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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/acme
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
1644-9665/$ - see frohttp://dx.doi.org/10
nCorresponding autE-mail address:
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 ofmaterial utilization and the waste heat recovery.
5.
Optimization of the cycle time, e.g. through the reductionof 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.
r e f e r e n c e s
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