Microstructure and properties of a low-carbon steel processed by equal-channel angular pressing

Materials Science and Engineering A 410–411 (2005) 312–315

Microstructure and properties of a low-carbon steel processed by equal-channel angular pressing Jing Tao Wang a,∗ , Cheng Xu b , Zhong Ze Du c , Guo Zhong Qu a , Terence G. Langdon b a

School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China b Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA c School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, PR China Received in revised form 29 April 2005

Abstract An ultrafine-grained low carbon steel (Fe–0.15 wt.% C–0.52 wt.% Mn) was fabricated by equal channel angular pressing (ECAP) at room temperature by pressing for up to a maximum of 10 passes using route C. There was an elongated substructure with a width of ∼0.2–0.3 ␮m after 10 passes of ECAP and the corresponding tensile strength was >1200 MPa. It is shown that subsequent annealing for 1 h at 773 K, which is below the recrystallization temperature of ferrite, leads to equiaxed grains with a size of ∼0.3–0.4 ␮m, an increase in the tensile elongation and a strength above 1000 MPa. Annealing for 1 h at 873 K, which is above the recrystallization temperature, gives a recrystallized equiaxed structure of fine grains with an average grain size of ∼7 ␮m, a strength of <500 MPa and stress–strain curves similar to those anticipated for a low-carbon steel. © 2005 Elsevier B.V. All rights reserved. Keywords: Equal-channel angular pressing; Steel; Strength; Ultrafine grains

1. Introduction It is well known that grain refinement is an effective route for achieving high strength in polycrystalline materials. This is especially true in an important engineering material such as steel and it has led to the successful development of the concept of Thermo-Mechanical Control Processing in steel production [1]. The same approach has been utilized in attempts to develop excellent strength and durability of conventional steels through processes such as mechanical milling and accumulative roll bonding [2–6]. It is now well established that processing through the application of severe plastic deformation is especially effective in refining the grain size of bulk polycrystalline solids [7]. Typically, a process such as equal-channel angular pressing (ECAP), in which a sample is pressed through a die constrained within a channel bent through an abrupt angle, is capable of producing a submicrometer grain size in a wide range of materials [8]. Furthermore, ECAP is an attractive procedure because it can be scaled up fairly easily to produce relatively large bulk samples [9]. ∗

Corresponding author. Tel.: +86 25 84314947; fax: +86 25 84315159. E-mail address: [email protected] (J.T. Wang).

0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.111

There have been several studies to date on the application of ECAP processing to various types of steel and with the processing conducted over a range of different conditions [10–17]. The present research was designed to extend these results by conducting experiments using a low-carbon steel. Specifically, the objectives were: (i) to investigate the grain refinement achieved by ECAP and (ii) to evaluate the mechanical properties of the steel in the as-pressed condition and after annealing treatments. 2. Experimental material and procedures A hot-rolled commercial low-carbon steel, containing 0.15 wt.% C, was used for these experiments: the chemical composition of the material is shown in Table 1. Samples for ECAP processing were cut from the hot-rolled plate along the longitudinal direction in the form of bars having dimensions of 15 mm × 15 mm × 60 mm. A square-shaped configuration was selected to permit easy rotation by 180◦ between passes and thus to prevent any undesirable rotation of the samples during ECAP. All of the ECAP was conducted at room temperature using a split die and a hydraulic press. Samples were pressed repetitively through a number of passes using processing route C where the sample is rotated by 180◦ between consecutive passes [18].

J.T. Wang et al. / Materials Science and Engineering A 410–411 (2005) 312–315 Table 1 Composition of the experimental alloy (wt.%) Element

Content

C Si Mn P S Cr Ni Cu Fe

0.15 0.17 0.52 0.019 0.021 0.10 0.10 0.10 Balance

In the present investigation, the angle of intersection between the two channels was Φ = 90◦ and the angle representing the outer arc of curvature where the two channels intersect was Ψ = 0◦ . It can be shown from first principles that these two angles lead to an imposed strain of ∼1 on each consecutive pass [19]. Samples were pressed for various numbers of passes up to a maximum of 10 passes corresponding to a maximum imposed strain of ∼10. Transmission electron microscopy (TEM) was used for microstructural characterization on the transverse cross-sections of the samples before and after ECAP from 1 to 10 passes. The foils were thinned with a twin-jet polishing facility using a solution of 5% HClO4 , and 95% C2 H5 OH. After thinning, a JCM-200CX electron microscope, operating at 160 kV, was used to examine the foils. Selected area electron diffraction (SAED) patterns were recorded using an aperture having a diameter of 2.5 ␮m. Tensile specimens were cut from the as-pressed billets with the tensile axes oriented parallel to the longitudinal direction. All tensile testing was conducted at room temperature using

