Yield performance is an important issue for any steel plant manager, since poor yield performance can adversely affect both steel plant output and production costs. In this article, the author discusses the topic of yield losses and elaborates on their importance.
What are yield losses?
A yield loss arises in any parts of the iron and steel making process, when for a particular step, the weight of the steel product made is less than the input of steel that is used. In a cold rolling mill for example, if the input – often called a charge weight – is 100 tonnes of hot rolled coil, and the output is 95 tonnes of cold-rolled steel, the yield performance equals 95%. Another way of expressing the yield factor is in terms of tonnes per tonne – in this case 100 divided by 95 or 1.053. Typical yield loss values are:
- in hot rolled steel coil production (slab to hot rolled coil), 3.5%
- in rebar production (for billet to bar), 5%
- in seamless tube making (billet to green pipe), 8.5%.
Why do the yield losses arise?
The yield losses arise for different reasons.
- For example, during the rolling process, there may be a break-out during which the steel bar comes out of its usual rolling trajectory, landing (often at speed) in an area adjacent to the rolling mill stands. This length of steel bar would not be further rolled; rather, it would be recycled as scrap so lowering the yield performance achieved on that particular rolling shift
- Scale will often be formed on a hot slab as it cools. This scale will later be removed from the surface as the slab is prepared for a subsequent process step
- When a steel coil is produced, the edge will often be trimmed on the final product, giving rise to a small yield loss
- When billet or slab is cast, the end is often cut. Irregularities arising during the casting process are often also ground away.
Each of the examples above reduces the final weight of steel product made, compared to the original charge weight of the steel.
Why are the yield losses important?
Yield losses are important for several reasons.
- First, it is important to understand yields in order to be able to predict the output of a mill. A mini mill making one million tonnes of crude steel may for example only be able to produce 850,000 tonnes of finished steel, if the cumulative yields from casting, hot rolling and cold rolling of the steel amount to 15%
- Second, yields are especially important also from the cost standpoint. This is because, whilst the scrap steel can generally be recycled, all the labour and energy costs associated with processing the wasted steel volumes are lost; and the value added (in terms of a final selling price) is not achieved. For some processes, the cost of the yield loss can actually exceed some of the other elements of conversion cost (such as the cost of consumables, electricity or other utilities). Management of yield performance therefore is an important aspect of overall steel mill cost control.
Understanding your own yield performance
A common way of understanding the yield performance of a particular steel plant is to compare its recent yield performance against that of similar plants. Typically, such plants will belong to competitors and may often be located in different countries. Technical visits to mills are however quite common in the steel industry; and are undertaken specifically so that learning benefits the industry.