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Materials Science and Engineering A 547 (2012) 51– 54

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering A

jo ur n al hom epage: www.elsev ier .com/ locate /msea

evealing microstructural and mechanical characteristics of cold-drawnearlitic steel wires undergoing simulated galvanization treatment

eng Fanga,∗, Xian-jun Hua,b, Shao-hui Chenb, Zong-han Xiec, Jian-qing Jianga

School of Materials Science and Engineering, Southeast University, Nanjing 211189, ChinaJiangsu Sha-Steel Group, Zhangjiagang City, Jiangsu Province 215625, ChinaSchool of Engineering, Edith Cowen University, Joondalup, WA 6027, Australia

r t i c l e i n f o

rticle history:eceived 1 June 2011eceived in revised form 1 February 2012ccepted 10 March 2012vailable online 29 March 2012

eywords:earlitic steel wireold-drawingnnealingementitepheroidization

a b s t r a c t

Spheroidization of lamellar cementite often occurs in cold-drawn pearlitic steel wires during galvanizingtreatment, leading to the degradation of mechanical properties. Therefore, it is important to understandeffects of galvanization process on microstructure and mechanical properties of cold-drawn wires. Inthis paper, cold-drawn steel wires were fabricated by cold drawing pearlitic steel rods from 13 mm to6.9 mm in diameter. Thermal annealing at 450 ◦C was used to simulate galvanizing treatment of steelwires. Tensile strength, elongation and torsion laps of steel rods and wires with, and without, annealingtreatment were determined. Microstructure was observed using scanning electron microscopy (SEM) andtransmission electron microscopy (TEM). In addition, differential scanning calorimetry (DSC) was used toprobe the spheroidization temperature of cementite. Experimental results showed that tensile strengthof wires increased from 1780 MPa to 1940 MPa for annealing <5 min, and then decreased. Tensile strengthbecame constant for annealing >10 min. Elongation of wires decreased for annealing <2.5 min, and thenrecovered slightly. It approached a constant value for annealing >5 min. Tensile strength and elongation of

wires were both influenced by the strain age hardening and static recovery processes. Notably, torsion lapsof wires hardly changed when annealing time was less than 2.5 min, and then decreased rapidly. Its valuebecame constant when the hold time is greater than 10 min. Lamellar cementite began to spheroidize atannealing >2.5 min, starting at the boundary of pearlitic grains, and moving inward. A broad exothermicpeak was found at temperatures between 380 ◦C and 480 ◦C, resulting primarily from the spheroidization

ich is

of lamellar cementite, wh

. Introduction

Cold drawing process can significantly enhance the tensiletrength of steel. Through it, high strength steel wires are man-factured and used in the construction of suspension bridges [1,2].o combat corrosion damage during service, hot dip galvanizing atbout 450 ◦C is typically applied in the manufacturing of wires. Asxpected, the microstructure and mechanical properties of steelires change considerably during the galvanizing process [3,4].

herefore, to develop high strength steel wires with excellent cor-osion resistance, it is necessary to understand the effect of the hotip galvanizing process upon the microstructure and mechanical

haracteristics of steel wires.

Partial dissolution of cementite has recently been identifiedn heavily deformed pearlitic steels [4–9]. Various mechanisms

∗ Corresponding author at: School of Materials Science and Engineering, Southeastniversity, JiangNing District, Nanjing, China. Tel.: +86 25 52090630;

ax: +86 25 52090634.E-mail address: fangfeng@seu.edu.cn (F. Fang).

921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.msea.2012.03.075

responsible for the degradation of torsion property of cold-drawn wires.© 2012 Elsevier B.V. All rights reserved.

that control cementite dissolution have been proposed, such ascementite decomposition due to the increase of its interfacial freeenergy [4] and segregation of carbon atoms around dislocations[5–7]. Notably, all these processes involve movement of the car-bon atoms through the interface between ferrite and cementite.Therefore, the decomposition of lamellar cementite might resumeduring the galvanization process, in which lamellar cementite mayalso spheroidize to minimize surface energy [3]. Consequently, themechanical properties are affected.

Thermal annealing is often used to explore the effect of galvaniz-ing treatment on the microstructure and mechanical properties ofcold-drawn steel wires [10–14]. For example, the shape of cemen-tite in cold drawn pearlite wires was studied at different annealingtemperatures, and strain age mechanism was revealed [10]. Leeet al. [11] studied the effect of annealing temperatures and holdtime on the delamination of wires during torsion tests. Languil-laume et al. [12,13] investigated the effect of annealing temperature

on the microstructure and strength of wires. Effects of anneal-ing treatment on the spheroidization behavior of cementite inmedium carbon steels following continuous shear drawing was alsoaddressed [14]. A fundamental understanding of the relationship

52 F. Fang et al. / Materials Science and Engineering A 547 (2012) 51– 54

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Fig. 1. Mechanical properties of cold drawn pearlitic steel wire after being hel

etween the microstructure and mechanical properties (especiallyorsional property, an important design parameter for steel wiressed in the construction of suspension bridges) during the hot dipalvanizing treatment is still lacking.

