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Journal of Materials Processing Technology 211 (2011) 1694– 1700

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

j o ur nal ho me p age : www.elsev ier .com/ locate / jmatprotec

ribological characteristics of high strength steel sheets under hot stampingonditions

. Ghiotti a,∗, S. Bruschib, F. Borsettoa

DIMEG, University of Padova, Via Venezia 1, 35131 Padova, ItalyDIMS, University of Trento, Via Mesiano 77, 38050 Trento, Italy

r t i c l e i n f o

rticle history:eceived 23 November 2010eceived in revised form 8 May 2011ccepted 15 May 2011vailable online 23 May 2011

a b s t r a c t

Nowadays, the hot stamping of high strength steel sheets is currently utilized to produce automotivecomponents characterized by a high strength-to-weight ratio and an increased resistance to impact. Inorder to avoid scaling and decarburization during the heating stage, the metal sheets are coated with aspecially developed Al–Si coating that have proved a relevant influence also on the tribological behaviourof the metal sheets during the forming stages. The paper presents a fundamental study about the sheet

eywords:ot stampingigh strength steeloatingriction

coating characteristics in terms of morphology, surface roughness and tribological behaviour as a functionof the process parameters typical of industrial hot stamping processes. The presented results show thatthe blank heating promotes the iron diffusion into the Al–Si coating giving form to an Al–Fe–Si ternaryalloy whose characteristics depend on the thermal cycle parameters; moreover, it is proved that thetribological characteristics of the metal blank surface in terms of friction coefficient depend on the blanktemperature and contact pressure.

© 2011 Elsevier B.V. All rights reserved.

. Introduction

In the last decade, the automotive industry has promoted theevelopment of innovative temperature-supported sheet form-

ng processes for producing components characterized by a hightrength-to-weight ratio, being pushed by the urgent demandf fuel consumption reduction and passengers safety increaseNeugebauer et al., 2006). Among these processes, hot stampingKarbasian and Tekkaya, 2010) represents a viable mean to man-facture structural parts of the car body-in-white made of hightrength steels, such as the quenchable boron steel 22MnB5, com-only available with the commercial name of Usibor® 1500P. The

rocess route of the hot stamping contemplates the heating of thelank inside a furnace, a holding time above the austenitizationemperature and its rapid transfer to the press, where the form-ng and the quenching take place simultaneously (Lorenz and Roll,005). The possibility to form the blank at elevated temperaturesuarantees lower stamping loads, higher formability and lowerpring-back phenomena compared to the same part obtainable

hrough traditional room temperature processes (Mori et al., 2005);t the same time, the simultaneous die quenching permits to obtain

fully martensitic part, thus characterized by extremely high level

∗ Corresponding author. Tel.: +39 0498276822.E-mail address: andrea.ghiotti@unipd.it (A. Ghiotti).

924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2011.05.009

of mechanical strength and crashworthiness properties. In orderto avoid surface oxidation and decarburization during the heat-ing stage, the metal blanks are covered with an Al–Si layer that hasproved to be resistant to temperature as well as to contribute to thefriction and die wear reduction (Borsetto et al., 2009). When heatedat elevated temperature, diffusive phenomena at the blank–layerinterface are activated and determine the formation of an Al–Fe–Siternary alloy, which presents a melting point considerably higherthan the original Al–Si layer (approximately 600 ◦C) (Fontalvo andMitterer, 2005).

Even if the diffusion mechanisms of both the iron and thealuminium in the coating and the substrate have been alreadyinvestigated and identified (Borsetto et al., 2008), the knowledgeof the influence that the thermal process parameters can have onthe Al–Si coating and, consequently, on the tribological behaviourat the blank–die interface is very limited. Other literature studiesreport preliminary results about the friction coefficient betweenAl–Si coated 22MnB blanks (Yanagida et al., 2010) and different toolsteels (Stoehr et al., 2008); as a function of the heating temperature,but either they do not replicate the hot stamping industrial condi-tions (Hardell and Prakash, 2008) or they do not propose a properexplanation of the investigated phenomena (Hardell et al., 2008).

