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High strength steels for light weighting automotive applications
Mark Rainforth Andrew Patterson Alfonce Chamisa Arjan Rijkenberg (Tata Steel)
high strength, low weight components for automotive applications
High strength steels- a success story!
Increase in car curb weight in Europe
FSV=future steel vehicle
300 400 500 600 700 800 900 1000
Elon
gatio
n A8
0, %
Tensile Strength, MPa
0
20
40
60
80
100
120
140
1100
30.000MPa.%
20.000MPa.%
Next
generation AHSS
MA
IF LC
CMn HSLA
The need to escape the ‘banana’ diagram
Design of nanoprecipitation steels to deliver strength and formability
Designers are specifying higher strength multiphase steels to meet weight targets. This also extends to the automotive chassis system.
However, processing and joining higher strength materials can prove challenging
The need is for:
High yield and tensile strength
Improved combination of hole-expansion and elongation
Good bendability
Good fatigue strength
Good weldability Edge splits waste a lot of time and money: 99% of splits on chassis parts are edge splits Courtesy Tata Steel
To achieve high high elongation and high hole expansion ratio, adopt a single-phase ferrite microstructure as the base-line
Hole
-exp
ansi
onra
tio
Total elongation
Single-phase ferrite
XPF grades
Single-phase ferrite
XPF gradesBS/MSBS/MS
HSLADP
FB/CP
HSLADP
FB/CP S550MC
650 MPa
HEC: 30-50%
HEC: 100-140% XPF steels
Courtesy Tata Steel
Nano-precipitation strenghtened single-phase ferrite
microstructure
MICROSTRUCTURE
Time
Tem
pera
ture
FERRITE
Schematic CCT diagram
MARTENSITE
BAINITE
600-650 oC
≥900 oC
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Fine ferrite grain size with either equiaxed or possibly
bainitic structures
S500
S420
FB590
CP800
CP1000
XPF650
XPF800
XPF1000
Nano precipitation steels- starting to break out of the ‘banana’ diagram
Courtesy Tata Steel
S700
CP800
XPF800
3mm data
800MPa grade offers superior elongation and HEC compared with CP800 and S700
800MPa grade offers increased yield strength compared with CP800
Nano precipitation steels- starting to break out of the ‘banana’ diagram
Courtesy Tata Steel
Balancing strength and formability
FORMABILITY: single-phase ferrite
STRENGTH: Nb,V, Mo nano-precipitation
100
200
300
400
500
600
0 2 4 6 8 10Precipitate size (nm)
σ ppt (M
Pa)
C N V Nb
Mo Al
Balance
High fraction Small size
prec
ipita
tion
grai
n siz
e
Nano-precipitation strenghtened steel
Nanoprecipitation steels- remarkable strengths >1GPa, ductility >25%
Nano precipitation of vanadium carbide in a ferrite matrix
(020)MC
(020)Ferrite
17-VMo Atom Probe
Left: Carbide distribution, showing interphase precipitates Bottom: Line scan across a carbide showing that it consists of V and Mo, with a trace of Nb
HE
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What does the future hold?
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Conclusions • There is no such thing as a traditional material- materials can evolve quickly to meet a demand
• Materials must be designed with resource efficiency in mind- both composition, manufacturing route, manufacturing waste and recyclability/re-use