The mechanical properties of medium to high-Mn austenitic TWIP/TRIP steels subjected to warm to hot working were studied. The steel samples were processed by plate rolling at various temperatures ranging from 500 ◦C to 1150 ◦C. The deformation microstructures depend significantly on the rolling temperature. The discontinuous dynamic recrystallization readily developed during hot rolling at temperatures of T ≥ 800 ◦C, whereas the work hardened microstructures with high dislocation densities evolved by warm rolling at temperatures below 800 ◦

Belyakov, A.

Kaibyshev, R.

Torganchuk, V.

English

DSpace at Belgorod State University

Vol. 134 (2018) ACTA PHYSICA POLONICA A No. 3Proceedings of the International Symposium on Physics of Materials (ISPMA 14), September 10–15, 2017, PragueDeformation Mechanisms Operating in TWIP/TRIP SteelsProcessed by Warm to Hot WorkingV. Torganchuk, A. Belyakov∗ and R. KaibyshevBelgorod State University, Pobeda 85, Belgorod, 308015, RussiaThe mechanical properties of medium to high-Mn austenitic TWIP/TRIP steels subjected to warm to hotworking were studied. The steel samples were processed by plate rolling at various temperatures ranging from 500 ◦Cto 1150 ◦C. The deformation microstructures depend significantly on the rolling temperature. The discontinuousdynamic recrystallization readily developed during hot rolling at temperatures of T ≥ 800 ◦C, whereas the workhardened microstructures with high dislocation densities evolved by warm rolling at temperatures below 800 ◦C.The warm to hot rolling strengthened the steels remarkably. The ultimate tensile strength in the range of 750–1250 MPa and the corresponding yield strength of 300–900 MPa could be obtained by processing at designatedtemperature.DOI: 10.12693/APhysPolA.134.640PACS/topics: 81.40.Lm1. IntroductionHigh-manganese austenitic steels with twinning-induced plasticity (TWIP) or transformation-inducedplasticity (TRIP) effects have attracted a great inter-est of materials scientists. Such steels exhibit a benefi-cial combination of mechanical properties. The productof tensile strength with total elongation of TWIP/TRIPsteels may exceed 50000 MPa×%, which is significantlylarger than that of other structural steels and alloys [1].The physical mechanisms responsible for the outstandingstrength-ductility combination have been the subject ofnumerous studies. It is generally agreed that the excel-lent combination of strength and ductility results frompronounced strain hardening caused by mechanical twin-ning in steels with stacking fault energy (SFE) rangingfrom 18 to 40 mJ/m2 or martensitic transformation insteels with lower SFE [2]. The grain subdivision by thintwins or martensite plates with thickness of 10–40 nm [3]decreases significantly the effective grain size and hardensthe steel in accordance with the Hall–Petch relationship.Mechanical properties of metallic materials includinghigh-Mn TWIP/TRIP steels are greatly affected by theirmicrostructures, which in turn depend on the processingmethod/condition. The important structural parametersaffecting the mechanical properties of steels and alloysare the grain size and the dislocation density [4]. Re-garding high-Mn steels, both a decrease in the grain sizeby static recrystallization and an increase in the dislo-cation density by cold to warm working result in sig-nificant strengthening, although plasticity degrades re-markably [5, 6]. An appropriate strength-ductility com-bination in the steels can be obtained owing to desiredstructural parameters, which are directly controlled by∗corresponding author; e-mail: belyakov@bsu.edu.ruthe conditions of warm to hot working [7]. However, theeffect of thermo-mechanical processing conditions on thedeveloping microstructures and mechanical behavior ofhigh-Mn TWIP/TRIP steels has not been studied in suf-ficient detail. The aim of the present paper, therefore, isto report our current results on the microstructure evo-lution during warm to hot rolling and mechanical prop-erties of advanced high-Mn steels.2. ExperimentalThe starting materials, Fe–12Mn–0.6C–1.5Al, Fe–18Mn–0.4C and Fe–18Mn–0.6C (all in wt%), were hotrolled at 1150 ◦C. Subsequently, the steel plates wereprocessed by rolling at various