2011_Hamid Azizi-Alizamini_Austenite Formation in Plain Low-carbon Steels

Austenite Formation in Plain Low-Carbon Steels

HAMID AZIZI-ALIZAMINI, MATTHIAS MILITZER, and WARREN J. POOLE

In this study, austenite formation from hot-rolled (HR) and cold-rolled (CR) ferrite-pearlitestructures in a plain low-carbon steel was investigated using dilation data and microstructuralanalysis. Different stages of microstructural evolution during heating of the HR and CRsamples were investigated. These stages include austenite formation from pearlite colonies,ferrite-to-austenite transformation, and final carbide dissolution. In the CR samples, recrys-tallization of deformed ferrite and spheroidization of pearlite lamellae before transformationwere evident at low heating rates. An increase in heating rate resulted in a delay in spheroidi-zation of cementite lamellae and in recrystallization of ferrite grains in the CR steel. Further-more, a morphological transition is observed during austenitization in both HR and CRsamples with increasing heating rate. In HR samples, a change from blocky austenite grains to afine network of these grains along ferrite grain boundaries occurs. In the CR samples, austeniteformation changes from a random spatial distribution to a banded morphology.

DOI: 10.1007/s11661-010-0551-5� The Minerals, Metals & Materials Society and ASM International 2010

I. INTRODUCTION

AUSTENITE formation occurs during many indus-trial heat treatments of steels. However, the importanceof this phase transformation has been undervaluedbecause austenite usually is not found in the micro-structure of final steel products. Furthermore, it ischallenging to characterize austenite microstructuresthat are present at high temperatures. These limitationsresulted in few studies of austenite formation comparedwith austenite decomposition. Nevertheless, a significantbody of work on austenite formation is available in theliterature.[1–5] However, with the development of ad-vanced high-strength steels such as dual phase (DP),transformation-induced plasticity (TRIP), and complexphase steels, there has been renewed interest in studyingaustenite formation. For example, intercritical anneal-ing is an essential processing step for these steels whenmanufactured as cold-rolled and coated sheets, primar-ily for automotive applications. In addition, austeniteformation is of major interest for microstructure evolu-tion in the heat-affected zone of welds. Microstructuresfrom which austenite formation has been investigatedinclude hot-rolled (HR) and cold-rolled (CR) ferrite-pearlite,[6,7] ferrite-spheroidized carbide,[8] and ferrite-martensite[9] structures. Investigating the austenite for-mation from ferrite-pearlite structures, Speich et al.[1]

observed three major transformation stages, i.e., (1)rapid pearlite-to-austenite transformation, (2) austeniteformation from proeutectoid ferrite, and (3) finalequilibrium via partitioning of Mn in austenite. Yanget al.[7] investigated the effect of initial cold reduction on

the kinetics of austenite formation and its morphology.They observed that recrystallization of deformed ferriteand spheroidization of pearlite lamellae can take placeprior to austenite formation in the CR sample. Usingdilatometry data, de Cock et al.[10] showed that ferriterecrystallization prior to phase transformation results indimension changes prior to austenite formation in lowand ultra-low carbon steels. The sequence of thesemicrostructural changes is dependent strongly on theemployed heating rate in both HR and CR struc-tures.[6,11,12] San Martın et al.[6] showed that there canbe an overlap between the first two stages of austeniteformation at sufficiently low heating rates (e.g., 0.05 K/s)in a HR low-carbon Nb microalloyed steel. Savranet al.[11] also reported that lamellar ferrite and cementitephases in pearlite colonies either can transform simul-taneously or consecutively depending on heating rate. Inthe CR structures, however, rapid heating results in anoverlap between ferrite recrystallization and austeniteformation. This interaction affects the morphology ofaustenite directly and, consequently, mechanical prop-erties of intercritically annealed multiphase steels.Huang et al.[12] investigated systematically the effect ofheating rate on the microstructure of a Mo-alloyed DPsteel. They showed that an overlap between ferriterecrystallization and austenite formation with increasingthe heating rate from 1 K/s to 100 K/s resulted in amorphological transition from a randomly distributedto a banded structure of martensite. As a result, asignificant change in mechanical properties wasrecorded.[13] The same morphological transition, fromrandom to fibrous distribution, was observed byGrange.[14] The extent of this overlap depends on thechemical composition of the steel and amount of coldreduction. Petrov et al.[15] and Huang et al.[12] reportedthat an ultrafast heating rate, i.e., in excess of 1000 K/s,is needed to view the overlap in the CMnSi TRIP steelsused in their studies. Kestens et al.[16] showed that thisoverlap is not viable even at 3000 K/s in an interstitial

HAMID AZIZI-ALIZAMINI, PhD Student, MATTHIASMILITZER and WARREN J. POOLE, Professors, are with theCentre for Metallurgical Process Engineering, The University ofBritish Columbia, Vancouver, British Columbia V6T 1Z4, Canada.Contact e-mail: [email protected]

Manuscript submitted March 29, 2010.Article published online December 3, 2010

1544—VOLUME 42A, JUNE 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A

free (IF) steel. Overall, there are a limited number ofstudies devoted to the effect of heating rate on theinteraction between ferrite recrystallization and austen-ite formation and its impact on the microstructuralevolution in CR steel products[12,15–17]

The aim of the current work is to investigatesystematically the effect of initial structure, both HRand CR, and heating rate on the dilation response, andmicrostructural changes during austenite formation in alow-carbon steel. In particular, this study is designed toquantify experimentally individual stages of austeniteformation and their potential interaction with ferriterecrystallization and spheroidization of cementite lamel-lae. Advancing knowledge in this area is critical toevaluate intercritical annealing strategies for advancedhigh-strength steels.

