Overcoming Strength-Ductility Trade-Off at Cryogenic Temperature of Low Carbon Low Alloy Steel Via Controlling Retained Austenite Stability

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Article Overcoming Strength-Ductility Trade-Off at Cryogenic Temperature of Low Carbon Low Alloy Steel via Controlling Retained Austenite Stability

Xuelin Wang 1, Zhenjia Xie 1,* , Chengjia Shang 1,* and Gang Han 2

1 Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China; [email protected] 2 Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China; [email protected] * Correspondence: [email protected] (Z.X.); [email protected] (C.S.)

Abstract: Stress–strain behavior of a low carbon low alloy multiphase steel with ferrite, tempered bainite, and retained austenite was studied at different cryogenic temperatures. Results indicated that both strength and ductility were enhanced with decreasing tensile testing temperature. The enhancement of both strength and ductility was attributed to the decreased mechanical stability of retained austenite with decreasing temperature, resulting in sufﬁcient transformation induced plasticity (TRIP) effect for increasing work hardening rate.

Keywords: multiphase steel; retained austenite stability; work hardening rate; ductility; low temperature property

Citation: Wang, X.; Xie, Z.; Shang, C.; Han, G. Overcoming 1. Introduction Strength-Ductility Trade-Off at High strength low alloy (HSLA) steels have attracted great interest for wide fields of Cryogenic Temperature of Low applications, due to their excellent strength–toughness balance, economy, and weldabil- Carbon Low Alloy Steel via ity [1–3]. With applied fields of HSLA steels extending to low temperature environment, Controlling Retained Austenite low temperature toughness is of significant importance in consideration of safety. Generally, Stability. Metals 2021, 11, 157. yield strength of HSLA steels increases dramatically to be higher than fracture strength https://doi.org/10.3390/met110 with decreasing temperature, thus sudden brittle fracture takes place without ductile plastic 10157 deformation [4]. This strength–ductility trade-off greatly limits their application in low temperature environments. Therefore, it is very important to simultaneously enhance yield Received: 4 December 2020 strength and impact toughness with decreasing temperature for HSLA steels by tailoring Accepted: 12 January 2021 Published: 15 January 2021 microstructure. Recently, it has been suggested that the strength–ductility trade-off can be overcome though the controlled deformation-induced martensitic transformation from

Publisher’s Note: MDPI stays neutral face-centered cubic to body-centered cubic via metastability engineering in CoCrFeNiMo with regard to jurisdictional claims in medium-entropy alloys [5]. The purpose of this work is to introduce multiphase microstruc- published maps and institutional affil- ture containing retained austenite to a low carbon low alloy steel for enhancing the work iations. hardening ability at low temperatures, resulting in high fracture strength and high ductility. A lot of efforts have been devoted to improving strength, ductility, and toughness of HSLA steels by tailoring microstructure, and progress has been made. It has revealed that refinement of prior austenite grains via increasing high angle grain boundaries (HAGBs) is helpful for decreasing ductile-to-brittle transition temperature (DBTT), because HAGBs can Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. retard brittle crack propagation [6,7]. Control of multiphase microstructure via producing This article is an open access article metastable retained austenite in low carbon low alloy steels was able to significantly im- distributed under the terms and prove ductility and low temperature toughness [2,8–11]. Metastable retained austenite was conditions of the Creative Commons suggested to afford TRIP effect during tensile or impact to enhance the plastic deformation Attribution (CC BY) license (https:// ability before fracture. The stability of retained austenite is crucial to performance in creativecommons.org/licenses/by/ service [12,13]. The main concern is that once retained austenite in the steel is unstable 4.0/). and transforms into martensite thermally or mechanically due to a small deformation in

