In-situ Neutron Diffraction and Small-Angle Neutron Scattering (SANS) are employed for the first time simultaneously in order to reveal the interaction between the austenite to ferrite phase transformation and the precipitation kinetics during isothermal annealing at 650 and at 700 °C in three steels with different vanadium (V) and carbon (C) concentrations. Austenite-to-ferrite phase transformation is observed in all three steels at both temperatures. The phase transformation is completed during a 10 h annealing treatment in all cases. The phase transformation is faster at 650 than at 700 °C for all alloys. Additions of vanadium and carbon to the steel composition cause a retardation of the phase transformation. The effect of each element is explained through its contribution to the Gibbs free energy dissipation. The austenite-to-ferrite phase transformation is found to initiate the vanadium carbide precipitation. Larger and fewer precipitates are detected at 700 than at 650 °C in all three steels, and a larger number density of precipitates is detected in the steel with higher concentrations of vanadium and carbon. After 10 h of annealing, the precipitated phase does not reach the equilibrium fraction as calculated by ThermoCalc. The external magnetic field applied during the experiments, necessary for the SANS measurements, causes a delay in the onset and time evolution of the austenite-to-ferrite phase transformation and consequently on the precipitation kinetics.

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The initial formation of athermal martensite was proven to accelerate the subsequent bainite formation kinetics during isothermal holdings below the martensite-start temperature (M_s). The presence of prior athermal martensite (PAM) within the phase mixture is expected to modify the overall mechanical response of these newly-designed multiphase steels. Differences stem not only from the balance of product phases, but also from the effect of tempering of the PAM with variations in the applied holding time. This work investigates the effect of tempering time on the mechanical behaviour of the PAM and, as consequence, on the overall mechanical response of these microstructures. Results show that, for short holding times (several minutes), PAM yields similar to as-quenched martensite while, for longer holding times, its yielding behaviour becomes comparable to the one exhibited by typical tempered martensite. Furthermore, the use of Kocks-Mecking curves for the analysis of the mechanical performance confirms the bainitic character of the product phase isothermally formed below M_s. Tailoring the bainitic-martensitic microstructure with variations of the holding time below M_s enables to obtain advanced multiphase steels with comparable mechanical properties to those exhibited by conventional bainitic steels, but in shorter processing times due to the acceleration of bainite formation.

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Bainite formation in steels typically starts at austenite grain boundaries and continues through nucleation of bainite at newly formed bainitic ferrite/austenite interfaces. Recent experimental evidence has pointed out that austenite to bainite transformation can also proceed in the presence of martensite. Studies suggest that the presence of athermal martensite formed prior to bainite formation can accelerate the kinetics of bainite formation with the martensite/austenite interfaces acting as potential nucleation sites. In this work, a kinetic model based on the displacive mechanism of bainite formation is adapted to isolate the impact of martensite/austenite interfaces on the overall rate of bainite formation when bainite formation occurs in the presence of previously formed martensite. This adapted kinetic model is validated using dilatometer studies published in the literature on a silicon-containing low-carbon steel in which bainite formation experiments are performed both below and above the M_s temperature. The results suggest that the formalism of the existing kinetic theory can describe the effects of martensite/austenite interfaces on the bainite formation.

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A cold-rolled Ti-V high strength low alloy (HSLA) steel was isothermally annealed at 650 °C and 700 °C for different times. A unique combination of techniques including visible light microscopy (VLM), transmission electron microscopy (TEM), matrix dissolution, small angle neutron scattering (SANS) and hardness measurement has been employed to investigate the evolution of microstructure, hardness and precipitate composition, size and volume fraction. Results show that recrystallization is completed after annealing 8 h at 650 °C and 30 min at 700 °C. Three types of precipitates were identified: large Ti(C,N), medium-size (Ti,V)(C,N) and small (Ti,V)C. The Ti/(Ti+V) atomic ratio in the (Ti,V)C precipitates decreases with increasing radius in the 1–15 nm range, which can be explained by the initial nucleation of a TiC-rich core. The average size of the (Ti,V)C precipitates increases, whereas the number density decreases during annealing. The volume fractions of the three types of precipitates were separately determined by the matrix dissolution method. The volume fractions of (Ti,V)C precipitates obtained by matrix dissolution are comparable even slightly more accurate than those obtained by SANS. The hardness first increases and then decreases when annealing at both temperatures, which can be correlated well with the observed microstructural and precipitate evolution.

