Impurities are called chemical elements that have passed into the composition of steel in the process of its production as technological additives or as components of charge materials.
Impurities in steel are divided into permanent (ordinary), random and latent (harmful).
Permanent impurities in steel are manganese and silicon, which are present as impurities in almost all industrial steels. The manganese content in structural steels is usually in the range of 0.3-0.8% (if manganese is not an alloying element), in tool steels its content is somewhat lower (0.15-0.40%). The introduction of manganese as a technological additive in such quantities is necessary for the transfer of sulfur from iron sulfide to manganese sulfide. Silicon in well-deoxidized (calm) steels is usually contained in the range of 0.17-0.37%. In incompletely deoxidized low-carbon (£0.2% C) steels, it contains less: in semi-calm 0.05-0.017%, in boiling<0,07 %. В нержавеющих и жаропрочных, нелегированных кремнием сталях его может содержаться до 0,8 %.
Random impurities in steel can be practically any elements that accidentally get into steel from scrap, naturally alloyed ore or deoxidizers. Most often, these are Cr, Ni, Cu, Mo, W, A1, T1, etc. in quantities limited for impurities.
Latent impurities in steel are sulfur, phosphorus, arsenic and gases: hydrogen, nitrogen and oxygen. However, recently nitrogen, sulfur, and phosphorus are sometimes used as alloying additives to provide a number of special properties of steels.
According to the graded chemical composition of steel, it is possible to determine which elements are alloying additives, and which are impurities. If the graded chemical composition of the steel sets the lower (not less) and upper (not more) limits for the content of this element in the steel, then it will be alloying. As a rule, only the upper limit of the content is set for impurities. The only exceptions are manganese and silicon, the amount of which is regulated by the lower and upper limits, both for impurities and for alloying additives.
Harmful impurities of sulfur, phosphorus and gases are present in almost all steels and, depending on the type of steel, they can have a different effect on the properties.
At present, various technological processes and methods of steel production are widely used in metallurgy, as a result of which a significant reduction in metal contamination with non-metallic inclusions is achieved, and it becomes possible to control their composition, size and distribution. Such processes and methods include: refining remelting (electroslag, vacuum-arc), vacuum induction melting, out-of-furnace processing of steel with synthetic slags, degassing in a ladle, etc.
O2, S, P, N, H2, less often others (As) even in small quantities adversely affect the properties of iron and its alloys. Obtaining a metal of proper quality is achieved by reducing the content of harmful impurities to certain limits by various methods, depending on the characteristics of each impurity.
Steel deoxidation is the removal of dissolved oxygen. In oxidative refining, as impurities in the metal with a greater affinity for oxygen than iron decrease, the oxygen concentration increases. Carbon is oxidized later than other elements and the concentration of C in the metal determines the final. The oxidizability of the metal in the course of decarburization increases and is especially sharp when the carbon content is less than 0.2. The actual concentration of carbon in the metal at the end of refining for different grades of steel is in the range of 0.02-0.05%. When such a metal is cooled, the decarburization reaction with selective crystallization will continue. As crystallization proceeds, the remaining liquid gradually becomes enriched in impurities. In particular, carbon and [O], the concentrations of which all the time remain higher than the equilibrium for the decarburization reaction, which ensures its continuous flow. Part of the gas bubbles remains in the solidifying metal, making it bubbly. Due to the supersaturation of the metal with oxygen, iron oxides are released. As the temperature decreases, the solubility of O2 in iron drops sharply, which leads to an increase in the oxide phase at the grain boundary. This phenomenon is called red brittleness. An excessively high concentration of O2 in a metal is an example of an oxidative refining process. In addition, during the production and casting of steel, i.e. after being released from under the slag cover, it comes into contact with atmospheric air. Therefore, steel smelting always ends with deoxidation in order to remove oxygen from the metal to the limits that ensure complete or partial cessation of the decarburization reaction. Two groups of elements are used as deoxidizer elements - these are elements with a higher affinity for oxygen than iron Mn, Si. The second group - elements with a greater affinity for oxygen than carbon. They serve to completely “calm down” Al, Ti, B, Ca, Zirkoniy. Deoxidation is usually combined with alloying, i.e. with an increase to the required limits of useful impurities. There are three methods of deoxidation - precipitating, diffusion and evacuation.
