Artigo 1_17.03

Journal of Materials Processing Technology 210 (2010) 21032118

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

journa l homepage: www.e lsev ier .com/ l

A revie

H. KarbaInstitute of For aroper

a r t i c l

Article history:Received 29 OReceived in reAccepted 19 Ju

Keywords:Hot stampingHigh strength22MnB5

comknowble anaramriptioysicalthe tvestigproceepennmehe retentia

1. Introduction

Due to thand crashwmobile struapparent (patented (Gthat used t1984 Saab Aadopted a(Berglund,3 million pathe year 20and the numately107hot stampenents, like A(Fig. 1).

The hotvariants: thdirect hot stferred to thclosed tool (

CorresponE-mail add

ized by the use of a nearly complete cold pre-formed part which issubjectedonly to aquenching and calibrationoperation in thepress

0924-0136/$ doi:10.1016/j.e demand for reduced vehicleweight, improved safety,orthiness qualities, the need to manufacture auto-ctural components from ultra high strength steels iskerstrm, 2006). Hot stamping was developed andB1490535, 1977) by a Swedish company (Plannja),

he process for saw blades and lawn mower blades. Inutomobile AB was the rst vehicle manufacturer who

hardened boron steel component for the Saab 90002008). The number of produced parts increased fromrts/year in 1987 to 8 million parts/year in 1997. Since00 more hot stamped parts have been used in the carsmber of produced parts/year has gone up to approxi-millionparts/year in2007 (Aspacher, 2008). Theappliedd parts in the automotive industry are chassis compo--pillar, B-pillar, bumper, roof rail, rocker rail and tunnel

stamping process currently exists in two different maine direct and the indirect hot stamping method. In theampingprocess, a blank is heatedup in a furnace, trans-e press and subsequently formed and quenched in theFig. 2a). The indirect hot stamping process is character-

ding author. Tel.: +49 231 7557430; fax: +49 231 7552489.ress: [email protected] (H. Karbasian).

after austenitization (Fig. 2b) (Merklein et al., 2008). Full marten-site transformation in the material causes an increase of the tensilestrength of up to 1500MPa.

This paper includes the review over the research on hot stamp-ing. This starts with the description of the workpiece material usedin hot stamping. Then, the special characteristics of the processsteps in theprocess chain of hot stamping are described. Finally, thesubsequent processing of the hot stamped parts and the manufac-ture of the parts with tailored properties are presented. The paperincludes both the experimental and numerical investigations in theeld of hot stamping.

2. Material and coating

The investigations on ultra high strength steels by Naderi haveshown that boron alloys of 22MnB5, 27MnCrB5, and 37MnB4steel grades (Table 1) are the only steel grades which produce afully martensitic microstructure after hot stamping when a water-cooled tool is used (Naderi, 2007). Here, 22MnB5 steel grade is themost commonly used steel grade in hot stamping processes. Ini-tially, the material exhibits a ferriticpearlitic microstructure witha tensile strength of about 600MPa. After the hot stamping pro-cess, the component nally has a martensitic microstructure witha total strength of about 1500MPa (Fig. 3a). In order to achievesuch a microstructure and hardness transformation, the blank has

see front matter 2010 Published by Elsevier B.V.jmatprotec.2010.07.019w on hot stamping

sian , A.E. Tekkayaming Technology and Lightweight Construction, Dortmund University of Technology, B

e i n f o

ctober 2009vised form 15 July 2010ly 2010

steel

a b s t r a c t

The production of high strength steelpress hardening) requires a profoundnal part properties become predictaand their interaction. In addition to ptural parameters complicate the descessential for the explanation of all ph

In this article, the state of the art inof hot stamping are reviewed. The inhot stamping and subsequent furtherseveral gaps in the elds of forming-dthewhole process, correlation betweecation of some advanced processes. Tbackgrounds and shows the great posheet metal forming.ocate / jmatprotec

Str. 301, D-44227 Dortmund, Germany

ponents with desired properties by hot stamping (also calledledge and control of the forming procedures. In this way, thed adjustable on the basis of the different process parameterseters of conventional cold forming, thermal and microstruc-n of mechanical phenomena during hot stamping, which arephenomena of this forming method.

hermal, mechanical, microstructural, and technological eldsations of all process sequences, from heating of the blank tosses, are described. The survey of existing works has revealeddent phase transformation, continuous ow behavior duringchanical and geometrical part properties, and industrial appli-view aims at providing an insight into the forming procedurel for further investigations and innovation in the eld of hot

2010 Published by Elsevier B.V.

2104 H. Karbasian, A.E. Tekkaya / Journal of Materials Processing Technology 210 (2010) 21032118

Table 1Chemical components and mechanical properties of boron steels (Naderi, 2007).

Steel Al B C Cr Mn N Ni Si Ti

20MnB5 0.04 0.001 0.16 0.23 1.05 0.01 0.40 0.03422MnB5 0.03 0.002 0.23 0.16 1.18 0.005 0.12 0.22 0.0408MnCrB3 0.05 0.002 0.07 0.37 0.75 0.006 0.01 0.21 0.04827MnCrB5 0.03 0.002 0.25 0.34 1.24 0.004 0.01 0.21 0.04237MnB4 0.03 0.001 0.33 0.19 0.81 0.006 0.02 0.31 0.046

Steel Martensite start temperature in C Critical cooling rate in K/s Yield stress in MPa Tensile strength in MPa

As delivered Hot stamped As delivered Hot stamped

20MnB5 450 30 505 967 637 135422MnB5 410 27 457 1010 608 14788MnCrB3 * * 447 751 520 88227MnCrB5 400 20 478 1097 638 161137MnB4 350 14 580 1378 810 2040

* There is no possibility to have fully martensitic microstructure.

Fig. 1.

to be austewards, thewater-cooleblank and tthe cooling27K/s, at asitic transfofor the resual., 2008). Tsite start po(Somani et

The mecdependence

afterquenchingcanbecontrolledbyaproper adjustmentof the car-bon content. The alloying elements, such as Mn and Cr, are knownto have only a small inuence on the strength after quenching.However, since these elements have an inuence on hardenability,they are essential for shifting of existence elds. Thus, the desired

transe coohich

downartener aiateloxi

-coalSiect hhe inor ofHot stamped parts in a typical middle class car (N. N., 2009).

nitized in a furnace for at least 5min at 950 C. After-blank is formed and quenched simultaneously by the

phasefeasiblment wslowsto a m

Undimmedsurfaceare preis an Athe dirgated tbehavid die for 510 s. Due to the contact between the hothe cold tool, the blank is quenched in the closed tool. Ifrate exceeds a minimum cooling rate of approximatelytemperature of around 400 C, a diffusionless marten-rmation will be induced, which nally is responsiblelting high strength of the part (Fig. 3b) (Merklein et

he martensite transformation begins at 425 C (marten-int Ms) and ends at 280 C (martensite nish point Mf)al., 2001).hanical properties of steel after quenching change inon its carbon content and consequently, the strength

generated iof 10% silicocoatedblaninterface arAlSi coatinever, due toahighermeand quicklythe surfaceing layer froof hot stam

Fig. 2. Basic hot stamping process chains: (a) direct hot stampiformation and hardenability is achieved by technicallyling rates (Garcia Aranda et al., 2002). Boron is the ele-inuences the hardenability the most, whereas boronthe conversion into softer microstructures and leads

sitic microstructure over the cross-section of the part.ustenitization conditions, oxide scale formation occursy when the steel is in contact with air. In order to avoiddation and decarburization, most sheet metal blanksted with a protective layer. The widespread protectionlayer preventing scale formation on the steel duringot stamping operation. Borsetto et al. (2009) investi-uence of thermal process parameters on the chemicalthe coating of the AlSi layer. This metallic coating is

n a continuous hot-dip galvanizing process and consistsn, 3% iron, and87% aluminum.During the heating of thek, the steel diffusionprocess fromthe coatingsubstrateea to the coating surface is thermally activated. Theg has a melting point of approximately 600 C. How-the presence of Fe in the substrate, an AlFe alloy withlting point grows from the interfacewith the basemetalreaches the surface. The AlFe alloy, which migrates to

, has a higher melting point and this prevents the coat-m melting. For a heating temperature of 950 C typicalping processes, a sub-layer structure characterized byng, (b) indirect hot stamping.

