Transcript Design for Ultra High Strength Steel (UHSS)

Design for…
Advanced High Strength Steel
(AHSS)
MPD 575 – Cohort 7
Dave Berels
Bill Dowling
Steve McInally
John Robarge
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 manufactures are forced to pursue advanced
materials to balance: weight, cost, fuel efficiency, safety,
options and performance
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Outline
•
•
•
•
•
•
•
•
Driving Factors
What is AHSS?
Material Properties
AHSS Applications
Manufacturing Methodology
Design Details
Business Case
Summary
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Driving Forces
• 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 Forces (cont.)
• Crash safety specifications & requirements
• Over the past 10 years, several changes in the industry
have driven added requirements to vehicle crash
specifications:
– Increase of SUV and truck sales raised bumper heights
• Varying load conditions during side impact, resulting in different
failure modes, forced changes to test requirements
– Insurance Institute of Highway Safety (IIHS) has become a major
influence
– Change in government regulations
• FMVSS 216 Vehicle Rollover: Increases loading to 2.5x (from 1.5x)
vehicle weight for static loading
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Driving Forces
IIHS Bumper
Impact Area
FMVSS Bumper
Impact Area
(White Mesh)
(Black)
Rail-side
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Driving Forces (cont.)
Rising Fuel Prices
• Rising fuel prices 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 spend money on
more fuel efficient vehicles
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Driving Forces (cont.)
Gas Price Trends for the Next 7 Years
6.00
y = 9E-08x 2 - 0.006x + 106
5.00
Cost per Gallon
4.00
3.00
2.00
y = 0.0002x - 5.7666
1.00
0.00
08/11/87
05/07/90
01/31/93
10/28/95
07/24/98
04/19/01
01/14/04
10/10/06
07/06/09
04/01/12
12/27/14
09/22/17
Date
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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
Roll stability
Crash avoidance
Rear view cameras
Etc.
The support architecture (electronics and attaching hardware) adds
significant complexity & weight to the vehicle.
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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
• As mentioned previously, today’s vehicles are
adding more content
• Things like…
– Large moon roofs
– Safety devices (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 thread is continuous through-out 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 families 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
HSLA
270 MPa
to
550 MPa
Bake Harden
270 MPa
to
340 MPa
500 MPa
to
1500 MPa
Dual Phase & Complex Phase
500 MPa
to
1000 MPa
TRIP
500 MPa
to
800 MPa
Martensite
900 MPa
to
1500 MPa
Boron Heat Treat
1300 MPa
to
1450 MPa
Advance High Strength Steel
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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
– More ductile and easier to form than dual phase
– Drawback: poor welding properties
• Boron Heat Treated
– Special stamping process needed for forming
– High strength of material limits geometric complexity
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What is AHSS?
Low Strength
Ultra High Strength
Steels (<210MPa)
Steels (>550MPa)
High Strength
Steels
Elongation (%)
70
60
50
Conventional HSS
40
AK
30
Advanced HSS
BH
20
Mild Steels
10
0
0
200
400
600
800
1000
Lower Yield Strength (MPa)
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What is AHSS?
AUSTENITE
Temperature
Ar3
FERRITE
PEARLITE
BAINITE
Ms
MARTENSITE
Microstructure
Legend
Austenite
Martensite
Ferrite
Bainite
Time
MARTENSITE
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DUAL
PHASE
TRIP
COMPLEX
PHASE
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Microstructures of AHSS
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Ferrite (F)
Martensite (M)
Dual Phase (F + M)
TRIP (F + B/M + RA)
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What is AHSS?
Before 1986
Vehicles are already
trending towards AHSS
Mild Steel
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CHSS
AHSS
After 1986
Project 2010
Mild Steel
CHSS
AHSS
Mild Steel
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CHSS
AHSS
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AHSS Applications
The Ultra-Light Steel Auto Body Example
By: U.S. Steel
Background: US Steel designed a steel body made of
AHSS and UHSS to demonstrate the benefits of high
grade steels
• Benchmarking was conducted to determine the weight of
other body-in-whites
• USS’s body was designed using several different AHSS
materials
• CAE analysis of major test modes were completed
–
–
–
–
Crash Worthiness: 35 MPH Front and 35 MPH Rear Impact
Rollover Protection
Torsion Rigidity
Bending Rigidity
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AHSS Applications
Ultra-Light Steel Auto Body: Exploded View
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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
• Some seat belt supports are
cantilevered from the floor
• High moment load results from
a cantilevered seat back
Seat Belt Load
Vector (F)
d=1/2 m
– Using M = d x F results in a
moment of 4,448 Nm
• These types of bending loads
drive the need for heavier
gauge material
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2nd Row Seat Structure
(Side View)
23
AHSS Applications
Seat Example (continued)
• Using a standard beam, we can illustrate the benefits of a 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 C-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
24
AHSS Applications
Seat Example (continued)
HSLA
• Using these values, we can now find the stress for 3 mm HSLA
using a simple stress formula
 HSLA
M *C

I HSLA
 HSLA  344.9MPa
AHSS
• Since weight is such a large factor in vehicle/seat design today, an
engineer may want to use a AHSS steel
• The result of doing this same calculation with a steel thickness of 1
mm in martensite steel is as follows:
 AHSS
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M *C

I AHSS
 AHSS  943.1MPa
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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
345MPa
948MPa
HSLA
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Martensite
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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
Conclusions
• Substantial weight saves can be obtained by
using AHSS in key areas of designs
• This is an extreme example of weight reduction
because many applications would have other
design limitations, like threads on a hole
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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:
• Increased tool wear
• Large presses (increase tonnage) are needed to form
higher strength materials
• Traditional stamping techniques limit design/contour
complexity
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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
Draft
Angle
Flange
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
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Manufacturing Methods of AHSS
A few points about 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
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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
• Martinsitic 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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Summary
• Increased strength/stiffness can be obtained with
reduced cross-sections with the use of AHSS
• Special processes are required for AHSS in complex
shape applications
• Offers exceptional opportunity in the weight, strength,
and stiffness tradeoff balancing act
• Can provide solutions to compensate for added vehicle
content such as sunroofs & larger window geometry
• Depending on the application, its reasonable to expect a
weight save and cost save
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Sources
•
•
•
•
•
•
http://www.thefabricator.com/ToolandDie/ToolandDie_Article.cfm?ID=869
http://www.steel.org
http://usssautomotive.com
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
HH SLA 3 4mm
TH SLA 3mm
 HSLA 
 HSLA  344.9MPa
SH SLA 3mm
EH SLA 2 07G P a
 AHSS 


A rea  b  d  HH SLA b  TH SLA
2
A rea  2 22mm
2
Note: Centroid was found using FEA

CHSLA=13.9 mm
2 b  SH SLA  HH SLA TH SLA

2 b  d  2 HH SLA b  TH SLA
CAHSS=14.9 mm
y extreme  1 3.9 05mm
3
I 
3
2 SH SLA b  HH SLA TH SLA
3
6
I  8 .06 1 0
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M *C
I AHSS
 AHSS  943.1MPa
2
y extreme  b 
M *C
I HSLA

2
 A rea b  y extreme
Lm
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