Transcript
Institut für Eisenhüttenkunde der RWTH Aachen
New Microalloyed Forging Steels
CBMM, 13th July London
Prof. Dr.-Ing. Wolfgang Bleck
2
Introduction
Strip Products
• Continuous casting of slabs or thin slabs
• Thermomechanical rolling on HSM or CSM
• Cold rolling and batch or continuous annealing
→ in general: well defined process routes
Forging Products
• Semi-product: long products via numerous different
process routes
• Forging: numerous different product forms and
different temperature and deformation cycles
→ difficult process control
→ many constraints: tooling, forces
→ limited control of cooling rates
Niobium in Forging Steels
• High C contents; Martensitic of Bainitic microstructure
→ Transformation Kinetics; robust processes
→ Grain refinement of austenite/martensite?/bainite?
→ Enhanced mechanical properties
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Projects
Current Nb related Industry Projects at IEHK
• Case-Hardening Steels (Al-reduced)
DFG-Project, Cluster of Excellence WP 5200 Demon
• Precipitation Hardening Ferritic/ Pearlitic Steels
AViF-Project, A 228
• High-Strength Ductile Bainitic Steels
BMWI-Project, IGF 260 ZN
• TRIP-Forging Steels
BMWI-Project, IGF 374 ZN
• Forging Simulation of Nb-Microalloyed Steels
BMWI-Project, IGF 17246 N
• Damage Tolerant Microstructures of Highly Stressed
Components for Mechanical Engineering
DFG-AiF-Project, HiPerComp
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Microalloyed Gear Steels
Project 1: Case hardening steel for
high temperature carburising processes
Project 2: Al-reduced case hardening steel
for high temperature carburising processes
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High temperature carburizing
decreases the production
duration and costs of gear
components.
Grain size control of austenite is
needed.
Grain growth can be prevented by
microalloying of case
hardening steels.
Prediction and optimization of
particle size and amount is
performed by numerical
simulation of particle evolution.
Microalloyed Gear Steels
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Microalloyed Gear Steels Experimental Procedure
Material: 25CrMo4 + Nb/Ti
ST
EM
C Si Mn Cr Mo Ni V Al N Ti Nb
0.24 0.22 0.89 0.92 0.43 0.18 0.008 0.023 0.016 0.009 0.034
ST
EM
ST
EM
ST
EM
ST
EM
Ro
llin
g
Fo
rgin
g
FP-Annealing HT-Case hardening
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EFTEM / during Processing
N Nb
Al Ti
NbC
100 nm
BG-I, cold formed,
930 °C – 75 min
• Different particles can be found:
AlN, NbC and Ti(C,N)
• Shape factor for AlN >2 and
NbC - Ti(C,N) ~ 1
AlN Ti(C,N)
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Simulation Results
• Faster ripening of Al-nitrides in comparison to (Ti,Nb)-carbonitrides
• Higher pinning force for (Ti,Nb)-carbonitrides in comparision to Al-nitrides
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Simulation Results
• Prediction of pinning force
using calculation of particle
evolution along different
process chains
• Small improvement of pinning
force via shortening of
austenitization time
• Decrease of pinning force by
increase of austenitization
temperature up to 1200 °C
equal to increasing the case
hardening temperature to
1100 °C
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Microalloyed Gear Steels Grain Size Control
Austenite initial grain size ←
start microstructure,
heating conditions,
particle state
Pinning force ←
particle size, amount and
distribution
Description of pinning force
due to Zener force
Increase of particle amount
and decrease of particle
size needed
cmradiusparticler
amountparticlef
cmJenergysurface
cmJforceZenerZ
r
fZ
ZZZ
NCNbTiAlN
NCNbTiAlN
,
/ ,
/ ,
2
3
2
3
),)(,/(
),)(,(
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Al reduced Case Hardening Steel
• Al reduction is requested for cleanliness improvement.
• The same pinning force at 1050 °C in Nb modified 25CrMo4 steel can be
obtained by an increase of Nb content to 850 ppm.
• Additionally, an increase in solution temperature is needed.
• Conclusion: a combined development of alloy and process parameters is
needed in order to realize this new steel concept.
