Influence of the hot rolling process on the mechanical behavior of dual phase steels [Elektronische Ressource] / Mehdi Asadi

Lucy

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Influence of the Hot Rolling Process on the Mechanical Behavior of Dual Phase Steels Dissertation Zur Erlangung des Grades eines Doktors der Ingenieurwissenschaften vorgelegt von Mehdi Asadi aus Teheran / Iran genehmigt von der Fakultät für Natur- und Materialwissenschaftlichen der Technischen Universität Clausthal Tag der mündlichen Prüfung: 21.10.2010 Bibliografische Information Der Deutschen NationalbibliothekDie Deutsche Nationalbibliothek verzeichnet diese Publikation in der DeutschenNationalbibliografie; detaillierte bibliografische Daten sind im Internet überhttp://dnb.d-nb.de abrufbar.Vorsitzender der Prüfungskommission: Prof. Dr.-Ing. Albrecht WolterHauptberichterstatter: Prof. Dr.-Ing. Heinz PalkowskiBerichterstatter: Prof. Dr.-Ing. Bruno De CoomanD 104© PAPIERFLIEGER VERLAG GmbH, Clausthal-Zellerfeld, 2011.Telemannstraße 1 38678 Clausthal-Zellerfeldwww.papierflieger.euAlle Rechte vorbehalten. Ohne ausdrückliche Genehmigung des Verlagesist es nicht gestattet, das Buch oder Teile daraus auf fotomechanischemWege (Fotokopie, Mikrokopie) zu vervielfältigen.1. Auflage, 2011ISBN 978-3-86948-148-7 Acknowledgements Acknowledgements I would like to express my sincere thanks to my supervisor Prof. Dr.-Ing. Heinz Palkowski for his strong support, guidance and giving me the opportunity to pursue my Ph.D.

Informations

• Publié par: technische_universitat_clausthal
• Publié le: 01 janvier 2011
• Nombre de lectures: 43
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Influence of the Hot Rolling Process on the Mechanical Behavior of Dual Phase Steels

Dissertation

Zur Erlangung des Grades eines Doktors der Ingenieurwissenschaften vorgelegt von Mehdi Asadi

aus Teheran / Iran genehmigt von der Fakultät für Natur- und Materialwissenschaftlichen der Technischen Universität Clausthal Tag der mündlichen Prüfung: 21.10.2010

Bibliografische Information Der Deutschen Nationalbibliothek Die Deutsche Nationalbibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http://dnb.dnb.de abrufbar.

Vorsitzender der Prüfungskommission:

Hauptberichterstatter:

Berichterstatter:

D 104

©

Prof. Dr.Ing. Albrecht Wolter

Prof. Dr.Ing. Heinz Palkowski

Prof. Dr.Ing. Bruno De Cooman

PAPIERFLIEGER VERLAG GmbH, ClausthalZellerfeld, 2011 . Telemannstraße 1 38678 ClausthalZellerfeld www.papierflieger.eu

Alle Rechte vorbehalten. Ohne ausdrückliche Genehmigung des Verlages ist es nicht gestattet, das Buch oder Teile daraus auf fotomechanischem Wege (Fotokopie, Mikrokopie) zu vervielfältigen.

1. Auflage, 2011

ISBN 9783869481487

 Acknowledgements

Acknowledgements

I would like to express my sincere thanks to my supervisor Prof. Dr.-Ing. Heinz Palkowski for his strong support, guidance and giving me the opportunity to pursue my Ph.D. degree at TU Clausthal. Furthermore, my deep gratitude is given to Prof. Dr.-Ing. Bruno De Cooman for his scientific support during my stay at GIFT in Pohang and for agreeing to be on my thesis committee despite his extremely busy schedules and the long travelling-distance. Moreover, the author is indebted to Prof. Dr.-Ing. Georg Frommeyer. The achievement of this work was a result of his wide ranging knowledge and support.

