Abstract
Ultra-fast annealing (UFA) is a viable alternative for processing of 3rdgeneration advanced high strength steels (AHSS). Use of heating rates up to 1000°C/s shows a significant grain refinement effect in low carbon steel (0.1 wt.%), and creates multiphase structures containing ferrite, martensite, bainite and retained austenite. This mixture of structural constituents is attributed to carbon gradients in the steel due to limited diffusional time during UFA treatment. Quasi-static (strain rate of 0.0033s-1) and dynamic (stain rate 600s-1) tensile tests showed that tensile strength of both conventional and UFA sample increases at high strain rates, whereas the elongation at fracture decreases. The ultrafast heated samples are less sensitive to deterioration of elongation at high strain rates then the conventionally heat treated ones. Based on metallographic studies was concluded that the presence of up to 5% of retained austenite together with a lower carbon martensite/bainite fraction are the main reason for the improved tensile properties. An extended stability of retained austenite towards higher strain values was observed in the high strain rate tests which is attributed to adiabatic heating. The extension of the transformation induced plasticity (TRIP) effect towards higher strain values allowed the UFA-samples to better preserve their deformation capacity resulting in expected better crashworthiness.
Highlights
Driven by stringent regulations for passenger safety, CO2 emissions and fuel consumption, the most recently developed advanced high strength steels (AHSSs) for the automotive industry offer a good combination of strength and ductility [1,2]
The initial microstructure is primarily composed of cold deformed ferrite and pearlite but is transformed after heat treatment into a microstructure consisting of a mixture of ferrite and martensite (Fig. 2)
In samples heated at 400 °C/s and 1000 °C/s, the martensite fraction reaches a maximum of ∼45% at ∼803 °C and ∼783 °C, respectively, and decreases slightly for the highest peak temperatures
Summary
Driven by stringent regulations for passenger safety, CO2 emissions and fuel consumption, the most recently developed advanced high strength steels (AHSSs) for the automotive industry offer a good combination of strength and ductility [1,2]. These properties are maintained at the high strain rates typically met in a car crash. A third generation of AHSSs, which does not require, or at least contains, less expensive alloying elements which is still the main drawback of the 2nd generation, is considered as realistic approach To this generation among the others, belong steel processed via quenching and partitioning (Q&P), medium Mn steel and steels processed via ultra-fast annealing (UFA) ( known as rapid heating or “flash” annealing). Until now all results for strength and elongation of UFA steels are obtained via static or quasistatic tensile test on small, standard size samples [5] but there is no data about the behaviour of the UFA steel in the dynamic loading conditions which are more realistic in case of car accident
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