Abstract: Q345B hot-rolling seamless steel pipe has a typical external folding defect, which seriously affects the mass production. Scanning electron microscope, energy spectrum analyzer and metallographic microscope were used to analyze the morphology, composition and structure of the defect area. The external folding defect is determined by the decarburized layer and the oxidation spots on the two sides of the rolling pipe crack. The high casting temperature, the mismatch between the performance of slag and the pulling speed, and the uneven cooling in the secondary cooling zone are the main reasons leading to the surface cracks of the cast billet. By stably controlling the superheat of the molten steel within 20 ℃-30 ℃, increasing the basicity of slag to 1.05, reducing the viscosity of slag to 0.85 Pa·s, and reducing the total specific water of the secondary cooling zone to 0.20 L/kg, the surface cracks of the cast billet have been basically eliminated in the subsequent production.