Exploitation of the synergetic effect of Mo and Nb on high strength quenched and tempered boron steels

In response to the demanding strength and impact resistance market requirements, plates and pipes are usually quenched and tempered (Q&T) for several applications. Regarding the production of these high strength steels, the direct quenching process offers operational and economic advantages compared to the conventional quenching route. In this study, the applicability of the direct quenching strategy is evaluated. Moreover, the addition of boron as an alloying element is a common practice in high strength steels to ensure hardenability and promote bainitic and martensitic microstructures. In some cases, the addition of boron is not enough to ensure full martensite formation, and thus, the addition of Nb and Mo can increase the efficiency of boron. This thesis, is in the frame of an industrial project developed thanks to the collaboration of the International Molybdenum Association (IMOA), Dillinger and Ceit. This thesis is focused on the study of the addition of Nb, Mo and NbMo in boron high strength steels in terms of microstructure and mechanical properties. The results extracted during this project were useful for the development of new steel grades that fulfil the most demanding market requirements. Successful results were achieved from the industrial trials performed at Dillinger. With the purpose of analysing the impact of chemical composition, the applied strategy on hot working behaviour, phase transformation and mechanical properties, several thermomechanical treatments were completed. By means of different laboratory tests, such as torsion, dilatometry and plane strain compression tests, plate hot rolling and Q&T process were simulated. This project is divided in three main tasks and each of the task is in line with the different steps involved in a real industrial process. The first task is focused on the hot working behaviour of the studied steels and multipass and double-pass torsion tests were done. Multipass torsion tests were performed in order to define the critical temperatures such as the non-recrystallization temperature (Tnr). Additionally, double-pass torsion tests were carried out to analyse the softening kinetics and to validate different approaches available regarding recrystallization kinetics. Furthermore, plate hot rolling simulations were performed in torsion, with the purpose of analysing dynamic recrystallization behaviour in more depth. The second task is focused on the phase transformation analysis. Direct quenching (DQ) and conventional quenching (CQ) processing routes were simulated by dilatometry tests and from the dilatometry curves, Continuous Cooling Transformation (CCT) diagrams were built. In the third task, the relationship between microstructure and the resulting mechanical properties were analysed. To that end, plane strain compression tests were performed for simulating quenching (Q), as well as quenching and subsequent tempering (Q&T). From the obtained samples, tensile and Charpy specimens were machined to analyse the tensile and toughness properties. Regarding tensile properties, the contribution of different strengthening mechanism to yield strength (solid solution, grain size, dislocation density, carbon in solid solution and fine precipitation) were quantified. Likewise, the impact of different microstructural parameters (grain size, solid solution, dislocation density, presence of carbides, carbon in solid solution, fine precipitation and microstructural heterogeneity) on toughness were evaluated. Furthermore, an existing equation able to predict the impact transition temperature (ITT50%) for ferrite-pearlite and bainitic microstructures was extended to tempered martensitic microstructures. Regarding microstructural characterization, the obtained microstructures in each task were characterized using advanced characterization techniques, such as optical microscopy, field emission gun scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM). The microstructural characterization was completed by the electron backscattered diffraction (EBSD) technique, in order to quantify the crystallographic unit sizes and dislocation densities.