Steel is an alloy of iron and carbon in a percentage of the latter element variable between 0.008% and 2.11% by mass of its composition. The branch of metallurgy that specializes in producing steel is called steelmaking or steelmaking.
The steel produced before the detonation of the first atomic bombs is low-bottom steel, uncontaminated by radionuclides. Currently, the main source of this steel, required for the construction of very sensitive radiation measurement equipment, are the sunken ships that were built before the Trinity test, the most famous being the German warships sunk in Scapa Flow during the First World War.
It should not be confused with iron, which is a hard and relatively ductile metal. Carbon is a nonmetal of smaller diameter (dA = 1.54 Å), soft and brittle in most of its allotropic forms (except in the diamond form). The diffusion of this element in the crystal structure of the previous one is achieved thanks to the difference in atomic diameters, forming an interstitial compound.
The main difference between iron and steel is in the percentage of carbon: steel is iron with a carbon percentage of between 0.008% and 2.11%; Above this percentage alloys are called castings.
Depending on the rate at which the molten metal cools, a process called tempering, the microstructure of steel changes, and therefore also its mechanical properties. Some of the phases that can be found in steel are martensite, perlite, cementite, bainite and ferrite. To know the phases that steel has according to its composition and tempering, an iron-carbon phase diagram is used.
Steel retains the metallic characteristics of iron in its pure state, but the addition of carbon and other elements, both metallic and non-metallic, improves its physicochemical properties. However, if the alloy has a carbon concentration greater than 2.11%, castings are produced, which are much more brittle than steel and cannot be forged, but have to be molded.
There are many types of steel depending on the alloying element or elements that are present. The definition in the percentage of carbon corresponds to carbon steels, in which this non-metal is the only alloy, or there are others but in lower concentrations.
Other specific compositions receive particular names depending on multiple variables such as the elements that predominate in their composition (silicon steels), their susceptibility to certain treatments (cementing steels), some enhanced characteristics (stainless steels) and even depending on their use (structural steels). Usually, these iron alloys are included under the generic name of special steels, which is why here the definition of the common or “carbon” has been adopted that in addition to being the first manufactured and the most used, served as the basis for the others. This wide variety of steels led Siemens to define steel as “a composite of iron and another substance that increases its strength”.
As the strength of the steel is increased by adding carbon or subjecting it to different heat treatments, the elasticity and plasticity also decreases significantly. Therefore, it is necessary to find a balance, depending on the application that will be given to the material, between strength and flexibility.
History of Steel
The term steel comes from the Latin “aciarius“, and this from the word “acies“, which is how the edge of a knife is called in this language. “Aciarius” would therefore be the right metal, due to its hardness and resistance, to put it in the cutting part of weapons and tools. The exact date when the technique for obtaining iron from smelting minerals was discovered is unknown.
However, the first archaeological remains of iron utensils date back to 3000 BC and were discovered in Egypt, although there are vestiges of earlier ornaments. Quintus Horace Flaccus points out that steel weapons such as the falcata were used in the Iberian Peninsula as early as the first century BC, while noric steel (in Latin: chalybs noricus) was used by Roman troops. During the Han Dynasty of China, steel was produced by melting wrought iron with cast iron around the first century BC.
They also adopted production methods for the creation of wootz steel, a process that emerged in India and Sri Lanka from about 300 BC and exported to China towards the fifth century. This early method used a wind furnace, blown by the monsoons. Also known as Damascus steel, it was an iron alloy with a large number of different materials, including traces of other elements in concentrations less than 1000 parts per million or 0.1% of the composition of the rock. Studies by Peter Paufler suggested that its structure included carbon nanotubes, which could explain some of the qualities of this steel -such as its durability and ability to maintain an edge-, although due to the technology of the time it is possible that they have been obtained by chance and not by a premeditated design.
Between the ninth and tenth centuries, crucible steel was produced in Merv, in which steel was obtained by heating and cooling iron and coal by different techniques. During the Song dynasty of the eleventh century in China, steel production was carried out using two techniques: the first produced low-quality steel because it was not homogeneous – the “berganesque” method – and the second, precursor of the Bessemer method, removes the coal with repeated forgings and subjects the piece to abrupt cooling.
