Abstract
High-strength medium-carbon martensitic steel was heat treated through a quenching-partitioning-tempering (Q-P-T) treatment. Both the mechanism for improved ductility and the high temperature stability of austenite were investigated. The Q-P-T martensitic steel showed good products of strength and elongation (PSE) at various deformation temperatures ranging within 25–350 °C. The optimum PSE value (>57,738 MPa%) was achieved at 200 °C. The microstructure of the Q-P-T steel is constituted of laths martensite with dislocations, retained austenite located within lath martensite and small niobium carbides (NbC), and/or transitional ε-carbides that precipitated in the lath martensite. The good ductility can be mainly attributed to the laminar-like austenite that remained within the lath-martensite. The austenite can effectively enhance ductility through the effect of dislocation absorption by the retained austenite and through transformation-induced plasticity. The relationship between the microstructures and mechanical properties was investigated at high deformation temperatures.
Highlights
The quenching and partitioning (Q&P) process [1,2] has been put forward and applied in advanced high-strength steels (AHSSs) to achieve excellent ductility and strength
The Q&P process has gradually evolved to quenching-partitioning-tempering (Q-P-T) [3], direct quenching and partitioning (DQ&P) [4,5], and quenching and tempering-associated partitioning (Q-T&P) [6]
The main differences between the Q&P and Q-P-T processes are as follows: (i) The microalloying elements (Nb and V) are included in the Q-P-T process, which can result in grain refinement and carbides precipitation, but they are not included in the Q&P process. (ii) The tempering time and temperature depend on the proper carbides precipitation rather than the carbon partitioning process, and tempering includes partitioning
Summary
The quenching and partitioning (Q&P) process [1,2] has been put forward and applied in advanced high-strength steels (AHSSs) to achieve excellent ductility and strength. The core concept is based on the carbon distribution process from supersaturated martensite to untransformed austenite. The process is conducted to provide sufficient time at either the initial quenching temperature or above, and to stabilize the retained austenite to an ambient temperature. The Q-P-T treatment emphasized the application of precipitation strengthening to the Q&P process. (ii) The tempering time and temperature depend on the proper carbides precipitation rather than the carbon partitioning process, and tempering includes partitioning. The metallography of such steels consists of lath martensite, laminar-like retained austenite located within lath-martensite, and fine carbides dispersed throughout the martensite matrix
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