Wiliam Regone et al. Metalurgia & Materiais · charque, seguido por vários passes de forjamento, e, fi-nalmente, há um resfriamento controlado para definir a microestrutura e propriedades

303REM: R. Esc. Minas, Ouro Preto, 57(4): 303-311, out. dez. 2004

Wiliam Regone et al.

Metalurgia & Materiais

ResumoOs aços microligados são usados em peças automo-

tivas forjadas, tais como girabrequins e bielas. Essas pe-ças são trabalhadas a quente em uma seqüência de está-gios, que inclui o aquecimento até a temperatura de en-charque, seguido por vários passes de forjamento, e, fi-nalmente, há um resfriamento controlado para definir amicroestrutura e propriedades mecânicas. Nesse traba-lho, foram investigados o comportamento termomecâni-co e a evolução microestrutural de um aço microligadoTi-V na região de transição de fase. Os testes de torçãoforam feitos em múltiplos passos com deformação verda-deira de 0,26 em cada passo. Após cada passo de torção,as mostras foram resfriadas continuamente por 15 segun-dos para simular as condições de forjamento a quente.Esses testes forneceram resultados da temperatura deinício de transformação de fase e permitiram analisar asmudanças de microestrutura. Foram, também, realizadosos testes de trabalhabilidade para analisar a evolução mi-croestrutural por microscopia ótica e eletrônica de varre-dura. Os resultados dos testes de torção mostraram que atemperatura de início de transformação de fase está emtorno de 700°C. Os testes de trabalhabilidade feitos a 700°Cseguidos por resfriamento em água apresentaram microes-truturas com características distintas: regiões encruadas ede recristalização estática e dinâmica. Os testes de traba-lhabilidade de 700°C seguidos por resfriamento ao ar mos-traram uma complexa microestrutura de ferrita, bainita emartensita, enquanto que os testes feitos em 650 e 600°Cseguidos por resfriamento em água apresentaram uma mi-croestrutura com ferrita alotriomórfica presente nos con-tornos de grão da austenita.Palavras-chave: Aço microligado, decomposição daaustenita, análise numérica, evolução microestrutural.

AbstractMicroalloyed steels are used in the forging of many

automotive parts like crankshafts and connecting rods.They are hot worked in a sequence of stages that includesthe heating to the soaking temperature, followed byforging steps, and finally the controlled cooling to definethe microstructure and mechanical properties. In thiswork it was investigated the thermomechanical behaviorand the microstructural evolution of a Ti-Vmicroalloyed steel in the phase transition region.Torsion tests were done with multiple steps with truestrain equal to 0.26 in each step. After each torsion stepthe samples were continuous cooled for 15 seconds tosimulate hot forging conditions. These tests providedresults for the temperature at the beginning of the phasetransformation, and allowed to analyze themicrostructural changes. Also, workability tests wereheld to analyze the microstructural evolution by opticaland scanning electron microscopy. Results from thetorsion tests showed that the temperature for thebeginning of phase transformation is about 700 oC.Workability tests held at 700 oC followed by water-cooling presented microstructures with different regions:strain hardened, and static and dynamic recrystallized.Workability tests at 700 oC followed by air-coolingshowed a complex microstructure with ferrite, bainiteand martensite, while tests at 650 and 600 oC followedby water-cooling showed a microstructure withallotriomorphic ferrite present in the grain boundariesof the previous austenite.

Keywords: Microalloyed steel, austenite decomposition,numerical analysis, and microstructural evolution.

Effects of deformation on themicrostructure of a Ti-V microalloyed steel

in the phase transition regionWiliam Regone

Associate Researcher. State University of Campinas. School of Mechanical Engineering - Campinas, SP, BrazilE-mai: [email protected]

Sérgio Tonini ButtonAssistant Professor. State University of Campinas. School of Mechanical Engineering. Campinas, SP, Brazil

E-mail: [email protected]

REM: R. Esc. Minas, Ouro Preto, 57(4): 303-311, out. dez. 2004304

Effects of deformation on the microstructure of a Ti-V microalloyed steel in the phase transition region

1. IntroductionHot forging of microalloyed steels

is used to the manufacturing of manyautomotive components. The processflowchart begins with the heating tosoaking temperatures in the austeniticregion, followed by forging steps thatdefines the product geometry anddimensions. Finally, these products arecooled to obtain the desired properties [1].

