Study On Behavior Characteristics And Industrial Application Of Rare Earth In Heavy Rail Steel

With the rapid development of China’s railway industry in recent years,the quality and performance of steel rails are more and more demanding.Heavy rail steel with high strength and high cleanliness is an important research direction of heavy rail steel.In this paper,the production process,key production technology,inclusions and casting defects of heavy rail steel are investigated,and it is found that the quality defects of heavy rail steel are caused by inclusions in steel.Non-metallic oxides and sulfides are easy to form continuous large particles and irregular inclusions in heavy rail steel.At present,the lack of inclusions control means in the production process leads to frequent quality problems of heavy rail steel.In this paper,through industrial tests and theoretical analysis,the inclusion scheme of rare earth in the production process is improved,and the evolution process of the inclusions in rare earth heavy rail steel is studied,and the mechanism of action of rare earth on the improvement of inclusions in heavy rail steel is analyzed.Finally,the impact of rare earth on the production of heavy rail steel is evaluated.The effects of rare earth on the production of heavy rail steel were analyzed by physicochemical properties and comprehensive mechanical properties,which provided theoretical and practical guidance for industrial production of rare earth heavy rail steel.Firstly,the quality problems and characteristic inclusions in heavy rail steel production are analyzed,and the possibility of reaction between rare earth and inclusions in heavy rail steel is calculated theoretically by means of thermodynamic calculation and first-principle calculation.The thermodynamic calculation results show that rare earth can react with O,S and Al₂O₃in heavy rail steel to produce rare earth aluminate（REAlO₃）,sulfur oxide（RE₂O₂S）and rare earth oxide（RE₂O₃）successively under the conditions of steelmaking.The first principle calculation results show that the reaction products can help reduce the influence of inclusions on the quality of heavy rail steel.Secondly,the effects of adding rare earth after LF and VD in rare-earth heavy rail steel were studied by comparative test.The results show that a large amount of rare earth is lost during VD treatment when LF is added,and the average yield of rare earth is 2.83%.The content of rare earth in the casting billet produced by the scheme of adding rare earth after VD increased,and the average yield of rare earth in the casting billet was 11.73%.According to the analysis results of the field production indexes of multiple furnaces,compared with the scheme of adding rare earth after VD,the improved method of adding rare earth in production significantly increases the number of continuous pouring furnaces,and has no obvious influence on high-magnification test and component segregation.After the addition of rare earth,the inclusions of rare-earth heavy rail steel are reduced to different degrees.The grade of Class A inclusion in the billet produced by adding rare earth after VD decreases.The grade of coarse inclusion B decreases,and the grade of fine inclusion B decreases after LF and VD,respectively.The rating of Class D and Ds inclusions is reduced to 0 after the addition of rare earth.The inclusions in heavy rail steel produced by adding rare earth after VD were studied in the whole process.The number density of inclusions larger than 1μm²can be decreased from 88.44 inclusions/mm²to 78.58 inclusions/mm²by adding rare earth in heavy rail steel.Rare earth can also improve the size distribution of inclusions,so that the number density of large particle inclusions in heavy rail steel decreases,the number density of small particle inclusions increases,and the inclusion distribution in steel is more uniform.The number density of inclusions with an area of 5-10μm²,10-20μm²and an area of>20μm²decreased from 13.76,6.28 and 1.34 inclusions/mm²to 4.95,1.91 and 0 inclusions/mm²,respectively.The results show that a large number of aluminum-containing inclusions with an area of less than 1μm²are formed after the addition of rare earth,and then gradually decrease.The number and size of aluminum-containing inclusions in the cast blank sample decrease obviously,and the number density of aluminum-containing inclusions with an area of1～5μm²decrease the most.The number density of inclusions in other areas also decreased from 16.43/mm²to 3.00/mm².The effect of rare earth on MnS inclusions shows that the length to width ratio of MnS in heavy rail steel is reduced,and the quantity and size of MnS are significantly reduced.It is also found that under the influence of rare earth,more CaO-MgO-Al₂O₃-SiO₂-REO·MnS composite inclusions are formed in the smelting process of heavy rail steel,and the composite inclusions are easily removed by floating.Compared with U75V,the density of MnS inclusions in cast U76CrRE billet samples of rare earth heavy rail steel decreased from 118.83/mm²to 73.00/mm²,a decrease of 38.55%.The test results of heavy rail steel and rare earth heavy rail steel produced in recent years are analyzed.The results show that there is no significant difference in oxygen,nitrogen content,microstructure and grain size between heavy rail steel and rare earth heavy rail steel,but the hydrogen content is greatly reduced.The mechanical properties of U76CrRE of RE heavy rail steel are significantly improved compared with U75V of heavy rail steel.The addition of rare-earth makes the yield strength of heavy rail steel increase from 1035MPa to 1114MPa.Tensile strength increased from 1256MPa to 1332MPa;Elongation increased from 12.1%to 12.5%;Brinell hardness increased from 303HBW to327HBW;The thrust energy increases from 8.99J to 9.98J,and the tensile fracture analysis results show that rare earth can promote the transition from brittle fracture to ductile fracture of heavy rail steel.