Surface conditioning by reactive gases during continuous annealing of sheet steel

The focus of this work is to change surface chemistry and properties without deteriorating the bulk behaviour of deep-drawing steels. Therefore, possibilities and limits of surface modification by reactive gases such as CO–H2–N2 and NH3–H2–N2 atmospheres during continuous annealing of interstitial free (IF) steel grades were investigated. By this, the surface hardness, the bake-hardening properties, the proneness to secondary work embrittlement (SWE), and the surface reactivity in regard to coating processes should be affected. The study showed that the input of interstitials into the surface proceeded within seconds in accordance with the continuous annealing process. If a carburizing treatment was applied after recrystallization took place, an increase in strength combined with good deep-drawability and a still high r-value of two could be obtained. The resistance against SWE improved significantly. Bake-hardening capability could be achieved combined with moderate ageing tendency. Applying very strong carburizing treatments with carburizing times of 60 s, the chemical composition of the surface layer could be altered into a 20 μm thick austenite layer, thus leading to bainitic or martensitic transformations during cooling. Nitriding modified the surface chemistry affecting the Zn–Fe reactivity during galvannealing.