The main production processes for large-diameter seamless steel pipes are hot-rolled large-diameter seamless steel pipes and hot expanded diameter seamless steel pipes. The maximum specification for hot expanded seamless steel pipes is 325mm * 1220mm and the thickest is 120mm. Hot expanded seamless steel pipes can be produced in non national standard sizes. Hot expanded seamless steel pipes are commonly referred to as hot expanded pipes. Steel pipes with low density but strong shrinkage are subjected to a cold rolling or drawing process to expand the diameter of the pipe material. Thickening steel pipes in a short period of time can produce non-standard and special types of seamless pipes with low cost and high production efficiency, which is currently the development trend in the international pipe rolling field.

The steel formed by cold drawing, cold rolling and other cold processing of large-diameter seamless steel pipes, which is delivered directly without any heat treatment, is called the cold drawing or cold rolling state. Compared with the hot-rolled (forged) state, the steel in the cold drawn (rolled) state has high dimensional accuracy, good surface quality, low surface roughness, and higher mechanical properties. Due to the lack of oxide skin covering the surface of the steel delivered in the cold drawn (rolled) state and the presence of significant internal stress, it is highly susceptible to corrosion or rust. Therefore, the packaging, storage, and transportation of steel in the cold drawn (rolled) state have strict requirements and generally need to be stored in the warehouse. Attention should be paid to temperature and humidity control in the warehouse

Large caliber seamless steel pipes are not subjected to special heat treatment after hot rolling or forging, and are delivered directly after cooling, which is called the hot rolling or hot forging state. The termination temperature of hot rolling (forging) is generally 800-900 ℃, and then it is generally naturally cooled in air. Therefore, the hot rolling (forging) state is equivalent to normalizing treatment.

The difference is that the termination temperature of hot rolling (forging) varies, unlike the strict temperature control of normalizing heating, so the fluctuation of steel structure and properties is greater than that of normalizing. At present, many steel companies adopt controlled rolling. Due to strict temperature control during final rolling and forced cooling measures after final rolling, the grain size of steel is refined, and the delivered steel has high comprehensive mechanical properties. The reason why non twist controlled hot and cold rolled wire rods have superior performance compared to ordinary hot rolled wire rods is that the steel delivered in the hot-rolled (forged) state has a certain degree of corrosion resistance due to a layer of oxide scale on the surface, and the requirements for storage and transportation are not as strict as those for steel delivered in the cold drawn (rolled) state. Large and medium-sized steel and medium thick steel plates can be stored in open-air freight yards or covered with tarpaulins.