Considerations for the Development of New Ultra … Steel Europe Considerations for the Development of New Ultra-High Strength Steels ExperiencesfromExperiments andSimulationsof HydrogenEmbrittlement

ThyssenKrupp�Steel�EuropeThyssenKrupp�Steel�EuropeThyssenKrupp�Steel�EuropeThyssenKrupp�Steel�Europe

Considerations�for�the�Development�of�New�Ultra-High�Strength�Steels

Experiences from Experiments�and Simulations ofHydrogen�Embrittlement

Dr.�R.G.�Thiessen,�Dr.�O.�Rott08.10.2014

HE�– Multi-Scale Modelling and Measurement�:�NPL,�UK

08.10.2014

Dr.�R.G.�Thiessen

2222 ThyssenKrupp�Steel�EuropeThyssenKrupp�Steel�EuropeThyssenKrupp�Steel�EuropeThyssenKrupp�Steel�Europe

Considerations�for�the�Development�of�New�UHSSOutline

• Introduction

• Material�characterization

− Mechanical properties

− Microstructural features &�Hydrogen�Management

• Modelling to Understand Test�Results

− Model�Construction,�Constraints and Assumptions

− Stresses�and Strain Distribution�at�Critical�Stages

• Summary,�Conclusions,�&�Remarks


08.10.2014



Introduction:�High�Demands on�High-Strength Steels

• High�demands on�strength,�but�also�deformation

− Improved efficiency/performance

− Higher�safety

• In�many cases,�only materials that canundergo complex forming steps can yield theadvantage of higher strength � structuralstiffness

• No single material�can satisfy all�demands(not�to mention cost considerations)��various materials developed for specificapplications

Ductility and Formability required

hole-expansionbending +�stretching

strength


08.10.2014



Introduction:�High�Demands on�High-Strength Steels

• High�demands on�strength,�but�also�deformation

− Improved efficiency/performance

− Higher�safety

• In�many cases,�only complex forming stepscan take advantage of higher strength �

structural stiffness

• No single material�can satisfy all�demands(not�to mention cost considerations)��various materials developed for specificapplications

Ductility and Formability requiredPlastic strain - Membrane

Major�Stress�– Outer Layer


08.10.2014



Introduction:�Some Typical Testing Methods for Susceptibility to HEVarious demands cannot be combined into a�single test

Cup�test�(uncoated)

• 10�cups��(ß�=�2,0�/�ßmax)

• 5%�NaCl

• Visual�control

no�cracks

(4�weeks)

Constant�load testwith RSW�under corrosion

(uncoated /�EG)

no�cracks

(240�hrs�at�70%shear�strength)

• 5�samples

• RSW�according�to�SEP�1220/2

• Joining�Imax

• Cataphoretic painting

• Corrosion�exposure�by�INKA*-spray

*Audi:�Ingolstädter Korrosions- und�Alterungstest

Test�with RSW-stress�under corrosion(uncoated /�EG)

• 3 samples

• RSW-stress�by�distance-plates

• Corrosion�exposure�by�5%�NaClor�climate�change�(DIN�EN�ISO�11997/1B)

no cracks

(4�weeks NaCl��or 6�weeks cycliccorrosion test)


08.10.2014




• Introduction









08.10.2014



Material�Characterisation

Au�Au�Au�Au�

[%][%][%][%]

A50�A50�A50�A50�

[%][%][%][%]

Rp02�Rp02�Rp02�Rp02�

[MPa][MPa][MPa][MPa]

RmRmRmRm

[MPa][MPa][MPa][MPa]

DP�Steel�(T) 7.9 12.3 602 1185

TDP�Steel�(T) 1.7 5.2 1230 1250

• Both�materials�have�identical�chemistry�(C-Mn w�Ti)�and�a�UTS�of�1200�MPa (±50�MPa)

• DP-Steel�has�a�much�lower�yield�stress

• Uniform�elongation�is�much�lower�in�TDP-Steel�(elongation�after�necking�only�slightly�lower�:�3.5�to�4.4%)

DP-Steel�(T)DP-Steel�(L)TDP-Steel�(T)TDP-Steel�(L)

Mechanical Properties�– Tensile Properties�


08.10.2014




• Higher�bending�angle�achieved�with�TDP-Steel��(α (Fmax) for TDP=94°,�for DP=82°)

• Hole�expansion�more�than�5-times�higher�for�TDP-Steel�(λ(ISO-TS) [%]�for TDP=54%,�for DP=10%)

Mechanical Properties�– Forming Properties�

DP

TDP

0 20 40 60 80 1000

1000

2000

3000

4000

5000

6000

7000

172602L1

172602L2

172602Q1

172602Q2

F, N

α, °

172603L1

172603L2

172603Q1

172603Q2

DP-Steel�(T,�L)

TDP-Steel�(T,L)


08.10.2014




TDP-Steel

DP-SteelMicrostructural Features�- SEM

• DP-Steel�consists�of�martensite�with�a�fine�sub-structure�and�ferrite

• Fine�carbides�are�visible�in�ferrite

• TDP-Steel�is�comprised�of�tempered�(visibly�decomposed�with�large�carbides)�and�smaller�ferritic grains