Metals 2021, 11, 157. https://doi.org/10.3390/met11010157 https://www.mdpi.com/journal/metals Metals 2021, 11, 157 2 of 8

ducing metastable retained austenite in low carbon low alloy steels was able to signifi‐ cantly improve ductility and low temperature toughness [2,8–11]. Metastable retained austenite was suggested to afford TRIP effect during tensile or impact to enhance the plas‐ Metals 2021, 11, 157 tic deformation ability before fracture. The stability of retained austenite is crucial to2 per of 8‐ formance in service [12,13]. The main concern is that once retained austenite in the steel is unstable and transforms into martensite thermally or mechanically due to a small defor‐ mation in low temperature service, sudden brittle fracture takes place, resulting in disas‐ low temperature service, sudden brittle fracture takes place, resulting in disaster. Hence, ter. Hence, the objectives of the present work are to study the stability of retained austenite the objectives of the present work are to study the stability of retained austenite in a in a low carbon low alloy multiphase steel produced by the two‐step intercritical heat low carbon low alloy multiphase steel produced by the two-step intercritical heat treat- treatment, and to elucidate the effect of retained austenite stability on stress–strain behav‐ ment, and to elucidate the effect of retained austenite stability on stress–strain behavior at cryogenicior at cryogenic temperature. temperature.

2.2. MaterialMaterial andand MethodsMethods ofof ExperimentsExperiments TheThe chemicalchemical compositioncomposition ofof thethe experimentalexperimental steelsteel waswas Fe-0.07C-0.36Si-1.78Mn-Fe‐0.07C‐0.36Si‐1.78Mn‐ 0.51Ni-0.3Mo-0.78Nb-0.02Ti0.51Ni‐0.3Mo‐0.78Nb‐0.02Ti in weight weight percent percent (wt%). (wt%). The The steel steel was was melted melted in invacuum vacuum in‐ inductionduction furnace furnace and and cast cast into into ingot ingot of of weight weight 25 25 kg. The ingot was reheatedreheated toto 12001200 ◦°CC andand heldheld forfor 22 h,h, thenthen forgedforged intointo rodsrods withwith diameterdiameter of of 16 16 mm,mm, andand airair cooledcooled to to ambientambient temperature.temperature. The forged rods were intercriticallyintercritically annealed at 740 ◦°CC for 3030 min,min, subse-subse‐ quentlyquently airair cooledcooled toto ambientambient temperature.temperature. Next,Next, thethe annealedannealed specimenspecimen waswas temperedtempered at at 680680 ◦°CC forfor 3030 min,min, andand ﬁnally finally air air cooled cooled to to ambient ambient temperature. temperature. Mechanical Mechanical properties properties inin termsterms ofof strengthstrength andand elongationelongation ofof thethe treatedtreated samplessamples ((ϕφ55 mmmm standardstandard tensiletensile testtest samples)samples) werewere measuredmeasured atat anan extensionextension raterate ofof 2.52.5 ×× 1010−−3 3s−s1− with1 with an an extensometer extensometer of of25 25mm mm at cryogenic at cryogenic temperatures temperatures of − of80,− −80,120,− −120,160,− and160, − and196 °C,−196 respectively.◦C, respectively. The reason The reasonwhy the why tensile the tensile test starts test startsat −80 at °C− 80is ◦thatC is the that material the material has excellent has excellent room room temperature tempera- tureproperties properties and andgood good low lowtemperature temperature toughness toughness [8]. The [8]. Themodel model of cryogenic of cryogenic tensile tensile test‐ testinging machine machine is SANS is SANS CMT5000. CMT5000. The The specimen specimen was was cooled cooled to to the the set temperaturetemperature withwith liquidliquid heliumhelium (he), then kept for for 10 10 min, min, and and then then tensile tensile test test was was carried carried out, out, as as shown shown in inFigure Figure 1.1 .

FigureFigure 1.1. SchematicSchematic diagramdiagram ofof cryogeniccryogenic tensiletensile testtest process.process.

