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Microstructure and mechanical properties of cold drawn pearlitic steel wires: Effects of drawing-induced heating
Materials Science & Engineering A 784 (2020) 139341
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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Microstructure and mechanical properties of cold drawn pearlitic steel wires: Effects of drawing-induced heating Dasheng Wei a, Xuegang Min b, Xianjun Hu c, Zonghan Xie d, Feng Fang a, ** a
Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing, 211189, China Jiangsu Baosteel Fine Wire & Cord Co., Ltd., Haimen, 226114, China c Jiangsu Sha Steel Group, Zhangjiagang, 215625, China d School of Mechanical Engineering, University of Adelaide, SA, 5005, Australia b
A R T I C L E I N F O
A B S T R A C T
Keywords: Pearlitic steel wires Cold drawing Cryogenic temperature Cementite recrystallization Drawing heat
To investigate the effects of drawing-generated heating on the microstructural and mechanical properties of cold drawn pearlitic steel wires, cryogenic and room temperature (RT) drawing were carried out. Microstructure evolution of the steel wires was examined by scanning electron microscope (SEM), transmission electron mi croscope (TEM) and X-ray diffraction (XRD) analysis. Experimental results show that with the increase of cold drawing strain interlamellar spacing of pearlite was reduced, while the crystalline cementite was gradually transformed into amorphous state. Only slight difference in the microstructure and mechanical properties was identified between the RT and cryogenic drawings. Specifically, as the drawing strain increased, the cryogenic drawn wires have lower strength but better ductility. For example, the tensile strength of cryogenic drawn wires (ε ¼ 2.2) is about 2050 MPa, about 90 MPa lower than that of RT drawn wires. On the other hand, cryogenic drawing wires have produced a lower volume fraction of nano-crystalline cementite particles. As the nanocrystalline cementite particles could hinder the movement of dislocations, a lower fraction of nano-crystalline cementite in cryogenic drawn wires is believed to be responsible for observed lower strength and better ductility.
1. Introduction High strength steel wires have been widely used in automobiles, suspension bridge building and mining industry as they possess high strength and good ductility [1,2]. The steel wires are manufactured by cold drawing process. It was reported that the tensile strength of heavily cold drawn pearlitic wires can reach up to 7 GPa [3]. During the cold drawing, the interlamellar spacing of pearlite de creases [4,5], and the <110> texture is formed in ferrite, accompanied by an increase in dislocation density [6–8]. On the other hand, the microstructure evolution of cementite and its effect on mechanical properties have attracted much attention. Cementite platelets could be transformed into nanocrystalline or even amorphous state in drawing process [9–12]. After heavily cold drawing, cementite would be decomposed [13–15]. For example, as the drawing strain ε exceeded 5.1, most of cementite in cold drawn pearlitic steel wires decomposed [16]. Following severe cold drawing, the microstructure of cementite became unstable. Annealing at relatively low temperatures could result in the conspicuous carbon segregation and precipitation of intermediate
carbides, even the formation of nanocrystalline cementite, resulting in an increase in the strength of the wires [10,17]. The presence of nano crystalline cementite could however lead to the occurrence of torsion delamination [18,19]. During rapid cold drawing, the steel wires undergo adiabatic process [20], in which considerable heat would be generated [21], causing a drastic increase in temperature in the steel wires. As the strength of the steel wires continues to increase with cold drawing, its deformation resistance increases, more heat is produced [22]. In addition, the higher drawing speeds brings about higher temperature rises [23]. It leads to a greater temperature effect upon the structure and properties of the steel wires. For example, the rise of drawing temperature was reported to cause the deterioration of the lubrication property, which in turn resulted in surface delamination and cracks [24,25] and reduced the ductility of the wires [22,26]. Moreover, excessive temperature rise followed by fast cooling can lead to local hardening of the wires up to the point of where martensite structure forms [27]. Such martensite in clusions cause brittleness and wire fracture in subsequent drawing. The mobility of dislocations decreases at low temperature. Body-centered
* Corresponding author. School of Materials Science and Engineering, Southeast University, Jiangning District, Nanjing, 211189, China. E-mail address: [email protected] (F. Fang). https://doi.org/10.1016/j.msea.2020.139341 Received 29 February 2020; Received in revised form 30 March 2020; Accepted 31 March 2020 Available online 3 April 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
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cubic (bcc) metal materials, such as pearlitic steel, will become brittle, resulting in a higher strength and lower ductility [28,29]. Low-temperature embrittlement of metals can even cause Ductile-to-Brittle Transition [30]. Researchers have studied the effect of drawing heat on the macro-mechanical properties of steel wires [31–33]. However, a correlation between the microstructure and properties of steel wires remains elusive, which hinders the development of higher strength steel wires. In the present work, the microstructure evolution of pearlitic steel wires was interrogated under cryogenic and RT drawing conditions, and the effect of drawing heat on the microstructure-property relationship was ascertained.
Fig. 1. Schematic diagram of the tensile specimen geometry. The ‘d’, ‘L0’, ‘Lc’ and ‘Lt’ refers to diameter, original gauge length, free length and total length, respectively.
2. Experimental
could be observed between RT and cryogenic drawings. As the drawing strain (ε) increased to 2.2, the tensile strength (about 2143 MPa) of the RT drawing wires is about 90 MPa higher than that (2057 MPa) of cryogenic drawing wires. Fig. 2b shows the engineering stress-strain curves of steel wires prepared with different drawing strains at ε ¼ 0, ε ¼ 0.5, and ε ¼ 2.2. The uniform elongation (i.e., ductility) of pearlitic steel rods (ε ¼ 0) is found to be about 8.3%. For the drawing strain ε ¼ 0.5, no clear difference could be observed in the mechanical response between the steel wires drawn at different temperatures. However, with the drawing strain increased to ε ¼ 2.2, it is interesting to see that cryogenic drawn wires exhibit lower tensile strength but higher uniform elongation than RT drawn wires.
