Q195, Q234, Q235 and Q345 are all grades of carbon structural steel and low - alloy high - strength structural steel, and the numbers correspond to their yield strengths respectively.

Q195 is inexpensive, has relatively low strength, but good plasticity and weldability, and is suitable for applications that are sensitive to cost, such as manufacturing foundation bolts, ploughshares, chimneys and other light - duty structural parts.

Compared with Q195, Q234 has higher strength and hardness, and its price is also slightly higher than that of Q195. It is suitable for structural parts in buildings that require certain strength, such as beams and columns.

Q235 is widely used in building and engineering structures, such as steel bars, factory building frames, bridges, etc. Its price is moderate and it has very good cost - performance ratio.

Q345 is usually used for structural parts that need to bear larger loads, such as bridges, vehicles, ships, pressure vessels, etc., because of its high strength and durability. Its price is relatively high and it contains a relatively large amount of manganese and other alloy elements.