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Scripta Materialia 65 (2011) 528–531

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Effect of nitrogen on the critical strain for dynamic strain agingin high-manganese twinning-induced plasticity steel

Sangwon Lee, Jinkyung Kim, Seok-Jae Lee⇑ and Bruno C. De Cooman

Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology,

Pohang 790-784, Republic of Korea

Received 18 April 2011; revised 9 June 2011; accepted 10 June 2011Available online 16 June 2011

The effect of nitrogen on the dynamic strain again behavior of a Fe–18% Mn–0.6% C twinning-induced plasticity steel was inves-tigated by means of in situ infrared thermography during tensile testing. The addition of nitrogen affected the initiation of the Porte-vin–Le Chatelier bands and the characteristic shape of the serrations on the stress–strain curve. Also, nitrogen additions resulted inan increase in the critical strain for dynamic strain aging.� 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Nitrogen effect; Dynamic strain aging (DSA); Portevin–Le Chatelier (PLC) band; Twinning-induced plasticity (TWIP)

High-manganese and -carbon twinning-inducedplasticity (TWIP) FeMnC steels are of interest for theautomobile industry because of an excellent combina-tion of strength and ductility due to a high rate of workhardening. The work hardening rate is controlled bystrain-generated twins [1]. The passage of Shockley par-tial dislocations on every close-packed plane in lowstacking fault energy (�20 mJ m�2) FeMnC austeniticsteels result in the formation of mechanical twins. Theaustenite grains are subdivided continuously by themechanical twinning during deformation. The twinboundaries form strong barriers for dislocations and im-pede their slip movement. This additional dislocationstorage mechanism increases the strain hardening. Dy-namic strain aging (DSA) is also known to affect thestrain hardening mechanism and the propagation ofPortevin–Le Chatelier (PLC) bands leads to a serratedstress–strain curve, a phenomenon often referred to asjerky flow [2]. The effect of the DSA on the increase instrain hardening and the resulting deterioration of form-ability is caused by the interaction between mobileobstacles, e.g. interstitial atoms, and dislocations [3].PLC bands are nucleated and propagate when the con-ditions for dynamic strain aging are met, i.e. when tem-porarily stopped dislocations are pinned by point defectsand released simultaneously. This process results in a

1359-6462/$ - see front matter � 2011 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2011.06.017

⇑Corresponding author. E-mail: seokjaelee@postech.ac.kr

localized plastic deformation and a negative strain ratesensitivity [4–6].

It is well known that DSA is influenced by several fac-tors. At high temperatures, the mobility of the point de-fects can become high enough to lead to a morefrequent recapture of dislocations resulting in an en-hanced DSA effect [7]. An increased strain rate suppressesthe DSA effect because the interaction between disloca-tions and mobile obstacles becomes increasingly difficultat high strain rates [8]. The jerky flow on stress–straincurves associated with the DSA has been shown to bestrongly influenced by the grain size [9]. The critical strainof the DSA where the localized plastic deformation andband serration begin increases with increasing grain size[10,11]. The influence of alloying elements on the DSA ef-fect has also been studied in detail, and C, N, Al and Vadditions have been reported to affect the serrations onthe stress–strain curve in various steels [11–16]. Kimet al. [11] reported the effect of N on the DSA of austeniticstainless steel containing 12 mass% Ni and 18 mass% Cr.They explained that the addition of N retards the DSA ef-fect by the interaction between N and Cr, resulting in a de-crease in the diffusivity of Cr to dislocations. Bracke [12]showed that C atoms promote the DSA effect, while Natoms suppress DSA in austenitic Fe–22% Mn–(0.4–0.6)% C TWIP steel. He suggested that N atoms interferewith point defect complexes involving interstitial C, suchas Mn–C dipoles, which could be the source of DSA. As aconsequence, N additions result in the increase in the crit-ical strain. Brindley and Barnby [13] reported the effect of

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Table 1. The chemical composition of the steels used in the presentwork (in mass%).

