Large-diameter seamless steel pipe, the main production process is hot-rolled large-diameter seamless steel pipe and heat-expanded seamless steel pipe, the maximum specification of heat-expanded seamless steel pipe is 325mm-1220mm and the thickest is 200mm, heat-expanded seamless steel pipe can produce non-national standard size, heat-expanded seamless pipe is often said to be hot-expanded pipe. A finishing process in which steel pipes with relatively low density but very strong shrinkage are expanded by diagonal rolling or drawing. In a short time to make the steel pipe thickening, can produce non-standard, special types of seamless pipe, and low cost, high production efficiency, is the development trend in the field of international rolled pipe.
The steel pipe with outer diameter greater than or equal to 76mm and wall thickness ≥14mm on GB5310 (seamless steel pipe for high-pressure boilers) shall be subjected to Charpy V longitudinal impact test at 20℃; The average value of the impact work of the three samples shall not be less than 40J, allowing the impact work of one sample to be lower than the average value, but not less than 70% of the average value. For steel pipes with outer diameter greater than 219.1mm and wall thickness ≥25mm, transverse mechanical properties test can be used instead of longitudinal mechanical properties test. When Charpy V transverse impact test is carried out at 20℃, it is more convenient to test the transverse mechanical properties of large diameter pipe.
The state of cold-processed steel formed by cold drawing and cold rolling without any heat treatment is called cold drawing or cold rolling state. Compared with the hot rolling (forging) state, the steel in the cold drawing (rolling) state has high dimensional accuracy, good surface quality, low surface roughness, and high mechanical properties. Because the steel delivered in the cold drawing (rolling) state is not covered by oxide, and there is a great internal stress, it is easy to suffer corrosion or rust, so the steel in the cold drawing (rolling) state is easily corroded. Its packaging, storage and transportation have more strict requirements, generally need to be kept in the warehouse, and should pay attention to the temperature and humidity control in the warehouse.
Large caliber seamless steel pipe steel is no longer special heat treatment after hot rolling or forging, and is delivered directly after cooling, called hot rolling or hot forging state, the termination temperature of hot rolling (forging) is generally 800 ~ 900℃, and then it is generally cooled naturally in the air, so the hot rolling (forging) state is equivalent to normalizing treatment.
The difference is because the hot rolling (forging) termination temperature is high and low, unlike the normalizing heating temperature is strictly controlled, so the fluctuation of steel structure and performance is larger than the normalizing. Many iron and steel enterprises use controlled rolling, because the final rolling temperature control is very strict, and take forced cooling measures after the final rolling, so the grain refinement of steel, steel delivery has a higher comprehensive mechanical properties. No twist control cold and hot rolled wire is superior to ordinary hot rolled wire performance is this reason, hot rolled (forged) state delivery of steel, because the surface is covered with a layer of oxide, so it has a certain corrosion resistance, storage and storage requirements are not as strict as cold drawn (rolled) state delivery of steel, large and medium-sized steel, medium thick steel can be stored in the open yard or after the cover.
Large caliber seamless steel pipe steel is annealed before leaving the factory, and this delivery state is called annealing state. The purpose of annealing is mainly to eliminate and improve the organizational defects and internal stresses left over from the previous process, and to prepare for the organization and performance of the subsequent process, alloy structural steel, to ensure the hardenability of structural steel, cold heading steel, bearing steel, tool steel, turbine blade steel, iron wire type stainless heat-resistant steel steel commonly delivered in annealing state.
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