CPF
Forming of High Strength Steels
(HSS & A/UHSS) in the Automotive Industry
Dr. Taylan Altan, Professor & Director,
Eren Billur, Graduate Research Associate,
Center for Precision Forming (CPF) and ERC/NSM
The Ohio State University, Columbus, OH
www.cpforming.org / www.ercnsm.org
- Prepared for -
AIDA-America, Dayton, OH
June 13-14, 2012
Center for Precision Forming (CPF)
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Outline
CPF
1. Introduction
2. Material Properties
3. Formability
4. Presses
5. Tribology
6. Springback
7. Summary
Center for Precision Forming (CPF)
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Background
CPF
Potential advantages of HSS
 Weight savings in auto bodies, 15% to 25%
 Increase in crash resistance and safety.
[ “Structural Materials in Automotive Industries: Applications
and Challenges”, GM R&D Center]
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Introduction
CPF
INCREASED STRENGTH
DECREASED FORMABILITY
Ref: Sadagopan 2004
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Material properties of HSS/AHSS/UHSS
CPF
Sheet properties (flow stress) determination
 In common practice, the uniaxial tensile test is used to determine the properties/flow
stress of sheet metal.
 Tensile test does not emulate biaxial deformation conditions observed in stamping.
 Due to early necking in tensile test, stress/strain data (flow stress) is available for
small strains.
Necking begins
Engineering Stress-Strain Curve
True Stress-Strain Curve = Flow stress
In bulge test, flow stress over large strain can be obtained in biaxial stress state
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CPF
Material Properties
Flow Stress
n-value, as defined in
Hollomon’s Equation:
  k n
is not constant.
Challenges:
1) Predicting uniform
elongation,
2) Input of flow stress
into FEA codes.
Ref: World Steel Association, 2009.
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CPF
Material Properties
Determination of Flow Stress
Tensile Test
0.15
Ref: Nasser et al 2010
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Material Properties
CPF
Determination of Flow Stress
Bulge Test
Ref: Nasser et al 2010
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Material properties of HSS/AHSS/UHSS
CPF
Schematic of viscous pressure bulge test setup at CPF (OSU)
Clamping force
• Die diameter = 4
inches (~ 100 mm)
Bulge/
Dome height (h)
• Die corner radius =
0.25 inch (~ 6 mm)
Pressurized
medium
Initial Stage
Testing stage
Pressure (P)
Methodology to estimate material properties
from VPB test, developed at CPF (OSU)
Measurement
• Pressure (P)
FEM based
inverse technique
• Dome height (h)
Material properties
• Flow stress
• Anisotropy
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Material properties of HSS/AHSS/UHSS
CPF
Bulge test samples
Before bursting
After bursting
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CPF
Material Properties
Determination of Flow Stress
Bulge Test
0.49
Challenges:
1) Tensile test gives a very limited information,
2) Bulge test gives more reliable strain-stress data.
Ref: Nasser et al 2010
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Material properties of HSS/AHSS/UHSS
CPF
Bulge test for quality control of incoming sheet material
Graph shows dome height comparison for SS 409 sheet material from eight
different batches/coils [5 samples per batch].
Highest formability  G , Most consistent  F
Lower formability and inconsistent  H
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Material properties of HSS/AHSS/UHSS
CPF
Drawability of AHSS steels
Cugy et al 2006
New generation AHSS steels (X-IP steel) have higher drawability than conventional
mild steels.
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Material properties of HSS/AHSS/UHSS
CPF
Loading and Unloading modulus of AHSS steels
[ULSAB-AVC Report/AISI Training Session
document, 2002]
[Pervez et al 2005]
 Springback (elastic recovery) of the formed part is proportional to stress.
 Decrease in Young’s modulus with strain in AHSS steel results in higher springback.
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CPF
Material Properties
Apparent Modulus Variation
Material - DP780
220
Challenge:
Apparent Modulus
changes with plastic
strain
215
210
Apparent Modulus (GPa)
205
200
195
190
185
Unloading
Loading
180
175
170
165
160
155
150
145
140
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11
0.12
True Strain (mm/mm)
Ref: Kardes et al 2010
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Material Properties
CPF
Inconsistency of Material Properties
AHSS are performance based grades.
TRIP 800
Challenges:
1) Strength, elongation, weldability may vary,
2) Material properties are inconsistent from supplier to
supplier, even batch to batch.
Ref: Choi et al 2009.
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CPF
Formability
Local Failures
Significant
Stretching
Moderate Stretching and
Bending
High Hole Expanding and
Bending
Challenges:
1) Local failures do not correlate with n-value, R-value or
elongation,
2) Materials has to be tested under various stress states.
Ref: Sung et al 2007; Dykeman et al 2009.
