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

Outline Introduction Material Properties Formability Presses Tribology Springback Summary

Potential advantages of HSS Background Potential advantages of HSS Weight savings in auto bodies, 15% to 25% Increase in crash resistance and safety. [ “Structural Materials in Automotive Industries: Applications and Challenges”, GM R&D Center]

Introduction INCREASED STRENGTH DECREASED FORMABILITY Ref: Sadagopan 2004

Engineering Stress-Strain Curve True Stress-Strain Curve = Flow stress Material properties of HSS/AHSS/UHSS Sheet properties (flow stress) determination 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. Necking begins Engineering Stress-Strain Curve True Stress-Strain Curve = Flow stress In bulge test, flow stress over large strain can be obtained in biaxial stress state

Material Properties Flow Stress n-value, as defined in Hollomon’s Equation: is not constant. Challenges: Predicting uniform elongation, Input of flow stress into FEA codes. Ref: World Steel Association, 2009.

Material Properties 0.15 Determination of Flow Stress Tensile Test Ref: Nasser et al 2010

Material Properties Determination of Flow Stress Bulge Test Ref: Nasser et al 2010

Material properties of HSS/AHSS/UHSS Schematic of viscous pressure bulge test setup at CPF (OSU) Clamping force Die diameter = 4 inches (~ 100 mm) Die corner radius = 0.25 inch (~ 6 mm) Bulge/ Dome height (h) Pressurized medium Initial Stage Testing stage Pressure (P) Methodology to estimate material properties from VPB test, developed at CPF (OSU) Measurement Material properties FEM based inverse technique Pressure (P) Dome height (h) Flow stress Anisotropy

Material properties of HSS/AHSS/UHSS Bulge test samples Before bursting After bursting

Material Properties Determination of Flow Stress Bulge Test 0.49 Challenges: Tensile test gives a very limited information, Bulge test gives more reliable strain-stress data. Ref: Nasser et al 2010

Material properties of HSS/AHSS/UHSS Bulge test for quality control of incoming sheet material Graph shows dome height comparison for SS 409 sheet material from eight different batches/coils [5 samples per batch]. Highest formability  G , Most consistent  F Lower formability and inconsistent  H

Material properties of HSS/AHSS/UHSS Drawability of AHSS steels Cugy et al 2006 New generation AHSS steels (X-IP steel) have higher drawability than conventional mild steels.

Material properties of HSS/AHSS/UHSS Loading and Unloading modulus of AHSS steels [ULSAB-AVC Report/AISI Training Session document, 2002] [Pervez et al 2005] 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.

Material Properties Apparent Modulus Variation Challenge: changes with plastic strain Ref: Kardes et al 2010

Material Properties Inconsistency of Material Properties AHSS are performance based grades. TRIP 800 Challenges: Strength, elongation, weldability may vary, Material properties are inconsistent from supplier to supplier, even batch to batch. Ref: Choi et al 2009.

Formability Local Failures Challenges: Significant Stretching Moderate Stretching and Bending High Hole Expanding and Bending Challenges: Local failures do not correlate with n-value, R-value or elongation, Materials has to be tested under various stress states. Ref: Sung et al 2007; Dykeman et al 2009.

Formability Stretching (a) (b) (c) Challenges: Higher Stretchability Challenges: Stretchability decreases with strength {(a) and (b)}, Inconsistency is present in stretching (c). Ref: SSAB and Uddeholm 2008, Keeler and Ulnitz 2009, Dykeman et al 2009

Formability Bending Challenges: Bendability decreases with strength, Higher Bendability Elongation in bending does not correlate to elongation in tension test: DP980 failed at 14% elongation in tensile, 40% elongation in bending. Challenges: Bendability decreases with strength, Failure at bending cannot be predicted by tensile data. Ref: World Steel Association 2009, Yan 2009

Formability Stretch Bending Challenge: This type of fracture cannot be predicted using conventional Forming Limit Curve (FLC). DP980 B-pillar inner DP780 Underbody structural part Ref: Shi and Chen 2007

Formability Stretch Bendability A suggested test method: Angular Stretch Bending (ASB) Achievable heights of several steels: as strength increases, stretch bendability decreases. Ref: Sadagopan and Urban 2003, Wu et al 2006

