By smelting method steels are divided into open-hearth steels, converter steels, electric steels and steels produced by special smelting methods.
By structure Steels are divided into pearlitic, austenitic, ferritic and carbide.
By purpose There are structural steels, tool steels and steels with special properties. Columns, trusses, bridges, machine parts, etc. are made from structural steels; various tools are made from tool steels: cutting tools (cutters, drills, milling cutters, chisels, etc.), stamping tools (dies for cold and hot stamping) and measuring tools ( calipers, micrometers, rulers, calibers, etc.). Steels with special properties include heat-resistant, scale-resistant, stainless (corrosion-resistant) and steels with special physical properties: magnetic (hard and soft magnetic), with high electrical resistance, with special thermal and elastic properties.
By quality Steels are divided into ordinary quality steel, high quality steel, high quality steel and especially high quality steel. The quality of steel is determined by the content of harmful impurities (sulfur and phosphorus), non-metallic inclusions, etc. For example, in steel of ordinary quality, a sulfur content of no more than 0,05 , phosphorus 0,04 , quality - respectively 0,03 And 0,035 and high quality - 0,02 And 0,03 %.
By degree of deoxidation steels are made boiling, calm and semi-quiet.
In accordance with GOSTs, the following main types of carbon steels are smelted: low-carbon (less than 0.3% C), medium-carbon (0.3-0.7% C) and high-carbon (more than 0.7% C); by purpose: for structural ordinary quality and high-quality (including cemented, improved, high-strength and spring-spring), instrumental for cutting and measuring tools, as well as cold (less than 200 ° C) and hot pressing dies.
Carbon steel of ordinary quality for construction is smelted in accordance with GOST 380-85 and supplied to the consumer in the form of rods, sheets and other rolled profiles. Depending on the purpose and the characteristics guaranteed by the metallurgical plant, steel is divided into three groups: A, B, C, which, in turn, are divided into categories.
Group A steel is supplied according to mechanical properties and is manufactured in the following grades: St0, St1 kp (sp), St2 kp (ps and sp), St3 kp (ps, gps, gsp), St4 kp (ps), St5 ps, St6sp (ps ).
Group B steel is supplied according to a guaranteed chemical composition and is manufactured in the following grades: BSt0, BSt1, BSt2, BSt3, BSt4, BSt5, BSt6.
Group B steel is supplied with guaranteed mechanical properties and chemical composition and is manufactured in the following grades: VSt1, VSt2, VSt3, VSt4, VSt5.
Knowledge of the chemical composition is necessary in the case when the consumer’s steel is subjected to hot stamping, and the parts made from it are subjected to heat treatment, since the heating temperature is selected depending on the carbon content in the steel.
According to the degree of deoxidation, steel of all groups with numbers 1, 2, 3, 4 is produced boiling, calm and semi-calm, and with numbers 5 and 6 - only calm and semi-quiet. Steels St0 and BSt0 are not distinguished by the degree of deoxidation. Steel grades VSt1, VSt2, VSt3 of all degrees of deoxidation are supplied with a guarantee of weldability.
Explanation of brands:
a) the letters B and B before the letters St - steel group; group A is not indicated, for example St3, BSt3, VSt3;
b) letters St - steel, numbers from 0 to 6 - conventional brand number; As the number increases, the carbon content in steel and its strength increase. For example, in steels St3 and St5 the carbon content is respectively: 0.14-0.22 and 0.23-0.37%; temporary resistance σ B: 380-490 (38-49) and 560-640 (56-64) MPa (kgf/mm 2);
c) the letters added after the brand number indicate the degree of deoxidation: kp - boiling, ps - semi-calm, sp - calm, for example St3kp;
d) letter G - increased manganese content (St3Gps, VSt3Gsp);
High-quality structural steel melted according to GOST 1050-88, supplied according to the chemical composition and mechanical properties of the following grades: 08, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60. The grade means the average content (mass fraction) of carbon in hundredths of a percent. In addition to the above, steel grades 05 and 58 are supplied (55 pp - steel of reduced hardenability).
By deoxidation steels are smelted: boiling steel (kp) - 05 kp, 08 kp, 10 kp, 15 kp, 20 kp; semi-quiet (ps) - 08 ps, 10 ps, 15 ps, 20 ps (sheet steel for cold stamping); calm (sp) - 08, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60 (sp index is not included in the stamp).
By condition steel is produced without heat treatment, heat-treated T (annealed, highly tempered or normalized) and cold-worked H (calibrated, silver).
By purpose steel subgroups are distinguished: a - for hot pressure treatment; b - for cold machining (turning, milling, gouging, etc.); c - for cold drawing.
Springs and springs are made from steels smelted in accordance with GOST 14959-79 (carbon and alloy spring steel). Carbon spring steel is supplied in the form of round, square and profile bars, strips and coils of the following grades: 65, 70, 75, 80 and 85.
Carbon tool steel smelted according to GOST 1435-90, supplied according to chemical composition and mechanical properties (hardness). According to the chemical composition, steel is divided into high-quality and high-quality. High-quality steels contain harmful impurities of sulfur no more than 0.03 and phosphorus 0.035%. In high-quality steels there is no more than 0.02% sulfur and 0.03% phosphorus, less non-metallic inclusions than in high-quality steels, and the limits of silicon and manganese content are also more narrowed. The steel is supplied in an annealed state with a hardness of NV 187-217. Hardness after quenching H.R.C. 62.
Steel grades: quality - U7, U8, U9, U10, U11, U12, U13; high-quality - U7A, U8A, U9A, U10A, U11A, U12A, U13A. Steels with a high manganese content are also produced, grades U8G and U8GA, in which the manganese content is in the range of 0.35-0.60%.
In the brand designation, the letter U means carbon tool steel, the numbers mean the average mass carbon content in tenths of a percent, the letter A means high-quality steel, the letter G means high manganese content.
Chisels, hammers, screwdrivers, lathe centers (U7, U7A);
Punches, dies, scissors, saws (U8, U8A);
Cores, woodworking tools (U9, U9A);
Cutters, taps, reamers, cutters (U10, U10A);
Cutting dies, saws, molds (U11, U11A);
Cutters, drills, cutters, taps (U12, U13, U13A).
Automatic steel is smelted according to GOST 1414-75 of the following grades: A11, A12, A20, A30, A35E, A40G. Steel contains harmful additives of sulfur 0.08-0.25 and phosphorus 0.06-0.15%. To improve machinability by cutting, lead (up to 0.3%), manganese (up to 1.5%) and selenium (up to 0.1%) are introduced into steel (AC14, AC35G and A35E).
Application area:
Fastening parts (bolts, nuts);
Bushings, rollers, engine parts.
Foundry steel is smelted according to GOST 977-79 of the following grades: 15L, 20L, ..., 55L.
Alloy steels, their types and grades
Alloy steels differ from carbon steels:
Increased heat resistance, corrosion resistance;
Significant impact strength;
High values of σ t and γ;
High electrical resistance;
They have better hardenability;
Increase the amount of retained austenite.
