The effects of non-isothermal deformation on martensitic transformation in 22MnB5 steel

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Materials Science and Engineering A 487 (2008) 445–455

The effects of non-isothermal deformation on martensitictransformation in 22MnB5 steel

M. Naderi a,∗, A. Saeed-Akbari b, W. Bleck b

a Department of Materials Science and Engineering, Faculty of Engineering, Arak University, Shariati Street, Arak, Iranb Department of Ferrous Metallurgy, RWTH Aachen University, Aachen, Germany

Received 19 June 2007; received in revised form 12 October 2007; accepted 21 October 2007

bstract

In the present paper, the effects of process parameters on phase transformations during non-isothermal deformations are described and discussed.on-isothermal high temperature compressive deformations were conducted on 22MnB5 boron steel by using deformation dilatometry. Cylindrical

amples were uniaxially deformed at different strain rates ranging from 0.05 to 1.0 s−1 to a maximum compressive strain of 50%. Qualitative anduantitative investigations were carried out using surface hardness mapping data as well as dilatation curves. It was observed that a higher initialeformation temperatures resulted in a higher martensite fraction of the microstructure, while a variation in the martensite start temperature

as negligible. Another conclusion was that by applying larger amounts of strain as well as higher force levels, not only the martensite start

emperature, but also the amount of martensite was reduced. Moreover, it was concluded that using surface hardness mapping technique andilatometry experiments were very reliable methods to quantify and qualify the coexisting phases.

2007 Elsevier B.V. All rights reserved.

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eywords: Martensitic transformation; Dilatation; Boron steel; Surface hardne

. Introduction

Hot stamping is a non-isothermal high temperature formingrocess, in which complex ultra high strength parts are produced,ith the goal of no spring back. During the process, the blanks

re austenitized and subsequently formed and quenched in theie [1–4]. Forming and phase transformations are uniquely per-ormed in a single step. Therefore, studying the mutual effects ofeformation parameters and the possible phase transformationsuring the process on the final properties of the material is ofnterest.

It is well known that the forming process has to be extended inhe austenite phase region. Furthermore, the favorite phase trans-ormations which result in ultra high strengths are martensitics well as bainitic in their nature.

It is evident that the mentioned phase transformations dependn several factors such as austenization schedules, chemicalomposition and prior plastic deformations. The influence of

∗ Corresponding author at: Department of Ferrous Metallurgy, RWTH Aachenniversity, Intzestr. 1, 52072 Aachen, Germany. Tel.: +49 241 80 95822;

ax: +49 241 80 92253.E-mail address: [email protected] (M. Naderi).

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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2007.10.057

p; Mechanical stabilization

ustenization treatment [5–9] and plastic deformation of austen-te before its transformation to martensite and bainite have beenreviously reported [10–15].

In the performed research work, the influence of austenizationoaking time, initial deformation temperature, amount of strain,train rate, rate of cooling and amount of applied force on thehase transformations during non-isothermal compression testsere investigated.

. Experimental

The investigated material is a 22MnB5 steel industrially pro-essed to as hot rolled boron steel plates with a thickness of0 mm. The chemical composition is given in Table 1. Theicrostructure at as-delivered plates consists of 78 vol.% (±5%)

errite and 22 vol.% (±5%) pearlite.

.1. CCT diagram designation

The continuous cooling transformation (CCT) diagram,ig. 1, was determined by dilatometry tests, metallographic

nvestigations and hardness measurements. The samples wereustenitized at 900 ◦C for 5 min. The circled numbers indicate

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Table 1Chemical composition of the investigated 22MnB5 steel (in mass%)

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he values of final hardness, given in the macro Vickers HV10cale. For a heating-up rate speed of 5 ◦C/s, the first temperaturet which austenite start temperature (Ac1) is 720 ◦C, and the startemperature of primary ferrite to austenite transformation (Ac3)as determined to be 880 ◦C. The martensite start and finish

emperatures, Ms and Mf, are 410 and 230 ◦C, respectively. Itan be seen from the CCT diagram that a cooling rate of morehan 25 ◦C/s resulted in a fully martensitic microstructure.

