Strain Hardening Behavior and Microstructure Evolution of High-Manganese Steel Subjected to Interrupted Tensile Tests

metals

Article Strain Hardening Behavior and Microstructure Evolution of High-Manganese Steel Subjected to Interrupted Tensile Tests

Adam Grajcar *, Aleksandra Kozłowska and Barbara Grzegorczyk

Faculty of Mechanical Engineering, Silesian University of Technology, 18a Konarskiego Street, 44-100 Gliwice, Poland; [email protected] (A.K.); [email protected] (B.G.) * Correspondence: [email protected]; Tel.: +48-32-237-2933

Received: 18 January 2018; Accepted: 7 February 2018; Published: 10 February 2018

Abstract: Strain hardening behavior and the corresponding microstructure evolution of the high-manganese steel with additions of Si and Al were investigated in this study. Thermomechanically processed and solution-heat-treated sheet steels were compared under conditions of interrupted tensile tests. Relationships between microstructure and strain hardening were assessed for different strain levels using light microscopy and scanning electron microscopy techniques. It was found that the deformation of both steels at low strain levels was dominated by dislocation glide before the occurrence of mechanical twinning. The amount of twins, slip lines, and bands was increasing gradually up to the point of necking. As the strain level increased, dislocation density within twinning areas becomes higher, which enhances the strength, the work hardening exponent, and the work hardening rate of the investigated high-manganese sheet steels.

Keywords: strain hardening; microstructure; twinning; interrupted tensile test; high-Mn steel

1. Introduction A beneﬁcial strain hardening behavior of high-manganese steels depends signiﬁcantly on the extent of TRIP (Transformation Induced Plasticity) and TWIP (Twinning Induced Plasticity) effects. These phenomena can take place both at a forming stage or later during an exploitation stage [1,2]. High-manganese steels are characterized by a pure austenitic microstructure due to a manganese content ranging typically between 20% and 30% [3–5]. A stimulation of mechanical and other properties can be done using various combinations of Al, Si, Cr, and N. These steels are strengthened due to a complex interaction of dislocation glide, mechanical twinning, and α/ε martensite formation. The martensite platelets and mechanical twins act as obstacles to dislocation motion and block the movement of dislocations, and in this way they affect the work hardening behavior of high-manganese steels. The mentioned microstructural effects determine the mechanical behavior of steel, resulting in the delay of local necking, which allows for large uniform elongations [6–11]. A type of the deformation mode depends on a chemical composition of steel and temperature and strain rate. It is related to stacking-fault energy (SFE) of the austenite. With increasing SFE, the mechanisms change from a martensitic transformation (occurs when SFE is lower than 25 mJ/m2) to a mechanical twinning (SFE in a range of 25–60 mJ/m2). If SFE value is higher than 60 mJ/m2, the steel is strengthened mostly by the dislocation glide [12]. The strengthening process is also strongly related to grain size [13–16]. Yuan et al. [17] reported that deformation twins form easily during tensile deformations when initial grain size increases. The effect of grain size on the mechanical properties of their steels was also detected—the yield and ultimate tensile strengths decrease with the increase in grain size, while the elongation increases. Dini et al. [18] reported that in TWIP steels the onset of mechanical twinning shifts to a lower strain as grain size increases. Moreover, they noted that, during

Metals 2018, 8, 122; doi:10.3390/met8020122 www.mdpi.com/journal/metals Metals 2018, 8, 122 2 of 12 tensile stresses, the occurrence of mechanical twins increases with increasing strain, meaning that the work hardening increases due to the presence of mechanical twin boundaries (the dynamic Hall-Petch effect). Unfortunately, fatigue resistance of coarse-grained microstructures is lower than fine-grained steels [19]. A chemical composition of steel affects the SFE. Aluminum addition strongly increases SFE, whereas silicon causes an opposite effect. Park et al. [20] reported that, regardless of the SFE and/or the Al content, deformation was achieved by planar glide of dislocations before mechanical twinning occurred. The planar glide became more evident as the SFE increased by suppressing or delaying mechanical twinning. As the SFE is increased by Al addition, the critical stress for initiation of the mechanical twinning becomes higher [20]. It was also reported [21,22] that Al additions lead to a lower strain hardening rate and decrease the frequency of mechanical twins formation, resulting in a decrease in tensile strength. Allain et al. [23] noted that the addition of approximately 3 wt % Si to high-Mn TWIP steel has a remarkable effect on dislocation mobility by significantly reducing the activation volume and mean free path, affecting the yield strength and work-hardening rate. Pierce et al. [24] indicated that manganese addition significantly affects the SFE. Strength and ductility decrease when the SFE is higher than 39 mJ/m2 corresponding to a reduction in mechanical twinning. Usually, the strain hardening behavior is reported for the whole deformation extent. A very limited number of papers addressed the microstructure evolution and corresponding progressive work hardening response of high-manganese steels during an interrupted tensile test. Therefore, the goal of the current study is to link the microstructure—mechanical property relationships of the microalloyed high-Mn steel containing additions of Si and Al.

2. Materials and Methods The investigations were carried out on X4MnSiAlNbTi27-4-2 type steel, which belongs to the second generation of advanced high strength steels (AHSS). Table1 shows the chemical composition of the investigated material.

Table 1. Chemical composition of investigated steel in wt %.