This thesis would not be what it is without the support and cooperation of the members of the department of “Werkstoffumformung” as well as the technical staff of the Institute of Metallurgy. I would like to express my thanks to the past and present scientific colleagues of the department for their continuous help, friendship and kindness: Thank you Kai-Michael Rudolph, Mohamed Soliman, Nicole Schlosser, Olga Sokolova, Hanaa Mostafa, Lu Yu, Mithat Akdesir and Marcus Kühn.

Moreover, I would like to acknowledge Salzgitter Flachstahl GmbH as well as Salzgitter Mannesmann Forschung GmbH, here especially Dr.-Ing. Thomas Evertz and Dr.-Ing. Markus Krieger, regarding to scientific supports and provision of the materials.

Thank is owed to my friend Dr.-Ing. Ali Aghajani at Ruhr-Universität Bochum for providing the facilities of TEM tests as well as to my students, especially Reza Kaboli and Oliver Steinbis, who supported me during my activities.

Special thanks are given to my family and to my parents. Without their wisdom and guidance, I would not have been able to reach the goals I have to strive and be the best to reach my dreams.

Finally, I wish to record my deep sense of appreciation for my lovely wife for relentlessly supporting me during the entire period, in spite of all the hardship. This work is dedicated to my wife, BEHNAZ, a loyal friend and compassionate critic.

i

________________________________________________________________

ii

 Table of Contents

Table of Contents Page  Nomenclature.............................................................................................. vi 1 Introduction.................................................................................................. 1 2 Theoretical Background.............................................................................. 4 Phase (DP) High Strength Steels2.1 Dual ............................................ 4  2.1.1 Alloying Elements in DP Steels ........................................................... 4  2.1.2 Martensite in DP Steels....................................................................... 5  2.1.3 Properties of DP Steels ....................................................................... 7 2.2 Thermomechanical Controlled Processing (TMCP)....................... 9  2.2.1 Effect of Deformation, Temperature and Strain Rate............................ 10  2.2.2 Effect of Cooling Rate......................................................................... 11  2.2.3 Effect of Microalloying Elements ....................................................... 12 Evolution after TMCP2.3 Microstructure ............................................ 13  2.3.1 Grain Structure ................................................................................. 13  2.3.2 Dislocation Generation and Recovery ............................................... 14 2.4 Mechanisms of Bake Hardening.................................................... 16  2.4.1 Snoek Rearrangement or Ordering ................................................... 18  2.4.2 Cottrell Atmosphere Formation ......................................................... 18  2.4.3 Precipitation of Coherent Carbides ................................................... 20 Factors of Bake Hardening Mechanisms2.5 Metallurgical ............... 21  2.5.1 Effect of Solute Carbon ..................................................................... 21  2.5.2 Effect of Grain Size ........................................................................... 22  2.5.3 Effect of Alloying Elements................................................................ 23  2.5.4 Effect of Temper Rolling and Prestraining ......................................... 26  2.6 Bake Hardening in Multiphase Steels............................................ 27 3 Experimental Methods and Details........................................................... 33  3.1 Materials........................................................................................... 33  3.2 Simulation of Roughing Rolling Process...................................... 33  3.3 Simulation of Finishing Rolling Process...................................... 34  3.4 Characterisation of Microstructure................................................ 36  3.4.1 Light Optical Microscopy ................................................................... 36  3.4.2 Thermal Etching ................................................................................ 37  3.4.3 Transmission Electron Microscopy (TEM)......................................... 38  3.4.4 Saturation Magnetization Measurements .......................................... 38  3.5 Tensile Testing................................................................................ 39  3.6 Bake Hardening Experiments........................................................ 40 4 Hot Deformation Parameters: Results and Discussion.......................... 41  4.1 Introduction..................................................................................... 41  4.1.1 Aim of the Study ................................................................................ 41  4.2 Thermomechanical Controlled Processing.................................. 42  4.2.1 Estimation ofT nRX .............................................................................. 42  4.2.2 Simulation of Finishing Rolling .......................................................... 45  4.3 Results............................................................................................ 48 FC  4.3.1 Phase Transformation Behavior and DefiningT............................ 48  4.3.2 Microstructure Evolution.................................................................... 50 iii

