High Product Of Strength And Elongation Of Steels Treated By Quenching-PartitioningTempering Process And The Mechanism Of Strength And Ductility

The requirements of the resource and energy savings and the reduction of carbon emission are pushing the development of low-cost Fe-Mn-Si based advanced high strength steel (AHSS). In order to reach a higher product of strength and elongation (PSE) than 25000 MPa% of the advanced high strength steels (AHSS), even to 30000 MPa% of the next generation AHSS predicted by Matlock and Speer, both the composition and process of low and medium carbon steels have been designed in this work based on the novel heat treatment manner, quenching-partitioning-tempering (Q-P-T), proposed by T. Y. Hsu (Xu Zuyao) several years ago. Attentions were paid to understand the relationship between mechanical property and microstructure, in particular the enhancement effect of retained austenite on ductility of Q-P-T steels. The purpose of this dissertation attempts to investigate the mechanism of high PSE (product of strength and elongation) in designed Q-P-T steels with optimized processing parameters. The mechanical performance at different temperature from -85°C to 450°C was also studied to understand the potential application range of the proposed Q-P-T steel. The main achievements are expressed below.1. Four kinds of Fe-Mn-Si based steels with the addition of different carbon (nominal 0.2 or 0.4wt.% respectively) and niobium (nominal 0.03 or 0.08wt.% respectively) were designed in the present research. Based on the constrained carbon equilibrium (CCE) theory proposed by Speer et al., the maximum fraction of retained austenite as a function of quenching temperature (Tq) was calculated, and the optimized parameters of Q-P-T process were determined. The traditional quenching and tempering (Q&T) process was also employed to these steels designed for comparing with Q-P-T process. The results indicate that medium-carbon Q&T steel has higher ultimate tensile strength and lower elongation than low-carbon Q&T steel, which shows that the effect of carbon content follows the general behavior of high strength-low ductility. However, medium-carbon Q-P-T steel has the ultimate tensile strength of 1558 MPa and elongation of 20.3% (uniform elongation of 12.2%), which reach the theoretical value predicted by Matlock and Speer. Obviously, a novel Q-P-T process makes the effect of carbon content be different from the general behavior of high strength-low ductility. Besides, the strength and elongation of medium-carbon Q-P-T steel also are much higher than 1222 MPa and 15.2% (uniform elongation of 6.13%) of low-carbon Q-P-T steel. These results indicate that a novel Q-P-T process produced a positive effect of carbon content on both strength and ductility.2. The origin of different mechanical properties between Q-P-T steels and Q&T steels was revealed by microstructural characterization with XRD, SEM and TEM. The only difference between Q&T and Q-P-T treatment is the quenching temperature (Tq) selected, and Tq in Q-P-T process is much higher than that (room temperature) in Q&T process. Microstructural analysis indicates that the high Tq correspond to the high fraction of retained austenite and low stress caused by quenching. These are favorable for the transformation induced plasticity (TRIP) caused by retained austenite and the low formation probability of micro-crack caused by quenching. The increase of carbon content in Q-P-T specimens is responsible for the high strength due to higher density of dislocation in martensite, smaller packet size of martensite and more carbides distributed in martensite matrix. Meanwhile, the increase of carbon content is responsible for the high ductility due to stronger TRIP effect. These are why Q-P-T steels exhibit much better in both strength and elongation than Q&T steels.3. The volume fractions of retained austenite as a function of strain in medium-carbon Q-P-T steels were determined by XRD. The results indicate that the retained austenite fraction decreases with increasing strain, which verifies the occurrence of TRIP effect. While, the low-carbon Q-P-T steel demonstrates a weak TRIP effect owing to less retained austenite in it. The work-hardening exponent-true strain curves of medium-carbon and low-carbon Q-P-T steels clearly demonstrate the difference of their TRIP effects.4. The average dislocation densities in both martensite and retained austenite in medium-carbon Q-P-T steel were calculated by X-ray linear profile analysis (XLPA). The XLPA results indicate that the average dislocation density (Mρ) in martensite falls from 6.65×1014 m–2 (0% strain) to 5.25×1014 m–2 (3% strain), while the the average dislocation density (Aρ) in retained austenite rapidly raises from 4.21×1014 m–2 (0% strain) to 8.77×1014 m–2 (3% strain) which exceeds that in martensite. The phenomenon can be explained by the dislocation absorption by retained austenite (DARA) effect proposed in this work, namely, the plenty of dislocations in martensite move into the neighboring retained austenite, in other words, the dislocations are'absorbed'by nearby retained austenite. Furthermore, the amount of dislocation transported to retained austenite is larger than the amount of dislocation multiplication in martensite, as a consequence, the comprehensive result leads to the decrease of Mρand the rapid increase of Aρ. Since DARA effect makes the martensite be an"undeformation state", the deformation ability of hard phase martensite is intensified. With further increase of strain, when the amount of transported dislocation to retained austenite is smaller than the dislocation multiplication in martensite, the competitive result leads to the gradual increase of Mρ. While in the case of soft retained austenite phase, Aρstill sharply increases and shows high work–hardening rate. The DARA effect is indirectly verified by TEM observation in which the dislocations athwart from martensite to retained austenite at their interfaces can be clearly observed. Therefore, the mechanism of ductility enhancement by retained austenite can be briefly described as follows. In uniform deformation stage, DARA effect makes the martensite be an"undeformation state", the deformation ability of hard phase martensite is intensified. With increasing strain, when the stress caused by high dislocation density in local area reaches certain critical value, the strain–induced martensitic transformation will occur, which effectively relaxes the stress concentration in this area and avoids the formation of microcracks, which is the well–known TRIP effect. In sequent larger strain condition, once microcracks form in some high stress concentration areas and propagate, the retained austenite can block the propagation of microcracks, which is the well–known blocking microcrack propagation (BMP) effect.5. The retained austenite in low-carbon and medium-carbon Q-P-T steels exhibits excellent thermal stability since it still does not transform to martensite at -70℃, especially the characteristic parameter ( M sσ) of mechanical stability of retained austenite is about -20℃, exhibiting good mechanical stability. In addition, the product of strength and elongation at room temperature for medium-carbon Q-P-T steel can be kept at 300℃, as a result, medium-carbon Q-P-T steel studied in this work can be employed in the range from -20℃to 300℃. Low Msσis favorable for avoiding the occurrence of stress-induced martensitic transformation in carrying or manufacturing of workpieces, and thus strong TRIP effect will be produced by strain-induced martensitic transformation from retained austenite above Msσ. While Md≈200℃, it indicates that the retained austenite in workpieces at/below this temperature exhibits the optimum combination of thermal stability of retained austenite and TRIP effect.6. After Q-P-T treatment, both low-carbon and medium-carbon specimens were deformed at different temperature from -85°C to 450°C. a temperature zone with best strength and elongation values were found for both compositions: 100℃～300℃. Within this zone, the PSE of medium-carbon specimen always higher than that of low-carbon specimen, especially at 200°C, the PSE of medium-carbon specimen reaches 57738MPa%! In addition to the further strengthening by carbide precipitation at this temperature, a better plasticity in medium-carbon specimen again verifies the beneficial effect of retained austenite at an elevated temperature.7. The addition of Si can suppress the formation of brittle cementite at room temperature, but cannot suppress the formation of cementite by decomposition of retained austenite or by transformation of transition carbide in relative high temperature, such as at 350°C.