1. Forming Advanced High Strength Steels
Jay Weiner, B.S., M.Sc.(Eng)
BiggerBoat Solutions Ltd.
FEA, CAE/CAD & Die Engineering
April 28, 2011

2. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
HSS Application Areas
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3. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
Reasons For Using AHSS
Advanced High Strength Steels (AHSS) have been developed to meet new demands in the automotive industry.
Satisfy and Surpass Safety Standards
Better Crash Energy Absorption
Fatigue Resistance
Cost Savings
Reduced Weight
Better Fuel Economy
Reduced CO2 Emissions
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4. Families of High Strength Steels
High Strength Steel (HSS)
Yield Strength = 210MPa MPa
High Strength Low Alloy (HSLA)
Interstitial Free
Bake Hardenable
Advance High Strength Steel (AHSS)
Yield Strength => Overlap and Exceed HSS
Dual Phase (DP) & Complex Phase (CP)
Transformation Induced Plasticity (TRIP)
Martensitic (Mart)
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5. Dual Phase (DP) Properties and Microstructure
2 Phases – soft ferrite with islands of hard martensite
Higher percentage martensite => higher strength
Ferrite strains causing a high work hardening rate
n-value increases rapidly at low strain rates => drops to HSLA levels
Higher initial n-value restricts the onset of strain localization & large strain gradients
Reducing strain gradients reduces localized thinning
YS to TS ratio of about 0.6
Lower YS at a given TS translates to better elongation and formability
No Yield Point Elongation (increase in strain without increase in stress)
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6. Transformation Induced Plasticity (TRIP) Properties and Microstructure
2 Phases – soft ferrite with islands of martensite, retained austenite
Ferrite strains causing a high work hardening rate
As strain increases retained austenite transformed in to martensite
Volume & shape change in microstructure
Strain is accommodated and ductility is increased
Work hardening rate increases as strain increases
Slower initial n-value increase compared to Dual Phase
Increase of n-value continues at higher levels of strain
Increasing n-value with increasing strain restricts the onset of strain localization & large strain gradients
Reducing strain gradients reduces localized thinning
Significant stretch forming properties
YS to TS ratio of about 0.6
Lower YS at a given TS translates to better elongation and formability
No Yield Point Elongation (increase in strain without increase in stress)
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7. Martensitic (MS) Properties and Microstructure
Martinsite matrix with small amounts of ferrite and/or bainite
Minimum Tensile Strengths of 900MPa to 1700 MPa
Limited elongation
Used for simple cross sections
More complex shapes can be created by hot forming
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AHSS
MS / HSLA
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DP600
TRIP
MARTENSITIC
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AHSS – FEA Modeling
Simulation and Material Data
Mild and HSLA steels have well accepted material models
Use “Power Law” to create stress-strain curve for simulation software
Use measured values of YS, TS, n
Theoretical curves closely match empirical curves
AHSS steels can’t be force-fit to these predictive models
Variable n-value (work hardening exponent)
Variable Modulus of Elasticity
Plastic strain ratios r0, r45, r90 are not equal
Modeling of AHSS requires complete data
Stress - strain curve based on test data
Plastic strain ratios r0, r45, r90 directions from test data
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Green = Test Curve Data
Red = Power law
HSLA 440
DP 600
DP 980
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12. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
AHSS – Springback
Caused by the elastic recovery of the part
AHSS does not obey common formability rules
Material curls easily and part twist is common
Springback for AHSS is > HSLA steels of same Yield Strength
Springback for AHSS is £ HSLA steels of same Tensile Strength
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Types of Springback
3 common modes of springback
Wall angular change
Caused by stress difference in the sheet thickness direction when a sheet metal bends
Residual stresses create a bending moment
Sidewall curl
Uneven stress distribution or stress gradient through the thickness of the sheet metal
Generated during the bending and unbending process.
Part twist
Unbalanced residual stresses cause torsion moments in the cross-section
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14. Springback Correction
Most common method of correction is compensation by overbending
Works for AHSS materials as well as HSLA
Amount of compensation will generally be approximately 5% - 15% > for HSLA
Form as much finished shape as possible as early as possible
Immediately compensate for any springback modes encountered
Correct for springback in each station as you go
DO NOT rely on restrike to correct shape problems
Work hardened part will be very difficult to re-form
Use restrike to add darts, stiffening beads, etc.
Consider COUNTERMEASURES in addition to compensation
Replace draw operations with bending forms when possible
Keep a die clearance at approximately 1.3t to minimize sidewall curl
Sidewall curl reaches the maximum at 1t clearance
AHSS requires some counterintuitive thinking to control material flow
Stretch flanges to change stress gradients
Tensile/compressive elastic stress gradients become all tensile elastic stress gradients
Will minimize sidewall curl
Die radii should be ³ 5t (for DP600)
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15. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
Springback Case Study
Bracket formed in 3 forming stages
Crash form
Draw
Wipe
Evaluated Springback after 1st and 3rd stage
Evaluated 3 different materials including:
DP600,
HSLA340
HSLA550
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Springback Case Study: Material Curves
DP600
YS = 410MPa
TS = 685 MPa
n = 0.13
r = 1.02
HSLA340
YS = 403 MPa
TS = 480 Mpa
n = 0.12
r = 1.0
HSLA550
YS = 587 Mpa
TS = 697 MPa
n = 0.09
r = 0.83
DP600
HSLA340
HSLA550
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1st Form – Crash
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2nd Form – Draw
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3rd Form – Wipe
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20. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
1st Form – Springback
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21. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
Springback after 1st Form - Material Comparison
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22. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
DP600 vs. HSLA340 vs. HSLA550
Springback (X,Z) after 3rd form
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DP600
Springback (X,Z) after 1st form (no compensation)
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1st Form – Compensation
springback results used to develop a compensated shape
tool shape is modified to help remove the twist from the part.
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25. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
DP600
Deviation From Nominal - after 1st form with compensation
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26. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
3rd Form – Springback
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27. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
Springback after 3rd Form Springback Comparison
Amounts of springback for each is very simliar.
Proper counter measures => springback of Dual Phase can be controlled as easily as HSLA
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28. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
Springback after 3rd Form: DP600 with 1st Form Compensation
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29. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
DP600
Springback (X,Z) after 3rd form
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30. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering
Thank You.
BiggerBoat Solutions Ltd.
FEA, CAE/CAD & Die Engineering
Jay Weiner B.S. M.Sc.(Eng)