H. Karbasian, A.E. Tekkaya / Journal of Materials Processing Technology 210 (2010) 21032118 2105

ram (G

an alternati(Borsetto ettective layeforming limin the initiasheets cannsuitable forprotection,formed parcomponentmet by metheating andwith the baorder to milayer into tbe used forlayer must bence. Anoth(Goedicke eing processadditionalthe combinprocess. Inoblended witribologicalcold forminadditional l

The latesing the sheeand Ito (200nace couldstudied. Thea cooling exment. The aof lubricatio

3. Heating

The hotup to the auprocess winblanks durimartensiticLechler andature and tia full metall

urrinweritizant shareresu

stenib) onenoue temciened s. Win timis dAl

entw(Stosing.7), aed dinveure ocessgenemant thtion

ller h

the eFig. 3. Mechanical properties of 22MnB5 and CCT diag

ng variation of the chemical AlFe percentages appearsal., 2009). In the direct hot stamping process, this pro-r prevents the formation of scales. Due to the lowerits of the AlSi layer compared to the base materiall state at room temperature, the hot-dip aluminizedot be used for the indirect process and they are notcold forming. This coating does not provide cathodiclike zinc, but a high barrier protection. Similar to coldts, a cathodic protection is desirable for hot stampeds. These demands of the automotive industry could beallic coatings with cathodic protection, e.g. zinc. Duringhot stamping, the hot-dip galvanized zinc layer reactsse material to form intermetallic zinciron-phases. Innimize the propagation of microcracks in the coatinghe base material, hot-dip galvanized 22MnB5 can onlyindirect hot stamping. After hot stamping, the oxidee removed by shot peening to avoid a bad paint adher-er established protective coating for 22MnB5 is x-tec

t al., 2008) which is applied as a varnish in a coil coat-for the direct and indirect hot stamping process with

active corrosion protection. This coating is based onation of m-scaled materials according to the solgelrganic and organic materials are linked together and

th aluminum particles to form a protective coating. Thebehavior of this 7m thick protective coating in theg operation allows a controllable material ow withoutubrication (Paar et al., 2008).t method for the prevention of oxidation involves coat-ts with preventive oil, as described in the work of Mori9). The oxidation of heated sheets in an electrical fur-be prevented and the effect of two different oils waseffect of the oxidation preventive oil was evaluated in

the occblanksaustendiffere470HV

Thethe au(Fig. 4homogfurnacbe sufquench470HVduratioblanksternarytreatmabilityprocesal., 200exceed

Theprocedtheproa homomain ddiffereconduc

3.1. Ro

On

periment without forming and in a hot bending experi-nalysis of the sheet surface has shown that the numberns (up to 4 times) reduces the surface oxidation.

stamping process begins with the heating of the blankstenitization temperature. For the determination of thedow regarding a homogeneous austenitization of the

ng hot stamping as a pre-condition for the desired fullytransformation, annealing tests were performed byMerklein (2008) considering austenitization temper-

me. In these tests, the samples were quenched throughic contact pressure of 40MPa on both sides. To evaluate

furnaces orthese furnaheated. Alrequire a spbetween ba

The furnlengths ofinvestmentthe blanks.

The cyclon the die caustenitizerequired thmization oreduction inarcia Aranda et al., 2002).

g phase transformations, the hardness of the quenchede measured according to Vickers HV10. The minimumtion time at different austenitization temperatures andeet thicknesses to achieve the maximum hardness ofshown in Fig. 4.lts of this study show the signicant dependency oftization temperature (Fig. 4a) and the sheet thicknessthe minimum heat treatment time with respect to as austenitization of the quenchable steel 22MnB5. At aperature of 950 C, a dwell time of 3min was found tot to obtain the maximum martensitic content in theamples with a maximum hardness of approximatelyth decreasing furnace temperature, the austenitizatione increases. The upper time limit of AlSi pre-coated

etermined by means of the thickness of the alloyingSiFe layer (Austerhoff and Rostek, 2002) during heatith the objective to ensure a sufciently accurateweld-

pp et al., 2007) of the hot stamped parts during postAccording to the experiences from industry (Stopp etlayer thickness of approximately 40m should not beuring austenitization in the furnace.stigationsof Lechler (2009)have shownthat theheatingf the blank has a great inuence on the part properties,time, and the cost-efciencyofhot stamping. Therefore,ous blank temperature and a short heating time are thend on the heating system. The blank can be heated usingermalphenomena: radiation ina furnace, induction, and(Fig. 5).

earth furnace

xisting lines, the blank is usually heated in roller heath

walking beam furnaces. The size and connected load ofces depend on the through-put and the material to beSi coated materials used for preventing scale formationecial heating curve due to a necessary diffusion processse material and the coating (Suehiro et al., 2003).aces of the existing hot stamping lines already reach3040m. The high space requirement and the risingcosts show the need for alternative approaches to heat

e time for press-hardened parts is mainly dependentlosing time and the furnace residence time required tothe material and, in the case of coating, to achieve therough-alloying. With regard to die closing time, opti-f die cooling or the tool steels used could provide a

cycle time. A reduction in furnace residence time can


Fig. 4. Inuence of austenitization temperature, time (a), and sheet thickness (b) on the minimum austenitization time to achieve the maximum hardness of 470HV (Lechlerand Merklein,

only be achet al., 2008alaboratorytion.

3.2. Conduc

An alterheating protrodes (Mometal part.part. The coJoules Lawcircuit is prof power duthe conductinsulating ptance and tof these conical for the2008).

An impoefciency fresistance.in comparisused for coas pipes, rodof this heatilength of th

of thex ge

ducti

lastle, aheatondg antry olativcy.ce octric; oneing

ng of(Korgye ofnd t

ming

rder2008).

ieved by the following faster heating concepts (Lenze,b). These methods are in the development phase, and

investigations must verify them for industrial applica-

tion heating

native heating system is conduction heating. For thecess, a blank is clamped between the two pairs of elec-ri et al., 2009). The current ows through the sheetThe resistance of the material causes the heating of thenduction heating of the metallic substances is based on, according to which the heat generated in an electricoportional to the power of the electric circuit. The losse to the electrical resistance of the conductors results inors themselves being heated. A low surface quality andollution layers on the component increase the resis-

hus the heat generation in the contact area. The designtacts and the control of the contact pressure are crit-homogenous heating of the component (Kolleck et al.,

rtant basis for the use of conduction heating is theactor. This parameter directly depends on the partsBecause of the higher resistance of long componentson to short components, conduction heating is mainlymponents with a favorable length/diameter ratio, such

cationcompl

3.3. In

Theprincipcan becorrespformingeomeeld reefcieninuenthe eleanteedwhile bjammisystemthe enebecausgases a

