Investigation on the Design and Strength-Ductility Enhancement of Plain Quenching-Partitioning-Tempering Steel


Student thesis: Doctoral Thesis

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Awarding Institution
  • Jian LU (Supervisor)
  • Nailu CHEN (External person) (External Supervisor)
Award date20 Dec 2021


Quenching-Partitioning-Tempering (Q-P-T) is a promising process to treat ultra-high strength steels with good strength-ductility. Since its proposition of Q-P-T process by academician Xu Zuyao in 2007, low-C and medium-C low alloy Q-P-T martensitic steels were systematically researched. Strength-elongation of Q-P-T steels were enhanced with rising carbon content ranging from low-C to medium-C. Whether both strength and elongation of Q-P-T steels can be further enhanced by increasing carbon up to high carbon range or not, is a good question for further research. Besides, the Q-P-T process still lacks theoretical model for process design, as constrained carbon equilibrium (CCE) thermodynamic model cannot ensure prediction accuracy for the retained austenite (RA) fraction or its stability. Therefore, in this thesis, a concise QPT-LE (Local Equilibrium) thermo-kinetic model with dual interfaces (martensite/carbide and martensite/austenite) migration was established for the design. A plain high-C Q-P-T steel with excellent cost performance was developed and the mechanism for high strength-ductility was revealed. The main conclusions are as follows:

Firstly, the QPT-LE model with dual interfaces (martensite/carbide and martensite/austenite) migration was established to predict the evolution of austenite fraction and C content based on consideration of austenite decomposition (AD) and carbide precipitation (CP). The QPT-LE model can better predict the RA volume fraction (VRA) and its carbon content (Cγ) in a high-C Q-P-T steel, comparing CCE thermodynamic model without consideration of CP and interface migration (IM) or QP-LE thermo-kinetic model without consideration of CP. Besides, the effects of CP on VRA and Cγ was revealed by the QPT-LE, while the prediction accuracy of carbide fraction can be further improved by considering the effect of carbon segregation to dislocation.

Then, high-C Q-P-T steels was designed and treated by process. A 170 °C Q-P-T steel demonstrated 1,600 MPa strength, 28.8% elongation and 46 GPa% product of strength and elongation (PSE). Besides, it exhibited excellent performance/cost (PSE/raw material cost). During research on the “plain” high-C Q-P-T steel, phenomenon of dislocations across martensite/austenite interface (DAMAI) and its effect on ductility enhancement was found again, which was named “dislocation absorption by retained austenite” (DARA) to emphasize effect of RA on enhancement ductility. Here, we rename DAMAI to present straightly the essence of the phenomenon. The DAMAI effect was directly verified by in-situ TEM observation. Besides, DAMAI effect was further verified by molecular dynamics (MD) simulation theoretically. DAMAI effect makes martensite “softening” and thus evidently raises the martensite deformability accompanying with noticeable ductility enhancement of the steel. Furthermore, a new strategy for ductility enhancement of high-strength martensitic steels based on DAMAI effect is proposed, namely, balancing RA volume fraction and mechanical stability to enhance DAMAI effect and reduce strain-induced martensitic transformation (SIMT).

Thirdly, microstructural amount (including RA and carbide fraction) and Cγ in low-C and medium-C Q-P-T steels are quantitatively characterized for further validation on the universality of the QPT-LE model. In the same way, the prediction accuracy of microstructural amount and Cγ by QPT-LE model is better than those predicted by CCE thermodynamic model without consideration of CP and IM or QP-LE thermo-kinetic model without consideration of CP. Moreover, an important conclusion is obtained, that is, CP affects the VRA, while IM affects the Cγ. Besides, when the carbon content at the carbide/martensite interface is set to zero, the QPT-LE model degenerates to QP-LE model. Therefore, the QPT-LE model can be utilized for designing, and this is another university of QPT-LE model.

In general, the QPT-LE model established by us will be a novel tool in the design of process and microstructure of both Q-P-T and Q&P steels. Strategy based on DAMAI effect for ductility enhancement of high strength martensitic steel provides new route for development of other high performance steels.