Development of Formable Steel Grades Through Alternative Steelmaking Technologies
摘要
Electric arc furnace steelmaking has many environmental benefits compared to the blast furnace-basic oxygen furnace route but typically produces steel with higher nitrogen levels, presenting a particular challenge when aiming to produce interstitial free (IF) steel, which typically has a nitrogen content of <30 ppm. Whilst free nitrogen is known to have a deleterious effect on the formability of IF steel, the effect of higher levels of nitrogen combined in TiN precipitates is not clearly established. This work aims to investigate the impact of increasing nitrogen levels on the properties of IF steel. A route has been established for making lab-scale casts of IF steel with varying nitrogen contents and fixed excess Ti. The InTRAP technique has been used, in which smaller lab casts are inserted into a larger transfer bar before hot rolling, allowing processing parameters more representative of those at an industrial scale, and ensuring casts undergo the same heat treatment. Once processed, casts have been tensile tested and the microstructure analysed to investigate the impact of different nitrogen levels on properties such as formability. The casts show no clear deterioration in mechanical properties with increasing nitrogen content, which may be a result of the fast cooling rate, preventing the formation of large TiN precipitates. This may suggest that a high cooling rate, such as those on a thin slab caster, can minimise the impact of increasing nitrogen levels in IF steel, although it will be useful to compare results from casts with different cooling rates.