Forming Advanced High Strength Steels Jay Weiner, B.S., M.Sc.(Eng) BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com April 28, 2011

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering HSS Application Areas BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

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 BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

Families of High Strength Steels High Strength Steel (HSS) Yield Strength = 210MPa - 550 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) BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

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) BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

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) BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

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 BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering AHSS MS / HSLA BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering DP600 TRIP MARTENSITIC www.arcelormittal.com BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 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 BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering Green = Test Curve Data Red = Power law HSLA 440 DP 600 DP 980 BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

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 BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 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 BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

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) BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

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 BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 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 BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 1st Form – Crash BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 2nd Form – Draw BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 3rd Form – Wipe BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 1st Form – Springback BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering Springback after 1st Form - Material Comparison BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering DP600 vs. HSLA340 vs. HSLA550 Springback (X,Z) after 3rd form BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering DP600 Springback (X,Z) after 1st form (no compensation) BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 1st Form – Compensation springback results used to develop a compensated shape tool shape is modified to help remove the twist from the part. BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering DP600 Deviation From Nominal - after 1st form with compensation BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 3rd Form – Springback BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

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 BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering Springback after 3rd Form: DP600 with 1st Form Compensation BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering DP600 Springback (X,Z) after 3rd form BiggerBoat Solutions Ltd. FEA, CAE/CAD & Die Engineering 905-707-5860 jayw@biggerboatsolutions.com

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) 905-707-5860 jayw@biggerboatsolutions.com