How Stainless Steel Raw Coils are Processed : From Mill to Finished Product

Stainless steel is a widely used material, used in various industries from construction, automobile manufacturing to daily necessities. However, what we see in daily life are finished stainless steel products, such as stainless steel sheets, stainless steel coils, stainless steel cups, etc., and we know little about the formation of stainless steel materials. This blog mainly shares the series of processes of stainless steel from steel mills to the market, from billets to stainless steel coils.

1.Steelmaking (Primary Production)

• Raw Materials: The process begins with the melting of raw materials such as iron ore, scrap steel, and ferroalloys (which provide alloying elements like chromium and nickel).
• Electric Arc Furnace (EAF): Most modern stainless steel production uses an Electric Arc Furnace, where scrap steel is melted using electrical energy. Some plants may use a blast furnace for the primary iron production, which is then refined.
• Alloying: Elements like chromium (typically 10–30%), nickel (8–10%), and molybdenum (if required) are added to create different grades of stainless steel, which will affect properties like corrosion resistance, strength, and heat resistance.

2.Casting

• Continuous Casting: The molten metal is poured into a continuous caster where it solidifies into billets or slabs. This is typically done using water-cooled molds that form the shape of the steel (rectangular, square, or circular).
• Ingot Casting: Some older processes still use ingot casting, where molten metal is poured into molds to form large blocks (ingots) that need further processing.

3.Hot Rolling (Reheating and Rolling)

• Reheating: The steel slabs or billets are reheated in a furnace to around 1,100–1,250°C (2,000–2,300°F) to make them malleable.
• Rolling: The reheated slabs or billets are passed through a series of rollers to reduce their thickness. This process creates hot-rolled coils of stainless steel. The rolling process also refines the grain structure, improving the mechanical properties of the steel.
• Thickness Control: The thickness of the coil is controlled as it passes through rollers to achieve the desired specification.

4.Pickling and Descaling

• Pickling: After hot rolling, the steel surface typically has a scale (oxidized material) from the heat treatment. This scale can be removed by an acid bath in a pickling process, which uses a mixture of hydrochloric or sulfuric acid.
• Descaling: This step removes the oxides from the surface, ensuring the final surface finish is smooth and clean for further processing.

5.Cold Rolling

• Cold Rolling: In this step, the hot-rolled stainless steel is passed through rollers at room temperature to further reduce its thickness and achieve the desired final gauge. Cold rolling improves the mechanical properties of the steel, including its hardness, strength, and surface finish.
• Annealing: After cold rolling, the steel is annealed (heated and then cooled) to relieve stresses and improve ductility, which helps maintain its desired mechanical properties.

6.Surface Finishing

Polishing/Brushing: The stainless steel coils may undergo various surface treatments, such as polishing, brushing, or coating, to give the desired finish. Common finishes include:

• 2B Finish: A standard mill finish that’s smooth but not shiny.
• No. 4 Finish: A brushed or satin finish often used for architectural applications.
• Mirror Finish: A highly polished, reflective surface.

7.Coiling and Slitting

• Coiling: The final product is wound into coils, ready for shipment or further processing. The coils can be in various weights, ranging from a few hundred to thousands of pounds.
• Slitting: If necessary, the coils are slit into narrower widths using specialized slitting equipment. These slitted coils can then be sold in smaller, more manageable rolls or sheets.

8.Final Inspection and Packaging

• Inspection: The coils undergo rigorous testing for physical, mechanical, and chemical properties to ensure they meet required standards. Common tests include surface inspection, tensile strength, and corrosion resistance tests.
• Packaging: Once the coils are finished and inspected, they are carefully packaged to prevent damage during shipping. Packaging typically includes protective coatings, labeling, and sometimes additional handling like wooden crating for export.

In conclusion, the transformation of stainless steel raw coils from the mill to finished products involves a series of meticulous processes that enhance their properties and usability. This journey not only highlights the importance of stainless steel in modern manufacturing but also underscores the intricate craftsmanship involved in producing high-quality stainless steel items.