In the phase diagram, Fe - the alloying element Ni and Mn - expand the region of existence of the γ-phase; Mo, Ti - narrow the region of existence of the γ-phase; Si, Al, W, Sn, Mo and Ti - expand the α-phase region. The main alloying elements in steel are Cr, Ni, Si, Mn. Nickel - increases the ductility and toughness of steel; reduces the cold brittleness threshold temperature; reduces the sensitivity of steel to stress concentration. Chromium increases the heat resistance and corrosion resistance of steel; increases electrical resistance; reduces the linear expansion coefficient; increases the hardenability of steel; slows down the decomposition of martensite. Silicon increases the heat resistance of steel; complicates the formation and growth of cementite particles; used as a deoxidizing agent in steel melting.
W, Mo, V, Ti, B - additionally improve the properties of steel. Mo and W - increase the hardenability of steel (+ Ni); promote grain grinding; suppress the temper brittleness of steel.
V, Ti, Ni, Zr - form carbides that are sparingly soluble in austenite; (up to 0.15%) grind the grains; reduce the threshold of cold brittleness.
IN- increases the strength and hardenability of steel (0.001-0.005%).
The effectiveness of alloy elements is achieved with their optimal content in steel.
Alloy steels are classified:
According to the type of equilibrium structure;
Structure after normalization;
Chemical composition;
Purpose.
Alloy steels are classified as: hypoeutectoid (ferrite + alloyed pearlite); hypereutectoid (alloyed pearlite + carbides); eutectoid.
Steels are divided into 3 main classes:
Pearlitic (sorbitol, trostite and bainite);
Martensitic (in alloyed);
Austenitic (in high alloys).
Alloy steels are divided into:
By chemical composition: for chromium; manganese; chromium-nickel; chromium-nickel-molybdenum, etc.;
By the total amount of alloying elements in them: for low alloy (up to 2.5%); alloyed (2.5-10%); highly alloyed (over 10%);
By purpose: for structural (cemented, improved); instrumental; with special properties (“automatic” spring, ball-bearing, wear-resistant, corrosion-resistant, heat-resistant, heat-resistant, electrical, etc. steels).
Marking of alloy steels: A - nitrogen, B - niobium, B - tungsten, G - manganese, D - copper, E - selenium, T - titanium, K - cobalt, N - nickel, M - molybdenum, P - phosphorus, P - boron, C - silicon, F - vanadium, X - chromium, C - zirconium, Ch - rare earth, Yu - aluminum.
Machine-building case-hardening steels contain 0.1-0.3% carbon and 0.2 - 4.4% alloying elements. After saturation with carbon, hardening and low tempering, parts made from such steels have high surface hardness (up to 58-63 H.R.C.) with a viscous central part. Steels 15ХФ, 15Х, 20Х (with a yield strength of up to 700 MPa) are used for the manufacture of small loaded parts that experience moderate alternating and impact loads.
Steels 12ХНЗА, 20ХНЗА, 20ХН4А (with a yield strength of more than 700 MPa) are used for the manufacture of medium and large-sized parts operating under conditions of intense wear and increased loads. Particularly critical parts, for example, gear wheels of aircraft and ship engines, are made from steels 18Х2Н4МА, 18Х2Н4ВА. Economically alloyed steels 18KhGT, 30KhG, 25KhGT have a hereditary fine-grained structure, which makes it possible to reduce the technological cycle of part processing. Such steels are used for the manufacture of critical parts for large-scale and mass production.
Machine-building improved alloy steels contain 0.3-0.5% carbon and up to 5% alloying elements. They are used mainly after improvement (hardening and high tempering at a temperature of 500 - 600 °C for sorbitol). The main application is critical machine parts operated under cyclic or shock loads. For the manufacture of moderately loaded small parts of machines and mechanisms without significant dynamic loads, chromium steels 30Х, 38Х, 40Х, 50Х are used.
With increasing carbon content, the strength of these steels increases, but their toughness and ductility decreases somewhat. From chromium-nickel steels 40KhN, 50KhN, as well as from chromium-silicon-manganese steels 30KhGSA, 35KhGSA, which have high strength and toughness properties, critical parts that operate under the influence of dynamic loads are made.
Chromium-nickel-molybdenum steels 40ХНМА, 38ХМЗМА have increased mechanical properties at temperatures up to 450 °C.
Maraging high-strength steels(with a tensile strength of 1800-2000 MPa) - carbon-free (no more than 0.03% C) alloys of iron and nickel, alloyed with cobalt, molybdenum, titanium and other elements. High mechanical properties of steels HI8K9M5T, H12KI5M10 are achieved by combining the martensitic g ® a transformation, martensite aging and solid solution alloying. These steels retain high mechanical characteristics at low temperatures down to liquefied gas temperatures. Such steels are heat resistant up to temperatures of 500 - 700 °C. They are used for critical parts in aviation and shipbuilding.
Wear-resistant structural steels have high resistance to contact fatigue and abrasion due to high hardness, uniformity of structure, minimal content of non-metallic inclusions and metallurgical defects. Heat treatment (quenching and low tempering) of ShKh15GS steel ensures their hardness H.R.C. 60-66. For parts operating in aggressive environments (sea water, weak acid solutions, alkalis), corrosion-resistant high-carbon steel 95X18 is used.
Parts operated under the influence of shock loads, causing their surface hardening and, consequently, a decrease in the wear resistance of conventional steels, are made of austenitic high-manganese steel G13L. For the manufacture of parts operated under sliding friction conditions, graphitized steel is used, which has the structure of a ferrite-cementite mixture and graphite. The latter plays the role of a lubricant that prevents the contacting parts from seizing.
Corrosion-resistant steels and alloys resistant to corrosion in air, water (including sea water), and a number of acids, salts and alkalis. Chromium steels X25T, X28, which have a ferritic structure, are used to make parts used in highly aggressive environments, for example, in boiling nitric acid. Chromium-nickel steels 04Х18Н10, 08Х18Н10, 12Х12Н10Т, which have an austenitic structure, are used in aircraft and shipbuilding.
Heat-resistant steels and alloys ensure operation of parts at temperatures above 500 °C. For parts operated in an environment with a temperature of 500 - 580 °C, use low-carbon steels having a lamellar pearlite structure, alloyed with cobalt, molybdenum, vanadium, in particular 16M, 25ХМ, 12Х1МФ. Loaded parts operated in an environment with temperatures up to 450-470 °C are made of high-chromium steels 15X11NMF, 1HKVNMF, which have a sorbitol or troostite structure, depending on the tempering temperature.
Steel is the most common material in mechanical engineering. The creation of new, more advanced machines stimulates the creation of steel grades with properties that meet modern requirements in mechanical engineering. At the same time, previously created steel grades, taking into account new technologies for their production, continue to be in demand by designers when creating new and improving existing machines. It is customary to distinguish the following groups of steels:
- carbon steels, which account for approximately 80% of the total volume,
- alloyed structural and tool steels,
- steels with special properties for special purposes, etc.
