The Fe-0.44%C-1.8%Si-1.3%Mn-0.82%Cr-0.28%Mo steel treated by the quenching-partitioning partitioning process showed a product of strength and elongation of 30 GPa % with yield stress of 1350 MPa. Such a combination of high ultimate tensile strength and good ductility is attributed to a high portion of retained austenite (≥20%) transforming to martensite under tension. The high yield stress is provided by carbon supersaturation of austenite and a high dislocation density in this phas

Borisova, Yu. I.

Erokhin, M. N.

Gaidar, S. M.

Kaibyshev, R. O.

Mishnev, R. V.

DSpace at Belgorod State University

349ISSN 1028-3358, Doklady Physics, 2023, Vol. 68, No. 10, pp. 349–353. © Pleiades Publishing, Ltd., 2023.Russian Text © The Author(s), 2023, published in Doklady Rossiiskoi Akademii Nauk. Fizika, Tekhnicheskie Nauki, 2023, Vol. 512, pp. 73–77.The Significance of Retained Austenite in the High Strengthand Plasticity of Medium Carbon Q&P SteelR. V. Mishneva,b,*, Yu. I. Borisovaa,b, Academician M. N. Erokhina,S. M. Gaidara, and R. O. KaibyshevaReceived March 16, 2023; revised March 16, 2023; accepted May 10, 2023Abstract—The Fe–0.44%C–1.8%Si–1.3%Mn–0.82%Cr–0.28%Mo steel treated by the quenching–parti-tioning process showed a product of strength and elongation of 30 GPa % with yield stress of 1350 MPa. Sucha combination of high ultimate tensile strength and good ductility is attributed to a high portion of retainedaustenite (≥20%) transforming to martensite under tension. The high yield stress is provided by carbon super-saturation of austenite and a high dislocation density in this phase.Keywords: steel, mechanical properties, microstructure, phase transformationDOI: 10.1134/S1028335823100063The key characteristics of promising high-strengthsteels in advanced high strength steel (AHSS) arestrength and ductility [1]. Materials can be strong orductile, but the combination of high strength and goodductility is rare. A generally accepted criterion charac-terizing the combination of strength and ductility isthe product of strength and elongation: σBδ (MPa %)[1, 2]. For third generation AHSS steels, the index σBδshould be from 20 to 40 GPa % [1]. Another require-ment for these steels is the value of the yield strength(σ0.2) ≥ 1000 MPa [1]. Combinations of such high σ0.2and σBδ were not achieved in steels of the first and sec-ond generation [1].The contradiction between strength and ductility insteels can be overcome by heat treatment developedalmost 20 years ago, which was called “Quenching andPartitioning,” or Q&P [1–4]. This treatment involvesheating above the temperature Ac3, complete austen-itization followed by accelerated cooling in molten saltheated to a temperature Tq, between the start tempera-ture (Ms) and end (MTo) martensitic transformation.Quenching in heated salt ensures the formation of pri-mary martensite and retained austenite [1–5]. Thenext operation, “partitioning,” is annealing, which isalso usually carried out in molten salt heated to thetemperature Tp, as a rule, above the temperature Ms;hence, Tp > TMs [1–5]. During this operation, carbon isredistributed from primary martensite to retained aus-tenite, which is enriched in carbon. At the same time,during “partitioning,” a bainite transformation canoccur, but the chemical composition of the steel andthe temperature and time of partitioning are selectedin such a way as to minimize the specific volume of thebainite being formed. It should be noted that therelease of cementite is not allowed during the “parti-tioning” operation. This is achieved due to the highcontent (≥1.5 wt %) of Si in Q&P steels. The last oper-ation in Q&P processing is quenching from the “par-titioning” temperature, which leads to partial transfor-mation of retained austenite into secondary marten-site, which differs from primary martensite in its highcarbon content.Low-alloy steels for Q&P processing with a carboncontent from 0.4 to 0.56% are characterized by a