Basically, steel consists of iron and carbon, although it is the amount of carbon, as well as the level of impurities and alloying elements that determine the properties of each type of steel.
The carbon content of steel can range from 0.1 percent to 1.5 percent, but the most commonly used steel types contain between 0.1 to 0.25 percent carbon. Elements such as manganese, phosphorus and sulfur are found in all steel types, and while manganese produces beneficial effects, phosphorus and sulfur are harmful to the strength and durability of steel.
Different types of steel are produced according to the characteristics required for different uses, and different character systems are used to separate steel based on these characteristics.
According to the American Institute of Iron and Steel (AISI), steel can to a high degree be categorized into four groups based on their chemical compositions:
• Carbon steel
• Alloyed steel
• Stainless steel
• Tool steel
The table below shows the typical features of steel at room temperature (25°C). The wide areas of fracture strength, tensile strength and hardness are largely due to different heat treatment.
Carbon steel contains traces of alloying elements and accounts for 90 percent of total steel production. Carbon steel can be further categorized into three groups depending on the carbon content:
• Low carbon steel contains up to 0.3 percent carbon
• Medium carbon steel contains 0.3-0.6 percent carbon
• High carbon steel contains more than 0.6 percent carbon
Alloy steel contains alloying elements (such as manganese, silicon, nickel, titanium, copper, chromium and aluminum ) in varying proportions to manipulate the steel's properties, such as hardenability, corrosion resistance, strength, formability, weldability or ductility.
• Carbon is used to strengthen the steel, but at the same time this reduces toughness and weldability
Silicon or aluminum is used to prevent the melt from bubbling and forming pores in the steel
• Magnesium is used in the same way as carbon, but has less effect. However, it increases the hardness of the steel
• Niob is used to increase strength, malleability and ductility
• Vanadium in small quantities increases the strength of carbon steel
• Nickel, together with chromium, provides increased hardenability, higher impact strength and better fatigue abilities. If a lot of nickel is used, there is a risk of hydrogen cracks
• Chromium improves hardenability
• Molybdenum improves hardenability more than chromium and is often used with nickel to improve mechanical abilites
• Manganese increases strength, but is not as effective as carbon. It also reduces ductility and hardenability
Stainless steel* usually contains between 10 to 20 percent chromium as the main alloy element and is valued for its high corrosion resistance.
With over 11% chromium, stainless steel is about 200 times more corrosion resistant than low-alloy carbon steel. These steels can be divided into three groups based on their crystalline structure:
• Austenitic (304, 316, 6Mo, 904 etc): Austenitic steel is non-magnetic and non-heat treated, and usually contains 18 percent chromium, 8 percent nickel, and less than 0.8 percent carbon. Austenitic steel accounts for the largest share of the global stainless steel market
• Ferritic (22% Cr and 25% Cr Duplex): Ferritic steel contains traces of nickel, 12-17 percent chromium, less than 0.1 percent carbon, along with other alloying elements, such as molybdenum, aluminum or titanium. These magnetic steels cannot be hardened by heat treatment, but can be strengthened by cold forming
• Martensitic (13 Cr, 17-4PH etc): Martensitic steel contains 11 to 17 percent chromium, less than 0.4 percent nickel and up to 1.2 percent carbon. These are magnetic and heat treated
A type of stainless steel containing molybdenum in addition to chromium and nickel. The structure is austenitic and a further development of the 18-8 steel (see austenitic above). The acid-resistant variant of this contains 17-19% chromium, 8-13% nickel, 2-2.5% molybdenum and 2% manganese. Acid-resistant steels are particularly resistant in corrosive environments, such as harsh maritime environments, chemical and petrochemical industries.
*No steel types are rust-free, but some steel types are more rust-resistant than others
Tool steel contains tungsten, molybdenum, cobalt and vanadium in varying quantities to increase heat resistance and durability, making them ideal for cutting and drilling equipment.
Steel products by shape and use:
- Long and tubular products, such as shafts, rods, bars, rails, wire ropes, angles, tubes, shapes and sections
- Flat products include plates, sheets, coil and strips
• Other products include valves, fittings and flanges
Steel used for erecting buildings, warehouses, steel halls, sports halls, steel buildings and the like is most often carbon steel. Steel suitable for use as a structural material is most often defined as a weldable alloy of iron and certain (standardised) small amounts of other elements.
Structural steel is used for parts or structures, both with or without welding, where strength requirements are moderate. Low carbon weldable structural steel has a chemical composition that is easy to weld, e.g. without the need for preheating the material. Medium carbon machine steels have poorer weldability but can be heat treated to achieve a wider range of properties. In addition, they have better machinability than low carbon steels.
Structural steel is delivered in hot-rolled and straightened condition. However, larger dimensions can be normalized to improve swath toughness.
Corten is a type of special steel with an alloy that allows it to quickly get an outer film of rust . This is done with the intention that a further rust process will be delayed. It is also called stainless steel.
The steel is temperature resistant, and rusts to a lesser extent than other steels. In addition to hot areas, the material is used for facades, landscaping and works of art. In Norway, corten steel is often used for plant boxes, beds and garden edging.
