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Deformation of dual-structure medium carbon steel in cold drawing
Materials Science & Engineering A 583 (2013) 78–83
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Deformation of dual-structure medium carbon steel in cold drawing Feng Fang a,n, Xian-jun Hu a,b, Bi-ming Zhang a, Zong-han Xie c, Jian-qing Jiang a a b c
School of Materials Science and Engineering, Southeast University, Jiangning District, Nanjing 211189, PR China Jiangsu Sha-Steel Group, Zhangjiagang City, Jiangsu Province 215625, PR China School of Mechanical Engineering, University of Adelaide, SA 5005, Australia
art ic l e i nf o
a b s t r a c t
Article history: Received 23 October 2012 Received in revised form 14 June 2013 Accepted 18 June 2013 Available online 6 July 2013
In this paper, extremely high strength was obtained in medium carbon steel having a carbon content of 0.35% by weight through cold drawing. Experimental results showed that the tensile strength of the steel increased by nearly three folds from the original value 615 MPa to 1810 MPa corresponding to drawing strain of 3.0. To reveal the mechanisms that govern the strengthen increase, the microstructural evolution was analyzed during cold drawing, with respect to the change of the deformation resistance (measured by micro-hardness) of micro-constituents (i.e., primary or proeutectoid ferrite and pearlite) in the material. The proeutectoid ferrite became elongated and, at the same time, increasingly hardened while the pearlite maintained equiaxed shape after initial drawing. With the increase of the drawing strain, the pearlite was stretched parallel to drawing direction, accompanied by an increase in the 〈110〉 texture intensity and dislocation density in the ferrite phase. Under heavy drawing, a laminate structure formed, consisting of alternating pro-eutectoid ferrite and pearlite both parallel to the drawing direction. The 〈110〉 texture intensity in the ferrite phase became saturated as ε41.2. High density dislocation zones further spread in the ferrite phase. The interlamellar spacing between ferrite and cementite phases in the pearlite decreased. Based upon these observations, mechanistic models were constructed to provide insight into the deformation and strengthening mechanisms of this steel. & 2013 Elsevier B.V. All rights reserved.
Keywords: Medium carbon steel Wire Dual-structure Drawing Deformation
1. Introduction Medium carbon steel, also known as hypoeutectoid steel, exhibits a microstructure that comprises primary (or pro-eutectoid) ferrite and pearlite micro-constituents. Owing to a good combination of ductility, strength and wear resistance, it has been used for many applications including large parts, forging and automotive components [1–3]. Compared to high carbon steel with a pearlitic structure, the hypoeutectoid steel possesses limited strength (about 600 MPa, hot roll). In recent years considerable efforts have been invested to displace relatively expensive pearlitic steel by using hypoeutectoid steel in the manufacture of cold drawn wires with high strength, in doing so, costs associated with raw materials and processing can be significantly reduced. High strength hypoeutectoid steel wires have potential applications for bridge cables, tire cords and springs [4–6], bringing significant economic benefits to manufacturers. Unlike ferrite–cementite assembly in pearlitic steels having a coherent relationship in crystallography [7], ferritic–pearlitic structure in pro-eutectoid steels exhibits no such coherent relationship. Consequently, the deformation and strengthening mechanisms are expected to be different for these two types of steel. Deformation of ferrite–
n
Corresponding author. Tel.: +86 25 52090630; fax: +86 25 52090634. E-mail address: [email protected] (F. Fang).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.06.081
cementite lamellar structure in pearlitic steels during cold drawing process has been examined intensively in recent years [8], aided by high resolution electron microscopy. For example, partial cementite dissolution was observed [9–11], in order to maintain the integrity of the ferrite–cementite interface under heavy drawing. In contrast, the mechanistic understanding of the deformation of pro-eutectoid steels is much more limited, which may explain why effective strategies are currently lacking to strengthen pro-eutectoid steels to a great extent. In this work hypoeutectoid steel with 0.35 wt% of carbon content was deformed through cold drawing process up to a strain of 3.0. The tensile strength and deformation structure of the wires drawn to different strains were investigated to establish a direct link between deformation structure and mechanical strength of the steel during cold drawing. Assisted by the model analysis, the critical factors that control the deformation and strength of hypoeutectoid steel were identified. A three times increase in the tensile strength was also achieved for the steel samples used in this study and paves the way for the development of high strength hypoeutectoid steel.
