Steel: definition, classification, chemical composition and application

How often do we hear the word steel. And pronounce it not only professionals in the field of metallurgical production, but also the townsfolk. No solid construction can do without steel. In fact, when we talk about something metallic, we mean a product made of steel. We learn what it consists of and how it is classified.

Definition

Steel is perhaps the most popular alloy based on iron and carbon. Moreover, the share of the latter ranges from 0.1 to 2.14%, and the former cannot be lower than 45%. The simplicity of production and the availability of raw materials are crucial in the distribution of this metal to all spheres of human activity.

The main characteristics of the material vary depending on its chemical composition. The definition of steel as an alloy consisting of two components, iron and carbon, cannot be called complete. It may include, for example, chromium - to give heat resistance, and nickel, to provide resistance to corrosion.

Mandatory material components contribute to additional benefits. So, iron makes the alloy malleable and easily deformable under certain conditions, and carbon makes strength and hardness at the same time as brittle. That is why its share is so small in the total mass of steel. The determination of the alloy production method led to the content of manganese in the amount of 1% and silicon - 0.4%. There are a number of impurities that appear during the melting of a metal and which they are trying to get rid of. Along with phosphorus and sulfur, oxygen and nitrogen also degrade the properties of the material, making it less durable and changing ductility.

Classification

The definition of steel as a metal with a certain set of characteristics, of course, is beyond doubt. However, it is precisely its composition that makes it possible to classify the material in several directions. So, for example, metals are distinguished by the following characteristics:

• in chemical;
• structurally;
• by quality;
• by appointment;
• according to the degree of deoxidation;
• by hardness;
• on steel weldability.

The definition of steel, marking and all its characteristics will be described later.

Marking

Unfortunately, there is no world designation for steels, which greatly complicates trade between countries. In Russia, an alphanumeric system is defined. Letters denote the name of the elements and the method of deoxidation, and numbers indicate their number.

Chemical composition

There are two methods for dividing steel by chemical composition. The definition given by modern textbooks makes it possible to distinguish between carbon and alloyed materials.

The first sign defines steel as low-carbon, medium-carbon and high-carbon, and the second - low-alloy, medium-alloyed and high-alloyed. Low-carbon is called metal, which can include, according to GOST 3080-2005, in addition to iron, the following components:

• Carbon - up to 0.2%. It contributes to thermal hardening, due to which the temporary resistance and hardness are doubled.
• Manganese in an amount of up to 0.8% actively enters into a chemical bond with oxygen and does not allow the formation of iron oxide. The metal better withstands dynamic loads and is more malleable to thermal hardening.
• Silicon - up to 0.35%. With it, mechanical characteristics, such as viscosity, strength, weldability, become better.

According to GOST, the definition of steel as low-carbon is given to a metal that contains, in addition to useful, a number of harmful impurities in the following quantity. It:

• Phosphorus - up to 0.08% is responsible for the occurrence of cold brittleness, worsens endurance and strength. Reduces the toughness of the metal.
• Sulfur - up to 0.06%. It complicates the processing of metal by pressure, increases the temper brittleness.
• Nitrogen. Reduces the technological and strength properties of the alloy.
• Oxygen. Reduces durability and prevents the processing of tools during cutting.

It should be noted that low- or low-carbon steels are particularly soft and ductile. They are well deformed both in hot and in cold state.

The definition of medium carbon steel as well as its composition, of course, differ from the material described above. And the biggest difference is the amount of carbon, which ranges from 0.2 to 0.45%. Such a metal has a low viscosity and ductility along with excellent strength properties. Mild steel is usually used to make parts used under normal power loads.

If the carbon content is more than 0.5%, then such steel is called high carbon. It has increased hardness, reduced viscosity, ductility, and is used in the stamping of tools and parts by hot and cold deformation.

In addition to identifying the carbon present in the steel, characterization of the material is possible through additional impurities in it. If, in addition to ordinary elements, chromium, nickel, copper, vanadium, titanium, nitrogen in a chemically bound state are purposefully introduced into the metal, then it is called alloyed. Such additives reduce the risk of brittle fracture, increase corrosion resistance and strength. Their number also indicates the degree of alloying of steel:

• low alloyed - has up to 2.5% alloying additives;
• medium alloyed - from 2.5 to 10%;
• highly alloyed - up to 50%.

