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Press hardening — An innovative andchallenging technology

R. Neugebauera, F. Schieckb, S. Polsterb, A. Mosela, A. Rautenstraucha,J. Schonherrb,n, N. Pierschela

aChemnitz University of Technology, Institute for Machine Tools and Production Processes IWP, Reichenhainer Straße 70, 09126 Chemnitz,

GermanybFraunhofer Institute for Machine Tools and Forming Technology IWU, Reichenhainer Straße 88, 09126 Chemnitz, Germany

a r t i c l e i n f o

Article history:

Received 27 April 2012

Accepted 29 April 2012

Available online 4 May 2012

Keywords:

Press hardening

Hot sheet metal forming

Simulation

Tribology

Forming tool

nt matter & 2012 Politec.1016/j.acme.2012.04.013

hor. Tel.: þ49 371 5397 18Julia.Schoenherr@iwu.fr

a b s t r a c t

In view of the growing demand for high-strength, press-hardened sheet metal components

and the increasing need for energy and resource efficient process-chains, the optimization

of the press hardening process chain is a complex, multi-layered and challenging task. The

aim of the present paper is to show the potential for optimization in the press-hardening

process chain and to demonstrate initial implementation variants.

& 2012 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z.o.o. All rights

reserved.

Fig. 1 – Future demand for press-hardened components [2].

1. Motivation

Press-hardened, crash-relevant components such as side-impact

and bumper cross members have been used since the mid-

1980s. The trend towards the use of high-strength materials in

innovative bodywork concepts that began with this continued

ever since (Fig. 1). The number of press-hardened components

produced increased from about 3 million units in 1987 to around

124 million in 2010. On this basis the forecast is that the

production of high-strength body panels will increase to approxi-

mately 350 million components per year by 2015 [2].

In order to meet the requirements of this rapid technological

development, reliable and practical production strategies must

be developed. A successful example is the forming process of

press hardening, which combines the forming and heat treat-

ment of the sheet metal component in a single process step.

Fig. 2 shows a scheme of the direct process chain for the

production of press-hardened structural components. The

hnika Wrocławska. Publis

08; fax: þ49 371 5397 6180aunhofer.de (J. Schonherr

trimmed sheet metal blank made from heat-treatable

manganese–boron steel (22MnB5) is austenitized at 950 1C in a

roller hearth furnace. The heating process is followed by the hot

forming of the sheet metal on a cooled tool; the so-called press

hed by Elsevier Urban & Partner Sp. z.o.o. All rights reserved.

8.).

Fig. 2 – Process chain — direct press hardening.

a r c h i v e s o f c i v i l a n d m e c h a n i c a l e n g i n e e r i n g 1 2 ( 2 0 1 2 ) 1 1 3 – 1 1 8114

hardening or form-hardening step. The necessary high cooling

rate of at least 27 K/s results in a hardening by means of

transformation to a martensitic structure. Depending on the

material used, this gives the components a tensile strength of

1000–1900 N/mm2. The final geometry of the press-hardened

component is realized by the process steps mechanical trimming

(cutting of hardened material) or laser cutting.

The possibility of producing high-strength as well as light-

weight components offers enormous potential, particularly for

the reduction of vehicle mass in series production within the

automotive industry. For example, up to 20 kg of material can be

saved for a mid-sized vehicle through the use of press-hardened

body parts [8]. On one hand, this reduces the amount of steel

required by about 68 kg per body shell in the manufacture of the

vehicle; on the other hand, it reduces fuel consumption by about

0.1 l per 100 km when the vehicle is in use [3]. However, the use

of press-hardened components not only reduces the weight of

vehicles; the higher component strength significantly increases

the energy absorption capacity and therefore the crash perfor-

mance of structural components. Furthermore, the press-hard-

ening process allows different material properties to be set in a

controlled manner – by tailored tempering, for example – thereby

enabling the further improvement of crash-relevant compo-

nents. Other advantages of press hardening, from the perspec-

tive of manufacturing technology, are lower forming forces and

high dimensional accuracy. These are critical factors in the

dimensioning and selection of a suitable press. Furthermore,

the combination of forming, hardening and heat treatment

processes in a single tool, together with good formability, reduces

both the number of forming steps in the production of a

component and the tooling costs for the overall process.

900

950

1000

105022MnB5

ratu

r [°C

]

300

900

950

1000

1050

atur

e [°

C]

100 30 10 3 1[K/s]

2. Challenges in press hardening

In order to make use of the described potentials of press

hardening, a variety of technical production challenges has to

be overcome. Hence, in addition to technological process

optimization, research challenges include the design of

forming dies and thermo-mechanical FE simulation, as well

as present questions regarding tool tribology and the energy

and resource efficiency of the processes.

Characteristic starting points for process optimization are:

850 Ac3

Tem

pe 850

empe

r

1.

