Steel is an alloy of iron and other elements. Some elements are intentionally added to iron for the purpose of attaining certain specific properties and characteristics. Other elements are present incidentally and cannot be easily removed. Such elements are referred to as “trace” or “residual” elements.
Many product specifications have mandatory requirements for reporting certain elements and these vary. Most mills routinely provide heat analysis which includes the elements below. Although it is possible to analyze for other elements this is most often not practical or necessary unless they are additions (e.g. Pb – Lead, Sb – Antimony or Co - Cobalt).
C – Carbon
Mn – Manganese
P – Phosphorous
S – Sulphur
Si – Silicon
Cu – Copper
Ni – Nickel
Cr – Chromium
Mo – Molybdenum
V – Vanadium
Cb (Nb) – Columbium (Niobium)
Ti – Titanium
Al – Aluminum
N – Nitrogen
B - Boron
Sn - Tin
Ca – Calcium
There are thousands of steel alloys and their categorization is complex and varies by governing body. Most, however, can be broadly grouped into Plain Carbon Steel, Ultra Low Carbon (ULC) Steel, High Strength Low Alloy (HSLA) Steel, Alloy Steel, High Alloy Steel (including Stainless Steel and Tool Steel) and Electrical Steel. Advanced High Strength Steel (AHSS) is the newest classification of steels.
Alloying elements often serve different purposes in different steels. For example, Manganese contributes to steel’s strength and hardness in the as rolled condition but another important characteristic is its ability to increase hardenability which is critical in heat treating.
The effect of alloying elements on steel properties is a huge subject. The following is a very cursory summary of the influence of the above elements in common flat rolled products. More information may be found on the websites of governing bodies and materials information societies such as ASM International.
Carbon is the principal hardening element in steel. Hardness and strength increase proportionally as Carbon content is increased up to about 0.85%. Carbon has a negative effect on ductility, weldability and toughness. Carbon range in ULC Steel is usually 0.002 – 0.007%. The minimum level of Carbon in Plain Carbon Steel and HSLA is 0.02%. Plain Carbon Steel grades go up to 0.95%, HSLA Steels to 0.13%.
Manganese is present in all commercial steels as an addition and contributes significantly to steel’s strength and hardness in much the same manner but to a lesser degree than carbon. Manganese improves cold temperature impact toughness. Increasing the Manganese content decreases ductility and weldability. The typical Manganese content is 0.20 – 2.00%.
Phosphorus is most often a residual but it can be an addition. As an addition it increases hardness and tensile strength. It is detrimental to ductility, weldability and toughness. Phosphorus is also used in re-phosphorized high strength steel for automotive body panels. Typical amounts as a residual are less than 0.020%.
Sulphur is present in raw materials used in iron making. The steelmaking process is designed to remove it as it is almost always a detrimental impurity. A typical amount in commercial steel is 0.012%, and 0.005% in formable HSLA.
Silicon can be an addition or a residual. As an addition it has the effect of increasing strength but to a lesser extent than Manganese. A typical minimum addition is 0.10%. For post galvanizing applications the desired residual maximum is 0.04%.
Copper, Nickel, Chromium (Chrome), Molybdenum (Moly) and Tin are the most commonly found residuals in steel. The amount in which they are present is controlled by scrap management in the steelmaking process. Typically the specified maximum residual quantities are 0.20%, 0.20%, 0.15% and 0.06% respectively for Copper Nickel, Chromium and Molybdenum but the acceptable limits depend mainly on product requirements. Copper, Nickel, Chromium and Molybdenum, when they are additions, have very specific enhancing effects on steel. A Tin residual maximum is not usually specified but its content in steel is normally kept to 0.03% or less due to its detrimental characteristics.
Vanadium, Columbium and Titanium are strengthening elements that are added to steel singly or in combination. In very small quantities they can have a very significant effect hence they are termed micro-alloys. Typical amounts are 0.01 to 0.10%. In Ultra Low Carbon Steel Titanium and Columbium are added as “stabilizing” agents (meaning that they combine with the Carbon and Nitrogen remaining in the liquid steel after vacuum degassing). The end result is superior formability and surface quality.
Aluminum is used primarily as a deoxidizing agent in steelmaking, combining with oxygen in the steel to form aluminum oxides which can float out in the slag. Typically 0.01% is considered the minimum required for “Aluminum killed steel”. Aluminum acts as a grain refiner during hot rolling by combining with Nitrogen to produce aluminum-nitride precipitates. In downstream processing aluminum-nitride precipitates can be controlled to affect coil properties.
Nitrogen can enter steel as an impurity or as an intentional addition. Typically the residual levels are below 0.0100 (100 ppm).
Boron is most commonly added to steel to increase its hardenability but in low carbon steels it can be added to tie up Nitrogen and help reduce the Yield Point Elongation thus minimizing coil breaks. At the same time, when processed appropriately, the product will have excellent formability. For this purpose it is added in amounts up to approximately 0.009%. As a residual in steel it is usually less than 0.0005%.
Calcium is added to steel for sulphide shape control in order to enhance formability (it combines with Sulphur to form round inclusions). It is commonly used in HSLA steels especially at the higher strength levels. A typical addition is 0.003%.