Immersion Pyrometry and Open Hearth Furnace Productivity FASTER STEELMAKING AT RAVENSCRAIG

It is nowadays generally accepted that controlling the tapping temperature of steel by immersion pyrometry is an important means of improving steel quality. Broadly speaking, if the steel is too hot when tapped the ingots tend to crack, and if too cool they tend to suffer from “shell” on the surface and non-metallic inclusions beneath. From this point of view it is now simply a question of establishing the optimum tapping temperature for each quality of steel and for each ingot size, and ensuring that this is regularly attained. This temperature is, of course, closely linked with teeming speed in determining ingot quality. For instance at Ravenscraig, with 250-ton open-hearth furnaces, the same type of steel gives very similar overall results at the rolling mill either by tapping at 1590°C and teeming an &ton ingot through a large nozzle in 18 minutes, or by tapping at 1605°C and teeming this ingot through a small nozzle in 64 minutes. Intermediate teeming speeds give worse results than either of these. Once the optimum temperature for each quality is established, the aim is to ensure that the last temperature in each charge is taken within ten minutes of tapping, and that it is within 5°C of the specified figure. Whereas the effect of tapping temperature on steel quality is widely recognised, and sufficient temperatures are usually taken to ensure the correct tapping temperature, the value of taking immersion temperatures during the melting and early refining period is less generally realised.