Bainitic steel with 30 GPa% characteristics
The ability of a steel to be formed is often measured as the product of the ultimate tensile strength and total elongation (GPa%). The aim of the work presented in this thesis was to produce a bainitic steel by continuous cooling transformation, with a ultimate tensile strength exceeding 1000 MPa and yet having a ductility of 30%. The challenge associated with the development is that the steel must be spot-weldable, which means that the carbon concentration needs to be kept low enough to avoid the formation of martensite in the heat-affected zones of resistance spot welds. Fundamental phase transformation theory is used for alloy design, followed by alloy manufacture and validation using a variety of microstructural and mechanical character- ization techniques. Progress is made in understanding the onset of plastic instability by critically examining the role of retained austenite in enhancing ductility. The literature on the transformation induced plasticity (TRIP) steels and factors affecting the strength and ductility in TRIP–assisted steels has been reviewed. A model employing deformation induced martensitic transformation coupled with per- colation theory has been revisited with fresh data collated from published work to estimate the elongation of TRIP-assisted steels. A novel quantitative estimation of the stability of austenite required for optimising ductility has been developed and demonstrated to explain the new data. Two alloys emerged from the sequence of alloy and process design, which successfully broke the 30 GPa% barrier while maintaining the ability for mass production. Furthermore, tempering of the steel at a low temperature for few hours improved ductility by softening the small amount of martensite formed during cooling to ambient temperature. “Premature” failure during mechanical testing is usually due to some sort of heterogeneity in the manufactured steel. Such behaviour can sometimes be mitigated by refining the microstructure. This was investigated to see whether the required combination of strength and ductility can be achieved by reducing the scale of the parent austenite grains to achieve greater structural uniformity. However, a detailed investigation showed that lowering the austenitization temperature led to a deterioration in the properties due to microstructural banding. Mechanical stabilization during bainitic transformation from deformed austenite in- creased the volume fraction of martensite thus making the microstructure unfavourable for optimum mechanical properties. Therefore bainite should be generated from a strain–free recrystallised austenite grain structure. The Habit plane, orientation relationship and associated shape strain were characterized simultaneously for the first time on individual bainite plates. The irrational habit plane of bainite measured by the two surface analysis was found to be close to {457}γ , which is not far off the {557}γ lath martensite habit plane reported in the literature. Pole figure analysis of three plates of bainite showed consistent results. The shear strain associated with the bainite plate is consistent with the value estimated by the phenomenological theory of martensite.
Bainite, Advanced high strength steels, Bainite crystallography, Habit plane, Atomic force microscopy, Prior austenite grain reconstruction, EBSD, Tensile tests, TRIP work hardening, Mechanical stability, Hardness entropy
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