Title: "EFFECT OF MICROSTRUCTURE AND PRECIPITATION ON THE HARDENING OF STEELS AND DEVELOPMENT OF MODELS BASED ON MICROSTRUCTURAL PARAMETERS"
Author: MIREN ALAZNE ALTUNA ALBERDI
Date: 2008-04-11
Director: Dr. María Isabel Gutiérrez Sanz
Abstract:
The current work is the result of CEIT´s participation in a project initiated in 2003 and supported by the Euopean Coal and Steel Community (ECSC). The main objective of the research entitled “Mechanical property models for high strength complex microstructures”, is to reach a deeper understanding on the hardening mechanisms in materials to develop a one parameter microstructure-based model to predict the mechanical behaviour of the steels. The project is focused on the relationship between microstructure and mechanical behaviour of steels, in order to developed predictive models and design products with the appropriate properties.
The thesis is divided into two sections: in the first part, physical models have been developed in monocomponent´s steels (ferrite, bainite and martensite), whereas in the second part the effect of precipitation has been studied in microalloyed steels.
For the first section, different heat-treatments have been carried out to generate a broad range of microstructures, which are characterized microstructurally and mechanically. Additionally, the effect of grain size and the presence of carbon in solid solution on mechanical properties have been studied in ferritic steels. On the other hand, the influence of the thermal treatment´s conditions has been analized in bainitic steels.
A physical based-model has been developed in order to predict the tensile behaviour of different microstructures, taking into account the concept of the dislocation mean free path, defined as the distance dislocations can glide before they interact with an obstacle. In ferrites, the obstacles which have a greater influence on the movement of dislocations are grain boundaries, so the main parameter for this microstructure is the ferrite grain size. However, in bainites and martensites, the presence of dislocations is very high and their contribution is higher than the grain boundaries. Therefore, the main parameter for these microstructures is the dislocation density. Additionally, the dislocation density is related to the thermal treatment´s temperature in bainites and to the Ms (Martensite start temperature) in martensites.
On the other hand, in order to study the effect of second phase particles and simulate industrial rolling process, plane strain compression tests have been carried out in steels microalloyed by Nb and Nb+V. The samples obtained have been characterize microestructurally and mechanically. Moreover, the same samples are analized by Transmission Electron Microscopy (TEM) in order to understand the precipitation effect.
The effect of different parameters has been analized in the present research: coling temperature, coling maintenance time, cooling rate and the effect of vanadium. The presence of an abundane of fine precipitates has been found in some samples, which results in an elevated hardening. The hardening is higher in steels microalloyed with vanadium.
Finally, a model has been developed to describe the tensile behaviour of microalloyed steels, taking into account the grain boundaries and the precipitates.
