Investigations were conducted on low-carbon steel and the steel with same chemical composition with addition of microalloying element niobium. While tensile testing was carried out, the thermographic measurement was tacking place simultaneously. A specific behavior of niobium microalloyed steel was noticed. Test results have shown that, in the elastic deformation region, thermoelastic effect occurs, which is more pronounced in niobium microalloyed steel. Start of plastic flow in steel which is not microalloyed with niobium begins later in comparison to the microalloyed steel, and it is conducted so that, at the point of maximum stress, deformation zone is formed within which stresses grow. In steel microalloyed with niobium after proportionality limit, comes the occurrence of the localized increase in temperature and the occurrence of Lüders band, which propagate along the sample forming a deformation zone.

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In this study, an analysis of the influence parameters measured by the static tensile test, thermography and digital image correlation was performed during formation and propagation of the Lüders bands. A new approach to the prediction of stresses, maximum temperature changes and strains during the Lüders band formation and propagation is proposed in this paper. Application of the obtained mathematical models of influence parameters gives a clear insight into the behavior of niobium microalloyed steel at the beginning of the plastic flow, which can improve product quality and reduce costs during the forming of microalloyed steels with the appearance of the Lüders bands. The obtained models of influential parameters during formation and propagation of the Lüders bands have been developed by the regression analysis method. The proposed mathematical models showed low deviations of calculated results ranging from 1.34% to 12.37%.The local stress amounts, important in the forming of microalloyed steels since indicating surface roughness and plastic flow possibilities during the Lüders band propagation, are obtained by the mathematical model. It was found that stress amounts increase during the Lüders band propagation in the area behind the Lüders band front. The difference in stress amount between the start of the Lüders band propagation and advanced Lüders band propagation is 25.53 MPa.

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Thermo-graphic investigations were carried out on niobium microalloyed steel. During tensile testing temperature changes of samples, due to elastic and plastic deformation, were measured using infrared camera. The results of temperature measurements indicate that the specimen's temperature is lowered during the elastic deformation. This temperature drop is in other studies associated with so-called thermo-elastic effect which occurs in the metals during elastic deformation. Parallel with the infrared camera, surface of samples was recorded with digital imaging camera. Analysis of recorded images was analyzed using digital image correlation method. This is used to determine the deformation that occurs during stretching. The results show that during elastic deformation of samples the volume of samples is slightly increased. The volume change of samples and temperature drop, due to thermo-elastic effect, are compared for all samples. The analysis of experimental results explains that the thermo-elastic effect is directly related to the change of specimen volume during the elastic deformation of steel.