2009 Volume 49 Issue 6 Pages 837-842
Technology to precisely control Mg content has been developed to improve hot-workability of NW2201 Ni alloy. Relatively high standard deviation of Mg contents, 0.007, out of average Mg content of 0.02 mass% was problem among heats. This brought poor hot-workability leading to sliver-like surface defects on the strip when Mg content was relatively high or low.
Laboratory-scale experiments were performed to understand the behavior of Mg in terms of equilibrium and mass-transfer of the slag/metal reaction of 3(MgO)+2Al=(Al2O3)+3Mg. Experiments were firstly carried out to understand Mg content in molten Ni equilibrated with CaO–Al2O3–MgO–CaF2 system slags. It was found that Mg content in liquid Ni depended on Al content and C/A (CaO/Al2O3 ratio in mass%) in the slag. The measured Mg contents mostly agreed with thermodynamic calculation. Further, kinetic analysis was also undertaken experimentally by approaching from both higher and lower Mg contents to the equilibrium Mg content. As a result, mass-transfer of Mg in molten Ni is considered as the rate-determining step for the above reaction.
By applying the countermeasures, introduced from the view points of both equilibrium and kinetics, for practice, the standard deviation of Mg content was improved from 0.007 to 0.003 among heats. Consequently, the defects have been improved successfully.