SCIENCE CHINA Technological Sciences May 2013 Vol.56 No.5: 1139–1146 doi: 10.1007/s11431-013-5184-7
Microstructure and texture evolution in fully pearlitic steel during wire drawing GUO Ning1,2, LUAN BaiFeng2, WANG BingShu3 & LIU Qing2* 1 2
Department of Materials Science and Engineering, Southwest University, Chongqing 400715, China; Department of Materials Science and Engineering, Chongqing University, Chongqing 400045, China; 3 Department of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China Received January 1, 2013; accepted February 1, 2013; published online March 27, 2013
The evolution of morphology of pearlite and crystallographic texture of ferrite matrix in fully pearlitic steels during wire drawing were quantitatively investigated. The study revealed that a fiber structure of the pearlite morphology and a fiber texture of the ferrite matrix begin to take shape and develop gradually with increasing strain. The growth rates of the fiber structure and the texture are different in different regions within the wires with increasing drawing strain. There is a close relationship between the pearlite morphology and the crystalline texture during wire drawing. The pearlite interlamellar spacing (ILS) and thickness of cementite lamellae (T) decrease gradually both in longitudinal and transverse sections. The definition of pearlite colony should be reconsidered for describing microstructure of the wire drawing deformed pearlitic steels. pearlitic steel, wire drawing, microstructure, texture Citation:
Guo N, Luan B F, Wang B S, et al. Microstructure and texture evolution in fully pearlitic steel during wire drawing. Sci China Tech Sci, 2013, 56: 11391146, doi: 10.1007/s11431-013-5184-7
1 Introduction Cold drawing pearlitic steel wires have been studied extensively as a topic of considerable amounts of scientific researches over the past several decades because of their high strength as well as acceptable level of ductility [1–9]. They are widely used as strands, cords and cable wires. It is a common sense that the higher the strength of the cable wires the lower the weight of cable wires for the same load, which will reduce the self-weight of bridge as well as cut the total cost. In order to satisfy the need of suspension bridges with 2000 m main span or cable-stayed bridges with 1000 m main span the tensile strength has been enhanced from 1600 to 1860 MPa or even 2000 MPa in the past three decades.
*Corresponding author (email:
[email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2013
The pioneering study of the microstructure and strengthening mechanism of the pearlitic steel during wire drawing was carried out by Embury and Fisher [1], which described the forming and developing of the fiber structure and fiber texture, and proposed boundary strengthening mechanism in the cold drawing pearlitic steel wire. Many following researches [2–9] demonstrated Embury and Fisher’s work. Langford [2] pointed out that the microstructure developed during wire drawing was influenced by forming of the fiber texture. Toribio et al. [6] proposed an evolution mechanism of the pearlite morphology which was based on the analysis of different deformation behaviors of pearlite colonies of different configurations. Toribio’s evolution mechanism was improved by considering the deformation behavior of global/rod-like pearlite particle during wire drawing [8]. Since the report that dissolution of cementite lamellae would occur in heavily cold drawing pearlitic steel wires, most researchers have begun to focus tech.scichina.com
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on the dissolution mechanisms or characterization methods [10–13]. However, most of the previous works on the evolutions of microstructure and texture in fully pearlitic steel during wire drawing were qualitative analysis, and the experimental results of the microstructure were generally obtained only from the longitudinal section of the drawing wires. Reports on quantitative analysis of the evolutions of microstructure and crystallographic texture are rare. In this work, cold drawing wires with fully pearlite for producing bridge cables, which were drawn from low to medium strain, were studied both in longitudinal section and transverse section in order to follow the evolutions of the pearlite morphology and the crystallographic texture.