highvalue of the parameter σBδ ≥ 30 GPa %, which is twotimes or more higher than in low-alloy auto steelsbelonging to the first generation AHSS, in combina-tion with σ0.2 > 1000 MPa. A feature of these steels isthe presence in the structure of 12 to 30% retainedaustenite in the form of blocks after Q&P treatment[6–8]. The high ductility of these steels is associatedwith the TRIP effect (Transformation induced plas-ticity, plasticity induced by transformation) [9]. Undertension, almost 90% of the retained austenite trans-forms into martensite [10]. At the same time, thenature of the high yield strength in steels with a highcontent of retained austenite enriched in carbon afterQ&P treatment is unclear and little studied. It isbelieved that retained austenite has a strength almosttwo times lower than the yield strength of Q&P steels[5], the high strength of which can be due to the sum-mation of the strength of high-strength martensite andTECHNICAL SCIENCEaRussian State Agrarian University, Moscow Timiryazev Agricultural Academy, Moscow, RussiabBelgorod State University, Belgorod, Russia*e-mail: mishnev91@mail.ru350DOKLADY PHYSICS  Vol. 68  No. 10  2023MISHNEV et al.low-strength austenite according to the additivelaw [1, 4].The purpose of this study is to establish the reasonsfor the high value of the yield strength, relative elonga-tion, and parameter σBδ in the new Fe-0.44%C steelafter Q&P treatment.The studies were carried out on steel Fe–0.44%C–1.81%Si–1.33%Mn–0.82%Cr–0.28%Mo (wt %),which was obtained by induction melting in air fol-lowed by electroslag remelting. Next, the steel wasannealed at 1150°C for four hours, followed by forging.Samples with a thickness of 3 mm were subjected to athree-stage Q&P heat treatment, including austeniti-zation at a temperature of 900°C for 5 min, followedby quenching in a salt bath heated to various tempera-tures Tq from 140 to 300°C with holding for 15 s andsubsequent “partitioning” at the temperature Tp =400°C for 60 s in a salt bath, followed by air cooling.Tensile tests were carried out on samples with aworking length of 35 mm and a cross section of 7 ×3 mm on an Instron 5882 testing machine at roomtemperature. Microstructural studies were carried outusing a FEI Quanta 600 FEG scanning electronmicroscope equipped with an electron backscatter dif-fraction (EBSD) pattern analyzer. The dislocationdensity was determined from EBSD data using themethod described in [11]. The volume fraction ofretained austenite was calculated by X-ray diffraction(XRD) using a Rigaku Ultima IV diffractometer witha step size of 0.02 deg, 40 kV, and 40 mA.The average carbon concentration in retained aus-tenite was calculated using the formula given in [12]:(1)where CCRA is the carbon concentration in the retainedaustenite and aγ is the austenite lattice parameter.In tensile testing, the relative elongation variesfrom 17.5 to 21.4% in the quenching temperature rangeof 140–250°C, and the tensile strength varies fromσB = 1780 MPa at Tq = 140°C to 1580 MPa at Tq =250°C. As a result, at a quenching temperature≤250°C, the value of σBδ is >30 GPa % (Fig. 1), whichis two times higher than in auto steels belonging to thefirst generation AHSS [1]. The value σ0.2 varies from1370 MPa at Tq = 140°C to 1170 MPa at Tq = 250°C.There is no yield plateau observed on the σ–ε curves,and strain hardening begins after reaching the yieldpoint and continues until necking is formed. Conse-quently, parts made of this steel can be produced bysheet stamping [1]. Increasing the quenching tem-perature from 260 to 300°C (Ms = 270°C) leads to asharp decrease in plasticity and the parameter σBδ,which is accompanied by a significant increase in boththe yield strength and the tensile strength. The failureof a sample hardened at 300°C occurs with slightdeformation after reaching the yield point.