besieger. The most common deoxidation method, which consists in introducing a deoxidizer directly into the metal, where a heterogeneous reaction occurs. The deoxidizer oxide must have the properties of "precipitates", i.e. be insoluble in iron and able to easily stand out from the melt. In order for the reaction to proceed, it is necessary that the scavengers have a greater affinity than C and Fe and that their oxides are stronger than FeO. When a deoxidizer is introduced into steel, O2 decreases due to the deoxidation reaction until the equilibrium of the reaction is reached, therefore, the minimum residual concentration of O2 in the metal corresponds to equilibrium in the reaction and serves as a measure of the relative affinity of the deoxidizer for oxygen or a measure of the deoxidizing ability of deoxidizing elements. The mechanism of precipitating deoxidation includes the following stages: dissolution of the solid deoxidizer and its uniform distribution in the volume of the metal, chemical decarburization reaction, removal of the reaction products from the melt - non-metallic slag inclusions. The most important link is the third stage, because. the amount of non-metallic inclusions that pollute the metal and lower its quality depends on it. These non-metallic slag inclusions have a lower density than metal and float to its surface. The higher the ascent rate, the cleaner the steel is from non-metallic inclusions. The ascent rate of small spherical particles with a diameter of less than 1 mm. It follows from the equations that as f decreases, the speed decreases. The process accelerates with a decrease in the density of slag inclusions, an increase in temperature, a decrease in the viscosity of steel and, as a result, with an increase in the time of solidification and the time of ascent of the inclusions. The decisive role belongs to the particle size. The larger they are, the cleaner the steel. Particle enlargement occurs more easily in liquid form than in solid form. Therefore, for the complete purification of metals from deoxidation products, a high steel temperature of more than 1600 ° C and a low melting point are required.
Diffusion deoxidation consists in the formation of a process of diffusion of oxygen from the metal into the slag due to a decrease in its oxidation, due to the introduction of deoxidizers into it, which, by reducing iron oxides in the slag, lowers their concentration and, consequently, the concentration of oxygen. The advantage of this method over the precipitating one is that the interaction takes place in the slag and the metal is not contaminated. The disadvantage is the low speed of the process. Directly in units, mainly in electric furnaces in the production of special steels. Varieties of diffusion deoxidation is the processing of steel in ladles with synthetic slags with a low FeO content.
Vacuuming. This is a relatively new and promising method that makes it possible to obtain not only oxygen, but also hydrogen and nitrogen, i.e. carry out degassing of steels. For H and N, the solubility in the layer decreases with decreasing T. It is especially low in the solid state. When steel cools, hydrogen is released into micropores and forms flocks - these are small cracks that degrade mechanical strength. Nitrogen increases the hardness of steel, but at the same time increases brittleness and reduces ductility. The structure of gases in iron, i.e. absorption is accompanied by a change in its molecular state. Given that the gases in the metal are infinitely dilute solutions, the activity coefficient is approximately equal to 1. Therefore, the solubility of gases in the metal is a function of two quantities - temperature and pressure. The effect of temperature on solubility is determined by the value of deltaH. H2 and N2 dissolve with the absorption of heat. Therefore, overheating of the metal increases the gas saturation, this is especially true for EP. In addition, alloying elements form hydrides and nitrides, which contributes to an increase in gas saturation. Titanium and zirconium are especially prone to hydrides, and chromium and vanadium are especially prone to the formation of nitrides. If the ladle of metal is placed in a closed system and the gas pressure above the metal is significantly reduced, then the pressure in the gas bubbles contained in the melt will decrease significantly, especially in the upper layer of the metal. This will ensure the transition of bubbles of those impurities that can be released from the steel in a gaseous state and intensive ascent of bubbles. Carbon has a significant affinity for oxygen and can serve as a deoxidizer, but its use for this at normal pressure is very limited, because. the remaining CO leads to loose ingots, i.e. causes blisters in the metal. To prevent this, the introduction of a deoxidizer with a greater affinity for oxygen than carbon contaminates the metal with non-metallic inclusions. Vacuuming allows you to significantly reduce the oxygen concentration in steel without the use of deoxidizers and, therefore, without additional contamination of the metal. In this case, the role of carbon as a deoxidizer increases significantly, since during evacuation, CO is most completely removed from it and blistering of the metal is excluded.
