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Steel Mini Mill. A short introduction

Steel Making Process

Steel is an iron alloy that has specific additives that give it increased strength, resistant to corrosion and other properties (depending on the metals added in it).

Steel gives a better life and more durability and has replaced nearly all kinds of iron works. An ancient art, steel manufacturing goes back to ancient Rome, China and even India.

Steel production, however, was done on a limited basis as the process of refining iron ore and the tight quality control required was difficult.

This changed with the Bessemer process, where concentrated oxygen (99% purity) replaced normal air. Since air is nearly 78% nitrogen and is chemically neutral in the process, it would take a lot of time and effort to refine the ore.

Introduction of pure oxygen through the Bessemer process largely increased the efficiency and reduced the time required, making steel a commercial viability. Overtime, other processes have been introduced, which are much more efficient.

Steel Mini Mill & Electric Arc Furnace

Once a secondary process, minimills mostly consists of recycling scrap metal to create steel. Modern steel making industry has changed this, with many industries using it exclusively.

The most common method of steel making in minimills is using and Electric Arc Furnace (EAF). EAFs consist of a large vat known as the furnace, which is lined with corrosion resistant chemicals.

Scrap iron placed inside the furnace in a layered fashion (bottom layer of small pieces, middle of large and then the top of smaller scrap pieces).

How Graphite Electrodes are used

Three thick graphite rods are lowered into the furnace and are electrically charged. When the graphite cores touch the metal, they create an electric arc which heats the scrap.

As the top scrap melts, the graphite rods are lowered and the electric potential increased. After a certain depth, the rods are slightly raised, allowing the liquid metal to pool around it and conduct the heat and electricity more efficiently.

The process is highly controlled and to decrease the time for smelting, the furnace can be with heated through a gas/fuel fire or even charged with pre melted material from the last batch.

Once the scrap is melted and impurities float on top, the furnace is tilted and tapped, allowing for the metal to flow down into a ladle furnace, where further chemicals can be added according to customer requirements.

Since the electric arc furnace is a batch process that can be started up or shut down with ease, it has become a very popular choice as it allows steel manufacturers to follow the market demand and cut down on production without incurring process losses.

Sources

https://books.google.de/books?id=FAud8CE5stsC&pg=PA361&redir_esc=y#v=onepage&q&f=false

https://books.google.de/books?id=FAud8CE5stsC&pg=PA361&redir_esc=y#v=onepage&q&f=false

http://geo.msu.edu/extra/geogmich/minmills.html#:~:text=In%20a%20mini%2Dmills%2C%20electricity,melt%20(primarily)%20scrap%20steel.&text=Minimills%20make%20steel%20from%20scrap,into%20the%20semi%2Dfinished%20forms.

https://www.britannica.com/technology/steel/Electric-arc-steelmaking

http://www.cim.mcgill.ca/ (link below)

Click to access TM18-4_as_published.pdf

 

 

 

 

Primary Steelmaking for Beginners

Steelmaking history

Steelmaking has existed for nearly a thousand years, with modern techniques introduced in the 19th century. The process of manufacturing steel involves removal of impurities such as sulfur and silicon, with introduction of chromium and nickel to produce different grades of steel.

The history of steel making is very old, with it being found in ancient China, India, Iran and even Rome. Before the invention of the Bessemer process in 1850s, steel was only produced in small quantities, suitable for demands of small cities and states. The industrial revolution fueled the need of large scale production, with the Bessemer the only method of making steel in such large volumes.

Primary steel making either involves the use of pig iron or converted into steel, or the use of electric arc furnaces to melt steel scrap and recycle the material.

Oxygen Steelmaking

For using pig iron, the basic oxygen steelmaking method is used. This involves blowing oxygen in a carbon heavy melted iron. The oxygen is blown inside the furnace using a hollow pipe called the lance. The lance is liquid cooled to prevent its melting and its mouth is placed a few feet above the surface of the molten iron. The pressurized oxygen reacts with the carbon. The reaction ignites the carbon and an exothermic reaction raises the temperature to around 1,600 Celsius. Burnt lime or dolomite is introduced, which reacts with other impurities such as silicon and forms a layer on the top of the molten liquid, called slag.

After the purification is complete, the vessel is tilted and the molten iron is poured into another ladle furnace, where other metals and chemicals are added (nickel, chromium etc.) and mixed to produce the exact grade of steel required.