750

800

Ac1 750

800 T

Realization of zero-defect production, e.g. high process

safety, for example — and the determination of suitable

process windows.

2.

0.1700 700

Time [s]1 10 100 1000 10000

Minimization of the used resources, e.g. through opti-

mized component geometry and the reduction of energy

consumption.

3.

Fig. 3 – TTA diagram for material 22MnB5.

Energy-efficient tools, machinery and equipment, e.g. intelli-

gent tool systems and suitably adapted tool coatings.

4.

Reduction of losses, e.g. by increasing the degree of

material utilization and the waste heat recovery.

5.

Optimization of the cycle time, e.g. through the reduction

of downtime and idle time and the use of multiple

impression dies.

These optimization approaches can be put into practice

using various technological measures or innovative research

strategies.

3. Research strategies and solutionapproaches

This chapter describes the challenges of press hardening

in terms of process control, tool design, thermo-mechanical

coupled simulations, tribology and also energy and resource

efficiency. It also presents the research strategies as well as

the developed solution approaches.

3.1. Determination of thermo-mechanical parameters fortechnological process design

The process chain of hot sheet metal forming is designed by

determining the thermo-mechanical parameters. The aim,

besides the reduction of process temperature, is to shorten

the process time and to determine the limits of the process

for improved process reliability. The basic thermo-mechan-

ical studies are performed by varying different parameters

such as forming temperature, extent of deformation, heating

rate, cooling rate, austenitizing temperature and process

time. This reveals the influence of various factors on the

process and specifies the process window as a function of the

required component properties.

Dilatometric studies form the basis of the thermo-mechanical

analysis. In addition to determining a process-reliable heating

temperature for the austenitization of the material used, the

tests allow conclusions to be drawn regarding the production

of components with graded properties. The austenitizing tem-

perature is determined using a quenching and deformation

dilatometer at various heating rates from 1 K/s to 300 K/s, where

the recorded changes in the length of the samples provide

a r c h i v e s o f c i v i l a n d m e c h a n i c a l e n g i n e e r i n g 1 2 ( 2 0 1 2 ) 1 1 3 – 1 1 8 115

information regarding the beginning of the austenite transfor-

mation, Ac1, and the end of the austenite transformation Ac3.

The results are summarized in a time–temperature austenitiza-

tion (TTA) diagram (Fig. 3).

Component-specific properties can be adjusted in hot sheet

metal forming by varying the cooling rate. The time–

temperature–transformation (TTT) diagram is helpful in this

regard (Fig. 4). As with the TTA diagram, this is determined

using the dilatometric studies in which the samples were first

austenitized and then, after a predetermined holding time,

cooled at different cooling rates ranging between 0.2–60 K/s.

The mechanical properties of the heat-treated samples are

subsequently examined. This allows conclusions to be drawn

regarding their component-specific properties [4].

0.1 1 10 100 1000 100000

200

400

600

800

1000 Cooling rate curves Experimental points Transformation start and finish lines

Tem

pera

ture

[°C

]

Time [sec]

0.2

23

5

8

1012

15

20

25

30

60

A + F

A + P

BM START

M END

A

22MnB5

Fig. 4 – TTT diagram for material 22MnB5.

Fig. 6 – Design of the

Fig. 5 – Feasibility analysis (left: tool design, ri

3.2. Thermo-mechanical FE simulation for tool design

Thermo-mechanical coupled simulation is an important tool

for process planners and designers for reproducing and

evaluating the complex factors affecting the press hardening

process, such as temperature, strain rate, heat transfer and

material behavior. The special-purpose AutoForm FE simula-

tion program with its integrated hot-forming module makes

it possible, for example, to make early statements regarding

the manufacturability of components for the hot-forming

process. The shell-modeling approach chosen for this per-

mits the evaluation of the calculation results with respect to

the sheet thickness distribution/thinning, wrinkling and

formability, among other things, (Fig. 5) and is used for

process and tool design.

When considering and designing cooling channels, it is also

necessary to mesh the tools and integrated cooling channel

structures as solid elements in the FE Analysis. In this case

the commercial FE Software LSDyna was used in this project.

Calculations were performed with different thermal bound-

ary conditions, such as tool temperature Tt and the heat

transfer coefficient h, for an equivalent number of cycles, to

describe the temperature management in the B-pillar base

demonstrator tool. For the analysis of the cooling systems in

the forming tool (Fig. 6), the cooling channels on the left side

were cooled with water (20 1C), while the cooling channels on

the right side were switched off or heated with warm oil

(300 1C). This way the material characteristics can be locally

adapted in the component [5].

