Tube billet piercing process
Piercing is the first process in the deformation of hot-rolled seamless steel pipes, and its function is to Pierce solid tube billets into hollow rough tubes. Due to the fact that the surface defects or eccentricity (uneven wall thickness) of the perforated rough tubes are difficult to eliminate or alleviate in the subsequent deformation process, the quality of the perforated rough tubes has an extremely important impact on the quality of hot-rolled seamless steel pipes.
There are three methods for piercing tube blanks: pressure punching, push rolling piercing and cross rolling piercing.
1. Pressure punching
Pressure punching involves loading a heated billet or wavy steel ingot into a circular die, and then using a press to drive the punch to punch out an inner hole in the center of the tube billet. Generally, the area of the inner hole punched is equivalent to or slightly larger than the gap between the blank and the circular die, so the deformation is very small, and the elongation coefficient generally does not exceed 1.1.
2. Push and roll piercing
Push-rolling punching can be regarded as an improved form of pressure punching, that is, the fixed circular die used in pressure punching is changed to a pair of rolls with circular hole shapes, and the rolls are driven by motors. When the roll rotates to bite the tube blank into the die and roll it, the punch fixed at the center of the die penetrates it into a hollow rough tube. To promote rolling, a rear thrust needs to be added at the tail end of the billet, so it is called push rolling piercing.
Pushing and rolling perforation was a method that required the use of square continuous casting billets for perforation and pipe rolling at a time when the circular continuous casting process was not yet mature. Although it was a significant improvement over pressure punching, the deformation was still small, so the rough tubes were short and thick, and it was particularly prone to large uneven wall thickness. Therefore, after piercing, a cross-rolling extension machine needs to be set up to reduce the wall thickness of the rough tube, extend the length of the rough tube, and decrease the degree of uneven wall thickness of the rough tube. However, with the continuous casting billet process becoming increasingly mature, this method has gradually been replaced by cross-rolling piercing.
3 Oblique rolling piercing
The cross-rolling punching machine can be classified into two-roll cross-rolling punching machine and three-roll cross-rolling punching machine according to the number of rolls. It is based on the principle that the round tube blank is bitten into by mutually inclined and rotating rolls and moves forward in a spiral manner. The tube blank is punched into a hollow rough tube through the hole shape formed by the rolls, guide plates (or guide rolls, guide discs) and the top end.
In fact, the round tube blank is bitten in by the rolls, rotated and compressed to form a spiral forward. Before coming into contact with the top end, the plastic deformation in the center area of the tube blank under the repeated action of tensile and compressive stresses gradually develops into porosity. As the porosity gradually intensifies, it will lead to central rupture and form a "cavity" (also known as the "transverse forging effect"). Therefore, the front end of the top head should be adjusted to the position where the tube blank is loose but no "hole cavity" has been formed. At this time, the perforating force can be consumed less, the tool wear is less, and the quality of the perforated rough tube is good. If the tube blank comes into contact with the head and deforms after a "cavity" has been formed, it is very easy to form an "internal fold" in the inner hole of the rough tube.
15358968703