γ=CRA – 3.547 /0.) ,( 046C aQ&P treatment in the quenching temperaturerange of 140–250°C leads to the formation of a struc-ture consisting of primary martensite, secondary mar-tensite, and retained austenite (Fig. 2). Martensite hasthe typical morphology of lath martensite: initial aus-tenite grains, blocks, packets, and laths [13]. Retainedaustenite grains have an irregular shape and arelocated between martensite blocks (Fig. 2). The aver-age grain size of retained austenite is 2.65 μm (Tq =200°С). It is important to note that the structure ofsteel after quenching is characterized by a high densityof geometrically necessary dislocations, which char-acterizes the elastic bending of the lattice and is deter-mined from EBSD pictures. In this case, the disloca-tion density in retained austenite (ρ = 6.2 × 1014 m–2)and martensite are approximately the same at Tq =200°C, which contradicts early ideas about the low dislo-cation density in retained austenite in Q&P steels [5].The volume fraction of retained austenite increasesfrom 16 to 25% with increasing Tq from 140 to 250°С(Fig. 3). A subsequent increase in the quenching tem-perature from 250 to 300°C leads to a decrease in thevolume fraction of retained austenite to 5%. Thus, ifprimary martensite is not formed, then during “parti-tioning” there is no enrichment of retained austenitewith carbon and it transforms into martensite whencooled from the temperature Tp. Reducing the contentof retained austenite by five times with increasing thequenching temperature above Ms correlates with adecrease in plasticity and a decrease in the parameterσBδ by almost ten times, despite an increase in the ten-sile strength by more than 40%. It is obvious that thehigh plasticity and high values of σBδ of this steel,hardened at temperatures from 140 to 250°C, are asso-ciated with strain hardening during tension. The studyof samples after tension in the fracture region (neck)using X-ray diffraction analysis showed that the vol-ume fraction of retained austenite in the structuredoes not exceed 3%. Thus, more than 80% of theretained austenite is transformed into tensile deforma-tion martensite, which provides tensile strain harden-ing, since the strength of martensite is higher than thestrength of austenite at the same carbon content. Thisconfirms the assumption that the high ductility of steelafter Q&P treatment is associated with the TRIPeffect.To establish the nature of the high value of the yieldstrength of Fe-0.44%C type steel, it is necessary toestimate the yield strength (σ0.2) of the retained aus-tenite. This can be done based on the rule about theadditive contribution of various hardening mecha-nisms to the overall yield strength for one structuralcomponent [14]:(2)where σ0 is the lattice friction stress ~63.5 MPa in pureiron austenite [14, 15], σSS is the solid solutionstrengthening, σHP is the structural strengthening, andσ = σ + σ + σ + σ0.2 0 SS HP disl,DOKLADY PHYSICS  Vol. 68  No. 10  2023THE SIGNIFICANCE OF RETAINED AUSTENITE 351σdisl is the dislocation strengthening. Solid solutionstrengthening (σSS) is determined by the content of theelements of introduction and replacement and can becalculated using the following relation [14, 15]:(3)The carbon concentration CCRA = 1.38% wasobtained by X-ray diffraction from Eq. (1), and the( )( )σ = × + ×+ × + ×SS 356.5 %С 20.1 %S( ) i3.7 %Cr 1( 4.6) %Mo .content of substitution elements in austenite was takenequal to the content in steel, since the redistributionbetween structural components does not occur duringQ&P processing [3]. Structural strengthening (σHP) canbe estimated using the Hall–Petch equation [14, 16, 17]:(4)where KHP is the Hall–Petch coefficient. For this cal-culation we used KHP = 160 MPa μm0.5, obtained forσ = –0.5HP HP , K DFig. 1. (a) Engineering stress–strain curves, (b) the effect of quenching temperature on tensile mechanical properties (σB, σ0.2,relative elongation), (c) dependence of the product of strength and elongation σBδ on the quenching temperature of the steelunder study.1000150020005005 10 15Тq = 140СТq = 200СТq = 300СТq = 250СТq = 275С20Strain  , %Stress ,  МPа0102030400160 180 200 220 240 260 280 300 320 В,  GPa %140100015002500(а) (b)(c)В0.22000500150 200 300250 350Quenching temperature  Тq, СQuenching temperature  Тq, СStress   В,  0.2, МPаElongation , %1002520151050Fig. 2. Microstructure of Fe–0.44C steel after Q&P treatment at a quenching temperature of 200°C: (a) SEM and (b) EBSDanalysis demonstrating the phase composition, (c) KAM picture for the martensitic phase, (d) KAM picture for the austenitephase (KAM is kernel average misorientations, elastic bending of the lattice).