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CPF
Center for Precision Forming (CPF) 1
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
CPF
Center for Precision Forming (CPF) 2
1. Introduction
2. Material Properties
3. Formability
4. Presses
5. Tribology
6. Springback
7. Summary
Outline
CPF
Center for Precision Forming (CPF) 3
Background
[ “Structural Materials in Automotive Industries: Applications
and Challenges”, GM R&D Center]
Potential advantages of HSS
Weight savings in auto bodies, 15% to 25%
Increase in crash resistance and safety.
CPF
Center for Precision Forming (CPF) 4
Introduction
Ref: Sadagopan 2004
INCREASED STRENGTH DE
CR
EA
SE
D F
OR
MA
BIL
ITY
CPF
Center for Precision Forming (CPF) 5
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.
Sheet properties (flow stress) determination
Necking begins
Engineering Stress-Strain Curve True Stress-Strain Curve = Flow stress
Material properties of HSS/AHSS/UHSS
In bulge test, flow stress over large strain can be obtained in biaxial stress state
CPF
Center for Precision Forming (CPF) 6
Material Properties
Flow Stress
nk
Ref: World Steel Association, 2009.
Challenges:
1) Predicting uniform
elongation,
2) Input of flow stress
into FEA codes.
n-value, as defined in
Hollomon’s Equation:
is not constant.
CPF
Center for Precision Forming (CPF) 7
Material Properties
Tensile Test
Ref: Nasser et al 2010
Determination of Flow Stress
0.15
CPF
Center for Precision Forming (CPF) 8
Material Properties
Ref: Nasser et al 2010
Determination of Flow Stress
Bulge Test
CPF
Center for Precision Forming (CPF) 9
Bulge/
Dome height (h)
Initial Stage Testing stage
• Die diameter = 4
inches (~ 100 mm)
• Die corner radius =
0.25 inch (~ 6 mm)
Clamping force
Pressurized
medium
Measurement
• Pressure (P)
• Dome height (h)
FEM based
inverse technique
Material properties
• Flow stress
• Anisotropy
Methodology to estimate material properties from VPB test, developed at CPF (OSU)
Pressure (P)
Schematic of viscous pressure bulge test setup at CPF (OSU)
Material properties of HSS/AHSS/UHSS
CPF
Center for Precision Forming (CPF) 10
Before bursting After bursting
Bulge test samples
Material properties of HSS/AHSS/UHSS
CPF
Center for Precision Forming (CPF) 11
Material Properties
Challenges:
1) Tensile test gives a very limited information,
2) Bulge test gives more reliable strain-stress data. Ref: Nasser et al 2010
Determination of Flow Stress
0.49
Bulge Test
CPF
Center for Precision Forming (CPF) 12
Highest formability G , Most consistent F
Lower formability and inconsistent H
Graph shows dome height comparison for SS 409 sheet material from eight
different batches/coils [5 samples per batch].
Bulge test for quality control of incoming sheet material
Material properties of HSS/AHSS/UHSS
CPF
Center for Precision Forming (CPF) 13
New generation AHSS steels (X-IP steel) have higher drawability than conventional
mild steels.
Cugy et al 2006
Material properties of HSS/AHSS/UHSS
Drawability of AHSS steels
CPF
Center for Precision Forming (CPF) 14
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.
[ULSAB-AVC Report/AISI Training Session
document, 2002]
[Pervez et al 2005]
Material properties of HSS/AHSS/UHSS
Loading and Unloading modulus of AHSS steels
CPF
Center for Precision Forming (CPF) 15
140
145
150
155
160
165
170
175
180
185
190
195
200
205
210
215
220
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
Ap
pa
ren
t M
od
ulu
s (
GP
a)
True Strain (mm/mm)
Material - DP780
Unloading
Loading
Challenge:
Apparent Modulus
changes with plastic
strain
Apparent Modulus Variation
Ref: Kardes et al 2010
Material Properties
CPF
Center for Precision Forming (CPF) 16
TRIP 800
Ref: Choi et al 2009.
Inconsistency of Material Properties
AHSS are performance based grades.
Challenges:
1) Strength, elongation, weldability may vary,
2) Material properties are inconsistent from supplier to
supplier, even batch to batch.
Material Properties
CPF
Center for Precision Forming (CPF) 17
Formability
Ref: Sung et al 2007; Dykeman et al 2009.
Local Failures
Challenges:
1) Local failures do not correlate with n-value, R-value or
elongation,
2) Materials has to be tested under various stress states.
Significant
Stretching
Moderate Stretching and
Bending
High Hole Expanding and
Bending
CPF
Center for Precision Forming (CPF) 18 Ref: SSAB and Uddeholm 2008, Keeler and Ulnitz 2009, Dykeman et al 2009
Stretching DC06 DP600 DP800 DP1000 DP1200 DP1400
(b) (c)
Hig
her
Str
etch
abil
ity
(a)
Challenges:
1) Stretchability decreases with strength {(a) and (b)},
2) Inconsistency is present in stretching (c).
Formability
CPF
Center for Precision Forming (CPF) 19
Formability
Ref: World Steel Association 2009, Yan 2009
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.