2 Materials and methods Near-eutectoid steels with chemical composition of Fe0.83C-0.38Si-0.92Mn-0.23Cr in wt% were used for this study. The steels were hot-rolled at approximate 1123 K and quenched by Stelmor conveyor line to obtain rods with full pearlite. The rods were cold drawn to final wires though eight paths for manufacturing bridge cables. Wires with various diameters were obtained from a real manufacturing process by stopping the production chain (supplied by Fasten Group Company, Jiangyin, China). Four wires at different drawing strains taken a various passes of the overall wire drawing process from as-transformed rod (11.62 mm) to final wire (5.09 mm) were chosen as experimental samples for the current study. These four wires are refereed to as rod, wire 3, wire 6 and wire 8, respectively as shown in Table 1. The digits represent the numbers of wire drawing steps undergone. Figure 1 plots the true stress-strain curves Table 1 Information of samples investigated in this study. Wire diameter (Di), drawing strain () Samples Di (mm) Rod 11.62 0.00 Wire 3 8.48 0.63 Wire 6 6.12 1.28 Wire 8 5.09 1.65
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of the samples. Specimens for both transverse section views (hereinafter referred to as TS) and longitudinal section views (hereinafter referred to as LS) were cut respectively. Metallographic preparation of the specimens involved hot mounting in conductive bakelite (at 160°C for 8 min) followed by automated grind using silicon carbide papers from 120# to 400#. Then the specimens were polished with 9 and 3 m water based diamond suspensions on napless clothes with liquid lubricant. Final polish was carried out by the use of vibratory polishing (Vibromet2, Buehler) with 0.06 m colloidal silica polishing suspension for about 1 hour. The microstructures were observed through a field emission gun scanning electron microscope (FEG-SEM, FEI Nano Nova400) equipped with a backscattered electron imaging (BSEI) detector operating at a 10 kV voltage. Electron backscattered diffraction (EBSD, NordlysF detector, HKL, Oxford Instrument) measuring with a 0.2 m step size was performed at 20 kV. Secondary electron imaging in FEG-SEM was also adopted to reveal the microstructure of the specimens etched in 3% natal solution after fining BSEI and EBSD. Transmission electron microscope (TEM) observations were conducted by using a ZEISS Libra 200FE microscope with accelerating voltage of 200 kV. Additionally, the specimens for TEM examination were ground with SiC papers from 400# to 1200# and final thinning was used a double-jet electropolisher. Circular line method (CLM) and perpendicular line method (PLM) [14, 15] were utilized respectively for determining the mean size of pearlite interlamellar spacing (ILS) of different wire specimens with different strains. Thickness of cementite lamella (T) was determined by the use of PLM in TEM micrographs. In this work, the statistical analysis of at least 24 BSEI images and 30 TEM bright field micrographs resulted in ILS and T of each specimen. In order to quantify the fiber structures after drawing, lamellar pearlite were divided into several categories according to the angle i between cementite lamella and drawing direction (0–10°, 10–20°, 20–30°, 30–40°, 40–50°, 50–60°, 60–70°, 70–80° and 80–90°). For each wire specimen, area percentages of each type of i in the center, middle and surface regions within the specimen were statistically investigated from at least 10 BSEI micrographs respectively in order to quantify the fiber structure of the pearlite morphology.
3 3.1
Figure 1
True stress-strain curves of the wire samples.
Results and discussion Evolution of pearlite morphology
The cementite lamellae with fine thickness or high residual stress might be destroyed during the etching process of specimen preparation. Thus, microstructural characteristics of the samples were observed with both SEI method for
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etched specimens and BSEI method for that without etching. BSEI method was used for statistically studying LS specimens, while SEI method was used for characterizing TS specimens. More details can be referred to in the references which focus on comparing BSEI method and SEI technique for characterizing undeformed and deformed pearlitic steels
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[16, 17]. In this investigation, the evolution of pearlite morphology actually represents the deformation of cementite lamellae. Figure 2 shows the changes of morphology of pearlite in the center regions of the wire specimens. Figures 2(a), (b), (c) and (d) are the BSEI micrographs observed in LS, in
Figure 2 Evolution of pearlite morphology with increasing drawing strain ((a), (b), (c) and (d) are BSEI images observed in LS with the cementite and ferrite phases appearing as bright and dark regions respectively; (e), (f), (g) and (h) are SEI images observed in TS with the cementite and ferrite phases appearing as dark and bright regions respectively): = 0 ((a) and (e)); = 0.63 ((b) and (f)); = 1.28 ((c) and (g)); = 1.65 ((d) and (h)). The micrographs were observed in the center regions. DD means drawing direction.
