Effects of Nb and Mn on Microstructure and Toughness for Simulated HAZ for 590MPa Class Low Carbon Bainitic Steels

The effects of Nb and Mn on the microstructure and the toughness of simulated HAZ of 590MPa class low carbon bainitic steels were investigated. It was found that the toughness of the low carbon bainitic steel, of which hardenability is increased by adding Mn, is superior to the that increase gained by adding Nb. It was also revealed that the facets of Charpy impact surface in the Mn addition steel are more refined than those in the Nb addition steel. Therefore it was supposed that the coarse facets impaired the toughness of the Nb addition steel. The observations of the microstructures also clarified that the bainite blocks of the Mn addition steel are more refined than those of the Nb addition steel. The reason why adding Nb coarsened the bainite blocks may be explained by a decrease of the interfacial energy on austenite grain boundaries.