II. EXPERIMENTAL

A plain, low-carbon steel received as industrially hot-rolled material was used for this study. The detailedchemical composition is presented in Table I. The HRsteel was then 80 pct cold rolled, i.e., from 9.8 mm to1.8 mm using a laboratory rolling mill (roll diameter:130 mm). For austenite formation studies, test couponsof 10 9 60 9 1.8 mm were cut from the HR and CRsheets with longitudinal direction of the test coupon beingaligned with the rolling direction. A Gleeble 3500(Dynamic Systems Inc., Poestenkill, NY) thermome-chanical simulator was employed for all heat treatments.The temperature was controlled using a type K thermo-couple spot welded on the center of the sample. Dilatom-etry tests were conducted under high vacuum, �0.26 Pa(2.0 9 10�3 Torr). Continuous heating tests were per-formed with heating rates ranging from 1 K/s to 900 K/s.A dilatometer was attached to the center of the samples tomeasure the change in width during heating. The volumefraction of austenite was determined via analyzing thedilatometric data using the lever rule. Details of theprocedure for this measurement can be found else-where.[18] To analyze the microstructure during heating,additional samples were then subjected to interruptedheating tests and water quenching. The cooling rate wasapproximately 1000 K/s to ensure complete austenite-to-martensite transformation after quenching. For thesetests, the test chamber was back filled with inert Ar gasafter a high vacuum had been achieved.

A microstructural analysis was carried out along thetransverse direction. The microstructures were charac-terized using optical and electron microscopy. A HitachiS2300 (Hitachi Science Systems Ltd., Tokyo, Japan)scanning electron microscope (SEM) with a secondaryelectron detector and an energy-dispersive X-ray systemfor chemical analysis was used. AHitachi H-800 (HitachiScience Systems Ltd., Tokyo, Japan) transmission

electron microscope (TEM) operated at 200 kV wasemployed for TEM observation. Scanning Auger micro-scopy was used for the characterization of carbideparticles using a Microlab 350 system (Thermo ElectronCorp.) equipped with field emission source (10 keV and3.5 nA) and hemispherical energy analyzer in a vacuumof 2910�7 Pa. A secondary electron detector attached tothe equipment was used to characterize selected carbideparticles. LePera etching[19] was employed to revealmartensite and to measure the volume fraction ofaustenite (martensite at room temperature) from opti-cal micrographs. To reveal prior austenite grain bound-aries, the following procedure was followed: First,as-quenched samples were tempered in a tube furnaceat 823 K (550 �C) for 15 hours in an Ar atmospherefollowed by water quenching. Then, an etching solutioncomposed of aqueous picric acid with sodium dodecyl-benzene and a few droplets of Triton X-100 (Sigma-Aldrich, St. Louis, MO) as a surface active agent at atemperature range between 333 K and 353 K (60 �C and80 �C) were used to reveal austenite grain boundaries.Grain size measurements were based on the equivalentarea diameter approach and at least 500 grains wereanalyzed using SEM. The quantitative measurementswere conducted using Clemex image analysis software(Clemex Technologies Inc., Longueuil, PQ, Canada).For SEM analyses, samples were electropolished in

95 pct acetic acid and 5 pct perchloric acid solution, andthen etched with 3 pct Nital. To reveal cementiteparticles inside martensite islands, two-step etchingwas employed. After light etching with 2 pct Nital, deepetching using 4 pct Picral for 10 to 15 seconds wasperformed.[20] These samples were also used for Augermeasurements. For TEM observations, thin foils wereprepared by twin-jet polishing technique using a mixtureof 95 pct acetic acid and 5 pct perchloric acid at anapplied potential of 40 V at 293 K (20 �C).

III. RESULTS

A. Initial Structures

Figure 1(a) shows the initial HR ferrite-pearlite struc-ture consisting of approximately 20 vol pct pearlite and80 vol pct ferrite. Ferrite grain size and pearlite lamellarspacing are approximately 7 lm and 240 nm, respec-tively. Individual cementite particles were also observedat ferrite grain boundaries (indicated by arrows in theinset in Figure 1(a)). The HR microstructure is banded,which can be related to the segregation of Mn. Energy-dispersive X-ray analysis confirmed that concentrationof Mn in ferrite region was close to the nominal value inthe steel, however, it indicated segregation inside pearl-ite colonies with an average Mn content of approxi-mately 1.0 wt pct and an average distance of 12 lmbetween pearlite bands. Figure 1(b) shows the CR

Table I. Chemical Composition of the Steel Used in this Study in Weight Percent

Element Fe C Mn P S Si Al N

Wt pct Balanced 0.17 0.74 0.009 0.008 0.012 0.04 0.0047

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structure, where the alignment of ferrite grains andpearlite colonies into the rolling direction is evident inthe microstructure. The higher magnification inset in thefigure shows fragmentation and bending (arrows) ofcarbide plates. After 80 pct cold reduction, there was aninhomogeneous distribution of deformation in ferriteand pearlite. A measurement of the average thickness ofpearlite colonies before and after deformation showedthat the amount of cold reduction is only 70 pct inpearlite. The details of the procedures for strain mea-surements can be found elsewhere.[13]

B. Dilation Response and MicrostructuralCharacteristics in the HR and CR Materials

Figure 2 shows dilation curves for the HR and CRsamples (black and gray lines, respectively) heated at1 K/s into the single austenite phase region. Using thefirst derivative of the dilation curves, several evolutionsteps can be distinguished. In region (1), lattice expan-sion in ferrite-pearlite structures in both HR and CRsamples takes place at heating. The recovery ofdeformed structure can also occur in the CR sample.In region (2), deviation from linear thermal expansioncan be observed in the CR sample, whereas the dilationresponse in the HR steel remains unaffected. Both ofthese two stages are prior to austenite formation.

A sharp drop in the dilation curve is evident in bothHR and CR samples in stage (3), which is related topearlite-to-austenite transformation. Austenite forma-tion continues in region (4) for both steels. The dilationin this region can be related to ferrite-to-austenitetransformation. Finally, in region (5), linear latticeexpansion of austenite after completion of transforma-tion is observed. In detail, these stages can be rational-ized in the following sections.