4. For

In o

s, wires, and bands (Kolleck et al., 2008). Disadvantageng system is the inhomogeneous temperature along thee component. Another problem for the industrial appli-

must be trapress. Furthning of the

Fig. 5. Heating systems: (a) roller hearth furnace, (b) induction his heatingmethod is the difculty of heating blankswithometries homogeneously (Behrens et al., 2008).

on heating

heating system is the inductive heating of the blank. Inll electrically conducting or semiconducting substancesed by induction, and the resulting area of application isingly large for this technology: melting of metals, bulkd tempering, and assembly and packaging industry. Thef the inductor determines the position of the magnetice to the workpiece, which causes different degrees ofThe distance between inductor and sheet also has ann the efciency of the heating system. On the one hand,al insulation between inductor and sheet must be guar-the other hand, shaped blanks tend to go out of shapeheated. A small distance to the inductor can cause thethe heated blank with the risk of damaging the heating

lleck et al., 2009a,b). Compared to roller heath furnaces,efciency of induction heating is up to two times higher,the higher losses of the roller heath furnace by exhausthe rollers.

to avoid cooling of the part before forming, the blank

nsferred as quickly as possible from the furnace to theermore, forming must be completed before the begin-martensite transformation. Therefore, a fast tool closing

eating, (c) conduction heating.


and formincontrol. Afteis cooled byIn order toholder andstamping to

Anothermedia. Theformingandopportunitynology (Neuthe positionthe formingthe work ofLindkvist etpressure ofparison to cat the beginlower contaa homogenforming ofbe the applworking me

The currmetal formto shortenapplicationet al., 2008)2007). By th(2008) withtime could

4.1. Cooling

The quennot only inpropertiesdesign is torate of at letem is econows throunent. The hheat transfewithin the tFor an opticontact surf

tivity within the tool can be considerably inuenced by the choiceof the tool material. Another important factor with respect to heatdrain is the design of the cooling ducts, which is dened by the size,location, anddistribution of the cooling ducts. The heat drain can be

atede thd thecoo

d, thof thl dele. Ain threstrethoeredwitcoson t

ear o

wea8) uraturSi co

. Foron aras ouiltto fo

stripexporatiosurfaf theuitariboraturatingpin-chievparre, tmu

gatio

nchFig. 6. Tool design for the hot stamping process.

g process are the precondition for a successful processr forming, thepart is quenched in the closed tool,whichwater ducts to transfer the heat out of the tool system.avoid the quenching of the blank between the blankthe die during the forming process, most of the hotol systems work with a distance blank holder (Fig. 6).process variant is hot stamping by means of workinguse of temperature as a process parameter in hot gasa simultaneousquenchingof the formedparts offer theto increase the applicationeldof this innovative tech-gebauer et al., 2009). The formingprocedure startswithing of the prole or the blank. After the tool is closed,step is performed using the working media (Fig. 7). InNeugebauer et al. (2009), nitrogen, and in the work ofal. (2009), air is used as the working medium with a

up to 600bar. The advantage of hot gas forming in com-onventional hot stamping is the free forming of the partning of the forming process. In addition, because of thect time between the part and the tool during forming,ous blank temperature distribution leads to a uniformthe blank. Another interesting eld in gas forming mayication of thermally insulating and/or incompressibledia.ent strong demand for a higher efciency of hot sheeting processes inevitably leads to the question of howthe process cycle. Cooling can be accelerated by theof tool steels with improved heat conductivity (Casasand/or more efcient cooling systems (Hoffmann et al.,e application of the tool steels developed by Casas et al.a thermal conductivity of up to 66W/mK, the holding

accelerincreastool an

ThemethodesignoptimapossibpipesThe unthis mthe lassurfaceis veryeffects

4.2. W

Theal. (200tempethe AlradiusadhesiAlSi wThese bknownof the

Theingopein thewear oapply swith. TtempePVD cousing awere a

Comthermotextureinvesti

5. Quebe reduced from 10 to 2 s.

ducts

ching operation during the hot stamping process doesuence the economy of the process but also the nalof the component. The objective of the cooling ductsquench the hot part effectively and to achieve a coolingast 27K/s while martensite forms. The die cooling sys-omical if a uid coolant, such as water, is used, whichgh cooling ducts around the contours of the compo-eat ow in the formed component is dependent on ther from the component to the tool, the heat conductivityool, and the heat transfer from the tool to the coolant.mum heat transfer between component and tool, theace should not exhibit a scale or a gap. The heat conduc-

After theature rangemartensitecooling ratsite microsincrease ofite (fcc) intinuences tpletedescriof the resultphases, theafter coolin

For the mthe strain inthermal, anplasticity stby using a coolant with a low temperature in order toe temperature difference between the coolant and therefore the resulting heat ux (Steinbeiss et al., 2007).

ling holes can be drilled in the forming tool. For thise machining restrictions should be considered in thee hole position (Steinbeiss et al., 2007). Therefore, an

sign of the cooling system as to heat transfer is notnother alternative is to provide the cooling holes ase casting mold of the tool (Kuhn and Kolleck, 2006).icted design of the cooling system is the advantage ofd. Alternatively, the tools can be manufactured usingblank segments, which are screwed and form the toolh integrated cooling holes (Freieck, 2007). This methodt-effective, but the lamellar design can have negativehe part surface and the heat transfer within the tool.

f the tool surface

r resistance of the tool has been measured by Dessain etsing a strip drawing equipmentwhich is adapted to highe tests. The resistance heating device allows heating upated 22MnB5 strip. The heated strip slides over the diethis test, the contact surface was made of abrasion andeas. The die abrasion was predominant and adhesion ofbserved at the end of the contact area with the blank.layers, formed very early at the beginning of the test, arerm compacted layers on the tool surface during sliding(Hardell and Prakash, 2008) and exhibited low wear.sureof the tool steel tohigh temperatures inahot form-ncan result in large variations in frictiondue to changesce topography, removal of oxide layers and excessivetool. One approach to overcome friction problems is toble surface treatments and/or coatings to the tool steellogical studies by Hardell and Prakash (2008) at roome and 400 C on one plasma nitrided tool steel and twos (CrN and TiAlN) sliding against UHSSwere carried outon-disc machine. The best results as to wear resistanceed by the TiAlN coating.

able studies for another sheet coating do not exist. Fur-he austenitization time and its inuence on the surfacest be considered in design of experiments for furtherns.

ing

forming of the heated blank in the austenitic temper-, the part is quenched in the closed tool until the entiretransformation of the part structure is completely. Ae of more than 27K/s is necessary for a full marten-tructure of 22MnB5. Martensite evolution leads to anthe ow stress (Fig. 8). The transformation from austen-o martensite (bct) causes an increase in volume, whichhe stress distribution during quenching. Only the com-ptionof the transformationbehaviorallowsapredictioningmaterial properties, the volume fraction of differentresidual stresses, and the distortion of the workpieceg (Neubauer et al., 2008).odeling of the thermoplastic transformation behavior,crement can be described by the sum of elastic, plastic,d isotropic transformation and transformation-inducedrains (Table 2). Due to the different lattice structures of


Fig. 7. Gas forming of prole (a) and sheet (b).

austenite and the resulting microstructures ferrite, pearlite, bai-nite and martensite, a volume change occurs during the phasetransformation that can be described by means of the isotropictransformation strains. This effect causes only a change in volume,like the thermal strain increment. Additionally, the morphologiesof these microstructural components are very different, and con-sequently tof materialtion probleand the defal., 2007).

