Analysis and process control of longitudinal cracks on surface of 55SiCr spring steel casting blank
Through hot acid washing inspection of 55SiCr spring steel billets,it was found that there were longitudi-nal crack patterns on the inner arc surface of the 8th batch of billets in the 8th batch of casting.Energy spectrum analysis was conducted on the inclusions inside the crack,and it was found that the content of elements such as Na and F was relatively high,indicating that the inclusion of crystallizer protective slag was involved.To understand the causes of cracks,scanning electron microscopy was used to inspect the attached samples on the inner wall of the nozzle,and combined with the control of the steelmaking production process,research and analysis were conducted.The results indicate that the high content of Als in molten steel is the fundamental reason for the occurrence of longi-tudinal cracks on the surface of castings.Due to the tight production rhythm,the steel ladle used in this pouring is an aluminum containing steel turnover ladle.Before production,no aluminum free steel is used for ladle cleaning,resulting in a high content of molten steel with an Als content of≤0.005 0％.The actual control is between 0.005 5％and 0.008 0％.During the casting process,Al2O3 inclusions formed by secondary oxidation accumulate on the inner wall of the nozzle,gradually forming large adherents.Due to the continuous erosion of high-temperature molten steel,the adherents on the inner wall of the nozzle suddenly fall off,causing a sharp change in the liquid level of each flow crystallizer.The liquid level of the 8th stream crystallizer is required to be controlled within the range of(70±5)mm,but at the end of casting,the liquid level of the crystallizer is controlled between 34-85 mm,and the casting speed of the billet changes from a constant drawing speed of 1.80 m/min to 1.4-2.0 m/min,with excessive fluctuations.The liquid level of the crystallizer loses control for a long time,lasting for about 30 minutes,causing slag inclusion on the surface of the casting billet.When the surface of the casting billet is subjected to tensile stress,stress concentration occurs at the slag inclusion position,resulting in longitudinal cracks.By implementing strict low aluminum control measures for alloy materials,refractory materials,and strengthening continuous casting pro-tection pouring,there has been no problem of longitudinal cracks caused by slag inclusion on the surface of the bil-let,and the improvement effect is significant.
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