CPF
Center for Precision Forming (CPF) 20
Formability
Ref: Shi and Chen 2007
Stretch Bending
DP780
Underbody structural part
DP980
B-pillar inner
Challenge:
This type of fracture cannot be predicted using
conventional Forming Limit Curve (FLC).
CPF
Center for Precision Forming (CPF) 21
Formability
Ref: Sadagopan and Urban 2003, Wu et al 2006
Stretch Bendability
A suggested test method:
Angular Stretch Bending (ASB) Achievable heights of several
steels: as strength increases,
stretch bendability decreases.
CPF
Center for Precision Forming (CPF) 22
Formability
DC06 DP600 DP800 DP1000 DP1200 DP1400
(a) (b)
Ref: SSAB and Uddeholm 2008, World Steel Association 2009
Deep Drawing
DC06 DP600 DP800 DP1000 DP1200 DP1400
(a) (b)
Challenges:
1) Higher strength, results with
less deep drawability.
2) Sidewall curls and local
fractures are observed
CPF
Center for Precision Forming (CPF) 23
Formability
Ref: Palaniswamy and Altan 2006
Deep Drawing 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
CPF
Center for Precision Forming (CPF) 24
Formability
Ref: Sadagopan 2004, Sung et al 2007
Flanging / Edge Stretching
Hole Expansion Test
Cracked Sample
CPF
Center for Precision Forming (CPF) 25
Formability
Ref: SSAB and Uddeholm 2008
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
Flanging / Edge Stretching
Effect of hole blanking
Worn Tool
Sharp Tool
CPF
Center for Precision Forming (CPF) 26
Presses
Ref: Keeler and Ulnitz 2009
Required Load and Energy
Challenge:
Due to higher strength, required press load and energy
are higher.
CPF
Center for Precision Forming (CPF) 27
Press and tooling for forming HSS/AHSS/UHSS
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
Press slide force and energy requirements
IISI, 2006 IISI, 2006
CPF
Center for Precision Forming (CPF) 28
• 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.
Blank holder force requirements
Noel et al , 2005
Press and tooling for forming HSS/AHSS/UHSS
CPF
Center for Precision Forming (CPF) 29
Modification in transfer press for forming AHSS steel
• 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.
Haller , 2006
Press and tooling for forming HSS/AHSS/UHSS
CPF
Center for Precision Forming (CPF) 30
Presses
Ref: Miles 2004, Boerger 2008
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.
CPF
Center for Precision Forming (CPF) 31
Modification in blanking press for AHSS steel
• 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.
Haller , 2006
Blanking force
Linkage drive kinematics for
blanking press
Esher et al , 2004
Press and tooling for forming HSS/AHSS/UHSS
CPF
Center for Precision Forming (CPF) 32
Tooling for forming AHSS steel
• 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.
Haller , 2006 Esher et al , 2004
Press and tooling for forming HSS/AHSS/UHSS
Parting line of tool
steel inserts
CPF
Center for Precision Forming (CPF) 33
Lubrication and Friction
Ref: Kim et al 2009
Contact area with die
Challenges:
1) Higher contact pressure and higher temperature are detrimental for
lubricants,
2) Temperature and pressure additives are needed
CPF
Center for Precision Forming (CPF)
Evaluation of Lubricants Using The Cup Drawing Test (CDT)
(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)
34
CPF
Center for Precision Forming (CPF) 35
Tool Materials, Treatments, Coatings
Ref: Liljengren et al 2008
0
10,000
20,000
30,000
40,000
50,000
Vancron 40
Calmax + Nitr. +
PVD CrN
Sleipner + Nitr.
Weartec Vanadis 6
Sleipner D2
Too
l Lif
e /
Nu
mb
er
of g
oo
d p
arts
sta
mp
ed
Tool Material and Coatings
DP600
CPF
Center for Precision Forming (CPF) 36
Tool Materials, Treatments, Coatings
Ref: Young et al 2009
0
10,000
20,000
30,000
40,000
50,000
60,000
AISI D2 + CVD
TiC
Carmo + Nitr +
PVD CrN
AISI D2 Vanadis 4E
AISI D2 + PVD AlTiN
AISI M2 AISI M4 AISI M4 + CVD
TiC
AISI M2 + PVD CrN
AISI M4 + Hard
Cr
Too
l Lif
e /
Nu
mb
er o
f go
od
par
ts s
tam
ped
Tool Material and Coatings
DP980
CPF
Center for Precision Forming (CPF) 37
Product development using HSS/AHSS/UHSS
FLC based failure prediction not accurate – Need a better and reliable
failure prediction criteria for die engineering and analysis
Stoughton et al 2006
Failure prediction in forming AHSS steel
CPF
Center for Precision Forming (CPF) 38
Springback
Higher springback
HSLA350/450DP350/600
Ref: World Steel Association 2009
CPF
Center for Precision Forming (CPF) 39
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
Springback
CPF
Center for Precision Forming (CPF) 40
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.
Studies on forming of HSS/AHSS/UHSS
CPF
Center for Precision Forming (CPF) 41
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.
Summary
CPF
Center for Precision Forming (CPF) 42
Summary
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,
CPF
Center for Precision Forming (CPF) 43
Summary
2. Formability
a. Local failures are common and these do not correlate to n-
value, 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.
CPF
Center for Precision Forming (CPF) 44
Summary
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
Recommended