Steel is the most famous alloy in the world. In fact, when talking about iron structures and objects, we are talking about products (or their production) from one or another steel. 99% of the alloy is classified as a structural steel, so there are virtually no tools or equipment that use it.

In this article we will try to touch upon such topics as the classification of grades, the price of steel, its properties and use in construction.

Steel is an alloy of iron and carbon. In normal cases, the proportion of carbon ranges from 0.1 to 2.14%. But, given that alloy steels can contain many additional ingredients, today steel means an alloy in which the share of iron is at least 45%.

This video will tell you what steel is and how it is produced:

Concept and features

The main attractive qualities of steel are high strength with the availability of raw materials and a relatively simple production method. It is this combination that puts iron alloys in the position of absolute leader. Today there is simply no area of ​​the national economy where steel does not occupy the position of a structural material.

• Iron and carbon are essential components of an alloy. Among them is viscosity, due to which steel is classified as a deformable, malleable alloy. And carbon is hardness and strength, since hardness is always combined with fragility. The carbon addition is small and even in specialized formulations does not exceed 3.4%.
• In addition, due to the production method, steel always contains some proportion of manganese - up to 1%, and up to 0.4%. These impurities have little effect on the properties of the composition if they do not exceed the specified norm. For the same reasons, the composition also contains harmful impurities - phosphorus, sulfur, unbound nitrogen and oxygen. During the melting and alloying process, they try to get rid of these ingredients, since they reduce the strength and ductility properties of the alloys.
• Other additives are artificially introduced into the alloy in order to change the quality of the material. Thus, the addition of chromium gives steel heat resistance, corrosion resistance and toughness.
• An extremely useful quality of iron alloys is that the change in properties is influenced by very small additions of other substances by weight. This allows you to significantly diversify the quality of the material. In addition, the properties of the alloy are greatly influenced by the method of manufacturing the product itself - cold deformation, hot deformation, quenching, and so on.

Relation to cast iron

It is closest to steel in properties and composition. Some of the material is made from cast iron. However, in practice, the differences in characteristics are quite noticeable:

• steel is stronger and harder than cast iron;
• and has a lower melting point. The massiveness of cast iron products creates a misleading impression, since it is less durable;
• steel is easier to machine due to its low carbon content. ;
• cast iron has lower thermal conductivity, that is, it stores heat better than steel;
• Cast iron cannot be subjected to a procedure such as hardening. And the latter can significantly increase the strength of the material.

Advantages and disadvantages

It is quite difficult to describe the pros and cons of the material. In practice, we deal with steel products, and from an alloy of various grades, and, therefore, properties. And one of the features of the material is precisely that the method of manufacturing a product from it also affects its properties. The quality of a welded pipe cannot be compared with the characteristics of a cold-rolled steel pipeline.

In general, we can talk about the following advantages of steel:

• high strength and hardness - characteristic of all types;
• a huge variety of properties due to different composition and different processing methods;
• viscosity and elasticity sufficient for use in all areas where resistance to impact, static and dynamic loads in the absence of residual deformation is required;
• ease of machining – welding, cutting, bending;
• very high wear resistance compared to other structural materials and, accordingly, durability;
• the prevalence of raw materials and an economically viable production method, which determines the affordable cost of alloys.

The disadvantages include the following:

• The biggest drawback of the material is its lack of resistance to corrosion. To avoid damage, special types of steel are produced - stainless steel, but their cost is noticeably higher. More often, the problem is solved by coating steel products with a protective layer of metal or polymer;
• the alloy accumulates electricity, which significantly increases electrochemical corrosion. Any large structures - machine bodies, pipelines - require special protection;
• the alloy is not lightweight, steel structures are heavy and significantly make objects heavier;
• The manufacture of steel products is a multi-stage process. Shortcomings and errors at any stage result in a significant reduction in quality.

Types of metal

Calculating the number of alloys known and used today is a very difficult task. Classifying them is no less difficult: the properties of the material depend on the composition, production method, nature of the additives, processing method, and so on.

The most commonly used classifications are:

• according to the chemical composition of steels - carbon and alloyed;
• by structural composition - austenitic, ferritic, and so on;
• according to the content of impurities - regular quality, high-quality, and so on;
• by processing method - thermal hardening - annealing, thermomechanical - forging, chemical-thermal - nitriding;
• by purpose - tool, structural, special steels and so on.

This video will tell you about stainless steel:

Chemical composition

An alloy is essentially a solid solution. Moreover, the component dissolves in a solid base material according to different laws than in a liquid. The basis for the production of all iron alloys is the ability of iron to polymorphism, that is, the formation of different structural phases at different temperatures. Thanks to this, carbon and other elements dissolved in iron at high temperatures do not precipitate when the temperature drops, as happens with ordinary liquids, but form a joint structure.

According to their composition, steels are divided into carbon and alloy.

Carbon

Carbon - the main, that is, the alloying component that determines the properties, is carbon. There are 3 types:

• low carbon– less than 0.3%. The alloys are distinguished by their malleability and resistance to dynamic loads;
• medium carbon– the share of carbon varies from 0.3 to 0.7%;
• high carbon contain more than 0.7% carbon. They are distinguished by higher strength and hardness.

This division is associated with the transformations that occur in alloys. Up to a carbon content of 0.8%, the alloy retains a hypoeutectoid structure, that is, it has a ferrite-pearlite structure. As the proportion of carbon increases, the structure changes to eutectoid and hypereutectoid, which corresponds to pearlite and cementite. The phase ratio largely determines the strength characteristics.

The user is faced not so much with low- or high-carbon steel, but with the composition of a certain grade. The brand is determined by the ratio of several criteria, and not just carbon content.

There are 3 groups based on their purpose:

• A – mechanical qualities are standardized. The group is divided into 3 categories and 6 brands. The grade St is designated from 0 to 6. St0 is steel rejected for some indicators, used in insignificant structures. St6 – most closely corresponds to the concept of high-quality steel;
• B – standardized according to its chemical composition, divided into 2 categories and 6 grades, designated BST from 0 to 6. As the number increases, the strength and fluidity of the material increases;
• group B is standardized both in terms of mechanical parameters and composition. It is divided into 5 marks, designated VSt.

An additional classification based on manganese content is applied. I – with normal content of the element, that is, 0.25–0.8%, and II – with increased content, up to 1.2%

Alloyed

Alloyed steels are steels into which additional ingredients are specially introduced to impart other qualities to the composition. Classification is made according to the total volume of all alloying additives - not manganese or phosphorus impurities.

There are 3 types:

• low alloy– with a total volume of additives up to 2.5%;
• medium alloyed– contains from 2.5 to 10% impurities;
• V highly alloyed the share of additives exceeds 10%.

Alloying significantly complicates the structure of the solid solution, which leads to the emergence of a complex classification based on structural composition. Brands are marked by composition: the proportion of carbon must be indicated. And then the proportions of alloying additives are indicated in decreasing order. If the proportion of impurity is less than 1%, the substance is not indicated.

Both non-metals and metals are used as additives.

• Manganese– increases the strength and hardness of the material, improves cutting properties. But at the same time it contributes to an increase in grain, which reduces resistance to impact loads.
• Chromium– improves resistance to shock and static loads, and also increases heat resistance. With a large proportion of chromium, the material becomes stainless.
• – increases the elasticity of the alloy. With significant content, it imparts corrosion resistance and heat resistance to steel.
• Molybdenum– increases the hardness of the alloy, but at the same time reduces brittleness.

The best known of alloy steels is, of course, stainless steel. Most often it is chrome-nickel and chromium steel with a chromium content of up to 27%.

Phase and structural composition

Making steel is a complex and controversial process. Its peculiarity is that during melting the alloy undergoes phase transformations, which determine the combination of strength and elasticity.

Carbon alloying occurs in 2 stages. In the first stage, when heated to 725 C, iron combines with carbon, forming carbide, that is, a chemical compound called cementite. At normal temperatures, steel contains a mixture of cementite and ferrite. When the temperature rises above 725 C, cementite dissolves in iron, forming another phase - austenite.

This feature is associated with the classification of the alloy according to its structural composition in a normalized form:

• pearlite– these are mainly low-carbon and low-alloy steels;
• martensitic– with a high content of additives;
• authenite– highly alloyed.

In the annealed state, the following structural classes are distinguished:

• hypoeutectoid,
• hypereutectoid,
• ledeburite,
• ferritic,
• austenitic.

