Study On The Occurrence State Of Rare Earth Ce In Steel And Its Influence Mechanism Of Mechanical Properties And Corrosion Properties Of Steel

Science and technology are changing with each passing day.The downstream manufacturing industry has become more and more demanding on steel quality.The development of high strength and high toughness,and high corrosion resistant steel grades will be an important direction for the development of steel materials in the future.The addition of trace rare earth to steel can significantly improve its mechanical properties and corrosion resistance.Current research is mainly based on experimental studies,with few theoretical studies,especially the lack of understanding on the microscopic scale,which makes the nature and internal mechanism of rare earth unclear.The lack of theoretical guidance is the biggest bottleneck in the development of the widespread application of rare earth in steel.In this paper,theoretical calculations combined with experimental research are used to explore the microscopic mechanism of the role of rare earth Ce in steel.The mechanical properties and corrosion resistance of steel are studied from the two dimensions of solid solution Ce and Ce inclusions.The main research content and conclusions are as follows.Based on density functional theory,first-principles calculations are used to study the occupation of Ce atoms in?-Fe and the influence of common elements in steel on Ce solid solution from the perspective of energy,and its mechanical properties and electronic structure are calculated and analyzed.The results show that Ce atoms can replace Fe atoms to occupy the?-Fe supercell,and cannot exist in the matrix in the form of interstitial solid solution.In the system,Ce atoms lose electrons,which is reduced the radius of Ce atoms and produces a polarization effect,which is conducive to the solid solution of Ce in Fe.The doping of Cu,Mg,and C is not conducive to the solid solution of Ce in Fe.Cr,Mn,and Ni have little effect on the solid solution of Ce.Si,Al,Ti,V,Co,W,Mo,and Nb promote the solid solution of Ce,and Si is the strongest and Nb is the weakest.The mutual attraction between O,S,N and Ce is strong,while Mg,C and Ce are mutually exclusive.Ce doping reduces the incompressibility,shear strain resistance,stiffness and Vickers hardness of the Fe-Ce doped system,but improves the toughness and workability of the system.Ce doping leads to a decrease in the strength of the metallic bond of the system,which is the main reason for the decrease in the incompressibility,rigidity and hardness of the system.In addition,Ce increases the density of the electron cloud in the system,which is conducive to the improvement of the toughness of the doped system.Taking Ce treated clean steel(IF steel)as the research object,the basic physical parameters of Ce inclusions such as lattice parameters,elastic constants,mechanical properties and thermal expansion coefficient are calculated,and the effect of the physical properties of the inclusions on the mechanical properties of the steel matrix is quantitatively analyzed.The calculation results of the formation enthalpy show that the preferential and stable inclusions in the steel are Ce₂O₃,Ce₂O₂S,CeAlO₃ inclusions,which are consistent with the thermodynamic analysis results.The energy spectrum analysis and the calculation of the formation enthalpy confirm that Ce can be solubilized in Ti N inclusions.Solid solution Ce does not change the morphology and size of the original Ti N inclusions,but reduces the rigidity,incompressibility and hardness of the Ti N inclusions.At the same time,it improves the toughness and increases the coefficient of thermal expansion of inclusions.Ti N,Al₂O₃ and CeAlO₃inclusions have larger bulk modulus,Young's modulus,shear modulus and Vickers hardness values,showing greater rigidity and hardness,and are characterized by brittleness,while Ce₂O₃ and Ce₂O₂S inclusions are characterized by toughness.Compared with Al₂O₃ and Ti N,the thermal expansion coefficients of Ce₂O₃ and CeAlO₃ inclusions are close to that of the iron matrix,while the thermal expansion coefficient of Ce₂O₂S inclusion is slightly larger than that of the iron matrix.The difference between Ce inclusions and matrix in terms of incompressibility,rigidity,hardness,toughness and brittleness,and thermal expansion is small,and the consistency of plastic deformation of the steel matrix is improved,which is beneficial to delay the initiation of micro voids and micro cracks,and helps to improve the mechanical properties of steel.Based on the first-principles calculation framework,the surface model,anodic dissolution,and cathodic oxygen absorption reaction and other related electrochemical corrosion models of Fe-Ce doped system are constructed,and the internal mechanism the influence of Ce on the corrosion resistance of steel in the Cl medium environment is analyzed.The results show that the subsurface Ce increases the electronic work function of Fe(100)₁,reduces the surface energy density of Fe(100)₁,and makes the substrate balance absolute potential increase.While enhancing the electrochemical stability of Fe(100)₁,it also reduces the anodic dissolution rate of the Fe matrix,thereby improving the corrosion resistance of the Fe matrix.When the chlorine coverage gradually increases from 1/3 ML to 1 ML,the combining capacity between the first layer of Fe(100)-Ce^surf system and the inner layer of Fe atoms gradually increases,which indicates that the Ce solid solution in the first surface layer atoms effectively hinder the surface Fe atoms away from the Fe matrix,increase corrosion resistance and delay the occurrence of corrosion.When 1 ML of chlorine is adsorbed on the surface,the Fe(100)-Ce^sub system has a strong combining capacity,indicating that in a high concentration of chlorine environment,the Ce atoms in the subsurface layer effectively increase the difficulty of Fe atoms detaching from the matrix.Using scanning Kelvin probe force microscope,conductivity atomic force microscope,and first-principles calculations to monitor and analyze the micro area electrochemical information(such as surface potential and current distribution)and work functions of inclusions.And the internal mechanism of Ce inclusions inducing pitting corrosion in the marine environment is discussed.The results show that the surface potential of Ce₂O₃ inclusions is higher than that of Fe matrix,while the surface potentials of Ce₂O₂S and CeAlO₃ inclusions are lower than that of Fe matrix.However,Ce₂O₃,Ce₂O₂S,and CeAlO₃ are all insulators,and the steel matrix around the Ce₂O₃,Ce₂O₂S,and CeAlO₃ inclusions does not form a significant lattice distortion zone.Therefore,there is no galvanic corrosion between the Ce inclusions and the Fe matrix.The order of the electronic work function of inclusions and Fe matrix from small to large is:Ce₂O₂S<Ce₂O₃<CeAlO₃<Fe matrix<Al₂O₃,which is consistent with the dissolution sequence of inclusions and Fe matrix in the immersion experiment.Ce₂O₃,Ce₂O₂S,CeAlO₃ inclusions dissolve by themselves and dissolve preferentially to the Fe matrix,which is the main reason for the initiation of corrosion.