1. Carbon steel of ordinary quality
They are among the cheapest and most widely used. From them, up to 70% of all rolled products are obtained - hot-rolled, long- and shaped thick- and thin-sheet, wide-rolled and cold-rolled thin-sheet. These steels are used to make pipes, forgings, stampings, tape, wire, metal products (hardware): nails, ropes, mesh, bolts, nuts, rivets, as well as light and medium-loaded parts; pins, washers, keys, covers, casings, and from steel numbers 4-6 - shafts, screws, gears and spindles. Ordinary quality steels weld well.
Depending on the purpose, carbon steels of ordinary quality are divided (GOST 380-94) into three groups:
- A – supplied according to mechanical properties,
- B – supplied according to chemical composition,
- B – supplied according to mechanical properties and chemical composition.
Depending on the standardized indicators (strength characteristics, chemical composition), the steel of each group is divided into categories:
- group A – 1, 2 and 3;
- group B – 1st, 2nd;
- group B – 1, 2, 3, 4, 5, 6th.
Letters St means "steel", numbers from 0 before 6 – conventional brand number characterizing the mechanical properties of steel. As the grade number increases, the tensile strength σ in and the yield strength σ t increase and the relative elongation δ decreases. To indicate the degree of deoxidation, indices are placed after the brand number: kp– boiling, ps- semi-calm, joint venture– calm (for example: StZkp, StZps, StZsp; Tables 1 and 2).
The mechanical properties of carbon steel of ordinary quality of group A and the approximate purpose of carbon steel of ordinary quality are given in table. 1.
Table 1. Carbon steels, their mechanical properties and purpose
| steel grade | Properties | Approximate purpose | ||
| σ in, MPa | σ t, MPa | δ, % | ||
| St0 | No less | – | 23 | Unresponsible building structures, gaskets, washers, casings. Weldability is good |
| St1kp St1ps, St1sp | 300-390 | – | 35 | Lightly loaded parts of metal structures – rivets, washers, cotter pins, gaskets, casings. Weldability is good |
| St2kp St2ps, St2sp | 320-410 | 215 | 33 | Details of metal structures - frames, axles, keys, rollers, cemented parts. Weldability is good |
| StZkp StZps, StZsp StZGps | 360-460 | 235 | 27 | Bogie frames, cemented and cyanidated parts, which require high surface hardness and low core strength, crane hooks, rings, cylinders, connecting rods, caps |
| St4kp St4ps, St4sp | 400-510 | 255 | 25 | Shafts, axles, rods, pins, hooks, bolts, nuts, parts with low strength requirements |
| St5ps, St5sp | 490-630 | 285 | 20 | Shafts, axles, sprockets, fasteners, gears wheels, connecting rods, parts with increased strength requirements |
| St6ps | No less | 315 | 15 | Shafts, axles, hammer heads, spindles, couplings cam and friction, chains, high strength parts |
To be able to recognize steel grades during storage, rolled products are marked with indelible paint. To do this, regardless of the group and degree of deoxidation of the steel, use paint of the colors indicated in the table. 2.
Table 2. Marking color of carbon steel of ordinary quality
| steel grade | Marking color | steel grade | Marking color |
| St0 | Red and green | StZGps | Red and blue |
| St1 | White and black | St4 | Black |
| St1Gps | White and red | St4Gps | Black and red |
| St2 | Yellow | St5 | Green |
| St2Gps | Yellow and red | St6Gps | Green and white |
| St3 | Red | St6 | Blue |
2. High-quality carbon steel for construction
They are the main metal for the manufacture of machine parts (shafts, spindles, axles, gears, keys, couplings, flanges, friction discs, screws, nuts, stops, rods, hydraulic cylinders, eccentrics, chain sprockets, etc.), which, when interacting in a working machine, they perceive and transmit loads of varying magnitude. These metals are well processed by pressure and cutting, cast and welded, and are subjected to thermal, thermomechanical and chemical-thermal treatment.
Various special types of processing provide the toughness, elasticity and hardness of steels, making it possible to make parts from them that are viscous in the core and hard on the outside, which dramatically increases their wear resistance and reliability. High-quality carbon structural steels are used to produce rolled products, forgings, calibrated steel, silver steel, long steel, stampings and ingots.
Table 3. Basic properties of high-quality carbon structural steel
| Brand | Mechanical properties | Physical properties | Technological properties | ||||||||||
| σ t | σ in | δ, % | a n J/cm 2 | NV | γ, g/cm 3 | λ, W/(m °С) | α·10 6.1/°С | treatments- productivity cutting | welding | interval temperatures | plastic cold processing | *hot- | |
| MPa | |||||||||||||
| 08 | 196 | 324 | 33 | – | 126 | 7,83 | 811 | 11,6 | IN | BB | 800-1300 | BB | * |
| 10 | 206 | 321 | 31 | – | 140 | 7,83 | 811 | 11,6 | IN | BB | 800-1300 | BB | * |
| 15 | 225 | 373 | 27 | – | 145 | 7,82 | 770 | 11,9 | IN | BB | 800-1250 | BB | * |
| 20 | 245 | 412 | 25 | – | 159 | 7,82 | 770 | 11,1 | IN | BB | 800-1280 | IN | * |
| 25 | 274 | 451 | 23 | 88 | 166 | 7,82 | 732 | 11,1 | IN | BB | 800-1280 | IN | * |
| 30 | 294 | 490 | 21 | 78 | 175 | 7,817 | 732 | 12,6 | IN | IN | 800-1250 | IN | * |
| 35 | 314 | 529 | 20 | 69 | 203 | 7,817 | 732 | 11,09 | IN | IN | 800-1250 | IN | * |
| 40 | 321 | 568 | 19 | 59 | 183 | 7,815 | 596 | 12,4 | IN | U | 800-1250 | U | ** |
| 45 | 363 | 598 | 16 | 49 | 193 | 7,814 | 680 | 11,649 | IN | U | 800-1250 | U | ** |
| 50 | 373 | 627 | 14 | 38 | 203 | 7,811 | 680 | 12,0 | U | U | 800-1250 | U | ** |
| 55 | 382 | 647 | 13 | – | 212 | 7,82 | 680 | 11,0 | U | N | 800-1250 | N | ** |
| 60 | 402 | 676 | 12 | – | 224 | 7,80 | 680 | 11,1 | U | N | 800-1240 | N | ** |
| Note. N – low, U – satisfactory, V – high, BB – very high. | |||||||||||||
High-quality structural steels have higher mechanical properties (GOST 1050-88) than steels of ordinary quality, due to their lower content of phosphorus, sulfur and non-metallic inclusions. According to the type of processing, they are divided into hot-rolled, forged, calibrated and silver (with special surface finishing).