In the present work, effects of annealing time at 450 ◦C onechanical properties and microstructure of cold-drawn pearlitic

teel wires were investigated. In particular, a direct link betweenorsional property and the spheroidization of lamellar cementiteas established.

. Experimental procedure

Pearlitic steel used in this work were supplied by Sha-steelroup Company in the form of Stelmor-cooled rods. Their chemicalompositions are shown in Table 1. After pickling and phosphating15], hot-rolled rods (13 mm in diameter) were successively drawno a diameter of 6.9 mm with a total reduction of 72% (ε = 1.27). Theverage reduction per pass was about 14%.

The annealing treatment of the cold drawn steel wires was usedo simulate the galvanizing process. It was carried out in nitrogentmosphere at about 450 ◦C at different hold times of 30 s, 1 min,.5 min, 5 min, 10 min, 20 min and 40 min.

Tensile strength of wires was determined by tensile testst room temperature using a CMT5105 type universal testingachine, operating at a constant speed of 2 mm/min. The elonga-

ion of wires was measured during tensile tests by an extensometerttached to the testing machine. The tensile tests were performedccording to the Chinese national standard GB/T228-2002. Theorsion laps of wire samples (sample’s length is about 100 timests’ diameter) were measured at a rate of one lap per minutey using CTT500 type torsion testing machine. The torsion testsere conducted in accordance with the Chinese national standardB/T10128-1998. Microstructure of wires was characterized using

scanning electron microscope (SEM, FEI Siron-400) and transmis-ion electron microscope (TEM, JEM 2000EX).

Thermal analysis was undertaken using STA449 F3 differ-ntial scanning calorimetry (DSC) (NETZSCH-Gerätebau GmbH,ermany) at heating rate 20 ◦C/min in a flowing Ar atmosphere

16]. Following the DSC analysis, the microstructure of samples wasbserved by SEM to correlate new microstructural features with thendothermic or exothermic peaks.

able 1hemical composition of steel wires used in this work.

C Mn Si Cr V S P

0.82 0.76 0.23 0.26 0.04 0.005 0.010

ifferent time at 450 ◦C: (a) tensile strength and elongation; (b) torsional laps.

3. Results and discussion

3.1. Mechanical properties

The change of tensile strength with the annealing hold time,presented in Fig. 1a, can be described according to the slope ofthe curve. Initially, the tensile strength of wires increases from1780 MPa to 1940 MPa with increasing hold time and reaches max-imum strength when the hold time is 2.5 min, while the elongationdecreases from 8% to 6%. Such age hardening behavior may resultfrom the diffusion and dislocation pinning of dissolved carbonatoms in lamellar ferrite [17]. With increasing hold time, the tensilestrength of the wires decreases, then it reaches a plateau (about1600 MPa) for hold time >10 min, while the elongation of wiresincreases, then it plateaus (about 7%) for hold time >5 min. Suchage softening behavior occurs, presumably due to the spheroidiza-tion of lamellar cementite and the occurrence of recovery of ferrite,and it would be examined in the following section.

Fig. 1b shows the relationship between torsion lap of steel wiresand annealing hold time at 450 ◦C. The torsion lap of steel wires isfound to changes very little for hold time <2.5 min. As the hold timeincreases, it decreases rapidly from about 19 to 12 circles and levelsoff at about 11circles when the hold time exceeds 10 min.

3.2. Evolution of microstructure

Fig. 2 shows the microstructures obtained from the longitudi-nal section of pearlite steel rods, cold drawn wires, cold-drawnwires annealed at 450 ◦C for 2.5 min and for 10 min, respectively.As shown in Fig. 2a, the pearlitic steel rod has a two-phased,lamellar structure composed of alternating layers of (bright) fer-rite and (dark) cementite layers with the interlamellar spacingabout 100 nm; cementite forms a straight interface with ferrite,in which dislocation density is low. Fig. 2b shows the deformedmicrostructures of as-drawn wires. The interlamellar spacing ofpearlite is reduced to about 25–30 nm, apparently due to the draw-ing process. One can also note the very high density of extinctioncontours inside the ferrite lamellae, indicative of the high levelof dislocation density. Short hold time (under 1 min) at 450 ◦Cdoes not result in any observable change in the microstructure ofcold drawn wires (not shown here). However, when the annealingtime increases to 2.5 min, marked changes in the microstructureof wires occur. While a majority of the deformed lamellar cemen-tite still maintains its straight shape, those near the boundary ofpearlitic grains starts to spheroidize, as shown in Fig. 2c. At the

same time, extinction contours inside the ferrite lamellae are foundto fade out, which means the recovery of ferrite happened [11,12].The spheroidization of lamellar cementite is easily observed insidepearlite grains in Fig. 2d, indicating that a large amount of lamellar

F. Fang et al. / Materials Science and Engineering A 547 (2012) 51– 54 53

F ples: (a) rods; (b) cold drawn wires; (c) cold drawn wires annealed at 450 ◦C for 2.5 min;(

cFdea(stsf

3

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0 100 200 300 400 500

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ig. 2. TEM micrographs acquired from the longitudinal section of pearlitic steel samd) cold drawn wires annealed at 450 ◦C for 10 min.

ementite has spheroidized in the wires held for 10 min at 450 ◦C.urthermore, extinction contours inside the ferrite lamellae almostisappeared, suggesting the recovery of ferrite has completed. Thisxplains why the tensile strength of cold-drawn wires decreasednd the elongation increased when the hold time exceeds 5 minFig. 1a). Interestingly, it can be seen that the torsion propertytarted to decrease when the hold time was 2.5 min (Fig. 1b), whenhe spheroidization of lamellar cementite started. Therefore, thepheroidization of lamellar cementite is believed to be responsibleor the degradation of torsion property of the wires.

