Transcript weaverjm.faculty.udmercy.edu

Design for…
Advanced High Strength Steel
(AHSS)
MPD 575 – Cohort 7
Dave Berels
Bill Dowling
Steve McInally
John Robarge
Edits and Improvements From
• Joe Torres, Beatriz Dhruna, Spencer
Dinkins, Dwayne Mattison, Kevin
O’Callaghan, Norm Opolsky, Raghavan
Setlur, Keith Warner, Mac Lunn, Dave
Minock
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Introduction
As customer expectations continue to rise, automotive
manufacturers must continue to improve their vehicles:
• What was acceptable in vehicle performance yesterday,
is often uncompetitive today
• Passenger safety continues to grow in priority
• Government regulations drive increasing vehicle
demands (crashworthiness)
• Customers are demanding more features/content.
Automotive manufacturers are forced to pursue
advanced materials to balance weight, cost, fuel
efficiency, safety, options and performance.
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Outline
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Driving Factors
What is AHSS?
Material Properties
AHSS Applications
Manufacturing Methodology
Design Details
Business Case
Summary
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Driving Factors
• Crash safety specifications & requirements
• Need for weight reduction
– Rising fuel prices
– Offset additional vehicle features/content
• Demand for performance
• Structural stiffness / NVH
• Vehicle packaging demands
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Driving Factors (cont.)
• Crash safety specifications & requirements
• Over the past 10 years, several changes in the industry have
driven added requirements to vehicle crash specifications:
– Increase in sales of SUV and trucks with raised bumper heights
• Varying load conditions during side impact, resulting in different
failure modes, forced changes to test requirements
– Growing influence of the Insurance Institute of Highway Safety
(IIHS)
– Change in government regulations
• FMVSS 216 Vehicle Rollover: Increases loading to 2.5x (from 1.5x)
vehicle weight for static loading
• FMVSS 214 Vehicle-to-Pole Side Impact
– Increase of Global vehicle production (applicability of additional local
standards/regulations)
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Driving Factors
IIHS Bumper
Impact Area
FMVSS Bumper
Impact Area
(White Mesh)
(Black)
Rail-side
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Driving Factors (cont.)
Rising Fuel Prices
• Rising fuel prices and CAFÉ (corporate average
fuel economy) standards are forcing vehicle
manufacturers to rethink their business case
– Vehicle weight directly impacts fuel economy
– Manufacturers are now more willing to spend
more money on exotic materials to save
weight
– Customers are willing to pay a premium for
more fuel efficient vehicles
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Driving Forces: US Retail Gas
Prices
http://gasbuddy.com/gb_retail_price_chart.aspx
Driving Forces: Trade-offs
• Crash safety requires high strength / heavier gauges
• Fuel economy requires lower weight / lighter gauges
• Conflict between Safety & Fuel Economy / CO2 Emissions.
Demand for Fuel Economy
Markets facing high fuel prices are moving to smaller vehicles yet demanding equal
capability…
Examples:
• 2013 Ford Escape
– Must be capable of towing 4,600 lbs in Europe and Asia
– This is double what an American customer would tow
– Power must be delivered in a smaller package space
• 2015 F150
– Customers demand high torque with fuel economy
• Advent of 3.5L GTDI (Gasoline Twin-Turbo Direct Injection) V6
• Aluminum body construction instead of steel
– Higher payload and trailer capability to achieve target GCWR (Gross Combined
Weight Rating)
• 2011 Ford Explorer
– Shared with Flex and MKT (D-Class Vehicles)
– Outgoing product had V8 and body-on-frame
– Customers don’t care -> New must be as capable as old in their eyes
Bottom Line: To achieve gains in fuel economy, towing and payload, increased GCWR’s
can only be achieved by curb weight reductions…Thus lighter-weight materials such as
AHSS now become viable options.
Additional Feature Content
Vehicles in the 1950s were equipped with very few features. Today’s customers…
• Demand more standard features
– Power locks and windows
– Cruise control
– Safety features: Airbags, side airbags
– Air conditioning
– Anti-lock brakes
• Today’s vehicles are also packed with electronics
– Navigation systems
– Multi-speaker audio systems
– Roll stability
– Crash avoidance
– Rear view cameras
– Etc.
The support architecture (electronics and attaching hardware) adds complexity and weight to the
vehicle
• The weight of electrical systems on luxury cars is about 150 lbs vs. 4500 lbs total vehicle weight.
• Not a significant amount of weight vs. the total weight load of other vehicle systems such as the
body, powertrain, and NVH countermeasures.
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Demand for Performance
In combination with additional content, customers demand
higher levels of performance from their vehicles than
ever before
Examples:
• Convertibles in the past were plagued with cowl shake
and torsion instability
• Today’s customers are not willing to make trade-offs. A
topless vehicle can no longer be a rattle trap.
