SFB 761 „Stahl - ab...

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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



• 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


6000

4000

2000


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



• 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

T

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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


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


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


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)


HCT780C 0,18 0,080 0,015 2,20 2,00 0,80 0,005 0,20 0,15 1,00


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)


430-550 >690 >23% ~0,90 ~0,22 ~23


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

THANK YOU!

SFB 761 „Stahl - ab...

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