Forming Advanced High Strength Steels

Forming Advanced High Strength Steels

April 28, 2011April 28, 2011

P4171064b_webinar

BiggerBoat Solutions Ltd.

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Jay Weiner, B.S., M.Sc.(Eng)

Jay Weiner, B.S., M.Sc.(Eng)

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Advanced High Strength Steels (AHSS) have beendeveloped to meet new demands in the automotiveindustry.

Satisfy and Surpass Safety Standards

oBetter Crash Energy Absorption

oFatigue Resistance

Cost Savings

oReduced Weight

oBetter Fuel Economy

oReduced CO2 Emissions

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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)

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prop_table2

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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 & largestrain 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)

Dual Phase (DP) Properties and Microstructure

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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 & largestrain 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)

Transformation Induced Plasticity (TRIP) Properties and Microstructure

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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

Martensitic (MS) Properties and Microstructure

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MS / HSLA

AHSS

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DP600

www.arcelormittal.com

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TRIP

MARTENSITIC

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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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$D:\BBSL\temp\Altair_Webinar\ref_docs\images\HSLA440CURVE_vs_HSLA440PW.jpg$

$D:\BBSL\temp\Altair_Webinar\ref_docs\images\DP600CURVE_vs_DP600PW.jpg$

$D:\BBSL\temp\Altair_Webinar\ref_docs\images\DP980CURVE_vs_DP988PW.jpg$

Green = Test Curve Data

Red = Power law

HSLA 440

DP 600

DP 980

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◦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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◦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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◦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  5t (for DP600)

BiggerBoat Solutions Ltd.

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◦Bracket formed in 3 forming stages

1.Crash form

2.Draw

3.Wipe

◦Evaluated Springback after 1st and 3rd stage

◦Evaluated 3 different materials including:

DP600,

HSLA340

HSLA550

Springback Case Study

Springback Case Study

BiggerBoat Solutions Ltd.

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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

Springback Case Study: Material Curves

Springback Case Study: Material Curves

DP600

HSLA340

HSLA550

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form01_tool_kin

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form02_tool_kin

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form03_tool_kin

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sprbk01b

1st Form – Springback1st Form – Springback

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springback_mat_comparison

Springback after 1st Form - Material Comparison

Springback after 1st Form - Material Comparison

BiggerBoat Solutions Ltd.

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◦Springback (X,Z) after 3rd form

$D:\BBSL\temp\Altair_Webinar\reports\SB_ DP600_HSLA340_HSLA550_after_3rd_form.jpg$

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◦Springback (X,Z) after 1st form (no compensation)

$D:\BBSL\temp\Altair_Webinar\ref_docs\images\SB_ DP600_nocomp_after_1st_form.jpg$

BiggerBoat Solutions Ltd.

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form01_10_and_10a_tool_compensation

1st Form – Compensation1st Form – Compensation

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springback results used to develop a compensated shape

tool shape is modified to help remove the twist from the part.

◦Deviation From Nominal - after 1st form with compensation

$D:\BBSL\temp\Altair_Webinar\ref_docs\images\compensated_springback_after_1st_form.jpg$

BiggerBoat Solutions Ltd.

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sprbk04b

3rd Form – Springback3rd Form – Springback

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Springback after 3rd Form Springback Comparison

Springback after 3rd Form Springback Comparison

3rd_form_sprbk_compare

BiggerBoat Solutions Ltd.

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 Amounts of springback for each is very simliar.

 Proper counter measures => springback of Dual Phase canbe controlled as easily as HSLA

final_sprbak_compensation_comparison

Springback after 3rd Form: DP600 with 1st Form Compensation

Springback after 3rd Form: DP600 with 1st Form Compensation

BiggerBoat Solutions Ltd.

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jayw@biggerboatsolutions.com

◦Springback (X,Z) after 3rd form

$D:\BBSL\temp\Altair_Webinar\ref_docs\images\SB_ DP600_nocomp_vs_comp_after_3rd_form-B.jpg$

BiggerBoat Solutions Ltd.

FEA, CAE/CAD & Die Engineering

905-707-5860

jayw@biggerboatsolutions.com

BiggerBoat Solutions Ltd.

FEA, CAE/CAD & Die Engineering

Jay Weiner B.S. M.Sc.(Eng)

905-707-5860

jayw@biggerboatsolutions.com