The metallurgical mechanismspresent during these processes are thestrain hardening, the softening, theprecipitation, and phase transformations.Strain hardening is caused bydeformation and at a proper processtemperature can lead to the thermalsoftening, competing with the particlesprecipitation that will affect the materialstrength. Therefore, during hot forging,these metallurgical mechanisms willcontrol the microstructural evolution ofthe austenitic grains. The strainhardening causes the increase in thedislocations density [2]. Dynamic recoveryinvolves the annihilation of thesedislocations generating cells and sub-grains. Increasing the strain hardening, theequiaxial grains will elongate and sitesfavorable to the nucleation of new grainswill appear in the boundaries of theseelongated grains [3].

Dynamic recrystallization beginswhen the first nuclei restore themicrostructure only locally, while the restof the material keeps on hardening. Therecrystallized nuclei grow just after thedeformation is ended by metadynamicrecrystallization [4].

Static recovery takes place after thedeformations and partially restores themicrostructure. The total softening onlyoccurs with the recrystallization thatbegins after a necessary incubation timewith the nucleation of new grains free ofdeformation [5].

The final properties of hot forgedproducts are basically determined by themicrostructure defined at the end of theprocess. Therefore it is very importantto know how the phase transformationsoccur during the cooling stage.

The transformation of the austenitepresents two mechanisms: onediffusional with the nucleation andgrowth of new phases, like with theallotriomorphic ferrite that nucleates inthe grain boundaries of the austenite,forming layers that follow the limits ofthese boundaries. [6]. The secondmechanism is the martensitictransformation that is associated to highcooling rates and is caused by theshearing of the microstructure [7].

2. Material and methodsA great variety of microalloyed

steels with low and medium carboncontents have been used to themanufacturing by hot forging of manyautomotive components. The interest inforged parts with microalloyed steels isrelated to the thermomechanicalprocessing of these steels, because it ispossible to achieve high mechanicalstrength and toughness simply with aircooling directly after forging, thuseliminating the usual normalizing heattreatment, reducing process times andproduction costs.

In this work it was analyzed thethermomechanical behavior and themicrostructural evolution of a V-Timicroalloyed steel in the region of phasetransition. A sequence of torsion testswas carried out with controlleddeformations and continuos cooling.Also, workability tests were held in manytemperatures followed by air-cooling orwater-cooling to reveal themicrostructures present in theworkpieces.

The material studied in this workwas a commercial medium carbonmicroalloyed steel used in themanufacturing of automotivecomponents by hot forging followed bycontrolled air cooling. The chemical

composition of this steel is shown inTable 1. This material in the condition"as hot rolled and air-cooled" was re-rolled between 1150 and 1200°C to barswith 18 mm in diameter. No subsequentheat treatment was held in order topreserve material properties common toindustrial hot forging operations [8].

The tests with multiple deformationsteps (ε1, ε2,..., εn) by hot torsion withcontinuous cooling applied a methodsimilar to that used by many authors [9-11]. These tests were done with theheating of the workpieces to thetemperature of 1050 or 1200°C, at asoaking time of 15 minutes.

Then these workpieces werecontinuously cooled at the cooling rateof 1°C/s, as shown in Figure 1. Each testhad 19 deformation steps, with intervaltime of 30 seconds, mean strains of 0.3and mean strain rate equal to 1 s-1.

With the results obtained in thetorsion tests the mean equivalent stresswas calculated with the Expression (1)for each deformation step to plot a curvesimilar to that shown in Figure 2 [12]:

∫−=

b

a

ε

εeqeq

abeq dεσ

εε1σ (1)

Where (εb-εa) corresponds to theequivalent strain of the deformation step.

