Effect of Nitrogen Impurity on the Tensile Ductility and Impact Toughness of Al-Bearing DQP Steels

Abstract

The influence of small contents of nitrogen present as an impurity in 0.3C Al-bearing steels, which were processed through thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP), was examined in respect of room temperature tensile ductility and impact toughness. Two similar chemical compositions (in wt.%): Fe-0.3C-0.6Si-1.1Al (High-Al) with different N contents of 10 and 30 ppm were selected for this study. In addition, two other DQP steels with compositions: Fe-0.3C-1.0Si (High-Si) and Fe-0.3C-0.5Si-0.5Al (Al-Si), both containing about 30 ppm nitrogen, were also included in the study to compare the properties. Detailed metallographic studies using FESEM-EDS, TEM, EPMA and XRD combined with tensile testing and fractographic analysis indicated that already 30 ppm of nitrogen could impair tensile ductility of TMR-DQP processed High-Al steel in comparison to that with 10 ppm nitrogen. Similarly, the effect was adverse also in Al-Si steel (30 ppm N) despite its reduced Al content (0.5 wt.%), but High-Si steel (Al < 0.002 wt.%, N 30 ppm) did not show any such detrimental effect on tensile ductility. Extensive material characterization verified that even 30 ppm of nitrogen could impair ductility of Al-bearing steels, essentially due to the presence of AlN inclusions, despite that TMR-DQP processing enabled stabilization of 6–10% retained austenite (RA) in the steels. The capacity of RA in promoting improved ductility and strain hardening capacity was impaired by the presence of these inclusions. In contrast, impact toughness transition temperature T28J was not clearly affected with Al-Si when compared to low-N High-Al steel, although excessive splitting in Al-Si caused pronounced scatter in the results and increase in upper shelf impact toughness.