1. Thermal expansion of ferrite-pearlite structureIn region (1), below �773 K (500 �C), lattice expan-

sion of ferrite-pearlite structure takes place in both HRand CR structures after heating. In the CR sample,recovery of the deformed structure can proceed viarearrangement and annihilation of dislocations, but thiscannot be observed with dilatometry. The microstruc-tural features of the CR steel remain unaffected (com-pare Figures 3(a) and 1(b)). The measured linearthermal expansion coefficient of ferrite-pearlite structureis �16.0 9 10�6 K�1 that is in good agreement with thedata available in the literature.[21]

2. Recrystallization of ferrite in the CR sampleIn region (2), the linear thermal expansion continues in

the HR sample, but it deviates from linearity in the CRsteel. This deviation starts at �773 K (500 �C) andreaches up to 0.1 pct at 923 K (650 �C). Microstructuralobservations, which are shown in Figure 3(b), indicatethat there are two major changes during this process: (1)recrystallization of deformed ferrite grains and (2)

Fig. 1—(a) Initial HR structure and (b) after 80 pct cold reduction.The insets show higher magnification images. ND, normal direction;RD, rolling direction. (F: ferrite, P: pearlite, C: cementite).

Fig. 2—Dilation curves and their first derivatives for the HR andCR steels during heating at 1 K/s.


spheroidization of cementite particles. The deviationfrom a linear coefficient of thermal expansion could arisefrom several mechanisms, i.e., (1) volume change causedby the loss of dislocation density during recovery andrecrystallization, (2) change in crystallographic texture,(3) dissolution/spheroidization of carbides, and (4) relax-ation of residual stresses from cold rolling. Subsequentexaminations, which are presented in detail in theAppendix, revealed that ferrite recrystallization is themain mechanism responsible for the observed deviation.

3. Pearlite-to-austenite transformationThe first stage of austenite formation in the HR and

CR steel consists of pearlite-to-austenite transformation(stage (3)). The sharp drop in the first derivative of thedilation curve in Figure 2 indicates a relatively fastphase transformation rate. Closer observation revealsthat the transformation started earlier in the CRsamples. The lower level of the first derivative curvefor the CR sample in this stage suggests a slowertransformation rate as compared with the HR sample.Figure 4(a) shows martensite (formerly austenite at hightemperature) sweeping lamellar pearlite colonies in theHR material. There is almost no sign of spheroidizationof cementite lamellae inside pearlite colonies prior toaustenitization. It has been reported that austenitenucleates mainly at ferrite-pearlite interfaces as well as

the interface between pearlite colonies.[22] In addition,simultaneous nucleation of austenite at ferrite grainboundaries, especially at carbide particles, has also beenreported at low heating rates.[6]

However, the situation is different in the CR steel.Figure 4(b) represents the early stages of austeniteformation in the CR steel at 1003 K (730 �C). It canbe observed that cementite lamellae were mostly sphero-idized (�90 to 95 pct) even though some small fractionof the lamellar structures are still preserved in themicrostructure (shown by arrow P in Figure 4(b)). Acloser examination of these latter regions reveals aninteresting observation. The bright field TEM imageshown in Figure 4(c) provides an example for a ferriterecrystallization front moving into the deformed lamel-lae leaving behind rows of spheroidized carbides. Thisprovides evidence that this is one mechanism to producespheroidized carbides, which is not available in theundeformed case. These carbides are visible clearly inFigure 4(b) and (d) (indicated by arrows C). At the sametime, deformed ferrite layers inside pearlite coloniesrecrystallize and form fine equiaxed grains that arepinned by carbide particles (Figure 4(d)). Recrystalliza-tion is complete prior to austenite formation except forthe small regions in the pearlite colonies (e.g., P inFigure 4(b)). Nucleation and growth of austenite occurfrom the ferrite-spheroidized cementite aggregates.Growth of austenite continues until complete consump-tion of the aggregate. Figure 5(a) shows the presence ofparticles inside martensite (austenite at high tempera-ture) in the CR sample at 1013 K (740 �C). The presenceof the particles inside martensite after completion ofpearlite-to-austenite transformation was also observedfor the HR steel. To confirm that these particles arecarbides, Auger electron microscopy was used. Prior totaking Auger spectra, surface contaminations wereremoved by sputtering with Ar ions. Figure 5(b) pro-vides an example of Auger electron spectra where thecarbon content of the particle (black line) can becompared with that in the martensite matrix (gray line).A sharp increase in carbon content is observed in theparticle compared to the matrix. A subsequent analysison the particles using differentiation spectrum comparedwith the reference data[23] indicates that the carbon ispresent in the form of a carbide that is most probablycementite.Figure 6(a) shows the microstructure of the CR

structure quenched at 1013 K (740 �C). It can beobserved that subsequent growth of austenite grainstakes place at ferrite grain boundaries. A closer exam-ination using the inset reveals that undissolved carbideparticles are mostly spheroidized inside the ferritematrix. The volume fraction of austenite at this tem-perature is 0.22, which is almost equivalent to the initialpearlite content in the steel. The temperature at whichthe first contraction, which results from pearlite-to-austenite transformation, finishes was defined as Ach, asindicated in the derivative curve in Figure 2. It wasshown by San Martın et al.[6] that the Ach temperaturewas a good metric to separate the pearlite-to-austeniteand ferrite-to-austenite transformations at low heat-ing rates, e.g., 0.05 K/s, but at higher heating rates,

Fig. 3—Microstructural evolution of the CR structure quenched at(a) 783 K (510 �C) (point A in Fig. 2(b)) and (b) 943 K (670 �C)(point B in Fig. 2(b)) during heating at 1 K/s.


austenite formation at ferrite grain boundaries can alsooccur below the Ach temperature.[6]

4. Ferrite-to-austenite transformationAnother increase in the austenite volume fraction

occurs by austenite growth into proeutectoid ferritegrains, stage (4) in Figure 2. Dissolution of carbideparticles is completed during this stage, and no carbideparticles were evident at 1033 K (760 �C) in ferrite orinside austenite.

