Development of a new deep drawing quality grade combining low ageing sensitivity, bake hardening and high n-value

This project has studied the effect of steel composition and processing parameters in order to develop a soft deep drawing quality which combines the following characteristics: high n-value (> 0.22), low ageing sensitivity (AI < 20MPa), sufficient bake hardening (BH2 > 35MPa) and good deep drawability (high r-value). The work was divided between five different research partners: CRM, voestalpine, Corus, IRSID and ILVA. This research has shown that ELC grades are the better solution for the continuous annealing processing route or for the hot dip galvanising route, on the condition that an overageing section is included in the line. It is very difficult to make the ULC steels ageing-free while retaining a sufficiently high bake-hardening index. Nevertheless, hot dip galvanised bake hardenable ULC steels are produced industrially but this requires a very precise control of the alloying elements (Ti is added for the bake-hardening effect and Nb is often added for the control of the stretcher strain marks). Optimal properties were obtained for ELC steels with a total carbon content below 0.025 %. The overageing temperature seemed to be the most important parameter for controlling the bake hardening effect. It seemed to be extremely difficult to improve the n-value by varying the conventional processing parameters. A very interesting product was obtained by adding B to a conventional ELC steel. This B steel resulted in an n-value which was higher than 0.22 and the fulfillment of all other product requirements. However, this is only achievable when the atomic ratio B/N is around 1, which is not so easy to control in an industrial environment.