Thermo-mechanical coupled FE simulations show that the

component properties can be influenced by the tool tempera-

ture distribution and heat transfer coefficient, in this way,

parts with tailored properties can be achieved. For example, it

is possible to produce softer component zones (i.e., high

cooling system [5].

ght: AutoForm FE Simulation–formability).

a r c h i v e s o f c i v i l a n d m e c h a n i c a l e n g i n e e r i n g 1 2 ( 2 0 1 2 ) 1 1 3 – 1 1 8116

bainite phase fraction) by using tool tempering with hot oil at

300 1C (Fig. 7).

3.3. Modern tool design

In current industrial practice the two following cooling

concepts are used in press-hardening tools: shell structure

and cast-in cooling channels (Fig. 8).

These both variants were also selected for implementation in

the demonstrator tool B-pillar base. For the punch of the tool

the shell variant, consisting of a punch base substructure (Fig. 9)

and a punch shell, was selected; while for the die, the variant

with cast-in cooling pipes was chosen. Both tool designs were

made of spheroidal iron (EN-JS1070-GGG70). The template was

also made by using EN-JS1070 in the casting process. The cast-

in cooling pipes were made of 1.4571 stainless steel (Fig. 9).

Fig. 7 – Bainite phase distribution in the demonstration

component according to the FE simulation (left side of the

tool: water-cooled; right side of the tool: oil tempering) [5].

Fig. 8 – Schematic diagrams o

Fig. 9 – Production of the B-pil

At approximately 1500 1C, the melting temperature of the

stainless steel is greater than the melting temperature of the

casting material. Only under this condition it can be ensured

that the tubes will not collapse during the casting process.

The cooling systems in the punch and the die consist of

several individual cooling channels, each of which can be

separately controlled. This allows a more precise control of

the cooling and at the same time a more homogeneous

cooling using the counterflow principle.

3.4. Tool tribology as a control variable for processstability

Tool tribology is a process control variable that has a great

influence on process stability. Firstly, the friction coefficient

between tool and sheet has a significant effect on the forming

result, i.e., whether an acceptable part is produced or a work-

piece failure occurs. A further objective of the tribology

studies is to achieve a service life extension of the tools so

as to increase service intervals and avoid frequent stoppages.

In this respect, a common problem in series production is the

adhesion of the AlSi (aluminum silicon) sheet coating to the

tool surface.

The critical wear mechanisms during press hardening are

symptoms of fatigue due to cyclical thermal stress, which can

manifest themselves through spalling and cracking. Adhe-

sion phenomena can be seen to increase with rising process

temperatures. Abrasion such as that caused by particles

of worn-off work-piece coating can either increase when

compared to cold forming, or can decrease due to smaller

forming forces. Furthermore, the high temperatures cause

f the cooling concepts [6].

lar base demonstrator tool.

Friction values of the combinations tool coating-sheet metal (≈610-620°C)

0.49

0.24

0.55

0.23

0.46

0.23

0.48

0.20

0.00

0.10

0.20

0.30

0.40

0.50

0.60

cet-x+5BnM225BnM22

Tool Coatings

Ave

rage

d Fr

ictio

n Va

lue

Uncoated CrVN NbTiAlN TiZrCrN

Fig. 12 – Average friction values for the tool coating/sheet

pairings (average values for the final 20 mm of the drawing

path at approx. 610–620 1C).

a r c h i v e s o f c i v i l a n d m e c h a n i c a l e n g i n e e r i n g 1 2 ( 2 0 1 2 ) 1 1 3 – 1 1 8 117

microstructural transformations and diffusion processes in

the tool material and coating. Erosion sites, which are a sign

of a reduction in wear resistance, are symptomatic of such a

local influence on materials [7].

Three mechanically resistant coatings and one uncoated

tool steel were tested for suitability for press hardening tools

using a tempered strip drawing test with 901 deflection of

the sheet material, developed at the Fraunhofer IWU. Fig. 10

shows the drawing edge geometry with a die radius of 8 mm,

which represents the tool in this model experiment. The

experiments, using five strips each of the sheet materials

22MnB5, 22MnB5þAlSi, 22MnB5þx-tecs with a sheet thick-

ness of 1.5 mm, were performed with the following PVD

coatings for the drawing edge: CrVN, TiNbAlN and TiZrCrN.

The NbTiAlN and TiZrCrN coating systems were both applied

as monolayer coatings on the substrate (Boehler W360 Iso-

bloc, hardness: 56.5 HRC). The CrVN coating was applied after

the deposition of a Cr adhesion-promoting layer.

Adhesion of uncoated 22MnB5 resulting from the experi-

ments led to a drawing-edge roughness (Rz) of 16.5 mm, as

shown in Fig. 11, when using the uncoated drawing edge. In

contrast to this, very small Rz values of 1.1–1.7 mm were

measured for the PVD-coated drawing edges. The measured

values for 22MnB5 with x-tecs show very low roughness for the

CrVN coating, whereas the uncoated drawing edge achieved

better results than the (TiZrCr)N and (NbTiAl)N coatings. The

results of the tribology studies make it clear that the adhesions

of uncoated 22MnB5 onto the tool can be avoided by using PVD

Roughness Measurement

123

4

12

3

4

Reference Roughness(without load)

Fig. 10 – Measuring points on the drawing edges for the

roughness measurements.

