Stainless steels - Production

The stainless steel production process, regardless of the final product, consists of four essential steps.

The melting of raw materials (scrap metal, chromium feedstock, and possibly nickel and molybdenum) is carried out in an electric arc furnace with a capacity of approximately 150 tons.

The melting process is followed by a refining operation, which aims to reduce the initial carbon content of the molten metal from approximately 2% to as low as 0.020% or even lower. This process is most often carried out in an AOD (Argon-Oxygen Decarburization) converter and, less commonly, in a vacuum chamber, or via the VOD (Vacuum-Oxygen Decarburization) process, when even lower carbon contents are desired.

Today, casting is performed almost exclusively using a continuous casting machine. This process transforms the metal from a liquid to a solid state by using a bottomless mold in a vertical position, within which the molten metal gradually solidifies. For flat products, curved machines are used, with the product gradually unwinding from the mold, followed by its extraction in a horizontal position (slab). The thickness of a slab can vary from 150 to 250 mm and its width from 700 to 1,600 mm. For long products, the machines are often of the vertical type and operate on multiple strands. To minimize vertical space requirements, vertical-curved or curved machines have also been developed. The cross-section of the blooms produced in this way ranges from 100 × 100 mm to 250 × 250 mm. Ingot casting is now used only for grades whose chemical composition or size is incompatible with continuous casting.

Hot rolling of slabs has evolved significantly since the late 1980s. For historical reasons, hot rolling was performed on strip mills, which were designed for carbon steels but were sometimes unsuitable for processing stainless steels. For this reason, a rolling mill was adapted for the hot rolling of stainless steels: the Steckel mill. It is a single-stand reversible mill, equipped with two coil-winding furnaces on either side. It is generally preceded by a reversible quarto mill known as a roughing mill. This configuration, consistent with the equipment described earlier, allows for the rolling of virtually all grades of stainless steel and achieves an annual production of approximately one million tons.

For the sake of completeness, we should also mention continuous casting of thin strips, which allows for a direct transition from molten metal to a hot strip just a few millimeters thick. This process, which is particularly attractive because it eliminates the hot rolling operation, was the subject of numerous developments in the 1990s, but it has not reached the industrial stage for stainless steels. Since...