Effects of boron, niobium and titanium on grain growth in ultra high purity 18% Cr ferritic stainless steel

Ultra-high purity 18% Cr ferritic stainless steel exhibits superior corrosion resistance. But, grain enlargement is likely to occur during their production and heat treatment, and this causes practical drawbacks in terms of surface qualities and mechanical properties. This study examines the effect of boron (B) addition on the grain growth of the ferritic stainless steel subjected to annealing after cold rolling. Since high Cr ferritic stainless steels often contain Nb or Ti, the effect of simultaneous addition of B with Nb or Ti is also examined as well as B single addition. It was found that B single addition retains grain growth after the recrystallization of the cold-rolled steel. The simultaneous addition of B and Nb leads to finer grain structure than Nb single addition, while the simultaneous addition of B and Ti leads to coarser grain structure than Ti single addition. When B is added with Nb, simultaneous precipitation of niobium carbide (Nb(C, B)) and boron nitride (BN) was found, where NbC is refined by the B addition and the total density of the precipitates along with BN is increased as compared with that in only Nb added steel. When B is added with Ti, on the other hand, coarse M-23(C,B)(6) was found to precipitate primarily on titanium nitride (TiN), and hence, the total density of the precipitates in Ti+B steel becomes lower than that in only Ti added steel which contains both TiN and titanium carbide (TiC). These grain structure development observed in the present study is explained primarily by the pinning effect of precipitates, and the precipitation behavior observed is found to be supported by thermodynamic calculation of phase diagrams using Thermo Calc.