metals
Article Effect of Heterogeneous Microstructure on Refining Austenite Grain Size in Low Alloy Heavy-Gage Plate
Shengfu Yuan 1, Zhenjia Xie 2 , Jingliang Wang 2, Longhao Zhu 3, Ling Yan 3, Chengjia Shang 2,3,* and R. D. K. Misra 4 1 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China; [email protected] 2 Colarborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (Z.X.); [email protected] (J.W.) 3 State Key Laboratory of Steel for Marina Equipment and Application, Anshan Steel, Anshan 114021, China; [email protected] (L.Z.); [email protected] (L.Y.) 4 Laboratory for Excellence in Advanced Steel Research, Department of Metallurgical and Materials Engineering, University of Texas at El Paso, El Paso, TX 79912, USA; [email protected] * Correspondence: [email protected]; Fax: +8610-6233-2428
Received: 18 November 2019; Accepted: 13 January 2020; Published: 16 January 2020
Abstract: The present work introduces the role of heterogeneous microstructure in enhancing the nucleation density of reversed austenite. It was found that the novel pre-annealing produced a heterogeneous microstructure consisting of alloying elements-enriched martensite and alloying-depleted intercritical ferrite. The shape of the martensite at the prior austenite grain boundary was equiaxed and acicular at inter-laths. The equiaxed reversed austenite had a K-S orientation with adjacent prior austenite grain, and effectively refined the prior austenite grain that it grew into. The alloying elements-enriched martensite provided additional nucleation sites to form equiaxed reversed austenite at both prior austenite grain boundaries and intragranular inter-lath boundaries during re-austenitization. It was revealed that prior austenite grain size was refined to ~12 µm by pre-annealing and quenching, while it was ~30 µm by conventional quenching. This is a practical way of refining transformation products by refining prior austenite grain size to improve the strength, ductility and low temperature toughness of heavy-gage plate steel.
Keywords: alloying element enrichment; heterogeneous microstructure; nucleation site; grain refinement
1. Introduction Low carbon low alloy steels have been widely used in fields of marine engineering, engineering machines, and other structural applications due to their high strength, excellent toughness, good weldability and low cost [1–3]. With the structural components becoming large and the need for cost effectiveness, there is a significant demand for heavy gauge plate steel. However, for low carbon low alloy heavy-gage plate steel manufactured by thermomechanical controlling process (TMCP), austenite grains in the center of the plate gradually become larger due to insufficient reduction in thickness during controlled rolling. This leads to the deterioration of mechanical properties, especially low-temperature toughness [4–6]. Therefore, a heat treatment process should be applied to optimize the mechanical properties of heavy plate. Austenite grain refinement is well recognized as being of great importance in enhancing mechanical properties of high strength low alloy steels [7–9]. Several efforts have been made towards the refining of austenite grain size. Grange [10] reported that the thermal cycling process between martensite and austenite is helpful in refining austenite grain size by reversed transformation. The effect of the
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Metals 2020, 10, 132 2 of 11 effect of the two-step quenching (TSQ) and one-step quenching (OSQ) heat treatment process on austenite grain size was studied [11], and the results showed that the prior austenite grain size of TSQtwo-step heat-treated quenching specimen (TSQ) andwas one-step 50% finer quenching than OSQ (OSQ) heat heattreatment. treatment Further process studies on austenite[12–14] found grain twosize morphologies was studied [11 of], reversed and the resultsaustenite showed from marten that thesite prior or bainite: austenite equiaxed grain size austenite of TSQ and heat-treated acicular austenitespecimen during was 50% thermal finer than cycling OSQ heatprocess treatment. and multi- Furtherstep studiesheat treatment [12–14] found process. two In morphologies addition, the of mechanismreversed austenite of prior from austenite martensite grain or bainite:refinement equiaxed was attributed austenite and to acicularthe enhancement austenite during of nucleation thermal densitycycling processof equiaxed and austenite multi-step by heat thermally treatment activat process.ed nucleation In addition, or static the mechanism recrystallization of prior [15]. austenite grainThere refinement is a vast was literature attributed [13,14 to the,16–21] enhancement reporting ofthat nucleation reversed density austenite of equiaxednucleates austeniteat the prior by austenitethermally grain activated boundary, nucleation intersection or static recrystallizationof multiple-grains, [15 ].packet boundary, block boundary and cementite.There Miyamoto is a vast literature [22] pointed [13,14 ,out16– 21that] reporting austenite that preferentially reversed austenite nucleates nucleates at the athigh the angle prior boundaryaustenite grainof pearlite-ferrite. boundary, intersection Studies [23,24] of multiple-grains, on the crystallographic packet boundary,characteristics block determined boundary that and acicularcementite. austenite Miyamoto [had22] pointedKurdjumov–Sachs out that austenite (K-S) preferentially orientation nucleates relationship at the high ((111) anglefcc boundary//(011)bcc, [of−101] pearlite-ferrite.fcc//[−1–11]bcc) Studieswith the [23 matrix,,24] on while the crystallographic equiaxed austen characteristicsite nucleated determinedat prior austenite that acicular grain boundariesaustenite had grew Kurdjumov–Sachs into the adjacent (K-S) prior orientation austenite relationship grain at high ((111) angle// misorientation(011) ,[ 101] [23].