The extrusion of seamless steel tubes is an essential part of the production process. The extrusion method of large-diameter seamless steel pipe and small-diameter seamless steel pipe is basically the same, and is similar. The so-called extrusion is a pressure processing method of applying pressure to one end of a metal placed in an extrusion cylinder to form it through a die hole.
According to the difference between the metal flow direction and the direction of movement of the extrusion rod during extrusion, it is divided into positive extrusion and reverse extrusion. The metal flows in the same direction, that is, the positive extrusion, and vice versa, the reverse extrusion. The anti-extrusion is that the metal has no relative movement with the inner wall of the extrusion cylinder, and the pressing force is low, but the length of the extruded product is limited by the equipment, and thus the reverse extrusion is mainly used for the perforation of the blank. If a hollow billet is used, after the mandrel is inserted therein, the metal flows out of the annular gap at the die hole, and the tube is extruded.
There are more than 60 sets of extrusion units in the world. Except for a few for profile extrusion, 70% of the units are used for the production of steel pipes. The main type is stainless steel pipes. Compared with the rolling (longitudinal and cross-rolling) production methods, the extrusion method is characterized by three-way compressive stress during metal deformation. Under this optimal stress state, for stainless steel tubes with large steel numbers and large deformation resistance, good deformation, internal and external surface quality and metallographic structure can be obtained.
The improvement of the vertical hydraulic perforation process equipment can make the deviation accuracy of the wall thickness of the waste pipe after extrusion reach 5%~7%. The extrusion die is simple, easy to manufacture and replace, and is suitable for the production of small batch and multi-size stainless steel pipes. The extrusion unit can also produce steel pipes with a wide range of specifications. One set of 50MN (5000t) extrusion units can produce steel pipes with a specification of Φ25~245mm, which is far more adaptable to market demand than other rolling mill units.
The biggest advantage of the extrusion method is that the continuous casting billet can be directly used as raw material. The product quality is stable and the variety is flexible. It can directly produce hot extruded product tubes, and can also produce various shaped stainless steel tubes. However, the disadvantage of producing steel pipes by extrusion is that the finished product rate is low. In order to increase the yield, foreign manufacturers have improved the production process. For the high-demand varieties, the production process of pre-drilling, hydraulic punching machine reaming and extrusion into tubes is generally adopted.
For the general requirements of the cold-processed raw material pipe, different production processes can be adopted according to the specific conditions: the small-size stainless steel seamless pipe is perforated and extruded into a pipe directly on the vertical hydraulic punching machine; the blank used in the medium specification is pre-drilled as small as possible. The vertical hydraulic punching machine reams and extrudes into a tube; after the large size of the blank used for pre-drilling the larger hole, it directly enters the extruder and is extruded into a tube. Obviously, the production of finished pipes by extrusion and the provision of waste pipes for cold processing are a relatively economical and practical production method widely used in foreign hot-processed stainless steel pipes.