If the phmaterial reume is obthe parentunder an exirreversible(Greenwooticity whicfraction. Thplastic straThe commoity is the moto numerica

The descied usingunder differgram (FCCT

forming condition and is subsequently cooled at a predeterminedrate. The degree of transformation is measured.

6. FE simulation

stamed pand

es es reldepenicalatio

musticrosike the phburgta wss an8).a cos oft md teopyatioheir mechanical properties differ. Therefore, the studymacroscopic behavior becomes a difcult homogeniza-m due to the fact that new phases continuously evolveormation history must be accounted for (kerstrm et

ase transformation occurs without applied stress, thesponse is purely volumetric and an increase in vol-served due to the compactness difference betweenand product phase. When the transformations occurternal stress, transformation-induced plasticity causesdeformation. The GreenwoodJohnson mechanism

d and Johnson, 1965) describes the transformation plas-h depends on the austenite and the resulting phaseerefore, micro-stresses are introduced that generateins in the austenite when deviatoric stress is applied.nly used model considering the transformation plastic-del by Leblond et al. (1989), which was further appliedl modeling by kerstrm (2006).ribed analyticalmodel for the owbehavior can be ver-the continuous cooling transformation diagrams (CCT)ent forming modes. To determine the forming CCT dia-), the heated specimen is formeduntil it reaches the test

Hotintendhistorymixturheat imore,mechadeformlationand mistics land th(Oldention dahardneal., 200

FormationelemenelevateanisotrdeformFig. 8. Flow curves and microstructure of 22MnB5 duping is a thermo-mechanical forming process withhase transformation. Depending on the temperaturemechanical deformation, different phases and phase

volve. During the solid-state phase transformations,eased, which inuences the thermal eld. Further-nding on the mixture of micro constituents, both theand thermal properties vary with temperature and

n. Consequently, a realistic FE model for process simu-consider interaction between the mechanical, thermal,tructural elds (Fig. 9). This requires process character-e heat transfer coefcient, the material ow behavior,ase transformation under process-relevant conditions, 2008). Due to the transfer of microstructural evolu-ithin the process simulation, nal properties, such asd tensile strength, can be properly modeled (Jeswiet et

nsistent analysis of coupled thermo-mechanical defor-metals, Ghosh and Kikuchi (1988) developed a niteethod, which models the ow behavior of metals atmperatures. The proposed model considers the initialand temperature dependence of metals for large-

n forming processes.ring hot stamping.


Table 2Strain rate components in hot stamping.

Total strain rate ij = elij + plij

+ thij

+ trij

+ tpij (1)

elij: elastic component, pl

ij: plastic

component, tpij: trans. plasticity, th

ij, tr

ij:

thermal and isotropic transformationcomponent

Thermal and isotropictransformation component(Olle et al., 2008)

dth+trij

= dthij

+ dtrij

=(at+tn

(Tt+t

)atn (Tt )

1

)ij (2)

an: lattice constant, t: time step, ij:Kronecker delta, T: temperature

Transformation plasticity rate(kerstrm, 2006)

tpij

= 3th12h

(y

)z ln (z)

y1

(p1, T

) sij (3)h

(

y

)= 1 y 12

Volume frac(Koistinen a1959)

In recenspecializinghot stampina thermal atransfer of gcalculationconcepts arwithin the Fited data trathe accuracal., 2007). Ais the applForm, andP

Fig. 9. Intera(Bergman, 199

the denition and description of the physical processes. For exam-ple, the analysis with the FE software LS-DYNA can be performedusing thermal shell elements coupled with mechanical shell ele-ments for the metal sheet (Bergman and Oldenburg, 2004). Thethermal prothe mechantion methoadvantagescome contacan be modet al., 2008a

The predthe tools pladependentplastic defoblank andradiation frite to martehot formingthermo-me

tion

erma

prey mermar coepecteratiperhickAs thongly x + 3.5( y

12

)

y>

12

th12: difference in compactness betweenaustenite and the resulting phase, y1:current yield stress of austenite, z: volumefraction of resulting phase, sij: deviatoricstress, h: correction function according tothe nonlinearity in the applied stress, :current effective stress, y: current globalyield stress

tion of martensitend Marburger,

zM (t) = 1 e(

a(TMST))

(4)

TMS: martensite start temperature, a, :

simula

6.1. Th

Theparts bthe thetransfethe resing opthe tem(scale t2007).are strcoefcients

t years, several concepts of coupling two FE programsin their own respective domains for theperformanceofg have been developed. The coupled systems consider

nd a mechanical model, which are linked to achieve theeometrical and physical data. Because of the separateof thermal and mechanical phenomena, the couplinge very exible and efcient for parameter adjustmentsE models. The disadvantage of this method is the lim-nsfer between the two FE models, which can inuence

y of the results of the simulation (Hein, 2005; Tekkaya etnother alternative for the FE simulation of hot stampingication of special purpose programs: LS-DYNA, Auto-amStamp. The FEmodels vary in the existing options for

ction between thermal, mechanical, and microstructural elds9).

important peling of the

For the ding tool hasquenched bpressure. Dcontact plateters, the hon the contNewtons c

T(t) = (T0 A: contact svolume, t:perature,

The evalthe increasthe applied(Fig. 10). Inheat transfetact surfacein particulamore and ming direct henergy betwal., 2008a,b

6.2. Flow b

The owa conductito determiprocess-relblem is solved by the implicit time integration whileical problem is processed by the explicit time integra-d. This feature within LS-DYNA allows to combine theof each integration rule and, at the same time, over-ct solution stability and thermal convergence. The toolseled as rigid bodies with thermal behavior (Karbasian,b).iction of the temperature distribution in the blank andys a very important role in this process. A temperature-hardening function is also required to characterize thermation and to take into account the heat between thethe tools as well as heat loss due to convection andom the blank. The phase transformation from austen-nsite must also be considered in order to simulate theprocess (kerstrm et al., 2007). In the following, the

chanical properties and their determination for the FEof hot stamping are described.

l characteristics

diction of the mechanical properties of hot stampedans of FE simulation requires an accurate modeling ofl phenomena during forming and quenching. The heatfcient h is responsible for the thermal behavior andive cooling of the blanks throughout the whole form-on and can be inuenced by the contact pressure andature of the steel sheet as well as the surface conditionness, roughness, coating thickness, etc.) (Forstner et al.,e mechanical properties of the base material 22MnB5dependent on the temperature, this is one of the mostarameters to be taken into account regarding FE mod-rmally assisted forming.etermination of the heat transfer coefcient, a quench-been developed by Hoff (2007). The heated blank wasetween two water-cooled plates at a dened contacturing the test, the temperatures of the blank and of bothes were recorded. On the basis of the measured param-eat transfer coefcient was calculated in dependenceact conditions using an analytical method according toooling law.

T)e(h(A/cpV)t) + T (5)urface, cp: heat capacity, h: heat transfer coefcient, V:time, T0: initial temperature, T: environmental tem-: densityuation of the heat transfer coefcient h as a function ofing contact pressure shows the signicant inuence ofload on the heat exchange between workpiece and diecreasing the contact pressure leads to an increase of ther. This effect is related to the rise in the effective con-between the two contact partners through smoothing,r of the case in the AlSi coated sheets. Consequently,ore real metallicmetallic contact areas occur enforc-eat conductance effects, through which more thermaleen the two contact bodies is transferred (Karbasian et

).

ehavior

behavior of steel 22MnB5 was characterized usingve hot tensile test by Merklein and Lechler (2006)ne the material thermo-mechanical properties underevant conditions (Fig. 11). This investigation showed


Fig. 10. Heat transfer coefcients in hot sheet forming.

that not only strain but also strain rate temperature and temper-ature rate have the greatest inuence on the ow properties of22MnB5 at elevated temperatures in the austenitic state.

