Direct quenching and double tempering obtain high strength and toughness of Cu-bearing HSLC martensitic steel

Abstract

Cu-bearing high strength low carbon (HSLC) steel possesses high strength due to the existence of finely dispersed Cu-rich phases within the matrix. However, such high precipitation strengthening results in the loss of toughness. To break this strength-toughness trade-off, we propose a new strategy of direct quenching followed by double tempering (DQ-TT). Over four times higher energy was found in the DQ-TT sample (74 J) at −84^oC compared to the other samples with single tempering (DQ-T, 13 J) and reheated quenching (RQ-TT, 6 J) without the sacrifice of strength. This high toughness is proved to be derived from: the higher cleavage fracture stress (${\sigma }_F$) and the higher crack initiation and propagation energy. The former comes from the refined effective grain sizes (EGS, 5.66 μm) and the optimized element segregation, where Cu (low enrichment ratio of 2.75) and Mo (high enrichment ratio of 35.20). The reduced segregation of Cu and the enrichment of Mo at the lath boundaries can effectively enhance the interfacial bonding strength. The latter is related to the orientations of the materials. It is measured that the DQ-TT sample has higher <110>//RD (47.8%) and <001>//ND (20.5%) deformation textures, which correspond to the {001} cleavage plane parallel to the RD and {110} slip plane parallel to the ND. The weak {001}//RD and strong {110}//ND ensure crack initiation and propagation along weak interfaces while preventing significant cleavage fracture. Oscilloscope impact tests reveal that DQ-TT process possesses higher crack initiation and propagation energy, which are 20 J and 37 J, respectively.