Iron for industrial use was discovered around 1500 BC, in Metsamor and Mount Ararat, in Armenia. Iron technology was long kept secret, spreading widely around 1200 BC.
There are no records that temperability was known until the Middle Ages. The old methods for the manufacture of steel consisted of obtaining mild iron in the furnace, with charcoal and air draft, with a subsequent expulsion of the slag by hammering and carburizing the mild iron to cement it. Then cementation was perfected by melting cemented steel in clay crucibles and in Sheffield (England) crucible steels were obtained from 1740. The technique was developed by Benjamin Huntsman.
In 1856, Henry Bessemer developed a method for producing steel in large quantities, but since iron containing phosphorus and sulfur in small proportions could only be used, it was set aside. The following year, Carl Wilhelm Siemens created another, the Martin-Siemens procedure, in which steel was produced from the decarburization of mild iron and iron oxide smelting as a product of heating with oil, coke gas, or a mixture of the latter with blast furnace gas. This method also fell into disuse.
Although in 1878 Siemens was also the first to employ electricity to heat steel furnaces, the use of electric arc furnaces for commercial production began in 1902 by Paul Héroult, who was one of the inventors of the modern method of melting aluminum. In this method, an electric arc is passed inside the furnace between steel scrap whose composition is known and large carbon electrodes located on the roof of the furnace.
In 1948 the basic oxygen process L-D was invented. After the Second World War, experiments began in several countries with pure oxygen instead of air for steel refining processes. Success was achieved in Austria in 1948, when a steel mill located near the city of Linz, Donawitz developed the basic oxygen process or L-D.
In 1950 the continuous casting process was invented, which is used when it is necessary to produce rolled steel profiles of constant section and in large quantities. The process consists of placing a mold with the shape that is required under a crucible, which with a valve can dose molten material to the mold. By gravity the molten material passes through the mold, which is cooled by a water system; By passing the molten material through the cold mold it becomes pasty and acquires the shape of the mold. Subsequently, the material is formed with a series of rollers that at the same time drag it to the outside of the system. Once the material is formed with the necessary shape and with the appropriate length, the material is cut and stored.
At present, some metals and metalloids are used in the form of ferroalloys, which, together with steel, provide it with excellent qualities of hardness and strength.
Currently, the steelmaking process is completed by so-called secondary metallurgy. At this stage, the liquid steel is given the desired chemical properties, temperature, gas content, level of inclusions and impurities. The most common unit of secondary metallurgy is the ladle furnace. The steel produced here is ready to be subsequently cast, in conventional form or in continuous casting.
The intensive use that steel has and has had for the construction of metal structures has known great successes and resounding failures that have at least allowed the advancement of materials science. Thus, on November 7, 1940, the world witnessed the collapse of the Tacoma Narrows Bridge as it resonated with the wind. Already during the first years of the Industrial Revolution, there were premature breakages of railway axles that led William Rankine to postulate the fatigue of materials and during the Second World War there were some unforeseen sinkings of the American freighters Liberty when the steel was weakened by the mere decrease in temperature, a problem initially attributed to welding.
In many regions of the world, steel is of great importance to the dynamics of population, industry and commerce.
Ordinary steels. Alloy or special steels.
Alloyed or special steels contain other elements, in addition to carbon, that modify their properties. These are classified according to their influence:
Elements that increase hardness: phosphorus, nickel, copper, aluminum. Especially those that retain hardness at high temperatures: titanium, vanadium, molybdenum, tungsten, chromium, manganese and cobalt. Elements that limit the growth of grain size: aluminum, titanium and vanadium. Elements that determine in hardenability: increase hardenability: manganese, molybdenum, chromium, nickel and silicon. Decreases hardenability: cobalt. Elements that modify the resistance to corrosion or oxidation: increase the resistance to oxidation: molybdenum and tungsten. Promotes corrosion resistance: chromium. Elements that modify the critical transformation temperatures: The critical points rise: molybdenum, aluminum, silicon, vanadium, tungsten. Critical temperatures decrease: copper, nickel and manganese. In the particular case of chromium, critical points are raised when steel is high in percentage carbon but decreased when steel is low carbon.
