Understanding recovery behavior of advanced high strength automotive steels

Abstract

Tailoring of cold rolling to promote recovery and recrystallization process during annealing in single phase steels is well understood. In contrast, HSS/AHSS consists of ferrite-pearlite as the initial cold rolled microstructure. The recovery and recrystallization behavior in the ferrite might be different from pearlite. Understanding the behavior of individual phases is important in optimizing the properties and design of AHSS. This study investigates the early recovery, recovery behavior at 300, 500 ºC and partial recrystallization at 700 ºC for various soaking times after 80% cold rolling. Mainly pearlite fraction and its distribution were varied in three HSS/AHSS to investigate the role of pearlite at different annealing temperatures in comparison to single ferrite consisting steel. It is quite challenging to investigate recovery with only microscopy techniques due to subtle changes in structure. So we provide a comprehensive understanding of the correlative investigation across the multiple length scales using SEM, EBSD, TEM and nanoindentation. This study reveals the temperature and composition-driven recovery kinetics at 300 and 500 ºC. A combination of electron microscopy and nanoindentation and a simple kinetics model enables new insights on what drives the kinetics and/or the dominating mechanisms at different temperatures. It was found that at 300 ºC, single ferrite consisting of steels resulted in recovery, and interestingly ferrite-pearlite consisting HSS resulted in increasing hardness from the cold rolled state. Hardness increases show a clear dependence on the pearlite fraction and soaking time. Cottrell kinetics suggest and obey and#119905;2/3 in the case of banded pearlite distribution. Whereas in non-banded pearlite newlinedistribution the amount of free carbon available decreases due to the presence of microalloying element Nb and the kinetics deviate and#119905;2/3 law. Upon annealing at 500 ºC, a significant decrease in hardness is observed in single ferrite consisting of steels mainly due to the onset of partial