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which the dark phase is cementite and white phase ferrite. Whereas Figures 2(e), (f), (g) and (h) are the SEI micrographs observed in TS, in which the dark phase is ferrite and white phase cementite. The microstructure of the rod quenched by Stelmor line is very complicated (Figures 2(a) and (e)). Three categories of pearlite morphology can be observed, which are straight lamellar pearlite, curly lamellar pearlite and global/rod-like pearlite particle, but on the whole the microstructure still displays equiaxed pearlite colonies (Figures 2(a) and (e)). With increasing strain, the lamellae are elongated and aligned to the drawing direction. With further increasing strain a fibrous structure with pearlite fibers parallel to the drawing direction is developed when observed in LS (Figures 2(a), (b), (c) and (d)), while the pearlite lamellae become wavier and wavier when observed in TS (Figures 2(e) (f), (g) and (h)). Zelin [5] reported that the fiber structure was formed at a drawing strain ranging from 1.5 to 2.0. Langford [2] reported that the wavy microstructure in TS was developed because of the fiber texture of ferrite. The observations indicate that the formation of the fiber structure of pearlite morphology occurs due to a combination of the tensile stress along the drawing direction and the lateral compression perpendicular to the drawing direction. The fibrous morphology observed in LS is the main embodiment of effect of the tensile stress, while the wavy morphology observed in TS is the effect of the lateral compression. Figure 3 shows the quantitative analysis of the fiber structure evolution during wire drawing. It can be seen that differences in area percentage among the 9 types of i in original rod are very small, indicating that the microstructure of the rod is isotropic. As strain increases, the type of 0–10° increases sharply and all the others gradually decrease only with the exception that the type of 80–90° keeps constant. These data illustrate that most lamellar pearlite lamellae tend to align to the drawing direction with 0–10° angle deviation (about 77% in wire 8) with the increasing drawing strain.
Figure 3 Quantitative analysis of the fiber structures in the center regions of the wire specimens observed in LS.
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In addition, the cementite lamellae exhibit perfect deformability during cold drawing from low to medium strain. Kink or shear bands which are always perpendicular to the cementite lamellae can be easily found in wire 3 with a strain of 0.63 (Figure 2(b)). The angles between the kink or shear directions and the drawing direction are among 0° to 45° in LS. After kink or shear deformation, the cementite lamellae change from straight morphology to kink morphology. When it comes to wires 6 and 8 with medium strains (1.28 and 1.65), less bands are found in LS. Additionally, the angles between the kink or shear directions and the drawing direction decrease to 0–30°. The schematic illustrations in Figure 4 which describe the evolution of pearlite morphology with strain were established by an analysis of more than 200 experimental SEM micrographs (see Figure 2 as examples). It demonstrates how the pearlite morphologies of different types change systematically in TS and LS in response to the increasing strain. In the original state, most of the cementite lamellae are straight and relatively thick and the cementite lamellae have a random distribution (see Figures 4(a) and (d)). As shown in Figure 4(a), the pearlite lamellas can be simply divided into three types: lamellae parallel (or have a small angle of i) to the drawing direction (colony 1), lamellae perpendicular (or have a large angle of i) to the drawing direction (colony 2), and lamellae with a medium angle to the drawing direction (colony 3). After being drawn to a low strain state as shown in Figures 4(b) and (e), the cementite lamellae in colony 1 are lengthened along the drawing direction and thinned in the thickness direction