The designation of the steel grade is made up of the word “Steel” and a two-digit number that indicates the average carbon content in hundredths of a percent. For example, Steel 25 contains 0.25% carbon (permissible amount of carbon - 0.220.30%), Steel 60-0.60% (permissible amount -0.57-0.65%). The degree of deoxidation is not reflected in grades of calm steels, but in grades of semi-quiet and boiling steels, as well as steels of ordinary quality, it is designated by the letters “ps” and “kp”, respectively. In high-quality structural steels of all grades, the sulfur content is allowed to be no more than 0.040% and phosphorus – no more than 0.035%.
The main properties of high-quality carbon structural steel are given in table. 3, main purpose - in table. 4. Marking colors are given in table. 5.
Table 4. High-quality carbon steel for structural purposes, their main purpose
| steel grade | Main purpose |
| Steel 08kp, 10 | Parts produced by cold stamping and cold heading, tubes, gaskets, fasteners, caps. Cemented and cyanidated parts that do not require high core strength (bushings, rollers, stops, copiers, gears, friction discs) |
| Steel 15, 20 | Lightly loaded parts (rollers, pins, stops, copiers, axles, gears). Thin parts exposed to abrasion, levers, hooks, traverses, liners, bolts, couplers, etc. |
| Steel 30, 35 | Parts experiencing low stress (axles, spindles, sprockets, rods, traverses, levers, disks, shafts) |
| Steel 40, 45 | Parts that require increased strength (crankshafts, connecting rods, ring gears, camshafts, flywheels, gears, pins, ratchets, plungers, spindles, friction discs, axles, couplings, racks, rolling rollers, etc.) |
| Steel 50, 55 | Gear wheels, rolling rollers, rods, bandages, shafts, eccentrics, lightly loaded springs and leaf springs, etc. Used after hardening with high tempering and in a normalized state |
| Steel 60 | Parts with high strength and elastic properties (rolling rolls, eccentrics, spindles, spring rings, clutch springs and discs, shock absorber springs). Apply after hardening or after normalization (large parts) |
Table 5. Quality carbon steel marking colors
3. Carbon tool steel
Tool carbon steels are used to produce hot-rolled, forged and calibrated steel, silver steel, steel for cores, as well as ingots, sheets, strip, wire and other products. These steels are used to make cutting tools for processing metals, wood and plastics, measuring tools, and dies for cold deformation.
The heat resistance of tool carbon steels does not exceed 200°C; when heated above this temperature, they lose their hardness, and therefore their cutting properties and wear resistance.
Carbon tool steels can be divided into two groups (GOST 1435-99):
- quality steels U7, U8, U8G, U9, U10, U11, U12 and U13;
- high-quality brands U7A, U8A, U8GA, U9A, U10A, U NA, U12A and U13A.
In high-quality tool carbon steels, the allowed content is 0.03% sulfur and 0.035% phosphorus, in high-quality steels - 0.02% sulfur and 0.03% phosphorus. Steels produced by electroslag remelting contain up to 0.015% sulfur. Depending on the content of chromium, nickel and copper, carbon tool steels are divided into five groups:
- 1st – high-quality steels of all grades, intended for the manufacture of products of all types (except for patented wire and tape);
- 2nd – high-quality steels of all grades, intended for the same purposes as steels of the first group;
- 3rd – steel grades U10A and U12A for the manufacture of cores;
- 4th – steel of all grades for the production of patented wire and tape;
- 5th – steel grades U7÷U13 for the production of hot- and cold-rolled sheets and strips, including heat-treated ones with a thickness of up to 2.5 mm (except for patented tape), as well as steel of these grades for the production of hot-rolled and forged section steel and cold-drawn polished steel (silver).
Tool steel must have high hardness (63÷64 HRC 3), significantly exceeding the hardness of the material being processed, wear resistance and heat resistance (the ability to maintain properties at high temperatures).
A measuring instrument made of such steel must be durable (a = 590÷640 MPa) and maintain the specified dimensions and shape for a long time. Working parts of dies and rolling rollers for cold deformation (drawing, bending, upsetting, punching holes, knurling, rolling) made from this steel must have high hardness and wear resistance with sufficient toughness. All this is achieved by hardening and tempering, and for measuring instruments, through artificial aging. In table 6 shows the properties of carbon tool steel, table. 7 – approximate purpose of tool carbon steel.
Table 6. Properties of carbon tool steel (GOST 1435 - 74)
| steel grade | Mechanical properties | ||||
| σ t | σ in | δ, % | J/cm 3 | HRС | |
| U7A | 630 | 21 | – | 63 | |
| U8A | – | 590 | – | – | 63 |
| U10A | – | 590 | 23 | – | 63 |
| UNA | – | – | – | – | 63 |
| U12A | – | 640 | 28 | – | 64 |
| U13A | – | – | – | – | 64 |
Table 7. Approximate purpose of carbon tool steel
| steel grade | Purpose |
| U9 | Woodworking cutting tools (drills, cutters, knives) and hacksaw blades for steel processing |
| U10, U11 and U12 | Metal-cutting tools (shaped cutters, drills, taps, dies, reamers, cutters, files and lead screws of precision machines) |
| U13 | Razor knives, bladed surgical instruments and files |
| U7 and U8 | Bench hammers, chisels, vice jaws, templates, staples |
| U8, U9 and U10 | Micrometer tool parts, smooth and threaded gauges, collets, friction discs, springs, etc. |
As a rule, the manufacture of a tool is preceded by annealing for granular cementite, which promotes better machinability and reduces warping of parts during hardening.
Carbon steels contain carbon up to 2.14%, manganese (up to 0.8%), silicon (up to 0.35%), sulfur (up to 0.06%) and phosphorus (up to 0.07%). The listed elements are always present in steel, and therefore they are classified as permanent impurities. Manganese and silicon are introduced into steels for the purpose of deoxidation; the presence of sulfur and phosphorus is explained by the difficulty of removing them during smelting.
Silicon dissolves in ferrite and greatly strengthens it, while reducing ductility and significantly increasing the yield strength. This reduces the ability of steel to draw and cold heading. Therefore, in steels intended for cold stamping, the silicon content should be reduced.
Manganese increases the strength of ferrite and reduces the red brittleness of steel caused by sulfur. With iron, sulfur forms FeS sulfide, which is practically insoluble in iron and forms a eutectic with it (Fe + FeS), which melts at a temperature of 988°C. During crystallization, this eutectic is located around the grains in the form of rims. During hot working, when heated above 1000°C, the eutectic melts, which leads to disruption of the bond between grains and tears and cracks appear in the metal during deformation. This phenomenon is called red brittleness become. In the presence of manganese in steel, instead of iron sulfide, manganese sulfide MnS is formed with a melting point of 1620°C, thereby eliminating the phenomenon of red brittleness.
Sulfur compounds reduce mechanical properties, especially impact strength and ductility, sharply reduce the work of ductile crack development and fracture toughness K 1C. Sulfides impair weldability and corrosion resistance.
Phosphorus dissolves in small quantities in iron, forming a solid solution. Dissolving in ferrite, phosphorus reduces its ductility and toughness and sharply increases the cold brittleness threshold of steel. Every 0.01% of phosphorus increases the transition temperature of cold brittleness by 20...25 o C. At an increased content, phosphorus and iron form phosphides Fe 3 P and Fe 2 P, which, as part of the eutectic, are located along the grain boundaries and reduce the strength of steel.