HeatHeat treatedtreated samplessamples werewere mechanicallymechanically polishedpolished andand etchedetched withwith 4%4% nitalnital usingusing standard metallographic procedures for TESCAN MIRA3 field emission scanning electron standard metallographic procedures for TESCAN MIRA3 field emission scanning elec‐ microscopy (SEM, TESCAN, Brno, Czech Republic). To obtain information on distribution tron microscopy (SEM, TESCAN, Brno, Czech Republic). To obtain information on distri‐ and morphology of retained austenite, electron backscatter diffraction (EBSD, Oxford In- bution and morphology of retained austenite, electron backscatter diffraction (EBSD, Ox‐ struments, Oxford, UK) analysis was carried out after mechanical and electrolytic polishing ford Instruments, Oxford, UK) analysis was carried out after mechanical and electrolytic using TESCAN MIRA3 field emission scanning electron microscope (FE-SEM, TESCAN, polishing using TESCAN MIRA3 field emission scanning electron microscope (FE‐SEM, Brno, Czech Republic) at an acceleration voltage of 20 kV, with a step size of 60 nm. EBSD TESCAN, Brno, Czech Republic) at an acceleration voltage of 20 kV, with a step size of 60 data was post-processed by HKL CHANNEL 5 flamenco software (Oxford Instruments, nm. EBSD data was post‐processed by HKL CHANNEL 5 flamenco software (Oxford In‐ Oxford, UK) to acquire necessary information. The amount of retained austenite was also struments, Oxford, UK) to acquire necessary information. The amount of retained austen‐ evaluated using magnetic measurements [14]. For these measurements, samples were cut with approximately 150 mg. The M(H) curves were obtained at 300 K using a PPMS (Phys- ical Property Measurement System, Quantum Design, San Diego, USA). The hysteresis loops were carried out using the following sweep rates: 0.4 kOe/min for |H| < 0.5 kOe, 1.8 kOe/min for 0.5 < |H| < 5 kOe and 10 kOe/min for 6 < |H| < 15 kOe. Metals 2021, 11, 157 3 of 8

ite was also evaluated using magnetic measurements [14]. For these measurements, sam‐ ples were cut with approximately 150 mg. The M(H) curves were obtained at 300 K using a PPMS (Physical Property Measurement System, Quantum Design, San Diego, USA). The Metals 2021, 11, 157 3 of 8 hysteresis loops were carried out using the following sweep rates: 0.4 kOe/min for |H| < 0.5 kOe, 1.8 kOe/min for 0.5 < |H| < 5 kOe and 10 kOe/min for 6 < |H| < 15 kOe.

3.3. Results Results and and Discussion Discussion RepresentativeRepresentative SEMSEM micrographmicrograph of of the the heat-treated heat‐treated sample sample is given is given in Figure in Figure2a. Mul- 2a. Multiphasetiphase microstructure microstructure consisting consisting of ferrite, of ferrite, tempered tempered bainite bainite (TB), (TB), and retainedand retained austenite aus‐ tenite(RA) was(RA) obtained. was obtained. Polygonal Polygonal ferrite ferrite (PF) with (PF) diameter with diameter of ≈10 ofµ ≈m10 was μm observed was observed along alongprior austeniteprior austenite grain grain boundary. boundary. Lath Lath tempered tempered bainite bainite (TB, (TB, tempered tempered at 680 at 680◦C) °C) and and in- intercriticaltercritical ferrite ferrite composited composited as as lamellar lamellar structure. structure. TheThe distributiondistribution and morphology of retainedretained austenite austenite are are presented presented by by EBSD image image in in Figure 22b.b. ItIt waswas seenseen thatthat retainedretained austeniteaustenite distributed distributed mainly mainly in in the the lamellar lamellar structure structure with with fine ﬁne blocky and thin film ﬁlm-like‐like morphologies.morphologies. The The stabilization stabilization of of retained retained austenite austenite as a a result result of of two two-step‐step heat heat treatment treatment waswas reported reported in our previous work [8,15]. [8,15].