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an Instron machine operating at a constant rate of cross-head displacement and with an initial strain rate of 1.0 × 10−3 s−1 . Some samples were annealed for 1 h at temperatures of either 773 or 873 K, where the recrystallization temperature of ferrite is ∼823 K. 3. Experimental results and discussion 3.1. Microstructures before and after ECAP Fig. 1 shows optical microstructures of the low-carbon steel (a) in the as-received condition, (b) after processing by ECAP for 10 passes via route C, and after processing by ECAP for 10 passes via route C and then annealing for 1 h at (c) 773 K and (d) 873 K, respectively. Inspection of Fig. 1(a) shows a homogenous coarse-grained ferrite structure with some pearlite in the as-received hot-rolled sample: the ferrite grain size was measured as ∼25 ␮m. It is apparent from Fig. 1(b–d) that the pearlite is severely distorted by the imposition of severe deformation during ECAP. Furthermore, no grain boundaries or other structural details are visible by optical microscopy in Fig. 1(b) immediately following the ECAP processing. There appear to be some traces of deformation in the optical micrograph in Fig. 1(c) where the sample was annealed for 1 h at 773 K but again it is not possible to fully characterize the microstructure using optical microscopy. In Fig. 1(d), corresponding to annealing for 1 h at 873 K, there is a typical recrystallized fine-grained ferritic microstructure with a measured average grain size of ∼7 ␮m. In order to characterize the microstructural details after ECAP, it is necessary to examine the samples using TEM. Fig. 2 shows two examples of the microstructures in TEM for (a) the as-pressed sample after 10 passes of ECAP and (b) after ECAP

Fig. 1. Optical microstructures in three conditions: (a) in the as-received condition, (b) after 10 passes of ECAP via route C, (c) after 10 passes of ECAP and annealing for 1 h at 773 K and (d) after 10 passes of ECAP and annealing for 1 h at 873 K.

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Fig. 2. Observations by TEM: (a) after processing by 10 passes of ECAP at room temperature via route C and (b) after 10 passes of ECAP and annealing for 1 h at 773 K.

for 10 passes and annealing for 1 h at 773 K where this temperature is below the recrystallization temperature of ferrite. It can be seen in Fig. 2(a) that the as-pressed microstructure consists of nearly parallel bands of elongated grains having a width of the order of ∼0.2–0.3 ␮m. It is also apparent that the as-pressed microstructure is in a non-equilibrium condition with ill-defined boundaries and high densities of dislocation tangles. These ill-defined boundaries are generally interpreted as high-energy non-equilibrium boundaries and they are a consistent feature of samples processed by severe plastic deformation [20–23]. It is important to note that the SAED pattern in Fig. 2(a) shows diffraction spots arranged in rings with typical azimuth spreading, indicating the presence of boundaries having high angles of misorientation. Upon annealing for 1 h at 773 K, there is a reasonably recovered microstructure as shown in Fig. 2(b) with fewer intragranular dislocations and more equiaxed grains with a grain size of ∼0.3–0.4 ␮m which is only slightly larger than the widths of the elongated grains in the as-pressed condition. The boundaries also appear to be closer to an equilibrium configuration after annealing at 773 K and there is no evidence for recrystallization.

3.2. Mechanical properties at room temperature Fig. 3 shows the stress–strain curves obtained in tension for samples tested in three different conditions: (a) after ECAP for various numbers of passes without any subsequent annealing, (b) after ECAP for various numbers of passes and annealing for 1 h at 773 K and (c) after ECAP for various numbers of passes and annealing for 1 h at 873 K: the samples labeled “ECAP 0P” refer to the as-received and unpressed condition. It is apparent from Fig. 3(a) that the strength increases with increasing numbers of passes and there is a very high strength of >1200 MPa after 10 passes of ECAP where this is more than three times higher than for the as-received condition. Inspection of the tensile samples showed that the samples processed by ECAP exhibited immediate necking after yielding. The elongation to failure decreases drastically after four passes of ECAP by comparison with the as-received condition but there is a gradual increase in the elongation to failure with increasing numbers of passes. For 10 passes of ECAP, the elongation is similar to the as-received condition but the deformation in the as-pressed sample is heterogeneous and occurs during the necking pro-

Fig. 3. Tensile stress–strain curves at room temperature: (a) after ECAP for various numbers of passes, (b) after ECAP and annealing for 1 h at 773 K and (c) after ECAP and annealing for 1 h at 873 K.

