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On the Plasticity of Interstitial-Free Steel Subjected toCryogenic Rolling Followed by AnnealingGautam Anand a , Arijit Sinha b & Partha Protim Chattopadhyay aa Department of Metallurgy and Materials Engineering, Bengal Engineering and ScienceUniversity, Howrah, Indiab School of Materials Science and Engineering, Bengal Engineering and Science University,Howrah, IndiaAccepted author version posted online: 16 Jul 2012.Published online: 07 Mar 2013.

To cite this article: Gautam Anand , Arijit Sinha & Partha Protim Chattopadhyay (2013) On the Plasticity of Interstitial-FreeSteel Subjected to Cryogenic Rolling Followed by Annealing, Materials and Manufacturing Processes, 28:3, 242-248, DOI:10.1080/10426914.2012.667891

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On the Plasticity of Interstitial-Free Steel Subjected

to Cryogenic Rolling Followed by Annealing

Gautam Anand1

, Arijit Sinha2

, and Partha Protim Chattopadhyay1

1Department of Metallurgy and Materials Engineering, Bengal Engineering and Science University, Howrah, India2School of Materials Science and Engineering, Bengal Engineering and Science University, Howrah, India

Microstructure and tensile deformation behavior of interstitial-free (IF) steel subjected to near-cryogenic (�150�C to �100�C) rollingfollowed by annealing have been investigated. Twining has been found to be the dominant mode of deformation during rolling. Both

the rolled and annealed sample exhibited limited work hardening due to high matrix resistance. In the rolled sample cleavage, nuclei acted

as potential fracture initiation site. Annealing has resulted in effective blunting of cleavage nuclei giving rise to large total elongation due to

prolonged damage growth process.

Keywords Cryogenic rolling; Deformation twinning; Fracture; IF steel; Recovery.

INTRODUCTION

Cryogenic rolling has been recently explored as one ofthe practical methods for manufacturing of bulk materi-als with nanocrystalline=ultrafine grain size [1, 2]. As theslip-controlled processes are suppressed during defor-mation at such low temperature, twinning is triggeredas the alternative deformation mode [2, 3]. Cryogenicrolling also provides the opportunity for processing ofbulk samples of larger size which are amenable to thecharacterization by conventional techniques.Recently, cryogenic rolling of copper has drawn

special attention due to formation of bimodal micro-structure after appropriate annealing of the deformedsample, which results in excellent combination ofstrength and ductility [4–6].Extensive studies have been conducted on deformation

behaviour of FCC materials at low temperature [7–9]. Ithas been demonstrated that deformation twinning at lowtemperature governs the plastic deformation, parti-cularly in the case of low and medium stacking faultenergy materials like copper [10]. In contrast to FCCmaterials, experimental studies on low-temperaturedeformation behavior of BCC materials are limited[11–13]. However, the possibilities and concernedmechanisms have been investigated theoretically [14,15]. It is known that Peierls stress increases with thedecrease in the temperature whereas the twinning stressis almost invariant, with temperature or exhibit onlynominal temperature dependence [16]. Therefore, inBCC materials, lowering the temperature of deformationpromotes deformation twinning at the expense of

dislocation slip. It is well known that mechanism of defor-mation twinning in the cases of BCC and FCC metalsvary in several important aspects. Firstly, in contrast toFCC materials, dislocation slip is not pre-requisite forthe deformation twinning in BCC materials [17]. More-over, twin bands in FCC materials are much thinner incomparison with the BCC materials where twin bandsare several microns thick [18]. The other interesting fea-ture is the initiation of microcrack at the intersection oftwin bands and the termination of same at the obstacles,which is the common feature in BCC materials [19].In view of the above, an attempt has been made to

study the microstructural evolution in the case of highstacking fault energy material at low temperature duringcryogenic rolling due to possible interplay of dislocationslip and deformation twinning. Annealing was done inrecovery regime to study the change in the structure oftwin boundary due to thermally agitated motion of dislo-cation. Ti-stabilized interstitial-free (IF) steel was chosendue to its high stacking fault energy and to avoid compli-cation arising from dynamic strain aging or yield pointphenomena, as absence of interstitial solutes and otheralloying elements leads to the representative BCC beha-vior. The explicit values of stacking fault energy ofBCC and FCCmaterials, which shows deformation twin-ning is given in Table 1 [20–22]. The tensile deformationproperties of samples have been suitably correlated withthe corresponding microstructure to understand theunderlying mechanism contributing to the plasticity ofsamples.