Figure 6(b) shows the ferrite-martensite DP structureresulting when quenching from 1053 K (780 �C) in theCR sample. Here, martensite islands are distributedrandomly. It can be observed that a necklace shapestructure of these islands covers most ferrite grainboundaries. The volume fraction of austenite in bothHR and CR structures calculated via lever rule usingdilatometry curves together with metallographic mea-surements is presented in Figure 7. In addition to theexperimental results, the paraequilibrium (PE) austenitefraction calculated from Thermo-Calc (Thermo-CalcSoftware, Stockholm, Sweden) using the FE-2000 data-base is shown for comparison (Note: paraequilibriumindicates a constraint equilibrium without partitioningof substitutional alloying elements). It can be observedthat the volume fraction obtained from dilatation data isin good agreement with the metallographic observations

except at about 20 vol pct of austenite. At this point, thedilation data underestimates the actual transformedfraction. This point also coincides with complete pearl-ite-to-austenite transformation. This discrepancy wasalso observed by Oh et al.[24] in the early stage ofaustenite formation experiments. They attributed this tothe redistribution of carbon atoms in austenite grains.The carbon content in the austenite phase in the steelused in this study can reach up to �0.75 wt pct at thebeginning of austenite formation based on paraequilib-rium calculation. The lattice parameter and, thus, thespecific volume of austenite is a function of carboncontent.[21] This effect was not considered in the simpli-fied lever-rule calculations based on the linearizedthermal expansion of austenite with 0.17 wt pct carboncontent. However, a correction to take into accountthe carbon content of austenite using the proposedapproaches[24] can be misleading in the current casebecause of the remnant of carbide particles insideaustenite (Figure 5(a)). One would need to determinethe volume fraction of cementite particles inside aus-tenite grains and thereby estimate the actual carboncontent of austenite.

5. Thermal expansion of austeniteSubsequent heating into the single austenite phase

region, stage (5), consists of linear thermal expansion of

Fig. 4—Microstructure of HR steel heated at 1 K/s to 1008 K (735 �C) (a) and of CR steel heated at 1 K/s to 1003 K (730 �C) (b), (c) and(d) (F: ferrite, M: martensite, P: pearlite, C: cementite).


austenite grains. Both HR and CR materials expandlinearly in single austenite phase region with a thermalexpansion coefficient of �23.0 9 10�6 K�1 that is inagreement with the reported data for low-carbonaustenite.[21,25]

C. Effect of Heating Rate on Dilation Responseand Microstructure Evolution

In this section, the effect of heating rate on thedilation curves and microstructural evolution is studied.Different heating rates of 1 K/s, 10 K/s, 100 K/s,300 K/s, and 900 K/s were employed for the investiga-tion. For dilation experiments, all samples were heatedinto the single-phase austenite region.

Figure 8 shows dilation curves and their first deriv-atives for both HR and CR materials at different heatingrates. Table II summarizes the critical temperatures inboth HR and CR samples at different heating rates. Ae1(equilibrium austenite formation start temperature) andAe3 (equilibrium austenite formation finish temperature)are 977 K and 1096 K (704 �C and 823 �C), respec-tively. The effect of Mn segregation on the equilibriumtemperatures remains marginal as it decreases Ae1 andAe3 by 9 K and 7 K, respectively. It is evident that

raising the heating rate will increase the Ac1 temperature(start of austenite formation upon heating), the Ac3temperature (finish of austenite formation at heating),and the Ach temperature in the HR material (Fig-ure 8(a)). For example, the Ac1 and Ac3 temperaturesincrease from 1003 K to 1063 K (730 �C to 790 �C) andfrom 1127 K to 1200 K (854 �C to 927 �C), respectively,when the heating rate is raised from 1 K/s to 900 K/s.However, the overall trend in the shape of dilationcurves and their derivatives remains similar. From thederivative of the dilation curves, two distinct andperhaps overlapping stages can be observed, pearlite-to-austenite transformation with a relatively sharp slopefollowed by ferrite-to austenite transformation for allthe heating rates. Dilation curves for the CR structures,however, reveal two distinct and rather interestingobservations in comparison with their HR counterparts(Figure 8(b)): First, it can be observed that increasingthe heating rate from 1 K/s to 900 K/s resulted in thedisappearance of the deviation from linear thermalexpansion before phase transformation. This effect ismore pronounced in the derivative curves in which thecusp for 1 K/s at approximately 923 K (650 �C) isgradually shifting up toward the line representing thelinear thermal expansion coefficient of the ferrite-pearl-ite structure (�16.0 9 10�6 K�1). Second, as the heatingrate increases, a negligible change occurs in the start

Fig. 5—(a) Microstructure of the CR steel heated at 1 K/s to revealcementite particles inside martensite; arrows show carbide particles(F: ferrite, M: martensite) and (b) Auger spectra for carbide particlesand martensite.

Fig. 6—Microstructure of the CR steel continuously heated with1 K/s heating rate followed by water quenching at (a) 1013 K(740 �C) (the arrows inside the inset depict spheroidized carbide par-ticles) and (b) 1053 K (780 �C) (F: ferrite, M: martensite).


temperature of austenite formation represented by thesharp drop in the derivative of dilation curves. It can beobserved that all these sharp drops collapse essentiallyonto the same line. A similar behavior is observed forthe Ach temperature. However, the change in the Ac3temperature is similar to that observed for the HRmaterial, i.e., the Ac3 temperature increases from about1125 K (852 �C) to 1183 K (910 �C) when the heatingrate ramps up from 1 K/s to 900 K/s.

