Thermal Stability of Retained Austenite and Properties of a Multi-Phase Low Alloy Steel

Article Thermal Stability of Retained Austenite and Properties of A Multi-Phase Low Alloy Steel

Zhenjia Xie *, Lin Xiong, Gang Han, Xuelin Wang and Chengjia Shang *

Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (L.X.); [email protected] (G.H.); [email protected] (X.W.) * Correspondence: [email protected] (Z.X.); [email protected] (C.S.); Tel.: +86-10-62332428 (Z.X. & C.S.)

Received: 16 September 2018; Accepted: 29 September 2018; Published: 9 October 2018

Abstract: In this work, we elucidate the effects of tempering on the microstructure and properties in a low carbon low alloy steel, with particular emphasis on the thermal stability of retained austenite during high-temperature tempering at 500–700 ◦C for 1 h. Volume fraction of ~14% of retained austenite was obtained in the studied steel by two-step intercritical heat treatment. Results from transmission electron microscopy (TEM) and X-ray diffraction (XRD) indicated that retained austenite had high thermal stability when tempering at 500 and 600 ◦C for 1 h. The volume fraction was ~11–12%, the length and width remained ~0.77 and 0.21 µm, and concentration of Mn and Ni in retained austenite remained ~6.2–6.6 and ~1.6 wt %, respectively. However, when tempering at 700 ◦C for 1 h, the volume fraction of retained austenite was decreased largely to ~8%. The underlying reason could be attributed to the growth of austenite during high-temperature holding, leading to a depletion of alloy contents and a decrease in stability. Moreover, for samples tempered at 700 ◦C for 1 h, retained austenite rapidly transformed into martensite at a strain of 2–10%, and a dramatic increase in work hardening was observed. This indicated that the mechanical stability of retained austenite decreased.

Keywords: thermal stability; retained austenite; multi-phase microstructure; low alloy steel

1. Introduction It has been recognized that steels with retained austenite processes a good combination of strength and ductility due to the transformation-induced plasticity (TRIP) effect of retained austenite [1–3]. Retained austenite has become an essential component in the development of new-generation, advanced, high-strength steels in the automobile industry. In addition, stable retained austenite has been suggested to be helpful for the improvement of low-temperature toughness by lowering ductile–brittle transition temperature (DBTT). Much attention has been paid to the development of high-strength, high-toughness steels via the introduction of stable retained austenite. Significant progress has been made. In quenched and tempered nickel-rich (5–9 wt %) steels, high toughness at a cryogenic temperature of −196 ◦C was achieved due to the existence of stable film-like retained austenite along grain boundaries and martensite laths [4,5]. By combination of the TRIP effect and the martensite aging effect, an ultra-high strength (1.5 GPa) with a good combination of ductility and toughness was obtained in maraging steel [6]. In ultra-fine super-bainite steels, a very high strength (2.5 GPa) and good toughness was achieved by obtaining stable retained austenite [7]. In recent years, retained austenite has been introduced to low-carbon (<0.1 wt %), low-alloy (~2–3 wt % Mn) steels [8,9]. A high yield strength (~500–700 MPa) with excellent ductility and high toughness at low temperatures was obtained with a multi-phase microstructure containing a stable film-like retained austenite [10,11], which has great potential for structural engineering

