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David K. Matlock, John G. Speer, and Emmanuel De Moor
Advanced Steel Processing and Products Research Center*Colorado School of Mines
Golden, Colorado
Recent AHSS Developments for Automotive Applications:
Processing, Microstructures, and Properties
*ASPPRC: An NSF Industry/University Cooperative Research Center - Est. 1984
http://aspprc.mines.edu/
Workshop : Addressing Key Technology Gaps in ImplementingAdvanced High-Strength Steels for Automotive Lightw eighting
February 9 - 10, 2012USCAR Offices, Southfield, MI
Scope of Today ’s Presentation
• AHSS historical perspective – very brief
• Status of recent AHSS developments
• Identify opportunities• Enhanced first generation AHSS• New Third generation processing routes
• Observations and suggestions
DP Developments – 1970 ’s to ……
• 1960’s – 1970’s: HSLA steel• Thermomechancial processing +
microalloy additions
• 1970”s – 1980’s: Dual-phase steel• Fundamentals identified• Basis for AHSS developments
• 1990’s --- TRIP (Transformation I nduced Plasticity) and other AHSS developments, leading to “3 rd Generation AHSS ”
Dual-Phase Steels: “The Early Days ”
Significant Parallel Research Activities
•Nippon Steel• 1975: S. Hayami and
T. Furukawa, Microalloying ‘75
•GM• 1976: M.S. Rashid,
SAE + others
•Ford• 1978: R.G. Davies,
Met. Trans. + others
M.S. Rashid,SAE, (1977)
Time
Tem
pera
ture
"Dual Phase"
Ferrite +Martensite
A1
A3
Overage/Galvanize
DP Steel Processing
• Initially conceived based on “simple”processing routes for hot and cold rolled steels
• Microstructures more complex than simply ferrite + martensite and contained retained austenite + ??
0 1 2 3 4 5 6Retained Austenite (pct)
8
12
16
20
24
28
32
Elo
ngat
ion
(pct
)
0.10C - 1.32Mn - 0.54Si - 0.11V0.12C - 1.50Mn - 0.61Si - 0.055V
(a)
0 4 8 12 16 20 24Uniform Strain (pct)
0
1
2
3
4
5
6
Ret
aine
dA
uste
nite
(pct
) 0.12C - 1.50Mn - 0.61Si - 0.055V
(b)
“… the ductility could be further improved by increasi ng the amount of retained austenite …” A. Marder (1977)
…Lessons Learned from “Early ” DP Steels
A. Marder, “Factors Affecting the Ductility of Dual -Phase Alloys,” TMS, (1979)
• Processed as dual-phase steel• Observed microstructures more complex than
ferrite + martensite
However, in a different study: “… retained austenite did not have a measureable effect” due in part “to the fact that the first few percent pla stic strain eliminated … austenite from the microstructure.” Eldis, TMS, (1979)
0 0.1 0.2 0.3 0.4True Strain
0
0.2
0.4
0.6
0.8
1
Vo
lum
e F
ract
ion
Mar
ten
site
A
D
C
B
AusteniteStability
Condition
Schematic Austenite Stability Comparison*
…Lessons Learned from “Early ” DP Steels
Time
Tem
pera
ture
Ferrite-Bainite-Austenite
"TRIP"
"Dual Phase"Ferrite +Martensite
A1
A3
TRIP Steel Processing – “modified DP ”
• Isothermally transform or control cool
• Austenite in high strength matrix (e.g. fine grain ferrite, martensite, bainite, …)
• Strength = f(MVF , ferrite grain size)
• Basic TRIP steel requirements identified
• Control amount of retained austenite at room temperature.
