Experimental Study on Primary Scale Formation and Descalability on Steels Containing Ni and Ni plus Si

Nickel and silicon are attractive alloying elements for high-strength low-alloyed (HSLA) steel production. However, it is well known that the presence of Ni and Si in the steel can impair the surface quality, making it unsuitable for certain markets. The combined effects of Ni+Si on the oxide scale formation are still relatively unknown. Literature is dealing mostly with steels containing combinations of Ni and Si, with either traces of nickel (0.1%) or very high (8-16%) nickel levels. At Tata Steel we explored the effect of an optimum composition selected to achieve steel properties (0.15%Si and 1%Ni) on the formation of oxide in the reheating furnace and its descalability. Pilot hydraulic descaling trials were performed on blocks of three steel grades, applying reheating and hydraulic descaling in conditions closely resembling the industrial practice. The oxidation experiments show that synergistic effects occurring during the oxidation of alloys containing Ni (1.1%) are already obvious at relatively low levels of Si of 0.05%. This effect is even enhanced at higher Si levels of 0.15% and consists of increasing the adherence of oxide scale to the steel substrate by forming an entangled layer with oxidic pegs. In order to maximize descalability of (Ni,Si)-alloyed steels slabs, the metal/scale entanglement has to be minimised. In this respect, it was found that the slab surface temperature is the most important parameter. A gentle, smooth reheating process is required in which slab surface temperatures exceeding 1 300 degrees C should be avoided.