Metals 2018, 8, 807; doi:10.3390/met8100807 www.mdpi.com/journal/metals Metals 2018, 8, 807 2 of 11 Metals 2018, 8, x FOR PEER REVIEW 2 of 11 applications.galvanization, However,the heat effect retained zone austenite during welding is inevitably, and fire exposed-resistant to highapplications temperatures [12]. This in certainbrings processes,about risks such due asto galvanization,the deteriorated the stability heat effect of retained zone during austenite. welding, This and suggest ﬁre-resistants that retained applications austenite [12]. Thisdecompose brings abouts into risksthermodynamically due to the deteriorated stable ferrite stability and of cementite, retained austenite. leading Thisto a suggestssudden deterioration that retained austenitein toughness decomposes and an increase into thermodynamically in DBTT, known stable as tempering ferrite and embrittlement cementite, leading [13–15]. to Therefore, a sudden deteriorationunderstanding in the toughness thermal and stability an increase of retained in DBTT, austenite known upon as tempering tempering is embrittlement an essential topic [13–15 to]. Therefore,design low understanding-carbon, low-alloy the steel thermal with stability retained of austenite. retained austenite upon tempering is an essential topicIn to this design work, low-carbon, a multi-phase low-alloy microstructure steel with containing retained austenite. retained austenite was produced via two- step In intercritical this work, heat a multi-phase treatment in microstructure a low-carbon, containing low-alloy retained steel, as austenite our previous was work produced [8]. The via two-stepinfluence intercriticalof tempering heat at 500 treatment–700 °C inon athe low-carbon, multi-phase low-alloy microstructure steel, as was our studied previous by workscanning [8]. Theelectron inﬂuence microscopy of tempering (SEM), at transmission 500–700 ◦C on electron the multi-phase microscopy microstructure (TEM), and wasX-ray studied diffraction by scanning (XRD) electronwith particular microscopy emphasis (SEM), on transmission the thermal electron stability microscopy of retained (TEM), austenite. and X-rayMoreover, diffraction the effect (XRD) of with the particularstability of emphasis retained austenite on the thermal on properties stability was of retained investigated. austenite. Moreover, the effect of the stability of retained austenite on properties was investigated. 2. Experimental Material and Procedure 2. Experimental Material and Procedure The experimental steel was designed with a chemical composition of 0.1C-2.0Mn-1.5(Si + Al)- 1.0CuThe-1.0Ni experimental-0.3Mo. High steel silicon was and designed aluminum with was a chemical designed composition to suppress ofcarbide 0.1C-2.0Mn-1.5(Si formation and + Al)-1.0Cu-1.0Ni-0.3Mo.increase the stability of the High retained silicon austenite. and aluminum Molybdenum was designed was added to suppress mainly carbidefor solid formation-solution andstrengthening increase the and stability retarding of the recovery retained during austenite. high Molybdenum-temperature was tempering added mainly [12]. The for solid-solution experimental strengtheningsteel was smelted and by retarding a 50 kg vacuum recovery induction during high-temperature furnace (ZGXL-0.05, tempering Jinzhou, [China)12]. The, forged experimental into rod steeldiameter was smeltedof 14 mm. by In a order 50 kg vacuumto eliminate induction the effect furnace of forging, (ZGXL-0.05, the forged Jinzhou, rods China),were held forged at 980 into °C rod for diameter20 min, and of 14water mm. was In orderthen quenched to eliminate to room the effect temperature. of forging, The the two forged-step rods intercritical were held heat at 980treatmen◦C fort 20and min, subsequent and water tempering was then process quenched are to illustrated room temperature. in Figure The1. The two-step quenched intercritical rods were heat re- treatmentheated to and780 ° subsequentC for 30 min tempering, and water process cooled are to illustratedambient temperature in Figure1.. TheThe quenchedrods were rodsthen wereheld re-heatedat 680 °C for to 78030 min◦C, for and 30 air min, cooled and waterto room cooled temperature. to ambient Finally, temperature. these heat The-treated rods were samples then were held attempered 680 ◦C for at 30500, min, 600, and and air 700 cooled °C for to1 h room to investigate temperature. the thermal Finally, stability these heat-treated of retainedsamples austenite were and temperedthe effect on at 500,mechanical 600, and properties. 700 ◦C for 1 h to investigate the thermal stability of retained austenite and the effect on mechanical properties.

Two-step intercritical treatment

780 °C, 30 min Tempering 700 °C, 1 h 680 °C, 30 min

600 °C, 1 h

500 °C, 1 h Temperature

Air cooling

Time FigureFigure 1.1. Schematic diagram of the heat treatment in this work. Standard tensile tests with a diameter of 5 mm and gauge length of 25 mm were conducted Standard tensile tests with a diameter of 5 mm and gauge length of 25 mm were conducted at at room temperature on longitudinal samples, machined according to ASTM using computerized room temperature on longitudinal samples, machined according to ASTM using computerized universal testing system at a strain rate of 1 × 10−3 s−1. SEM samples with a dimension of universal testing system at a strain rate of 1 × 10−3 s−1. SEM samples with a dimension of φ14 × 4 mm ϕ14 × 4 mm were cut from the edge of tensile specimens for different tempering temperatures. were cut from the edge of tensile specimens for different tempering temperatures. After mechanical grounding and polishing, SEM samples were etched with 3% nital and characterized by a ZEISS ULTRA-55 field emission scanning electron microscope (FE-SEM, ZEISS, Jena, Germany) operated at

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After mechanical grounding and polishing, SEM samples were etched with 3% nital and characterized Metals 2018, 8, x FOR PEER REVIEW 3 of 11 by a ZEISS ULTRA-55 ﬁeld emission scanning electron microscope (FE-SEM, ZEISS, Jena, Germany) 20operated kV. After at 20SEM kV. observation, After SEM observation,samples were samples ground were and groundelectron and-polished electron-polished to determine to the determine volume fractionthe volume of the fraction retained of austenite the retained by X-ray austenite diffraction by X-ray(XRD) diffraction. The solution (XRD). was ethanolThe solution/perchloric was acidethanol/perchloric/glycerol = 8.5:1:0.5, acid/glycerol with a constant = 8.5:1:0.5 voltage, with a of constant 18 V and voltage a current of 18 V of and ~1.2 a current A. Quantitative of ~1.2 A. assessmentsQuantitative of assessments retained austenite of retained were austenite carried were out carried by XRD out (Smartlab by XRD (Smartlab 9KW, Tokyo, 9KW, JapanTokyo,) Japan)using CuKαusing CuKradiation.α radiation. The volume The volume fraction fraction of retained of retained austenite austenite was was estimated estimated by by measuring measuring the the pea peakk intensityintensity of of (200)α, (200)α, (211)α, (211)α, (200)γ, (200)γ, (211)γ (211)γ,, and and (311)γ. (311)γ. Moreove Moreover,r, e electronlectron back back-scattered-scattered diffraction diffraction (EBSD(EBSD,, EDAX EDAX Inc., Inc., Hikari, Hikari, NJ, NJ, USA) USA in) the in theSEM SEM operated operated at 20 kVat 20 was kV also was used also to studyused to the study evolution the evolutionof retained of austenite retained with austenite different with tensile different strains. tensile The stepstrai sizens. T washe step 0.1 µ sizem. Standard was 0.1 3μm mm. Standard TEM discs 3 mmwere TEM prepared discs were by a twin-jetprepared polishing by a twin technique-jet polishing using technique an electrolyte using an of 10%electrolyte perchloric of 10% acid perchloric and 90% acidethanol. and Microstructures90% ethanol. Microstructures after tensile tests after were tensile observed tests were in a observed JEOL JEM-2100FS in a JEOL TEM JEM (JEOL,-2100FS Tokyo, TEM (JEOL,Japan) Tokyo, with an Japa energyn) with dispersive an energy X-ray dispersive spectrometer X-ray (EDS) spectrometer operated (EDS) at 200 operated kV. at 200 kV.