• Control austenite mechanical stability
• Microstructures are “complex”
• “First Generation AHSS ”
…Lessons Learned from “Early ” DP Steels
σσσσT = Vfσσσσf + VMσσσσM
Microstructural Classes:1st Generation AHSS
Elo
ngat
ion
(%)
Tensile Strength (MPa)
0
10
20
30
40
50
60
70
0 600 1200300 900 1600
DP, CP
TRIP
MART
HSLA
IF
MildIF-HS
BHCMn
ISO
Elo
ngat
ion
(%)
600
-Conventional HSS
First Generation AHSSISO
Mild Steels
AISI: www.steel.org (2006)
1800
1600
1400
1200
1000
800
600
400
200
00.20.1 0.3 0.4
Tru
e S
tres
s (M
Pa)
True Strain0
Martensite
HSLA320
TRIP800
DP590
DP980
IF
Compiled from:Huang 1989Bruce 2003
Cornette 2005Säglitz 2008
Elo
ngat
ion
(%)
Tensile Strength (MPa)
0
10
20
30
40
50
60
70
0 600 1200300 900 1600
DP, CP
TRIP
MART
HSLA
IF
MildIF-HS
BHCMn
ISO
Elo
ngat
ion
(%)
600
-
TWIP
AUST. SS
L-IP
ISO
Third Generation AHSS:
Affordable Multiphase Steels:
Unique Microstructures**Goal: Formable steels with
Increased strength and ductility
Future Opportunities for AHSS**Predicted to contain:
• High strength constituent• Retained austenite with controlled stability
AISI: www.steel.org (2006)
Processing Opportunities for AHSSE
long
atio
n (%
)
Tensile Strength (MPa)
0
10
20
30
40
50
60
70
0 600 1200300 900 1600
DP, CP
TRIP
MART
IF
MildIF-HS
BHCMn
ISO
-
BH
TWIP
AUST. SS
L-IP
Future Opportunity
Third Generation AHSSHSLA
$$Enhanced TRIP with Modified γ
“Q&P”
B-Modified Hot Formed
TWIP with Lower $$
EnhancedDP
D.K. Matlock and J.G. Speer, “Third Generation of A HSS: Microstructure Design Concepts,” Microstructur e and Texture in Steels and Other Materials, ed. by A. Haldar, S. Su was, and D. Bhattacharjee, Springer, London, 2009, pp. 185-205.
Enhanced Dual-Phase Steels
Example approaches:• Control heating rate
• Thermal cycling to refine grain size
• Alter ferrite strength to control ferrite/martensite strength ratio
• …other….
D. Mumford, G. Fourlaris, A. Smith, and N. Silk, “T he Interaction of Recrystallization and Transformation during the Rehe ating of Multiphase Steels,” AISTech, 2008.
Heating Rate: CMnSiCr DP600 Steel
Heating Rate = 2 oC/s Heating Rate = 200 oC/s
20 µm
Heated to 800 oC; Soak for 30 s; Quenched
20 µm
UFG DP
Process Control: Ultra -Fine Grain DP Steels
Martensite TemperedMartensite
Steel: 0.17C, 0.74Mn
H. Azizi-Alizamini, M. Militzer, W.J. Poole, Procee dings AHSS Conf., AIST Orlando (2008)
UFG DPSteel: 0.17C, 0.74Mn
ND
RD
5 µµµµm
H. Azizi-Alizamini, M. Militzer, W.J. Poole, Procee dings AHSS Conf., AIST Orlando (2008)
Process Control: Ultra -Fine Grain DP Steels
UFG DP
ND
RD
5 µµµµm
H. Azizi-Alizamini, M. Militzer, W.J. Poole, Procee dings AHSS Conf., AIST Orlando (2008)
Process Control: Ultra -Fine Grain DP Steels
Steel: 0.17C, 0.74Mn
ND
RD5 µµµµm
MVF = 0.42Island size ≈ 2 µm
Advanced DP Steels - Optimization
• Processing for higher strength & enhanced performance
• High volume fraction of martensite uniformly dispersed as fine particles
• May need to control martensite strength
• Fine grain ferrite
• Modified to control martensite to ferrite strength ratio
• Minimize banding
• However, Third Generation AHSS not achievableby enhanced DP steels!!!
Multi-phase Modelingto Design “Third Generation AHSS ”
(1) Constituent volume fractions(2) Constituent properties(3) Austenite Stability
Approaches:• Ideal Composite Model• Micromechanical Models
• FEA Based• …other…
Application of Composite Model to IdentifyNew Multiphase AHSS Steels (2006)
True Strain
Tru
e S
tres
s Composite
Component 2
Component 1
εU2εU1 εU-C
CompositeInstability
REF: “The Tensile Strength and Ductility of Contin uous Fibre Composites” S.T. Mileiko, J. Mat. Sci., (1969)
• Assume each “constituent” described by flow curve:
σσσσ = Kεεεεnεεεεm
• Apply rule of mixtures for composites (assumes “Isostrain ”) σσσσT = σσσσ1V1 + σσσσ2V2 + ….