Steel Mn C N Fe

FeMnC 18.0 0.58 0.0023 BalanceFeMnCN 18.4 0.53 0.0900 Balance

Figure 1. True stress–strain curves of the FeMnC and FeMnCN TWIPsteels. The critical strain is determined as the strain where a serrationof value n appears. Enlarged stress–strain curves show the distinctdifference in the stress leap behaviors depending on nitrogen content.

S. Lee et al. / Scripta Materialia 65 (2011) 528–531 529

N additions on the DSA in mild steel strained in the tem-perature range 200–300 �C. The large serrations at strainsof 8–10% occurred in a mild steel with a 40 ppm additionof N, whereas no obvious serrations were present on thestress–strain curve of a mild steel containing 240 ppm N.Shun et al. [14,15] reported that the addition of Al in-creases the activation energy for the onset of serrationsby lowering the diffusivity of C in austentic Fe–30%Mn–1% C–Al TWIP steels. Al additions have been foundto be very effective in suppressing DSA in FeMnC TWIPsteels. It has also been reported that the addition of V sup-presses the DSA effect because the carbide precipitation(VC) removes solute carbons from the matrix [16].

Very few studies are available on the effect of alloyingelements on the critical strain for the onset of DSA inhigh-manganese austenitic TWIP steels. In the presentwork, the relationship between the addition of nitrogenand the critical strain of an austenitic TWIP steel con-taining high Mn and C was investigated by means ofin situ infrared (IR) thermography during a conven-tional uniaxial test. In addition, the initiation of thePLC bands was compared for different high-Mn austen-itic steels with the consideration of the relationship be-tween the alloying elements and the critical strain.

The chemical composition of the as-received hot-rolledhigh-Mn and -C sheets is listed in Table 1. Two TWIPsteels with different N levels were used for the presentstudy. Hot-rolled samples were cold-rolled to a thicknessof 1.25 mm, corresponding to a thickness reduction of50%. The samples were annealed at 800 �C for 104 s,and air cooled to room temperature. ASTM E8-type ten-sile test samples of the cold-rolled steel with their tensileaxis aligned along the rolling direction were tested atroom temperature at a strain rate of 5 � 10�3 s�1 usinga Zwick universal tensile testing machine. A CEDIP Silver420 M IR camera and a Vialux Autogrid high-resolutionoptical strain analysis system were used to study the PLCbands during the tensile tests. Square grids of 1 mm2 werechemically etched on the surface of the tensile specimensto measure the strain distribution. In the case of the IRthermography, the tensile specimens were covered witha layer of carbon black to obtain a high and homogeneousthermal emissivity.

Figures 1a and b compare the true stress–strain curvesof the FeMnC austenitic TWIP steel and the FeMnCNnitrogen-added TWIP steel. The yield strength and ulti-mate tensile strength are both slightly increased due tothe solid solution strengthening effect of N in the FeM-nCN steel. The enlarged part of Figure 1a indicates thatthe N addition also increases the critical strain. Whilethe serrations occur directly after yielding in the FeMnCsteel, the serrations start at a critical strain of 12% in thecase of the FeMnCN steel. The stress–strain curve ofthe FeMnCN steel has many more serrations than thestress–strain curve of the FeMnC TWIP steel, as shownin Figure 1b. Whereas large step-like serrations occur inthe stress–strain curve of the FeMnC steel, the stress–strain curve of the FeMnCN steel has small stress spikes.In addition, the stress–strain curve was continuously ser-rated in the case of the FeMnCN alloy.

The initiation and propagation of the PLC bands forthe FeMnC and FeMnCN steels are compared by meansof the in situ IR thermography analysis shown in Fig-

ures 2a and b. The IR thermography can reveal thebehavior of individual PLC bands because the pro-nounced strain localization associated with a PLC bandresults in a measurable increment of the local tempera-ture. The maximum temperature increment due to thestrain localization is approximately 15 �C. The time–strain and time–stress curves can be directly related tothe observation of the initiation and propagation ofthe PLC bands across the tensile specimens. In thetime–strain curves of Figures 2a and b, the measuredstrain is seen to increase sharply when a PLC bandpasses within the gauge length of the extensometer be-cause the entire sample deformation is localized in thePLC band. In contrast, the measured homogeneoussample strain increment is very small when the PLCband has moved out of the gauge length of the exten-someter. As a consequence, the time–strain curves havedistinct strain plateaus. The repeated nucleation of thePLC bands outside the gauge section of the extensome-ter and their initial propagation outside the section ofthe specimen monitored by the extensometer are clearlythe cause of the serrations associated with a step-like in-crease in stress (type A serrations) in the case of theFeMnC steel. The step-like nature of the discontinuitiesis therefore entirely due to the motion of the PLC bandoutside the gauge length of the extensometer in the caseof the FeMnC steel.

