Steel marking according to Russian, European and American systems. Chemical composition and classification of steels by purpose Brand composition of steels and classification by purpose

Understanding such an issue as the classification of carbon steels is very important, as this allows you to get a complete understanding of the characteristics of one or another type of this popular material. , like any other, is no less important, and a specialist must be able to understand it in order to choose the right alloy in accordance with its properties and chemical composition.

Distinctive characteristics and main categories

Carbon steels, which are based on iron and carbon, include alloys containing a minimum of additional impurities. The 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 (up to 2%).

In addition to decent technical characteristics, it should be noted affordable cost, which is important for a material widely used for the production of a wide variety of products.

The most significant advantages of carbon steels of various grades include:

• high plasticity;
• good workability (regardless of the heating temperature of the metal);
• excellent weldability;
• maintaining high strength even with significant heating (up to 400°);
• good tolerance to dynamic loads.

Carbon steels also have disadvantages, among which it is worth highlighting:

• a decrease in the ductility of the alloy with an increase in the carbon content in its composition;
• deterioration in cutting ability and decrease in hardness when heated to temperatures exceeding 200°;
• high susceptibility to the formation and development of corrosion processes, which imposes additional requirements on products made of such steel, which must be coated with a protective coating;
• weak electrical characteristics;
• tendency to thermal expansion.

The classification of carbon alloys by structure deserves special attention. The main influence on transformations in them is exerted by the quantitative carbon content. Thus, steels classified as hypoeutectoid have a structure based on ferrite and pearlite grains. The carbon content in such alloys does not exceed 0.8%. With an increase in the amount of carbon, the amount of ferrite decreases, and the volume of pearlite, accordingly, increases. According to this classification, steels containing 0.8% carbon are classified as eutectoid; the basis of their structure is predominantly pearlite. With a further increase in the amount of carbon, secondary cementite begins to form. Steels with this structure belong to the hypereutectoid group.

An increase in the amount of carbon in the steel composition to 1% leads to the fact that such properties of the metal as strength and hardness are significantly improved, while the yield strength and ductility, on the contrary, deteriorate. If the amount of carbon in steel exceeds 1%, this can lead to the formation of a coarse network of secondary martensite in its structure, which has a negative effect on the strength of the material. That is why in steels classified as high-carbon, the amount of carbon, as a rule, does not exceed 1.3%.

The properties of carbon steels are seriously influenced by the impurities contained in their composition. Elements that have a positive effect on the characteristics of the alloy (improving the deoxidation of the metal) are silicon and manganese, while phosphorus and sulfur are impurities that worsen its properties. Phosphorus in high content in carbon steel leads to the fact that products made from it become covered with cracks and even break when exposed to low temperatures. This phenomenon is called cold brittleness. Typically, steels with a high phosphorus content, if they are in a heated state, lend themselves well to welding and processing using forging, stamping, etc.

In products made from carbon steels that contain significant amounts of sulfur, a phenomenon called red brittleness may occur. The essence of this phenomenon is that metal, when exposed to high temperatures, becomes difficult to process. The structure of carbon steels, which contain a significant amount of sulfur, consists of grains with fusible formations at the boundaries. Such formations begin to melt as the temperature rises, which leads to a disruption of the bond between grains and, as a consequence, to the formation of numerous cracks in the metal structure. Meanwhile, the parameters of sulfur carbon alloys can be improved if they are microalloyed with zirconium, titanium and boron.

Production technologies

Today, there are three main technologies used in the metallurgical industry. Their main differences are the type of equipment used. This:

• converter type melting furnaces;
• open hearth units;
• melting furnaces powered by electricity.

In converter plants, all components of the steel alloy are melted: cast iron and scrap steel. In addition, the molten metal in such furnaces is additionally processed using technical oxygen. In cases where the impurities present in the molten metal need to be converted into slag, burnt lime is added to it.

The process of producing carbon steel using this technology is accompanied by active oxidation of the metal and its waste, the value of which can reach up to 9% of the total volume of the alloy. The disadvantage of this technological process is that it produces a significant amount of dust, and this necessitates the use of special dust cleaning units. The use of such additional devices affects the cost of the resulting product. However, all the shortcomings that characterize this technological process are fully compensated by its high productivity.

