardening and grain refining mechanisms. Elongation is extremely decreased to 12% after first CGP pass, and
Fig. 5. Variation of tensile properties of CGPed low carbon steel sheets versus annealing temperature and CGP pass number.
Fig. 6. The effect of annealing temperature on elongation of CGPed steel sheets.
then the rate of its reduction is decreased. After third pass, it reaches to 10.4%. Fig. 5 shows the tensile strength of groove pressed low carbon steel sheets versus annealing temperature at different CGP pass numbers. As can be seen, for all passes the tensile strength is firstly increased with increasing temperature and then it is decreased. On the other hand, firstly with increasing the temperature, the tensile strength is preserved and may be increased up to 400 ◦ C and further annealing leads to strength drop that these variations are more obvious for specimens after one and two CGP passes. As an example for specimen after one CGP pass, the strength is increased from 418 MPa at room temperature (as-pressed state) to 445 MPa at 400 ◦ C annealing temperature. At this stage, increase of grain boundary density due to partial restoration phenomenon may compensate the decrease of dislocation density. Thus, the strength is not decreased sharply unless the temperatures above 400 ◦ C used for annealing of specimens. Through annealing at temperatures higher than 400 ◦ C, due to the existence of large amount of strain in CGPed specimens, recrystallization and grain growth phenomena occur extremely which lead to loss of main strength. For example, the strength of one pass CGPed sheet annealed at 600 ◦ C is decreased to 325 MPa. From the tensile strength variations of sheets with annealing temperature, it can be seen that there is a thermal stability up to annealing temperature of 400 ◦ C. Also, from this figure it can be seen that with increasing CGP pass number at a constant annealing temperature, strengths of CGPed steel sheets are decreased. This trend may be due to the fact that the driving force for recrystallization and subsequent grain growth is increased with increasing the stored strain energy at specimens. The effect of post annealing phenomena on microstructural evolution of SPDed steel sheets will be explained in the microstructure section (Section 3.3).
Fig. 7. The profile of Vickers hardness along the length of the as-received specimen.
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Fig. 8. The Vickers hardness profiles along the length of the first pass specimens with different annealing temperatures.
Fig. 9. The Vickers hardness profiles along the length of the second pass specimens with different annealing temperatures.
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Fig. 11. Mean hardness value versus CGP pass number at room temperature.
Fig. 12. Variation of mean hardness with annealing temperature in each pass.
Fig. 13. Variation of I.F. with annealing temperature and CGP pass number. Fig. 10. The Vickers hardness profiles along the length of the third pass specimens with different annealing temperatures.
Fig. 6 shows the effect of post annealing treatment on the elongation of CGPed steel sheets. As can be seen, the annealing phenomena can effectively improve the elongation of as-pressed sheets. From this figure, it can be seen that the elongation is continuously increased with increasing the annealing temperature for all passes. As an example for third pass specimen, after
annealing at 100 ◦ C elongation is 10.4%, and after annealing at 600 ◦ C it is improved to 21%. Elongation improvement of heavily deformed specimens versus post annealing temperature is common and it is due to decreasing of dislocation density and also occurring of recrystallization and grain growth phenomena with annealing.
Table 1 Chemical composition of the steel sheet (wt.%). Fe
C
Si
Mn
P
S
Cr
Ni
Mo
Base
0.0527
0.0229
0.203
0.006
0.0031
0.0088
0.0281
0.0024
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Fig. 14. Microstructure of as-received specimen.
3.2. Hardness The hardness distribution across the length of as-received specimen on the surface plane is shown in Fig. 7. It can be seen from this figure that the hardness distribution along the length of as-received sheet is more uniform with an average hardness of 98 VHN.
The hardness distributions across the length of the CGPed specimens annealed at 25 (as-pressed), 100, 200, 300, 400, 500 and 600 ◦ C are shown in Figs. 8–10, respectively. As can be seen in these figures, in each pass the hardness is firstly increased with increasing the post annealing temperature. But in the following, with increasing the annealing temperature more than 400 ◦ C, the hardness is decreased. After annealing at 600 ◦ C, a sharp decrease in hardness distribution occurs for all passes. Also, it can be seen in these figures that with increasing the CGP pass number, the hardness values are decreased at constant post annealing temperature. Using the data presented in Figs. 8–10, the uniformity of hardness is quantitatively discussed later. Fig. 11 shows the mean values of hardness as a function of strain and CGP pass number at room temperature. As can be seen, the hardness values of sheets are rapidly increased from 98 VHN to 163 VHN through first pass. However in the following passes, the rate of increase in hardness is reduced and after three passes reaches to 180 VHN. Fig. 12 illustrates the mean hardness values of CGPed sheets after annealing at 25 (as-pressed) to 600 ◦ C for different passes. As can be seen, mean hardness values for all passes are preserved or slightly increased up to 400 ◦ C and therefore samples with higher post-annealing temperatures show higher hardness values. For example, hardness of the sheet after three CGP passes is increased from 158.3 VHN at annealing temperature of 100 ◦ C to 170.5 VHN at that of 400 ◦ C. Annealing at temperature higher than 500 ◦ C shows a sharp decrease in hardness value and tensile strength due
Fig. 15. The microstructures of CGPed steel sheets after post annealing at 400 ◦ C: (a) one pass, (b) two passes and (c) three passes.