.2. Thermo-mechanical experiments

All deformation tests and dilatometry testing were conductedn a Baehr 805 dilatometer with a deformation unit designedo perform a simple uniaxial compression test. The geome-ry of the sample used was, in all cases, cylindrical with aength of 10 mm and a diameter of 5 mm. In all tests the tem-erature was measured using a surface mounted Pt/Pt–Rh10%hermocouple located at the mid-length, relative to the spec-men. Non-isothermal deformations were established througheveral simultaneous forming and quenching tests at tempera-ures between 600 and 850 ◦C and at strain rates of 0.05 s−1 ≤˙ ≤ 1.0 s−1. Regarding the duration of the experiments, the sam-les were deformed to compressive strains of −0.1 to −0.5 insingle step. Unless otherwise mentioned, all the samples wereustenitized at 900 ◦C for 5 min and quenched to the compres-

ion temperatures at a cooling rate of 50 ◦C/s. In order to fixhe initial deformation temperature TiD, the samples were keptor 1 s at the specific TiD, and then the deformation was carriedut.

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Fig. 1. CCT diagram of the 22MnB5 steel. Samp

ngineering A 487 (2008) 445–455

.2.1. Austenitizing soaking timeThe effect of austenitizing soaking time on the phase trans-

ormations was investigated by setting all of the deformationarameters as constant values. Four samples were austenitizedt 900 ◦C, which is 20 ◦C above the Ac3 temperature for 5, 10, 15nd 20 min, respectively. After austenization, each sample wasooled down to the initial deformation temperature of 800 ◦C andubsequently deformed and quenched down to room tempera-ure. The rate of deformation was ε = 0.1 s−1, and the amountf strain was εpl = −0.2. The cooling rate during the process wasept constant to 50 ◦C/s.

.2.2. Initial deformation temperature (TiD)The effect of TiD on phase transformations was studied

y starting non-isothermal deformations at different tempera-ures of 850, 800, 750 and 700 ◦C, respectively. These selectediD values are in the range of the possible initial deformation

emperatures during hot stamping process. The samples wereustenitized at 900 ◦C for 5 min. The amount of strain was 10%,pl = 10% and the strain rate was ε = 0.05 s−1 and the cool-ng rate during the simultaneous forming and quenching was0 ◦C/s.

.2.3. Amount of strainTo study the effects of the applied strains, on the phase trans-

ormations of the investigated steel, the samples were subjectedo strains ranges from 0.1 to 0.5. This was done by setting allther forming variables as constant values. In this respect, aust-nization was performed at 900 ◦C for 5 min. Moreover, a strainate of 0.1 s−1 was applied, the compressions were started at thenitial deformation temperature of 800 ◦C.

.2.4. Strain rateThe effect of strain rate on phase transformations was con-

idered by applying a constant strain of ε = −0.4 to the samples

les were austenitized at 900 ◦C for 5 min.

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M. Naderi et al. / Materials Science

t different strain rates of 0.07, 0.1, 0.2, 0.4, 0.7 and 1.0 s−1,espectively. It is evident that the above-mentioned deforma-ions were carried out at different periods of time. The samplesere austenitized at 900 ◦C for 5 min, and the deformation was

tarted at 800 ◦C.

.2.5. Cooling rateIn some cases, samples were cooled down at different cool-

ng rates between 25 and 100 ◦C/s to consider the influence ofooling rate on the transformations. These samples were austen-tized at 900 ◦C for 5 min, and the deformation was initiated at00 ◦C. The strain rate was ε = 0.1 s−1 and the maximum strainas ε = 0.2.

.2.6. Amount of the applied forceThe maximum amounts of applied forces were taken from

he peak values in the force versus time diagrams. The peakepresents the amount of force at the end of deformation. Ithould be noted that due to the compressive deformation, theorces have a negative sign. However, the mentioned values areonsidered to be positive for simplicity.

The above-mentioned processes as well as an examplef the force–time–temperature diagrams are illustrated inig. 2.

ig. 2. Non-isothermal forming and cooling plan; (a) the schedule of experi-ents and (b) an example of force and temperature evolution during the tests.

iD stands for the initial deformation temperature.

