Carbon steel grades. Classification, GOST, application

Steel is a product of ferrous metallurgy, the main structural material. It produces building reinforcement, metal products of various profiles, pipes, parts, mechanisms and tools.

Steel production

Iron and steel industry is engaged in the production of cast iron and steel. Cast iron is a solid but not durable material. Steel is a strong, reliable, ductile, alloy- prone metal used in foundry, rolling, forging and stamping.

There are several methods for steelmaking:

1. Converter. Equipment: oxygen converter. Charge (starting materials): white cast iron, steel scrap metal, limestone. Only carbon steels are produced.
2. Martenovsky. Equipment: open-hearth furnace. Charge: molten iron, steel scrap, iron ore. It is universal for both carbon and alloy steels.
3. Electric arc. Equipment: electric arc furnace. Charge: steel scrap metal, cast iron, coke, limestone. Universal method.
4. Induction. Equipment: induction furnace. Charge: steel and cast iron scrap, ferroalloys.

The essence of the steel production process is to reduce the number of negative chemical inclusions in order to obtain a metal that is popularly called "iron", or rather, an iron-carbon alloy with a carbon content of not more than 2.14%.

Deoxidation processes

For steel at the final stage of smelting, a boiling process is characteristic, which is affected by its inherent nitrogen, hydrogen, carbon monoxide. Such a solidified alloy has a porous structure that is removed by rolling. It is soft and ductile, but not strong enough.

The deoxidation process consists in the deactivation of boiling impurities by introducing ferromanganese, ferrosilicon, aluminum into the alloy. Depending on the amount of residual gases and deoxidizing elements, the steel can be semi-calm or calm.

Finished steel of the required degree of deoxidation is poured into molds for crystallization and use at subsequent technological stages of manufacturing finished steel products.

Carbon steel classification

All steel existing on the world market can be divided into carbon and alloy. All grades of carbon steel are divided into different groups of classifier and designation features.

Based on the main classification features, there are:

1. Carbon structural steels. In them, carbon fiber is less than 0.8%. They are used for the manufacture of fittings, rolled products and castings.
2. Carbon tool steels that contain carbon in an amount of 0.7% to 1.3%. They are used for tools, equipment devices.

By deoxidation methods:

• boiling - deoxidizing elements (RE) in the composition of less than 0.05%;
• semi-calm - 0.05% ≤RE≤0.15%;
• calm - 0.15% ≤ RE ≤ 0.3%.

By chemical composition:

• low carbon (0.3% ≤);
• medium carbon (0.3 C 0 0.65%);
• high carbon (0.65≤≤1.3%).

Steels containing carbon in an amount above 1.3% are not used in industry.

Depending on the microstructure:

• hypereutectoid - in such carbon steel, the composition is less than 0.8%;
• eutectoid - these are steels with a carbon content of 0.8%;
• hypereutectoid - steels with a carbon content of more than 0.8%.

By quality:

1. The usual quality. Sulfur here contains less than 0.06%, phosphorus - not more than 0.07%.
2. Quality steel. They do not contain sulfur and phosphorus greater than 0.04%.
3. High quality. The amount of sulfur here does not exceed 0.025%, and phosphorus - not more than 0.018%.

According to the main standard, carbon steel grades are divided into:

• constructional usual quality;
• constructional quality;
• instrumental quality;
• instrumental high quality.

Features of marking structural steel of ordinary quality

Steel of ordinary quality contains: C - up to 0.6%, S - up to 0.06%, P - up to 0.07%. Let's look at how this carbon steel is labeled. GOST 380 defines the following nuances of designation:

• A, B, C - group; A - not indicated in stamps;
• 0–6 after the letters “St” - the serial number in which the chemical composition and (or) mechanical properties are encrypted;
• G - the presence of Mangan Mn (manganese);
• kp, ps, cn - degree of deoxidation (boiling, semi-calm, calm).

The numbers from 1 to 6 after denoting the degree of deoxidation through a dash are categories. Moreover, the first category is not indicated in any way.

The letters M, K at the beginning of the brand can mean the metallurgical method of production: open-hearth or oxygen-converter. Incidentally, carbon steel of ordinary quality is represented by the quantitative composition of grades, approximately 47 pieces.

Classification of structural steels of ordinary quality

Carbon steels of ordinary quality are divided into groups.

• Group A: steels that must exactly match the specified mechanical properties. They are delivered to the consumer most often in the form of sheet and multi-profile rolled products (sheets, brands, I-beams, fittings, rivets and casings). Stamps: St0, St1 - St6 (CP, PS, SP), categories 1-3, including St3Gps, St5Gps.
• Group B: steels, which should be regulated by the necessary chemical composition and properties. Casting and rolling are made, which will be subjected to additional mechanical processing by pressure in the hot state (forging, stamping). Brands: BST0, BST1 (CP-SP), BST2 (CP, PS), BST3 (CP-SP, including BST3GPS), BST4 (CP, PS), BST 6 (ps, SP), categories 1 and 2.
• Group B: steels that must meet the desired chemical, physical, mechanical and technological properties. This group is characterized by a variety of grades from which plastic sheet metal is made, durable reinforcement for working in areas of significant temperature differences, critical parts (bolts, nuts, axles, piston fingers). All products of various composition, properties and brands of this group are united by good technological weldability. Stamps: BCt1-BCt6 (CP, PS, SP), BC5 (PS, SP), including BCt3Gps, categories 1-6.

