Melting & Refining
Stainless Steels are usually melted by charging scrap and alloying elements (e.g. ferrochrome, ferronickel) into an EAF. After the initial melting the metal is transferred to an AOD or VOD/VD vessel for refining. Advanced steels for demanding and critical applications require vacuum melting in a VIM furnace followed by VAR or ESR remelting.
Primary Melting
Electric Arc Furnace (EAF)
Stainless steel is usually produced in an EAF. Scrap and various alloying elements (e.g. ferrochrome, ferronickel) are charged into the vessel. The charged material is heated by an electric arc (by passing current through carbon electrodes) until the metal is melted. An EAF also allows to charge 100 % scrap but in this case it is difficult to control the exact chemical composition. the typical tap-to-tap times are, depending on the power of the transformer and vessel size, between 50 – 80 minutes.
Induction Furnace (IF)
This is an electric furnace using electromagnetic induction to heat the charged scrap. This process for melting stainless steel is widely used in India and China because investment costs are lower than in an EAF. The major drawback of an IF is the lack of refining possibilities and size limits. The quality of the melted steel heavily depends on the quality of the charged scrap which should be clean of oxidation and the chemical composition must be known. As IF’s tend to be smaller than EAF furnaces, their productivity is limited. The biggest IF’s are today 50 t while state of the art EAF’s are 3 times bigger (150t).
Vacuum Induction Melting (VIM)
This is a melting process under vacuum conditions using electromagnetic induction. It is used for the melting of high purity metal and alloys.
Secondary Melting / Refining
Argon Oxygen Decarburization (AOD)
This is a special converter, in which the carbon content of the stainless steel heats is reduced to the targeted specifications by injection blowing of argon and oxygen mixtures. Also final alloy additions can be made in the AOD to reach the exact chemistry. Usually this vessel is also used to ‘build up’ the liquid steel volume by approx. 10%. AOD’s provide a better Cr yield than VOD’s.
Ladle Furnace (LF)
This furnace is positioned between the melting and continuous casting facilities. It is used to exactly adjust the steel temperature and composition to meet the casting demands. It can also be used to transfer metal between the primary and secondary melting units and a ‘bumper vessel’ to build up volume for sequence casting. Ladle furnaces have a power supply via electrodes and are able to slightly increase the liquid steel temperature if required.
Vacuum Oxygen Degassing (VOD) / Vacuum Degassing (VD)
This process reduces the gas content, particularly hydrogen and carbon, as well as reducing non- metallic inclusions by subjection the molten metal to a vacuum. VOD’s and VD’s are used when very low C-contents have to be reached.
Vacuum Arc Remelting (VAR)
This is a secondary melting process where steel ingots (also called electrodes) are slowly remelted by an arc under vacuum to produce advanced steels and superalloys for the most demanding and critical applications, e.g. for aerospace or nuclear applications.
Electro Slag Remelting (ESR)
This is a process used for the remelting and refining of steels and special alloys which are used for critical applications e.g. in medical implants, aerospace components or power plants. Like VAR, the ESR process involves remelting electrodes, but a slag pool instead of a vacuum is used to isolate the melt from the atmosphere.
Casting
After melting, the liquid metal is either cast directly into slabs, blooms or billets in the continuous casting process or into blocks/ingots in the ingot casting process.
Continuous Casting
The melted steel from the ladle furnace is cast directly into semi-finished shapes (slabs, blooms or billets). It is a casting process for the continuous, high-volume production of metal sections with a constant cross-section. It allows lower-cost production of metal sections with better quality, due to automated control throughout the process.
Ingot Casting
The melted steel is poured into an ingot mould. After the metal solidifies into ingots, the moulds are stripped. Ingots are required to produce large forgings but they can also be transferred into slabs, blooms, or billets on a roughing mill. Ingot casting is an economic solution for producing small volumes of a specific alloy but not recommendable for high volume production of commodity steels due to lower yields compared with continuous casting.