2. Experimental procedure Medium carbon steel wires used in this work are produced by Shasteel Group Company, PR China. The chemical compositions of the
F. Fang et al. / Materials Science & Engineering A 583 (2013) 78–83
79
2000
Mn
Si
Al
S
P
0.35
0.74
0.12
0.005
0.005
0.015
samples are shown in Table 1. After pickling and phosphating [12], hot-rolled wire rod (10 mm in diameter) were successively drawn to a diameter of 2.2 mm with a total reduction of 95% (i.e., ε¼3.0). The average reduction per pass was about 14%. The tensile strength of the resulting wires was determined at room temperature by using a universal testing machine (CMT5105 type, Zhuhai Suns-Tech Electric Equipment Co. LTD., China) operating at a constant speed of 2 mm/min. The tests were performed as per the Chinese National Standard GB/T228-2002. The deformation resistance of micro-constituents in the steel samples was also measured using a micro-hardness sclerometer (FM-700, FUTURETECH, Japan) with the maximum load 10 gf. The indentation tests were performed parallel to the drawing direction. Microstructure and texture evolution of cold drawn wires was examined using a FEI Siron-400 scanning electron microscope (SEM) interfaced with a EDAX-TSL system for electron backscatter diffraction (EBSD). Step size of EBSD scans was 100 nm, and data were processed with OIM analysis software suite and no data cleanup was applied on any of the scans. The dislocation and density in cold drawn wires were detected with transmission electron microscope (TEM) (JEM 2000EX, JEOL Lmt., Japanese).
1500
1000
500
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
True Strain, Fig. 1. Tensile strength with different true strain.
Ferrite Pearlite
400
Microhardness, Hv
C
Tensile Strength, b/MPa
Table 1 Chemical composition of medium carbon steel wires used in this work.
350 300 250 200 150
3. Results and discussion 3.1. Mechanical properties The tensile strength of cold drawn steels as a function of true strain is presented in Fig. 1. It increases from the original 615 MPa to 1810 MPa that corresponds to a true strain of 3.0. According to the variation of Δs/Δε with true strain, the drawing process can be divided into three stages: (1) ε≤1.0, Δs/ Δε ¼450; (2) 1.0 oε≤1.8: Δs/Δε ¼230; (3) 1.8 oε≤3.0: Δs/Δε ¼ 530. A rapid increase in strength can be seen in the first and third stages, whereas the strength increases slowly in the second stage. The steel wire became too brittle to be drawn for ε 4 3.0. To gain a deep insight into the mechanical processes that regulate the strength of the steel wires during cold drawing, the deformation resistance of micro-constituents (i.e., proeutectoid ferrite and pearlite) in the samples prepared at different strains was measured by micro-hardness tests and displayed in Fig. 2. The change of tensile strength of the proeutectoid ferrite with drawing strain exhibits a striking similarity to that of the cold drawn steels. In contrast, a linear increase in the tensile strength is observed for pearlite. The initial micro-hardnesses values of ferrite and pearlite are 160 HV and 230 HV, respectively. The micro-hardness of ferrite increases quickly up to ε ¼1.0, beyond that the difference in micro-hardness between these two micro-constituents gradually decreases and lies within 10 HV at ε ¼ 3.0. 3.2. Evolution of microstructure Before cold drawing, the sample obtained from the longitudinal section (Fig. 3a) exhibits similar microstructural features as that of the transverse sections (Fig. 3). Both consist of two micro-constituents, namely, ferrite and pearlite (a two-phased, lamellar structure composed of alternating layers of ferrite and cementite). The orientation of the lamellar structure in pearlite is random.
100
0.0
0.5
1.0
1.5
2.0
2.5
3.0
True strain/ Fig. 2. Relationship between micro-hardness and true strain.
The microstructure characteristics observed from the longitudinal section of the steel wires drawn to different strains are shown in Fig. 4. As shown in Fig. 4a, the pro-eutectoid ferrite was stretched and became elongated, while the pearlite maintained an equiaxed shape after initial drawing. With the increase of the drawing strain, the pearlite was deformed, in which cementite aligned and extended towards the drawing direction through a combined rotating and shearing process, as shown in Fig. 4b. Under heavy drawing (Fig. 4c and d), pearlite was further elongated along the drawing direction, presumably through the shearing of cementite platelets. A laminate structure consisting of alternating layers of pro-eutectoid ferrite and pearlite results. Following heavy drawing (ε≥3.0) the transverse section of the steel wire is also examined (Fig. 5). Micro-voids are visible at the interface between pro-eutectoid ferrite and pearlite. It may result from the asymmetrical distribution of deformation in proeutectoid ferrite and pearlite. Apparently, the deformation of (softer) pro-eutectoid ferrite is greater than that of pearlite during cold drawing. This inharmonious deformation results in the strain mismatch at the interface of pro-eutectoid ferrite and pearlite, leading to the formation of micro-voids. On the other hand, proeutectoid ferrite and pearlite are assumed to have different Poisson's ratio [13]. Consequently, different (lateral) strains were induced in them perpendicular to the drawing direction, resulting in tensile stress mismatch at the interface of these two microconstituents. It also contributes to the formation of micro-voids. The presence of these structural defects may explain the low tolerance of the heavily drawn wires to mechanical damage.
