| Reinforced and plasticized steel during with quenching and partitioning（Q&P）treatment（based on carbon partitioning）and transformation induced plasticity（TRIP）effect are the research hotspots of the third generation of advanced high strength steels（AHSS）.Although increasing carbon content is the most effective method to promote carbon enrichment in austenite and reduce Ms point,it will injure the cold and hot processing properties and corrosion resistance of steel plate to a certain extent.Therefore,it is of great significance in both theory and engineering application to develop a new path which can not only overcome the adverse effects of increasing carbon content but also improve the strength and toughness of steel（especially in the field of nickel-saving stainless steel）.Like as carbon,nitrogen has attracted more and more attention for its beneficial effects on steel strength,work hardening rate,austenite stability（including thermal and mechanical stability）,resistance to corrosion,to wear,to cavitation,to abrasion and to fatigue.However,it is still not so clear about the thermodynamic and kinetic of N partitioning behavior in steels,the regulation methods of microstructure and properties of N partitioning TRIP steel,and the microscopic deformation mechanism of N partitioning enhanced and plasticized stainless TRIP steel.In this thesis,the microstructure characteristics of austenite and martensite,and TRIP effect are controlled by the Q&P process during nitrogen partitioning instead of carbon partitioning for an ultra-low carbon chromium manganese nitrogen stainless steel（00Cr13Mn8Mo N）.By means of thermodynamic calculation,dynamic expansion analysis（DEA）,X-ray diffraction（XRD）,transmission electron microscopy（TEM）,etc.The evolution characteristics of microstructures,mechanical properties and microscopic deformation mechanism of multi-component microstructure during the Q&P treatment were studied.The main experimental results are as follows:The pseudo binary equilibrium phase diagram,the mole fraction and elemental composition of the equilibrium phase in the tested steel at different temperatures were obtained.During the equilibrium cooling process,the transition ofδferrite to austenite（1200℃）,precipitation of Cr2N phase（850℃）,precipitation of M23C6phase（780℃）,precipitation ofσphase（650℃）,the beginning（638℃）and end（540℃）of transformation ofγtoαferrite and precipitation of M6C phase（332℃）occurred in sequence.The measured phase transformation temperatures such as Ac1,Ac3and Ms are540℃,635℃and 125℃,respectively.According to DEA results and migration of interstitial atoms during the Q&P treatment,a new mathematical model was established to evaluate the volume fraction of retained austenite（RA）in the partitioning and subsequent cooling process of tested steel,and the results were verified by that of XRD analysis.Real-time,rapid and accurate characterization of retained steel content（RAC）during the Q&P treatment and subsequent cooling was achieved.For the samples treated with one-step partitioning including Q90P and Q30P,RAC in the steel increases first and then decreases with the increase of quenching holding temperature（QT）.When QT was 80℃,RAC reached the peak of 28.5 vol%after the first partitioning.For the sample treated with two-step partitioning（Q90P30P）,the content of RA in the steel is higher than that in the sample Q30P,which proves that the two-step partitioning can more effectively enrich interstitial atoms in the RA.The amount of martensite,RA and nitrogen content in the tested steel can be significantly changed by one-step and two-step partitioning.The RAC in both the one-step and two-step partitioning Q&P treatments was significantly higher than that in the original quenched samples（7.8vol%）,indicating that RA with different stability and morphology such as block,granular and thin film were obtained in the samples.Compared with quenched（Q）samples,the mechanical properties of the tested steel can be improved obviously after one-step and two-step partitioning processes.Under the condition of no sacrifice of tensile strength of material,the elongation and product of strength and elongation（PSE）after the partition treatment were significantly increased.The yield strength,tensile strength,elongation and strong plastic volume of Q90P30P samples reached 671 MPa,1144 MPa,31%and 35.5 GPa·%,respectively.The necessary microstructure condition for obtaining excellent mechanical properties of the samples after two-step partitioning treatments is multiple in-situ composite microstructure.It can enable RA to gradually produce TRIP effect in a wide range of stress,that is,heterogeneous TRIP effect.The typical feature of heterogeneous TRIP effect is that the work hardening curve decreases gradually at a steady rate in the“uniform deformation”stage,and the corresponding second-derivative curve fluctuates slightly around zero. |
