Relation between microstructure and mechanical properties of a low-alloyed TRIP steel

摘要

Steels with transformation induced plasticity (TRIP) offer a good balance of ductility and strength, which is a result of gradual transformation of metastable austenite into martensite during straining. A thorough understanding of the relation between microstructure and martensitic transformation and the related austenite stability is the key for a successful improvement of the mechanical properties. The present low-alloyed TRIP steel has a rather heterogeneous texture and microstructure on the microscopic level due to the intense austenite banding. This makes microstructure characterization particularly important and difficult. As EBSD is used as the main microstructure characterization tool particular attention is given to the statistical reliability of EBSD-based results. In addition, a new and fully automated mapping technique, that helps to cover larger areas and probe more grains while keeping the particular advantages of a small step size, is presented. Lastly, the comparison of results obtained by EBSD and XRD revealed that the EBSD technique is reliable and representative provided that sampling and sample preparation are adequately done. After identifying the optimum sampling conditions the deformation and transformation of austenite during interrupted ex-situ bending tests were studied by EBSD investigations. The effects of size and shape of austenite grains on the strain-hardening of the present TRIP steel have been interpreted by a simple rule of mixture for stress-partitioning and a short fiber reinforced composite model. The composite model of Cox was used to investigate the effect of grain shape on load partitioning. Moreover, the grain average misorientation (GAM) analysis of the EBSD results revealed that at the initial stage of deformation mainly larger grains deform. These larger grains, however, do not reach the same strain level as the smaller grains because they transform into martensite at an early stage of deformation. The texture development during tension and compression of the present TRIP steel is studied by analysing the results of bending experiments. Here, the change in the retained austenite texture was due to both deformation and transformation into martensite, whereas the ferrite texture changed due to deformation only. A theoretical relation between the deformation of FCC and BCC crystals was derived by a phenomenological constitutive deformation model. By comparing the experimental observations to the theoretical relation, the changes in the austenite texture due to slip were distinguished from the changes due to transformation. These comparisons have shown that the strong decrease in the Brass and Goss components of austenite are mainly due to the martensitic transformation. Lastly, a detailed analysis of the effects of grain size, grain shape and crystallographic orientation on austenite stability is presented. The experiments revealed that the retained austenite is not transforming continuously, but rather in a distinct 3-stage behaviour. Accordingly, the grains can be grouped into 3 categories; grains that are transforming with a high rate at low strain, those that transform at high strain with a low rate and lastly a strain gap between these two types where the austenite does not transform. Grains that are transforming at low strains tend to be large, have low Taylor factor and a strongly elongated shape, while the remaining stable grains are small, more spherical in shape and have high Taylor factor. The grain size effect is quantitatively explained by a diffusion model that shows that larger grains are not fully stabilized due to insufficient carbon diffusion. The grain shape effect is related to the strain distribution that is found in short-fiber composite materials. The Taylor factor indicates the ease of plastic deformation and a low Taylor factor, therefore indicates a large abundance of possible nuclei for martensitic transformation according to strain-induced transformation mode. The strain-gap between the early and late transforming grains is explained by the lack of grains with medium Taylor factor. Finally, a logistic regression model was used to quantify the relative contribution of each of the microstructure-related parameter. The results of this model showed that the grain size is the most important parameter as its significance is about an order better than the second most important parameter, the Taylor factor. This thesis shows, firstly, how to obtain a reliable and representative description of rather heterogeneous microstructures using the EBSD technique. The presented concepts can be used to improve the statistical reliability of all possible types of EBSD-based microstructure analysis. Secondly, a comprehensive analysis of the relation between microstructure and macroscopic mechanical properties of a low alloyed TRIP steel is presented. These results provide a useful basis for further development and improvement of TRIP-assisted steels.