The chemical composition of steel is reported on all material test certificates to EN10204. This article briefly covers the effect that each element has on the steel and why it is important or really not wanted. The focus is very much of carbon steel to EN10025, EN10028 and EN10225 – that is steels for structural engineering, boiler and pressure vessels and offshore use.
Chemical Composition of Steel – Elements in the Periodic Table
Carbon – C
Carbon is added to iron to make steel. In its pure form iron is quite soft and adding up to 2% carbon gives it toughness and strength. Structural steels plates typically contain about 0.15 to 0.3% Carbon. As the amount of carbon increases in the steel the strength increases but the ductility decreases. So iron with a lot of carbon added to it becomes very brittle and is unable to respond elastically to dynamic loading.
Silicon – Si
Silicon is added to carbon steels to help deoxidise, or kill, them. That is the silicon helps to remove bubbles of oxygen from the molten steel. It is also useful in increasing strength and hardness but is less effective than Manganese in doing so. Negatively for many uses it also increases grain size so there is usually an upper limit on it.
Manganese – Mn
Manganese is probably the second most important alloying element after carbon on steel. Like Carbon has a large impact on strength, ductility and hardenability. Manganese helps to reduce oxides and also counteract the presence of Iron Sulphide. Steelmakers however had to be careful that the level of Carbon and Manganese doesn’t get too high or the steel becomes too brittle and decreases weldability
Phosphorous – P
In structural steel Phosphorous is normally considered to be an unwanted residual element. This is because most applications require very low or low phosphorus requirements. Phosphorus increases steel embrittlement which reduces the toughness and ductility of the metal. In uses this generally appears as cracks and fractures. High phosphorus in steel is a contributing factor to HIC cracking in wet H2S environments.
Sulphur – S
Sulphur is another residual element in structural and pressure vessel steels. Sulphur decreases notch impact toughness, reduces weldability and decreases ductility. It generally appears as sulphide inclusions in the steel which decreases its strength.
Nitrogen – N
Nitrogen is a residual element for hot rolled steel plates. Generally high levels of nitrogen will give the plate inconsistent mechanical properties and make welding more difficult by increasing embrittlement in the heat affected zone (HAZ).
Copper – C
In structural steels copper is primarily used as an alloying element as it will improve atmospheric corrosion resistance and help paint bond the the steel. It also has a small impact on hardenability.
Niobium – Nb
Niobium is a key grain refining element in steel production. That is because it makes the grain size smaller it simultaneously improves strength, toughness and ductility.
Vanadium – V
Vanadium when added in the steelmaking process helps to remove oxides and thus increases the yield strength and tensile strength of steel plates
Titanium – Ti
Titanium in steel helps to keep grain size small and also helps manage inclusions by making them rounder.
Chromium – Cr
Chromium as an alloying element in steel helps to increase its corrosion and oxidation resistant properties. When the % of chromium in the steel exceeds 1.1% a surface layer is formed that helps protect the steel against oxidation
Nickel – Ni
Nickel is used to improve the steels corrosion resistance properties. It is a key component in stainless steels but at the low concentrations found in carbon steels it helps to increase impact strength and hardenability.
Molybdenum – Mo
Molybdenum is used to increase the strength of boiler and pressure vessel steels at typical boiler operating temperatures of 400°C. Typically it is used in conjunction with Chromium to provide strength and corrosion resistance at high temperature as well as increased creep strength.
Boron – B
Boron is added to fully killed fine grained steel to increase hardenability. This gives a benefit to the yield strength and toughness if the steel is fully hardened before tempering.
Zirconium – Zr
Zirconium is added to steel to modify the shape of inclusions. It helps them to become rounder (as opposed to elongated). The result is that toughness and ductility are improved when the plate is fabricated into a shell.
Impact of Chemical Composition Of Steel on its Price
We had a bit of fun and took the chemical composition of some common steel grades and then looked up how much the elements going in cost. We used Wikipedia for this and to be honest the prices of elements were all over the place – covering retail purchase of Sulphur to bulk purchase of Copper. Some things were too expensive and some too cheap. And the prices came from a period between 2005 and 2012…. And of course some were metal oxides rather than the raw element. Still this table should give you a small idea of how the cost of different steel is determined by the chemical composition of the steel