3.3. Results Results and and D Discussioniscussion Figure2 presents the SEM microstructure for the experimental steel before and after Figure 2 presents the◦ SEM microstructure for the experimental steel before and after tempering attempering 500–700 ° atC. 500–700After twoC.-step After intercritical two-step heat intercritical treatment, heat a multi treatment,-phase a microstructure multi-phase microstructure consisting of intercriticalconsisting of ferrite, intercritical martensite/retained ferrite, martensite/retained austenite was austenite obtained, was obtained, as shown as in shown Figure in Figure2a. The2a. microstructuralThe microstructural development development and and stabilization stabilization of retained of retained austenite austenite was was elucidated elucidated in in our our previous previous ◦ studiesstudies [ [88––1111].]. It It waswas suggested suggested that, that during, during intercritical intercritical annealing annealing at 780 at C, 780 reverted °C, reverted austenite austenite formed formeduniformly uniformly along prior along austenite prior austenite grain boundaries grain boundaries and martensite and martensite matrix lath matrix boundaries. lath boundaries. With the Withgrowth the of growth reverted of reverted austenite austenite during holding, during holding, alloying alloying elements elements including including C, Mn, andC, Mn Ni, thatand areNi thatpartitioned are partitioned into the into reverted the reverted austenite austenite form anform adjacent an adjacent annealed annealed martensite martensite matrix, matrix, leading leading to a tomixture a mixture microstructure microstructure of of alloying-enriched alloying-enriched austenite austenite and and alloying-depleted alloying-depleted annealedannealed martensite martensite (intercritical(intercritical ferrite). ferrite). Due Due to tothe the hig highh hardenability hardenability of the of thealloying alloying-enriched-enriched reverted reverted austenite, austenite, the austenitethe austenite transformed transformed to martensite to martensite during during the the subsequent subsequent water water cooling. cooling. Hence, Hence, a a dual dual-phase-phase microstructuremicrostructure consisting consisting of of intercritical intercritical ferrite ferrite and and martensite martensite was was obtained obtained by by the the first ﬁrst interc intercriticalritical ◦ ◦ annealingannealing at at 780 780 °CC.. By By the the second second step step,, intercritical intercritical annealing annealing at at 680 680 °CC,, reverted reverted austenite austenite formed formed atat the the alloying alloying-enriched-enriched martensite, martensite, and and was was further further enriched enriched by byC, Mn C,, Mn, and andNi. Because Ni. Because of the of high the enrichmenthigh enrichment of alloying of alloying elements elements and fine and grain ﬁne grainsize of size reverted of reverted austenite, austenite, retained retained austenite austenite was obtained,was obtained, leading leading to a multi to a multi-phase-phase microstructure microstructure consisting consisting of intercritical of intercritical ferrite, ferrite, martensite martensite,, and ◦ retainedand retained austenite. austenite. After tempering After tempering at 500–700 at 500–700°C for 1 h,C there for 1 is h, no there obvious is no change obvious in the change multi in- phthease multi-phase microstructure microstructure consisting consistingof intercritical of intercritical ferrite and ferrite tempered and temperedmartensite/retained martensite/retained austenite. Theaustenite. multi-phase The multi-phase microstructure microstructure remained a remainedlamina morphology. a lamina morphology. It indicated that It indicated the multi that-phase the microstructuremulti-phase microstructure had good thermal had good stability thermal upon stability high-temp uponeratur high-temperaturee tempering. tempering. a b Intercritical ferrite

Martensite/RA 2 μm 2 μm

Figure 2. Cont.