Figure 9 shows austenite fractions in both HR andCR steels as a function of temperature at differentheating rates measured using dilatometry. Prediction byparaequilibrium calculation is also included in thegraphs. It can be observed that increasing the heatingrate increases both the Ac1 and Ac3 temperatures, andshifts the transformation to higher temperatures in theHR steel below equilibrium as explained previously.Figure 9(b) also represents the lever-rule results foraustenite fractions for different heating rates in the CRsteel. A comparison between the lever-rule results andmetallographic measurements for 300 K/s heating rate isgiven in Table III. It is evident that the lever-ruleanalysis of dilation data for the early stages of austeniteformation is unsatisfactory, but there is a good agree-ment between the lever-rule calculation and metallo-graphic measurements for austenite fractions above 0.2at 300 K/s. Similar observations were made for 1 K/sheating rate; see Section III–B. Furthermore, the dila-tion measurements suggest that up to 20 vol pct aus-tenite fraction, the transformation is independent of theheating rate. However, beyond this point, increasing theheating rates shifts the transformation gradually tohigher temperatures. Table IV represents the tempera-ture for the formation of 0.5 volume fraction of aus-tenite (T0.5) for different heating rates for both HR andCR samples. These data indicate a reduced temperaturedependency of austenite fraction as a function of heatingrate in the CR steel. For example, the shift in T0.5 when

Fig. 7—Austenite fraction in the HR and CR steels as a functionof temperature for continuous heating at 1 K/s and PE austenitefraction.

Fig. 8—Dilation curves and their first derivatives for (a) HR and (b) CR steels during heating with different heating rates.


increasing the heating rate from 1 K/s to 900 K/s is92 K in the HR steel, whereas this shift is 33 K for theCR sample.

Figure 10 shows the microstructures for the HR steelheated at 1 K/s and 300 K/s to 1023 K (750 �C)followed by water quenching. It can be observed thatincreasing the heating rate has changed the morphologyand distribution of austenite grains from a large blocky

one to a network of fine austenite grains along ferritegrain boundaries.Figure 11 shows the microstructural evolution of the

CR structure heated at 300 K/s followed by immediatewater quenching at 973 K, 1013 K, and 1053 K (700 �C,740 �C, and 780 �C), respectively. Several major differ-ences exist between the microstructures of rapid-heatedsamples in comparison with their low-heating-ratecounterparts in the CR steel shown in Figure 6.

(a) Figure 11(a) shows the microstructure at 973 K(700 �C). This temperature is just before the startof austenite formation, but some regions in theferrite structure are not yet recrystallized. Theseregions are distributed mostly between closelyspaced pearlite colonies, depicted by arrows.Approximately 12 pct of ferrite remained unrecrys-tallized at this stage. At the same time, the lamellarpattern of cementite particles in the pearlite colo-nies is almost unaffected, and the spheroidizationof cementite lamellae in the pearlite colonies is atearly stages in contrast to the 1 K/s heating rateexperiment.

(b) At 1013 K (740 �C) (Figure 11(b)), it can beobserved that the elongated austenite grains areformed that are not observed during intercriticalannealing at 1 K/s (Figure 6(a)). A closer observa-tion of Figure 11(c) reveals that austenite forma-tion starts at pearlite colonies and ferrite grainboundaries. The latter nucleation sites are shown

Table II. Summary of the Critical Transformation Temperatures (Ac1, Ach, and Ac3 in K (�C)) at Different Heating Rates

Heating Rate (K/s) 1 10 100 900

HR sample Ac1 1003 (730) 1012 (739) 1033 (760) 1063 (790)Ach 1036 (763) 1051 (778) 1065 (792) 1085 (812)Ac3 1127 (854) 1143 (870) 1188 (915) 1223 (950)

CR sample Ac1 992 (719) 993 (720) 997 (724) 1007 (734)Ach 1044 (771) 1050 (777) 1033 (760) 1043 (770)Ac3 1125 (852) 1127 (854) 1160 (887) 1183 (910)

Fig. 9—Austenite fraction in (a) HR and (b) CR steels as a function of temperature for different heating rates and paraequilibrium (PE) austen-ite fraction.

Table III. Comparison of Austenite Volume FractionMeasured Using Metallographic Analysis and the Lever Rule

for the CR Steel Heated at 300 K/s

Temperature,[K (�C)]

993(720)

1033(760)

1053(780)

1073(800)

1103(830)

Lever rule 0.02 0.19 0.30 0.42 0.67Metallography 0.14 0.26 0.30 0.38 0.70

Table IV. Effect of Heating Rate on T0.5 in the HRand CR Samples

Heatingrate (K/s) 1 10 100 900

T0.5 (HR),K (�C)

1067 (794) 1073 (800) 1113 (840) 1159 (886)

T0.5 (CR),K (�C)

1067 (794) 1071 (798) 1083 (810) 1100 (827)


by arrows in Figure 11(c). The austenite grainsformed at ferrite grain boundaries are substantiallysmaller than those formed at pearlite colonieselongated in rolling direction. Figure 11(d) showsthe microstructure at 1053 K (780 �C) with0.3 volume fraction martensite. It can again beobserved that banded martensite islands are elon-gated in rolling direction. This is in contrast to therandom distribution observed for lower heatingrates (Figure 6(b)). Similar to the sample quenchedfrom 1013 K (740 �C), large numbers of isolatedfine austenite grains are distributed at the ferritegrain boundaries. Unlike in the HR samples andthe CR samples heated at 1 K/s, no network ofaustenite grains formed along ferrite grain bound-aries. Furthermore, measurements of austenitegrain sizes on samples heated at 1 K/s and 300 K/s,respectively, to just above the Ac3 temperature fol-lowed by water quenching revealed a grain sizereduction from 11 lm to 6 lm as a result ofincreasing the heating rate (Figure 12).

