| The residual large inclusions in steel material can lead to the development of cracks and impair its mechanical properties,while the coarse grains also lead to a significant reduction in the toughness of the material.In the last century,the technique of "oxide metallurgy" was proposed to solve the above problems,which is to form the fine dispersed inclusions in the material as heterogeneous nucleation points to induce intragranular ferrite nucleation.With the continuous development of external addition technique and equipment,the addition of second-phase particles with suitable composition into the molten steel by spraying method can promote grain refinement and fine inclusions.Previously,the addition of nanoscale second-phase particles to steel has been investigated,and the inclusions and microstructure of the steel material have been refined to some extent.However,due to the characteristics of large specific surface area and high surface energy of nanoparticles,the particles tend to agglomerate and float to the steel surface after entering into the molten steel,resulting in a significant decrease in the efficiency of using nanoparticles in the steel.The agglomeration phenomenon between nanoparticles is soft agglomeration,and the traditional physical means cannot essentially eliminate the interparticle forces,so it is necessary to change the surface properties of nanoparticles for steelmaking.Firstly,a new type of nanoparticles with core-shell structure for steelmaking was prepared by chemically modifying the surface of MgO nanoparticles,and based on the characterization results,the carbonized particles had a carbon layer of 10 nm thickness on the surface with excellent monodispersity in solution.Under helium atmosphere,the wetting angle of the original MgO nanoparticles reached 130° in the high temperature molten steel,while the wetting angle of the surface modified MgO@C nanoparticles was only 50°.The surface-modified nanoparticles exhibited better wettability with a smaller wetting angle.The high temperature pre-experimental segmental sampling and the determination of the alloying element content were calculated to show that the yield of modified nanoparticles in the test steel reached 65%,which was much higher than that of the original nanoparticles.By using the chemical surface treatment method,the yield of nanoparticles for steelmaking was improved,and the key technical problem of additive nanoparticle technology was solved.Then,the modified nanoparticles for steelmaking were applied to design high-temperature smelting experiments,and it was found that nanoparticles had a great influence on the characteristics of non-metallic inclusions in steel.According to the results of Factsage thermodynamic simulation software and SEM-EDS analysis,it was known that a large number of irregularly shaped TiN inclusions were formed in the tested steel with nanoparticle addition,and the MgAl2O4 spinel also gradually replaced the single-phase Al2O3 inclusions in the original steel.The number density of fine inclusions in the test steel containing modified nanoparticles was also higher than that in the test steel containing original nanoparticles.In particular,the proportion of submicron-scale inclusions in the test steel containing 0.03%modified MgO@C nanoparticles reached 77.2%of the total number of inclusions.The fine and stable inclusions could hinder the migration of prior austenite grains and induce acicular ferrite nucleation.According to the theory of heterogeneous nucleation of ferrite induced by inclusions,the equivalent critical nucleation diameter of TiN inclusions was thermodynamically calculated to be 0.346 ?m.The modified nanoparticles also had a significant effect on the evolution of microstructure in the tested steels under different cooling conditions.In low-carbon high-alloy steels,an increase in cooling rate reduced the proportion of polygonal ferrite in the test steels and generated bainite phases.The greater cooling rate provided higher subcooling for the ferrite phase transformation.Meanwhile,the fine dispersed inclusions in the nano-test steels had a pinning effect on the prior austenite grain boundaries,and the fine grains also promoted acicular ferrite nucleation.In the in-situ observation experiments,ferrite side plate always formed on the boundary prior to acicular ferrite formation on the intragranular inclusions.When the cooling rate increased up to-15?/s,the starting transformation temperature of both ferrite side plate and acicular ferrite decreased,and the proportion of acicular ferrite increased.Meanwhile,the length of acicular ferrite was linearly proportional to time within a certain temperature range,indicating that the driving force of acicular ferrite nucleation during the phase transition process was basically constant with time.When the cooling rate was the same,the starting nucleation temperature of the acicular ferrite in the nano-test steel was higher than that of the original steel,and the nucleation rate was greater.Finally,the external nanoparticle addition technology was combined with deformation strengthing technology to investigate the evolution of microstructure and mechanical properties in the experimental steel under the double strengthening by controlling the hot compression deformation parameters.The compression deformation experiments showed that the larger compression led to a decrease in the average size of both ferrite and martensite phases in the steel.A large amount of chain-like deformation induced ferrite and interlaced acicular ferrite were formed in the test steel,which greatly enhanced the interlacing of microstructures in the steel and improved the strength and toughness of the material.At the same deformation temperature,the stress peaks in the test steel were always higher than those in the original steel.When the deformation temperature was 750?,the maximum stress peak of the nano-test steel was 516 MPa,which was 28.4%higher than that of the original steel. |
PDF Full Text Request