Abstract
This study investigated the effects of varying phosphorus content (between 20 and 350ppm) in ETP copper by process monitoring using Statistical Process Control (SPC) graphs. Differences in electrical conductivity between as-cast slabs of low phosphorus and those of high phosphorus, as well as how this affects the mechanical behaviour of copper strip were investigated. The study overall focuses on optimising phosphorus as a deoxidiser in copper melts.
Copper slabs were cast through a semi continuous vertical casting process. After casting, conductivity was measured using the eddy current resistance tester on the slabs. The slabs then underwent a hot rolling process, whereby the thickness was reduced by 83% (140 mm – 24 mm) over multiple passes. The slabs were then scalped, to take off the oxide layer that forms during hot rolling. The slabs were then cold rolled, and the slab thickness was further reduced by 88% (24mm – 3mm). Phosphorus reacts with the dissolved oxygen in molten copper to form phosphorus oxide. Results showed that phosphorus decreases as the slab is being cast, resulting in the full slab having various electrical conductivity values. There was also a variation in phosphorus content in the button samples taken from the molten metal. It was found that 6mm must be cut off from the button samples for more accurate chemical results and that phosphorus must be given at least 1200 seconds to diffuse in the 20-ton channel induction furnace. After the rolling processes were complete, the strips were taken for mechanical testing. Hardness was measured using macro-Vickers hardness tester, The hardness values for strip with low and high phosphorus was found to be between 70HV and 90HV. The strength of the strip was evaluated using a tensile testing machine and low phosphorus strip had yield strengths ranging from 190MPa to 215MPa. The increase in phosphorus drastically decreased the yield strength of the strip, ranging from 145Mpa to 180MPa. Hot rolling reduced the phosphorus content in the copper plates and cold rolling increased the phosphorus content. Phosphorus highly affects the electrical conductivity of copper, and the results also showed that an increase in phosphorus resulted in grain refinement of copper strip, and this improved mechanical properties.