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cc dd

2 2 μmμm 2 2 μmμm

Figure 2. SEM images of the studied steel before (a) and after tempering at different temperatures: FigFigureure 2 2.. SEM SEM images images of of the the studied studied steel steel before before ( (aa)) and and after after tempering tempering at at different different temperatures: temperatures: ( (bb)) (b) 500 ◦C, (c) 600 ◦C, (d) 700 ◦C. 500500 °°CC,, ((cc)) 600 600 °°CC,, ((dd)) 700700 °°CC. . The detailed microstructure of retained austenite for samples before and after tempering at TheThe detaileddetailed microstructuremicrostructure ofof retainedretained austeniteaustenite forfor samplessamples beforebefore andand afterafter tempertemperinging atat 500500–– 500–700 ◦C was characterized by TEM. Figure3 presents the TEM microstructure of sample after the 700700 °°CC waswas characterizedcharacterized byby TEM.TEM. FigFigureure 33 presentspresents thethe TEMTEM microstructuremicrostructure ofof samplesample afterafter thethe twotwo-- two-step intercritical treatment without the third step of tempering. The TEM bright-field image of stepstep intercritical intercritical treatment treatment without without the the third third step step of of tempering. tempering. TheThe TEMTEM bright bright--fieldfield image image of of microstructure revealed that retained austenite (the dark areas in Figure3a) manifested a film-like microstructuremicrostructure revealedrevealed thatthat retainedretained austeniteaustenite (th(thee darkdark areasareas inin FigFigureure 3a)3a) manifestedmanifested aa filmfilm--likelike morphology trapped between ferritic matrix laths and presented a granular shape along prior austenite morphologymorphology trapped trapped between between ferritic ferritic matrix matrix laths laths and and presented presented a a granular granular shape shape along along prior prior grain boundaries, as did the corresponding dark-field image for retained austenite (Figure3b) and austeniteaustenite graingrain boundaries,boundaries, asas diddid thethe correspondingcorresponding darkdark--fieldfield imageimage forfor retainedretained austeniteaustenite ((FigureFigure selected area diffraction pattern (SADP) analysis with an austenite zone axis of [1 0 0] (Figure3c). 3b)3b) andand selecselectedted area area diffraction diffraction pattern pattern (SADP) (SADP) analysis analysis with with anan austeniteaustenite zone zone axisaxis of of [1 [1 0 0 0] 0] ( (FigureFigure After tempering at 500 ◦C, the retained austenite remained thermodynamically stable within the 3c).3c). AfterAfter temperingtempering atat 500500 °°CC,, thethe retainedretained austeniteaustenite remainedremained thermodynamicallythermodynamically stablstablee withinwithin thethe matrix, and no decomposition of retained austenite into nano-sized carbides and ferrite was observed, matrix,matrix, andand nono decomposition decomposition of of retained retained austenite austenite into into nano nano--sizsizeded carbides carbides and and ferrite ferrite was was as shown in Figure4a,b. SADP analysis results (Figure4c) confirmed that the dark areas in the observed,observed, asas shownshown inin FigFigureure 4a4a,,b.b. SADPSADP analysisanalysis resultsresults ((FigureFigure 4c)4c) confirmedconfirmed thatthat thethe darkdark areasareas inin TEM bright-field image (Figure4a) and light areas in the TEM dark-field image (Figure4b) were thethe TEMTEM brightbright--fieldfield imageimage ((FigureFigure 4a)4a) andand lightlight areasareas inin thethe TEMTEM darkdark--fieldfield imageimage ((FigureFigure 4b)4b) werewere face-centered-cubic (FCC) retained austenite under an austenite zone axis of [1 1 0]. When the faceface--centeredcentered--cubiccubic (FCC) (FCC) retainedretained austenite austenite under under anan austeniteaustenite zone zone axis axis of of [1[1 1 1 0]. 0]. When When thethe tempering temperature was up to 600 or 700 ◦C, the multi-phase microstructure of the studied steel temperingtempering temperaturetemperature waswas upup toto 600600 oror 700700 °°CC,, thethe multimulti--phasephase microstructuremicrostructure ofof thethe studiedstudied steelsteel was also thermally stable. No retained austenite decomposed into carbides or ferrite, as shown in waswas alsoalso thermallythermally stablestable.. NNoo retainedretained austeniteaustenite decomposeddecomposed intointo carbidescarbides oror ferrite,ferrite, aass shownshown inin Figure4d,e, respectively. Retained austenite kept in film-like morphology within the ferritic laths and FigFigureure 4d 4d,,e,e, respectively. respectively. Retained Retained austenite austenite kept kept in in film film--likelike morphology morphology within within the the ferritic ferritic laths laths and and granular type at prior austenite grain boundaries. granulargranular typetype atat priorprior austeniteaustenite graingrain boundaries.boundaries.

aa bb cc

11 μμmm 11 μmμm 55 1/nm1/nm

Figure 3. a FFigureigure 33. . (((aa))) Bright Bright-ﬁeld Bright--fieldfield TEM TEM TEM images images images of of of retained retained retained austenite austeniteaustenite in in in the the studied studied steel steel afteafter afterr the the two-steptwo two--stepstep intercritical treatment before tempering; (b) dark-ﬁeld TEM images of retained austenite for the red intercriticalintercritical treatmenttreatment beforebefore tempering;tempering; ((bb)) ddarkark--fieldfield TEMTEM imagesimages ofof retainedretained austeniteaustenite forfor thethe redred rectangle area; (c) corresponding diffraction pattern of retained austenite under the zone axis of [1 0 0]. rectanglerectangle area;area; ((cc)) ccorrespondingorresponding diffractiondiffraction patternpattern ofof retainedretained austeniteaustenite underunder thethe zonezone axisaxis ofof [1[1 00 0].0].

