Q235 belongs to carbon structural steel and Q345 belongs to low alloy and high strength structural steel. 

Q in Q235 and Q345 indicates the first letter of yield strength "yield strength", 235 and 345 indicate "yield strength level", which generally refers to "lower yield point", and their most important difference is the strength (including yield strength and tensile strength) Q345. Higher than Q235, the content of manganese (Mn) in the chemical composition of the Q345 series is higher than that of the Q235 series, and the impact toughness (impact work) Q345 series is higher than that of the Q235 series.