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ngineering A 487 (2008) 445–455 447

. Fundamentals

Phase transformations can be well studied using quantitativend qualitative microstructural investigations as well as by tak-ng the dilatation data into consideration. The first method helpso recognize present phases and the fraction of each phase in the

icrostructure. The latter not only aids in specifying the startnd finish temperatures of the phase transformations, but canlso be used to quantify the volume fraction of the transformednd produced phases.

Based on the above-mentioned reasons, the microstructuralnvestigations by using light optical and scanning electron

icroscopy techniques were carried out. The problem which haso be taken into account is that the compressive deformation iseterogeneous, i.e., the effective strain and the cooling rate alonghe horizontal centerline of the samples vary. Consequently, theistribution of the produced phases is not homogeneous. Thiseans that light optical or scanning electron microscopy inves-

igations on only a few selected points of the heterogeneouslyeformed surfaces are not proper techniques to identify the phaseractions inside the experimental specimen.

It is well known that each phase has its own hardness levels.ased on this fact, an innovative surface hardness mapping tech-ique was developed in the Department of Ferrous Metallurgy atWTH Aachen University. By this technique, the surface of theample is scanned using an indenter which exerts a 0.8 g forcen the surface of the sample and records the hardness of theoints on Vickers or Rockwell hardness scales. In the presentesearch work, surface hardness measurements were performedn the previously deformed samples. For this, the deformed sam-les were cut lengthwise. The Vickers hardness of the wholeeformed surface was measured in 0.3 mm steps. Afterwards,he surface hardness map of the sample was plotted.

The samples were mounted after cutting. Surface hardnesseasurements were started 1 mm into the sample, i.e., from

he polymeric mount. The hardness of mount material wasetected as 999, which is ignored. Due to the boundary condi-ions between the mount material and the sample, the hardnessalues of the edge of sample must not be quantified. Accord-ngly, quantitative measurements by surface hardness mappingere performed from the reliable hardness data, which is taken

rom inside the sample.This technique gives the best physical understanding of phase

eterogeneity using hardness criterion.The complete surface hardness mapping of a sample which

as deformed ε = −0.1 strain is plotted in Fig. 3a. The hardnesscale is 25HV0.8. With this very low hardness scale, the hetero-eneity of the microstructure can be well observed. Additionally,wo light optical microscopy images, representing different

icrostructures in different parts of the sample, are given inig. 3b and c. The microstructure in Fig. 3b is fully martensitichile bainite is the predominant phase in Fig. 3c.In the present study, hardness values greater than 400HV0.8

ere assumed to be martensite. Hardness between 200 and00HV0.8 was assumed to be bainite. A hardness less than00HV0.8 to be ferrite. These assumptions were calculated usinghe CCT diagram, microscopic investigations and several reports

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Fig. 3. The deformed sample with εmax = −0.1, ε = 0.05 s−1, TiD = 750 ◦C; (a)tea

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he complete surface hardness map with the scale of 25HV0.8 representing het-rogeneity, (b and c) light optical microscopy images representing martensitend/or bainite in zones ‘X’ and ‘Y’ as mentioned in (a).

16–18]. Pearlite was not considered, because it was not detected

n the final microstructure, using LOM and SEM images.

It is well known that the dilatation observed during marten-itic transformation is principally anisotropic due to Bain strainsnd the related stresses [19,20]. This makes it difficult to asses

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ngineering A 487 (2008) 445–455

he fraction of transformation using one-dimensional dilatomet-ic measurements. Due to the sharp dilatation curves regardinghe martensitic transformation in all of the experiments, the

artensite phase fractions within the deformed samples werealculated using the dilatation data. The calculation is based onsimple mathematical method. In this method, the amount of

ilatation due to the martensitic transformation in one of theamples in the CCT diagram was assumed as a reference. In thisase, the sample was cooled down with the experienced coolingate of approximately 50 ◦C/s. A fully martensitic microstruc-ure caused a dilatation of around 0.26%. All other calculations –egarding the martensite phase percent resulting from the dilata-ion data in the rest of the specimens – were completed usinghis reference value.