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a b

1 μm 1 μm

1 μm

Figure 4.4. (a,,c,,d) Bright Bright-ﬁeld-field TEM images of retained retained austenite austenite (RA) (RA) for for samples samples tempered tempered at 500, 600 600,, and 700 ◦°CC,, respectively; (b) dark-ﬁelddark-field images of RA in ((a).).

The volume fraction of retained austenite in experimental steel before and after tempering at 500–700 °C was determined by XRD, and the obtained XRD spectra and calculated results are presented in Figure 5. The specimen before tempering contained the maximum volume fraction of

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The volume fraction of retained austenite in experimental steel before and after tempering atMetals 500–700 2018, 8◦, Cx FOR was PEER determined REVIEW by XRD, and the obtained XRD spectra and calculated results6 of are 11 presented in Figure5. The specimen before tempering contained the maximum volume fraction of ~14%. After tempering at 500 and 600 °C, the volume fraction of retained austenite slightly decreased ~14%. After tempering at 500 and 600 ◦C, the volume fraction of retained austenite slightly decreased to ~12 and 11%, respectively. It can be deduced that retained austenite in the studied steel obtained to ~12 and 11%, respectively. It can be deduced that retained austenite in the studied steel obtained by the two-step intercritical treatment had good thermal stability upon tempering at 500–600 °C. by the two-step intercritical treatment had good thermal stability upon tempering at 500–600 ◦C. However, after tempering at 700 °C, the volume fraction of retained austenite was reduced largely to However, after tempering at 700 ◦C, the volume fraction of retained austenite was reduced largely to ~8%. The decrement in volume fraction of retained austenite should be attributed to the growth of ~8%. The decrement in volume fraction of retained austenite should be attributed to the growth of austenite during holding at a high-temperature tempering of 700 °C, leading to a decreased chemical austenite during holding at a high-temperature tempering of 700 ◦C, leading to a decreased chemical stability of austenite due to the depleted alloying elements and grain coarsening. stability of austenite due to the depleted alloying elements and grain coarsening.

2 1 1  2 0 0  2 0 0  2 2 0  3 1 1  f =8% Tempering at 700°C 

f =11%

Tempering at 600°C 

Intensity, a.u. Intensity, f =12% Tempering at 500°C 

f =14% Before tempering 

50 60 70 80 90 100 2 degree

Figure 5. XRD spectra for samples before and after tempering at 500–700 ◦C. Figure 5. XRD spectra for samples before and after tempering at 500–700 °C. The dimension in length and width of austenite areas was measured from corresponding TEM micrographsThe dimension of samples in length before and afterwidth different of austenite tempering areas treatments.was measured Each fro datam corresponding was obtained fromTEM atmicrographs least 50 measurements, of samples andbefore the obtainedand after resultsdifferent are plottedtempering in Figure treatments.6. It can Each be seen data that was the obtained length andfrom width at least of austenite 50 measurements, for sample and before the temperingobtained results and samples are plotted tempered in Figure at 500 6. and It can 600 be◦C seen are similar,that the andlength they and are width ~0.77 of and austenite ~0.21 µ m,for respectively. sample before After tempering tempering and at samples 700 ◦C, tempered the dimension at 500 of and austenite 600 °C areasare similar, largely and increased, they are the ~0.77 length and and ~0.21 width μm, wererespectively. up to ~1.7 After and tempering 0.28 µm, respectively. at 700 °C, the In dimension addition, EDSof austenite analysis wasareas carried largely out increased, to detect the concentrations length and width of the alloyingwere upelements to ~1.7 and (Mn 0.28 and μ Ni)m, respec in austenite.tively. EachIn addition, data was EDS determined analysis was from carried at least out 10 measurements,to detect concentrations and the analyzedof the alloying results elements are presented (Mn and in FigureNi) in6 austenite.. After the Each two-step data intercriticalwas determined treatment, from at manganese least 10 measurements and nickel in, austenite and the analyzed were enriched results toare as presented high as ~6.8 in andFigure 1.7 wt6. %, After respectively. the two-step After int temperingercritical treatment, at 500 and 600manganese◦C, the concentration and nickel in ofaustenite manganese were and enriched nickel into as austenite high as remained~6.8 and 1.7 similar wt % to, respectively. the sample before After tempering tempering, at ~6.2–6.6 500 and and 600 ~1.6°C, wtthe %, concentration respectively, of whereas manganese the concentrations and nickel in of austenite manganese remained and nickel similar for samplesto the sample tempered before at 700tempering,◦C decreased ~6.2–6.6 to ~5.4and and~1.6 ~1.4wt % wt, respectively %, respectively., whereas the concentrations of manganese and nickel for samples tempered at 700 °C decreased to ~5.4 and ~1.4 wt %, respectively.

