Different grades of stainless steel require different subsequent processing. The way different grades of steel are formed, processed, and finished has a significant impact on their appearance and performance.
Before you can create a deliverable steel product, you must first create a molten alloy.
For this reason, most steel grades have common starting steps.
Step 1: Melt
The production of stainless steel begins by melting scrap metal and additives in an electric arc furnace (EAF). The EAF uses high-powered electrodes to heat the metals over a period of hours to create a molten, fluid mixture.
Because stainless steel is 100% recyclable, many stainless steel orders contain up to 60% recycled steel. This not only helps control costs, but also helps reduce environmental impact.
The specific temperatures will vary depending on the grade of steel being produced.
Step 2: Remove Carbon Content
Carbon helps increase the hardness and strength of iron. However, too much carbon can also cause problems, such as carbide precipitation when welding.
Before the molten stainless steel is cast, the carbon content must be calibrated and reduced to the appropriate level.
There are two ways foundries control carbon content.
The first is through argon oxygen decarburization (AOD). Injecting an argon mixture into the molten steel reduces the carbon content while minimizing the loss of other essential elements.
Another method is vacuum oxygen decarburization (VOD). In this method, the molten steel is transferred to another chamber where oxygen is injected into the steel while it is heated. A vacuum then removes the outgassing gases from the chamber, further reducing the carbon content.
Both methods allow for precise control of carbon content to ensure proper blending and accurate characteristics of the final stainless steel product.
Step 3: Adjustments
After carbon reduction, temperature and chemical balance and homogenization are finally achieved. This ensures that the metal meets the requirements of its intended grade and that the composition of the entire batch of steel is consistent.
Samples are tested and analyzed. Adjustments are then made until the mixture reaches the required standards.
Step 4: Forming or Casting
Once the molten steel is complete, the foundry must create the original shape for cooling and processing the steel. The exact shape and size will depend on the final product.
Common shapes include:
Blooms
Bills
Slabs
Rods
Tubes
The forms are then marked with identifiers to track the batch through the various processes that follow.
From here, the steps will vary depending on the intended grade and end product or function. Slabs become plates, strips, and sheets. Billets and billets become bars and wire.
Depending on the grade or format ordered, the steel may go through several of these steps to develop the desired appearance or characteristics.
The following are the most common steps:
Hot Rolling
This step is performed at temperatures above the recrystallization temperature of the steel and helps determine the approximate physical dimensions of the steel. Precise temperature control throughout the process keeps the steel soft enough to be processed without changing its structure.
The process slowly adjusts the dimensions of the steel through repeated rolling. In most cases, this will involve multiple passes through the mill to achieve the desired thickness.
Cold Rolling
Cold rolling is often used where precision is required and is performed below the recrystallization temperature of the steel. The steel is shaped using multiple support rolls. This process produces a more aesthetically pleasing, uniform surface.
However, it also changes the structure of the steel, so heat treatment is usually required to recrystallize the steel to its original microstructure.
Annealing
After rolling, most steels go through an annealing process. This involves controlled heating and cooling cycles. These cycles help soften the steel and relieve internal stresses.
The specific temperatures and times involved depend on the grade of steel, and both the heating and cooling rates affect the final product.
Descaling or Pickling
As steel is processed through its various steps, a layer of scale often builds up on its surface.
This scale buildup is not only unsightly, it also affects the steel’s stain resistance, durability, and weldability. Removing this scale is essential to forming the oxide barrier that gives stainless steel its unique corrosion and stain resistance.
Descaling or Pickling can remove 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 be re-rolled or extruded for further processing. This is followed by repeated annealing stages until the desired properties are achieved.
Cutting
Once the steel is processed and ready, the batches are cut as required by the order.
The most common methods are mechanical, such as cutting with a guillotine knife, circular knife, high-speed blade, or punching with a die.
However, for complex shapes, flame cutting or plasma jet cutting can also be used.
The best choice depends on the grade of steel required and the shape required for the delivered product.
Finishing
Stainless steel has a variety of surface finishes, from matte to mirror. Finishing is one of the last operations in the manufacturing process. Common techniques include acid or sand etching, sandblasting, belt grinding, belt polishing, and belt buffing.
At this point, the steel is collected in its final form and ready to be shipped to the customer. Coils and coils are a common method of storing and shipping 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 unique to the order.