Apart from the signicant impact of the temperature and strainrate on the ttests, the decould be deof about 80plastic behanisotropy

6.3. Materi

For theof semi-emstress havehave shownof 22MnB5data by HocThe experimobtainedbydeformatioeter estimapublished ithose of Joh(2006) and

Hochholthe experimmodel. Fig.stress increNortonHo

The values of the ow stress at relatively large strains representthe main disadvantage of the JohnsonCook model in the work ofDurrenberger et al. (2008, 2009) (Fig. 12b). The predictions of theVoceKocks relation are in good agreement with the experimental

he mf ab

erimeproperaametheof thtionsmentontines th

ion, iby k-me

strucion pThe ca hasnB5

rmin

descraditFLC.at dihermo-mechanical properties in the conductive tensilependency of the temperature on the plastic anisotropytected (Merklein and Lechler, 2008). At a temperature0850 C, the sheet metal exhibits an almost isotropicavior. Because of austenitization, the inuence of thecan be neglected (Merklein and Lechler, 2008).

al models

thermo-mechanical forming processes, a large varietypirical as well as physically based models for the owbeen proposed. The existing models in Table 3, thata good capability of representing the ow behavior

in recent publications, are tted to the experimentalhholdinger et al. (2009) and Durrenberger et al. (2009).ental data for the determination of the ow stress areanupsetting test,whichwas conducted in ahigh-speed

n dilatometer (Hochholdinger et al., 2009). The param-tion of some of the material models in Table 3 are notn the cited references. But the other material data likensonCook andNortonHoff can be found in kerstrmLechler (2009).dinger et al. (2009) have shown that the best t ofental data can be achieved with the TongWahlen12a shows the failure in the saturation of the owases at higher values of the effective plastic strain inff and the NematNasser model.

data, tstrain oin expity to ror temnal parstudy,rationpredicexperi

A cdescribevolutlationthermophaseformat2005).tal datof 22M

6.4. Fo

Theing is tcurvessheetFig. 11. (a) Experimental setup and (b) ow curves (Merkodel predicts an early saturation of the ow stress at aout 0.06,while a slight strain-hardening is still observedents. The MolinariRavichandran model has the capac-duce history effects, such as rapid changes in strain rateture history, by means of the evolution law of the inter-ter. However, for the material 22MnB5 analyzed in thisMolinariRavichandran model predicts an early satu-e ow stress after a plastic strain of about 0.10. Theof the Ghost model are in good agreement with theal data of 22MnB5 up to a plastic strain of about 0.05.uous model of the ow behavior, which includes ande phase transformation up to the end of the martensites developed and implemented in a nite element simu-erstrm et al. (2007). The created model describes thechanical material properties on the basis of the actualture considering latent heat, volume change and trans-lasticity during phase transformation (kerstrm et al.,omparison of thematerialmodelswith the experimen-shown that the realistic modeling of the ow behaviorin the austenite condition is possible.

g limit curve

ription of thematerial formability by sheetmetal form-ionally achieved through the approach of forming limitThis curve shows necking and fracture in the deformedfferent stress states on sheet samples from balancedlein and Lechler, 2006).


Table 3Material models for 22MnB5.

kerstrm (2006) Nemat-Nasser (1999) f = 0

{1 [

kTG0

(ln

0+ ln f (p, T)

)]1/q}1/pf (p, T) + 0a np (6)

f (p, T) = 1 + a0[1 (

TTm

)2]1/2p

0: effective stress, k: Boltzmann constant p, q: shape of energy barrier, 0: referencestrain, G0: magnitude of activation energy, T: temperature

Johnson and Cook (1983) f = (A + Bn)[1 + C ln

(

0

)][1 (

T T0Tf T0

)m]with T T0 (7)

A, B, C, n, m: coefcients of the model, 0: reference strain rate, T: temperature, T0:reference temperature, T: melting temperature

Hochholdinger et al. (2009) Norton (1929) and Hoff (1954) f = K(b + )n0 exp[cn(TT0)]m0 exp[cm(TT0)] exp(

T

)(8)

K, , b, n0, m0, cn , cm: coefcients of the model, T: temperature, T0: reference temperature

Tong et al. (2005) f = A[m1(TT0) exp

(m2Q

RT

)][1 exp

(Nnp

)](9)

A, m1, m2, N, : constants of the model, R: gas constant, Q: activation energy, T:temperature, p: strain rate

Durrenberger et al. (2009) Ghost (Bouaziz et al., 2004) =(0 + Mab

)(

1 + kTb3va

arsh

[

0exp

(Q

RT

)])(10)

M: Taylor factor, : dislocation parameter, : shear modulus, b: Brgers vector, 0:dislocation density, k: Boltzmann constant, R: gas constant, 0: friction resistance, va:

feren

with

eferengrain

S) exp

ss, 0

biaxial to pvated tempnot only bymicrostruct2009).

Severalpose, withcommonlymetastablements, thatdifferent paing and fracbetween thspherical or

Formingin the NakaHere, for th

it cuore, s, theup.egrinemeenitee thshear stress, 0: re

Molinari and Ravichandran (2005) f = 0(

0

)1/m: strain rate, 0: rrate sensitivity, d:

VoceKocks (1976) f = S +[(0

s: saturation stre

ure shear. However, when forming take places at ele-eratures, the material formability is greatly inuencedthe strain but also by temperature, strain rate, and

ural evolution during deformation (Pellegrini et al.,

tests have been developed and applied for this pur-the Marciniak and the Nakajima tests being the mostapplied ones. In both tests, the sheet metal in the

ing limTherefpunchthe set

Pellimprovof austture araustenitic phaseundergoes thermo-mechanical treat-are carried out at different temperatures and followths of strain and strain rate until the onset of neck-ture occurs (Bariani et al., 2008). The main difference

ese tests is the shape of the punch,which is either hemi-at (Dahan et al., 2007).limit curves of the material 22MnB5 were measured

jima tests at elevated temperatures in different studies.e determination of the temperature-dependent form-

during hotof the analytion tests at(2009) ensuduring thecult contrtransfer to tleads to a lea good cool

Fig. 12. Flow curves of different models for 22MnB5 ace strain rate

0 = (d)(

0

)(11)

ce strain rate, 0: intrinsic resistance, m: instantaneous materialsize, T: temperature(

r

)](12)

: initial yield stress, r: relaxation strain

rves, the traditional test equipment has to be modied.pecial heating cartridges have to be introduced in thedie, and the blank holder to control the temperature of

i et al. (2009) have shown that the thermally inducednt of the material and higher values of the slip systemsin comparison to the initial ferrite-pearlitemicrostruc-

e cause of the evolution of the mechanical properties

stamping (Fig. 13). The main reasons for the scatteringzed results are the different procedure and the evalua-elevated temperatures. The heating method by Lechlerres a uniform distribution of the blank temperatureheating phase in the furnace, but implies a more dif-ol of the temperature during cooling due to the manualhepress. The inductionheatingof Pellegrini et al. (2009)ss uniform distribution of the temperature but also toing control.

nd the experiments.


