Development of Heavy Section 3.5% Ni Steel Plates with Good Mechanical Properties after Stress Relieving

摘要

The conventional normalized type of 3.5% Ni steel can hardly secure the required strength and toughness after SR treatment because of its high susceptibility to SR embrittlement. The investigation indicated that the mechanism of SR embrittlement was caused by coalescing and coarsening of carbides at the grain boundary and that it is effective for the suppression of SR embrittlement to reduce a carbon content and to give a structure in which fine carbides are uniformly dispersed. Based on the results of this investigation, a normalized type 3.5% Ni steel was developed having a low carbon content and low contents of alloying elements such as Cu, Cr and Mo and showing an excellent resistance to SR embrittlement. As an example, performance test carried out with the base metal and welded joints of 100 mm thick steel plate clearly has a strength and toughness fully satisfying the Standards even after the strict SR treatment, as well as an excellent workability.