Formation mechanism of large-size CaO-Al2O3-SiO2 inclusion in high-speed railway wheel steel
To investigate the formation mechanism of large-size CaO-Al2O3-SiO2 inclusions in high-speed railway wheel steel,the characteristics of inclusions were analyzed by scanning electron microscopy(SEM)and energy dis-persive spectrometer(EDS).Additionally,the formation process was calculated using FactSage thermodynamic software.The results reveal that the inclusions in the continuous casting billet of wheel steel are mainly CaO-Al2O3-SiO2,categorized into two different types.The first type is the small-size solid and semi-solid inclusion formed dur-ing the cooling and solidification process of liquid steel.The second type is the large-size liquid inclusion originating from the refining process.Notably,among these liquid inclusions,up to 43％exceed 10 pm in size and can be traced back to the calcium treatment stage during the refining process.Following calcium treatment,the liquid steel under-goes a reaction leading to the formation of liquid CaO-Al2O3-SiO2 inclusions.Due to their low contact angle with the liquid steel,only about 40°,these inclusions are difficult to remove and consequently persist in continuous casting billet.The thermodynamic calculation results reveal that only controlling Al content is insufficient in inhibiting the formation of liquid CaO-Al2O3-SiO2 inclusions.It is necessary to control the appropriate Ca content to avoid the ap-pearance of such inclusions.During the calcium treatment stage of liquid steel,under the condition of Al mass frac-tion of 0.013％,when the Ca mass fraction is maintained at 0.000 7％,the inclusions with a liquid phase ratio of 20％can be formed.These inclusions do not affect the continuous casting of liquid steel and can be easily removed.Achieving such accurate Ca content necessitates precise calcium treatment,as well as appropriate Al content and re-fining slag composition.The research results provide some guidance for the precise control of large-size inclusions in wheel steel.
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