The two main components of steel are found in abundance in nature, which favors its large-scale production. This variety and availability makes it suitable for numerous uses such as the construction of machinery, tools, buildings and public works, contributing to the technological development of industrialized societies. Despite its density (7850 kg/m³ density compared to 2700 kg/m³ of aluminum, for example) steel is used in all sectors of industry, including aeronautics, since parts with higher stresses (either impact or fatigue) can only withstand with a ductile and tenacious material such as steel, in addition to the advantage of its relatively low cost.
Other elements in steel
Alloying elements of steel and improvements obtained with the alloy
Standard classifications of steels such as AISI, ASTM and UNS, establish minimum or maximum values for each type of element. These elements are added to obtain certain characteristics such as hardenability, mechanical strength, hardness, toughness, wear resistance, weldability or machinability. Listed below are some of the effects of alloying elements on steel:
- Aluminum: used in some high hardness Cr-Al-Mo nitriding steels in concentrations close to 1% and in percentages below 0.008% as a deoxidizer in high alloy steels.
- Boron: in very small quantities (from 0.001 to 0.006%) it increases hardenability without reducing machinability, as it combines with carbon to form carbides providing a hard coating. It is used in low alloy steels in applications such as plow blades and wires of high ductility and surface hardness. Also used as a nitrogen trap, especially in drawing steels, to obtain N values less than 80 ppm.
- Cobalt: very hardening. Decreases hardenability. Improves strength and hardness in hot. It is an unusual element in steels. Increases the magnetic properties of steels. It is used in tool high-speed steels and refractory steels.
- Chromium: Forms very hard carbides and communicates to the steel greater hardness, strength and toughness at any temperature. Alone or alloyed with other elements, it improves corrosion resistance. Increases the penetration depth of hardening by thermochemical treatment such as carburization or nitriding. It is used in stainless steels, tool steels and refractories. It is also used in trim coatings or hard coatings with high wear resistance, such as plungers, shafts, etc.
- Molybdenum: it is a common element of steel and greatly increases the depth of hardening of steel, as well as its toughness. Austenitic stainless steels contain molybdenum to improve corrosion resistance.
- Nitrogen: Added to some steels to promote austenite formation.
- Nickel: it is a gammagen element allowing an austenitic structure at room temperature, which increases toughness and impact resistance. Nickel is widely used to produce stainless steel, because it increases corrosion resistance.
- Lead: lead does not combine with steel, it is found in it in the form of tiny blood cells, as if it were emulsified, which favors easy machining by chip removal, (turning, brushing, drilling, etc.) since lead is a good cutting lubricant, the percentage ranges between 0.15% and 0.30% and the carbon content must be limited to values below 0.5% because it makes tempering difficult and decreases toughness in hot. It is added to some steels to greatly improve machinability.
- Silicon: moderately increases hardenability. It is used as a deoxidizing element. Increases the strength of low-carbon steels.
- Titanium: used to stabilize and deoxidize steel, keeps the properties of steel stable at high temperature. Its high affinity for carbon is used to prevent the formation of iron carbide when welding steel.
- Tungramen: also known as tungsten. It forms with iron very complex carbides stable and very hard, withstanding high temperatures. In percentages of 14 to 18%, it provides fast steels with which it is possible to triple the cutting speed of carbon tool steels.
- Vanadium: has an energetic deoxidizing action and forms complex carbides with iron, which provide steel with good fatigue resistance, tensile strength and shear power in tool steels.
- Niobium: It is used to give hardness, flexibility and elasticity to steel, mainly used for structural steel and for automotive steels.
Impurities are all undesirable elements in the composition of steels. They are found in steels and also in foundries as a result of being present in minerals or fuels. Efforts are made to eliminate them or reduce their content because they are detrimental to the properties of the alloy. In cases where disposal is impossible or too costly, their presence is allowed in minimal quantities.
