Steel is an alloy of iron with carbon being the main alloying element. Reinforcing Steel Bars (Rebars) conforming to European quality standards (EN 10080:2005) typically contain 0.22 (as a % of mass) of Carbon and traces of elements like Sulphur, Phosphorus, Nitrogen and Copper. Other elements may be added to steel to improve its resistance to corrosion without compromising its strength like Boron, Chromium, Molybdenum, Titanium etc. However these “special” steels are classed as alloying steels and are very rarely used in the form of Rebars.
The various element contents together with the manufacturing process will determine the final steel mechanical and physical properties. A higher Carbon content will increase the material’s axial strength however at the same time produce harder steel that is less ductile and weaker in bending. Hence the right balance between strength and ductility needs to be achieved to acquire a strong material that can withstand axial stresses of 500-650Mpa and at the same time be ductile enough to withstand bending forces like the ones produced in a building subjected to an earthquake for example.
The manufacturing process is equally important in determining the physical characteristics of steel. Modern production processes utilize techniques like tempering (heat treatment) and quenching (rapid cooling) to achieve steels with complicated internal structures that are both very strong and ductile. An example of these processes is TEMPCORE.
Reinforcing Steel Bars or Rebars is an important component of reinforced concrete. It comes in two forms; straight bars (typically 12m) 6-40mm dia. and coiled 6-16mm dia. It is given ridges to improve adhesion with concrete; once concrete cures and adheres around the steel bars, the two materials behave as one composite material. Concrete on its own is very weak in tension but very strong in compression, hence steel is added to strengthen it. The combination of the two works very well because the two compliment one another, they share similar thermal expansion properties, adhere very well with one another while at the same time concrete protects Rebar from fire and corrosion.
Most European national standards like BS 4449:2005 (Britain), CYS 302:2010 (Cyprus), ELOT 1421-3 (Greece) and DIN 488 (Germany) specify three categories of reinforcing steel the B500A, B500B and B500C with the latter being the most ductile and earthquake resistant. It is very important for engineers to be able to understand the differences of these steels when specifying for a new structure and for buyers to procure the correct quality that conforms to the requirements of the building.
There are two basic production routes for steel being utilized today by manufacturers, the Blast Furnace Route and the Electric Arc Furnace route (EAF). The former makes use of iron ore that has been processed by a sintering plant whereas the latter uses almost exclusively recycled steel also known in the industry as scrap steel.
Blast Furnace plants are larger than EAFs due to the process employed that requires higher volumes at a time to be cost efficient. Typically these plants will produce 1-4 million tonnes per annum.
The first part of the process involves heating the iron ore in the Blast Furnace using Coke (a fuel made from Coal with high Carbon content) to temperatures exceeding 1500°C in a process called reduction. During this process the Carbon from the Coke combines with oxygen found in the oxygen rich iron ore (iron oxide) to produce CO2, leaving the iron ore free from oxygen but still with quite a few impurities (AKA Pig Iron).
The hot metal is then transferred into a basic oxygen burner called a converter where pure oxygen is blown into the material in order to burn unwanted elements and impurities thus transforming the material into steel.
The final stage involves casting the steel into various shapes that can be used in later stages of production like ingots (to be re-melted), Blooms (used for long structural members), Billets (used to roll Rebar) and Plates (used to roll flat products). A Billet for example will be received by a rolling mill, heat-treated and rolled under controlled temperatures to produce Reinforcing Steel Bars. The chemical composition of the Billet will affect the final rolled product. More often than not the rolling mill is directly connected to the casting unit of the plant to produce finished goods in one continuous production line saving energy and time.
Electric Arc Furnace Route
The smaller sized EAFs are typically able to produce 0.5-1.5 million tonnes per annum by using almost exclusively recycled steel (scrap). These are powered by electric energy and hence release fewer pollutants into the surrounding environment.
A designed mix of various scrap steel qualities is added into the Furnace where it is heated to 1800°C by two graphite electrodes that are able to produce an electric arc that reaches 3500°C. The molten material is then transferred into a ladle furnace in order to fine-tune the final chemical composition by introducing various alloying elements.
EAFs are able to produce all kinds of steels and due to their lower capacity and the flexibility they provide makes them increasingly popular in Europe. Further process steps like casting and rolling are comparable to the Blast Furnace route.