Difúzní charakteristiky vodíku v moderních konstrukčních ocelích

Abstract

The presented dissertation thesis is devoted to the study of hydrogen diffusion characteristics in the TRIP (Transformation Induced Plasticity) steels, belonging to the group of AHSS (Advanced High Strength Steels). These steels are mainly used in the automotive industry, especially for their mechanical properties, where high values of the tensile strength are required in the range of 500-1000 MPa, high elongation at fracture in the range from 20 to 40% and the ability to absorb large amounts of dynamic energy at high strain rates (e.g. during traffic accidents). These properties can be achieved thanks to the multiphase microstructure, which consists of ferrite, ferritic bainite and residual austenite, which can be transformed into deformation induced martensite as a result of the plastic deformation. During the surface treatment of these steels in order to increase their corrosion resistance, hydrogen may entry the steel, which may cause hydrogen embrittlement. Hydrogen embrittlement of steels is closely related to the diffusion characteristics of hydrogen in steels, and it is the hydrogen diffusion characteristics of the TRIP steels that form the main content of this dissertation thesis. The thesis studies three variants of the TRIP steels with different chemical composition, namely TRIP 800 steels based on C-Mn-Si, then based on C-Mn-Si-Al and a variant of the C-Mn-Si-P type. Steels are studied in three different states, in the state without deformation, then in the states after 5% and 10% cold tensile deformation. The need to distinguish between different states stems from the fact that TRIP steels are used in car body construction for structural elements, which are usually finished by cold forming with uneven deformation distribution, where some structural elements are essentially free of significant deformation, while other parts can be cold deformed quite significantly. The microstructural characteristics of the investigated steels are described by light microscopy, scanning electron microscopy and transmission electron microscopy. The content of residual austenite in the initial state and in the states after 5% and 10% tensile deformation is evaluated by X-ray diffraction analysis. The hydrogen diffusion characteristics are studied by the method of electrochemical permeation of hydrogen using two experimental protocols: one with two ascending branches of the transition curve, the other according to Zakroczymski with three stages of hydrogenation. Analogous trends are observed for both experimental protocols, in principle in all aspects, the observed differences can be attributed to the experimental complexity of the method and the number of parameters that affect the experiment. The obtained results present a significant contribution to a comprehensive study of the action of hydrogen in the TRIP steels. In particular, the obtained hydrogen diffusion coefficients can be included among the few that have been obtained so far in the TRIP steels, even on an international scale.