What's 'Good Quality' Steel and How Do You Achieve it?Shawn Martin | April 18, 2018
Production of high-quality steel begins with recycling. In fact, annually there are nearly 80 million tons of steel being recycled in North America alone and according to the Steel Recycling Institute (SRI) it is the world's most recycled material.
There are two basic processes used to produce steel: one involves the use of a basic oxygen furnace (BOF) and the other an electric arc furnace (EAF).
Feedstocks used to produce steel in a BOF include molten pig iron and scrap steel. Pig iron is produced from iron ore in a blast furnace and contains approximately 92 to 94 percent iron and 3 to 5 percent carbon. The BOF method requires about 70 percent pig iron while scrap steel is added to lower production costs and decrease the carbon footprint. A flux is also added to remove impurities.
The BOF doesn’t require a traditional fuel source, but rather a lance is used to feed pure oxygen. Oxygen reacts with carbon in the molten pool raising the furnace temperature to 1,700 degrees Celsius. Impurities are oxidized and the carbon content is reduced. It produces a liquid steel that can be further refined through secondary steel-making processes.
An EAF is not charged with pig iron. A small amount of pig iron or direct reduced iron (DRI), may be used to balance the chemistry while scrap steel is the predominant feedstock. Approximately 35 million watts of electricity are used to produce an arc across two electrodes, melting the scrap steel and reaching temperatures as high as 1,600 degrees Celsius. Like the BOF method, fluxes are added to remove impurities.
DRI, like pig iron, is produced from an iron ore, but rather than being mixed with coke or coal in a blast furnace it is reduced at lower temperatures in the presence of hydrocarbon-rich gases.
The costs to produce DRI is decreasing as newer technologies like Tenova HYL and Danieli & C’s ENERGIRON reduction technology come to market. Their unit is highly flexible. It accommodates a diverse range of reducing gas sources and iron ores and yields a valuable feedstock at lower production costs. The chemical composition of the DRI produced is also well controlled with carbon content reduced down to 2 to 5 percent.
Factors Affecting Quality
In order to produce high-quality steel, it is imperative that scrap metal is well sorted. It is also imperative that any coating is removed before entering the furnace as these coatings typically contain tramp metals, which cannot be removed by pyro-metallurgical processes in a high-temperature furnace.
Tramp elements include copper, tin, antimony and lead. When introduced into a BAF or EAF, they become dissolved in the steel melt. They have a low affinity for oxygen. Whereas an oxygen lance is used to blow out carbon and oxidize silicon, manganese and aluminum that may be present in other feedstocks, tramp elements remain in the steel melt pool and contaminate the liquid steel.
Scrap purification processes used to remove tramp elements including electrolytic reactions, low-temperature alkaline reactions, selective vaporization and other chemical reactions as physical treatments are, for the most part, ineffective.
Alongside scrap purification, accurate identification of scrap alloys is equally as important. The premium paid for steel scrap varies dramatically and is based on material composition. Above costs, however, industries rely on accurate identification as certain alloying elements need to be present in the melt when producing a given alloy.
To improve sorting methods and/or increase quality control X-ray fluorescent (XRF), analyzers like Thermo Scientific’s Niton™ portable XRF analyzers are used to accurately verify trace elements present. The non-destructive testing technique offers fast response times and does not require sample preparation.
As scrap steel plays a dominant role in today’s steel-making processes, scrap purification and sorting are the two largest factors affecting liquid steel quality. As EAF methods increase market share, this trend will continue even while it is accompanied by a growing market for DRI.
To produce a high-quality steel, feedstocks and processes need to be optimized and whether you are charging your furnace with pig iron or DRI, the accompanying scrap will help reduce costs and decrease the carbon footprint so long as it is well sorted and pre-treated.