Authors: Manuel Gómez, S.F. Medina, J.I. Chaves
Abstract: The austenite static recrystallization kinetics at several temperatures and the
recrystallization-precipitation-time- temperature (RPTT) diagrams of a medium-carbon vanadium
microalloyed steel have been determined for a strain ε = 0.35. Unlike many other studies carried out
previously on V microalloyed steels, the recrystallized fraction against time curves showed the
formation of a double plateau that indicates two stages of inhibition of recrystallization due to the
formation of different types of strain induced precipitates. This work makes use of transmission
electron microscopy to study the nature and size distribution of these precipitates capable of inhibiting
recrystallization. The values of driving and pinning forces for static recrystallization are calculated
and an analysis of the relationship between the net balance of these forces, the precipitation state and
the progress or inhibition of the recrystallization is accomplished. A value of driving force that
decreases as recrystallized fraction grows during isothermal holding time is estimated and helps to
interpret the behavior of austenite after deformation.
417
Authors: J.I. Chaves, S.F. Medina, Manuel Gómez, L. Rancel, Pilar Valles
Abstract: 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.
405
Authors: S.F. Medina, Manuel Gómez, J.I. Chaves, P.P. Gómez, P. Adeva
Abstract: The intragranular nucleation of ferrite has been studied in a V-microalloyed steel
(C=0.102; Mn=1.479; V=0.140; N=0.016, %wt). By means of hot torsion tests, recrystallizationprecipitation-time-temperature (RPTT) diagrams have been drawn which show the precipitation kinetics and the recrystallization-precipitation interaction at any temperature. RPTT diagrams were determined at two strains, 0.20 and 0.35 respectively. Deformation tests were carried out at 890°C and different holding times, corresponding to moments before the start of precipitation, during
precipitation and after precipitation had ended, respectively. In order to relate the precipitation state with the intragranular nucleation, the strengthening of austenite was measured taking into account the non-recrystallized austenite fraction prior to the phase transformation. In this way, the possibility of V-nitrides acting as nucleation sites was evaluated by comparison of ferrite grain size versus holding time. Thus it was found that the precipitate size and precipitated volume are influencing the intragranular nucleation, although this is not a strong influence.
371
Authors: Manuel Gómez, S.F. Medina
Abstract: The values of recrystallisation driving (FR) and pinning forces (FP) during the hot rolling of a low Nb-microalloyed steel were calculated using several expressions found in the literature. A comparative study of the hypothesis into the interaction between precipitates and migrating grain boundaries was carried out, and the methods for estimating volume fractions of precipitates and dislocation density were assessed. Though the criterion selected greatly influences the values obtained for both forces, FP grows faster than FR as the rolling temperature drops.
147
Authors: Manuel Gómez, S.F. Medina, Pilar Valles, Alberto Quispe
Abstract: 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).
489
Authors: S.F. Medina, M.I. Vega, Manuel Gómez
Abstract: This work has studied the influence of different Ti and N compositions on hot
deformation strength by determining the peak stress of flow curves and the activation energy (dynamic recrystallisation). It has also assessed their influence on static recrystallisation by means of the statically recrystallised fraction versus time and the activation energy. A precipitate study performed by SEM and TEM has yielded a better understanding of the influence of the Ti/N ratio and precipitation state in hot deformation (dynamic and static recrystallisation). A correlation was
found between for the finer distribution of precipitates, Ti/N ratio close to 1.5, smaller austenite grain, maximum activation energy for hot deformation (dynamic recrystallisation) and maximum activation energy for static recrystallisation.
1205
Authors: S.F. Medina, Alberto Quispe
1139
Authors: Manuel Gómez, S.F. Medina, Alberto Quispe
1535