Effects of strain rate and adiabatic heating on mechanical behavior of medium manganese Q&P steels

In this work, the mechanical behavior and properties of four different multiphase steels was studied in tension at strain rates of 10-4, 10-2, 0.5 and 800 s-1. The four materials include a medium manganese (3%) steel grade overcritically and intercritically annealed and Q&P heat treated and two industrially produced TRIP-assisted steels, DH800 and TRIP700 steels, which have different retained austenite morphology. The temperature and strain of the specimens were studied using high speed infrared thermography (IRT) and digital image correlation (DIC). The mechanical response of the Q&P steels had considerably higher tensile strength than the two industrially produced steels. The Q&P steel with a higher austenite volume fraction strain hardened significantly more than the other steels. The DH800 steel and the intercritically annealed Q&P steel heated less with Delta T of 25 degrees C during uniform deformation than the TRIP700 steel and the overcritically annealed Q&P steel with Delta T of 35 degrees C. However, the industrially produced steels DH800 and TRIP700 had higher uniform elongation of 0.12 mm/mm and 0.14 mm/mm whereas the Q&P steels reached only 0.09 mm/mm, meaning that the heating rate of the Q&P steels was considerably steeper. In addition, the stronger necking of the DH800 and TRIP700 steels led to much higher maximum temperatures before failure (max. 260 degrees C) than those observed for the Q&P steels (max. 140 degrees C). The Taylor-Quinney coefficients of the Q&P steels were large in the beginning of the plastic deformation (0.65-0.95) but decreased as a function of plastic deformation, whereas the Taylor-Quinney Coefficients of the DH800 and TRIP700 steels were lower (0.5-0.6) but increased gradually as a function of plastic deformation.