Round steel refers to a solid long steel bar with a circular cross-section. Its specifications are expressed in diameter in millimeters (mm). For example, "50mm" means round steel with a diameter of 50 mm.

1. Classification by process

Round steel is divided into three types: hot rolled, forged, and cold drawn. The specifications of hot-rolled round steel are 5.5-250 mm. Among them: small round steel of 5.5-25 mm is mostly supplied in straight strips and bundles, and is commonly used as steel bars, bolts, and various mechanical parts; round steel larger than 25 mm is mainly used to manufacture mechanical parts and seamless steel pipe blanks.

2. Classification by chemical composition

Carbon steel can be divided into low-carbon steel, medium-carbon steel, and high-carbon steel according to its chemical composition (i.e. carbon content).

(1) Mild steel

Also known as mild steel, low carbon steel with carbon content from 0.10% to 0.30% is easy to accept in various processing such as forging, welding, and cutting, and is often used to make chains, rivets, bolts, shafts, etc.

(2) Medium carbon steel

Carbon steel with a carbon content of 0.25% to 0.60%. There are many products including killed steel, semi-killed steel, boiled steel, and so on. In addition to carbon, it can also contain a small amount of manganese (0.70% ~ 1.20%). According to product quality, it is divided into ordinary carbon structural steel and high-quality carbon structural steel. Thermal processing and cutting performance are good, but the welding performance is poor. The strength and hardness are higher than low-carbon steel, but the plasticity and toughness are lower than low-carbon steel. Hot-rolled materials and cold-drawn materials can be used directly without heat treatment, or they can be used after heat treatment. Medium carbon steel after quenching and tempering has good comprehensive mechanical properties. The highest hardness that can be achieved is about HRC55 (HB538), and σb is 600~1100MPa. Therefore, among various uses with medium strength levels, medium carbon steel is the most widely used. In addition to being used as a building material, it is also widely used in the manufacture of various mechanical parts.

(3) High carbon steel

Often called tool steel, it contains carbon from 0.60% to 1.70% and can be hardened and tempered. Hammers, crowbars, etc. are made of steel with a carbon content of 0.75%; cutting tools such as drills, taps, reamers, etc. are made of steel with a carbon content of 0.90% to 1.00%.

3. Classification according to steel quality

According to the quality of steel, it can be divided into ordinary carbon steel and high-quality carbon steel.

(1) Ordinary carbon structural steel, also known as ordinary carbon steel, has wider restrictions on carbon content, performance range, and content of phosphorus, sulfur, and other residual elements. In China and some countries, it is divided into three categories according to the delivery guarantee conditions: Class A steel (Class A steel) is steel with guaranteed mechanical properties. Class B steel (Class B steel) is steel with guaranteed chemical composition. Special type steel (C type steel) is steel that ensures both mechanical properties and chemical composition and is often used to manufacture more important structural parts. The most produced and used steel in China is A3 steel (Class A No. 3 steel) with a carbon content of about 0.20%, which is mainly used for engineering structures.

Some carbon structural steels also add trace amounts of aluminum or niobium (or other carbide-forming elements) to form nitride or carbide particles to limit grain growth, strengthen the steel, and save steel. In China and some countries, in order to adapt to the special requirements of professional steel, the chemical composition and properties of ordinary carbon structural steel have been adjusted, thus developing a series of ordinary carbon structural steel for professional steel (such as bridges, buildings, Steel bars, steel for pressure vessels, etc.).

(2) Compared with ordinary carbon structural steel, high-quality carbon structural steel contains lower sulfur, phosphorus and other non-metal inclusion content. According to different carbon content and uses, this type of steel is roughly divided into three categories: ① Less than 0.25% C is low carbon steel, especially 08F, 08Al, etc., which have a carbon content of less than 0.10%. Due to its punchability and weldability, it is widely used as deep drawing parts such as automobiles, can making, etc. 20G is the main material for manufacturing ordinary boilers. In addition, low-carbon steel is also widely used as carburizing steel in the machinery manufacturing industry. ②0.25~0.60%C is medium carbon steel, which is mostly used in the tempered state to make parts in the machinery manufacturing industry.

(3) More than 0.6% C is high-carbon steel, which is mostly used to make springs, gears, rollers, etc. According to the different manganese content, it can be divided into two steel groups: ordinary manganese content (0.25~0.8%) and higher manganese content (0.7~1.0% and 0.9~1.2%). Manganese can improve the quench-hardenability of steel, strengthen ferrite, and increase the yield strength, tensile strength, and wear resistance of steel. Usually the mark "Mn" is added after the grade of steel with high manganese content, such as 15Mn, or 20Mn, to distinguish it from carbon steel with normal manganese content.

4. Classification by use

According to use, it can be divided into carbon structural steel and carbon tool steel.

Carbon tool steel has a carbon content between 0.65 and 1.35%. It can obtain high hardness and high wear resistance after heat treatment. It is mainly used to manufacture various tools, cutting tools, molds, and measuring tools (see tool steel).

Carbon structural steel is divided into 5 grades according to the steel yield strength: Q195, Q215, Q235, Q255, Q275.

Each grade is divided into grades A, B, C, and D due to different qualities. There are four grades at most, and some have only one grade. In addition, there are differences in the deoxidation methods of steel smelting.