(a) (b)

Figure 2. (a) SEM micrograph showing multiphase microstructure and (b) EBSD image showing morphology and distribu- Figure 2. (a) SEM micrograph showing multiphase microstructure and (b) EBSD image showing morphology and distri‐ tion of retained austenite (RA) in the multiphase microstructure of the studied steel, in which red areas represent retained bution of retained austenite (RA) in the multiphase microstructure of the studied steel, in which red areas represent re‐ austenite. (PF: polygonal ferrite, TB: tempered bainite). tained austenite. (PF: polygonal ferrite, TB: tempered bainite). Figure3a shows the engineering stress–stain curves of the studied steel measured at Figure 3a shows the engineering stress–stain curves of the studied steel measured at temperatures of −80, −120, −160, and −196 ◦C. It was found that both yield strength and temperatures of −80, −120, −160, and −196 °C. It was found that both yield strength and tensile strength were increased with decreasing tensile temperature. The yield strength and tensile strength were increased with decreasing tensile temperature. The yield strength tensile strength were 507 and 805 MPa at −80 ◦C, respectively. When the test temperature and tensile strength were 507 and 805 MPa at −80 °C, respectively. When the test temper‐ was decreasing to −196 ◦C, the yield strength and tensile strength were increased to 846 atureand 1083 was decreasing MPa, respectively. to −196 °C, More the importantly,yield strength the and uniform tensile strength and total were elongations increased also to 846presented and 1083 a trend MPa, to respectively. increase with More decreasing importantly, the tensile the uniform temperature, and total except elongations that there also was presenteda slight decrease a trend of to total increase elongation with decreasing at −160 ◦C. the The tensile studied temperature, steel exhibited except high that uniform there waselongation a slight of decrease 16.1–19.1% of total and elongation high total at elongation −160 °C. The of 30.0–33.3% studied steel at tensileexhibited temperature high uni‐ formrange elongation of −80 to −of196 16.1–19.1%◦C. This atypicaland high phenomenon total elongation of stress–strain of 30.0–33.3% behavior at tensile in thistempera work‐ turemay range be ascribed of −80 toto − the196 presence °C. This of atypical the TRIP phenomenon effect of retained of stress–strain austenite, behavior rather than in this the workclassical may deformation be ascribed mechanism to the presence by dislocation of the TRIP glide effect and of dislocationretained austenite, interactions rather [16 than,17]. the classicalTo understand deformation the ductility mechanism of the studiedby dislocation steel at glide low temperatures, and dislocation work interactions hardening [16,17].behavior at examined temperatures was studied. The work hardening rates are plotted in Figure3b, according to the calculation results from the Hollomon equation [ 18,19]. Three-stage work hardening behavior was characterized for samples tensile tested at all temperatures, namely the work hardening rate firstly decreased, then increased, and finally decreased to failure with increasing strain, as observed in TRIP-aided steels at room temperature [20,21]. It can be found that the decrease of work hardening rate in the first stage was stopped at the end of the yield plateau for all four testing temperatures. This suggested that the first stage of work hardening behavior was dominated by dislocation glide in ferrite due to breaking away from Cottrell atmosphere. The increase in the second stage was ascribed to the TRIP effect of retained austenite [21]. In the third stage, the downtrend of work hardening rate became slow, leading to delaying the occurrence of Metals 2021, 11, 157 4 of 8

necking and enhancing the ductility. This indicated that retained austenite manifested variations in mechanical stability under different loading temperatures. At relative high temperature of −80 ◦C, some of the retained austenite had high mechanical stability, they were stable to supply the TRIP effect till failure. With decreasing tensile testing temperature, the mechanical stability of retained austenite decreased, such that more retained austenite

Metals 2021, 11, 157 afforded the TRIP effect during strain to provide sustainable work hardening rate.4 This of 8 was the reason for the atypical phenomenon of increase in ductility with decreasing temperature.

(a) (b)

Figure 3.3. ((aa)) EngineeringEngineering stress–strain stress–strain curves curves and and (b ()b true) true stress–strain stress–strain curves curves and and work work hardening hardening rate rate of the of studied the studied steel atsteel low at temperatures. low temperatures.