Smelting in an open-hearth furnace is another popular technology that is used to produce carbon steels of various grades. All the necessary raw materials (steel scrap, cast iron, etc.) are loaded into that part of the open-hearth furnace, which is called the melting chamber, which is heated to the melting temperature. Complex physical and chemical interactions take place in the chamber, in which molten metal, slag and a gaseous environment take part. The result is an alloy with the required characteristics, which is discharged in a liquid state through a special hole in the rear wall of the furnace.

Steel produced by smelting in electric furnaces, due to the use of a fundamentally different heating source, is not exposed to an oxidizing environment, which makes it cleaner. Various grades of carbon steel produced by smelting in electric furnaces contain less hydrogen. This element is the main reason for the appearance of flakes in the structure of alloys, which significantly worsen their characteristics.

No matter how the carbon alloy is smelted and no matter what category in the classification it belongs to, the main raw materials for its production are cast iron and metal scrap.

Methods for improving strength characteristics

If the properties of grades are improved by introducing special additives into their composition, then the solution to this 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 the use of this technology, a structure consisting of martensite is formed in the surface layer of the metal, the hardness of which is 9.5 GPa (in some areas it reaches 11.5 GPa).

Surface plasma hardening also leads to the formation of metastable retained austenite in the metal structure, the amount of which increases if the percentage of carbon in the steel composition increases. This structural formation, which can transform into martensite when running in a carbon steel product, significantly improves such characteristics of the metal as wear resistance.

One of the effective ways to significantly improve the characteristics of carbon steel is chemical-thermal treatment. The essence of this technology is that a steel alloy, heated to a certain temperature, is subjected to chemical action, which can significantly improve its characteristics. After such treatment, which can be applied to carbon steels of various grades, the hardness and wear resistance of the metal increases, and its corrosion resistance to wet and acidic environments improves.

Other classification parameters

Another parameter by which carbon alloys are classified is the degree of their purification from harmful impurities. Steels that contain a minimum amount of sulfur and phosphorus have better mechanical characteristics (but also higher cost). This parameter became the basis for the classification of carbon steels, according to which alloys are distinguished:

• ordinary quality (B);
• qualitative (B);
• increased quality (A).

Steels of the first category (their chemical composition is not specified by the manufacturer) are selected based only on their mechanical characteristics. Such steels are characterized by minimal cost. They are not subjected to heat or pressure treatment. For high-quality steels, the manufacturer stipulates the chemical composition, and for high-quality alloys, the mechanical properties. What is important is that products made 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 according to their main purpose. Thus, a distinction is made between structural steels, from which parts for various purposes are produced, and tool steels, used in full accordance with their name - for the manufacture of various tools. Tool alloys, when compared with structural alloys, are characterized by increased hardness and strength.

In the marking of carbon steel you can find the designations “sp”, “ps” and “kp”, which indicate the degree of its deoxidation. This is another parameter for classifying such alloys.
The letters “sp” in the marking indicate quiet alloys, which may contain up to 0.12% silicon. They are characterized by good impact strength even at low temperatures and are characterized by high uniformity of structure and chemical composition. Such carbon steels also have disadvantages, the most significant of which are that the surface of products made from them is of lower quality than that of boiling steels, and after welding work, the characteristics of parts made from them deteriorate significantly.

Semi-quiet alloys (denoted by the letters “ps” in the marking), in which silicon can be contained in the range of 0.07–0.12%, are characterized by a uniform distribution of impurities in their composition. This ensures the consistency of the characteristics of products made from them.

In boiling carbon steels containing no more than 0.07% silicon, the deoxidation process is not completely completed, which causes the heterogeneity of their structure. Meanwhile, they are distinguished by a number of advantages, the most significant of which include:

• low cost, which is explained by the insignificant content of special additives;
• high plasticity;
• good weldability and machinability using plastic deformation methods.

How are carbon steel alloys marked?

Understanding the principles of marking carbon steel is as easy as understanding the basis for its classification: they are not much different from the rules for designating steel alloys of other categories. In order to decipher such markings, you don’t even need to look at special tables.

