Study On Mechanism Of Yttrium-based Rare Earth On Inclusions Behavior And Microstructure And Properties Of EH36 Shipbuilding Steel

With the rapid development of marine economy,the performance requirements of steel for marine engineering are getting higher and higher,especially the low temperature impact properties.Making use of the unique properties of rare earth metal elements and according to the technical idea of"Oxide Metallurgy",this paper takes EH36 shipbuilding steel as the research object to explore the behavior law of yttrium-based rare earth on inclusions and the action mechanism of microstructure and properties of shipbuilding steel.The technical means of thermodynamic and kinetic calculation analysis,first-principles calculation,laboratory experiment and industrial test research are adopted.This paper focuses on the thermodynamic analysis of the formation of inclusions in shipbuilding steel,the study of the effect of rare earth on the behavior of inclusions in shipbuilding steel,and the effect of rare earth on the microstructure and properties of shipbuilding steel.the regulation mechanism of rare earth oxide metallurgy in shipbuilding steel is analyzed,and the action mechanism of yttrium-based rare earth on inclusion behavior and microstructure and properties of shipbuilding steel is expounded,which provides a theoretical basis for improving the low temperature impact properties of shipbuilding steel.The main conclusions are made as follows:1.Through thermodynamic analysis,the thermodynamic conditions for the formation of rare earth oxides,sulfides,oxygen sulfides,alumina and manganese sulphide in yttrium-based rare earth EH36 steel and the relationship between equilibrium solubility product and actual solubility product are revealed.It is known that rare earth oxides and rare earth sulfur oxides can be formed in rare earth ship plate steel,and rare earth sulfides are only formed when the content of rare earth in steel is 0.028 wt.%.After adding yttrium-based rare earth to EH36steel,rare earth converts Al₂O₃into rare earth oxide and rare earth sulfur oxide.Mn S in EH36steel begins to precipitate after entering the austenite region.2.In laboratory experiments,EH36 ship plate steels with different yttrium-based rare earth contents were made by 25 kg medium frequency induction furnace.The effects of rare earths on inclusions and low temperature impact properties of EH36 shipbuilding steels were studied by means of scanning electron microscope（SEM）and energy dispersive spectroscopy（EDS）.It is found that rare earths modify the inclusions in EH36 steel into fine spherical rare earth inclusions.Compared with EH36 steel without rare earths,when the rare earth content in steel is 0.018 wt.%,the transverse impact properties at-40°C and-60°C are increased by59.4%and 65.7%respectively,and the low temperature impact properties are significantly improved.When the content of rare earth in steel increases to 0.038 wt.%,the low temperature impact property of steel is lower than that of the sample without rare earth,indicating that rare earth has an inhibitory effect on the impact property of EH36 steel.3.In the industrial test,the experimental steels with rare earth contents of 0 wt.%,0.009wt.%,0.020 wt.%and 0.028 wt.%were prepared.The inclusions in EH36 steel plates with different yttrium-based rare earth contents were detected by scanning electron microscope and energy spectrum analysis,combined with automatic statistical analysis of ASPEX inclusions and non-aqueous solution electrolysis.It is found that the inclusions in EH36 steel plate without rare earth addition are mainly Al₂O₃and Mn S.After adding appropriate amount of rare earth,these inclusions are modified into spherical or quasi-spherical RE₂O₃-RE₂O₂S composite phase inclusions,and the size of inclusions is refined to less than 2μm.In addition,the inclusions in the EH36 shipbuilding are finely distributed,in which 0.020wt.%rare earth content EH36 steel inclusions less than 3μm account for 91.95%of the total,spherical or quasi-spherical dispersion distribution in the steel,can achieve the rare earth oxide metallurgy function,can effectively refine the microstructure of EH36 steel and improve the impact properties of EH36 shipbuilding steel.4.The effect of yttrium-based rare earth on the impact properties of EH36 steel was studied by impact test.The austenite grain growth behavior of different samples at hot rolling temperature was studied by high temperature laser confocal microscope,and the process of rare earth particles pinning grain boundary in steel was investigated.The grain characteristics of EH36 steel were analyzed by electron backscatter diffraction,and the existence form of rare earth in steel was analyzed by first principle calculation.The effect of yttrium-based rare earth on the microstructure and properties of EH36 steel was studied.It is found that compared with the EH36 steel without rare earth,after adding 0.020 wt.%yttrium-based rare earth,the longitudinal impact energy absorption of EH36 steel at-80℃is increased by245.1%,the transverse impact energy absorption is increased by 361.6%,and the pearlite is refined by 53.8%.Through in-situ observation,rare earth particles are dispersed at the grain boundary of 0.020 wt.%rare earth content EH36 steel,which can pin the grain boundary and make the grain boundary migrate slowly.at the same time,when the grain size of 0.020 wt.%rare earth content EH36 steel is completely austenitized at 1200℃,the grain size of 0.020wt.%rare earth content EH36 steel is the smallest and the austenite size is reduced by 62.5%.According to EBSD analysis,the proportion of large-angle grain boundaries in 0.020 wt.%rare earth content EH36 steel increased by 26.75%,and the average effective grain size decreased by 9.72%.When the material was impacted,it could absorb more energy.According to the first-principle calculation,the particle formation energy of rare earth atoms inα-Fe is the lowest at the grain boundary and tends to segregate in the grain boundary region.During the hot rolling and heating process of EH36 steel,more rare earth particles are pinned to the grain boundary,the grain growth is restrained,the microstructure is refined,and the impact property of steel is improved.5.Combined with laboratory experiments and industrial experiments,it can be obtained that when the content of yttrium-based rare earth in EH36 steel is 0.020 wt.%,EH36 steel can get the best low temperature impact property.