Lucía Rancel

Effects of Nb, V, Ti and Al on Recrystallisation/Precipitation Interaction in Microalloyed Steels

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.

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.

Microstructural Unit Controlling Cleavage Crack Propagation in High Strength Bainitic Steels

The strengthening mechanisms which are operative in bainite are very well known: small bainite packet, small width of the laths, dislocation density and size and number of carbide particles (Fe3C), among others. Bainite packet size has been traditionally considered as the value measured by optical microscopy (OM), as electron back scattered diffraction (EBSD) technique is relatively recent. In a V-microalloyed steel with bainitic microstructure of C=0.38%, V=0.12% and N= 0.0214% the average length and width of ferrite laths and of cementite carbides were measured. On the other hand, the bainite packet size was measured by OM and EBSD with a misorientation of 15o. These values of the microstructural units have been taken in account to calculate the effective surface energy γp given by Griffith’s model for cleavage fracture. It was concluded that bainite packet size determined by EBSD with a misorientation angle criterion of 15o was the microstructural parameter that controls cleavage crack propagation. Given the relationship between the average unit crack path (UCP) and the bainite packet size, it was concluded that the effective surface energy of cleavage fracture (γp) would be between 71.6 and 82.6 J m-2.