Table of Contentsn

4.3.3 4.3.4 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.5

5.1 5.1.1 5.2 5.3 5.3.1 5.3.2 5.3.3 5.3.4 5.4 5.4.1 5.4.2 5.4.3

Mechanical Properties....................................................................... 54 Bake Hardening Behavior ................................................................. 55 Discussion....................................................................................... 60 Influence of the Hot Deformation Schedules on the Phase Transformation Behavior ................................................................... 60 Influence of the Hot Deformation Schedules on the Microstructure .. 63 Influence of the Hot Deformation Schedules on the Mechanical Properties .......................................................................................... 65 Influence of the Hot Deformation Schedules on the Bake Hardening Behavior........................................................................... 67 Influence of the Prestraining and Baking Condition on the Bake Hardening Behavior .......................................................................... 69 Conclusions.................................................................................... 71 5 Martensite Content and Cooling Rate: Results and Discussion............ 73 Introduction..................................................................................... 73 Aim of the Study ................................................................................ 73 Simulation of Finishing Rolling..................................................... 73 Results............................................................................................ 75 FC Phase Transformation Behavior and DefiningT............................ 75 Microstructure Evolution.................................................................... 78 Mechanical Properties....................................................................... 79 Bake Hardening Behavior ................................................................. 80 Discussion....................................................................................... 85 Influence of Cooling Rate on the Phase Transformation Behavior .... 85 Influence of Cooling Rate and Martensite Volume Fraction on the Microstructure......................................................................... 86 Influence of Cooling Rate on the Mechanical Properties and Bake Hardening Behavior ................................................................. 88 Influence of Martensite Volume Fraction on the Mechanical Properties and Bake Hardening Behavior ......................................... 89 Conclusions.................................................................................... 91 6 Chemical Composition: Results and Discussion.................................... 93 Introduction..................................................................................... 93 Aim of the Study ................................................................................ 94 Investigated Materials and Alloying Concept............................... 94 Simulation of Roughing Rolling..................................................... 95 Simulation of Finishing Rolling..................................................... 96 Results............................................................................................ 97 FC Phase Transformation Behavior and DefiningT............................ 97 Microstructure Evolution.................................................................. 101 Mechanical Properties..................................................................... 104 Bake Hardening Behavior ............................................................... 106 Microstructure Evolution after Prestraining and Baking Process ..... 111 Discussion..................................................................................... 113 Influence of Alloying Elements on the Phase Transformation Behavior .......................................................................................... 113 Influence of Alloying Elements on the Microstructure...................... 116 Influence of Alloying Elements on the Mechanical Properties ......... 117

iv

5.4.4

5.5

6.1 6.1.1 6.2 6.3 6.4 6.5 6.5.1 6.5.2 6.5.3 6.5.4 6.5.5 6.6 6.6.1 6.6.2 6.6.3

 Table of Contents

6.6.4 Influence of Reheating Temperature on the Microstructure and  Mechanical Properties in Nb Microalloyed DP Steels...................... 120  6.6.5 Influence of Alloying Elements on the Bake Hardening Behavior.... 121 6.6.6 Influence of the Prestraining and Temperature on the Bake  Hardening Behavior......................................................................... 124  6.7 Conclusions.................................................................................. 133 7 Summary................................................................................................... 135 References................................................................................................ 138

v

Dynamic recrystallization

Carbon

Bake hardening [MPa]

Chromium

Ferrite/martensite

Cold rolled annealed

Field emission gun

Bake hardening coefficient

Body centered tetragonal

Nomenclature

Continuous dynamic recrystallization

C

bct

BH

CDRX

vi

FEG

F/M

F/F

d α,α’,γ

DRX

f α

f γ

DP

D

Cr

CRA

Ferrite fraction [%]

Upper limit temperature ofα+γphase field under equilibrium [°C]

