Piercing is the first process of deformation of hot-rolled seamless steel pipe, and its function is to pierce the solid tube blank into a hollow capillary tube. Due to the surface defects or eccentricity (uneven wall thickness) of the threaded capillary, it is difficult to eliminate or reduce it in the subsequent deformation process. Therefore, the quality of the perforated capillary has an extremely important influence on the quality of the hot-rolled seamless steel pipe. The tube blank piercing methods include pressure punching, push rolling piercing and skew rolling piercing.
1 Pressure punching
Pressure punching is to put the heated square billet or corrugated steel ingot into the circular die, and then use the press to drive the punch to punch out the inner hole in the center part of the tube blank. Generally, the area of the punched inner hole 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 rolling piercing
Push rolling piercing can be regarded as an improved form of pressure punching, that is, the fixed round die during pressure punching is changed to a pair of rolls with a circular pass, and the rolls are driven by a motor. When the roll rotates to bite the tube blank into the pass and roll it, the punch fixed at the center of the pass penetrates it into a hollow capillary tube. In order to push rolling, a back thrust is added to the tail end of the billet, so it is called push rolling piercing.
The advantages of push-roll piercing are as follows:
1) The center of the billet is in a state of compressive stress. The process is a combination of punching and longitudinal rolling, which will not cause inward folding defects of two-roll cross rolling. The inner surface of the capillary is of good quality and has lower requirements for the quality of the billet;
2) The average unit pressure on the punch is about 50% smaller than that of pressure piercing, so the tool consumption is small;
3) During the piercing process, the metal in the central part of the blank is mainly deformed, which makes the coarse and loose structure in the center well processed and densified. At the same time, under the action of compressive stress, the inner and outer surfaces of the capillary are not easy to crack.
4) The productivity is higher than that of pressure piercing, up to two per minute;
Push-rolling piercing was a method in which the circular continuous casting process was not yet mature at that time, and square continuous casting slabs were needed to perforate and roll pipes. Although it has been greatly improved compared with pressure punching, the amount of deformation is still small, so the capillary is short and thick, and it is especially prone to large uneven wall thickness. Therefore, after piercing, it is necessary to set up a cross-rolling stretching machine to reduce the capillary wall thickness and extend the capillary length, and reduce the unevenness of the capillary wall thickness. However, with the maturity of the circular continuous casting billet technology, this method has been gradually replaced by cross-rolling piercing.
3 cross rolling piercing
Cross-rolling piercing is based on the fact that the round tube blank is bitten by two mutually inclined and co-rotating rolls and advances spirally. The tube blank is pierced into a hollow capillary tube through the pass formed by the roll, guide plate (or guide roll, guide plate) and the plug.
In fact, the round tube blank is bitten by the roll, rotated and compressed and deformed and spirally advanced. Before contacting with the plug, the plastic deformation of the central area of the tube blank under repeated tensile and compressive stresses gradually develops into looseness. As the looseness gradually increases, the center will rupture and form a “cavity” (also known as “transverse forging effect”) .Therefore, it is necessary to adjust the front end of the plug to the position where the tube blank is loose and does not form a “cavity”. At this time, the perforation force consumption is low, the tool wear is less, and the quality of the capillary tube is good.If the tube blank is deformed in contact with the plug after the “cavity” has been formed, it is easy to form an “inward fold” in the inner hole of the capillary tube.