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10 2.4 Length of austenite Width of austenite 2.1 Mn in austenite 8 Ni in austenite 1.8 Average concentration of

alloying element,

m 1.5 6 

1.2 4 0.9

wt.% Average size, Average 0.6 2 0.3

0.0 0 25 500 600 700 Tempering temperature, C

Figure 6. Statistic results of dimension (solid symbol) and alloying elements (Mn and Ni) concentration Figure 6. Statistic results of dimension (solid symbol) and alloying elements (Mn and Ni) (blank symbol) of retained austenite by TEM images for samples before and after tempering at concentration (blank symbol) of retained austenite by TEM images for samples before and after 500–700 ◦C. tempering at 500–700 °C. On the basis of our previous studies [8,11] on microstructure evolution and mechanism of retained On the basis of our previous studies [8,11] on microstructure evolution and mechanism of austenite during the two-step intercritical treatment, the thermal stability of retained austenite in the retained austenite during the two-step intercritical treatment, the thermal stability of retained present study upon tempering can be interpreted as follows. It was revealed that the first step of austenite in the present study upon tempering can be interpreted as follows. It was revealed that the intercritical annealing decreased Ac1 temperature (designated as Ac1’ temperature) of the experimental first step of intercritical annealing decreased Ac1 temperature (designated as Ac1’ temperature) of the steel due to the enrichment of alloying elements in martensite. Therefore, reverted austenite was experimental steel due to the enrichment of alloying elements in martensite. Therefore, reverted able to form at a relative low temperature in the range of Ac1’-Ac1 and was stabilized and retained austenite was able to form at a relative low temperature in the range of Ac1’-Ac1 and was stabilized because of a further enrichment of alloy content and fine grain size [16–18]. When the third tempering and retained because of a fur◦ ther enrichment of alloy content and fine grain size [16–18]. When the temperature (500 and 600 C) was lower than Ac1’. Retained austenite was stable and no further third tempering temperature (500 and 600 °C) was lower than Ac1’. Retained ◦austenite was stable and growth was observed in this work. When the tempering temperature (700 C) was higher than Ac1’, no further growth was observed in this work. When the tempering temperature (700 °C) was higher an obvious growth of austenite was observed both in thickness and length. With the growing of than Ac1’, an obvious growth of austenite was observed both in thickness and length. With the austenite, the alloy content in austenite was depleted, and the stability of austenite was thus reduced. growing of austenite, the alloy content in austenite was depleted, and the stability of austenite was Furthermore, the increase in grain size of austenite also largely decreased the stability of austenite. thus reduced. Furthermore, the increase in grain size of austenite also largely decreased the stability As a result, some retained austenite developed into martensite and the volume fraction of retained of austenite. As a result, some retained austenite developed into martensite and the volume fraction austenite decreased to 8% after tempering at 700 ◦C for 1 h in this study. of retained austenite decreased to 8% after tempering at 700 °C for 1 h in this study. On the other hand, retained austenite exhibited high thermal stability on tempering at 500 and On the other hand, retained austenite exhibited high thermal stability on tempering at 500 and 600 ◦C, and no obvious decomposition into ferrite and carbides was observed. This was different 600 °C, and no obvious decomposition into ferrite and carbides was observed. This was different from the retained austenite in conventional CMnSiAl TRIP steel as reported in [18], in which retained from the retained austenite in conventional CMnSiAl TRIP steel as reported in [18], in which retained austenite decomposed into ferrite and carbides when tempered at 300–500 ◦C and the kinetics of austenite decomposed into ferrite and carbides when tempered at 300–500 °C and the kinetics of the the decomposition was accelerated when the tempering temperature was increased. It has been decomposition was accelerated when the tempering temperature was increased. It has been suggested that the thermal stability of retained austenite is strongly affected by carbon content [19,20]. suggested that the thermal stability of retained austenite is strongly affected by carbon content It was found that, because of its lower carbon content, large block-type retained austenite had a high [19,20]. It was found that, because of its lower carbon content, large block-type retained austenite had thermal stability compared to small film-type retained austenite [21], leading to a lower driving force a high thermal stability compared to small film-type retained austenite [21], leading to a lower for carbide formation. Based on our previous study using atom probe tomography with a similar driving force for carbide formation. Based on our previous study using atom probe tomography with steel by this two-step intercritical treatment, the carbon content in the retained austenite was ~0.4 wt a similar steel by this two-step intercritical treatment, the carbon content in the retained austenite was %[9]. The carbon concentration was much lower than the ~1 wt % in the retained austenite in the ~0.4 wt % [9]. The carbon concentration was much lower than the ~1 wt % in the retained austenite conventional TRIP steel [22,23]. In addition, high silicon and aluminum (~1.5 wt % in total) were added in the conventional TRIP steel [22,23]. In addition, high silicon and aluminum (~1.5 wt % in total)