2.1. Preparation of the steel wires The commercial pearlitic steel rods were used in this work. The chemical compositions of steel rods are shown in Table 1. The rods were cold drawn at a speed of 1.9 � 101–4.4 � 101 s 1 at cryogenic tem perature (about 77 K) and room temperature (about 298 K). Wires with the initial diameter of 5.5 mm were drawn successively to 1.8 mm, achieving a total drawing strain of ε ¼ 2.2. And the average reduction per pass was about 20%. Molybdenum disulfide (MoS2) was used for lubrication during drawing. 2.2. Materials characterization The tensile testing was measured at room temperature with the help of universal materials testing machine (CMT 5105) at a strain rate of 2.3 � 10 3 s 1. Tensile tests were performed according to the Chinese Na tional Standard GB/T228-2002. The schematic diagram of tensile specimen geometry is shown in Fig. 1. Three samples were tested in each case to ensure the accuracy and reliability of the experimental results. Before microstructural observation, the samples were ground and pol ished along the drawing direction and then immersed in etching solution (4% nitric acid þ 96% alcohol, volume fraction) for 3 s. A FEI Sirion-400 scanning electron microscope (SEM) was used observe the microstruc ture of samples. Thin foils for transmission electron microscope (TEM) observations were prepared by mechanical polishing and then thinning by GATAN 691 Precision Ion Polishing System (PIPS). Samples were thinned at a cryogenic temperature in PIPS to avoid the temperature rise. To avoid the microstructure damage, ion angle and energy were set to be 3.5� and 2.5 keV at the final stage of TEM sample preparation. The microstructure of the wires was characterized using JEOL-2100 F TEM operating at an accelerated voltage of 200 kV. X-ray diffraction (XRD) analysis was performed (Rigaku D-max 2100) using a diffractometer with a Cu target. The step size and scanning speed was set at 0.001� and 0.5� /min, respectively.
3.2. Microstructure characterization 3.2.1. Original microstructure of the wires prior to drawing As shown in Fig. 3a, the steel rods (ε ¼ 0) were composed of lamellar arrangement of ferrite and cementite. The orientation of lamellar pearlite was random. Fig. 3b shows that pearlitic structures comprise alternating layers of ferrite and cementite. The lamellar spacing between ferrite and cementite is about 120 nm, and the thickness of cementite lamellae is about 12 nm. Selected area electron diffraction (SAED) is shown as inset in Fig. 3b. The diffraction patterns of cementite indicate that cementite is made of single crystalline. The cementite exhibits a platelet-like shape, as inset shown in Fig. 3b. 3.2.2. Deformation of pearlitic structure As shown in Fig. 4a, pearlite colonies in the RT drawn wires corre sponding to ε ¼ 0.5 were stretched along the drawing direction, accompanied by the decrease of the interlamellar spacing in pearlite. Fig. 4b shows the wires (ε ¼ 0.5) drawn at 77 K. Compared with the steel wires drawn at 298 K, no difference could be observed between the RT and cryogenic drawings. Fig. 4c shows the wires (ε ¼ 2.2) drawn at 298 K. After severe deformation, pearlite was almost parallel to the drawing direction (D.D.) and the interlamellar spacing further reduced. Some broken cementite segments can be observed. For the wires (ε ¼ 2.2) drawn at 77 K, the pearlite also shows fibrous structure (Fig. 4d). No difference could be observed in wires (ε ¼ 2.2) corresponding to the RT and cryogenic drawings. Fig. 5a shows the lamellar structures in pearlitic steel wires (ε ¼ 2.2) drawn at 298 K. Many cementite particles could be observed in the cementite lamella (Fig. 5b), further revealed by high resolution trans mission electron microscope (HRTEM) observation presented in Fig. 5c. In the cementite region, a number of small particles with lattice fringes can be observed. The particle size is about 3 nm, and disordered struc tures are visible between the particles. It indicates that amorphous cementite was partially transformed into nanocrystalline state during severe cold drawing. Therefore, the resultant cementite lamellae consist of nanocrystalline cementite surrounded by amorphous cementite. The Fast Fourier Transformation (FFT) pattern of the cementite was obtained from the rectangle box, as shown in Fig. 5c. It reveals (121) cementite
3. Results 3.1. Mechanical properties Fig. 2a shows the tensile strength of pearlitic steel wires with different drawing temperatures (room temperature and cryogenic tem perature) as a function of drawing strain. The tensile strength of pearlitic steel rods (ε ¼ 0) is found to be about 1350 MPa. With the increase of the drawing strain, the tensile strength of the wires gradually increased. For the drawing strain ε < 1.1, no clear difference in the tensile strength Table 1 Chemical composition of high carbon steel rods. Elements
C
Mn
Si
Cr
P
S
Cu
Fe
wt.%
0.92
0.30
0.17
0.20
0.009
0.009
0.006
Bal.
2
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Fig. 2. Mechanical properties of pearlitic steel wires drawn at different temperatures. a) Tensile strength vs drawing strain; b) Engineering stress-strain curves.
¼ 2.2) drawn at 77 K. The lamellar structures consist of alternating ferrite and cementite. Fig. 5e shows fewer cementite particles are pre sent in the cementite lamellae. No lattice fringes were observed in the cementite regions (Fig. 5f), which are dominant in the cementite lamellae. It is worth noting that the boundaries between ferrite and cementite became blurred with the increase of drawing strain [12,34]. 3.3. XRD analysis Fig. 6a shows X-ray diffraction patterns of three types of wires, i.e., pearlitic steel rods (ε ¼ 0), RT drawn wires (ε ¼ 2.2, 298 K) and cryo genic drawn wires (ε ¼ 2.2, 77 K). The diffraction peaks, corresponding to α-Fe (110), α-Fe (200) can be easily identified. The diffraction peaks of (110) α-Fe and (200) α-Fe of wires drawn at 298 K and 77 K show significant broadening, which is understood to result from nanostructuring and high residual stresses. It indicates that a refinement of ferrite lamellae and an increase in dislocation density took place during the cold drawing process [35,36]. Fig. 6b presents truncated strong (110)α-Fe diffraction peak and diffraction peaks from cementite phase. As the wires without drawing (ε ¼ 0), (121) cementite, (211) cementite and (221) cementite diffraction peaks can be observed. While in the wires (ε ¼ 2.2) drawn at 298 K, cementite diffraction peaks were weakened and broadened. It may be due to the deformation and partial decomposition of cementite platelets and the emergence of nano-crystalline particles in the steel structures. By comparison, for the wires prepared at lower temperature (77 K), almost no peaks related to cementite can be observed. It suggests that the cementite exist mostly in amorphous form under cryogenic drawing condition.
Fig. 3. Microstructure of high carbon steel rods. a) SEM image; b) TEM image with SAED and DF as inset.
