Microstructure and mechanical properties of a low-carbon HDQ&P steel

Yufei Lu a , Jilan Yang a , Jun Xu a , Zhenghong Guo a, and Jianfeng Gu b

a Institute of Materials Modification and Modelling, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, People’s Republic of China;

b State Key Laboratory for Laser Manufacturing, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, People’s Republic of China

ABSTRACT

The microstructure and mechanical properties of Fe-0.19C-1.46Mn-1.50Si, a low-carbon hotrolling direct quenching and partitioning steel, were investigated, and particular emphasis was placed on the effects of the rolling temperature and the subsequent cooling method. Once rolling occurred above the recrystallization temperature, the grain size of prior austenite could be reduced from ∼88.2 to ∼11.8 μm. The corresponding packet and block sizes of martensite could be reduced from ∼17.6 to ∼4.3 μm and from ∼9.0 to ∼1.8 μm, respectively, after a typical two-step quenching and partitioning (Q&P) treatment following rolling immediately. Once rolling occurred below the recrystallization temperature, deformed prior austenite with a grain length and width of ∼150 and ∼60 μm, respectively, were obtained. Consequently, only the block size would be reduced to ∼1.9 μm. The volume fraction of retained austenite (RA), which was ∼11% in the specimens, was not significantly affected by the rolling temperature. Further, if the typical two-step Q&P treatment was replaced by air cooling after the rolled specimen was quenched to the martensitic start temperature, the volume fraction of RA in the specimen was reduced to ∼7.4%, and the corresponding carbon depletion degree in the martensitic matrix was decreased as well. Tension and impact tests indicated that the cooling method was more critical than the rolling temperature for determining the mechanical properties. The presence of sufficient amounts of RA and carbon depletion in the refined martensitic matrix contribute to the strong strain hardening and energy absorption capabilities of the steel together. As a result, rolling at a temperature that was slightly higher than the recrystallization temperature combined with the typical two-step Q&P treatment could be an optimized process to obtain excellent comprehensive mechanical properties. In particular, the ductile-to-brittle transition temperature (DBTT) could be reduced from −10°C to −30°C.

KEYWORDS

Hot-rolling direct quenching and partitioning steel; refinement of martensitic microstructure; retained austenite; mechanical property; ductile-to-brittle transition temperature