– Consumers want coupe performance out of a convertible
•
Suspension components are handling larger loads
– Vehicle handling is continually improving on all vehicles
– Sports cars are continually advancing the suspension
components to obtain an advantage over their competitors
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Vehicle Packaging Demands
• Today’s vehicles are adding more content...
– Luxury
• Large moon roofs
• Voice controlled media centers
– Increased Safety
• air curtains
• side airbags
• These added features are constraining the cross
sections of critical structural supports
• Vehicles are not getting larger, so stronger
materials are required to maintain strength
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Summary of Driving Forces
One main theme is continuous throughout all the changes
in customer demands and specifications…
– The purpose of the introduction of
AHSS is to reduce weight and
maintain or increase strength!
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What is AHSS?
Three main classifications of steels are used in the
automotive applications.
Steel Types
Tensile Strength
Mild Steel
100 MPa
To
270 MPa
Conventional High Strength Steel
270 MPa
to
550 MPa
High-strength low-alloy (HSLA)
270 MPa
to
550 MPa
Bake Hardened
270 MPa
to
340 MPa
Advanced High Strength Steel
500 MPa
to
1500 MPa
Dual Phase & Complex Phase
500 MPa
to
1000 MPa
TRIP (Transformation Induced
Plasticity)
500 MPa
to
800 MPa
Martensite
900 MPa
to
1500 MPa
Boron Heat Treat
1300 MPa
to
1450 MPa
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AHSS Types
• Dual Phase / Complex Phase
– Mixture of ferrite and martensite
– Bake hardens during e-coating process
– High formability at low strength
• TRIP: Transformation Induced Plasticity
– The material is work-hardened during the forming process
– Retained austenite transforms to martensite during plastic
deformation
– More ductile and easier to form than dual phase
– Drawback: poor welding properties
• Boron Heat Treated
– Special stamping process needed for forming
– High material strength limits geometric complexity
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AHSS Types
• Advanced High-Strength Steels
– 1st Generation AHSS ⇒ need to improve formability
– 2nd Generation AHSS ⇒ need to develop commercially available
manufacturing processes
– 3rd Generation AHSS development methodology
• Identify fundamental mechanisms at quantum mechanical and
micromechanical level that determine overall strength and
ductility of steel alloys
• Analyze the effect of alloy compositions and distribution of
various hard phases present in steels on their thermomechanical properties
• Investigate the efficacy of various additives to design a novel
3rd generation AHSS alloy with improved strength and
formability
AHSS Types
• 1st Generation Advanced High-Strength Steels
– Dual Phase (DP)
AHSS Types
• 1st Generation Advanced High-Strength Steels
– Ferritic-Bainitic (FB)
AHSS Types
• 1st Generation Advanced High-Strength Steels
– Complex Phase (CP)
AHSS Types
• 1st Generation Advanced High-Strength Steels
– Martensitic (MS)
AHSS Types
• 1st Generation Advanced High-Strength Steels
– Transformation-induced Plasticity (TRIP)
AHSS Types
• 1st Generation Advanced High-Strength Steels
– Hot Formed (HF)
AHSS Types
• 2nd Generation Advanced High-Strength Steels
– Twinning-induced Plasticity (TWIP)
Upfront Design Considerations
• Various design suggestions found in
http://machinedesign.com/article/advanced
-high-strength-steels-add-strength-andductility-to-vehicle-design-0503, such as
strategies for reducing springback,
optimizing radii, improving structural
efficiency, making thin plates, and
producing strong joints.
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What is AHSS?
Third Generation Opportunity
Some Definitions
Ferrite - Fe3, pure iron, Body Center Cubic
configuration.
Austenite - Above 900C start temperature, phase
transition of Ferrite to Face Centered Cubic.
Ferrite
Martensite - rapidly quenched Austenite,
FCC structure phase changed to BCC with
highly strained, supersaturated carbon
atoms.
Austenite
Pearlite - Below 700C start temperature, slowly cooled. Iron
largely precipitated. Orthorhombic. ferrite + cementite (Fe3C).
Bainite - Between Pearlite and Martensite. Less Iron diffusion than
Martensite, more than Pearlite.
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Continuous Cooling
Transformation Phase Diagram
AUSTENITE
Transformation
Start
Temperature (C)
800
FERRITE
PEARLITE
BAINITE
400
MARTENSITE
Cooling Time
MARTENSITE
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DUAL
PHASE
TRIP
COMPLEX
PHASE
MPD 575 - DF AHSS
Microstructure
Legend
Austenite
Martensite
Ferrite
Bainite
Microstructures of AHSS
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Ferrite (F)
Martensite (M)
Dual Phase (F + M)
TRIP (F + B/M + RA)
MPD 575 - DF AHSS
Usage of AHSS
Vehicles trending towards
increasing % use of AHSS
*source autosteel.org
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Benefits of AHSS?