3.4. Cryogenic drawing wires subjected to annealing In order to analyze the effect of drawing heat on the microstructure of cementite, the steel wires (ε ¼ 2.2) drawn at 77 K were annealed at 473 K for 30 min and its microstructure was analyzed. As shown in Fig. 7a, the lamellar structures remain. Many cementite particles could be observed in the cementite lamellae (Fig. 7b). The particle size is about 2–5 nm. The existence of large cementite particles may be due to the non-uniform growth of fine cementite particles during the annealing treatment, which bears a similarity to the cementite morphology of RT drawn wires. Supposedly, a rapid temperature increase could take place in steel wires during cold drawing at RT [26,37], which triggered the nucleation and growth of cementite within a short time.
Fig. 4. SEM micrographs of as-drawn pearlitic steel wires. a) ε ¼ 0.5, 298 K; b) ε ¼ 0.5, 77 K; c) ε ¼ 2.2, 298 K; and d) ε ¼ 2.2,77 K. Drawing direction (D. D.).
crystal planes. The Inverse Fast Fourier Transformation (IFFT) image of the (121) cementite is presented in Fig. 5c. The spacing between cementite atomic layers is 0.239 nm, confirming the existence of the (121) planes. Fig. 5d shows the nanostructure of pearlitic steel wires (ε
4. Discussion The steel wires are usually drawn at room temperature. The tem 3
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Fig. 5. TEM micrographs of as-drawn pearlitic steel wires. a) ε ¼ 2.2, 298 K, BF; b) DF image of a); c) HRTEM image of a); d)ε ¼ 2.2, 77 K, BF; e) DF image of d); f) HRTEM image of d); insets in c) and f) show the FFT and IFFT images of marked boxes.
Fig. 6. (a) X-ray diffraction patterns of pearlitic steel wires prepared at different drawing temperatures; (b) Truncated (110) peaks from cementite phase. The ε ¼ 0 refers to the rods prior to drawing.
α-Fe
diffraction peak and diffraction
perature of steel wires rises rapidly due to fast plastic deformation and surface friction [22,23]. For analyzing the effect of drawing heat on the microstructure of steel wires, it is important to estimate the temperature rise in wires. To calculate the drawing heat generated from internal plastic deformation and friction at the die-wire interface, the following heat balance equation is can be utilized [22,38,39]: � � 1 4 f2 ⋅ υ ⋅ ρ ⋅c ⋅z ⋅ðTdie; out Tdie; in Þ ¼ F ⋅ km þ pffiffi f2 ⋅ α ⋅ kfm þm ⋅Q ⋅km ⋅ μ ⋅ υ⋅z A 3 3 (1) where f2 is the transverse area corresponding to the wire at die outlet, υ is the speed of cold drawing, ρ is the density of steel wire, c is the specific heat of steel wire, z is the cold drawing deformation time of steel wire, Tdie,out is the temperature of the steel wire at die outlet, Tdie,in is the temperature of the steel wire at die inlet, A is the conversion factor, F is the deviation of the transverse area of the steel wire between die inlet and die outlet, km is the average deformation resistance during the cold
Fig. 7. TEM micrographs of pearlitic steel wires (ε ¼ 2.2, 77 K) annealed at 473 K for 30 min. a) BF and b) DF. SAED image is shown as inset in a).
4
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drawing deformation, α is the half die angle, kfm is the mean yield strength of the steel wire before and after cold drawing deformation, m is the coefficient of heat partition, Q is the area of contact between steel wire and die, and μ is the friction coefficient between steel wire and die. From equation (1), the wire temperature at the exit of the drawing die can be determined as [22,38,39]. � � 1 4 Tdie;out ¼ Tdie;in þ F ⋅ km þ pffiffi f2 ⋅ α ⋅ kfm þ m ⋅ Q ⋅ km ⋅ μ (2) A⋅f2 ⋅ρ⋅c 3 3 In the present work, the value of the heat partition coefficient (m) (0 < m < 1) is identified as 0.8, which means the fraction of the frictional heat transferred to the wire. The coefficient of friction (μ) between the high carbon steel wire and the die is assumed as 0.06 [22]. The density of the wire (ρ) is 7.9 � 103 kg m 3 and the specific heat of the wire (c) is 0.46 � 103 J/(kg⋅� C). The calculated and measured wire temperature values at the exit of the die are plotted in Fig. 8. The wire temperature at the exit of the die is about 453–493 K. The calculation value is slightly higher than the actual measurement. This discrepancy may be due to the heat loss of wires at the die exit [22,38]. With the increase of drawing strain, the interlamellar spacing of pearlite decreased, and cementite deformed and partially decomposed [13,40]. Under RT drawing condition, cementite platelets may be transformed into nano-crystalline or even amorphous state [9–12]. As shown in Fig. 5b and c, a mixture of nano-crystalline and amorphous states could be observed in cementite lamellae. However, under cryo genic drawing condition, the amorphous state was dominant in cementite lamellae. Compared with RT drawn wires, no cementite diffraction peaks were observed in the X-ray diffraction pattern of the cryogenic drawn wires. During the RT drawing process, the wire tem perature at the exit of the die is about 453–493 K, which could drive the cementite transformation from amorphous to nano-crystalline state [10]. As shown in Fig. 7, the steel wires (ε ¼ 2.2, 77 K) were annealed at 473 K, leading to the cementite transformed from the amorphous to nano-crystalline. It could be inferred that cementite was transformed into amorphous state during RT deformation before it was recrystallized under drawing heat. Cryogenic drawing could offset the effect of drawing heat and inhibit the recrystallization of amorphous cementite. Recrystallized cementite can serve as physical barriers against dislocation motion [4,10,41]. Fig. 9a shows the dislocation morphology of pearlitic steel wires (ε ¼ 2.2) drawn at 298 K. Curved dislocation lines are presented in lamellar ferrite. The cementite crystal plane spacing is 0.239 nm, revealing the existence of cementite (121) planes (SAED inset in Fig. 9a). Bright spots in Fig. 9b represent cementite nanocrystals formed under RT drawing. In particular, the spots indicated by the
Fig. 9. TEM micrographs of cold drawn pearlitic steel wires (ε ¼ 2.2, 298 K) a) BF; b) DF. Note that the white arrows in BF point out the dislocation lines within a ferrite platelet. The white arrows in DF indicate cementite nanocrystals impeding dislocation motion.
