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In this paper a new insight into fundamentals of static recrystallization, precipitation and their interaction during sub-critical annealing of three cold-rolled low-carbon microalloyed steel grades is presented. The grades under investigation are a base grade containing V as a microalloying element, a Ti+ grade containing Ti as microalloying element added into the base grade, and a Ti+Mn+ grade containing additional Mn added into the Ti+ grade. The cold-rolled steels are sub-critically annealed inside a muffle furnace to simulate industrial continuous annealing parameters in order to investigate the interaction between recrystallization and precipitation across transient stages of the annealing process as a function of temperature and time. The Zener pinning of precipitates and solute drag force of Mn on the recrystallization process are calculated and compared with measured values obtained from experimental studies on the recrystallization kinetics. Results suggest that the recrystallization kinetics is fastest in the base grade. For the Ti+ grade, fine (< 15 nm) (Ti,V)(C/N) particles retard the recrystallization kinetics. For the Ti+ and Ti+Mn+ grades, solute drag effect of Mn solute atoms for dwell time longer than 2 min at annealing temperature of 800 oC is negligible.

In the present study, cold-rolled low-carbon steel is annealed at three different conditions: 700 oC for 0 s, 800 oC for 0 s and 800 oC for 2 min at the heating rate of ~10 oC/s. Recrystallization behaviour on sample surface is studied using a heated stage Scanning Electron Microscopy and Electron Backscattered Diffraction. For the lower annealing temperature of 700 oC with no dwell, almost no recrystallization is observed and microstructure resembles the as-received deformed material with the exception of occasional sub-micron sized nuclei. For the annealing conditions of 800 oC 0 s and 800 oC 2 min, onset and evolution of recrystallization is observed in-situ as a function of the initial as-cold rolled texture. Slower recovery rate of alpha fibre than gamma fibre is observed and confirmed by lower drop in average geometrically necessary dislocation (GND) density for un-recrystallized alpha fibres (1.1E+14 m-2 for 700 oC 0 s , 1.4E+14 m-2 for 800 oC 0 s and 4.5E+14 m-2 for 800 oC 2 min) than for un-recrystallized gamma fibre grains (3.0E+14 m-2 for 700 oC 0 s , 6.2E+14 m-2 for 800 oC 0 s and 9.8E+14 m-2 for 800 oC 2 min) during annealing. Strong gamma texture in recrystallized matrix is found for annealing conditions of 800 oC 0 s and 800 oC 2 min. From TEM characterisation it was shown that sub-grain boundaries are decorated with fine precipitates (diameter d < 15 nm) of titanium-vanadium carbides (Ti,V)C for the annealing condition of 700 oC 0 s, which suggests that these precipitates play a major overall role in retardation of the recrystallization kinetics.

A new generation of low-carbon microalloyed High Strength Low Alloy (HSLA) steels has been developed to utilize a combination of single-phase ferritic microstructures and optimized interphase precipitation to provide high level strength and exceptional formability. The interphase precipitation reaction is a transient process lending itself strongly to take advantage of in-situ characterization techniques. The austenite/ferrite interface kinetics during isothermal transformation at 1003 K is measured using HT-CSLM, the pre-exponential effective mobility constant was found to be mobility 0.822 (m J)/(mole s). The V interphase precipitation is characterised using TEM at isothermal transformation temperatures of 923 and 973 K as having inter-sheet spacing of 22±7 and 32±9 nm respectively. Interphase precipitation inter-sheet-spacing is simulated using a revised Quasi-Ledge model and qualitatively predicts the observed trends observed for inter-sheet spacing. The results of in-situ characterisation and modelling suggest that it is possible to optimize the strengthening potential of the precipitation processes by controlling the thermal processing of microalloyed HSLA.

In-situ characterization techniques have been applied to elucidate the influence of γ/α transformation upon the extent of interphase precipitation in a low-carbon, vanadium-HSLA steel. Electron Back-scattered diffraction analyses of the γ/α orientation relationship with continuous cooling at 2 and 10 K/s suggest that the proportion of ferrite likely to hold interphase precipitation varies little with cooling rate. However, TEM analyses show that the interphase precipitation refines with increasing cooling rate in this cooling range. With cooling rates in excess of 20 K/s, interphase precipitation is increasingly suppressed due to the increasingly diffusional-displacive nature of the Widmanstatten γ/α transformation that is activated. The present study illustrates that the extent and dimensions of interphase precipitation can be controlled through controlled cooling.

Nanosteels are used in automotive applications to accomplish resource-efficiency while providing high-tech properties. Quantitative data and further understanding on the precipitation kinetics in Nanosteels can contribute to fulfil this goal. Small-Angle Neutron Scattering measurements are performed on a Fe-C-Mn-V steel, previously heat-treated in a dilatometer at 650°C for several holding times from seconds to 10 hours. The evolution of the precipitate volume fraction, size distribution and number density is calculated by fitting the experimental Small-Angle Neutron Scattering curves. The effect of phase transformation on precipitation kinetics is also discussed. Complementary Transmission Electron Microscopy, Scanning Electron Microscopy and Inductively Coupled Plasma Optical Emission Spectroscopy measurements are performed to support the Small-Angle Neutron Scattering data analysis.

This paper discusses practical EBSD strategies for identification and partitioning of phase constituents in DP and TRIP microstructures including martensite, retained-austenite, bainite, intercritical and epitaxial ferrite. EBSD data is complemented with nano-indentation analysis, providing evidence of indentation-induced phase transformation of retained-austenite in TRIP steel and micro-crack initiation at the interface between ferrite and mechanically transformed martensite.