• Fine�carbides�are�also�visible�in�ferrite

Steel�&�Hydrogen�2014

05.05.2014



Material�CharacterisationSTE�Microscopy (metal-foil)�:�DP-Steel�and TDP-Steel

602 603

1301_011444

• overview of ferrite and martensite�in�both microstructures

• martensite�distinguished by highly faceted,�darker appearance

DP-Steel TDP-Steel


05.05.2014



Material�CharacterisationSTE�Microscopy (metal-foil)�:�DP-Steel�and TDP-Steel

602

0

603

1301_011450

• comparable dislocation density in�DP�and TDP�microstructure

− dislocations pinned (bake-hardening)�in�TDP

DP-Steel TDP-Steel


05.05.2014




TDP-Steel

DP-SteelHydrogen�Management:�Solubility and Binding

• TDA�with�increasing�heating�rate�allowed�estimation�of�effective�binding�energy�(Choo &�Lee�Model)

− DP�and�TDP�have�comparable�binding�energies

• Permeation�experiments�provided�indication�of�effective�coefficient�of�diffusion

− higher�diffusion�in�TDP�than�in�DP

• Diffusible�(RT�upto 350°C)�is�comparable�in�DP�and�TDP

Binding�Energy

- 33�- 36kJ/mol

Diffusion

- instat.:1.47E-7�cm2/s�

- stat:�6.61E-7�cm2/s

Diff.�H:�0.63�ppm

Binding�Energy

- 34�- 35kJ/mol

Diffusion

- instat.:2.20E-7�cm2/s�

- stat:�9.16E-7�cm2/s

Diff.�H:�0.65�ppm

TDA

TDA


05.05.2014



Mini-Summary�of Mechanical/Microstructure CharacterisationMechanical Properties�relevant�for Hydrogen�Embrittlement?

• Inhomogeneity�between�mechanical�properties�of�phases�is�significantly�higher�in�DP�than�TDP�

− DP:�50/50�Ferrite/Martensite

− TDP:�10/90�bake-hardened�Ferrite/tempered�Martensite

• Excellent�uniform�elongation�in�DP�based�on�localized�plastic�flow�in�ferrite

− Micro-localisation of�strain�can�be�deleterious�in�forming�operations�that�superpose�a�macro-localisation of�the�material�deformation�� exaggerated�localisation leads�to�overloading�of�ferrite�and�crack�initiation

• Improved�bending�and�hole-expansion�properties�in�TDP�due�to�reduced�disparity�between�mechanical�properties�of�phases

• Dislocations�in�TDP�pinned�by�carbon�(bake-hardening�effect)


08.10.2014




• Introduction









05.05.2014



0,5�mm

Simulation�Construction:�Boundary�ConditionsSymmetry�and�Free�Edges�(Sample�Thickness:�1�mm)

Symmetry

Symmetry

(2D-Symm)

Symmetry�(smallest�cross-section)

Stress-Free

Edge�(2D-Free)Edge�for�

Line-Evaluations

Region�of�Simulation

2D-Symm�≈�plane�strain

2D-Free�≈�plane�stress


05.05.2014



Simulation�Construction :�Material�Description�Flow�Curves and Fits ( ) ( ) ( )( )ξασσασα −−−++= ∞ exp1

YYHK

�� (MPa) � (MPa) �� (MPa) �

DP 623 1969 1129 132

TDP 1108 50 1240 84


05.05.2014



Simulation�Construction:�Assumptions

TDP-Steel

DP-Steel

Motivation�for using same�material�stress-strain relation with and without H

• 1h�precharging in�20%�NH4SCN

• Samples�unnotched

• Strain�rate:�2E-6�s-1�

• Straining�in�medium�(continued�charging�during�straining)

• H-charged�samples�follow�the�stress-strain�path�of�the�uncharged�samples

AIRAIRAIRAIR

NHNHNHNH4444SCNSCNSCNSCN


05.05.2014



Test�Results of SSRT�(notched samples;�air +�NH4SCN)Basis�for Simulation

0

200

400

600

800

1000

1200

1400

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8

Stress

�base

d�on�smalle

st�Cross

Stress

�base

d�on�smalle

st�Cross

Stress

�base

d�on�smalle

st�Cross

Stress

�base

d�on�smalle

st�Cross

-- -- Section��

Section��

Section��

Section��(M

Pa)

(MPa)

(MPa)

(MPa)

Grip�Displacement�(mmGrip�Displacement�(mmGrip�Displacement�(mmGrip�Displacement�(mm))))

DP

TDP

H-Fracture in�TDP�(after�ca.�650s,�637,3�MPa)�

H-Fracture in�DP�(after�ca.�500s,�481,8�MPa)


05.05.2014



Comparison of the von�Mises Equivalent StressMid-Plane�at�Notch Area

1200TM09:�F/Amin =�637,3�MPa1200M04:�F/Amin =�481,8�MPa


05.05.2014



Comparison of the Effective Plastic StrainMid-Plane�at�Notch Area



05.05.2014



Comparison of the Hydrostatic StressMid-Plane�at�Notch Area



05.05.2014



Comparison of the Hydrostatic Stress�on�Plane�of Smallest Cross-Section



05.05.2014



Comparison of Hydrostatic Stress�At�Straining Stage�causing Failure in�DP�(500s)