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Interphase precipitation occurring during solid-state phase transformations in micro-alloyed steels is generally studied through transmission electron microscopy, atom probe tomography, and ex situ measurements of Small-Angle Neutron Scattering (SANS). The advantage of SANS over the other two characterization techniques is that SANS allows for the quantitative determination of size distribution, volume fraction, and number density of a statistically significant number of precipitates within the resulting matrix at room temperature. However, the performance of ex situ SANS measurements alone does not provide information regarding the probable correlation between interphase precipitation and phase transformations. This limitation makes it necessary to perform in situ and simultaneous studies on precipitation and phase transformations in order to gain an in-depth understanding of the nucleation and growth of precipitates in relation to the evolution of austenite decomposition at high temperatures. A furnace is, thus, designed and developed for such in situ studies in which SANS measurements can be simultaneously performed with neutron diffraction measurements during the application of high-temperature thermal treatments. The furnace is capable of carrying out thermal treatments involving fast heating and cooling as well as high operation temperatures (up to 1200 °C) for a long period of time with accurate temperature control in a protective atmosphere and in a magnetic field of up to 1.5 T. The characteristics of this furnace give the possibility of developing new research studies for better insight of the relationship between phase transformations and precipitation kinetics in steels and also in other types of materials containing nano-scale microstructural features.

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Advanced High Strength Steels (AHSS) have been used extensively for the last three decades in the automotive industry as they exhibit an enhanced combination of strength and ductility which has successfully allowed the weight reduction of structural components. This breakthrough has been highly beneficial for the environment, as lighter vehicles have reduced the CO2 emissions during use. In the last decade, the development of AHSS has been focused on the design of complex microstructures containing high strength phases, such as bainite and martensite, as well as a softer phase providing ductility and strain hardening, such as austenite. However, the thermomechanical processing of these multiphase steels requires long, complex, and energy-intensive thermal treatments with a high environmental footprint. New alternative processing routes are being developed for producing these multiphase steels sustainably, without compromise on strength and ductility, thus achieving reduced CO2 emissions throughout the lifecycle of steel. In this framework, a new thermal treatment consisting of a rapid cooling below the martensite start temperature (Ms) followed by an isothermal treatment at the same quenching temperature is proposed as a promising environmentally sustainable alternative for the production of such multiphase steels. This Ph.D. thesis investigates, from a scientific point of view, the phase transformations and the interactions between the phases formed during the above-described novel isothermal treatment below Ms in a low-carbon high-silicon steel. The thermal treatment is applied in different combinations of quenching temperature and isothermal holding time in order to stimulate the formation of diverse phase fraction mixtures. The research also elucidates the effects of the formation of each of the phases on the microstructure-property relationships of these multiphase steels.@en

In-situ Small-Angle Neutron Scattering (SANS) is used to determine the time evolution of the chemical composition of precipitates at 650 °C and 700 °C in three micro-alloyed steels with different vanadium (V) and carbon (C) concentrations. Precipitates with a distribution of substoichiometric carbon-to-metal ratios are measured in all steels. The precipitates are initially metastable with a high iron (Fe) content, which is gradually being substituted by vanadium during isothermal annealing. Eventually a plateau in the composition of the precipitate phase is reached. Faster changes in the precipitate chemical composition are observed at the higher temperature in all steels because of the faster vanadium diffusion at 700 °C. At both temperatures, the addition of more vanadium and more carbon to the steel has an accelerating effect on the evolution of the precipitate composition as a result of a higher driving force for precipitation. Addition of vanadium to the nominal composition of the steel leads to more vanadium rich precipitates, with less iron and a smaller carbon-to-metal ratio. Atom Probe Tomography (APT) shows the presence of precipitates with a distribution of carbon-to-metal ratios, ranging from 0.75 to 1, after 10 h of annealing at 650 °C or 700 °C in all steels. These experimental results are coupled to ThermoCalc equilibrium calculations and literature findings to support the Small-Angle Neutron Scattering results.