//[ This1–11] also) fcc bcc − fcc − bcc suggestedwith the matrix, that the while equiaxed equiaxed austenit austenitee was nucleatedbeneficial in at refining prior austenite austenite grain grain boundaries size. In addition, grew into some the literaturesadjacent prior [25–28] austenite published grain that at high cementite angle misorientation was the core [of23 ].austenite This also nucleation. suggested However, that the equiaxed in this study,austenite the was content beneficial of carbon in refining is 0.08 austenite wt. %, grain which size. was In addition,too low to some grow literatures to the [core25–28 of] publishedaustenite nucleation.that cementite In contrast, was the corethe cement of austeniteite was nucleation. easily decomposed. However, inIn thisthis study,study, the a novel content pre-annealing of carbon is process0.08 wt. prior %, which to quenching was too low was to growintroduced to the coreto increase of austenite nucleation nucleation. density In contrast,of equiaxed the cementitereversed austenitewas easily and decomposed. refine austenite In this grain study, size a novel in the pre-annealing core of heavy process plate. priorMicrostructure to quenching evolution was introduced during heatto increase treatment nucleation was studied density to of elucidate equiaxed the reversed mechanism austenite of andaustenite refine grain austenite refinement grain size by in pre- the annealingcore of heavy and plate.quenching. Microstructure evolution during heat treatment was studied to elucidate the mechanism of austenite grain refinement by pre-annealing and quenching. 2. Experimental Material and Procedure 2. Experimental Material and Procedure The chemical composition of experimental steel was C 0.08, Si 0.23, Mn 1.21, P 0.011, S 0.002, Ni 1.1, (CuThe + chemicalCr + Mo) composition < 2.0, Nb 0.022, of experimental B 0.0012 and steel Ti 0.014 was in C 0.08,weight Si 0.23,percent Mn (wt. 1.21, %). P 0.011,The A SC1 0.002, and A NiC3 temperatures1.1, (Cu + Cr +wereMo) measured< 2.0, Nb by 0.022, dilatometry B 0.0012 to and be Ti 715 0.014 and in 863 weight °C, respectively. percent (wt. Three %). The specimensAC1 and (denotedAC3 temperatures as 1#, 2#, were 3#) with measured dimensions by dilatometry of 10 mm to (rolling be 715 direction) and 863 ◦C, × 3 respectively. mm (transverse Three direction) specimens × 10(denoted mm (thickness) as 1#, 2#, were 3#) with cut dimensionsfrom the center of 10 of mm the (rolling experimental direction) plate 3steel mm which (transverse was hot direction) rolled to × × 10010 mm mm (thickness) by TMCP, were because cut from of inferior the center mechanical of the experimental properties plate of center steel which plate. was Heat hot treatments rolled to 100 were mm carriedby TMCP, out because as shown of inferior in Figure mechanical 1. Specimen properties 1# wa ofs reheated center plate. to 900 Heat °C treatments for 30 min were for carriedcomplete out re- as austenitization,shown in Figure 1and. Specimen then quenched 1# was reheated to room to 900temp◦Cerature for 30 min(Figure for complete 1a). To re-austenitization,isolate the effect and of intercriticalthen quenched annealing, to room temperaturespecimen 2# (Figurewas isothermally1a). To isolate held the for effect 30 ofmin intercritical at 740 °C annealing,followed specimenby water quenching2# was isothermally to room temperature held for 30 min(Figure at 740 1b),◦C to followed distinguish by water the microstructure quenching toroom after temperatureintercritical annealing.(Figure1b), In to the distinguish case of specimen the microstructure 3#, two-step after heat intercritical treatment annealing.process: intercritical In the case annealing of specimen at 740 3#, °Ctwo-step for 30 heatmin followed treatment by process: quenching intercritical and reheating annealing to at900 740 °C◦ Cfor for 30 30 min min followed followed by by quenching, quenching was and applied,reheating as to shown 900 ◦C in for Figure 30 min 1b. followed by quenching, was applied, as shown in Figure1b.
Figure 1. Heat treatment scheme of experimental specimens: (a) conventional quenching process; and Figure 1. Heat treatment scheme of experimental specimens: (a) conventional quenching process; and (b) intercritical annealing, and intercritical annealing + conventional quenching processes. (b) intercritical annealing, and intercritical annealing + conventional quenching processes.
