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CPF Center for Precision Forming (CPF) 1 Forming of High Strength Steels (HSS & A/UHSS) in the Automotive Industry Dr. Taylan Altan, Professor & Director,

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Presentation on theme: "CPF Center for Precision Forming (CPF) 1 Forming of High Strength Steels (HSS & A/UHSS) in the Automotive Industry Dr. Taylan Altan, Professor & Director,"— Presentation transcript:

1 CPF Center for Precision Forming (CPF) 1 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 / - Prepared for - AIDA-America, Dayton, OH June 13-14, 2012

2 CPF Center for Precision Forming (CPF) 2 1.Introduction 2.Material Properties 3.Formability 4.Presses 5.Tribology 6.Springback 7.Summary Outline

3 CPF Center for Precision Forming (CPF) 3 Background [ Structural Materials in Automotive Industries: Applications and Challenges, GM R&D Center] Potential advantages of HSS Weight savings in auto bodies, 15% to 25% Increase in crash resistance and safety.

4 CPF Center for Precision Forming (CPF) 4 Introduction Ref: Sadagopan 2004 INCREASED STRENGTH DECREASED FORMABILITY

5 CPF Center for Precision Forming (CPF) 5 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. Sheet properties (flow stress) determination Necking begins Engineering Stress-Strain Curve True Stress-Strain Curve = Flow stress Material properties of HSS/AHSS/UHSS In bulge test, flow stress over large strain can be obtained in biaxial stress state

6 CPF Center for Precision Forming (CPF) 6 Material Properties Flow Stress Ref: World Steel Association, Challenges: 1)Predicting uniform elongation, 2)Input of flow stress into FEA codes. n-value, as defined in Hollomons Equation: is not constant.

7 CPF Center for Precision Forming (CPF) 7 Material Properties Tensile Test Ref: Nasser et al 2010 Determination of Flow Stress 0.15

8 CPF Center for Precision Forming (CPF) 8 Material Properties Ref: Nasser et al 2010 Determination of Flow Stress Bulge Test

9 CPF Center for Precision Forming (CPF) 9 Bulge/ Dome height (h) Initial Stage Testing stage Die diameter = 4 inches (~ 100 mm) Die corner radius = 0.25 inch (~ 6 mm) Clamping force Pressurized medium Measurement Pressure (P) Dome height (h) FEM based inverse technique Material properties Flow stress Anisotropy Methodology to estimate material properties from VPB test, developed at CPF (OSU) Pressure (P) Schematic of viscous pressure bulge test setup at CPF (OSU) Material properties of HSS/AHSS/UHSS

10 CPF Center for Precision Forming (CPF) 10 Before bursting After bursting Bulge test samples Material properties of HSS/AHSS/UHSS

11 CPF Center for Precision Forming (CPF) 11 Material Properties Challenges: 1)Tensile test gives a very limited information, 2)Bulge test gives more reliable strain-stress data. Ref: Nasser et al 2010 Determination of Flow Stress 0.49 Bulge Test

12 CPF Center for Precision Forming (CPF) 12 Highest formability G, Most consistent F Lower formability and inconsistent H Graph shows dome height comparison for SS 409 sheet material from eight different batches/coils [5 samples per batch]. Bulge test for quality control of incoming sheet material Material properties of HSS/AHSS/UHSS

13 CPF Center for Precision Forming (CPF) 13 New generation AHSS steels (X-IP steel) have higher drawability than conventional mild steels. Cugy et al 2006 Material properties of HSS/AHSS/UHSS Drawability of AHSS steels

14 CPF Center for Precision Forming (CPF) 14 Springback (elastic recovery) of the formed part is proportional to stress. Decrease in Youngs modulus with strain in AHSS steel results in higher springback. [ULSAB-AVC Report/AISI Training Session document, 2002] [Pervez et al 2005] Material properties of HSS/AHSS/UHSS Loading and Unloading modulus of AHSS steels

15 CPF Center for Precision Forming (CPF) 15 Challenge: Apparent Modulus changes with plastic strain Apparent Modulus Variation Ref: Kardes et al 2010 Material Properties

16 CPF Center for Precision Forming (CPF) 16 TRIP 800 Ref: Choi et al Inconsistency of Material Properties AHSS are performance based grades. Challenges: 1)Strength, elongation, weldability may vary, 2)Material properties are inconsistent from supplier to supplier, even batch to batch. Material Properties

17 CPF Center for Precision Forming (CPF) 17 Formability Ref: Sung et al 2007; Dykeman et al Local Failures Challenges: 1)Local failures do not correlate with n-value, R-value or elongation, 2)Materials has to be tested under various stress states. Significant Stretching Moderate Stretching and Bending High Hole Expanding and Bending

18 CPF Center for Precision Forming (CPF) 18 Ref: SSAB and Uddeholm 2008, Keeler and Ulnitz 2009, Dykeman et al 2009 Stretching (b)(c) Higher Stretchability (a) Challenges: 1)Stretchability decreases with strength {(a) and (b)}, 2)Inconsistency is present in stretching (c). Formability

19 CPF Center for Precision Forming (CPF) 19 Formability Ref: World Steel Association 2009, Yan 2009 Bending Elongation in bending does not correlate to elongation in tension test: DP980 failed at 14% elongation in tensile, 40% elongation in bending. Challenges: 1)Bendability decreases with strength, 2)Failure at bending cannot be predicted by tensile data.

20 CPF Center for Precision Forming (CPF) 20 Formability Ref: Shi and Chen 2007 Stretch Bending DP780 Underbody structural part DP980 B-pillar inner Challenge: This type of fracture cannot be predicted using conventional Forming Limit Curve (FLC).

