Steel as an alloy
The term 'Steel' covers a wide range of ferrous alloys consisting of iron, carbon and other elements according to the required properties.
Most metals used in the building industry are alloys, which are combinations of elements, thus altering their individual properties and behaviour. The major alloys are iron and carbon, but the volume of the latter must be calculated precisely in order not to affect the microstructure of the material and even those elements which may be regarded as impurities, such as phosphorous, silicon and carbon, can be desirable as alloying elements in the right proportions.
The two major categories of structural and non-structural steels are defined by carbon content.
Structural steels are described as medium carbon steels with typically between 0.12% and 0.24% carbon. They include all the basic steels used for structural engineering, often in the form of rolled sections. Non-structural steels fall above and below this category.
Steels with less than 0.15% carbon are described as low carbon steels and are flexible enough for pressing into sheets for cladding and being drawn into wire. However such steels are relatively weak.
The strength and rigidity of such panels and sheets is therefore largely dependent upon the shapes pressed into them. These shapes are also designed to be stress-relieving, that is they have no right angle junctions. To improve stiffness they may be used as part of a composite panel system in which case adhesion technology can become an important factor.
Steels with between 0.5% and 1.5% carbon are described as high carbon steels and are used for casting as well as machining. Casting steels for special applications require careful quality control to check for small impurities, which require making good by welding.
Casting steels for special applications, for instance joint details for nodal connections between structural elements, need good quality control and during their manufacture should be checked (often by X-ray or ultrasonic equipment) in case there are inclusions - small impurities within the metal - which have to be made good by welding.
