​​​​​​​​​​​​​​​​​​​​​​​Producing stainless steel

​​​​​Outokumpu Handbook of Stainless steel​

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​Steel is not a single product. There are many thousands of different grades of steel with a variety of properties – physical, chemical, and mechanical. Added to that, each grade can be produced in several different forms.

 

Stainless steel product forms

Starting from a geometrical point of view, steel product forms can be divided into flat and long products. An alternative way to describe product forms is to follow the production route, starting with semi-finished products, rolled products, and products finished by additional operations.

 

Flat products

A flat product is a plate product or a (hot or cold) rolled coil product. Typically, it is rolled in a rolling mill to produce the final thickness. Plate products vary in thickness from 5 mm to 200 mm and thin flat rolled products from 50 μm to 5 mm. In addition there is an intermediate thickness range covered by hot-rolled coiled flat products. Plate products are used for construction, large diameter welded pipes, and boiler applications. Thin flat products are used in the chemical process industry, domestic white-good products, welded tubes, and a host of other products, from minute springs to huge heat exchangers and storage tanks.

 

Long products

The long products include a variety of forms such as rods, bars, wires, rails, tubes, and sections. Many of the products originate from hot-rolled rod coil that is further processed into wires or cold-drawn bar. The rod coil itself can have a round, square, hexagonal, or octagonal shape. Final products include reinforcing bars for concrete, engineering products such as gears, and tools.

 

Semi-finished products

Steel products such as ingots, billets, blooms, and slabs fall into the semi-finished product category. These forms can be made by direct continuous casting of liquid steel or by pouring the liquid steel into ingots, which are then hot-rolled into semi-finished products.

  

The production process

 

Raw materials

Stainless steel is the most recycled material in the world and it is estimated that 82% of the stainless steel in use today will be recycled into new steel. When recycled, the stainless steel is not downgraded – recycled steel has the same quality and properties as the original steel. That’s why, on average, recycled steel makes up around 60% of the raw material used to make new stainless steel. For Outokumpu the recycled steel content of our raw material is between 85 and 90%.

To handle such large volumes of steel scrap, rigorous purchase routines are needed. At arrival, before entering the melt shop site, the steel scrap is tested to ensure that no radioactive components are present. It is then tested, analyzed, and sorted according to its alloying content to ensure that as little virgin material as possible is needed to get the right chemical composition of the stainless steel produced.

 

The melt shop

Processing steel scrap and alloys in the melt shop is the first step in the production of stainless steel. The configuration of the melt shop depends on the capabilities of the steel works, but typically there are four main steps: 

1) Melting of raw materials in an electric arc furnace (EAF)
The most conventional method is to melt the raw materials, scrap (alloyed and unalloyed) and ferro-alloys in an electric arc furnace, where powerful electric arcs start to melt the scrap and alloys.

During the melting process, the arc reaches temperatures of up to 3,500 °C, and the molten steel can reach up to 1,800 °C. The additional injection of chemical energy, in the form of carbon, ferrosilicon, oxygen, or fuel gas mixtures, speeds up the melting process.

2) Removal of carbon, sulfur, and possibly nitrogen, in a steel converter
After melting, the steel is further processed in an AOD (Argon Oxygen Decarburization) converter. The main objective of this step is to reduce the carbon content to a target amount by injecting an oxygen-argon mixture, but also to supply additional alloying elements. After the AOD treatment, the molten material is poured into a ladle.

When liquid ferrochrome is used, an iron-rich scrap mix with low alloy content is melted in the arc furnace. Nickel and molybdenum, together with the liquid ferrochrome, are then added to the AOD converter.

An alternative decarburization process is VOD (Vacuum Oxygen Decarburization), which is especially suitable when very low carbon or nitrogen content is required, for instance in the case of some high chromium ferritic stainless steels.

3) Tuning of the steel composition and temperature in ladle treatment
The quality requirements for most stainless steels make a secondary metallurgical treatment necessary. This is done in a ladle station, ladle furnace, or as a vacuum treatment of the liquid steel. The goal of this process step is to make the final adjustment to the chemical composition. The steel is slowly stirred to remove unwanted inclusions and to homogenize both the temperature and chemistry of the molten material, maintaining a tightly specified composition within exact temperature limits.

4) Casting of slabs or ingots
The liquid steel ladle is then transported to the continuous casting machine or, in some cases, to the ingot casting area. Continuous casting is a process in which molten steel is converted into slabs of manageable size.

From the ladle, the molten steel is tapped into the tundish – an intermediate vessel designed to maintain a constant weight and allow for flying ladle changes during casting. The steel then flows into the water-cooled copper mold. The flow is often automatically controlled to maintain a constant level in the mold as the bottom of the mold is slowly retracted. Arc-shaped top and bottom rollers support the solidified strand shell. Molten casting powder is used to reduce friction and ensure good contact between the mold and the shell. Starting immediately below the mold and between the rollers, intensive water spray cools the shell, solidifying the hot melt inside to form a solid strand of steel. The strand is continuously cooled and shrinks in volume as it is passed to the cutting station.

At this stage, the steel is still hot and glowing, but is sufficiently solid to enable the strand to be cut with movable oxygen lances into manageable pieces called slabs. Slabs serve as the feedstock for flat hot rolling mills. The process is similar for the semi-finished casting products like blooms and billets, which are feedstock for long product production.

