Q345 is a steel material. It is a low alloy steel,overall performance is good, low-temperature performance, the performance of cold stamping,welding performance and good machinability  widely used in bridges, vehicles, ships,construction and pressure vessel.q represents the yield of this material, behind the 345, the yield refers to the value of this material, in the 345 or so. As the material thickness will increase its yield value decreases.