The main production processes of large-diameter seamless steel pipes are hot-rolled large-diameter seamless steel pipes and heat-expanded-diameter seamless steel pipes. The maximum specification of hot-expanded seamless steel pipes is 325mm-1220mm and the thickest is 200mm. It is used for steel pipes with relatively low density but strong shrinkage. A waste tube finish rolling process in which the diameter of the tube is enlarged by the cross-rolling method or the drawing method. In a relatively short period of time, thickening steel pipes can produce non-standard and special types of seamless pipes, with low cost and high production efficiency, which is the development trend of the international pipe rolling field.
Weight calculation formula of large-diameter seamless steel pipe:
[(Outer diameter-Wall thickness)*Wall thickness]*0.02466 = kg/m (weight per meter)
General specifications of large-diameter seamless steel pipes:
People with an outer diameter between (89mm-1200mm) are used to calling them large-diameter seamless pipes because of their relatively large diameter.
The wall thickness (5mm-200mm) can actually be divided into: large-diameter thick-walled seamless pipe, large-diameter thin-walled seamless pipe.
The state in which large-diameter seamless steel pipes are cold-drawn, cold-rolled and other cold-formed steels, directly delivered without any heat treatment, is called cold-drawn or cold-rolled state. Compared with the hot-rolled (forged) state, the cold-drawn (rolled) state of the steel has high dimensional accuracy, good surface quality, low surface roughness, and higher mechanical properties. Because of the cold-drawn (rolled) state of delivery of steel The surface is not covered by oxide scale, and there is a large internal stress, which is easily corroded or rusted. Therefore, the packaging, storage and transportation of cold-drawn (rolled) steel have strict requirements, and generally need to be kept in a warehouse. And should pay attention to the temperature and humidity control in the warehouse.
Large-diameter seamless steel pipes are no longer subjected to special heat treatment after hot rolling or forging, and they are delivered directly after cooling, which is called hot rolling or hot forging state. The termination temperature of hot rolling (forging) is generally 800 to 900 ℃. Afterwards, it is generally cooled naturally in the air, so the hot-rolled (forged) state is equivalent to normalizing.
The difference is that the end temperature of hot rolling (forging) is high or low, unlike normalizing, which has strict heating temperature control. Therefore, the fluctuation of steel structure and properties is greater than that of normalizing. Many iron and steel enterprises adopt controlled rolling. Due to the strict control of the final rolling temperature and forced cooling measures 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 non-twist controlled cold and hot-rolled wire rod has better performance than ordinary hot-rolled wire rod. Steel delivered in hot-rolled (forged) state has a certain degree of corrosion resistance because the surface is covered with a layer of oxide scale. It is stored and transported. The requirements are not as strict as the steel delivered in the cold drawn (rolled) state. Large and medium-sized steel and medium-thick steel can be stored in the open yard or after being covered.
Large-diameter seamless steel pipes are annealed and heat-treated before they leave the factory. This delivery state is called annealed state. The purpose of annealing is mainly to eliminate and improve the structural defects and internal stress left by the previous process, and to prepare the organization and performance for the subsequent process, alloy structural steel, structural steel with guaranteed hardenability, cold heading steel, bearing Steels, tool steels, steam turbine blade steels, and iron cable-shaped stainless heat-resistant steels are usually delivered in annealed state.
Round tube billet → heating → piercing → three-roll cross rolling, continuous rolling or extrusion → stripping → sizing (or reducing diameter) → cooling → straightening → hydraulic test (or flaw detection) → marking → warehousing
The mechanical properties of large-diameter steel pipes are an important index to ensure the final use performance (mechanical properties) of large-diameter steel pipes, and it depends on the chemical composition and heat treatment system of the steel pipe. Therefore, according to different application requirements, the mechanical properties of large-diameter spiral steel pipes are specifically introduced from the aspects of tensile strength, yield point, and elongation.
1 Tensile strength
In the tensile process, the maximum force (Fb) that the sample bears when it breaks is the stress (σ) obtained from the original cross-sectional area (So) of the sample, which is called the tensile strength (σb), and the unit is N/mm2 (MPa). It represents the maximum ability of a metal material to resist damage under tensile force.
2 Yield point
For a metal material with a yield phenomenon, the stress at which the sample can continue to elong without an increase in the force during the stretching process (remains constant) is called the yield point. If the force drops, the upper and lower yield points should be distinguished. The unit of yield point is N/mm2 (MPa).
3 Elongation after breaking
In the tensile test, the percentage of the length of the gauge length increased after the sample is broken to the original gauge length is called the elongation. Expressed by σ, the unit is %.