under the tensile stress along the drawing direction. The lamellae in colony 3 have rotated and become parallel to the drawing direction. For the cementite lamellae in colony 3, it can be speculated that rotating, rather than thinning, might be the predominant deformation. It seems that kink or shear deformation orientated from 0 to 45 degrees with respect to the drawing direction preferentially occurs in the plats in colony 2. The probable reason for forming of kink or shear in this case is that the cementite lamellae behave as stronger particles inside the ferrite matrix and exhibit a higher transverse resistance which produces a kinking effect [6]. Under the kinking effect, the slip on {110} or {112} plane initially takes place in the ferrite matrix and then transfers into the cementite lamellae. As a result, the kink or shear bands are developed in colony 2 at a low strain state [8]. The similar bands formed by slip during orientating from 30 to 45 degrees with respect to the drawing direction were observed in Zelin’s work [5], while from 0 to 27 degrees were observed in Zhang et al.’s work [18]. After kink or shear deformation the lamellae become wavy with two ends have a propensity to align to the drawing direction, as shown in colony 2 in Figure 4(b). When being drawn to a medium strain state as shown in Figures 4(c) and (f), the lamellae in colony1and colony 3 as well as the two ends of the lamellae in colony 2
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Figure 4 Schematic illustration of evolution of the pearlite morphology with increasing drawing strain, with (a), (b) and (c) observed in LS, and (d), (e) and (f) observed in TS: (a) (d) original state; (b) (e) low strain; (c) (f) medium strain.
are lengthened due to the result that most of the lamellae rotate to the drawing direction. With the drawing strain changing from 1.28 to 1.65 in this work, the fiber structure of pearlite morphology, highly elongated pearlite lamellae aligned with the drawing direction, begins to take shape and gradually develops. The typical morphology of the cementite lamellae observed by TEM is shown in Figure 5 as an example. An essential assumption on the deformation or strengthening mechanism of wire drawing is that the plastic strain of cold drawing is homogeneous [1], and ILS and T are reduced in proportion to the wire diameter (D), as shown in the following: ILSi Tθ i Di , ILS0 Tθ 0 D0
(1)
where ILSi, Ti and Di relate to pearlite interlamellar spacing, cementite thickness and wire diameter of a drawn wire and ILS0, T0 and D0 relate to those of the initial rod. Additionally, it is known that the true strain () in drawing deformation is defined as
Figure 5
ln
Di 2 . D0 2
(2)
Thus, the theoretical values of ILS and T could be calculated by combining eqs. (1) and (2). Figure 6 plots the ILS and T as functions of the drawing strain. It illustrates that both ILS and T in LS and TS of the rod are similar to each other, indicating ILS and T are isotropic before drawing. Moreover, these isotropic features of ILS and T are not changing with the increasing drawing strain. 3.2
Evolution of crystalline texture
Three regions including center, middle and surface in LS of the wire specimens with different strains were characterized by EBSD, respectively (Figure 7). Inverse pole figure coloring with the drawing direction is conducted here for displaying micro-textures evolution of the wires during cold drawing process. In the original state, all those three regions have equiaxed pearlite nodules/grains and random textures. With the increasing drawing strain the pearlite nodule/grain
TEM bright field micrographs of wire 8 with a strain of 1.65: (a) Observed in LS; (b) observed in TS.
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Figure 6 Pearlite interlamellar spacing (ILS) and thickness of cementite lamellae (T) vs. drawing strain: (a) ILSt and ILSl are determined in TS and LS, respectively; (b) Tt and Tl are determined in TS and LS, respectively. The intervals are 95% confidence intervals on the average value.