On the basis of these limits, the paper presents fundamentalinvestigations with a twofold objective: (i) to study in depth theAl–Si coating evolution in terms of chemical composition, mor-phology and topography as a function of the hot stamping thermal

A. Ghiotti et al. / Journal of Materials Processing Technology 211 (2011) 1694– 1700 1695

Table 1Chemical composition of 22MnB5 (in wt%).

ctp

tiefhrcfc

2

2c

ptsa

ts

cytofttoEa

3c

cit

C Mn Si Cr Ti B

0.25 1.40 0.35 0.30 0.05 0.005

ycle parameters; (ii) to identify the friction coefficient betweenhe 22MnB5 blanks and the dies as a function of the most relevantrocess parameters.

The first part of the paper is devoted to the investigation ofhe influence that the thermal parameters exert on the Al–Si coat-ng during hot stamping; scanning electron microscopy (SEM),nergy-dispersive X-ray spectroscopy (EDX) and white light inter-erometry (WLI) analyses are utilized. In the second part, theigh-temperature friction experiments are described and theesults in terms of friction coefficient dependency on the pro-ess parameters are outlined. Explanations of the characteristicrictional behaviour as a function of the testing temperature andontact pressure are proposed, supported by SEM observations.

. Materials

The material object of the investigation is the boron steel2MnB5 provided in 1.5 mm thick sheets, covered with an Al–Sioating. The chemical composition of the steel is reported in Table 1.

In the as-received state, the material presents a soft ferritic-earlitic microstructure with low strength but high ductility; onhe other hand, after forming, the structure becomes fully marten-itic, with ultimate tensile strength up to 1500 MPa, but with stillcceptable ductility.

The chemical composition of the blank coating was identifiedhrough the EDX by the employing the area as a method of mea-urement, with a measuring accuracy of 8%.

Before the heating and forming stages, the Al–Si coating layer,ontaining approximately 10–13% of silicon according to EDX anal-sis, appears as shown in the optical image of Fig. 1. In this figure,hree different zones can be outlined: the coating, where aggregatesf silicon are embedded in the aluminium matrix (zone I), the inter-ace area between the coating and the steel substrate (zone II), andhe steel substrate (zone III). The interface zone is a ternary sys-em containing iron, aluminium and silicon in weight percentagesf 39%, 53% and 8%, respectively according to SEM–EDX analysis.ven if the coating thickness varies significantly along the blank,n average value of 25 ± 10 �m can be considered.

. Chemical and morphological evolution of the Al–Sioating

In this section the Al–Si coating evolution in terms of chemicalomposition and morphological characteristics, referring both tots thickness and its surface, will be evaluated as a function of thehermal parameters that can be varied during a hot stamping pro-

Fig. 2. Experimental set-up for thermal experime

Fig. 1. Optical image of the Al–Si coating thickness at room temperature.

cess. In the following paragraphs, the experimental apparatus andthe results of experiments are presented and discussed.

3.1. Thermal experiments

In order to evaluate the influence of the thermal parameters onthe chemical and morphological evolution of the Al–Si coating, adedicated experimental set-up (Fig. 2 left) was designed and real-ized where the metal sample is subjected to the same thermal cycletypical of the hot stamping process. The experimental set-up isarranged on a MTSTM machine equipped in the tensile mode, wherea tensile-type sheet sample is heated by a frontal inductor, andmaintained at the target temperature for a certain amount of time(holding time) in order to get a complete microstructure homog-enization; the sample is finally quenched to room temperature bymeans of compressed air nozzles. The thermal profile applied in theexperiments is shown in the right part of Fig. 2. In order to assure thecontrol of the heating and the cooling down steps, a k-type thermo-couple is spot-welded on the specimen in the middle of the heatedzone. The cooling system is capable to reproduce different coolingrates by means of compressed air or mixed water-compressed airmedium in the range from 5 ◦C/s to 300 ◦C/s.

The heating temperature, holding time and cooling rate are thethermal parameters that are varied according to the experimentalplans reported in Tables 2–4.

3.2. Influence of the thermal parameters on the coating behaviour

3.2.1. Heating temperatureThe influence of the blank heating temperature on the coat-

ing characteristics was evaluated through backscattered electrons

nts (left) and applied thermal cycle (right).

1696 A. Ghiotti et al. / Journal of Materials Process

Table 2Experimental plan to investigate the influence of the heating temperature.

Heatingtemperature(◦C)

Heating rate(◦C/s)

Holding time(s)

Cooling rate(◦C/s)

600 ± 10 7.3 ± 0.1 180 ± 0.1 300 ± 5900 ± 10 7.3 ± 0.1 180 ± 0.1 300 ± 5

Table 3Experimental plan to investigate the influence of the soaking time.

