Phosphorus in Steels

Phosphorus in Steels  Phosphorus (P) (atomic number 15 and atomic weight 30.974) has density of 1.82 gm/cc. It has a melting point of 44.1 deg C and boiling point of 280 deg C. The iron (Fe) – P phase diagram is shown in Fig 1. Fig 1 Fe- P phase diagram P is normally considered an undesirable impurity in steels. It is present in varying concentrations in iron ore, is retained in hot metal, but is eliminated early in the steelmaking process. P oxidizes readily and is removed from steel as P2O5, which is taken up by the oxidizing slag, before the oxidation of carbon takes place. Carryover of any P2O5 containing oxidizing slag can result in P reversion to the steel in subsequent steelmaking operations. In normal commercial steels, residual P content is usually  at a level of 0.05 % max, but concentrations as low as 0.005 % are not unusual. P is readily removed only in basic steelmaking processes. Acidic processes must therefore begin with low P raw materials. It was the ability to remove this element that led to the widespread adoption of the steelmaking by the basic open hearth, electric arc furnace, basic Bessemer converter and subsequently basic oxygen furnace (BOF) processes. P is sometimes added intentionally to the steel to improve strength, machinability and atmospheric corrosion resistance. P is added to the steel in the form of ferro-phosphorus (Fe-P), containing 23 % to 26 % P. Fe-P fines are usually briquetted, after using a binder. Fe-P is capable of oxidizing the residual silicon to silica, thus enabling it to float out to the ladle slag during steel making. The intent is to reduce the concentration of residual siliceous inclusions, which are detrimental to machinability. Fe-P is normally added to the...