Complex carbonitrides precipitated in base metal and heat-affected zone (HAZ) in Nb-Ti hot-rolled microalloyed steel plates have been identified to be Ti-rich (Nb, Ti)(C, N). As the reheating temperature is decreased from 1,200 to 1,150 A degrees C, the average particle size in base metal is decreased from 40 to 20 nm. The morphology of complex carbonitrides in the HAZ, however, is transformed from cuboidal to rectangle shape with length of over 500 nm. Reheating at low temperature 1,150 A degrees C may improve the toughness of HAZ by reducing the austenite size at large heat input welding.