Post on 30-Mar-2020
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Quantenmechanisch geführtes Design neuer Eisenbasis-Werkstoffe
Lecture 1.3.2012
„Cold Formable Steels “
SFB 761 „Stahl - ab initio““““
Outline
• Terms and Definitions
• Description of cold formability
• Steel grades
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Sheet-metal forming
Cold forging Warm forging Hot forging
Forging
Forming
Source: Hirschvogel Automotive Group
Terms of Forming
2. Verfahren: Möglichkeiten und Grenzen
Hot forging:
• Work temperature is above the recrystallisation temperature• High formability of the materials• Low forming forces• No change in strength in the formed workpiece
Warm forging:
• Work temperature for steel at: 750 - 950°C• No or very little scaling on the surface• Lower forming forces than cold forging• Tighter dimensional tolerance than hot forging
Cold forging:
• Work temperature at room temperature (no additional heating)• Tight dimensional tolerances• No scaling on the surface• Increase of strength and reduction of strain by strain hardening
Definitions
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2. Verfahren: Möglichkeiten und Grenzen
The forming temperature influences decisively
• the material-strength (kf)• the forming capacity the material (ϕ B)• the scale formation by oxidation of the material
Flow stress, total elongation andscale thickness depending on thetemperature for different steels
Source: Hirschvogel Automotive Group
Influence of forming temperature
Example for sheet metal forming: beverage cans
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Deep drawing Wall Thinning Bottom shaping
The DWI – process: Drawing Wall Ironing
Important thin sheet forming processes
• Shearing
• Bending• Stretch forming• Deep drawing
Possible failure mechanisms• Fracture• Necking• Wrinkles• Dimensional deviation
Basics of thin sheet metal forming
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The cold formability of a thin sheet is the ability to reach the form of a hollow body or of a profile without a defect under a certain strain.
A successful forming is not decided by the material alone, but by the complex interaction of the process, the forming machinery, the tools and the material.
The technological property cold formability describes the plastic behaviour during forming without external heat supply. Three stages can be identified:
Yielding – Strain hardening – Ductile failure
Definitions
Outline
• Terms and Definitions
• Description of cold formability
• Steel grades
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Material parameters for description of sheetformability
• Mechanical properties: YS, YS/TS-ratio, A u, A80
• Flow curve: σσσσ = c*ϕϕϕϕn
σσσσ = σσσσ00 - (σσσσ00 - σσσσy) * exp (- ϕϕϕϕ/ ϕϕϕϕ0)σσσσ = c*ϕ
• Strain hardening behaviour: n,d σσσσ / dϕϕϕϕ -analysis
• Anisotropy: r m, ∆∆∆∆r
• Strain Rate Sensitivity: m-value
• Forming Limit Diagram: FLD 0, LDH, strain path
• Limiting Drawing Ratio: LDR
• Temperature Sensitivity
• Grain size, phase distribution, γγγγr stability
Yield Criterionaccording to von Mises
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Yield Locus Curves by Barlat andderivation of model parameters
Barlat YLD2000:
my
mmmSSSSSS σ2~~~~~~
1332211 =−+−+−=Φ
• 8 parameter model• convex yield surface• can be used for FE simulations• tensile test or Bulge test: σ0, σ45, σ90, σb r0, r45, r90, rb
Barlat YLD2005:
• 18 parameter model• Improved description of anisotropic yielding for most sheet steels compared to
Barlat YLD2000 • Derivation of yield stresses and r values in 15° steps• tensile test or Bulge test: σ0, σ15, σ30, σ45, σ60, σ75, σ90 σb
Figure of the yield locusaccording to Barlat YLD 2005
Nor
mal
ized
stre
sses
, yy
r90r75 r90
r0
r45
Normalized stresses, xx
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Flow curves of cold rolled sheet steels
1000
800
600
400
200
0
0 0,1 0,2 0,3True strain ϕϕϕϕ
x uniform elongation
True
str
ess
σσ σσ, M
Pa
TRIP
DP
IF-HS
IF
FeP04
Strain hardening of cold rolled sheet steels
0,2
IF
4000
3000
2000
1000
00 0,1 0,3
True strain ϕϕϕϕ
x uniform elongation
Str
ain
hard
enin
g ra
te
θθ θθ, M
Pa
TRIP
DP
IF-HS
FeP04
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Temperature effect on strain hardeningof austenitic stainless steel 1.4571
True
str
ess
σσ σσ, M
Pa
0
1600
1200
800
400
00 0,2 0,4 0 0,2 0,4
True strain ϕϕϕϕ