The mechanical properties in carbon steels are affected by the carbon content. With an increase in carbon content, strength, hardness and wear resistance increase, but ductility and toughness decrease, and weldability deteriorates.
Change in the strength of steel depending on the carbon content.
Ferrite(solid solution of carbon in iron) - very plastic and viscous, but fragile.
Perlite, a mechanical mixture of fine plates of ferrite and cementite, gives strength. Cementite very hard, brittle and statically strong. With an increase in the carbon content in the steel (up to 0.8%), the perlite content increases and the strength of the steel increases. However, along with this, its ductility and impact strength decrease. At a content of 0.8% C (100% perlite), the strength of the steel reaches its maximum.
Manganese injected into any steel for deoxidation (i.e., to eliminate harmful inclusions of ferrous oxide). Manganese dissolves in ferrite and cementite, so its detection by metallographic methods is impossible. It increases the strength of the steel and greatly increases the hardenability. The content of manganese in carbon steel of certain grades can reach 0.8%.
Silicon, like manganese, is a deoxidizer, but acts more efficiently. In boiling steel, the silicon content should not exceed 0.07%. If there is more silicon, then silicon deoxidation will occur so completely that the liquid metal will not “boil” due to carbon deoxidation. Quiet carbon steel contains 0.12 to 0.37% silicon. All silicon dissolves in ferrite. It greatly increases the strength and hardness of steel.
Sulfur- harmful impurity. During the steelmaking process, the sulfur content is reduced, but it cannot be completely removed. In open-hearth steel of ordinary quality, the sulfur content is allowed up to 0.055%.
The presence of sulfur in large quantities leads to the formation of cracks during forging, stamping and hot rolling, this phenomenon is called red brittleness. In carbon steel, sulfur reacts with iron, resulting in iron sulfide FeS. In the process of hot plastic deformation along the grain boundaries, hot cracks are formed.
If a sufficient amount of manganese is introduced into the steel, then the harmful effects of sulfur will be eliminated, since it will be bound into refractory manganese sulfide. MnS inclusions are located in the middle of the grains, rather than along their boundaries. During hot working, the MnS inclusions are easily deformed without cracking.
Phosphorus, like sulfur, is a harmful impurity. Dissolving in ferrite, phosphorus sharply reduces its ductility, increases the transition temperature to a brittle state, or otherwise causes cold brittleness of steel. This phenomenon is observed when the phosphorus content is above 0.1%.
Areas of the ingot with a high content of phosphorus become cold brittle. In open-hearth steel of ordinary quality, no more than 0.045% R is allowed.
Sulfur and phosphorus, causing brittleness of steel and at the same time lowering mechanical properties, improve machinability: the surface finish increases, the time between regrinding cutters, cutters, etc. increases. Therefore, for a number of non-critical parts subjected to machining, so-called automatic steels with a high sulfur content (up to 0.30%) and phosphorus (up to 0.15%).
Oxygen- harmful impurity. Iron oxide, like sulfur, causes steel brittleness. Very hard oxides of aluminium, silicon and manganese sharply impair the machinability of steel by cutting, quickly dulling the cutting tool.
In the process of smelting carbon steel from scrap metal, nickel, chromium, copper and other elements can get into it. These impurities impair the technological properties of carbon steel (in particular, weldability), so they try to minimize their content.
Steel marking
In carbon steels of ordinary quality, the content of harmful impurities is allowed, as well as gas saturation and contamination with non-metallic inclusions. And depending on the purpose and the complex of properties, they are divided into groups: A - supplied with guaranteed mechanical parameters, B - supplied with guaranteed chemical parameters, C - supplied with guaranteed chemical and mechanical parameters.
