Effect Of Rare Earth Ce And Heat Treatment On Inclusions In U75V Steel

In this paper,U75V heavy rail steel with national standard composition and U75V steel with13 ppm and 440 ppm rare earth Cehave been successfully smelted by vacuum induction furnace.The effects of Cecontent and heat treatment system on inclusions and mechanical properties of the test steel have been studied.The research process includes literature investigation,thermodynamic calculation,batching calculation,vacuum induction furnace smelting experimental steel,composition testing,machining,test steel rolling,heat treatment,inclusion analysis,microstructure analysis,Gleeble hot compression test,impact test and hardness testing.Using Fact Sage 7.3 thermodynamic calculation software,according to the actual measured composition,the phase precipitation of U75V smelted with Cecontent of 0ppm,13 ppm and 440ppm was calculated respectively.The inclusions in 0ppm test steel are mainly MnS,Al₂O₃,13 ppm test steel are mainly MnS,Al₂O₃,Al CeO₃,Ce₂S₃,440ppm test steel are mainly CeN,Al N,Ce₂O₃,CeS.The morphology,size and composition of inclusions in U75V steel treated by different processes were detected by scanning electron microscope and energy dispersive spectrometer.The size and length-width ratio of inclusions were analyzed by Image-J software.The results show that there are mainly three kinds of inclusions in the original U75V steel:pure MnS;Al₂O3-SiO₂-CaO-MnO composite inclusion;The average length-width ratio and the average size of Al₂O₃-SiO₂-CaO inclusion semi-wrapped in MnS are 3.3 and 4.5?m.After adding 13 ppm rare earth Ce,the species and composition of inclusions were complicated.In addition to pure MnS,three other Ce-containing inclusions with different morphology were found.Compared with the original U75V,the spheroidization effect of inclusion after adding Ceis similar to that of annealing treatment.The average length-width ratio is reduced to 1.4,which is about 58%.However,the average size reduction is only 9%.Furthermore,after the test steel with added Cewas held at 1200?for 2 h and then cooled with water,the spheroidization rate of inclusion did not change,that is,the average lengthwidth ratio was still 1.4.However,compared with the original U75V steel,the average size of inclusions decreases by 1.4?m(about 31%).The average size of inclusions is also reduced by 1?m(about 24%)compared with the steel with only Ceaddition.After adding 440 ppm rare earth Ce,the number of MnS,Al₂O₃ and other conventional inclusions in the steel decreases sharply compared with the original U75V steel,and the probability of containing rare earth Cein the steel inclusions is greater.The average size of the inclusions decreases by 1.88?m(about 42%),and the average length-width ratio decreases by about 59%when it is 1.37.At 1200?for 2h,the average size and the average length-width ratio of the 440 ppm steel with the original U75V inclusions are reduced by 79%,53%and 1.56 respectively.The average size of inclusions in steel is reduced by 25.33%by adding REE,21.66%by adding REE,59.72%by adding REE and heat treatment.The average size of inclusions can be reduced by adding rare earth or heat treatment alone,and the combined action of the two is the most obvious.The average length-width ratio of inclusions decreases by 53.48%with the addition of rare earth,and the average length-width ratio of inclusions decreases by 28.30%with the addition of rare earth and heat treatment,and the average size of inclusions decreases by56.61%with the combined action of rare earth and heat treatment.The average length-width ratio of inclusions can be reduced by single addition of rare earth or single heat treatment,and the effect of the two treatments is the most obvious.In this experiment,the average size and the average length-width ratio of inclusions decrease most obviously in the experimental steel with 440 PPM rare earth element at 1200?for 2 h.However,it is difficult to find MnS and its related inclusions due to the preferential combination of Ceand S at this rare earth content.The true stress-strain curve of the test steel was measured by Gleeble 1500 hot compression tester.Under the condition of T=950?and?=0.1,?440 ppm increased by 39.06%compared with 0ppm in this experiment.Under this condition,the test steel has the strongest resistance to plastic deformation.JB-300B impact testing machine was used to test the impact of the test steel.The addition of rare earth elements will reduce the impact work of the experimental steel at room temperature.The impact work of the experimental steel with the addition of 9 ppm rare earth elements is increased by8%compared with that without the addition of rare earth elements.The impact work of the experimental steel with the addition of 9 ppm rare earth elements is increased by 24%compared with that without the addition of 440ppm rare earth elements.The low temperature impact property of experimental steel is improved by trace rare earth.HR-500 hardness tester was used to test the hardness of the test steel.Compared with the experimental steel without overheating treatment,the hardness value of the experimental steel with 13ppm rare earth is reduced by 61.2%.Compared with the experimental steel with 13 ppm rare earth,the hardness value of the experimental steel with 440 ppm rare earth is reduced by 11.14%,and the total reduction ratio is 65.54%.The hardness value decreases with the increase of rare earth content.The hardness value of the experimental steel at 1300?is 52.87%higher than that of the experimental steel at 1200?.Increasing the heat treatment temperature is beneficial to increase the hardness of the experimental steel.With the increase of holding time,the hardness value generally has a downward trend,adding trace rare earth hardness value has little change,comprehensive comparative analysis,adding 13 ppm rare earth experimental steel at 1200?for 1h of the highest hardness value.The microstructure of the test steel was observed by STEMI-508 microscope.When 0ppm,13ppm and 440ppm are added to the experimental steel,the grain size of pearlite decreases obviously with the increase of rare earth content,the number of granular pearlite decreases and the number of flake pearlite increases,which is beneficial to improve the mechanical properties of the experimental steel.