What is the meaning of such a division? The fact is that alloying additives have different effects on different steel structures. Thus, the dissolution of alloying elements in ferrite leads to an increase in tensile strength, with the exception of manganese and silicon, which strengthen the alloy. When alloying austenite, the yield strength decreases with a relatively high strength. As a result, the material is easily and quickly strengthened during deformation - work hardening.

Classification by deoxidizing agent

When melting metals, a common problem is the gas dissolved in them - oxygen, nitrogen, hydrogen; to remove it, they resort to deoxidation. Depending on the completeness of the process, there are 3 types:

• calm– the metal does not contain ferrous oxide. The alloy is completely free of gases, so its properties are most stable and uniform. It is used for critical structures, since the technology for its production is expensive;
• semi-calm– hardens without boiling, but is accompanied by the release of gases. Some gases remain, but can be removed by rolling the alloy. Typically, semi-quenched steel is used for structural purposes;
• boiling– contains dissolved gases. This affects the properties: the material is prone to cracking during welding, for example, but since the production of boiling steel requires the least cost, such an alloy is also produced for many simple structures.

Classification by purpose

A rather arbitrary division of steels according to the areas of application of steel.

• Construction– alloys of regular quality and low-alloy, designed for high static and, in some cases, dynamic loads. The main requirement for them is good weldability. In fact, depending on the nature of the construction project, materials of very different quality are used.
• Instrumental– usually high-carbon and high-alloy, used in the manufacture of tools. There are stamped alloys, cutting alloys and steels for measuring instruments. Cutting materials are distinguished by their hardness and heat resistance, and the material for measuring instruments is highly wear-resistant.
• Structural– low in manganese. These are cemented, high-strength, automatic, ball-bearing, wear-resistant and so on, used for the manufacture of a wide variety of components and structures. Such a huge variety of properties is achieved through alloying.
• Sometimes they highlight special steels- heat-resistant, heat-resistant, acid-resistant, but in fact they are a type of structural.

Steel can contain useful impurities, that is, alloying elements, and harmful ones. Based on the content of harmful substances, 4 groups are distinguished:

• privates– or ordinary quality, with a share of sulfur not exceeding 0.06% and phosphorus not exceeding 0.07%;
• quality– the allowed proportion of sulfur is not more than 0.04% and phosphorus is not more than 0.035%. The process of their manufacture is more expensive, but the mechanical properties of steel are also higher;
• high quality– the proportion of sulfur does not exceed 0.025%, and phosphorus – 0.025%. Alloys are produced mainly in electric furnaces to achieve high purity;
• especially high quality– are smelted in electric furnaces using special methods. This way, only high-alloy steels with a sulfur content of up to 0.015% and phosphorus - 0.025% are produced.

Alloy production

The process of manufacturing the alloy comes down to the processing of cast iron, during which excess impurities are annealed and alloying elements are introduced. Several methods are used for this.

• Martenovsky– molten or solid cast iron with ore is smelted in an open-hearth furnace at 2000 C to anneal off excess carbon. Additives are introduced at the end of the melt. The steel is poured into ladles and transported to the rolling mill.
• Oxygen converter– more productive. Air or a mixture of air and oxygen is blown through the cast iron in the furnace, achieving faster and more complete annealing.
• Electric melting– melting is carried out in a closed furnace at 2200 C, which prevents gases from entering the alloy. An expensive method that only produces high-quality compounds.
• Direct method– in a shaft furnace, pellets obtained from iron ore are blown with combustion products of natural gas – a mixture of oxygen, carbon monoxide, ammonia, at a temperature of 1000 C.

The steel making process does not end there. In cases where it is necessary to obtain the most durable material, additional processing is resorted to.

Thermal method

Thermal methods include:

• annealing – heating and slow cooling of different types and at different speeds;
• hardening - heating above the critical temperature, which causes recrystallization of the alloy, and faster cooling;
• tempering is a procedure carried out after hardening in order to reduce the stress of the metal;
• normalization is the same annealing, but carried out not in an oven, but in air.

Thermo-mechanical method

Thermomechanical methods combine mechanical and thermal effects:

• high-temperature TMT - hardening - hardening, hardening, is carried out immediately after heating, while the alloy retains its austenitic structure. The change due to plastic deformation during rolling or stamping is retained by 70% and after cooling the steel turns out to be stronger;
• for low-temperature TMT – cold-rolled steel. The alloy is heated to achieve an austenitic state, cooled below the recrystallization points to achieve the appearance of a martensitic phase - within 400–600 C. Then hardening is carried out - hardening, rolling. When cooled, the effect is completely preserved.

Thermochemical treatment

Thermochemical treatment is the heating of alloys and holding them in certain chemical environments. The most well-known methods include:

• carburization – saturation of the alloy surface with carbon. In this way a wear-resistant top layer is obtained;
• nitriding – saturation of steel with nitrogen. The goal is the same - to obtain a top wear-resistant layer, but compared to carburization, nitriding provides higher corrosion resistance;
• nitrocarburization and cyanidation - saturation of the surface layer with both carbon and nitrogen. Provides higher speed and productivity of the process.

Material cost

The cost of the material is no less varied than the number of brands. Standard steel on the London Metal Exchange in December 2016 costs $325 per ton. Stainless steel costs are noticeably higher, with cold-rolled 304 grade stainless steel priced between $1,890 and $1,925 per ton in December.

Steel is the most popular and most widespread metal alloy in the world. When talking about the national economy, we mean specifically various steel alloys.

To see how steel melts, watch the video below:

Steel is the most common alloy. The variety of applications results in a large number of varieties with different requirements, both in terms of mechanical and chemical characteristics of steel. Different grades of steel imply not only a variety of chemical composition, but also manufacturing technology.

The variety of alloys is based on the chemical composition of the metal, since the alloying components determine the final result, and the manufacturing and processing technology only emphasizes and highlights individual characteristics. Some elements included in the composition may impair performance, so individual elements of the label may indicate the absence or low content of such substances.

Deciphering the markings allows you to determine the content of the main elements of the alloy and, in part, the production technology, as well as evaluate the technical characteristics, and with them the scope of possible application.

In addition to differences in composition and processing, steel is also divided into categories based on mechanical strength. There are 5 categories, which differ in test methods for compliance with mechanical strength. Tensile and impact strength tests are carried out on control samples.

Types of steels and features of their markings

Various areas of application of steel require that it have strictly defined properties - physical, chemical. In one case, the highest possible wear resistance is required, in others, increased resistance to corrosion, in others, attention is paid to magnetic properties.

There are many types. The bulk of the smelted metal goes into the production of structural steel, which includes the following types:

When deciphering the designations, it is necessary to take into account that each of the types corresponds to a strictly defined letter in the marking.

Classification by chemical composition

The main alloying additives are metals. By varying the quantitative composition of additives and their mass fraction, a wide variety of steel grades are obtained. Pure iron itself has low technical properties. Low mechanical strength and high susceptibility to corrosion require the introduction of additional substances into the alloy composition, which are aimed at improving one of the qualities, or several at once.

Often, the improvement of some characteristics entails the deterioration of others. Thus, high-alloy stainless steels may have low mechanical strength, while high-quality carbon steels, along with high strength, receive weakened corrosion properties.

As mentioned above, one of the classifications of steel grades is its chemical composition. The main components of all steels without exception are iron and carbon, the content of which should not exceed 2.14%. Depending on the amount and proportions of additives, the iron content in the composition should be at least 50%.

Based on the amount of carbon contained, steels are classified into three groups:

• Low-carbon – carbon content less than 0.25%;
• Medium carbon – 0.25-0.6% carbon;
• High carbon, with a carbon content of more than 0.6%.

Increasing the percentage of carbon increases the hardness of the metal, but at the same time its strength decreases.

To improve performance, a certain amount of chemical elements is introduced into the alloy. Such steels are called alloyed. For alloy steels there is also a division into three groups:

• Low alloy, containing additives up to 2.5%;
• Medium alloyed, which contain from 2.5 to 10% alloying elements;
• Highly alloyed. The content of alloying impurities varies from 10 to 50%.

The marking of steels reflects the presence and percentage of alloying additives. When deciphered, each element is assigned a specific letter, next to which there is a number corresponding to its content as a percentage. The absence of numbers indicates that the additive is present in the alloy in an amount of less than 1-1.5%. The presence of carbon in the composition is not reflected, since it is included in all compositions, but its content is indicated at the very beginning of the labeling.

The marking can also indicate the purpose of the alloy. Since this classification also uses letter designations, the order of their arrangement is regulated - at the beginning, middle and end of the marking.

