Toughness Enhancement of High Strength Low Alloy Strip Steels

207 Pages·2014·7.38 MB·English

by  Rachel Bridget Punch

Toughness Enhancement of High Strength Low Alloy
Strip Steels
By
Rachel Bridget Punch

A thesis
submitted to The University of Birmingham
for the degree of DOCTOR OF PHILOSOPHY

School of Metallurgy and Materials
College of Engineering and Physical Sciences
University of Birmingham
September 2013
University of Birmingham Research Archive

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Abstract

High strength strip steels with a yield strength of ~700 MPa are used in the yellow goods
industry, with recent trends requiring thicker strips with an improved toughness. Strip steel
was produced to thicknesses of 10 – 17 mm, with thinner strips showing improved Charpy
impact toughness compared to the thicker strip. It was found that all samples (different
thicknesses and test orientations) showed splits on the fracture surfaces in the upper transition
region.

The hot rolled strip steels showed a predominantly fine-grained (~4 µm ECD) ferrite
microstructure, with some coarse grain patches (grains ≥5 µm grouped together). No
significant differences were noted for the grain size, yield stress, tensile strength, hardness
and texture for the different compositions. Splits were characterised and produced
successfully by low blow Charpy testing with acoustic emission (AE) sensors indicating the
presence of splits which occurred in the upper transition region predominantly without main
crack propagation. Analysis by x-ray tomography and sectioning was carried out showing
splits propagate by transgranular cleavage, preferentially following coarse-grained regions,
with the 10 mm strip producing deeper splits which was related to elongated grains (high
aspect ratio).

Therefore the presence of deep splits improved toughness through a reduction of the impact
transition temperature, which was achieved by ensuring the presence of a bimodal grain
structure, containing coarse elongated grain patches.
Publications

R. Punch, M.Strangwood and C.L Davis, “Split formation during low blow Charpy
testing of high strength strip steels”, Metallurgical and Materials Transactions A,
December 2012, Volume 43, Issue 12, pp 4622-4632

R. Punch, A.G. Kostryzhev, M.Strangwood and C. L. Davis “The effect of
microstructure on the origin and propagation of splits in high strength strip steel”
EuroMat, Sept 2011 (Poster)

R. Punch, A.G. Kostryzhev, M.Strangwood and C. L. Davis “Investigation into the
initiation and propagation of splits in high strength strip steel” ATCOM 2011, Ranchi,
India, July 5-7th, 2011

A.G. Kostryzhev, R. B. Punch, C. L. Davis, and M. Strangwood Acoustic emission
monitoring of split formation during Charpy impact testing of high strength steel,
Material Science and Technology, 2012,(2), 28,pp 240-242
Acknowledgements

There are many people to who I owe a lot for their help during my time as a PhD student.
Firstly I need to thank my supervisors Professor Claire Davis and Dr Martin Strangwood, for
their constant support and encouragement. I have learnt a great deal from both of them and
could not have asked for better supervisors. Thank you to my industry supervisor, Dr Dave
Crowther from Tata Steel, for his support and advice throughout the project, along with the
Materials and Metallurgy department, in particular Dr Carl Slater, who was always on hand
to discuss latest test results or just grab a morning coffee.

I could not have carried out this project if it wasn’t for the financial support of the
Engineering and Physical Sciences Research Council (EPSERC) and Tata Steel. Thank you
for giving me the opportunity.