temperatures from 500 ◦Cto 1100 ◦C to a total rolling reduction of 60%. Aftereach rolling pass with about 10% reduction, the sam-ples were re-heated to the designated rolling tempera-ture. The structural observations were carried out us-ing a JEOL JEM 2100 transmission electron microscope(TEM) and a Quanta 600 scanning electron microscope(SEM) equipped with an electron back scattering diffrac-tion (EBSD) pattern analyzer incorporating an orienta-tion imaging microscopy (OIM) system. The mean grainsize was measured using the linear intercept method onthe OIM images counting the distance between high-angle boundaries along the normal direction (ND). Thetensile tests were performed using an INSTRON 5882testing machine at ambient temperature under a strainrate of 10−3 s−1 with dog bone specimens cut along therolling direction (RD).3. Results and discussionTypical deformation microstructures evolved in thehigh-Mn steels during warm to hot rolling are shownin Fig. 1. Two types of the microstructures are de-veloped. The uniform microstructures consisting of al-most equiaxed grains with numerous annealing twins(640)Deformation Mechanisms Operating in TWIP/TRIP Steels Processed by Warm to Hot Working 641Fig. 1. Typical OIM micrographs for deformation mi-crostructures evolved in the Fe–12Mn–0.6C–1.5Al (a–c) and Fe–18Mn–0.6C (d–f) steels during hot to warmrolling at 1000 ◦C (a, d), 800 ◦C (b, e), and 600 ◦C (c, f).The colors indicate the normal direction (vertical in allpictures).Fig. 2. Effect of the rolling temperature on the trans-verse grain size in the high-Mn steels.Fig. 3. Stress-elongation curves of the high-Mn steelssubjected to rolling at the indicated temperatures.evolve in the steels during hot rolling at temperatures ofT ≥ 800 ◦C. These microstructures result from discontin-uous dynamic recrystallization (DRX), which commonlytakes place in low SFE steels during hot working [8]. Adecrease in the deformation temperature should suppressdiscontinuous DRX. Indeed, the deformation microstruc-tures composed of flattened original grains evolve duringwarm rolling at T < 800 ◦C. The transverse grain sizedecreases from 50–80 to 5–10 µm with a decrease in therolling temperature from 1150 to 800 ◦C in the range ofhot working accompanied by discontinuous DRX (Fig. 2).Further decrease in the rolling temperature below 800 ◦Cdoes not lead to remarkable variation of the transversegrain size, which seems to be dependent on the originalgrain size and the total rolling reduction.642 V. Torganchuk, A. Belyakov, R. KaibyshevFig. 4. Effect of the rolling temperature on the yieldstrength (σ0.2), the UTS, and total elongation (δ) of thehigh-Mn steels.Series of tensile stress-elongation curves for the warmto hot rolled steels are shown in Fig. 3 and the mechani-cal properties, i.e., yield strength (σ0.2), ultimate tensilestrength (UTS), and total elongation (δ), are representedin Fig. 4. Generally, a decrease in the rolling temperatureresults in an increase in strength and a decrease in plas-ticity. It should be noted that the 18%Mn steels exhibithigher strength and elongation than the 12%Mn steel af-ter rolling at each temperature within the studied range.The temperature effect on the strength/ductility prop-erty of the rolled steels is more pronounced for the warmworking domain than for the hot working conditions, es-pecially, for the 18%Mn steels. The yield strength pro-Fig. 5. TEM images of microstructures after tensiletests at room temperature of the Fe–18Mn–0.4C (a) andFe–18Mn–0.6C (b) steels processed by rolling at 800 ◦C.The insets are dark field images showing deformationmartensite in (a) and twining in (b).gressively increases with a decrease in the rolling tem-perature, and the rate of strengthening accelerates as thetemperature decreases. Similarly, UTS slightly increaseswith decrease of the rolling temperature from 1150 to700 ◦C followed by substantial increase as the rolling tem-perature decreases to 500 ◦C, although the 12%Mn steelis more susceptible to strengthening as compared to the18%Mn steels at all the rolling temperatures. The totalelongations of the Fe–12Mn–0.6C–1.5Al and Fe–18Mn–0.6C steels almost linearly decrease with a decrease in therolling temperature. In