Steels are marked with a combination of the letters St and a number (from 0 to 6) showing the grade number, and not the average carbon content in it, although as the number increases, the carbon content in steel increases. Steels of groups B and C have the letters B and C in front of the brand, indicating their belonging to these groups. Group A steels are used in the delivered state for products whose manufacture is not accompanied by hot working. In this case, they retain the normalization structure and mechanical properties guaranteed by the standard.
Group B steels are used for products manufactured using hot working (forging, welding and, in some cases, heat treatment), in which the original structure and mechanical properties are not preserved. For such parts, information about the chemical composition is important to determine the hot working mode.
Steels are the most common materials. They have good technological properties. Products are obtained as a result of processing by pressure and cutting.
The advantage is the ability to obtain the desired set of properties by changing the composition and type of processing. Steels are divided into carbon and alloy steels.
Effect of carbon and impurities on the properties of steels
Carbon steels are the main ones. Their properties are determined by the amount of carbon and the content of impurities that interact with iron and carbon.
The influence of carbon.
The influence of carbon on the properties of steels is shown in fig. 10.1
Fig.10.1. Effect of carbon on the properties of steels
With an increase in the carbon content in the steel structure, the amount of cementite increases, while the proportion of ferrite decreases. Changing the ratio between the components leads to a decrease in ductility, as well as to an increase in strength and hardness. The strength increases to a carbon content of about 1%, and then it decreases, as a coarse mesh of secondary cementite is formed.
Carbon affects the viscous properties. Increasing the carbon content increases the cold brittleness threshold and reduces the impact strength.
Electrical resistance and coercive force increase, magnetic permeability and magnetic induction density decrease.
Carbon also affects the technological properties. An increase in the carbon content worsens the casting properties of steel (steels with a carbon content of up to 0.4% are used), workability by pressure and cutting, and weldability. It should be taken into account that steels with a low carbon content are also poorly machined.
Influence of impurities.
There are always impurities in steels, which are divided into four groups. one. Permanent impurities: silicon, manganese, sulfur, phosphorus.
Manganese and silicon are introduced in the process of steel smelting for deoxidation, they are technological impurities.
The manganese content does not exceed 0,5…0,8 %. Manganese increases strength without reducing ductility, and sharply reduces the red brittleness of steel caused by the influence of sulfur. It helps to reduce the content of iron sulfide FeS, since it forms a compound of manganese sulfide with sulfur MnS. Particles of manganese sulfide are located in the form of separate inclusions, which are deformed and become elongated along the rolling direction.
Located near the grains, increases the brittle transition temperature, causes cold brittleness, reduces the work of crack propagation, Increasing the phosphorus content for each 0,01 % raises the cold brittleness threshold by 20…25ºС.
Phosphorus has a tendency to segregation, therefore, in the center of the ingot, individual sections have a sharply reduced viscosity.
For some steels, it is possible to increase the phosphorus content up to 0,10…0,15 %, to improve machinability.
Sulfur reduces ductility, worsens weldability and corrosion resistance.
The sulfur content in steels is 0,025…0,06 %. Sulfur is a harmful impurity that enters steel from cast iron. When interacting with iron, it forms a chemical compound - sulfur sulfide FeS, which, in turn, forms a low-melting eutectic with iron with a melting point 988ºС. When heated for rolling or forging, the eutectic melts, and the bonds between the grains are broken. During deformation, tears and cracks appear at the locations of the eutectic, the workpiece is destroyed - the phenomenon red brittleness.
Red brittleness - increased brittleness at high temperatures
Sulfur reduces mechanical properties, especially impact strength a and ductility (u), as well as the endurance limit. It impairs weldability and corrosion resistance.
2. Hidden impurities- gases (nitrogen, oxygen, hydrogen) - enter the steel during smelting.
Nitrogen and oxygen are in steel in the form of brittle non-metallic inclusions: oxides ( FeOSiO 2 ,Al 2O 3) nitrides ( Fe2N), in the form of a solid solution or in a free state, located in defects (cavities, cracks).
