Effect Of P On Microstructure And Mechanical Properties Of Steel Plates For Pressure Vessel (A516)

Steel plates for pressure vessel have been used to manufacture the components of pressure vessel like shell, shell cover and so on, which refers to high quality carbon steel, high quality low-alloy steel and alloy steel with high strength, toughness and weldability. In addition, the chemical componets and the quality of steel plate for pressure vessel are strictly demanded. To save cost and improve quality, the effect of P on steel plates for pressure vessel receives a great deal of attention. Segregation of P occurs during the process of molten steel's solidification, and this results in the reduction of impact toughness of the steel. And also, substitutional solid solution appears due to the replacement of Fe by the P. The result is that the strength of the steel is improved with slight reducing of elongation. At the same time, the corrosion resistance from air and cutting machinability are improved if there is P in steels. For example, the interaction between P and Nb, different working process, and heat treatment process all have effects on the segregation of P. So the effect of P on the microstructures of steel plates for pressure vessel is needed, so that P can play an active role in the steels, and the cost is reduced, at the same time, the theoretical foundation can be provided for the manufacturing of steel plates for pressure vessel A516.This dissertation was completed with the support of "Effect of Phosphorus on Microstructure and Mechanical Properties of Steel Plates for Pressure vessel (A516)" project cooperation with POSCO. Three kinds of experimental steels were prepared, with phosphorus content of 0.0092%,0.017% and 0.029% respectively. Static and dynamic continuous cooling transformation curves of experimental steels with different phosphorus content were attained. The effects of cooling rate, deformation and phosphorus content on the continuous cooling transformation were compared. The effects of phosphorus content and heat treatment process on the microstructures and the mechanical properties of the experimental steels were researched. The experimental steels welding properties have been also researched. The main results as following:(1) When the cooling rate ranging from 0.5?/s to 20?/s, the CCT curves of the steel include three parts:ferrite transformation, pearlitic transformation, bainite transformation. The volume of the bainite increased the volume of ferrite, and pearlite decreased as the cooling rate increased. The deformation makes the CCT curve move to the top left as the deformation promotes the pro-eutectoid ferrite transformation. The P inhibits transformation, which leads to the CCT curve moving to lower right.(2) The microstructure including pearlite, ferrite, and band structure after hot rolling. Heat treatment minimized the band structure, and refined the grain size. The strength of the steel decreased as the P content increased from 0.00092% to 0.017%. The strength of the steel increased as the P content increased to 0.029%. The elongation and the impact toughness decrease as the P content increased from 0.0092% to 0.029%.(3) No.P3 steel welding metal contains a large number of acicular ferrite, improving the toughness of the material; overheated zone include the eutectoid ferrite and martensite. The fracture in the tension specimen located in the base metal, the mechanical properties of test plate is same with the ones in the base metal.(4) When the P content increased to 0.029%, after the heat treatment, the tensile strength is 510MPa, the elongation is 33.1%. The Charpy transverse impact energy 102J at-50?. After welding, the tensile strength is 475MPa, the elongation is 36.2%, and the Charpy transverse impact energy of the overheated is 77J at -50?. The performance can fully meet ASTM A516/A516M (65 levels) requirements.