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The microstructure was characterized by opticaloptical microscope (OM) (OLYMPUS, Beijing, China), fieldfield emissionemission scanning scanning electron electron microscope microscope (FEG-SEM; (FEG-SEM; ULTRA ULTRA 55) and 55) electron and backscatterelectron backscatter diffraction (EBSD,diffraction Oxford (EBSD, Instruments, Oxford England, Instruments, UK). For En OM,gland, the specimensUK). For were OM, metallographically the specimens polished were andmetallographically etched by Picric acidpolished (China and electron etched microscope by Picric scientific acid (China instrument, electron China) microscope consisting scientific of 10 mL detergentinstrument, (Diao China) Pai madeconsisting in Nice of Group),10 mL detergent 2 mL CCl4, (Diao 0.2 g Pai NaCl, made 4.5 in g picricNice Group), acid and 2 60 mL mL CCl4, deionized 0.2 g waterNaCl, in4.5 50–55 g picric◦C water acid bathand 60 in ordermL deionized to observe water the equiaxed in 50–55 austenite °C water grain bath size. in order The specimens to observe were the etchedequiaxed by austenite 4% nital for grain SEM size. observation. The specimens The local were chemical etched by composition 4% nital for in SEM specimen observation. 2# was examined The local bychemical energy composition dispersive x-ray in specimen spectrometer 2# was (EDS) examined facility availableby energy with dispersive transmission x-ray electronspectrometer microscope. (EDS) Thefacility austenite available transformation with transmission vs temperature electron micr duringoscope. reheating The austenite was simulatedtransformation by dilatometer vs temperature at a heatingduring reheating rate of 0.1 was◦C/s simulated for the one-step by dilatometer and two-step at a processes.heating rate The of specimens0.1 °C/s for for the EBSD one-step analysis and weretwo- groundstep processes. and electrolytically The specimens polished for EBSD in an analysis electrolyte were of ground 10% perchloric and electrolytically acid, 5% glycerol polished and in 85% an ethanolelectrolyte at 15 of V 10% for 30 perchloric s. The EBSD acid, measurements 5% glycerol were and performed 85% ethanol at a voltageat 15 V of 20for kv30 with s. The a step EBSD size ofmeasurements 0.1 µm. were performed at a voltage of 20 kv with a step size of 0.1 μm.
3. Results and Discussion 3.1. Refining Prior Austenite Grain 3.1. Refining Prior Austenite Grain Figure2 shows the e ffect of two-step heat treatment on the refinement of austenite grain size Figure 2 shows the effect of two-step heat treatment on the refinement of austenite grain size in in the core of heavy-gage plate processed by TMCP. The austenite grain size of hot rolled specimen the core of heavy-gage plate processed by TMCP. The austenite grain size of hot rolled specimen was was ~105 µm in the core, as presented in Figure2a, because the cooling rate was lower in the core of ~105 μm in the core, as presented in Figure 2a, because the cooling rate was lower in the core of the the hot rolled plate; the microstructure was composed of bainite with large martensite and austenite hot rolled plate; the microstructure was composed of bainite with large martensite and austenite constituents (M/A). After conventional one-step quenching of hot rolled plate (specimen 1#), the prior constituents (M/A). After conventional one-step quenching of hot rolled plate (specimen 1#), the prior austenite grain was inhomogeneous, the large austenite grain (γb: ~78 µm) was surrounded by small austenite grain was inhomogeneous, the large austenite grain (γb: ~78 μm) was surrounded by small austenite grain (γa: ~29 µm), as shown in Figure2b. For the specimen 2# (intercritical annealed at austenite grain (γa: ~29 μm), as shown in Figure 2b. For the specimen 2# (intercritical annealed at 740 740 C for 30 min), as shown in Figure2c, where it can be seen that austenite grain size was as large as °C for◦ 30 min), as shown in Figure 2c, where it can be seen that austenite grain size was as large as that of the hot rolled sample (~106 µm), but there was fresh martensite formed at the prior austenite that of the hot rolled sample (~106 μm), but there was fresh martensite formed at the prior austenite grain boundary (G-M) and the inter-lath (L-M). The microstructure of specimen 2# was composed grain boundary (G-M) and the inter-lath (L-M). The microstructure of specimen 2# was composed of of G-M, L-M and intercritical ferrite (I-F); as indicated in Figure2c, G-M and L-M was transformed G-M, L-M and intercritical ferrite (I-F); as indicated in Figure 2c, G-M and L-M was transformed from from equiaxed reversed austenite (ERA) and distributed at the prior austenite grain boundary, and equiaxed reversed austenite (ERA) and distributed at the prior austenite grain boundary, and acicular acicular reversed austenite (ARA) was present at bainite-lath. However, after the two-step heat reversed austenite (ARA) was present at bainite-lath. However, after the two-step heat treatment treatment (specimen 3#), fine and uniform austenite grains were obtained, as shown in Figure2d. The (specimen 3#), fine and uniform austenite grains were obtained, as shown in Figure 2d. The average average grain size was ~12 µm, which was twice as fine as specimen 1#. It can be seen that the novel grain size was ~12 μm, which was twice as fine as specimen 1#. It can be seen that the novel pre- pre-annealing heat treatment process enhanced the nucleation of reversed austenite and refined the annealing heat treatment process enhanced the nucleation of reversed austenite and refined the coarse prior austenite grains. coarse prior austenite grains.
Figure 2. Cont.