C% Mn% Si% Al% S% P% Nb% Ti% N% O% 0.040 27.50 4.18 1.96 0.017 0.004 0.033 0.010 0.0028 0.0007

The investigated steel was melted in a vacuum furnace (VSB 50 Balzers, Asslar, Germany). After melting and casting, the ingot was hot-forged and roughly hot-rolled in a temperature range between 900 and 1200 ◦C to a thickness of ca. 4.5 mm. Then, the sheet was hot-rolled in three passes to a ﬁnal thickness of 2 mm (obtained at 850 ◦C). Next, the samples were cooled in water to room temperature. Two groups of samples were prepared (Figure1):

1. Thermomechanically processed; 2. Thermomechanically processed and annealed at 900 ◦C within 20 min. Metals 2018, 8, 122 3 of 12 Metals 2018, 1, x FOR PEER REVIEW 3 of 12

Figure 1. Parameters of thermomechanical processing to obtain (a) hot-rolled steel (Steel I) and (b) Figure 1. Parameters of thermomechanical processing to obtain hot-rolled steel (Steel I) and solution- solution-heat-treated steel (Steel II). heat-treated steel (Steel II).

The detailed conditions of the hot rolling are listed in Table 2. The detailed conditions of the hot rolling are listed in Table2. Table 2. Conditions of the thermomechanical rolling. Table 2. Conditions of the thermomechanical rolling. Deformation Sheet Thickness Sheet Thickness Absolute Relative Strain Pass TemperatureDeformation beforeSheet a ThicknessPass Sheetafter Thickness a Pass AbsoluteReduction RelativeStrain StrainRate NumberPass (°C)Temperature (mm)before a Pass after(mm) a Pass Reduction(mm) Strain(%) Rate(s−1) Number ◦ −1 1 1050 ( C) 4.5 (mm) (mm) 3.4 (mm) 1.1 (%)25 (s )8.3 2 1950 1050 3.4 4.5 2.5 3.4 1.1 0.9 2525 8.3 10 3 2850 950 2.5 3.4 2.0 2.5 0.9 0.5 2520 10 10 3 850 2.5 2.0 0.5 20 10

Room temperature uniaxial tensile tests were performed using 2 mm thick samples machined fromRoom the thermomechanically temperature uniaxial rolled tensile sheet tests in werethe rolling performed direction. using Tensile 2 mm tests thick were samples interrupted machined at fromdefined the strain thermomechanically values of 5%, 10%, rolled and sheet20% and in the at a rolling final rupture direction. at a Tensile strain rate tests of were 5 × 10 interrupted−3 s−1. The test at −3 −1 deﬁnedwas performed strain values to monitor of 5%, 10%, the andmicrostructure 20% and at a evol ﬁnalution rupture at atthe a straindifferent rate ofstrain 5 × 10levels.s Based. The teston wasresults performed of the tensile to monitor tests, the the microstructure yield stress evolution(YS), tensile at the strength different (UTS), strain and levels. uniform Based onand results total ofelongation the tensile were tests, determined. the yield stress Moreover, (YS), tensile the work strength hardening (UTS), rate and and uniform the work and total hardening elongation exponent were determined.n were calculated. Moreover, the work hardening rate and the work hardening exponent n were calculated. Polished with AlAl22O3 and etched using 5% Nital, samples were optically examined after thethe samplessamples werewere deformeddeformed toto predeterminedpredetermined strainstrain levels.levels. The specimens for opticaloptical observationobservation were takentaken fromfrom the the middle middle of of the the deformed deformed specimen specimen length, length, according according to the to tensile the tensile direction. direction. The optical The observationsoptical observations were performed were performed using a Zeissusing Axio a Zeiss Observer Axio Observer Z1m optical Z1m microscope optical microscope (Carl Zeiss (Carl AG, Jena,Zeiss Germany).AG, Jena, Detailed Germany). microstructure Detailed microstructure analysis was performed analysis usingwas aperformed scanning electron using microscopea scanning Zeisselectron SUPRA microscope 25 (Carl Zeiss Zeiss SUPRA AG, Jena, 25 (Carl Germany) Zeiss operatingAG, Jena, atGermany) 20 kV. operating at 20 kV.

3. Results and Discussion 3. Results and Discussion 3.1. Initial Material 3.1. Initial Material An optical micrograph of the thermomechanically processed sample in the initial state is An optical micrograph of the thermomechanically processed sample in the initial state is shown shown in Figure2a. The occurrence of annealing twins inside austenite grains elongated in the in Figure 2a. The occurrence of annealing twins inside austenite grains elongated in the direction of direction of hot rolling can be observed. The microstructure is relatively coarse-grained due hot rolling can be observed. The microstructure is relatively coarse-grained due to relatively low to relatively low deformation levels; the average grain size was estimated to be about 70 µm. deformation levels; the average grain size was estimated to be about 70 µm. Elongated sulfide Elongated sulﬁde inclusions were also observed. Figure2b shows the micrograph of a hot-rolled inclusions were also observed. Figure 2b shows the micrograph of a hot-rolled and subsequently solution-heat-treated specimen. The microstructure is characterized by a more fine-grained

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Metals 2018, 1, x FOR PEER REVIEW 4 of 12 and subsequently solution-heat-treated specimen. The microstructure is characterized by a more ﬁne-grainedmicrostructure microstructure due to a relatively due to low a relatively annealing low temperature. annealing temperature. The average The grain average size was grain estimated size was estimatedto be about to 37 be µm. about 37 µm.

