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　　When the seamless steel pipe is sizing by micro tension reducing mill, the wall of the steel pipe is not uniformly thickened due to the transverse deformation (radial flow) of the metal, which makes the inner hole of its cross section "hexagonal", commonly known as "inner hexagonal". "Inner hexagon" is the result of metal transverse non-uniform deformation, and it is also a special form of non-uniform wall thickness of seamless steel pipe.

　　For hot rolled steel pipe, the maximum reduction rate of single stand of rolling mill is 3.5%, and the average tension coefficient is less than 0.48. The diameter and wall thickness of seamless steel pipe can reach the final product size by adopting micro tension rolling, three roll 14 stands and two motors centralized differential drive micro tension reducing mill for sizing. However, when rolling small diameter thick wall seamless steel pipe, this set of equipment is prone to "inner hexagonal" defects, which seriously affects the wall thickness and inner diameter accuracy. The production process can effectively reduce the "inner hexagonal" defects by adjusting the reducing ratio and modifying the pass parameters of the reducing mill.

　　By reducing the total reducing rate and single stand reducing rate and selecting reasonable pass design, the "inner hexagon" defect of micro tension reducing seamless steel pipe can be reduced or even eliminated.

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