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  Removal of impurities     6 of 11

In addition to high levels of oxygen, the crude steel tapped from the converter also contains impurities which are not desired in the final steel chemistry; the steelmaking process relies on raw materials from a number of sources such as iron ore, coke, limestone and recycled steel scrap. These materials contain impurities which when added to the steelmaking process inevitably result in low levels of elements not desired in the final steel chemistry being found in the steel.

Modern primary and secondary steelmaking practices can reduce the level of these impurities to a very low level, but usually a residual impurity level remains. One of the trace elements that is present during the steel making process and is generally undesirable is sulphur. The presence of free sulphur in a steel product is detrimental to its properties, especially toughness.

Modern steel making processes and selection of raw materials means that the levels of sulphur present in a steel are very low. Any remaining sulphur can be removed through the use of alloying additions of manganese which reacts with the sulphur to form MnS. MnS inclusions are relatively soft, especially at elevated temperature, and can become elongated during hot rolling, forging or drawing processing, (see types of inclusions).

The presence of elongated MnS inclusions in the rolled / forged / drawn steel can present a problem for the manufacturing / fabrication / finishing stage of steel processing. This is especially true for strip steels that are going to be deep drawn into drinks cans. The reason for this is that the deep drawing process occurs at room temperature where the MnS inclusions are no longer soft enough to deform. As the steel blank is drawn into the can shape the wall thickness of the can is reduced to ~6 m. MnS inclusions can be several micron in length and this can cause failure of the can through tearing. Any increase in can reject number will cost the manufacturer in terms of time retrieving the damaged can and revenue for the rejected can. This means that rejects from inclusions cannot be tolerated and extremely clean steels are required. This is achieved through precise control of the steel making stage of processing to ensure that there are virtually no MnS inclusions present in the steel destined to be strip steel used for making drinks cans. Also additions can be made to the steel which modify any MnS inclusions present to a composition which remains globular and does not deform to give long MnS stringers during steel rolling. Due to the increased costs associated with specifying such high levels of cleanness for this product grade of steel, other less stringent product grades are not specified to such a high level and MnS inclusions will be seen (although at a low level due to the general level of steel cleanness achievable today).


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