Seamless steel tubes are divided into hot-rolled (extrusion) seamless pipes and cold drawn (rolled) seamless pipes due to their different manufacturing processes. Cold drawn (rolled) tubes are divided into two types: round tubes and shaped tubes.
The biggest difference between straight-line high-frequency electric resistance welded steel (Electric Resistance Welding, ERW for short) and seamless steel pipe is that ERW has a welded seam, which is the key to the quality of erw steel pipe. The modern ERW steel pipe production process and equipment, due to the unremitting efforts of the international, especially the United States, have made the ERW steel pipe seamlessly satisfactory. Some people have divided the seamlessness of ERW steel pipes into geometric seamless and physical seamless. Geometric seamlessness is to remove the internal and external burrs of the ERW steel pipe. Due to the continuous improvement and improvement of the structure and cutter of the internal burr removal system, the removal of the inner burr of the large and medium diameter steel pipe has been well treated. The internal burr can be controlled from -0.2mm to +O.5mm. Physical seamlessness means that there is a difference between the metallographic structure inside the weld and the base metal, which leads to a decrease in the mechanical properties of the weld zone, and measures need to be taken to make it uniform and uniform. The high-frequency welding heat process of the ERW steel pipe causes a temperature distribution gradient near the edge of the tube blank, and forms a characteristic area such as a melting zone, a semi-melting zone, an overheated structure, a normalized zone, an incomplete normalized zone, and a tempered zone. In the superheated zone structure, the austenite grains grow sharply due to the welding temperature above 1000 °C, and a hard and brittle coarse crystal phase is formed under cooling conditions, and the existence of a temperature gradient causes welding stress. In this way, the mechanical properties of the weld zone are formed to be lower than that of the base metal. The physical seamless steel pipe is heated by the local heat treatment process of the weld to the AC3 (927 ° C) by the intermediate frequency induction heating device, and then The air cooling process with a length of 60m and a speed of 20m/min is re-cooled when needed. The use of this method achieves the purpose of eliminating stress, softening and refining the structure, and improving the comprehensive mechanical properties of the weld heat affected zone. At present, the world’s advanced ERW units have generally adopted this method to treat the welds, and have achieved good results. The high-quality ERW steel pipe can not only identify the weld seam, but also achieve a weld seam coefficient of 1, which achieves the matching of the weld zone structure and the base metal.
At the same time, the ERW steel pipe has the use of hot-rolled coil as raw material, and the wall thickness can be controlled to be ±0.2mm. The second end of the steel pipe is in accordance with the American APl standard or GB/T9711.1 standard, trimming the beveling, and the length is delivered. Etc. In recent years, various natural gas pipeline network engineering and gas companies have widely adopted ERW steel pipes as the main pipe of urban pipe network.