Abstract: Effect of trace boron on the impact toughness of 440MPa deposited metal has been investigated.It was found that the best impact toughness was obtained by maintaining boron contents within 30ppm when Ti content is 0.01%~0.04% and 40ppm~90ppm when Ti content is 0.04%~0.07% respectively.With the boron contents in these ranges,the microstructure of the deposited metal is almost fine acicular ferrite.Otherwise there will be large volume fraction of pro-eutectoid ferrite or upper bainite and granular bainite along the prior austenite grain boundary which will deteriorate the impact toughness.