The letter “U” at the very beginning of the alloy brand designation indicates that it belongs to the tool category. The letters “A”, “B” and “C” written at the very end of the marking indicate which quality group carbon steel belongs to. The amount of carbon contained in the alloy is indicated at the very beginning of its marking. Moreover, for steels of high quality (group “A”), the amount of this element will be indicated in hundredths of a percent, and for alloys of groups “B” and “C” - in tenths.

In the marking of individual carbon steels you can find the letter “G” after the numbers indicating the quantitative carbon content. This letter indicates that the metal contains an increased amount of an element such as manganese. The designations “sp”, “ps” and “kp” indicate what degree of deoxidation carbon steel corresponds to.

Carbon alloys, due to their characteristics and low cost, are actively used for the production of elements of building structures, machine parts, tools and metal products for various purposes.

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Steel is a malleable and wrought alloy of iron and carbon (as a permanent impurity). Also contains other alloying elements and other harmful impurities. The carbon content should not exceed 2.14%. By changing the chemical composition of this alloy using carbon concentration and adding alloying elements, it is possible to obtain a wide range of different grades of this metal that will have different properties. This is what allows this material to be used in most industries.

Principles of steel classification

Classification and marking of steel occurs according to the following parameters:

By chemical composition

Depending on the chemical composition, this metal is divided into two types: carbon and alloy. In its turn, carbonaceous are divided into:

• low-carbon (carbon content below 0.2%);
• medium-carbon (carbon content in the range of 0.2% - 0.45%);
• high-carbon (carbon content above 0.5%).

Alloy steels are classified according to the total total amount of alloying elements (the carbon content is not summed up; manganese begins to be considered an alloying element when its content in the alloy is more than 1%, silicon - more than 0.8%). The following are distinguished:

• low alloy (below 2.5%);
• medium alloyed (within 2.5% - 10%);
• highly alloyed (more than 10%).

By structure

Such a classification feature as the structure of the material is considered less stable, since it depends on the cooling rate, alloying, heat treatment method and some other variable factors. However, the structure of the finished material still allows for an objective assessment of its quality. Classification of steel by structure in the annealing and normalization states. In the annealing state, the following are distinguished:

After the normalization process, steel is divided into the following classes:

• pearlitic - contain a low amount of alloying elements, structure after normalization: pearlite, pearlite + ferrite, pearlite + hypereutectoid carbide;
• martensitic - contain a high amount of alloying elements, as well as a relatively low critical hardening rate;
• austenitic - characterized by a high content of alloying elements, structure: austenite, austenite + carbide.

By purpose

For reasons such as appointment e steels are divided into structural, tool and special purpose(having special properties).

Structural ones are used for the manufacture of all kinds of parts in devices, machines, and elements of building structures. They are divided into:

• ordinary quality;
• improved;
• cemented;
• automatic;
• high strength;
• spring-spring.

Tools are used for the manufacture of cutting, measuring and other tools. They are divided into the following groups:

• for the manufacture of cutting tools;
• for the manufacture of measuring instruments;
• for the manufacture of stamping and pressing equipment.

Special purpose are alloys with special physical and/or mechanical properties. There are:

By quality and production method

In this case, quality is understood as the entire set of properties of the metal, which are determined by the metallurgical process of its manufacture. The quality of steel is determined by the presence of harmful impurities in it. First of all, these are the chemical elements sulfur and phosphorus. Depending on their content they are divided into:

• ordinary quality - containing up to 0.06% sulfur and 0.07% phosphorus;
• high-quality - up to 0.035% sulfur and 0.035% phosphorus;
• high-quality - no more than 0.025% sulfur and 0.025% phosphorus.
• especially high quality - no more than 0.015% sulfur and 0.025% phosphorus.

According to the degree of deoxidation

Deoxidation is the process of removing oxygen from a liquid alloy. Undeoxidized steel has relatively low ductility and is more susceptible to brittle fracture during heat treatment under pressure. According to the degree of deoxidation they are divided into:

• calm;
• semi-calm;
• boiling.

