The effect of Al and N on the notch toughness and tensile strength of heavy section pressure vessel steel plates is investigated. Notch toughness of steel A533B (Mn-Mo-Ni), which has mixed microstructure of ferrite and bainite, is drastically changed by the ratio of sol.N/sol.Al. With metallurgical observations, it is revealed that AlN morphology is influenced by the ratio of sol.N/sol.Al through the level of solute Al (CAl). At the heat treatment of heavy section steel plate, AlN shows OSTWALD ripening and its speed depends upon CAl. When Al is added (Al≥0.010%) in steel and sol.N/sol.Al≤0.5, CAl remains low. This prevents AlN ripening, and brings fine austenite grain size and high toughness. On the other hand, when sol.N/sol.Al<0.5, CAl becomes high and this gives poor toughness through coarse AlN precipitates and coarse austenite grain. Therefore, controll of sol.N/sol.Al over 0.5 is favorable to keep high toughness in A533B steel. In steel A387-22 (Cr-Mo) which has full bainitic microstructure, too fine austenite grain brings about poor hardenability, and polygonal ferrite, which brings about both poor strength and toughness, appears in mi crostructure. Then sol.N/sol.Al<0.5 is better to give high hardenability in steel A387-22.
