Evolution of Microstructure and Crystallographic Texture During Welding for 1.5 GPa Ultra-high Strength Steel Toughened by an Elongated Grain Structure With Intensive Texture

The advanced microstructure of the elongated grain structure with intensive texture, which mainly consists of {112}⟨110⟩_α and {332}⟨113⟩_α components, significantly improves the toughness of 1.5 GPa ultra-high strength steel by delamination toughening. The elongated grain structure with texture affects the microstructure evolution of heat affected zone (HAZ), resulting in the refinement of coarse-grained HAZ(CGHAZ), the separate distribution of granular bainite and lath martensite in fine-grained HAZ (FGHAZ) and the formation of clustered blocky martensite in intercritical HAZ (ICHAZ). In ICHAZ, the sharp {112}⟨100⟩_α of untransformed grains does not undergo reorientation for high thermal stability. For transformed grains, initial {112}⟨110⟩_α and {332}⟨113⟩_α components are reproduced by texture heredity. However, with the increase of peak temperature, the heredity tendency of texture weakens, causing the reduction of texture intensity. In CGHAZ, the Goss and {112}⟨110⟩_α texture are obtained because of grains oriented growth. It is found that serious and heterogenous HAZ softening is a significant feature. FGHAZ becomes the weakest zone due to the massive formation of granular bainite. Due to the disappearance of {332}⟨113⟩_α and the formation of Goss texture, CGHAZ shows the least modulus.