Softening Reasons for Coarse Grain Heat Affected Zone of Ti Microalloyed SQ700MCD High Strength Steel

The microstructure and the behavior of dissolution and precipitation of the second phase particles in base metal and thermal simulating coarse grain heat affected zone (CGHAZ) of Ti microalloyed SQ700MCD high strength steel were studied by thermal simulation test, and the reasons of CGHAZ softening were analyzed. The results show that one of reasons for softening was the microstructure change of CGHAZ from lath martensite to granular bainite with the increase of cooling time(t8/5). The second phase particle in base metal was carbonitride, and it had the average size of less than 10 nm and distributed diffusely, but a phenomenon of re-dissolution of most carbonitride particles in CGHAZ was found, the disappearance of dispersive precipitation strengthening was the another reason. The size of particles without complete re-dissolution increased with the increase of t8/5, and the degree of softening also increased gradually.