Rz and Ra of the drawing edges after the tests with 22MnB5 and22MnB5+x-tec

6.7

16.5

1.4 2.41.11.6

7.1

1.7

3.8

9.8

1.52.8

0.30.2

2.7

1.0

0.3 0.1

0.5

0.2

1.3

0.30.2

1.7

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

Measuring Point 4(Reference)

Measuring Point 3 (22MnB5) Measuring Point 3(22MnB5+x-tec)

Rz

[μm

]

0.0

0.3

0.7

1.0

1.3

1.7

2.0

2.3

2.7

3.0

Ra

[μm

]

Rz_uncoated Rz_CrVN Rz_(NbTiAl)N Rz_(TiZrCr)NRa_uncoated Ra_CrVN Ra_(NbTiAl)N Ra_(TiZrCr)N

Fig. 11 – Roughness of drawing edges before and after the

tests with uncoated 22MnB5 and 22MnB5þx-tecs.

coatings. In this case the tool surface is protected using

mechanically resistant coatings and, as other studies have

shown, an improvement in component quality is also achieved.

The coefficients of friction are almost completely unaf-

fected by the tool coating on both sheet variants (uncoated

and x-tecs-coated; Fig. 12). However, a change of the initial

sheet material can have a significant influence on the

process. The use of an x-tecs coated sheet in place of an

uncoated sheet reduces friction by about a half, whereby the

formation of cracks due to excessive thinning during hot

forming can be prevented.

4. Evaluation of the energy and resourceefficiency of process chains

One milestone on the path to an energy and resource efficient

production process chains is the use of holistic balancing

methods, evaluation tools and planning software. In the

future, such tools will enable engineers, for instance in car

body manufacturing, to balance and evaluate process chains

in terms of energy and resource efficiency, to choose energy-

sensitive influencing factors and to detect technological

improvement approaches. Therefore, a method was developed

whereby energy-sensitive, technological factors and the

energy and material flows of a process are incorporated within

a ‘‘process energy balance’’ for accounting and evaluation. This

procedure for energy and material balancing, shortened to

PEMB, consists of four steps: analysis, modeling, balancing and

evaluation. PEMB is based on the SADT model (Structured

Analysis and Design Technique), which is connected with a

techno-economic classification of production process elements

and generates a comprehensive process model. This process

model enables, on the one hand the visualization of energy and

material flows (step 1), and on the other hand the systematic

modeling of all the process elements involved (step 2), such as

raw materials, auxiliary materials, machinery and equipment or

electrical energy (Fig. 13).

Based on the individual process models, process-specific

process energy balances are created, which are then sum-

marized as a process chain energy balance (step 3). These

process and process-chain balances can, for example, be

Fig. 13 – Energy and material flow during press hardening

(example).

a r c h i v e s o f c i v i l a n d m e c h a n i c a l e n g i n e e r i n g 1 2 ( 2 0 1 2 ) 1 1 3 – 1 1 8118

used to calculate energy and material consumption, deter-

mine the process efficiency of the processes or analyze the

influence of process parameters such as process temperature,

sheet thickness, tool geometry and tensile strength on energy

and resource efficiency. The last step of the method (step 4)

analyses and evaluates the process and process chain bal-

ances created, whereby improvement approaches can be

identified and quantified. This basis can provide guidance

for improvement activities as well as setting energy- and

resource-sensitive process parameters. This provides engi-

neers such as planners for bodywork manufacturing with

a method that allows the energy and material flows of

process chains to be identified and evaluated. In addition,

energy-sensitive influencing factors can be selected, and

thus energy and resource efficient process chains can be

designed [1].

5. Summary and outlook

In order to meet the large future demand for hot-formed

components and to cope with the increasing demands in terms

of energy and resource efficiency, a wide variety of technological

challenges must be overcome. The basis for this is formed

primarily through the use of thermo-mechanically coupled,

finite element simulation, the implementation of defined tribo-

logical application conditions and the quantitative determination

of the energy and material consumptions of processes. Funda-

mental studies with regard to material and process parameters

as well as the development of appropriate tooling concepts

have provided the first steps towards overcoming the technical

challenges of manufacturing. This also provides the basis for

addressing current complex issues such as the mechanical

trimming of press-hardened components or of graded press-

hardened components.

Acknowledgements

The Cluster of Excellence ‘‘Energy-Efficient Product and Pro-

cess Innovations in Production Engineering’’ (eniPROD) is

funded by the European Union (European Regional Develop-

ment Fund) and the Free State of Saxony.

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