• High strength – AHSS is up to four times harder than normal
high-strength steel, making it much stronger and stiffer
• Lightweight – AHSS can be formed in pieces that can be up to
10 to 15 times thinner than normal steel without sacrificing
strength, which enables weight reduction and improved fuel
economy
• Shapeability – AHSS can be formed into complex shapes that
can be welded into structural areas such as pillars and bumpers
in a car
AHSS Applications
The Ultra-Light Steel Auto Body Example
By: U.S. Steel
•
Background: US Steel designed a body made of AHSS and UHSS to
demonstrate the benefits of high grade steels
•
Benchmarking was conducted to determine the weight of other body-inwhites
•
USS’s body was designed using several different AHSS materials
•
CAE analysis of major test modes were completed
– CrashWorthiness: 35 MPH Front and 35 MPH Rear Impact
– Rollover Protection
– Torsion Rigidity
– Bending Rigidity
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AHSS Applications
Ultralight Steel Auto Body: Exploded View
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AHSS Applications
AHSS Applications
AHSS Applications
The Ultra-Light Steel Auto Body Results
• 25% weight reduction
• Increased strength performance in all categories
• Zero increase in cost
Conclusion: If AHSS is utilized from the beginning,
a large weight save and increase of strength can
be realized.
Note: Study completed by US Steel Corporation.
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AHSS Applications
Seat Example
• AHSS is making its way into seating applications
• Seats carry high loads from seat belt restraints that attach to the
structure
Seat Belt Tower
2nd Row Seat Structure
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AHSS Applications
Seat Example (continued)
• Seat belt point loads can
exceed 2,000 lbs (8918 N)
• Some seat belt supports are
cantilevered from the floor
• High moment load results from
a cantilevered seat back
Seatbelt Load
Vector (F)
d=1/2 m
– Using M = d x F results in a
moment of 4,459 Nm
• These types of bending loads
drive the need for heavier
gauge material
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2nd Row Seat Structure
(Side View)
AHSS Applications
Seat Example (continued)
• Using a standard beam, we can illustrate the benefits of AHSS
– Assuming a C-channel stamping that has a thickness of 3 mm (t=3),
width of 40 mm (d=40), and a height of 20 mm (b=20)
– Finding the moment of inertia for this Channel is 8.06 x 10-6 kg m2
– Using a simple moment calculation, with a 500 N load, at a length
(L=400 mm) it results in a load of 200 Nm.
d
L
t
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b
AHSS Applications
Seat Example (continued)
HSLA
• Using these values, we can now find the stress for 3 mm HSLA
using a simple stress formula
AHSS
• Since weight is such a large factor in vehicle/seat design today, an
engineer may want to use AHSS
• The result of doing this same calculation with a steel thickness of 1
mm in C-Channel steel is as follows:
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AHSS Applications
FEA of a simple beam structure
– Verifies the stress of the hand calculations
– 345 MPa is at the 340 MPa yield strength of the
HSLA
– 945 MPa is within the allowable range of martensite
steel
– Elongation must be monitored if deflection is critical
345 MPa
948 MPa
HSLA
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Martensite
MPD 575 - DF AHSS
AHSS Applications
The Seat Example (continued)
• HSLA C-channel at 3 mm thickness has a mass of 0.694 kg
• Martensite C-channel at 1 mm thickness has a mass of 0.2439 kg
• This is a ~280% weight reduction in the part
•
•
•
•
This is an extreme example as noted.
A follow-up analysis could generate 3 estimates: a low, medium,
and high example.
If you’re selling a solution from a supplier, then you would use
the high estimate
Engineers like to see the range probability of achieving this
improvement.
Conclusions
•
•
Substantial weight saves can be obtained by using AHSS in key areas of
designs
This is an extreme example of weight reduction as noted.
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Manufacturing Methods for AHSS
The increased strength of AHSS over the conventional HS and mild steels has
driven the development of new manufacturing methods.
Some of the issues driving the new technology are:
• Tools wear more quickly
• Large presses (increased tonnage) are needed to form higher strength
materials
• Traditional stamping techniques limit design/contour complexity
• Extreme care must be taken when designing with AHSS steel
– Traditionally sub-assemblies (i.e. seat structures) are welded together to
make a complete frame.
– The carbon content that contributes to the added strength is also
detrimental to the weld process.
– Added heat during the weld process can contribute to brittleness of the
steel, thus causing abrupt failures.
