No. 20 steel belongs to “high-quality carbon structural steel” GB/T699-1999, and the number represents the carbon content. The physical and mechanical properties and chemical composition of No. 20 steel are close to those of Q235. Close to Q235 steel is No. 20 steel, No. 20 steel is high-quality carbon steel, when Q235 steel is used, there is no problem.
Q345 steel belongs to “low-alloy high-strength structural steel” GB/T1591-94, and the number represents yield strength. The old grades of Q345 are 16Mn, 12MnV, etc. The load-bearing structural steel is generally carbon structural steel and low-alloy high-strength structural steel, which has tensile strength, elongation, yield strength, and sulfur and phosphorus content, as well as carbon content. Q345 has good comprehensive mechanical properties, low temperature performance, good plasticity and weldability, and is used as a structure, mechanical parts, building structures, general metal structural parts that bear dynamic loads such as medium and low pressure containers, oil tanks, vehicles, cranes, mining machinery, power stations, bridges, etc., and can be used in a hot rolled or normalized state, and can be used for various structures in cold areas below -40 °C.
No. 20 steel belongs to ordinary low-carbon steel, Q345 belongs to ordinary low-alloy steel, the tensile strength of the two is not the same different classification of steel, No. 20 steel refers to the carbon content, Q345 refers to the yield strength of steel.
20 steel: The steel is carbon steel (carbon steel) with a carbon content of about 0.20%, which belongs to low-carbon steel and is of high-quality quality. The steel has low strength and hardness, good plasticity and toughness, good carburizing effect and good weldability. The steel is commonly used to make pressure machined parts (such as gaskets, chains), carburized parts (gears, cams), welded parts, plastic molds, etc.
Q345: The steel is alloy steel, belongs to the alloy steel in the low-alloy structural steel, the corresponding old national standard in the grade of 16Mn, the carbon content is about 0.16%, the manganese content is less than 1.5%, the quality is ordinary quality, the steel is mainly used in engineering structures, such as the roof beams of the plant, bridges, tower cranes, etc., its strength, hardness, plasticity, toughness, weldability, etc. are quite good in the steel used in engineering structures, the steel is mostly supplied in the state of profiles, without heat treatment, directly used.
In terms of use, it can be summarized as 20 steel, which is mainly used to make parts and can be heat treated; Q345 is mainly used for the manufacture of engineering structural parts, which are usually not heat treated.
The above answer 20 steel is almost the same, because the carbon content is small, the ductility is good, and it can be stamped into a cup and plate, and then it is used for carburizing and nitriding.
Q345 is an ordinary mechanism steel, unlike 20 steel, he does not emphasize the composition of the steel, but its yield strength must be above 345MPa. Because the composition is not fixed, it is difficult to heat treat (the temperature and time control of heat treatment are related to the composition, and different processes are used for different ingredients). It is mainly used as a structural part.
20# is Q235, the yield point is 235, and the yield point of Q345 is 345, Q345 is the previous 16Mn, and the hardness is greater than 20#