Understanding such an issue as the classification of carbon steels is very important, as this allows you to get a complete understanding of the characteristics of one or another type of this popular material. The marking of such steels, like any other, is no less important, and a specialist must be able to understand it in order to choose the right alloy in accordance with its properties and chemical composition.
A huge range of rolled metal products are produced from carbon steels
What are carbon steels?
Carbon steels are an iron alloy with a carbon content of up to 0.6%. The amount of sulfur and phosphorus depends on the quality of the metal. Alloying elements are present in small quantities. Qualitative characteristics depend on the amount of carbon, sulfur, phosphorus, manganese and silicon.
- hardness;
- weldability;
- strength;
- viscosity;
- elasticity.
The more carbon, the higher the hardness, brittleness and worse weldability.
Heat-resistant steel Inox
For the production of its products, TMF uses heat-resistant high-alloy stainless steel Inox with a chromium content of 13% and a temperature for the onset of scale formation from 750°C to 900°C, which is confirmed by certificates from manufacturers.
The use of heat-resistant Inox chromium steel made it possible to reduce the wall thickness to 2 mm and eliminate the outdated contradiction: the thinner the walls of the furnace, the better it heats, but the faster it will burn out. The service life of a stove made of thin heat-resistant steel is no less, if not longer, than the service life of a stove made of thick ordinary steel. At the same time, the mass of a furnace made of heat-resistant steel is 2 times less, and the “rate of fire” is 2 times higher. The walls of the stove quickly heat up and just as quickly begin to warm up the air of the steam room, the stones and adjacent rooms of the bathhouse through the open doors of the steam room.
In addition, heat-resistant steel, due to the oxide film formed, does not enter into an oxidative reaction with atmospheric oxygen, preventing it from “burning out.”
General characteristics
Carbon steel is divided into 3 groups according to the requirements for chemical composition and mechanical properties. Letter designation. The defining factors for the group are:
- A - mechanical properties;
- B - chemical composition;
- B - the chemical composition and basic mechanical properties are strictly maintained.
Group B alloys are chemically tested, and a sample is taken from the ladle during a spill to check mechanical properties - tensile and flexural strength, impact strength. Hardness is controlled by heat treatment.
Application area
As mentioned above, carbon steel alloys according to their main purpose are divided into two large categories: instrumental and structural. Tool steel alloys containing 0.65–1.32% carbon are used in full accordance with their name - for the production of tools for various purposes. In order to improve the mechanical properties of tools, they turn to such a technological operation as hardening carbon steel, which is performed without any particular difficulties.
Areas of application of carbon tool steels
Structural steel alloys are used very widely in modern industry. They are used to make parts for equipment for various purposes, structural elements for mechanical engineering and construction purposes, fasteners and much more. In particular, such a popular product as carbon wire is made from structural steel.
Carbon wire is used not only for domestic purposes, for the production of fasteners and in the construction industry, but also for the manufacture of such critical parts as springs. After carburization, structural carbon alloys can be successfully used for the production of parts that, during operation, are subject to severe surface wear and experience significant dynamic loads.
Of course, carbon steel alloys do not have many of the properties of alloy steels (in particular, stainless steel), but their characteristics are quite sufficient to ensure the quality and reliability of the parts and structures that are made from them.
Composition of chemical elements
The main element is iron. The relationship to the group is determined by the amount of carbon. The content of non-metallic inclusions of phosphorus and sulfur impairs the mechanical properties. They contribute to red brittleness and cold brittleness, the formation of cracks in hot and cold metal.
Corrosion resistance is ensured by low carbon content and the addition of chromium. The amount of chemical elements in carbon steel, manganese and silicon, depends on the deoxidation method and quality class. Manganese can be present within 1.2% in normal quality alloys, up to 1.8% in high quality alloys. The silicon content does not exceed 0.3%.
High-quality steels of group B are checked for their properties and chemical composition. The permissible amount of non-metallic inclusions is 0.03–0.0018%.
The amount of carbon determines the hardness of steel, its ability to harden and weld.
The lower the carbon index, the better the metal is welded. St 40X requires heating before welding, St 6 requires heating to 700⁰ and post-welding tempering. Hardenability is the opposite. Up to St4 alloys do not heat up and do not change their hardness. 40x steel can crack when exposed to sudden cooling in water.
Heating steel
Other classification parameters
Classification of carbon steels is possible according to the level of purification from harmful impurities. The following groups of alloys are distinguished:
- ordinary quality (B);
- qualitative (B);
- increased quality (A).
Category B includes steels that meet certain mechanical characteristics. They are more affordable and are not subject to pressure or heat treatment. Certificates of categories A and B can be subjected to various deformations, and for them the manufacturer prescribes the composition and all the properties.
There is a classification according to the scope of application:
- structural – used for the manufacture of products for various purposes;
- instrumental - used for the manufacture of various tools.
