| 321 stainless steel belongs to chromium-nickel austenitic stainless steel,which is a common and economical structural material,which is mainly used in chemical,petrochemical and nuclear industries.Due to the addition of Ti element in 321 stainless steel,the precipitation of chromium carbide at the grain boundary is effectively inhibited under high temperature conditions,and the generation of chromium-depleted areas is reduced,so that 321 stainless steel has strong resistance to intergranular corrosion at high temperatures.However,because the added Ti has a strong affinity for both O and N in the steel,it is easy to form Ti-containing inclusions such as TiN and TiOx.These inclusions can cause nozzle clogs and connections in the production process of 321 stainless steel.The quality defects of the billet,especially the continuous casting of 321 stainless steel billet.Therefore,studying the precipitation mechanism and control measures of titanium-containing inclusions during the production of 321 stainless steel billets is of great significance for improving the continuity of 321 stainless steel billet production and product quality.In the research of this subject,firstly,through the analysis of nozzle clogs,defective billets and the changes in the content of Al,Ti and N elements in the production process,it was found that TiN and various titanium-containing oxides in the steel make 321 stainless steel billet produced nozzle clogs and continuous casting billet quality defects,these inclusions have a larger number and larger size Subsequently,a research method combining industrial production site sampling and laboratory high-temperature experiments was adopted,and a reasonable experimental plan was designed.Combines experimental equipment and software such as scanning electron microscopy,energy-dispersive spectroscopy,oxygen,nitrogen and hydrogen analyzer,inductively coupled plasma spectrometer,inclusion automatic analysis system,Factsage thermodynamic calculation software,Matlab calculation software,etc.The precipitation mechanism,influencing factors and control measures of TiN and various titanium-containing oxides in 321 stainless steel are analyzed and studied.The main research contents and results are as follows:Combined with laboratory experiment and detection data of automatic inclusion analysis system,the precipitation behavior of TiN inclusions in molten steel was quantitatively evaluated.The study found that for 321 stainless steel with Ti and N contents of 0.23 wt%and 0.017 wt%,respectively,at a temperature of 1600?,the number of TiN inclusions in molten steel does not change significantly over time.The average size has been maintained at about 2 ?m,and the number of TiN inclusions below 3 ?m exceeds 90%of the total.In the process of reducing the temperature of molten steel from 1600? to 1500?,the number of TiN inclusions in the steel decreased,and the average size increased by 3?m,reaching about 5 ?m.At the same time,the proportion of the number of TiN inclusions above 5 ?m has increased from less than 1%to 50%.Compared with the water quenching cooling method,the air cooling method increases the size of the TiN inclusions in the steel,but the average size increase does not exceed 1?m.However,under the condition of cooling with the furnace(5?/min),the increase in the size of TiN inclusions in steel is very obvious,and the average increase in size reaches 7?m.In addition,the proportion of the number of TiN inclusions with a size of more than 5?m increased from less than 1%in the water quenched sample at 1600? to more than 80%.The influence of the content of Al and Ti and the order of addition on the evolution of inclusions under the conditions of the molten steel system of 321 stainless steel was discussed.The study found that before Al or Ti was added,the main inclusions in the molten steel were CrOy-MnO-SiO2 in liquid phase.Thermodynamic calculations show that when the Al content in the molten steel exceeds 0.0085 wt%or the Ti content exceeds 0.036 wet%,the original liquid inclusions in the molten steel can be completely reduced to solid Al2O3 or TiOx inclusions.Due to the high concentration of Ti and N,TiN inclusions can be precipitated on the surface of the original oxide in the molten steel.After the oxide is completely wrapped by TiN,the TiN layer on the oxide surface prevents the oxide from being further reduced by Al or Ti.Combined with the results of the mechanism analysis,adding Al before Ti alloying can effectively improve the yield of Ti.The effect of Ca and Ce on the modification of titanium-containing inclusions in 321 stainless steel was studied.For Ca treatment,it was found that the change of Ca content has an important effect on the composition and morphology of oxides in steel.The results of thermodynamic calculation and analysis of the evolution of oxides showed that when the Ti content is 0.2 wt%,the content of Al and Ca are less than 0.1 wt%and 0.0005 wt%,respectively,and it is beneficial for the formation of liquid inclusions in molten steel.For Ce treatment,it is found that the addition of Ce can effectively remove Al2O3 inclusions are modified to contain Ce oxides.When the Ce content is 0.0012 wt%,the oxides in the steel can be completely modified to Ce-contain oxides.As the Ce content in the molten steel increases,the oxides in the molten steel gradually transform into Ce-rich oxides.The order of evolution of the oxides is;Al2O3?CeAl11O18?CeAlO3?Ce2O3,but after the outer layer of the oxide is completely wrapped by TiN,the addition of Ce has little effect on the oxide,because the TiN in the outer layer of the oxide can prevent the modification of the oxide by Ce in the molten steel.The influence of the re-oxidation of molten steel from the ladle to the tundish on the titanium-containing inclusions was analyzed.The study found that during the re-oxidation process,the average increase of T.O content in molten steel was about 0.0020 wt%,and the average increase of T.N content was about 0.0015 wt%.For the[O]entering the molten steel,due to the high content of[Al],it will not react with the Ti in the molten steel,but with the[Ca]and[Al]in the molten steel,resulting in a large number of(Ca-Al)rich-Ti-Mg-O inclusions rich in CaO and Al2O3.For the[N]entering the molten steel,because its concentration has reached the thermodynamic conditions for forming TiN with Ti in the molten steel,and the tundish has good kinetic conditions,the newly precipitated TiN inclusions can self nucleate to form homogeneous TiN inclusions,and the original oxides can be used as nucleation core to form heterogeneous TiN inclusions. |
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