To studyunderstand thermal the and ductility mechanical of the stability studied of steel retained at low austenite, temperatures, the volume work hardening fraction of behaviorretained austeniteat examined was temperatures determined by was saturation studied. magnetizationThe work hardening (Ms) for rates the are undeformed plotted in Figureand uniform 3b, according deformed to the area calculation of samples results after from testing the at Hollomon different equation temperatures. [18,19]. The Three ob-‐ stagetained work magnetization hardening behavior (M) curves was as characterized a function of for applied samples magnetic tensile ﬁeldtested (H at) all are temper shown‐ atures,in Figure namely4. It can the bework seen hardening that the rateMs wasfirstly identical decreased, for then samples increased, without and strain finally at dif-de‐ creasedferent temperatures. to failure with This increasing indicated strain, that as retained observed austenite in TRIP in‐aided the studied steels at steel room had temper high‐ − − ◦ aturethermal [20,21]. stability It can in temperaturebe found that range the decrease of 80 to of work196 C.hardening After the rate maximum in the first uniform stage wasdeformation, stopped at the theM sendincreased of the obviously.yield plateau There for wasall four a tendency testing thattemperatures. the increment This of sugMs‐ gestedincreased that with the first decreasing stage of testing work temperature.hardening behavior This effect was was dominated attributed by todislocation the decreasing glide of non-ferromagnetic phase of retained austenite. Hence, it can be concluded that the in ferrite due to breaking away from Cottrell atmosphere. The increase in the second stage mechanical stability of retained austenite in the studied steel decreased with decreasing was ascribed to the TRIP effect of retained austenite [21]. In the third stage, the downtrend temperature. Therefore, more retained austenite transformed to martensite during strain, of work hardening rate became slow, leading to delaying the occurrence of necking and resulting in sustainable work hardening rate due to sufﬁcient TRIP effect. Given that all enhancing the ductility. This indicated that retained austenite manifested variations in retained austenite transformed to martensite at −196 ◦C, the M at −196 ◦C was taken as mechanical stability under different loading temperatures. At relatives high temperature M (f ) for the austenite-free sample. Therefore, based on the calculation method in [14], the of −s 80 °C, some of the retained austenite had high mechanical stability, they were stable value of the last point of the magnetization curves (M (c)) obtained at different conditions to supply the TRIP effect till failure. With decreasing stensile testing temperature, the me‐ is used to calculate the volume fraction of retained austenite (fγ). The calculation method chanical stability of retained austenite decreased, such that more retained austenite af‐ is presented in Equation (1). forded the TRIP effect during strain to provide sustainable work hardening rate. This was

the reason for the atypical phenomenonMs( fof) − increaseMs(c) in ductility with decreasing tempera‐ fγ = × 100% (1) ture. Ms( f ) To study thermal and mechanical stability of retained austenite, the volume fraction of retainedThus, theaustenite volume was fraction determined of retained by saturation austenite magnetization can be calculated (Ms) asfor≈ the21.1% unde for‐ ◦ formedsamples and undeformed uniform deformed at −80 to area−196 of samplesC. After after maximum testing at uniform different deformation temperatures. at −The80 ◦ obtainedand −120 magnetizationC, the volume (M fraction) curves of as retained a function austenite of applied was magnetic≈4.5–5.2%. field When (H) theare testingshown ◦ intemperature Figure 4. It decreased can be seen to that−160 theC, M thes was volume identical fraction for samples of retained without austenite strain decreased at different to temperatures.≈1.8%. This conﬁrmed This indicated that the that enhancement retained austenite of both strengthin the studied and ductility steel had with high decreasing thermal stabilitytemperature in temperature was ascribed range to of the −80 decreased to −196 °C. mechanical After the maximum stability ofuniform retained deformation, austenite, theresulting Ms increased in sufﬁcient obviously. transformation There was induced a tendency plasticity that the (TRIP) increment effect forof M sustainables increased work with decreasing testing temperature. This effect was attributed to the decreasing of non‐ferro‐ magnetic phase of retained austenite. Hence, it can be concluded that the mechanical sta‐ bility of retained austenite in the studied steel decreased with decreasing temperature. Therefore, more retained austenite transformed to martensite during strain, resulting in sustainable work hardening rate due to sufficient TRIP effect. Given that all retained aus‐ tenite transformed to martensite at −196 °C, the Ms at −196 °C was taken as Ms(f) for the austenite‐free sample. Therefore, based on the calculation method in [14], the value of the last point of the magnetization curves (Ms(c)) obtained at different conditions is used to

Metals 2021, 11, 157 5 of 8

calculate the volume fraction of retained austenite (fγ). The calculation method is pre‐ sented in Equation (1).

