First, what are the steelmaking details of S355J2H seamless steel pipe?
1) Raw material selection and proportioning for S355J2H seamless steel pipe: The first step in producing S355J2H seamless steel pipe is to carefully select 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. 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 for S355J2H seamless steel pipe: 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) from the molten iron. Electric arc furnace steelmaking uses electrical energy to generate high temperatures to melt raw materials, allowing for more precise control of the steel’s composition. During steelmaking, strict control of operating parameters such as temperature, oxygen blowing volume, and slag formation is crucial to ensure the purity and accurate chemical composition of the molten steel. The refining process is also critical, such as using a ladle refining furnace (LF) to refine the molten steel. The LF furnace further deoxidizes and desulfurizes, adjusts the composition and temperature of the molten steel, and homogenizes its composition, improving steel quality.

Secondly, what are the rolling processes for S355J2H seamless steel pipes?
1) S355J2H seamless steel pipe billet preparation: The steel ingots or continuously cast billets obtained from steelmaking need to be forged or rolled into billets. 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 can lead to overheating and burning of the tube blank, 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 of S355J2H Seamless Steel Pipe: Hot rolling is commonly used for S355J2H seamless steel pipe. 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 on a tube rolling mill to achieve the specified wall thickness and outer diameter. During hot rolling, parameters such as rolling speed, rolling pressure, and roll gap must be precisely controlled according to the dimensional and quality requirements of the S355J2H seamless steel pipe. For example, excessively high rolling speeds may cause excessively high surface temperatures in the S355J2H seamless steel pipe, resulting in surface defects; improper roll gaps will affect the wall thickness accuracy of the S355J2H seamless steel pipe.
3) To ensure the quality of S355J2H seamless steel pipes, sizing or reduction processes may be used. Sizing ensures the outer diameter of the S355J2H seamless steel pipe reaches a precise dimension, while reduction appropriately reduces the outer diameter to meet different market demands while maintaining quality.

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

Fourth, the finishing process of S355J2H seamless steel pipes:
1) Cutting: S355J2H seamless steel pipes are cut according to customer requirements. Various cutting methods are available, such as sawing, flame cutting, and plasma cutting. Sawing is suitable for applications requiring high precision, while flame cutting and plasma cutting are suitable for larger S355J2H 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 S355J2H seamless steel pipes may bend or deform during rolling or heat treatment, straightening is necessary. Pressure straightening or roller straightening methods are commonly used. Pressure straightening applies pressure to the bent parts of the S355J2H seamless steel pipe; roller straightening uses multiple straightening rollers to continuously straighten the pipe. During the straightening process, straightening parameters, such as straightening pressure and the spacing of the straightening rollers, should be adjusted according to the degree of curvature and dimensions of the S355J2H seamless steel pipe to achieve good straightness.
3) Surface Treatment: The surface of the S355J2H 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 S355J2H seamless steel pipe, making the surface smoother; shot blasting uses high-speed shots to impact the surface of the S355J2H seamless steel pipe, removing surface impurities and creating a certain compressive stress on the surface, improving surface hardness and fatigue resistance.