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F. Fang et al. / Materials Science & Engineering A 583 (2013) 78–83
Fig. 3. Microstructure of medium carbon steel rods (a) longitudinal section and (b) transversal section.
Fig. 4. Microstructure of medium carbon steel wires in drawing (longitudinal section) (a) ε¼ 0.5, (b) ε ¼1.20, (c) ε ¼1.9 and (d) ε¼ 2.6.
Fig. 5. Microstructure of medium carbon steel wire after heavily drawing (ε¼ 3.0) (a) transverse section image and (b) magnification of (a).
F. Fang et al. / Materials Science & Engineering A 583 (2013) 78–83
The textural change of the ferrite phase in the steel samples drawn to different strains was measured in terms of inverse pole figure measured in the drawing direction. Fig. 6 shows the inverse pole figure of the steel wires corresponding to the true strain (ε) of 0, 0.5, 1.2, 1.9 and 2.6. The color in the inverse pole figures represents the intensity of texture. Expectedly, the original steel rod (corresponding to a true strain of 0) had an almost random texture, as seen in Fig. 6a. With the increase of the drawing strain, the orientation 〈110〉 intensified and became dominant, accompanied with the appearance of other textures of low intensity including 〈215〉, 〈112〉 and 〈103〉. The intensity of 〈110〉 fiber texture was also quantified and is shown in Fig. 7. It reveals that, as the drawing strain increased, the 〈110〉 texture in the ferrite phase increased and became saturated when the drawing strain reaches 1.5, which correlates to the third stage of the hardening process in Fig. 1. Interestingly, compared with pearlitic steels [14], the critical drawing strain with which the 〈110〉 texture becomes saturated in the ferrite phase is higher for the hypoeutectoid steel. Such difference can be attributed to the presence of pro-eutectoid ferrite phase. During the initial drawing, both {110} 〈1—1—11〉 and {112} 〈—1—11—1—1〉 slip systems are active in pro-eutectoid ferrite phase to absorb deformation. The deformation (or elongation) of pro-eutectoid grains relaxes the demand applied to hard pearlite to accommodate plastic deformation in the material [15].
Therefore, pearlite barely deforms except that cementite platelets rotate and align towards the drawing direction. Through this mechanism, larger strain can be tolerated in the pro-eutectoid steel, rendering the 〈110〉 texture more dominant than the pearlitic steel. For strains over the critical value ( 1.5), the 〈110〉 texture increases slowly, coinciding with a linear increase in the tensile strength of the drawn steels as in Fig. 1.
5
<110> Fiber Texture Intensity
3.3. Texture analysis
81
4
3
2
1 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Drawing Strain, Fig. 7. Calculated intensity of 〈110〉 fiber texture for ferrite phase in drawing.
Fig. 6. Calculated 001 inverse pole figure for ferrite phase of medium carbon steel wires at (a) ε¼ 0, (b) ε¼ 0.5, (c) ε¼ 1.2, (d) ε ¼1.9 and (e) ε ¼2.6. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.
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F. Fang et al. / Materials Science & Engineering A 583 (2013) 78–83
3.4. TEM analysis The microstructural changes of the drawn steel wires corresponding to the true strain (ε) of 0, 0.5, 1.2 and 2.6 are also observed using TEM. As shown in Fig. 8a, a limited number of dislocations exist within the pro-eutectoid ferrite in the original steel rod. In comparison, almost no dislocations can be found in pearlite micro-constituent prior to drawing. Fig. 8b shows the deformation microstructure of the as-drawn wire (ε ¼ 0.5). High density dislocation zones appear in the pro-eutectoid ferrite, apparently due to the drawing process. The presence of dislocations in the ferrite in the pearlite structure means the plastic deformation also occurred in the pearlite during the drawing process. Moreover, the deformation strain in pro-eutectoid ferrite is higher than in the pearlite (ε ¼2.6 in Fig. 8c). With further drawing, the number of high density dislocation zones in proeutectoid ferrite increased significantly and the interlamellar spacing of the pearlite also decreased (Fig. 8d). By coupling the change of mechanical properties with the microstructural evolution, a direct link between the strength increase and deformation microstructure in the hypoeutectoid steel can be established. In the initial drawing, plastic deformation can easily take place in pro-eutectoid ferrite having a low dislocation density. As a result, soft pro-eutectoid ferrite deformed along the drawing direction and hard pearlite maintained its equiaxed
shape. With the increase of the drawing strain, the strength of proeutectoid ferrite increased rapidly because of work hardening. Consequently, the plastic deformation was activated in the pearlite colonies, in which cementite platelets progressively aligned with the drawing direction through rotating and shearing. In addition, hard cementite platelets almost maintained a straight shape during deformation, accommodated by the soft pro-eutectoid ferrite. In contrast, the pearlite steel lacks soft pro-eutectoid ferrite to absorb increasing strain, bending or fracturing was often observed in cementite platelets that have high angles with the drawing direction [16]. This leads to the loss of structural integrity of the pearlitic steel at relatively small drawing strain. With a further increase in the drawing strain, the deformation mismatch between pro-eutectoid ferrite and pearlite exacerbated, resulting in the formation voids or micro-crack at the interface of two different micro-constituents. The initiation of these structural defects would set the limits for the drawing ability (i.e., drawability) of the hypoeutectoid steel and the tensile strength that it can achieve through drawing process. To impart high strength to medium carbon steel through cold drawing and, at the same time, prevent the formation of structural defects in the material, it is vital to regulate the deformation in the steel and to minimize the strain mismatch between two different micro-constituents. Consequently, design strategies that significantly boost the strength of pro-eutectoid ferrite may be used, for
Fig. 8. TEM images of medium carbon steel wires in drawing (a) ε¼ 0, (b) ε¼ 0.5, (c) ε¼ 1.2 and (d) ε¼ 2.6.