- Sulfur: approximate maximum limit: 0.04%. Sulfur with iron forms sulfide, which, together with austenite, gives rise to a eutectic whose melting point is low and which, therefore, appears on grain edges. When cast steel ingots must be hot-rolled, the eutectic is in a liquid state, which causes the material to shell. The presence of sulfide is controlled by the addition of manganese. Manganese has a higher affinity for sulfur than iron so instead of FeS it forms MnS which has high melting point and good plastic properties. The Mn content must be approximately five times the concentration of S for the reaction to occur. The final result, once the causative gases have been eliminated, is a less porous casting, and therefore of higher quality. Although it is considered a harmful element, its presence is positive to improve machinability in machining processes. When the percentage of sulfur is high it can cause pores in the weld.
- Phosphorus: approximate maximum limit: 0.04%. Phosphorus is harmful, either by dissolving in ferrite, as it decreases ductility, or by forming FeP (“iron phosphide”). Iron phosphide, together with austenite and cementite, forms a ternary eutectic called “stearite”, which is extremely fragile and has a relatively low melting point, which is why it appears on grain edges, transmitting its fragility to the material.
Although it is considered a detrimental element in steels, because it reduces ductility and toughness, making it brittle, it is sometimes added to increase tensile strength and improve machinability.
Classification of steels
According to the mode of manufacture
- Cast steel
- Calmed steel
- Effervescent steel
- Fried steel
- Drawn steel
According to the way of working it
According to composition and structure
- Ordinary steels
- Alloy or special steels
Alloyed or special steels contain other elements, in addition to carbon, that modify their properties. These are classified according to their influence:
- Elements that increase hardness: phosphorus, nickel, copper, aluminum. Especially those that retain hardness at high temperatures: titanium, vanadium, molybdenum, tungsten, chromium, manganese and cobalt.
- Elements that limit the growth of grain size: aluminum, titanium and vanadium.
- Elements that determine the hardenability: manganese, molybdenum, chromium, nickel and silicon increase the hardenability, while cobalt decreases it.
- Elements that modify the resistance to corrosion or oxidation: increase the resistance to oxidation: molybdenum and tungsten. Promotes corrosion resistance: chromium.
- Elements that modify the critical transformation temperatures: The critical points rise: molybdenum, aluminum, silicon, vanadium, tungsten. Critical temperatures decrease: copper, nickel and manganese. In the particular case of chromium, critical points are raised when steel is high in percentage carbon but decreased when steel is low carbon.
According to the uses
- Magnet or magnetic
- Self-tempering steel
- Construction steel
- Fast-cutting steel
- Finishing steel
- Cutting steel
- Tool steel
- Spring steel
- Refractory steel
- Bearing steel
According to the application
- Construction steels
- General purpose steels
- Cemented steels
- Hardening and tempering steels
- Stainless or special purpose
- Steels for cutting and machining tools
Mechanical properties and chemical physics of steel
Although it is difficult to establish the physical and mechanical properties of steel because these vary with the adjustments in its composition and the various heat, chemical or mechanical treatments, with which steels with combinations of characteristics suitable for countless applications can be achieved, some generic properties can be cited:
- Its average density is 7850 kg/m³.
- Depending on the temperature, the steel can contract, expand or melt.
- The melting point of steel depends on the type of alloy and the percentages of alloying elements. That of its main component, iron is about 1510 °C in its pure (unalloyed) state, however, steel frequently has melting temperatures of around 1375 °C, and in general the temperature required for melting increases as the percentage of carbon and other alloys increases. (except eutectic alloys that melt at once). On the other hand, fast steel melts at 1650 °C.
- Its boiling point is around 3000 °C.
- It is a very tenacious material, especially in some of the alloys used to make tools.
- Relatively ductile. With it thin threads called wires are obtained.
- It is malleable. Thin sheets called tinplates can be obtained. Tinplate is a sheet of steel, between 0.5 and 0.12 mm thick, coated, usually electrolytically, by tin.