• At InstabilityUd
d σσσσ====εεεεσσσσ
Increase MVF
Constituent propertiesfrom the literature
• High Mn Austenite(assumed stable )• Martensite
Constituent UTS (MPa)
Uniform True
Strain
Ferrite 300 0.3
Martensite 2000 0.08
Austenite 640 0.6
200 400 600 800 1000 1200 1400 1600Ultimate Tensile Strength (MPa)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Un
ifo
rm E
ng
inee
rin
g S
trai
n
Ferrite + Martensite
Austenite + Martensite
50%
30%
40%
60%
% MVF
Property Assessment: Stable Austenite (2006)
D.K. Matlock and J.G. Speer, “Design Considerations for the Next Generation of Advanced High Strength Sheet Steels,” Proceedings of the 3 rd Int. Conf. on Structural Steels , ed. by H.C. Lee, The Korean Institute of Metals and Materials, Seoul, Korea, 2006, pp. 774-781.
Comparison to “3 rd Generation ” AHSS (2006) E
long
atio
n (%
)
Tensile Strength (MPa)
0
10
20
30
40
50
60
70
0 600 1200300 900 1600
DP, CP
TRIP
MART
IF
MildIF-HS
BHCMn
ISO
-
BH
TWIP
AUST. SS
L-IP
Future Opportunity
Third Generation AHSSHSLA
Austenite + Martensite
Ferrite + MartensiteFerrite + Martensite
50% MVF
Stable Austenite+ Martensite
UPostUTotal eee −−−−++++====
D.K. Matlock and J.G. Speer, “Design Considerations for the Next Generation of Advanced High Strength Sheet Steels,” Proceedings of the 3 rd Int. Conf. on Structural Steels , ed. by H.C. Lee, The Korean Institute of Metals and Materials, Seoul, Korea, 2006, pp. 77 4-781.
Initial Microstructure: Ferrite + AusteniteInitial Metastable Austenite = 0 to 85 %
Property Assessment: Metastable Austenite
Constituent UTS (MPa)
Uniform True
Strain
Uniform Engineering
Stain
Ferrite 300 0.3 0.35
Austenite 640 0.6 0.82
Martensite 2000 0.08 0.08
Calculation Approach
• Assume austenite stability described by saturation function
• Assume function depends on alloying, microstructure, T, etc.
• Consider only “static hardening”
• Vary initial austenite content in ferrite-austenite composite
Saturation function reference: e.g. G.B. Olson, in Deformation, Processing, and Structure , ed. by G. Krauss, ASM, Metals Park, OH, 1984, pp. 391-425.
0 0.1 0.2 0.3 0.4True Strain
0
0.2
0.4
0.6
0.8
1
Vo
lum
e F
ract
ion
Mar
ten
site
A
D
C
B
AusteniteStability
Condition
D.K. Matlock and J.G. Speer: [presented at ICASS-2 006, Korea, August, 2006 and MATS 2008, Jamshedpur, India, Febr uary, 2008]; published as “Third Generation of AHSS: Mic rostructure Design Concepts,” Microstructure and Texture in Stee ls and Other Materials, ed. by A. Haldar, S. Suwas, and D. Bhattacharjee, Springer, London, 2009, pp. 185-205.