There are so-called hidden impurities in steels, which include oxygen 0.002...0.008%), nitrogen (0.002...0.007%), hydrogen (0.0001...0.0007%). These impurities can be present in steel in the form of brittle non-metallic inclusions (FeO, Al 2 O 3, Fe 4 N) or a solid solution, and can also be free in defective areas of the metal (cracks, cavities, etc.). When melted, they dissolve in the steel and then precipitate upon cooling, mainly along the grain boundaries, which reduces the resistance to brittle fracture. In addition, non-metallic inclusions are stress concentrators. The presence of hydrogen causes the appearance of flakes in alloy steels (metal micro-discontinuities with a diameter of up to 10...15 mm in the central part of the forging).
Non-metallic inclusions are brittle and break during rolling, arranged in the steel in the form of chains. This creates microscopic stress concentrators, which reduces fatigue characteristics and toughness.
Some impurities enter steel during smelting from scrap and are called random. Such impurities include chromium, nickel, copper in the presence of up to 0.3%. Their influence in such quantities on the properties of steels is insignificant.
Carbon has the greatest influence on the properties of steel. Figure 6 shows the dependence of the strength and ductility of steel on the carbon content in it. It can be seen that carbon very sharply increases strength properties while simultaneously reducing ductility and toughness. This is explained by the fact that cementite inclusions inhibit the movement of dislocations in ferrite and, naturally, as the number increases, their influence increases.
As the amount of carbon increases, the transition temperature of cold brittleness of steel increases sharply. Every 0.1% C increases the temperature of transition from ductile to brittle fracture by 20 o C.
Carbon also affects other physical properties of steel, in particular, with an increase in the amount of carbon, electrical resistance and coercive force increase, and magnetic permeability decreases.
Carbon steels are divided according to the production method depending on the melting units used converter, open-hearth and electric steel. At the same time, according to the method of deoxidation, steel can be boiling(deoxidized only with manganese), semi-calm(deoxidized with manganese and silicon) and calm(deoxidized with manganese, silicon and aluminum).
Figure 6 - Dependence of the mechanical properties of steel (a) and
phase composition (b) on carbon content
1.4.2.1 Classification and marking of carbon steels
According to the structure in the equilibrium state, they are distinguished hypoeutectoid, eutectoid and hypereutectoid steels. Hypoeutectoid steels contain carbon from 0.025 to 0.8%, their structure consists of ferrite and pearlite. The carbon content in eutectoid steel is 0.8% C with a completely pearlitic structure. In hypereutectoid steels, along with the pearlite component, cementite inclusions are formed, and the carbon content can vary from 0.8 to 2.14%.
The most common classification of carbon steels is by quality, which is determined by the content of sulfur and phosphorus. In accordance with this characteristic, steels are classified ordinary quality, high quality and high quality.
Carbon steels of ordinary quality (Table 1) are marked with letters St, which means steel. After St followed by a conventional brand number from 0 to 6, which reflects the chemical composition of the steel. The degree of deoxidation of steel is indicated by letters kp, ps, sp, which mean, respectively, boiling (deoxidized by manganese), semi-calm (deoxidized by manganese and silicon), calm (deoxidized by manganese, silicon and aluminum). The mass fraction of sulfur in steels of all grades is £ 0.050%, phosphorus – £ 0.040%, in St0 sulfur – £ 0.060%, phosphorus – £ 0.070%.
Quite often you can also find markings from previous years, according to which all steel of ordinary quality is divided into three groups.
Group A – marked St0, St1, St2, St3, St4, St5, St6.
Group B - marked with the letters M, K, B (which indicates the production method - open-hearth, converter, Bessemer), and then St0, St1, St2, St3, St4, St5, St6.
Group B – marked VSt1, VSt2, VSt3, VSt4, VSt5, VSt6.
Group A steels are supplied with guaranteed mechanical properties. They are not amenable to hot processing. The higher the number, the higher the strength, but the lower the ductility of the steel.
Group B steels are supplied with a guaranteed chemical composition and can be subjected to hot processing (for example, forging and heat treatment) at the consumer's site.
Group B steels are supplied with guaranteed mechanical properties and chemical composition (used for welded structures).
Table 1 - Chemical composition of ordinary carbon steels
quality
Steels of all groups with grade numbers 1, 2, 3, 4 according to the degree of deoxidation are produced boiling, semi-calm, and calm, and steels with grades 5 and 6 are semi-calm and calm.
Carbon quality steels differ from ordinary quality steels in having a lower content of sulfur (not more than 0.04%) and phosphorus (not more than 0.035%), as well as a smaller amount of non-metallic inclusions. The chemical composition of these steels is limited to a narrower range. High-quality carbon steels are marked with the word steel and a subsequent two-digit number, which shows the average carbon content in steel in hundredths of a percent, for example, 08, 10, 15, etc. (Table 2).
Table 2 - Composition and mechanical properties of high-quality carbon steels
| steel grade | WITH, % | Mn,% | Si, % | Cr, % | s 0.2, MPa | s in, MPa | δ,% | y, % | KCU, J/cm 2 |
| 0,05-0,12 | 0,35-0,65 | 0,17-0,37 | 0,10 | - | |||||
| 0,07-0,14 | 0,35-0,65 | 0,17-0,37 | 0,15 | - | |||||
| 0,12-0,19 | 0,35-0,65 | 0,17-0,37 | 0,25 | - | |||||
| 0,17-0,24 | 0,35-0,65 | 0,17-0,37 | 0,25 | - | |||||
| 0,22-0,30 | 0,50-0,80 | 0,17-0,37 | 0,25 | ||||||
| 0,27-0,35 | 0,50-0,80 | 0,17-0,37 | 0,5 | ||||||
| 0,32-0,40 | 0,50-0,80 | 0,17-0,37 | 0,25 | ||||||
| 0,37-0,45 | 0,50-0,80 | 0,17-0,37 | 0,25 | ||||||
| 0,42-0,50 | 0,50-0,80 | 0,17-0,37 | 0,25 | ||||||
| 0,47-0,55 | 0,50-0,80 | 0,17-0,37 | 0,25 | ||||||
| 0,52-0,60 | 0,50-0,80 | 0,17-0,37 | 0,25 | - | |||||
| 0,57-0,65 | 0,50-0,80 | 0,17-0,37 | 0,25 | - |
When designating boiling or semi-calm steel, the degree of deoxidation is indicated in letters at the end of the grade kp, ps. In the case of mild steel, the degree of deoxidation is not indicated. High-quality carbon steels also include steels with a high manganese content (0.7 - 1.0%). Such steels have the letter at the end of the grade G.
Used for critical products high quality steel with lower sulfur content (up to 0.025%) and phosphorus (up to 0.025%). When designating high-quality steels, the letter A is added to the end of the grade.