Electric Arc Furnace

Electric arc furnaces are usually used for melting scrap iron and steel. The process involves the use of a lined vessel that is fed with scrap and a three graphite rods are lowered, touching the surface. A high voltage electrical current is passed, which creates arcs and produce heat, melting the steel. To assist the process, some pre melted steel may also be added prior to the arcing and sometimes even gas burners are used to bring the temperature up to speed.

As in all steel making, after the melting is complete, the vessel is tilted to remove the liquefied steel, ensuring that the impurities that are floating on top stay behind to be removed later.

Sources

https://books.google.de/books?id=FAud8CE5stsC&pg=PA361&redir_esc=y#v=onepage&q&f=false

https://books.google.de/books?id=FAud8CE5stsC&pg=PA361&redir_esc=y

https://en.wikipedia.org/wiki/Steelmaking#cite_note-3

https://www.britannica.com/technology/steel/Electric-arc-steelmaking

 

 

 

 

Electric Arc Furnace for Beginners

Long History of Electric Arc Furnace Steel Making

Electric Arc Furnaces (EAF) have been used since the 19th century to melt iron. Different attempts were made but the first successful electric arc furnace was developed and patented by James Burgess Readman in 1888. The furnace was specifically crafted for the production of phosphorus.

EAFs also played a pivotal role during World War 2, used primarily of the production of different steel alloys. After the war, the adoption of a mini mill concept integrated with EAFs had helped many European war ravaged countries to start production of electric steel.

Melting Metal with Graphite Electrodes

EAFs are basically a huge electrical circuit that produces heat to melt metal. The construction consists of heat and corrosive resistant vessel with a lid that has three graphite electrodes.

The vessel is “charged” with scrap, light metal pieces sandwiching heavy pieces. The electrodes are lowered and when they touch the metal, low electric voltage is passed. The arc is struck and the graphite electrodes press down, going into the scrap and the electrical energy creating enormous amounts of heat to melt the scrap metal.

As the electrodes bore further into the metal, the electrical voltage is increased since the arcs formed will not be able to touch the sides of the vessel and damaging it. The electrodes are then raised back slightly, allowing a space in which the molten metal can pool up easily.

Advantages of Electric Arc Furnace Steelmaking

Once the impurities that float on the top are removed, the vessel is tilted to pour out the purified liquid metal into pre heated ladles to be cooled off.

EAFs offer advantages over other methods as steel can be made from 100% scrap. The overall energy requirements from making steel from ores is minimal. Another advantage is that unlike ore production of steel, EAFs can be rapidly started or shut down, allowing for batch operations. There is not much wear and tear involved, with usually the graphite electrodes being worn away. The electrodes are manufactured in a modular way where more pieces of electrodes are added as the old ones erode.

 

Sources for Electric Arc Furnace Steelmaking 

United States Patent and Trademark office: %252Fnetahtml%252FPTO%252Fpatimg.htm

https://www.britannica.com/technology/steel/Electric-arc-steelmaking

Modeling and Control of an Electric Arc Furnace, Benoit Boulet, Gino Lalli and Mark Ajersch, Centre for Intelligent Machines, McGill University, 3480 University Street, Montréal, Québec, Canada H3A 2A

Click to access TM18-4_as_published.pdf

http://www.industrialmetalcastings.com/foundries_electric_arc_furnace.html#:~:text=Advantages%3A%20Electric%20arc%20furnaces%20are,required%20to%20produce%20the%20steel.

https://en.wikipedia.org/wiki/Arc_welding

 

The Electric Arc Furnace – Part 4 (EAF vs the others)

This is the fourth part of our EAF-series. It is about the differences and communalities of EAF, Indiction Furnaces, Ladle furnaces, Plasma Furnaces & Vacuum Arc Furnaces.

EAF vs other electrical furnaces

Arc furnaces differ from induction furnaces in that the charge material is directly exposed to an electric arc and the current in the furnace terminals passes through the charged material.

For steelmaking, direct current (DC) arc furnaces are used, with a single electrode in the roof and the current return through a conductive bottom lining or conductive pins in the base.

The advantage of DC is lower electrode consumption per ton of steel produced, since only one electrode is used, as well as less electrical harmonics and other similar problems.

The size of DC arc furnaces is limited by the current carrying capacity of available electrodes, and the maximum allowable voltage. Maintenance of the conductive furnace hearth is a bottleneck in extended operation of a DC arc furnace.

In a steel plant, a ladle furnace (LF) is used to maintain the temperature of liquid steel during processing after tapping from EAF or to change the alloy composition. The ladle is used for the first purpose when there is a delay later in the steelmaking process.