Figure 2. Austenitic microstructures of (a) thermomechanically rolled steel and (b) Figure 2. Austenitic microstructures of (a) thermomechanically rolled steel and (b) solution-heat-treated steel. solution-heat-treated steel.

3.2. Microstructure Changes during Interrupted Tensile TestsTests Steel samples were subjected to interrupted tensil tensilee tests at different strain levels of 5%, 10%, and 20% and at thethe pointpoint ofof rupture.rupture. Microstructure evolution was observed at each deformation step. Figure Figure 33a,ba,b showshow thatthat thethe samplessamples deformeddeformed upup toto aa 5%5%strain. strain. SlipSlip lineslines andand bandsbands werewere observed bothboth inin the the microstructure microstructure of thermomechanicallyof thermomechanically processed processed and solution-heat-treatedand solution-heat-treated steels. Nosteels. deformation No deformation twins twins were observedwere observed (Figure (Figure4a,b). 4a,b). Dislocation Dislocation activity activity always always occurs occurs before before the twinningthe twinning mechanism mechanism [20,25 [20,25].]. The twinningThe twinning deformation deformation will start will whenstart when the stress the concentration,stress concentration, which iswhich provided is provided by piled-up by piled-up dislocations, dislocations, surpasses surpasses the critical the stress critical for stress deformation for deformation twins [21]. twins The stress [21]. neededThe stress for needed twins to for nucleate twins to is farnucleate more thanis far that more for th twinsan that to growfor twins [25]. to Park grow et al.[25]. [20 Park] reported et al. that[20] inreported Fe-22Mn-0.6C that in Fe-22Mn-0.6C solution-heat-treated solution-heat-treate steel, fromd the steel, initial from stage the ofinitial deformation stage of deformation (5%), mechanical (5%), twinningmechanical occurred twinning and occurred its population and its continuously population continuously increased with increased increasing with stain. increasing They indicated stain. They that planarindicated glide that dominated planar glide deformation dominated before deformation the activation before of the mechanical activation twinning of mechanical for all twinning investigated for steelsall investigated regardless steels of the regardless SFE. Mazurkiewicz of the SFE. Mazu [26] observedrkiewicz [26] the dominantobserved the character dominant of slipcharacter lines inof X8MnSiAlNbTi25-1-3slip lines in X8MnSiAlNbTi25-1-3 steel deformed steel updeformed to 10%. up The to 10%. mechanical The mechanical twins were twins observed were observed at higher at deformationhigher deformation levels—above levels—above 20%. 20%. We alsoWe also observed observ thated that the the dislocation dislocation glide glide occurs occurs at at aa lower deformation level, before twinning initiation.

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Figure 3. Microstructures of the investigated steel deformed at 5% (a and b), 10% (c and d) and 20% Figure 3. Microstructures of the investigated steel deformed at 5% (a,b), 10% (c,d) and 20% (e,f) strain (e and f) strain levels and at the point of rupture (g and h). levels and at the point of rupture (g,h).

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Figure 4. Scanning electron microscope (SEM) images of the investigated steel deformed at 5% (a and Figure 4. Scanning electron microscope (SEM) images of the investigated steel deformed at 5% (a,b) b) and 20% (c and d) strain levels and at rupture (e and f). and 20% (c,d) strain levels and at rupture (e,f). The microstructures of the investigated steels strained to 10% (Figure 3c,d) are characterized by The microstructures of the investigated steels strained to 10% (Figure3c,d) are characterized by austenite grains elongated according to the stretching direction. The grains are more elongated in austenite grains elongated according to the stretching direction. The grains are more elongated in the the case of hot-rolled steel. The presence of numerous crossed deformation lines and bands was case of hot-rolled steel. The presence of numerous crossed deformation lines and bands was revealed revealed in both hot-rolled and solution-heat-treated steel. Moreover, single mechanical twins were in both hot-rolled and solution-heat-treated steel. Moreover, single mechanical twins were observed observed in the hot-rolled specimen. Park et al. [20] reported that, in solution-heat-treated inFe-0.6C-22Mn-3Al the hot-rolled specimen. steel, Parkthe mechanical et al. [20] reported twins that,were inobserved solution-heat-treated at 10% strain, Fe-0.6C-22Mn-3Al whereas in steel,Fe-0.6C-22Mn-6Al the mechanical steel twins twins were appeared observed at athigh 10% strains strain, over whereas 40%. inThis Fe-0.6C-22Mn-6Al indicates that Al steeladdition twins appearedstronglyat increases high strains the overSFE, 40%.resulting This indicatesin the retardation that Al addition of twinning strongly occurrence. increases Peng the SFE, et al. resulting [21] inobserved the retardation in Fe-1.3C-20Mn of twinning TWIP occurrence. steel that Peng a etlarge al. [21number] observed of mechanical in Fe-1.3C-20Mn twins at TWIP 11% steelstrain. that a largeHowever, number mechanical of mechanical twins were twins not at observed 11% strain. in the However, Fe-1.3C-20Mn-3Cu mechanical TWIP twins steel. were The not twinning observed inmechanism the Fe-1.3C-20Mn-3Cu occurred in this TWIP case steel. at 20% The strain. twinning This was mechanism due to the occurred higher SFE in this of the case steel at 20% with strain. Cu Thisaddition. was due The to occurrence the higher of SFE twinning of the steelis strongly with Cu related addition. to the The SFE, occurrence which depends of twinning on the chemical is strongly composition of steel. Aluminum concentrations in the steel investigated in the present work are

