2019年10月16日星期三

How is Steel Made? Converter Steelmaking

converter furnace


How is Steel Made? Converter Steelmaking


In life, steel products can be seen everywhere, but what we usually call steel and iron is not pure iron, but a general term for the two "alloys" of iron. The main component of pig iron and steel is iron, which also contains small amounts of carbon, silicon, manganese, sulfur, phosphorus, and other elements. The main difference between pig iron and steel is carbon content, performance, and use.

The carbon content of pig iron is more than 2%, the carbon content of industrial pig iron is generally between 2.11% and 4.3%, and contains silicon, manganese and a small number of elements such as sulfur and phosphorus. Pig iron is a product smelted by iron ore through a blast furnace. The carbon content of steel is between 0.02% and 2.11%, and also contains a small amount of silicon, manganese and a very small amount of sulfur, phosphorus, and other elements.

Pig iron can be cast, but it is not suitable for forging, and the steel has high strength, high hardness, strong toughness, good ductility, and easy machining.

How to Make Raw Iron into Steel?


The main purpose of steelmaking is to reduce the excessive carbon content of pig iron. For steel, phosphorus is a harmful element, which reduces the plasticity and toughness of the steel, and appears cold and brittle (the strength of the metal material increases with the decrease of temperature, and the toughness decreases), and the content should be limited to 0.05% or less. At the same time, sulfur is also a harmful element, which makes the steel hot brittle, and its content should be limited to less than 0.05%. However, manganese can increase the firmness, strength and wear resistance of steel. In addition, silicon also contributes to the strength of the steel. Therefore, in steelmaking, it is necessary to remove elements such as phosphorus and sulfur in the pig iron that affect the properties of the steel, while retaining or increasing elements such as silicon and manganese and adjusting the ratio between the elements.

The steelmaking furnaces required for steelmaking are divided into open-hearth furnaces, converter furnaces, and electric arc furnaces.

electric arc furnace


Take converter steelmaking as an example. The shape of the converter is similar to that of a pear. The inner wall is made of refractory bricks. The parts that are blown into the furnace according to the gas are divided into bottom blowing, top blowing, and side blowing converter. The basic tasks of converter steelmaking are decarburization, dephosphorization, desulfurization, deoxidation, removal of harmful gases and non-metallic inclusions, temperature and composition adjustment.

Converter steelmaking mainly involves oxygen blasting into molten pig iron to oxidize impurities such as silicon and manganese. A large amount of heat is released during the oxidation process (containing 1% silicon to raise the temperature of the pig iron by 200 degrees Celsius) to achieve a sufficiently high temperature in the furnace. Therefore, converter steelmaking does not require an external heating source.

Converter Steelmaking Process


At the beginning of steelmaking, the slag in the furnace is first removed, and liquid pig iron of 1200~1300 °C and a certain amount of scrap steel and quicklime are injected into the converter, and then air is blown into the converter to make it stand upright. In this process, the surface of the liquid pig iron will react violently, and the silicon and manganese elements will be oxidized to form slag, and the reaction will be fully utilized by convection. After a period of time, when only a small amount of silicon and manganese remain in the molten steel, the carbon begins to oxidize, generating carbon monoxide and releasing a large amount of heat, causing the molten steel to boil vigorously, and the carbon monoxide overflowing the furnace mouth will burn to generate a huge flame. Finally, the phosphorus element also undergoes an oxidation reaction and further produces ferrous phosphate. Ferrous phosphate reacts with quicklime to form calcium phosphate and calcium sulfide to form slag. When the phosphorus and sulfur elements are gradually reduced, the flame subsides, and the brown ferroferric oxide steam appears at the mouth of the furnace, the steel is already refined. Finally, the converter is turned to a horizontal position, the molten steel is poured into the ladle, and deoxidizer is added for deoxidation.

converter furnace


It is worth mentioning that iron making and steel making are two completely different processes. First of all, the raw materials of the two are different. The ironmaking raw materials are iron ore, coke, etc., and the steelmaking raw materials are pig iron and scrap steel. Secondly, the equipment is different. Usually, the equipment for iron making is a blast furnace, and the steelmaking is a converter, an electric furnace, and the like. Steelmaking and ironmaking are two stages in steel production. Ironmaking is the upstream process of steel production, and pig iron obtained from ironmaking is the raw material for steelmaking. Generally, the iron is first refined, then steel is produced, and finally, various steels are obtained.



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electric arc furnace


2019年10月14日星期一

What is Secondary Refining Technology?

secondary refining


What is Secondary Refining Technology?