F. Fang et al. / Materials Science & Engineering A 583 (2013) 78–83
example, decreasing the grain size in pro-eutectoid ferrite colonies and/or alloying pro-eutectoid ferrite with silicon. In doing so, the strength of pro-eutectoid ferrite would be lifted to a level close to that of pearlite. A coherent relationship between tailored proeutectoid ferrite and pearlite is thus expected to develop during cold drawing, providing the resulting steel wire extremely high strength and damage tolerance. This may open realistic pathways to developing extremely strong, damage tolerant medium carbon steel wire with relative better toughness and lower costs. 4. Conclusions Medium carbon steel having a carbon concentration of 0.35% by weight was used in this work to develop a deep picture of microstructure-property relationship during cold drawing process. The tensile strength of the steel with 50% of primary ferrite was found to increase impressively by nearly three folds from 615 MPa to 1810 MPa corresponding to a drawing strain of 3.0. The drawing processes can be divided into three phases: the first and third phase signifies a rapid increase of the tensile strength with drawing strain, while the second stage is characterized by a relatively slow rise in strength. Micro-hardness tests were used to probe the mechanical response of the micro-constituents in the steel during drawing. Compared to pearlite in the steel that exhibited a steady, linear rate of increase in the strength, the deformation resistance of primary ferrite increased rapidly to the drawing strain (ε)≈1.0. Beyond that, the strength of primary ferrite increased slowly and approached that of the pearlite. The roles of key structure components were identified in regulating the strength of cold drawn steel wires. During the initial drawing pro-eutectoid ferrite was deformed and extended along drawing direction, while the pearlite maintained its original equiaxed shape. With the increase of drawing strain, the intensity of 〈110〉 texture in the ferrite phase increased rapidly and became saturated at 1.5. High density dislocation zones appeared in both pro-eutectoid ferrite and the ferrite in pearlite. Pearlite was also deformed, in which cementite platelets aligned with the drawing direction, presumably through rotation and shearing. Under heavy drawing, high density dislocation zones spread in the ferrite and the interlamellar spacing in the pearlite decreased. A laminate structure that consists of alternating layers of pro-eutectoid ferrite
83
and pearlite resulted, leading to an appreciable increase in the tensile strength. In addition, micro-voids appeared in the heavily drawn steel wire, as a result of strain mismatch at the interface between pro-eutectoid ferrite and pearlite. These structure defects may be prevented by controlling the deformation of the proeutectoid ferrite. By doing so, the resulting steel wires are expected to exhibit not only high strength, but also immunity to mechanical damage.
Acknowledgments The authors would like to thank Prof. Fan Li in the Jiangsu Key Lab for Advanced Metallic Materials, Southeast University for the help on EBSD analysis. This work was financially supported by the Natural Scientific Foundation of Jiangsu Province (Grant no. BK2011616) and the Southeast University Research Project, and was partially funded by the Jiangsu Province Industry-University Strategic Research Program (Grant no. BY2011144). The samples were prepared in Jiangsu Province Institute of Research of Iron and Steel, which is partially supported by the Zhangjiagang City Scientific & Technological Project under Grant no. ZKJ1013.
