Papers by Author: Isabel Gutiérrez

Abstract: The microstructural refinement induced when the holding time between last deformation pass and accelerated cooling is reduced, affects the mechanical properties in low carbon Nb and Nb-Mo microalloyed steels. Plane strain compression tests were performed and mechanical property samples machined in order to quantify this effect using tensile and Charpy impact tests. A complete microstructural characterization was carried out using electron backscattered diffraction (EBSD) measuring unit size distributions and homogeneity of complex microstructures. The synergetic combination of Nb and Mo elements modifies the final microstructures and, therefore, affects the contribution of different strengthening mechanisms, such as substructure, precipitation hardening and dislocation density. Even though strength is not clearly affected by the reduction of the holding time after the last deformation pass, Charpy properties are considerably improved in the case of the Nb steel. The presence of MA islands in the Nb-Mo steel limits the beneficial effect of the microstructural refinement and toughness remains unmodified.

Abstract: 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 γ_p 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 (γ_p) would be between 71.6 and 82.6 J m^-2.

Abstract: Due to the increased complexity of steel microstructures, when considering the application of available Hall-Petch type equations for yield strength prediction, a number of difficulties raises. For example, the correlation between grain size measurements by EBSD technique and optical microscopy (OP) in complex microstructures is required in order to integrate data to the traditional equations developed for OP results and ferrite-pearlite microstructures. Besides, the introduction of some additional terms to the equations to account for precipitation, C in solution and forest dislocation contributions presents some difficulties that need to be overcome to improve prediction accuracy. Different microstructures (ferrite-pearlite, bainite, quenched and Q&T) have been produced by thermal and thermomechanical treatments, followed by microstructural characterisation and mechanical testing. A Hall-Petch coefficient dependent on the boundary misorientation distribution is proposed. This approach allows dealing in a similar way ferritic, bainitic and martensitic microstructures. The Hall-Petch coefficient, thus defined, corresponds to the previously proposed by Pickering for ferrite, while bainitic microstructures give a smaller value. Additionally, the equation used to express the fracture appearance transition temperature of ferritic-pearlitic microstructure has been generalized from the developments made in the calculation of the yield stress.

Abstract: A reversion of the strain produces a modification of the static recrystallization kinetics. Initially, the reversion increases the recrystallization time, that reaches a maximum at a certain strain, and decreases again for increasing reverse strains. This transient on recrystallization kinetics develops over a strain interval similar to that of the microstructural and stress-strain transients. At strains beyond the transient, the reversion can be regarded as a shift on the strain axis. However, at the authors knowledge there is no formulation able to describe the material behaviour during the transient. The present work introduces an equivalent strain concept based on the substructural dissolution/build-up processes taking place as a result of the strain reversal. This formulation allows including the effect of the strain path on recrystallization models.

Abstract: In extra-low carbon steels, the generation of specific nucleation sites in the deformed microstructure determines the formation of an optimum recrystallization texture. In particular, during the cold rolling of the steel sheets, transition bands are generated in order to accommodate the different deformation paths followed by the deformation bands (DB-s) within the grains. -fibre grains (ND-fibre grains) are, in general terms, more fragmented than -fibre grains (RD-fibre grains). Consequently, the higher orientation gradients and stored energy levels of the -fibre grains determine the ND-fibre annealing texture. Nevertheless, during recrystallization of the ELC steel, nucleation in different type of transition bands (TB-s) between α-fibre components has also been observed in the present work. From a previous crystallographic classification of transition bands done by the authors, the effect of the lattice curvature, either by gradual or sharp orientation gradients, is studied during early recrystallisation stages. Specifically, the crystallographic characteristics of recrystallized nuclei formed at transition bands generated between α-fibre components are analyzed using electron back-scatter diffraction (EBSD). This technique enables the orientation of deformation bands, the misorientation across them, the orientation of the new recrystallized grains and the misorientation of those grains with the adjacent matrix grains to be determined. Recrystallization of components different to α at the expense of α components has also been found.

Abstract: The recovery and recrystallization kinetics of a cold rolled interstitial free (IF) steel were studied during isothermal annealing. Magnetic methods based on coercive field measurements, hardness tests and metallography were applied so as to follow the kinetics experimentally. The coercive field measurement technique reveals a higher degree of resolution for monitoring recovery than conventional hardness determination and also allows the recrystallization progress to be monitored. The results obtained are compared to those previously determined for a non-stabilized extra low carbon (ELC) steel. The observed differences are discussed in terms of the presence of microalloying elements, Ti and Nb, which slow down recovery and delay recrystallization.

Abstract: Nb is added to C-Mn steels in order to use the solute drag and/or strain induced precipitation as a useful tool to condition the austenite in the hot rolling mill and produce during the subsequent cooling a refined ferrite grain size. The highest degree of refinement is obtained in conventional rolling mills by accumulating the deformation in austenite during the last passes, followed by early cooling in the run out table to produce a high density of nucleated ferrite grains. However, the maximum refinement is to a certain extent attenuated due to the ferrite grain coarsening taking place during the transformation. The present work analyses the different aspects limiting the final achievable ferrite grain refinement.

Abstract: A combination of monotonic and reverse tests has been carried out in order to assess the strain path effects on an austenitic stainless steel hot deformed by torsion. Microstructural results have been obtained by EBSD. The misorientation average parameter measured at different step size scans, the Kernel parameter and the orientation spread average parameter, provide a picture of the in-grain curvature developed during the different strain paths. The results show that these parameters are sensitive to the strain path.

Abstract: It has been demonstrated recently that when a reversion of the strain is applied during the hot working of a Nb-microalloyed steel, the subsequent static recrystallization kinetics is significantly affected. However, depending on the magnitude of the reversion, the static softening kinetics can be accelerated or delayed. This relates to the substructure dissolution taking place by the effect of the reversal. In the present work, new microstructural results obtained by EBSD on an austenitic stainless steel hot deformed by torsion is used to explain better the observed effect of the reversion of the strain.

Abstract: During annealing of cold rolled steel sheets, the evolution of the texture, to produce the characteristic texture of a recrystallised material, depends on a series of factors, including the starting deformation texture, composition and processing conditions. In the present work, electron backscattered diffraction (EBSD) techniques have been used to investigate the deformation substructures being developed by cold rolling into grains belonging to different texture components. The strain localisation, the deformation banding, the spread in orientation into grains with different orientations are some of the aspects that have been considered. Annealing cycles at different temperatures have also been carried out in order to promote recovery and initial stages of recrystallisation . The obtained substructures have been compared to the as-cold rolled ones. It has been observed that recovery induces the dislocation structures to arrange into subgrains. Associated to the ND fibre, a network of relatively high angle boundaries develop within the subgrain structure. The recrystallisation nuclei have been observed to evolve from such a network.