This master’s thesis investigates the kinetics of static softening in the high-strength structural steel S690QL. The study is based on double-hit compression tests performed using a Gleeble thermomechanical simulator, enabling controlled simulation of hot deformation conditions. In the first deformation step, specimens were compressed at selected temperatures, strain rates and strain levels, followed by an isothermal holding period intended to promote static softening processes. Subsequently, a second deformation step was applied to evaluate the mechanical response of the material.
The degree of softening was determined from the analysis of stress–strain curves and used as a quantitative measure of the extent of softening during the interpass holding time. The influence of temperature, strain and strain rate on softening behavior was systematically analyzed. The results show that the degree of softening increases with increasing temperature and holding time, while strain rate has a secondary effect on the softening kinetics, with temperature being the dominant parameter. Furthermore, the results indicate that softening in S690QL steel proceeds more slowly compared to lower-alloyed steels, which can be attributed to the presence of microalloying elements that retard recovery and recrystallization processes.
The findings of this work contribute to a better understanding of static softening in high-strength microalloyed steels under conditions relevant to industrial thermomechanical processing. The obtained results provide a useful basis for the optimization of hot forming parameters for S690QL steel and for further modeling of softening kinetics in high-strength structural steels.
Key words: High-strength low-alloy steel, S690QL, static softening, double-hit compression test, Gleeble simulator, thermomechanical processing, softening kinetics.
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