Formability Deep Drawing Challenges: Higher strength, results with less deep drawability. Sidewall curls and local fractures are observed Ref: SSAB and Uddeholm 2008, World Steel Association 2009

Formability Deep Drawing One solution to this problem is: Deep Drawing Optimizing blankholder pressure, including multi-point cushion systems. Ref: Palaniswamy and Altan 2006

Formability Flanging / Edge Stretching Hole Expansion Test Cracked Sample Ref: Sadagopan 2004, Sung et al 2007

Formability Flanging / Edge Stretching Effect of hole blanking Worn Tool Sharp Tool Effect of hole blanking Challenges: Edge cracks cannot be predicted by FLC and are related to sheared edge quality, Higher strength reduces the hole expansion ratio (HER), HER gets even worse with worn tools Ref: SSAB and Uddeholm 2008

Presses Required Load and Energy Challenge: Due to higher strength, required press load and energy are higher. Ref: Keeler and Ulnitz 2009

Press and tooling for forming HSS/AHSS/UHSS Press slide force and energy requirements IISI, 2006 IISI, 2006 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 and tooling for forming HSS/AHSS/UHSS Blank holder force requirements Noel et al , 2005 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.

Press and tooling for forming HSS/AHSS/UHSS Modification in transfer press for forming AHSS steel Haller , 2006 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.

Presses Reverse Load in Blanking Challenge: Due to higher strength, blanking load (forward tonnage) would be higher, resulting in higher reverse load. Solutions: Use stepped punches, Keep the punches in good shape, Reduce blanking speed, Use hydraulic dampers. Ref: Miles 2004, Boerger 2008

Press and tooling for forming HSS/AHSS/UHSS Modification in blanking press for AHSS steel Linkage drive kinematics for blanking press Blanking force Haller , 2006 Esher et al , 2004 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.

Press and tooling for forming HSS/AHSS/UHSS Tooling for forming AHSS steel Parting line of tool steel inserts Haller , 2006 Esher et al , 2004 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.

Lubrication and Friction Challenges: 1) Higher contact pressure and higher temperature are detrimental for lubricants, 2) Temperature and pressure additives are needed Ref: Kim et al 2009

Evaluation of Lubricants Using The Cup Drawing Test (CDT) (in cooperation with HONDA and several lubricant companies) Performance evaluation criteria (cups drawn to same depth): Higher the Blank Holder Force (BHF) that can be applied without fracture in the drawn cup, better the lubrication condition Smaller the flange perimeter, better the lubrication condition (lower coefficient of friction)

Tool Materials, Treatments, Coatings Ref: Liljengren et al 2008

Tool Materials, Treatments, Coatings Ref: Young et al 2009

Product development using HSS/AHSS/UHSS Failure prediction in forming AHSS steel Stoughton et al 2006 FLC based failure prediction not accurate – Need a better and reliable failure prediction criteria for die engineering and analysis

Springback Higher springback Ref: World Steel Association 2009

Springback Higher springback Springback compensation: Over forming, Locally deforming / bottoming, Stretching by higher forces. Modeling of springback is a challenge: Flow stress equations do not fit, Unloading modulus may vary, More Bauschinger effect is observed. Ref: Sung et al 2007

Studies on forming of HSS/AHSS/UHSS 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.

Summary 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 Material Properties Flow stress equations cannot be expressed in simple form (σ=kεn), Flow stress data determined with tensile test is very limited (~0.1-0.2 true strain), Unloading modulus may vary with plastic strain, Material properties are not consistent,

Summary 2. Formability Local failures are common and these do not correlate to n-value, R-value or elongation, Various tests (hole expansion, stretch bending, etc.) are required. 3. Presses Higher load and energy required, Higher reverse loads are observed in blanking.

Summary 4. Friction / Lubrication Higher loads are temperatures observed, Lubricants, tool materials, treatments and coatings have to be selected carefully. 5. Springback Higher springback is observed, Prediction of springback requires more sophisticated analyses

Questions / Comments Contact information: Taylan Altan, Professor and Director Center for Precision Forming (CPF) www.cpforming.org / www.ercnsm.org The Ohio State University, Columbus, OH Email: altan.1@osu.edu, Ph: (614) 292 5063