5th International High Manganese Steel Conference - HMNS 2022

In a thermal cycle consisting of a quenching and partitioning (Q&P) process, followed by a subsequent galvannealing process, the effect of Si and Al especially on the retained austenite in the microstructure was investigated by two alloy concepts 0.2C-4Mn-1.5Si and 0.2C-4Mn-1.5Al. Thereby the influence of a replacement of the alloy element Si by Al and their carbide retarding effect was investigated in the Q&P-temperature region until 400°C and also at higher galvannealing temperatures up to 560°C. Thereby samples were annealed by dilatometry to investigate the microstructure evolution. Afterwards retained austenite was measured by saturation magnitude magnification and X-ray diffraction method and microstructure was investigated by light optical microscopy, scanning electron microscopy and electron backscattered diffraction. As Si and Al have a different effect on the tempering behavior, respectively carbide precipitation in martensite, derivation of relative length change was calculated from dilatometry martensite tempering tests and heat flow was measured by differential scanning calorimetry starting from fully martensitic microstructure.
During traditional Q&P, Si-concept was more efficient in stabilizing high amounts of lath like and blocky RA compared to Al. This correlates well with the tempering experiments of martensite, as more carbide precipitation takes place around the Q&P temperature, respectively lower carbon partitioning potential for the Al-concept. At higher temperatures, during the galvannealing process, Al showed considerable benefit compared to Si, as it retards effectively the retained austenite decomposition into pearlite. For the Al-concept annealing at galvannealing temperatures results also in a good work hardening behavior due to the still effective TRIP-effect, whereby for the Si-concept RA was decomposed. These investigations showed that Si-alloyed grade in particular possesses excellent properties after Q&P process. In contrast, the Al-concept proved to be more suitable to fulfill the desired requirements subsequent to galvannealing.