The 20 in steel refers to the carbon content, which is 0.2%, classifying it as low carbon steel.
Steel can be divided into low carbon steel, medium carbon steel, and high carbon steel.
Low carbon steel - generally has a carbon content of less than 0.25%;
Medium carbon steel - generally has a carbon content between 0.25% and 0.60%;
High carbon steel - generally has a carbon content greater than 0.60%.
In addition to containing the element carbon (C) and a certain amount of silicon (Si) for deoxidation (generally not exceeding 0.40%), manganese (Mn) (generally not exceeding 0.80%, but can be as high as 1.20%), it does not contain other alloying elements (except for residual elements).
Steel with a carbon content of less than 2.1% is classified as steel, while a carbon content higher than 2.1% is classified as iron.
The higher the carbon content in steel, the worse its toughness; conversely, the lower the carbon content, the better its toughness.
(1) The characteristics of grade 20 steel are basically similar to those of grade 15 steel but with slightly higher strength. Applications: Suitable for manufacturing parts that are not too critical in automotive, tractor, and general machinery manufacturing industries, such as handbrake shoes on cars, lever shafts, gearbox forks, driven gears, camshafts on tractors, suspension balance shafts, inner and outer bushings of balance weights; used in hot-rolled or normalized conditions for various mechanical parts that do not bear much load but require high toughness; in heavy and medium machinery manufacturing industries for forged or pressed components like pull rods, hooks, levers, sleeves, fixtures etc.; widely used in turbine and boiler manufacturing industries for pipes, flanges, connecting boxes and various fasteners working under non-corrosive media with pressure ≤6N/square meter and temperature ≤450℃; also used for making castings like crossheads and pistons on railways and locomotives.
(2) The characteristics of grade 25 steel are similar to those of grade 20 steel but with slightly higher strength; its carbon content lies between low and medium levels providing certain strength along with good plasticity, toughness, weldability and cold stamping performance; it has good machinability but lower hardenability and hardenability; small cross-section parts can achieve better strength and toughness after quenching followed by low or medium temperature tempering without temper brittleness; generally used after hot rolling or normalizing. Applications: Suitable for making welded structural components in general machinery manufacturing as well as parts that undergo forging or hot stamping without being subjected to high stress such as shafts, rollers, connectors and fasteners; used in boiler manufacturing for making components under pressure <600N/mm² at temperatures <450℃.
(3) The characteristics of grade 30 steel: Compared to low-carbon steels it has higher strength and hardness while maintaining good toughness; this type of steel is mostly used in normalized conditions but can also undergo quenching treatment. Applications: Suitable for producing small cross-section parts that do not bear much load at working temperatures ≤150℃ using hot forging or hot pressing methods such as screw rods, pull rods, shaft keys, lifting rings, gears and sleeves; good cutting performance makes it widely applicable in automatic machine tools processing bolts and screws etc.; also suitable for making cold-headed forged parts and welded components; can also be used for carburized or carburizing-nitrogen co-carburized parts.
(35) The characteristics are similar to those of grade 30 steel with good plasticity and appropriate strength; it has good machinability while weldability is acceptable (weldability decreases when the carbon content is at its upper limit); high cold deformation plasticity allows wire drawing under cold conditions along with local upsetting; plates can undergo cold stamping; this type of steel has low hardenability with critical hardening diameters typically ranging from about10~23mm when quenched in water; when cross-sectional dimensions exceed >50mm mechanical properties are similar between tempered state versus normalized state hence larger cross-section parts like crankshafts , rotating shafts , levers , connecting rods , beams , wheel rims along with bolts , screws processed on automatic machine tools can be produced without heat treatment. | 