Initial Microstructure: Ferrite + AusteniteInitial Metastable Austenite = 0 to 85 %
Constituent UTS (MPa)
Uniform True
Strain
Uniform Engineering
Stain
Ferrite 300 0.3 0.35
Austenite 640 0.6 0.82
Martensite 2000 0.08 0.08
Elo
ngat
ion
(%)
Tensile Strength (MPa)
0
10
20
30
40
50
60
70
0 600 1200300 900 1600
DP, CP
TRIP
MART
IF
MildIF-HS
BHCMn
ISO
-
BH
TWIP
AUST. SS
L-IP
Future Opportunity
Third Generation AHSSHSLA
A
B
CD
A
C
D
B
Elo
ngat
ion
(%)
Tensile Strength (MPa)
0
10
20
30
40
50
60
70
0 600 1200300 900 1600
DP, CP
TRIP
MART
IF
MildIF-HS
BHCMn
ISO
-
BH
TWIP
AUST. SS
L-IP
Future Opportunity
Third Generation AHSSHSLA
A
B
CD
A
C
D
B
10%25%
40%
0 0.1 0.2 0.3 0.4True Strain
0
0.2
0.4
0.6
0.8
1
Vo
lum
e F
ract
ion
Mar
ten
site
A
D
C
B
AusteniteStability
Condition
Austenite Amount
Property Assessment: Metastable Austenite
D.K. Matlock and J.G. Speer: [presented at ICASS-2 006, August, 2006 and MATS 2008, Jamshedpur, India, February, 20 08]; published as “Third Generation of AHSS: Microstruc ture Design Concepts,” Microstructure and Texture in Steels and Other Materials, ed. by A. Haldar, S. Suwas, and D. Bhatt acharjee, Springer, London, 2009, pp. 185-205.
H.N. Han, C.S. Oh, G. Kim, O. Kwon, “Design method for TRIP-aided multiphase steel based on a microstr ucture-based modeling for transformation-induced plasticity and mechanically induced martensitic transformation,” Ma terials Science and Engineering A, 499 (2009) 462–468.
Micromechanical Modeling: Korea (2009)• Assumed TRIP, multi-phase steel – varied vol. fracti ons• Applied a general kinetic model for the mechanically
induced transformation of austenite to martensite
K.S. Choi, A. Soulami, W.N. Liu, X. Sun. M.A. Khale el, “Influence of various material design parameter s on deformation behaviors of TRIP steels,” Computational Materials S cience 50 (2010) 720–730.
14 % Austenite
14 % Austenite
44 % Austenite 44 % Austenite
Micromechanical Modeling: PNNL (2010)• FEA Model - utilize “Representative Volume Element”
(RVE) to describe TRIP Steel microstructure
• RVE based on actualmicrostructures
Summary of Model Predictions
• “Big -Picture” predictions independent of modeling approach
• New (e.g.“3 rd Generation ”) AHSS will require:• High strength constituents
e.g. Martensite, fine grained ferrite, bainite…
• Austenite with controlled stability• e.g. transformation to martensite
• Additional Predictions: Steels with improved resistance to localized fracture obtained by• Refined microstructures• Minimized property gradients
Advances in Steel Processing
New 3rd Generation AHSS
Processing Examples
(1) “Enhanced ” TRIP Steels with high austenite volume fractions
(2) Q&P Steels produced by Advanced Processing = f (T, t)
Design Approach:
• Add alloying elements• Mn or N or C or …. to stabilize austenite
• Increase ferrite strength• Decrease Ferrite Grain Size by TMCP
(Thermomechanical processing)
• Increase austenite volume fraction > 20%• Fine and uniformly distributed
•Acknowledge: NSF, CMMI Grant # 0729114
Mn-Modified TRIP Steels
P. J. Gibbs, E. De Moor, M. J. Merwin, B. Clausen, J. G. Speer, D. K. Matlock, “Austenite Stability Effects on Tensile Behavior of Manganese Enriched Austenite TRIP Steel,” Met. and Mat. Trans., vol. 42 , (2011), pp. 3691-3702.
Experimental Approach
wt pctC Mn Si
0.099 7.09 0.13
Mn
C
Si
P. J. Gibbs, E. De Moor, M. J. Merwin, B. Clausen, J. G. Speer, D. K. Matlock, Met. and Mat. Trans., vol. 42 (2011) pp. 3691-3702.
•Material – 7.1 Mn
•Partition Mn:long-time intercritical heat treatments
•Predict heat treatments•Use ThermoCalc: TCFE2 Data (1999)
•Austenite composition•Austenite fraction
Step 1
Microstructure
• Heat treat for 1 week• 575 to 675 oC
• Retained austenite measured by neutron diffraction (Lujan Center at the Los Alamos National Laboratory)
Prediction
P. J. Gibbs, E. De Moor, M. J. Merwin, B. Clausen, J. G. Speer, D. K. Matlock, Met. and Mat. Trans., vol. 42 (2011) pp. 3691-3702.
