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FORMABILITY CHARACTERIZATION OF ADVANCED HIGH-STRENGTH STEELS

Sriram SadagopanIspat Inland Inc.

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AISI/DOEFunding

AISI/AAC member companiesSteels

Ispat Inland technician staffExperimentation

ACKNOWLEDGEMENTS

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ACKNOWLEDGMENTS

Ispat Inland Team• Bernard Levy• Sriram Sadagopan• Mai Huang• Benda Yan

AISI Steering Committee• Ron Soldaat – Dofasco• Wei Wang – SeverStal NA• Alex Konieczny – US Steel

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OUTLINE

Introduction

ResultsSheared edge stretching limitsBending under tension limitsSpringback and Curl

Conclusions

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INTRODUCTION

Formability

Part/Die geometry

P

μP

Friction

0

20

40

60

80

100

120

-40 -30 -20 -10 0 10 20 30 40

Minor strain (%)

Maj

or s

trai

n (%

)

Material

0

20

40

60

80

100

120

-40 -30 -20 -10 0 10 20 30 40

Minor strain (%)

Maj

or s

trai

n (%

)

Process

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1600Tensile Strength (MPa)

0 600 1200300 900

Elo

ngat

ion

(%)

0

10

20

30

40

50

60

70 Low StrengthSteels (<270MPa)

Ultra High StrengthSteels (>700MPa)

High StrengthSteels

DP, CP

TRIPAHSS

MART

HSLA

IF

MildIF-HS

BHCMn

ISO

Conventional HSS

INTRODUCTION

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PROJECT SCOPE

Mechanicalbehavior

Splittinglimits

Simulativetests

Formability Characterizationof AHSS

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Mechanicalbehavior

•ASTM tensile testing•Instantaneous work hardening•Medium strain rate mechanical behavior•Bauschinger effect

Splittinglimits

•Forming limit curves•Sheared edge stretching limits•Bending under tension limits

Simulativetests

•Stretch Formability•Stretch Drawability•Springback & Curl

PROJECT SCOPE

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STEELS EVALUATED

Baseline SteelsDQSK, DDQ+

Exposed Panel SteelsBH210, BH280, IF-Rephos, GA340BH, DP500

Structural SteelsBH300, HSLA350, HS440W, DP600, TRIP600, DP800, DP980, RA830, M190

Multiple lots for each steel grade

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RESULTS

Sheared Edge Stretching limitsHole extrusion test

Bending Under Tension limitsAngular stretch bend test

Springback & CurlU Channel Draw test

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SHEARED EDGE STRETCHING LIMITS

ApplicationFinishing operations

• Curved flanging

Test MethodHole extrusion test%HE (λ) before thickness crack

d0 df

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SHEARED EDGE STRETCHING LIMITS

Ultimate Tensile Strength (MPa)

0

20

40

60

80

100

120

140

160

180

0 200 400 600 800 1000 1200 1400 1600

% H

EBHIFDPDQSKHS440WHSLA350IF-RephosM190RA830TRIP600GA340BH

DP980

HSS

DP,TRIP

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STRETCH FLANGING

( )LRLRRHE

−−−

=0

00 )(HE

HERL

+=

10

900

R0

L

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STRETCH FLANGING

•Need to reduce stretch flange length for AHSS

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 200 400 600 800 1000 1200 1400 1600

Ultimate Tensile Strength (MPa)

L/R 0

DP980

Fail

Safe

HSS

DP, TRIP

BHIFDPDQSKHS440WHSLA350IF-RephosM190RA830TRIP600GA340BH

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BENDING UNDER TENSION LIMITS

ApplicationTight tool radius+stretching

Relationship between max. stretching and tool radius

Test MethodAngular Stretch Bend test

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ASB TEST: TOOLING

IncreasingR/tIncreasingR/t

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0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 200 400 600 800 1000 1200

UTS (MPa)

Max

imum

true

saf

e si

dew

all t

hinn

ing

stra

in

R/t = 5

R/t=3

R/t=1

max. thinning strain in sidewall

ASB TEST: STRETCH BENDABILITY

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0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 200 400 600 800 1000 1200

UTS (MPa)

Max

imum

true

saf

e si

dew

all t

hinn

ing

stra

in

R/t = 5

R/t=3

R/t=1Fixed geometry lines

max. thinning strain in sidewall

ASB TEST: STRETCH BENDABILITY

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SPRINGBACK

Curl

Springback

DP600HSLA350

IF

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SPRINGBACK: CHANNEL DRAW TEST

Drawbead

Punch Die block

Binder

SampleTool gap

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SPRINGBACK: DOE

Process Variables (factors)Tooling radiiTool gapDrawbead penetration

Two levels for each Process Variable

Tooling Radii Tool Gap Bead Penetration Small 3.0 or 6.0 mm (R/t <5) 1t+0.05mm Quarter Large 10.0 mm (R/t>5) 1.5t Maximum

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SPRINGBACK: STATISTICAL ANALYSIS

Tooling radii, tool gap, and drawbead penetration are the most influential factors

Interactions of these factors are not significant

Drawbead penetration and tooling radii are more important than tool gap

Penetration > Tool Radius > Tool Gap

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SPRINGBACK: EFFECT OFDRAWBEAD PENETRATION

0

4

8

12

16

20

24

28

32

125 375 625 875 1125Yield Strength (MPa)

Sprin

gbac

k A

ngle

* t

HSS_large HSS_smallDP_large DP_smallTRIP_large TRIP_smallSkin Panel_large Skin Panel_smallBaseline_large Baseline_small

Large and Small Bead Penetration

DP980

DP600DP800

HSSHSLA TRIP600

0

4

8

12

16

20

24

28

32

125 375 625 875 1125Yield Strength (MPa)

Sprin

gbac

k A

ngle

* t

HSS_large HSS_smallDP_large DP_smallTRIP_large TRIP_smallSkin Panel_large Skin Panel_smallBaseline_large Baseline_small

Large and Small Bead Penetration

DP980

DP600DP800

HSSHSLA TRIP600

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SPRINGBACK - DP600 VS. HSLA350

Tool gap

3

4

5

6

7

8

9

t+0.05mm 1.1t 1.5tTool Gap

Sprin

gbac

k (d

egre

e)

Tool Radii

3

4

5

6

7

8

9

3.0mm 6.0mm 10.0mmPunch Radius

Sprin

gbac

k (d

egre

e)

DP600

HSLA350

DP600

HSLA350

3

4

5

6

7

8

9

Quarter Half FullDrawbead Penetration

Sprin

gbac

k (d

egre

e)

Drawbead penetration

DP600

HSLA350

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CONCLUSIONS

Stretch FlangingGuidelines

• Reduce flange length for AHSS• Gainers in addendum

Bending Under TensionGuideline

• Reduce sidewall stretching strain for tight bending radii

Springback of DP600 > HSLA350 Guideline

• 1~20 difference