Nitrogen Effects in Dual-Phase Sheet Steel

The effects of variations in free nitrogen content on the microstructure and room-temperature tensile properties of laboratory processed dual-phase (DP) sheet steels with a base composition (wt pct) of 0.1C-2.0Mn-0.2Mo-0.2Cr were evaluated. Systematic variations in aluminum and nitrogen content along with coiling temperature were used to create a range of interstitial nitrogen contents and microstructure variations after hot rolling. Two cold rolling reductions of 40 and 70 pct were applied before intercritical annealing. Tensile properties show increasing strength and decreasing total elongation with increasing nitrogen content. Increasing the coiling temperature and adding Al diminished the N effects, while increased cold reduction increased tensile strength in some instances. Microstructural observations indicate an increase in martensite fraction with increased prior cold reduction, while increased coiling temperature decreased the fraction of martensite.