Microstructure Evolution and Its Effects on the Mechanical Behavior of Cold Drawn Pearlite Steel Wires for Bridge Cables

ZHANG Fan, MAO Xinping, BAO Siqian,3＊, ZHAO Gang,3, ZHAO Sixin,DENG Zhaojun, HE Meng,3, HUANG Fangyu,3, QU Xi,3

(1. The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China;2. Baosteel Central Research Institute, Wuhan 430080, China; 3. Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China)

Abstract: The microstructure evolution and its effects on the mechanical performance of 2000 MPa bridge cable steel wires were investigated by transmission electron microscope (TEM), electron backscatter diffraction (EBSD), X-ray diffractometer (XRD) and mechanical tests. Experimental results reveal that, with the increasing strain from 0 to 1.42, a fiber structure and a <110> fiber texture aligned with the wire axis are gradually developed accompanied by cementite decomposition and the formation of sub-grains; the tensile strength increases linearly from 1 510 to 2 025 MPa, and the reduction of the area is stable with a slight decline from 44% to 36%. After annealing at 450 ℃ for different times, pronounced changes in the microstructure occur. Cementite lamella fragment into coarser globules corresponding to a remarkable spheroidization process,while ferrite domains recover and recrystallize, and this process is associated to modifications in the mechanical properties. Furthermore, based on the observations on dislocation lines crossing through cementite lamellae, a possible mechanism of cementite decomposition is discussed.

Key words: pearlitic steels; cold drawing; annealing; microstructure; mechanical properties

1 Introduction

Cold-drawn pearlitic steel wires are widely used in bridge construction due to their excellent strength and a reasonable level of toughness[1,2]. The wires for bridge cables are made of high carbon steel wire rods by a successive cold drawing process and hot-dip galvanizing treatment; the latter is mainly intended to prevent corrosion.

Due to the strong demand for cold-drawn pearlitic steel wires in engineering applications, researchers have studied the microstructural and mechanical properties of cold-drawn wires and have found that the manufacturing of pearlitic steel wires leads to a microstructural evolution associated to the modification of mechanical properties[3]. The microstructure of pearlitic steel wires consists of alternating layers of body-centered cubic ferrite and orthorhombic cementite(soft and hard phases in pearlite, respectively). During the cold drawing process, the lamellar pearlitic structure becomes thin and aligned with the wire axis[4]. And researchers have found that cementite lamellae exhibit remarkable ductility despite the expected problem of slips in the orthorhombic crystal structure of carbides[5].Some interesting phenomena have also been found: at room temperature the cementite dissolves into ferrite matrix during cold drawing which is supposed to be thermodynamically stable and the carbon solubility in ferrite is small, and this is also considered to be the main reason for the loss of steel ductility[6-14].

Previous studies on cold-drawn pearlitic ferrite have primarily focused on the microtextural evolution[15-18]. It is found that the <110> fiber texture strengthens with the increasing strain, and the {110}<110> circular texture weakens the torsion ability of wires. In addition, some studies have indicated that,in cold-drawn pearlitic steel wires, fibrous lamellar cementite inhibits the dynamic recovery of ferrite lamellae, and this phenomenon is quite different from the cold-drawing behavior of copper wires and other single-phase materials[6]. Some studies have investigated the effects of warm deformation and annealing on the microstructure of pearlitic wires and found the phenomena of the spheroidization of lamellar cementite and the recovery and recrystallization of lamellar ferrite[19,20]. However, the microstructure evolution and its effects on the mechanical performance of cold-drawn wires are yet not clearly understood[21,22].

The purpose of the present work was to illustrate the microstructure evolution and its effects on the mechanical performance of 2 000 MPa bridge cable steel wires (the highest strength grade in engineering applications). The properties of steel wire rods, colddrawn wires, and annealed wires were examined by transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), X-ray diffractometry(XRD), and mechanical tests. Meanwhile, with respect to the observation of dislocation lines crossing the cementite lamellae, the most possible mechanism for cementite dissolution was discussed.

