Microstructure of vanadium micro-alloyed steels for automotive applications: Interaction of precipitation with austenite-to-ferrite phase transformation studied by SANS and neutron diffraction

The focus of the present work is on the micro-alloying element vanadium, which is well known for providing precipitation strengthening to steels and which has, therefore, attracted a lot of interest the last decades. Vanadium carbide precipitation can take place in the migrating austenite/ferrite interface during the austenite-to-ferrite phase transformation, i.e. interphase precipitation, and in ferrite. Due to the beneficial contribution of the vanadium carbides to the mechanical properties of the steel and the necessity to make optimum use of vanadium, it is critical to understand and quantify the vanadium carbide precipitation and its interaction with the austenite-to-ferrite phase transformation.
We study the precipitation kinetics of vanadium carbides and its interaction with the phase transformation kinetics in vanadium micro-alloyed steels that differ in vanadium and carbon concentrations and that have undergone different isothermal annealing treatments. In Chapter 1, the introduction to the research topic and the scope of this thesis are described. The novelty of our research is the use of advanced neutron scattering techniques i.e. Neutron Diffraction and Small-angle Neutron Scattering (SANS), coupled to Atom Probe Tomography (APT) and Transmission Electron Microscopy (TEM), to study model vanadium micro-alloyed steels during heat-treatments. The combination of neutron diffraction and SANS to study, simultaneously and in-situ, the interaction between the phase-transformation and precipitation kinetics is unique, as is the furnace that is designed and developed for these in-situ measurements. The results provide fundamental insight into the role of vanadium on the phase-transformation and precipitation kinetics, which is deemed essential for the development of micro-alloyed steels with reduced amounts of alloying elements without compromising properties.
In Chapter 2, the vanadium carbide precipitation kinetics and its interaction with the phase transformation kinetics is investigated. 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 are studied. Dilatometry is used for heat-treating the specimens and studying the phase-transformation kinetics during isothermal annealing at 900 °C, 750 °C and 650 °C for up to 10 h. Samples annealed for different holding times are used for ex-situ SANS, TEM and APT to study the precipitation kinetics. Vanadium carbide precipitation is only observed during or after the austenite-to-ferrite phase transformation at 650 °C and not during annealing at 900 °C and 750 °C. The precipitate volume fraction and mean radius continuously increase as the holding time increases, while the precipitate number density starts to decrease after 20 min, which corresponds to the time at which the phase transformation has finished. This indicates that nucleation and growth are dominant during the first 20 min, while later precipitate growth and coarsening take place. TEM indicates the presence of spherical/slightly ellipsoidal precipitates in all steels after annealing at 650 °C and 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.
Chapter 3 introduces a custom-made furnace designed and built by our group at TU Delft. It is able to facilitate in-situ and simultaneous neutron diffraction and SANS measurements during heat-treatments of metals. In-situ and simultaneous studies on phase-transformation and precipitation kinetics are necessary 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. Precipitation, occurring during solid-state phase transformations in micro-alloyed steels, is generally studied through TEM, APT and ex-situ SANS measurements. The advantage of SANS over the other two characterization techniques is that it allows for the quantitative determination of size distribution, volume fraction, and number density of a statistically significant number of precipitates within the resulting iron matrix at room temperature. However, individual ex-situ SANS measurements do not provide information regarding the correlation between interphase precipitation and phase transformations. The presented furnace is, thus, developed for in-situ studies in which SANS measurements can be performed simultaneously to neutron diffraction measurements during typical high-temperature thermal treatments for steels. 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-transformation and precipitation kinetics in steels and also in other types of materials containing nano-scale microstructural features.
In Chapter 4, in-situ 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 and carbon concentrations. The evolution of the ratio of the nuclear to magnetic SANS component is used for this analysis. The samples are heat-treated in the furnace presented in Chapter 3. Precipitates with a distribution of sub-stoichiometric carbon-to-metal ratios in all steels are detected. The precipitates have a high iron content at the early stages of annealing, which is gradually being substituted by vanadium during isothermal holding. 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. We found that the addition of vanadium and carbon to the steel has an accelerating effect on the evolution of the precipitate composition. Addition of vanadium to the nominal composition of the steel increases the concentration of vanadium in the precipitates, reduces the iron concentration and leads to a smaller carbon-to-metal ratio. APT measurements prove 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 studied steels.
In Chapter 5, in-situ neutron diffraction and 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 in-situ in vanadium micro-alloyed steels. The neutron scattering measurements are performed in three steels with different vanadium and carbon concentrations during isothermal annealing treatments at 650 °C and 700 °C for 10 h. The furnace introduced in Chapter 3 is used for the heat treatments. The austenite-to-ferrite phase-transformation and precipitation kinetics are quantified and the interaction between these two phenomena is explained. We show that the phase transformation is completed during the 10 h annealing treatment in all cases and that it is faster at 650 °C than at 700 °C for all alloys. Our analysis shows that additions of vanadium and carbon to the steel composition cause a retardation of the phase transformation and the effect of each element is explained through its contribution to the Gibbs free energy dissipation. The phase transformation is found to initiate the vanadium carbide precipitation. The presence of ellipsoidal precipitates is confirmed by TEM, contributing to the SANS data analysis. Larger and fewer precipitates are detected at the higher temperature in all three steels, and a larger number density of precipitates is detected in the steel with higher concentrations of vanadium and carbon. The effect of the precipitation kinetics to the phase-transformation kinetics is also discussed. An important outcome is that the external magnetic field applied during the experiments, necessary for the SANS measurements, causes a delay in the onset and time evolution of the phase transformation and consequently on the precipitation kinetics.