Effect Of Trace Rare Earth On Inclusions,Microstructure And Mechanical Properties Of A572.Gr65 High Strength Structural Steel

A572 Gr65 high strength structural steel has high strength,good impact toughness and welding performance,which is widely used in power,building,bridge and other engineering fields.In industry,Nb,V,Ti microalloying and controlled rolling and controlled cooling processes are used to produce A572Gr65 steel.However,Nb,V,Ti and other elements are lack of resources and expensive,resulting in high cost of steel.There are abundant and cheap rare earth resources in our country,and they can purify molten steel,modify inclusions,refine grains and microalloy in steel.If the expensive microalloy elements are replaced by cheap rare earth elements,the production cost of A572Gr65 steel can be effectively reduced on the premise of ensuring its good properties.Therefore,in this paper,A572 Gr65 steel is taken as the research object,the idea of adding rare earth and reducing alloy is used to produce the A572 Gr65hot rolled steel plate containing rare earth in a factory.The effects of rare earth elements and their contents on inclusions,microstructure and mechanical properties of A572 Gr65 steel,as well as the substitution of rare earth elements for microalloyed elements were analyzed by means of metallographic microscope,SEM,tensile tester and Charpy impact tester.The main research contents and conclusions are as follows:(1)The goal of replacing part of Nb with appropriate amount of RE is achieved.0.005%of Nb is replaced by 0.0043%of RE,and 0.007%of Nb is replaced by 0.0057%of RE,the design requirements are met.The O content and S content of A572 Gr65 steel decrease with the increase of RE content,which shows that RE can obviously improve the cleanliness of liquid steel.(2)When RE was added to A572 Gr65 steel,Al₂O₃inclusions and Al₂O₃-CaO inclusions with large size and angularity are modified into RE₂O₂S inclusions.The inclusion size decreases from 5?m to 1?m,the shape changes from irregular to spherical,distributed in the steel matrix dispersedly.(3)The addition of RE can improve the solidification structure of A572Gr65 steel,increase the proportion of equiaxed grains,at the same time,the composition distribution is uniform without segregation,and the microstructure of hot rolled plate can be refined.(4)With the increase of RE content in the steel,the room temperature strength and low temperature impact toughness of A572 Gr65 steel are increased.RE modified inclusions and refined microstructure are the main factors to improve the comprehensive mechanical properties of the steel.(5)The properties of A572 Gr65 steel have not decreased but improved after the replacement of some microalloyed elements Nb with appropriate amount of RE,which provides a way to develop low-cost and high-quality RE microalloyed steel.