1. The lower limit of yield strength of Q235 is 235MPa, and the lower limit of yield strength of Q345 is 345MPa(the meaning of Q is the word "qu" in Chinese characters, and the value behind represents the lower limit of yield strength).

2, the alloy content of the two is different :Q235 is ordinary carbon steel, Q345 is low alloy steel

Q235 is carbon structural steel, compared with the old standard GB700-79 grade A3, C3

Q345 is a low alloy structural steel, compared with the old standard 1591-88 grade 12MnV, 16Mn 16MnRE, 18Nb, 14MnNb

Q235- Metal structural parts, carburized or cyanide parts with low core strength requirements, tie rods, connecting rods, hooks, couplings, bolts and nuts, sleeves, shafts and welds

Q345- Good comprehensive mechanical properties, low temperature performance can also be good plasticity and weldability, used as low-pressure vessels, oil tanks, vehicles, cranes, mining machinery, power stations, Bridges and other dynamic load bearing structures, mechanical parts, building structures, general metal structural parts, hot rolled or normalizing state use, can be used for a variety of structures in cold areas below -40℃

Q235 steel pipe is carbon structural steel (GB/700-2006) This kind of steel is generally smelted by converter or open-hearth furnace, its main raw material is hot metal plus scrap steel, sulfur and phosphorus content in steel is higher than high-quality carbon structural steel, generally sulfur ≤0.050%, phosphorus ≤0.045%. The content of other alloying elements brought into the steel by the raw material, such as chromium, nickel and copper, generally does not exceed 0.30%, according to the composition and performance requirements, the grade of such steel is indicated by Q195, Q215A, B, Q235A, B, C, D, Q255A, B, Q275 and other steel grades.