EXPERIMENTAL DETAILS

The chemical composition of the steel is given inTable 2. The bar stock of IF steel was forged to obtainbar of 10mm thickness for subsequent rolling. Beforerolling, samples were annealed at 900�C for 15min toobtain equiaxed grains. The cold rolling was done at

Received July 20, 2011; Accepted October 28, 2011

Address correspondence to Partha Protim Chattopadhyay,Department of Metallurgy and Materials Engineering, BengalEngineering and Science University, Shibpur, Howrah-711103, India;E-mail: ppc@metal.becs.ac.in

Materials and Manufacturing Processes, 28: 242–248, 2013Copyright # Taylor & Francis Group, LLCISSN: 1042-6914 print=1532-2475 onlineDOI: 10.1080/10426914.2012.667891

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near-cryogenic temperature (�150�C to�100�C). Duringcryogenic rolling, temperature was measured after eachpass using Lutron TC-920 equipment before soakingwithin the liquid nitrogen bath. Soaking was continueduntil the samples attained the temperature of liquid nitro-gen bath. The temperature measured after each pass wasfound to increase with increased amount of deformationand varied between �150�C to �100�C. The increase intemperature is presumably due to internal heating underinhomogeneous deformation of the sample. It is reason-able to conjecture that at later stages of rolling internalheating continue to increase due to increase amount ofaccumulated inhomogeneous strain in the samples. 20%deformation was achieved in 22 passes. The approximatestrain rate during rolling was estimated to vary between9.0588� 10�3 to 1.1159� 10�2 s�1. Subsequently, a setof rolled samples were annealed at 400�C for 5, 10, and60min, respectively. Microstructures of the equiaxed,rolled, and annealed samples were studied by OpticalMicroscope (Olympus CK40M) and Scanning ElectronMicroscope (Hitachi 3400N operated at 15 kV). Theaverage grain size of equiaxed material and grain size dis-tribution of annealed material was estimated using Ima-geJ software. Tensile testing of cryogenically rolled andannealed sample (10min) was carried out using anInstron - 4204 testing machine with a crosshead velocityof 1.0mm=min as per ASTM Standard (E8M �96).The imposed strain rate during tensile testing was1.4� 10�3 s�1. Features of the fracture surface andsub-surface regions of the fractured end were investigatedby Scanning Electron Microscopy.Sub-surface region was investigated to study the

location and nature of instability. The fractured end oftensile samples was carefully grounded to the requireddepth to remove the unevenness and standard metallo-graphic method was followed to obtain the micrograph.

RESULTS AND DISCUSSION

Figure 1 shows the SEM micrograph of the as receivedequiaxed material having average grain size of 18mm.

Figure 2a shows the optical microstructure of the cryogeni-cally rolled sample. The micrograph does not exhibit theusual directionality of the polycrystalline aggregate alongthe rolling direction by crystal rotation and formation ofgeometrically necessary dislocations (GND). However, incase of cryogenic rolling due to constrained slip, the latticerotation due to deformation twinning and crystal bendingis accommodated by micro-void nucleation, as shown byarrow in Fig. 2a and enlarged view being shown in inset.The deformed grains are frequently intersected by the twinbands, most of which span across the entire grain, unlessintercepted by another twin band or an obstacle.Figures 2b–d show the SEM micrograph of the samplesrolled at cryogenic temperature. The images reveal themicrostructural features delineated by the variation indepth or surface relief. Figure 2b reveals frequent facetingof the crystals, which corresponds to crystal bending that,takes place by transmission of shear through grain bound-aries under constraints [23]. At low temperatures, thedeformation strain is transmitted from grain to grain bytwo-dimensional straining at one side of grain boundary,which is relieved by deformation on the other [24]. Underconstrained slip at low temperature, the microscopicchange in crystal (i.e., geometry of coherent domain) isrestricted as such in polycrystalline aggregate to maintaincontinuity along grain boundary rather than elongationof grains in rolling direction. Such conjecture thatlow-temperature deformation situation leads to lack ofdirectionality are substantiated by the presence of promi-nent faceting in grains in which twinning is more promi-nent (Fig. 2b).Figure 2c shows the formation of wedge shaped fea-