Every slab is given a unique identity number and is carefully tracked. This identity (slab/coil number) follows the material through the whole production line and is written in the material certificate. This makes it possible to track the material all the way back to the melt shop. Recorded production parameters are linked to the identity number to ensure that the individual end product fulfills the customer’s requirements. All slabs are inspected to ensure a high quality.

 

Hot rolling

Hot rolling is a metalworking process that occurs at a temperature above the recrystallization temperature of the material. The starting material is usually semi-finished casting products, such as slabs, blooms, or billets. The cast microstructure is broken down and deformed during processing and the deformed grains recrystallize, which maintains a microstructure in which the grains have approximately the same dimensions in all directions and prevents the steel from hardening.

While the finished product is of good quality, the surface is covered in mill scale, which is an iron- and chromium-rich oxide that forms at high temperatures. It is usually removed in the annealing and pickling line, which restores the smooth metallic surface.

 

Cold rolling, annealing, and pickling

The cold rolling of stainless steel is predominately carried out in cluster mills. These mills have small-diameter working rolls, each backed by two or three layers of supporting rolls, and are very suitable for cold-rolling wide sheets of stainless steel to close tolerances and improved surface finishes.

The mills are equipped to measure strip thickness, profile, and flatness, and have various actuators to control the strip profile and flatness. The rolling schedule defines the number of passes the strip has to go through and which thickness reduction per pass will be used when rolling from the starting gauge to the desired final gauge. During the cold rolling process, the material deforms and strain hardens to the extent that only a few percent of the original tensile elongation remains. Typical cold rolling reduction is 50–90% and depends on the work hardening behavior of the steel grade.

To restore the material properties after cold rolling, a heat treatment is needed to recover and recrystallize the deformed microstructure. This is done in a continuous annealing and pickling line. The majority of the cold rolled products have a 2B/2BB surface, which means that no mechanical scale breaking is used; all oxide scale is removed by chemical means. This is achieved by passing the strip through several consecutive mixed acid pickling baths, and then finishing off with high pressure water rinsing. Many of these lines also have some type of flatness improvement equipment installed.

Another route after cold rolling is to use a bright annealing line (BA-line) to restore the material properties. In such a line, the strip is heat treated in a protective atmosphere, often hydrogen or a mixture of hydrogen and nitrogen. As no oxygen is available inside the furnace, no additional oxide scale is formed and whatever oxide remains on the strip is reduced to metal. If the cooling is also done in a protective environment, the strips can, after skin pass rolling, exhibit a mirror like finish, referred to as 2R or BA (Bright Annealed).

Skin pass rolling (also called temper rolling) is a very light cold-rolling treatment that involves the least amount of reduction (0.5–1%). The purpose of skin passing is to improve the strip shape and remove minor surface blemishes, create the desired surface finish, and to produce the appropriate mechanical properties.


Final processing

The final operations before sending the product to the customer are called finishing. This is where coils and plates are tailored to customer requirements, which can include:

  • Leveling a plate or coil with a roller leveler or a stretch leveler, or a combination of both techniques
  • Edge trimming of the coil to the desired width
  • Slitting a coil into several narrow coils
  • Cutting a coil into tailored or standard length sheets/plates in dedicated cut-to-length (CTL) lines
  • Shearing or cutting into an order-sized square/rectangular plate or customized specially shaped plate from a larger quarto mother plate.
  • Applying a plastic film coat to provide surface protection for further processing by the customer
  • Edge preparation for special welding requirements
  • Surface preparation such as grinding, brushing, pattern rolling, or embossing

 

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 Glossary

Bar
Products supplied in straight lengths, never in coils, which differentiates them from rods. Bars can be supplied in hot-rolled, and cold worked condition and have different geometries, such as round, square, hexagon, and octagonal shape.

AOD
Argon Oxygen Decarburization. A method of reducing the carbon content in the stainless steel melt.

Vacuum Oxygen Decarburization
A method of reducing the carbon content in the stainless steel melt.

Recrystallization
Heat treatment to soften steel previously hardened by cold work, and to control the grain structure in the final product.

Scale
When the thick oxide layer formed, for example, in furnace atmospheres breaks down and spalls off due to growth stresses, the material “scales”.

Annealing
Heat treatment that alters the microstructure of a material causing changes in properties such as strength, hardness, and ductility.

Pickling
Removal of surface oxides by chemical treatment in a pickling bath, often consisting of a mixed-acid bath containing nitric (HNO3) and hydrofluoric acid (HF).

2B surface finish
Surface finish designation according to EN 10088-2 and ASTM A480 of material that is cold rolled, heat treated, pickled, and skin passed, where skin passing may be done by tension leveling.

2BB surface finish
The Outokumpu designation of material that is cold rolled, heat treated, bright pickled, and skin passed.

Mixed acid
Pickling bath often containing a mixture of nitric acid (HNO3) and hydrofluoric acid (HF).

Bright annealing
Heat treatment of cold-rolled stainless steel in an inert gas atmosphere, often hydrogen or hydrogen/nitrogen, which protects the steel from getting oxidized.

2R surface finish
Surface finish designation according to EN 10088-2 of cold rolled material that is bright annealed, i.e. heat treated under inert gas atmosphere.

BA surface finish
Surface finish designation according to ASTM A480 of material that is bright annealed, i.e. heat treated under inert gas atmosphere.

Slitting
A way to cut a coil into several narrow coils suitable for the end- users following production steps.

Mother plate
The original plate from which several smaller plates are cut.

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