Notch Base

0

200

400

600

800

1000

1200

0 0,5 1 1,5 2 2,5 3 3,5 4

hydrostatisc

he�Spannung�(MPa)

hydrostatisc


hydrostatisc


hydrostatisc


xxxx----Koordinate�(mm)Koordinate�(mm)Koordinate�(mm)Koordinate�(mm)

1200M04�(0,00%Ti) 1200TM09�(0,00%Ti),�500s

DP

TDP


05.05.2014



Comparison of Hydrostatic Stress�At�Straining Stages�causing Failure in�DP�(500s)�and TDP�(650s)

0

200

400

600

800

1000

1200

0 0,5 1 1,5 2 2,5 3 3,5 4

hydrostatisc


hydrostatisc


hydrostatisc


hydrostatisc



1200M04�(0,00%Ti) 1200TM09�(0,00%Ti)

Empirically related to the local hydrogen�concentration

Notch Base

DP

TDP


05.05.2014



0

0,5

1

1,5

2

2,5

3

3 3,1 3,2 3,3 3,4 3,5 3,6 3,7 3,8

effektive

�pl.�Dehnung�(%)

effektive


effektive


effektive



1200M04�(0,00%Ti) 1200TM09�(0,00%�Ti),�500s max.�hydr.�Sp.�(DP) max.�hydr.�Sp.�(TM,�500s)

Comparison of Effective Plastic StrainAt�Straining Stage�causing Failure in�DP�(500s)

Notch Base

DP TDP

��̅ 0.035 ��̅ 0.016

max.�hydrostatic stress


05.05.2014



0

0,5

1

1,5

2

2,5

3

3 3,1 3,2 3,3 3,4 3,5 3,6 3,7 3,8

effektive


effektive


effektive


effektive



1200M04�(0,00%Ti) 1200TM09�(0,00%�Ti) max.�hydr.�Spannung�(DP) max.�hydr.�Spannung�(TM)

Comparison of Effective Plastic StrainAt�Straining Stages�causing Failure in�DP�(500s)�and TDP�(650s)

Notch Base

DP TDP

��̅ 0.035 ��̅ 0.040

max.�hydrostatic stress


05.05.2014



Mini-Summary�of Simulation�Results

• DP-Steel

− max.�strain (air)�Φmax =�0.077�(=7.7%)

− max.�pl. deformation at�notch base 1.8%

− max.�hydrostatic stress�ca.�710�MPa

− eff.�pl. deformation at�max.�hydrostatic stress�ca.�0.75%

− at�fracture (500s),�ave.�εPL in�DP�=�0.035�(TDP�=�0.016)�&�max.�σH =�713MPa�(in�TDP=825MPa)

� TDP�doesn‘t fail due�to lower eff.�Pl.�Strain???

• TDP-Steeel

− max.�strain (air)�Φmax =�0.030�(=3.0%)

− max.�pl. deformation at�notch base 2.5%

− max.�hydrostatic stress�ca.�1014�MPa

− eff.�pl. deformation at�max.�hydrostatic stress�ca.�0.50%

− at�fracture (650s),�average εPL in�TDP�=�0.040�&�max.�σH =�1015MPa


08.10.2014




• Introduction









08.10.2014



Summary�and�Conclusions�(i)

• Two materials (DP�+�TDP)�with the same�chemistry have been engineered with lab�annealing cycles

• Advantages�during uniaxial loading in�DP�are due�to large�disparity in�mechanical properties ofinduced phases;�TDP�is designed to have lower disparity

• Both materials host�transformation-induced dislocations,�but�TDP�has pinned dislocations

• Both materials exhibit similar H-management�in�undeformed state

• TDP�endures a�higher H-enrichment at�notch base due�to higher hydrostatic stress

• plastic deformation (locally and macroscopically)�in�TDP�is greater than in�DP

� TDP,�the material�with (more)�homogeneous mechanical properties at�the microscopic level,�exhibits a�higher resistance to hydrogen�embrittlement

Combining Material�Characterisation &�Testing /�Simulation�Results


08.10.2014



Summary�and�Conclusions�(ii)

• Open�question:�

− observations based on�ferritic microstructures (high�diffusibility,�low solubility)�� valid�forfundamentally different�microstructures?

• When developing rankings,�please combine apples with apples

− comparing loss of ductility (delta-factors)�can lose�sight of the higher absolute�elongation

− rankings should be developed between materials that satisfy requirements for an�application(multiple�properties,�such�as yield stress,�strength,�elongation,�bending &/or HER)

Open�Questions and Requests


08.10.2014



Thank-you for yourattention…

Questions?

Research�for this publication was�supported in�part by theRFCS�grant number RFSR-CT-2010-00020

Documents

Considerations for the Development of New Ultra … Steel Europe Considerations for the Development of New Ultra-High Strength Steels ExperiencesfromExperiments andSimulationsof HydrogenEmbrittlement