arrows correspond to the dislocation line marked by the arrows in Fig. 9a. It shows that the dislocations in ferrite may be pinned by the cementite nanocrystals. This phenomenon was also observed in another study [10]. In addition, cementite recrystallization may take place along the original cementite sites or lamellar ferrite boundaries [42]. As the nano-crystalline cementite precipitated at the grain boundaries, the interfacial energy would be reduced, so that the boundaries can be stabilized and serve as barriers against dislocation movement [3,17]. However, during cryogenic drawing process, the initial deformation temperature was low, which partially offset the drawing heat effect. It thus inhibited the cementite transformation from the amorphous to nano-crystalline, resulting in weak blockage effect on dislocation mo tion. Therefore, the wires drawn at cryogenic temperature have a rela tively low strength and improved ductility. For the pearlitic steel wires prepared at low drawing strains (ε < 1.1), the deformation resistance (km) are low. Therefore, the wire temperature at the exit of the die is lower than high drawing strain conditions according to equation (2). In addition, the cementite did not undergo heavy deformation and decomposition under low drawing strains. Even under the higher drawing temperature (453–493 K), the cementite is not easily recrys tallized, let alone those at lower drawing temperature. Therefore, for low drawing strains, there is almost no difference in the mechanical behavior between the RT and cryogenic drawn wires. Based on the above experimental results and analysis, it seems apparent that the transformation of cementite from the amorphous to nano-crystalline state would be inevitable during the cold drawing process if the drawing heat is not suppressed. Nano-crystalline cementite would pin dislocations, resulting in an increase in the strength of wires. In addition, the occurrence of torsion delamination was closely related with the presence of nano-crystalline cementite [18]. To avoid it, two approaches are given here for consideration: 1) ultra-low temperature cooling. However, the ferrite has a body-centered cubic structure, which may cause low-temperature brittleness or even ductile-brittle transition at ultra-low temperature [20,28,29]. 2) Lower drawing strain at each drawing step. It can reduce the drawing heat and cementite deformation and decomposition so that cementite is not easily recrystallized. How ever, the strength increase would be limited at each drawing and achieving the high strength would require more drawing steps to be implemented that cause an increase in process time and die number/ costs. Finding a solution to achieve a combination of high strength and ductility in heavily drawn pearlitic steel wires while keeping the process costs low remains an interesting and challenging task. 5. Conclusion The effect of drawing heat on the microstructure and mechanical properties of cold drawn pearlitic steel wires was explored. The following conclusions can be drawn:
Fig. 8. A comparison of the calculated and measured wire temperatures at the exit of the die. 5
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1) No marked difference in the microstructural and mechanical prop erties exists between RT and cryogenic drawings. As the drawing strain increased, the cryogenic drawn wires have lower strength but better ductility. The tensile strength of cryogenic drawn wires (ε ¼ 2.2) is about 2050 MPa, about 90 MPa lower than that of RT drawn wires. 2) As the drawing strain increased, interlamellar spacing of pearlite was decreased, and crystalline cementite was gradually transformed into amorphous state. Compared with RT drawing wires, nano-crystalline cementite particles in cryogenic drawing wires have a lower volume fraction and exhibit in smaller sizes. 3) Nano-crystalline cementite particles could hinder the movement of dislocations. Compared with RT drawn wires, the lower volume fraction of nano-crystalline cementite in cryogenic drawn wires might be responsible for observed lower strength and better ductility.
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Data availability statement All data included in this study are available upon request by contact with the corresponding authors. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Dasheng Wei: Writing - review & editing. Xuegang Min: Method ology. Xianjun Hu: Investigation. Zonghan Xie: Writing - review & editing. Feng Fang: Writing - review & editing, Supervision. Acknowledgement This work was funded by the Science and Technology Advancement Program of Jiangsu Province (BA2017112) and the Industry-University Research Cooperation Project of Jiangsu Province, PR China (BY2018194). The study was also partly funded by 333 Projects of Jiangsu Province, PR China (BRA2018045). Authors thank X. Shen, L. Li, S. Cui and K. Yang for assistance with preparation the samples, and A. Xu and C. Jin for support of TEM analysis. Z. Xie acknowledges the support provided by the Australian Research Council Discovery Projects. References [1] D. Raabe, P.P. Choi, Y.J. Li, A. Kostka, X. Sauvage, F. Lecouturier, K. Hono, R. Kirchheim, R. Pippan, Metallic composites processed via extreme deformation: toward the limits of strength in bulk materials, MRS Bull. 35 (12) (2010) 982–991. [2] C. Borchers, R. Kirchheim, Cold-drawn pearlitic steel wires, Prog. Mater. Sci. 82 (2016) 405–444. [3] Y.J. Li, D. Raabe, M. Herbig, P.-P. Choi, S. Goto, A. Kostka, H. Yarita, C. Borchers, R. Kirchheim, Segregation stabilizes nanocrystalline bulk steel with near theoretical strength, Phys. Rev. Lett. 113 (10) (2014) 106104. [4] N. Maruyama, T. Tarui, H. Tashiro, Atom probe study on the ductility of drawn pearlitic steels, Scripta Mater. 46 (8) (2002) 599–603. [5] J.D. Embury, R.M. Fisher, The structure and properties of drawn pearlite, Acta Metall. 14 (2) (1966) 147–159. [6] X.D. Zhang, A. Godfrey, N. Hansen, X.X. Huang, Hierarchical structures in colddrawn pearlitic steel wire, Acta Mater. 61 (13) (2013) 4898–4909. [7] X.D. Zhang, A. Godfrey, X.X. Huang, N. Hansen, Q. Liu, Microstructure and strengthening mechanisms in cold-drawn pearlitic steel wire, Acta Mater. 59 (9) (2011) 3422–3430. [8] F. Fang, L.C. Zhou, X.J. Hu, X.F. Zhou, Y.Y. Tu, Z.H. Xie, J.Q. Jiang, Microstructure and mechanical properties of cold-drawn pearlitic wires affect by inherited texture, Mater. Des. 79 (2015) 60–67. [9] C. Borchers, T. Al-Kassab, S. Goto, R. Kirchheim, Partially amorphous nanocomposite obtained from heavily deformed pearlitic steel, Mater. Sci. Eng., A 502 (1–2) (2009) 131–138.
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[40] X.D. Zhang, A. Godfrey, W. Liu, Q. Liu, Study on dislocation slips in ferrite and deformation of cementite in cold drawn pearlitic steel wires from medium to high strain, Mater. Sci. Technol. 27 (2) (2011) 562–567. [41] Y.J. Li, P. Choi, S. Goto, C. Borchers, D. Raabe, R. Kirchheim, Evolution of strength and microstructure during annealing of heavily cold-drawn 6.3 GPa hypereutectoid pearlitic steel wire, Acta Mater. 60 (9) (2012) 4005–4016.
