The influence of niobium content and initial microstructure of steel on the occurrence of Lüders band at the start of the plastic flow during cold deformation

In this paper, the results of a research on influence of niobium and initial microstructure on start of plastic flow of a material and formation of Lüders bands during cold plastic deformation in niobium microalloyed steel are presented. Research was carried out on hot rolled strips from low carbon steel and three steels with the same base chemical composition but with different amount of niobium: 0.035% Nb, 0.048% Nb, 0.060% Nb. Niobium microalloyed steel strips are rolled with selected parameters of thermomechanical treatment and additionally heat treated with the goal of achieving inhomogeneous microstructure. Static tensile testing was performed on samples, while simultaneously being recorded with thermographic camera. The results of thermographic measurements were analysed with software package Irbis 3 professional. The initial microstructure is tested for all samples. During the start of plastic flow in all microalloyed steel samples, with fine-grained homogeneous microstructure, inhomogeneous deformations are created. These were not found on samples which do not contain microalloying element niobium. In area of inhomogeneous deformations, Lüders bands are formed and propagate through the deformation zone. Lüders band was not found in microalloyed steels with coarse grain. The tests show that the addition of micro-alloying element niobium affects Lüders bands formation but the content of niobium in steel has no effect on the appearance of the Lüders bands. Additionally, it was found that the initial microstructure has a great influence on Lüders bands occurrence.