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CPF
Formability
Higher Stretchability
Stretching
(a)
DC06
DP600
DP800
(b)
DP1000
DP1200
DP1400
(c)
Challenges:
1) Stretchability decreases with strength {(a) and (b)},
2) Inconsistency is present in stretching (c).
Ref: SSAB and Uddeholm 2008, Keeler and Ulnitz 2009, Dykeman et al 2009
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Formability
CPF
Bending
Elongation in bending does not
correlate to elongation in tension
test:
DP980 failed at 14% elongation
in tensile, 40% elongation in
bending.
Challenges:
1) Bendability decreases with strength,
2) Failure at bending cannot be predicted by tensile data.
Ref: World Steel Association 2009, Yan 2009
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Formability
CPF
Stretch Bending
Challenge:
This type of fracture cannot be predicted using
conventional Forming Limit Curve (FLC).
DP780
Underbody structural part
Ref: Shi and Chen 2007
DP980
B-pillar inner
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Formability
CPF
Stretch Bendability
A suggested test method:
Angular Stretch Bending (ASB)
Achievable heights of several
steels: as strength increases,
stretch bendability decreases.
Ref: Sadagopan and Urban 2003, Wu et al 2006
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CPF
Formability
Deep Drawing
Challenges:
1) Higher strength, results with
less deep drawability.
2) Sidewall curls and local
fractures are observed
DC06
DP600
DP800 DP1000
(a)
(b)
DC06
DP600
DP800 DP1000 DP1200 DP1400
Ref: SSAB and Uddeholm 2008, World Steel Association 2009
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CPF
Formability
Deep Drawing
One solution to this problem is:
Optimizing blankholder pressure, including multi-point
cushion systems.
Al 6111-T4, t=1 mm
BH210, t=0.8mm
DP500, t=0.8mm
Ref: Palaniswamy and Altan 2006
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Formability
CPF
Flanging / Edge Stretching
Hole Expansion Test
Cracked Sample
Ref: Sadagopan 2004, Sung et al 2007 Center for Precision Forming (CPF)
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CPF
Formability
Flanging / Edge Stretching
Worn Tool
Sharp Tool
Effect of hole blanking
Challenges:
1) Edge cracks cannot be predicted by FLC and are related to
sheared edge quality,
2) Higher strength reduces the hole expansion ratio (HER),
3) HER gets even worse with worn tools
Ref: SSAB and Uddeholm 2008
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Presses
CPF
Required Load and Energy
Challenge:
Due to higher strength, required press load and energy
are higher.
Ref: Keeler and Ulnitz 2009
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Press and tooling for forming HSS/AHSS/UHSS CPF
Press slide force and energy requirements
IISI, 2006
IISI, 2006
Presses with higher force and energy capacity required for forming AHSS steels due to
its higher strength and higher strain hardening compared to mild steels
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Press and tooling for forming HSS/AHSS/UHSS CPF
Blank holder force requirements
Noel et al , 2005
• Higher blank holding force required due to its higher strength and relatively thin gage
used compared to conventional steel to form the part.
• Hydraulic cylinders / Nitrogen gas springs built in the die to provide higher blank holder
force required to form AHSS steels.
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Press and tooling for forming HSS/AHSS/UHSS CPF
Modification in transfer press for forming AHSS steel
Haller , 2006
•
Higher load in forming AHSS steels results in large tilting of transfer press slide. 
reduction in part accuracy and press life.
•
Double slide transfer press with independent slide for lead press /drawing stage is
preferred option.
•
Double action hydraulic press with cushion in press bed preferred for lead press 
flexibility in choosing slide depending on die size.
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Presses
CPF
Reverse Load in Blanking
Challenge:
Due to higher strength,
blanking load (forward
tonnage) would be
higher, resulting in
higher reverse load.
Solutions:
• Use stepped punches,
• Keep the punches in good shape,
• Reduce blanking speed,
• Use hydraulic dampers.
Ref: Miles 2004, Boerger 2008
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Press and tooling for forming HSS/AHSS/UHSS CPF
Modification in blanking press for AHSS steel
Linkage drive kinematics for
blanking press
Blanking force
Esher et al , 2004
Haller , 2006
•
Higher snap-through force in blanking AHSS steels  Detrimental to press life
•
Blanking press with linkage drive are introduced to reduce the velocity close to BDC
to reduce snap-through forces.
•
Soft-shock – add on to the blanking press to reduce the impact force on the press
and increase press life.
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Press and tooling for forming HSS/AHSS/UHSS CPF
Tooling for forming AHSS steel
Parting line of tool
steel inserts
Esher et al , 2004
Haller , 2006
•
Conventional monoblock design from cast iron material not preferred for AHSS
forming.
•
Cast iron tool with tool steel inserts are used for improved strength and wear
resistance.
•
Cooling channels incorporated in dies to release heat quickly and increase stroking
rate.