ture at twin-matrix interface, marked by arrows, whichis potential precursor for cleavage initiation. The mech-anism of wedge shaped cleavage nuclei has been illu-strated in the inset of Fig. 2c. In this case, twodislocation come together to lower their strain energyby providing a pure edge dislocation lying on (001) plane[25]. The presence of cleavage nuclei at twin-matrixinterface can be explained, considering the fact thatthe temperature and=or strain rate for the plastic

FIGURE 1.—SEM micrograph of as-received IF steel sample.

TABLE 1.—Stacking fault energy of various BCC and FCC materials which

shows deformation twinning.

Metal or alloy Crystal structure Stacking fault energy (mJ=m2)

a - Fe [20] BCC 250

Mo[21] BCC 607

Cu[22] FCC 40

Au[22] FCC 16

Al[22] FCC 135

TABLE 2.—Chemical composition of IF steel.

Alloyingelements C Mn S P Si Al N Nb Ti

Weight

percentage

0.002 0.4 0.006 0.014 0.02 0.0==6 0.0033 0.011 0.05

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deformation are comparable to those for activation ofdeformation twinning and cleavage nucleation [26]. Inother words, such condition imply the inability of simul-taneous operation of at least five independent slip systemto maintain the continuity at grain boundary required insuch ferritic alloys. When five independent slip systemsfail to operate, there is a strain mismatch that can beaccommodated by grain boundary cracking or twinningdue to the geometric necessity. The stress will build up, ifnecessary, until either of these relieving mechanismsoperates. When twinning and cleavage initiation requiresimilar stresses as crystal slip, there is an easy switchingof the mode of accommodation and the locally favorablemode operates. Now impingement of twin on grainboundary can easily nucleate cleavage crack, becausethe alternative, continued slip in the next grain, is moredifficult. Similarly, twin-twin intersection may findcleavage initiation as the easier stress-relieving alterna-tive, as depicted in Fig. 2d. The figure shows the forma-tion of a prominent cleavage crack at twin–twinintersection and termination of twin at grain boundary.The cleavage feature in Fig. 2d is marked with an arrow.Figures 3a–d show the micrographs of the sample

rolled at cryogenic temperature and subsequentlyannealed at 400�C for different times. The micrographof the sample annealed for 5min, as shown in Fig. 3a,reveals the recovery of deformed structure along withthe presence of deformation twins in the microstructure.Figure 3b shows the micrograph of the sample annealedfor 60min. In addition to the recovery of the deformedgrains, the microstructure exhibits the presence of

cross-twins, which is the common feature of deformationtwins and it implies that the twin structure is inheritedfrom the deformation microstructure [26]. Figure 3bshows the absence of crystal bending and frequent pres-ence of deformation bands. Moreover, the micrographshows evidences of the recovery of the surface reliefpresent in the micrograph of the rolled sample (Fig. 2b).Figures 3c and d shows the SEM micrographs of the