Finally, and most importantly to my family. To my parents, for always supporting my
decisions, and being there for me. To my family and friends for the fun times and words of
encouragement, and last but not least to Simon Holland. Thank you for your support, I
couldn’t have done it without you.
CONTENTS
1. INTRODUCTION ........................................................................................................... 20
1.1. High strength low alloy (HSLA) steel strip ........................................................... 20
1.2. Processing of HSLA steels .................................................................................... 22
1.2.1. Reheating ........................................................................................................... 24
1.2.2. Hot Deformation ................................................................................................ 27
1.3. Recrystallisation .................................................................................................... 29
1.3.1. Cooling and Coiling Temperature ..................................................................... 33
1.4. Toughness ..................................................................................................................... 35
1.5. Strengthening mechanisms ........................................................................................... 36
1.5.1. Grain size ........................................................................................................... 36
1.5.2. Solid Solution Strengthening ............................................................................. 38
1.5.3. Phase Balance Strengthening ............................................................................. 39
1.5.4. Precipitation Strengthening ................................................................................ 43
1.5.5. Work Hardening................................................................................................. 44
1.6. Alloying Elements ................................................................................................. 44
1.6.1. Effects of Chromium.......................................................................................... 47
1.6.2. Effects of Nickel ................................................................................................ 47
1.6.3. Effects of Molybdenum ..................................................................................... 47
1.6.4. Effects of Titanium ............................................................................................ 48
1.6.5. Effects of Vanadium .......................................................................................... 49
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1.6.6. Effects of Niobium ............................................................................................. 51
1.7. Inclusions ............................................................................................................... 53
1.8. Ductile to Brittle Transition Temperature during Charpy testing ......................... 55
1.8.1. Predicting ITT .................................................................................................... 58
1.9. Effect of coarse TiN particles on toughness .......................................................... 59
1.10. Effect of Texture .................................................................................................... 61
1.11. Splits (laminations, fissures).................................................................................. 63
1.11.1. Effect of splits on the tensile tri-axiality ahead of the Charpy notch ............. 64
1.11.2. Effect of Charpy specimen thickness on split formation ............................... 71
1.12. Formation of split cracks ....................................................................................... 73
1.12.1. Role of texture ................................................................................................ 73
1.12.2. Role of inclusions ........................................................................................... 75
1.12.3. Role of cementite particles and carbides on grain boundaries ....................... 79
1.12.4. Grain elongation ............................................................................................. 80
1.12.5. Role of banded microstructure ....................................................................... 84
1.12.6. Role of specimen orientation (longitudinal and transverse specimens) ......... 86
1.12.7. Role of yield stress ......................................................................................... 89
1.13. Summary of literature ............................................................................................ 90
1.14. Aims and objectives………………………………………………….…………. 92
2. MATERIALS, PROCESSING AND EXPERIMENTAL PROCEDURE ...................... 94
2.1. Materials Studied ................................................................................................... 94
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2.2. Material Processing ............................................................................................... 95
2.3. Experimental Techniques ...................................................................................... 96
2.3.1. Optical microscopy and image analysis ............................................................. 96
2.3.2. Scanning Electron Microscopy (SEM) analysis ................................................ 96
2.3.3. Electron Backscattered Diffraction (EBSD) ...................................................... 97
2.3.4. Thermodynamic modelling ................................................................................ 97
2.3.5. Hardness testing ................................................................................................. 97
2.3.6. Impact testing ..................................................................................................... 98
2.3.7. Acoustic Emissions (AE) testing ..................................................................... 100
2.3.8. X-ray tomography and Matlab image reconstruction ...................................... 100
2.3.9. Split characterisation……………………………………………………..…..101
3. CHARACTERISATION OF AS-RECEIVED STRIPS ................................................ 102
3.1. Mechanical properties .......................................................................................... 102
3.2. Hardness .............................................................................................................. 103
3.3. Microstructure ..................................................................................................... 104
3.3.1. Contributions to yield stress............................................................................. 110
3.3.2. EBSD ............................................................................................................... 111
3.3.3. TiN ................................................................................................................... 113
3.3.4. Thermo-Calc predictions for TiN .................................................................... 116
3.3.5. Charpy impact testing ...................................................................................... 117
3.4. Summary of section 3 .......................................................................................... 121
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4. FRACTURE ANALYSIS .............................................................................................. 122
4.1. TiN Inclusions ..................................................................................................... 122
4.1.1. Summary of effect of TiN on fracture ............................................................. 125
4.2. Splits/ Fissures/ Laminations ............................................................................... 126
4.3. Split Fracture Mode ............................................................................................. 127
4.4. Split Crack Initiation ........................................................................................... 129
4.4.1. Low blow Charpy testing ................................................................................. 129
4.4.2. Acoustic Emission testing ................................................................................ 129
4.4.3. X-ray tomography ............................................................................................ 134
4.5. Summary of Section 4 ......................................................................................... 137
5. SPLITS - COMPARISON OF LONGITUDINAL 10 AND 16.8 MM THICK STRIPS.... 138
5.1. Charpy impact toughness of the 16.8 and 10 mm thick strips in the longitudinal
orientation. ......................................................................................................................... 139
5.2. Split Crack Initiation ........................................................................................... 140
5.3. Split Crack Propagation Relative to the Microstructure ...................................... 144
5.3.1. Texture in the longitudinal 10 and 16.8 mm thick strip ................................... 149
5.3.2. Effect of yield strength, carbide thickness and second phases on the ITT ...... 151
5.3.3. Aspect ratio ...................................................................................................... 152
5.3.4. Grain boundary misorientation ........................................................................ 154
5.4. Sub-size Charpy Specimens ................................................................................ 155
5.5. Summary of section 5 ............................................................................................... 155
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6. SPLITS - COMPARISON OF LONGITUDINAL AND TRANSVERSE
ORIENTATIONS .................................................................................................................. 157
6.1. Microstructure ..................................................................................................... 158
6.2. Comparison between the ITT Values for the Transverse and Longitudinal
Orientations ........................................................................................................................ 159
6.3. Low Blow Testing for the 16.8 and 10 mm Thick MAD Strips in the Transverse
Orientation ......................................................................................................................... 161
6.4. Summary of section 6 .......................................................................................... 167
7. STEEL X ........................................................................................................................ 169
7.1. Steel X Microstructure ......................................................................................... 170
7.2. TiN Inclusions ..................................................................................................... 171
7.3. Charpy Impact Curves ......................................................................................... 173
7.4. Fracture Analysis ................................................................................................. 176
4.1 Steel X - Texture / Mis-orientation ...................................................................... 176
7.5. Splits in Steel X ................................................................................................... 178
7.6. Summary of section 7 .......................................................................................... 179
8. CONCLUSIONS............................................................................................................ 181
9. FUTURE WORK ........................................................................................................... 185
10. APPENDIX .................................................................................................................... 187
10.1. Code for Matlab programme. .............................................................................. 187
11. REFERENCES .............................................................................................................. 192

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