The process of deoxidizing still steels in a smelting furnace/or ladle using manganese, aluminum and silicon. Solidification in the mold occurs quietly, without gas evolution. A shrinkage cavity is formed in the upper part of the ingots. This type has anisotropy, that is, the mechanical properties are different and depend on the direction - the plastic properties in the transverse direction (along the rolling direction) are significantly lower than in the longitudinal direction. In addition, in the upper part of the ingot the content of sulfur, phosphorus and carbon is increased, and in the lower part it is reduced. This significantly worsens the properties of the product, sometimes even to the point of rejection.

Deoxidation in boiling water occurs only due to manganese. Excess oxygen during solidification partially reacts with carbon, releasing gas bubbles (carbon monoxide). This is where the impression of “boiling” is created. In this type there are practically no non-metallic inclusions arising from deoxidation products. It is a low-carbon alloy, with a minimum silicon content and a high content of gaseous impurities. Used in the manufacture of car body parts, etc. It has good cold formability.

Semi-quiet steels occupy a middle position between calm and boiling steels. Deoxidation is carried out in two stages: partly in the melting furnace and ladle, and finally in the mold. In the mold, deoxidation occurs due to the carbon contained in the metal.

Decoding steels in materials science

Belongs to the class: structural carbon quality. Chemical composition: carbon - 0.17−0.24%; silicon - 0.17−0.37%; manganese - 0.35−0.65%; sulfur - up to 0.04%; phosphorus - up to 0.04%. Widely used in boiler making, for pipes and heating pipelines for various purposes; in addition, the industry produces rods and sheets.

HVG transcript

Belongs to the class: alloyed instrumental. Used for the manufacture of measuring and cutting tools, taps, broaches.

Steel is the main metal material used in the production of machines, tools and appliances. Its widespread use is explained by the presence in this material of a whole complex of valuable technological, mechanical and physicochemical properties. In addition, steel has a relatively low cost and can be produced in large quantities. The production process of this material is constantly being improved, thanks to which the properties and quality of steel can ensure trouble-free operation of modern machines and devices at high operating parameters.

General principles for classifying steel grades

The main classification characteristics of steels: chemical composition, purpose, quality, degree of deoxidation, structure.

• Become by chemical composition divided into carbon and alloy. Based on the mass fraction of carbon, both the first and second groups of steels are divided into: low-carbon (less than 0.3% C), medium-carbon (C concentration is in the range of 0.3-07%), high-carbon - with a carbon concentration of more than 0.7%.

Alloyed steels are those that contain, in addition to permanent impurities, additives introduced to increase the mechanical properties of this material.

Chromium, manganese, nickel, silicon, molybdenum, tungsten, titanium, vanadium and many others are used as alloying additives, as well as a combination of these elements in various percentages. By number of additives Steels are divided into low-alloy (alloying elements less than 5%), medium-alloy (5-10%), and high-alloy (contain more than 10% additives).

• According to its purpose Steels can be structural, tool, and special-purpose materials with special properties.

The most extensive class are structural steels, which are intended for the manufacture of building structures, parts of devices and machines. In turn, structural steels are divided into spring-type, improved, cemented and high-strength.

Tool steels are distinguished depending on the purpose of the tool produced from them: measuring, cutting, hot and cold deformation dies.

Special purpose steels are divided into several groups: corrosion-resistant (or stainless), heat-resistant, heat-resistant, electrical.

• By quality Steels are of ordinary quality, high-quality, high-quality and especially high-quality.

The quality of steel is understood as a combination of properties determined by the process of its manufacture. Such characteristics include: uniformity of structure, chemical composition, mechanical properties, manufacturability. The quality of steel depends on the content of gases in the material - oxygen, nitrogen, hydrogen, as well as harmful impurities - phosphorus and sulfur.

• According to the degree of deoxidation and the nature of the solidification process, steels are calm, semi-calm and boiling.

Deoxidation is the operation of removing oxygen from liquid steel, which provokes brittle fracture of the material during hot deformation. Mild steels are deoxidized with silicon, manganese and aluminum.

• By structure They separate steels in the annealed (equilibrium) state and in the normalized state. Structural forms of steels are ferrite, pearlite, cementite, austenite, martensite, ledeburite and others.