(c) Figure 13(a) shows the distribution of carbide par-ticles inside martensite islands in the samplequenched from 1013 K (740 �C) after heating at300 K/s. It can be observed that the process ofspheroidization of cementite lamellae is almostcomplete. A few remaining elongated particles are

not yet spheroidized; these particles are indicatedby arrows in Figure 13. A closer examinationshows that the size of the carbide particles is finerthan in the samples heated at 1 K/s to 1013 K(740 �C). The average size for carbide particlesdecreases from 190 nm to 130 nm when increas-ing the heating rate from 1 K/s (Figure 5(a)) to300 K/s (Figure 13).

D. Cold-Rolled and Recrystallized Structures

Starting with the CR structures, it was shown that theinitial structure before austenite formation can bedifferent depending on the heating rate (Figures 3(b)and 11(a)), which makes it difficult to compare theresults systematically. Thus, in a next step, all CRsamples were heated to 943 K (670 �C) with 1 K/sheating rate (point B in Figure 2(b)), and then heatedabove the Ac3 temperature with different heating ratesof 1 K/s, 10 K/s, 100 K/s, and 900 K/s. In this scenario,the initial structure will be a recrystallized (REX) ferritematrix with partially spheroidized pearlite colonies(Figure 3(b)), which is referred to hereafter as CR+REX structure. Per definition, for heating at 1 K/s theCR+REX sample coincides with the CR one. Fig-ure 14(a) shows the dilation curves and their firstderivatives for the CR+REX samples. Unlike for theCR structures (Figure 8(b)), the shift in the Ac1 tem-peratures via increasing the heating rate is now evident.Figure 14(b) represents the austenite fraction vs tem-perature at different heating rates for CR+REXstructures. With increasing the heating rate, the curvesshift toward higher temperature, resembling the trendobserved in the HR structures.This trend is summarized in Figure 14(c) in which the

Ac1 temperature represented by a sharp drop in thederivative of the dilation curves is shown for the HR,CR, and CR+REX samples. As discussed, the shift inthe Ac1 temperature for the CR samples is just 15 Kwhen increasing the heating rate by almost three ordersof magnitude. In contrast, this shift is approximately60 K for both HR and CR+REX samples, whichconfirms the importance of the overlap between recrys-tallization and austenite formation. The start tempera-tures for austenite formation in the CR+REX samplesare approximately 10 K lower than those for the HRsamples at all employed heating rates.

IV. DISCUSSION

The observations made during high-heating-rateexperiments can be explained in terms of the kineticsof the events such as recrystallization, spheroidization,and austenite formation, and their interactions. In theHR steel, austenite formation is the main process,whereas in the CR samples, spheroidization of cementitelamellae, recrystallization of deformed ferrite grains,and austenite formation are the major events that takeplace during heating. The heating rate dictates availabletime for these events and their interactions.

Fig. 10—Microstructures of the HR steel continuously heated at(a) 1 K/s and (b) 300 K/s followed by water quenching from 1023 K(750 �C) (F: ferrite, M: martensite).


In the HR samples, an increase in austenite formationstart and finish temperatures, the Ac1 and Ac3 temper-atures (Figure 8(a)), can then be understood in theaforementioned context. When increasing the heatingrate, increasingly less time is available for austeniteformation as a thermally activated process to take place,which leads to a shift to higher transformation temper-atures that is well documented in the literature.[26] Thisincrease of the superheating for austenite formationresults in an increased nucleation site density foraustenite grains such that a finer network of austenitegrains forms (Figure 10).

In the CR steel, at low heating rates, e.g., 1 K/s, thereis sufficient time for recrystallization of ferrite to takeplace before austenite formation starts. This can beobserved in Figure 3(b) in which fully recrystallizedgrains exist without any sign of austenite formation, i.e.,presence of martensite at room temperature. In thisscenario, austenite forms at ferrite–cementite interfacesand then grows into ferrite grains. Nucleation at ferritegrain boundaries as a competitive event results information of a network of austenite at higher temper-atures. This trend suggest similarities between micro-structural evolution during intercritical annealing inboth HR and CR samples heated at lower heating rate,e.g., 1 K/s (Figures 6 and 10).

However, with increasing the heating rate in the CRsteel, the possibility of having unrecrystallized ferrite

grains at the beginning of austenite formation increases.The extent of this increase is proportional to theemployed heating rate. These unrecrystallized grainswith a rather high stored energy will then be suitableplaces for austenite formation. Furthermore, frag-mented pearlite lamellae and/or spheroidized carbidesprovide an increased nucleation density compared withHR samples. Similar to the HR samples, increasing theheating rate in the CR samples results in a delayedaustenite formation as a thermally activated process.The balance of these effects leads to a similar dilationresponse independent of heating rate in the CR samplesat the early stages of austenite formation, i.e., the Ac1temperature is essentially independent of heating rate(Table II). Because a network of austenite grains doesnot exist at ferrite grain boundaries, the dominantgrowth mechanism at high heating rates will be the onethat requires short-range carbon redistribution, i.e.,austenite nucleates at the interfaces of pearlite coloniesthat are elongated in the rolling direction, transformsthese colonies, and then grows laterally into the unre-crystallized ferrite grains that are mostly locatedbetween these colonies. Growth of austenite grainsnucleated at ferrite grain boundaries will then behampered because of limited carbon supply. In thisscenario, austenite grains will inherit the distribution ofpearlite colonies elongated in the rolling direction(Figure 11(d)). This leads to a transition from a random

Fig. 11—Microstructural evolution of the CR steel continuously heated at 300 K/s followed by water quenching from (a) 973 K (700 �C), (b, c)1013 K (740 �C) and (d) 1053 K (780 �C) (F: ferrite, M: martensite, P: pearlite).


distribution of martensite islands for lower heating ratesto a banded distribution for higher heating rates,compare Figures 6(b) and 11(d). As shown in Figure 15,the formation of banded austenite structures from thepearlite colonies is pronounced particularly at thehighest heating rate employed, i.e., 900 K/s.