[42] J. Takahashi, M. Kosaka, K. Kawakami, T. Tarui, Change in carbon state by lowtemperature aging in heavily drawn pearlitic steel wires, Acta Mater. 60 (1) (2012) 387–395.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Microstructure and mechanical properties of cold drawn pearlitic steel wires: Effects of drawing-induced heating Dasheng Wei a, Xuegang Min b, Xianjun Hu c, Zonghan Xie d, Feng Fang a, ** a
Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing, 211189, China Jiangsu Baosteel Fine Wire & Cord Co., Ltd., Haimen, 226114, China c Jiangsu Sha Steel Group, Zhangjiagang, 215625, China d School of Mechanical Engineering, University of Adelaide, SA, 5005, Australia b
A R T I C L E I N F O
A B S T R A C T
Keywords: Pearlitic steel wires Cold drawing Cryogenic temperature Cementite recrystallization Drawing heat
To investigate the effects of drawing-generated heating on the microstructural and mechanical properties of cold drawn pearlitic steel wires, cryogenic and room temperature (RT) drawing were carried out. Microstructure evolution of the steel wires was examined by scanning electron microscope (SEM), transmission electron mi croscope (TEM) and X-ray diffraction (XRD) analysis. Experimental results show that with the increase of cold drawing strain interlamellar spacing of pearlite was reduced, while the crystalline cementite was gradually transformed into amorphous state. Only slight difference in the microstructure and mechanical properties was identified between the RT and cryogenic drawings. Specifically, as the drawing strain increased, the cryogenic drawn wires have lower strength but better ductility. For example, the tensile strength of cryogenic drawn wires (ε ¼ 2.2) is about 2050 MPa, about 90 MPa lower than that of RT drawn wires. On the other hand, cryogenic drawing wires have produced a lower volume fraction of nano-crystalline cementite particles. As the nanocrystalline cementite particles could hinder the movement of dislocations, a lower fraction of nano-crystalline cementite in cryogenic drawn wires is believed to be responsible for observed lower strength and better ductility.
1. Introduction High strength steel wires have been widely used in automobiles, suspension bridge building and mining industry as they possess high strength and good ductility [1,2]. The steel wires are manufactured by cold drawing process. It was reported that the tensile strength of heavily cold drawn pearlitic wires can reach up to 7 GPa [3]. During the cold drawing, the interlamellar spacing of pearlite de creases [4,5], and the <110> texture is formed in ferrite, accompanied by an increase in dislocation density [6–8]. On the other hand, the microstructure evolution of cementite and its effect on mechanical properties have attracted much attention. Cementite platelets could be transformed into nanocrystalline or even amorphous state in drawing process [9–12]. After heavily cold drawing, cementite would be decomposed [13–15]. For example, as the drawing strain ε exceeded 5.1, most of cementite in cold drawn pearlitic steel wires decomposed [16]. Following severe cold drawing, the microstructure of cementite became unstable. Annealing at relatively low temperatures could result in the conspicuous carbon segregation and precipitation of intermediate
carbides, even the formation of nanocrystalline cementite, resulting in an increase in the strength of the wires [10,17]. The presence of nano crystalline cementite could however lead to the occurrence of torsion delamination [18,19]. During rapid cold drawing, the steel wires undergo adiabatic process [20], in which considerable heat would be generated [21], causing a drastic increase in temperature in the steel wires. As the strength of the steel wires continues to increase with cold drawing, its deformation resistance increases, more heat is produced [22]. In addition, the higher drawing speeds brings about higher temperature rises [23]. It leads to a greater temperature effect upon the structure and properties of the steel wires. For example, the rise of drawing temperature was reported to cause the deterioration of the lubrication property, which in turn resulted in surface delamination and cracks [24,25] and reduced the ductility of the wires [22,26]. Moreover, excessive temperature rise followed by fast cooling can lead to local hardening of the wires up to the point of where martensite structure forms [27]. Such martensite in clusions cause brittleness and wire fracture in subsequent drawing. The mobility of dislocations decreases at low temperature. Body-centered
* Corresponding author. School of Materials Science and Engineering, Southeast University, Jiangning District, Nanjing, 211189, China. E-mail address: [email protected] (F. Fang). https://doi.org/10.1016/j.msea.2020.139341 Received 29 February 2020; Received in revised form 30 March 2020; Accepted 31 March 2020 Available online 3 April 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
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cubic (bcc) metal materials, such as pearlitic steel, will become brittle, resulting in a higher strength and lower ductility [28,29]. Low-temperature embrittlement of metals can even cause Ductile-to-Brittle Transition [30]. Researchers have studied the effect of drawing heat on the macro-mechanical properties of steel wires [31–33]. However, a correlation between the microstructure and properties of steel wires remains elusive, which hinders the development of higher strength steel wires. In the present work, the microstructure evolution of pearlitic steel wires was interrogated under cryogenic and RT drawing conditions, and the effect of drawing heat on the microstructure-property relationship was ascertained.
Fig. 1. Schematic diagram of the tensile specimen geometry. The ‘d’, ‘L0’, ‘Lc’ and ‘Lt’ refers to diameter, original gauge length, free length and total length, respectively.
2. Experimental
could be observed between RT and cryogenic drawings. As the drawing strain (ε) increased to 2.2, the tensile strength (about 2143 MPa) of the RT drawing wires is about 90 MPa higher than that (2057 MPa) of cryogenic drawing wires. Fig. 2b shows the engineering stress-strain curves of steel wires prepared with different drawing strains at ε ¼ 0, ε ¼ 0.5, and ε ¼ 2.2. The uniform elongation (i.e., ductility) of pearlitic steel rods (ε ¼ 0) is found to be about 8.3%. For the drawing strain ε ¼ 0.5, no clear difference could be observed in the mechanical response between the steel wires drawn at different temperatures. However, with the drawing strain increased to ε ¼ 2.2, it is interesting to see that cryogenic drawn wires exhibit lower tensile strength but higher uniform elongation than RT drawn wires.