Henceforth, the martensite fraction given in the dilatationiagrams is the result from the calculations with the mathemat-cal method. The percentages given in the diagrams, includinghe martensite start (Ms) and finish (Mf(%)) temperatures, werealculated using the surface hardness mapping method. Mf(%) issed instead of Mf (which must be mentioned as an “approximatef”), because the indicated finish temperature is not related to

he 100% martensite formation.In the following section, the results relating and the discus-

ions regarding the effects of austenization soaking time, initialeformation temperature, amount of strain, strain rate, rate ofooling and amount of applied force are considered separately.

. Results and discussions

.1. Austenitizing soaking time

Dilatation curves and the variation in the martensite start (Ms)nd finish (Mf) temperatures are plotted in Fig. 4. It is seen thathe Ms temperature decreases of about 37 ◦C by increasing theime period of austenization. This finding agrees with Umem-to and Owen’s [6] and Ankara et al.’s [7] observations. Theyescribed that the lath shaped martensite transformation is oftenssociated with grain boundaries. Therefore, a nucleation argu-ent would suggest that the finer grain sizes result in higher Ms

emperatures, i.e., easier nucleation since grain boundary areancreases. Based on this, the longer soaking time resulted in aoarser primary austenite grain size, i.e., lower Ms temperature.

As stated before, martensite fraction governs the coexistinghases in the microstructure calculated by using two methods.he first method, which is incorporated in Fig. 4a, was estimatedathematically using dilatation values in terms of the dilatation

f a fully martensitic dilatation value. The second measurementsn Fig. 4b were calculated using the results from surface hard-ess maps. The obtained results from these two estimations areomparable. The difference of the values is between 2 and 6%artensite.Microstructural inhomogeneity of the samples can be illus-

rated by means of surface hardness measurements. For instance,

wo surface hardness maps related to the austenization at 900 Cor 5 min and 20 min are plotted in Fig. 5. The first map showshat approximately 28% of the sample has a hardness valuereater than 400HV0.8, which is assumed to be martensite, while

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M. Naderi et al. / Materials Science and Engineering A 487 (2008) 445–455 449

Fig. 4. The effects of austenization soaking time on phase transformations;(a) dilatation diagrams and corresponding martensite fractions; (b) varia-tion of martensite start (Ms) and finish (Mf(%)) temperature and the fractionost

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he rest is bainite. In the second map, 23% of the surface has aardness value greater than 400HV0.8. Meanwhile, about 1%errite, which is the white region in the corner, with hardnessalues of less than 200HV0.8 were detected.

.2. Initial deformation temperature

One of the critical issues during hot stamping is the initialeformation temperature. The hot blanks are transferred fromhe furnace into the dies and during this transfer, the tempera-ure of the hot blank decreases. As a result, deformation is noterformed at the austenization temperature, but instead at loweremperatures.

Due to this, the initial deformation temperatures wereelected below the austenization temperature in 50 ◦C intervals.ther forming variables were not changed. Austenization was

arried out at 900 ◦C for 5 min. The chosen strain rate and the

mount of strain were ε = 0.05 s−1 and ε = −0.1, respectively.

Dilatation curves related to the martensitic transformation atifferent initial deformation temperatures are plotted in Fig. 6a.he values of the dilatation due to the martensitic transformation

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ig. 5. Surface hardness maps corresponding to the deformed samples;

iD = 800 ◦C, ε = 0.1 s−1 and εmax = 0.2 after austenization at 900 ◦C for; (a)

aus = 5 min and (b) taus = 20 min.

nd the estimated martensite content using dilatation percent-ges are listed in the inset table. Except for TiD equal to 750 ◦C,ll of the other temperatures approximately exhibit the sameagnitude of dilatation. Consequently, they comprise almost

he same amount of martensite.The variations in Ms and Mf(%) are negligible, see Fig. 6b. The

stimation of the martensite fraction at the initial deformationemperatures of 700, 800 and 850 ◦C are in good agreement withhe values calculated from dilatation curves. On the contrary, theraction of estimated martensite by surface hardness measure-ents of the sample, where the compressive deformation was

nitiated at a temperature of 750 ◦C, is about 78 vol.%, whereashe calculated result using dilatation values yielded only about7 vol.% martensite.

It can be concluded that a higher austenite deformation tem-erature reduces the effect of plastic strain. This is consistentith less mechanical stabilization, and hence, a greater fractionf martensite was achieved.