High-quality carbon steels are divided into low-, medium- and high-carbon steels depending on the carbon content. Low-carbon steels of high ductility and low strength include steels 08, 08kp, 10, 10kp, 15, 15G..., 25G, which are used for the manufacture of lightly loaded parts (cam shafts, axles, bushings). Heat treatment (hardening and tempering, carburization) significantly increases the strength and toughness of products made from these materials, which allows you to create lighter structures and save metal. Medium-carbon steels (with a carbon content of 0.3...0.55%), depending on the required mechanical properties, are used after normalization, hardening with high-temperature tempering, high-frequency hardening and low-temperature tempering. Shafts, gears, connecting rods, spindles, etc. are made from these steels.
High-carbon steels contain carbon from 0.6 to 0.85% and are characterized by high strength and elastic properties and increased wear resistance. After quenching and tempering or hardening with high-frequency heating, parts made of these steels can work under friction conditions in the presence of high static and vibration loads. These steels are used to make rope wire, as well as spring wire after patenting.
Carbon steels, which contain 0.7...1.3% C, are used for the manufacture of impact and cutting tools. They are marked U7...U13, Where U denotes carbon steel, and the number is the carbon content in tenths of a percent.
The positive qualities of carbon steels include their fairly high set of mechanical properties, which is ensured by heat treatment. Carbon steels have good technological properties. They are not scarce and cheap.
The main disadvantage of carbon steels is their low hardenability (up to 15 mm).
Cast iron
1.4.3.1 General information
Cast irons are alloys of iron with carbon, the amount of which exceeds 2.14%. A significant portion of the cast iron produced is remelted into steel, but at least 20% of the cast iron produced is used to make cast parts.
Cast irons are distinguished by high casting properties and are one of the main modern casting materials. About 75% of all castings are made from cast iron. A lower melting point compared to steels and completion of crystallization at a constant temperature (formation of eutectic) provide higher casting characteristics: fluidity and mold fillability, shrinkage and less tendency to form shrinkage cracks.
Due to their low ductility, cast iron is not subject to pressure treatment.
Depending on the chemical composition and crystallization conditions, carbon in cast iron can be in a chemically bound state in the form of cementite or in a free state in the form of graphite. In accordance with this, they distinguish white cast iron (carbon is in the form of cementite) and gray(carbon is in the form of graphite inclusions).
In white cast iron, phase transformations occur in accordance with the diagram Fe-Fe 3 C. Depending on the carbon content, they are divided into hypoeutectic (2.14...4.3% C), eutectic (4.3% C) and hypereutectic (4.3...6.67% C).
In hypoeutectic cast irons, the structural components at room temperature are pearlite, ledeburite and cementite; in eutectic – ledeburite; in hypereutectic - ledeburite and cementite.
White cast irons have high hardness (450...550HB and above), due to the presence of a large amount of cementite in them. Along with high hardness, white cast iron is characterized by high fragility, which precludes its use for the manufacture of machine parts. Castings from white cast iron are used, which are used to produce parts from malleable cast iron by carrying out graphitizing annealing. Castings with a surface layer (12...30 mm) of white cast iron and a core of gray cast iron are also used. The presence of a “bleached” surface layer ensures high wear resistance of such a casting.
Gray cast irons, in which carbon is in the form of graphite inclusions, are of industrial importance, and therefore the conditions for their formation, i.e., the graphitization process, become important.
Graphite contains 100% carbon, while the carbon concentration in cementite is only 6.67%. The crystal structures of austenite and graphite are significantly different, while the crystal structures of austenite and cementite are more similar in structure. Therefore, the formation of cementite from the liquid phase and from austenite should proceed more easily than graphite, since the work of formation of the nucleus and the diffusion processes necessary for this are not so significant.
However, the mixture ferrite + graphite or austenite + graphite has less free energy than a mixture ferrite + cementite or austenite + cementite Therefore, thermodynamic factors contribute to the formation of graphite rather than cementite.
Due to the above circumstances, with rapid cooling and hampering diffusion processes, the formation of cementite occurs, and with slow cooling, the determining factor is the desire to minimize free energy, which leads to the formation of graphite.
Gray cast irons differ in the shape of graphite inclusions. Graphite, which is formed in cast iron during the process of crystallization and subsequent cooling, has a lamellar shape, and cast iron with such graphite is called gray.
The formation of graphite due to the decomposition of cementite occurs not only during crystallization and cooling, but also when white cast iron is heated to high temperatures. This phenomenon is used in the production of so-called malleable cast iron. In this case, the graphitization centers grow more or less evenly in all directions and flocculent graphite inclusions are formed. Cast iron with such graphite is called malleable cast iron.
Cast iron with spherical graphite, which is obtained by modification with magnesium and cerium, is called high strength cast iron.
Cast irons, like steels, are multicomponent alloys containing Fe, C, Si, Mn, P and S.
Carbon has a decisive influence on the quality of cast iron, changing the casting properties and the number of graphite inclusions. The higher its concentration, the more graphite precipitation and the lower the mechanical properties of cast iron, therefore the carbon content in industrial cast iron does not exceed 3.8%. The lower limit of carbon content is 2.4% and is limited by the need to ensure sufficient castability.
Silicon has a strong graphitizing effect; it promotes the release of graphite during the solidification process and the decomposition of already formed cementite. The silicon content in cast iron ranges from 0.3 to 5%.
Manganese complicates the graphitization processes and slightly improves the mechanical properties of cast iron. The amount of manganese in cast iron can vary within 0.5...1%.
Sulfur's bleaching ability is 5 to 6 times greater than manganese. In addition, sulfur reduces fluidity, increases shrinkage and increases the tendency to cracks. Therefore, sulfur is a harmful impurity and its content in cast iron does not exceed 0.15%.
Phosphorus has virtually no effect on graphitization. Its maximum solubility in ferrite is 0.3%. At a higher content, phosphorus forms a triple phosphide eutectic with iron and carbon with a melting point of 950 o C, which increases the fluidity of cast iron. However, this eutectic has high hardness and brittleness, so an increased phosphorus content in castings up to 0.7% is allowed only if it is necessary to ensure high wear resistance. For artistic casting, cast irons with a phosphorus content of up to 1% are used.
Of the alloying elements, the degree of graphitization is increased by nickel and copper, and chromium complicates the process of graphite formation.
Graphite inclusions affect the mechanical properties of castings, since they can be considered as voids of the corresponding shape, near which stresses are concentrated. The magnitude of these stresses is greater, the sharper the defect, therefore, the metal softens to the greatest extent in the presence of plate-shaped graphite inclusions, the flake-like form of graphite is less dangerous, and the most acceptable is the spherical form of graphite. Graphite inclusions have the greatest influence on the fracture resistance of materials under severe loading methods (impact and tensile) and have virtually no effect under compressive loads. Cast irons with lamellar graphite have the lowest ductility (δ = 0.2...0.5%), intermediate (δ = 5...10%) - with flake graphite and the highest - with spherical graphite (δ £ 15%).