Metals 2018, 8, 122 7 of 12 related to the SFE, which depends on the chemical composition of steel. Aluminum concentrations in the steel investigated in the present work are close to those reported by Park et al. [20]. Hence, the strain level needed to activate the twinning mechanism was similar: 10% and 11%, respectively. Ghasri-Khouzaen et al. [27] investigated the relationship between strengthening mechanisms and carbon content in Fe-0-0.6C-22Mn steels. They found that the deformation products were mechanical twins for the 0.6C alloy and ε-martensite for the 0C and 0.2C alloys. For the 0.4C alloy, both mechanical twins and ε-martensite were identiﬁed during deformation. An increase in the deformation level to 20% results in an increase in the amount of deformation twins in the thermomechanically rolled steel (Figures3e and4c). Moreover, deformation twins occur in the solution-heat-treated specimen (Figures3f and4d). The amount of deformation twins is higher in the hot-rolled steel when compared to the solution-heat-treated sample. Crossed deformation lines, bands, and twins were also observed. Twin population continuously increased with increasing strain from 10% to 20%. Deformation twins are nucleated preferentially at grain boundaries, which act as sources for stacking faults. Most grains develop one main twinning system in addition to the dislocation glide. In TWIP steels, long primary twins often occur and span a grain from one grain boundary to another. Secondary twins may be nucleated within the twins of the primary system [28]. Wang et al. [29] observed the microstructure evolution in 316 stainless steel in a range of deformation levels: 10–30%. Deformation features evolved from planar dislocations to a single system twinning, then to multi-system twinning and intersections among twins, with increasing strain. Ding et al. [25] reported that, for deformation strains between 0.35 and 0.45, crossed twins occurred. Moreover, martensite transformation ε→α occurred at the place of these crossed twins, and martensite grew along the thickness of the twinned regions. No martensites ε or α’ were observed in the investigated microstructure (Figures3a–h and4a–h) . It was confirmed in our earlier research using X-ray diffraction [8]. Such behavior is typical for steels characterized by an SFE >20 mJ/m2, resulting from the chemical composition. Samples deformed to rupture are characterized by numerous crossed slip lines and bands (Figure3g,h). The amount of crossed slip lines, bands, and deformation twins is higher than in the samples deformed to smaller deformation levels (Figure4e,f). Intensively elongated austenitic grains are visible in the case of the thermomechanically rolled sample (Figure3g). The grain size of this austenitic steel has a great effect on the formation and morphology of deformation twins. With the increase in grain size, deformation twins become easier to form. Yuan et al. [17] reported that deformed tensile samples with a grain size of 2.2 µm showed no deformation twins. In contrast, in samples with a grain size of 5.4–28.7 µm, deformation twins were easily found. Dini et al. [18] proved that in TWIP steels the onset of mechanical twinning shifts to a lower strain as grain size increases. In our case, for the solution-heat-treated steel, average grain size was estimated to be about 37 µm (Figure2b), whereas the average grain size of hot-rolled steel was about 70 µm (Figure2a). As mentioned before, a coarse-grained microstructure enhanced the occurrence of mechanical twins, so mechanical twins nucleated at lower strain levels in the case of the hot-rolled steel.

3.3. Strain Hardening Behavior The interrupted tensile tests were performed at strain levels of 5%, 10%, 20% and at rupture in order to identify deformation-induced strengthening mechanisms. Mechanical properties of the hot-rolled steel were compared to the solution-heat-treated steel. The results of the tensile tests are shown in Table3. The thermomechanically processed steel shows higher UTS and TE values (808 MPa and 44.8%, respectively) when compared to the solution-heat-treated specimen (727 MPa and 29.5%, respectively). The hot-rolled steel shows a YS/UTS ratio ca. 0.68, which indicates its relatively high strengthening potential. The solution-heat-treated specimen shows a lower YS/UTS ratio (0.59) due to the lower yield stress (it is lower by above 100 MPa). However, the more ﬁne-grained microstructure results in its lower ultimate tensile strength because of the hampering effect of the smaller grain size on twin formation. The hot-rolled steel is also characterized by higher uniform elongation (UE) due to the Metals 2018, 8, 122 8 of 12

Metals 2018, 1, x FOR PEER REVIEW 8 of 12 more coarse-grained microstructure. The same effect was observed by Yuan et al. [17]. Yield stress (YS) valuesGenerally, were the determined thermomechanically at each deformation rolled steel level sh (Tableows 3a). higher Generally, YS than the thermomechanically the solution-heat-treated rolled steelsteel shows due to a higherthe higher YS than dislocation the solution-heat-treated density after hot steelrolling. due The to the microstructure higher dislocation of the density steel after after hotsolution rolling. heat The treatment microstructure contains of a the smaller steel afterdislocation solution density heat treatment and finally contains a smaller a smaller YS value. dislocation density and ﬁnally a smaller YS value. Table 3. The mechanical characteristics of the investigated steels. Table 3. Steel Type UTS (MPa)The mechanical characteristics YS (MPa) of the investigated YS/UTS steels. UE (%) TE (%) Average DeformationSteel Type level UTS (MPa) - 5% 10% YS 20% (MPa) Max. YS/UTS- UE- (%) TE (%)- value DeformationHot-rolled level -808 5%550 10% 530 20% 600 Max.550 Average557 value 0.68 - 26.7 - 44.8 - Hot-rolled 808 550 530 600 550 557 0.68 26.7 44.8 Solution-heat-treated 727 433 440 420 428 430 0.59 23.7 29.5 Solution-heat-treated 727 433 440 420 428 430 0.59 23.7 29.5