(а) (b) (c) (d)MartensiteMartensiteAusteniteAustenitePhase2 m austenitemartensite5 m 5 m 5 m352DOKLADY PHYSICS  Vol. 68  No. 10  2023MISHNEV et al.austenitic steel in [14]. Dislocation strengthening(σdisl) can be expressed by the Taylor dependence [14,16, 17]:(5)where α is the dislocation strengthening coefficient(about 0.24 for materials with an fcc lattice [16]), M isthe Taylor coefficient = 3.06 obtained by calculationfrom misorientation maps, G is the shear modulus(= 75 GPa [17]), b is the Burgers vector (= 0.25 nm),and ρ is the dislocation density, which at Tq = 200°C isρ = 6.2 × 1014 m–2.According to the calculation according to Eq. (2),σ0.2 for steel after Q&P at 200°C is σ0.2 = 63.5 + 536.5 +98.3 + 343.4 = 1041.7 MPa, which coincides with theexperimental value of σ0.2 = 1120 MPa with high accu-racy.Consequently, the enrichment of retained austen-ite with carbon during the “partitioning” operationand the high dislocation density in retained austeniteplay a major role in achieving high values of σ0.2 inmedium carbon Q&P steel. There is every reason tobelieve that plastic deformation in the multiphasestructure, which is formed in Fe-0.44%C steel  afterQ&P treatment, begins in the least strong phase,which is retained austenite. Q&P treatment provideshigh strength of retained austenite, which is the reasonfor the high yield strength of medium-carbon Q&Psteels with a large specific volume of this phase with ablock morphology.Thus, Fe-0.44%C steel after Q&P treatment hashigh values of the parameter σB δ > 30 GPa %, andyield strength σ0.2 > 1000 MPa. This is due to the high-volume fraction of retained austenite ~25%, which hasa block morphology, high carbon content (~1.3%),and high density of lattice dislocations (~6 × 1014 m–2).The morphology of retained austenite causes themajority of it to transform into martensite during ten-sion (TRIP effect), which provides a high value ofσ = α ρ0.5disl ,MGbstrain hardening, and this, in turn, allows one toachieve high values of ultimate strength and relativeelongation at the same time. A combination of highstrength and ductility can be obtained in Q&P steelswith a high content of retained austenite if this phase,in addition to the ability to transform into deformationmartensite, has high strength due to the high carboncontent and high dislocation density.ACKNOWLEDGMENTSThe studies were carried out on the equipment of theJoint Scientific Center for Technologies and Materials ofBelgorod State National Research University.FUNDINGThis research was funded by the Ministry of Science andHigher Education of the Russian Federation, grant no. 075-15-2021-572.CONFLICT OF INTERESTThe authors of this work declare that they have no con-flicts of interest.REFERENCES1. N. Fonstein, Advanced High Strength Sheet Steels(Springer Int., Cham, 2015).2. Zh. Xiong, P. J. Jacques, A. Perlade, and Th. Pardoen,Metall. Mater. Trans. A 50, 3502 (2019). https://doi.org/10.1007/s11661-019-05265-23. J. Speer, D. K. Matlock, B. C. de Cooman, andJ. G. 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The significance of retained austenite in the high strength and plasticity of medium carbon Q&P steel

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The significance of retained austenite in the high strength and plasticity of medium carbon Q&P steel

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