Based on the structure of the metal base, gray, malleable and high-strength cast irons are divided into ferritic, ferritic-pearlitic and pearlitic.
The metal base in cast iron provides the greatest strength and wear resistance if it has a pearlite structure. The presence of ferrite in the structure, without increasing the ductility and toughness of cast iron, reduces its strength and wear resistance. Gray ferritic cast iron has the lowest strength.
As a structural material, cast iron has the following positive properties. The presence of graphite improves cutting performance because chips break at graphite inclusions. Compared to steel, cast iron has better anti-friction properties, due to the fact that graphite inclusions themselves are a lubricant. Cast iron perfectly dampens vibrations and has increased cyclic viscosity due to microvoids that are filled with graphite. Cast iron parts are not as sensitive to external stress concentrators (grooves, holes, etc.) compared to steel parts. Cast irons are cheaper than steels due to simpler production technology.
Carbon steel, the grades of which are described below, is widely used in various industries. The choice of a specific grade of carbon steel is made based on the specific purpose for which it will be used. This is due to the fact that each brand has different characteristics.
Steel classification
All carbon steels, depending on the area of application, are divided into low-carbon, medium-carbon and high-carbon steels, and are divided according to several parameters:
- Deoxidation method.
- Composition of chemical elements.
- Microstructure.
- Quality.
According to basic standards, carbon steels are divided into:
- Structural conventional.
- Structural quality.
- Instrumental quality.
- High quality instrumental.
Manufacturing technology
Steel production in the metallurgical industry is done in a variety of ways. Each production method is different, depending on the equipment used. Thus, all equipment for the production of carbon steels can be divided into three types:
- Converter melting furnaces.
- Open hearth furnaces.
- Electric ovens.
Converter
Converter furnaces melt the entire composition of the alloy. With this method, the molten mass is treated with technical oxygen. To clean the hot mass from various impurities, lime is added to it. This makes it possible to turn impurities into slag. During the production process, the metal oxidation process actively occurs. This provokes the release of a large amount of waste.
The production of carbon steels in converter-type furnaces has a significant drawback. This includes the fact that during operation a large amount of dust is released. This leads to the need to install additional filtration units, which entails the cost of money. Despite this, the converter method has high productivity and is widely used in metallurgy.
Open-hearth
The production of various grades of carbon steel using open-hearth furnaces makes it possible to obtain a high-quality final product. The production process occurs as follows:
- The alloy components are loaded into a specialized furnace compartment: cast iron, steel scrap, etc.;
- The entire composition is heated to a high temperature;
- Under the influence of temperature, all components turn into a homogeneous hot mass;
- During melting, all components of the iron and carbon alloy interact;
- The material resulting from the chemical reaction leaves the furnace.
Electrical
The method for producing various grades of carbon steel in electric furnaces differs from those listed above. Its difference lies in the method of heating the composition. The use of electricity to heat components reduces the oxidation of the metal. This significantly reduces the amount of hydrogen in the metal, which improves the structure of the alloy and affects the quality of the final product.
Use of steel
Carbon steel of various grades is used to make structures in many industries. Depending on the application of the product, certain brands are used.
Regular quality
The amount of foreign impurities in finished products is regulated by GOST 380-2005. Regular quality carbon steel is used to produce:
- St0– sheathing, fittings, etc.;
- St1– channels, T-beams and I-beams. It has low hardness but good viscosity;
- St2– parts of non-critical structures. It is a highly plastic material;
- St3– rolled metal used for the construction of building structures, bodies, automotive rims, etc.;
- St5– bolts, nuts, levers, pins, axles, etc.;
- St6– high-strength parts for woodworking and metalworking machines.
High quality
The following are produced from high-quality steel grades:
- Pipes and parts that are applicable in boiler building.
- Products with high ductility - bolts, nuts, etc.
- Parts designed to create welded structures.
- Various kinds of pipes, pins, axles.
- Gears, clutches of trucks, buses and other equipment.
- Spring washers, rings.
Instrumental
Carbon tool steels of various grades have increased strength and high impact toughness. They are used to create all kinds of tools and replacement elements. During production, products are subjected to repeated exposure to high temperatures, which improves their physical properties. The products are resistant to rapid temperature changes and are highly resistant to corrosion.
Steel marking
According to the marking, all carbon steels are divided into three categories:
- Group A. This includes alloys that meet strictly specified mechanical properties;
- Group B. The steels of this group clearly correspond in chemical composition;
- Group B. Products in this group must meet mechanical, physical and chemical properties simultaneously.
For ordinary quality steel, the letters St. appear at the beginning of the designation. Following the letters St in the marking there is a digital designation. The number in the marking indicates the metal grade number. Next, after the number, the type of alloy is entered. The alloy type designation is as follows:
- KP- boiling;
- PS– semi-calm;
- JV- calm.
Directly before the letter designation of the alloy there is a letter indicating the steel group. If the product belongs to group A, then the letter is not affixed.
To quickly identify the brand, the manufacturer applies the appropriate stripes with specialized paint:
- St0– green stripe + red.
- St1– one yellow + one black.
- St3Gsp– brown + blue.
- St3– red.
- St4– black.
- St5Gps– brown + green.
- St5– green.
- St6– blue.
The degree of carbon presence in the material is determined at the very beginning. The amount of carbon for a group A metal is indicated in hundredths of a percent. For B and C – in tenths. In some cases, after these numbers the manufacturer puts the letter G. This means that the product contains a large amount of manganese.
Quality steel categories
High-quality steels of different markings can be divided into several categories:
- 08ps, 08kp– have high plasticity. Well suited for cold rolling;
- From 10 to 25– used for hot stamping or rolling;
- From 60 to 85– used for making critical structures such as springs, springs, clutches;
- 30, 50, 30G, 50G– increased strength, withstanding heavy loads.
Exceptions to notation
Quality steels have some exceptions in the designations. These include:
- 15K, 20K, 22K– used in the construction of boilers;
- 20-PV– contains 0.2 percent carbon and copper with chromium. Pipes for heating systems are made from it;
- OSV– contains additives of nickel, chromium and copper. The axles of railway cars are made from it;
- A75, ASU10E, AU10E– applicable for parts in watch movements.
From the above, it follows that before using a carbon steel product, you need to pay attention to its markings. This way you can determine its physical and chemical properties and area of purpose. Knowing the meaning of marking metal products, there will be no difficulties in selecting a specific type for any purpose.
Carbon steel is characterized by a carbon content of up to 2.14% without the presence of alloying elements, a small amount of impurities in the composition, and a small content of magnesium, silicon and manganese. This in turn affects the properties and features of application. It is the main product of the metallurgical industry.
Compound
Depending on the amount of carbon, carbon and alloy steel are divided. The presence of carbon gives the material strength and hardness, and also reduces viscosity and ductility. Its content in the alloy is up to 2.14%, and the minimum amount of impurities due to the manufacturing process allows the bulk to consist of iron up to 99.5%.
High strength and hardness are what characterize carbon steel.
