Iron ore is obtained in the usual way: by open or underground mining and subsequent transportation for initial preparation, where the material is ground, washed and processed.
Ore is poured into a blast furnace and blasted with hot air and heat, which turns it into molten iron. It is then removed from the bottom of the furnace into molds known as pigs, where they cool to produce cast iron. It is converted into wrought iron or processed into steel in several ways.
What is steel?
In the beginning there was iron. It is one of the most common metals in the earth's crust. It can be found almost everywhere, in combination with many other elements, in the form of ore. In Europe, the beginning of work with iron dates back to 1700 BC.
In 1786, French scientists Bertollet, Marge and Vandermonde precisely determined that the difference between iron, cast iron and steel is due to different carbon contents. Nevertheless, steel made from iron quickly became the most important metal of the industrial revolution. At the beginning of the 20th century, world steel production amounted to 28 million tons, which is six times more than in 1880. By the beginning of World War I, its production amounted to 85 million tons. For several decades, it has practically replaced iron.
The carbon content affects the characteristics of the metal. There are two main types of steel: alloyed and unalloyed. Steel alloy refers to chemical elements other than carbon added to iron. Thus, an alloy of 17% chromium and 8% nickel is used to create stainless steel.
Currently, there are more than 3,000 cataloged brands (chemical compounds), not counting those that are created to meet individual needs. All of them contribute to the transformation of steel into the most suitable material for solving the problems of the future.
Raw materials for steelmaking: primary and secondary
The smelting of this metal using many components is the most common mining method. Burden materials can be both primarily used and recycled. The basic composition of the charge, as a rule, is 55% of cast iron and 45% of the remaining scrap metal. Ferroalloys, converted cast iron and technically pure metals are used as the main element of the alloy, as a rule, all types of ferrous metal are secondary.
Iron ore is the most important and basic raw material in the steel industry. About 1.5 tons of this material is required to produce a ton of pig iron. About 450 tons of coke are used to produce one ton of pig iron. Many metallurgical plants even use charcoal.
Water is an important raw material for the steel industry. It is mainly used for hardening coke, cooling blast furnaces, producing steam at the doors of a coal furnace, operating hydraulic equipment and removing wastewater. About 4 tons of air are required to produce a ton of steel. The flux is used in a blast furnace to extract contaminants from the smelting ore. Limestone and dolomite combine with the extracted impurities to form slag.
Both blast furnaces and steel furnaces are lined with refractories. They are used for facing furnaces designed for smelting iron ore. Silica or sand is used for molding. Non- ferrous metals are used to produce steel of various grades : aluminum, chromium, cobalt, copper, lead, manganese, molybdenum, nickel, tin, tungsten, zinc, vanadium, etc. Among all these ferroalloys, manganese is widely used in steelmaking.
Iron waste obtained from dismantled structures of factories, machinery, old vehicles, etc., is recycled and widely used in this industry.
Cast iron for steel
Steel is smelted using cast iron much more often than with other materials. Cast iron is a term that usually refers to gray iron, but it is also identified with a large group of ferroalloys. Carbon is from about 2.1 to 4 wt.%, While silicon is usually from 1 to 3 wt.% In the alloy.
Smelting of iron and steel takes place at a melting point between 1150 and 1200 degrees, which is about 300 degrees lower than the melting point of pure iron. Cast iron also exhibits good fluidity, excellent machinability, and resistance to deformation, oxidation, and casting.
Steel is also an alloy of iron with a variable carbon content. The carbon content in steel is from 0.2 to 2.1 wt.%, And it is the most economical alloying material for iron. Iron smelting is useful for various engineering and structural purposes.
Iron ore for steel
The steelmaking process begins with the processing of iron ore. The rock containing iron ore is ground. Ore is mined using magnetic rollers. Fine-grained iron ore is processed into coarse lumps for use in a blast furnace. Coal is purified from impurities in a coke oven, which gives an almost pure form of carbon. The mixture of iron ore and coal is then heated to produce molten iron or iron from which steel is made.
In the main oxygen furnace, molten iron ore is the main raw material and is mixed with various amounts of steel scrap and alloys to produce various grades of steel. In an electric arc furnace, recycled steel scrap is melted directly into new steel. About 12% of the steel is made from recycled material.
Smelting technology
Smelting is the process by which a metal is obtained either as an element or as a simple compound from its ore by heating above the melting point, usually in the presence of oxidizing agents such as air, or reducing agents such as coke.
In steelmaking technology, a metal that combines with oxygen, such as iron oxide, is heated to a high temperature, and the oxide is formed in combination with carbon in the fuel, which is released as carbon monoxide or carbon dioxide.
Other impurities, collectively called veins, are removed by the addition of a stream with which they combine to form slag.
In modern steel melts, a reflective furnace is used. Concentrated ore and stream (usually limestone) are loaded into the upper part, and molten matte (a combination of copper, iron, sulfur and slag) is pulled from below. A second heat treatment in a converter furnace is necessary to remove iron from the dull surface.
