Advanced High-Strength Steels for Automotive Applications

© Dr.-Ing. habil. M. Schaper 04/2013

Advanced High-Strength Steels for Automotive Applications

Dr.-Ing. habil. M. Schaper

Leibniz Universität Hannover Institute of Materials Science

Advanced High-Strength Steels for Automotive Applications © Dr.-Ing. habil. M. .Schaper


Seite 2

steel of my dreams

the ideal sheet: • plastic deformation starts at low stress • no Lüders bands occur • constant and high strain hardening • high elongation without necking • no springback • further deformation during crash • low density • high stiffness

strain e in %

stre

ss s

in M

Pa

ultimate tensile strength

yield strength elongation without necking

coil inspection



Seite 3

steel grades

100

90

80

70

60

50

40

30

20

200 0

ma

xim

um

elo

ng

ati

on

in

%

300 400 500 600 700 800 900 1000 1100 ultimate strength in MPa

10

Fe-Mn-Al-C-steel

Fe-Mn-Al-Si- TWIP-steel

Fe-Mn-Al-Si- Trip-steel

conv. Trip-steel DP-steel

CP-steel

DC06

MS-steel

IF(HS)

DC01-05

ZStE

BH-steel

austenitic stain less steel

22MnB5* air hardening steel*

* in final condition



Seite 4

Dual Phase Steel

Complex Phase Steel

Bake-Hardening Steel

annealed

steels

TRIP / TWIP

steels

conventional

steels

overview

TRIP Steel

grades with high manganese

content

- TRIP-effect

- TWIP-effect

- others

Hot Formed Steel

Post-Forming Heat-

Treatable Steel

good formability

strong strain hardening

very good formability

high toughness

best formability

very high toughness

(after hardening)



Seite 5

overview

Dual Phase Steel

Complex Phase Steel


annealed steels

TRIP / TWIP steels

conventional

steels

TRIP Steel

grades with high manganese content

- TRIP-effect

- TWIP-effect

- others

Hot Formed Steel

Post-Forming Heat-Treatable Steel

good formability



high toughness

best formability

very high toughness (after hardening)



Seite 6

Dual Phase Steel requirement: strong strain hardening

ferritic with 5% to 10% martensite toughness: Re~ 300 - 600 MPa Rm~ 500 - 800 MPa BH2 ~ 40 MPa max. elongation: up to 20% alloy: C<0,2%; Mn<2,5%; Si<0,8%; Al<1,5% Cr+Mo<1%

microstructure DP 600

advantages: cheep strong strain hardening

current research subjects: bigger ultimate strength

50µm 50µm50µm50µm 50µm50µm

microstructure (schematic)

ferritic-martensiteic structure by cooling from austenite/ferrite stage

strain j

flow

stre

ss k f

in M

Pa

50 µm

ferrite

martensite



Seite 7

Bake-Hardening-Steels requirement: strong strain hardening after deformation

advantage: strain hardening after

deformation current research subjects:

interaction with new steel grades

R

Work-Hardening R p

R p2,0

R eL

eH

straine in %

170°C 20 min.

T T

T

T

T T

T

T

Bake-Hardening

T

interstitial C and/or N

work hardening => additional dislocations

blocking the dislocations

170°C 20 min.

stre

ss s

in M

Pa

micro structure: ferritic with interstitial C toughness: Re~ 180- 360 MPa Rm~ 300 - 480 MPa BH2 ~ 40 MPa max. elongation: 26% to 34% alloy: C<0,1%; Mn<0,8%; Si<0,5%; P<0,12%



Seite 8

Complex Phase Steel

requirements: high toughness, strong strain hardening

microstructure: ferritic-bainiteic matrix with martensite toughness: Re up to 900 MPa; Rm up to 1200 MPa maximum strain: 10% BH2: 70 MPa alloy: C <0,17%; Mn 2,2%; Si 0,8%; Al 1,2%; Nb+Ti<0,2%; V<0,2%

microstructure CP Steel

deformation mechanism: dislocation slip

hardening mechanism: solid solution hardening


elevated toughness by: grain fining

recrystallization precipitation of micro-alloying elements

solid solution hardening precipitation hardening

B-Pillar made of CP-Steel


ferrite

bainite martensite



Seite 9

DP-Steel


CP-Steel

50 µm

Complex Phase Steel

Requirement: high toughness, strong strain hardening

microstructure: ferritic-bainiteic matrix with martensite toughness: Re up to 900 MPa; Rm up to 1200 MPa maximum strain: 10% BH2: 70 MPa alloy: C <0,17%; Mn 2,2%; Si 0,8%; Al 1,2%; Nb+Ti<0,2%; V<0,2%

deformation mechanism: dislocation slip

hardening mechanism: solid solution hardening

elevated toughness by: grain fining

recrystallization precipitation of micro-alloying elements

solid solution hardening precipitation hardening

B-Pillar made of CP-Steel


ferrite

bainite martensite



Seite 10

overview

Dual Phase Steel

Complex Phase Steel


annealed steels

TRIP / TWIP

steels

conventional steels

TRIP Steel

grades with high manganese

content

- TRIP-effect

- TWIP-effect

- others

Hot Formed Steel

Post-Forming Heat-Treatable Steel

good formability



high toughness

best formability

very high toughness (after hardening)