Tensile Properties – Room Temperature
(33 %)
(26%)
(40%)
(43.5%)
(< 2%)•Constant engineering strain rate
•ASTM E-8 sub-sized samples•32 mm reduced section
•Initial austenite contents in (%)
P. J. Gibbs, E. De Moor, M. J. Merwin, B. Clausen, J. G. Speer, D. K. Matlock, Met. and Mat. Trans., vol. 42 (2011) pp. 3691-3702.
Strain -dependent Austenite Transformation
P. J. Gibbs, E. De Moor, M. J. Merwin, B. Clausen, J. G. Speer, D. K. Matlock, Met. and Mat. Trans., vol. 42 (2011) pp. 3691-3702.
(33 %)
(26%)
(40%)
(43.5%)(< 2%)
(33 %)
(26%)
(40%)
(43.5%)(< 2%)
• Intercritical annealing:• Enhances Mn partitioning• Increases austenite stability
• Cost efficient ways to enhance partitioning need to be identified
• ….Research is ongoing ……..
• Acknowledge: NSF, CMMI Grant # 0729114
Ferrite -Austenite: Mn -Modified TRIP Steels
Quenching and Partitioning“Q&P ”
Time
Tem
pera
ture
Ferrite-Bainite-Austenite
"TRIP"
"Dual Phase"Ferrite +Martensite
A1
A3
TRIP Steel Processing
• Isothermally transform or control cool
• Austenite in high strength matrix (e.g. fine grain ferrite, martensite, bainite, …)
Interrupted Cooling: The “Q&P ” Process
J.G. Speer, et al. ASPPRC, Colorado School of Mines, ( 2003 - 2011)Time
Tem
pera
ture A1
A3
MS
MF
` 2-Step Q&P
1-Step Q&P
• Novel method to produce high strength material with significant amounts of retained austenite
• Processing path to 3 rd Generation AHSS
Quenching and Partitioning (Q&P)
J.G. Speer, D.K. Matlock, B.C. De Cooman, and J.G. Schroth, “Carbon Partitioning Into Austenite after Martensite Transformation,” Acta Materialia , vol. 51, 2003, pp. 2611-2622.
• Fundamentally new approach developed by Prof. John G. Speer at ASPPRC/CSM with automotive collaborators
• Designed to produce enhanced AHSS with retained austenite in controlled martensite/ferrite matrix
(1) Annealing temperature controls amount of initial ferrite
(2) Quench temperature (QT): controls amount of ini tial martensite
(3) Alloying controls carbide stability(4) Partitioning temperature and time: control aust enite
stability(5) Result: unique combinations of martensite + aus tenite
+ ferrite + ..other..
MF
Ac 3
QTTem
per
atu
re
C = CA3γ
MS
Ac 1
γγγγγ
γγγγαααα
Time TimeTime
MF
Ac 3
QTTem
per
atu
re
C = CA3γ
MS
Ac 1
γγγγγ
γγγγαααα
Time TimeTime
Q&P: Unique Designed Microstructures
J.G. Speer, D.K. Matlock, B.C. De Cooman, and J.G. Schroth, “Carbon Partitioning Into Austenite after Martensite Transformation,” Acta Materialia , vol. 51, 2003, pp. 2611-2622.
“Q&P ” Alloying and Processing• Alloying: e.g. Mn Si Al C Mo
• … to control critical temperatures: M s Mf …..
• … to suppress cementite formation
1 10 100 1000Partitioning Time (s)
0
0.05
0.1
0.15
0.2
0.25V
olu
me
Fra
ctio
n R
etai
ned
Au
sten
ite
High Si1.54 wt pct
Low Si0.45 wt pct
Steel: 0.2C, 3.5Mn QT = 240 oC PT = 350 oC
M.J. Santofimia, et al., “Experimental Investigatio n of Q&P on Two Low-Carbon Steels with Different Silicon Contents,” Proc. Int. Conf. on New Developme nts in AHSS , ed. by J.G. Speer, B. Nelson, and R. Pradhan, AIST, Warrendale, PA, 2008.