Classification by purpose

The classification of steel types by purpose has already been given above. Marking of structural steels includes the following designations:

• Construction - denoted by the letter C and numbers characterizing the yield strength.
• Bearing - designated by the letter Ш. Next comes the designation and content of alloying additives, mainly chromium.
• Instrumental unalloyed - denoted by the letter U and carbon content in tenths of a percent.
• High-speed - denoted by the letter P and symbols of alloying components.
• Unalloyed structural steel has the symbols Cn and a number indicating the carbon content in tenths or hundredths of a percent.

The remaining varieties, including tool grades made of alloy steels, do not have special designations other than their chemical composition, so the decoding and purpose of individual types can only be determined from reference literature.

Classification by structure

The structure of steel refers to the internal structure of the metal, which can vary significantly depending on heat treatment conditions and mechanical influences. The shape and size of grains depend on the composition and ratio of alloying additives and production technology.

The basis of steel grains is a crystal lattice of iron, which includes atoms of impurities - carbon, metals. Carbon can form solid solutions in the crystal lattice, or it can create chemical compounds, carbides, with iron.

Metal additives exist in the form of solutions, and many of them affect the state of the carbon solution.

The structure of steel changes with temperature changes. These changes are called phases. Each phase exists in a certain temperature range, but alloying additives can significantly shift the boundaries of the transition of one phase to another.

The following are the main phases of the state of the metal:

• Austenite. Carbon atoms are located inside the iron crystal lattice. This phase exists in the range of 1400-700 °C. If the composition contains from 8 to 10% nickel, the austenite phase can persist at room temperature.
• Ferrite. Solid solution of carbon in iron.
• Martensite. Supersaturated carbon solution. This phase is characteristic of hardened steel.
• Bainite. The phase is formed by rapid cooling of austenite to a temperature of 200-500 °C. Characterized by a mixture of ferrite and iron carbide.
• Perlite. Equilibrium mixture of ferrite and carbide. It is formed when austenite is slowly cooled to a temperature of 727 °C.

The phases of the metal structure characterize its physical properties, depending on which the class of steel is determined - structural, foundry, and so on.

Quality classification

Alloyed and unalloyed steel within each grade differs in quality, which depends on the production technology and the quality of the starting materials.

The quality of steel is particularly affected by impurities that remain in it during the reduction of iron from ore concentrates. Phosphorus and sulfur mainly negatively affect the quality of steel. According to their content, steel of ordinary quality and high-quality steel are classified, at the end of which there is the letter A. The phosphorus content in high-quality steel does not exceed 0.025%.

Classification by deoxidation method

When steel is smelted, a certain amount of oxygen remains in it as part of the iron oxides. To reduce the amount of oxygen and restore iron from oxides, a deoxidation reaction is used, in which compounds that are more active in interacting with oxygen than iron are added to the molten metal. During the reaction, the released oxygen also reacts with carbon, resulting in the formation of carbon dioxide, which is released in the form of bubbles.

Depending on the amount of deoxidizing agents and the duration of the process, three types of the final alloy can be distinguished:

• Boiling steel. As a result of the minimal use of additives and reaction time, the yield of finished products is increased, which, at the same time, is of low quality;
• Calm steel. Metal that has completely undergone deoxidation processes. It is of high quality, but expensive to produce due to the high cost of reagents and reduced product yield;
• Semi-quiet steel. An intermediate option with the optimal combination of quality and cost.

When producing an assortment of steel grades from metal of varying degrees of deoxidation, special marking of materials is used, respectively, with the symbols “sp”, “kp” and “ps”.

Marking of steels according to Russian standards

Marking steel according to Russian standards makes it possible to determine the composition of the metal and, in part, whether it belongs to a certain type.

If the presence of carbon in steel is more than 1%, its amount is not indicated in the marking. The steel grade includes letter designations of alloying additives indicating their quantity in tenths and hundredths of a percent, but if the component content is less than 1.5%, then only the letter designation is present in the marking.

In addition to the chemical composition, the marking contains symbols characterizing the purpose of the steel and the degree of its quality.

Marking of steels according to American and European systems

Marking of steels produced domestically and in the post-Soviet space makes it possible to approximately determine the composition, purpose and characteristics without resorting to reference literature. In American and European standards, such decoding is, for the most part, absent. This is due to the large number of organizations involved in the standardization of metal products.

For the most part, the designation of steel according to American and European standards does not indicate the chemical composition. Types of steel according to their intended purpose are characterized by an alphabetic or digital code, which can be deciphered using reference literature.

Only in the European standard EN10027 there is an option for marking alloys by chemical composition, which is closely similar to domestic designations.

Designations of alloying elements

In order to recognize the qualitative and quantitative composition from the markings, letter designations are used for alloying elements. Basically, Russian letters correspond to the names of elements, although there are exceptions, since there are elements that begin with the same letters. The table of alloying elements is as follows.

As can be seen from the table, it contains two non-metals - silicon and nitrogen, but no carbon. The presence of carbon is implied in the composition of any steel, therefore the designation indicates only its content

Color coding

Color marking of steels is used to indicate rolled products. This is convenient when storing materials in warehouses and transporting them. Steels are marked with marks in the form of dots or stripes made with indelible paint. The color of the designations is selected from the table according to the purpose of the steel. In this case, the steel group and the degree of its deoxidation are not taken into account.

Examples of decoding markings

To make the decoding clearer, some of the most striking examples of marking should be given. Based on the examples, determining the steel grade in comparison with already known ones will not be a difficult task. Here are some types of steel with decoding symbols:

• 30ХГСА - decoding of the steel grade indicates that the alloy contains 0.3% carbon, as evidenced by the number at the beginning of the designation. Steel contains chromium (X), manganese (G), silicon (C), but their content is less than 1.5%. The symbol “A” at the end of the designation indicates that the steel is high quality.
• U8GA – tool steel with a carbon content of 0.8%. High quality with added manganese.
• R6M5F2K8 – high-speed steel. Contains 5% molybdenum, 2% vanadium, 8% cobalt. Chromium is contained in all high-speed steels in an amount of about 4%, so it is not included in the designation. Tungsten is also always present, but its content can vary, so in this brand its amount is 6%.
• St3sp5 - unalloyed structural steel, completely deoxidized - calm, category 5, that is, it can be used for the manufacture of load-bearing welded structures.
• HVG – HVG steel contains chromium, tungsten and manganese in an amount of about 1% and additional alloying elements, but their content is less than 0.5%.

By structure:

< С, тем >perlite, steel is stronger.

By purpose:

1)

QUESTION 14. Classification of steels according to production method and quality.

According to the production method:

1) Sour method;

2) The main method is non-deoxidized steel KP, calm SP, if there are no letters after the brand, then it is calm steel, if not completely deoxidized, then ps.

By quality:

Depending on the content of harmful impurities: sulfur and phosphorus, steel is divided into:

Ordinary quality steel, content up to 0.06% sulfur and up to 0.07% phosphorus. Ordinary quality steel is also divided into 3 groups based on supplies:

1. steel group A supplied to consumers based on mechanical properties (such steel may have a high sulfur or phosphorus content);

2. steel group B - by chemical composition;

3. steel Group B- with guaranteed mechanical properties and chemical composition.

1. High quality- up to 0.035% of sulfur and phosphorus each separately.

2.High quality- up to 0.025% sulfur and phosphorus.

3. Particularly high quality, up to 0.025% phosphorus and up to 0.015% sulfur.

Alloy steels. Alloying elements. Marking l/s.

Alloy steels are widely used in tractor and agricultural engineering, in the automotive industry, heavy and transport engineering, and to a lesser extent in machine tool building, tool and other types of industry. This steel is used for heavily loaded metal structures.

Steels in which the total amount of alloying elements does not exceed 2.5% are classified as low-alloy, those containing 2.5-10% are alloyed, and more than 10% are classified as high-alloy (iron content more than 45%).

Low-alloy steels are most widely used in construction, and alloy steels are most widely used in mechanical engineering.

Alloyed structural steels are marked with numbers and letters. The two-digit numbers given at the beginning of the brand indicate the average carbon content in hundredths of a percent; the letters to the right of the number indicate the alloying element. For example, steel 12Х2Н4А contains 0.12% C, 2% Cr, 4% Ni and is classified as high-quality, as indicated by the letter IАI at the end of the grade.

Construction low alloy steels

Low alloy steels are those containing no more than 0.22% C and a relatively small amount of non-deficient alloying elements: up to 1.8% Mn, up to 1.2% Si, up to 0.8% Cr and others.

