Free cutting steel is an alloy steel that adds a certain amount of one or more free cutting elements such as sulfur, phosphorus, lead, calcium, selenium, and tellurium to the steel to improve its cutting properties. Also known as steel for automatic machine tool processing, referred to as automatic steel. This type of steel can be cut at higher cutting speeds and deeper depths of cut. Due to the easy-cutting elements added to the steel, the cutting resistance of the steel is reduced. At the same time, the characteristics of the easy-cutting elements themselves and the compounds formed function to lubricate the cutting tools, break chips easily, reduce wear, and thus reduce the surface quality of the workpiece. roughness, improving tool life and production efficiency.
According to the easy-cutting elements, it can be divided into:
⑴Sulfur free cutting steel
Sulfur forms manganese sulfide inclusions with manganese and iron in steel. Such inclusions can interrupt the continuity of the base metal, prompt chip breakage to form a small and short curl radius during cutting, and are easy to remove, reducing tool wear and reducing the machined surface. roughness and improve tool life. Generally, the machinability of steel increases as the sulfur content in the steel increases. However, the longitudinal and transverse mechanical properties of steel are quite different, the transverse plasticity and toughness are poor, and the fatigue and corrosion resistance properties are also reduced. When the sulfur content in steel is too high, it will cause thermal brittleness, cause difficulties in hot processing of steel, and deteriorate the mechanical properties of steel. Usually the sulfur content is 0.08% ~ 0.30%, and some can be increased to 0.4%. The sulfur content in free-cutting tool steel and stainless steel should be between 0.06% ~ 0.10%.
Phosphorus is mostly added to steel in combination with sulfur, usually with a phosphorus content of 0.04% to 0.12%. The solid solution of phosphorus in ferrite will increase the hardness and strength, reduce the toughness, and make the chips easy to break and remove, thereby obtaining good results. The machined surface roughness is good, but too high phosphorus content will significantly reduce the plasticity and increase the hardness, which will have a harmful effect on the machinability of the steel.
⑵ Lead free cutting steel
Lead is in the form of fine metal particles in steel, evenly distributed or attached to the surrounding sulfides. Due to the low melting point of lead, the melt oozes out during cutting to lubricate, reduce friction and improve machinability, but does not affect the mechanical properties at room temperature. The lead content in steel is generally 0.10% to 0.35%. Because lead has a large specific proportion, if the content is too high, it will easily cause severe segregation and form large particle inclusions, which will instead reduce the beneficial effect of lead on cutting processing. Lead and sulfur are combined into low carbon structural steel to improve the cutting effect of the steel more significantly.
⑶Calcium free cutting steel
Calcium in steel combines with aluminum and silicon to form low melting point composite oxides (mainly CaO·Al2O3·SiO2). During high-speed cutting, the calcium oxides adhere to the surface of the cutting tool to lubricate and reduce friction, thus improving the use of the tool. life. If it contains elements such as sulfur and lead at the same time, their composite effect will make the cutting effect better.
With the improvement of cutting tools since the 1980s, tools coated with T|N coating on calcium free-cutting steel have a significant effect on gear processing tools such as hobbing cutters and gear shaper cutters with high tool costs.
⑷Se, tellurium and bismuth free cutting steel
The content of tellurium and bismuth is about 0.03% to 0.10%, and the content of selenium can reach 0.15%. Selenium exists in steel in the form of selenides such as FeSe and MnSe. Its role is similar to that of sulfur. For steels that require both high machinability and good plasticity, it is better to add selenium to steel than sulfur. Tellurium can be added alone or simultaneously with lead or sulfur to form composite inclusions to reduce cutting resistance and cutting heat, make chips easy to remove, significantly improve the machinability of steel, and obtain good machined surface roughness. However, adding tellurium will slightly reduce the plasticity and toughness of steel. Selenium and tellurium are generally used in alloy steel. Bismuth acts similarly to lead in steel, appearing as inclusions of fine metal particles, evenly distributed or attached around sulfides.