2 Experimental

The chemical composition of the commercial steel used in the current experiment is presented in Table 1. A wire rod with a diameter of 14.0 mm was produced by hot rolling and the stelmor process in Baoshan Iron & Steel Co. Ltd. In order to obtain a fine pearlitic microstructure, the wire rod was quenched at 550 ℃ after austenitizing at 930 ℃ and experienced a complete isothermal pearlitic transformation. After pickling and phosphating, the rod was subjected to eight stages of the industrial cold drawing process with a true strain ofε= 1.42, and the as-produced wire with 6.9 mm diameter was then galvanized under industrial condition (annealing in zinc bath at 450 ℃ for 60 seconds) to a diameter of 7.0 mm[23]. And for further research, the cold drawn wires at strain of 1.42 have also been treated by laboratory annealing at 450 ℃ for different times (more than 60 seconds).

Table 1 Chemical composition of the steel materials used in this work/wt%

Steel wire rods, cold-drawn, and annealed wires were subjected to the following mechanical tests.Tensile tests were performed on a universal material testing machine at a constant speed of 2 mm/min to obtain the data of tensile strength and area reduction.According to the Chinese National Standard GB/T 239-1999, unidirectional torsion tests were conducted on a CTT1000 torsion testing machine, and specimens with a length of 700 mm were tested with a torsion velocity of 30 r/s. Mechanical tests under each condition were successfully repeated at least three times.

The TEM analysis was carried out by a JEOL JEM-2100F instrument operating at a voltage of 200 kV, and the morphological characteristics of ferrite were investigated by a FEI Quanta 450FE electron backscatter diffractometer (EBSD). X-ray diffraction(XRD) measurements were conducted on a Rigaku SmartLab diffractometer using a copper tube(kα=0.154 nm,kβ=0.139 nm) under a conventional goniometer mode (θ-2θ) at 45 kV and 200 mA.

3 Results and discussion

3.1 Mechanical properties

The tensile strengths and area reductions of the investigated wires as a function of the drawing strain(ε) are presented in Fig.1. With the increasing drawing strain from 0 to 1.42, the tensile strength increased linearly from 1 510 to 2 025 MPa and the reduction of the area was stable with a slight decline from 44%to 36%, indicating that the pearlitic steel wires had a combination of excellent strength and an acceptable level of toughness. The macroscopic and microscopic morphologies of the wires during tensile fracture at a strain of 1.42 are displayed in Figs.2(a) and 2(b), and the tensile fracture morphology was characterized by a ductile rupture with an obvious necking phenomenon(Fig.2(a)) and a large number of dimples (Fig.2(b)).

Fig.1 The variation tendency of tensile strength and reduction of the area during the cold drawing process

Fig.3 presents the number of twists and tensile strength of the wires at a strain of 1.42 after annealing at 450 ℃ for different times. When the annealing time reached 5 min, the tensile strength increased from 2 025 MPa to the maximum value of 2 140 MPa, and the torsion number decreased rapidly from 35 to the minimum value of 4 with delaminated fractographs.With the continuous increase of the annealing time, the tensile strength decreased to less than 2 000 MPa, and the torsion number increased gradually to about 10.This phenomenon can be attributed to the variation of the carbon concentration and dislocation density in the ferrite phase, cementite decomposition, and spheroidization[3].

Fig.2 Tensile fractograph of the wires at a strain of 1.42: (a)macroscopic; (b) microscopic morphologies

Fig.3 Tensile strength and torsion number of cold drawn wires (ε= 1.42) after annealing at 450 ℃ for different times

