FUNDAMENTALS OF MATERIALS MODELLING
FOR HOT STAMPING OF UHSS PANELS WITH
GRADED PROPERTIES
by
NAN LI
A thesis submitted for the degree of Doctor of Philosophy of Imperial College London and the Diploma of Imperial College London
Department of Mechanical Engineering Imperial College London November 2013
DECLARATION OF AUTHORSHIP
I, Nan Li, declare that the work presented in this thesis titled ‘fundamentals of materials modelling for hot stamping of UHSS panels with graded properties’ is my own, all else is appropriately referenced.
The copyright of this thesis rests with the author and is made available under a Creative
Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work.
I
ABSTRACT
The aim of this study is to develop the fundamentals of materials modelling to enable effective process control of hot stamping for forming UHSS panels with graded properties for optimised functional performance. A selective heating and press hardening strategy is adopted to grade the microstructural distribution of a press hardened component through differential heat treatment of the blank prior to forming. Comprehensive material models, to enable prediction of austenite formation and deformation behaviours of boron steel under hot forming conditions, as well as the dynamic response of a press hardened part with tailored properties in collision situations, have been developed based on experimental investigations and mechanism studies. The research work is concerned with four aspects: feasibility of the selective heating and press hardening strategy, austenite formation in boron steel during selective heating, thermo-mechanical properties of boron steel under hot stamping, and mechanical properties of boron steel with various microstructures at room temperature.
Feasibility studies for the selective heating and press hardening strategy were carried out through a designed experimental programme. A lab-scale demonstrator part was designed and relevant manufacturing and property-assessment processes were defined. A heating technique and selective-heating rigs were designed to enable certain microstructural distributions in blanks to be obtained. A hot stamping tool set was designed for forming and quenching the parts. Test pieces were formed under various heating conditions to obtain demonstrator parts having variously graded microstructures. Microstructural distributions in the as-formed parts were determined through hardness testing and microstructural observation. Ultimately, the structural performance of the parts was evaluated through bending tests.
II
Heat treatment tests were performed to study the formation of austenite in boron steel during selective heating. Characterisation of the effects of heating rate and temperature on transformation behavior was conducted based on the test results. A unified austenite formation model, capable of predicting full or partial austenite formation under both isothermal and non- isothermal conditions, was developed, and determined from the heat treatment test results.
Hot tensile tests were performed to study the thermo-mechanical properties of the austenite and initial phase (ferrite and pearlite) of boron steel. The viscoplastic deformation behaviours of the both phase states were analysed in terms of strain rate and temperature dependence based on the test results. A viscoplastic-damage constitutive model, capable of describing the thermo- mechanical response of boron steel in a state corresponding to hot stamping after selective heating, was proposed. Values of constants in the model for both the austenite and initial phase were calibrated from the hot tensile test results.
Dynamic and quasi-static tensile testes combined with hardness testing and microstructural observation were carried out to study the mechanical properties of press hardened boron steel with various microstructures at room temperature. Based on the results, the strain rate sensitivity of the martensite and initial phase of boron steel was characterised; the relationships between mechanical properties (true ultimate tensile strength, 0.2% proof stress, elongation, and hardness) and phase composition (volume fraction of martensite), for boron steel with various microstructures, were rationalised. Finally, a viscoplastic-damage constitutive model, capable of predicting the mechanical response of a press hardened boron steel part with graded properties being subjected to crash situations in automobiles, were developed, and determined from the test results.
III
ACKNOWLEDGEMENTS
First and foremost, I would like to express my deep gratitude to my supervisors Professor
Jianguo Lin and Dr Daniel S. Balint for their continuous guidance, support, and encouragement throughout my PhD study. Their discipline, expertise, and inspiration have been indispensable to my growth as a researcher. I am also very grateful to Professor Trevor A. Dean of the University of Birmingham for his valuable advice and great help on my work, papers, and thesis.
Financial support from the industrial sponsor SAIC Motor UK Technical Centre is acknowledged.
