In-depth understanding of the relationship between dislocation substructure and tensile properties in a low-carbon microalloyed steel

Abstract

At present, the microstructure-tensile property relationship of low carbon microalloyed steel has been widely investigated, mainly with the effects of microstructure type and its simple morphology including size and shape etc. The correlation between dislocation substructure and tensile properties, however, seems more essential and needs in-depth analysis. In this study, four typical Mn containing (from 1.08 to 1.77 wt%) low-carbon microalloyed steels were prepared by Thermo Mechanical Control Process, and the correlations among Mn content, dislocation substructure and tensile properties were investigated. A mixed microstructure consisting of polygonal ferrite, degenerated pearlite, granular bainitic ferrite and martensite/austenite (M/A) constituents formed in each steel, with highly complex dislocation configuration and distribution. For the 1.08% Mn steel, the dislocation substructures appeared mainly in dislocation wall distributing in the ferrite matrix of degenerated pearlite, and dislocation tangle in the granular bainitic ferrite around the M/A constituents and the bainitic ferrite lath boundary. As the Mn content increased from 1.45% to 1.77%, the dislocation cells appeared and increased. Simultaneously with the rising of Mn content, the mean equivalent diameter (MED) of all the ferrite matrix decreased. The ferrite grains whose MED was defined by the misorientation tolerance angles (MTAs) from 2 to 8°, featured a variety of dislocation substructures including dislocation cell boundaries, dislocation walls, bainitic ferrite lath boundaries, and dislocation tangles. They effectively governed the yield strength due to their determined close Hall–Petch relationship. Moreover, the significant increase in the amount of M/A constituents enhanced the strain hardening capacity, leading to an elevated tensile strength. Dislocation tangles exhibiting around the M/A caused a microstrain concentration, promoted additional microvoids and deteriorated the elongation.