Heatingtemperature(◦C)

Heating rate(◦C/s)

Holding time(s)

Cooling rate(◦C/s)

900 ± 10 7.3 ± 0.1 10 ± 0.1 300 ± 5900 ± 10 7.3 ± 0.1 180 ± 0.1 300 ± 5

Table 4Experimental plan to investigate the influence of the cooling rate.

Heatingtemperature(◦C)

Heating rate(◦C/s)

Holding time(s)

Cooling rate(◦C/s)

(tbad

fWac

difference in the coating sub-layer evolution can be noticed: in the

900 ± 10 7.3 ± 0.1 180 ± 0.1 50 ± 5900 ± 10 7.3 ± 0.1 180 ± 0.1 300 ± 5

BSE) SEM cross sections analyses, carried out on metal samplesested according to the experimental plan presented in Table 2. Thelank chemical composition was also determined through SEM EDXnalysis, with measurements carried out in the coating thicknessirection.

The results of the above mentioned analyses are shown in Fig. 3or samples processed at 20 ◦C, 600 ◦C and 900 ◦C, respectively.

hen analyzed at room temperature, the Al matrix and the Siggregates cannot be distinguished since their atomic weights areomparable. Once the blank reaches the heating temperature of

Fig. 3. Influence of the heating temperature on the c

ing Technology 211 (2011) 1694– 1700

600 ◦C, the iron starts to diffuse from the interface area to the coat-ing surface (zones B and C in Fig. 3) giving origin to an Al–Fe–Siternary alloy. As a consequence of the iron diffusion, the furtherfrom the interface zone the lower the iron amount (Fe wt% in zoneB = 46%, Fe wt% in zone C = 70%). When the blank is heated up toits austenitization temperature, the coating layer develops a sub-layer structure consisting of approximately 5 zones (A–E). ZonesA–D are characterized by an alternating variation of the aluminiumand iron percentages, while zone E, representing the interface withthe substrate, by a very low amount of aluminium.

The chemical evolution of the coating with the heating tem-perature does correspond to an evolution of its topography aswell, as depicted in the WLI images reported in Fig. 4. At roomtemperature, the Al–Si coating is characterized by large plateauareas corresponding to the Al matrix, with small cavities represent-ing the Si precipitates, and an average surface roughness Sa equalto 1.54 ± 0.1 �m. With regards to the surface topography, whenthe diffusion step starts taking place at 600 ◦C, the large plateauareas change into dense small peaks, which determine a higherroughness (Sa = 3.42 ± 0.01 �m). By a further increase of the heatingtemperature up to austenitization, the surface continues evolving,with the growth of the surface structure, giving raise to an overalllower roughness (Sa = 2.37 ± 0.01 �m). The new surface topogra-phy derives from the iron diffusion into the coating, with the newAl–Fe–Si ternary alloy that takes origin within the diffusion step.

3.2.2. Holding timeIn Fig. 5 the influence that the holding time at elevated temper-

ature has on the coating morphology and topography is evidencedin the SEM cross sections and in the WLI images of metal samplestested according to the experimental plan of Table 3. A substantial

case of the shorter holding time, the five sub-layer structure has notenough time to develop and therefore the coating appears morehomogeneous, if compared to the sample subjected to a higher

oating morphology and chemical composition.

A. Ghiotti et al. / Journal of Materials Processing Technology 211 (2011) 1694– 1700 1697

Fig. 4. Influence of the heating temperature on the coating topography.

rature

hatlt(

l(ticbv

Fig. 5. Influence of the holding time in tempe

olding time. These observations are confirmed by the SEM–EDXnalyses (see Fig. 6) on the sample processed with a shorter holdingime, due to the presence of surface zones (such as zone A in theeft part of Fig. 6) not completely reached by the iron diffusion; onhe other hand a longer holding time shows a five layer structuresee the right part of Fig. 6).

From the WLI analyses, the sample subjected to aower holding time presents a lower average roughnessSa = 1.79 ± 0.01 �m) compared to the sample kept longer inemperature (Sa = 2.34 ± 0.01 �m). Moreover, the former sample

s characterized by a Svk value that is approximately doubleompared to the one of the other sample, since it is characterizedy much larger valley areas, while the roughness parameter Spkalue is almost the same. These topographical characteristics

Fig. 6. SEM–EDX analyses for samples subjected to 10 s (

on the coating morphology and topography.

can be attributed again to the iron diffusion step, not completelydeveloped in the case of the sample subjected to a lower holdingtime in temperature.

