Papers by Author: Manuel Gómez

Abstract: Under certain conditions of temperature, time and deformation, static recrystallization of austenite in microalloyed steels can be temporarily inhibited by means of the strain-induced precipitation of nanoparticles that cause a pinning effect on austenite grain boundaries in motion. This inhibition can be seen by the formation of a “plateau” in the curves of static recrystallization of austenite obtained from double-deformation tests carried out under isothermal conditions. In this work, several microalloyed steels with different compositions are studied by hot torsion tests in order to characterize the kinetics of recrystallization and its inhibition. The precipitation state in austenite is studied in several samples by means of transmission electron microscopy. The influence of the type of microalloying element (Al, Nb, V) and the mean size of the precipitates on the duration time of the plateau is studied and relationships between these variables can be obtained. Particularly, it is seen that Al-alloyed steels present a much coarser particle size and a considerably shorter plateau compared to Nb and V-microalloyed steels.

Abstract: Starting from the expression of Dutta and Sellars for the beginning of strain induced precipitation in microalloyed steels, the influence of temperature on t_0.05 parameter has been studied. Although the equation given by these authors reproduces well the typical “C” shape of the curve of precipitation start time P_s, the expression is not reliable for all cases. The precipitation-time-temperature (PTT) diagrams have been plotted thanks to a new experimental study carried out by means of hot torsion tests on approximately twenty microalloyed steels having different contents of Nb, V and Ti. Mathematical analysis of results recommends the modification of some parameters such as supersaturation ratio (K_s) and constant B, which is no longer a constant but a function of K_s when the latter is calculated at the nose temperature (T_N) of curve P_s. The value of parameter B is deduced from the minimum point or nose of the P_s curve, where ∂t_0.05/∂T is equal to zero, and it can be demonstrated that B cannot be a constant. The new expressions for these parameters derive from the latest studies developed by the authors and this work supposes an attempt to improve the model. The expressions are now more consistent and predict with remarkable accuracy the PTT curves.

Abstract: 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.

Abstract: Recrystallisation/precipitation interaction in four steels having Nb, V, Ti, and Al, respectively, as microalloying elements has been studied by means of hot torsion tests. Remarkable differences were found in the results obtained for each steel. Nb and V-microalloyed steels presented long inhibition plateaus, but the steel with Al displayed a very short plateau. Finally the steel with Ti did not show plateau. This means that Nb and V precipitates (nitrides and carbides) can inhibit the static recrystallization but this does not happen for Al and Ti (which form nitrides). The difference between activation energies allows to predict the efficiency of different precipitates to strengthen the austenite during hot rolling. RPTT diagrams showed the interaction between both phenomena, along with the strain induced precipitation kinetics and precipitate coarsening. It is found that AlN particles nucleate and grow faster than NbCN or VN.

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.

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.

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.

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.

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).

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.