Large diameter seamless pipe
generally refers to seamless steel pipe with an outer diameter of more than 402mm, including thin-walled seamless pipe and thick-walled seamless steel pipe. The manufacturing process of large-diameter seamless steel pipe is hot rolling. Compared with the original hot rolled steel pipe, the production cycle of seamless steel pipe can be shortened and the production cost of seamless steel pipe can be reduced compared with that of the original hot rolled steel pipe. In terms of quality, the hot expanded seamless pipe is not as good as the original hot-rolled seamless steel pipe. At present, the largest diameter of hot-rolled seamless pipe in China is 760mm.
The large-diameter seamless steel pipe will not be subject to special heat treatment after hot rolling or forging, and will be delivered directly after cooling, which is called hot rolling or hot forging state. The termination temperature of hot rolling (forging) is generally 800 ~ 900 ℃, and then it will be cooled naturally in the air. Therefore, the hot rolling (forging) state is equivalent to normalizing treatment.
The steel of large-diameter seamless steel pipe is annealed before leaving the factory. This delivery state is called annealing state. The purpose of annealing is mainly to eliminate and improve the structural defects and internal stress left by the previous process, and make preparations for the structure and performance of the next process. The steels of alloy structural steel, structural steel with guaranteed hardenability, cold heading steel, bearing steel, tool steel, steam turbine blade steel and iron cable type stainless and heat-resistant steel are usually delivered in annealing state.