Two surface hardness maps related to the initial deforma-ion temperatures of 700 and 800 ◦C are plotted in Fig. 7. Nohases with a hardness less than 200HV0.8 were detected. As aesult, ferrite is not present, which leads to the conclusion thathe microstructure consists only of bainite and martensite.

.3. Amount of strain

True stress versus logarithmic strain curves are plotted inig. 8. The cooling rate for every experiment was fixed at 50 ◦C/s.herefore, larger strain values were achieved at lower temper-tures at the final state of deformation. This sustains the fact

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450 M. Naderi et al. / Materials Science and Engineering A 487 (2008) 445–455

Fig. 6. The effect of initial deformation temperature on phase transformations;(a) dilatation diagrams and corresponding martensite fractions; (b) variationoeM

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It is well established [13,22–24] that martensitic transfor-mation involves the coordinated movement of atoms. Suchmovements cannot be sustained against strong defects, suchas grain boundaries. Less drastic defects, such as isolated

f martensite start (Ms) and finish (Mf(%)) temperature and the fraction ofach phase corresponding to surface hardness map calculations. B = bainite and

= martensite.

f finding more secondary phases, like ferrite and/or bainite,ecause these regions are passed through, during forming. Forxample, in the flow stress–strain curve based on 50% deforma-ion, the slope changes after a strain of ε = −0.4, respectively.he temperature in this region is between 600 and 550 ◦C, which

s the temperature region of bainite. As a result, bainitic trans-ormation is initiated during deformation.

Dilatation curves corresponding to different strain values asell as the reference dilatation curve related to the non-deformed

ample are plotted in Fig. 9a. The dilatation values are listedn the inset table. Without deformation, a martensitic transfor-

ation – which leads to the fully martensitic microstructure –esulted expansion of 26 vol.% of the sample. Conversely, it cane concluded that lower dilatation values result in lower amountf martensite. Based on this conclusion, the amounts of marten-ite produced during the mentioned experiments are estimated.he values are listed in the merged table.

The variation in the martensite start (Ms) and finish (Mf(%))

emperatures and the fraction of the present phases calculatedrom the surface hardness values are represented in Fig. 9b. Anptical microscopy image, corresponding to the 30% deformedample, is given in Fig. 10. The microstructure is mostly bai-

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ig. 7. Surface hardness maps corresponding to the deformed samples;˙ = 0.05 s−1 and εmax = 0.1; austenization at 900 ◦C for 5 min; (a) TiD = 800 ◦Cnd (b) TiD = 700 ◦C.

ite and the rest is martensite. By increasing the amount ofeformation, the martensite start temperature (Ms) as well ashe fraction of martensite within the microstructure is reduced.he Ms temperature decreases by about 32 ◦C, and with respect

o the amount of martensite, the finish temperature of the marten-itic transformation is increased by about 48 ◦C. These resultsualitatively agree with the earlier observations that the com-ressive stresses decrease the Ms and the dilatation values12,21].

ig. 8. True stress–logarithmic strain curves related to different amounts ofeformation; TED stands for the temperature at the end of deformation.

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Fig. 9. The effect of the amount of strain on phase transformations; (a) dilatationdiagrams and corresponding martensite fractions—values close to the curvesindicate the amount of strain; (b) variation of martensite start (Ms) and finish(Mf(%)) temperature and the fraction of each phase corresponding to surfacehardness map calculations. TiD = 800 ◦C.

Fig. 10. Light optical microscopy image corresponding to the deformed sample;ε = 0.1 s−1 and TiD = 800 ◦C; austenization at 900 ◦C for 5 min and εmax = 0.3.

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islocations, also hinder the progression of such displacive trans-ormations. During deformation, the amount of strain in theustenite phase region becomes sufficiently large. Therefore,he motion of glissile interfaces becomes impossible and causeshe blockage of martensitic transformation to halt. Consequently,

ore driving force for the martensitic transformation is requiredhich can be provided at lower temperatures. Hence, the Ms

emperature is decreased to lower temperatures. As a result, themount of martensite is also decreased. There are some reportsn which the effect of plastic deformation on the austenite to fer-ite transformation was considered [16,25–27]. In this case, ashe stored energy of deformation increases, both the start and fin-sh temperatures of the austenite to ferrite transformation wereound to increase, and the temperature range of the austenite toerrite transformation is reduced.