References [1] T. Tarui, S. Nishida, A. Yoshie, H. Ohba, Y. Asano, I. Ochiai, Nippon Steel Tech. Rep. 80 (1999) 44. [2] H. Tashiro, S. Nishida, T. Tarui, S. Ohashi, S. Sasaki, K. Nakamura, Nippon Steel Tech. Rep. 80 (1999) 38. [3] S. Allain, O. Bouaziz, Mater. Sci. Eng. A 496 (2008) 329–336. [4] H. Huh, S.B. Kim, J.H. Song, J.H. Lim, Int. J. Mech. Sci. 50 (2008) 918–931. [5] T. Waterschoot, K. Verbeken, B.C. De Cooman, ISIJ Int. 46 (2006) 138–146. [6] F. Fang, X.J. Hu, S.H. Chen, Z. Xie, J.Q. Jiang, Mater. Sci. Eng. A 574 (2012) 51–54. [7] D.S. Zhou, G.J. Shiflet, Metall. Trans. A 23 (1992) 1259–1269. [8] G. Langford, Metall. Trans. A 8 (1977) 861–875. [9] J. Languillaume, G. Kapelski, B. Baudelet, Acta Mater. 45 (1997) 1201–1212. [10] V.A. Shabashov, A.G. Mukoseev, V.V. Sagaradze., Mate. Sci. Eng. A 307 (2001) 91–97. [11] V.G. Gavriljuk, Scr. Mater. 46 (2002) 175–177. [12] F. Fang, J.H. Jiang, S.Y. Tan, A.B. Ma, J.Q. Jiang, Surf. Coat. Technol. 204 (2010) 2381–2385. [13] G.N. Greaves, A.L. Greer, R.S. Lakes, T. Rouxel, Nat. Mater. 10 (2011) 823–837. [14] P. Kumar, N.P. Gurao, A. Haldar, Satyam Suwas, ISIJ Int. 51 (2011) 679–684. [15] P. Van Houtte, Mater. Sci. Eng. A 55 (1982) 69–77. [16] W.J. Nam, C.M. Bae, Mater. Sci. Eng. A 203 (1995) 278–285.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Deformation of dual-structure medium carbon steel in cold drawing Feng Fang a,n, Xian-jun Hu a,b, Bi-ming Zhang a, Zong-han Xie c, Jian-qing Jiang a a b c
School of Materials Science and Engineering, Southeast University, Jiangning District, Nanjing 211189, PR China Jiangsu Sha-Steel Group, Zhangjiagang City, Jiangsu Province 215625, PR China School of Mechanical Engineering, University of Adelaide, SA 5005, Australia
art ic l e i nf o
a b s t r a c t
Article history: Received 23 October 2012 Received in revised form 14 June 2013 Accepted 18 June 2013 Available online 6 July 2013
In this paper, extremely high strength was obtained in medium carbon steel having a carbon content of 0.35% by weight through cold drawing. Experimental results showed that the tensile strength of the steel increased by nearly three folds from the original value 615 MPa to 1810 MPa corresponding to drawing strain of 3.0. To reveal the mechanisms that govern the strengthen increase, the microstructural evolution was analyzed during cold drawing, with respect to the change of the deformation resistance (measured by micro-hardness) of micro-constituents (i.e., primary or proeutectoid ferrite and pearlite) in the material. The proeutectoid ferrite became elongated and, at the same time, increasingly hardened while the pearlite maintained equiaxed shape after initial drawing. With the increase of the drawing strain, the pearlite was stretched parallel to drawing direction, accompanied by an increase in the 〈110〉 texture intensity and dislocation density in the ferrite phase. Under heavy drawing, a laminate structure formed, consisting of alternating pro-eutectoid ferrite and pearlite both parallel to the drawing direction. The 〈110〉 texture intensity in the ferrite phase became saturated as ε41.2. High density dislocation zones further spread in the ferrite phase. The interlamellar spacing between ferrite and cementite phases in the pearlite decreased. Based upon these observations, mechanistic models were constructed to provide insight into the deformation and strengthening mechanisms of this steel. & 2013 Elsevier B.V. All rights reserved.
Keywords: Medium carbon steel Wire Dual-structure Drawing Deformation
1. Introduction Medium carbon steel, also known as hypoeutectoid steel, exhibits a microstructure that comprises primary (or pro-eutectoid) ferrite and pearlite micro-constituents. Owing to a good combination of ductility, strength and wear resistance, it has been used for many applications including large parts, forging and automotive components [1–3]. Compared to high carbon steel with a pearlitic structure, the hypoeutectoid steel possesses limited strength (about 600 MPa, hot roll). In recent years considerable efforts have been invested to displace relatively expensive pearlitic steel by using hypoeutectoid steel in the manufacture of cold drawn wires with high strength, in doing so, costs associated with raw materials and processing can be significantly reduced. High strength hypoeutectoid steel wires have potential applications for bridge cables, tire cords and springs [4–6], bringing significant economic benefits to manufacturers. Unlike ferrite–cementite assembly in pearlitic steels having a coherent relationship in crystallography [7], ferritic–pearlitic structure in pro-eutectoid steels exhibits no such coherent relationship. Consequently, the deformation and strengthening mechanisms are expected to be different for these two types of steel. Deformation of ferrite–
n
Corresponding author. Tel.: +86 25 52090630; fax: +86 25 52090634. E-mail address: [email protected] (F. Fang).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.06.081
cementite lamellar structure in pearlitic steels during cold drawing process has been examined intensively in recent years [8], aided by high resolution electron microscopy. For example, partial cementite dissolution was observed [9–11], in order to maintain the integrity of the ferrite–cementite interface under heavy drawing. In contrast, the mechanistic understanding of the deformation of pro-eutectoid steels is much more limited, which may explain why effective strategies are currently lacking to strengthen pro-eutectoid steels to a great extent. In this work hypoeutectoid steel with 0.35 wt% of carbon content was deformed through cold drawing process up to a strain of 3.0. The tensile strength and deformation structure of the wires drawn to different strains were investigated to establish a direct link between deformation structure and mechanical strength of the steel during cold drawing. Assisted by the model analysis, the critical factors that control the deformation and strength of hypoeutectoid steel were identified. A three times increase in the tensile strength was also achieved for the steel samples used in this study and paves the way for the development of high strength hypoeutectoid steel.
