New insights into the contribution of grain boundaries on fire resistance of 690 MPa ultra-heavy plate steel

Abstract

In this study, novel insights into the contribution of grain boundary (GB) density on fire resistance is elucidated. We systematically analyzed the relationship between grain boundary density and fire resistance of 690 MPa ultra-heavy plate steel from the perspective of crystallography. The yield strength (YS) at elevated-temperature (ET) of 600 ℃ at the surface and quarter of the plate were 542 and 518 MPa, respectively, while at the center it was only 455 MPa. EBSD analysis indicated that the surface exhibited finer Bain group and higher density of high angle grain boundaries (HAGBs) as compared to the center. The HAGBs can reduce the slip of grain boundaries (GBs) and provide obstacles to the movement of dislocations at ET, which thereby significantly improves the fire resistance. The deformation and cooling rate were decreased rapidly from the surface to the center, which led to Bain group coarsening and drastically decreased HAGBs, resulting in a significant reduction of fire resistance of steel in the center of ultra-heavy plate.