21 CPF Center for Precision Forming (CPF) 21 Formability Ref: Sadagopan and Urban 2003, Wu et al 2006 Stretch Bendability A suggested test method: Angular Stretch Bending (ASB) Achievable heights of several steels: as strength increases, stretch bendability decreases.

22 CPF Center for Precision Forming (CPF) 22 Formability Ref: SSAB and Uddeholm 2008, World Steel Association 2009 Deep Drawing Challenges: 1)Higher strength, results with less deep drawability. 2)Sidewall curls and local fractures are observed

23 CPF Center for Precision Forming (CPF) 23 Formability Ref: Palaniswamy and Altan 2006 Deep Drawing One solution to this problem is: Optimizing blankholder pressure, including multi-point cushion systems.

24 CPF Center for Precision Forming (CPF) 24 Formability Ref: Sadagopan 2004, Sung et al 2007 Flanging / Edge Stretching Hole Expansion Test Cracked Sample

25 CPF Center for Precision Forming (CPF) 25 Formability Ref: SSAB and Uddeholm 2008 Challenges: 1)Edge cracks cannot be predicted by FLC and are related to sheared edge quality, 2)Higher strength reduces the hole expansion ratio (HER), 3)HER gets even worse with worn tools Flanging / Edge Stretching Effect of hole blanking Worn Tool Sharp Tool

26 CPF Center for Precision Forming (CPF) 26 Presses Ref: Keeler and Ulnitz 2009 Required Load and Energy Challenge: Due to higher strength, required press load and energy are higher.

27 CPF Center for Precision Forming (CPF) 27 Press and tooling for forming HSS/AHSS/UHSS 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 slide force and energy requirements IISI, 2006

28 CPF Center for Precision Forming (CPF) 28 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. Blank holder force requirements Noel et al, 2005 Press and tooling for forming HSS/AHSS/UHSS

29 CPF Center for Precision Forming (CPF) 29 Modification in transfer press for forming AHSS steel 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. Haller, 2006 Press and tooling for forming HSS/AHSS/UHSS

30 CPF Center for Precision Forming (CPF) 30 Presses Ref: Miles 2004, Boerger 2008 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.

31 CPF Center for Precision Forming (CPF) 31 Modification in blanking press for AHSS steel 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. Haller, 2006 Blanking force Linkage drive kinematics for blanking press Esher et al, 2004 Press and tooling for forming HSS/AHSS/UHSS

32 CPF Center for Precision Forming (CPF) 32 Tooling for forming AHSS steel 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. Haller, 2006 Esher et al, 2004 Press and tooling for forming HSS/AHSS/UHSS Parting line of tool steel inserts

33 CPF Center for Precision Forming (CPF) 33 Lubrication and Friction Ref: Kim et al 2009 Challenges: 1) Higher contact pressure and higher temperature are detrimental for lubricants, 2) Temperature and pressure additives are needed

34 CPF Center for Precision Forming (CPF) 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): i.Higher the Blank Holder Force (BHF) that can be applied without fracture in the drawn cup, better the lubrication condition ii.Smaller the flange perimeter, better the lubrication condition (lower coefficient of friction) 34

35 CPF Center for Precision Forming (CPF) 35 Tool Materials, Treatments, Coatings Ref: Liljengren et al 2008

36 CPF Center for Precision Forming (CPF) 36 Tool Materials, Treatments, Coatings Ref: Young et al 2009

37 CPF Center for Precision Forming (CPF) 37 Product development using HSS/AHSS/UHSS FLC based failure prediction not accurate – Need a better and reliable failure prediction criteria for die engineering and analysis Stoughton et al 2006 Failure prediction in forming AHSS steel

38 CPF Center for Precision Forming (CPF) 38 Springback Higher springback Ref: World Steel Association 2009

39 CPF Center for Precision Forming (CPF) 39 Higher springback Springback compensation: 1)Over forming, 2)Locally deforming / bottoming, 3)Stretching by higher forces. Modeling of springback is a challenge: 1)Flow stress equations do not fit, 2)Unloading modulus may vary, 3)More Bauschinger effect is observed. Ref: Sung et al 2007 Springback

40 CPF Center for Precision Forming (CPF) 40 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. Studies on forming of HSS/AHSS/UHSS

41 CPF Center for Precision Forming (CPF) 41 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 & Youngs 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

42 CPF Center for Precision Forming (CPF) 42 Summary 1.Material Properties a.Flow stress equations cannot be expressed in simple form (σ=kε n ), b.Flow stress data determined with tensile test is very limited (~ true strain), c.Unloading modulus may vary with plastic strain, d.Material properties are not consistent,

43 CPF Center for Precision Forming (CPF) 43 Summary 2. Formability a.Local failures are common and these do not correlate to n- value, R-value or elongation, b.Various tests (hole expansion, stretch bending, etc.) are required. 3. Presses a.Higher load and energy required, b.Higher reverse loads are observed in blanking.

44 CPF Center for Precision Forming (CPF) 44 Summary 4. Friction / Lubrication a.Higher loads are temperatures observed, b.Lubricants, tool materials, treatments and coatings have to be selected carefully. 5. Springback a.Higher springback is observed, b.Prediction of springback requires more sophisticated analyses

45 CPF Center for Precision Forming (CPF) 45 Questions / Comments Contact information: Taylan Altan, Professor and Director Center for Precision Forming (CPF) / The Ohio State University, Columbus, OH Ph: (614)


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