Metals 2018, 8, x FOR PEER REVIEW 8 of 11

Metals 2018, 8, 807 8 of 11 were added in the studied steel, which can suppress carbide formation during tempering. Therefore, the high thermal stability of retained austenite was obtained when tempered at 500 and 600 °C. in the studiedUniaxial steel, tensile which tests canwere suppress carried out carbide at room formation temperature during to tempering. reveal the mechanical Therefore, thestability high of thermalretained stability austenite of retained and its austeniteeffect on wasductility obtained for samples when tempered before and at 500 afte andr tempering. 600 ◦C. The received strUniaxialess–strain tensile curves tests are were plotted carried in outFigure atroom 7. From temperature these engineering to reveal the stress mechanical–strain curves, stability yield of retainedstrength, austenite tensile and strength, its effect uniform on ductility and total for elongation samples before were and determined. after tempering. As seen The from received Figure 7, stress–strainsamples before curves and are plotted after tempering in Figure7 a.t From 500 these and 600 engineering °C possessed stress–strain similar curves, stress– yieldstrain strength, behavior, tensileexhibiting strength, a yield uniform strength and of total ~649 elongation–664 MPa, a were tensile determined. strength of As~766 seen–773 from MPa, Figure and a 7high, samples ductility before(a uniform and after elongation tempering of at ~18 500–20% and and 600 ◦aC total possessed elongation similar of stress–strain ~35–36%). This behavior, finding exhibiting implies that a yieldretained strength austenite of ~649–664 after it MPa, is tempered a tensile strengthat 500 and of ~766–773600 °C has MPa, a mechanical and a high stability ductility similar (a uniform to that elongationbefore tempering. of ~18–20% Unlike and athe total thermal elongation stability of ~35–36%). of retained This austenite, ﬁnding the implies mechanical that retained stability austenite is largely afterdepende it is temperednt on chemical at 500 and composition 600 ◦C has and a mechanical the grain stabilitysize of retained similar toaustenite that before, shape tempering., and distributio Unlike n the[2 thermal4], partitioning stability of of strain retained between austenite, phases the [25 mechanical,26]. The similar stability mechanical is largely stability dependent could on be chemical attributed compositionto their similar and the chemical grain size composition of retained and austenite, grain size shape, for and samples distribution before [and24], partitioningafter tempering of strain at 500 betweenand 600 phases °C. However, [25,26]. The samples similar tempered mechanical at 700 stability °C had could a different be attributed stress– tostr theirain behavior, similar chemical as shown compositionin Figure 7. and For grainsamples size tempered for samples at 700 before °C, it and manifested after tempering a low yield at 500 strength and 600 of ~580◦C. However, MPa and a sampleshigh tensile tempered strength at 700 of◦ ~877C had MPa, a different presenting stress–strain rapid work behavior, hardening as shown to fracture. in Figure The7. low For yield samples stress temperedcould be at attributed 700 ◦C, it to manifested severe recovery a low yielddue to strength high-temp of ~580erature MPa aging. and aThe high rapid tensile work strength hardening of ~877route MPa, may presenting be caused rapid by the work decrease hardening in the tomechanical fracture.The stability low yieldof retained stress couldaustenite be attributeddue to the tolow severealloy recovery content, dueleadi tong high-temperature to dramatic transformation aging. Thes to rapid martensite work hardening during early route strain. may be caused by the decrease in the mechanical stability of retained austenite due to the low alloy content, leading to dramatic transformations to martensite during early strain.

900

800

700

600

500

Before Tempering

400 Tempering at 500C Tempering at 600C 300 Tempering at 700C

EngineeringMPa Stress, 200

100

0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Engineering strain

Figure 7. Engineering stress–strain curves for samples before and after tempering at 500–700 ◦C. Figure 7. Engineering stress–strain curves for samples before and after tempering at 500–700 °C. In the present work, EBSD studies were conducted to investigate the mechanical stability of ◦ retainedIn austenite the present for work, samples EBSD tempered studies at were 700 conductedC. The analyzed to investigate EBSD t resultshe mechanical are presented stability in of Figureretained8a–c, austenite corresponding for samples to specimens tempered after at 700 stains °C. The of 2,analyzed 5, and EBSD 10%. Asresults seen are in presented Figure8a, in after Figure ε an8a,b early,c, stagecorresponding of deformation to specimens ( = 2%), after the stains retained of 2, austenite 5, and 10%. was As uniformly seen in Figure distributed 8a, after between an early matrixstage lathsof deformation in film-like (ε shape = 2%), and the retained at grain boundariesaustenite was in uniformly blocky morphology. distributed Darkbetween areas, matrix which laths werein film considered-like shape hard and martensite at grain boundaries [27] with ain low block bandy morphology contrast (BC). Dark value, areas were, which observed were considered nearby bothhard types martensite of retained [27 austenite] with a (film-likelow band type contrast and block (BC) type). value, This were finding observed implies nearby that the both retained types of austeniteretained was austenite partially (film transformed-like type and into block martensite type). This at low finding strain. implies Similar that results the retained were reportedaustenite inwas conventionalpartially transformed TRIP steel [24 into]. Whenmartensite strain at was low increased strain. Similar to 5% (Figure results8 b), were the reported martensite in (dark conventional area) increased,TRIP steel and [2 the4]. When retained strain austenite was increa decreased.sed to Quantitative5% (Figure 8b) analysis, the martensite results indicated (dark area) that ~50%increased of , theand retained the retained austenite austenite transformed decreased. into martensiteQuantitative after analysis a strain results of 5% indicated in comparison that ~50% with of the the sample retained afteraustenite a strain transformed of 2%. When into strain martensite was furtherafter a strain increased of 5% to in 10%, comparison almost allwith retained the sample austenite after a was strain transformed into martensite, and little of the retained austenite at the grain boundaries remained,