Workability upsetting tests werecarried out with workpieces machinedwith 15 mm in diameter and 20 mm high,and deformed in a hydraulic press. Theupper die similar to an edge was chosento promote a severe strain gradient insidethe workpiece. This upper die wasmoved at 15 mm/s with a mean nominalstrain rate of 0.93 s-1. The workpieceswere heated at 1100°C held for 15 minutesand cooled with a mean rate of 4.5°C/s to

Table 1 - Chemical composition of the microalloyed steel (wt.%).

C Mn Si Al S P Ti V N

0.32 1.51 0.66 0.024 0.031 0.016 0.028 0.099 0.006



700, 650 and 600°C. Then theworkpieces were deformed and air-cooled or water-cooled. Figure 3Ashows the dies and workpiece beforethe deformation, and Figure 3B, theworkpiece after hot forged.

Workpieces from the torsion testsand from the upsetting tests wereanalyzed by optical and electronscanning microscopy.

To reveal the microstructures thesamples were grinded and polished withalumina (granulometry 1 and 0,5 µm.

Water-cooled samples were etchedwith an aqueous solution saturated ofpicric acid and detergent to reveal theaustenitic grains. All samples wereimmersed in this solution heated to 80°Cfor 60 to 120 seconds, followed by a lightpolishing with alumina 0.3 µm to betterobserve the austenite grain boundaries.This procedure was repeated many timestill satisfactory results were obtained [13].

Air-cooled samples were etchedwith nital 2% solution to reveal themicrostructures formed from theaustenite decomposition.

3. Results anddiscussion3.1 Torsion tests withmultiple deformation stepswith continuous cooling

To analyze the thermomechanicalbehavior of a V-Ti microalloyed steel,many deformation sequences withcontinuous cooling were carried out.Figures 4A and 4B show the resultsobtained in these tests for the equivalentstress as a function of the effective

Figure 1 - Sequence of torsion steps (ε1, ε2, ..., εn) withcontinuous cooling.

εn

ε2

1 oC/s

TtorsionTem

pera

ture

Time

15 min

Tsoakingε1

Figure 2 - Equivalent flow stress as a function of effectivestrain in torsion steps.

Effective strainεi+1

εi εbεa

Equi

vale

nt s

tress σi+1

σi

σ

Figure 3 - A) - Workpiece and dies before the deformation. B) Workpiece after hot forged. Mean dimensions in mm.

Flat die

Workpiece

Edge die

A) B)



strain. Figure 5A and 5B show resultsfor the equivalent mean stress (EMS) asa function of the temperaturecharacteristic of each torsion step.

The results in Figure 4Acorrespond to workpieces heated to1200ºC for 15 minutes, cooled at a coolingrate of 1ºC/s, and then deformed in asequence of 16 torsion steps before therupture of the workpieces, with aninterval time of 30 seconds, mean strainof 0.3, and mean strain rate of 1s-1.

Results in Figure 4B were obtained witha similar procedure, but with a initialtemperature of 1050ºC, and only 10torsion steps before the rupture.

Figures 4A and 4B show that mostof the deformation steps were held in theaustenitic region and few steps were heldin the phase transition as shown by thematerial rupture. The aspect of the flowcurves indicated that strain hardeningprevails during the passes and that inthe interval time static recovery and static

recrystallization must take placesoftening the material and causing asmall reduction in the equivalent stress.After several torsion steps effectivestrain is high enough to initiate thedynamic recrystallization and thereduction in the equivalent stress is morepronounced [14-15].

In Figures 5A and 5B it can beobserved in the austenitic region thatEMS increases with the temperature falltill the beginning of the phase transition

Figure 4 - A) Flow curves of the torsion steps. Initial temperature: 1200ºC. B) Flow curves of the torsion steps. Initial temperature:1050ºC.

0 1 2 3 4 5 6 70

50

100

150

200

250

300

Step #6, 1050 OC

Rupture

Step #1, 1200 OC

Equi

vale

nt s

tress

(MPa

)

Effective strain0 1 2 3 4 5 6 7

0

50

100

150

200

250

300

Step #6, 901 0C

Rupture

Step #1, 1050 0C

Equi

vale

nt s

tress

(MPa

)

Effective strain

A) B)

Figure 5 - A) EMS variation plotted from the results of Figure 4A. B) EMS variation plotted from the results of Figure 4B.

