Steel is the most adaptable and important material in our industrial society. This is a material consisting mainly of iron and containing less than 2% carbon. In addition, there are alloying materials that steel often contains unintentionally or which can be added deliberately for specific steel applications.
Non-metallic elements often contained in steel include silicon, phosphorus or sulfur. Metallic elements found in many steel grades are often manganese, chromium, nickel or molybdenum.
Steel was produced and industrially used as early as the 19th century. The advantages of the material became more and more obvious starting no later than 1900, causing the amount of world crude steel production to increase steadily. In the 20th century, it shaped the entire industrial society. And the success continues: since the 2010s, 1,600 million tons of steel or more have been produced worldwide every year – and the trend is still rising.
Due to its versatility, steel is now used in almost all major industrial sectors. Typical properties of steel are:
- High and high-strength,
- Hot and cold formable,
- weldable,
- Hard, tough and wear-resistant,
- Corrosion-resistant,
- High- and high temperature resistant
Iron has a purity of 99.8% to 99.9%. Although steel consists mainly of this element, the most important part is carbon. Its share has a crucial impact on the mechanical and technological properties of the material. Steel is therefore an alloy, or a mixed product, which is mostly made of iron, a small proportion of carbon and other non-metallic or metallic elements.
In order to produce steel or pig iron, you need an iron carrier, i.e. iron ore or scrap, fuels and reducing agents such as coke, coal, oil, gas and certain supplements, such as lime or corresponding alloying agents.
Pure iron does not occur in nature. Only chemical compounds, so-called iron ore, can be extracted and used for the production of steel. The most common compounds are iron oxides, i.e. iron with oxygen and other impurities. These are mainly present in Australia, Brazil and Russia – and are being mined there.
The starting material for steel is iron. In order to obtain pure iron from natural iron ore, the chemical compound must be dissolved and the oxygen separated from the ore by means of reduction with all other impurities. This requires a reducing agent, such as coke, which has a stronger bond to oxygen than the iron itself.
The pig iron is then heated to 1,536°C. At this temperature, the material melts. Carbon and alloying elements are then added for the production of steel. The speed of the subsequent cooling of the steel is decisive for the extent of the carbon diffusion, which has a direct effect on the specific steel hardness.
Steel is produced in a blast furnace. Hot air is blown into the furnace so that the pure oxygen is supplied with the carbon or coke. This reaction is highly exothermic and releases enormous energy – the blast furnace heats up “itself” through chemical reactions.
Without going into detail, we do a small, three-part chemical excursion:
- Oxygen and coke react to carbon dioxide gas (heat release)
C + O2 -> CO2
- Carbon dioxide reacts at high temperatures with coke Carbon monoxide (heat release)
CO2 + C -> 2 CO
- Carbon monoxide absorbs the oxygen of the ores and pig iron is produced
Fe3O4 + 4 CO -> 3 Fe + 4 CO2
Classification criteria are chemical composition, requirements for use, manufacturing process, intended use or mechanical properties. These criteria give rise to the different types of steel.
Alloyed and unalloyed steels are used in accordance with specified limits for the mass fractions of the alloying elements differences. While alloyed steels exceed the limit of at least one element, unalloyed steels do not meet any of these specified mass limits.
Stainless steels are alloy steel grades with a carbon content not greater than 1.2%. In addition, it includes at least 10.5% chromium. This proportion ensures that the steel cannot be attacked by surrounding media and, among other things, does not develop rust.
The chromium content forms something like an invisible protective cover. This so-called passive layer forms spontaneously in air or in an liquid medium and can adapt to the environment. After a mechanical surface injury, it generally regenerates on its own – so it is self-healing.
Quality steel is a collective term for steels that meet specific requirements. These can be requirements like fracture toughness, grain size or formability that can be placed on the steel. Quality steels can be alloyed as well as unalloyed steels.
High-grade steels have a higher degree of iron purity than quality steels. In addition, the chemical composition and careful production have improved its properties. By the way: A high-grade steel does not necessarily have to meet the requirements of a stainless steel. Nevertheless, in everyday life, only high-grade steels are usually referred to as stainless steels. The alloy content of low- or high-alloy stainless steel is precisely defined.
Steel develops various types of structures through controlled heating and cooling, which manifest themselves through a corresponding hardness state. The decisive factor for this is carbon.
The most important types of structures are:
- Ferrite
- Perlite
- Bainit
- Martensite
- Austenite
- Cementite
Martensite is the hardest structural component in steels. This hardness is caused by the particularly solid solution of carbon, which develops through rapid temperature changes in the manufacturing process. Martensitic steels are also called knife steels. Due to their special hardness and their stainless properties, they are ideally suited as material for edge tools, such as cutlery or for the production of surgical instruments.
Austenitic steels are characterized by their high resistance to aggressive environmental conditions – they are corrosion-resistant. Due to the nickel content of more than 8%, they achieve good toughness. Austenitic steels can be deformed even at comparatively low temperatures. They are often used for the production of punching tools and for medical applications.