2. Experimental procedure Medium carbon steel wires used in this work are produced by Shasteel Group Company, PR China. The chemical compositions of the
F. Fang et al. / Materials Science & Engineering A 583 (2013) 78–83
79
2000
Mn
Si
Al
S
P
0.35
0.74
0.12
0.005
0.005
0.015
samples are shown in Table 1. After pickling and phosphating [12], hot-rolled wire rod (10 mm in diameter) were successively drawn to a diameter of 2.2 mm with a total reduction of 95% (i.e., ε¼3.0). The average reduction per pass was about 14%. The tensile strength of the resulting wires was determined at room temperature by using a universal testing machine (CMT5105 type, Zhuhai Suns-Tech Electric Equipment Co. LTD., China) operating at a constant speed of 2 mm/min. The tests were performed as per the Chinese National Standard GB/T228-2002. The deformation resistance of micro-constituents in the steel samples was also measured using a micro-hardness sclerometer (FM-700, FUTURETECH, Japan) with the maximum load 10 gf. The indentation tests were performed parallel to the drawing direction. Microstructure and texture evolution of cold drawn wires was examined using a FEI Siron-400 scanning electron microscope (SEM) interfaced with a EDAX-TSL system for electron backscatter diffraction (EBSD). Step size of EBSD scans was 100 nm, and data were processed with OIM analysis software suite and no data cleanup was applied on any of the scans. The dislocation and density in cold drawn wires were detected with transmission electron microscope (TEM) (JEM 2000EX, JEOL Lmt., Japanese).
1500
1000
500
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
True Strain, Fig. 1. Tensile strength with different true strain.
Ferrite Pearlite
400
Microhardness, Hv
C
Tensile Strength, b/MPa
Table 1 Chemical composition of medium carbon steel wires used in this work.
350 300 250 200 150
3. Results and discussion 3.1. Mechanical properties The tensile strength of cold drawn steels as a function of true strain is presented in Fig. 1. It increases from the original 615 MPa to 1810 MPa that corresponds to a true strain of 3.0. According to the variation of Δs/Δε with true strain, the drawing process can be divided into three stages: (1) ε≤1.0, Δs/ Δε ¼450; (2) 1.0 oε≤1.8: Δs/Δε ¼230; (3) 1.8 oε≤3.0: Δs/Δε ¼ 530. A rapid increase in strength can be seen in the first and third stages, whereas the strength increases slowly in the second stage. The steel wire became too brittle to be drawn for ε 4 3.0. To gain a deep insight into the mechanical processes that regulate the strength of the steel wires during cold drawing, the deformation resistance of micro-constituents (i.e., proeutectoid ferrite and pearlite) in the samples prepared at different strains was measured by micro-hardness tests and displayed in Fig. 2. The change of tensile strength of the proeutectoid ferrite with drawing strain exhibits a striking similarity to that of the cold drawn steels. In contrast, a linear increase in the tensile strength is observed for pearlite. The initial micro-hardnesses values of ferrite and pearlite are 160 HV and 230 HV, respectively. The micro-hardness of ferrite increases quickly up to ε ¼1.0, beyond that the difference in micro-hardness between these two micro-constituents gradually decreases and lies within 10 HV at ε ¼ 3.0. 3.2. Evolution of microstructure Before cold drawing, the sample obtained from the longitudinal section (Fig. 3a) exhibits similar microstructural features as that of the transverse sections (Fig. 3). Both consist of two micro-constituents, namely, ferrite and pearlite (a two-phased, lamellar structure composed of alternating layers of ferrite and cementite). The orientation of the lamellar structure in pearlite is random.
100
0.0
0.5
1.0
1.5
2.0
2.5
3.0
True strain/ Fig. 2. Relationship between micro-hardness and true strain.