3.2.3. Cooling rateThe BSE SEM cross sections shown in Fig. 7 demonstrate that the

cooling rate imposed to the sample after the thermal cycle does notaffect the sub-layer structure developing when the blank is heatedat 900 ◦C. The five zones of the sub-layer structure, indicated withthe letters A–E, and representing different concentrations of alu-

minium and iron, maintain the same distribution and the samethickness in the analyzed sections. Moreover, the chemical com-position does not significantly change as shown by the SEM–EDXanalysis results reported in Fig. 8.

left) and 180 s (right) holding time in temperature.

1698 A. Ghiotti et al. / Journal of Materials Processing Technology 211 (2011) 1694– 1700

Fig. 7. Influence of the cooling rate

Ff

4

ivts

4

bttos

ig. 8. SEM–EDX analysis of the sub-layers structure chemical composition as aunction of the sample cooling rate.

. Tribological behaviour

In this section the tribological behaviour of the Al–Si coatings evaluated as a function of the thermal parameters that can bearied during a hot stamping process. In the following paragraphs,he experimental apparatus and the results of experiments are pre-ented and discussed.

.1. High-temperature pin-on-disk experiments

The evaluation of the tribological characteristics was carried outy performing pin-on-disk experiments, on a universal mechanical

ester (UMT), specially equipped to work at elevated tempera-ures. Fig. 9(a) shows the overall experimental setup and detailsf the pin and heating chamber. The pin shown in Fig. 9(b) (repre-enting the tool) is connected to a both vertical and lateral linear

Fig. 9. UMT configured for the high-tem

on the coating morphology.

motion system, while the disc (representing the blank) is fixed toa lower rotary drive. Ultra-accurate strain-gauge sensors performsimultaneous measurements of load and torque in the vertical andhorizontal directions. A normal-load sensor provides feedback tothe vertical motion controller, actively adjusting the sample posi-tion to ensure the application of a constant load during testing. Asuspension is mounted to the force sensor permitting the upperspecimen to follow height variations of the disk during its rotation.To perform high-temperature pin-on-disc experiments, the UMTis equipped with a chamber (see Fig. 9(c)), specifically designed toassure the tribo-system heating up to 1000 ◦C. To precisely con-trol the chamber temperature, a thermocouple located inside thechamber gives the control feedback to a computerized temperaturecontroller.

In order to reproduce the tribo-system typical of hot stamping,the material used for the pin is the hardened hot working tool steelAISI h11, while the counter material is a disc made of the 22MnB5with the Al–Si coating. The metal disc is heated up to 900 ◦C in orderto have its complete austenitization inside the heating chamber ata heating rate of about 0.43 ◦C/s; then, it is cooled down to thetarget temperature with a constant cooling rate equal to 0.33 ◦C/s.Even if the applied cooling rate is lower than the industrial one, it isshown in §3.2.3 that the cooling rate does not influence the 5 lay-ers structure coating formation. In order to simulate the industrialhot stamping conditions where the dies are kept at room tempera-ture thanks to specifically designed water channels, the pin is keptoutside the chamber during the sample heating; all the tests were

carried out in normal atmosphere conditions. Once the disk hasreached the target temperature, the pin is moved inside the cham-ber and brought into contact with the specimen. The whole test iscarried out in air as testing atmosphere in order to closely repli-

perature pin-on-disc experiments.

A. Ghiotti et al. / Journal of Materials Processing Technology 211 (2011) 1694– 1700 1699

Table 5DOE experimental plan to investigate friction conditions.

Factors Low level High level

Normal pressure (MPa) 5 25Temperature (◦C) 500 800Sliding velocity (mm/s) 1 10Surface condition Turned Polished

Fc

ctvDOp(iffa

4

ssatbvavioaA

Fig. 11. Influence of the contact pressure on the friction coefficient at the lowertemperature (500 ◦C).

ig. 10. Influence of the testing temperature on the friction coefficient at the lowerontact pressure (5 MPa).

ate the industrial conditions. No lubricant was adopted accordingo the industrial practise. To evaluate the effects of the most rele-ant process parameters and investigate their reciprocal influence,OE techniques were applied for designing the experimental plan.n the basis of the main parameters affecting hot stamping, fourrocess parameters (factors) were considered: (i) normal pressureii) blank temperature (iii) sliding velocity, and (iv) tool surface fin-shing. A full factorial plan was developed with two levels for eachactor and repeatability equal to two. Low level and high level ofactors, shown in Table 5, were initially chosen on the basis of thectual process conditions (Turetta, 2008).

