چكيده به لاتين
The development and use of lightweight steels in the automotive industry have gained significant attention due to their potential for reducing the weight of structures, fuel consumption, environmental pollution, and increasing the strength of vehicle bodies. Therefore, in the present study, the strain rate sensitivity behavior of a dual-phase lightweight steel with a composition of Fe-30.6Mn-8.6Al-0.07C was investigated through tensile testing at room temperature under quasi-static strain rates ranging from 10-5 to 10-1 s-1. As the strain rate increased from 10-4 to 10-1 s-1, the yield stress increased from 457 to 520 megapascals. Furthermore, high and positive values of strain rate sensitivity (m~0.12) were obtained even at low applied strains (0.15), whereas it is expected that the yield strength and strain-hardening behavior would not be sensitive to changes in strain rate at room temperature. Furthermore, the values of elongation to fracture at room temperature increase from 20.9% to 37.7% with an increase in the applied strain rate. Recent observations, contrary to the typical trend observed in conventional steels, have raised discussions regarding the changes in deformation mechanisms at ambient temperature with a decrease in the strain rate. In this regard, variations in the steel's strain hardening behavior and instantaneous hardening power at different stages of hardening were evaluated. The number of hardening stages and the hardening potential of the material decrease with a decrease in the applied strain rate. The observed different stages and peaks of hardening were attributed to the mechanisms induced by strain (substructure development, twinning, microstructural changes, and phase transformation). Substructure development in the ferrite phase (which constitutes 85% of the initial structure) was identified as the main deformation path at higher strain rates, resulting in the formation of subgrains and subboundaries, leading to increased formability and hardenability of the steel. Under lower strain rates, the development of subboundaries is not possible due to the large initial grain size of ferrite (approximately 1 millimeter) and the presence of high energy barriers in its arrangement. Under a strain rate of 10-1 per second, grain rotation of the ferrite phase towards the (111) planes as the preferred orientation for substructure development can be observed. Under lower strain rates, the formation of preferred austenite texture towards the (110) planes is detectable. With a decrease in the strain rate, conditions are provided for load transfer from ferrite to austenite and the occurrence of austenite transformation to ferrite induced by strain. However, due to the discontinuous network and low volume fraction of the austenite phase (approximately 15%), the strain-induced transformation is not capable of significantly increasing the effective strain rate and hardenability compared to conditions where substructure development prevails in ferrite.
Keywords: lightweight steel, hardness, strain-hardening behavior, substructural transformations, strain-induced transformation.