What does it mean? For example, an increase in any properties of steel is provided as follows:

1. The addition of chromium. It positively affects the mechanical characteristics in the amount of 2% of the total.
2. The introduction of Nickel from 1 to 5% increases the temperature reserve of viscosity. And reduces cold brittleness.
3. Manganese works the same way as nickel, although much cheaper. However, it increases the sensitivity of the metal to overheating.
4. Tungsten is a carbide forming additive that provides high hardness. Since it prevents grain growth during heating.
5. Molybdenum is an expensive supplement. Which increases the heat resistance of high speed steels.
6. Silicon. Increases acid resistance, elasticity, scale resistance.
7. Titanium. May contribute to the formation of a fine-grained structure if combined with chromium and manganese.
8. Copper. Increases anticorrosion properties.
9. Aluminum. Increases heat resistance, scale, toughness.

Structure

The determination of the composition of steel would be incomplete without a study of its structure. However, this symptom is unstable, and may depend on a number of factors, such as: heat treatment mode, cooling rate, degree of alloying. According to the rules, the steel structure should be determined after annealing or normalization. After annealing, the metal is divided into:

• hypereutectoid structure - with excessive ferrite;
• eutectoid, which consists of perlite;
• hypereutectoid - with secondary carbides;
• ledeburite - with primary carbides;
• austenitic - with a face-centered crystal lattice;
• ferritic - with a cubic body-centered lattice.

The determination of the class of steel is possible after normalization. It is understood as a type of heat treatment, which includes heating, exposure and subsequent cooling. Pearlitic, austenitic and ferritic classes are distinguished here.

Quality

The determination of types has become possible in quality in four ways. It:

1. Of ordinary quality are steels with a carbon content of up to 0.6%, which are smelted in open-hearth furnaces or in converters using oxygen. They are considered the cheapest and inferior in characteristics to the metals of other groups. An example of such steels are St0, St3sp, St5kp.
2. Qualitative. Bright representatives of this type are St08kp, St10ps, St20. They are smelted using the same furnaces, but with higher requirements for the charge and production processes.
3. High-quality steels are melted in electric furnaces, which guarantees an increase in the purity of the material for non-metallic inclusions, that is, an improvement in mechanical properties. These materials include St20A, St15H2MA.
4. High-quality - made by the method of special metallurgy. They are subjected to electroslag remelting, which provides purification from sulfides and oxides. These steels include St18HG-Sh, St20HGNTR-Sh.

Structural steels

This is perhaps the simplest and most understandable sign for the average person. Distinguish structural, tool and special steel. Structural accepted to divide into:

1. Construction - these are carbon steels of ordinary quality and representatives of a low alloyed range. Several requirements are imposed on them, the main of which is weldability at a fairly high level. An example is StS255, StS345T, StS390K, StS440D.
2. From cementitious products are made that work under conditions of surface wear and at the same time experience dynamic loads. These include low-carbon steels St15, St20, St25 and some alloyed: St15H, St20H, St15HF, St20HN, St12HNZA, St18H2N4VA, St18H2N4MA, St18HGT, St20HGR, St30HGT.
3. For cold stamping, foliage rolled from high-quality low-carbon samples is used. Such as St08Yu, St08ps, St08kp.
4. Improved steels that undergo improvement during hardening and high tempering. These are medium-carbon (St35, St40, St45, St50), chromium (St40X, St45X, St50X, St30XRA, St40XP) steels, as well as chromium silicon manganese, chromium-nickel-molybdenum and chromium-nickel.
5. Spring springs have elastic properties and retain them for a long time, as they have a high degree of resistance to fatigue and fracture. These are carbon representatives of St65, St70 and alloy steel (St60S2, St50HGS, St60S2HFA, St55HGR).
6. High-strength specimens are those that have a strength twice that of other structural steels, achieved by heat treatment and chemical composition. The bulk are alloyed medium-carbon steels, for example, St30KhGSN2A, St40KhN2MA, St30KhGSA, St38KHN3MA, StOZN18K9M5T, St04KHIN9M2D2TYU.
7. Ball-bearing steels are characterized by special endurance, a high degree of wear resistance and strength. Requirements are made to them for the absence of various kinds of inclusions. These samples include high-carbon steels with a chromium content in the composition (STShH9, STShH15).
8. Automatic steel definitions have the following. These are samples for use in the manufacture of non-essential products such as bolts, nuts, screws. Such spare parts are usually machined. Therefore, the main task is to increase the machinability of parts, which is achieved by introducing tellurium, selenium, sulfur and lead into the material. Such additives contribute to the formation of brittle and short chips during processing and reduce friction. The main representatives of automatic steels are designated as follows: STA12, StA20, StA30, StAS11, StAS40.
9. Alloy steels with a chromium content of about 12% are classified as corrosion-resistant, since it forms an oxide film on the surface that prevents the occurrence of corrosion. Representatives of these alloys are St12H13, St20Kh17N2, St20Kh13, St30Kh13, St95Kh18, St15Kh28, St12Kh18NYuT,
10. Wear-resistant samples are used in products that work with abrasive friction, impact and strong pressure. An example is the details of railway tracks, crushing and tracked vehicles, such as St110G13L.
11. Heat resistant steels can work with high heat. They are used in the manufacture of pipes, gas and steam turbine spare parts. These are mainly high-alloy low-carbon samples, which necessarily have nickel, which may contain additives in the form of molybdenum, niobium, titanium, tungsten, and boron. An example would be St15HM, St25H2M1F, St20HZMVF, St40HYUS2M, St12H18N9T, StHN62MVKYU.
12. Heat-resistant are particularly resistant to chemical damage in air, gas and furnace, oxidizing and carburizing environments, but exhibit creep under severe loads. Representatives of this type are St15Kh5, St15Kh6SM, St40Kh9S2, St20Kh20N14S2.