2.1. Preparation of the steel wires The commercial pearlitic steel rods were used in this work. The chemical compositions of steel rods are shown in Table 1. The rods were cold drawn at a speed of 1.9 � 101–4.4 � 101 s 1 at cryogenic tem perature (about 77 K) and room temperature (about 298 K). Wires with the initial diameter of 5.5 mm were drawn successively to 1.8 mm, achieving a total drawing strain of ε ¼ 2.2. And the average reduction per pass was about 20%. Molybdenum disulfide (MoS2) was used for lubrication during drawing. 2.2. Materials characterization The tensile testing was measured at room temperature with the help of universal materials testing machine (CMT 5105) at a strain rate of 2.3 � 10 3 s 1. Tensile tests were performed according to the Chinese Na tional Standard GB/T228-2002. The schematic diagram of tensile specimen geometry is shown in Fig. 1. Three samples were tested in each case to ensure the accuracy and reliability of the experimental results. Before microstructural observation, the samples were ground and pol ished along the drawing direction and then immersed in etching solution (4% nitric acid þ 96% alcohol, volume fraction) for 3 s. A FEI Sirion-400 scanning electron microscope (SEM) was used observe the microstruc ture of samples. Thin foils for transmission electron microscope (TEM) observations were prepared by mechanical polishing and then thinning by GATAN 691 Precision Ion Polishing System (PIPS). Samples were thinned at a cryogenic temperature in PIPS to avoid the temperature rise. To avoid the microstructure damage, ion angle and energy were set to be 3.5� and 2.5 keV at the final stage of TEM sample preparation. The microstructure of the wires was characterized using JEOL-2100 F TEM operating at an accelerated voltage of 200 kV. X-ray diffraction (XRD) analysis was performed (Rigaku D-max 2100) using a diffractometer with a Cu target. The step size and scanning speed was set at 0.001� and 0.5� /min, respectively.
3.2. Microstructure characterization 3.2.1. Original microstructure of the wires prior to drawing As shown in Fig. 3a, the steel rods (ε ¼ 0) were composed of lamellar arrangement of ferrite and cementite. The orientation of lamellar pearlite was random. Fig. 3b shows that pearlitic structures comprise alternating layers of ferrite and cementite. The lamellar spacing between ferrite and cementite is about 120 nm, and the thickness of cementite lamellae is about 12 nm. Selected area electron diffraction (SAED) is shown as inset in Fig. 3b. The diffraction patterns of cementite indicate that cementite is made of single crystalline. The cementite exhibits a platelet-like shape, as inset shown in Fig. 3b. 3.2.2. Deformation of pearlitic structure As shown in Fig. 4a, pearlite colonies in the RT drawn wires corre sponding to ε ¼ 0.5 were stretched along the drawing direction, accompanied by the decrease of the interlamellar spacing in pearlite. Fig. 4b shows the wires (ε ¼ 0.5) drawn at 77 K. Compared with the steel wires drawn at 298 K, no difference could be observed between the RT and cryogenic drawings. Fig. 4c shows the wires (ε ¼ 2.2) drawn at 298 K. After severe deformation, pearlite was almost parallel to the drawing direction (D.D.) and the interlamellar spacing further reduced. Some broken cementite segments can be observed. For the wires (ε ¼ 2.2) drawn at 77 K, the pearlite also shows fibrous structure (Fig. 4d). No difference could be observed in wires (ε ¼ 2.2) corresponding to the RT and cryogenic drawings. Fig. 5a shows the lamellar structures in pearlitic steel wires (ε ¼ 2.2) drawn at 298 K. Many cementite particles could be observed in the cementite lamella (Fig. 5b), further revealed by high resolution trans mission electron microscope (HRTEM) observation presented in Fig. 5c. In the cementite region, a number of small particles with lattice fringes can be observed. The particle size is about 3 nm, and disordered struc tures are visible between the particles. It indicates that amorphous cementite was partially transformed into nanocrystalline state during severe cold drawing. Therefore, the resultant cementite lamellae consist of nanocrystalline cementite surrounded by amorphous cementite. The Fast Fourier Transformation (FFT) pattern of the cementite was obtained from the rectangle box, as shown in Fig. 5c. It reveals (121) cementite
3. Results 3.1. Mechanical properties Fig. 2a shows the tensile strength of pearlitic steel wires with different drawing temperatures (room temperature and cryogenic tem perature) as a function of drawing strain. The tensile strength of pearlitic steel rods (ε ¼ 0) is found to be about 1350 MPa. With the increase of the drawing strain, the tensile strength of the wires gradually increased. For the drawing strain ε < 1.1, no clear difference in the tensile strength Table 1 Chemical composition of high carbon steel rods. Elements
C
Mn
Si
Cr
P
S
Cu
Fe
wt.%
0.92
0.30
0.17
0.20
0.009
0.009
0.006
Bal.
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Fig. 2. Mechanical properties of pearlitic steel wires drawn at different temperatures. a) Tensile strength vs drawing strain; b) Engineering stress-strain curves.
¼ 2.2) drawn at 77 K. The lamellar structures consist of alternating ferrite and cementite. Fig. 5e shows fewer cementite particles are pre sent in the cementite lamellae. No lattice fringes were observed in the cementite regions (Fig. 5f), which are dominant in the cementite lamellae. It is worth noting that the boundaries between ferrite and cementite became blurred with the increase of drawing strain [12,34]. 3.3. XRD analysis Fig. 6a shows X-ray diffraction patterns of three types of wires, i.e., pearlitic steel rods (ε ¼ 0), RT drawn wires (ε ¼ 2.2, 298 K) and cryo genic drawn wires (ε ¼ 2.2, 77 K). The diffraction peaks, corresponding to α-Fe (110), α-Fe (200) can be easily identified. The diffraction peaks of (110) α-Fe and (200) α-Fe of wires drawn at 298 K and 77 K show significant broadening, which is understood to result from nanostructuring and high residual stresses. It indicates that a refinement of ferrite lamellae and an increase in dislocation density took place during the cold drawing process [35,36]. Fig. 6b presents truncated strong (110)α-Fe diffraction peak and diffraction peaks from cementite phase. As the wires without drawing (ε ¼ 0), (121) cementite, (211) cementite and (221) cementite diffraction peaks can be observed. While in the wires (ε ¼ 2.2) drawn at 298 K, cementite diffraction peaks were weakened and broadened. It may be due to the deformation and partial decomposition of cementite platelets and the emergence of nano-crystalline particles in the steel structures. By comparison, for the wires prepared at lower temperature (77 K), almost no peaks related to cementite can be observed. It suggests that the cementite exist mostly in amorphous form under cryogenic drawing condition.