Furnace secondary refining technology refers to metallurgical technology that improves the physical and chemical properties of molten steel in a more economical and effective way on the basis of the production of molten steel in smelting furnaces (converters, electric furnaces). include:

(1) In the ladle, the temperature, composition, gas, harmful elements and inclusions of the molten steel are further adjusted and purified to achieve the purpose of being clean, uniform and stable.

(2) Promote gas and inclusion floating in the tundish to stabilize the molten steel temperature during the full casting process (also known as tundish metallurgy).

(3) Removing inclusions in the steel in the crystallizer to promote nucleation and uniform crystallization.


furnace refining


Technology for Secondary Refining:



(1) Use gas permeable bricks or insert a spray gun for gas agitation, or use electromagnetic stirring.

(2) Adding alloying elements, deoxidizers, and slag modifiers through the transfer system.

(3) A solid material including an alloy is sprayed through a spray gun.

(4) Feed the alloy wire into the ladle.

(5) Various vacuum degassing techniques.

(6) Heating the molten steel in a ladle furnace.

(7) Protection technology for steel flow.


Furnace secondary refining is divided into two categories: vacuum and non-vacuum. Slag washing and argon blowing are the simplest vacuum refining methods. LF refining is the most commonly used non-vacuum refining method. Vacuum processing is the most suitable means of high-quality steel refining. RH and VD are the most commonly used. Among the different steelmaking vacuum treatment technologies, AOD and VOD are common non-vacuum and vacuum refining methods for smelting stainless steel.


Ladle refining furnace


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2019年10月11日星期五

The Specific Process and Steps of Steelmaking

steelmaking


Steelmaking is the process of getting the pig iron into a steelmaking furnace and melting it according to a certain process. According to the survey online, steel products include steel ingots, continuous casting billets, and direct casting into various steel castings. Generally speaking, steel generally refers to steel that is rolled into various steel materials. Steel is a ferrous metal but steel is not exactly equal to ferrous.

Introduce the Specific Process and Steps of Steelmaking


Slag Formation: The operation of adjusting the slag composition, alkalinity, and viscosity and its reaction ability in steel and iron production. The purpose is to produce a metal having the desired composition and temperature by a slag-metal reaction. For example, oxygen top-blown converter slagging and oxygen blowing operations are to produce slag with sufficient fluidity and alkalinity, which can transfer enough oxygen to the molten metal surface to reduce sulfur and phosphorus below the upper limit of the planned steel grade. The amount of splashing and spilling during oxygen blowing is minimized.


Slag Discharge: slagging or slag operation taken during smelting according to different smelting conditions and purposes during electric arc furnace steelmaking. When smelting with the single slag method, the oxidized slag must be slag at the end of the oxidation; when the slag is reduced by the double slag method, the original oxidized slag must be completely released to prevent phosphorus from being returned.

Molten Pool Stirring: Supplying energy to the molten metal pool to cause movement of the molten metal and slag to improve the kinetic conditions of the metallurgical reaction. The molten pool agitation can be achieved by means of gas, mechanical, electromagnetic induction and the like.

electric arc furnace


The Bottom Blowing of the Electric Arc Furnace: N2, Ar, CO2, CO, CH4, O2, and other gases are blown into the molten pool of the furnace according to the process requirements by nozzles placed at the bottom of the furnace to accelerate the melting and promote the metallurgical reaction process. The bottom blowing process can shorten the smelting time, reduce the power consumption, improve the dephosphorization and desulfurization operations, increase the amount of residual manganese in the steel, and increase the yield of metals and alloys. It can make the molten steel composition and temperature more uniform, thus improving steel quality, reducing cost and increasing productivity.

Melting Period: The melting period of steelmaking is mainly for open-hearth and electric arc furnace steelmaking. The electric arc furnace steelmaking is called the melting period from the start of the energization until the charge is completely melted, and the open hearth steelmaking is completed from the completion of the molten iron to the completion of the charge. The task of the melting period is to melt and heat the charge as soon as possible, and to make the slag in the melting period.

Oxidation Period and Decarbonization Period: The oxidation period of ordinary power arc furnace steelmaking usually refers to the process stage of melting, sampling, and analysis of the furnace to the completion of oxidation. It is also believed to start with oxygen blowing or decarburization. The main task of the oxidation period is to oxidize carbon and phosphorus in the molten steel; remove gases and inclusions; and uniformly heat the molten steel. Decarburization is an important process in the oxidation phase. In order to ensure the purity of the steel, the amount of decarburization is required to be greater than about 0.2%. With the development of refining technology outside the furnace, most of the oxidative refining of the electric arc furnace is moved to a ladle or a refining furnace.

steelmaking process diagram


Refining Period: The steelmaking process through the slagging and other methods to remove some of the elements and compounds that are harmful to the quality of the steel, chemically selected into the gas phase or discharged into the slag to remove it from the molten steel process.

