Large-diameter seamless steel pipes are steel products formed by cold processing such as cold drawing and cold rolling. The state in which they are delivered directly without any heat treatment is called cold drawn or cold rolled state. Compared with the hot-rolled (forged) state, steel in the cold-drawn (rolled) state has high dimensional accuracy, good surface quality, low surface roughness, and higher mechanical properties. Because steel is delivered in the cold-drawn (rolled) state The surface is not covered by oxide scale, and there is great internal stress, making it very susceptible to corrosion or rust. Therefore, cold-drawn (rolled) steel has strict requirements on its packaging, storage, and transportation, and generally needs to be kept in a warehouse. And attention should be paid to the temperature and humidity control in the warehouse.


Large-diameter seamless steel pipes no longer undergo special heat treatment after hot rolling or forging. They are delivered directly after cooling, which is called a hot rolling or hot forging state. The termination temperature of hot rolling (forging) is generally 800~900℃. Afterward, it is generally cooled naturally in the air, so the hot rolling (forging) state is equivalent to normalizing treatment.


The difference is that the end temperature of hot rolling (forging) can be high or low, unlike the normalizing heating temperature control, so the fluctuations in the steel structure and properties are greater than in normalizing. Many steel companies use controlled rolling. Since the final rolling temperature is strictly controlled and forced cooling measures are taken after the final rolling, the grains of the steel are refined and the delivered steel has higher comprehensive mechanical properties. This is the reason why cold and hot-rolled wire rods without twist control have superior performance than ordinary hot-rolled wire rods. Steel delivered in a hot-rolled (forged) state has a certain corrosion resistance because the surface is covered with a layer of iron oxide scale. Storage and transportation The requirements are not as strict as those for steel delivered in the cold-drawn (rolled) state. Large and medium-sized steel sections and medium-thick steel plates can be stored in the open yard or after being covered.


Large diameter seamless steel pipes are annealed and heat-treated before leaving the factory. This delivery state is called the annealed state. The purpose of annealing is mainly to eliminate and improve the structural defects and internal stress left over from the previous process, and to prepare the structure and performance for the subsequent process. Alloy structural steel, structural steel with guaranteed hardenability, cold heading steel, bearings Steel, tool steel, turbine blade steel, and cable-type stainless heat-resistant steel are usually delivered in an annealed state.