J.T. Wang et al. / Materials Science and Engineering A 410–411 (2005) 312–315

cess. It is apparent from Fig. 3(b) that there is a small decrease in the strength through annealing for 1 h at 773 K although a strength of >1000 MPa is retained after 10 passes of ECAP. The stress–strain curves exhibit short periods of uniform flow without necking and there is also a sharp yield point for the sample in the as-received condition. These curves show that the necking resistance increases through annealing at a temperature of 773 K although the necking resistance is insufficient to permit a long region of uniform deformation. The curves in Fig. 3(c) show the effect of annealing for 1 h at 873 K where this temperature is above the recrystallization temperature for ferrite. It is apparent that the strengths are lower after ECAP with a maximum strength of <500 MPa after 10 passes of ECAP. However, the samples subjected to ECAP show small increases in both the yield and tensile strengths by comparison with the sample in the as-received condition. In addition, the shapes of the stress–strain curves in Fig. 3(c) are typical of the curves anticipated for a lowcarbon steel including the presence of sharp yielding, although it is apparent that the yielding phenomenon is gradually lost as samples are subjected to higher numbers of passes in ECAP. The curves in Fig. 3 demonstrate the potential for manipulating the strength and nature of the stress–strain curves through appropriate annealing treatments. 4. Summary and conclusions (i) A low-carbon steel was processed by ECAP for up to a maximum of 10 passes at room temperature. In the as-pressed condition, the microstructure consisted of elongated grains with widths of ∼0.2–0.3 ␮m. The tensile strength increased by a factor of 3 to >1200 MPa after ECAP for 10 passes although there was immediate necking after yielding. (ii) Annealing for 1 h at 773 K, which is below the recrystallization temperature of 823 K for ferrite, gives a reasonably equiaxed microstructure with a grain size of ∼0.3–0.4 ␮m. The tensile strength is >1000 MPa after 10 passes of ECAP and there is a good combination of high strength and reasonable ductility. (iii) Annealing for 1 h at 873 K leads to a microstructure consisting of recrystallized equiaxed grains with a grain size of ∼7 ␮m. The stress–strain curves in this condition are typical of low-carbon steels with a maximum strength of <500 MPa.

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Acknowledgements This work was supported by the National Nature Science Foundation of China under Grant No. 50474028 and the National Science Foundation of the United States under Grant No. DMR0243331. References [1] F.B. Pickering, Constitution and Properties of Steels, VCH, New York, NY, 1992, p. 186. [2] Y. Kimura, S. Takaki, S. Suejima, R. Uemori, H. Tamehiro, ISIJ Intl. 39 (1999) 176. [3] Y. Kimura, S. Suejima, H. Goto, S. Takaki, ISIJ Intl. Suppl. 40 (2000) S174. [4] H. Hidaka, T. Tsuchiyama, S. Takaki, Scripta Mater. 44 (2001) 1503. [5] S. Takaki, K. Kawasaki, Y. Kimura, J. Mater. Proc. Technol. 117 (2001) 359. [6] S.H. Lee, H. Utsunomiya, T. Sakai, Mater. Trans. 45 (2004) 2177. [7] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103. [8] R.Z. Valiev, N.A. Krasilnikov, N.K. Tsenev, Mater. Sci. Eng. A 137 (1991) 35. [9] Z. Horita, T. Fujinami, T.G. Langdon, Mater. Sci. Eng. A 318 (2001) 34. [10] D.H. Shin, B.C. Kim, Y.S. Kim, K.T. Park, Acta Mater. 48 (2000) 2247. [11] D.H. Shin, B.C. Kim, K.T. Park, W.Y. Choo, Acta Mater. 48 (2000) 3245. [12] D.H. Shin, C.W. Seo, J. Kim, KT. Park, W.Y. Choo, Scripta Mater. 42 (2000) 695. [13] K.T. Park, Y.S. Kim, J.G. Lee, D.H. Shin, Mater. Sci. Eng. A 293 (2000) 165. [14] D.H. Shin, I. Kim, J. Kim, K.T. Park, Acta Mater. 49 (2001) 1285. [15] D.H. Shin, Y.S. Kim, E.J. Lavernia, Acta Mater. 49 (2001) 2387. [16] K.T. Park, Y.S. Kim, D.H. Shin, Metall. Mater. Trans. A 32 (2001) 2373. [17] Y. Fukuda, K. Oh-ishi, Z. Horita, T.G. Langdon, Acta Mater. 50 (2002) 1359. [18] M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A 257 (1998) 328. [19] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 143. [20] J. Wang, Z. Horita, M. Furukawa, M. Nemoto, N.K. Tsenev, R.Z. Valiev, Y. Ma, T.G. Langdon, J. Mater. Res. 8 (1993) 2810. [21] J.T. Wang, Y. Iwahashi, Z. Horita, M. Furukawa, M. Nemoto, R.Z. Valiev, T.G. Langdon, Acta Mater. 44 (1996) 2973. [22] Z. Horita, D.J. Smith, M. Furukawa, M. Nemoto, R.Z. Valiev, T.G. Langdon, J. Mater. Res. 11 (1996) 1880. [23] Z. Horita, D.J. Smith, M. Nemoto, R.Z. Valiev, T.G. Langdon, J. Mater. Res. 13 (1998) 446.