Static and dynamic strain aging in 'interstitial-free' steels

The effect of chemical composition and cooling rate on the static strain aging (SSA) and dynamic strain aging (DSA) behaviors of four IF steels was investigated after reheating to different temperatures and cooling to room temperature at various rates. In the case of DSA, tensile tests were carried out over the range from room temperature to 450°C and at strain rates of 10-4 to 10-1 s-1 . The DSA behavior was also studied in torsion in the austenite and ferrite regions. Aging index (AI) tests were carried out to evaluate the response to SSA and to estimate the amount of carbon in solution after employing various cooling rates. In all cases, although the atomic Ti/C ratio was greater than one, even still air cooling (3°C/s) led to a supersaturated/unstabilized material and to the occurrence of both DSA and SSA. By contrast, there were no signs of SSA and DSA after furnace cooling (0.05°C/s).;The three Ti and the Ti-Nb IF grades studied displayed serrated flow behavior at all strain rates in the blue brittleness temperature range (100 to 300°C). A simple model is described that predicts whether or not DSA will occur at the strain rates and temperatures involved in the processing of IF steels. It was found that the higher the Ti/S ratio, the higher the solute C. Steels with different chemistries but equal Ti/S ratios displayed the same aging behavior. Internal friction measurements were executed to establish a calibration between the AI values and solute C levels. Two novel techniques for the strengthening of IF steels by dynamic bake hardening (BH) are presented. These techniques lead to much higher BH values compared to those produced by conventional BH methods. The observations also show that when C supersaturation results from cooling after coiling, lower annealing temperatures after cold rolling lead to higher solute C levels.