| According to the Quenching-Partitioning-Tempering (Q-P-T) process proposed by T. Y. Hsu (Xu Zuyao), new types of low carbon high strength steels with adequate plasticity and toughness were obtained through different Q-P-T processes. Meanwhile, the effects of tempering parameters on the microstructures and mechanical properties of Q-P-T steels were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Gleeble thermal simulator and Zwick/Sans universal testing machines, and the work-hardening behaviors during plastic deformation were also analyzed. Additionally, the characteristics of microstructurures and the retained austenite stability of Q-P-T steels under different tensile temperatures were revealed. Lastly, a possibility on the application of Q-P-T processes in engineering was explored, and two proper Q-P-T processes for 12 and 20 mm thick hot-rolled plates were designed, respectively. The main research achievements are described as follows.Firstly, a low carbon Q-P-T steel was designed, and its composition was measured as 0.256C-1.2Si-1.48Mn-1.5Ni-0.05Nb (wt.%). Based on the"Constrained Carbon Paraequilibrium"(CCE) theory proposed by Speer et al. and the related experimental data,proper Q-P-T process parameters were determined: initial quenching temperature was set to 290℃, and partitioning / tempering temperature was ranged from 350 to 450℃with the corresponding time ranged from 15 to 3600 s. The results indicated that this Q-P-T steel exhibited the highest ultimate tensile strength (UTS) (1500 MPa), sufficient ductility (14%) and product of strength (21000 MPa%) for tempering at 350℃for 30 s; and the best ductility (17%) and PSE (22000 MPa%) with superior UTS (1265 MPa) for tempering at 425℃for 30 s. The microstructures of the Q-P-T steel consisted of dislocation-type lath martensite and flake-like retained austenite with dispersively distributed carbides in martensite matrix, but the carbides changed from HCP-typeεtransition carbides to FCC-type (B1) NbC carbides with increasing tempering temperature. Especially for tempering at 425℃for 3600 s, the properties of the steel were deteriorated due to the formation of cementite (Fe3C) from the decomposition of retained austenite. In addition, the work-hardening behaviors of the Q-P-T steel during plastic deformation were also analyzed. The results showed that work-hardening rate decreased as tensile strain increased and its downward trend slowed down to a platform at higher strain, while work-hardening exponent presented a three-stage change including a first rapid decreasing stage, a plateau stage before necking and the last slow-decrease stage. The work-hardening behaviors mentioned above were attributed to the transformation induced plasticity (TRIP) effect of retained austenite.Secondly, the microstructures and mechanical properties of the steel subjected to Q-P-T process (tempering at 425℃for 30 s) under different tensile temperatures were investigated in detail. When deformed at -85~25℃, samples exhibited good low-temperature properties almost the same as that deformed at room temperature. Samples deformed at 25~300℃showed excellent combinations of strength and ductility, and the mechanical properties reached the peak values at 200℃with the UTS of 1300 MPa, the yield strength (YS) of 940 MPa, the ductility of 22% and the PSE of 28600 MPa%. However, when tensile temperature increased to over 300℃, the mechanical properties tended to lower. By comparing with microstructure deformed at room temperature, it was found that the microstructure deformed at -85℃almost kept unchanged,which was the reason that Q-P-T steel exhibited good low-temperture properties. When the tensile temperature was set to 200℃, both the TRIP effect of retained austenite and the precipitation strengthening effect ofεtransition carbides contributed to the best combination of strength and ductility. However, when the tensile temperature rose to 400℃, the temper-softening of lath martensite, the decomposition of retained austenite and the formation of Fe3C occurred simultaneously, which led to the deterioration of mechanical properties. In addition, the stability of retained austenite in Q-P-T samples under different tensile temperatures was analyzed in detail, and its four characteristic temperatures were determined combining with experimental data, namely, Ms<-85℃, MT= 300℃, Msσ= 0℃and Md = 473℃.Lastly, the possibility of Q-P-T processes applied in engineering was explored, and a new Q-P-T process with"pre-cooling + water quenching + air cooling"method for the continuous quenching process of hot-rolled plates was developed. According to this method, two proper Q-P-T processes for 12 and 20 mm thick hot-rolled plates were designed, respectively. The results showed that the microstructures of the as-treated plates consisted of martensite, retained austenite and NbC carbides, and some bainite formed near the core of the plates as the dimension increased from 12 mm to 20 mm. These two types of hot-rolled plates exhibited good mechanical properties with YS higher than 900 MPa, UTS higher than 1200 MPa and elongation more than 15%, which reached the design objective. The exploration of the Q-P-T process with"pre-cooling + water quenching + air cooling"method provided a reference for the applications of Q-P-T processes in engineering.
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