Another interpretation is the detraction of the martensite startemperature. It is reported that the Ms temperature is mainly aunction of carbon content, i.e., by increasing the carbon con-ent, the Ms temperature is decreased. This can be explained asconsequence of ferrite and/or bainite formation, in which car-on becomes enriched in the remaining austenite. Therefore,emaining austenite transforms into martensite at somewhatower temperatures.

.4. Strain rate

After austenitizing at 900 ◦C for 5 min and at constant initialeformation temperature of 800 ◦C and maximum deformationf 40%, some samples were non-isothermally deformed withifferent strain rates of 0.07 s−1 ≤ ε ≤ 1.0 s−1. The flow curvesnd the variation of maximum stress at 40% deformation areiven in Fig. 11. It is evident that for the above-mentioned condi-ions, the higher deformation rate finishes the Ms transformationt higher temperatures (Fig. 11b). For instance, by using a strainate of 1.0 s−1, deformation will be completed at 780 ◦C, i.e.,0 ◦C lower than the initial deformation temperature. Further-ore, this temperature decreases in an applied strain rate of

.07 s−1 is about 300 ◦C. When the deformation is completed atemperatures higher than 700 ◦C, the flow curves fit well to eachther. But, as soon as the temperature at the end of deforma-ion is decreased to 600 or 500 ◦C, the flow curves show a rapidncrease. In case of the lowest strain rate at 0.07 s−1, the slopef the curve is changed after the strain values of 30%, whichs due to bainitic transformation. This indicates that bainiticransformation initiates during deformation.

Variation in the martensite start temperature Ms and themount of martensite in the final microstructure, based on theesults of surface hardness mapping and dilatation curves, areiven in Fig. 12.

The martensite volume fraction in the final microstructure isfunction of the amount of generated dislocations or mechanicaltabilization, shifts in the bainite and ferrite zones to the left ando the temperature at the end of deformation. Taking this fact

nto consideration and by reviewing the results, two differenthenomena were identified.

At temperatures below 700 ◦C, higher strain rates resulted inore martensite in the final microstructure. This is the effect

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Fig. 12. The effect of strain rate on phase transformations; (a) dilatation dia-grams and corresponding martensite fractions—values close to the curvesindicate strain rates; (b) variation of martensite start (Ms) and finish (Mf(%))tm

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ig. 11. The effect of strain rate on (a) flow stress and (b) maximum stresst 40% deformation. Austenization at 900 ◦C for 5 min. TiD = 800 ◦C; coolingate = 50 ◦C/s.

f avoiding or hindering bainitic transformation. Thus, the Msemperature increases of about 20 ◦C.

On the contrary, at temperatures above 700 ◦C, higher ratesf deformation yield less martensite. In this temperature range,here transformation does not occur and the microstructure is

ully austenitic, higher strain rates generate more dislocations.onsequently, due austenite becoming more mechanically sta-ile at higher strain rates, the martensite start temperature Ms asell as martensitic transformation is reduced. The Ms tempera-

ure decreases by about 23 ◦C.In conclusion, the effect of mechanical stabilization is domi-

ant at temperatures above 700 ◦C, while at lower temperatures,hase transformation is dominant.

Two surface hardness maps corresponding to strain ratesf 0.2 and 1.0 s−1 are plotted in Fig. 13. For ε = 1.0 s−1, seeig. 13b, deformation was abandoned at 780 ◦C due to a cool-

ng rate of 50 ◦C/s. This temperature is high enough to cross theerrite nose. As a consequence approximately 1 vol.% primaryerrite is present in the final microstructure.

.5. Cooling rate

Four samples were selected to study the effect of coolingates. The samples were deformed for 2 s with the strain rate

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emperature and the fraction of each phase corresponding to surface hardnessap calculations. TiD = 800 ◦C; T = 50 ◦C/s.

f 0.1 s−1 to obtain 20% of compressive strain. Accordingly,ifferent cooling rates imply different temperatures at the end ofeformation as presented in Fig. 14. In this regard, deformationst different cooling rates of 25, 50, 75 and 100 ◦C/s are completedt temperatures of 750, 700, 650 and 600 ◦C, respectively. As aesult, higher cooling rates during non-isothermal deformationsn the austenite region yield higher strength levels. Additionally,he same phenomena occur that were mentioned in Section 4.4.s the temperature at the end of deformation decreases below00 ◦C, the flow curves increase steeply to higher strength levels.

