Understand this issue, as a classification of carbon steels, very important, as it allows you to get a complete picture of the characteristics of a variety of this popular material. Marking of such steels, like any other, no less important, and the specialist must be able to understand it, to choose the right alloy according to its properties and chemical composition.
A huge range of rolled metal products is produced from carbon steels
Excellent characteristics and main categories
To carbon steel, based on iron and carbon, include alloys, containing a minimum of additional impurities. Quantitative carbon content is the basis for the following classification of steels:
- low carbon (carbon content within 0,2%);
- medium carbon (0,2-0,6%);
- high carbon (to 2%).
Norms of content of chemical elements in carbon steel
In addition to decent technical characteristics, the affordable cost of carbon steels should be noted, which is important for the material, widely used for the production of various products.
The most significant advantages of carbon steels of different brands include:
- high ductility;
- good workability (regardless of the heating temperature of the metal);
- excellent weldability;
- maintaining high strength even with significant heating (to 400 °);
- good tolerance of dynamic loads.
Carbon steels also have disadvantages, among which it is worth noting:
- reducing the ductility of the alloy with an increase in its carbon content;
- deterioration of cutting ability and decrease in hardness at heating to temperatures, exceeding 200 °;
- high susceptibility to the formation and development of corrosion processes, which imposes additional requirements on such steel products, on which a protective coating must be applied;
- weak electrical characteristics;
- tendency to thermal expansion.
The classification of carbon alloys by structure deserves special attention. The main influence on the conversion in them has a quantitative carbon content. So, become, belonging to the category of pre-eutectoid, have a structure, which is based on grains of ferrite and perlite. The carbon content in such alloys does not exceed 0,8%. As the amount of carbon increases, the amount of ferrite decreases, and the volume of perlite, in accordance, increases. Become, which contains 0,8% carbon, according to this classification are eutectoid, the basis of their structure is mainly perlite. With a further increase in the amount of carbon, secondary cementite begins to form. Steels with this structure belong to the eutectoid group.
The microstructure of steels is formed during crystallization and depends on the content in the carbon alloy
The increase in the composition of the amount of carbon to 1% leads to, what are the properties of the metal, as strength and hardness, significantly improved, and the yield strength and ductility, vice versa, deteriorating. If the amount of carbon in the steel exceeds 1%, this can lead to, that in its structure the rough grid from secondary martensite will be formed, which in the most negative way affects the strength of the material. That is why in steels, belonging to the category of high-carbon, the amount of carbon, usually, does not exceed 1,3%.
Impurities also have a serious effect on the properties of carbon steels, contained in their composition. Elements, which have a positive effect on the characteristics of the alloy (improve metal deoxidation), there is silicon and manganese, and phosphorus and sulfur are impurities, deteriorating its properties. Phosphorus with a high content of carbon steel leads to that, that its products are covered with cracks and even break when exposed to low temperatures. This phenomenon is called cold brittleness. What is characteristic, steels with high phosphorus content, if they are heated, well give in to welding and processing by means of forging, stamping, etc..
The content of chemical elements in carbon steel of different brands
In products made of those carbon steels, which contain a significant amount of sulfur, such a phenomenon may occur, as redness. The essence of this phenomenon is, that the metal under the influence of high temperature begins to be poorly processed. Structure of carbon steels, which contain a significant amount of sulfur, is a grain with low-melting formations at the borders. Such formations begin to melt with increasing temperature, which leads to a violation of the connection between the grains and, Consequently, to the formation of numerous cracks in the metal structure. Meanwhile, the parameters of carbon sulfur alloys can be improved, if you perform their microalloying with zirconium, titanium and boron.
production technologies
To date, the metallurgical industry uses three main technologies for the production of carbon steel. Their main differences are the type of equipment used. it:
- melting furnaces of converter type;
- open-hearth installations;
- melting furnaces, working on electricity.
In converter plants, all components of the steel alloy are melted: cast iron and steel scrap. in addition, molten metal in such furnaces is further treated with technical oxygen. In those cases, when impurities, present in molten metal, must be converted into slag, burnt lime is added to it.
Furnace for converter smelting of steel
The process of obtaining carbon steel by this technology is accompanied by active oxidation of the metal and its soot, the value of which can reach 9% of the total alloy. The disadvantage of this technological process should include that, that it passes with the formation of a significant amount of dust, and this necessitates the use of special dust extraction units. The use of such additional devices affects the cost of production. However, all the disadvantages, which characterizes this technological process, fully compensated by its high performance.
Smelting in an open-hearth furnace is another popular technology, which is used to produce carbon steels of various grades. In that part of the open-hearth furnace, which is called the melting chamber, all necessary raw materials are loaded (steel scrap, cast iron, etc.), Which is heated to the melting point. Complex physicochemical interactions take place in the cell, in which molten metal is involved, slag and gaseous medium. The result is an alloy with the required characteristics, which in the liquid state is removed through a special hole in the rear wall of the furnace.