Similar(2009), Chablank tempstrain in pla

In the eaccomplishLechler (200distributionHowever, tduring coolhand, the inleads to a lgood coolinthe difcultions at higphase transformabilityan alternatiure criteriorate, tempe2009).

6.5. Friction

The frictstamping catesting metand the toolcoefcient a

An experimentalanalyticalnumerical evaluation method wasused by Sthr et al. (2008) to determine the friction coefcientunder process-relevant conditions. Here, the cup deep drawingtest (Fig. 14a), following the timetemperature prole of the hotstamping process, acted as experimental basis. To reveal differ-ent thermal conduction rates during the experiments with theobjective to investigate the impact of the temperature on thetribological conduction rates, the temperature of the tool wasvaried. In this work, the friction values determined for AlSicoated 22MnB5 specimens were calculated by Siebels approach(Siebel and Beisswnger, 1955). The results (Fig. 15) show asignicant decrease of the friction coefcient with rising tem-perature, with the values ranging from 0.6 to 0.3 (Sthr et al.,2008).

The pin-on-disc test (Fig. 14b) was performed by Ghiotti et al.(2009a,b) and by Hardell and Prakash (2008) to investigate theinuence of the process parameters (i.e. temperature, pressure,sliding velocity and roughness) on the friction at the interface

en the sheet metal blank and the dies. The analysis hastha

re ismpiinteressuetallhenb). Tpin-ot me

g. 15et mion.therg te. Thethent dired ffor tonstffereal br a h

lastg (FimactedFig. 13. Forming limit curves of 22MnB5.

to cited works of Pellegrini et al. (2009) and Lechlerstel et al. (2008) have shown that the higher the initialerature or the sheet thickness, the higher the criticalne strain.xisting works, heating and cooling of the blank areedbydifferentmethods.On theonehand, in theworkby9), the heating method ensures a uniform temperatureon the blank during the heating phase in the furnace.

his implies a more difcult control of the temperatureing due to themanual transfer to the press. On the otherductionheating used in thework byBariani et al. (2008)ess uniform distribution of temperature but allows ag control, ensuring very high cooling rates. Therefore,ty in carrying out tests under real isothermal condi-h enough cooling rates to ensure the avoidance of anyformation is evident. The issueof evaluating sheetmetalat elevated temperatures can be accomplished throughve approach, based on the application of a proper fail-n that can be developed as a function of strain, strainrature, and microstructural evolution (Pellegrini et al.,

coefcient

ion characteristics of the coated material used in hotn be evaluated by different test methods (Fig. 14). The

betweshownpressuhot staof themal printermtion w2009a,in thein shee0.8 (Fiin shecondit

Anodrawin(2008)area ofdifferemeasuresultsand a cthat difrictiontrue fo2008).

Thedrawintestingintegrahods vary in the contact condition between the sheet,which is important for thedeterminationof the frictionnd must be similar to hot stamping conditions.

the blank hcited invest0.6 increase

Fig. 14. Principle of the test methods for the evaluation ot the interaction between the temperature and thethe most relevant one for the friction coefcient in

ng. Both studies have shown that the friction valueface decreases with increasing temperature and nor-re. This fact may be explained by the formation of theic FeAl, which is responsible for the friction reduc-increasing normal pressure is applied (Ghiotti et al.,he contact condition between the blank and the tooln-disc method is different from the contact conditiontal forming. Therefore, the determined values of up to) using this method are not suitable for the applicationetal forming under the corresponding surface contact

method for the determination of the friction is thest (Fig. 14c), which was carried out by Dessain et al.heated sheet is drawn through the convex contact

tool with a simultaneous measurement of the forces inrections. The friction coefcient is calculated from theorces according to Pawelski (1964). A comparison of thehe friction coefcient at a constant pressure of 10MPaant austenitization time in the furnace of 390 s revealsnt temperature valueshaveonly a small inuenceon theehavior of the tribological system (Fig. 15). The same isigher surface pressure value of 25MPa (Dessain et al.,

method used to evaluate the friction coefcient is stripg. 14d), applied by Yanagida and Azushima (2009). Thehine consists of a furnace and a drawing device with anblank holder. The friction coefcient is calculated fromolder force and the tension force. In contrast to otherigations, the calculated friction coefcient of 0.5 up tos with increasing temperature at a constant pressure of

f friction characteristics.


Fig. 15. Friction coefcient against temperature.

10MPa (Fig. 15). The variation of the contact pressure between 7and 14MPa has shown that the friction coefcient is independentof the conta2009).

The resucient of Acondition dvalues. In coconditions adrawing unhas not bee

7. Final pr

The martensile streferent wormeasuremeinvestigatiois the precomaterial. Duformation,process con

In a conthe relevan(2008) usedsimulation,characteristthe therma

The owrates showowbehaviare varied atime, the st

et al. (2009)have shownthehigh shapeaccuracyof thehot stampedparts with minimal springback.

For the description of the thermo-mechanical phenomena dur-ing hot stamping and their background, the analysis of residualstress is required. In further investigations, the analysis of thestress fractions due to thermo mechanical and microstructuralprocedures can be the dominant process parameters with regardto the part properties. This knowledge is also essential for thedistortion-free manufacturing of hot stamped parts with tailoredproperties.

7.1. Corrosion

The same cathodic protection of cold formed parts is desirablefor hot stamped parts. The surface of the FeAl alloy phase of hot-dip aluminized sheets after the heat treatment is rough, and thanksto an anchoring effect of the surface, its paintability is goodwithoutchemical treatment. The FeAl alloy phase exhibits a better corro-

sistaost eheetoatinarriethe cat thnoting pint aardeme

traforformdueandter

superfaceellowtectectioeel h

subemeiede forermZn cvoidyedct pressure within this range (Yanagida and Azushima,

lts of the different investigations on the friction coef-lSi coated 22MnB5 (Fig. 15) show that the contacturing the test has a great inuence on the calculatedmparison to the real hot stamping process, the contactre similar to those in deep drawing (Fig. 14a) and stripder bending (Fig. 14e). The strip drawing under bendingn used for the determination of the friction coefcient.

operties

tensite evolution during quenching causes an increasedngth of up to 1500MPa, which is veried in dif-ks using tensile tests (Naderi, 2007) and hardnessnts (kerstrm, 2006). The subsequent microstructurens have shown that the full martensite transformationndition for the increased mechanical strength of thee to the effect of the cooling rate and the phase trans-

the nal mechanical properties are dependent on thetrol.tinuous cooling process, cooling rate and hardness aret parameters. In a practical approach, Erhardt and Bke

the cooling gradients, calculated in the hot formingto determine a value of hardness and other mechanicalics of the part. The mechanical properties depend onl and forming history of the part.curves of 22MnB5 at different temperatures and strain

the strong inuence of these process parameters on theor of thematerial. Although temperature and strain ratelong the part surface and process in the course of theudies of Yanagimoto and Oyamada (2007) and Kusumi

sion rean almsteel sAlSi chigh b

Foried thand isannealand papress h

Thephs-uling peThis iscoatingAlso ining isthe suucts (ythe procross-sbase st2009).

