Mechanism Of Ductility Enhancement Of An Advanced High Strength-ductility Q-P-T Steel And Its Dynamic Mechanical Properties

In an international attention of reducing carbon emissions, it is urgently required toinvestigate and develop the third generation of advanced high strength steels (AHSS) forsaving energy and raw materials as well as protecting environment. Therefore, accordingto the novel quenching-partitioning-tempering (Q-P-T) process proposed by T. Y. Hsu(Xu Zuyao) for the development of the third AHSS, both the composition and proper Q-P-T process of low carbon Q-P-T steel have been designed in this work. To compare withthe novel Q-P-T process, the low carbon steel designed has also been treated by thetraditional quenching and tempering (Q&T) process. Scanning electron microscopy (SEM),transmission electron microscopy (TEM), X-ray diffraction (XRD) and mechanical tests(quasi-static tensile test, dynamic tensile test and impact test) were employed to study theeffects of two heat treatment processes on mechanical properties and microstructures, andto study the deformation temperature dependence of mechanical properties andmicrostructures for Q-P-T steel. Based on the measurement of average dislocationdensities in both Q-P-T steel and bainitic steel combined with TEM observation, the effectof dislocation absorption by retained austenite (DARA) is verified in the low carbon steel,similar to that in the medium carbon steel proposed recently. More importantly, theDARA effect is also found in bainitic steel, from which the mechanism of retainedaustenite on ductility enhancement of high strength steel is clarified. The mainachievements are expressed below.(1) In this work, a low carbon Fe-0.25C-1.48Mn-1.20Si-1.51Ni-0.05Nb (wt.%) steelwas designed. Based on the “Constrained Carbon Paraequilibrium”(CCE) thermodynamicmodel proposed by Speer et al, the relationship of the theoretical retained austenite fraction at room temperature (20°C) and quenching temperature (Tq) was predicted, andsubsequently the proper Q-P-T process was designed. The tensile test results indicate thatthe designed steel through the proper Q-P-T process exhibits a high ultimate tensilestrength of1322MPa and a quite high total elongation of16.9%, accompanying a highproduct of strength and elongation of22342MPa%. However, the steel treated by thetraditional Q&T process exhibits worse comprehensive mechanical properties than Q-P-Tsteel. Moreover, the results of impact tests at room temperature show that Q-P-T steel hasbetter impact property than Q&T steel. The microstructural characterization by TEMreveals that the high strength of the Q-P-T steel results from dislocation-type martensitelaths and fcc NbC carbides and hcp ε carbides precipitated dispersively in martensitematrix, while excellent ductility is attributed to the significant transformation inducedplasticity (TRIP) effect due to considerable amount of retained austenite. However, theplasticity of the Q&T steel is very poor due to a little amount of retained austenite.(2) The only difference between the traditional Q&T process and the novel Q-P-Tprocess is the Tqchosen, and the Tqselected in the novel Q-P-T process is much higherthan that (room temperature,20°C) in the traditional Q&T process. Since the higher Tqcorresponds to the higher volume fraction of retained austenite and lower internal stresscaused by quenching and martensitic transformation, the Q-P-T steel exhibits betterductility and toughness. Besides, the higher Tqhas lower driving force of martensitictransformation and results in more uniform sizes of martensite blocks and laths, and theyare favorable for the increase of strength and toughness, which partially compensate thedecrease of Q-P-T steel’ strength due to the amount of martensite being lower than that inQ&T steel. This is why Q-P-T steel exhibits much better ductility and toughness thanQ&T steel, while the strength of Q-P-T steel is somewhat lower than that of Q&T steel.(3) The results of tensile tests and XRD at different deformation temperatures from70°C to400°C indicate that retained austenite exhibits high thermal stability atdeformation temperatures from70°C to300°C accompanying good mechanicalproperties. Therefore, the Q-P-T steel studied can be considered to be applied in thetemperature range from70°C to300°C. In deformation temperature range from70°C to300°C, the high ultimate tensile strength of the Q-P-T steel is attributed to martensite lathswith high density of dislocation and two kind of carbides (fcc NbC carbides and hcp ε carbides) dispersively precipitated from martensite matrix, and the good ductility resultsfrom the significant TRIP effect from considerable amount of retained austenite. However,in deformation temperature range over300°C, the weak TRIP effect and the formation ofbrittle cementite lead to the poor mechanical properties of the Q-P-T steel. The research inthis work reveals that the addition of Si in Q-P-T steels can suppress the formation ofbrittle cementite (Fe3C) in martensite matrix at deformation temperatures from70°C to300°C, but cannot prevent from the formation of Fe3C by decomposition of retainedaustenite or by transformation of transitional ε carbides at temperatures above300°C.(4) The retained austenite fractions as a function of strain in both the lower carbon Q-P-T steel and the bainitic steel were all measured by XRD. The XRD results indicate thatthe TRIP effect both occurs, which is verified by the decrease of retained austenite fractionwith the increase of strain and the appearance of twin-type martensite plates duringdeformation. Based on the measurement of average dislocation densities in bothmartensite (or bainite) and retained austenite by using X-ray diffraction line profileanalysis (XLPA) combined with TEM observation, the DARA effect in the low carbonsteel is verified, similar to that in the medium carbon steel proposed recently. Moreimportantly, the DARA effect is also found in the bainitic steel, which means thatdislocations in bainite move into the nearby retained austenite, namely, the dislocations inbainite are “absorbed” by the neighbouring retained austenite, causing the averagedislocation density in bainite laths to decrease. Such a DARA effect makes the hard phasemartensite (or bainite) exist in a “soft state” or a “non-work hardening state”, and thus theharmonious deformation ability of hard phase martensite (or bainite) with soft phaseaustenite is intensified.(5) The two conditions of DARA effect were proposed, namely, the sufficient amountof retained austenite and the coherent or semi-coherent interface between martensite (orbainite) and retained austenite.(6) The mechanism of retained austenite on ductility enhancement of high strengthsteel can be summarized as three successive effects during deformation: DARA effect,TRIP effect and BCP (Blocking crack propagation) effect, and these three effects one afterthe other enhance the harmonious deformation ability of hard phase martensite (or bainite)with soft phase retained austenite. (7) The dynamic mechanical properties of Q-P-T steel and Q&T steel at variousstrain rates were measured. It indicates that the tensile strength and ductility of Q-P-T steelare both enhanced with increasing strain rate. In contrast, the tensile strength of Q&T steelis enhanced with increasing strain rate, but the ductility slightly decreases. At the samestrain rate, the comprehensive mechanical properties of Q-P-T steel is much better thanthose of Q&T steel. Experiments show that the Q-P-T steel studied can be considered tobe applied in the strain rate range from quasi-static (104/s) to dynamic tension (103/s).(8) The cause of strength and ductility enhancement at high strain rate was analyzedfor the Q-P-T steel. The activation of a large number of dislocation sources and the partialsuppression of the dislocation cross-slip at high strain rate result in the increase of strength.Meanwhile, both the change of fracture mode and adiabatic temperature rise effect at highstrain rate contribute to the enhancement of ductility, but the weakening of DARA effect,TRIP effect and BCP effect of retained austenite at high strain rate reduces the ductility ofthe Q-P-T steel, and thus their combination gives rise to the improvement of ductility atsome extent comparing with low strain rate (quasi-static rate).