Analysis On The Evolution Of Inclusions In U76CrRE Steel

The Work Plan for Promoting the Development of Multimodal Transport,Optimizing and Adjusting the Transport Structure（2021-2025）issued by the General Office of the State Council,put forward the key points of national transportation in the next few years.Among them,there were clear requirements for railway transportation—by 2025,national railway freight volume will increase about 10%,compared with 2020.Mainly in the Beijing-Tianjin-Hebei region,the Yangtze River Delta region,the Guangdong-Hong Kong-Macao Greater Bay Area and other areases where high-speed trains travel,as well as the main coal producing areas of Shanxi,Shaanxi and Mongolia,and complex traveling areas such as the"Belt and Road Initiative".Therefore,it is of great significance to the long-term development of the country to improve the performance of railway rails,ensure they can satisfy different usage environments.This test took the U76CrRE heavy rail steel developed by Baotou Steel Group as the research object,by analyzing the inclusions in the samples of different processes in the whole process of smelting,summarized the evolution of inclusions,to explore optimization measures;Compared it with U75V which without rare earth to explore the effect of rare earths to inclusions;Heating treatment experiments with different heating temperatures and holding times were carried out on the billets to explore better heat treatment conditions.Analysed of inclusion behavior in the whole processes of U76CrRE heavy rail steel,found that with the progress of the steelmaking process,the optimization effect of removing various inclusions in steel was obvious,the effect of dephosphorization,denitrification and desulfurization was remarkable.The type of Mn S and Mg O-Al₂O₃-Si O₂-Ca O inclusions were controlled in the appropriate range in the finished rail.In the processes after rare earth is added,rare earth content was gradually decreased,indicated that the inclusions played a role in modification,converted them into rare earth inclusions that float or precipitate,then removed them,this played a certain role in the removal of inclusions in steel.In various inclusions,in addition to the number of oxygen-containing inclusions increasing in the later periods,other types of inclusions had significant transitions in number,size and morphology.The growth of oxygen-containing inclusions may be caused by exposure to oxygen in the air during rolling and heating treatment in the later stages of steelmaking.So,in the next processing,improvement in treatment process is necessary.In the comparative test of inclusions of U76CrRE and U75V,we found that rare earth had obvious optimization effect on inclusions.When rare earth was added to steel,it would combine with sulfur and oxygen to generate rare earth inclusions,then most of them were removed,the remaining small part did not cause much harm even if it remained in the steel;In addition,rare earth reduced the size of the inclusions,improved inclusion morphology.Therefore,adding rare earth to steel had an effect on improving the performance of steel.In the heat treatment test of U76CrRE billet,different heating temperatures and holding time were set as gradients.The results showed that when the heating temperature is 1100°C,with the increase of holding time,the size of Mn S and composite inclusions decreased gradually,the shape had been significantly improved,the microstructure was the best when kept for 1h;When the heating temperature is 1200°C,with the holding time increased,the shape of Mn S decreased,the size of composite inclusions increased,and the microstructure gradually deteriorated.