Curves showing true stress vs. true strain, work hardening exponent vs. true strain, and strain hardeningCurves rate showing vs. true true strain stress are vs. presented true strain, in Figu workres hardening 5–7, respectively. exponent Each vs. curve true strain, terminates and strain at a hardeningmaximum rateuniform vs. true elonga straintion. are Figure presented 5 shows in Figures the true5–7 ,stress respectively. vs. true Eachstrain curve curves terminates obtained at at a maximumdifferent strain uniform levels. elongation. Both steels Figure showed5 shows continuous the true stress and vs. gradual true strain strain curves hardening, obtained typical at different for strainhigh-manganese levels. Both steelsTWIP showedsteels continuous[30–32]. Necking and gradual strains strain (corresponding hardening, typical to uniform for high-manganese elongation) TWIPobtained steels for [30 –the32 ].hot-rolled Necking strainsand solution-heat-treated (corresponding to uniform steels elongation) were 0.24 obtained and 0.21, for therespectively hot-rolled and(Figure solution-heat-treated 5d). steels were 0.24 and 0.21, respectively (Figure5d).

FigureFigure 5.5. TrueTrue stress–truestress–true strainstrain curves of thermomech thermomechanicallyanically rolled rolled and and solution-heat-treated solution-heat-treated steels steels atat different different deformationdeformation levelslevels ofof ((aa)) 5%,(5%,(bb)) 10%,10%, and (c) 20% and at ( (d)) rupture. rupture.

In the case of the hot-rolled steel, the n exponent increases gradually to a maximum value of In the case of the hot-rolled steel, the n exponent increases gradually to a maximum value of 0.24 corresponding to the true strain ~0.2 (Figure 6). Then, it decreases up to the point of necking. A 0.24 corresponding to the true strain ~0.2 (Figure6). Then, it decreases up to the point of necking. slope of the n vs. true strain curve is lower in the case of the hot-rolled steel. This means that the steel A slope of the n vs. true strain curve is lower in the case of the hot-rolled steel. This means that is strengthened in a gradual way. Finally, it is reflected in a higher uniform elongation. The thesolution-heat-treated steel is strengthened steel in is a characterized gradual way. by Finally, a higher it israte reflected of strain in hardening. a higher uniform That is why elongation. the n Theexponent solution-heat-treated increases intensively steel isup characterized to 0.33 at the by true a higher strain rate ~0.17; of strainthen, the hardening. n exponent That rapidly is why thedecreases n exponent up to increases the point intensively of necking up (Figure to 0.33 6), at wh theich true is strainrelated ~0.17; to the then, difficult the n activation exponent rapidlyof the decreasesenhanced uptwinning to the in point a fine-grained of necking steel. (Figure 6), which is related to the difficult activation of the enhanced twinning in a fine-grained steel.

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Figure 6. WorkWork hardening exponent as a function of true strain.