400 800 1200 1600Ultimate Tensile Strength, MPa
0
20
40
60
To
tal E
lon
gat
ion
, %
TRIP (ULSAB)A
TRIP (GM)A
Conventional TRIPB
1-Step Q&PB
2-Step Q&PB
M (ULSAB)A
M (GM)A
M (Ispat-Inland)C
DP (ULSAB)A
DP (GM)A
DP (Ispat-Inland)C
7500C,180 s + WQB
8200C,180 s + WQB
9000C,180 s + WQB
TRIP, Austemper
DualPhase
Martensitic
Q&P
Note A: 80 mm gauge lengthNote B: 25.4 mm gauge length Note C: Gauge length unspecified
Properties of Q&PSummary and Comparison with AHSS
A.M. Streicher, J.G. Speer, D.K. Matlock, and B.C. De Cooman, Int. Conf. on AHSS for Automotive Applications Proc. , ed. by J.G. Speer, AIST, Warrendale, PA, 2004, pp . 51-62.
International Interest in the “Q&P ” Process
• Initially conceived, 2003
• Ongoing R&D at:
• Universities
• 21 in 13 countries
• Research Institutes
• 13 in 7 countries
• Companies
• 14 in 10 countries
Current Status of
AHSS Developments
Elo
ngat
ion
(%)
Tensile Strength (MPa)
0
10
20
30
40
50
60
70
0 600 1200300 900 1600
DP, CP
TRIP
MART
IF
MildIF-HS
BHCMn
ISO
-
BH
TWIP
AUST. SS
L-IP
Future Opportunity
Third Generation AHSSHSLA
Austenite + Martensite
Ferrite + MartensiteFerrite + Martensite
Stable Austenite+ Martensite
Example representative property combinations for automotive market
1000 MPa30%
1500 MPa20%
AISI: www.steel.org (2006)
AHSS Property Targets
D.K. Matlock and J.G. Speer, “Design Considerations for the Next Generation of Advanced High Strength Sheet Steels,” Proceedings of the 3 rd Int. Conf. on Structural Steels , ed. by H.C. Lee, The Korean Institute of Metals and Materials, Seoul, Ko rea, 2006, pp. 774-781.
De Moor, Gibbs, Speer, Matlock, and Schroth, “Strat egies for Third-Generation Advanced High-Strength Steel Development”, AIST Transactions , Iron & Steel Tech. Nov. 2010.
AHSS Development StatusEnhanced DP
Ultrafine DP27
Modified TRIP
Matsumura29,30
M icro-alloyed TRIP31
TRIP Type Bainite34
TRIP-DUAL35
Interrupted Quench36
Bainite
Bhadeshia-Edmonds47,48
M iihkinen-Edmonds49,50
Caballero37,38,44
Garcia-Mateo46
Quenching & Partitioning
Jun-Fonstein35
Streicher52
De Moor54,55,56,57
Li 58
W ang - ind. trial59
Rapid Heating and Cooling
flash process60,61,62
Lower Mn TW IP/TRIP
Frommeyer67
Dastur-Leslie68
M erwin69,70,71,72
800 1000 1200 1400 1600 1800 2000 2400
5
10
15
20
25
30
35
40
45
50T
otal
Elo
ngat
ion,
%
Tensile Strength, MPa
1000 MPa30%
1500 MPa20%
Ductility data adjusted to represent standard ASTM E-8 geometry
De Moor, Gibbs, Speer, Matlock, and Schroth, “Strat egies for Third-Generation Advanced High-Strength Steel Development”, AIST Transactions , Iron & Steel Tech. Nov. 2010.