The microstructure characteristics observed from the longitudinal section of the steel wires drawn to different strains are shown in Fig. 4. As shown in Fig. 4a, the pro-eutectoid ferrite was stretched and became elongated, while the pearlite maintained an equiaxed shape after initial drawing. With the increase of the drawing strain, the pearlite was deformed, in which cementite aligned and extended towards the drawing direction through a combined rotating and shearing process, as shown in Fig. 4b. Under heavy drawing (Fig. 4c and d), pearlite was further elongated along the drawing direction, presumably through the shearing of cementite platelets. A laminate structure consisting of alternating layers of pro-eutectoid ferrite and pearlite results. Following heavy drawing (ε≥3.0) the transverse section of the steel wire is also examined (Fig. 5). Micro-voids are visible at the interface between pro-eutectoid ferrite and pearlite. It may result from the asymmetrical distribution of deformation in proeutectoid ferrite and pearlite. Apparently, the deformation of (softer) pro-eutectoid ferrite is greater than that of pearlite during cold drawing. This inharmonious deformation results in the strain mismatch at the interface of pro-eutectoid ferrite and pearlite, leading to the formation of micro-voids. On the other hand, proeutectoid ferrite and pearlite are assumed to have different Poisson's ratio [13]. Consequently, different (lateral) strains were induced in them perpendicular to the drawing direction, resulting in tensile stress mismatch at the interface of these two microconstituents. It also contributes to the formation of micro-voids. The presence of these structural defects may explain the low tolerance of the heavily drawn wires to mechanical damage.
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Fig. 3. Microstructure of medium carbon steel rods (a) longitudinal section and (b) transversal section.
Fig. 4. Microstructure of medium carbon steel wires in drawing (longitudinal section) (a) ε¼ 0.5, (b) ε ¼1.20, (c) ε ¼1.9 and (d) ε¼ 2.6.
Fig. 5. Microstructure of medium carbon steel wire after heavily drawing (ε¼ 3.0) (a) transverse section image and (b) magnification of (a).
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The textural change of the ferrite phase in the steel samples drawn to different strains was measured in terms of inverse pole figure measured in the drawing direction. Fig. 6 shows the inverse pole figure of the steel wires corresponding to the true strain (ε) of 0, 0.5, 1.2, 1.9 and 2.6. The color in the inverse pole figures represents the intensity of texture. Expectedly, the original steel rod (corresponding to a true strain of 0) had an almost random texture, as seen in Fig. 6a. With the increase of the drawing strain, the orientation 〈110〉 intensified and became dominant, accompanied with the appearance of other textures of low intensity including 〈215〉, 〈112〉 and 〈103〉. The intensity of 〈110〉 fiber texture was also quantified and is shown in Fig. 7. It reveals that, as the drawing strain increased, the 〈110〉 texture in the ferrite phase increased and became saturated when the drawing strain reaches 1.5, which correlates to the third stage of the hardening process in Fig. 1. Interestingly, compared with pearlitic steels [14], the critical drawing strain with which the 〈110〉 texture becomes saturated in the ferrite phase is higher for the hypoeutectoid steel. Such difference can be attributed to the presence of pro-eutectoid ferrite phase. During the initial drawing, both {110} 〈1—1—11〉 and {112} 〈—1—11—1—1〉 slip systems are active in pro-eutectoid ferrite phase to absorb deformation. The deformation (or elongation) of pro-eutectoid grains relaxes the demand applied to hard pearlite to accommodate plastic deformation in the material [15].
Therefore, pearlite barely deforms except that cementite platelets rotate and align towards the drawing direction. Through this mechanism, larger strain can be tolerated in the pro-eutectoid steel, rendering the 〈110〉 texture more dominant than the pearlitic steel. For strains over the critical value ( 1.5), the 〈110〉 texture increases slowly, coinciding with a linear increase in the tensile strength of the drawn steels as in Fig. 1.
5
<110> Fiber Texture Intensity
3.3. Texture analysis
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4
3
2
1 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Drawing Strain, Fig. 7. Calculated intensity of 〈110〉 fiber texture for ferrite phase in drawing.
Fig. 6. Calculated 001 inverse pole figure for ferrite phase of medium carbon steel wires at (a) ε¼ 0, (b) ε¼ 0.5, (c) ε¼ 1.2, (d) ε ¼1.9 and (e) ε ¼2.6. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.
