Research On Microstructure And Properties Controlling And Corrosion Resistance Of Cargo Oil Tank D36

In recent years, major accidents of oil spill since oil tanks corrosion occur frequently. Oil tanks corrosion problems subject to increasing attention. Up to now, the IMO has completed performance standards of the cargo oil tanks (COT) corrosion-resistant steel and designed the test procedures. This standard as a sole solution equivalent alternatives the COT criterion standard and enforced. Japan has developed COT corrosion-resistant steel and tried in the large oil tankers and achieved initial success. However, our research in this area is still in the initial stage, our country urgently need to develop corrosion-resistant COT. This paper takes corrosion-resistant D36 steel for the cargo oil tanks as the research object and makes research on, microstructure and properties, rolling technology, corrosion resistance and corrosion mechanism. The main work and research results are involved as follows:(1) Transformation behavior of COT D36 steel by hot simulation experiments with different deformation parameters was researched. The continuous cooling transformation (CCT) diagrams were conducted by using thermal dilation method combined with microstructure observation. The results show that in continuous cooling conditions, with the cooling rate increasing, the microstructure gradually from polygonal ferrite, pearlite and a little bainite change to a small amount of acicular ferrite, granular bainite, finally change to bainite ferrite and martensite. The main parameters for rolling COT D36 steel determined by multi pass thermal simulation were as follows:finishing temperature 820?850?; finishing cooling temperature 600?650?/s, cooling rate 5?15?/s. In this range of process, a uniform different types of microstructure can be obtained.(2) The TMCP technics was applied to COT D36 steel in the laboratory rolling experiment. The influences of control rolling and control cooling process parameters on microstructure and properties of the tested steel was researched. The mechanical properties of Hot-rolled steel was tested with microstructure observation, tensile test and impact test. The results show that Good comprehensive mechanical properties of COT D36 steel can be obtain by controlling of the heat temperature in 1200?, two regions rolling, the finish rolling temperature in 810??830?, the final cooling temperature in 630?650?,and the cooling rate in 5?15?/s. Microstructure mainly are ferrite & granular bainite and ferrite & pearlite.(3) Through corrosion simulation of COT bottom plate experiment, annual corrosion rate of the hot-rolled steel for COT D36 was measured. Corrosion production morphology, phase composition and corrosion mechanisms was observed and analyzed. The results show that: Whit this alloy composition and rolling process the tested steel have good corrosion resistance. The annual corrosion rate is 0.21-0.26 mm/a, far less than the national standard of lmm/a; The surface of tested steel does not appear obvious pitting, and has a good pitting resistance. The corrosion products is ?-FeOOH, Fe3O4 and good stability ?-FeOOH; Inclusions in steel is mainly MnS, Al2O3 and (Mn, Ca)S composite inclusions. These inclusions are the sensitive parts of the pitting corrosion and have an adverse effect to corrosion resistance; Increasing the proportion of small-angle grain boundaries and reducing proportion of high angle grain boundaries help to improve corrosion resistance of the tested COT D36 steel.(4) Through electrochemical polarization curves measurement experiment and electrochemical impedance spectroscopy, the polarization curves and the Nyquist diagram and Bode diagram of electrochemical impedance spectroscopy were plotted. The equivalent circuits in corrosion process were proposed. The results show that:With Cl- concentration increases, the ability of Cl- to erosion the passive film enhance, the corrosion rate is accelerated; Adding Cr, Cu, W and other alloying elements improve corrosion potential of COT D36 steel, which will help formtion of surface passivation film production, so less prone to corrosion; uniform grain structure has better corrosion resistance than uneven grain structure. Compared with the bare steel, samples by simulating under floor corrosion test form corrosion product film which make the resistance value increases, and the resistance of the charge through corrosion product film increases. But as corrosion time increasing, localized corrosion is worsened, which make the corrosion product film damaged, the resistance of the charge through corrosion product film decreases, and the resistance value declines.