Microstructural changes are reflected in the work hardening behavior of of the the investigated investigated steels. steels. A progressiveprogressive character character of theof twinningthe twinning mechanism mechan allowsism forallows large for uniform large elongation uniform inelongation combination in withcombination the high with tensile the strength. high tensile The hot-rolledstrength. steelThe showedhot-rolled a greatersteel showed tendency a togreater twinning tendency than theto solution-heat-treatedtwinning than the solution-heat-treated steel, so higher uniform steel, elongation so higher and betteruniform strength elongation properties and werebetter obtained strength in thisproperties case (Table were3). obtained Xie et al. in [ 31 this] observed case (Table that the3). Xi interactionse et al. [31] between observed dislocations that the interactions and twin boundaries between introduceddislocations by and prior twin cold-drawing boundaries of introduced 316 austenitic by stainlessprior cold-drawing steel may benefit of 316 the austenitic deformation stainless uniformity steel andmay delaybenefit the the initiation deformation of fatigue uniformity cracking. and Liu delay et al. the [32 ]initiation reported of that fatigue the work cracking. hardening Liu et behavior al. [32] dependsreported alsothat onthe strainwork state.hardening They behavior reported depends that the strengthalso on strain and failure state. strainThey reported in tension that of the Fe-20Mn-1.2Cstrength and failure are higher strain than in those tension obtained of the for Fe-20Mn-1.2C the rolling and are shear higher states. than This those was obtained due to the for higher the twinningrolling and volume shear fractionstates. This in the was case due of to tension the higher stresses. twinning Under volume tensile deformation, fraction in the twinning case of tendstension to bestresses. initiated Under in grains tensile with deformation, the <111> orientation twinning in tends the tensile to be direction initiated [28 ].in grains with the <111> orientationAs can in be the seen tensile in Figure direction7, the [28]. d σ/d ε values decrease rapidly at the initial range of deformation; then,As the can work be seen hardening in Figure rate 7, the stabilizes dσ/dε values at a constant decrease levelrapidly due at tothe the initial combined range of occurrence deformation; of deformationthen, the work twinning hardening and deformationrate stabilizes slip. at a The constant strain hardeninglevel due to rate the of combined the thermomechanically occurrence of rolleddeformation and solution-heat-treated twinning and deformation steels rapidly slip. decreasedThe strain up hardening to a true strain rate of ~0.04the thermomechanically and then decreased monotonicallyrolled and solution-heat-treated down to ~1500 MPa. steels Liu et rapidly al. [32] reporteddecreased that up effects to a oftrue twinning strain onof strain~0.04 hardeningand then becomedecreased dominant monotonically when thedown twinning to ~1500 volume MPa. Liu fraction et al. reaches [32] reported about 5%that and effects evidently of twinning increases on withstrain increasing hardening strain. become Therefore, dominant the relativewhen the stability twinning of the volume strain hardeningfraction reaches rate above about true 5% strain and ~0.04evidently is probably increases related with toincreasing the increase strain. in twin Therefor volumee, the fraction. relative As stability the deformation of the strain level hardening increases, therate dislocationabove true density strain ~0.04 in the is twinning probably areas related becomes to the higher increase [20], in which twin canvolume promote fraction. the strength As the (Figuredeformation5), the level strain increases, hardening the dislocation exponent (Figuredensity6 in), andthe twinning the strain areas hardening becomes rate higher (Figure [20],7 )which of thecan investigatedpromote the materials. strength (Figure It was reported5), the strain [19,28 ,hardening33] that the exponent strain hardening (Figure 6), of high-Mnand the TWIPstrain steelshardening is controlled rate (Figure via 7) the of dynamic the investigated Hall-Petch materials. mechanism. It was Thereported formation [19,28,33] of twins that graduallythe strain reduceshardening the of dislocation high-Mn meanTWIP freesteels path, is controlle increasingd via fragmentation the dynamic of theHall-Petch grains viamechanism. twin lamellae The andformation finally of contributing twins gradually to enhanced reduces the dislocation dislocation storage mean and free thepath, subsequent increasing work fragmentation hardening of rate. the Bouazizgrains via et twin al. [33 lamellae] concluded and fina that,lly of contributing all possible deformation to enhanced mechanisms dislocation storage in TWIP and steels, the deformation subsequent twinningwork hardening had the rate. most Bouaziz beneficial et effectal. [33] on concluded strain hardening. that, of Theall possible shape of deformation the curves is mechanisms very similar upin toTWIP a true steels, strain deformation of ~0.08. The twinning work strengthening had the most rate bene of theficial solution-heat-treated effect on strain hardening. specimen The was shape slightly of higherthe curves over is a strainvery rangesimilar of up 0.1–0.2. to a Antrue enhanced strain of strengthening ~0.08. The work leads strengthening to a faster blocking rate of of the dislocation–twinningsolution-heat-treated interactions,specimen was leading slightly to faster higher neck over initiation a strain and range resulting of 0.1–0.2. in lower An uniform enhanced and totalstrengthening elongations. leads A moreto a gradualfaster blocking change inof thethe workdislocation–twinning hardening rate is moreinteractions, beneficial leading for obtaining to faster a betterneck initiation balance ofand ductility resulting and in strength lower uniform for the thermomechanically and total elongations. processed A more steel.gradual change in the work hardening rate is more beneficial for obtaining a better balance of ductility and strength for the thermomechanically processed steel.

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FigureFigure 7.7. Work-hardeningWork-hardeningrate rateas asa afunction functionof of true true strain. strain.

4.4. ConclusionsConclusions TheThe relationshipsrelationships betweenbetween thethe strainstrain hardeninghardening behaviorbehavior andand correspondingcorresponding microstructuremicrostructure evolutionevolution during during interrupted interrupted tensile teststensile for thermomechanicallytests for thermomechanically processed and solution-heat-treated processed and high-manganesesolution-heat-treated steels containinghigh-manganese increased steels amounts containing of Si and increased Al were amount determined.s of ItSi was and found Al were that thedetermined. progress of It workwas found strengthening that the is progress controlled of bywork the strengthening combination of is dislocation controlled slip by andthe deformationcombination twinning.of dislocation The dislocationslip and deformat slip occurredion twinning. before twinningThe dislocation at a lower slip strainoccurred level. before The twinningtwinning wasat a activatedlower strain at strainlevel. levelsThe twinning ca. 10% was and activated 20% correspondingly at strain levels for ca. the 10 and thermomechanically 20% correspondingly rolled for and the solution-heat-treatedthermomechanically steels.rolled Theand amountsolution-heat-treate of deformationd steels. twins The and amount slip bands of increaseddeformation gradually twins and up toslip the bands point ofincreased necking gradually for the hot-rolled up to the steel, point whereas of necking the increase for the in deformationhot-rolled steel, effects whereas was faster the forincrease the solution-heat-treated in deformation effects steel was with faster a smaller for the initial solution-heat-treated grain size. The twinning steel waswith enhanced a smaller for initial the coarse-grainedgrain size. The microstructure twinning was because enhanced the twins for the nucleated coarse-grained at a lower mi straincrostructure level for thebecause hot-rolled the twins steel, bringingnucleated a betterat a lower product strain of strength level for and the ductility. hot-rolled Further steel, researchbringing is a needed better product to complete of strength microscopic and observationsductility. Further by Transmission research is needed Electron to complete Microscopy microscopic and Electron observations Back-Scattered by Transmission Diffraction Electron results. TheMicroscopy strain hardening and Electron rate of bothBack-Sca investigatedttered Diffraction steels rapidly results. decreased The upstrain to a truehardening strain of rate ca. 0.04of both and theninvestigated decreased steels monotonically rapidly decreased due to the up complex to a true interaction strain of ca. of 0.04 deformation and then twinningdecreased and monotonically dislocation slip.due Theto the more complex gradual interaction progress of of the deformation strengthening twinning of the thermomechanically and dislocation slip. processed The more steel gradual leads toprogress higher mechanicalof the strengthening properties. of the thermomechanically processed steel leads to higher mechanical properties. Acknowledgments: This work was financially supported with statutory funds of the Faculty of Mechanical Engineering of Silesian University of Technology in 2018. Acknowledgments: This work was financially supported with statutory funds of the Faculty of Mechanical AuthorEngineering Contributions: of Silesian A.G.University conceived, of Technology designed, in and 2018. performed the experiments; A.K. performed the work hardening calculations, analyzed the corresponding results, and wrote the paper; B.G. linked the relationships betweenAuthor Contributions: microstructure andA.G. work conceived, hardening designed, behavior, and analyzed performed the the data, experiments; and reviewed A.K. the performed paper. the work Conflictshardening of calculations, Interest: The analyzed authors declarethe corresponding no conflict ofresult interest.s, and wrote the paper; B.G. linked the relationships between microstructure and work hardening behavior, analyzed the data, and reviewed the paper.

