Fundamental Research On The Application Of Nanoparticles Added From Outside In Non-quenched And Tempered Steel 35MnVs

Non-quenched and tempered steel(NQTS)produced by Chinese steel plants,often has many problems on the homogeneity of microstructure and properties because of the coarse grains.This has limited the development and application of NQTS seriously.The addition of nanoparticles from outside in steel(ANOS)proposed based on the oxide metallurgy technique,can decrease the inclusion size in steel,accelerate the inclusions to induce the acicular ferrites formation,and then improve the microstructure effectively.In this study,the ANOS was applied in the manufacture process of NQTS,which helped to resolve the problems of coarse grains.Based on this experiment,some key factors,including the nanoparticles yield,the nucleation site types of acicular ferrites,and the application of ANOS in the controlled rolling and controlled cooling process,were investigated.First,this research tried to apply the ANOS on the manufacture of NQTS 35MnVS through the Vacuum Induction Furnace(VIF).Based on the analysis about the nanoparticles yield,the method called vacuum induction levitation technology was proposed to improve the yield of nanoparticles.The results showed that after applying the ANOS on the manufacture of experimental steels through VIF,the inclusion size got effective refinement.These refined inclusions induced the acicular ferrites formation,and the microstructure got preliminary improvement.Then,the experimental steels were manufactured in vacuum induction levitation furnace(VILF),and the elemental yield of Al and Ti increased by 24.4%and 11.4%respectively,compared with that of the experimental steels in VIF.Moreover,the number of inclusion type also had an obvious increase,and the average size of inclusions decreased by 40.0%.In such experimental steels,the inducing rate of inclusions on acicular ferrites is close to 100%,and the structure acicular ferrites became the main microstructure type.Second,with the help of laser scanning confocal microscopy(LSCM),the inducing characteristics of different nucleation cores for acicular ferrites,and the nucleation properties of acicular ferrites during the cooling process of experimental steel,were observed and analyzed systematically.Also the influence of cooling rate on the nucleation properties of acicular ferrites,and austenite grain size were investigated.Based on the experimental results,the microstructure properties and its formation mechanism under different cooling rates,were analyzed.The results showed that the main inducing sites of acicular ferrites mainly included the inclusion TiN-MnS,retained austenite,and inclusion MgAl2O4.Among them,the inclusion TiN-MnS had the highest nucleation inducing ratios for acicular ferrites,which acted as the main nucleation type in steel.The microstructures properties under cooling rates 1.25?/s and 5?/s were better compared with that under other cooling rates.The differences on microstructures properties in steels under different cooling rates were mainly related to the characteristics of retained ?-Fe phases and the size of austenite grains.At last,the ANOS was used in the process of deformation induced ferrite transformation(DIFT)in experimental steel.In this part,the steels with ANOS application could be called the experimental steels with double strengthening(ESDS),and the steels without ANOS application could be called the experimental steels with single deformation strengthening(ESSDS).The differences on microstructure characteristics and mechanical poperties between these two experimental steels were compared,and the confirmation of key parameters during the deformation strengthening process was also investigated.The results showed that the microstructure types of the experimental steels with and without ANOS application were blocky ferrites and acicular ferrites,respectively.Under different deformation processes,the peak stress in ESDS was higher around 50%than that in ESSDS.The lower the austenitizing temperature and the larger the quantity of deformation,the smaller the microstructure grain size in experimental steels.When the deformation temperature was between Ad3 and Ar3,the driving force of ferrites nucleation in deformation process was relatively larger,and also the nucleation ratio of ferrites was higher.