Effect Of Nitrogen On Microstructure And Properties High Of Steel Containing Ce Strength Used For Marine Platform

The development of marine resources in the world is gradually moving from the shallow sea to the deep sea.Higher requirements are placed on the comprehensive performance of steel for offshore platforms.A large number of studies have shown that nitrogen is precipitated and strengthened in steel and rare earth plays a role in refining grains and purifying molten steel in steel which can improve various mechanical properties of steel and improve corrosion resistance of steel.In this experiment N and Ce are added to the high-strength offshore platform steel and the fine grain strengthening effect of the rare earth and the precipitation strengthening effect of the nitrogen are used to obtain better strengthening effect so that the mechanical and corrosion resistance of the steel for the offshore platform is Improved.The equilibrium precipitation phase existing in the temperature range of 0°C¹600°C was calculated using the Thermo-Calc software for experimental steel containing 0.027%nitrogen.The microstructure of the experimental steel was observed by Axiovert25 metallographic microscope.The morphology and composition of the precipitates of the experimental steel were observed and analyzed by scanning electron microscopy and energy spectrum analyzer.The influence of nitrogen on the precipitation phase of the experimental steel was analyzed.The mechanical properties of the experimental steel were tested by using Vickers hardness tester electronic universal testing machine and impact testing machine.The influence of nitrogen on the mechanical properties of the experimental steel was analyzed.The experiment of cyclic corrosion weight loss experiment and electrochemical experiment was carried out to study the addition of nitrogen and rare earth the effect of corrosion properties of steel.The experimental results show that after equilibrium analysis of the precipitation phase of experimental steel containing nitrogen in Thermo-Calc the equilibrium precipitates at 500°C⁷00°C are mainly Nb?C,N??V?C,N??VN?NbN and Cr₂₃C₆.At the same time the experimental steel with nitrogen content of 0.027%was tempered at different temperatures.It was found that the uniform tempering temperature at 640°C can obtain uniform Sorbite and polygonal ferrite structure which gives better comprehensive mechanical properties.The microstructure of the experimental steel after the tempering treatment makes the microstructure of the experimental steel fine and uniform with the addition of nitrogen and at this time the nitrogen acts to refine the crystal grains.With the increase of nitrogen content the precipitation phase of the experimental steel gradually increased and it was found that the carbonitride and CeO were combined and precipitated along the grain boundary and the experimental steel with a nitrogen content of 0.027%had a large amount of Nb?C,N?in a small spherical shape.Dispersion is distributed on the ferrite matrix and no formation of nitride and Cr₂₃C₆ is found in the experimental steel.After testing the mechanical properties of the experimental steel it was found that the strength and toughness of the experimental steel gradually increased with the increase of N content.When the nitrogen content is 0.027%the strength and toughness are the best.At this time nitrogen plays a role of precipitation strengthening in the steel and the rare earth Ce acts to promote precipitation strengthening in the steel.After testing the corrosion performance of the experimental steel it was found that the corrosion resistance of the experimental steel after adding N and Ce was obviously improved.The phase analysis of the experimental steel rust layer is mainly composed of Fe?OH?₂??-FeOOH??-FeOOH and Fe3O4 wherein?-FeOOH can effectively hinder the mass transfer of the corrosive medium and play a role on the steel matrix.Protective effects.The addition of nitrogen and rare earth has no effect on the rust layer composition of the experimental steel and the corrosion performance of the steel is improved by refining the grain of the structure.