6,6 7,2 7,8 8,4 9,0 9,6 10,2

40

80

120

160

200

240

EMS

(MPa

)

10000/T (K-1)

1200 1000 800

Ar3

reductionof EMS

Tnr

766 0C905 0C

T (0C)

6,6 7,2 7,8 8,4 9,0 9,6 10,2

40

80

120

160

200

240

EM

S (M

Pa)

10000/T (K-1)

1200 1000 800

reductionof EMS

Ar3

7630C

T (0C)A) B)



at the temperature Ar3 as indicated inthe graphs. Figure 5A can be dividedinto three different regions consideringthe metallurgical mechanisms actingduring the tests. Figure 5B shows onlytwo of these regions.

In the first region, shown before thenon-recrystallization temperature(Tnr~905°C), the main metallurgicalmechanisms are the strain hardening, thedynamic recovery and the dynamicrecrystallization during the deformation,

and the static recovery and staticrecrystallization in the interval betweenthe steps. In the second region, betweenTnr and Ar3 the precipitation induced bydeformation takes place and fine particlesare precipitated in the boundaries of theaustenite grains. These particles blockthe boundaries and inhibit that betweenthe deformation steps the static recoveryand recrystallization restore the material[16]. Therefore, the strength is increasedby the precipitation and by the

temperature fall. In the third region,defined after Ar3 at a temperature near to766°C, ferrite is formed in the austeniteboundaries reducing the materialstrength and causing its rupture.

Figure 5B does not show a definedTnr since the initial temperature of1050°C was not enough to solubilize theprecipitates into the austenite. Thetemperature to phase transformation Ar3is found 763°C almost the same found inFigure 5A.

Figure 6 - A) Sample deformed at 700°C and water-cooled - non-deformed region. B) Sample deformed at 700°C and water-cooled- region with low deformation.C) Sample deformed at 700°C and water-cooled - region with high deformation. D) Sample deformedat 700°C and water-cooled - region with high deformation. (optical microscopy).

A) B)

C) D)



3.2 Microstructural analysisof samples from theworkability tests

With the results from theworkability tests it was possible toevaluate the influence of the applieddeformation and of the temperature inmany regions of the workpiece, on themetallurgical mechanisms present in thedeformation and cooling stages.

Figures 6A to 9F show themicrostructures of samples heated to1100°C for 15 minutes, cooled at a meanrate of 4.5°C/s, then deformed at 700, 650or 600ºC, and finally water-cooled or air-cooled.

3.2.1 Samples deformed at 700°Cand water-cooled

Figures 6A to 6D show themicrostructures and obtained for thesesamples in three regions: withoutdeformation, low deformation, andsevere deformation.

Figure 6A shows non-deformedaustenite grains with a size of 50 µm thatrepresent the condition before thedeformation. Figure 6B shows hardenedelongated grains. In this region, thedeformation was not enough to initiatethe dynamic recrystallization. Figure 6Cshows small equiaxial grains generatedby dynamic and metadynamicrecrystallization. Figure 6D show adeformed region with a microstructuresimilar to martensite. It can be observedthat in the grain boundaries some tracesof allotriomorphic ferrite is alreadyformed.

3.2.2 Samples deformed at 700°Cand air-cooled

Figures 7A to 7F show themicrostructures of these samples for thesame regions analyzed in the item 3.2.1.

Since the workability tests wereheld near Ar3 it was observed a complexmicrostructure with differentmorphologies that difficult itsinterpretation. Figure 7 - Sample deformed at 700 °C and air-cooled.

B) Non-deformed region - scanningelectron microscopy.

A) Non-deformed region - opticalmicroscopy.

D) Strain hardened region - scanningelectron microscopy.

C) Strain hardened region - opticalmicroscopy.

F) Strain hardened region - scanningelectron microscopy.