In the marking of carbon steel, the designations “sp”, “ps” and “kp” are found. They indicate the degree of its oxidation and are another classification of alloys:
- “sp” – quiet alloys containing up to 0.12% silicon, characterized by impact strength, uniform texture and chemical composition; the main disadvantage is the lower quality surface of the products;
- “ps” – semi-quiet alloys with a silicon content of 0.07–0.12%, which are characterized by a uniform distribution of impurities;
- “kp” – boiling carbon steels with a silicon content of less than 0.07%, which are characterized by a heterogeneous structure.
Advantages of boiling steels:
- affordable cost (due to low additive content);
- high plasticity;
- good machinability and flexibility in processing by plastic deformation.
Classification by degree of deoxidation
According to the degree of deoxidation, carbon alloys are divided into the following types:
- boiling;
- calm;
- semi-calm.
Boiling alloys of ordinary quality are released from the furnace immediately after adding the deoxidizer. In some cases, deoxidation is carried out in a ladle. As a result, many air bubbles form under the crust.
For tool alloys, the deoxidation reaction begins before the spill and ends completely when poured into the ladle.
Boiling steels are used for the production of ingots, slabs and blooms - large-section rolled products. Subsequently, they are melted down into high-quality metal in electric furnaces or converted into rolled products of a smaller diameter - round, square. During processing, air escapes and the grain is pulled lengthwise, increasing the mechanical properties of the steel. Semi-quiet steels are characterized by increased malleability.
Production technologies
Today, there are three main carbon steel production technologies used in the steel industry. Their main differences are the type of equipment used. This:
- converter type melting furnaces;
- open hearth units;
- melting furnaces powered by electricity.
In converter plants, all components of the steel alloy are melted: cast iron and scrap steel. In addition, the molten metal in such furnaces is additionally processed using technical oxygen. In cases where the impurities present in the molten metal need to be converted into slag, burnt lime is added to it.
Converter steel furnace
The process of producing carbon steel using this technology is accompanied by active oxidation of the metal and its waste, the value of which can reach up to 9% of the total volume of the alloy. The disadvantage of this technological process is that it produces a significant amount of dust, and this necessitates the use of special dust cleaning units. The use of such additional devices affects the cost of the resulting product. However, all the shortcomings that characterize this technological process are fully compensated by its high productivity.
Smelting in an open-hearth furnace is another popular technology that is used to produce carbon steels of various grades. All the necessary raw materials (steel scrap, cast iron, etc.) are loaded into that part of the open-hearth furnace, which is called the melting chamber, which is heated to the melting temperature. Complex physical and chemical interactions take place in the chamber, in which molten metal, slag and a gaseous environment take part. The result is an alloy with the required characteristics, which is discharged in a liquid state through a special hole in the rear wall of the furnace.
Open hearth furnace workshop
Steel produced by smelting in electric furnaces, due to the use of a fundamentally different heating source, is not exposed to an oxidizing environment, which makes it cleaner. Various grades of carbon steel produced by smelting in electric furnaces contain less hydrogen. This element is the main reason for the appearance of flakes in the structure of alloys, which significantly worsen their characteristics.
No matter how the carbon alloy is smelted and no matter what category in the classification it belongs to, the main raw materials for its production are cast iron and metal scrap.
Production methods and quality differences
According to production methods, alloys are divided into three types:
- open hearth;
- convector;
- in electric furnaces.
The production method and quality division are indicated in the metal certificate and can be indicated letter-wise at the end of the marking. For example, VD is electric arc remelting, Sh is slag remelting.
Open hearth furnaces with the lowest quality are used for reworking and rolling of group A. Electric furnaces produce an alloy of high and very high quality.
Warranty for stoves made of Inox and Carbon steel
TMF became the first stove manufacturer in Russia to provide a 3-year warranty on sauna stoves made of heat-resistant Inox . Carbon structural steel stoves come with a standard 1 year .
The warranty covers not only the integrity of the metal, but also the welds of the firebox, and it is the seams that are the “sore spot” of many manufacturers. Naturally, the guarantee is valid subject to compliance with the operating rules set out in the instructions for each TMF sauna and heating stove.
Marking features
The marking of carbon steels has an alphanumeric meaning and is indicated at the end of the rolled product by a certain color. St at the beginning means normal quality. Then comes a number indicating the amount of carbon and the method of deoxidation.
For a material with increased quality, the designation begins with the word Steel, then carbon in hundredths and the letter designation of alloying elements.
High-quality ones are designated at the end with the letter A. Special, high-carbon, instrumental - U, high-speed - R.
Marked carbon steel
The principle of thermal uniform strength
Compare: unlike chrome-plated heat-resistant steel with an oxidation temperature of 750°C, the temperature at which scaling begins in structural and most alloy steels does not exceed 400°C, which is almost 2 times lower than the combustion temperature of birch logs.
But is everything so bad with structural steels? Does it mean that they will burn out twice as fast if their oxidation temperature is lower than Inox steel? No! We have found a solution!
