Q390D is a high-strength low-alloy structural steel, whose chemical composition primarily includes basic elements such as carbon (C), manganese (Mn), silicon (Si), phosphorus (P), sulfur (S), and may also contain trace alloying elements like vanadium (V), niobium (Nb), titanium (Ti), which help enhance the strength and toughness of the steel.

Carbon is the main element affecting the strength of steel. The carbon content in Q390D is typically controlled at a low level to ensure good plasticity and weldability. Manganese, as an alloying element, can increase the strength and hardness of the steel while improving its processing performance. Silicon helps enhance the elastic limit of the steel, while phosphorus and sulfur are generally regarded as impurities and need to be kept at low levels to avoid degrading the material's performance.

Trace alloying elements such as vanadium, niobium, and titanium may exist in Q390D in the form of fine carbides or nitrides, which can refine the grain structure and improve the steel's strength and impact resistance. The addition of these alloying elements enables Q390D to maintain high yield strength while also offering good low-temperature impact toughness and weldability, making it suitable for more demanding engineering applications.