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The precipitation kinetics of vanadium carbides and its interaction with the austenite-to-ferrite phase transformation is studied in two micro-alloyed steels that differ in vanadium and carbon concentrations by a factor of two, but have the same vanadium-to-carbon atomic ratio of 1:1. Dilatometry is used for heat-treating the specimens and studying the phase transformation kinetics during annealing at isothermal holding temperatures of 900, 750 and 650 °C for up to 10 h. Small-Angle Neutron Scattering (SANS) and Atom Probe Tomography (APT) measurements are performed to study the vanadium carbide precipitation kinetics. Vanadium carbide precipitation is not observed after annealing for 10 h at 900 and 750 °C, which is contrary to predictions from thermodynamic equilibrium calculations. Vanadium carbide precipitation is only observed during or after the austenite-to-ferrite phase transformation at 650 °C. The precipitate volume fraction and mean radius continuously increase as holding time increases, while the precipitate number density starts to decrease after 20 min, which corresponds to the time at which the austenite-to-ferrite phase transformation is finished. This indicates that nucleation and growth are dominant during the first 20 min, while later precipitate growth with soft impingement (overlapping diffusion fields) and coarsening take place. APT shows gradual changes in the precipitate chemical composition during annealing at 650 °C, which finally reaches a 1:1 atomic ratio of vanadium-to-carbon in the core of the precipitates after 10 h.@en

The accelerated formation of bainite in presence of martensite is opening a new processing window for the steel industry. However, for a feasible industrial implementation, it is necessary to determine the mechanical behaviour of the steels developed under such conditions. This study focuses on analysing the effects of the formation of athermal martensite, followed by the formation of bainitic ferrite, on the mechanical response of a low-C high-Si steel. For this purpose, microhardness measurements and tensile tests have been performed on specimens that were thermally treated either above or below the martensite-start temperature (M_s). Specimens isothermally treated below M_s exhibit a good combination of mechanical properties, comparable with that of the specimens heat treated by conventional treatments above M_s, where there was no prior formation of martensite. Investigations show an increase of the yield stress and a decrease of the ultimate tensile strength as the isothermal holding temperature is decreased below M_s. The formation of prior athermal martensite and its tempering during the isothermal holding leads to the strengthening of the specimens isothermally heat treated below M_s at the expense of slightly decreasing their strain hardening capacity.

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Advanced Multiphase High Strength Steels are generally obtained by applying isothermal treatments around the martensite start temperature (M_s). Previous investigations have shown that bainitic ferrite can form from austenite in isothermal treatments below M_s, where its formation kinetics is accelerated by the presence of the athermal martensite. That athermal martensite is tempered during the isothermal treatment, and fresh martensite may form during the final cooling to room temperature. The distinction between product phases present after the application of this type of heat treatments is difficult due to morphological similarities between these transformation products. The aim of this study is to characterize the structural and morphological features of the product phases obtained in isothermal treatments below the M_s-temperature in a low-carbon high-silicon steel. Multiphase microstructures, having controlled fractions of product phases, were developed by applying isothermal treatments above and below M_s, and were further studied by electron back scatter diffraction (EBSD) and scanning electron microscopy (SEM). The bainitic or martensitic nature of these product phases is discussed based on this characterization. Results showed that bainitic ferrite appears in the form of acicular units and irregularly shaped laths. Tempered martensite appears as laths with a sharp tip and as relatively large elongated laths with wavy boundaries containing protrusions.

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Thermomechanical processing of Advanced Multiphase High Strength Steels often includes isothermal treatments around the martensite start temperature (Ms). Investigations show that the presence of martensite prior to these isothermal treatments accelerates the kinetics of the subsequent transformation. The aim of this study is to determine whether the presence of a small volume fraction of martensite affects the nucleation kinetics of the subsequent transformation, as well as to qualitatively and quantitatively determine this contribution. For this purpose, dilatometry experiments were performed at different temperatures above and below Ms in a lowcarbon high-silicon steel. The isothermal product formed both above and below Ms has been identified as bainitic ferrite. The transformation kinetics analysis demonstrated that the bainitic ferrite formation is much faster below Ms than above Ms, with a difference of two orders of magnitude.@en