Crystallography analysis of toughness in high strength ultra-heavy plate steel

Abstract

A high strength ultra-heavy plate steel was studied to elucidate the mechanism responsible for the difference in strength and toughness between surface and core. The yield strength and tensile strength on the surface were 724 and 821 MPa, respectively, while the core had only 625 and 707 MPa. The −40 °C Charpy impact energy on the surface was 235 (±7) J, while the core was only 128 (±24) J. Electron backscattered diffraction (EBSD) indicated that the density of high angle grain boundaries (HAGBs) in the surface was significantly higher than that in the core. The density of HAGBs was greatly increased by the twin-related V1/V2 variant pairs, while the density of the low angle grain boundaries (LAGBs) in the core was higher because of more variant pairs such as V1/V4, V1/V8, etc. The transformation mode dominated by CP (closed-packed plane) grouping was formed on the surface because of high cooling rate, and lower cooling rate in the core resulted in a transformation mode dominated by Bain grouping.