Huang et al.[12] also observed a similar morphologicalshift in a Mo-alloyed DP steel in which increasing theheating rate from 1 K/s to 100 K/s resulted in bandedstructures. Their explanation for the transition wasbased on concurrent recrystallization of ferrite grainsand austenite formation. They speculated that recrys-tallization of ferrite grains during austenite formationencourages austenite grains to nucleate and grow ondeformed pearlite colonies elongated in the rollingdirection. Subsequent growth takes place via rapidlengthening and thickening of the austenite grains ratherthan nucleation on nonstationary ferrite grain bound-aries. However, at low heating rates, austenite has thepossibility of competitive formation on both ferritegrain boundaries as well as pearlite colonies leading toan equiaxed distribution of austenite grains. Theyshowed that in their steel, approximately 90 pct offerrite grains remained unrecrystallized at 100 K/s at thebeginning of austenite formation, whereas this volumefraction is much lower, �15 pct, in the steel used in thisstudy even at 900 K/s because of the relatively lean steelchemistry. Perhaps a heating rate of several thousanddegrees Kelvin per second is needed to view a significantoverlap of recrystallization and austenite formation inthe current steel. A morphological change via rapidheating was also reported by Grange.[14] They achievedfibrous DP steels via heating of initially CR ferrite-pearlite/martensite structures.The kinetics of spheroidization of cementite lamellae

is controlled mainly by the diffusion of carbon atoms.Essentially, the spheroidization of pearlite colonies canbe divided into fragmentation of carbide particles,rounding off the sharp edges and then growth ofparticles via Ostwald ripening and shape coarsening.These stages can proceed simultaneously.[27] In the HRstructure, the time available for the range of heatingrates employed in this study is not sufficient for theseprocesses to take place. Annealing times of the order ofhours just below the eutectoid temperature are neededfor complete spheroidization of cementite lamellae in theHR steel.[28] But, in the CR samples, the morphology ofcementite lamellae has been modified by cold rolling.Fragmentation and bending of cementite lamellae aswell as introducing crystal defects such as vacancies anddislocations into the ferrite grains and ferrite–cementiteinterfaces as fast diffusion paths stimulate the sphero-idization process. Thus, it is likely to observe thisprocess to take place in the CR samples. At low heatingrates, e.g., 1 K/s, sufficient time is available for almostcomplete spheroidization to occur before austeniteformation. Cementite particles can even coarsen tolarger carbide particles during the process, whereas athigh heating rates, cementite lamellae remain frag-mented without any subsequent coarsening. Thisexplains the finer distribution of carbide particles at300 K/s as compared with 1 K/s.Austenite grain refinement in the CR samples via

rapid heating can be related to an increase in austenitenucleation site density. These nucleation sites includepearlite colonies and ferrite grain boundaries(Figures 11(b) and (c)). Deformed pearlite coloniesprovide suitable nucleation sites for austenite grains.

Fig. 12—Revealing austenite grains in the CR samples heated at(a) 1 K/s and (b) 300 K/s to just above the corresponding Ac3 tem-peratures followed by water quenching.

Fig. 13—Revealing cementite particles inside martensite islands inthe CR sample heated at 300 K/s to 1013 K (740 �C).


Furthermore, a large population of ferrite grain bound-aries provides suitable sites for austenite nucleation.Simultaneously, rapid heating prevents extensive growthof austenite grains in the intercritical annealing region.The refinement of austenite grains via rapid heating wasalso reported by Andrade-Carozzo and Jacques.[17]

Lesch et al.[29] employed this idea to develop ultrafinegrained ferrite structure via rapid transformationannealing in low carbon steels. The process is essentiallybased on rapid heating of CR structures just above theAc3 temperature followed by rapid cooling to roomtemperature.The relative independence of austenite fraction in the

CR steel on heating rate in comparison with the HRcounterpart (Figure 9) can be understood based on theinitial structures prior to austenite formation in bothmaterials. In the HR structure, this initial structureremains unchanged. Thus, increasing the heating rateraises superheating until a network of austenite grainsforms at ferrite grain boundaries. In the CR sample,however, the situation is different. Similarly, increasingthe heating rate tends to increase the superheat, but theinitial structure prior to austenite formation will also bea function of the heating rate. In ferrite, it changes froma fully recrystallized structure to a partially recrystal-lized one as the heating rate increases, and in pearlite, itdelays the spheroidization of carbide particles. Thus,nucleation and growth scenarios for austenite formationwill change accordingly. The balance of these two effectswill then affect the kinetics of austenite formationas a function of heating rate. However, when firstrecrystallizing the CR sample, i.e., with the CR+REX

Fig. 14—(a) Dilatation curves and their first derivatives for CR+REX samples (heated at 1 K/s heating rate up to 943 K (670 �C) followed byheating into austenite single-phase region with different heating rates), (b) austenite fraction as a function of temperature for different heatingrates, and PE austenite fraction, (c) comparison of Ac1 temperatures at different heating rates for HR, CR and CR+REX samples.

Fig. 15—Microstructure of the CR steel continuously heated at900 K/s followed by water quenching from 983 K (710 �C) (F: fer-rite, M: martensite, P: pearlite).


structure, the similarity with the dilation response of theHR sample is restored (Figure 14), as the initial struc-ture in both materials mainly consists of recrystallizedferrite. The slightly different transformation start tem-peratures (Figure 14(c)) might be attributable to par-tially spheroidized pearlite lamellae in the CR+REXsamples. Because austenite nucleation commences at theinterface of ferrite and cementite in the pearlite colonies,the broken lamellae provide more interfaces, andconsequently, the possibility for austenite nucleationincreases resulting in lower Ac1 temperatures inthe CR+REX samples compared with their HRcounterparts.

