page 13
Secondary steel making
Graphs of steel specifications with roll over highlights
Low levels of S and P in the metal are acceptable to the customer but the levels of C, Si and Mn must suit the customer's requirements (in this case, for tinplate).
Adjustments
Adjustments can be made to the composition of the steel after it has been tapped from the BOS furnace. Adding an alloy of C, Si and Mn to the molten metal increases the levels of those elements and brings the steel up to the customer’s specification. Changing the composition of the steel after tapping it from the furnace is called secondary steelmaking.
Part of graphic of ladel arc furnace with rollover highlightsPart of graphic of ladel arc furnace with rollover highlights
Part of graphic of ladel arc furnace with rollover highlights
Part of graphic of ladel arc furnace with rollover highlights
Part of graphic of ladel arc furnace with rollover highlightsPart of graphic of ladel arc furnace with rollover highlights
Removing gases
Some oxygen dissolves in the steel when the furnace is blown. Adding small amounts of aluminium will remove the oxygen as aluminium oxide. This solid forms a slag on the surface of the steel that is removed before the metal is cast.

Some steels require very low levels of impurity gases. The gases can be removed by vacuum degassing. This involves shaking the molten metal at very low pressures, which allows dissolved impurity gases, such as oxygen, to escape from the molten metal.

Making it uniform
Next, the metal is stirred by blowing it with argon gas. This ensures that the composition of the metal is uniform. Stirring also helps to even out the temperature throughout the liquid metal. The temperature is a critical factor when the metal is being cast. The ladle arc furnace in the picture can be used to adjust the temperature of the metal before casting.

By the time that secondary steelmaking is complete, the metal should be within specification. In the bar chart at the top of the page, you can see how the composition of the steel that is ready for casting compares with the metal that the customer ordered.

Question 13

(a) Why do you think that argon gas is chosen for stirring the molten metal?

(b) Suggest how the metal in the ladle arc furnace is heated.