How is S355J2H seamless steel pipe produced and processed

First, what are the steelmaking steps for S355J2H seamless steel pipe?
(1) Raw material selection and proportioning: The production of S355J2H seamless steel pipe begins with the careful selection of raw materials. The main raw materials are molten iron and scrap steel. Molten iron generally comes from blast furnace ironmaking, and its purity and composition have a significant impact on the quality of the subsequent steel products. The addition of scrap steel can adjust the composition and cost of the steel. During proportioning, the ratio of molten iron to scrap steel must be precisely controlled according to the target chemical composition. For example, to ensure a suitable carbon content, the carbon content of the molten iron and the amount of carbon introduced from the scrap steel need to be considered. Some alloying elements, such as ferromanganese and ferrosilicon, are also added to adjust the content of manganese, silicon, and other elements in the steel to meet the chemical composition requirements of the S355J2H steel grade.
(2) Steelmaking process: Oxygen converter steelmaking or electric furnace steelmaking processes are typically used. In oxygen converter steelmaking, high-pressure oxygen is blown into the converter to oxidize and remove impurities (such as carbon and phosphorus) in the molten iron. Electric arc furnace steelmaking uses electrical energy to generate high temperatures to melt raw materials, which allows for more precise control of the steel composition. During steelmaking, strict control of operating parameters such as temperature, oxygen blowing volume, and slag formation is necessary to ensure the purity and accuracy of the chemical composition of the molten steel. The refining process is also crucial, such as using a ladle refining furnace (LF) to refine the molten steel. The LF furnace can further deoxidize and desulfurize, adjust the composition and temperature of the molten steel, and homogenize the composition, thus improving the quality of the steel.

Second, how is the rolling of S355J2H seamless steel pipes carried out?
(1) Billet preparation: The steel ingots or continuously cast billets obtained after steelmaking need to be processed into billets through forging or rolling. During this process, the heating temperature and rolling parameters must be carefully controlled to ensure the dimensional accuracy and internal quality of the billet. For example, excessively high heating temperatures may cause the tube blank to overheat or burn, affecting its subsequent processing performance; while excessive or insufficient rolling force will affect the dimensional accuracy and surface quality of the tube blank.
(2) Hot Rolling Process: For S355J2H seamless steel pipes, hot rolling is commonly used. After heating the tube blank to a suitable rolling temperature (generally around 1000-1200℃), it is pierced through a piercing mill to form a rough tube. Then, it is continuously rolled by a tube rolling mill to achieve the specified wall thickness and outer diameter of the rough tube. During the hot rolling process, parameters such as rolling speed, rolling pressure, and roll gap must be precisely controlled according to the size and quality requirements of the seamless steel pipe. For example, excessively high rolling speed may cause the surface temperature of the seamless steel pipe to be too high, resulting in surface defects; an unsuitable roll gap will affect the wall thickness accuracy of the seamless steel pipe. To ensure the quality of the seamless steel pipe, sizing or reduction processes may also be used. Sizing ensures that the outer diameter of the seamless steel pipe reaches the precise size, while appropriately reducing the outer diameter to meet different market demands, while ensuring the quality of the seamless steel pipe. 

Third, how to perform heat treatment on S355J2H seamless steel pipes?
(1) Normalizing treatment: S355J2H seamless steel pipes may require normalizing treatment. Normalizing is a heat treatment process in which the seamless steel pipe is heated to above the critical temperature (generally around 900-950℃) and then cooled in air. Normalizing can refine the steel grains, improve the strength and toughness of the steel, and eliminate residual stress generated during hot rolling. Through normalizing treatment, the mechanical properties of the seamless steel pipe are more uniform, which is beneficial to subsequent processing and use. During the normalizing process, the heating rate, holding time, and cooling rate must be strictly controlled. Too fast a heating rate may lead to an excessive temperature difference between the inside and outside of the seamless steel pipe, generating thermal stress; insufficient holding time will not be sufficient to refine the grains; and an unsuitable cooling rate will affect the final performance of the seamless steel pipe.
(2) Tempering treatment (if necessary): For some seamless steel pipes with special requirements, tempering treatment may also be required. Tempering is a heat treatment process in which seamless steel pipes are heated to below the critical temperature (generally around 550-650℃) and then slowly cooled. Tempering is mainly used to eliminate internal stresses generated by quenching or normalizing in seamless steel pipes, and to adjust the balance between hardness and toughness. For example, if the hardness of a seamless steel pipe is slightly high after normalizing, tempering can appropriately reduce the hardness and improve toughness, making its performance more suitable for use.

Fourth, how is the finishing process of S355J2H seamless steel pipe carried out?
(1) Cutting: The seamless steel pipe is cut according to customer requirements. There are various cutting methods, such as sawing, flame cutting, and plasma cutting. Sawing is suitable for applications requiring high cutting precision, while flame cutting and plasma cutting are suitable for cutting larger seamless steel pipes. During the cutting process, it is important to control the cutting speed and quality to ensure a smooth cut surface without obvious defects.
(2) Straightening: Since seamless steel pipes may bend and deform during rolling or heat treatment, straightening is necessary. Pressure straightening or roller straightening is commonly used. Pressure straightening straightens the seamless steel pipe by applying pressure to the bent parts; roller straightening uses multiple straightening rollers to continuously straighten the seamless steel pipe. During the straightening process, straightening parameters, such as straightening pressure and the spacing of the straightening rollers, must be adjusted according to the degree of bending and size of the seamless steel pipe to obtain good straightness.
(3) Surface treatment: The surface of the seamless steel pipe is treated to improve its surface quality and corrosion resistance. Surface treatment methods include pickling and shot blasting. Pickling can remove oxide scale and rust from the surface of the seamless steel pipe, making the surface smoother; shot blasting uses high-speed shots to impact the surface of the seamless steel pipe, removing surface impurities and creating a certain compressive stress on the surface, improving surface hardness and fatigue resistance.