Open-hearth furnaces shop
Steel, obtained by smelting in electric furnaces, due to the use of a fundamentally different heat source is not affected by the oxidizing environment, which allows you to make it cleaner. In different grades of carbon steel, obtained by smelting in electric furnaces, less hydrogen is present. This element is the main reason for the appearance in the structure of flocken alloys, 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 scrap metal.
Ways to improve strength characteristics
If the properties of alloy steel grades are improved by introducing special additives into their composition, then the solution of such a problem in relation to carbon alloys is carried out by performing heat treatment. One of the advanced methods of the latter is surface plasma hardening. As a result of using this technology, a structure is formed in the surface layer of the metal, consisting of martensite, whose hardness is 9,5 GPa (in some areas it reaches 11,5 GPa).
The equipment for plasma hardening is small, mobile and easy to operate
Surface plasma hardening also leads to this, that metastable residual austenite is formed in the metal structure, the number of which is growing, if the percentage of carbon in the steel increases. This structural formation, which can turn into martensite during the run-in of carbon steel products, significantly improves this characteristic of the metal, as wear resistance.
One of the effective ways, which can significantly improve the performance of carbon steel, there is a chemical heat treatment. The essence of this technology is, that steel alloy, heated to a certain temperature, subjected to chemical action, which allows to significantly improve its characteristics. After such processing, which can be subjected to carbon steels of various grades, increase the hardness and wear resistance of the metal, and also improves its corrosion resistance to wet and acidic environments.
Treatment of parts by chemical-thermal method in a vacuum furnace significantly increases the surface strength
Other classification parameters
Another parameter, according to which carbon alloys are classified, there is a degree of their purification from harmful impurities. The best mechanical characteristics (but also a higher cost) steel differs, which contain a minimum amount of sulfur and phosphorus. This parameter became the basis for the classification of carbon steels, according to which alloys are allocated:
- of normal quality (V);
- quality (B);
- high quality (A).
General principles of steel classification
Became the first category (their chemical composition is not specified by the manufacturer) choose, based only on their mechanical characteristics. Such steels have a minimum cost. They are not subjected to any heat treatment, no pressure treatment. For quality steels, the manufacturer determines the chemical composition, and for high-quality alloys - and mechanical properties. What is important, products from alloys of the first two categories (B and C) can be subjected to heat treatment and hot plastic deformation.
There is a classification of carbon alloys and their main purpose. So, distinguish structural steels, from which produce parts for various purposes, and instrumental, used in full accordance with their name - for the manufacture of various tools. Tool alloys, if you compare them with structural, differ in the increased hardness and durability.
Content of basic elements in tool steels
In the marking of carbon steel can be found the designation "sp", "Ps" and "kp", which indicate the degree of its deoxidation. This is another parameter of the classification of such alloys.
The letters "sp" in the label indicate calm alloys, which may contain up to 0,12% silicon. They are characterized by good toughness even at low temperatures and have a high homogeneity of structure and chemical composition. Such carbon steels have disadvantages, the most significant of which are, that the surface of products from them is less qualitative, than boiling steels, and after welding, the characteristics of the parts of them significantly deteriorate.
Semi-quiet alloys (are denoted by the letters "ps" in the label), in which silicon may be contained within 0,07-0,12%, characterized by uniform distribution of impurities in its composition. This ensures the consistency of the characteristics of their products.
Characteristics of common semi-quiescent steels
In boiling carbon steels, containing no more 0,07% silicon, the deoxidation process is not complete, which causes the heterogeneity of their structure. Meanwhile, they have a number of advantages, to the most significant of which should be attributed:
- low cost, due to the low content of special additives;
- high ductility;
- good weldability and machinability by means of plastic deformation methods.
How carbon steel alloys are marked
Understanding the principles of carbon steel labeling is just as easy, as well as in the bases of its classification: they differ little from the rules for designating steel alloys of other categories. In order to decipher such markings, you don't even need to look in special tables.
Examples of decoding markings
Letter «U», which stands at the very beginning of the alloy brand designation, If there is a number, that it belongs to the category of instrumental. About that, which quality group includes carbon steel, say the letters "A", "B" and "B", affixed at the very end of the marking. The amount of carbon, contained in the alloy, affixed at the very beginning of its marking. At the same time for steels, with high quality (group "A"), the amount of this element will be specified in hundredths of a percent, and for alloys of groups "B" and "B" - in tenths.
The letter "G" can be found in the marking of individual carbon steels, standing after the numbers, indicating the quantitative carbon content. This letter indicates, that the metal contains an increased amount of such an element, like manganese. To you, to what extent deoxidation corresponds to carbon steel, indicate the designation "sp", "Ps" and "kp".
Due to their characteristics and low cost, carbon alloys are actively used for the production of building elements, machine parts, tools and metal products for various purposes.