- It allows a good machining in machine tools before receiving a heat treatment.
- Some compositions and shapes of steel maintain greater memory, and deform when exceeding their elastic limit.
- The hardness of steels varies between that of iron and that which can be achieved by alloying or other thermal or chemical procedures among which perhaps the best known is the tempering of steel, applicable to steels with high carbon content, which allows, when it is superficial, to preserve a tenacious core in the piece that avoids fragile fractures. Typical steels with a high degree of surface hardness are those used in machining tools, called high-speed steels that contain significant amounts of chromium, tungsten, molybdenum and vanadium. The technological tests to measure hardness are Brinell, Vickers and Rockwell, among others.
- It can be welded easily.
- Corrosion is the biggest disadvantage of steels since iron oxidizes very easily increasing its volume and causing surface cracks that allow the progress of oxidation until the piece is completely consumed. Traditionally, steels have been protected by surface treatments Although there are alloys with improved corrosion resistance such as “corten” construction steels suitable for weathering (in certain environments) or stainless steels.
- It has a high electrical conductivity. Although it depends on its composition is approximately 3 · 10 S/m. In high-voltage overhead lines, aluminium conductors with steel core are frequently used, providing the latter with the necessary mechanical resistance to increase the spans between the towers and optimize the cost of installation.
- It is used for the manufacture of artificial permanent magnets, since a piece of magnetized steel does not lose its magnetization if it is not heated to a certain temperature. Artificial magnetization is done by contact, induction or by electrical procedures. As far as stainless steel is concerned, ferritic stainless steel is stuck to the magnet, but austenitic stainless steel does not stick to the magnet since the iron phase known as austenite is not attracted by magnets. Stainless steels contain mainly nickel and chromium in percentages of the order of 10% in addition to some alloys in smaller proportion.
- An increase in temperature in a steel element causes an increase in its length. This increase in length can be assessed by the expression: , being the coefficient of expansion, which for steel is approximately 1.2 · 10 (i.e. ). If there is freedom of expansion, there are no major subsidiary problems, but if this expansion is prevented to a greater or lesser degree by the rest of the components of the structure, complementary efforts appear that must be taken into account. Steel expands and contracts according to an expansion coefficient similar to the expansion coefficient of concrete, so its simultaneous use in construction is very useful, forming a composite material called reinforced concrete. Steel gives a false sense of security by being non-combustible, but its fundamental mechanical properties are severely affected by the high temperatures that profiles can reach in the course of a fire.
It is the physical degradation (loss or gain of material, appearance of cracks, plastic deformation, structural changes such as phase transformation or recrystallization, corrosion phenomena, etc.) due to the movement between the surface of a solid material and one or more contact elements.
Standardization of the different steel classes
To homogenize the different varieties of steel that can be produced, there are systems of standards that regulate the composition of steels and their performance in each country, in each steel manufacturer, and in many cases in the largest consumers of steels.
For example, in Spain they are regulated by the UNE-EN 10020:2001 standard and were formerly regulated by the UNE-36010 standard, both published by AENOR.
There are other steel regulatory standards, such as the AISI classification (much more widely used internationally), ASTM, DIN, or ISO 3506.
In the equilibrium diagram or iron-carbon phase diagram (Fe-C) (also iron-carbon diagram), the transformations that carbon steels undergo with temperature are represented, admitting that the heating (or cooling) of the mixture is carried out very slowly, so that the diffusion processes (homogenization) have time to complete. This diagram is obtained experimentally by identifying the critical points – temperatures at which successive transformations occur – by various methods.
Due to the ease of rusting steel when it comes into contact with the atmosphere or water, it is necessary and desirable to protect the surface of steel components to protect them from oxidation and corrosion. Many surface treatments are closely related to beautifying and decorative aspects of metals.
The most commonly used surface treatments are the following:
- Zinc: antioxidant surface treatment by a mechanical electrolytic process to which different metal components are subjected.
- Chrome plating: surface coating to protect from oxidation and beautify.
- Galvanizing: surface treatment given to steel sheet.