These steels include steels 09G2, 09GS, 17GS, 10G2S1, 14G2, 15HSND, 10KHNDP and many others. Steels in the form of sheets and shaped sections are used in construction and mechanical engineering for welded structures, mainly without additional heat treatment. Low-alloy low-carbon steels are weldable.

For the manufacture of large-diameter pipes, 17GS steel is used (s0.2=360MPa, sв=520MPa).

For the manufacture of parts strengthened by carburization, low-carbon (0.15-0.25% C) steels are used. The content of alloying elements in steels should not be too high, but should provide the required hardenability of the surface layer and core.

Chromium steels 15X, 20X are intended for the manufacture of small products of simple shape, cemented to a depth of 1.0-1.5mm. Chromium steels, compared to carbon steels, have higher strength properties with some lower ductility in the core and better strength in the cemented layer.

Steel production.

Compared to cast iron, steel contains less carbon, silicon, sulfur and phosphorus. To produce steel from cast iron, it is necessary to reduce the concentration of substances by oxidative smelting.

In the modern metallurgical industry, steel is smelted mainly in three units: convectors, open-hearth furnaces and electric furnaces.

Steel production in converters.

The converter is a pear-shaped vessel. The upper part is called a visor or helmet. It has a neck through which liquid cast iron and steel and slag are drained. The middle part has a cylindrical shape. In the lower part there is an attached bottom, which is replaced with a new one as it wears out. An air box is attached to the bottom, into which compressed air enters.

The capacity of modern convectors is 60 - 100 tons or more, and the air blast pressure is 0.3-1.35 Mn/m. The amount of air required to process 1 ton of cast iron is 350 cubic meters.

Before pouring cast iron, the convector is turned to a horizontal position, at which the tuyere holes are above the level of the poured cast iron. Then it is slowly returned to a vertical position and at the same time a blast is applied, which prevents the metal from penetrating through the holes of the tuyeres into the air box. In the process of blowing air through liquid cast iron, silicon, manganese, carbon and partially iron burn out.

When the required carbon concentration is reached, the convector is returned to a horizontal position and the air supply is stopped. The finished metal is deoxidized and poured into a ladle.

Bessemer process. Liquid cast iron with a fairly high content of silicon (up to 2.25% and higher), manganese (0.6-0.9%), and a minimum amount of sulfur and phosphorus is poured into the converter.

Based on the nature of the reaction occurring, the Bessemer process can be divided into three periods. The first period begins after the blast is started in the converter and lasts 3-6 minutes. Small drops of liquid cast iron fly out of the converter neck along with the gases, forming sparks. During this period, silicon, manganese and partially iron are oxidized according to the reactions:

2Mn + O2 = 2MnO,

2Fe + O2 = 2FeO.

The resulting ferric oxide partially dissolves in the liquid metal, promoting further oxidation of silicon and manganese. These reactions occur with the release of a large amount of heat, which causes the metal to heat up. The slag turns out to be acidic (40-50% SiO2).

The second period begins after almost complete burnout of silicon and manganese. The liquid metal is heated well enough that favorable conditions are created for the oxidation of carbon by the reaction C + FeO = Fe + CO, which occurs with the absorption of heat. Carbon combustion lasts 8-10 minutes and is accompanied by a slight decrease in the temperature of the liquid metal. The resulting carbon monoxide burns in air. A bright flame appears above the convector neck.

As the carbon content in the metal decreases, the flame above the neck decreases and the third period begins. It differs from previous periods in the appearance of brown smoke above the neck of the converter. This shows that silicon, manganese and carbon have almost completely burned out of the cast iron and very strong oxidation of iron has begun. The third period lasts no more than 2–3 minutes, after which the convector is turned over to a horizontal position and deoxidizing agents (ferromanganese, ferrosilicon or aluminum) are introduced into the bath to reduce the oxygen content in the metal. Reactions occur in the metal

FeO + Mn = MnO + Fe,

2FeO + Si = SiO2 + Fe,

3FeO + 2Al = Al2O3 + 3Fe.

The finished steel is poured from the convector into a ladle and then sent for casting.

To obtain steel with a predetermined amount of carbon (for example, 0.4 - 0.7% C), blowing the metal is stopped at the moment when the carbon has not yet burned out of it, or you can allow the carbon to completely burn out, and then add a certain amount of cast iron or carbon containing a certain amount of ferroalloys.

Most open hearth furnaces are heated with a mixture of blast furnace, coke and generator gases. Natural gas is also used. An open-hearth furnace running on fuel oil has generators only for heating the air.

Charge materials (scrap, cast iron, fluxes) are loaded into the furnace by a filled machine through filling windows. Heating of the charge, melting of the metal and slag in the furnace occurs in the melting space when the materials come into contact with a torch of hot gases. The finished metal is released from the furnace through holes located in the lowest part of the hearth. During melting, the outlet hole is clogged with refractory clay.

The smelting process in open hearth furnaces can be acidic or basic. In the acid process, the refractory masonry of the furnace is made of silica brick. The upper parts of the hearth are welded with quartz sand and repaired after each melt. During the smelting process, acidic slag with a high silica content (42-58%) is obtained.

During the main smelting process, the hearth and walls of the furnace are laid out from magnesite bricks, and the roof is made from silica or chromium-magnesite bricks. The upper layers of the hearth are welded with magnesite or dolomite powder and repaired after each melt. During the smelting process, acidic slag with a high content of 54 – 56% CaO is obtained.

Basic open-hearth process. Before starting smelting, the amount of raw materials (pig iron, scrap steel, limestone, iron ore) and the sequence of their loading into the furnace are determined. Using a pouring machine, a mold (special box) with a shaft is introduced into the melting space of the furnace and turned over, as a result of which the charge is poured onto the bottom of the furnace. First, small scrap is loaded, then larger scrap and then lump lime (3 - 5% of the metal weight). After heating the loaded materials, the remaining steel scrap and cast iron are fed in two or three portions.

To more intensively supply the metal bath with oxygen, iron ore is introduced into the slag. Oxygen dissolved in the metal oxidizes silicon, manganese, phosphorus and carbon according to the reactions discussed above.

By the time the entire charge melts, a significant part of the phosphorus passes into the slag, since the latter contains a sufficient amount of ferrous oxide and lime. To avoid the reverse transition of phosphorus into the metal, before the bath begins to boil, 40 - 50% of the primary slag from the furnace.

After the primary slag has been downloaded, lime is charged into the kiln to form a new and more basic slag. The heat load of the furnace increases so that the refractory lime quickly turns into slag, and the temperature of the metal bath increases. After some time, 15–20 minutes, iron ore is loaded into the furnace, which increases the content of iron oxides in the slag and causes a carbon oxidation reaction in the metal

[C] + (FeO) = Co gas.

Carbon monoxide is formed and is released from the metal in the form of bubbles, creating the impression of boiling, which contributes to the mixing of the metal, the release of metal inclusions and dissolved gases, as well as the uniform distribution of temperature throughout the depth of the bath. For a good boiling of the bath, it is necessary to supply heat, since this reaction is accompanied by the absorption of heat. The duration of the boiling period of the bath depends on the furnace capacity and steel grade, and ranges from 1.25 to 2.5 hours or more.

Typically, iron ore is added to the furnace during the first boiling period, called metal polishing. The rate of carbon oxidation during this period in modern large-capacity open-hearth furnaces is 0.3–0.4% per hour.

During the second half of the boiling period, iron ore is not fed into the bath. The metal boils with small bubbles due to iron oxides accumulated in the slag. The rate of carbon burnout during this period is 0.15 - 0.25% per hour. During the boiling period, monitoring the basicity and fluidity of the slag.

When the carbon content in the metal is slightly lower than required for the finished steel, the last stage of smelting begins - the period of finishing and deoxidation of the metal. A certain amount of lump ferromanganese (12% Mn) is introduced into the furnace, and then after 10 - 15 minutes ferrosilicon (12-16% Si). Manganese and silicon interact with oxygen dissolved in the metal, as a result of which the carbon oxidation reaction is suspended. An external sign of the release of the metal from oxygen is the cessation of the release of carbon monoxide bubbles on the surface of the slag.

During the main smelting process, partial removal of sulfur from the metal occurs through the reaction

+ (CaO) = (CaO) + (FeO).

This requires high temperature and sufficient basicity of the slag.

Acid open hearth process. This process consists of the same periods as the main one. The charge used is very pure in terms of phosphorus and sulfur. This is explained by the fact that the resulting acidic slag cannot retain these harmful impurities.

