Q235B square tubes are plain carbon steel, while Q345B square tubes are low-alloy steel; such square tubes generally refer to those produced in Wuxi.
The "Q" represents yield strength, which is "Qu" in Chinese; the minimum yield strength of Q235B square tubes is 235 MPa; the minimum yield strength of Q345B square tubes is 345 MPa.
In terms of application:
Q235B Square Pipe – Metal structural components, carbonitriding or cyaniding parts with low core strength requirements, connecting rods, pull rods, hooks, couplings, bolts and nuts, sleeves, shafts, and welded parts.
2. Q345B Square Pipe -- Offers excellent comprehensive mechanical properties, good low-temperature performance, and favorable plasticity and weldability. Suitable for structural components, mechanical parts, architectural structures, and general metal structures that bear dynamic loads in medium and low-pressure vessels, oil tanks, vehicles, cranes, mining equipment, power stations, and bridges. Available in hot-rolled or normalized condition, it can be used in various structures in cold regions below -40°C.

In selecting rectangular steel tubes, we recommend: for strength control in structure, consider using Q345B rectangular tubes; for stiffness control in structure, consider using Q235B rectangular tubes. The Q345B rectangular tubes are slightly more expensive, but due to their high strength, they can save on material costs.