Understanding Yield Loss in Iron and Steel Manufacturing

Introduction

Yield performance is an important issue for any steel plant manager, as poor performance can negatively affect both steel plant output and production costs. In this article, the author discusses the issue of performance loss and explains its importance.

What are performance losses?

A yield loss arises in any part of the iron and steel manufacturing process when, for a particular step, the weight of the steel product manufactured is less than the steel input that is used. In a cold rolling mill, for example, if the input, often referred to as charge weight, is 100 tons of hot-rolled coil and the output is 95 tons of cold-rolled steel, the throughput yield is 95%. Another way to express the yield factor is in terms of tons per ton, in this case 100 divided by 95 or 1,053. Typical yield loss values ​​are:

• in the production of hot-rolled steel coils (plates to hot-rolled coils), 3.5%
• in the production of rebar (from billet to bar), 5%
• in the manufacture of seamless tubes (billet to green tube), 8.5%.

Why do performance losses arise?

Yield losses arise for different reasons.

• For example, during the rolling process, there may be a break during which the steel bar falls out of its normal rolling path and lands (often at high speed) in an area adjacent to the rolling stands. This length of steel bar would no longer be rolled; rather, it would be recycled as scrap, reducing the throughput performance achieved on that particular rotating shift
• Scale will often form in a hot slab as it cools. This scale will later be removed from the surface as the slab is prepared for a later process step.
• When a steel coil is produced, the edge is often trimmed in the final product, resulting in little loss of yield.
• When billet or slab is cast, the end is often cut off. Irregularities that arise during the casting process are also often removed.

Each of the above examples reduces the final weight of the fabricated steel product, compared to the original load weight of the steel.

Why are yield losses important?

Yield losses are important for several reasons.

• First, it is important to understand yields in order to predict the output of a mill. For example, a mini-mill producing 1 million tonnes of crude steel can only produce 850,000 tonnes of finished steel if the cumulative yields from melting, hot rolling and cold rolling of steel amount to 15%.
• Second, yields are also especially important from a cost standpoint. This is because, while scrap steel can generally be recycled, all of the labor and energy costs associated with processing the scrap steel volumes are lost; and added value (in terms of final sale price) is not achieved. For some processes, the cost of yield loss may exceed some of the other conversion cost elements (such as the cost of consumables, electricity, or other services). Yield performance management is therefore an important aspect of overall steel mill cost control.

Understand your own performance performance

A common way to understand the throughput performance of a particular steel plant is to compare its recent throughput performance to that of similar plants. Such plants will typically be owned by competitors and may often be located in different countries. However, technical visits to plants are quite common in the steel industry; and are carried out specifically so that the learning benefits the industry.