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Figure 2. SEM images of hot rolled plate steel (a) and specimen 2# (c) by nital etching; optical microscope images of specimen 1# (b) and specimen 3# (d) by picric etching; M/A: martensite and Figure 2. SEMSEM images images of of hot hot rolled rolled plate plate steel steel ( aa)) and specimen 2# ( c) by nital etching;etching; optical austenite constituent; I-F: intercritical ferrite; L-M: martensite distributed at laths; G-M: martensite microscope images of specimen 1# ( b) and specimen 3# ( d) by picric etching; M/A: M/A: martensite and distributed at prior austenite grain boundary; γa and γb: austenite grain. austenite constituent; I-F: intercritical ferrite; L-M: martensite distributed at laths; G-M: martensite distributed at prior austenite grain boundary; γ and γ : austenite grain. 3.2. Crystallographicdistributed at prior Characteristic austenite grains of Pre-Annealedboundary; γa and Microstructure γb: austenite grain. 3.2. Crystallographic Characteristics of Pre-Annealed Microstructure 3.2. CrystallographicThe pre-annealed Characteristic specimens of Pre-Annealed(2#) was invest Microstructureigated by EBSD in order to analyze the crystallographicThe pre-annealed orientation specimen of fresh (2#) was martensite investigated at prior by EBSD austenite in order grain to analyzeboundary, the crystallographicas presented in The pre-annealed specimen (2#) was investigated by EBSD in order to analyze the Figureorientation 3. A typical of fresh fresh martensite martensite at prior ‘b’ between austenite two grain prior boundary, austenite as grains presented γ-A and in Figure γ–B was3. A selected, typical crystallographic orientation of fresh martensite at prior austenite grain boundary, as presented in asfresh shown martensite in Figure ‘b’ between 3a. From two the prior pole austenite figures, grainsFigureγ 3b–d,-A and itγ was–B was found selected, that the as shown position in Figureof “one”3a. Figure 3. A typical fresh martensite ‘b’ between two prior austenite grains γ-A and γ–B was selected, particularFrom the poleBain figures,group of Figure selected3b–d, fresh it was martensite found that in the the (100) position pole offigure “one” was particular identical Bain to “one” group Bain of as shown in Figure 3a. From the pole figures, Figure 3b–d, it was found that the position of “one” groupselected of freshprior martensite austenite grain in the γ (100)–B, which pole figure indicated was identicalthat the selected to “one” fresh Bain martensite group of prior held austenite the K-S particular Bain group of selected fresh martensite in the (100) pole figure was identical to “one” Bain orientationgrain γ–B, whichrelationship indicated with that the theprior selected austenite fresh grain martensite γ-B, but held no orientation the K-S orientation relationship relationship with the group of prior austenite grain γ–B, which indicated that the selected fresh martensite held the K-S withprior theaustenite prior austenite grain γ-A. grain In addition,γ-B, but nofrom orientation the pole figures, relationship Figure with 3b,c, the it priorwas found austenite that grain the freshγ-A. orientation relationship with the prior austenite grain γ-B, but no orientation relationship with the martensiteIn addition, between from the inter-laths pole figures, had Figure the 3sameb,c, it Bain was foundgroups that with the intercritical fresh martensite annealed between ferrite, inter-laths which prior austenite grain γ-A. In addition, from the pole figures, Figure 3b,c, it was found that the fresh suggestedhad the same that Bain the groups fresh martensite with intercritical kept the annealed K-S orientation ferrite, which relationship suggested with that thethe freshprior martensite austenite martensite between inter-laths had the same Bain groups with intercritical annealed ferrite, which grain.kept the The K-S fresh orientation martensite relationship between the with inter-laths the prior was austenite formed grain. by acicular The fresh reversed martensite austenite between when suggested that the fresh martensite kept the K-S orientation relationship with the prior austenite coolingthe inter-laths to room was temperature. formed by acicularThis illustrated reversed th austeniteat the acicular when cooling reversed to roomaustenite temperature. held the ThisK-S grain. The fresh martensite between the inter-laths was formed by acicular reversed austenite when orientationillustrated thatrelationship the acicular with reversed the prior austenite austenite held grai then. Sadovskii K-S orientation [29] showed relationship that it withis possible the prior to cooling to room temperature. This illustrated that the acicular reversed austenite held the K-S reconstructaustenite grain. the Sadovskiiprior austenite [29] showed by thatgrowth it is possibleand impingement to reconstruct of the acicular prior austenite austenite by growthduring orientation relationship with the prior austenite grain. Sadovskii [29] showed that it is possible to austenitization.and impingement of acicular austenite during austenitization. reconstruct the prior austenite by growth and impingement of acicular austenite during austenitization.
Figure 3. Cont.