M s ( f )  M s (c) fγ  100% (1) M s ( f ) Thus, the volume fraction of retained austenite can be calculated as ≈21.1% for sam‐ ples undeformed at −80 to −196 °C. After maximum uniform deformation at −80 and −120 °C, the volume fraction of retained austenite was ≈4.5–5.2%. When the testing temperature decreased to −160 °C, the volume fraction of retained austenite decreased to ≈1.8%. This Metals 2021, 11, 157 confirmed that the enhancement of both strength and ductility with decreasing tempera5 of 8‐ ture was ascribed to the decreased mechanical stability of retained austenite, resulting in sufficient transformation induced plasticity (TRIP) effect for sustainable work hardening hardeningrate. The main rate. reason The main why reason retained why austenite retained can austenite be stable can to be low stable temperature to low temperature (−196 °C) (is− 196ascribed◦C) is to ascribed the highly to the enrichment highly enrichment of Mn and of Mn C andduring C during the two the‐step two-step intercritical intercritical heat heattreatment. treatment. Atom Atom probe probe tomography tomography analysis analysis in our in ourprevious previous studies studies revealed revealed that that the theMn Mnand and C contents C contents were were ≈6.6≈ and6.6 and0.4 wt%. 0.4 wt%. In addition, In addition, the obtained the obtained retained retained austenite austenite was wasvery very fine, ﬁne, the average the average size sizewas was ≈300≈ nm300 in nm diameter. in diameter. Hence, Hence, the retained the retained austenite austenite was wasvery very stable stable thermally thermally up to up −196 to − °C196 [8,15].◦C[8 ,15].

FigureFigure 4.4. Magnetization ((MM)) curvescurves asas aa functionfunction ofof appliedapplied magneticmagnetic ﬁeldfield ((HH)) forfor samplessamples withoutwith‐ deformationout deformation and and after after uniform uniform deformation deformation in the in range the range of − of80 − to80− to196 −196◦C. °C.

ToTo reveal reveal the the failure failure mechanisms mechanisms at at different different testing testing temperatures, temperatures, the fracturethe fracture surfaces sur‐ werefaces examinedwere examined using SEM. using The SEM. macroscopic The macroscopic and microscopic and microscopic fractographs fractographs are shown are in Figuresshown 5in and Figures6. It can 5 and be seen6. It fromcan be the seen Figure from5 thatthe Figure with the 5 decreasethat with of the temperature, decrease of the tem‐ anisotropicperature, the fracture anisotropic morphology fracture graduallymorphology changes gradually to the changes isotropic to fracturethe isotropic morphology. fracture ◦ Whenmorphology. the temperature When the decreases temperature to −196 decreasesC, the fractureto −196 °C, surface the presentsfracture surface complete presents brittleness.complete This brittleness. phenomenon This can phenomenon be attributed can to the be changeattributed of microstructure to the change withof microstructure the decrease ofwith temperature the decrease and of the temperature conditions of and testing. the conditions From Figure of6 testing., it can be From seen Figure that several 6, it can main be cracksseen that with several edge stepmain were cracks always with edge observed step alongwere always the diameter observed direction. along the Full diameter ductile ◦ fracturedirection. with Full dimples ductile fracture was characterized with dimples for was samples characterized tensile tested for samples at −80 and tensile−120 testedC. Basedat −80 onand our −120 previous °C. Based study, on our this previous effect could study, be causedthis eﬀect by comprehensivecould be caused inﬂuence by compre of‐ texturehensive evolution, influence the of redistributiontexture evolution, of grain the boundaries, redistribution and of the grain state ofboundaries, three-dimensional and the stressstate of with three large‐dimensional tensile plastic stress deformation with large during tensile necking plastic [22 deformation]. When testing during temperature necking − − ◦ decreased[22]. When to testing160 andtemperature196 C, decreased the main crack to −160 was and not − obvious,196 °C, the and main some crack small was cracks not and brittle fracture were observed within the matrix. This indicated that after large uniform deformation, the plasticity during necking decreased at −196 ◦C. This may ascribe to the full consumption of retained austenite during uniform deformation. Therefore, without the help of TRIP effect from retained austenite to release the stress, the brittle fracture occurred during necking. This result suggested that some stable retained austenite that remained after uniform deformation could suppress brittle collapse and ensure safety in service. Metals 2021, 11, 157 6 of 8