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Lubrication and Friction
CPF
Contact area
with die
Challenges:
1) Higher contact pressure and higher temperature are detrimental for
lubricants,
2) Temperature and pressure additives are needed
Ref: Kim et al 2009
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Evaluation of Lubricants Using The
Cup Drawing Test (CDT)
CPF
(in cooperation with HONDA and several lubricant companies)
Performance evaluation criteria (cups drawn to same depth):
i. Higher the Blank Holder Force (BHF) that can be applied without fracture
in the drawn cup, better the lubrication condition
ii. Smaller the flange perimeter, better the lubrication condition (lower
coefficient of friction)
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Center for Precision Forming (CPF)
Tool Life / Number of good parts stamped
Tool Materials, Treatments, Coatings
50,000
CPF
DP600
40,000
30,000
20,000
10,000
0
Vancron Calmax + Sleipner Weartec Vanadis Sleipner
40
Nitr. +
+ Nitr.
6
PVD CrN
D2
Tool Material and Coatings
Ref: Liljengren et al 2008
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Tool Life / Number of good parts stamped
Tool Materials, Treatments, Coatings
60,000
CPF
DP980
50,000
40,000
30,000
20,000
10,000
0
AISI D2 Carmo AISI D2 Vanadis AISI D2 AISI M2 AISI M4 AISI M4 AISI M2 AISI M4
+ CVD + Nitr +
4E
+ PVD
+ CVD + PVD + Hard
TiC
PVD
AlTiN
TiC
CrN
Cr
CrN
Tool Material and Coatings
Ref: Young et al 2009
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Product development using HSS/AHSS/UHSS CPF
Failure prediction in forming AHSS steel
Stoughton et al 2006
FLC based failure prediction not accurate – Need a better and reliable
failure prediction criteria for die engineering and analysis
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CPF
Springback
Higher springback
DP350/600
Ref: World Steel Association 2009Center for Precision Forming (CPF)
HSLA350/450
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Springback
CPF
Higher springback
Springback compensation:
1) Over forming,
2) Locally deforming / bottoming,
3) Stretching by higher forces.
Modeling of springback is a challenge:
1) Flow stress equations do not fit,
2) Unloading modulus may vary,
3) More Bauschinger effect is observed.
Ref: Sung et al 2007
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Studies on forming of HSS/AHSS/UHSS
CPF
 Studies are conducted by:
 International Iron & Steel Institute (IISI) including programs such as ULSAB &
ULSAC [www.worldautosteel.org]
 Auto-Steel Partnership (A-SP) [www.a-sp.org]
 American Iron and Steel Institute (AISI) [www.autosteel.org]
 All major steel companies, [Mittal/Usinor, U.S. Steel, ThyssenKrupp, Nippon
Steel, POSCO, etc]
 Analysis of springback in forming of a AHSS is conducted by CPF in
cooperation with its member companies and universities in Germany and
Sweden.
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Summary
CPF
 Use of AHSS will continue to increase in the automotive industry.
 Low formability, high springback & high forces are primary concerns in forming AHSS.
 Yield stress (flow stress), n-value & Young’s modulus change with deformation
(strain).
 Non uniformity in incoming material a concern in forming high strength steels 
robust process design needed.
 Bulge test , a better test to estimate the flow stress of AHSS sheet materials over
large strain
 Higher forming forces requires increased attention to tool specifications (Tool material,
Heat treatment) & selection of die surface coatings. Die & process design requires
more engineering.
 In stamping of HSS, the requirements on stamping presses increase (higher forming
forces, better controls, increased stiffness & off center loading capacity).
 Prediction of potential failure locations and springback in die engineering and analysis
not reliable  Need more investigation on the AHSS material behavior in different
strain paths.
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Summary
CPF
1. Material Properties
a. Flow stress equations cannot be expressed in simple form
(σ=kεn),
b. Flow stress data determined with tensile test is very limited
(~0.1-0.2 true strain),
c. Unloading modulus may vary with plastic strain,
d. Material properties are not consistent,
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Summary
CPF
2. Formability
a. Local failures are common and these do not correlate to nvalue, R-value or elongation,
b. Various tests (hole expansion, stretch bending, etc.) are
required.
3. Presses
a. Higher load and energy required,
b. Higher reverse loads are observed in blanking.
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Summary
CPF
4. Friction / Lubrication
a. Higher loads are temperatures observed,
b. Lubricants, tool materials, treatments and coatings have to
be selected carefully.
5. Springback
a. Higher springback is observed,
b. Prediction of springback requires more sophisticated
analyses
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Questions / Comments
CPF
Contact information:
Taylan Altan, Professor and Director
Center for Precision Forming (CPF)
www.cpforming.org / www.ercnsm.org
The Ohio State University, Columbus, OH
Email: altan.1@osu.edu, Ph: (614) 292 5063
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