Materials Science Forum Vols. 500-501

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: This paper provides a detailed description of complex bainitic microstructures obtained during the recent development of low carbon linepipe steels with strengths in the range of X100 to X120. New experimental techniques based on a high resolution FEG-SEM and EBSD have been used to characterise and quantify the mixture of ultrafine bainitic ferrite and nanosize second phases in these steels. It was found that the occurrence of incomplete transformation generates new, previously unexplored bainitic microstructures with a wealth of microstructural features that is beyond classification based on conventional concepts. Clear differences in distributions of boundary misorientations and effective grain size were noted between upper, lower and granular bainites. Based on these results a new classification scheme and definition of bainite is proposed.

Abstract: A metallurgical model to describe the formation of ferrite, perlite and bainite during continuous cooling was developed. The model uses classical nucleation and early growth theories to calculate the beginning of the reaction, and Avrami-type equations to evaluate the transformation progress. The materials studied were plain carbon steels with different Mn contents, and microalloyed steels with Nb, Ti and/or V additions. The model predictions were adjusted and validated with experimental results obtained from dilatometric tests. The range of cooling rates considered was between 5°C/sec and 50°C/sec. From the comparison with the experimental data it was found that the model correctly describes the dependence of the reaction kinetic on the cooling rate, as well as on the steel chemistry.

Abstract: A model has been developed to predict the ferrite grain size distribution resulting after the austenite to ferrite transformation in slowly cooled low carbon steels. The model uses the austenite grain size distribution present before transformation as input and provides the size distribution of ferrite grains at any instant in the transformation range, whether the microstructure is partially either completely transformed. The model is based on empirical equations relating the mean austenite and ferrite grain sizes and log-normal shape grain size distributions. A validation of the model was carried out in the laboratory by torsion tests for Nb and Nb-V microalloyed steels.

Abstract: The addition of small quantities of vanadium in structural steels produces a significant refinement in the final ferrite microstructure. There are two different mechanisms contributing to refinement: enhancement of grain boundary ferrite nucleation and intragranular nucleation. The contribution of each mechanism depends on the vanadium content and heat treatment of the steel. In this study the contribution of both refining mechanisms has been evaluated for two V-microalloyed steels subjected to different heat treatments. The results confirm that this refinement is based on the enhancement of ferrite nucleation through particle-stimulated nucleation mechanisms, while other aspects, as the influence of vanadium slowing down the austenite-ferrite transformation kinetics, seem to exert a minor effect.

Abstract: The kinetics of isothermal bainite formation at 483 K, 503 K and 533 K in a bearing SAE 52100 steel (1,01% C – 1,36% Cr – 0,32% Mn – 0,25% Si (wt.%)) is investigated theoretically and experimentally. It is found that the bainite formation becomes significantly slower with decreasing temperature. To interpret the results, a physical model with a concept of reconstructive approach proposed in the literature is applied. The model gives a reasonable agreement with experimental results, although a certain deviation exists at the late stage of the transformation. This deviation is attributed to the diffusion coefficient used in the model without considering the effect of alloying elements, mainly chromium.

Abstract: The present work describes the correlation between the microstructure and the mechanical properties of multiphase steels processed through a Continuous Annealing Process Line (CAPL). The mechanical and microstructural properties of one of the former experimental steel compositions was also processed via Batch Annealing (BA) and the results have been included for comparison reasons.

Abstract: The present work describes the correlation between the microstructure and the mechanical properties of multiphase steels processed through a Continuous Annealing Process Line (CAPL). The mechanical and microstructural properties of one of the former experimental steel compositions was also processed via Batch Annealing (BA) and the results have been included for comparison reasons.

Abstract: It seems completely clear that the combination within the same microstructure of various steel phases (ferrite, bainite, martensite) and the existence of the TRIP effect (i.e. a mechanically induced martensitic transformation) improves in a large way the properties of strength and ductility of low-alloy steels. TRIP-aided multiphase steels have been studied for several years and arouse a growing industrial interest. This project deals with the influence of a large Nb addition on the microstructure and mechanical properties of TRIP-assisted multiphase steels. It is shown that Nb influences in a large way the different phase transformations as well as the recrystallisation of the ferrite matrix occurring during the heat-treatment of cold-rolled TRIP-aided steels. As a consequence, the mechanical properties also depend on the parameters of the thermomechanical process and on the content and stability of the retained austenite.