contrast, the Fe–18Mn–0.4C steelexhibits bimodal temperature dependence for total elon-gation, which is about 60% after rolling at 700–1150 ◦Cand decreases to 30% with a decrease in the rolling tem-perature to 500 ◦C.The steels of Fe–12Mn–0.6C–1.5Al and Fe–18Mn–0.6Care characterized by monotonous dependences of the me-chanical properties on the rolling temperature irrespec-tive of the operating mechanisms of microstructure evo-lution in the range of 500–1150 ◦C. Namely, the yieldstrength gradually decreases while the plasticity increaseswith increase of the rolling temperature. However, theFe–18Mn–0.4C steel demonstrates an apparent satura-Deformation Mechanisms Operating in TWIP/TRIP Steels Processed by Warm to Hot Working 643tion for the total elongation as the rolling temperatureincreases above 700 ◦C, although the strength contin-uously decreases. Such a behavior can be associatedwith the deformation mechanisms operating during ten-sile tests. The Fe–18Mn–0.4C steel specimen experi-enced ε-martensitic transformation during tensile tests(Fig. 5a), whereas the tensile deformation of the Fe–18Mn–0.6C steel specimen was accompanied by defor-mation twinning (Fig. 5b). This difference in deforma-tion mechanisms results from the difference in SFE. Thesteel with lower carbon content has lower SFE [9] and,thus, involves strain-induced martensite upon cold defor-mation. In spite of similarities in structural parametersof deformation martensite and twinning, the latter pro-vides much larger elongation during tensile tests. TheFe–18Mn–0.4C steel subjected to warm rolling at 700 ◦Cexhibits the maximal plasticity, which can be supportedby strain-induced martensitic transformation. Hence,further increase in the rolling temperature does not im-prove plasticity irrespective of increase of the DRX grainsize and decrease of the dislocation density, although theyield strength certainly depends on the grain size anddislocation density.4. SummaryThe desired combination of strength and plasticity ofmedium to high-Mn austenitic TWIP/TRIP steels, suchas Fe–(12–18)%Mn–(0.4–0.6)%C, can be obtained bywarm to hot rolling under appropriate conditions. Theultimate tensile strength in the range of 750–1250 MPa,the yield strength of 300–900 MPa, and correspondingtotal elongation of 80–20% are achieved in the steelsafter warm to hot rolling at 500–1150 ◦C. The yieldstrength of the steels gradually increases with a decreasein the rolling temperature irrespective of the mechanismsof microstructure evolution under different deformationconditions, i.e., discontinuous DRX or work hardeningduring hot or warm rolling, respectively. On the otherhand, an increase in the total elongation by increasingthe rolling temperature can be limited by deformationmechanisms. High-Mn TWIP steels with moderate SFEare characterized by increase of ductility above 80% withincrease of the rolling temperature. In contrast, steelswith rather low SFE exhibit TRIP effect, which do notprovide total elongation above approximately 60%.AcknowledgmentsThe financial support received from the Ministryof Education and Science, Russia, under grant No.11.3719.2017/PCh is gratefully acknowledged.References[1] P.S. Kusakin, R.O. Kaibyshev, Rev. Adv. Mater. Sci.44, 326 (2016).[2] O. Bouaziz, S. Allain, C.P. Scott, P. Cugy, D. Barbier,Curr. Opin. Solid State Mater. Sci. 15, 141 (2011).[3] Z. Yanushkevich, A. Belyakov, C. Haase,D.A. Molodov, R. Kaibyshev, Mater. Sci. Eng. A651, 763 (2016).[4] N. Hansen, Scr. Mater. 51, 801 (2004).[5] Z. Yanushkevich, A. Belyakov, R. Kaibyshev,C. Haase, D.A. Molodov, Mater. Charact. 112, 180(2016).[6] P. Kusaki, K. Tsuzaki, D.A. Molodov, R. Kaibyshev,A. Belaykov, Metall. Mater. Trans. A 47A, 5704(2016).[7] Z. Yanushkevich, S.V. Dobatkin, A. Belyakov,R. Kaibyshev, Acta Mater. 136, 39 (2017).[8] K. Huang, R.E. Loge, Mater. Des. 111, 548 (2016).[9] A. Saeed-Akbari, L. Mosecker, A. Schwedt, W. Bleck,Metall. Mater. Trans. A 43A, 1688 (2012).

Deformation mechanisms operating in TWIP/TRIP steels processed by warm to hot working

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Deformation mechanisms operating in TWIP/TRIP steels processed by warm to hot working

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