2.2.4. Non-Linearity

The non-linearity of the EAF process with respect to EE is caused by numerous factors. One of these factors is related to the various sub-processes of the EAF as well as the resulting total process time. For example, the heat loss through radiation and cooling losses are more prevalent during the refining stage, when the steel is molten, compared to the first charging and melting phases. Furthermore, the radiative heat transfer is proportional to *T*4, which adds to the non-linearity. By varying the refining times and melting times, the resulting contribution to the EE consumption can vary significantly between heats.

Delays imposed by up- and downstream processes as well as by the process itself add to the non-linearity of the process with respect to the EE consumption. The reason is because the process time is no longer a result of the total charged scrap weight or produced steel grade. Furthermore, the delays are very hard to predict and should therefore be considered to be an external non-linear factor imposed on the EAF at all times. The effects of delays have been extensively discussed in [1].

The charged scrap types also add to the non-linearity of the process since combining varying amounts of each scrap type will affect the melting behavior. For example, large and bulky pieces of steel scrap must be exposed to the steel melt for longer times than thinner pieces of steel scrap.

#### *Processes* **2020**, *8*, 1044

Larger fractions of heavy scrap increase the melting time, which in turn increases the EE consumption. The general strategy is to charge bulkier scrap types in the lower layers of the scrap bucket. This is regardless of the amount of bulky scrap in the heat. Failing to expose bulkier scrap to the hot heel and molten steel means that partially molten pieces of scrap will remain during the latter stages of the refining process. Hence, the melting time becomes the factor that determines the TTT of the heat and hence also the EE consumption.