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Figure 3. (a) Band contrast images of 2# specimen; two pole figures (b,c) corresponding to the prior FigureFigure 3.3. ((aa)) BandBand contrastcontrast imagesimages ofof 2#2# specimen;specimen; twotwo polepole figuresfigures ((bb,,cc)) correspondingcorresponding toto thethe priorprior austenite grain γ-A and γ-B in image (a); pole figures (d) corresponding to the selected sections of ‘b’ austeniteaustenite graingrainγ γ-A-A and andγ γ-B-B in in image image ( a();a); pole pole figures figures (d ()d corresponding) corresponding to to the the selected selected sections sections of ‘b’of ‘b’ in in image (a). imagein image (a). (a). 3.3.3.3. EnrichmentEnrichment ofof AlloyingAlloying ElementsElements 3.3. Enrichment of Alloying Elements TheThe distributiondistribution ofof alloyingalloying elementselements inin specimenspecimen 2#2# waswas examined,examined, as presented in Figure4 4.. The distribution of alloying elements in specimen 2# was examined, as presented in Figure 4. FromFrom intercriticalintercritical ferrite ferrite to to martensitemartensiteto to intercriticalintercritical ferrite ferriteon on thethe otherother side,side, lineline scanningscanning ofof alloyingalloying From intercritical ferrite to martensite to intercritical ferrite on the other side, line scanning of alloying elementselements (Figure(Figure4 4a)a) showedshowed thatthat thethe contentscontents of alloying elements, elements, Mn, Mn, Ni, Ni, Cu Cu and and Cr Cr increased increased in elements (Figure 4a) showed that the contents of alloying elements, Mn, Ni, Cu and Cr increased in inmartensite martensite and and decreased decreased in intercri in intercriticaltical ferrite ferrite (I-F), (I-F), as shown as shown in Figure in Figure 4b. Five-line4b. Five-line scanning scanning results martensite and decreased in intercritical ferrite (I-F), as shown in Figure 4b. Five-line scanning results resultsare shown are shownin Table in 1. Table It can1. be It seen can bethat seen the conc thatentration the concentration of alloying of elements alloying Mn, elements Ni, Cu Mn, and Ni, Cr are shown in Table 1. It can be seen that the concentration of alloying elements Mn, Ni, Cu and Cr Cuwas and significantly Cr was significantly higher in contrast higher to in their contrast nominal to their compositions, nominal compositions, which formed whichthe heterogeneous formed the was significantly higher in contrast to their nominal compositions, which formed the heterogeneous heterogeneousmicrostructure microstructure of alloying ofelement alloying enriched element enrichedmartensite martensite and depleted and depleted intercritical intercritical ferrite. ferrite. In microstructure of alloying element enriched martensite and depleted intercritical ferrite. In Incombination combination with with the the image image of of Figure Figure 2d,2d, the the heterogeneous microstructure isis relatedrelated toto finefine and and combination with the image of Figure 2d, the heterogeneous microstructure is related to fine and uniformuniform austeniteaustenite grainsgrains obtainedobtained byby thethe two-steptwo-step heatheat treatment.treatment. uniform austenite grains obtained by the two-step heat treatment.
4.0 4.0 Mn MnNi 3.5 (b) M NiCu 3.5 (b) M I-F/M interface CuCr 3.0 I-F/M interface M/I-F interface Cr 3.0 I-F M/I-F interface 2.5 I-F 2.5 I-F 2.0 I-F 2.0 1.5
Content (wt%) Content 1.5 Content (wt%) Content 1.0 1.0 0.5 0.5 0.0 0.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5Distance 1.0 (μm) 1.5 2.0 Distance (μm) FigureFigure 4.4. ExaminedExamined schematicschematic imageimage ofof freshfresh martensitemartensite afterafter intercriticalintercritical annealingannealing ((aa);); examinedexamined Figure 4. Examined schematic image of fresh martensite after intercritical annealing (a); examined resultsresults ofof alloyingalloying elementselementsdistribution distribution by by FEG-SEM FEG-SEM ( b(b).). results of alloying elements distribution by FEG-SEM (b). Table 1. Concentration of alloying elements in M for specimen 3#. Table 1. Concentration of alloying elements in M for specimen 3#. Table 1. Concentration of alloying elements in M for specimen 3#. PositionPosition Mn Mn Ni Ni Cu Cu Cr Cr Position Mn Ni Cu Cr 1# 1.891# 1.89 1.591.59 0.68 0.680.51 0.51 1# 1.89 1.59 0.68 0.51 2# 2.082# 2.08 1.671.67 0.85 0.850.68 0.68 3# 1.812# 2.08 1.561.67 0.85 0.720.68 0.56 3# 1.81 1.56 0.72 0.56 4# 1.953# 1.81 1.571.56 0.72 0.790.56 0.59 4# 1.95 1.57 0.79 0.59 5# 2.094# 1.95 1.741.57 0.79 0.820.59 0.65 Mean 1.965# 2.09 1.631.74 0.82 0.770.65 0.60 5# 2.09 1.74 0.82 0.65 Mean 1.96 1.63 0.77 0.60 Mean 1.96 1.63 0.77 0.60 3.4. Role of Heterogeneous Microstructure 3.4. Role of Heterogeneous Microstructure 3.4. RoleThe of role Heterogeneous of the heterogeneous Microstructure microstructure in phase transformation of austenite was studied by thermalThe role simulation of the heterogeneous (dilatometer) microstructure experiments, in as phase shown transformation in Figure5. of The austenite start temperature was studied The role of the heterogeneous microstructure in phase transformation of austenite was studied by thermal simulation (dilatometer) experiments, as shown in Figure 5. The start temperature and andby thermal finish temperature simulation (dilatometer) of austenite transformationexperiments, as wereshown 718 in ◦FigureC and 5. 877 The◦C, start respectively, temperature for and the finish temperature of austenite transformation were 718 °C and 877 °C, respectively, for the one-step finish temperature of austenite transformation were 718 °C and 877 °C, respectively, for the one-step
Metals 2020, 10, 132 6 of 11 Metals 2020, 10, x FOR PEER REVIEW 6 of 11 one-stepheat treatment heat treatment specimen, specimen, and for the and two-step for the heat two-step treatment heat specimen, treatment they specimen, were 695 they °C were and 881 695 °C,◦C andrespectively, 881 ◦C, respectively,as presented as in presented Figure 5a,c. in FigureWe can5a,c. see We that can from see thatthe plot from of the the plot rate of of the austenite rate of austenitetransformation transformation and heating and temperature, heating temperature, there was there only wasa peak only transformation a peak transformation rate for the rate one-step for the one-stepheat treatment heat treatment specimen specimen(Figure 5b (Figure), and there5b), and were there two were peak two rates peak for the rates two-step for the two-stepheat treatment heat treatmentspecimen (Figure specimen 5d). (Figure This indicated5d). This indicatedthat the allo thatying the elements-enriched alloying elements-enriched fresh martensite fresh martensite had lower hadtransformation lower transformation temperature temperature and alloying and alloying elements-depleted elements-depleted intercritical intercritical ferrite ferrite had had higher transformationtransformation temperature.temperature.
