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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