P. Valles

Characterization by Electron Diffraction of Two Thermodynamical Phases of Precipitation in Nb-Microalloyed Steels

Excellent mechanical properties (high strength and toughness) of microalloyed steels are mainly caused by induced precipitation during thermomechanical treatment (TMT) and grain refinement. It has been recently found that TMT of Nb-microalloyed steels can give rise to two different kinds of precipitates, manifested by the double plateau in the statically recrystallised fraction (Xa) against time curves. This work presents an electron diffraction study performed in a transmission electron microscope, equipped with an EDS analytical system. Lattice parameters of a great deal of particles, smaller than 200 nm and with face cubic centred structure, have been measured. Frequency distribution of the values of lattice parameters shows that these are grouped in two sets whose mean values are close. Comparison of these values with those found in the literature for carbides, nitrides and carbonitrides usually present in microalloyed steels demonstrates that they are Nb carbonitrides with slight stoichiometric differences (NbCxNy).

Pinning Forces Exerted by TiN Particles in Austenite of Structural Steels and Comparison with Driving Forces for Grain Growth and for Static Recrystallisation

In this work the pinning forces exerted by TiN particles in the austenitic phase in two Ti microalloyed steels have been determined and compared with the driving forces for austenite grain growth and for static recrystallisation between hot rolling passes, respectively. TiN precipitate sizes were measured by transmission electron microscopy (TEM) and the precipitated volumes were calculated. These results were then used to calculate pinning forces. The driving forces for recrystallisation were found to be approximately two orders of magnitude higher than the pinning forces, which explains why the austenite in these steels barely experiences hardening during rolling and why the accumulated stress prior to the austenite→ferrite transformation is insufficient (low dislocation density) to refine the ferritic grain.

Analysis of V(C, N|) nanoparticles in a medium carbon bainitic microalloyed steel and their influence on strengthening

Abstract Continuous cooling transformation diagrams were plotted for a microalloyed steel in dilatometric tests at different cooling rates, and maximum and minimum cooling rates for bainite formation were determined. Austenite grain and bainite packet sizes were measured in three bainitic microstructures obtained by continuous cooling from austenitisation temperatures of 950, 1 050 and 1 150 °C, respectively. The results show that bainitic packet growth cannot exceed the austenite grain boundaries where it has nucleated. V(C, N) particle size was measured using field emission gun scanning electron microscopy and transmission electron microscopy, respectively. Both techniques yielded very similar values, but transmission electron microscopy is preferable due to its higher resolution and the possibility to carry out electron diffraction and dispersive X-ray analyses. The presence of precipitates has contributed to raising the yield strength, which can be predicted by Orowans expression.

Study of the Microstructure and Strain Induced Precipitation during Thermomechanical Processing of Low Carbon Microalloyed Steels

A series of anisothermal multipass hot torsion tests were carried out to simulate hot rolling on three high-strength low-carbon steels with different amounts of Mn, Mo, Nb and Ti and designed for pipeline construction. Mean Flow Stress was graphically represented against the inverse of temperature to characterize the evolution of austenite microstructure during rolling. The effect of austenite strengthening obtained at the end of thermomechanical processing on the final microstructure obtained after cooling was studied. Higher levels of austenite strengthening before cooling promote a refinement of final microstructure but can also restrict the fraction of low-temperature transformation products such as acicular ferrite. This combined effect gives rise to a wide range of final microstructures and mechanical properties depending on the composition, processing schedule and cooling rates applied. On the other hand, the precipitation state obtained at diverse temperatures during and at the end of hot rolling schedule was evaluated by means of transmission electron microscopy (TEM) in two microalloyed steels. It was found that two families of precipitates with different morphology, composition and mean size can coexist in microalloyed steels.