To analyze the formation of multiple shear bands in HY-100 steel, we consider an infinitely extended layer of finite thickness subjected to shear loading. The perturbation approach, associated with numerical methods, is used to determine the instability modes. The criteria of Wright–Ockendon and Molinari are used to determine the shear band spacing. The hypothesis consisting in considering the proportion of plastic work dissipated as heat (quantified by the Taylor–Quinney coefficient ) as independent of the loading path is now recognized as highly simplistic. The present study attempts to provide a systematic approach to the inelastic heat fraction evolution for a general loading within the framework of thermoviscoplastic standard modeling, including a number of material parameters as strain hardening, strain rate sensitivity, and thermal softening. The effect of each material parameter on the shear band spacing is determined by using a power law constitutive relation. The Johnson–Cook and power law models are used to illustrate the influence of the constitutive relation on the results for the adiabatic shear band spacing by studying the response of HY-100 steel. We have compared our results with available experimental results in the literature over a very wide range of strain rates . In this study, we show that the variation in the Taylor–Quinney parameter as a function of shear strain is an important parameter that plays a significant role in the calculation of the shear band spacing.