E) Strain hardened region - opticalmicroscopy.



Figure 7A (optical microscopy) shows a non-deformedregion with a martensite microstructure and some idiomorphicferrite grains, confirmed by Figure 7B obtained by electronmicroscopy, where the ferrite grains were formed in theaustenite boundaries.

Figure 7C shows the microstructure of a deformed andstrain hardened region. This same region is shown in Figure7D by electron microscopy and it can be observed that therespective austenite grains were deformed but their interiorare free of deformation due to the dynamic recovery.

Figure 7E also shows a deformed region with a complexmorphology. Figure 7F (electron microscopy) shows that ferritewas formed in the austenite boundaries. In the interior of theaustenite grains it can be observed the formation of bainite.The primary bainite needles nucleated in the boundaries andthe secondary needles formed from the primary are found inthe interior of the grains.

3.2.3 Samples deformed at 650°C and water-cooledAgain, these samples were analyzed in three regions:

without deformation, low deformation, and high deformation.In these tests the material was in the region of transition fromaustenite to ferrite.

Figure 8A shows a non-deformed region with amicrostructure similar to martensite with allotriomorphic ferriticgrains in the boundaries of the previous austenite. Figure 8Bshows similar results but with elongated austenitic grains andferrite in the boundaries. Figure 8C shows austenitic grainsdeformed and recrystallized, with allotriomorphic ferrite in theaustenite boundaries.

3.2.4 Samples deformed at 600°C and water-cooledFigures 9A to 9F show the microstructures of three

different regions (non-deformed, low deformation and highdeformation) of samples upset in the workability tests at 600°Cand water-cooled. At this temperature the material shouldpresent two phases, austenite and ferrite.

Figure 9A shows the microstructure of the non-deformedregion with morphology similar to martensite withallotriomorphic ferrite in the previous austenite boundaries.The same microstructure can be observed by scanning electronmicroscopy in Figure 9B.

The region with low deformation is shown in Figures 9Cand 9D and the microstructure shows deformed austeniticgrains due to the intense strain hardening.

Figures 9E shows the microstructure of the region withhigh deformation. The morphology is similar to the martensitebut the shape of the previous austenitic grains is not welldefined because they could be strain hardened and elongatedor restored by dynamic recovery. Figure 9F shows the same

Figure 8 - Sample deformed at 650°C and water-cooled. A)Non-deformed region- optical microscopy. B) region with lowdeformation - optical microscopy. C) Region with high deformation- optical microscopy.

A)

B)

C)



Figure 9 - Sample deformed at 600°C and water-cooled. A) Non-deformed region, optical microscopy. B) Non-deformed region,scanning electron microscopy. C) Region with low deformation, optical microscopy. D) Region with low deformation, scanningelectron microscopy. E) Region with high deformation, optical microscopy. F) Region with high deformation, scanning electronmicroscopy.

A)

C)

E)

B)

D)

F)



region by electron microscopy and it canbe observed the previous austeniticmicrostructure with small bainite needlesin the interior of the grains.

4. ConclusionsResults from the torsion tests

showed that the temperature for thebeginning of phase transformation isaround 765°C for the microalloyed steelstudied in this work.

Workability tests at 700°C followedby water-cooling showed that strainhardening, static and dynamicrecrystallization can be present and formcomplex microstructures with ferrite andbainite mixed to the martensite. Testsheld at 650 and 600°C with water-coolingpresented a microstructure withallotriomorphic ferrite formed in theboundaries of the austenitic grains.

In the sequence of torsion testswith continuous cooling, the stresscurves in the austenitic region showedan increase in the equivalent stresscaused by the temperature fall and strainhardening.

The critical strain to promotedynamic recrystallization was reachedand confirmed by the decrease in theequivalent stress relative to the next step,despite the temperature fall.

In the torsion tests at 1200°C theTnr is near 905°C, while for 1050°C Tnr isnot easily defined. The temperature tophase transformation is near 765°C forboth temperatures 1200 and 1050°C.