3.2 Microstructural evolution

3.2.1 Cold drawing process

Fig.4 exhibits the TEM micrographs obtained from the longitudinal section of the cold-drawn wires at different strains. As seen in Figs.4(a) and 4(b), the initial microstructures of the steel wire rod exhibited a random orientation, the boundaries of pearlitic colonies are clear and distinguishable, and the inter-lamella spacing was approximately 180 nm. One interesting observation is that a handful of initial lamellae structure have been broken which is different from an almost perfect pearlitic lamellar structure[24]. And there are a few dislocations in ferrite lamellar matrix in initial microstructures (Fig.4(b)), which is supposed to generate on the cooling process from austenitizing temperature to room temperature. Fig.4(e) and 4(f)reveals that the microstructure of the as-drawn wires at ε = 0.79 changed unevenly during cold drawing. It is noticeable from Fig.4(c) that partial pearlitic colonies rotated to the drawing axis accompanied by a rapid decrease of the interlamellar spacing, and some of the pearlitic lamellae deviated from the drawing axis with a broader interlamellar spacing. It is observable from Fig.4(d) that there existed shear-bands (S-bands)in the microstructure that are known to be related to the angle between initial pearlite lamellar and the drawing direction[15]. Fig.4(e) and 4(f) are of as-drawn wire at strain of 1.42, Fig.4(e) shows that most of the pearlite colonies became aligned to the wire axis, and the inter-lamella spacing significantly decreased to approximately 100 nm accompanied by the thickness reduction of cementite lamellae and density increase of dislocation in the ferrite matrix. As shown in Fig.4(f),sub-grains which have been attributed to dislocation rearrangement were observed[25].

Fig.4 TEM micrographs of longitudinal section at different strains: (a) and (b) ε = 0; (c) and (d) ε = 0.79; (e) and (f)ε= 1.42

Fig.5 displays a bright-field image and a cementite dark-field image at the same region obtained from the longitudinal section of cold-drawn wires with a true strain of 1.42. To some degree, the microstructure still retained the lamellar structure in the bright-field image (Fig.5(a)). White particle spots located between adjacent ferrite lamellae were observed in the dark-field image (Fig.5(b)). It indicates that cementite lamellae were no longer continuous. A weak ring pattern was obtained from the microdiffraction in the region,and the dark-field image clearly indicates the grain fragmentation in cementite[6,26-27].

Fig.5 Longitudinal section (a) bright-field and (b) dark-field images of cold drawn wires at a strain of 1.42

3.2.2 Annealing process

Fig.6 presents the effects of the galvanization process on the microstructure of cold-drawn wires (ε= 1.42) in the longitudinal view. After annealing in the zinc bath at 450 ℃ for 60 seconds, most of the lamellar microstructure was retained (Fig.6(a)) and white particle spots were located in ex-cementite lamellae(Fig.6(b)). In comparison to Fig.5(b), both the number of white particle spots and the intensity of ring patterns increased significantly and the size of most spots was very fine as small as few nanometers (Fig.6(b)). The contrast indicates that the fragmentation of cementite plates was more obvious after the annealing process.

Fig.6 Longitudinal section (a) bright-field and (b) dark-field images of galvanized steel wire

Fig.7 depicts the effects of annealing time on the microstructure of cold-drawn wires (ε= 1.42) in the longitudinal view. The wires were annealed for 5 min(Figs.7(a) and 7(b)) and 30 min (Figs.7(c) and 7(d))at 450 ℃. Fig.7(a) expresses the occurrence of a high dislocation density in the elongated sub-grains. It is discernible from Fig.7(b) that with the distribution of a few cementite globules near ex-cementite lamellae,the majority of lamellar microstructures were still maintained. As the annealing time was raised to 30 min,some prominent changes occurred in the microstructural morphology. Partial lamellar structures completely disappeared with the recrystallization of ferrite domains and the fragmentation of lamellar cementite.The dislocation density in ferrite grains decreased significantly, and a portion of cementite globules was located at ferrite grain boundaries (Fig.7(c)); thus,it was difficult to distinguish the contour of lamellar structures. Moreover, more cementite lamellar ruptured into granular grains (Fig.7(d)).

