Deformation mechanism of low carbon bainitic steels

Abstract

The bainitic steel with low carbon content has become an important steel grade among advanced high strength steels (AHSS) due to its excellent combination of strength and toughness. In general, the bainitic steel has a lamellar microstructure with either full bainite lath microstructure or with metastable filmy retained austenite (RA) embedded in the bainitic lath matrix. This unique lamellar microstructure determines the overall mechanical performance of bainitic steel. Consequently, to reveal the unique deformation mechanisms in the bainitic steel, it is crucial to capture the evolution of key microstructure parameters during plastic deformation, which is the main target of present thesis. Since bainitic steel has an ultrafine-grain size, a unified dislocation-based model is firstly proposed to predict the mechanical behavior of various metals with grain sizes ranging from submicron to microns. The effect of grain boundary dynamic recovery (GB-DRV) on the evolution of dislocation density is considered in this model to explain the poor tensile ductility of ultrafine-grained metals.

In addition to the ultrafine bainitic lath thickness, the bainitic steel also has a high dislocation density. The respective contribution of dislocation forest strengthening and lath boundary strengthening to the yield strength of fully bainitic steels are determined by experiments. It is found that the dislocation density increases with the decrease of lath thickness. Moreover, the dislocation strengthening becomes dominant when the lath thickness is below one micron in bainitic steel. Besides of its lath morphology, the bainitic steel has an enrichment of dislocations at the bainitic boundary due to the displacive shear transformation. By mathematical modeling and detailed experiments, it is found that such dislocations enrichment at the bainitic boundary exhibits a significant influence on the multiplication and annihilation of dislocations during plastic deformation. An estimation on such dislocation enrichment phenomenon demonstrates a doubled dislocation density in a domain of 0-0.2 m in adjacent to the bainitic lath boundary, which is consistent with other experimental investigations. Different from the fully bainitic steel, the bainitic steels with decoration of retained austenite grains demonstrates an enhanced strain hardening behavior. The retained austenite could transform to martensite during plastic deformation, demonstrating a transformation induced plasticity (TRIP) effect. An in-situ neutron diffraction is utilized to reveal the improved strain hardening in bainitic steel with TRIP effect. It is found that the phase strength has a strong correlation with the intergranular stress and these stresses exhibit a very similar trend to that of fully bainitic steels. Therefore, the enhanced strain hardening should mainly stem from the increased martensite volume fraction and the enhanced stress partitioning of martensite. The effect of bainitic matrix strength on the mechanical stability of RA is also investigated. By a low-temperature recovery, the bainitic matrix strength is reduced due to dislocation annihilation while the other microstructure parameters are kept as constant. It is found that the reduced bainite strength results in an increased stress partitioning and a relaxed compressive stress state on RA, enhancing the TRIP effect in the bainitic steels with low-temperature recovery treatment.