V. CONCLUSIONS

In this study, austenite formation in a HR andCR plain low-carbon steel was investigated systemati-cally using dilation experiments and microstructuralcharacterization. A variety of scenarios can be rational-ized to explain the overall rate of austenite formationconsidering the overlap between austenite formation,recrystallization of ferrite-pearlite structure, and sphero-idization of Fe3C. The following can be concluded fromthis study.

The dilation response can, in the HR material, besubdivided into several stages: (1) thermal expansion offerrite/pearlite structure, (2) pearlite-to-austenite trans-formation, (3) ferrite-to-austenite transformation, and(4) austenite thermal expansion. Additionally, recrystal-lization of ferrite and spheroidization of pearlite cantake place in the CR sample. Recrystallization of ferriteresults in a change in the dilation response of the CRsteel.

Increasing the heating rate shifts the austeniteformation to higher temperatures in the HR sampleand results in formation of a network of finer austenitegrains. Increasing the heating rate in the CR material,however, results in formation a variety of initialstructures ranging from fully recrystallized to partiallyrecrystallized structures prior to austenite formation.This transition can be monitored using the dilationdata. Furthermore, the degree of spheroidization ofcarbide lamellae in pearlite colonies is reduced withincreasing the heating rate. As a result, a morphologicalshift is observed from randomly distributed to abanded structure of austenite. It can be noted thatthe banded feature of austenite could be controlledtheoretically by the processing parameters such as thelevel of cold work and the heating rate. A higherheating rate also resulted in austenite grain refinementin CR steels.

ACKNOWLEDGMENTS

The authors would like to acknowledge the Natu-ral Sciences and Engineering Research Council ofCanada (NSERC) for their financial support. We aregrateful to J.D. Embury for valuable discussions andsuggestions.

APPENDIX

To examine the effect of different factors on thedeviation from linearity in the dilation curves observedbefore austenite formation, see Section III–B; additionalinvestigations were carried out. Cold rolling can intro-duce residual stresses that are concentrated on thesurface of the samples.[30] Relieving residual stressesimposed by cold rolling can occur prior to austeniteformation in the CR steel. It can also contribute to thedimensional changes in the samples.[31] However, reduc-tion of residual stresses via thinning the samples down tohalf thickness by polishing shows no sign of change inthe deviation. Furthermore, dilatometry of 80 pct CRprespheroidized samples (annealed for 24 hours at963 K [690 �C] before cold rolling) revealed a dilationresponse similar to that of the CR sample. Thus,spheroidization was then ruled out to contribute todilation changes. A 80 pct CR Ti-added IF steel with thechemical composition given in Table A1 was used tosupplement the dilation study during heating for a casewhere no cementite is present, i.e., any effect frompearlite can be excluded. The initial microstructureconsists of elongated ferrite grains. To examine the starttemperature of ferrite-to-austenite transformation, asample was heated into the single austenite phase regionat 1 K/s, and an Ac1 temperature of 1193 K (920 �C)was measured. As shown in Figure A1, a thermal cyclewas then employed to investigate the dilation beforephase transformation. It includes a heating stage (solidblack line), fast He quenching (black dashed line), andreheating stage of the sample (gray line). The CR samplewas heated first at 1 K/s to 1123 K (850 �C) followed byrapid He quenching down to 473 K (200 �C) andimmediate reheating at 1 K/s to 1123 K (850 �C). Thecontraction in the dilation curve in the first heating pathis evident in Figure A1 (similar to the CR low-carbonsteel in Figure 2). The relative change length (DL/Lo) isapproximately 0.11 pct, which is similar to the CR low-carbon steel, i.e., 0.1 pct. However, in the next reheatingstage, the dilation curve shows no sign of contraction.Microstructural observations at 873 K (600 �C) and1123 K (850 �C) showed that the microstructure chan-ged from the CR to a completely recrystallized one

Table A1. Chemical Composition of the IF Steel in Weight Percent

Element Fe C Mn P S Si Ti B N

Wt pct Balanced 0.0026 0.16 0.011 0.008 0.01 0.068 0.0005 0.003


during the first heating stage. Thus, a reduction indislocation density and texture changes from an alphafiber to a gamma dominant texture[10,32] that occurduring ferrite recrystallization remain as possible expla-nation for the deviation. A reduction in specific volumeof 0.37 pct for 80 pct cold-drawn steel wires duringannealing and before austenite formation was reportedby Gridnev et al.[33] With an assumption of isotropicchange in length during dilation, one can estimate DL/Lo through DL/Lo � DV/3Vo for a small relative volumechange in which DL is the length change, Lo is the initiallength, DV is the volume change, and Vo is the initialvolume of the sample. Thus, the value for DL/Lo

concluded from the work by Gridnev et al.[33] is�0.12 pct, which is in good agreement with the obser-vations made in this study, i.e., 0.1 pct for the low-carbon steel and 0.11 pct for the IF steel. Gridnevet al.[33] attributed this observation to annihilation ofpoint and line defects as well as microcracks. De Cocket al.[10,32] also showed that the reduction in dislocationdensity during recrystallization can account for theamount of dilation (�0.05 pct) that is observed duringheating of 80 pct CR low and ultralow carbon steelsbefore austenitization at 10 K/s. Thus, it is concludedthat the deviation from linear thermal expansion isrelated to recrystallization of the sample during heatingand is consistent with the volume change caused by areduction in the average dislocation density.

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Fig. A1—Dilatation curves for IF steel during heating-cooling-heat-ing cycles. The cooling rate during rapid cooling was approximately200 K/s.


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2011_Hamid Azizi-Alizamini_Austenite Formation in Plain Low-carbon Steels