The samples deformed in theworkability tests at 700, 650 and 600°C

and water-cooled showed similarmicrostructures dependent on the localstrain and temperature.

Non-deformed regions presentedaustenitic grains similar to those foundin the material before deformation. Theregions with low deformation showedelongated austenitic grains withallotriomorphic ferrite formed in theboundaries. Regions with highdeformations presented mixedmicrostructures with elongated and finerecrystallized austenitic grains. Byscanning electron microscopy it waspossible to observe allotriomorphicferrite formed in the boundaries and aninterlaced structure of bainite needles inthe interior of the austenitic grains.

5. AcknowledgementsAuthors wish to thank CNPq -

Conselho Nacional de DesenvolvimentoCientífico e Tecnológico and FAPESP -Fundação de Amparo à Pesquisa doEstado de São Paulo, for the financialsupport to this work.

6. References[1] REGONE, W., NEVES, F.O., BUTTON,

S.T. Análise numérica do comportamentotermomecânico e microestrutural de umaço microligado ao V-Ti em processamentoanálogo ao forjamento a quente. In:CONFERÊNCIA INTERNACIONAL DEFORJAMENTO, 6. Gramado, RS, Anais...,p. 113-125, october, 2002.

[2] LE MAY, I. Principles of mechanicalmetallurgy. New York: Elsevier, 1981,Cap.6.

[3] MECKING, H., GOTTSTEIN, G. Recoveryand recrystallization during deformation.In: HAESSNER, F. (Dr. Riederer) (Ed.).

Recrystallization of Metallic Materials.Verlag, 1978. p. 195-222.

[4] PETKOVIC, R. A., LUTON, M. J., JONAS,J. J. Can. Met. Quart. v. 14, p. 137, 1975.

[5] McQUEEN, H. J., JONAS, J. J. Recoveryand recristalization during highttemperature deformation. In: ARSENAUT,R. J. (Ed.). Treatise on Materials Scienceand Technology, v. 6, p. 393-493. NewYork: Academic Press, 1976.

[6] REGONE, W., NEVES, F.O., BUTTON,S.T. Análise do comportamentomicroestrutural de um aço microligado porsimulação física análoga ao forjamento aquente. In: CBECIMAT, 15. Anais... Natal,november, 2002.

[7] GENTILE, F.C., REGONE, W., NEVES,F.O., BUTTON, S.T. Análise numérica eexperimental da evolução microestruturalem forjamento a quente de um açomicroligado ao V-Ti. In: CBECIMAT, 15.Anais... november, 2002.

[8] OLIVEIRA, M. A. F. São Carlos: FederalUniversity of São Carlos, 2001. (DoctorateThesis).

[9] JONAS, J. J. Mat. Scie. and Eng., v. A184,p. 155, 1994.

[10] BORATTO, F. et al. Thermec-88, ed. I.Tamura, Tokyo:1988. p. 383.

[11] SOUSA, R. C. São Carlos: FederalUniversity of São Carlos, 1996. (DoctorateThesis).

[12] BAI, D. Q., YUE, S., SUN, W. P., JONASJ. J. Metall. trans. v. 24A, p. 2151, October,1993.

[13] REGONE, W., BALANCIN, O.Modelagem e simulação de seqüências dedeformações a quente para descrever epredizer a evolução microestrutural duranteo processamento metalúrgico. REM -Revista Escola de Minas, v. 51, n. 1, p.27-33, 1998.

[14] CABRERA, J. M., AL OMAR, JONAS, J.J., PRADO J. M. Metall. trans., v. 28A,p.2233, November, 1997.

[15] ELWAZRI, A. M., WANJARA P., YUE S.Mat. Scie. and Eng., v. A339, p. 209, 2003.

[16] MEDINA, S. F. Mat. Scie. and Tech., v.14, p. 217, 1998.

Artigo recebido em 09/09/2004 eaprovado em 15/11/2004.

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Wiliam Regone et al. Metalurgia & Materiais · charque, seguido por vários passes de forjamento, e, fi-nalmente, há um resfriamento controlado para definir a microestrutura e propriedades