Theimprovor modsuitablhigh thtion ofis to aprealloFig. 16. B-pillar with tailored properties (Erhardt ance than a steel sheet without surface treatment andquivalent corrosion resistance to that of a galvannealed(Suehiro et al., 2003). It must be pointed out that theg does not provide cathodic protection like zinc but a

r protection.orrosion testswith x-tec coated blanks, it has been clar-e corrosion does not come from the steel basic materialproduced by iron diffused into the coating during therocess (Goedicke et al., 2008). To ensure weldabilitydherence, the x-tec coating has to be removed afterning by shot blasing (Paar et al., 2008).asured protection against perforation corrosion of arm (ZnFe coated) blank shows that the ZnFe coat-s signicantly better than conventional zinc coatings.to a slightly higher electrochemical potential in thismore stable corrosion products (Faderl et al., 2008).

ms of paint delamination, the discussed ZnFe coat-rior to conventional galvanized coatings as long asis properly conditioned. Red-colored corrosion prod-rust) may occur as a result of the iron content in

ive coating, analogously to galvannealed coatings. Then of the corroded sample clearly demonstrates that theas not been attacked for a very long time (Faderl et al.,

ject of the current investigations is the continuousnt of established coatings and the development of newmetallic coatings with an active corrosion protectionthe direct process. One of the goals is to combine theal stability of AlSi coatings with the cathodic protec-oatings for the direct and indirect process. Another goalintercrystalline cracks during hot stamping by usingzinc coatings (Paar et al., 2008).

nd Bke, 2008).


arts with tailored properties.

8. Subsequ

Becauseprocessingan adaptedIn the followquent proceapplicabilit

8.1. Cutting

Similar tare the nexing and, if ncutting hot

8.1.1. LaserBecause

laser cuttinstamped panot causeanto other cutis the fact ththe parts toby the stiffnpart. The lamovement

8.1.2. HardThe ach

severe blanconventionshown thatsurface andcess paramclearances,erties of theon the sheaall tested bpunch-die cthe fact thatincreases uclearance a

Anothershows that

cutting edge, but it becomes strongly sensitive to highdurinalonredubut tnce.ust

espe8).

Warmtherurinm cuge thlate

partsion,diff

can besigferrealsw008)mosis mparFig. 17. Different methods for hot stamping of the p

ent processing

of the special mechanical properties, the subsequentof the hot stamped parts requires a process analysis andprocess window in regard to the relevant parameters.ing, cutting and joining as the most important subse-ssing of hot stamped parts will be described and theiry in series production will be evaluated.

o conventional sheet metal forming, cutting or piercingt stage in the hot stamping process chain after form-ecessary, shot blasting. The different methods used forstamped parts are reviewed.

cuttingof the high strength of the material after hot stamping,g is the most commonly used method for cutting hotrts. Due to the contactless trimming laser, cutting doesy toolwearor any failureon the cuttingedge in contrastting methods. Another advantage of using laser cuttingat there are nearly no limits with regard to the shape ofbe trimmed. The achievable tolerances are inuencedess of the laser machine and the holding xture of the

ser cutting time depends on the part geometry and theof the laser machine (Kolleck et al., 2009a,b).

of theloadsoccurslightedge,resistamise mtools,al., 200

8.1.3.Ano

parts dof warting ed

Theof theformatLocallynentsrates dthe premateriet al., 2

Theing. Thdesiredcuttingieved high strength of press-hardened parts causesking tool wear and sometimes early tool failure withal mechanical blanking methods. So et al. (2009) havein the blanking process, the quality of the shearedthe dimension precision are inuenced by certain pro-

eters, such as punch speeds, blanking angles, punch-dietool cutting edge geometries, and the mechanical prop-material. In this study, no inuence of the punch speed

redgeometries and theblanking forces areobserved. Forlanking angles, the rollover increases with increasinglearance. Furthermore, a noticeable result is found inthe ratioof theburnishdepth increases as the clearance

ntil the burr forms, but decreases again with increasingfter burr formation (So et al., 2009).investigation on hard cutting by Picas et al. (2008)the high hardness of the punch induces low wear Fig. 18. Cooling its application since considerable microfracturesg the cutting edge. In the high-toughness punch, action of wear resistance is produced in the cuttinghis is compensated by a high toughness and fatigueTherefore, an optimal toughness-hardness compro-be found to improve the mechanical behavior of

cially in cutting of press-hardened parts (Picas et

cuttingalternative for cutting hot stamped parts is cutting ofg quenching at elevated temperatures. The advantagestting are a reduced cutting force and an optimized cut-rough a short process chain.st innovative procedure is the selective heat treatmentduring quenching. In order to avoid martensite trans-

the cooling rate in the cutting areas must be reduced.erentiated heat treatments in the hot stamped compo-e performed by means of differentiated heat transfer

ned for improvement of posterior cutting, and one ofdways to achieve this is trough the employment of toolithdifferent thermal conductivities (Maikranz-Valentin.t cost-effective cutting method is pre-developed blank-ethod requires a certain blank design to achieve thet outline after forming (Kolleck et al., 2009a,b). The tol-g curves for different tool temperatures (Ghiotti et al., 2009a,b).


Fig. 19. Mechanical properties at different austenitization temperatures (Sthr et al., 2009).

erances that can be achieved are smaller thanwith cutting after hotforming.

8.2. Joining

Because of the low formability, the joining by forming meth-ods cannot be applied for joining hot stamped parts. Therefore,weldabilityapplicationical composthe investigarc weldingreviewed.

Thealumheating befa highmeltiproduct is net al., 2003)

The rstweldable anpress-hardebehavior isafter the hoof weldinganti scalingshown the(Goedicke e

coated blanks has shown that the applied atmosphere inside thefurnace while heating the blanks has a great inuence on the spot-weldability. The O2 content was found to be the main driver here.Heating in air causes the formation of oxide layers so that spotwelding cannot be applied. Applying a nitrogen atmosphere leadsto very low oxide percentages inside the coated layer after heatingand forming, and thus supports spot welding (Braun and Fritzsche,2009).

ZnFed wurcessibll et alasef theuencetectl exacturo avt-diped ab

stam

fullyeadsof the hot stamped parts is the precondition for theof the parts in practice. The coating layer and its chem-ition cancause failuresduringwelding. In the following,ations on resistance spot welding, laser and gas metalof the material 22MnB5 with different coatings are

inumplating layerof theproduct is transformedduringore being press-formed into a FeAl alloy phase havingng point, and, thanks to this, the spot-weldability of theot affected by the presence of coating layers (Suehiro.generation scale protection, e.g. x-tec, is not spot-d must therefore be removed by sandblasting from thened parts before further processing. The reason for thisthat the electric resistance of the anti-scaling coatingt stamping process is too high to permit a sufcient owcurrent. The test of second and third generation x-tec

coatings by the integration of magnesium particles hassuitability of the coatings for resistance spot weldingt al., 2008). Resistance spot welding (RSW) of x-tec

Forproduca DC soalsopo(Fader

Forjoints ono inwereding wilmanuforder tthe horemov

9. Hot

Theparts lFig. 20. Hot stamped parts in automotive industre coatings, the best resistance spot welding results areith the double-pulse technology in combination with(Faderl et al., 2009). The other methods of joining are

e, suchasSGwelding, SGbrazing, laser andstudweldingl., 2009).r and gas metal arc welding, the cross-sections of thetested combinations show that the x-tec coating had

e on the welding behavior. No pores or other defectsed inside the jointswith andwithout overlap. This coat-ibly work also with different substrates, e.g. H430LA foring tailor welded blanks (Braun and Fritzsche, 2009). Inoid the diffusion of Al and Si into the weld seam, whenaluminized sheets are used, the coated layer must beout 2mm along the weld seam.

