The effects of different initial microstructures on the microstructural features, mechanical properties, and fracture appearance of a C-Mn dual phase (DP) steel are studied. It is revealed that the martensitic and normalize initial microstructures result in excellent combination of strength and ductility due to the good distribution of the martensite particles in the DP microstructure. Conversely, the conventional ferritic-pearlitic banded structure and spheroidized one show inferior work-hardening response and mechanical properties mainly due to the fewer effect of martensite on the deformation of larger ferritic areas that are present in the resultant microstructures. On the other hand, cooling from the austenitic region to intercritical range followed by quenching results in high strength and very high incremental work-hardening exponents. This is attributed to the presence of well-defined and isolated martensite fibers in the matrix, which is shown to behave like the deformation response of composites. The fracture appearances are in general agreement with the obtained results from the tension tests.
In this review, effects of initial microstructure before intercritical annealing and processing routes of dual phase (DP) high strength steels are discussed. Prior-applied deformation processes as cold rolling and severe plastic deformation (SPD) including their impacts on mechanical properties of DP steels are described. Influences of heating rate, recrystallization of ferrite, nucleation and grain growth of austenite on the microstructure evolution are also presented. The intercritical annealing and thermomechanical processes of DP steels are comparatively outlined. Furthermore, post-intercritical annealing treatments, i.e., tempering, bake hardening, strain and quench aging are argued. Effects of key alloying elements on the microstructure and mechanical properties of DP steels are briefly summarized. A further development of DP steels can properly benefit from this extensive review.
Herein, the effects of initial microstructure in combination with the severe plastic deformation on the developed microstructures and mechanical properties of dual‐phase (DP) steels are investigated. DP steels are produced by an intercritical annealing from the preliminarily heat‐treated samples with different initial microstructures. The constrained groove pressing (CGP) process is additionally applied before the final intercritical annealing step. It is found that the martensitic and tempered martensitic microstructures lead to DP steels with a good balance of strength and elongation because of finer and more uniformly dispersed martensitic islands. The ferrite/pearlite banded initial microstructure is not a proper choice for intercritical annealing as all mechanical properties become worse. The CGP can be employed before intercritical annealing for refining microstructure and enhancing damage tolerance of DP steels. Nevertheless, the impacts of the CGP process on the resulted strength of DP steels are obviously different depending on the initial microstructures. The amounts, sizes, and distributions of dimples and cleavage facets on the fracture surfaces of DP steels correlate well with the corresponding microstructures and tensile characteristics.
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