The hot extrusion process of seamless steel pipe is a pressure forming method, which is to place the tube blank in a pressure-resistant container, and apply external force to cause the tube blank to plastically deform under the state of three-dimensional compressive stress. During extrusion, the tube blank is in a state of three-way compressive stress, so it can obtain a larger plastic deformation than rolling and forging. Depending on the material and extrusion method, the extrusion elongation coefficient can reach 5~500, or even higher. Because the extruded seamless steel pipe has the characteristics of high dimensional accuracy, dense metal structure and stable mechanical properties, hot-extruded seamless steel pipe is widely used in nuclear power, thermal power, petrochemical, aviation, aerospace, transportation and other fields .
Hot extrusion process
Billet preparation → annular furnace preheating → vertical induction furnace heating → glass powder lubrication → vertical press punching (expanding) → phosphorus removal → vertical induction furnace reheating → glass powder lubrication → extrusion → cutting and pressing → cooling
When using hot extrusion technology to produce seamless steel pipes, the broken pipes must be hollow billets. One is a hollow billet for perforating a forged or cast billet, and the other is a hollow billet for centrifugal casting. There are 3 different piercing methods for perforating forging or casting billets according to their steel grades and specifications:
①The solid tube blank is perforated on the vertical perforator;
② Drill a small hole (also called a pilot hole) in the center of the broken pipe in advance, and perform perforation on a pressure perforator;
③ Drill a large hole with a diameter slightly larger than the extrusion core rod in the center of the broken pipe, and send it directly to the extruder for extrusion.
During extrusion, the glass mat between the tube and the extrusion die melts and acts as a lubricant. The outer diameter of the extruded seamless steel pipe depends on the inner diameter of the die hole, and the gap between the mandrel and the die hole determines the thickness of the steel pipe. After the extrusion is completed, the steel pipe is cooled and collected, and subjected to heat treatment and hardcover treatment
Technical Difficulties of Hot Extrusion Process of Seamless Steel Tubes
1. Oxidation-free heating of tube blanks
Typically, the temperature of the material should be maintained at 80°C-150°C below its "liquidus" when heated. Especially in the heating process of the material is very likely to appear "overburning" phenomenon, so the heating temperature of the material needs to be strictly controlled during the heating process. The current extrusion process usually adopts the method of preheating the ring furnace and heating the vertical induction furnace. The bad material is heated to a certain temperature (should not exceed 900°C) in the ring furnace, and then the billet is sent into the vertical induction heating furnace to be heated to the hot extrusion temperature of the material. This can not only increase the heating speed, but also effectively reduce the burning loss of the metal, thereby effectively improving the quality of heating. At the same time, the automatic control of the induction heating process and the establishment of the heating control model can also effectively ensure the temperature accuracy of the length direction and diameter direction of the bad material, so as to effectively ensure that the bad material is heated without oxidation. The billet can also be heated under protective atmosphere conditions during induction heating, and the metal is difficult to oxidize.
2. The service life of the mold
The service life of the mold usually refers to the service life of the extrusion cylinder, the perforating needle and the extrusion die, which are in contact with the hot metal and generate relative flow with the hot metal.
3. Lubrication problems
Lubrication has always been one of the difficulties in the application of seamless steel pipe hot extrusion technology.
Hot extrusion process
Billet preparation → annular furnace preheating → vertical induction furnace heating → glass powder lubrication → vertical press punching (expanding) → phosphorus removal → vertical induction furnace reheating → glass powder lubrication → extrusion → cutting and pressing → cooling
When using hot extrusion technology to produce seamless steel pipes, the broken pipes must be hollow billets. One is a hollow billet for perforating a forged or cast billet, and the other is a hollow billet for centrifugal casting. There are 3 different piercing methods for perforating forging or casting billets according to their steel grades and specifications:
①The solid tube blank is perforated on the vertical perforator;
② Drill a small hole (also called a pilot hole) in the center of the broken pipe in advance, and perform perforation on a pressure perforator;
③ Drill a large hole with a diameter slightly larger than the extrusion core rod in the center of the broken pipe, and send it directly to the extruder for extrusion.
During extrusion, the glass mat between the tube and the extrusion die melts and acts as a lubricant. The outer diameter of the extruded seamless steel pipe depends on the inner diameter of the die hole, and the gap between the mandrel and the die hole determines the thickness of the steel pipe. After the extrusion is completed, the steel pipe is cooled and collected, and subjected to heat treatment and hardcover treatment
Technical Difficulties of Hot Extrusion Process of Seamless Steel Tubes
1. Oxidation-free heating of tube blanks
Typically, the temperature of the material should be maintained at 80°C-150°C below its "liquidus" when heated. Especially in the heating process of the material is very likely to appear "overburning" phenomenon, so the heating temperature of the material needs to be strictly controlled during the heating process. The current extrusion process usually adopts the method of preheating the ring furnace and heating the vertical induction furnace. The bad material is heated to a certain temperature (should not exceed 900°C) in the ring furnace, and then the billet is sent into the vertical induction heating furnace to be heated to the hot extrusion temperature of the material. This can not only increase the heating speed, but also effectively reduce the burning loss of the metal, thereby effectively improving the quality of heating. At the same time, the automatic control of the induction heating process and the establishment of the heating control model can also effectively ensure the temperature accuracy of the length direction and diameter direction of the bad material, so as to effectively ensure that the bad material is heated without oxidation. The billet can also be heated under protective atmosphere conditions during induction heating, and the metal is difficult to oxidize.
2. The service life of the mold
The service life of the mold usually refers to the service life of the extrusion cylinder, the perforating needle and the extrusion die, which are in contact with the hot metal and generate relative flow with the hot metal.
3. Lubrication problems
Lubrication has always been one of the difficulties in the application of seamless steel pipe hot extrusion technology.