AHSS Development StatusEnhanced DP
Ultrafine DP27
Modified TRIP
Matsumura29,30
M icro-alloyed TRIP31
TRIP Type Bainite34
TRIP-DUAL35
Interrupted Quench36
Bainite
Bhadeshia-Edmonds47,48
M iihkinen-Edmonds49,50
Caballero37,38,44
Garcia-Mateo46
Quenching & Partitioning
Jun-Fonstein35
Streicher52
De Moor54,55,56,57
Li 58
W ang - ind. trial59
Rapid Heating and Cooling
flash process60,61,62
Lower Mn TW IP/TRIP
Frommeyer67
Dastur-Leslie68
M erwin69,70,71,72
Ductility data adjusted to represent standard ASTM E-8 geometry
800 1000 1200 1400 1600 1800 2000 2400
5
10
15
20
25
30
35
40
45
50T
otal
Elo
ngat
ion,
%
Tensile Strength, MPa
Matsumura et al. (1987, 1992)0.39C, 1.2Mn, 1.2-1.5Si
Q&PDe Moor et al.(2008, 2009)
Mn PartitionedGibbs et al. (2011)Merwin (2006)
Flexible Processing Linesand the Production of New AHSS
�DP�TRIP�Mart�Q&P
��DPDP��TRIPTRIP��MartMart��Q&PQ&P
Tem
pera
ture
Time
0 2 4 6 8 10 12 14 16 18 20 22 24 26200
400
600
800
1000
1200
1400
1600
HC1200/1500MS
HC950/1180MS
HC700/980MS
HC550/980DP
HC500/780DPHC420/780DP
Str
ess(
MP
a)
Strain(%)L. Wang, X. Jin, and H. Qian, “Recent Development o f Galvanizing Sheet Steels in Baosteel,” Proceedings Galvatech 2011 .
Baosteel's Special Flexible HSS Line
• Opened March 2009• Multi-purpose processing
• CR and GI• Flexible heating/cooling
Li Wang and Weijun Feng, “Development and Applicatio n of Q&P Sheet Steels,”Advanced Steels, Y. Weng et al. (eds), Springer-Verl ag, Berlin, (2011) .
Q&P1000: Composition (wt pct)0.2C, 1.5 Si, 1.8 Mn
Baosteel: Q&P from New Line
B pillar Trial
Formability Limited by Fracture
Tensile Properties … NOT the Whole Story …
Formability Limited by Fracture Edge Cracking
Shear Fracture
Hole Expansion Test Sample
Images Courtesy of Jim Fekete, GM (2006)
www.autosteel.org
Dünckelmeyer et al.,Slovika (2009)
Tensile Properties … NOT the Whole Story …
Banding: DP980
A. Hudgins, et al. ASPPRC 2010
10 µm
Void Nucleation: DP 740 MPa
Steel(780 MPa)
Tensile Elong.
Stretch-flangeability
(HER)
DP 19 % 60 %
TRIP 30 % 40 %
Lee et al. ASPPRC/Posco 2005after Cho, Pusan Nat. U., 200
SampleSampleSample
2 mm
Hole Expansion
Shear FractureTest
Shear FractureTest
FullyAustenitized
Hole Expansion Tests: Summary
• Hole Expansion Ratio (HER) = f(microstructure)• Improved microstructural homogeneity = higher HER
E. De Moor, C. Föjer, J. Penning and J.G. Speer, Proc. of IIDDRG 2009 Conference , ed. by B.S. Levy, D.K. Matlock and C.J. Van Tyne, Colorado Scho ol of Mines, Golden, CO, 2009, pp.413-424.
1.0 mm DP600
1.0 mm 2.5 mm 3.2 mm 5.0 mm 6.4 mm 12.7 mm
2.5 cm
Microstructural Effects on AHSS Fracture
Bending Under TensionFBACK
VPull , FFront
Increase Radius
Alex W. Hudgins, “Shear Failures in Bending of Advanced High Strength Steels,” PhD Thesis, Colorado School of Mines, Golden, CO, 2010.
0 2 4 6 8 10 12 14R/t
400
600
800
1000
Pea
k F
ron
t S
tres
s (M
Pa)
1.0 mm DP780
Commercial Steel Matrix
Bending Under Tension Data
(R/t)crit = measure of formability
(R/t)crit = formability (i.e. susceptibility to shear )
A.W. Hudgins, D.K. Matlock, J.G. Speer, J.R. Fekete, and M.S. Walp, “The Susceptibility to Shear Fracture in Bending of Advanced High Strength Sheet Steels,” Proceedings of Steel Properties and Applications Conference, edited by L.C. Oldham, AIST, Warrendale, PA, 2007, pp. 145-157.