In the production of TMF furnaces, the principle of thermal uniform strength . We increased the thickness of the stove parts subject to the greatest temperature and mechanical load. For stoves made of chrome-plated steel Inox - only 1.5 times to a thickness of 3 mm. And for furnaces made of structural steel - 2 times, and especially loaded ones - 3 times!!!
For example, the bottom of the heater of the Tunguska 2011 Carbon stove is made of 6 mm thick steel, and the lower segment of the closed heater of the Angara 2012 Carbon stove, made of 4 mm thick steel, is additionally protected by a 3 mm thick plate, which in total gives a thickness of 7 mm . In addition, the lower part of the stove fireboxes in the area of the grate is reinforced in a similar way. It is the heat-loaded areas that are protected, which allows you to increase the service life of the stove without practically reducing its thermal efficiency. This is why TMF structural steel furnaces are called “smart furnaces”.
How to decipher steel markings
The grade of carbon steel and its quality group can be determined by the type of marking. Each number and letter has its own meaning and shows quality requirements, the degree of deoxidation, and the presence of alloying elements.
For example, for a regular quality alloy:
- St 2 kp - normal quality with carbon content 0.09–0.15%, boiling, manganese 0.25–0.50%, silicon less than 0.05%;
- St3G PS - carbon content in the range of 0.14–0.22%, semi-quiet, manganese in the range of 0.80–1.1%, silicon not more than 0.15%.
High quality carbon steels are marked with numbers (carbon content in hundredths) and letters (alloying element). For example:
- 45 - 0.45% carbon;
- 40ХН - carbon 0.4%, chromium and nickel less than 2%.
The decoding of high-carbon grades has a letter indicating the type of material, its application and a number - the percentage of carbon in tenths. Tool alloys are designated U. For example:
- U8 - instrumental, 0.8% carbon;
- U12 - carbon content 1.2%.
The chemical composition can be more accurately determined from the table in the metallurgist's reference book.
The rolled product is marked with a colored stripe at the end:
- red - St3;
- yellow - St2;
- green - ST5;
- blue - St6.
Each type of steel has its own marking. Doped ones can contain up to 3 color stripes.
Features and main classification of material
The presence of carbon in steel ensures its reliability and strength, and also reduces the level of toughness and ductility. The bulk of the material contains up to 99.5% iron. Steel changes its characteristics due to heat treatment, during which the required hardness of the metal surface is achieved.
The main classification of carbon steel is based on the amount of carbon. There are three types of material:
- low-carbon (contain about 0.2% carbon);
- medium carbon (0.2–0.6%);
- high carbon (up to 2%).
The more carbon a material contains, the more durable and less ductile it will be; the toughness of steel also decreases and brittleness increases. Alloys that contain more than 2.4% carbon are already classified as cast iron.
Advantages of carbon steel:
- high plasticity;
- excellent weldability;
- good machinability regardless of the heating temperature of the metal;
- maintaining high strength characteristics even with high heating (up to 400 degrees);
- resistance to dynamic loads.
Disadvantages of carbon steel:
- with increasing carbon content, steel decreases its ductility;
- tends to expand when exposed to heat;
- has weak electrical characteristics;
- when heated to temperatures exceeding 200 degrees, it deteriorates its cutting ability and reduces hardness;
- susceptible to rust, which imposes serious requirements on steel products, for example, the need to apply a protective coating.
Carbon steel is also classified according to its structure into three groups:
- hypoeutectoid (their structure is based on ferrite and pearlite grains, the carbon content does not exceed 0.8%);
- eutectoid (the structure is based on pearlite, carbon content is 0.8%);
- hypereutectoid (contain secondary cementite and more than 0.8% carbon).
The structure of steel determines its quality condition.
The parameters of steel are seriously affected by the impurities it contains. Silicon and manganese have a positive effect on the metal (they increase its susceptibility to deoxidation), while phosphorus and sulfur have a negative effect (they worsen its properties).
The increased content of phosphorus causes steel products to crack and deform when exposed to cold air, this is called cold brittleness. When the temperature rises, steel with phosphorus lends itself well to processing (welding, forging, stamping).
Steel products with high sulfur concentrations are difficult to process at high temperatures. This phenomenon is called red brittleness. The structure of the material is sulfur grains with fusible boundaries. An increase in temperature breaks the connections between them, which leads to the appearance of cracks. The properties of carbon steel with a high sulfur content can be improved if it is alloyed with other chemical elements.
Which companies produce carbon steel?
The largest producer of carbon steel is the Mechel full-cycle metallurgical plant. It unites several large plants, ranging from coke production to various rolled products. In addition, rolled steel is produced by metallurgical plants:
- "Chelyabinsk";
- "Ukrainian Forge" - Chelyabinsk region;
- "Izhstal" - Udmurtia;
- Beloretsk Iron and Steel Works - Bashkortostan.
The metallurgical industry for the production of ferrous metal is located closer to iron ore and coal deposits. For non-ferrous casting factories, energy sources are more important.