are distorted and elongated, which results in the growth of fiber texture. It is well known that for most metals with body centered cubic (BCC), the fiber texture becomes prevailing under tensile or drawing strain [19]. Figure 8 shows quantitative analysis of the texture evolution in different regions based on the EBSD data. One important proviso: A fiber texture component in this work is defined as misorientation with deviation of 9° from an ideal fiber texture; the area fraction of each texture component in different samples calculated by Channel5 software is quantified with a same order which goes like , , , , and . The results indicate that the rod has random texture. In wire 3 with a strain of 0.63 the area fraction of the fiber component has increased compared to that of in the rod (Figures 7(d), (e), (f) and Figure 8(b)). The fiber component in the center region increases faster than that in the middle or surface region by comparing Figures 8(a) with (b). This phenomenon is also found at the strain of 1.28 in wire 6. With the strain increasing from 1.28 to 1.65, the fiber components have a fast growth in both middle region and surface region. In wire 8 with a strain of 1.65, the fiber components in the center, middle and surface regions have increased to 64%, 63% and 60%, respectively. All the other textures such as , and decrease to less than 18% in area fraction. 3.3
the texture in those three different regions, it can be illustrated that both deformations of the cementite lamellae and ferrite matrix are inhomogeneous among different regions in order to decrease the deformation resistance during wire drawing. The results also indicate that there is a close relationship between the pearlite morphology and crystalline texture of ferrite matrix during wire drawing, which should be considered as a deform integrity in drawing deformation. 3.4
Definition of pearlite colony in drawing wires
After drawing as shown in Figures 4(c) and (f), the original interface between the neighbor colonies have changed or even disappeared. In another word, under current definition of the pearlite colony which is defined according to the pearlite morphology [20, 21], the colony will grow with the increasing strain during the drawing process. Thus, we think that it is useless to measure the average size of the pearlite colony or evaluate the colony size effect on the mechanical properties of the drawing steel wires. This is why we calculated the area percentage rather than the colony numbers of each type of i in wire specimens. We believe that the current definition of colony is only suitable to describe pearlite transformation mechanism or evaluate the effect on the mechanical properties of the rod with straight cementite lamellae.
Relation between morphology and texture
The experimental results show that a fiber structure of the pearlite morphology and a fiber texture of the ferrite matrix begin to take shape and develop gradually with the increasing drawing strain. Thus, the features of microstructure and texture in the cold drawing pearlitic wires have transformed from isotropic to anisotropic. Evolution of the fiber structure (defined as the angle i between cementite lamella and drawing direction ranging from 0 to 10°) and changing of the fiber texture in different regions within the wire specimens are displayed in Figure 9. Based on comparison of the growth rates of the fiber structure and
4
Conclusion
Evolutions of microstructure and texture in fully pearlitic steels during wire drawing have been quantitatively investigated. The results show that a fiber structure of pearlite morphology accompanied by localized shearing, and a fiber texture of ferrite matrix begins to form and develops gradually with the increasing drawing strain. The growth rates of the fiber structure and the texture are not constant among different regions in the wires. Both pearlite interlamellar spacing (ILS) and thickness of
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Figure 7 Schematic diagram of placement of EBSD mapping and inverse pole figure coloring EBSD maps: = 0 ((a), (b) and (c)); = 0.63 ((d), (e) and (f)); = 1.28 ((g), (h) and (i)); = 1.65 ((j), (k) and (l)).
Figure 8
Quantitative analysis of texture evolution in different regions among the wire specimens: (a) = 0; (b) = 0.63; (c) = 1.28; (d) = 1.65.
Figure 9 Evolutions of fiber structure of pearlite morphology and fiber texture of ferrite matrix in different regions among the wire specimens with increasing drawing strain: (a) Fiber structure with an angle ranging from 0–10° on DD; (b) //DD texture.
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cementite lamellae (T) determined in transverse section and longitudinal section decrease gradually with the increasing strain. The definition of pearlite colony should be reconsidered for describing microstructure of the wire drawing deformed pearlitic steels. This work was supported by the Major Program of the National Natural Science Foundation of China (Grant No. 50890170) and the Scientific Research Foundation for Doctoral Scholars of Southwest University, China (Grant No. SWU112043). The authors are very grateful to Fasten Group Company for the supply of the pearlite steel wires and steel wire rods used in this investigation.
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