Impurities that are constantly included in the structure of carbon steel have a small content. Manganese and silicon do not exceed 1%, and sulfur and phosphorus are within 0.1%. An increase in the amount of impurities is characteristic of another type of steel, which is called alloyed.
The lack of technical ability to completely remove impurities from the finished alloy allows the following elements to be included in carbon steel:
- hydrogen;
- nitrogen;
- oxygen;
- silicon;
- manganese;
- phosphorus;
- sulfur
The presence of these substances is determined by the steel melting method: converter, open-hearth or other. And carbon is added on purpose. If the amount of impurities is difficult to regulate, then adjusting the level of carbon in the composition of the future alloy affects the properties of the finished product. When the material is filled with carbon up to 2.4%, steel is classified as carbon.
Characteristic
The characteristics and structure of the metal are changed using heat treatment, through which the required surface hardness or other requirements for the use of the steel structure are achieved. However, not all structural properties can be adjusted using thermal methods. Such structurally insensitive characteristics include rigidity, expressed by the elastic modulus or shear modulus. This is taken into account when designing critical components and mechanisms in various fields of mechanical engineering.
In cases where the calculation of the strength of an assembly requires the use of small-sized parts that can withstand the required load, heat treatment is used. This effect on “raw” steel makes it possible to increase the rigidity of the material by 2-3 times. The metal that is subjected to this process is subject to requirements regarding the amount of carbon and other impurities. This steel is called high quality.
Classification of carbon steels
According to the direction of application of products, carbon steel is divided into tool and structural.
The last of them is used for the construction of various buildings and frame parts. Tools are used to make durable tools for performing any work, including metal cutting. The use of metal products in the household required the classification of steel into different categories with specific properties: heat-resistant, cryogenic and corrosion-resistant.
According to the method of production, carbon steels are divided into:
- electric steel;
- open hearth;
- oxygen converter.
Differences in the structure of the alloy are due to the presence of different impurities characteristic of a particular smelting method.
The relationship of steel to chemically active environments has made it possible to divide products into:
- boiling;
- semi-calm;
- calm.
- hypereutectoid, in which the amount of carbon exceeds 0.8%;
- eutectoid, with a content of 0.8%;
- hypoeutectoid - less than 0.8%.
It is the structure that is a characteristic feature in determining the state of the metal. In hypoeutectoid steels, the structure consists of pearlite and ferrite. Eutectoid ones have pure pearlite, while hypereutectoid ones are characterized by pearlite with admixtures of secondary cementite.
By increasing the amount of carbon, steel increases strength and reduces ductility. The viscosity and brittleness of the material also have a great influence. As the percentage of carbon increases, the impact strength decreases and the fragility of the material increases. It is no coincidence that when the content is more than 2.4%, metal alloys are already classified as cast iron.
According to the amount of carbon in the alloy, steel is:
- low carbon (up to 0.29%);
- medium carbon (from 0.3 to 0.6%);
- high carbon (more than 0.6%).
Marking
When designating carbon steels of ordinary quality, the letters St are used, which are accompanied by numbers characterizing the carbon content. One digit shows the quantity multiplied by 10, and two digits by 100. When guaranteeing the mechanical composition of the alloy, B is added before the designation, and compliance with the chemical constituents is B.
At the end of the marking, two letters indicate the degree of deoxidation: ps - semi-quiet, kp - boiling state of the alloys. For calm metals this indicator is not indicated. An increased amount of manganese in the structure of the product is designated by the letter G.
When designating high-quality carbon steels used in the manufacture of tools, the letter U is used, next to which a number is written confirming the percentage of carbon in a 10-fold amount, regardless of whether it is two-digit or single-digit. To highlight higher quality alloys, the letter A is added to the designation of tool steels.
Examples of designation of carbon steels: U8, U12A, St4kp, VSt3, St2G, BSt5ps.
Production
The metallurgical industry produces metal alloys. The specificity of the process for producing carbon steel is the processing of cast iron billets with the reduction of suspended matter such as sulfur and phosphorus, as well as carbon, to the required concentration. The differences in the oxidation technique by which carbon is removed allows us to distinguish different types of smelting.
Oxygen converter method
The basis of the technique was the Bessemer method, which involves blowing air through liquid cast iron. During this process, carbon is oxidized and removed from the alloy, after which the iron ingots gradually turn into steel. The productivity of this technique is high, but sulfur and phosphorus remained in the metal. In addition, carbon steel is saturated with gases, including nitrogen. This improves strength but reduces ductility, making the steel more prone to aging and high in non-metallic elements.
Given the low quality of steel produced by the Bessemer method, it was no longer used. It was replaced by the oxygen-converter method, the difference of which is the use of pure oxygen, instead of air, when purging liquid cast iron. The use of certain technical conditions during purging significantly reduced the amount of nitrogen and other harmful impurities. As a result, carbon steel produced by the oxygen-converter method is close in quality to alloys melted in open-hearth furnaces.
The technical and economic indicators of the converter method confirm the feasibility of such smelting and make it possible to replace outdated methods of steel production.
Open hearth method
A feature of the method for producing carbon steel is the burning of carbon from cast iron alloys not only with the help of air, but also by adding iron ores and rusty metal products. This process usually takes place inside furnaces, to which heated air and combustible gas are supplied.
The size of such melting baths is very large; they can hold up to 500 tons of molten metal. The temperature in such containers is maintained at 1700 ºC, and carbon burning occurs in several stages. First, due to excess oxygen in flammable gases, and when slag forms above the molten metal, through iron oxides. When they interact, slags of phosphates and silicates are formed, which are subsequently removed and the steel acquires the required quality properties.
Steel melting in open hearth furnaces takes about 7 hours. This allows you to adjust the desired composition of the alloy when adding different ores or scrap. Carbon steel has long been manufactured using this method. Such stoves, in our time, can be found in the countries of the former Soviet Union, as well as in India.
Electrothermal method
It is possible to produce high-quality steel with a minimum content of harmful impurities by melting it in vacuum furnaces of electric arc or induction furnaces. Thanks to the improved properties of electric steel, it is possible to produce heat-resistant and tool alloys. The process of converting raw materials into carbon steel occurs in a vacuum, due to which the quality of the resulting workpieces will be higher than the previously discussed methods.
The cost of such metal processing is more expensive, so this method is used when there is a technological need for a high-quality product. To reduce the cost of the technological process, a special ladle is used, which is heated inside a vacuum container.
Application
Carbon steel, due to its properties, has found wide application in various sectors of the national economy, especially in mechanical engineering. The use of metal’s ability to resist loads and have high fatigue limits in design calculations makes it possible to manufacture from carbon steel such critical machine parts as: flywheels, gear drives, connecting rod housings, crankshafts, plunger pump pistons, and technological equipment for woodworking and light industry.
High-carbon steels with an increased amount of manganese are used for the manufacture of parts such as springs, leaf springs, torsion bars and similar components that require the elasticity of the alloy. Tool alloys of improved quality are widely used in the production of tools used to process metals: cutters, drills, countersinks.