6000
4000
2000
True strain ϕϕϕϕ
Str
ain
hard
enin
g θθ θθ,
MP
a
Temperature:
55 19085 115 125 195175165135
strain measurement system
Determination of a Forming Limit Diagram
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Impact of different strain patheson the maximum attainable local strain
Forming limit curves of different car body materials
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General description of forming limitsfor sheet metal deformation
Ductile crack initiation
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Characteristics of a ductile fracture
Macroscopic Microscopic Fractographic
Plastic deformation Crack progress with deflections Dimp les
Ductile fracture in a component
Crack initiation in the centerof a tensile sample
Microvoids and voids on the fracture surface
Relationship between the number of oxideinclusions and the oxygen content
Num
ber
of o
xide
s
Oxygen, ppm
Average inclusion length:
Steeltype:
Average inclusion length:
Steeltype:
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Models of damage mechanics
Growth description for each single using Voronoi cells within
the frame work of continuum damage mechanics (GTN model)
Failure predictionbased on a microstrutural analysis
Failure prediction using local void growth and coalescence description
criti
cal p
last
. str
ain
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Damage-curves depending ontype of steel and microstructure
Equ
ival
ent p
last
ic s
trai
n εε εεp
v
Stress triaxiality σσσσm /σσσσv
Influences on the damage curve
Influence of the Matrix toughness
Influence of the particle size and distribution, Matrix toughness
Equ
ival
ent p
last
ic s
trai
n εε εεp
v
Stress triaxiality σσσσm /σσσσv
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Outline
• Terms and Definitions
• Description of cold formability
• Steel grades
Steels in a car body
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Different forming zones in one panel
Source: voestalpine Stahl GmbH
hole expansion
Folding, flanging, bending
Deep drawing
edge/ notch effect
Laboratory tests for formability description
Source: voestalpine Stahl GmbH
Work area
Forming limit diagram
Notched tensile test
Bending test
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Sheet Forming: Tests and Parameters
Flow
Flow
StretchStretch
Stretch Bending
Stretch
Stretch
Shrink
Stretching
BendingStretch-flanging
Deep-drawing
Elongationr-valuen-value
n-value(at low ε)Elongation∆r → 0
λ -valueMicrostructuralhomogeneity
λ -valueMicrostructuralhomogeneity
Lochaufweitungsversuch (LAV)
Formänderungsanalyse:DP800Blechdicke: 2mmDi = 15mm
PrüfblechBlechhalter
Stempel
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Einteilung der Feinblech - Umformung
TiefziehenBiegen/Lochaufweitung
Quelle: voestalpine Stahl GmbH
Anforderungen?
Microstructure optimisation:strain hardening and failure
Strain hardening
• Significant hardness difference between hard and soft phase (GND: geometrically necessary dislocations)
• Homogenous distribution of small islands of the hard phase • Optimum volume fraction and optimum stability of retained austenite
Failure; crack development
• Small hardness difference between the phases• Small grain size/island diameters of hard phases• No alignment of hard phases• Retained austenite transformation should result in not too hard product
phases• Retained austenite transformation should proceed smoothly
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Strength-elongation relationships formild, HSS, AHSS and high Mn steels
Source: Modified from ″Advanced High Strength Steel (AHSS) Application Guidelines ″, IISI Comittee on Automotive Applications, March 2005
• Mild Steels� CQ Commercial Quality � DQ Drawing Quality� DDQ Deep Drawing Quality� EDDQ Extra Deep Drawing Quality � IF Interstial Free Steel (mild, high-strength)
• HSS High Strength Steels � HSLA High Strength Low Alloy steels,� BH Bakehardening steels� Rephos P – alloyed steels
• AHSS Advanced High Strength Steel � DP Dual Phase Steels, � TRIP Transforamtion Induced Plasticity Steels� CP Complex Phase Steels
Cold formable steelsfor the automotive industry
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Chemical compositions of some HSS
Steel type Designation C Mn Si Al P Cr Nb Ti V
BH H180BD 0.04 0.70 0.50 0.04 0.01 - - - -
″ 0.011) 0.70 0.50 0.04 0.01 - - - -
IF-HS H260YD 0.003 0.40 0.10 0.03 0.05 - 0.04 0.02 -