Interstitial impurities (nitrogen N, oxygen O) increase the cold brittleness threshold and reduce the resistance to brittle fracture. Non-metallic inclusions (oxides, nitrides), being stress concentrators, can significantly reduce the endurance limit and viscosity.
Hydrogen dissolved in steel is very harmful, which significantly embrittles the steel. It leads to the formation in rolled billets and forgings flocks.
Flockens- thin cracks of an oval or rounded shape, having the appearance of spots in a break - silver flakes.
Metal with flocks cannot be used in industry; during welding, cold cracks form in the deposited and base metal.
If hydrogen is in the surface layer, then it is removed as a result of heating at 150…180 , better in vacuum mm Hg. Art.
Vacuum is used to remove hidden impurities.
3. special impurities, which are specially introduced into steel to obtain the specified properties. Impurities are called alloying elements, and steels are called alloyed steels.
Assignment of alloying elements.
The main alloying element is chromium (0,8…1,2)%. It increases the hardenability, contributes to obtaining high and uniform hardness of steel. Cold brittleness threshold of chromium steels - (0…-100) ºС.
Additional alloying elements.
Boron - 0.003%. Increases hardenability and also raises the cold brittleness threshold (+20…-60) ºС.
Manganese - increases hardenability, but promotes grain growth and raises the cold brittleness threshold to (+40…-60) ºС.
Titanium (~0,1%) introduced for grinding grain in chromium-manganese steel.
Introduction of molybdenum (0,15…0,46%) in chromium steels increases hardenability, reduces the threshold of cold brittleness to –20…-120 ºС. Molybdenum increases the static, dynamic and fatigue strength of steel, eliminates the tendency to internal oxidation. In addition, molybdenum reduces the tendency for temper brittleness in steels containing nickel.
Vanadium in quantity (0.1…0.3) % in chromium steels, it refines the grain and increases strength and toughness.
The introduction of nickel into chromium steels significantly increases strength and hardenability, lowers the cold brittleness threshold, but at the same time increases the tendency to temper brittleness (this disadvantage is compensated by the introduction of molybdenum into the steel). Chrome-nickel steels have the best set of properties. However, nickel is in short supply and the use of such steels is limited.
A significant amount of nickel can be replaced by copper, this does not lead to a decrease in viscosity.
When alloying chromium-manganese steels with silicon, steels receive chromansil (20HGS, 30HGSA). Steels have a good combination of strength and toughness, are well welded, stamped and machined. Silicon increases impact strength and thermal toughness.
The addition of lead, calcium improves machinability. The use of hardening heat treatment improves the complex of mechanical properties.
Distribution of alloying elements in steel.
Alloying elements dissolve in the main phases of iron-carbon alloys (ferrite, austenite, cementite), or form special carbides.
The dissolution of alloying elements occurs as a result of the replacement of iron atoms by atoms of these elements. These atoms create stresses in the lattice, which cause a change in its period.
Changing the dimensions of the lattice causes a change in the properties of ferrite - strength increases, ductility decreases. Chromium, molybdenum and tungsten strengthen less than nickel, silicon and manganese. Molybdenum and tungsten, as well as silicon and manganese in certain quantities, reduce the viscosity.
In steels, carbides are formed by metals located in the periodic table to the left of iron (chromium, vanadium, titanium), which have a less completed d- electronic band.
In the process of carbide formation, carbon donates its valence electrons to fill d the electron band of the metal atom, while in the metal the valence electrons form a metallic bond, which determines the metallic properties of carbides.
When the ratio of atomic radii of carbon and metal is more than 0,59 typical chemical compounds are formed: Fe3c,Mn3c,Cr 23C6,Cr 7C 3 ,Fe3W 3C- which have a complex crystal lattice and, when heated, dissolve in austenite.
When the ratio of atomic radii of carbon and metal is less than 0,59 implementation phases are formed: Mo 2c,wc,VC,TiC,TaC,W2C- which have a simple crystal lattice and are difficult to dissolve in austenite.
All carbides have high hardness and melting point.
4. Random impurities.
Classification and marking of steels
Steel classification
Steels are classified according to many criteria.
- By chemical: composition: carbon and alloyed.
