Resumen de High strain rate mechanical behavior of advanced high strength steels
The requirements of lightweighting and optimizing fuel efficiency of vehicles promoted the development of advanced high strength steels (AHSSs), which are characterized by high strength and ductility. Until now, three generations of AHSSs have been developed, and significant body of research on this topic exists in the current literature. The vast majority of studies focused on the microstructural design to improve basic tensile mechanical properties of AHSSs. However, their dynamic behavior and impact resistance have not been systematically investigated, despite their significant relevance for automotive applications. This thesis focuses on the high strain rate performance of three AHSSs. These are a dual phase (DP) 1180 steel, a 304 stainless steel (304 SS) and a quenching and partitioning (Q&P) steel, belonging to the first, second and third generation AHSSs, respectively.
The main emphasis of this experimental work was laid on the impact resistance of the 1 mm thick AHSSs sheets subjected to drop weight impact testing. Mechanical behavior, microstructure evolution and mechanisms operating during high strain rate deformation were analyzed. Microstructure evolution was comprehensively characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. Special attention was paid to adiabatic heating (which was measured in situ) and its interplay with the deformation mechanisms. The experimental results indicated that the 304 SS has the best impact resistance (130 J), followed by the Q&P steel (110 J) and the DP 1180 steel (90 J). The impact resistance of the Q&P steel was enhanced by the combined effect of dislocation glide and TRIP phenomenon. These mechanisms along with twinning mechanism further increased the impact resistance of the 304 SS. Due to the combination of excellent mechanical properties and low cost, the Q&P steel appears to be more attractive for automotive application compared to the 304 SS and DP steel.
Additionally, tensile mechanical behavior of the Q&P steel was carefully examined in a wide range of strain rates (10−4 – 103 s−1) by employing split Hopkinson tensile bar (SHTB) system and digital image correlation (DIC) technique. It was shown that the yield strength (YS) of the Q&P steel in dynamic tests (500 – 1000 s−1) was by 200 MPa higher compared to the static tests (10−4 and 10−2 s−1), while the ultimate tensile strength (UTS) tended to increase linearly with strain rate. Retained austenite (RA) fraction decreased exponentially with the increase of plastic strain during both static and dynamic tensile testing. Comparative analysis of the outcomes of SHTB and drop weight impact testing showed that high strain rate biaxial deformation promoted transformation of retained austenite into martensite along with the ductility of the Q&P steel.