Ferritic steel is one of the stainless steels along with the martensitic and austenitic steels. Due to its high chromium content – an average of 12-18 % – ferritic steel is more corrosion-resistant than martensitic steel but has, however, a lower hardness. Ferritic steel is characterized by its magnetic properties and, like martensitic and austenitic steels, is used in the knife industry or chemical industry as soon as a magnetic effect is desired.
Damask steel is a multi-layer steel. It consists of several layers of steel laid on top of each other and welded together. Often a long piece of steel is folded and forged again and again, until finally a compact piece of steel consisting of many layers is created. This procedure eliminates impurities. In addition, the steel is enriched with carbon by welding and can be
harden better. Damask steel is characterized by enormous hardness and unique structures.
In contrast to the mono-steel, where the whole blade is made of the same material, multi-layer steel blades have a layered structure. The composite of the different layers is made during forging by the so-called fire welding. A hard core of high-carbon, hardable Steel ensures sharpness and cutting accuracy. For protection, this middle layer is surrounded by a sheath of softer iron or non-curable steel, which is forged in one or more layers. It gives the blade breaking strength.
Takefu is a place in Japan and is known worldwide for its blacksmiths. Takefu Special Steel Co. Ltd. stands for first-class multilayer steel and has made a name for itself in the knife industry. Takefu knives offer first-class quality. The typical, unique patterns that the blades receive through the numerous layers of steel are very appreciated by lovers of high-quality knife art – all over the world.
In general, metal powder is a fine powder of a metal that usually burns very well. Metal powders are usually obtained by grinding, i.e. mechanically. However, they can also be obtained by chemical, physical or electrolytic means.
Steel creates prospects for the future: industrial 3D printing is an additive manufacturing process, that is suitable f.e. for functional prototypes, complex constructions, the reduction of multi-part assemblies and end uses. Laser melting systems are used to manufacture complex metallic components. These are also called metal 3D printing and melt metal powder in order to “print” metallic shapes and designs from them.
Steels are divided into grades according to their degree of desired and intended quality. Steel grades therefore provide information on the quality of the steel.
Coils are treated as units in the steel industry. In German, the term is partly used as a technical term for strip steel rollers and steel wire rollers used as raw product. Strip steel rollers are a common form for the transport of wide-flat product of metals and alloys between manufacturers and the processing industry.
According to DIN EN 10027-1, there are so-called short names, which are divided into two main groups.
In the main group 1, short names are which statements about the use and the mechanical/physical properties of the steels.
The main group 2 says something about the chemical composition of the steels.
A short name consists of a main symbol and additional symbols, which are used when the main symbol alone is not sufficient to identify a steel.
Steels of the main group 1 are assigned to their use by the main symbol and then by additional symbols in terms of their mechanical and physical properties.
Main group 2 steels are divided into four groups in the main symbol with respect to their contained elements. The additional symbols provide additional information, for example regarding their suitability or the condition of treatment.
Steel can be used by means of various processing and treatment processes. Roughly summarized they are:
- casting forms
- forming
- cutting
- joining
- coating
- heat treatment
“Casting forms” mean that a solid body is formed from a previously shapeless material. This body is in the Steel industry the archetype of steel.
During forming, one existing shape is deliberately transformed into another. This happens when an external force is so large that the atoms of the material shift. Examples include the forging or rolling of steel.
When separating, the cohesion of the structure is removed locally and the steel is cut through. This is mainly done by cutting or shaving.
Joining creates or increases the cohesion between separate workpieces locally. The connection can be of a fixed or movable nature. If steel is added, two or more workpieces are brought together. This can be achieved by simple screw connections, but also by welding, gluing or mechanical joining.
In addition to iron and aluminium, cars are mostly made of steel. The reason for this is above all the hardness and the many different technical properties of the material. In addition, steel is a pioneer in terms of price-performance ratio and can be easily recyclable.
DIN EN ISO 9000 is a conceptual standard in which quality is defined. Quality means that a material meets certain requirements. The international standard DIN EN ISO 9001 is a Quality management system for companies to meet complex requirements. DIN EN ISO 9001: 2015 is therefore the revised quality management system from 2015.
Of course, CO2 emissions in the production of steel are unavoidable. Nevertheless, steel is far from a climate killer, as the material is indispensable for the production of some of the largest CO2 savings such as wind turbines, geothermal energy or the weight reduction of cars. Taken as a whole, the CO2 balance of steel is therefore positive.
The steel industry is facing the challenge of reducing CO2 emissions in steel production to the minimum. For example, solar power is mainly used to save CO2 in energy terms.
So, if we look at CO2 emissions in steel production, the CO2 savings and the contribution of the steel industry to climate protection, innovative steels were already able to save six times as much CO2 as their production causes since 2007. Steel in times of climate protection? YES, PLEASE!
