GB / T 688-1988 20 # steel chemical composition
C: 0.17-0.24
Si: 0.17-0.37
Mn: 0.35-0.65
S: <= 0.035
P: <= 0.035
Cr: <= 0.25
Ni: <= 0.25
Cu: <= 0.25

Carbon: carbon content less than 1.35%, in addition to iron, carbon and limited within the silicon, manganese, phosphorus, sulfur and other impurities, excluding steel other alloying elements. Carbon steel is mainly dependent on the performance of the carbon content. Carbon content increases, the strength of steel, the hardness increases, ductility, toughness and weldability decrease. Compared with other types of steel, carbon steel using the first, low cost, wide performance range, the largest amount. According to carbon content is usually divided into low-carbon steel (carbon content of 0.04% ~ 0.25%), carbon steel (carbon content of 0.25% ~ 0.6%), high-carbon steel (carbon content from 0.6% to 1.35%). Mass is divided into ordinary carbon steel, its harmful impurities phosphorus, sulfur content less than 0.05%, including the CPI steel (A type of steel, to ensure the mechanical properties), CPI (B) steel (Class B steel, guaranteed chemical composition) and special classes steel (class C steel, guarantee the mechanical properties and chemical composition); quality carbon steel, harmful impurities phosphorus, sulfur content less than 0.04%; High quality carbon steel, harmful impurities phosphorus, sulfur content of less than 0.03%. By end-use is divided into carbon structural steel and carbon tool steel, the former is mainly used in the manufacture of structural parts and machine parts, usually is a low carbon and medium carbon steel; latter is used to manufacture cutting tools, measuring tools, molds, etc. general is a high-carbon steel