The influence of carbon and alloying elements on the properties of steel

Industrial steels are chemically complex alloys of iron and carbon. In addition to these basic elements, as well as alloying components in alloy steels, the material contains permanent and random impurities. The main characteristics of steel depend on the percentage of these components.

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Carbon has a decisive influence on the properties of steel. After annealing, the structure of this material consists of ferrite and cementite, the content of which increases in proportion to the increase in carbon concentration. Ferrite is a low-strength and ductile structure, while cementite is hard and brittle. Therefore, an increase in carbon content leads to an increase in hardness and strength and a decrease in ductility and toughness. Carbon changes the technological characteristics of steel: workability by pressure and cutting, weldability. An increase in carbon concentration leads to deterioration in machinability due to hardening and reduced thermal conductivity. The separation of chips from high-strength steel increases the amount of heat generated, which causes a decrease in tool life. But low-carbon steels with low viscosity are also poorly processed, since chips that are difficult to remove are formed.

Steels with a carbon content of 0.3-0.4% have the best cutting machinability.

An increase in carbon concentration leads to a decrease in the ability of steel to deform in hot and cold states. For steel intended for complex cold forming, the amount of carbon is limited to 0.1%.

Low-carbon steels have good weldability. For welding medium- and high-carbon steels, heating, slow cooling and other technological operations are used to prevent the occurrence of cold and hot cracks.

To obtain high strength properties, the amount of alloying components must be rational. Excess alloying, excluding the introduction of nickel, leads to a decrease in toughness reserve and provokes brittle fracture.

• Chromium is a non-deficient alloying component and has a positive effect on the mechanical properties of steel at its content of up to 2%.
• Nickel is the most valuable and scarce alloying additive, introduced in a concentration of 1-5%. It most effectively reduces the cold brittleness threshold and helps to increase the temperature reserve of viscosity.
• Manganese, as a cheaper component, is often used as a substitute for nickel. Increases yield strength, but may make steel sensitive to overheating.
• Molybdenum and tungsten are expensive and scarce elements used to increase the heat resistance of high-speed steels.

Principles of steel marking according to the Russian system

In the modern metal products market there is no common steel marking system, which significantly complicates trading operations, leading to frequent errors when ordering.

In Russia, an alphanumeric designation system has been adopted, in which the names of the elements contained in steel are marked with letters, and their quantities are marked with numbers. The letters also indicate the method of deoxidation. The marking “KP” denotes boiling steels, “PS” – semi-calm steels, and “SP” – calm steels.

• Ordinary quality steels have an index St, after which a conditional grade number from 0 to 6 is indicated. Then the degree of deoxidation is indicated. The group number is placed in front: A – steel with guaranteed mechanical characteristics, B – chemical composition, C – both properties. As a rule, the group A index is not assigned. Example of designation – B Article 2 KP.
• To designate structural high-quality carbon steels, a two-digit number indicating the C content in hundredths of a percent is indicated in front. At the end - the degree of deoxidation. For example, steel 08KP. High-quality tool carbon steels have the letter U in front, and then a double-digit carbon concentration in tenths of a percent - for example, U8 steel. High-quality steels have the letter A at the end of the grade.
• In alloy steel grades, letters indicate alloying elements: “H” is nickel, “X” is chromium, “M” is molybdenum, “T” is titanium, “B” is tungsten, “Y” is aluminum. In structural alloy steels, the C content is indicated in hundredths of a percent at the front. In tool alloy steels, carbon is marked in tenths of a percent; if the content of this component exceeds 1.5%, its concentration is not indicated.
• High-speed tool steels are designated by the index P and an indication of the tungsten content in percent, for example, P18.

Marking of steels according to American and European systems

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In the United States, there are several steel marking systems developed by various standards organizations. For stainless steels, the AISI system is most often used, which is also valid in Europe. According to AISI, steel is designated by three numbers, in some cases followed by one or more letters. The first number indicates the class of steel, if it is 2 or 3, then it is austenitic class, if 4 it is ferritic or martensitic. The next two digits indicate the serial number of the material in the group. The letters stand for:

• L – low mass fraction of carbon, less than 0.03%;
• S – normal concentration of C, less than 0.08%;
• N means nitrogen has been added;
• LN – low carbon content combined with nitrogen addition;
• F – increased concentration of phosphorus and sulfur;
• Se – steel contains selenium, B – silicon, Cu – copper.