- Nickel plating: nickel bath with which a metal is protected from oxidation.
- Pavonado: surface treatment given to small pieces of steel, such as screws.
- Paint: used especially in structures, cars, boats, etc.
A suitable heat treatment process allows to significantly increase the mechanical properties of hardness, toughness and mechanical strength of steel. Heat treatments change the microstructure of the material, so the macroscopic properties of the steel are also altered.
Heat treatments that can be applied to steel without changing its chemical composition are:
They are heat treatments in which, in addition to changes in the structure of the steel, there are also changes in the chemical composition of the surface layer, adding different chemicals to a certain depth. These treatments require the use of controlled heating and cooling in special atmospheres. Among the most common objectives of these treatments are to increase the surface hardness of the parts leaving the core softer and more tenacious, reduce friction by increasing lubricating power, increase wear resistance, increase fatigue resistance or increase corrosion resistance.
- Cementation (C): increases the surface hardness of a piece of mild steel, increasing the concentration of carbon on the surface. It is achieved by taking into account the medium or atmosphere that surrounds the metal during heating and cooling. The treatment manages to increase the carbon content of the peripheral area, obtaining later, by means of tempering and tempering, a great surface hardness, resistance to wear and good toughness in the core.
- Nitriding (N): like cementation, it increases surface hardness, although it does so to a greater extent, incorporating nitrogen into the composition of the surface of the piece. It is achieved by heating the steel to temperatures between 400 and 525 °C, within a stream of ammonia gas, plus nitrogen.
- Cyanidation (C+N): surface hardening of small pieces of steel. Baths with cyanide, carbonate and sodium cyanate are used. Temperatures between 760 and 950 °C apply.
- Carbonitriding (C + N): like cyanidation, introduces carbon and nitrogen in a surface layer, but with hydrocarbons such as methane, ethane or propane; ammonia (NH3) and carbon monoxide (CO). The process requires temperatures of 650 to 850 °C and requires quenching and subsequent tempering.
- Sulfinization (S + N + C): increases the resistance to wear by the action of sulfur. The sulfur was incorporated into the metal by heating to a low temperature (565 °C) in a salt bath.
Among the factors that affect the heat treatment processes of steel are the temperature and the time during which the material is exposed to these conditions. Another determining factor is the way in which the steel returns to room temperature. Process cooling may include immersion in oil or use of air as a refrigerant.
The method of heat treatment, including its cooling, influences the steel to take its commercial properties.
According to this method, in some classification systems, it is assigned a prefix indicative of the type. For example, steel O-1, or A2, A6 (or S7) where the letter O is indicative of oil quenched, and A is the initial of air; the prefix S is indicative that the steel has been treated and considered shock resistant.
The steel that is used for the construction of metal structures and public works, is obtained through the rolling of steel in a series of standardized profiles.
The rolling process consists of pre-heating the molten steel ingots to a temperature that allows the deformation of the ingot by a stretching and roughing process that occurs in a chain of pressure cylinders called the rolling mill. These cylinders form the desired profile until the required measurements are achieved. The dimensions of the sections achieved in this way do not conform to the required tolerances and that is why rolled products often have to be subjected to machining phases to adjust their dimensions to the required tolerance.
Forging is the process that modifies the shape of metals by plastic deformation when the steel is subjected to pressure or a continuous series of impacts. Forging is usually carried out at high temperatures because this improves the metallurgical quality and mechanical properties of the steel.
The purpose of forging steel parts is to reduce as much as possible the amount of material that must be removed from the parts in their machining processes. In stamping forging, the flow of the material is limited to the cavity of the stamp, composed of two matrices that have engraved the shape of the piece to be achieved.
Corrugated steel is a kind of rolled steel used especially in construction, for use in reinforced concrete. These are steel bars that have bumps or “corrugas” that improve adhesion with concrete. It is endowed with a great ductility, which allows that when cutting and bending it does not suffer damage, and has a great weldability, all this so that these operations are safer and with a lower energy expenditure.