The different serration patterns on the stress–straincurves of the FeMnC and FeMnCN steels can be ex-plained by the location of the PLC band initiation.The PLC bands in the FeMnC steel are initiated at the

Figure 2. The tensile test time–engineering strain curves of (a) FeMnC steel and (b) FeMnCN steel, and the temperature change of the specimensurface observed by means of IR thermography during the tensile test. The red box indicates the position of the PLC band initiation and the redarrow indicates the direction of the band propagation.

Figure 3. The distribution of the true major strain measured beforeand after the PLC band initiation in the FeMnCN steel: (a) opticalstrain analysis of the entire tensile sample; (b) the major and minorstrain diagram for the strains in the areas labeled A and B in (a), priorto and after the passage of a PLC band.

530 S. Lee et al. / Scripta Materialia 65 (2011) 528–531

end of the gauge part outside the gauge length of theextensometer, while the PLC bands in the FeMnCNsteel are initiated at the center of the gauge part andpropagate to the outer edges of the sample. This initia-tion and propagation mechanism is repeated duringthe entire tensile test. The number of PLC bands andtheir propagation velocity in the time range from 60 to180 s were investigated. Twelve PLC bands are initiatedin the FeMnC steel, whereas 20 PLC bands are initiatedin the FeMnCN steel. The average velocity of the PLCbands for the FeMnCN steel and FeMnC steels is 10and 12 mm s�1, respectively. The influence of the addi-tion of 900 ppm N on the velocity of the PLC bands isnot significant, and the applied strain rate is not main-tained by the PLC band propagation time in the FeM-nCN steel, which is approximately half of that in theFeMnC steel. As the distance a PLC band moves acrossthe extensometer gauge length is influenced by the posi-tion of the PLC band initiation, the PLC band initiationinside the gauge length observed for the FeMnCN steelinduces the formation of a larger number of PLC bands.The different shape of the serrations is also related to theposition where a PLC band is initiated. As a PLC bandis initiated out of the extensometer in the FeMnC steelat one end of the tensile sample, the localized straindue to the PLC band formation is not measured andthe stress increases apparently without strain increment.In the time–strain curve, a strain plateau is observedwhich ends when the PLC band moves to the extensom-eter range. When the stress discontinuity associated withthe initiation of a PLC band occurs in the time–stresscurve of the FeMnCN steel, the strain increase is imme-diately recorded because the PLC band is initiated with-in the gauge length of the extensometer.

The critical value of the dislocation density for DSAto occur is important because dislocations act as sourcesfor pinning solute atoms [7]. It is known that the numberof dislocations is increased with increasing plastic strain.Figure 3a shows a map of the true major strain for a ten-sile specimen before and after a PLC band initiation inthe FeMnCN steel. The data were obtained by meansof high-resolution optical strain analysis. The true majorstrain of 11.5% corresponds to the critical strain forband initiation in the FeMnCN steel (see Fig. 1a) asmeasured at the gauge center. The first PLC band is

preferentially initiated at the center of the sample whenthe strain is increased to 12.5%. The first PLC band hasan inclination angle of 48o to the tensile axis. Figure 3bshows the major strain–minor strain distribution for theareas labeled A and B on the sample. The size of the areais 6 � 6 mm2. Before the initiation of the PLC band, thetrue minor and major strains are clustered around the(�0.055, +0.115) point before PLC band initiation be-cause the central gauge part is uniformly strained. Thestrain distribution becomes inhomogeneous when thePLC band is initiated at the center of the tensile speci-men. The average minor–major strain point for regionA moves to (�0.066, +0.138) locally, whereas the strainparameters for region B remain close to the(�0.060, +0.125) point. The difference in strain valuesfor regions A and B was used to determine the proper-ties of the first PLC band in the FeMnCN steel (Table 2).The results show that the band strain rate is approxi-mately 20 times higher than the applied strain rate,and that the band strain is associated with a consider-able thickness strain. The band normal anisotropy(eW =eT ) is about 0.86. These band parameters are similarto those reported by Kim et al. [17].