Fig.7 Longitudinal section TEM micrographs of cold drawn wires at a strain of 1.42 after annealing at 450 ℃ for different times: (a) and (b) 5 min; (c) and (d) 30 min

3.3 Microtexture analysis

The ferrite texture at the 1/4 diameter region of the longitudinal section was quantified by the inverse pole figure (different colors in Fig.8 represent the intensities of different texture levels). Fig.8(a)reveals that the distribution of crystallographic orientations in steel wire rods was random, and the amount of <110> fiber texture was little. Atε= 1.42,pearlitic colonies became stretched and rotated toward the drawing direction (Fig.4) accompanied by the formation of the <110> fiber texture (Fig.8(b)), thus the <110> fiber texture evolved as the major texture component in the inverse pole figure. After annealing in the zinc bath at 450 ℃ for 60 seconds, the <110>fiber texture became more dominant (Fig.8(c)). It is ascribed to the effects of inherited textures (prior cold deformation textures before annealing) on the recovery process in the microstructure[28].

Fig.8 Inverse pole figure (in wire drawing direction) obtained for ferrite phase at 1/4 diameter region: (a) patented steel wire rods (ε = 0); (b)cold drawn wires (ε = 1.42); (c) annealed wires (ε = 1.42, annealing at 450 ℃ for 60 seconds)

Fig.9 Misorientation angles from 2° to 15° of the longitudinal section at 1/4 diameter regions of pearlite colonies: (a)patented steel wire rods (ε = 0); (b) cold drawn wires (ε= 1.42); (c) annealed wires (ε = 1.42, annealing at 450 ℃for 60 seconds)

In order to quantitatively analyze the evolution zinc bath at 450 °C for 60 seconds) were measured as 6.323°, 5.899°, and 6.165° respectively (Fig.9). The misorientation distribution plot had a strong peak in the 2°-4.28° boundary regime, and its percentage significantly increased from 37.9% to 46.2% after cold drawing and decreased slightly from 46.2% to 43.5%after subsequent annealing.

4 Discussion

of dislocation density under different conditions,the misorientation angles from 2° to 15°, which are considered as low-angle grain boundaries (LAGB),were measured by EBSD at the 1/4 diameter region of the longitudinal section[29]. The average misorientation angles for steel wire rods (ε= 0), cold-drawn wires (ε= 1.42), and annealed wires (ε= 1.42, annealing in the

4.1 Microstructural evolution

The initial microstructures of the patented steel wire rods possessed a typical and imperfect lamellar structure that was characterized by a few dislocation lines in the ferrite matrix and a small amount of discontinuous cementite lamellae (Figs.4(a) and(b)), and initial pearlite colonies exhibited a random orientation (Fig.10). During cold drawing, the pearlite deformation was greatly influenced by the angle between initial cementite lamellae and the drawing direction, and numerous studies have been conducted to classify the initial pearlite morphology according to different angles: (A) parallel or have a small angle to the drawing axis, (B) perpendicular or have a large angle to the drawing axis, and (C) a medium angle to the drawing axis[30,31]. Type A pearlite lamellae that had a favorable orientation to the drawing axis were lengthened along the drawing direction accompanied by a rapid decrease of the interlamellar spacing at a low strain of 0.79 (shown in the top right corner of Fig.4(c)), and this spacing gradually became thinner as the strain increased. The deformation process of type B and C pearlite lamellas was divided into two stages:reorientation at a low strain and alignment with the wire axis at a large strain[32]. In the first stage, type B and C pearlite lamellae underwent a rigid body rotation rather than thinning as the predominant deformation accompanied by a slight decrease of the interlamellar spacing (shown in the bottom of Fig.4(c))[31]. In addition, shear-bands (S-bands) orientated from 0° to 45° with respect to the drawing direction were observed(Fig.4(d))[15]. These S-bands preferentially occurred in type B pearlite lamellae and were formed by the slip on the {110} or {112} plane (initially took place in the ferrite matrix and then transferred to cementite lamellae under shear stress)[30]. As a consequence of S-bands, the cementite lamellae presented wavy which is the evidence of its plastic deformation at room temperature, and fractured in some cases, as shown in Fig.4(d). Moreover, S-bands induced a rapid change in the local orientation of ferrite and then promoted the formation of high-angle boundaries of ferrite[15]. In the second stage, at a large strain, most lamellae became highly elongated and aligned with the drawing axis accompanied by the thinning of ferrite and cementite lamellae and the generation of an extremely high dislocation density in the ferrite matrix, and a part of cementite lamellae fractured (Fig.4(e)). According to previous investigations, the angle between S-bands and the drawing direction decreased with the increase of the strain, and at a strain of 1.5, about 97% of the lamellae rotated close to the drawing direction in the longitudinal section, and this percentage came to almost 100% at a higher strain[15,31].Fig.4 also displays the evolution of dislocation structures. During cold drawing, newly-generated dislocations nucleated from the ferrite-cementite interface intersected with previous dislocations and formed dislocation tangles(Fig.4(e)), and dislocation cells were formed and subsequently transformed into sub-grains by dislocation rearrangement under a higher dislocation density as cross-slip occurred easily in BCC crystals (Fig.4(f))[33].

