Abstract: In order to improve the quality of P91 steel products, causes of delaminate defects on P91 steel were analyzed by scanning electron microscopy (SEM) and ASPEX. The results showed that magnesiaalumina spinel and calcium aluminate of steelmaking process led to stress concentration in the machining process. Inclusion gathering place was easy to form crack and delamination. The low index of inclusion level and total oxygen content reflected problems upon steel cleanliness. By means of optimizing refining slag composition (w（CaO）=55 %－60 %，w（SiO₂）=10 %－15 %，w（Al₂O₃）=18 %－23 %), using nitriding technology and high quality magnesia carbon brick to improve anti-erosion on 10 furnaces trial-manufacture, the inclusion level was stably controlled below 1.0 grade and w（T.O） was reduced from 21.04×10^-6 to 16.73×10^-6. The delaminate defects reduced and the steel tube product yield increased by 15 %.

Key words: P91 steel, delaminate defects, cleanliness, refining slag, nitriding technology, magnesia carbon brick