Metals 2018, 8, x FOR PEER REVIEW 9 of 11 Metals 2018, 8, 807 9 of 11 of 2%. When strain was further increased to 10%, almost all retained austenite was transformed into martensite, and little of the retained austenite at the grain boundaries remained, as shown in Figure ◦ as shown8c. This in implies Figure8 c.that This the implies retained that austen the retainedite in sample austenites tempered in samples at tempered700 °C presented at 700 C a presentedrelative low a relativemechanical low stability mechanical compared stability to comparedthose in conventional to those in TRIP conventional steel [24]. TRIPTherefore, steel a [24 dramatic]. Therefore, increase a dramaticin work increasehardening in workwas observed hardening at was a strain observed of 2– at10%. a strain Nevertheless, of 2–10%. by Nevertheless, modifying bywork modifying hardening workbehavior hardening due behavior to strain due-induced to strain-induced martensite martensitetransformation transformation of retained of austenite, retained austenite, high ductility high (a ductilityuniform (a uniformelongation elongation of ~19% ofand ~19% a total and elongation a total elongation of ~33%) of was ~33%) obtained. was obtained.

a b

Deformation direction

FigureFigure 8. EBSD 8. EBSD micrographs micrographs of retained of retained austenite austenite for samplesfor samples tempered tempered at 700 at 700◦C at°C different at different tensile tensile strains:strains: (a) strain(a) strainε = ε 2%, = 2%, (b) ( strainb) strainε = ε 5%, = 5%, (c) ( strainc) strainε = ε 10% = 10% (red (red corresponds corresponds to retainedto retained austenite). austenite).

4. Conclusions4. Conclusions In thisIn this study, study, multi-phase multi-phase microstructure microstructure consisting consisting of intercriticalof intercritical ferrite, ferrite, martensite, martensite and, and ~14% ~14% volumevolume fraction fraction retained retained austenite austenite was was obtainedobtained by a a novel novel two two-step-step intercritical intercritical treatment treatment in 0.1C in - 0.1C-2.0Mn-1.5(Si,2.0Mn-1.5(Si, Al) Al)-1.0Cu-1.0Ni-1.0Cu-1.0Ni steel. steel. Tempering Tempering effects effects on on microstructure microstructure evolution, evolution, the stabilitystability of of retainedretained austenite,austenite, and and the the resultant resultant properties properties were were carefully carefully investigated investigated using using SEM, SEM, TEM, TEM, XRD, XRD, andand EBSD. EBSD. The The results results and and conclusions conclusions are are summarized summarized as follows:as follows: (1)(1) Retained Retained austenite austenite obtained obtained by by two-step two-step intercritical intercritical treatment treatment was was thermally thermally stable stable when when ◦ temperingtempering at 500 at 500 and and 600 600C ° forC for 1 h. 1 Theh. The volume volume fraction fraction of retainedof retained austenite austenite was was slightly slightly decreased decreased fromfrom ~14 ~14 to ~11–12%,to ~11–12%, and and alloy alloy contents contents in retainedin retained austenite austenite and and size size in lengthin length and and width width remained remained ◦ similarsimilar to thatto that before before tempering. tempering. In addition, In addition, the experimentalthe experimental steel steel after temperingafter tempering at 500 at and 500 600 andC 600 had°C similar had similar stress–strain stress–str behaviorain behavior to that to beforethat before tempering. tempering. This This implies implies that that there there was was no obviousno obvious changechange in thein the mechanical mechanical stability. stability. (2)(2) The The volume volume fraction fraction of theof the retained retained austenite austenite was was decreased decreased largely largely to ~8%to ~8% after after tempering tempering ◦ at 700at 700C ° forC for 1 h.1 h Results. Results from from TEM TEM characterization characterization suggestsuggest thatthat thethe underlyingunderlying reasonreason could be beattributed to the rapid growth of austenite during during holding holding at at high high temperature temperature.. In In addition, addition, the theretained retained austenite austenite rapidly rapidly transformed transformed into into martensite martensite at a at strain a strain of 2– of10%, 2–10%, and a and dramatic a dramatic increase increasein work in workhardening hardening was observed. was observed. This indicated This indicated that the that mechanical the mechanical stability stability of retained of retained austenite

Metals 2018, 8, 807 10 of 11 austenite was decreased. Nevertheless, by modifying work hardening behavior due to strain-induced martensite transformation of retained austenite, high ductility (a uniform elongation of ~19% and a total elongation of ~33%) was obtained.

Author Contributions: Z.X. analyzed the results and wrote the draft; L.X. and G.H. performed the experiments; X.W. reviewed the draft; C.S. designed and supervised the experiments. Funding: This research was funded by the National Natural Science Foundation of China (No. 51701012) and the Fundamental Research Funds for the Central Universities (No. FRF-TP-17-004A1). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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