The martensitic start temperature (Ms) decreases by about0 ◦C by increasing the cooling rates from 25 to 100 ◦C/s ashown in Fig. 15. Due to lower temperatures at the end ofeformation at higher cooling rates, the mechanical stabilityf austenite increases and as a result, a higher driving forceor martensitic transformation is needed. Higher driving forcesre provided at lower Ms temperatures, i.e., Ms decreases byncreasing the cooling rate during non-isothermal tests.

It is evident that higher cooling rates during non-isothermalxperiments generate more dislocations. These dislocationsill retard or hinder martensitic transformation and conversely,

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Fig. 15. The effect of cooling rate on phase transformations; (a) dilatationdiagrams and corresponding martensite fractions—values close to the curvesindicate cooling rates; (b) variation of martensite start (Ms) and finish (Mf(%))tm

ig. 13. Surface hardness maps corresponding to the deformed samples; amountf strain = 0.2 and TiD = 800 ◦C; austenization at 900 ◦C for 5 min; (a) ε = 0.2 s−1

nd (b) ε = 1.0 s−1.

ncourage thermally activated transformations. Hence, higherooling rates result in a lower martensite content in the finalicrostructure (Fig. 15b).Except for the cooling rate of 25 ◦C/s, the estimated marten-

ite content in the final microstructure based on dilatation curvesnd surface hardness measurements are in good quantitativegreement. The deviations of the results obtained by these twoethods vary between 1 and 8 vol.% martensite.It is well known that in the absence of deformation, i.e., static

ontinuous cooling transformation diagrams, higher coolingates facilitate martensitic transformation. However, in dynamicontinuous cooling transformation (DCCT) diagrams, higherooling rates do not essentially result in a higher amount of

ig. 14. True stress–logarithmic strain curves exhibit the effect of differentooling rates during non-isothermal compression experiments. CR, cooling rate.

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emperature and the fraction of each phase corresponding to surface hardnessap calculations. TiD = 800 ◦C, ε = 0.1 s−1 and εmax = 0.2.

artensite. This is confirmed for the given experimental con-itions, since higher cooling rates yielded a lower amount ofartensite. This is due to the continuation of deformation into

ower temperatures, which enhances the possibility of bainiticransformation. Accordingly, a higher amount of bainite resultedn lower amounts of martensite.

Therefore, it can be concluded that during non-isothermaleformations the optimum cooling rates must be employed tovoid accelerating the ferritic/bainitic transformations.

.6. Amount of applied force

The most important parameter in the stamping process, whichan be controlled and monitored, is the applied force. This isetermined by the load cell on the punch. Therefore, it can bessumed as an independent parameter.

Attempts were focused on finding a relationship between

he applied force and the martensitic transformation during theimultaneous forming and quenching process. The achievementsre represented in Fig. 16.

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454 M. Naderi et al. / Materials Science and E

Fig. 16. The effect of amount of applied force on phase transformations; (a)vov

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ariation of martensite start (Ms) and finish (Mf(%)) temperature and the fractionf martensite phase corresponding to surface hardness map calculations; (b)ariation of dilatation curves by force.

It was observed that regardless of the other process parame-ers, like strain rate, amount of deformation, and TiD, the appliedorce during compression and cooling in the austenite region hadignificant effects on the martensitic transformation.

The Ms temperature decreases by about 50 ◦C throughout theifferent applied forces between 0 and 14 kN, while the Mf(%)emperature increases about 50 ◦C. By applying higher forceevels, the dislocation density of the austenite matrix increases.onsequently, more driving force is needed to obtain martensitic

ransformation. Hence, the Ms temperature is reduced. However,t should be mentioned that the applied force in the elastic partstress-affected) accelerates martensitic and bainitic transforma-ion, because it is added to the driving force for transformation23].

