Research On The Application Of Composite Niobium-titanium Alloy In Automobile Beam Steel Based On Bayan Obo Mine

As the main alloying element in steel,niobium is widely used in the production of alloy high strength steel.Due to the rapid development of China's iron and steel industry and the lack of niobium resources,niobium resources rely heavily on imports.The development and utilization of domestic niobium resources is undoubtedly of great practical significance for the development of domestic iron and steel industry.The Bayan Obo mine is the largest niobium storage area in China.To solve the comprehensive utilization of low-grade composite niobium and titanium resources in the mine can alleviate the shortage of niobium resources in China to a certain extent.In this paper,commercial niobium and titanium and niobium and titanium produced in Bayan Obo are respectively used as alloying elements to smelt high strength automobile beam steel.Then,the stress-strain curves,isothermal and continuous cooling curves of the experimental steel were obtained by means of the thermal simulation compression experiment and the calculation software,and the rolling process window of the automobile beam steel was determined.Through the hardness test and tensile test of the experimental steel after rolling,the mechanical properties of the steel were obtained,and the microstructure of the experimental steel was observed with the help of scanning electron microscope,and the shape,size and distribution of the precipitated phases were studied.The experimental results provide a theoretical and technological reference for the application of niobium and titanium resources in iron and steel.In the case of the same thermal simulation process,the results of the thermal simulation study show that commercial ferroniobium and composite niobium-titanium smelted experimental steel have the same strain-stress curve.In the single pass compression experiments at 860 ?,950 ?,1020 ?,1080 ? and 1130 ?,there are two mechanisms of dynamic recovery and dynamic recrystallization in all experimental steels.At the above five temperatures,when the first pass deformation is25%?35%?40%?30%?20%,no dynamic recrystallization will occur in the first pass deformation regardless of the temperature and deformation.However,static recrystallization occurs when the deformation temperature is above 1020 ?.The thermodynamic calculation shows that commercial ferroniobium and niobium titanium composite have little effect on the TTT curve and CCT curve of the experimental steel,and bainite structure can be obtained in all the experimental steels at about 550 ?.The microstructure observation shows that the commercial NB Fe and Nb Ti composite steels have the same microstructure,inclusion characteristics and precipitates.The microstructure is ferrite,a small amount of pearlite and retained austenite.The grain size is 11.5?m and the average grain size is 6?m.There are square(Ti,Nb)(C,N)and ellipsoidal MNS composite inclusions with side length of 1.31 ? 3.64?m,and the second phase precipitated below 50 nm.The mechanical property test shows that the two kinds of alloyed steel have similar mechanical properties.The tensile strength of slow cooling after rolling is greater than550 MPa,and the tensile strength of water cooling after rolling is greater than 800 MPa,which meets the design requirements of high strength steel.