CPF Center for Precision Forming (CPF) 1 Forming of High Strength Steels (HSS & A/UHSS) in the Automotive Industry Dr. Taylan Altan, Professor & Director,

CPFCPF

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

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1. Introduction

2. Material Properties

3. Formability

4. Presses

5. Tribology

6. Springback

7. Summary

Outline

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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.

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Introduction

Ref: Sadagopan 2004

INCREASED STRENGTH DE

CR

EA

SE

D F

OR

MA

BIL

ITY

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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

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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.

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Material Properties

Tensile Test

Ref: Nasser et al 2010

Determination of Flow Stress

0.15

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Material Properties

Ref: Nasser et al 2010


Bulge Test

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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)


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Before bursting After bursting

Bulge test samples


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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


0.49Bulge Test

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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


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New generation AHSS steels (X-IP steel) have higher drawability than conventional mild steels.

Cugy et al 2006


Drawability of AHSS steels

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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]


Loading and Unloading modulus of AHSS steels

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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 plasticstrain

Apparent Modulus Variation

Ref: Kardes et al 2010

Material Properties

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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

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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

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Center for Precision Forming (CPF) 18Ref: 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

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Formability

Ref: World Steel Association 2009, Yan 2009

Bending

Hig

her

Ben

dabi

lity

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.

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Formability

Ref: Shi and Chen 2007

Stretch Bending

DP780Underbody structural part

DP980 B-pillar inner

Challenge:This type of fracture cannot be predicted using conventional Forming Limit Curve (FLC).

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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.

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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

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Formability

Ref: Palaniswamy and Altan 2006

Deep DrawingDeep 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

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Formability

Ref: Sadagopan 2004, Sung et al 2007

Flanging / Edge Stretching

Hole Expansion Test

Cracked Sample

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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

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Presses

Ref: Keeler and Ulnitz 2009

Required Load and Energy

Challenge:Due to higher strength, required press load and energy are higher.

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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

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• 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


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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


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Presses

Ref: Miles 2004, Boerger 2008

Reverse Load in Blanking

Challenge:Due to higher strength, blanking load (forwardtonnage) would behigher, resulting in higher reverse load.

Solutions:• Use stepped punches,• Keep the punches in good shape, • Reduce blanking speed,• Use hydraulic dampers.

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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


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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


Parting line of tool steel inserts

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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

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Center for Precision Forming (CPF)

Evaluation of Lubricants Using TheCup 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

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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

Tool

Life

/ N

umbe

r of g

ood

part

s sta

mpe

d

Tool Material and Coatings

DP600

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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

Tool

Life

/ N

umbe

r of g

ood

part

s sta

mpe

d

Tool Material and Coatings

DP980

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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

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Springback

Higher springback

HSLA350/450DP350/600

Ref: World Steel Association 2009

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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

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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

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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

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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,

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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

c. Higher load and energy required,

d. Higher reverse loads are observed in blanking.

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Summary

4. Friction / Lubrication

a. Higher loads are temperatures observed,

b. Lubricants, tool materials, treatments and coatings have to

be selected carefully.

5. Springback

c. Higher springback is observed,

d. Prediction of springback requires more sophisticated

analyses

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Questions / Comments

Contact information:

Taylan Altan, Professor and DirectorCenter for Precision Forming (CPF)

www.cpforming.org / www.ercnsm.orgThe Ohio State University, Columbus, OH

Email: [email protected], Ph: (614) 292 5063

Documents

CPF Center for Precision Forming (CPF) 1 Forming of High Strength Steels (HSS & A/UHSS) in the Automotive Industry Dr. Taylan Altan, Professor & Director,