Due to its excellent corrosion resistance, high strength and aesthetic appearance, stainless steel has a wide range of uses in both industrial and consumer markets.
But how does stainless steel go from a pile of scrap or refined ore to its final shape and application?
How Stainless Steel is Made?
The specific process of different grades of stainless steel will be different in the later stage. How a steel is formed, machined and finished plays an important role in determining its appearance and performance.
Before you can create deliverable steel products, you must first create molten alloys.
Therefore, most steel grades have common starting steps.
Step 1: Melt
Manufacturing stainless steel begins by melting scrap metal and additives in an electric arc furnace (EAF). EAF uses high powered electrodes that heat metal over the course of hours to create a molten fluid mixture.
Since stainless steel is 100% recyclable, many stainless steel orders contain as much as 60% recycled steel. This not only helps control costs, but also helps reduce environmental impact.
The exact temperature will vary depending on the grade of steel being produced.
Step 2: Remove carbon content
Carbon contributes to the hardness and strength of iron. However, too much carbon can cause problems, such as carbide precipitation during welding.
Before molten stainless steel is cast, the carbon content must be calibrated and reduced to the proper level.
There are two methods for foundries to control carbon content.
The first is by argon oxygen decarburization (AOD). Injecting an argon gas mixture into molten steel reduces the carbon content while minimizing loss of other essential elements.
Another method used is vacuum oxygen decarburization (VOD). In this method, molten steel is transferred to another chamber where oxygen is injected into the steel while it is heated. A vacuum is then removed from the chamber to remove the outgassing, further reducing the carbon content.
Both methods allow precise control of carbon content to ensure the proper blend and accurate characteristics of the final stainless steel product.
Step 3: Adjust
After carbon reduction, the temperature and chemistry are finally balanced and homogenized. This ensures that the metal meets the requirements of its intended grade and that the composition of the steel remains consistent throughout the lot.
Test and analyze samples. Adjustments are then made until the mixture meets the required standards.
Step 4: Molding or Casting
With molten steel produced, the foundry must now create the original shape used to cool and process the steel. Exact shape and size will depend on the final product.
Common shapes include:
*Blooms
*Billets
*Slabs
*Rods
*Tubes
The form is then marked with an identifier to track the batch through the various processes to be followed.
From here, steps will vary depending on the intended grade and end product or function. Flat plates become plates, strips and sheets. Billets and billets become bars and wire.
Depending on the grade or specification ordered, the steel may go through some of these steps multiple times to develop the desired appearance or properties.
The following steps are the most common.
Hot rolled
Performed at temperatures above the recrystallization temperature of the steel, this step helps to set the rough physical dimensions of the steel. Precise temperature control throughout the process makes the steel soft enough to work without altering the structure.
The process uses repeated passes to slowly adjust the size of the steel. In most cases this will involve rolling through multiple rolling mills over time to achieve the desired thickness.
Cold rolling
Typically used when precision is required, cold rolling occurs below the recrystallization temperature of the steel. Multiple support rolls are used to form the steel. This process creates a more attractive, uniform finish.
However, it also distorts the steel's structure and often requires heat treatment to recrystallize the steel to its original microstructure.
Annealing
After rolling, most steels are annealed. This involves controlled heating and cooling cycles. These cycles help to soften the steel and release internal stresses.
The exact temperatures and times involved will depend on the grade of steel, with both heating and cooling rates affecting the final product.
Descaling or pickling
As steel is processed through various steps, it often accumulates scale on the surface.
This accumulation is not just unattractive. It also affects the stain resistance, durability and weldability of the steel. Removing this scale is critical to forming the oxide barrier that gives stainless steel its characteristic corrosion and stain resistance.
Descaling or pickling removes this scale using an acid bath (called pickling) or by controlled heating and cooling in an oxygen-free environment.
Depending on the final product, the metal may return to rolling or extrusion for further processing. This is followed by repeated annealing stages until the desired properties are achieved.
Cutting
Once the steel is finished and ready, the lot is cut to meet the order.
The most common methods are mechanical methods such as cutting with guillotines, round knives, high-speed blades or stamping with dies.
However, for complex shapes, flame cutting or plasma jet cutting can also be used.
The best choice will depend on the grade of steel required and the desired shape of the delivered product.
Finishing
Stainless steel is available in a variety of finishes from matte to mirror. Finishing is one of the final steps involved in the manufacturing process. Common techniques include acid or sand etching, grit blasting, belt sanding, belt polishing, and belt polishing.
At this point, the steel is collected in its final form and ready to be shipped to the customer. Rolls and coil are a common method of storing and transporting large quantities of stainless steel for use in other manufacturing processes. However, the final form will depend on the type of steel required and other factors specific to the order.