Study on cleanliness of X80 pipeline steel

By the methods of metallographic observation,sample-electrolyzing and scanning electron microscopy(SEM)analysis,the nonmetallic inclusions in X80 pipeline steel were investigated.The results showed that inclusions had a significant downward trend from LF inlet to microscopic in casting process with a fall of 77.77%,the number of inclusions reduced most obviously in LF and RH refining process.The main types of inclusions in slab are CaO-CaS-Al2O3composite inclusions,xCaO/xAl2O3is less than 1.71 of 12CaO·7Al2O3,inclusion modification effect is poorer.From converter end to casting process,the average total oxygen content had a downward trend.From LF inlet to the calcium treatment process finished,the average mass fraction of T.O decreased less than 3×10-6,the average mass fraction of T.O reduced 2.27×10-6 in RH refining process.It needs to improve the level of LF refining operation and RH refining operation stability.RH outbound to the slab was a process of increasing nitrogen,the mass fraction of nitrogen increased 2.27×10-6,protective casting level was higher.The large inclusions quantity was 1.46 mg/10 kg,inclusions are mainly spherical calcium aluminate inclusions,silicon aluminate inclusions and composite inclusions with magnesia-alumina spinel.