Furnaces usually operate on solid charge. The amount of scrap is equal to 30–50% of the mass of the metal charge. No more than 0.5% Si is allowed in the charge. Iron ore cannot be fed into the furnace, since it can interact with the silica of the hearth and destroy it as a result of the formation of the low-melting compound 2FeO*SiO2. To obtain primary slag, a certain amount of quartzite or open-hearth slag is loaded into the furnace. After this, the charge is heated by furnace gases; iron, silicon, manganese are oxidized, their oxides are fused with fluxes and form acidic slag containing up to 40–50% SiO2. In this slag, most of the ferrous oxide is in silicate form, which makes it difficult to transfer from slag to metal. Boiling of the bath during the acid process begins later than during the main process, and occurs more slowly even with good heating of the metal. In addition, acidic slags have increased viscosity, which negatively affects carbon burnout.

Since steel is smelted under a layer of acidic slag with a low content of free ferrous oxide, this slag protects the metal from oxygenation. Before leaving the furnace, the steel contains less dissolved oxygen than the steel smelted in the main process.

To intensify the open-hearth process, the air is enriched with oxygen, which is supplied to the flame. This makes it possible to obtain higher temperatures in the flame, increase its emissivity, reduce the amount of combustion products and thereby increase the thermal power of the furnace.

Oxygen can also be introduced into the furnace bath. The introduction of oxygen into the torch and into the furnace bath reduces the melting periods and increases the furnace productivity by 25-30%. The production of chromium-magnesite vaults instead of dinas vaults makes it possible to increase the thermal power of furnaces, increase the overhaul period by 2-3 times and increase productivity by 6-10%.

Electron beam melting of metals. To obtain especially pure metals and alloys, electron beam melting is used. Melting is based on the use of the kinetic energy of free electrons accelerated in a high voltage electric field. A stream of electrons is directed at the metal, causing it to heat up and melt.

Electron beam melting has a number of advantages: electron beams make it possible to obtain a high heating energy density, regulate the melting speed within wide limits, eliminate contamination of the melt by the crucible material, and use the charge in any form. Overheating of the molten metal in combination with low melting speeds and deep vacuum create effective conditions for cleaning the metal from various impurities.

Electroslag remelting. A very promising method for producing high-quality metal is electroslag remelting. Drops of metal formed during remelting of the workpiece pass through a layer of liquid metal and are refined. When processing metal with slag and directed crystallization of the ingot from bottom to top, the sulfur content in the workpiece is reduced by 30–50%, and the content of non-metallic inclusions by two to three times.

Vacuuming steel. Vacuum melting is widely used to produce high-quality steel. The ingot contains gases and a certain amount of non-metallic inclusions. They can be significantly reduced if you use evacuation of steel during its smelting and casting. In this method, the liquid metal is kept in a closed chamber, from which air and other gases are removed. Evacuation of steel is carried out in a ladle before pouring into molds. The best results are obtained when steel, after evacuation in a ladle, is poured into molds also in vacuum. Metal smelting in vacuum is carried out in closed induction furnaces.

Refining steel in a ladle with liquid synthetic slag. The essence of this method is that steel is purified from sulfur, oxygen and non-metallic inclusions by intensively mixing the steel in a ladle with slag previously poured into it, prepared in a special slag melting furnace. Steel after treatment with liquid slag has high mechanical properties. By reducing the refining period in arc furnaces, the productivity of which can be increased by 10 - 15%. An open hearth furnace processed with synthetic slags is close in quality to the quality of steel smelted in electric furnaces.

Steel (from German Stahl) is an alloy (solid solution) of iron with carbon (and other elements), characterized by a eutectoid transformation. The carbon content in steel is no more than 2.14%. Carbon gives iron alloys strength and hardness, reducing ductility and toughness.

Considering that alloying elements can be added to steel, steel is an alloy of iron with carbon and alloying elements containing at least 45% iron (alloyed, high-alloy steel).

Applications

Steels with high elastic properties are widely used in mechanical and instrument making. In mechanical engineering they are used for the manufacture of springs, shock absorbers, power springs for various purposes, in instrument making - for numerous elastic elements: membranes, springs, relay plates, bellows, braces, suspensions.

Springs, machine springs and elastic elements of devices are characterized by a variety of shapes, sizes, and different operating conditions. The peculiarity of their work is that under large static, cyclic or shock loads, residual deformation is not allowed in them. In this regard, all spring alloys, in addition to the mechanical properties characteristic of all structural materials (strength, ductility, toughness, endurance), must have high resistance to small plastic deformations. Under conditions of short-term static loading, resistance to small plastic deformations is characterized by the elastic limit, and under long-term static or cyclic loading - by relaxation resistance.

Classification

Steels are divided into structural And instrumental. A type of tool steel is high-speed steel.

According to the chemical composition, steels are divided into carbon and alloy; including by carbon content - into low-carbon (up to 0.25% C), medium-carbon (0.3-0.55% C) and high-carbon (0.6-2% C); Alloyed steels, according to the content of alloying elements, are divided into low-alloyed - up to 4% of alloying elements, medium-alloyed - up to 11% of alloying elements and high-alloyed - over 11% of alloying elements.

Steels, depending on the method of their production, contain different amounts of non-metallic inclusions. The content of impurities is the basis for the classification of steels by quality: ordinary quality, high-quality, high-quality and especially high-quality.

Characteristics of steel

Density: 7700-7900 kg/m³,

Specific gravity: 75500-77500 N/m³ (7700-7900 kgf/m³ in the MKGSS system),

Specific heat capacity at 20 °C: 462 J/(kg °C) (110 cal/(kg °C)),

Melting point: 1450-1520 °C,

Specific heat of fusion: 84 kJ/kg (20 kcal/kg, 23 Wh/kg),

Thermal conductivity coefficient at a temperature of 100 °C. Chrome-nickel-tungsten steel 15.5 W/(m K)

Chromium steel 22.4 W/(mK)

Molybdenum steel 41.9 W/(mK)

Carbon steel (grade 30) 50.2 W/(mK)

Carbon steel (grade 15) 54.4 W/(mK)

Coefficient of linear thermal expansion at a temperature of about 20 °C: steel St3 (grade 20) 1/°C

stainless steel 1/°C

Rail steel 690-785 MPa

Steel production

The essence of the process of processing cast iron into steel is to reduce to the required concentration the content of carbon and harmful impurities - phosphorus and sulfur, which make the steel brittle and brittle. Depending on the method of carbon oxidation, there are various methods for processing cast iron into steel: converter, open-hearth and electrothermal.

Bessemer method

The Bessemer method processes cast iron that contains little phosphorus and sulfur and is rich in silicon (at least 2%). When oxygen is blown through, silicon is first oxidized, releasing a significant amount of heat. As a result, the initial temperature of cast iron from approximately 1300° C quickly rises to 1500-1600° C. Burnout of 1% Si causes an increase in temperature by 200° C. At about 1500° C, intense carbon burnout begins. Along with it, iron also intensively oxidizes, especially towards the end of the burnout of silicon and carbon:

Si + O2 = SiO2

2C + O2 = 2CO

2Fe + O2 = 2FeO

The resulting iron monoxide FeO dissolves well in molten cast iron and partially goes into steel, and partially reacts with SiO2 and in the form of iron silicate FeSiO3 goes into slag:

FeO + SiO2 = FeSiO3

Phosphorus completely transfers from cast iron to steel, so P2O5 with an excess of SiO2 cannot react with basic oxides, since SiO2 reacts more vigorously with the latter. Therefore, phosphorous cast iron cannot be processed into steel using this method.

All processes in the converter proceed quickly - within 10-20 minutes, since air oxygen blown through the cast iron reacts with the corresponding substances immediately throughout the entire volume of the metal. When blowing with oxygen-enriched air, the processes are accelerated. Carbon monoxide CO, formed when carbon burns out, gurgles upward and burns there, forming a torch of light flame above the neck of the converter, which decreases as the carbon burns out and then completely disappears, which serves as a sign of the end of the process. The resulting steel contains significant amounts of dissolved iron monoxide FeO, which greatly reduces the quality of the steel. Therefore, before casting, steel must be deoxidized using various deoxidizing agents - ferrosilicon, pheromanganese or aluminum:

2FeO + Si = 2Fe + SiO2

FeO + Mn = Fe + MnO

3FeO + 2Al = 3Fe + Al2O3

Manganese monoxide MnO as the main oxide reacts with SiO2 and forms manganese silicate MnSiO3, which goes into slag. Aluminum oxide, as a substance insoluble under these conditions, also floats to the top and turns into slag. Despite its simplicity and high productivity, the Bessemer method is now not widespread enough, since it has a number of significant disadvantages. Thus, cast iron for the Bessemer method must have the lowest content of phosphorus and sulfur, which is not always possible. With this method, a very large burnout of the metal occurs, and the yield of steel is only 90% of the mass of cast iron, and also a lot of deoxidizing agents are consumed. A serious disadvantage is the inability to regulate the chemical composition of steel.

