2003 Volume 21 Issue 3 Pages 397-403
It has been greatly interested that the development of a new high strength steel with grain refinement. Fine grain steel has the same component and the weldability as a mild steel, whereas conventional high strength steel has a low weldability due to its alloying elements. It is found the occurrence of softening caused by coarsening in the heat-affected-zone (HAZ) of weld parts of the fine grain steels and mechanical design of weld parts including weld HAZ softening becomes much important. In this paper, the production of an ultra-fine grain steel (NFG steel) produced by high reduction and high cooling rate is firstly introduced. Mechanical properties of the weld part is examined by considering the characteristics of tensile strength under the theory of the strength of the undermatched joints. The effects of the mechanical and geometrical heterogeneity on tensile strength of the HAZ softening joint are studied by using the large-deformation finite-element analysis. It is found that a design of the HAZ softening joint is possible considering welding conditions and tensile strength of the joint mostly overreach that of the base metal.