Transformations in TRIP-assisted Steels: Microstructure and Properties

A thesis submitted for the degree of Doctor of Philosophy
Sourabh Chatterjee
Darwin College, University of Cambridge
November, 2006

Abstract
Despite the presence of high-carbon martensite, TRIP-assisted steels
possess large uniform elongation. High-carbon martensite is normally brittle.
In this thesis, it has been demonstrated that this apparent anomaly is due
to the fine size of the martensite plates.
The mechanical properties of these steels are due to the transformation
of retained austenite into martensite during deformation and hence appear to
be dominated by the volume fraction and carbon content of retained austenite.
These parameters have been related to the chemical composition and
heat treatment of the steels with neural networks, using published data.
An optimum alloy was formulated by combining the neural network
with a genetic algorithm, to minimise the silicon addition whilst maximizing
the retained austenite fraction. This resulted in the creation of a radically
different microstructure, designated ?-TRIP.
Transformation of austenite into martensite during deformation ceases
beyond a critical strain. A theory has been developed to predict this limit.
Calculations using the theory indicate that the high-carbon austenite in these
steels may transform into martensite due to stress, rather strain.
These materials are often tested for stretch- flangeability, a measure of
formability. Neural network analysis of the published data revealed the ultimate
tensile strength to be the most important tensile parameter influencing
stretch- flangeability.

Contents
1 Introduction. .. .. .. .. .. .. .. .. .. .. .. .. .. . . 1
1.1 Scope of the research. .. .. .. .. .. . . 2
2 TRIP-assisted steels. .. .. .. .. .. .. . . 4
2.1 Martensite and TRIP steels. .. .. .. .. . . 5
2.2 Mode TRIP-assisted steels. .. .. .. .. . . 8
2.3 Microstructural evolution. .. .. .. .. .. . 9
2.4 Alloying elements in TRIP-assisted steels. .. .. . . 16
2.5 Mechanical performance. .. .. .. .. .. . 18
2.6 Factors aecting performance. .. .. .. .. . 20
2.6.1 Proportion of phases. .. .. .. .. . . 21
2.6.2 Stability of retained austenite. .. .. .. . 22
2.6.3 Test parameters. .. .. .. .. .. . 26
2.6.4 State of stress or strain. .. .. .. .. . 29
2.7 Strain-induced martensite formation. .. .. .. . 30
2.8 Special properties. .. .. .. .. .. .. . 33
2.8.1 Formability. .. .. .. .. .. . . 33
2.8.2 Crash-worthiness. .. .. .. .. .. . 36
2.8.3 Fatigue resistance. .. .. .. .. .. . 36
2.8.4 Bake hardening. .. .. .. .. .. . 38
2.9 Limitations. .. .. .. .. .. .. . . 39
2.10 Other variants. .. .. .. .. .. .. . . 41
2.11 Summary. .. .. .. .. .. .. .. . 42
3 Brittle martensite. .. .. .. .. .. .. . . 45
3.1 Hypothesis. .. .. .. .. .. .. .. . 45
3.2 Experiments. .. .. .. .. .. .. . . 46
3.3 Results and discussion. .. .. .. .. .. . 48
3.4 Simulating microstructural evolution. .. .. .. . 55
3.5 Martensite in TRIP-assisted steels. .. .. .. . . 60
3.6 Experiments. .. .. .. .. .. .. . . 61
3.7 Results and discussion. .. .. .. .. .. . 64
3.8 Summary. .. .. .. .. .. .. .. . 69
4 Microstructural Modelling. .. .. .. .. . . 71
4.1 Neural network modelling. .. .. .. .. . . 71
4.2 Model for retained austenite fraction. .. .. .. . 75
4.2.1 Database. .. .. .. .. .. .. . 75
4.2.2 Model characteristics. .. .. .. .. . . 76
4.2.3 Model predictions. .. .. .. .. .. . 78
4.3 Model for carbon in retained austenite. .. .. .. . 85
4.3.1 Database. .. .. .. .. .. .. . 85
4.3.2 Model characteristics. .. .. .. .. . . 85
4.3.3 Model predictions. .. .. .. .. .. . 86
4.4 Summary. .. .. .. .. .. .. .. . 93
5 ?-TRIP steel. .. .. .. .. .. .. . . 95
5.1 Optimisation. .. .. .. .. .. .. . . 96
5.2 Optimised TRIP-assisted steel. .. .. .. .. . 98
5.3 Thermodynamic calculations. .. .. .. .. . . 99
5.4 Experiments. .. .. .. .. .. .. . . 102
5.5 Results and discussion. .. .. .. .. .. . 104
5.6 Summary. .. .. .. .. .. .. .. . 121
6 Mechanical Stabilisation. .. .. .. .. .. . .125
6.1 Mechanical driving force. .. .. .. .. .. . 125
6.2 Role of strain. .. .. .. .. .. .. . . 127
6.3 Mathematical formulation. .. .. .. .. . . 129
6.4 Results and discussion. .. .. .. .. .. . 132
6.4.1 Austenitic stainless steels. .. .. .. .. . 133
6.4.2 TRIP-assisted steels. .. .. .. .. . . 137
6.4.3 Bainitic steels. .. .. .. .. .. . . 143
6.4.4 Athermal martensite. .. .. .. .. . . 145
6.5 Summary. .. .. .. .. .. .. .. . 148
7 Formability. .. .. .. .. .. .. .. .149
7.1 Stretch-flangeability. .. .. .. .. .. . . 149
7.2 Neural networks. .. .. .. .. .. .. . 150
7.3 Results and discussion. .. .. .. .. .. . 151
7.4 Summary. .. .. .. .. .. .. .. . 159
8 Conclusions. .. .. .. .. .. .. .. . 164

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