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With the purpose of evaluating the effect of niobium additions on the microstructures of the HAZ (heat-affected zone) in mild steels, a plain C-Mn steel (without niobium) and three niobium containing steels with 0.01, 0.02 and 0.04 wt% Nb respectively, were investigated through simulated HAZ experiments at heat inputs of 20, 50 and 80 kj/cm. The microstructures of simulated coarse-grained HAZ's have been examined by optical metallography and transmission electron microscopy. It was found that the addition of niobium had a significant effect on the transformation in the HAZ. For high energy heat inputs (80 and 50 kj/ cm), the addition of niobium retarded the pearlite formation, even in the case of the 0.01 wt%Nb containing steel. The microstructures of the coarse-grained HAZ of niobium-containing steels consisted mainly of secondary Widmanstätten ferrite, but that of the niobium-free steel contained a large amount of pearlite as well as secondary Widmanstätten ferrite. At the low energy heat input level (20 kj/cm), the microstructures of the coarse-grained HAZ of the niobium-containing steels were all similar and consisted mainly of interlocking ferrite plates with small amounts of bainite and Widmanstätten ferrite; while that of the niobium-free steel comprised Widmanstätten ferrite and martensite with a small quantity of pearlite. The results from Charpy impact tests indicated that the niobium-containing steels with a simulated HAZ heat input of 20 kj/cm possessed higher toughness than those treated at heat inputs of 50 and 80 kj/cm. It is proposed that the interlocking ferrite structure, which forms in the HAZ of the niobium-containing steels after simulation of a 20 kj/cm heat input level, improves the toughness property. [ABSTRACT FROM AUTHOR]