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Fig. 16. The microstructures of CGPed steel sheets after post annealing at 600 ◦ C: (a) one pass, (b) two passes and (c) three passes.
to rapid grain growth (that is described in next section). Consequently, the temperatures higher than 400 ◦ C are not appropriate for post-annealing of CGPed low carbon steel sheets. The variations of hardness versus annealing temperature show a thermal stability behavior for annealing up to 400 ◦ C temperature. As mentioned, the trends for hardness variations are similar to that for strength and the descriptions are the same. In order to investigate the uniformity of hardness distributions, the inhomogeneity factor (I.F.) is used. This can be calculated by [36]:
n I.F. =
i=1
¯ (Hi − H) ¯ H
2
0.5
/n − 1
(1)
where n is the number of hardness measurements on each spec¯ is the imen, Hi is the hardness value of ith measurement, and H mean hardness value. Less I.F. value indicates higher homogeneity of mechanical properties. In conventional metal forming processes, as the annealing temperature is increased, the inhomogeneity factor is decreased [37]. In contrast to the expectation, the results of this study show that there is an optimum post-annealing temperature for acquiring the least I.F. Fig. 13 shows the variation of I.F. value with post annealing temperature in different CGP passes. As reported in this figure, the I.F. value of as-received specimen is least, but after imposing of one CGP pass, it is increased. Also among CGPed sheets, the I.F. is decreased with increasing the CGP pass number indicating increase of strain homogeneity.
With increasing the annealing temperature, the I.F. value is rapidly increased at first and then it is decreased up to annealing at 400 ◦ C. In annealing at temperatures higher than 400 ◦ C, the I.F. value is increased again. As it is shown in Fig. 13, the least I.F. can be obtained by post annealing of CGPed specimens at 400 ◦ C. The increase in I.F. after annealing at temperatures higher than 400 ◦ C can be interpreted by abnormal grain growth presented in next section.
3.3. Microstructure In previous sections, it was discussed that abnormal grain growth leads to sharp strength and hardness drops in specimens annealed at elevated temperatures. The microstructure of the asreceived low carbon steel sheet from the thickness plane (Fig. 2) is shown in Fig. 14. As can be seen in this figure, the as-received steel consists of approximately 6 vol% pearlite (dark contrast) and the remainder is ferrite (bright contrast). As can be observed in Fig. 14, as-received low carbon steel sheet has an average grain size of 30 m. The microstructural evolutions of low carbon steel sheets during constrained groove pressing were studied in the work of present authors [16]. It was revealed that an ultrafine grain structure with cell size of 200–300 nm can be achieved in low carbon steel sheets by imposing the CGP method up to three passes. For example, the microstructures after post-annealing of specimens at two different annealing temperatures (400 ◦ C and 600 ◦ C)
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are shown in Figs. 15 and 16. Fig. 15a, b and c shows the microstructures of annealed sheets at 400 ◦ C after one, two and three CGP passes, respectively. The normal grain growth can be observed. As shown in Fig. 16, with increasing the postannealing temperature up to 600 ◦ C, the abnormal grain growth can be observed. Regarding the presence of strain inhomogeneity in CGPed sheets, driving force for restoration is not uniform, which leads to dissimilar grain sizes. Consequently, in the grain growth phenomenon occurs at elevated temperatures, the kinetic of growth results in abnormal grain growth. The same results for abnormal grain growth have been reported for CGPed copper [36]. As mentioned in previous sections, hardness and tensile strength variations show the thermal stability behavior at annealing temperatures up to 400 ◦ C temperature. From the microstructural investigations, a reliable thermal stability up to that temperature can be observed. In all of previous discussions, it was shown that with post annealing of CGPed sheets at 400 ◦ C, in addition to preserving the hardness and strength, elongation is significantly increased that this subject can improve the formability of the severely deformed steel sheets. The I.F. examinations show that hardness distributions of CGPed specimens are more uniform after annealing at 400 ◦ C. Also microstructural investigations show that no abnormal grain growth occurs in this temperature. Therefore, temperature of 400 ◦ C can be introduced as an optimum post annealing temperature for CGPed low carbon steel sheets. Also, it can be considered that severely deformed steel sheets annealed at this temperature have a thermal stability condition. This result is consistent with that reported in Section 1. 4. Conclusions In this research, method of constrained groove pressing (CGP) is carried out on the sheets of low carbon steel at the room temperature and the strain magnitudes up to 3.48 is imposed to these sheets. The effects of post-annealing phenomena on mechanical properties and microstructures of these CGPed sheets have been studied. The main results of this study can be presented as follows: 1. Tensile strength of the steel sheet is increased from 299 MPa to 428 MPa through imposing three passes of CGP at room temperature, and elongation is decreased from 47.3% to 10.4%. Also, the hardness is continuously increased from 98 VHN to 180 VHN with increasing the pass number. 2. Tensile strength and hardness of CGPed sheets are increased with increasing annealing temperature up to 400 ◦ C and higher annealing temperatures cause to strength and hardness drops. 3. Post annealing phenomena can effectively increase the elongation of severely deformed sheets. 4. The I.F. of CGPed and annealed sheets at 400 ◦ C is the lowest. In the other words, specimens in this temperature have the most homogenous mechanical properties and microstructure.
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