Unveiling the Deformation Mechanism of High-Strength Low-Alloy Structural Steel with Gradient Dislocation-Cell Structure

Abstract There is a pressing need for high-performance, high-strength low-alloy structural (HSLA) steels in various engineering fields, such as hydraulic components, engineering machinery, bridges, ships, and pressure vessels. In this study, a gradient dislocation-cell structure is introduced into an HSLA steel through ultrasonic severe surface rolling. The cell size is approximately 614 nm at the topmost surface layer, and increases with increasing the depth. Most of the cell walls have a misorientation ranging from 2° to 15°, indicating they belong to low angle grain boundaries (LAGBs), while some cell walls have a misorientation of less than 2°, corresponding to dense dislocation walls (DDWs). This unique gradient structure offers an exceptional combination of strength and ductility, with a high yield strength of 522.3±1.4 MPa and an accepted elongation of 25.5±1.7%. The morphology and size of the dislocation cells remain remarkably stable after uniaxial tension, demonstrating their efficacy as effective barriers hindering dislocation movement and thus enhancing strength and hardness. This gradient dislocation-cell structure facilitates inhomogeneous plastic deformation during uniaxial tensile loading, resulting in a pronounced accumulation of geometrically necessary dislocations (GNDs). These GNDs play a significant role in conferring favorable mechanical properties by inducing hetero-deformation-induced (HDI) strengthening effects and forest hardening effects. This study presents a promising avenue for achieving the desired mechanical properties in HSLA steel.