Oxygen convection method
The oxygen converter process is the leading steelmaking process in the world. The world production of converter steel in 2003 amounted to 964.8 million tons, or 63.3% of the total production. Converter manufacturing is a source of environmental pollution. The main challenges are reducing emissions, discharges and reducing waste. Their essence is the use of secondary energy and material resources.
Exothermic heat is generated by oxidation reactions during purging.
The main steelmaking process using its own reserves:
- Molten iron (sometimes called hot metal) from a blast furnace is poured into a large refractory lined container called a ladle.
- The metal in the ladle is sent directly to the main steel production or pre-treatment stage.
- High-purity oxygen at a pressure of 700-1000 kilopascals is introduced at a supersonic speed onto the surface of the iron bath through a water-cooled lance, which is suspended in a vessel and held a few feet above the bath.
The pre-treatment decision depends on the quality of the hot metal and the desired final quality of the steel. The very first removable bottom converters that can be disconnected and repaired are still in use. The spears used for blasting have been changed. To prevent the lances from sticking during blowing, slotted cuffs with a long tapering copper tip were used. The tips of the tip after combustion burn CO generated by blowing in CO 2 and provide additional heat. Darts, refractory balls, and slag detectors are used to remove slag.
Oxygen convection method: advantages and disadvantages
It does not require the cost of gas purification equipment, since dust generation, i.e., evaporation of iron, is reduced by 3 times. Due to a decrease in the yield of iron, an increase in the yield of liquid steel in 1.5 - 2.5% is observed. The advantage was that the purge intensity in this method increases, which makes it possible to increase the converter performance by 18%. The quality of steel is higher because the temperature in the purge zone is reduced, which leads to a decrease in nitrogen formation.
The disadvantages of this method of steelmaking led to a decrease in demand for consumption, as the level of oxygen consumption increases by 7% due to the large consumption for fuel combustion. An increased hydrogen content in the recycled metal is observed, due to which it is necessary to purge with oxygen for some time after the end of the process. Among all the methods, oxygen-converter has the highest slag formation, the reason is the inability to monitor the oxidation process inside the equipment.
Open-hearth method
The open-hearth method for most of the 20th century was the main part of the processing of all steel made in the world. William Siemens in the 1860s sought a means of raising the temperature in a metallurgical furnace, resurrecting an old proposal for the use of the waste heat generated by the furnace. He heated the brick to a high temperature, then used the same way to introduce air into the furnace. Preheated air significantly increased the temperature of the flame.
Natural gas or atomized heavy oils are used as fuel; air and fuel are heated to combustion. The furnace is loaded with liquid blast-iron and steel scrap along with iron ore, limestone, dolomite and fluxes.
The furnace itself is made of highly refractory materials such as magnesite brick for hearths. The weight of open-hearth furnaces reaches 600 tons, and they are usually installed in groups, so that the massive auxiliary equipment needed to charge the furnaces and process liquid steel can be effectively used.
Although the open-hearth process is almost completely replaced in most industrialized countries by the main oxygen process and electric arc furnace, it produces about 1/6 of all steel produced worldwide.
The advantages and disadvantages of this method
The advantages include ease of use and ease of production of alloy steel mixed with various additives that give the material various specialized properties. The necessary additives and alloys are added immediately before the end of the smelting.
The disadvantages include reduced efficiency, compared with the oxygen-converter method. Also, the quality of steel is lower compared to other methods of metal smelting.
Electric Steel Method
The modern method of steelmaking using its own reserves is a furnace that heats a charged material using an electric arc. Industrial arc furnaces range in size from small units with a payload of about one ton (used in foundries for the production of cast iron products) to 400 tons of units used in secondary metallurgy.
Arc furnaces used in research laboratories can have a capacity of only a few tens of grams. The industrial temperatures of an electric arc furnace can be up to 1800 ° C (3.272 ° F), while laboratory installations can exceed 3000 ° C (5432 ° F).
Arc furnaces differ from induction furnaces in that the charging material is directly exposed to an electric arc, and the current in the terminals passes through the charged material. The electric arc furnace is used for steel production, consists of a refractory lining, usually water-cooled, large, covered with a sliding roof.
The furnace is mainly divided into three sections:
- A shell consisting of side walls and a lower steel bowl.
- The hearth consists of a refractory that extends the lower bowl.
- The roof with a refractory lining or water cooling can be made in the form of a section of the ball or in the form of a truncated cone (conical section).
The advantages and disadvantages of the method
This method occupies a leading position in the field of steel production. The steelmaking method is used to create high-quality metal, which is either completely devoid of or contains a small amount of undesirable impurities, such as sulfur, phosphorus and oxygen.
The main advantage of the method is the use of electricity for heating, so that you can easily control the melting temperature and achieve an incredible rate of heating of the metal. Automated work will be a pleasant addition to the excellent opportunity for high-quality processing of various scrap metal.
The disadvantages include high power consumption.