.2. Friction coefficient evolution

The results from high-temperature pin-on-disk experimentshow that the friction coefficient is not affected by both the slidingpeed and surface quality of the pin. On the contrary, the temper-ture, contact pressure and, in particular, their interaction affecthe tribological behaviour of the Al–Si coated steel sheets. Thisehaviour is represented in Figs. 10 and 11, showing that, for lowalues of the contact pressure, the friction coefficient decreasess the temperature increases (Fig. 10); while for low temperaturealues, the friction coefficient decreases as the contact pressure

ncreases (Fig. 11). These results, conflicting with the analogousnes in the field of bulk forming processes at elevated temper-tures, could be explained by a change in the behaviour of thel–Si–Fe ternary alloy developing at high temperature on the blank

Fig. 13. Chemical composition of the blank surfac

Fig. 12. Top down BSE SEM image of a sample processed through high-temperaturepin-on-disk (T = 700 ◦C, p = 25 MPa).

surface at increasing values of the contact pressure and testing tem-perature. No adhesion phenomena at the pin–disc interface wereobserved, which could explain the increase in the friction coeffi-cient.

In order to explain this behaviour, first SEM EDX analysis wascarried out on the samples tested at different temperatures andcontact pressure, to verify if any change in the coating chemicalcomposition happened as a consequence of the different test-ing conditions. Both the zones in the wear track and in the nontested areas were analyzed (Fig. 12). Independently from the test-ing parameters, no chemical difference can be appreciated between

the two areas (Fig. 13), implying that the testing parameters do notaffect the coating chemical composition.

e coating in and out of the wear track zone.

1700 A. Ghiotti et al. / Journal of Materials Processing Technology 211 (2011) 1694– 1700

ples p

nopstcdtaatflcpt

sqafi

5

iofotaif

i

ties of quenchable high strength steels in hot stamping operations, PhD Thesis,University of Padova.

Fig. 14. Top down BSE SEM images of the wear track zones of sam

Both macro- and micro-mechanical approaches to friction phe-omena understanding were then followed to give an explanationf the coating tribological response to temperature and contactressure. On a macro level, the case of a soft coating on a hardubstrate was considered as representative of the investigatedribo-system: following this approach, friction depends on theoating shear strength and contact area, which is related to theeformation properties of the substrate. On a micro scale, fric-ion characteristics are related to the deformation of the surfacesperities of the softer material. In the case of low testing temper-ture (500 ◦C), as a consequence of the contact pressure increase,he asperities deformation and the consequent surface topographyattening become more significant (Fig. 14). Therefore, for higherontact pressure, the pin slides over a less rough surface if com-ared to the case of low normal load, thus causing a reduction ofhe friction coefficient.

On the other hand, the higher the sheet metal temperature theofter the coating, thus the lower its shear strength. As a conse-uence, when the pin slides over the coated surface, the asperitiesre more easily deformed provoking a decrease of the friction coef-cient.

. Conclusions

In the last decade, due to the increased importance of hot stamp-ng, special coatings have been developed in order to avoid blankxidation and decarburization, such as the Al–Si layer that is usedor the boron steel 22MnB5, commercially available with the namef Usibor® 1500P. The paper presents the Al-Si coating characteris-ics in terms of chemical composition, morphology and topographys a function of the hot stamping thermal cycle parameters and itsnfluence on the tribological behaviour during the process. It wasound that:

i. The heating temperature governs the diffusion phenomena ofthe iron to the coating surface giving origin to an Al–Fe–Siternary alloy. Furthermore, the surface topography and rough-ness vary with the heating temperature.

rocessed at T = 500 ◦C with p = 5 MPa (left) and p = 25 MPa (right).

ii. The holding time must be adequate to allow the iron diffusioninto the Al matrix and the formation of the Al–Fe–Si alloy sys-tem. Longer holding times allow more homogeneous surfacecharacteristics in terms of chemical elements and topography.

ii. The cooling rate does not affect the surface characteristics, inthe range variations typical of the hot stamping.

iv. The blank temperature and contact pressure are the processparameters that have the larger influence on the friction coeffi-cients as it was proved by experimental tests that replicate thetypical condition of industrial processes.

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