P H220PD 0.06 0.70 0.50 0.04 0.07 - - - -
HSLA H320LA 0.07 0.35 0.01 0.04 0.01 - 0.04 - -
DP H300X 0.10 1.00 0.50 0.04 0.01 - - - -
″ 0.10 1.00 0.10 0.04 0.05 - - - -
″ 0.08 1.50 0.10 0.04 0.07 0.40 - 0.03 -
H340X 0.10 1.20 0.10 0.04 0.01 0.80 0.04 - -
″ 0.07 1.40 0.10 0.04 0.04 0.04 - -
PM MS1000-W 0.17 2.20 2.00 0.04 0.01 1.00 - - -
TRIP RA700K 0.20 1.50 1.50 0.04 0.01 - - - -
″ 0.20 1.50 0.10 1.80 0.01 - - - -
″ 0.15 2.00 0.10 0.04 0.04 - - - -
″ 0.20 1.50 1.10 0.04 0.01 - 0.04 - -
RA800K 0.20 1.50 1.50 0.04 0.01 - - - 0.10
HMS TRIP 0.02 15 3 3 - - - - -
TWIP 0.03 25 3 3 - - - - -
TWIP 0.60 22 - - - - - - -
1) batch annealed
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ferrite pearlite / cementite martensite bainite austenite
mild steel
conventional high strength steel
Dual phase steel
TRIP steel
Microstructure of cold formable steelsbefore and after deformation
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DDQ steels: Microstructure andcharacteristic mechanical properties
max. C P S Mn Ti Al Si N
DC04 0,08 0,030 0,030 0,40 - 0,040 0,010 0,005
YS MPa UTS MPa A 80% r90 value n90 value Ag %
140-210 270-350 >38% >1,6 >0,18 ~22
Microstructure:• ferritic matrix• Carbides (few) and AlN
Mechanical properties:• Low yield strength and tensile strength• Low hardness• Very high total elongation• Great ductility and drawability
EN 10130:2006 (D)
Specified value Typical value
EDDQ steels: Microstructure andcharacteristic mechanical properties
Microstructure:• Ferritic matrix• Precipitates (few) of microalloy elements
Mechanical properties:• Low yield strength and tensile strength• High n-value• High r-value• High uniform elongation• Great ductility and drawability
max. C P S Mn Ti Al Si N
DC06 0,02 0,020 0,020 0,25 0,3 0,040 0,010 0,002
YS MPa UTS MPa A 80% r90 value n90 value Ag %
140-170 270-330 >41% >2,1 >0,22 ~25
EN 10130:2006 (D)
Specified value Typical value
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HSLA steels: Microstructure andcharacteristic mechanical properties
Microstructure:• Ferritic microstructure with a little pearlite• Precipitates of microalloy elements
Mechanical properties:• High yield strength • High tensile strength• Limited drawability and stretchability
EN 10292:2007 (D)
max. C P S Mn Ti Al (min) Si Nb
HX340LAD 0,11 0,030 0,025 1,00 0,15 0,015 0,50 0,09
YS MPa UTS MPa A 80% r90 value n90 value Ag %
340-420 410-510 >21% ~1,0 ~0,14 ~14
DP steels: Microstructure and characteristic mechanical properties
Microstructure:• Hard martensite islands in a soft
ferritic matrix• Inhomogeneous hardness
distribution
Mechanical properties:• Low yield strength• Low yield ratio (Rp0.2 / Rm)• High strain hardening• High n-value • High uniform and total elongation • Moderate hole expansion • Moderate bendability
EN 10338:2007(D)
max. C P S Mn Al Si B V Nb+Ti Cr+Mo
HCT500X 0,14 0,080 0,015 2,00 2,00 0,80 0,005 0,20 0,15 1,00
YS MPa UTS MPa A 80% r90 value n90 value Ag %
300-380 >500 >23% ~0,95 ~0,16 ~18
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CP steels: Microstructure andcharacteristic mechanical properties
Microstructure:• Mixture of different bainitic phases, ferrite
(and martensite)• Homogeneous hardness distribution
Mechanical properties:• High yield strength • High yield ratio (Rp0.2 / Rm)• Moderate strain hardening• Moderate n-value• Moderate uniform and total elongation • Excellent hole expansion• Excellent bendability
EN 10338:2007(D)
max. C P S Mn Al Si B V Nb+Ti Cr+Mo
HCT780C 0,18 0,080 0,015 2,20 2,00 0,80 0,005 0,20 0,15 1,00
YS MPa UTS MPa A 80% r90 value n90 value Ag %
500-700 >780 >10% ~0,70 ~0,085 ~8
TRIP steels: Microstructure andcharacteristic mechanical properties
Microstructure:• Bainite and retained austenite within a soft
ferritic matrix
Mechanical properties:• High yield strength • High yield ratio (Rp0.2 / Rm)• Very high strain hardening • Very high uniform elongation but small
post-uniform elongation• Moderate hole expansion• Moderate bendability
EN 10338:2007(D)
YS MPa UTS MPa A 80% r90 value n90 value Ag %
430-550 >690 >23% ~0,90 ~0,22 ~23
max. C P S Mn Al Si B V Nb+Ti Cr+Mo
HCT690T 0,32 0,120 0,015 2,50 2,00 2,20 0,005 0,20 0,20 0,60
20 µm
Specified value
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Steels for car bodies
Quantum-mechanics guided design of new Fe-based materials
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