The martensite contents, and in the same manner, the dilata-ion – as is seen in Fig. 16 – are decreased by increasing themount of applied forces. The microstructure, in absence of thepplied force, is fully martensitic, whereas in the case of 14 kNpplied force, the microstructure is almost fully bainitic. There-ore, lower applied forces during hot stamping process lead to a

ore successful martensitic transformation.It was also found that there is a minimum force, which does

ot alter the dilatation values and martensite content. This min-mum limit varies between 6 and 8 kN. The same conclusion

4

ngineering A 487 (2008) 445–455

s reported for 27MnCrB5 steel, which was deformed non-sothermally [14].

It was observed that the higher applied forces lead to moreecondary phase formation. Thus, increasing the applied forceevel in the austenite region, the nose of bainite and ferrite inhe CCT diagram not only shifts to the left, but also to loweremperatures (due to the decrease in the Ms temperature).

It can be interpreted that by applying higher force levels onamples with a constant shape or volume, more dislocations wille formed. Accordingly, these will accelerate thermally acti-ated phase transformations. Therefore, the nose of ferrite andainite phases shift more and more to the left. Moreover, thepplied force in the austenite region altered bainite nose morehan ferrite zone.

The above-discussed results give the best key points to con-rol the martensitic transformation during hot stamping process,ecause the applied force is very suitable parameter, which cane monitored and controlled during the process.

. Summary and conclusion

The effects of the process parameters on the martensitic trans-ormation during a specific non-isothermal deformation whichas planed very similar to the hot stamping process schedulesere studied. A large number of experiments in different defor-ation conditions have been performed to reveal the effects of

he relevant process parameters on the martensitic transforma-ion in the 22MnB5 steel grade.

It has also been demonstrated that the innovative surface hard-ess mapping technique as well as dilatometry experiments areery reliable methods to estimate the martensite content in aeterogeneous microstructure. Surface hardness mapping dataan also be used for the quantitative and qualitative estimationf other coexisting phases.

Considering the results achieved using these two methods, its concluded that:

. The discrepancy between phase fractions derived from thehardness and the dilatometric data could be explained by thefact that one-dimensional dilatometric measurements wereused.

. The longer austenization soaking time resulted in a lowermartensite start temperature, while the amount of martensitein the final microstructure remained approximately constant.By increasing the soaking time from 5 to 20 min, the Mstemperature decreased about 37 ◦C.

. The change of the martensite start temperature at differentinitial deformation temperatures was negligible. A higherinitial deformation temperature yielded a higher volumefraction of martensite. The initial deformation temperatureof 850 ◦C resulted in 94% martensite, while by decreasingthe temperature to 700 ◦C, 77% martensite was produced.It means by about 22% reduction which is considerable

value.

. Increasing the stored energy of deformation by imposinghigher amount of deformation of austenite prior to martensitetransformation decreases the martensitic start temperature,

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Institue of Metals, London, 1996, p. 489.

M. Naderi et al. / Materials Science

while the temperature regime of the transformation con-tracted, and the final fraction of martensite decreased.

. At temperatures below 700 ◦C, higher strain rates resultedin an increase of martensite in the final microstructure. Thisoccurs by diminishing bainitic transformation. Because ofthis, the Ms temperature increases by about 20 ◦C. On thecontrary, at temperatures above 700 ◦C, higher strain rates ofdeformation yield lower martensite content. In that temper-ature range in which transformation does not occur and themicrostructure is fully austenitic, higher strain rates gener-ate more dislocations. Consequently, due to more mechanicalstabilization of austenite at higher strain rates, the martensitestart temperature Ms as well as the possibility of a martensitictransformation is reduced.

. On the contrary to the static continuous cooling transfor-mation diagrams in which higher cooling rates enhancethe tendency of martensitic transformation, higher coolingrates during simultaneous forming and quenching resulted inlower martensite content. The martensite start temperaturedecreases with increasing the cooling rates.

. There is a minimum force limit which does not alter thedilatation values and the martensite content. On the otherhands, it was demonstrated that the higher applied force levelsreduce the possibility of martensitic transformation.

cknowledgement

The corresponding author has greatly profited, generouslyavished in the course of correspondence, from Prof. Bhadeshia,ambridge University.

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