Bessemer steel usually contains less than 0.2% carbon and is used as industrial iron for the production of wire, bolts, roofing iron, etc.

Thomas method

The Thomas method processes cast iron with a high phosphorus content (up to 2% or more). The main difference between this method and the Bessemer method is that the converter lining is made of magnesium and calcium oxides. In addition, up to 15% CaO is added to the cast iron. As a result, slag-forming substances contain a significant excess of oxides with basic properties.

Under these conditions, phosphate anhydride P2O5, which arises during the combustion of phosphorus, interacts with excess CaO to form calcium phosphate and goes into slag:

4P + 5O2 = 2P2O5

P2O5 + 3CaO = Ca3(PO4)2

The combustion reaction of phosphorus is one of the main sources of heat in this method. When 1% phosphorus is burned, the temperature of the converter rises by 150 ° C. Sulfur is released into the slag in the form of calcium sulfide CaS, insoluble in molten steel, which is formed as a result of the interaction of soluble FeS with CaO according to the reaction:

FeS + CaO = FeO + CaS

All the latter processes occur in the same way as with the Bessemer method. The disadvantages of the Thomas method are the same as those of the Bessemer method. Thomas steel is also low-carbon and is used as technical iron for the production of wire, roofing iron, etc.

Open hearth furnace

The open-hearth method differs from the converter method in that the burning of excess carbon in cast iron occurs not only due to atmospheric oxygen, but also the oxygen of iron oxides, which are added in the form of iron ore and rusty iron scrap.

An open-hearth furnace consists of a melting bath, covered with a refractory brick arch, and special regenerator chambers for preheating air and combustible gas. The regenerators are filled with a refractory brick packing. When the first two regenerators are heated by furnace gases, combustible gas and air are blown into the furnace through the red-hot third and fourth regenerators. After some time, when the first two regenerators heat up, the gas flow is directed in the opposite direction, etc.

The melting baths of powerful open-hearth furnaces are up to 16 m long, up to 6 m wide and more than 1 m high. The capacity of such baths reaches 500 tons of steel. Scrap iron and iron ore are loaded into the smelting bath. Limestone is also added to the mixture as a flux. The oven temperature is maintained at 1600-1650° C and above. Burnout of carbon and cast iron impurities in the first period of melting occurs mainly due to excess oxygen in the combustible mixture with the same reactions as in the converter, and when a slag layer forms above the molten cast iron - due to iron oxides

4Fe2O3 + 6Si = 8Fe + 6SiO2

2Fe2O3 + 6Mn = 4Fe + 6MnO

Fe2O3 + 3C = 2Fe + 3CO

5Fe2O3 + 2P = 10FeO + P2O5

FeO + C = Fe + CO

Due to the interaction of basic and acidic oxides, silicates and phosphates are formed, which turn into slag. Sulfur also goes into slag in the form of calcium sulfide:

MnO + SiO2 = MnSiO3

3CaO + P2O5 = Ca3(PO4)2

FeS + CaO = FeO + CaS

Open hearth furnaces, like converters, operate periodically. After casting the steel, the furnace is again loaded with charge, etc. The process of converting cast iron into steel in open hearths occurs relatively slowly over 6-7 hours. Unlike a converter, in open-hearth furnaces you can easily adjust the chemical composition of steel by adding scrap iron and ore to the cast iron in one proportion or another. Before the end of the smelting, the heating of the furnace is stopped, the slag is drained, and then acid oxides are added. Alloy steel can also be produced in open hearths. To do this, appropriate metals or alloys are added to the steel at the end of the melting process.

Electrothermal method

The electrothermal method has a number of advantages over the open-hearth method and especially the converter method. This method makes it possible to obtain very high quality steel and precisely regulate its chemical composition. Air access to the electric furnace is insignificant, therefore, much less iron monoxide FeO is formed, which pollutes the steel and reduces its properties. The temperature in the electric furnace is not lower than 2000° C. This allows steel to be melted using highly basic slags (which are difficult to melt), in which phosphorus and sulfur are more completely removed. In addition, due to the very high temperature in electric furnaces, it is possible to alloy steel with refractory metals - molybdenum and tungsten. But electric furnaces consume a lot of electricity - up to 800 kW/h per 1 ton of steel. Therefore, this method is used only for producing high-quality special steel.

Electric furnaces come in different capacities - from 0.5 to 180 tons. The furnace lining is usually made of the main one (with CaO and MgO). The composition of the charge may be different. Sometimes it consists of 90% scrap iron and 10% cast iron, sometimes it is dominated by cast iron with additives in a certain proportion of iron ore and scrap iron. Limestone or lime is also added to the mixture as a flux. The chemical processes during steel smelting in electric furnaces are the same as in open hearths.

Properties of steel

Physical properties

density ρ ≈ 7.86 g/cm3; coefficient of linear thermal expansion α = 11 ... 13 10−6 K−1;

thermal conductivity coefficient k = 58 W / (m K);

Young's modulus E = 210 GPa;

shear modulus G = 80 GPa;

Poisson's ratio ν = 0.28 ... 0.30;

resistivity (20 °C, 0.37-0.42% carbon) = 1.71 10−7 ohm m

Pearlite is a eutectoid mixture of two phases - ferrite and cementite, contains 1/8 cementite and therefore has increased strength and hardness compared to ferrite. Therefore, hypoeutectoid steels are much more ductile than hypereutectoid steels.

Steels contain up to 2.14% carbon. The foundation of the science of steel, as an alloy of iron and carbon, is the phase diagram of iron-carbon alloys - a graphical display of the phase state of iron-carbon alloys depending on their chemical composition and temperature. To improve the mechanical and other characteristics of steels, alloying is used. The main purpose of alloying the vast majority of steels is to increase strength by dissolving alloying elements in ferrite and austenite, forming carbides and increasing hardenability. In addition, alloying elements can increase corrosion resistance, heat resistance, heat resistance, etc. Elements such as chromium, manganese, molybdenum, tungsten, vanadium, and titanium form carbides, but nickel, silicon, copper, and aluminum do not form carbides. In addition, alloying elements reduce the critical cooling rate during quenching, which must be taken into account when assigning quenching modes (heating temperatures and cooling media). With a significant amount of alloying elements, the structure can change significantly, which leads to the formation of new structural classes compared to carbon steels.

Steel processing

Types of heat treatment

Steel in its initial state is quite plastic, it can be processed by deformation: forging, rolling, stamping. A characteristic feature of steel is its ability to significantly change its mechanical properties after heat treatment, the essence of which is to change the structure of the steel during heating, holding and cooling, according to a special regime. The following types of heat treatment are distinguished:

annealing;

normalization;

hardening;

Vacation.

The richer the steel is in carbon, the harder it is after heat treatment. Steel with a carbon content of up to 0.3% (technical iron) practically cannot be hardened.

Carburization (C) increases the surface hardness of mild steel due to increased carbon concentration in the surface layers.

QUESTION 13. Classification of steels by structure and purpose.

By structure:

1) hypoeutectoid (carbon 0-0.8) found in this structure. Ferrite and pearlite. How< С, тем >perlite, steel is stronger.

2) eutectoid (C=0.8). They have only pearlite in their structure, the steel is strong.

3) avtectoid (C 0.8-2.14). They have P and C second in their structure, they have become very hard, less viscous and plastic.

By purpose:

1) construction (C 0.8-2.14) these steels are quite strong, can be rolled and welded well.

2) Mechanical engineering (C 0.3-0.8). They have more perlite, so they are more TV than construction materials, although their viscosity and ductility are reduced.

3) Instrumental (C from 0.7-1.3). This is high carbon steel, very hard, not ductile.

4) Casting steels - alloys are used for steel castings. C=0.035. low carbon steels.

Steel- a common engineering material.

Steel refers to alloys of iron and carbon containing from 0.02 to 2.14% C. In addition to carbon, steel contains permanent impurities Mn, Si, S, P, etc., which affect its properties. Steels are classified by chemical composition, quality and application.

By chemical composition A distinction is made between carbon and alloy steels. Based on carbon content, both are divided into low (less than 0.25% C), medium (0.30 - 0.70% C) and high carbon (more than 0.7% C). Depending on the total content of alloying elements, low (less than 5%), medium (5.0 -10.0%) and high alloy (more than 10.0%) steels are distinguished.

