Study On Process Control Of Surface Sliver Defect Of Ultra Low Carbon Steel In Shougang Jingtang Company

The ultra low carbon steel was used widely for manufacturing industries,such as car and homily electrical appliance of drawing formation to take shape piece and surface panel piece.The customer had a very high request for surface quality of cold rolled strip,especially used for automobile outer panel of high-end brands.Slight quality defects would be magnified after painting process,which would seriously affect the appearance and performance of products and became the bottleneck of opening high-end customer market.Therefore,it is of great significance to study and analyze the formation and evolution of surface defects of ultra low carbon steel to improve its surface quality.Sliver defects appeared on surface of ShougangJingtang's ultra low carbon steel were studied by scanning electron microscope(SEM),and qualitatively analysis of dispersive spectrum(EDS).The results showed that the defect was caused mainly by Al2O3 inclusions,mould flux powder and FeO.There were two main types of strip defects caused by the Al2O3 inclusions.One was cluster A1203 inclusions in the steel,and the other was the bubbles with Al2O3 inclusions captured by the solidification shell,which was broken during rolling process.The main reason for sliver defects caused by mould slag was entrapment forming large size of inclusions,which was close to the surface of slab.The FeO defect was caused by the oxidation of the hot rolled plate.Aiming at the sliver defects caused byAl2O3and mould flux,this kind of defect was effectively controlled by the modification of ladle slag by reducing the top slag oxidation,reasonable pure cycle time of RH and increasing the sedation time of ladle ensuring inclusion floating,optimizing performance of mould powder and structure of nozzle,control speed,decreasing argon blowing to reducing the liquid level fluctuation,reducing the tapping temperature and water descaling system reform measures,cold rolled steel strip surface defects was reduced.In steel making process,the top slag oxidation was decreases to 5%by modification of ladle slag;In refining process,the sedation time and pure cycle time were determined,which were greater than 6min and from 25min to 45min;In continuous casting process,mould powder,nozzle,speed,argon blowing were optimized to reducing the liquid level fluctuation.Through the process optimization,the aggregation behavior of composite inclusions was improved,the number of inclusions in the large clusters was reduced,the nozzle clogging of the casting process was relieved,and defects on the surface of the ultra low carbon steel were reduced from 0.55%to 0.23%.