I would like to thank Dr Damian Dry and Mark Hillier of SAIC for their advice on my PhD project and provision of material. I am also indebted to my previous supervisor Professor Weimin
Gao of Tongji University for facilitating the project and being willing to help all the time.
Financial support from the ‘UK-China Scholarships for Excellence’ scheme is also acknowledged.
Sincere thanks to the technical staff of Department of Mechanical Engineering, Imperial College
London: Hugh MacGillivray, Dr Leonard Wanigasooriya, Suresh Viswanathan Chettiar, Amit
Choda, and Mark Holloway. Their assistance was important for the success of my experimental work. Very many thanks to my previous and current colleagues and officemates: Dr Awang Ngah,
Shamsiah, Dr Qian Bai, Dr Jingqi Cai, Guicai Chen, Shouhua Chen, Thomas Dunnett, Xuetao Li,
Dr Ali Mehmanparast, Dr Mohamed Saad Kamel Mohamed, Aditya Narayanan, Dr Panagiotis
Sphicas, Zhutao Shao, Dr Liliang Wang, Yi Wang, Dr Xiaoyu Xi, Dr Haoliang Yang, Kailun
Zheng and all who helped me and encouraged me in any respect during my PhD study. My special thanks are given to my lovely PhD-Village members: Qi Cao, Lei Ding, Ran Fei, Yeyi
Liu, Yijiang Wu and Lingling Zheng. The four-year journey has not been easy, but I have been
IV lucky to have them accompany. I would also like to express my appreciation to Dr Mengnan
Shen and Xiaofeng Ju for providing me support and strength from China.
Last but not least, I wish to express a heartfelt gratitude to my beloved parents for their endless trust, understanding, patience, and support. They hope me around, but they set me free and taught me to be strong. Without this, my PhD would not have been possible. Thank you, my father and mother, I own you so much.
V
NOMENCLATURE
α Ferrite
θ Cementite
ψ Initial phase (ferrite and pearlite)
γ Austenite
Ae1, Ae3 Temperature to start and complete austenite formation under
equilibrium conditions, K
Ac1, Ac3 Temperature to start and complete austenite formation during
continuous heating, K v Relative volume change
vψ0, vψ(T) Relative volume change of the initial phase at 873K and at any
temperature
vγ0, vγ(T) Relative volume change of austenite at 873K and at any temperature
Cψ, Cγ Thermal expansion coefficient of the initial phase and austenite, /K
N Austenite nucleation rate
G Austenite growth rate
R Gas constant, J/ mol·K
T Absolute temperature, K
T Heating rate, K/s
Tx Temperature corresponding to certain volume fraction of austenite, K
VI
e.g. T50% is the temperature when volume fraction of austenite reaches
50% t Instantaneous time, s (origin: time at temperature of 873K)
tAc1 Time to start austenite formation, s (same origin as t)
tX Time corresponding to certain volume fraction of austenite, s (same
origin as t)
e.g. t80% is the time when volume fraction of austenite reaches 80%
∆t Soaking time increment, s
∆tX1-X2 Time to increase volume fraction of austenite from x1 to x2 during
soaking, s
e.g. Δt80%fs-90%fs is the time to increase volume fraction of austenite from
80% of fs to 90% of fs during soaking
∆t' Time increment during continuous heating, s
' ∆t Ac1-X Time increment from starting austenite formation to reaching certain
volume fraction of austenite during continuous heating, s
fM Volume fraction of martensite
fA Volume fraction of austenite
' Extended volume fraction of austenite fA
fAs Saturated volume fraction of austenite
fP Volume fraction of pearlite
N Nuclei quantity of austenite per unit sample volume
VII
QN Activation energy related to nucleation of austenite, J/mol v Volume growth rate of an austenite nucleus
Ve Extended volume of austenite in a unit of real sample volume
Ve Growth rate of the extended volume of austenite
Qv Activation energy related to volume growth of austenite, J/mol m ,n Parameters related to the impingement mechanism of austenitization
Tm Melting temperature, K