Corrugated steel bars are standardized. For example, in Spain, they are covered by the UNE 36068:2011, UNE 36065:2011 and UNE 36811:1998 IN Standards.
Corrugated steel bars are produced in a range of diameters ranging from 6 to 40 mm, in which the section in cm² that each bar has as well as its weight in kg is cited.
Bars less than or equal to 16 mm in diameter can be supplied in bars or rolls, for diameters greater than 16 they are always supplied in the form of bars.
The corrugated product bars have technical characteristics that must be met, to ensure the corresponding calculation of reinforced concrete structures. Among the technical characteristics, the following stand out, all of them are determined by means of the tensile test:
- yield strength Re (Mpa)
- unit breaking load or tensile strength Rm (MPa)
- elongation of rupture A5 (%)
- elongation under maximum load Agt (%)
- the ratio between loads Rm/Re
- Young E module
Steel stamping consists of a machining process without chip removal where the steel plate is subjected by means of presses suitable for drawing and stamping processes to achieve certain metal parts. For this, the appropriate molds are placed in the presses.
Steel die-cutting consists of a machining process without chip removal where all kinds of holes are drilled in the steel plate by means of impact presses where they have placed their respective dies and dies.
Steel parts can be machined in chip removal processes in machine tools (drill, lathe, milling machine, CNC machining centers, etc.) then hardened by heat treatment and finished machining by abrasive procedures in the different types of grinding machines that exist.
The grinding process allows to obtain very good qualities of surface finish and measures with very narrow tolerances, which are very beneficial for the construction of quality machinery and equipment. But the size of the part and the displacement capacity of the grinding machine can present an obstacle.
On special occasions, steel heat treatment can be carried out before machining in chip removal processes, depending on the type of steel and the requirements that must be observed for a given part. With this, it must be taken into account that the tools necessary for such works must be very strong because they suffer hasty wear in their useful life. These peculiar occasions can occur when the manufacturing tolerances are so narrow that the induction of heat in treatment is not allowed because it alters the geometry of the work, or also because of the same composition of the batch of the material (for example, the pieces are shrinking a lot by being treated). Sometimes machining after heat treatment is preferable, since the optimum stability of the material has been achieved and, depending on the composition and treatment, the machining process itself is not much more difficult.
In some manufacturing processes that rely on electric discharge with the use of electrodes, the hardness of the steel does not make a noticeable difference.
In many situations, the hardness of the steel is decisive for a successful result, such as in deep drilling by ensuring that a hole maintains its position relative to the axis of rotation of the carbide bit. Or for example, if the steel has been hardened by being heat treated and by another subsequent heat treatment has been smoothed, the consistency may be too soft to benefit the process, since the path of the bit will tend to deviate.
Bending steel that has been heat treated is not highly recommended because the cold bending process of hardened material is more difficult and the material has most likely become too brittle to be bent; The bending process using torches or other methods to apply heat is also not recommended since when reapplying heat to the carbide metal, the integrity of this changes and can be compromised.
For use in construction, steel is distributed in metal profiles, these being of different characteristics according to their shape and dimensions and must be used specifically for a specific function, whether beams or pillars.
Steel in its different classes is overwhelmingly present in our daily lives in the form of tools, utensils, mechanical equipment and forming part of appliances and machinery in general as well as in the structures of the houses we inhabit and in the vast majority of modern buildings. In this context, there is the modern version of steel profiles called Metalcón.
Manufacturers of goods transport (trucks) and agricultural machinery are major consumers of steel.
Railway construction activities are also large consumers of steel, from the construction of road infrastructures as well as the manufacture of all types of rolling stock.
The same applies to the arms manufacturing industry, especially that dedicated to building heavy weapons, armored vehicles and armored vehicles.
Also consuming a lot of steel are the large shipyards that build ships, especially oil tankers, and gas or other tankers.
Prominent consumers of steel include car manufacturers because many of their significant components are made of steel.
Examples include the following automotive components that are made of steel:
- They are made of forged steel among other components: crankshaft, connecting rods, pinions, gearbox transmission shafts and steering articulation arms.
- Stamping sheet are the doors and other components of the body.