In Europe, the EN system is used, which differs from the Russian one in that it first lists all alloying elements, and then, in the same order, their mass fraction is indicated in numbers. The first number is the carbon concentration in hundredths of a percent.

If alloy steels, structural and tool, except for high-speed steels, include more than 5% of at least one alloying additive, the letter “X” is placed in front of the carbon content.

EU countries use the EN marking, in some cases indicating in parallel the national mark, but with the mark “obsolete”.

International analogues of corrosion-resistant and heat-resistant steels

Corrosion-resistant steels

Europe (EN)	Germany (DIN)	USA (AISI)	Japan (JIS)	CIS (GOST)
1.4000	X6Cr13	410S	SUS 410 S	08Х13
1.4006	X12CrN13	410	SUS 410	12Х13
1.4021	X20Cr13	(420)	SUS 420 J1	20Х13
1.4028	X30Cr13	(420)	SUS 420 J2	30Х13
1.4031	X39Cr13		SUS 420 J2	40Х13
1.4034	X46Cr13	(420)		40Х13
1.4016	X6Cr17	430	SUS 430	12Х17
1.4510	X3CrTi17	439	SUS 430 LX	08Х17Т
1.4301	X5CrNI18-10	304	SUS 304	08Х18Н10
1.4303	X4CrNi18-12	(305)	SUS 305	12Х18Н12
1.4306	X2CrNi19-11	304 L	SUS 304 L	03Х18Н11
1.4541	X6CrNiTi18-10	321	SUS 321	08Х18Н10Т
1.4571	X6CrNiMoTi17-12-2	316 Ti	SUS 316 Ti	10Х17Н13М2Т

Heat-resistant steel grades

Europe (EN)	Germany (DIN)	USA (AISI)	Japan (JIS)	CIS (GOST)
1.4878	X12CrNiTi18-9	321H		12Х18Н10Т
1.4845	X12CrNi25-21	310 S		20Х23Н18

High speed steel grades

steel grade			Analogues in US standards
CIS countries GOST	Euronorms
R0 M2 SF10-MP
R2 M10 K8-MP
R6 M5 K5-MP
R6 M5 F3-MP
R6 M5 F4-MP
R6 M5 F3 K8-MP
R10 M4 F3 K10-MP
R6 M5 F3 K9-MP
R12 M6 F5-MP
R12 F4 K5-MP
R12 F5 K5-MP

Structural steel

steel grade			Analogues in US standards
CIS countries GOST	Euronorms

Basic range of stainless steel grades

CIS (GOST)	Euronorms (EN)	Germany (DIN)	USA (AISI)
03 X17 N13 M2		X2 CrNiMo 17-12-2
03 X17 N14 M3		X2 CrNiMo 18-4-3
03 X18 N10 T-U
06 ХН28 MDT		X3 NiCrCuMoTi 27-23
08 X17 N13 M2		X5CrNiMo 17-13-3
08 X17 N13 M2 T		Х6 CrNiMoTi 17-12-2
		X6 CrNiTi 18-10
20 Х25 Н20 С2		X56 CrNiSi 25-20
03 X19 N13 M3
02 X18 M2 BT
02 X28 N30 MDB		X1 NiCrMoCu 31-27-4
03 X17 N13 AM3		X2 CrNiMoN 17-13-3
03 X22 N5 AM2		X2 CrNiMoN 22-5-3
03 X24 N13 G2 S
08 X16 N13 M2 B		X1 CrNiMoNb 17-12-2
08 X18 N14 M2 B	1.4583 X10 CrNiMoNb	X10 CrNiMoNb 18-12
		X8 СrNiAlTi 20-20
		X3 CrnImOn 27-5-2
		Х6 CrNiMoNb 17-12-2
		X12 CrMnNiN 18-9-5

Bearing steel

Spring steel

steel grade			Analogues in US standards
CIS countries GOST	Euronorms

Heat resistant steel

steel grade			Analogues in US standards
CIS countries GOST	Euronorms

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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.

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. The combustion of carbon 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.