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3.4. TEM analysis The microstructural changes of the drawn steel wires corresponding to the true strain (ε) of 0, 0.5, 1.2 and 2.6 are also observed using TEM. As shown in Fig. 8a, a limited number of dislocations exist within the pro-eutectoid ferrite in the original steel rod. In comparison, almost no dislocations can be found in pearlite micro-constituent prior to drawing. Fig. 8b shows the deformation microstructure of the as-drawn wire (ε ¼ 0.5). High density dislocation zones appear in the pro-eutectoid ferrite, apparently due to the drawing process. The presence of dislocations in the ferrite in the pearlite structure means the plastic deformation also occurred in the pearlite during the drawing process. Moreover, the deformation strain in pro-eutectoid ferrite is higher than in the pearlite (ε ¼2.6 in Fig. 8c). With further drawing, the number of high density dislocation zones in proeutectoid ferrite increased significantly and the interlamellar spacing of the pearlite also decreased (Fig. 8d). By coupling the change of mechanical properties with the microstructural evolution, a direct link between the strength increase and deformation microstructure in the hypoeutectoid steel can be established. In the initial drawing, plastic deformation can easily take place in pro-eutectoid ferrite having a low dislocation density. As a result, soft pro-eutectoid ferrite deformed along the drawing direction and hard pearlite maintained its equiaxed
shape. With the increase of the drawing strain, the strength of proeutectoid ferrite increased rapidly because of work hardening. Consequently, the plastic deformation was activated in the pearlite colonies, in which cementite platelets progressively aligned with the drawing direction through rotating and shearing. In addition, hard cementite platelets almost maintained a straight shape during deformation, accommodated by the soft pro-eutectoid ferrite. In contrast, the pearlite steel lacks soft pro-eutectoid ferrite to absorb increasing strain, bending or fracturing was often observed in cementite platelets that have high angles with the drawing direction [16]. This leads to the loss of structural integrity of the pearlitic steel at relatively small drawing strain. With a further increase in the drawing strain, the deformation mismatch between pro-eutectoid ferrite and pearlite exacerbated, resulting in the formation voids or micro-crack at the interface of two different micro-constituents. The initiation of these structural defects would set the limits for the drawing ability (i.e., drawability) of the hypoeutectoid steel and the tensile strength that it can achieve through drawing process. To impart high strength to medium carbon steel through cold drawing and, at the same time, prevent the formation of structural defects in the material, it is vital to regulate the deformation in the steel and to minimize the strain mismatch between two different micro-constituents. Consequently, design strategies that significantly boost the strength of pro-eutectoid ferrite may be used, for
Fig. 8. TEM images of medium carbon steel wires in drawing (a) ε¼ 0, (b) ε¼ 0.5, (c) ε¼ 1.2 and (d) ε¼ 2.6.
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example, decreasing the grain size in pro-eutectoid ferrite colonies and/or alloying pro-eutectoid ferrite with silicon. In doing so, the strength of pro-eutectoid ferrite would be lifted to a level close to that of pearlite. A coherent relationship between tailored proeutectoid ferrite and pearlite is thus expected to develop during cold drawing, providing the resulting steel wire extremely high strength and damage tolerance. This may open realistic pathways to developing extremely strong, damage tolerant medium carbon steel wire with relative better toughness and lower costs. 4. Conclusions Medium carbon steel having a carbon concentration of 0.35% by weight was used in this work to develop a deep picture of microstructure-property relationship during cold drawing process. The tensile strength of the steel with 50% of primary ferrite was found to increase impressively by nearly three folds from 615 MPa to 1810 MPa corresponding to a drawing strain of 3.0. The drawing processes can be divided into three phases: the first and third phase signifies a rapid increase of the tensile strength with drawing strain, while the second stage is characterized by a relatively slow rise in strength. Micro-hardness tests were used to probe the mechanical response of the micro-constituents in the steel during drawing. Compared to pearlite in the steel that exhibited a steady, linear rate of increase in the strength, the deformation resistance of primary ferrite increased rapidly to the drawing strain (ε)≈1.0. Beyond that, the strength of primary ferrite increased slowly and approached that of the pearlite. The roles of key structure components were identified in regulating the strength of cold drawn steel wires. During the initial drawing pro-eutectoid ferrite was deformed and extended along drawing direction, while the pearlite maintained its original equiaxed shape. With the increase of drawing strain, the intensity of 〈110〉 texture in the ferrite phase increased rapidly and became saturated at 1.5. High density dislocation zones appeared in both pro-eutectoid ferrite and the ferrite in pearlite. Pearlite was also deformed, in which cementite platelets aligned with the drawing direction, presumably through rotation and shearing. Under heavy drawing, high density dislocation zones spread in the ferrite and the interlamellar spacing in the pearlite decreased. A laminate structure that consists of alternating layers of pro-eutectoid ferrite
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and pearlite resulted, leading to an appreciable increase in the tensile strength. In addition, micro-voids appeared in the heavily drawn steel wire, as a result of strain mismatch at the interface between pro-eutectoid ferrite and pearlite. These structure defects may be prevented by controlling the deformation of the proeutectoid ferrite. By doing so, the resulting steel wires are expected to exhibit not only high strength, but also immunity to mechanical damage.
Acknowledgments The authors would like to thank Prof. Fan Li in the Jiangsu Key Lab for Advanced Metallic Materials, Southeast University for the help on EBSD analysis. This work was financially supported by the Natural Scientific Foundation of Jiangsu Province (Grant no. BK2011616) and the Southeast University Research Project, and was partially funded by the Jiangsu Province Industry-University Strategic Research Program (Grant no. BY2011144). The samples were prepared in Jiangsu Province Institute of Research of Iron and Steel, which is partially supported by the Zhangjiagang City Scientific & Technological Project under Grant no. ZKJ1013.
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