Reduction Period: In the ordinary power arc furnace steelmaking operation, the time from the completion of the oxidization at the end of the oxidation to the tapping is usually referred to as the reduction period. Its main task is to make reducing slag for diffusion, deoxidation, desulfurization, control of chemical composition and temperature adjustment. At present, the high-power and ultra-power electric arc furnace steelmaking operations have canceled the reduction period.

secondary refining


Secondary Refining: The steelmaking process in which the molten steel in the steelmaking furnace (converter, electric furnace, etc.) is transferred to another vessel for refining, also called secondary metallurgy. The steelmaking process is therefore divided into two steps: initial refining and refining. Primary refining: The charge is melted, dephosphorized, decarburized and main alloyed in an oxidizing atmosphere furnace. Refining: Degassing, deoxidizing, desulfurizing, removing inclusions and fine-tuning components in a vacuum, inert gas or reducing atmosphere vessel. The benefits of splitting steel into two steps are improved steel quality, shorter smelting time, simplified process and lower production costs. There are many kinds of secondary refining, which can be roughly divided into two types: refining under normal pressure and refining under vacuum. According to different treatment methods, it can be divided into ladle processing type secondary refining and ladle refining type secondary refining.

Stirring of Molten Steel: Stirring of molten steel during refining outside the furnace. It homogenizes the molten steel composition and temperature and promotes metallurgical reactions. Most metallurgical reaction processes are phase-interfacial reactions, and the rate of diffusion of reactants and products is a limiting step in these reactions. When the molten steel is at rest, the metallurgical reaction speed is very slow. For example, the desulfurization of the molten steel in the electric furnace takes 30 to 60 minutes. In the refining, it takes only 3 to 5 minutes to desulfurize by stirring the molten steel. When the molten steel is in a static state, the inclusions are floated and removed, and the removal speed is slow; when the molten steel is stirred, the removal rate of the inclusions increases exponentially and is related to the stirring strength, the type and the characteristics and concentration of the inclusions.

steel tapping


Ladle Feeding, ladle processing, ladle refining, inert gas treatment, composition control, silicon enhancement, endpoint control.

Steel Tapping: The operation of discharging molten steel when the temperature and composition of the molten steel reach the specified requirements of the steel grade.


Precautions

Care should be taken to prevent slag from flowing into the ladle when tapping. Additives for adjusting the temperature, composition, and deoxidation of molten steel are added to the ladle or tapping stream during the tapping process, also called deoxidation alloying.

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2019年10月10日星期四

Electric Arc Furnace Steelmaking Technology Advances in Recent Years

EAF steelmaking



Electric Arc Furnace Steelmaking Technology Advances in Recent Years

 
Electric arc furnace steelmaking technology has made great progress in recent years. Following the upsurge of compact small steel mills in the 1960s, the situation of the joint venture has changed. In recent years, some short-process steelmaking plants using advanced technology integration have been produced.
 
(1) Cremona ESP endless strip rolling plant, Alvidi, Italy. The company has built the world's first set of ArvediESP endless rolling strip plants, including a 250t arc furnace, two 250t ladle refining furnaces and ESP endless rolling strip production line. The designed annual production capacity is 2 million tons, which can produce strip steel with a thickness of 0.8-12.0 mm and a width of 1590 mm.

(2) Crawford Zweil Plant of Newcastle Company, USA. In the reformed EAF steelmaking workshop, there is no heating device between the caster and the hot rolling mill. The strip passes through the hot rolling mill stand, i.e. the finished strip is 0.7-2.0 mm thick and 2,000 mm wide, with an annual design capacity of about 50,000 tons. Castrip process has high productivity and fine microstructures due to its high cooling rate, short solidification time and high drawing speed (usually 8O-1500m/min).

(3) Mesabi Nugget ITmk3 Plant, Kobe Iron and Steel Company, Japan.ITmk3 (IronTeChnology Mark), known as the third generation ironmaking process, is a coal-based direct reduction technology developed jointly by Kobe Iron and Steel Company of Japan and Midland Company of the United States to produce high-quality pig iron in converter hearth furnace. Molten iron can be obtained directly from pulverized coal. Using this technology, iron ore powder can be smelted into slag-free, homogeneous and nearly pure granular iron within 10 minutes.


electric arc furnace



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