When the punch descends and contacts the 20# seamless steel pipe, the 20# seamless steel pipe bends at the beginning, and the material bulges, deforms and hardens at the fillet of the punch.
The punch continues to decline. At this time, 20# seamless steel pipe has two possibilities of deformation. One is that the blank at the bottom of the punch elongates and deforms under the influence of two-dimensional tensile stress, the general product increases and the thickness decreases, that is, bulging deformation; Second, under the influence of tangential compressive stress and radial tensile stress, the blank in the flange Department flows to the straight wall through the fillet of the die, resulting in general product reduction and thickness increase, that is, deep drawing deformation.
When the drawing deformation resistance of 20# seamless steel pipe is equal to the bulging deformation resistance, the bulging at the bottom will be carried out simultaneously with the drawing at the flange. If the drawing deformation resistance is less than the bulging deformation resistance, the blank at the bottom and wall will no longer deform, the blank at the flange will transfer to the straight wall to form the cylinder wall, and the drawing deformation can proceed smoothly until the deformation is completed. If the drawing deformation resistance is greater than the bulging deformation resistance at the bottom or seam, bulging deformation will occur. Because the bulging deformation level is limited, the change of this deformation style means the beginning of splitting.