Effect Of Rare Earth Ce On The Corrosion Resistance And Weldability Of High-strength Low-alloy Steels

With the development of marine resources,the demand for high-strength low-alloy steels is increasing in the field of marine engineering,and the higher requirements are put forward on its welding performance and corrosion resistance.Rare earth added in steel plays a significant role in deoxidization,desulfurization and inclusions modification.The formation of fine and disperse rare earth inclusions can improve the welding performance and corrosion resistance of steel.China is a large rare earth and steel producing country,but not a steel power.Therefore,it is of great strategic significance to combine rare earth resources with iron and steel resources,and utilize the advantages of rare earth to develop high-strength low-alloy steel with high performance.In this paper,the effect of rare earth content on particle characteristics,microstructure,welding performance and corrosion resistance of high-strength low-alloy steel has been systematic studied.The main conclusions are as follows.(1)In situ observation of grain refinement in the heat treatment of Ce-treated steel,the results reveal that the particles can pin austenite grain boundary,and thus retard the growth of austenite grain at high temperature.With the temperature decrease,the austenite was gradually transformed to the acicular ferrite and bainite.The acicular ferrites nucleate on the intragranular inclusion and grow randomly in multiple directions.The bainitic ferrite sheaves nucleate on the grain boundary and grow in the form of packets into the grain interior.The mixed microstructure of the acicular ferrite and bainites can divide the austenite grain into several regions,and thereby refining grain.In Ce-bearing steel,the inclusions of Ca-Al oxide with a layer of Ti N were modified to Ce Al O₃,and the size and number of inclusions decreased.Furthermore,Ce promoted the dissolve of Ti in steel,resulting in the finer precipitates with higher density formed in Ce-bearing steel.These precipitates can restrict the austenite grain growth based on the grain boundary pinning effect,which led to refine the grains in Ce-bearing steel.(2)The influence of Ce content on the microstructure and toughness in coarse-grained heat-affected zone of high-strength low-alloy steel has been studied under the100k J/cm heat input welding thermal simulation.The results indicated that Ce₂O₂S inclusions were gradually modified to Ce_xS_y-Ce P and Ce P with the Ce content increased from 0.012 wt.%to 0.086 wt.%.The higher fraction of acicular ferrite was formed in the0.012 wt.%-Ce-treated steel due to the lower mismatch between Ce₂O₂S and?-Fe.Furthermore,the fraction of M-A constituent decreased with the Ce content increased.Therefore,a superior toughness was obtained in the simulated CGHAZ of the 0.012 wt.%-Ce-treated steel due to the formation of the fine-grained microstructure with the higher density of acicular ferrite grains and lower fraction of M-A constituents.(3)Based on the first principle,the mechanism of inclusion-induced initial pitting corrosion of Ce-treated steel has been studied.The electronic work function of Ti N,Mn S,Ca O·2Al₂O₃,Ce₂O₂S,Ce Al O₃ and the matrix has been calculated.The results indicated that the pit initially occurred at the Mn S and Ce₂O₂S rather than the matrix due to their lower electronic work function compared to the matrix and other phases.Thus,Mn S and Ce₂O₂S as the anode phase of micro galvanic corrosion dissolved preferentially at the initial stage of pitting.In addition,the Ce-rich inclusions possessed the higher pitting corrosion resistance than the same type of inclusions due to their higher electronic work function.(4)The influence of Ce content on the corrosion resistance in high-strength low-alloy steel has been studied.The results indicated that Mn S,Ce₂O₂S,Ce_xS_ypossessing higher pitting sensitivity can induce the occurrence of pitting in 0 wt.%,0.012 wt.%and0.060 wt.%Ce-treated steel,respectively.The Ce P inclusion in 0.060 wt.%and 0.086 wt.%Ce-treated steel presented higher pitting corrosion resistance,and can inhibit the propagation of pit.However,a superior corrosion resistance was obtained in the 0.012wt.%-Ce-treated steel.Because the finer and dispersed inclusions were formed in the0.012 wt.%-Ce-treated steel,in which the size of inclusions smaller than 1?m were about46%.These fine inclusions were hard to induce the occurrence of pitting corrosion.While the size and number of Ce P inclusions were larger in 0.060 wt.%and 0.086 wt.%Ce-treated steel,resulting in the poor corrosion resistance.(5)The inhibition mechanism of Ce³⁺ on high-strength low-alloy steel in 3.5%NaCl solution has been studied.The results indicated that Ce³⁺as a corrosion inhibition,can be deposited on the surface of steel and formed the Ce oxides or hydroxides.They had a dense granular structure,which enhanced the resistance of corrosion product films.The diffusion of Cl^-and O to the surface of steel substrate thus became difficult,decreasing the corrosion rate of steel.Furthermore,Ce³⁺can react with O and form CeO₂.The consumption of O further protected the surface of steel.