ped parts with tailored properties

martensitic transformation during hot stamping of theto a tensile strength of up to 1500MPa and a low elon-y (N. N., 2008).


gation of about 5%. But an improved crash performance of a vehiclestructural component (such as a B-pillar) can be achieved by intro-ducing regions which have an increased elongation for improvedenergy absorption. The B-pillars shown in Fig. 16 have an excellentintrusion coin the lowe

The manbe carried otailor welde

Alternatthe coolingsitic microsforming prothe materiaplete austentherefore aare quenchtimetemp

9.1. Tool te

The reduthe temperconductionheat and coized regionof higher dunumerical ssible to creaprotection,absorption.during quethe materiameans of th

9.2. Tool m

The locatem can bedifferent thof tool seq(2008) withIn this wayfaces. Becausystem, theferent strokproperties m

9.3. Tool su

The heacondition btion by Geowith integrof the air gathe criticalof this metarea, whichmethod forturing. TheThe investitransfer coeof hot stam

9.4. Blank tempering

Heating above Ac3 can be done locally in the zones of the blankwhere a martensitic structure is to be achieved. In this way, the

ete tthesl fere usee emto b

to bethe tilityditio

vestinealiapp

ductraturtly loed sh

ilor w

mingdy tindin h

d proat-trrtenth inanksust

rmabonsid

plic

comal sorelevviewf theplicationsillar rem

en 1.

nclu

his rrengdiffestinghot sr devapp

epenilityion ding ain a uing pntrol in the upper section and a high energy absorptionr section.ufacturing of a single part with tailored properties canut using different process control strategies or usingd blanks (Fig. 17).ively, the thermal process canbe inuencedby reducingrate below the specic 27K/s to avoid a fully marten-tructure in dened areas of the component during thecess, or by reducing the annealing temperature belowl-specic Ac3 temperature, which leads to an incom-itization. Both methods implicate a lower strength andhigher ductility. The other areas of the component

ed, following the commonly known press hardeningerature prole (Sthr et al., 2009).

mpering

ction of the quench rate can be achieved by increasingature of the die, which will reduce the heat transfer bybetween the blank and the die surface. It is possible tool different regions of a die, which will generate local-s of high strength (fully martensitic) and other regionsctility (mixture of daughter phases). Experimental andtudies by Lenze et al. (2008a,b) indicate that it is pos-te a part with areas of very high strength for intrusionand others regions with increased ductility and energyThe selected tool tempering inuences the cooling ratenching and the nal phases of the material. Therefore,l properties of the parts can be selectively adjusted bye different tool temperatures (Fig. 18).

aterial

lly differentiated heat treatment within a tool sys-achieved by the application of tool materials with

ermal conductivities. A modular tool system consistsuences made of tool steels developed by Casas et al.thermal conductivities from 7W/mK up to 66W/mK.

, the heat transfer can be controlled along the part sur-se of different thermal boundary conditions within thetool sequences achieve the thermal steady state by dif-es. This thermal phenomenon and its effect on the partust be investigated in further studies.

rface

t transfer during quenching is affected by the contactetween the part and the tool. The numerical investiga-rge et al. (2009) on the thermal behavior of tool systemsated partial contact gaps has shown that in the regionsps, the cooling rates of the part can be reduced belowrate of the martensite transformation. A disadvantagehod is the free forming of the part in the contact gapcan decrease the shape accuracy of the part. Another

the local reduction of heat transfer can be surface struc-structured surface reduces the effective contact area.gation of the effect of structured surfaces on the heatfcient can be the focus of an innovativemanufacturingped parts with tailored properties.

complonly inoriginathat thand thappearit hasiors informaband, adThe inent anfor thelowertempenicanstamp

9.5. Ta

Foris alreamotiveblankstailorenon-hethe mastrengThe blzone mited foto be c

10. Ap

Thean idesafety-ular inareas otive approtecdoor pcross mbetwe

11. Co

In thigh state thethe exiena offurthe

Thepart dweldabformatstampto obtastamphermal cycle above austenitization would be appliede zones, whereas all the others zones would retain theritic-pearlitic structure. Ghiotti et al. (2009a,b) indicateof a furnace with areas kept at different temperatures

ployment of resistance heating are two approaches thate the most suitable ones for this application. However,taken into account that the different material behav-

wo areas inuence the forming process accordingly. Themaybe reduced in the sectionwitha lower temperaturenally, springback may occur (Erhardt and Bke, 2008).gation on the thermo-mechanical properties at differ-ng temperatures by Sthr et al. (2009) has shown thatlicability of lower annealing temperatures to achieve aility prole for components with tailored properties, ae lower than 825 C has to be chosen to assure a sig-wer strength and hardness than that of commonly hoteet metal (Fig. 19).

elded blanks

of tailorwelded blanks or proles at room temperaturehe state of the art in various applications in the auto-ustry. Similar to this, the application of tailor weldedot stamping allows the manufacturing of parts withperties. Here, the blanks made of heat-treatable andeatable steel grades will be hot stamped. Because ofsite evolution in the heat-treatable steel, the nal partcreases contrary to that of the non-heat-treatable steel.are laser welded. Before this, the coating of the weldbe removed. The position of the weld seam and its lim-ility during hot stamping are further important aspectsered in the process design of tailor welded blanks.

ations

bination of forming andhardeningmakes 22MnB5 steellution for the construction of structural elements andant components in the automotive industry, in partic-of the implementation of penetration protection in thepassenger cabin or motor (N. N., 2008). Some automo-tions of hot stamping areA-pillars, B-pillars, side impact, sills, frame components, bumpers, bumper mounts,einforcements, roof frames, tunnels, rear and front end

bers (Fig. 20). The sheet thickness in these parts varies0 and 2.5mm.

sion

eview, the recent investigations on hot stamping ofth steels are summarized with the objective to evalu-rent research results. This general survey has revealedgaps in the knowledge by describing specic phenom-tamping. Furthermore, some innovative procedures forelopments in the eld of hot stamping are identied.lication and subsequent processing of the hot stampedd on an efcient cutting system as well as the partand surface texture. In order to avoid the oxide scaleuring austenitization, most sheet metals used for hot

re pre-coated. The objective of current developments isniversal coatingmaterial for the direct and indirect hot

rocess with additional cathodic protection.


The cycle time for press-hardened parts is mainly dependenton the die closing time and the furnace residence time requiredto austenitize the material and, as to the coating, to achieve therequired through-alloying. With regard to die closing time, opti-mizing thea reductioncan only beor inductiv(conductionfuture.

Hot stamintended phfor the procthe mechanbe achievedtransfer coemation und

The comtal data hasof 22MnB5model of thmaterial prconsideringticity up tosheet metalthrough thebe developemicrostruct

The owrates showthe ow behigh shapeback withinstamping. Hstamped patigate the inwithin theproperties.

This thorgurationsstrength steof the physifor an optim

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A review on hot stampingIntroductionMaterial and coatingHeatingRoller hearth furnaceConduction heatingInduction heating

FormingCooling ductsWear of the tool surface

QuenchingFE simulationThermal characteristicsFlow behaviorMaterial modelsForming limit curveFriction coefficient

Final propertiesCorrosion

Subsequent processingCuttingLaser cuttingHard cuttingWarm cutting

Joining

Hot stamped parts with tailored propertiesTool temperingTool materialTool surfaceBlank temperingTailor welded blanks

ApplicationsConclusionReferences

Artigo 1_17.03

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Transcript of Artigo 1_17.03