A survey of the literature shows that the compositionof a TWIP steel affects the location of the PLC band ini-tiation and the serration behavior. Chen et al. [8] re-ported that PLC bands in a Fe–18% Mn–0.6% CTWIP steel were nucleated at the grip end of tensilespecimen. The serration was observed just after yielding

Table 2. Characteristics of the PLC band initiated first in the FeMnCN steel.

Length strain(eL)

Width strain(eW )

Thickness strain(eT )

Band velocity(mm s�1)

Band strainrate (s�1)

Inclinationangle (�)

Normal anisotropy(eW =eT )

+0.013 �0.006 �0.007 10 2.3 � 10�2 48 0.86

S. Lee et al. / Scripta Materialia 65 (2011) 528–531 531

of the TWIP steel, which means that the critical strainwas very close to the yield strain. The PLC bands occur-ring during deformation of the Fe–18% Mn–0.6% C–0.09% N TWIP steel were initiated at the center of gaugelength in the present study. The observed critical strainwas about 12%. Zavattieri et al. [18] reported that thelocation of band nucleation was close to the center ofthe gauge section for an Fe–17% Mn–0.6% C–1% AlTWIP steel, and the serration was observed at strainslarger than 15%.

Figure 4 shows the relationship between the composi-tion and the critical strain in high-Mn TWIP steels [18–20]. The critical strain is associated with the position ofthe PLC band initiation. If the FeMnC TWIP steel isconsidered as a reference, the addition of N and Al in-creases the critical strain and the center of gauge part be-comes the preferred position for PLC band initiation.On the other hand, the increase in C and Mn contentsdecreases the critical strain and induces the PLC bandto initiate at a stress concentration located in the grippart of the specimen.

To summarize, the relationship between the additionof N to an FeMnC TWIP steel and the DSA was inves-tigated. The addition of N results in an increase in thecritical strain for DSA and the initiation of the PLCbands is observed to occur in the gauge section ratherthan in the grip part of the sample during tensile tests.The position of the PLC band initiation influences theshape of the serrations. A continuously serrated stress–strain curve is obtained for the N-added TWIP steel,whereas the stress–strain curve of the N-free FeMnCsteel has step-like stress increments, i.e. type A serra-tions. The PLC band initiation location and the criticalstrain are influenced by the N alloying in a manner sim-ilar to what is observed when the Al content of anFeMnC TWIP steel is increased [8,17]. As DSA is influ-enced by point-defect interactions with dislocations, andthe N and Al are known to influence the stacking faultenergy of TWIP steel [21], the observations reported inthe present report point to a possible interaction be-

Figure 4. Relationship between the addition of alloying elements inhigh-Mn TWIP steel and the critical strain for the serrations observedon the stress–strain curve [18–20]. The position of the initial bandobserved on the gauge part of tensile specimen is influenced by thealloying elements.

tween the stacking fault and point defects. If the DSAis caused by the breaking away of stacking faults frompoint defects or point-defect complexes, an increase inthe stacking fault energy will reduce the defect–stackingfault interaction and the DSA will occur at a largerstrain when the dislocation density, and hence the stack-ing fault density, is increased. The center of the tensilespecimen is more strained than other positions andreaches the critical strain first, resulting in the formationof the PLC band close to the sample center. When thecritical strain is reduced to a value close to the yieldstrain by increasing the C or Mn content, the small localplastic strains can reach the critical strain uniformlyacross the whole tensile specimen. In that case, thePLC band preferentially initiates outside the samplegauge section due to the non-uniform stress concentra-tion in the grip section of the tensile specimen.

This research was supported by a WCU (WorldClass University) program through the National Re-search Foundation of Korea funded by the Ministry ofEducation, Science and Technology (R32-10147). Theauthors sincerely acknowledge the support of POSCO.

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