Effect Of Rare Earth Cerium On Cryogenic Vessel Steel Microstructure And Cryogenic Toughness

Due to China’s rich rare earth(RE)resources,the research and development of RE steels has always been one of the hot spots in the metallurgical field.The unique outer electronic structure of RE elements make them widely used in steel materials,adding it to steel can improve the mechanical properties.For cryogenic vessel steel,the research on the mechanism of the RE on the microstructure and cryogenic toughness is still limited,and it has not give a theoretical explanation in realizing industrial production and replacing relatively high prices elements.Therefore,this paper focuses on the microstructure and cryogenic toughness of C-Mn and Ni-containing cryogenic vessel steel.In the industrial production process of smelting and hot rolling,the influence of Ce on the inclusions,microstructure and properties of C-Mn cryogenic vessel steel is studied on the whole process;the mechanism of Ce after substitution of Ni on the microstructure and cryogenic toughness of Ni-containing cryogenic vessel steel is studied in the laboratory.This paper focuses on accelerating the R&D of industrial application of RE steel and explore the feasibility of replacing Ni with high abundance Ce.For the industrially smelted C-Mn cryogenic vessel steel,the formation rule of Ce inclusions is obtained through thermodynamic calculation and experimental analysis.The effect of different Ce content on inclusions,microstructure and slab quality is studied.When adding an amount of 0.0009%Ce,it has a beneficial effect on the size and morphology.Also,the formed high melting point Ce2O2S fine inclusions effectively become the heterogeneous nucleation core of the solidified primary phase,optimize the solidification structure,and improve the central segregation of the slab.Based on the thermal simulation compression and continuous cooling transformation experiments,the influence mechanism of Ce on deformation recrystallization and cooling phase transformation is studied.By comparing the stress-strain curve and microstructure characteristics of Ce added and RE-free C-Mn cryogenic vessel steel at different deformation temperatures,it is concluded that the addition of Ce can inhibit the recrystallization transformation,expand the non-recrystallization temperature transformation zone,and achieve the matrix microstructure refinement,while Ce has no obvious effect on the cooling phase.The matching heating system,rolling system and cooling system for industrial production of C-Mn cryogenic vessel steel are proposed,which provides guidance for industrial production and application.Through the analysis of experimental research results,it is found that with the increase of Ce,the transverse and longitudinal yield strength and tensile strength of C-Mn cryogenic vessel steel show a slow increase trend,while the elongation shows a trend of first increase and then decrease,that is,when the Ce content is at 0.0009%,the strong-plastic matching of the product is the best.However,the impact energy absorbed under different low temperature experimental conditions shows a trend of first increasing and then decreasing.When the Ce addition amount is 0.0009%,the impact energy absorbed is relatively high at-20℃,-40℃,-50℃ and-60℃.The comparison shows that when the Ce content is 0.0009%,there will be more fine ferrite and fine pearlite are obtained in the microstructure of the experimental steel,and the size of Ce containing inclusions formed is the smallest.Combined with the results of welding simulation experiments can be found that Ce improves the cryogenic toughness of the welding heat affected zone of C-Mn cryogenic vessel steel by refining the microstructure and modifying the inclusions.Comparing the impact energy absorbed by hot-rolled steel strip of the same thickness produced in industrial scale at-40℃,it is verified that the steel strip with 0.0009%(±0.000 1%)Ce has better cryogenic toughness than that without RE.For Ni-containing cryogenic vessel steel,the equilibrium phase diagram of binary alloy of Fe-Ce and Fe-Ni are calculated by FactSage software.The acquired structure is similar to Fe-Ni phase diagram by controlling the addition amount of Ce,the results show that it is feasible to replace Ni with Ce.It is found from the experimental results that on the basis of 7Ni steel,when 0.0026%Ce is added to replace 0.5%Ni,large angle grain boundaries and fine Ce containing inclusions can be formed,which effectively inhibit the initiation and propagation of cracks.The impact energy absorbed at-196℃ is similar to that of 7Ni steel.However,when 0.0265%Ce is added to replace 0.5%Ni,the number of large angle grain boundaries decrease,fine Ce containing inclusions aggregate growth,which destroys the continuity of the matrix,Ce is locally enriched in the matrix,with uneven distribution,and the morphology and size of reversed austenite decrease.The above mentioned research results of using 0.0265%Ce instead of 0.5%Ni clarify the reasons for the cryogenic toughness decline of the matrix.