Open-hearth steel combines the two requisites of (1) a very reliable steel made in large quantities, and (2) moderate cost of production. Steel made by this process is used for bridge material, ship plates and frames, axles, tires, springs, wire, steel castings and tools not requiring extremely hard-cutting edges.

Armor plate, common projectiles, gun forgings and boiler material are made of open-hearth steel from selected materials especially treated to insure a minimum of phosphorus in the steel.

High-carbon steel can be made by the open-hearth process, but there is difficulty in eliminating sulphur and phosphorus. Because of this condition, high-carbon steel is made mostly by the crucible process, in which the impurities can be carefully regulated.

The amount of phosphorus in iron ore determines whether the acid or the basic process must be used in making steel from the iron smelted from this ore. The basic process is the one used to reduce phosphorus down to safe limits, but there is always the risk that phosphorus may by some accident get back from the slag into the steel before tapping out.