.3. DSC analysis

The microstructural evolution of both pearlitic rods and cold-

rawn steel wires during heating was probed by means of DSCnalysis (Fig. 3). It is clear that neither endothermic nor exother-ic peaks appear during heating of the pearlitic rods, while a broad

xothermic peak appeared at temperatures extending from about

Temperature (ºC)

Fig. 3. DSC curves of cold-drawn wires and steel rods obtained at heating rate of20 ◦C/min.

54 F. Fang et al. / Materials Science and Engineering A 547 (2012) 51– 54

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Fig. 4. SEM micrographs revealing microstructure of (a) c

80 ◦C to 480 ◦C. The occurrence of exothermic peak during heat-ng of the cold drawn wires may be influenced by microstructurehanges. To gain a deeper picture of the microstructure evolution ofold drawn wires during the heating process, the microstructuresf both cold drawn steel wires and steel rods following DSC analysisere examined. Fig. 4a shows that a large amount of cementite in

old drawn wires has spheroidized, whereas the cementite in steelods still assumes a lamellar shape (Fig. 4b). Therefore, a direct linkan thus be established between the exothermic peak in the DSCurve and the spheroidization of lamellar cementite in the steelires. The broad exothermic peak over a wide temperature rangeay result from the fact that the spheroidization of lamellar cemen-

ite starts at the boundary of pearlitic grains (Fig. 2c), and thenxpands inward (Fig. 2d).

As mentioned earlier, during the cold drawing process par-ial dissolution of cementite occurred and dissolved carbon atoms

igrated to the dislocations of ferrite/cementite interfaces, coin-iding with the increase of the dislocation density in ferrite [4–9]. Asuch, the microstructure evolution of cold drawn wires during thennealing process can be summarized as follows: at the beginningf the annealing process, carbon atoms diffuse from regions of highoncentration (i.e., dislocations at ferrite/cementite interfaces) toeighboring dislocations within in ferrite and pin them [18]. Strainge hardening thus occurs. With the increase of hold time, the den-ity of dislocations decreases, since the static recovery of defectsenerated from cold drawing process takes place. Consequently,arbon atoms move back and re-settle at ferrite/cementite inter-aces. The tensile strength of the wires decreases and the elongationncreases. With a further increase in the hold time, the spheroidiza-ion of lamellar cementite occurs, driven by the minimization ofurface energy for lamellar cementite.

Here we demonstrate that the lamellar cementite in cold-drawnearlitic wires maintains the original shape, provided the hold time

s short (i.e., 2.5 min at 450 ◦C). Moreover, we show a small win-ow exists for strain age hardening, enabled by the dislocationinning in ferrite by dissolved carbon atoms migrating from fer-ite/cementite interfaces. It leads to a further increase in strength ofteel wires and only a very slight decrease in torsion property con-rolled by the spheroidization of lamellar cementite in the wires.

. Conclusions

The galvanization treatment of cold-drawn pearlitic steel wiresas simulated by thermal annealing at 450 ◦C in the laboratory. The

ffects of the annealing time on the microstructure evolution andechanical properties of the steel wires were clarified.Tensile strength of wires increases with increasing time for

old time <5 min, and then decreases and reaches a plateau for

[

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rawn wires and (b) steel rods following the DSC analysis.

hold time >10 min. The elongation of wires exhibits an oppositetrend. Tensile strength and elongation of cold-drawn wires are bothinfluenced by the strain age hardening and static recovery pro-cess. The torsion laps of wires changes very little for hold time<2.5 min, and then decreases rapidly. It plateaus when the holdtime is greater than 10 min. Spheroidization of lamellar cementiteis responsible for the degradation of torsion property of cold-drawnwires.

In addition, neither endothermic nor exothermic peaks weredetected during the heating of the pearlite rods, while a weakexothermic peak was identified for cold drawn pearlitic wires. Itresults from the spheroidization of lamellar cementite.

Acknowledgements

The authors would like to thank A. Q. Xu of the Jiangsu ProvinceKey Laboratory for Advanced Metallic Materials of Southeast Uni-versity for the assistance in TEM analysis. This work was financiallysupported by Natural Science Foundation of Jiangsu Province (grantno. BK2011616) and Southeast University, and was partially fundedby Jiangsu Province Industry-University Strategic Research Pro-gram (grant no. BY2011144). The samples were prepared at JiangsuProvince Institute for Research of Iron and Steel, which is partiallysupported by Zhangjiagang City Scientific & Technological Programunder grant no. ZKJ1013.

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