100 718℃ 12 (a) (b) 10 80
877℃ 8 60 6 40 4 Change length in 20 2
0 0
-2
-20 TransformationRate ofreversed austenite -4 0 100 200 300 400 500 600 700 800 900 740 760 780 800 820 840 860 TeTTeTatTTeT℃ Temperature/℃
120 4.0 695℃ 100 (c) 3.5 (d) 3.0 80 2.5 60 881℃ 2.0 40 1.5
20 1.0 Change inChange length 0 0.5
-20 0.0
Transformation Rate of reversed austenite reversed of Rate Transformation -0.5 -40 -1.0 0 200 400 600 800 1000 700 725 750 775 800 825 850 875 TeTTeTatTTeT℃ Temperature/℃
Figure 5.5. Gleeble simulated curves of one-step ( a) and two-steptwo-step ((c) heatheat treatment;treatment; thethe austeniteaustenite b d transformationtransformation raterate curvescurves ofof one-stepone-step ((b)) andand two-steptwo-step ((d)) heatheat treatment.treatment. The austenite nucleation during reheating for intercritically annealed specimen can be discussed The austenite nucleation during reheating for intercritically annealed specimen can be discussed from the viewpoint of thermodynamics. As is well known, the thermal activated nucleation needs to from the viewpoint of thermodynamics. As is well known, the thermal activated nucleation needs to overcome the energy barrier before the formation of embryo. According to the nucleation theory [30], overcome the energy barrier before the formation of embryo. According to the nucleation theory [30], under ideal state, the relationship of the nucleation energy barrier, interface energy and intrinsic energy under ideal state, the relationship of the nucleation energy barrier, interface energy and intrinsic difference of fcc phase and bcc phase can be expressed by: energy difference of fcc phase and bcc phase can be expressed by: 3 2 △GG0 0= = 1616πγπγ3//(3*((3*(△gg))2)) (1) (1) 4 4 where ∆G0 is nucleation energy barrier, γ is interface energy, ∆g is intrinsic energy difference of fcc where ∆G0 is nucleation energy barrier, γ is interface energy, ∆g is intrinsic energy difference of fcc phasephase andand bccbcc phasephase whenwhen bccbcc phasephase transformedtransformed intointo fccfcc phase.phase. The nucleation energy barrier is decreased when the interface energy is low (in ourour case,case, the interfaceinterface energy was low becausebecause of alloying elements enriched at the interface) and the intrinsic energy didifferencefference of fcc phasephase and bccbcc phasephase increased.increased. Thus, the necessary driving energy for nucleation was small, which enhancedenhanced the nucleation rate ofof reversedreversed austeniteaustenite viavia thermallythermally activatedactivated nucleation.nucleation.
∆g = gfcc − gbcc (2) ∆g = gfcc gbcc (2) where gfcc or gbcc represent intrinsic energy of α phase− or γ phase, respectively, at a temperature and whereat a givengfcc orchemicalgbcc represent composition. intrinsic energy of α phase or γ phase, respectively, at a temperature and at a givenThe chemical composition composition. of G-M/L-M and I-F after intercritical annealing at 740 °C was calculated by Thermo-calc 3.0 based on TCFE 7.0 according to the nominal composition (NC). The alloying element contents in G-M/L-M and I-F were 0.15C-0.21Si-2.03Mn-1.87Ni-0.52Cr-0.36Mo-0.80Cu (wt. %) and
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The composition of G-M/L-M and I-F after intercritical annealing at 740 ◦C was calculated by Thermo-calc 3.0 based on TCFE 7.0 according to the nominal composition (NC). The alloying element Metals 2020, 10, x FOR PEER REVIEW 7 of 11 contents in G-M/L-M and I-F were 0.15C-0.21Si-2.03Mn-1.87Ni-0.52Cr-0.36Mo-0.80Cu (wt. %) and 0.05C-0.24Si-0.79Mn-0.74Ni-0.43Cr-0.48Mo-0.35Cu0.05C-0.24Si-0.79Mn-0.74Ni-0.43Cr-0.48Mo-0.35Cu (wt.(wt. %),%), respectively.respectively. FigureFigure6 6 shows shows the the intrinsic intrinsic energyenergy didifferencefference ofof fccfcc phasephase andand bccbcc phasephase forfor thethe nominal composition, martensitemartensite (G-M(G-M/L-M)/L-M) andand I-F,I-F, respectively,respectively, during reheating. reheating. It It can can be be seen seen that that the the intrinsic intrinsic energy energy difference difference of G-M/L-M of G-M/L-M was washigher higher than thanNC when NC when the temperature the temperature was below was below 763 °C, 763 and◦C, the and intrinsic the intrinsic energy energy difference difference of I-F ofwas I-F greater was greater than thanNC NCas the as thetemperature temperature was was greater greater than than 815 815 °C.◦C. Thus, Thus, the heterogeneousheterogeneous microstructuremicrostructure cancan reducereduce thethe energyenergy barrierbarrier andand drivingdriving energyenergy forfor nucleation,nucleation, andand enhanceenhance thethe nucleationnucleation ofof equiaxedequiaxed reversedreversed austenite.austenite.