In Figs.5 and Fig.6, white particle spots located between adjacent ferrite lamellae indicate that partial cementite lamellae were deformed into planar nanoscale particles or amorphous structures during cold drawing, causing the fragmentation of cementite lamellae[26]. The corresponding selected area electron diffraction (SAED) patterns, including ferrite reflections and rings corresponding to cementite interplanar spacings, imply that a portion of the ferrite matrix was deformed into substructures accompanied by small misorientations, and rings were formed from small, randomly-oriented, and fragmented cementite grains. The fragmentation of cementite plates was more prominent after the annealing process.

After annealing at 450 ℃ for different times, a pronounced change in the morphology occurred (Fig.7).As the annealing time was raised from 5 to 30 min,elongated sub-grains became equiaxed accompanied by a decrease of the dislocation density, indicating the occurrence of recovery during annealing. Meanwhile,cementite lamellae fragmented into coarser globules due to a spheroidization process.

Fig.10 Inverse pole figure coloring maps of the longitudinal section at 1/4 diameter regions of patented steel wire rods

Fig.11 X-ray diffraction patterns of as-patented steel wires rods(ε = 0), as-drawn wires (ε = 1.42) and annealed wires (ε= 1.42, annealing at 450 ℃ for 5 and 30 min, respectively)

Fig.11 displays the XRD patterns of longitudinal specimens extracted from different states. As weaker cementite peaks were not visible, this analysis was carried out only on ferrite peaks. In comparison to the as-patented wire, the ferrite diffraction peaks of the asdrawn wire (ε = 1.42) manifest prominent broadening and a little shift to the left. The broadening of ferrite diffraction peaks occurred due to nano-structuring and a high residual internal stress, indicating a refinement of ferrite lamellae and an increase of the dislocation density during the cold drawing process (Fig.5), and the shift occurred due to an increase of lattice parameters,implying an increase in the carbon content of ferrite and cementite decomposition[34]. For the as-drawn wire, the width of ferrite diffraction peaks decreased and slightly shifted to the right as the annealing time increased,and it can be attributed to a decrease of internal elastic strains and cementite precipitation corresponding to the recovery process (Fig.7)[6,13].

Fig.8 presents the textural evolution in the wire specimens in different states. Patented steel wire rods had a nearly random texture, and for the cold-drawing strain of 1.42, the <110> fiber texture became dominant accompanied by the transformation of pearlite colonies from isotropic to anisotropic (Fig.4). The <110> fiber texture became more prevailing after annealing at 450℃ for 60 s, and this phenomenon corresponded to the recovery process[28]. The evolution of dislocation density under different conditions is exhibited in Fig.9.The cold-drawing strain of 1.42 resulted in dislocation multiplication and dislocation rearrangement, and this process was accompanied by a decrease of the average misorientation angle and an increase of the low-angle boundary regime (<4.28°). After annealing,the dislocation density decreased due to recovery by the rearrangement and annihilation of dislocations stored during cold drawing, and this phenomenon corresponded to grain growth, which was accompanied by an increase of the average misorientation angle and a decrease of the low-angle boundary regime (<4.28°)[6].