Seite 11

TRIP Steel requirements: high toughness, strong hardening, high energy absorption during crashes

Ferrit

Bainit

Restaustenit

Martensit

Ferrit

Bainit

Restaustenit

Martensit

microstructure: 50 % - 60 % ferrite 25 % - 35 % bainite 5 % - 10 % retained austenite < 5 % martensite toughness: Re up to 550 MPa; Rm about 900 MPa maximum elongation: < 60% alloy: C<0,3%; Mn 1,5-2%; Al+Si<2%

RA (g) ferrite (a)

martensite (e o. a´) bainite

Zeit

Te

mp

era

tur

Ferrit

Bainit

Ms

Perlit

Zeit

Te

mp

era

tur

Ferrit

Bainit

Ms

Perlit

advantage: strong hardening energy absorption during crash

current research subjects: embrittlement

schematic microstructure

ttt-diagram

tem

pera

ture

Time

stre

ss in

MP

a

strain in %

TRIP Steel

1200

1000

800

600

400

200

0 0 20 40 60 80 100

deformation mechanism: dislocation slip austenite transformation

e

e e



Seite 12

HSD-Steel (TWIP)

microstructure: austenitic toughness: Re 300 up to MPa; Rm up to 700 MPa maximum elongation: up to 100% alloy: Mn 24-30%; Al 3%; Si 3%

deformation mechanism: twinning SFE: from 20 to 25 mJ/m²

hardening mechanism: work hardening and twinning

requirement: very god formability and high toughness

Tem

pera

tur

Kohlenstoffgehalt

g-Fe

a-Fe

-Fe

Tem

pera

tur

Kohlenstoffgehalt

g-Fe

a-

-

-Feg-

Fe

Fe-C-Diagram with stabilized g-region

TWIP Steel TEM picture

twins



Seite 13

TWIP Steel

advantage: very good formability

current research subjects: embrittlement

stre

ss in

MP

a

strain in %

TWIP-Steel

TRIP Steel

1200

1000

800

600

400

200

0 0 20 40 60 80 100

HSD Steel (TWIP)

microstructure: austenitic toughness: Re 300 up to MPa; Rm up to 700 MPa maximum elongation: up to 100% alloy: Mn 24-30%; Al 3%; Si 3%

requirement: very god formability and high toughness



Seite 14

overview

Dual Phase Steel

Complex Phase Steel


annealed

steels

TRIP / TWIP steels

conventional steels

TRIP Steel

grades with high manganese content

- TRIP-effect

- TWIP-effect

- others

Hot Formed Steel

Post-Forming Heat-

Treatable Steel

good formability



high toughness

best formability

very high toughness

(after hardening)



Seite 15

post-forming heat-treatable steels (LH800)

microstructure: ferritic/perlitic; martensitic toughness: Re 800 MPa; Rm up to 1100 MPa maximum elongation: ~25% alloy: C 0,07-0,15%; Mn 2,1%; Si 0,8%; Cr 0,5-1%

softened microstructure LH800

advantages: very high toughness due to hardening after forming also after welding

current research subjects: decrease of distortion

10 µm 10 µm

requirements: most high toughness, good weldability

martensitic microstructure LH800

10 µm

10 µm



Seite 16

Bru

chde

hnun

g [%

]

Streckgrenze [MPa]

22MnB5

UmformenundHärten

ÜbergabedT/dtLuftAustenitisieren

T> AC3

dTmin ~ 30K/sT

Bru

chde

hnun

g [%

]

Streckgrenze [MPa]

22MnB5

UmformenundHärten

ÜbergabedT/dtLuftAustenitisieren

T> AC3

dTmin ~ 30K/sT

Hot Formed Steel (22MnB5) advantages:

very good formability in hot state

no spring back highest toughness

disadvantages: investment costs no galvanizing

current research subjects: coating higher toughness toughness taper

requirement: most high toughness (Rm 1600 MPa), no spring back

before heating: ferritic-perlitic

during deformation: austenitic

part: martensitic

in MPa

in %

yield strength

max

imum

elo

ngat

ion

austenitic stage

delivery

forming and

hardening


Leibniz Universität Hannover Institute of Materials Science Steel still offers a large

amount of possibilities

for brand new alloys. We will

explore all of them - but we will use only a

few ;-)

Advanced High-Strength Steels for Automotive Applications

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