NC 51000 G-M/L-M IF 48000
45000 )(J)
bcc 42000 -g fcc
(g 39000
36000
33000 763℃ 815℃ 30000 700 725 750 775 800 825 850 875 900 Temperature (℃) Figure 6. The intrinsic energy difference of fcc phase and bcc phase when bcc phase transformed into Figure 6. The intrinsic energy difference of fcc phase and bcc phase when bcc phase transformed into fcc phase for the compositions of NC, G-M/L-M and I-F calculated by thermo-calc 3.0 based on TCFE 7. fcc phase for the compositions of NC, G-M/L-M and I-F calculated by thermo-calc 3.0 based on TCFE 3.5. Nucleation7. Rate of Reversed Austenite The effect of heterogeneous microstructure on reaustenitization during reheating was studied 3.5. Nucleation Rate of Reversed Austenite via step by step experiments. Experiments were designed every 20 ◦C in the temperature range of 720–860The◦ Ceffect for one-stepof heterogeneous and two-step microstructure heat treatment on processes,reaustenitization and the during results reheating are shown was in Figure studied7. Twovia step optical by microscopestep experiments. images showExperiments the distribution were de ofsigned equiaxed every reversed 20 °C in austenite the temperature grain (Figure range7a,b) of for720–860 the one-step °C for one-step and two-step and two-step heat treatment heat treatmen processes.t processes, It can beand seen the thatresults the are equiaxed shown reversedin Figure austenite7. Two optical grain microscope was large and images mainly show distributed the distribu at priortion of austenite equiaxed grain reversed boundary austenite forone-step grain (Figure heat treatment7a,b) for the specimen one-step (Figureand two-step7a); however, heat treatment the fine proc equiaxedesses. It reversed can be seen austenite that the grain equiaxed obtained reversed by two-stepaustenite heat grain treatment was large was and present mainly at thedistributed prior austenite at prior grain austenite boundary grain and boundary intragranular for one-step (Figure heat7b). Thetreatment results specimen in Figure (Figure7b showed 7a); however, the heterogeneous the fine equiaxed microstructure reversed austenit can enhancee grain the obtained nucleation by two- of equiaxedstep heat reversedtreatment austenite. was present The numberat the prior of equiaxed austenite reversed grain boundary austenite grainsand intragranular for different (Figure intercritical 7b). annealingThe results temperature in Figure 7b were showed counted, the asheterogeneou shown in Figures microstructure7c (note: the statisticalcan enhance grain the size nucleation was for an of areaequiaxed greater reversed than 2 squareaustenite. micro-meters; The number for theof two-stepequiaxed experimentalreversed austenit specimens,e grains the for grain different below 820intercritical◦C was tooannealing small to temperature count). The were austenite counted, grain as number shown forin Figure the two-step 7c (note: specimen the statistical was higher grain thansize was for thefor an one-step area greater specimen than for2 square every micro-meters; intercritical annealing for the two-step temperature, experimental which specimens, indicated that the thegrain nucleation below 820 density °C was of too equiaxed small to grain count). for two-stepThe austenite specimen grain was number higher for in the contrast two-step to that specimen of the one-stepwas higher specimen. than for the one-step specimen for every intercritical annealing temperature, which indicated that the nucleation density of equiaxed grain for two-step specimen was higher in contrast to that of the one-step specimen.
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400 one-step:760~860℃ (interval:20℃) two-steps: 740℃(30min) (c) 350 +820~860℃ (interval:20℃)
300
eTsed aTstentie gTain 250
200
150
100 NTTbeT of eqTiaxed Tev 760 780 800 820 840 860 Annealing teTTeTatTTe
Figure 7. TheThe optical optical microscope microscope images images of of one-step one-step (a) ( aand) and two-step two-step (b) ( bat) 820 at 820 °C ◦holdingC holding 1 min; 1 min; (c) (c) the number of statistically equiaxed reversed austenite grain (within the size of 165 114 square the number of statistically equiaxed reversed austenite grain (within the size of 165 × 114 square× micro micro meter). meter). In a summary, a schematic image of austenitized behavior during one-step and two-step In a summary, a schematic image of austenitized behavior during one-step and two-step austenitization is proposed, as shown in Figure8. For the one-step heat treatment specimen, the austenitization is proposed, as shown in Figure 8. For the one-step heat treatment specimen, the equiaxed reversed austenite (ERA) grains only occurred at the prior austenite grain boundary and grew equiaxed reversed austenite (ERA) grains only occurred at the prior austenite grain boundary and fast during austenitization (Figure8b) to form the austenite grain γa (Figure8c), the acicular reversed grew fast during austenitization (Figure 8b) to form the austenite grain γa (Figure 8c), the acicular austenite (ARA) grew and impinged to reconstitute the prior austenite γb (Figure8c), as presented reversed austenite (ARA) grew and impinged to reconstitute the prior austenite γb (Figure 8c), as in Figure2b. For the two-step specimen, the heterogeneous microstructure of alloying elements presented in Figure 2b. For the two-step specimen, the heterogeneous microstructure of alloying enriched fresh martensite (including G-M and L-M) and depleted intercritical ferrite was obtained after elements enriched fresh martensite (including G-M and L-M) and depleted intercritical ferrite was intercritical annealing (Figure8d), which enhanced the equiaxed reversed austenite grain nucleated at obtained after intercritical annealing (Figure 8d), which enhanced the equiaxed reversed austenite the intragranular and prior austenite grain boundary during reaustenitization (Figure8e). Finally, fine grain nucleated at the intragranular and prior austenite grain boundary during reaustenitization and uniform austenite grain size was obtained after complete austenitization, (Figures8f and2d). (Figure 8e). Finally, fine and uniform austenite grain size was obtained after complete austenitization, (Figures 8f and 2d).
