In microalloyed steels, plastic instabilities often appear which have been found to be associated with changes in the microstructure. In this paper, research was carried out on the influence of the microstructure in different areas of the deformation zone during the formation of Lüders bands in niobium microalloyed steel. Thermography and digital image correlation during static tensile testing were used to research deformation behavior and the area before and during the formation of the Lüders band. Different local values of temperature changes, i.e., stress changes, and strains in the examined areas during the formation of the Lüders band were determined. The highest values of the temperature changes and strains during the formation of the Lüders band were measured in the area of the initial appearance of the Lüders band. In order to clarify the observed changes, a microstructural analysis, using X-ray diffraction, scanning electron and transmission electron microscopy methods, was used. It was established that the observed temperature, i.e., stress, and strain changes are related to changes in the microstructure. The analyses of changes in the microstructure, arrangement and interaction of dislocations with precipitates revealed significant changes in the movement of dislocations and their interaction with fine niobium-containing precipitates during the formation of Lüders bands. The influence of microstructural parameters on the Lüders band formation in niobium microalloyed steel has been established based on this research.

Characterization of the Lüders bands that occur in niobium microalloyed steel is carried out in different points of their formation and propagation. During static tensile testing infrared thermography is simultaneously used to record temperature changes and digital image correlation to determine the strain amounts in specific points of Lüders bands formation and propagation. Temperature changes which present stress changes were taken at the different strain amounts 0.005 mm/mm, 0.013 mm/mm, 0.026 mm/mm and 0.032 mm/mm. Performed static tensile tests showed the influence of different strain rates 1.810⁻³ s⁻¹, 710⁻³ s⁻¹ and 1.8510⁻² s⁻¹ on the stress changes during Lüders bands formation and propagation. In different specific points such as Lüders band front, behind the Lüders band front (deformed area) and in front of the Lüders band front (undeformed area) tests were carried out. The studies revealed maximum stress changes behind Lüders band front and their increase with increasing strain rate. At the highest strain rate in the point of maximum stress behind the Lüders band front the greatest temperature changes was measured.

In order to determine the Lüders band propagation along the deformation zone length and width as affected by the force action direction, a detailed line analysis is performed by applying thermography and digital image correlation (DIC) during static...