On the characterization procedure to quantify the contribution of microstructure on mechanical properties in intercritically deformed low carbon HSLA steels

Abstract

The mechanical properties of intercritically rolled microstructures have been scarcely reported in literature. Although the strengthening effect of intercritical rolling is generally recognized, there is no a clear opinion on its effect on toughness. Therefore, a greater knowledge of how different process parameters affect the mechanical properties during intercritical deformation is required. With the aim of evaluating the relationship between microstructure and mechanical properties on intercritically deformed low carbon steels, plane strain compression tests were carried out. Plane strain compression tests allow for both the characterization of the microstructural features and the evaluation of mechanical properties, via tensile and Charpy tests. Firstly, the intercritically deformed microstructures were characterized using the EBSD technique, and then a discretization methodology was used to distinguish both intercritically deformed and non-deformed ferrite populations. Next, strength and toughness properties were measured by means of tensile and Charpy tests. The results indicate that the reduction of the deformation temperature leads to an increment of yield strength for both steels, but at the same time toughness properties worsen. Deformed ferrite fractions higher than 25% result in a very pronounced loss of ductility. The yield strength was predicted by estimating the contribution of different strengthening mechanisms (solid solution, grain size refinement, dislocation density) corresponding to each ferrite population by considering a nonlinear law of mixtures. Similarly, the impact of different microstructural parameters (solid solution, grain size, microstructural heterogeneity, contribution of dislocation density and secondary phases) on toughness was evaluated and a new equation able to predict ductile to brittle transition temperature for intercritically deformed microstructures was developed.