ReferencesConflicts of Interest: The authors declare no conflict of interest.

1. Mesquita, R.A.; Schneider, R.; Steineder, K.; Samek, L.; Arenholz, E. On the austenite stability of a new References quality of twinning induced plasticity steel, exploring new ranges of Mn and C. Metall. Mater. Trans. A 2013, 1. 44Mesquita,, 4015–4019. R.A.; [CrossRef Schneider,] R.; Steineder, K.; Samek, L.; Arenholz, E. On the austenite stability of a new 2. Kuc,quality D.; of Hadasik, twinning E.; induced Niewielski, plasticity G.; Schindler, steel, exploring I.; Mazancova, new ranges E.; Rusz,of Mn S.; and Kawulok, C. Metall. P. StructuralMater. Trans. and A mechanical2013, 44, 4015–4019. properties of laboratory rolled steels high-alloyed with manganese and aluminium. Arch. Civ. 2. Mech.Kuc, D.; Eng. Hadasik,2012, 12 ,E.; 312–317. Niewielski, [CrossRef G.; Schindler,] I.; Mazancova, E.; Rusz, S.; Kawulok, P. Structural and mechanical properties of laboratory rolled steels high-alloyed with manganese and aluminium. Arch. Civ. Mech. Eng. 2012, 12, 312–317.