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Figure 8. Schematic of direct austenitization (a–c) or partition and reaustenitization (a,d–f) for hot Figure 8. Schematic of direct austenitization (a–c) or partition and reaustenitization (a,d–f) for hot rolled experimental steel. ERA: equiaxed reversed austenite; ARA: acicular reversed austenite. rolled experimental steel. ERA: equiaxed reversed austenite; ARA: acicular reversed austenite. 4. Conclusions 4. Conclusions In this study, coarse prior austenite grains in the core of heavy plate processed by TMCP were studiedIn this by two-stepstudy, coarse heat treatment,prior austenite and thegrains conclusions in the core are of concluded heavy plate as follows:processed by TMCP were studied by two-step heat treatment, and the conclusions are concluded as follows: The heterogeneous microstructures of alloying elements-enriched fresh martensite and -depleted • The heterogeneous microstructures of alloying elements-enriched fresh martensite and -depleted intercritical ferrite were obtained after intercritical annealing. The fresh martensite was distributed intercritical ferrite were obtained after intercritical annealing. The fresh martensite was at prior austenite grain boundary (G-M) and inter-lath (L-M). G-M was transformed by equiaxed distributed at prior austenite grain boundary (G-M) and inter-lath (L-M). G-M was transformed reversed austenite and the L-M was formed by acicular reversed austenite, when the reversed by equiaxed reversed austenite and the L-M was formed by acicular reversed austenite, when the austenite obtained during intercritical annealing was quenched to room temperature. reversed austenite obtained during intercritical annealing was quenched to room temperature. The heterogeneous microstructure increased the intrinsic energy difference of fcc phase and bcc • The heterogeneous microstructure increased the intrinsic energy difference of fcc phase and bcc phase below 763 C or above 815 C, and the interface energy decreased because of alloying phase below 763 °C◦ or above 815 °C,◦ and the interface energy decreased because of alloying element enrichment at the interface, which reduces the nucleation energy barrier according to the element enrichment at the interface, which reduces the nucleation energy barrier according to the nucleation theory under ideal state. Therefore, the nucleation driving energy of equiaxed reversed nucleation theory under ideal state. Therefore, the nucleation driving energy of equiaxed austenite is less. The heterogeneous microstructure can enhance the equiaxed reversed austenite reversed austenite is less. The heterogeneous microstructure can enhance the equiaxed reversed nucleation at intragranular and prior austenite grain boundary during reheating, which effectively austenite nucleation at intragranular and prior austenite grain boundary during reheating, which refined the coarse prior austenite grains in the core of hot rolled heavy plate processed by TMCP. effectively refined the coarse prior austenite grains in the core of hot rolled heavy plate processed The prior austenite grains in the core of heavy plate processed by TMCP was very large (~105 • by TMCP. µm). The austenite grains were inhomogeneous when the hot rolled specimen was reheated • The prior austenite grains in the core of heavy plate processed by TMCP was very large (~105 by one-step heat treatment process. However, for the two-step heat treatment process, fine and μm). The austenite grains were inhomogeneous when the hot rolled specimen was reheated by uniform austenite grain size (~12 µm) was obtained, which was two times finer compared to the one-step heat treatment process. However, for the two-step heat treatment process, fine and one-step heat treatment. An effective way of improving strength, ductility and low temperature uniform austenite grain size (~12 μm) was obtained, which was two times finer compared to the toughness in alloy steel is to refine the prior austenite grain size. This study provides a possible one-step heat treatment. An effective way of improving strength, ductility and low temperature way of effectively refining prior austenite grain size. toughness in alloy steel is to refine the prior austenite grain size. This study provides a possible way of effectively refining prior austenite grain size. Author Contributions: S.Y.: conceptualization, data curation, formal analysis, investigation, writing—original Authordraft; Z.X.: Contributions: review & editing; S.Y.: conceptualization, J.W.: formal analysis, data reviewcuration, & editing;formal analysis, L.Z.: data investigation, curation; L.Y.: writing—original dcuration; C.S.: draft;conceptualization, Z.X.: review & funding editing; acquisition, J.W.: formal supervision, analysis, review review & & editing; editing; L.Z.: R.D.K.M.: data curation; review & L.Y.: editing. dcuration; All authors C.S.: conceptualization,have read and agree funding to the publishedacquisition, version supervision, of the manuscript. review & editing; R.D.K.M.: review & editing. All authors have read and agree to the published version of the manuscript.
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Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.
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