The ladle furnace consists of a refractory roof, a heating system, and, when applicable, a provision for injecting argon gas into the bottom of the melt for stirring. Unlike a scrap melting furnace, a ladle furnace does not have a tilting or scrap charging mechanism.[citation needed]

Electric arc furnaces are also used for production of calcium carbideferroalloys and other non-ferrous alloys, and for production of phosphorus. Furnaces for these services are physically different from steel-making furnaces and may operate on a continuous, rather than batch, basis.

Continuous process furnaces may also use paste-type, Søderberg electrodes to prevent interruptions due to electrode changes. Such a furnace is known as a submerged arc furnace because the electrode tips are buried in the slag/charge, and arcing occurs through the slag, between the matte and the electrode. A steelmaking arc furnace, by comparison, arcs in the open.

The key is the electrical resistance, which is what generates the heat required: the resistance in a steelmaking furnace is the atmosphere, while in a submerged-arc furnace the slag or charge forms the resistance. The liquid metal formed in either furnace is too conductive to form an effective heat-generating resistance.

Amateurs have constructed a variety of arc furnaces, often based on electric arc welding kits contained by silical blocks or flower pots. Though crude, these simple furnaces can melt a wide range of materials, create calcium carbide, etc.

A plasma arc furnace (PAF) uses plasma torches instead of graphite electrodes. Each of these torches consists of a casing provided with a nozzle and an axial tubing for feeding a plasma-forming gas (either nitrogen or argon), and a burnable cylindrical graphite electrode located within the tubing. Such furnaces can be referred to as “PAM” (Plasma Arc Melt) furnaces. They are used extensively in the titanium melt industry and similar specialty metals industries.[10]

Vacuum arc remelting (VAR) is a secondary remelting process for vacuum refining and manufacturing of ingots with improved chemical and mechanical homogeneity.

In critical military and commercial aerospace applications, material engineers commonly specify VIM-VAR steels. VIM means Vacuum Induction Melted and VAR means Vacuum Arc Remelted. VIM-VAR steels become bearings for jet engines, rotor shafts for military helicopters, flap actuators for fighter jets, gears in jet or helicopter transmissions, mounts or fasteners for jet engines, jet tail hooks and other demanding applications.

Most grades of steel are melted once and are then cast or teemed into a solid form prior to extensive forging or rolling to a metallurgically sound form. In contrast, VIM-VAR steels go through two more highly purifying melts under vacuum. After melting in an electric arc furnace and alloying in an argon oxygen decarburization vessel, steels destined for vacuum remelting are cast into ingot molds.

The solidified ingots then head for a vacuum induction melting furnace. This vacuum remelting process rids the steel of inclusions and unwanted gases while optimizing the chemical composition. The VIM operation returns these solid ingots to the molten state in the contaminant-free void of a vacuum.

This tightly controlled melt often requires up to 24 hours. Still enveloped by the vacuum, the hot metal flows from the VIM furnace crucible into giant electrode molds. A typical electrode stands about 15 feet (5 m) tall and will be in various diameters. The electrodes solidify under vacuum.

For VIM-VAR steels, the surface of the cooled electrodes must be ground to remove surface irregularities and impurities before the next vacuum remelt. Then the ground electrode is placed in a VAR furnace. In a VAR furnace the steel gradually melts drop-by-drop in the vacuum-sealed chamber.

Vacuum arc remelting further removes lingering inclusions to provide superior steel cleanliness and further remove gases such as oxygen, nitrogen and hydrogen. Controlling the rate at which these droplets form and solidify ensures a consistency of chemistry and microstructure throughout the entire VIM-VAR ingot.

This in turn makes the steel more resistant to fracture or fatigue. This refinement process is essential to meet the performance characteristics of parts like a helicopter rotor shaft, a flap actuator on a military jet or a bearing in a jet engine.

For some commercial or military applications, steel alloys may go through only one vacuum remelt, namely the VAR. For example, steels for solid rocket cases, landing gears or torsion bars for fighting vehicles typically involve the one vacuum remelt.

Vacuum arc remelting is also used in production of titanium and other metals which are reactive or in which high purity is required.

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Sources

https://en.wikipedia.org/wiki/Electric_arc_furnace

Degner, M. et ali, Steel Institute VDeh (2008), Steel Manual, Düsseldorf Verlag Stahleisen GmbH.

httpswww.shutterstock.comdeimage-vectormetallurgy-iron-steel-production-electric-arc-1391332520