As the name suggests, stainless steel is a high-alloy steel with "rust and corrosion resistance" as its core feature. Its surface usually maintains a bright metallic luster and can resist erosion from weak corrosive media such as air, moisture, and steam for a long time without additional electroplating or coating. In practical applications, steels resistant to weak corrosion are customarily called "stainless steel", while those resistant to corrosion from chemical media such as strong acids, alkalis, and salts are called "acid-resistant steel". Although there are differences in composition between the two, they both rely on the "corrosion-resistant armor" formed by alloying
Elements.
Stainless steel's corrosion resistance is mainly attributed to chromium, a key element. When the chromium content is ≥10.5%, an extremely thin and dense chromium oxide passive film forms on the steel surface, isolating external corrosive media and achieving "self-protection". On this basis, adding elements such as nickel and molybdenum can further improve corrosion resistance, strength, and temperature resistance. Generally, the chromium content in stainless steel can be as high as 26%, which is sufficient to cope with most corrosive environments.
There are many types of stainless steel with different properties. Common classification methods include:
- Classification by steel structure: Such as martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, and duplex stainless steel.
- Classification by main chemical composition or characteristic elements in steel: Such as chromium stainless steel, chromium-nickel stainless steel, chromium-nickel-molybdenum stainless steel, as well as ultra-low carbon stainless steel, high-molybdenum stainless steel, and high-purity stainless steel.
- Classification by steel performance characteristics and applications: Such as nitric acid-resistant (nitric acid grade) stainless steel, sulfuric acid-resistant stainless steel, pitting-resistant stainless steel, stress corrosion-resistant stainless steel, and high-strength stainless steel.
- Classification by steel functional characteristics: Such as low-temperature stainless steel, non-magnetic stainless steel, free-cutting stainless steel, and superplastic stainless steel.
Currently, the most commonly used classification methods are based on steel structure, chemical composition, or a combination of both.
Stainless steel can be divided into five categories: coil, plate, section steel, steel pipe, and components. The most important are coil and plate.
- Section steel refers to materials of various shapes made from plates, including angle steel, flat steel, I-steel, and channel steel.
- Steel pipes mainly refer to seamless steel pipes; the opposite is welded pipes, with the difference lying in whether they are formed in one piece.
- Components mainly include elbows, flanges, and other small parts.
Coils and plates are essentially the same, only differing in factory form. Coils are supplied in rolls, while plates are in individual sheets. Plates supplied directly from the factory are also called original plates, usually thick plates—since very thick steel cannot be coiled (generally, steel over 16mm thick cannot be coiled).
Based on the rolling process of the mill (the machine used to roll plates), stainless steel plates are divided into hot-rolled and cold-rolled:
- Hot-rolled plates are usually marked as NO.1.
- Cold-rolled plates are marked as 2B or BA (BA has a better, brighter surface than 2B, close to a mirror; the best is a mirror finish, which is not available in stock and requires processing).
For example, 304 stainless steel is often purchased as plates, meaning coils must be flattened into plates using a flattening machine. Domestic cold-rolled plates are generally below 3mm thick, while hot-rolled plates are above 3mm. Plates of 3mm thickness can be either hot-rolled or cold-rolled, but imported cold-rolled plates can be up to 4mm or even 6mm thick.
Hot-rolled plates between 3mm and 12mm are called medium plates, and those above 12mm are thick plates (up to over 120mm thick), collectively referred to as hot-rolled medium and heavy plates. Cold-rolled plates are known as cold-rolled thin plates.
There is also a simpler classification: stainless steel can be roughly divided into three series: 200, 300, and 400.
- The 300 series is the most commonly used.
- The 200 and 400 series are substitutes for the 300 series to a certain extent. Strictly speaking, the 400 series is not stainless steel but "stainless iron"—it contains no nickel, so it can be attracted by a magnet.
- The 200 and 300 series contain nickel, so they are non-magnetic and cannot be attracted by magnets.
304 is the most common grade in the 300 series, so changes in 304 prices can generally indicate the overall price trend of stainless steel. The 200 series has low nickel content, the 400 series has no nickel, and the 300 series has the highest nickel content—thus, the 300 series is most affected by nickel prices.
The 300 series can be simply divided into 304, 304L, 316, 316L, 321, 309S, 310S, etc. The distinction is based on the content of various metal elements, which leads to different properties. The difference between 304 and 304L lies in carbon (C) content—those marked with "L" indicate low carbon content (also called low-carbon stainless steel). The same applies to the difference between 316 and 316L.
As a professional manufacturer of continuous bright annealing
Furnaces, we specialize in producing equipment tailored for annealing various types of stainless steel. Our furnaces ensure precise temperature control, maintain the bright surface of stainless steel products, and enhance their corrosion resistance and mechanical properties—perfectly matching the processing needs of stainless steel coils, plates, pipes, and components.