Fig.12 TEM micrographs of cold drawn wires under ε = 1.42(dislocation lines pass through the cementite lamellae)

Two mechanisms have been proposed to illustrate the phenomenon of cementite decomposition[6,7]. The first mechanism is attributed to the higher binding enthalpy between dislocations and carbon atoms in ferrite than that between carbon and iron atoms in cementite (also known as dislocation drag mechanism).The second mechanism is ascribed to the increase of the cementite/ferrite interface free energy arising from the strong microstructural refinement during cold drawing, leading to cementite destabilization and dissolution. Based on the experimental data obtained from thermomagnetic analyses, Mössbauer spectroscopy, internal friction, Atom probe field ion microscopy (APFIM), and X-ray photoelectron spectroscopy (XPS) which has been widely used as a surface analysis technique[35], it is proposed that the first mechanism is the most probable reason for cementite decomposition[7]. Fig.12 also supports the first mechanism. Mobile dislocations crossing through cementite lamellae collected and dragged carbon atoms from cementite into adjacent ferrite and induced cementite decomposition.

4.2 Mechanical properties

Fig.1 displays the relationship among tensile strength, reduction of area, and cold-drawing strain. The increase of the tensile strength and the decrease of the reduction of area can be attributed to microstructural refinement, dislocation multiplication, and the increase of carbon content in ferrite respectively corresponding to boundary strengthening, dislocation strengthening,and solid solution hardening[2]. The cup and conetype fracture and small dimple patterns in the fracture surface shown in Fig.2 indicate that cold-drawn wires were still ductile. Therefore, it can be inferred that cold-drawn 2 000 MPa bridge cable steel wires had a combination of excellent strength and an acceptable level of toughness.

Fig.3 illustrates the effects of annealing time on tensile strength and the number of twists. The initial increase in the tensile strength and the decrease in the number of twists seemed to be associated with the locking of mobile dislocations by carbon atoms in ferrite. With the increase in the annealing time(greater than 5 min), annealing softening became predominant accompanied by a substantial change in the microstructure (Figs.7 and 11). The softening can be attributed to numerous processes, including a drop in the carbon concentration in ferrite, recovery and recrystallization, and cementite spheroidization.

5 Conclusions

The microstructure evolution and its effects on the mechanical behavior of cold-drawn pearlite steel wires were investigated by TEM, EBSD, XRD, and mechanical tests. The main observations are presented below.

a) During the cold drawing process, microstructural morphologies changed unevenly and were gradually transformed from a state with no preferred orientation to an aligned state with the wire axis accompanied by microstructural refinement, dislocation multiplication, the growth of <110> texture, and cementite decomposition. Local shear bands, waved and fragmented cementite lamellae, and sub-grains were formed with the increasing drawing strain.

b) After annealing at 450 ℃ for different times,pronounced changes in the microstructure occurred.Cementite lamella fragmented into coarser globules due to a remarkable spheroidization process. Further,ferrite domains recovered and recrystallized that was accompanied by a decrease in dislocation density, and the <110> fiber texture became more dominant.

c) Dislocation lines crossing through cementite lamellae supported the dislocation drag mechanism of cementite dissolution. Mobile dislocations crossing through cementite lamellae collected and dragged carbon atoms from cementite into adjacent ferrite and induced cementite decomposition.

d) With the increasing cold-drawing strain from 0 to 1.42, the tensile strength increased linearly from 1 510 to 2 025 MPa, and the reduction of area was stable with a slight decline from 44% to 36%. For cold-drawn wires (ε= 1.42), when the annealing time reached 5 min, the tensile strength increased from 2 025 MPa to the maximum value of 2 140 MPa, and the torsion number decreased rapidly from 35 to the minimum value of 4 with delaminated fractographs.With the continuous increase of the annealing time(greater than 5 min), the tensile strength decreased to less than 2 000 MPa, and the torsion number increased gradually to about 10.