obvious, and some small cracks and brittle fracture were observed within the matrix. This indicated that after large uniform deformation, the plasticity during necking decreased at −196 °C. This may ascribe to the full consumption of retained austenite during uniform deformation. Therefore, without the help of TRIP effect from retained austenite to release Metals 2021, 11, 157 the stress, the brittle fracture occurred during necking. This result suggested that some 6 of 8 stable retained austenite that remained after uniform deformation could suppress brittle collapse and ensure safety in service.

Figure 5. SEMFigure images 5. SEM images showing showing the macroscopicthe macroscopic fractographs fractographs of of samples samples tested tested at different at different temperatures: temperatures: (a) −80 °C, (b ()a ) −80 ◦C, Metals −2021120, °C,11, 157(c) − 160 °C, and (d) −196 °C. 7 of 8 (b) −120 ◦C, (c) −160 ◦C, and (d) −196 ◦C.

Figure 6. SEMFigure images 6. SEM images showing showing the the microscopic microscopic fractographs fractographs of ofsamples samples tested tested at different at different temperatures: temperatures: (a) −80 °C, (b ()a ) −80 ◦C, −120 °C, (c) −160 °C, and (d) −196 °C, corresponding to the marked area in Figure 5. (b) −120 ◦C, (c) −160 ◦C, and (d) −196 ◦C, corresponding to the marked area in Figure5. 4. Conclusions The stability of retained austenite was studied in a low carbon low alloy steel pro‐ cessed by the two‐step intercritical heat treatment. The retained austenite exhibited high thermal stability in range of −80 to −196 °C. Whereas, mechanical stability decreased with decreasing temperature from −80 to −196 °C. The decrease in mechanical stability of re‐ tained austenite was helpful for enhancing the uniform elongation via sufficient TRIP ef‐ fect during uniform deformation, resulting in simultaneous enhancement of strength and ductility at low temperatures. However, the full consumption of retained austenite during uniform strain may raise risk of sudden brittle collapse during necking.

Author Contributions: X.W. contributed primarily to the writing and compilation of manuscript and data analysis; Z.X. was heavily involved in the project conceptualization, administration, and planning; C.S. was in charge of reviewing and editing the manuscript; G.H. contributed primarily to carrying out the main experiments and data collection. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (No. 51701012) and the Fundamental Research Funds for Central Universities (No. FRF‐TP‐19‐052A2). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article or supplementary material. Acknowledgments: S. van der Zwaag at Delft University of Technology is acknowledged for his valuable comments and suggestions. Conflicts of Interest: The authors declare no conflict of interest.

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4. Conclusions The stability of retained austenite was studied in a low carbon low alloy steel processed by the two-step intercritical heat treatment. The retained austenite exhibited high thermal stability in range of −80 to −196 ◦C. Whereas, mechanical stability decreased with decreasing temperature from −80 to −196 ◦C. The decrease in mechanical stability of retained austenite was helpful for enhancing the uniform elongation via sufﬁcient TRIP effect during uniform deformation, resulting in simultaneous enhancement of strength and ductility at low temperatures. However, the full consumption of retained austenite during uniform strain may raise risk of sudden brittle collapse during necking.

Author Contributions: X.W. contributed primarily to the writing and compilation of manuscript and data analysis; Z.X. was heavily involved in the project conceptualization, administration, and planning; C.S. was in charge of reviewing and editing the manuscript; G.H. contributed primarily to carrying out the main experiments and data collection. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (No. 51701012) and the Fundamental Research Funds for Central Universities (No. FRF-TP-19-052A2). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Acknowledgments: S. van der Zwaag at Delft University of Technology is acknowledged for his valuable comments and suggestions. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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