Investigation On The Design And Strength-Ductility Enhancement Of Plain Quenching-Partitioning-Tempering Steel

Quenching-Partitioning-Tempering（Q-P-T）,as a modified process of quenching and partitioning（Q&P）,is a promising process to treat ultra-high strength steels with a good balance of strength and ductility.Since the proposition of Q-P-T process by academician Xu zuyao in our group in 2007,low carbon and medium carbon low alloy Q-P-T martensitic steels were systematically researched.Both strength and ductility of Q-P-T martensitic steels were enhanced with rising carbon content ranging from low carbon to medium carbon.Whether both the strength and ductility 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 the constrained carbon equilibrium（CCE）thermodynamic model for Q&P process 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 of Q-P-T steels.And a plain high carbon Q-P-T steel with highest 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 its carbon content based on the consideration of austenite decomposition and carbide precipitation.The QPT-LE model can better predict the volume fraction of RA(V_RA)and carbon content in RA（C_γ）in a high carbon Q-P-T steel,comparing with CCE thermodynamic model without consideration of carbide precipitation and interface migration or QP-LE thermo-kinetic model without consideration of carbide precipitation.Besides,the effects of carbide precipitation on V_RA and C_γwas 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,a high carbon Fe-0.67C-1.48Mn-1.53Si-0.038Nb（wt.%）steel was designed and treated by Q-P-T process.The high carbon steel was austenitized for360 s at 810℃,followed by quenching in saltbath at 170℃,then partitioning/tempering at 400℃ for 600 s in another saltbath,and finally quenching to room temperature（RT）in water.This sample was short called 170℃ Q-P-T steel.The 170℃ Q-P-T steel demonstrated 1,600 MPa strength,28.8%elongation and 46GPa%product of strength and elongation（PSE）.Besides,the high carbon Q-P-T steel exhibited excellent performance/cost（PSE/raw material cost）.At the same time,strength-ductility trade-off is solved only by increasing the low cost carbon content from low carbon to high carbon in Q-P-T steels,which is the century-long pursuit of researchers.During research on the“plain”high carbon 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 the effect of austenite on enhancement ductility.Here,we rename DAMAI to present straightly the essence of the phenomenon.The DAMAI effect was directly verified by in situ dynamic transmission electron microscopy observation during tension.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 both the volume fraction and mechanical stability of RA to enhance DAMAI effect and reduce strain-induced martensitic transformation（SIMT）.Thirdly,microstructural amount（including RA and carbide fraction）and C_γin low and medium carbon 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 C_γin low and medium carbon Q-P-T steels by QPT-LE model is better than those predicted by CCE thermodynamic model without consideration of carbide precipitation and interface migration or QP-LE thermo-kinetic model without consideration of carbide precipitation.Moreover,an important conclusion is obtained,that is,carbide precipitation affects the V_RA,while interface migration affects the C_γ.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 used for the design of Q-P-T and Q&P steels,which 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 steels and Q&P steels.The new strategy based on DAMAI effect for ductility enhancement of high strength martensitic steel provides a new pathway for developing other high strength-ductility martensitic steels.