- By carbon content:
a) low-carbon, with a carbon content up to 0,25 %;
b) medium carbon, with carbon content 0,3…0,6 %;
c) high-carbon, with a higher carbon content 0,7 %
- According to the equilibrium structure: hypoeutectoid, eutectoid, hypereutectoid.
- By quality. A quantitative indicator of quality is the content of harmful impurities: sulfur and phosphorus:
a) carbon steels of ordinary quality:
b) quality steels;
c) high-quality steels.
- Smelting method:
a) in open-hearth furnaces;
b) in oxygen converters;
c) in electric furnaces: electric arc, induction, etc.
- By appointment:
a) structural - used for the manufacture of parts of machines and mechanisms;
b) tool - used for the manufacture of various tools;
c) special - steels with special properties: electrical, with special magnetic properties, etc.
Steel marking
Accepted alphanumeric designation of steels
Carbon steels of ordinary quality (GOST 380).
Marked: St.2kp., Bst.3kp, Vst.3ps, Vst.4sp.
St is the index of this steel group. Numbers from 0 before 6 - this is the conditional number of the steel grade. With an increase in the grade number, the strength increases and the ductility of steel decreases. According to the guarantees upon delivery, there are three groups of steels: A, B and C. For steels of group A, mechanical properties are guaranteed upon delivery, the index of group A is not indicated in the designation. For group B steels, the chemical composition is guaranteed. For group B steels, both mechanical properties and chemical composition are guaranteed upon delivery.
The indexes kp, ps, cn indicate the degree of deoxidation of steel: kp - boiling, ps - semi-calm, cn - calm.
Quality carbon steels
Quality steels are supplied with guaranteed mechanical properties and chemical composition (group B). The degree of deoxidation is mostly calm.
Structural quality carbon steels. They are marked with a two-digit number indicating the average carbon content in hundredths of a percent. The degree of deacidification is indicated if it differs from calm.
Steel 08 kp, steel 10 ps, steel 45.
Tool quality carbon steels are marked with the letter Y (carbon tool steel) and a number indicating the carbon content in tenths of a percent.
U8 steel, U13 steel.
Tool high-quality carbon steels. They are marked similarly to high-quality tool carbon steels, only at the end of the brand they put the letter A to indicate high quality steel.
Steel U10A.
Quality and high quality alloy steels
The designation is alphanumeric. Alloying elements have conventional designations, are designated by the letters of the Russian alphabet.
Designations of alloying elements:
X - chromium, H - nickel, M - molybdenum, V - tungsten, K - cobalt, T - titanium, A - nitrogen (indicated in the middle of the brand), G - manganese, D - copper, F - vanadium, C - silicon, P - phosphorus, P - boron, B - niobium, C - zirconium, Yu - aluminum.
Alloy structural steels
Steel 15Kh25N19VS2
At the beginning of the stamp, a two-digit number is indicated, showing the carbon content in hundredths of a percent. The alloying elements are listed below. The number following the symbol of the element shows its content in percentage,
If the number is not set, then the content of the element does not exceed 1,5 %.
The indicated steel grade contains 0,15 % carbon, 35% chrome, 19 % nickel, up to 1,5% tungsten, up to 2 % silicon.
To designate high-quality alloy steels, the symbol A is indicated at the end of the grade.
Alloy tool steels
Steel 9XC, steel HVG.
At the beginning of the brand, a single-digit number is indicated, showing the carbon content in tenths of a percent. When the carbon content is more than 1%, the number is not indicated,
All alloyed tool steels are of high quality.
Some steels have non-standard designations.
High speed tool steels
P is the index of this group of steels (from rapid - speed). The carbon content is more than 1%. The number shows the content of the main alloying element - tungsten.
In the indicated steel, the tungsten content is 18 %.
If steels contain an alloying element, then their content is indicated after the designation of the corresponding element.
Ball bearing steels
Steel SHKH6, steel SHKH15GS
Ш is the index of this group of steels. X - indicates the presence of chromium in the steel. The following number shows the chromium content in tenths of a percent, in the indicated steels, respectively, 0,6 % and 1,5 %. The alloying elements included in the composition of the steel are also indicated. The carbon content is more 1 %.
