Abstract: Described are the production process and product quality of CS-Q780 steel with converter smelting used for the high strength arm frame tube in a crane. The chemical composition of CS-Q780 steel produced with the process "Mg based liquid iron desulfurization→80 t converter smelting→LF refining→VD furnace vacuum treatment" can be accurately controlled with high steel purity and less gasses and inclusions contained in steel. The seamless steel tube,used for the crane arm frame,made from CS-Q780 steel produced with the process can fully meet the mechanical property and welding performance required by the crane arm frame.