| The exploitation of marine resources and the development of marine defense construction are based on ocean engineering steels with high-strength and high corrosion-resistant.But in the corrosion environment with high humidity and high chloride ion concentration,pitting induced by inclusions is a serious threat to the safety and stable operation of marine engineering structures.In this study,titanium was used as an alloying element in steel instead of the more expensive niobium,which was conducive to reduce the production cost.Simultaneously,titanium alloying combined with aluminum deoxidization and calcium treatment was adopted to control the characteristics of inclusions,which can improve the casting performance and corrosion resistance of Ti-containing high strength steel.This study can provide the theoretical basis for the development and production of low-cost Ti-bearing high strength corrosion resistant structural steel.The main findings are as follows:(1)Firstly,the experiments with different titanium content(0.032%?0.153%,mass percent)were performed to research the characteristics and precipitation behavior of inclusions in Ti-containing high strength steel.The results were as follows:The inclusions were predominantly spherical composite oxides(Al-Ti-Mg-Ca-(Mn)-O-(S))and square titanium nitrides in Ti-containing high strength steel with calcium treatment.With the increase of titanium content,the number of inclusions increased significantly and the average size of inclusions increased slightly.Ti in inclusions could be reduced by calcium,then gradually diffuse into the surface of the inclusion,which was conducive to the heterogeneous nucleation of TiN and the number increase of nitrides in steel.The size of TiN could be effectively reduced when the Ti content was less than0.04%and N content was less than 0.0018%;when the Ti content was more than0.1%,the number of fine oxides in the steel obviously increased which was conducive to refine and disperse the distribution of TiN inclusions of heterogeneous nucleation and reduce the risk of large-size TiN inclusions.The inclusions in the steel were high melting point Al2O3 or MgAl2O4 when the content of Ti was less than 0.045%,which might cause nozzle clogging;while the Ti content was more than 0.106%,the inclusions were changed to easy-removing liquid TiAl2O5.The high melting point inclusions in low titanium steel could be modified by calcium to low melting point calcium aluminate,reducing the risk of nozzle clogging.(2)Secondly,the effect of different Al,Ti,Ca content on the corrosion property of steel with Al-Ti deoxidized and Ca treatment in 3.5wt%NaCl solution was studied.The findings are listed as followed:Inclusions were an important source of pitting,the induced pitting capacity of sulfide inclusions were higher than other inclusions.With the aluminum content decreasing(from 0.026%to 0.008%),the number of inclusions increased and the size of inclusions decreased in steel,which was conducive to refine and disperse the distribution of sulfide inclusions of heterogeneous nucleation and to reduce the corrosion sensitivity of inclusions.In addition,the higher Ti content in the inclusions in low Al steel also help improve the corrosion resistance of steel.With increasing the Ti content(from 0.106%to 0.153%),the number increase of nitrides and the Ti content increase of oxides enhance the corrosion potential and decrease the corrosion current of steel,which help improve the corrosion resistance of steel.With the increase of calcium content(from 0.0017%to 0.0039%),TiO_x content decreases and CaO content increases in inclusions,resulting in the increase of corrosion sensitivity of inclusions,which reduce the corrosion resistance of steel.(3)Finally,the mechanism of pitting corrosion induced by different types of inclusions and the influence of inclusion characteristics on pitting were discussed by experiments and theoretical calculations.The results show that:The standard potential of inclusions was generally higher than that of the iron matrix.When the inclusions formed galvanic corrosion with the iron matrix,the iron matrix would dissolve prior to inclusions.Owing to the conductivity of oxide inclusions was far worse than that of sulfide inclusions,the corrosion current at the sulfides is larger and the corrosion induced by sulfides was also more serious.The higher modified degree of alumina by calcium,the higher the sulfur capacity of the inclusions,resulting in the increase of corrosion activity of the inclusions.Therefore,the corrosion resistance of low calcium steel was better.For the same inclusions,pitting occurred first in areas where the radius of curvature is small.So,the closer to the spherical shape the morphology of the inclusions,the lower the corrosion sensitivity.The impact of the size of inclusions on pitting was insignificant when the inclusions were corrosion resistance to steel;while it was significant when the inclusions are corrosion susceptibility to steel.In addition,the cluster-like inclusions were more likely to induce pitting than the separate inclusions,and the corrosion was also more serious. |
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