sample annealed at 400�C for 10min after cryogenicrolling. Figure 3c demonstrates the accommodation ofcrystal bending by recovery of disoriented segments ofdeformed grains resulting in formation of sub-grains. Incase of the incoherent boundaries, dislocations are lockedat interface during the formation of twins. Annealingresults in relief of the accommodated stress due toenhanced mobility leading to increased coherency attwin-matrix interface attained by local atomic rearrange-ment. Figure 3d shows the blunted cleavage microcracks.The blunting of cleavage microcrack (Fig. 3d) may beexplained in terms of enhanced dislocation mobility atthe annealing temperature. The micrograph (Fig. 3d) alsoshows the accommodation of the large scale crystal bend-ing by disintegration of microcracks facilitated by theclimb of edge dislocations along the planes parallel tothe twin boundary. The mechanism of disintegration isschematically presented in the inset of Fig. 3d. The micro-cracks are blunted due to the concomitant thermally acti-vated dislocation emission=absorption at the crack tip.Such cracks do not trigger the cleavage as the dominantmode of fracture. The grain size distribution in case ofsample annealed at 400�C for 10min was estimated using

FIGURE 2.—Optical micrographs of cryogenically rolled sample showing (a) twin bands and microvoids, SEM micrograph of cryogenically rolled sample

and enlarged view of microvoids (inset), (b) faceting of crystals, (c) wedge shaped cleavage nuclei at twin-matrix interface, and mechanism of cleavage

nuclei formation (inset), and (d) cleavage crack at cross-twins.

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ImageJ software. Figure 4 shows the estimated grain sizedistribution. The abnormal subgrain growth behavior, asobserved in the above case, can be defined as one of thetypical cases of abnormal subgrain growth observed inthermally activated evolution of deformed substructuresin high stacking fault energy which contains the boundaryhaving low energy and high mobility [27].Figure 5 shows the engineering stress-strain curve for

as received equiaxed, cryogenically rolled, and annealedsamples. The corresponding tensile properties are shown

FIGURE 3.—Optical micrograph of sample cryogenically rolled and annealed at 400�C for, (a) 5 minutes, (b) 60 minutes, (c)–(d) SEM micrograph of sam-

ple cryogenically rolled and annealed at 400�C for 10 minutes and mechanism of blunting of cleavage nuclei (inset).

FIGURE 4.—Estimated grain size distributions in cryogenically rolled and

annealed sample at 400�C for 10 minutes.

FIGURE 5.—Engineering stress-strain curve of as-received equiaxed, cryo-

genically rolled and annealed sample at 400�C for 10 minutes.

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in Table 3. It is evident from Fig. 5 that for both the sam-ples deformation instability sets in just beyond the yieldpoint; while the rolled sample failed after only about0.9% strain, the annealed sample exhibited substantialelongation after instability. The observed tensile behav-iour in the case of cryogenically rolled samples, withand without subsequent annealing, is essentially contrib-uted by the activities of microvoids and cleavage nucleipresent in the samples under the applied strain, ratherthan the usual slip controlled plasticity. The limited duc-tility in the as rolled specimen may be attributed to theformation of cleavage nuclei as evidenced in Fig. 2c. Dur-ing the course of uniaxial deformation in tension, cleav-age nuclei eventually link up and result in fracture at theearly stage of straining. In contrast, significant elonga-tion (50%) has been achieved after annealing of the cryo-genically rolled sample at 400�C for 10min. It is alsoevident that softening is the dominant process for thewhole range of straining. Blunting of cleavage nuclei dur-ing annealing of the sample allows the straining under thedominant role of void growth and coalescence (Fig. 3d).

Therefore, the role of active cleavage nuclei and micro-voids predominantly accounts for the softening in thecase of as rolled and annealed samples, respectively. Itis also imperative to mention that the high stacking faultenergy of IF steel results in the recovery during annealingleading to the paucity of mobile dislocations and, hence,limited hardening. Therefore, it is also conspicuous thatthe plasticity is predominantly governed by softeningand=or damage growth [28, 29] or stable void growthmechanisms without perceptible hardening.Figures 6a–d present different features concerning

fracture of the tensile samples. The fractograph of theas rolled sample (Fig. 6a) demonstrates primarily cleav-age fracture with presence of isolated microvoids. In con-trast, the fractograph of the sample annealed at 400�Cfor 10min as shown in Fig. 6b reveals the formationand coalescence of microvoids without the dominantpresence of cleavage facets. Therefore, the tensile curveand fractograph of the annealed sample show perceptibletotal elongation in the regime of softening and damagegrowth.In order to understand the microstructural mechanism

related to softening, the sub-surface regions of the frac-tures end of the tensile samples were examined and therelevant SEM images are presented in Figs. 6c and d.Figure 6c reveals the prominent bending of twins with

the fine scale elevations at the twin matrix interface.These fine scale elevations are designated as ‘‘tongues,’’which evidences the deflection of crack at twin-matrixinterface during tensile testing [26]. The bending of twintakes place due to the interaction of shear bands formedin the course of tensile test with the pre-existing