Fig. 3. Microstructure of high carbon steel rods. a) SEM image; b) TEM image with SAED and DF as inset.
3.4. Cryogenic drawing wires subjected to annealing In order to analyze the effect of drawing heat on the microstructure of cementite, the steel wires (ε ¼ 2.2) drawn at 77 K were annealed at 473 K for 30 min and its microstructure was analyzed. As shown in Fig. 7a, the lamellar structures remain. Many cementite particles could be observed in the cementite lamellae (Fig. 7b). The particle size is about 2–5 nm. The existence of large cementite particles may be due to the non-uniform growth of fine cementite particles during the annealing treatment, which bears a similarity to the cementite morphology of RT drawn wires. Supposedly, a rapid temperature increase could take place in steel wires during cold drawing at RT [26,37], which triggered the nucleation and growth of cementite within a short time.
Fig. 4. SEM micrographs of as-drawn pearlitic steel wires. a) ε ¼ 0.5, 298 K; b) ε ¼ 0.5, 77 K; c) ε ¼ 2.2, 298 K; and d) ε ¼ 2.2,77 K. Drawing direction (D. D.).
crystal planes. The Inverse Fast Fourier Transformation (IFFT) image of the (121) cementite is presented in Fig. 5c. The spacing between cementite atomic layers is 0.239 nm, confirming the existence of the (121) planes. Fig. 5d shows the nanostructure of pearlitic steel wires (ε
4. Discussion The steel wires are usually drawn at room temperature. The tem 3
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Fig. 5. TEM micrographs of as-drawn pearlitic steel wires. a) ε ¼ 2.2, 298 K, BF; b) DF image of a); c) HRTEM image of a); d)ε ¼ 2.2, 77 K, BF; e) DF image of d); f) HRTEM image of d); insets in c) and f) show the FFT and IFFT images of marked boxes.
Fig. 6. (a) X-ray diffraction patterns of pearlitic steel wires prepared at different drawing temperatures; (b) Truncated (110) peaks from cementite phase. The ε ¼ 0 refers to the rods prior to drawing.
α-Fe
diffraction peak and diffraction
perature of steel wires rises rapidly due to fast plastic deformation and surface friction [22,23]. For analyzing the effect of drawing heat on the microstructure of steel wires, it is important to estimate the temperature rise in wires. To calculate the drawing heat generated from internal plastic deformation and friction at the die-wire interface, the following heat balance equation is can be utilized [22,38,39]: � � 1 4 f2 ⋅ υ ⋅ ρ ⋅c ⋅z ⋅ðTdie; out Tdie; in Þ ¼ F ⋅ km þ pffiffi f2 ⋅ α ⋅ kfm þm ⋅Q ⋅km ⋅ μ ⋅ υ⋅z A 3 3 (1) where f2 is the transverse area corresponding to the wire at die outlet, υ is the speed of cold drawing, ρ is the density of steel wire, c is the specific heat of steel wire, z is the cold drawing deformation time of steel wire, Tdie,out is the temperature of the steel wire at die outlet, Tdie,in is the temperature of the steel wire at die inlet, A is the conversion factor, F is the deviation of the transverse area of the steel wire between die inlet and die outlet, km is the average deformation resistance during the cold
Fig. 7. TEM micrographs of pearlitic steel wires (ε ¼ 2.2, 77 K) annealed at 473 K for 30 min. a) BF and b) DF. SAED image is shown as inset in a).
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drawing deformation, α is the half die angle, kfm is the mean yield strength of the steel wire before and after cold drawing deformation, m is the coefficient of heat partition, Q is the area of contact between steel wire and die, and μ is the friction coefficient between steel wire and die. From equation (1), the wire temperature at the exit of the drawing die can be determined as [22,38,39]. � � 1 4 Tdie;out ¼ Tdie;in þ F ⋅ km þ pffiffi f2 ⋅ α ⋅ kfm þ m ⋅ Q ⋅ km ⋅ μ (2) A⋅f2 ⋅ρ⋅c 3 3 In the present work, the value of the heat partition coefficient (m) (0 < m < 1) is identified as 0.8, which means the fraction of the frictional heat transferred to the wire. The coefficient of friction (μ) between the high carbon steel wire and the die is assumed as 0.06 [22]. The density of the wire (ρ) is 7.9 � 103 kg m 3 and the specific heat of the wire (c) is 0.46 � 103 J/(kg⋅� C). The calculated and measured wire temperature values at the exit of the die are plotted in Fig. 8. The wire temperature at the exit of the die is about 453–493 K. The calculation value is slightly higher than the actual measurement. This discrepancy may be due to the heat loss of wires at the die exit [22,38]. With the increase of drawing strain, the interlamellar spacing of pearlite decreased, and cementite deformed and partially decomposed [13,40]. Under RT drawing condition, cementite platelets may be transformed into nano-crystalline or even amorphous state [9–12]. As shown in Fig. 5b and c, a mixture of nano-crystalline and amorphous states could be observed in cementite lamellae. However, under cryo genic drawing condition, the amorphous state was dominant in cementite lamellae. Compared with RT drawn wires, no cementite diffraction peaks were observed in the X-ray diffraction pattern of the cryogenic drawn wires. During the RT drawing process, the wire tem perature at the exit of the die is about 453–493 K, which could drive the cementite transformation from amorphous to nano-crystalline state [10]. As shown in Fig. 7, the steel wires (ε ¼ 2.2, 77 K) were annealed at 473 K, leading to the cementite transformed from the amorphous to nano-crystalline. It could be inferred that cementite was transformed into amorphous state during RT deformation before it was recrystallized under drawing heat. Cryogenic drawing could offset the effect of drawing heat and inhibit the recrystallization of amorphous cementite. Recrystallized cementite can serve as physical barriers against dislocation motion [4,10,41]. Fig. 9a shows the dislocation morphology of pearlitic steel wires (ε ¼ 2.2) drawn at 298 K. Curved dislocation lines are presented in lamellar ferrite. The cementite crystal plane spacing is 0.239 nm, revealing the existence of cementite (121) planes (SAED inset in Fig. 9a). Bright spots in Fig. 9b represent cementite nanocrystals formed under RT drawing. In particular, the spots indicated by the
Fig. 9. TEM micrographs of cold drawn pearlitic steel wires (ε ¼ 2.2, 298 K) a) BF; b) DF. Note that the white arrows in BF point out the dislocation lines within a ferrite platelet. The white arrows in DF indicate cementite nanocrystals impeding dislocation motion.