Ae 3

Ae 1

Nomenclaturen

Atom probe tomography

Grain size (ferrite, martensite, prior austenite) [m]

2 -1 Diffusion coefficient for carbon [m ·s ]

AHSS

A

Abbreviations

Interaction parameter defining the strength of the atmosphere

Burgers vector [m]

Body centered cubic

Temperature at which conversion of austenite to ferrite is completed [°C]

Temperature at which austenite begins to convert to ferrite [°C]

b

Ar 3

APT

bcc

Ar 1

b BH

Ferrite/ferrite

Austenite fraction [%]

Advanced high strength steels

Dual phase steel

Lower limit temperature ofα+γphase field under equilibrium [°C]

HSLA

LAGB

HAADF

Hot dip galvanized

Martensite start temperature [°C]

Molybdenum

Concentration of C in solution [ppm]

Metadynamic recrystallization

Manganese

Nitrogen

Number of C atoms, diffusing to dislocations in a unit volume in time t

Non-recrystallization

High angle annular dark field

Low angle grain boundaries

Actual length of the sample measured during the transformation [m]

Graduate Institute of Ferrous Technology

vii

-23 -1 Boltzmann’s constant [1.38·10 J·K ]

Shear modulus [GPa]

Finishing temperature [°C]

M S

Mo

MVF

n

High strength low alloy

 Nomenclature

Low carbon

NoRX

n 0

Nb

Numerical constant [-]

1/2 Hall-Petch coefficient [MPa ·m ]

Strength coefficient [MPa]

Light optical microscopy

Taylor factor [-]

MFS

MDRX

N(t)

Mn

N s

N

Niobium

F T

G

GIFT

HDG

K

Work hardening coefficient [-]

Mean flow stress [MPa]

k y

k B

K d

l

m

LC

LOM

Martensite volume fraction [%]

-2 Initial dislocation density [m ]

Yield strength [MPa]

Recrystallization

Scanning transmission electron microscopy

Recrystallization stop temperature

Austenitization time [s]

Thermomechanical controlled processing

Austenitization temperature [°C]

Salzgitter Flachstahl GmbH

Nomenclaturen

Recrystallization time temperature

FC T

-1 -1 Gas constant [8.314 J·mol ·K ]

Transmission electron microscopy

Room temperature [°C]

Zener-Hollomon temperatur [K]

Phosphorus

Numerical constant [-]

Tensile strength [MPa]

R p0.2

R e

R

R m

viii

TMCP

Saturation magnetization

T nRX

Lower yield strength [MPa]

T z

RTT

RUB

RT

RX

TEl

TEM

t A

SM

RXST

Si

S

PS

Q def

P

Fast cooling start temperature [°C]

Holloman-Jaffe temperature [K]

Total elongation [%]

Absolute temperature [K]

Silicon

Temperature [°C]

-1 Activation energy for deformation [J·mol ]

Ruhr-Universität Bochum

STEM

SZFG

T HJ

T abs

T A

Prestrain [%]

Non-recrystallization temperature [°C]

T

Transformation induced plasticity

-2 Generated dislocation density [m ]

Universal testing machine

Ultra low carbon

Martensite

Vanadium

Greek Symbols

Transformed austenite fraction [-]

Austenite

Friction stress [MPa]

Corresponding stress for 2 % plastic strain [MPa]

-2 Annihilated dislocation density [m ]

Engineering strain, deformation [%]

Slip distance of the dislocation [m]

Critical true strain [-]

Angle [°]

Total true strain [-]

X

Zener-Hollomon parameter [-]

Work hardening [MPa]

True stress [MPa]

-1 Strain rate [s ]

Yiled stress [MPa]

Z

True strain [-]

γ

θ

λ

ε

-2 Dislocation density [m ]

 Nomenclature

Reheating temperature [°C]

ρ ann

UTS

ULC

σ

ρ gen

ρ

φ c

φ t

WH

V

T R

Proportionality constant [-]

TRIP

β

α’

Ferrite

α

ix

σ y

ϕ &

φ

σ 2.0

σ 0