Tool steel

In this group, alloys are divided into die alloys for cutting and measuring tools. Steel for stamps come in two forms.

• Material for cold deformation should have a high degree of hardness, strength, wear resistance, heat resistance. But to have sufficient viscosity (StH12F1, StH12M, StH6VF, St6H5VMFS).
• The material for hot deformation is characterized by good strength and toughness. Along with wear resistance and scale resistance (St5KhNM, St5KhNV, St4KhZVMF, St4Kh5V2FS).

Steel for measuring tools, in addition to wear resistance and hardness, should be distinguished by the constancy of size and easy to grind. These alloys are used for making gauges, staples, templates, rulers, scales, and tiles. An example would be alloys StU8, St12H1, StHVG, StH12F1.

The determination of steel groups for cutting tools is quite easy. Such alloys must have cutting ability and high hardness for a long time, even if they are heated. These include carbon and alloy tool, as well as high speed steels. Here we can name the following outstanding representatives: STU7, STU13A, St9HS, StHVG, STR6M5, STRYUK5F5.

Alloy deoxidation

The determination of steel by the degree of deoxidation implies three of its types: calm, semi-calm and boiling. The very same concept means the removal of oxygen from a liquid alloy.

In the case of mild steel, almost no gases are released during solidification. This is due to the complete removal of oxygen and the formation of a shrinkable shell on top of the ingot, which is then cut off.

In semi-quiet steel, gases are partially released, that is, more than in calm, but less than in boiling. There is no sink, as in the previous case, but bubbles form at the top.

Boiling alloys emit a large amount of gas during solidification, and in the cross section it is enough to simply notice the difference in chemical composition between the upper and lower layers.

Hardness

This concept refers to the ability of a material to resist harder penetrating into it. Hardness determination was made possible using three methods: L. Brinell, M. Rockwell, and O. Vickers.

According to the Brinell method, a hardened steel ball is pressed into the ground surface of the sample. Studying the diameter of the print, determine the hardness.

Rockwell Steel Hardness Test Method. It is based on calculating the penetration depth of the tip in the form of a diamond cone with an angle of 120 degrees.

According to Vickers, a diamond tetrahedral pyramid is pressed into the test sample. With an angle of 136 degrees at the opposite sides.

Is it possible to determine the grade of steel without chemical analysis? Specialists in the field of metal science are able to recognize the grade of steel by spark. Determination of the components of the metal is possible during its processing. For example:

• HVG steel has dark raspberry sparks with yellow-red specks and bunches. At the ends of the branched threads bright red stars with yellow grains in the middle appear.
• Steel P18 is also determined by dark raspberry sparks with yellow and red bunches at the beginning, however, the strands are straight and have no branches. At the ends of the beams there are sparks with one or two light yellow grains.
• Steel grades , , 15, 9 have yellow sparks with light stars. And red grains on the branches.
• Steel U12F is distinguished by light yellow sparks with thick and large stars. With a few red-yellow bunches.
• Steel 15 and 20 have bright yellow sparks, many branches and stars. But few beams.

The determination of steel by spark is a fairly accurate method for specialists. However, the townsfolk cannot characterize the metal, having studied only the color of the spark.

Weldability

The property of metals to form a compound under a certain action is called the weldability of steels. The definition of this indicator is possible after the content of iron and carbon is detected.

It is believed that low carbon steels are well bonded by welding. When the carbon content exceeds 0.45%, weldability deteriorates and becomes the worst with a high carbon content. This also happens because the inhomogeneity of the material increases, and sulfide inclusions are precipitated at the grain boundaries, which lead to cracking and an increase in internal stress.

Alloying components also act, impairing the compound. The most unfavorable for welding are called such chemical elements as chromium, molybdenum, manganese, silicon, vanadium, phosphorus.

However, adherence to the technology when working with low alloy steels provides a good percentage of weldability without the use of special measures. The determination of weldability is possible after evaluating a number of important qualities of the material, including:

• Cooling rate.
• Chemical composition.
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