There are always impurities in steels, which are divided into four groups. one. Permanent impurities: silicon, manganese, sulfur, phosphorus.
Manganese and silicon are introduced in the process of steel smelting for deoxidation, they are technological impurities.
The manganese content does not exceed 0,5…0,8 %. Manganese increases strength without reducing ductility, and sharply reduces the red brittleness of steel caused by the influence of sulfur. It helps to reduce the content of iron sulfide FeS, since it forms a compound of manganese sulfide with sulfur MnS. Particles of manganese sulfide are located in the form of separate inclusions, which are deformed and become elongated along the rolling direction.
Located near the grains, increases the brittle transition temperature, causes cold brittleness, reduces the work of crack propagation, Increasing the phosphorus content for each 0,01 % raises the cold brittleness threshold by 20 ... 25 o C.
Phosphorus has a tendency to segregation, therefore, in the center of the ingot, individual sections have a sharply reduced viscosity.
For some steels, it is possible to increase the phosphorus content up to 0,10…0,15 %, to improve machinability.
S– reduced ductility, weldability and corrosion resistance. P-distorts the crystal lattice.
The sulfur content in steels is 0,025…0,06 %. Sulfur is a harmful impurity that enters steel from cast iron. When interacting with iron, it forms a chemical compound - sulfur sulfide FeS, which, in turn, forms a low-melting eutectic with iron with a melting point 988 o C. When heated for rolling or forging, the eutectic melts, and the bonds between the grains are broken. During deformation, tears and cracks appear at the locations of the eutectic, the workpiece is destroyed - the phenomenon red brittleness.
Red brittleness - increased brittleness at high temperatures
Sulfur reduces mechanical properties, especially impact strength a and ductility
(δ and ψ), as well as the endurance limit. It impairs weldability and corrosion resistance.
2. Hidden impurities- gases (nitrogen, oxygen, hydrogen) - enter the steel during smelting.
Nitrogen and oxygen are in steel in the form of brittle non-metallic inclusions: oxides ( FeO, SiO 2 , Al 2 O 3)nitrides ( Fe2N), in the form of a solid solution or in a free state, located in defects (cavities, cracks).
Interstitial impurities (nitrogen N, oxygen O) increase the cold brittleness threshold and reduce the resistance to brittle fracture. Non-metallic inclusions (oxides, nitrides), being stress concentrators, can significantly reduce the endurance limit and viscosity.
Hydrogen dissolved in steel is very harmful, which significantly embrittles the steel. It leads to the formation in rolled billets and forgings flocks.
Flockens- thin cracks of an oval or rounded shape, having the appearance of spots in a break - silver flakes.
Metal with flocks cannot be used in industry; during welding, cold cracks form in the deposited and base metal.
If hydrogen is in the surface layer, then it is removed as a result of heating at 150…180 , better in vacuum ~10 -2 ... 10 -3 mm Hg. Art.
Vacuum is used to remove hidden impurities.
3. Special impurities- specially introduced into steel to obtain the specified properties. Impurities are called alloying elements, and steels are called alloyed steels.
hard-worked steel
Wire, thin sheets have found wide application in the economy. These types of products are obtained in metallurgy by rolling, drawing in a cold state. As a result of such processing, the metal is strengthened due to a phenomenon called hardening. Due to room temperature, the hardening is not removed. This type of processing is called hardening.
Hardening of steel strongly depends on the degree of work hardening and on the carbon content (Fig. 7).
Record values of σw were obtained for reduction up to 90% in steel 1.2% C at wire ∅ 0.1 mm.
Hardening is an inevitable process of any plastic deformation. Hardening (hardening) is accompanied by an increase in strength and hardness and a significant decrease in ductility.
Therefore, after rolling or drawing in a cold state, sheets, channels, pipes are hardened.
Most often, this is a desired property change. Sometimes it's undesirable. For example, you can't make chasing on a cold-worked copper sheet - it will tear. It is necessary to remove hardening by heat treatment (annealing).
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