- Rolled steel are the profiles that make up the frame.
- All the springs that incorporate are made of steel, such as; valve springs, seat springs, clutch press, shock absorbers, etc.
- Made of high-quality steel are all the bearings that are mounted on cars.
- Die-cut sheet are the rims of the wheels, except the high-end ones that are made of aluminum alloys.
- Steels are all screws and nuts.
It should be noted that when the car goes to scrapping due to its age and deterioration, all the steel parts are separated, converted into scrap and recycled back into steel by electric furnaces and rolling mills or iron castings.
Mechanical testing of steel
When a technician designs a metal structure, designs a tool or a machine, he defines the qualities and performance that the constituent materials must have. As there are many different types of steels and, in addition, their performance can be varied with heat treatments, a series of mechanical tests are established to verify mainly the surface hardness, the resistance to the different stresses that may be subjected, the degree of finishing of the machining or the presence of internal cracks in the material, which directly affects the material as fractures or breaks can occur.
There are two types of testing, some that can be destructive and others non-destructive.
All steels have standardized reference values for each type of test to which they are subjected.
Non-destructive testing is as follows:
- Microscopic test and surface roughness: microscopes and roughness meters.
- Ultrasonic tests.
- Tests by penetrating liquids.
- Magnetic particle tests.
- Hardness test (Brinell, Rockwell, Vickers); by durometers.
The destructive tests are as follows:
- Tensile test with specimen
- Resilience test.
- Compression test with standardized specimen.
- Shear test.
- Bending test.
- Torsion test.
- Bending test.
- Fatigue test.
Steel production and consumption
Evolution of world steel consumption (2005)
World consumption of finished steel finished products in 2005 exceeded one billion tonnes. The evolution of consumption is extremely different between the main geographical regions. China recorded an increase in apparent consumption of 23% and now accounts for almost 32% of world steel demand. In the rest, after a year 2004 marked by a significant increase in stocks caused by forecasts of price increases, 2005 was characterized by a phenomenon of stock reduction, with the following trend registered: −6% in Europe (EU25), −7% in North America, 0% in South America, +5% in CIS, +5% in Asia (excluding China), +3% in the Middle East.
World steel production (2005)
World crude steel production in 2005 amounted to 1129.4 million tonnes, an increase of 5.9 percent over 2004. This evolution was uneven in the different geographical regions. The increase is mainly due to Chinese steel companies, whose production increased by 24.6 percent to 349.4 million tonnes, representing 31 percent of world production, up from 26.3 percent in 2004. An increase was also observed in India (+16.7%). The Japanese contribution has remained stable. Asia as a whole currently produces half of the world’s steel. While the production volume of European and North American steel companies fell by 3.6% and 5.3% respectively.
The distribution of steel production in 2005 was as follows according to figures estimated by the International Iron and Steel Institute (IISI) in January 2006:
|North and Central America
· United States
|Rest of the world||39,3|
|Data in millions of tons|
World production of steel
Available data on world steel production in 2019, in millions of tonnes per year :
|Rank||Country||Million tons of steel|
Steel, like other metals, can be recycled. At the end of their useful life, all elements built in steel such as machines, structures, ships, cars, trains, etc., can be scrapped, separating the different component materials and originating selected waste commonly called scrap. It is pressed into blocks that are sent back to the steel mill to be reused. This reduces the expenditure on raw materials and energy that must be spent on steel manufacturing. Recycled scrap is estimated to cover 40% of global steel needs (2006 figure).
The recycling process is carried out under the rules of prevention of occupational and environmental risks. The furnace in which the scrap is melted has a high electricity consumption, so it is usually turned on when the demand for electricity is lower. In addition, radioactivity detectors are placed at different stages of recycling, such as at the entrance of trucks transporting scrap metal to recycling industries.
Beware of handling scrap metal
Personnel handling scrap metal should always be vaccinated against tetanus infection, as they can become infected when wounded by scrap metal. Anyone who suffers a cut with a steel element, should go to a medical center and receive this vaccine, or a booster if previously received.