Metals 2018, 8, 122 11 of 12

3. Grajcar, A.; Kuziak, R. Softening kinetics in Nb-microalloyed TRIP steels with increased Mn content. Adv. Mater. Res. 2011, 314–316, 119–122. [CrossRef] 4. Dobrzanski, L.A.; Borek, W. Hot-working behaviour of advanced high-manganese C-Mn-Si-Al steels. Mater. Sci. Forum 2010, 654–656, 266–269. [CrossRef] 5. Jablonska, M.B. Mechanical properties and fractographic analysis of high manganese steels after dynamic deformation tests. Arch. Metall. Mater. 2014, 59, 1193–1197. [CrossRef] 6. Steineder, K.; Schneider, R.; Krizan, D.; Beal, C.; Sommitsch, C. Investigation on the microstructural evolution in a medium-Mn steel (X10Mn5) after intercritical annealing. HTM J. Heat Treat. Mater. 2015, 70, 19–25. [CrossRef] 7. Kowalska, J.; Ratuszek, W.; Witkowska, M.; Zielińska-Lipiec,A.; Kowalski, M. Microstructure and texture evolution during cold-rolling in the Fe-23Mn-3Si-3Al alloy. Arch. Metall. Mater. 2015, 60, 1789–1794. [CrossRef] 8. Grajcar, A.; Kciuk, M.; Topolska, S.; Plachcinska, A. Microstructure and corrosion behavior of hot-deformed and cold-strained high-Mn steels. J. Mater. Eng. Perform. 2016, 25, 2245–2254. [CrossRef] 9. Kucerova, L.; Opatova, K.; Kana, J.; Jirkova, H. High versatility of niobium and alloyed AHSS. Arch. Metall. Mater. 2017, 62, 1485–1491. [CrossRef] 10. Tehovnik, F.; Steiner-Petrovic, D.; Vode, F.; Burja, J. Influence of molybdenum on the hot-tensile properties of austenitic stainless steels. Mater. Tehnol. 2012, 6, 649–655. 11. Radwanski, K. Application of FEG-SEM and EBSD methods for the analysis of the restoration pocesses occurring during continuous annealing of Dual-Phase steel strips. Steel Res. Int. 2015, 86, 1379–1390. [CrossRef] 12. Mazancova, E.; Mazanec, K. The stacking fault energy evaluation of the TWIP and TRIPLEX alloys. Kovove Mater. 2009, 47, 353–358. 13. Grajcar, A.; Plachcinska, A.; Topolska, S.; Kciuk, M. Effect of thermomechanical treatment on the corrosion behavior of Si and Al-containing high-Mn austenitic steel with Nb and Ti microaddition. Mater. Tehnol. 2015, 49, 889–894. [CrossRef] 14. Gorka, J. Microstructure and properties of the high-temperature (HAZ) of thermo-mechanically treated S700MC high-yield-strength steel. Mater. Tehnol. 2016, 50, 617–621. [CrossRef] 15. Kurc-Lisiecka, A.; Lisiecki, A. Laser welding of the new grade of advanced high-strength steel Domex 960. Mater. Tehnol. 2017, 51, 199–204. [CrossRef] 16. Gutierrez-Urrutia, I.; Raabe, D. Grain size effect on strain hardening in twinning-induced plasticity steels. Scr. Mater. 2012, 66, 992–996. [CrossRef] 17. Yuan, X.; Chen, L.; Zhao, Y.; Di, H.; Zhu, F. Dependence of grain size on mechanical properties and microstructures of high manganese austenitic steel. Proc. Eng. 2014, 81, 143–148. [CrossRef] 18. Dini, G.; Najafizadeh, A.; Ueji, R.; Monir-Vaghefi, S.M. Tensile deformation behavior of high manganese austenitic steel: The role of grain size. Mater. Des. 2010, 31, 3395–3402. [CrossRef] 19. Ma, P.; Qian, L.; Meng, J.; Liu, S.; Zhang, F. Fatigue crack growth behavior of a coarse- and a fine-grained high manganese austenitic twin-induced plasticity steel. Mater. Sci. Eng. A 2014, 605, 160–166. [CrossRef] 20. Park, K.T.; Jin, K.G.; Han, S.H.; Hwang, S.W.; Choi, K.; Lee, C.S. Stacking fault energy and plastic deformation of fully austenitic high manganese steels: Effect of Al addition. Mater. Sci. Eng. A 2010, 527, 3651–3661. [CrossRef] 21. Peng, X.; Zhu, D.; Hu, Z.; Yi, W.; Liu, H.; Wang, M. Stacking fault energy and tensile deformation behavior of high-carbon twinning-induced plasticity steels: Effect of Cu addition. Mater. Des. 2013, 45, 518–523. [CrossRef] 22. Shen, Y.S.; Qiu, C.H.; Wang, L.; Sun, X.; Zhao, X.M.; Zuo, L. Effects of cold rolling on microstructure and mechanical properties of Fe-30Mn-3Si-4Al-0.093C TWIP steel. Mater. Sci. Eng. A 2013, 561, 329–337. [CrossRef] 23. Allain, S.; Bouaziz, O.; Chateau, J.P. Thermally activated dislocation dynamics in austenitic FeMnC steels at low homologous temperature. Scr. Mater. 2010, 62, 500–503. [CrossRef] 24. Pierce, D.T.; Jiménez, J.A.; Bentley, J.; Raabe, D.; Wittig, J.E. The influence of stacking fault energy on the microstructural and strain hardening evolution of Fe–Mn–Al–Si steels during tensile deformation. Acta Mater. 2015, 100, 178–190. [CrossRef] Metals 2018, 8, 122 12 of 12

25. Ding, H.; Ding, H.; Song, D.; Tang, Z.; Yang, P. Strain hardening behavior of TRIP/TWIP steel with 18.8% Mn. Mater. Sci. Eng. A 2011, 528, 868–873. [CrossRef] 26. Mazurkiewicz, J. Structure and Properties of High Manganese MnSiAlNbTi-25-1-3 Steels Increased Store of Cold Plastic Deformation Energy; International OCSCO World Press, Open Access Library: Gliwice, Poland, 2013. 27. Ghasri-Khouzani, M.; McDermid, J.R. Effect of carbon content on the mechanical properties and microstructural evolution of Fe-22Mn-C steels. Mater. Sci. Eng. A 2015, 621, 118–127. [CrossRef] 28. De Cooman, B.C.; Estrin, Y.; Kim, S.K. Twinning-induced plasticity (TWIP) steels. Acta Mater. 2018, 142, 283–362. [CrossRef] 29. Wang, S.J.; Jozaghi, T.; Karaman, I.; Arroyave, R.; Chumlyakov, Y.I. Hierarchical evolution and thermal stability of microstructure with deformation twins in 316 stainless steel. Mater. Sci. Eng. A 2017, 694, 121–131. [CrossRef] 30. Jiménez, J.A.; Frommeyer, G. Analysis of the microstructure evolution during tensile testing at room temperature of high-manganese austenitic steel. Mater. Charact. 2010, 61, 221–226. [CrossRef] 31. Xie, X.; Ninga, D.; Sun, J. Strain-controlled fatigue behavior of cold-drawn type 316 austenitic stainless steel at room temperature. Mater. Charact. 2016, 120, 195–202. [CrossRef] 32. Liu, F.; Dan, W.J.; Zhang, W.G. The effects of stress state on the strain hardening behaviors of TWIP steel. J. Mater. Eng. Perform. 2017, 26, 2721–2728. [CrossRef] 33. Bouaziz, O.; Guelton, N. Modelling of TWIP effect on work-hardening. Mater. Sci. Eng. A 2001, 319, 246–249. [CrossRef]

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