Application of Nanoindentation Techniques
Dis
tanc
e (m
icro
ns)
5
10• 0.1C-1.0Mn-0.3Si DP
• Vary intercritical T
• Vary MVF and martensite hardness
• Determine “hardness ratio”
• Result: R/t critical decreases with a decrease in the martensite to ferrite hardness ratio
A.W. Hudgins, D.K. Matlock, J.G. Speer, J.R. Fekete , and M.S. Walp, “The Susceptibility to Shear Fracture in Bending of Advanced High Strength Sheet Steels,” Proceedings of Steel Properties and Applications Conference, edited by L.C. Oldham, AIST, Warrendale, PA, 2007, pp. 145-157.
•Formability (as measured by R/t) increased with a decrease in hardness ratio between martensite and ferrite.
•Other variables evaluated
• Martensite morphology (limited effect)
• Banding (effect orientation dependent)
Summary: Shear Fracture Susceptibility
A.W. Hudgins, D.K. Matlock, J.G. Speer, J.R. Fekete , and M.S. Walp, “The Susceptibility to Shear Fracture in Bending of Advanced High Strength Sheet Steels,” Proceedings of Steel Properties and Applications Conference, edited by L.C. Oldham, AIST, Warrendale, PA, 2007, pp. 145-157.
Alternate Processing Routes:
Press Hardening
Time
Tem
pera
ture A1
A3
MS
MF
` Press Hardened
Deform Austenite
Press Hardened Steels (PHS)
• Production of high strength (1500 MPa) parts• Martensitic microstructures; boron alloys
http://www.interlaken.com/hotstamping (2011)
T. Altan, 2007www.stampingjournal.com
Press Hardened Steels (PHS)
kam-stampingguru.blogspot.com/2011www.cartype.com/pics/6272 (2011)
http://www.batz.com/stamping-dies/press-hardening.php (2011)
Heated Blank Blank in Die
PHS for High StrengthStructural Components
Press Hardened Steels (PHS)
Press Hardening
• Enhanced press hardening steels still maturing
• Cost (e.g. heating, etc) and surface finish continue to be issues for consideration
• Steel developments leading to alternate processing routes for similar strength parts are of significant interest
• e.g. Q&P, bainitic , …• Cold or warm forming
AHSS Closing Comments
• Approaches to potentially produce Third Generation AHSS have been identified
• Need optimized production routes to control:
• Constituent volume fractions, size, distribution
• Constituent mechanical properties• Austenite stability• …other
• Possibility for national research processing facility???
• AHSS: Additional Important Points• Welding and Joining• Formability – Springback• Surface Finish• Coating• Hydrogen effects• ……..• TWIP steels???
• Significant government funding in Germany and Korea
AHSS Closing Comments
• AHSS developments need to optimize time to implementation!!!
• …DP Steels, 35 years + ….• “ Most Important ” AHSS
• …TRIP Steels, >20 years• …Q&P ≈ 7+ years • ………
• …The Competition Continues …
AHSS Closing Comments
AHSS -- “Beyond Automotive ”
• Multiple inquires received at ASPPRC -Examples:
• Lightweight military vehicles• DOD/DOE workshop
Detroit, July 2011
• Blast resistant surfacing – US Army
• Metal storage cabinets (shipping costs!)
• Personal hand tools
• ….others …..
Steel ProducersAK SteelArcelorMittal SteelBaosteelEssar Steel Algoma Inc.Evraz Inc., N.A.GerdauNucor Steel Co.POSCOSABICSeverstal NA Inc.SSAB Enterprises, LLCTata Steel EuropeThe Timken Co.United States Steelvoestalpine Stahl GmbH
Heavy Equipment Mfg.Caterpillar Inc.Deere & Co.Automotive ManufacturesChrysler Group LLC General Motors Co.ToyotaSuppliersBekaertJohnson ControlsTube Investments of IndiaOtherEast Metals N. Amer. (Vanadium)Los Alamos National LabPrecision Cast Parts Corp.Reference Metals (Niobium)
The support of the Advanced Steel Processing and Products Research Center corporate partners,
listed below, is gratefully acknowledged .
http://aspprc.mines.edu /
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