| Nowadays,in order to adapt to the sustainable exploitation of offshore oil and gas resources,high quality steels for offshore platform are great demanded,however,there are obvious shortcomings in the current domestic offshore platform steel using the existing quenched and tempered steel,such as high production cost and poor low-temperature toughness.In order to overcome this shortcoming,now we intend to reduce costs by substituting "Ni" with "Mn".Through the design of the composition with low-C medium-Mn,Complex microstructure of tempered martensite and reversed austenite is obtained,then obtained a high strength and high toughness offshore platform steel.Because the sea water is strong electrolyte solution,it can cause strong electrochemical corrosion to the steel plate then cause corrosion failure event.Therefore,it is indispensable to make a research on corrosion behavior of medium-Mn offshore platform steel.The study on corrosion behavior of medium-Mn steel can provide technical parameters for the industrial production of medium-Mn offshore platform steel so as to obtain excellent comprehensive performance medium-Mn steel.This paper has studied microstructure,mechanical properties and corrosion behavior in different marine environment of medium-Mn offshore platform steel.The main research contents of this paper are as follows:(1)The critical phase transformation point,static CCT and dynamic CCT curve are mensurated by Formastor-FII dilatometer and MMS-300 multifunctional thermal simulation testing machine.The results show that Ac1 and Ac3 temperature of medium-Mn steel decrease significantly,austenite region is expended.In the process of continuous cooling transformation of austenite,it only has martensitic transformation and the microstructure is not sensitive to the change of cooling rate.(2)The results of rolling and heat treatment experiment research of medium-Mn steel shows that tempered martensite and reversed austenite can be obtained when online quenching after rolling and tempering in the intercritical region.Different tempering process research results show that the impact toughness of test steel is improved with the increase of tempering temperature and tempering time but strength decreased.Optimum comprehensive mechanical properties can be obtained when second initial rolling temperature is 960 ?,finishing rolling temperature is 900 ? and then tempering at 650? for 30min.The yield strength of experimental steel is much higher than 690MPa and longitudinal impact energy at-60 ? is more than 200J.(3)The corrosion effects of medium-Mn steels in marine splash zone is simulated by drying-wetting cycles corrosion test.Corrosion behavior and corrosion mechanism of experimental steel are analysed by Scanning electron microscope(SEM),Electron probe micro-analyzer(EPMA)and X-ray diffractometer(XRD).Results show that:The change of corrosion rate of low-C medium-Mn steel exhibits three distinguishable stages.The corrosion rate rises quickly at the beginning,then decreases subsequently and tends to be stable finally.Corrosion products are gradually transformed into a dense and thick corrosion rust from the loose and porous morphology with the extension of the corrosion time,and the protective ability of rust layer is also gradually enhanced.In the environment of high concentration of chloride ion,chloride ions promote the formation of y-FeOOH.The corrosion products contain manganese oxide and iron manganese oxide,these manganese compounds can increase defect density in the rust layer and deteriorate the protective ability of rust layer.C steel shows batter corrosion resistance among three kinds of experimental steels.(4)Static immersion corrosion test has been carried out to simulate C steel corrosion effects in seawater full immersion zone.Results show that the change of corrosion rate exhibits three distinguishable stages with the extension of the corrosion time.In the first stage,corrosion rate is high and then decreases rapidly.The corrosion rate increases slightly in the second stage and decreases subsequently and tends to be stable in the last stage.The corrosion products are mainly composed of Fe3O4 and FeCr2O4.FeCr2O4 is more compact and stable,it can hinder the mutual diffusion of oxygen and metal ions,inhibit electrochemical reaction process and retard the degree of corrosion. |
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