| Duplex stainless steel,which consists of the ferrite phase and austenitic phase approximately accounting for half.Duplex stainless steel has high strength,better toughness and excellent corrosion reisitance.It has been widely used in energy,petroleum,chemical and marine due to its excellent performance.With the continuous improvement of smelting technology and the development of material design,the application field of duplex stainless steel is also expanding and the requirement of corrosion resistance is also increasing.Although the duplex stainless steel has better corrosion resistance,but the local corrosion will occur in specific conditions.So the corrosion problem of duplex stainless steel can not be ignored.The pitting corrosion mainly occurs on the surface of stainless steel where there are defects,such as lattice defects or inclusions.At present,researches on the mechanism of pitting corrosion mainly focus on inclusions,expecilly the MnS inclusions,but few researches on the influence of oxide inclusions on corrosion potential.The formation of sulfide inclusions is directly related to S content in steel.While the deep desulfurization process of duplex stainless steel needs to be improved.In addition,as a duplex stainless steel with high alloy content,the effect of different alloy addition on ite microstructure and corrosion resisitance is also to be studied.Heat treatment process is an indispensable part in the production of duplex stainless steel.The influences of solution temperature and solution time on the corrosion resistance of duplex stainless steel should be further studied.Therefore,in this paper,the in-situ observation is used to analyze the changes of compositions and morphology of inclusions during the chemical and electrochemical corrosion process more intuitively,which furtherly interprets the pitting corrosion mechanism of duplex stainless steel.In order to reduce the content of S in steel,the surfur capacity of different slag systems was calculated.The Factsage software was applied to calculate the physical properties.Th deep desulfurization of duplex stainless steel was studied by experiments.In order to study the microstructure and corrosion resistance of duplex stainless steel with different alloying elements,the addition of Mn and Ti was studied in this paper.In addition,the Thermo-calc software was applied to calculate the precipitated phase of duplex stainless steel,and the compositions and precipitation temperature of the precipitated phase were given.The effect of differen heat treatment condition on the microstruction and corrosion resistance were verified by experiments.The research results are as follows:1)The stability of different type of inclusions is quite different in corrosion solution.The(Mn,Si,Mg,Al)oxides are the most active,which were completely corroded off after two hours.The(Ba,Si,Ca,V)oxides and MnS were gradually corroded with corrosion time.The corrosion of these inclusions leave a large of corrosion pits,which destroy the resistance to corrosion of duplex stainless steel.However,the(Mn,Al,Ti)oxides are very stable and change little after 96 hour in corrosion solution.These inclusions have little influence on the resistance to corrosion.Durnng the corrosion process of stainless steel,the inclusions that are easy to dissolve are corroded firstly.When the inclusions are completed corroded,the exposed steel matrix will to be corroded.When the large-size inclusions are corroded away,the surface area of the matrix exposed to the solution is larger,and the effect on the matrix is much larger than that of the small-size inclusions.Therefore,the large-size inclusions have a worse impact on the corrosion resistance of the duplex stainless steel.2)In the desulfurization slag system,it is feasible to use CaO-MgO-A12O3-S1O2 to replace CaO-SiO2-CaF2 slag system for deep desulfurization.According to the calculation of surfur capacity,CaO-MgO-A12O3-SiO2 slag system has the advantages of low melting point,high sulfur capacity and low pollution.The addition BaO can improve the desulfurization efficiency,but the melting point will also increase.The oxygen activity in molten steel affects the sulfur distribution ratio between slag and steel.High temperature is conductive to desulfurization and deoxidization of molten steel and low oxygen potential is conductive to deep desulfurization.Therefore,A1 deoxidization prior to desulfurization can effectively improve desulfurization efficiency.When the dissolved oxygen is lower than 30 ppm,CaO-MgO-Al2O3-SiO2 slag system can reduce sulfer content in steel to 5 ppm.It is the guarantee of high desulfurization to reasonably control the composition of reduced slag and ensure the low oxygen activity of molten steel.3)The addition of Mn can improve the ratio of austenite in duplex stainless steel.With the increase of Mn content,the corrosion rate increases,pitting potential and corrosion resistance decrease.Differen inclusions have different effects on the corrosion rate.(Mn,Si)oxides are easily dissolved by corrosion solution,leaving pits,which furtherly accelerat the corrosion of steel matrix.However,(Cr,Mn,Al)oxides are insoluble in etching solution,which has little effect on the pitting resistance of duple stainless steel.During the electrochemical corrosion process,the presence of(Mn,Si)oxides will worsen the corrosion environment,accelerate the formation of pitting and reduce the resistance to pitting corrosion of duplex stainless steel.In addition,the corrosion of mattix is the major part of weight loss.4)The addition of Ti accelerates the generation of TiC,which plays a role in refining anstenite grains.Moreover,TiC inclusions are easily dissolved in(Mn,Si)oxides to form composite inclusions.The addition of Ti will reduce the proportion of austenite and has little effect on the type,quantity,and size of inclusions in steel.The addition of Ti can reduce the corrosion rate,increase the pitting potential and improve the corrosion resistance of duplex stainless steel.5)The calculation of Thermo-calc software shows that the content of ferrite and austenite ratio is 1:1 under the temperature of 1075 ℃.As the temperature rising,the content austenite decreases and ferrite increses.In addition,the precipitation temperature of a phase is 950 ℃.The precipitation temperature of carbide is 855 ℃.The precipitation temperature of MnS is 1360℃.The results show that with the increase of the solution time,the ferrite content increases,the austenite decreases and the corrosion resistance increases.With the increase of solution temperature,the ferrite content rise,but not the higher the temperature,the better the performance of corrosion resistance.The suitable solid solution temperature is between 1050-1075 ℃. |
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