arrows correspond to the dislocation line marked by the arrows in Fig. 9a. It shows that the dislocations in ferrite may be pinned by the cementite nanocrystals. This phenomenon was also observed in another study [10]. In addition, cementite recrystallization may take place along the original cementite sites or lamellar ferrite boundaries [42]. As the nano-crystalline cementite precipitated at the grain boundaries, the interfacial energy would be reduced, so that the boundaries can be stabilized and serve as barriers against dislocation movement [3,17]. However, during cryogenic drawing process, the initial deformation temperature was low, which partially offset the drawing heat effect. It thus inhibited the cementite transformation from the amorphous to nano-crystalline, resulting in weak blockage effect on dislocation mo tion. Therefore, the wires drawn at cryogenic temperature have a rela tively low strength and improved ductility. For the pearlitic steel wires prepared at low drawing strains (ε < 1.1), the deformation resistance (km) are low. Therefore, the wire temperature at the exit of the die is lower than high drawing strain conditions according to equation (2). In addition, the cementite did not undergo heavy deformation and decomposition under low drawing strains. Even under the higher drawing temperature (453–493 K), the cementite is not easily recrys tallized, let alone those at lower drawing temperature. Therefore, for low drawing strains, there is almost no difference in the mechanical behavior between the RT and cryogenic drawn wires. Based on the above experimental results and analysis, it seems apparent that the transformation of cementite from the amorphous to nano-crystalline state would be inevitable during the cold drawing process if the drawing heat is not suppressed. Nano-crystalline cementite would pin dislocations, resulting in an increase in the strength of wires. In addition, the occurrence of torsion delamination was closely related with the presence of nano-crystalline cementite [18]. To avoid it, two approaches are given here for consideration: 1) ultra-low temperature cooling. However, the ferrite has a body-centered cubic structure, which may cause low-temperature brittleness or even ductile-brittle transition at ultra-low temperature [20,28,29]. 2) Lower drawing strain at each drawing step. It can reduce the drawing heat and cementite deformation and decomposition so that cementite is not easily recrystallized. How ever, the strength increase would be limited at each drawing and achieving the high strength would require more drawing steps to be implemented that cause an increase in process time and die number/ costs. Finding a solution to achieve a combination of high strength and ductility in heavily drawn pearlitic steel wires while keeping the process costs low remains an interesting and challenging task. 5. Conclusion The effect of drawing heat on the microstructure and mechanical properties of cold drawn pearlitic steel wires was explored. The following conclusions can be drawn:
Fig. 8. A comparison of the calculated and measured wire temperatures at the exit of the die. 5
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1) No marked difference in the microstructural and mechanical prop erties exists between RT and cryogenic drawings. As the drawing strain increased, the cryogenic drawn wires have lower strength but better ductility. The tensile strength of cryogenic drawn wires (ε ¼ 2.2) is about 2050 MPa, about 90 MPa lower than that of RT drawn wires. 2) As the drawing strain increased, interlamellar spacing of pearlite was decreased, and crystalline cementite was gradually transformed into amorphous state. Compared with RT drawing wires, nano-crystalline cementite particles in cryogenic drawing wires have a lower volume fraction and exhibit in smaller sizes. 3) Nano-crystalline cementite particles could hinder the movement of dislocations. Compared with RT drawn wires, the lower volume fraction of nano-crystalline cementite in cryogenic drawn wires might be responsible for observed lower strength and better ductility.
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Data availability statement All data included in this study are available upon request by contact with the corresponding authors. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Dasheng Wei: Writing - review & editing. Xuegang Min: Method ology. Xianjun Hu: Investigation. Zonghan Xie: Writing - review & editing. Feng Fang: Writing - review & editing, Supervision. Acknowledgement This work was funded by the Science and Technology Advancement Program of Jiangsu Province (BA2017112) and the Industry-University Research Cooperation Project of Jiangsu Province, PR China (BY2018194). The study was also partly funded by 333 Projects of Jiangsu Province, PR China (BRA2018045). Authors thank X. Shen, L. Li, S. Cui and K. Yang for assistance with preparation the samples, and A. Xu and C. Jin for support of TEM analysis. Z. Xie acknowledges the support provided by the Australian Research Council Discovery Projects. References [1] D. Raabe, P.P. Choi, Y.J. Li, A. Kostka, X. Sauvage, F. Lecouturier, K. Hono, R. Kirchheim, R. Pippan, Metallic composites processed via extreme deformation: toward the limits of strength in bulk materials, MRS Bull. 35 (12) (2010) 982–991. [2] C. Borchers, R. Kirchheim, Cold-drawn pearlitic steel wires, Prog. Mater. Sci. 82 (2016) 405–444. [3] Y.J. Li, D. Raabe, M. Herbig, P.-P. Choi, S. Goto, A. Kostka, H. Yarita, C. Borchers, R. Kirchheim, Segregation stabilizes nanocrystalline bulk steel with near theoretical strength, Phys. Rev. Lett. 113 (10) (2014) 106104. [4] N. Maruyama, T. Tarui, H. Tashiro, Atom probe study on the ductility of drawn pearlitic steels, Scripta Mater. 46 (8) (2002) 599–603. [5] J.D. Embury, R.M. Fisher, The structure and properties of drawn pearlite, Acta Metall. 14 (2) (1966) 147–159. [6] X.D. Zhang, A. Godfrey, N. Hansen, X.X. Huang, Hierarchical structures in colddrawn pearlitic steel wire, Acta Mater. 61 (13) (2013) 4898–4909. [7] X.D. Zhang, A. Godfrey, X.X. Huang, N. Hansen, Q. Liu, Microstructure and strengthening mechanisms in cold-drawn pearlitic steel wire, Acta Mater. 59 (9) (2011) 3422–3430. [8] F. Fang, L.C. Zhou, X.J. Hu, X.F. Zhou, Y.Y. Tu, Z.H. Xie, J.Q. Jiang, Microstructure and mechanical properties of cold-drawn pearlitic wires affect by inherited texture, Mater. Des. 79 (2015) 60–67. [9] C. Borchers, T. Al-Kassab, S. Goto, R. Kirchheim, Partially amorphous nanocomposite obtained from heavily deformed pearlitic steel, Mater. Sci. Eng., A 502 (1–2) (2009) 131–138.
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