By quality There are steels of ordinary quality, high-quality, high-quality and especially high-quality. This classification determines the conditions of metallurgical production of steels and, above all, the content of harmful impurities in them.

Ordinary quality steels include carbon steels containing up to 0.6% - C, up to 0.060% - S and up to 0.070% - P. Hot-rolled long products are made from them: beams, rods, channels, angles, pipes, etc., as well as cold rolled sheet steel.

In accordance with GOST 380-88, three groups (A, B and C) of ordinary quality steels are produced.

Group A includes steels supplied according to their mechanical properties without specifying their chemical composition. Steels of this group are designated by the letters St (steel) and the numbers 0, 1, 2...6.

The higher the number, the higher the carbon content and strength (σ in, MPa) and the lower the ductility (δ,%). These steels are used in the as-delivered condition without subsequent hot forming or heat treatment. Examples of steel in this group are the following grades: St0, St1, St4.

Group B - steels supplied with a guaranteed chemical composition. The designation of the steel grade of this group is preceded by the letter B, for example, BSt0, BSt1, etc.

Group B represents steels supplied with guaranteed chemical composition and mechanical properties. Group B is introduced into the designation of the steel grade of this group, for example, VSt1, VSt5. The chemical composition of the steel is the same as that of the corresponding grade of group B, and the mechanical properties are the same as those of group A.

Steels of groups B and C are used in cases where steel must be subjected to hot deformation or strengthened by heat treatment.

Steels of ordinary quality are further divided into calm, semi-quiet and boiling.

Mild steels are deoxidized during the smelting process with manganese, silicon, aluminum, and titanium. They contain a minimal amount of oxygen and various oxides. Silicon content is usually 0.15 - 0.35%. Quiet steels are designated by the letters "sp", for example, St3sp, BSt5sp, VSt4sp, etc.

Boiling steels are deoxidized during the smelting process only with manganese, the silicon content is no more than 0.1% (traces). Before pouring, they contain an increased amount of oxygen, which interacts with carbon to form CO bubbles. The release of bubbles from the metal gives the impression that it is boiling. Some of them remain in the metal, forming its honeycomb-like structure. Boiling steels are additionally designated by the letters “kp”, for example, BStZkp, St2kp, VSt4kp.

Semi-quiet steels, in terms of the degree of deoxidation, occupy an intermediate position between calm and boiling steels and contain up to 0.17% silicon (preliminarily deoxidized with manganese). Semi-quiet steels are additionally designated by the letters “ps”, for example, St1ps, St2ps, VSt5ps, etc. Due to its greater homogeneity compared to boiling steel, semi-mild steel has properties close to those of mild steel. Mild steel is used for the production of rolled products and shaped castings; semi-calm and boiling - for rental.

High quality steel. In terms of chemical composition, these are carbon alloy steels, the content of sulfur and phosphorus in which should not exceed 0.035% each. Fluctuations in carbon content within the grade should not exceed 0.08%.

High quality steels. These are carbon and alloy steels, smelted primarily in electric and acidic open-hearth furnaces. The content of sulfur and phosphorus is no more than 0.025% each, and fluctuations in carbon within the brand are no more than 0.07%.

Especially high-quality steels are alloy steels smelted in electric furnaces with electroslag remelting and contain sulfur and phosphorus of no more than 0.015% each.

By application The following classes of steels are distinguished: construction, general-purpose machine-building, special-purpose machine-building, tool, with special chemical and physical properties. In this work, we will limit ourselves to considering construction, general-purpose engineering and tool steels, and the rest will be studied in the Materials Science course.

Marking of construction and engineering steels for general purposes. The marking of carbon steels of ordinary quality was discussed above.

High-quality carbon steels according to GOST 1050-88 are marked with numbers 08, 10, 15, 20... 85, which indicate the average carbon content in hundredths of a percent. Depending on the degree of deoxidation, these steels can be calm or boiling (08 and 08kp, 10 and 10kp).

Alloy steels are marked with numbers and letters, for example, 15X; 45HF; 18HGT; 12ХН3А; 20Х2Н4А; 14G2 25G2S, etc. The two-digit numbers at the beginning of the mark indicate the average carbon content in hundredths of a percent; the letters to the right of the number indicate the alloying element: A - nitrogen, B - niobium, B - tungsten, G - manganese, D - copper, K - cobalt, N - nickel, M - molybdenum, P - phosphorus, P - boron, C - silicon, T - titanium, F - vanadium, X - chromium, C "zirconium, Yu - aluminum, U - rare earth. The numbers after the letter (element symbol) indicate the approximate content of the corresponding alloying element in whole percentages, the absence of a number indicates that it is about 1% or less. The letter A at the end of the designation indicates that the steel is high-quality (12ХИ3А), at the beginning - automatic steel (A15, A30), in the middle - nitrogen. For steels used in cast form, the letter L is placed at the end of the mark ( for example, 25L, 35GL).

Construction steel is used for welded structures, main oil and gas pipelines, for reinforcing reinforced concrete structures, etc. For these purposes, low-carbon and low-alloy high-quality steels, and steels of ordinary quality (VStZsp, VSt3Gps, VSt5Gps, 14G2, 17GS, 15HSND, etc.) are widely used.

General purpose engineering steel is divided into three groups: steels used without hardening heat treatment; case-hardened low-carbon (up to 0.25% C) and improved medium-carbon (from 0.30-0.50% C) steels. These are, as a rule, carbon and low-alloy steels.

Steels used without hardening heat treatment. These are steels supplied in sheets for subsequent stamping, deep drawing, etc. In terms of chemical composition, steels are low-carbon with low silicon content (kp, ps) and low-alloy (08kp, 08ps, 15kp, 20Khkp, etc.).

Cementable steels are used for products subjected to surface saturation with carbon. After carburizing, hardening and low tempering, parts made from these steels have a hard surface (HRC 58-62), good wear resistance, and a tough, strong core (HRC 20-30). For small non-critical products, steel grades 10, 15, 20, 15X, 20X are widely used. For more critical and large products, alloyed high-quality and high-quality steels are used, for example, 18KhGT, 12KhN3A, 20Kh2N4A, 20KhGR, 18Kh2N4VA, etc.

Upgradeable Machine-building steels are used after hardening and high tempering (improvement). For products with a small cross-section or operating under low loads, steel grades 35, 40, 45, 50 are used. For parts with a larger cross-section, low- and medium-alloy steels are used, which have high hardenability and provide high mechanical properties throughout the entire cross-section, for example, 40Х, 30ХГТ, 50Г2 , 40ХН, 40ХНМА, ЗОХН2ВФ, etc.

Tool steels designed for the manufacture of cutting, measuring, cold-formed and hot-formed tools. These are, as a rule, high-carbon steels containing over 0.70% C (with the exception of steels for hot-forming tools, which are classified as medium-carbon steels). These include high-quality and high-quality steels, carbon, alloy and high-speed. They are marked accordingly.

Carbon tool steels are designated by the letter U and numbers indicating the average carbon content in tenths of a percent (U7, U8, U10, U12A, etc.).

Alloyed tool steels 9ХС, X, 5ХВГ, 3Х8В2, etc. marked with a number showing the average carbon content in tenths of a percent, if it is less than 1.0%. If the carbon content is 1.0% or higher, then the figure is most often missing. The letters indicate alloying elements (see above), and the numbers following them indicate the content in whole percent of the corresponding alloying element.

High-speed steels are marked with the letter P (R14F4). The number following it indicates the content of the main alloying element (tungsten) in whole percent. The carbon content in high-speed steels is 0.75-1.15%, chromium - 3.8-4.2% is not indicated in the designation of the steel grade. In addition, all high-speed steels contain vanadium; if it is less than 2.2%, then it is not indicated in the brand.

For cutting tools, carbon steels U8, U10, U8A, U12 GOST 1435-90 are used, alloyed 9ХС, ХВГ, Х (GOST 5950-73), as well as high-speed high-alloy steels grades R18, R12, R6MZ, R6M5, R10K5 (GOST 19265- 73). A distinctive feature of tool steels for cutting tools is their high carbon content (from 0.70 to 1.5%), which makes it possible to obtain high hardness IKS 60-65 after quenching and tempering.

For the manufacture of cold-formed tools, carbon and alloy steels for cutting tools are often used. This is explained by the fact that the operating conditions of cutting dies and cutting tools are very close. The best steels for cold-forming tools are X12F1, X12M, X6VF, etc.