FIGURE 6.—SEM micrograph of tensile fractured surface of, (a) cryogenically rolled sample, (b) sample annealed at 400�C for 10 minutes, SEM

micrograph of sub-surface of fractured end of, (c) cryogenically rolled sample, and (d) sample annealed at 400�C for 10 minutes.

TABLE 3.—Tensile properties as evaluated from tensile test.

Sample

Tensile stressat yield

(0.2% offset) (MPa)

Tensile stressat maximumload (MPa)

Tensile strainat fracture

Equiaxed structure 125 279.2 0.54

20% as-rolled 310 316.4 0.009

20% rolled and

annealed at 400�Cfor 10min

275.2 296.1 0.50

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deformation twins [30]. There is also evidence of theshear at the region away from the core. Therefore, it isreasonable to conjecture that the presence of the cleav-age nuclei (Fig. 2c) acts as the potential site for cleavageinitiation and propagation that leads to essentially cleav-age fracture.Figure 6d shows the sub-surface of the sample

annealed at 400�C for 10min. In contrast with the rolledsample, the presence of tongue at the twin-matrix inter-face is almost absent at the core region of the fracturedsurface. Moreover, there is signature of initiation ofvoids at the boundary of the curved twins. The outerregion of fractured region exhibits the evidences ofshearing at an angle of 45�, with the tensile axis.The observed contrast in the sub-surface images

between the as-rolled and annealed sample clearly indi-cates that the brittle fracture in the case of as-rolled sam-ple is primarily due to the active cleavage nuclei at twin-matrix interface. On the other hand, annealing results inblunting of cleavage possibly by thermally activatedemission=absorption of dislocation in the course ofannealing. This necessitates the accommodation of plas-tic strain by formation and coalescence of voids ratherthan nucleation of fresh microcracks at twin-matrixinterface, which is energetically less favorable, in theabsence of active cleavage nuclei.

SUMMARY

The results of the above investigation may be sum-marized as the following.

1. In BCC materials, constraint plasticity and low tem-perature, as in the case of present study, resulted inthe formation of deformation twins as well as cleav-age nuclei resulting in limited tensile plasticity.Microcleavage is the potential mechanism of strainmismatch in cryogenically rolled material, which linkup after the initial straining (0.9%) because of deflec-tion at twin boundaries. Twin boundaries curve dur-ing tensile deformation, possibly due to shear activityat the tip of fracture end in the course of deformation.

2. Annealing has resulted in improved coherency attwin-matrix interface, as manifested by the decreasein the thickness of twin boundary and blunting ofthe cleavage nuclei because of enhanced dislocationactivity at these stress concentration sites facilitatingenhanced tensile plasticity.

3. Very low-strain hardening prior to necking reflectshigh matrix resistances in both the cryogenicallyrolled and annealed samples. Sub-boundary harden-ing appears as a distinct possibility for annealed sam-ple. Additional twinning during tensile deformationseems not to be prominent; otherwise, necking couldhave been postponed.

4. In the case of rolled samples, presence of cleavagenuclei at incoherent twin-matrix interfaces results infracture in cleavage manner. Presence of void attwin-matrix interface does not play any significantrole. In contrast, in annealed sample, formation

and coalescence of voids during plastic strainingresults in ductile fracture.

ACKNOWLEDGMENTS

The authors are thankful to Dr. S.K. Ray for helpfuldiscussions and acknowledge TATA Steel Limited,Jamshedpur, India for providing steel for presentinvestigation.

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