Structural steels of ordinary quality are alloys having wide diverse use in industry.

Carbon steel marking

The carbon content in the steel of the named quality is from 0.05% to 0.6%. The smelting of metal of this classification group occurs in open-hearth or electric arc methods. A wide range of carbon presence diversifies the mechanical properties: low-carbon - ductile, medium-carbon - durable.

High-quality carbon steels in the composition of S and P are not more than 0.04%, respectively.

Marking (GOST 1050-88):

• numbers 05-60 - the encrypted presence of carbon (minimum - 0.05%, maximum - 0.6%);
• kp, ps, cn - the degree of deoxidation ("cn" is not indicated);
• G, Yu, F - contain manganese, aluminum, vanadium.

Labeling Exceptions

Carbon quality steels in their marking have exceptions:

• 15K, 20K, 22K - high-quality steels, applicable in boiler building;
• 20-PV - carbon - 0.2%, steel is applicable in the manufacture of pipes by hot rolling, in boiler construction and installation of heating systems, contains copper and chromium;
• OSB - steel for the manufacture of car axles, contains nickel, chromium, copper.

All grades of high-quality steels are characterized by the possible need for the use of thermal (for example, normalization) and chemical-thermal treatment (for example, cementation).

Classification of carbon steel steels

This type of carbon steel can be divided into 4 groups:

1. Highly ductile material applicable for cold machining (rolling), sheet and pipe rolling. Grades - steel 08ps, steel 08, steel 08kp.
2. The metal used in hot rolling and stamping, which will work in thermally aggressive conditions. Grades - from steel 10 to steel 25.
3. Steel, which has found application in the manufacture of critical parts, including springs, springs, couplings, bolts, shafts. Grades - from steel 60 to steel 85.
4. Steel, requiring reliable operation in aggressive conditions (for example, a chain of a caterpillar tractor). Grades steel 30, steel 50, steel 30G, steel 50G.

It is also possible to divide into 2 groups all the well-known carbon steel grades from the high-quality class: structural ordinary and structural manganese-containing.

The use of carbon structural steel

Carbon tool steels are characterized by high strength and toughness. They are necessarily subject to multi-stage heat treatment.

Carbon content in steel: 0.7 - 1.3%. For high-quality - up to 0.03%, phosphorus - up to 0.035%. And for high-quality instrumental: sulfur - up to 0.02%, phosphorus - up to 0.03%.

Brand designation (GOST 1435-74):

• U - carbon instrumental;
• 7 -13 - the carbon content in it is 0.7-1.3%, respectively;
• G - the presence of manganese;
• A - high quality.

Exceptions to the basic principles of marking carbon tool steels are material for parts of watch mechanisms A75, ASU10E, AU10E.

Requirements for Carbon Tool Steels

In accordance with GOST, tool steels must meet a number of characteristics.

Necessary physico-chemical and mechanical properties: qualitative indicators of hardness, impact strength, strength, resistance to temperature changes during operation (during cutting, drilling, impact loads), corrosion resistance.

Specified technological properties:

• resistance to negative processes of cutting technology (chip sticking, riveting);
• good machinability by turning and grinding;
• heat treatability;
• resistance to overheating.

To improve the quality of mechanical and technological parameters, tool steels are subjected to multi-stage heat treatment:

• annealing of the source material before the manufacture of tools;
• quenching (cooling in salt solutions) and subsequent tempering of finished products (mainly low tempering).

The properties obtained are determined by the chemical composition and the resulting microstructure: martensite with cementite and austenitic inclusions.

Use of carbon tool steels

The described steels are used for the manufacture of all kinds of tools: cutting, percussion, auxiliary.

• Steel U7, U7A - hammers, chisels, axes, chisels, sledge hammers, chisels, fishing hooks.
• Steel U8, U8A, U8G - saws, screwdrivers, center punches, countersinks, milling cutters, pliers.
• Steel U9, U9A - metalwork tools, tools for cutting wood.
• Steel U10, steel U10A, U11, U11A - rasps, taps, twist drills, an auxiliary tool for stamping and calibration.
• U 12, U12A - reamers, taps, measuring tools.
• U13, U13A - files, razor and surgical instruments, stamping punches.

A rational choice of carbon steel brand, its heat treatment technology, understanding of its properties and features is the key to long-term service of manufactured, processed or used structures or tools.