Steels for dies that deform metal in a hot state must have high mechanical properties (strength, toughness) at elevated temperatures and have fire resistance, i.e. withstand repeated heating and cooling (thermal cycles) without cracking. These are, as a rule, low- and medium-alloy steels containing carbon from 0.35 to 0.60%, such as 5ХНМ, 5ХНМА, 4Х5В2ФС, ЗХ2В8Ф, etc.

Steels for measuring instruments must have high hardness, wear resistance and maintain dimensional stability. For this purpose, high-carbon low-alloy steels of grades X, 9ХС, ХВГ, etc. are usually used. In addition, for flat tools (rulers, staples, templates, etc.) low-carbon structural steels 15, 15Х, 20Х, etc., subjected to surface saturation, are often used carbon followed by hardening.

Steel is an alloy of iron and carbon, in which the mass fraction of carbon is 2.14% (theoretically). In practice, the carbon concentration is no more than 1.5%. In addition to carbon, steel contains permanent impurities: silicon, manganese, sulfur, phosphorus and other chemical elements. Steel production involves the recycling of pig iron by various methods: open-hearth, converter, electric smelting, etc. The essence of steel production is the removal of carbon and other chemical elements in the process of melting a charge consisting of liquid or pig iron, scrap steel, iron ore and limestone . Melting is carried out in various steelmaking units: open-hearth furnaces, converters, electric arc, electric induction and other metallurgical units.

Steel is also the main structural material in mechanical engineering and other industrial sectors.

Under normal conditions, simple carbon steels are used; at high temperatures and active environments - special alloy steels (for example, for the manufacture of a pump for pumping acids, mechanisms operating in sea water, etc.).

In this regard, the ferrous metallurgy of our country produces steel with various physical, chemical and mechanical properties. All industries receive steel of various grades, assortments and names from metallurgists. It is almost impossible to remember this variety of steels supplied by metallurgists, therefore the science of metals - metal science - classifies all produced steels according to various criteria (Fig. 5.10).

According to the chemical composition, steels are divided into two large groups: carbon and alloy.

Rice. 5.10.

Carbon steels contain iron, carbon and permanent impurities inherent in iron-carbon alloys. There are no other chemical elements in carbon steels. Carbon steels, based on the mass fraction of carbon, are divided into low-carbon (up to 0.3% carbon), medium-carbon (0.3...0.6% carbon) and high-carbon (more than 0.6% carbon).

Alloyed Steels, in addition to carbon, contain various chemical elements, both metals and non-metals. These elements are introduced during the smelting process to obtain higher physicochemical and mechanical properties compared to carbon steels. To alloy means to alloy, to combine, therefore chemical elements introduced into steel are called alloying elements, and steels alloyed with them are called alloy steels.

The quality of steels depends on the characteristics of metallurgical processes, processed raw materials, type of smelting and other factors that determine the chemical composition of steels and the presence of harmful impurities in them - sulfur and phosphorus, as well as various gases: nitrogen, hydrogen and oxygen. Harmful impurities and gases present in them give steels negative physical, chemical, mechanical and technological properties, i.e. deteriorate their quality. In this regard, according to the quality of steel, both carbon and alloy, they are divided into four groups: steel of ordinary quality, high-quality, high-quality, and especially high-quality.

Become ordinary quality contain 0.045...0.060% sulfur, 0.04...0.07% phosphorus.

Quality steels are made with a mass fraction of sulfur not exceeding 0.04%, phosphorus - 0.035...0.040%. High-quality steels come in both carbon and alloy types.

High quality carbon and alloy steels contain no more than 0.02% sulfur and 0.03% phosphorus.

Particularly high quality steels have a mass fraction of sulfur of no more than 0.015%, phosphorus - no more than 0.025%. Alloyed especially high-quality steels are produced by electroslag or vacuum-arc remelting.

According to their purpose, carbon and alloy steels are divided into structural, tool and special.

Structural steels, both carbon and alloyed, are used for the manufacture of various machine parts, welded building structures, etc. These steels are subject to certain requirements for chemical composition, mechanical, technological, operational and chemical properties. These can be case-hardened, improveable and high-strength steels. Some of these steels are subjected to chemical-thermal treatment, others - only heat treatment. According to technological characteristics, structural steels are divided into stamped, welded, foundry and highly machinable by cutting (automatic). According to their intended purpose, these steels can be spring-spring, ball-bearing, magnetic, electrical, construction, etc.

Steels of this group according to their chemical properties are divided into stainless, acid-resistant, scale-resistant, etc., and depending on the chemical resistance they are structural and special-purpose.

Structural carbon steels include ordinary quality steels (grades STO, St1, etc.), as well as high-quality steels (grades 05, 10, 15, etc.). Alloyed structural steels include a large group of low- and medium-alloy steels subjected to chemical-thermal and heat treatment (for example, 20Х, 15Г, 15ХФ, 40Х, 45ХН, etc.).

Instrumental Carbon and alloy steels are used for the manufacture of cutting, measuring and impact tools, dies for deformation in hot and cold conditions. These steels are subject to high requirements for hardenability, red-hardness, durability (working time from sharpening to sharpening), etc.

Special Alloy steels are, as a rule, structural materials with special properties. These include stainless (corrosion-resistant), heat-resistant, magnetic, electrical, high electrical resistance, heat-resistant and other steels. This group consists of high-alloy steels with a mass fraction of alloying elements exceeding 10%. Chromium, nickel, manganese, etc. are used for alloying. The use of certain alloying elements is determined by the required properties. For example, corrosion-resistant steels must have a mass fraction of chromium of at least 13%, heat-resistant steels - depending on the required temperature - 9... 17% chromium, 2% silicon. Some brands, in addition, contain nickel or titanium (for example, 40Х9С2, 06Х17Г, etc.).

According to the method of deoxidation, steels are divided into three categories: boiling, calm and semi-quiet.

Deoxidation is the process of removing liquid iron oxide (IeO) from steel, which is formed during the smelting process and gives the steel an active tendency to corrosion. In addition, the deoxidation process removes nitrogen and hydrogen from the steel in the liquid state. Deoxidation is carried out by adding silicon, manganese or aluminum to the casting ladle before releasing steel, depending on the required degree of deoxidation.

It has been practically established that in the presence of oxygen in steel that has reacted with iron (FeO), high brittleness is formed during hot deformation. In addition, iron oxide contributes to a decrease in strength at low temperatures and is highly prone to intergranular corrosion.

Boiling steel is deoxidized with manganese. When steel cools, gases are released in the molds, which create the false impression that the steel is boiling as it solidifies. Boiling steels are produced both of ordinary quality and of high quality. As a rule, these steels are low-carbon.

Calm steel is deoxidized with aluminum, manganese and silicon. In these steels, oxygen almost completely reacts with deoxidizing agents, floats to the top and is removed with the slag. When cooled, they solidify quietly, without gas evolution. All high-quality alloy and carbon steels are produced calm.

Semi-calm steels occupy an intermediate position between boiling and calm steels. They are deoxidized with manganese and aluminum. Semi-quiet steels are produced only in carbon steels.

The structure of steel is greatly influenced by the mass fraction of carbon, alloying elements and the state of delivery. In this regard, according to their structure, steels are classified into annealed (equilibrium) and normalized states.

IN annealed state The structure of steels is divided into six classes:

• hypoeutectoid - ferrite and pearlite structure;
• eutectoid - pearlite structure;
• hypereutectoid - structure of pearlite and cementite;
• ledeburite - the structure of primary ledeburite or carbide;
• austenitic - the structure of solid solutions supersaturated with carbon;
• ferritic - the structure of solid solutions with slightly saturated carbon.

Carbon steels have the structure of the first three classes, alloy steels have the structure of all six classes. Ledeburite, austenitic and ferritic classes of structures are formed by introducing nickel, vanadium, tungsten and other alloying elements into the composition. With a certain combination, the formation of intermediate classes of structures is possible, for example, semi-ferritic, semi-austenitic, etc.

IN normalized state Steels have four classes of structures: ferritic, pearlitic, martensitic and austenitic.

The structure of ferritic steel is unstable. Depending on the cooling rate in air, this steel can acquire the structure of pearlite, troostite or sorbitol. All carbon and low-alloy steels belong to the ferritic class.

Low-carbon steels with a mass fraction of carbon up to 0.15%, alloyed with chromium (12...15%), form a stable ferrite structure. When heated and cooled, this class of steel does not change its structure.

Martensitic steels have high stability and, when cooled, form a hard, finely dispersed structure. This class includes medium and high alloy steels.

Austenitic steels are formed with a high mass fraction of nickel and manganese in combination with chromium. Steels of this class have high impact toughness.