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Experimental results of grain size
distribution
• High grain stability of Al-reduced grade for 1050 °C and 1100 °C
• No abnormal growth at 1100 °C
Al Nb Ti N
Ref 227 ppm 337 ppm 89 ppm 166 ppm
Mod 87 ppm 850 ppm 16 ppm 160 ppm
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Materials Design of
High-Strength Forging Steels
• Project 1: Microalloyed precipitation hardening
ferritic/pearlitic steels (PHFP-M)
• Project 2: High-strength ductile bainitic steels (HDB)
• Project 3: TRIP-Forging Steels
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Materials Design of Forging Steels Motivation
Commonly used alloy concepts for forging components:
• Quenched and Tempered (Q&T) forging steels (e.g. 42CrMo4)
Need of additional heat treatment, danger of distortion
• Ferritic / Pearlitic precipitation hardening forging steels (e.g. 38MnVS6)
Limited in strength and especially toughness
Necessity for advanced mechanical properties and easy processing
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Material Design of Forging Steels Alloy Design
C Si Mn S Cr Mo B Nb Ti V N
PHFP 1 0.38 0.60 1.40 0.05 0.04 0.03 <0.0005 <0,001 <0,001 0.10 0.010
PHFP-M 1 0.36 0.68 1.44 0.03 0.15 0.03 <0.0005 0.029 0.022 0.19 0.021
PHFP-M 2 0.30 0.62 1.44 0.03 0.29 0.04 <0.0005 0.049 0.020 0.19 0.012
HDB 1 0.30 1.56 1.52 0.02 1.23 0.08 0.0025 0.029 0.023 <0,001 0.012
HDB 2 0.22 1.47 1.50 0.01 1.31 0.09 0.0025 0.035 0.026 <0,001 0.011
Effect of Alloying Elements:
Nb, V: increase strength by precipitation hardening
Cr: decreases vcrit and increases tensile strength
B, Ti, N: B decreases vcrit in solute state, therefore Ti is added to
form TiN (refinement austenite grain) instead of BN
Si: Si > 1% suppresses the formation of cementite
chemical composition in wt.%
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Materials Design of Forging Steels PHFP-M
C Si Mn P S Cr Mo Ni Cu N Al Nb Ti V
low Nb 0,35 0,64 1,40 0,008 0,030 0,16 0,06 0,16 0,01 0,0154 0,021 <0,001 0,02 0,11
medium Nb 0,35 0,62 1,41 0,008 0,029 0,17 0,06 0,16 0,01 0,0144 0,023 0,03 0,02 0,10
high Nb 0,35 0,65 1,41 0,009 0,030 0,17 0,06 0,17 0,01 0,0141 0,026 0,06 0,02 0,10
chemical composition in wt.%
Nb(C,N)
MnS
AlN
Increasing Nb content leads to increasing strength, especially yield strength
Amount of mass fraction of
microalloying precipitates: Strength properties:
Precipitation hardening ferritc/pearlitic steels:
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Correlation Heat Treatment / Microstructure /
Mechanical Properties in HDB Steels
Blocky
M/A
Lath
wid
th in
mm
Str
en
gth
in
MP
a
To
tal
elo
ng
ati
on
in
%
To
un
gh
ne
ss
at
RT
in
J
Transformation temperature in °C
bainitic ferrite
elongated
retained
austenite
blocky
M/A
Mechanical Properties HDB 1 steel:
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Toughness dependent on Yield Strength
+ MAE
bainitic
microstructure
amount retained austenite after
continuous cooling: ~7,8%
(measured by EBSD analysis)
Common Rail
(HDB 2)
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Materials Design of Forging Steels Mechanical Properties
PHFP-M
pearlite lamellae spacing l
Ti, V, Nb (C,N)
ferrite fraction
HDB
bainitic ferrite FB + retained austenite R
+ Nb (C,N)
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Effect of Nb and Mn on the transformation
behaviour of steel 18CrNiMo7
• The variation of small amounts of Mn (+0,5 wt.-%) and Nb
(+0,025 wt.-%) results in a change of transformation behavior
• Therefore the mechanical properties change dramatically especially at very
slow cooling rates
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Forging of Microalloyed Steels
Simulation the Flow-Behaviour and
Microstructure Evolution
during Multi-Hit Forging of Microalloyed Steels
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Hot Forging: Process and Materials Simulation
Modeling the Flow-Behaviour and
Microstructure Evolution
Hot-Deformation Flow Stress
σ=f(ε,T,ε) .
Microstructure
Dynamic/ Static
Recovery and Recrystallization
Precipitation of
Microalloying Elements Ti,V,Nb
Grain Growth
σ=f(ε,T,S) .
ρ=g(ε,T,ρ)
Dislocation Density
Empirical Approaches
Internal Variable
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ρcrit
Critical
Dislocation Density
ρcrit
FV;R
Interconnection of the Sub-Modules
Dislocation Density Evolution/
Static Recrystallization
fconv kdyn,c8,c1,n
Precipitation Model
Temperature,
Strain Rate,
Grain Size,
Chem.Comp.
Dm ρ σ
Dm ρ X
Y/N
CC
CN
CV
kstat
Hot Forging: Process and Materials Simulation
Dislocation Density Evolution/
Dynamic Recrystallization
Temperature,
Strain Rate,
Grain Size,
Chem.Comp.
Temperature,
Strain Rate,
Grain Size,
Chem.Comp.
Temperature,
Strain Rate,
Grain Size,
Chem.Comp.
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FEM-Simulation in FORGE
Courtesy GmbH
Hot Forging: Process and Materials Simulation
50%
45%
40%
35%
30%
25%
20%
15%
10%
5%
0
0.30
0.27
0.24
0.21
0.18
0.15
0.12
0.09
0.06
0.03
0
High Grade
of Deformation
High Volume
Fraction of DRX
XDXN /-- PHI /--
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FEM-Simulation in FORGE Austenite Grain Size
Hot Forging: Process and Materials Simulation
l=32
l=27
l=32
l=42 l=47
l=29
DM /µm
55.0
50.0
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
5.5
l PAG in µm
Courtesy GmbH Courtesy GmbH
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Model Implementation in eesy-2-form
Courtesy GmbH
Simulation of the Flow Stress as
Function of the Dislocation Density
Translation of the Model in
Material Flow Simulation
Meshed geometry of an uniaxial
forging sample
Local distribution of flow stress
(status after 0.35s process time)
Flow stress of the middle element
Hot Forging: Process and Materials Simulation
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Niobium in New Forging Steels
Multi-Tasking
New use
of Nb in a great
variety of steels
Grain Refinement
Crack Resistance
Increase of:
Strength
Toughness
Ductility
Applications
Short Processing
Fast Precipitation
Kinetics
Multifunctional
Precipitation
Hardening
Al,V Nb
Alloy Design
Economic/ Efficient
Processing
Wide Fields of
Applications
Improved Materials
Properties and
Design
Institut für Eisenhüttenkunde der RWTH Aachen
New Microalloyed Forging Steels
CBMM, 13th July London
Prof. Dr.-Ing. Wolfgang Bleck
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