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Manufacturing Methods for AHSS
Quick lesson in stamping terminology
• Draw depth – How deep a formation can be
made in a material
• Draft angle – Departure angle of a formed piece
of material from being perfectly square
Flange
Draft
Angle
Draw Depth
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Manufacturing Methods for AHSS
Traditional methods of forming metals:
• Progressive dies
• Line dies
• Transfer dies
New generation of forming process for AHSS:
• Hot stamping
• Roll forming with high speed piercing operations
• Stamping films added to material thickness
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Manufacturing Methods for AHSS
Complex Stamping
for a Car pillar
Simple Beam
Cross Section
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Manufacturing Methods of AHSS
Large coils of steel are fed through a
series of rollers.
Roll Forming
Part
Sample Part
Rollers
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Manufacturing Methods of AHSS
Sample Part
Hot Stamping
B-pillar
Ambient
Steel
Heated
Steel
Stamping
Press
Steel
Oven
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Stamping Die
MPD 575 - DF AHSS
Manufacturing Methods of AHSS
Welding:
• MIG welding isn’t recommended for 700 MPa
strength materials and above
– Weld fill material isn’t available at high strengths
– Weld material should not be your weakest part of the
joint
• Resistance welding is sensitive to material
thickness
• Laser welding is the best solution for AHSS, but
is a relatively new and developing technology,
and is currently higher cost
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Stress corrosion cracking (SCC) of AHSS steel,
Manufacturing Considerations
• If the presence of hydrogen sulfide causes entry of hydrogen into
the component, the cracking phenomenon is often termed “sulphide
stress cracking (SSC)”
• All steels are affected by hydrogen, as is evidenced by the influence
of hydrogen on corrosion fatigue crack growth.
• Hydrogen may be produced by corrosion reactions such as rusting.
• Hydrogen embrittlement under static load is only experienced in
steels of relatively high strength.
• Hydrogen may be introduced into the steel by a number of routes,
including welding, pickling, electroplating, exposure to hydrogencontaining gases and corrosion in service.
• Hydrogen embrittlement is not a permanent condition.
– The effects of hydrogen introduced into components prior to service may
be reduced by baking for a few hours at around 200 °C.
• In use, cathodic protection reduces the corrosion rate
Source: http://events.nace.org/library/corrosion/Forms/embrittlement.asp
Design Details
• Complexity of parts may be limited due to manufacturing capabilities
– Use of hot stamping processes are a must for parts with deep
draws
– Multiple operations such as hot stamping and roll forming may
be required
• Some AHSS parts are limited to simple roll forming sections
– Utilizing multiple parts is often required for attachment to
adjacent parts
– Easily formed materials can incorporate attachment brackets
– For example, roll forming does not allow for integrated joints
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Business Case
• Piece price impact
– Cycle times will increase due to multiple
progressive operations needed for AHSS
– Cost per pound increase on an average of
33% for AHSS
• Martensitic CR Gd is ~$0.43 per lb
• HSLA CR 340 XF is ~$0.35 per lb
• Tooling
– Adding a hot stamping procedure with the
ovens will increase tooling by as much as a
100%
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Business Case
Revisiting the C-channel example
• The weights for the different examples were 0.69 kg
(1.52 lbs) for the HSLA and 0.24 kg (0.52 lbs) for the
martensitic material:
– Material for the two parts cost is $0.53 for the HSLA and $0.23.
for the martensite
– Assuming an added 20% increase due to cycle time, the
martensite C-channel is $0.28
– Tooling for a C-channel is the same for a roll formed part, but
adding a hot stamp operation will add $300K
• The point is that material may be more expensive, but
the reduced amount material used nets a cost save
• Tooling cost needs be analyzed based on part volume of
the application
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Possible Service Concerns
• The material is thinner, meaning difficult to shape and
remove dents
• MIG welding (most common body shop method) of
AHSS is difficult
• Joint Integrity could be affected by repairs
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Summary
• AHSS results in increased strength/stiffness with
reduced cross-sections
• AHSS requires special processes in complex shape
applications
• AHSS offers exceptional opportunity in the weight,
strength, and stiffness tradeoff balancing act
• AHSS provides potential solutions to compensate for
added vehicle content such as sunroofs & larger window
geometry
• AHSS enables combined weight and cost savings ,
depending on the application
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Sources
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http://www.thefabricator.com/ToolandDie/ToolandDie_Article.cfm?ID=869
http://www.steel.org
http://usssautomotive.com
http://www.autosteel.org/
Paul Geek, Advance Engineering for Ford Motor Company
Ken Chereson, Ford Purchasing
Nassos A. Lazaridis - Ispat International,, “Designing and Manufacturing AHSSIntensive Vehicles: The AHS Steel Grades and their Characteristics”, October
2004
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Appendix
• Sample Calculations
Note: Centroid was found using FEA
CHSLA=13.9 mm
CAHSS=14.9 mm
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