High-strength steels, characterized by an excellent combination of mechanical properties, such as ductility and formability, with satisfactory weldability and cost, are in continuous demand in most industries, especially the automotive industry, where reducing vehicle weight is an important means of improving both fuel efficiency and passenger safety. A new generation of nanostructured bainitic steels with high carbon-silicon content, known as carbide-free bainitic ferrite (CFB), has been developed to meet the above demand. However, high-C nanostructured CFB steel needs to be heat-treated, sometimes for several days, to attain the required microscopic structural features. Furthermore, the drawbacks of high carbon content are weldability problems, where cracks can be developed in the heat-affected zones due to the formation of untempered martensite. In this context, low- or medium-C nanostructured CFB steel could be an excellent alternative as it overcomes the shortcomings of high-C nanostructured CFB steels. However, obtaining nanostructured CFB in low- and medium-C steel can be challenging because reducing the steel’s carbon content increases the martensite start temperature and diminishes the difference between bainite and martensite start temperatures, ultimately resulting in losing the benefits of performing bainite transformation at a low temperature.
Among the prominent parameters that control the bainitic ferrite plate thickness is the strength of the austenite from where the bainite grows and the driving force of the transformation. Therefore, thermomechanical treatments such as ausforming can be a possible solution to overcome the limitations of using high carbon steel. The work hardening of supercooled austenite, owing to deformation prior to bainite transformation, resists the thickening growth of bainitic ferrite plates. Besides, the kinetics of bainite transformation, microstructure morphology, and retained austenite stability can be affected. Therefore, in order to achieve the above objectives, a more fundamental understanding of the mechanisms involved in the evolution of the microstructure as a result of the ausforming process in medium-C high Si steels is required. In particular, the impact of ausforming on bainitic transformation and the characteristics of bainitic laths needs to be understood and related to the ausforming parameters.
The present Ph.D. project aims to address the above question with the view to ultimately determine optimum ausforming conditions in order to obtain nanosized bainitic plates in a medium carbon high Si steel. To this end, three major objectives were identified: The first part of the study provides the details of how the stability of deformed retained austenite was modified according to the amount of deformation at 600 oC prior to isothermal bainitic transformation at 325 to 400 oC. The analysis showed that the main factor contributing to retained austenite's stability was bainitic isothermal temperature regardless of percent deformation. For the studied steel, bainitic transformation above Ms and below 350 oC was the threshold temperature above which the retained austenite within the CFB matrix decomposed dramatically to martensite as percent deformation increased. CFB microstructure obtained at 325 oC had the highest retained austenite thermal stability among other bainite transformation temperatures.
In the second part of the study, the anisotropy in the morphology of CFB is correlated to the transformation plasticity strain (TP) as a function of ausforming temperature (above or below bainite start temperature, (Bs). The microstructural change observed by SEM and EBDS analysis were compared with TP strains evolution calculated by axial and radial strains during bainite transformation at 325 oC after deforming supercooled austenite at 600, 400, and 325 oC. Ausforming at 600 oC had a limited effect on microstructural changes and TP strains and comparable with the pure isothermal condition. An alignment in bainitic ferrite plates was observed when a compressive plastic deformation was applied below Bs. The TP strain intensified with severe alignment in the microstructure when the ausforming was applied at the isothermal transformation temperature (325 oC). Moreover, nanostructured morphology with ~100 nm bainitic ferrite plate thickness was obtained, and the hardness reached a level similar to that of a martensitic state (~550 HV).
Finally, the response of the CFB microstructure to tempering process, obtained via pure isothermal transformation (P-bainite), i.e., no deformation applied prior to bainitic transformation, and ausforming followed by isothermal transformation (A-bainite), was investigated. The aim of this study was done in order to determine the underlying mechanisms of observed dilatometric and microstructural changes during isothermal tempering at 400 and 500 oC. The outcome of this investigation showed that the P-bainite condition had minimal dilatometric changes for all examined tempering temperatures. In addition, no microstructural changes, such as cementite precipitation, revealed either by metallography or XRD examination, expect a mild increase in bainitic ferrite plats by about 20%. Consequently, the hardness level before and after tempering has not changed much (6% decrease). Likewise, ausformed sample (A-bainite) showed minimum dilatometric and microstructural change during tempering at 400 oC.
On the other hand, a large contraction in dilatometric signal was observed in the ausformed bainitic microstructure that tempered at 500 oC. The change in dilatation started at the early stage of the heating step at around 400 oC. The observed contraction was linked to cementite precipitation due to either decomposition of bainitic ferrite and/or film-like retained austenite. The cementite precipitates could be revealed by SEM and XRD techniques with an amount of about 6%. The presence of these precipitates prevents the large coarsening of bainite ferrite, thereby producing hardness levels similar to that of non-tempered conditions (14% decrease). Moreover, the calculation of carbon content in bainitic ferrite showed a remarkable decrease, from 0.19 to 0.08 wt.% after tempering at 500 oC, compared with other investigated conditions. This can be related to altering the supersaturated bainitic ferrite crystal (BCT) to itsequilibrium state (BCC). The significant effect of tempering in the A-bainite sample is due to the enhancement of carbon diffusion through the existence of high dislocation density.
| Date | 17 Nov 2021 |
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| Original language | American English |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Mohammad Jahazi (Supervisor) & Carlos Garcia-Meteo (Co-supervisor) |
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