High-quality carbon structural steel: In addition to carbon (C) elements and a certain amount of silicon (Si) (generally no more than 0.40%) and manganese (Mn) (generally no more than 0.80%) for deoxidation, the steel can be as high as 1.20% %) alloying elements, without other alloying elements (except residual elements). Such steels must guarantee both chemical composition and mechanical properties. The content of sulfur (S) and phosphorus (P) impurity elements is generally controlled below 0.035%. If it is controlled below 0.030%, it is called high-grade high-quality steel, and “A” should be added after its grade, such as 20A; if P is controlled below 0.025% and S is controlled below 0.020%, it is called super-high-quality steel, and its grade should be followed by adding "E" for distinction. For other residual alloy elements brought into the steel from raw materials, such as chromium (Cr), nickel (Ni), copper (Cu), etc., the content is generally controlled at Cr≤0.25%, Ni≤0.30%, Cu≤0.25%. Some grades of manganese (Mn) content reach 1.40%, called manganese steel.

High-quality carbon structural steel relies on adjusting the carbon (C) content to improve the mechanical properties of the steel. Therefore, according to the level of carbon content, such steel can be divided into:

Low carbon steel - carbon content is generally less than 0.25%, such as 10, 20 steel, etc.;

Medium carbon steel - carbon content is generally between 0.25 and 0.60%, such as 35, 45 steel, etc.;

High carbon steel - carbon content is generally greater than 0.60%. This type of steel is generally not used to make steel pipes.

In fact, there is no clear boundary between their carbon content.

This type of steel has a large output and a wide range of uses. Generally, it is rolled (forged) into round, square, flat and other profiles, plates and seamless steel pipes. Mainly used in the manufacture of general structural and mechanical structural parts and components, as well as building structural parts and pipelines for conveying fluids. According to the requirements of use, it may be used after heat treatment (normalizing or tempering).

The sulfur and phosphorus content of high-quality carbon structural steel is less than 0.035%, and it is mainly used to manufacture more important parts. In engineering, it is generally used to produce steel wires, steel strands and anchors for prestressed concrete, as well as high-strength bolts and steel castings for important structures.

According to GB/T 699-1999, the grade of high-quality carbon structural steel is represented by two digits, that is, the ten-thousandth of the average carbon content in the steel. For example, No. 20 steel represents a high-quality carbon steel with an average carbon content of 0.20%. For boiling steel, add F at the end, such as 10F, 15F, etc. 08, 10, 15, 20, 25 and other grades are low carbon steels, which have good plasticity and are easy to draw, stamp, extrude, forge and weld.

Among the high-quality carbon structural steels, grades 08, 10, 15, 20, and 25 belong to low-carbon steels, which have good plasticity and are easy to draw, stamp, extrude, forge and weld. Among them, 20 steel is the most widely used, and is often used to make screws, nuts, washers, small shafts, stamping parts, welding parts, and sometimes carburized parts. 30, 35, 40, 45, 50, 55 and other grades belong to medium carbon steel. Due to the increase in the pearlite content in the steel, the strength and hardness are higher than before, and the hardness after quenching can be significantly increased. Among them, 45 steel is the most typical, it not only has high strength and hardness, but also has good plasticity and toughness, that is, excellent comprehensive performance. 45 steel is the most widely used in mechanical structures, and is often used to manufacture shafts, screws, gears, connecting rods, sleeves, keys, important screws and nuts. 60, 65, 70, 75 and other grades belong to high carbon steel. After quenching and tempering, they not only increase in strength and hardness, but also have excellent elasticity. They are often used to manufacture small springs, springs, wire ropes, rolls, etc.