Study Of Mechanism Of Rare-earth Elements La?Ce On Microstructure And Properties Of The Steel Of Offshore Platforms

To demand the harsh conditions of service in the marine environment,the steel of offshore platforms demand not only high toughness but also excellent corrosion resistance.The trace rare earth addition is an effective way to obtain these comprehensive properties.In the industrialized production of steel,in order to avoid the phenomenon of nodulation in continuous casting process,micro rare earth elements can only be added in steel.So,its performance-optimization is remarkable,and the action mechanism needs to be improved.In this experiment,rare earth lanthanum and cerium?less than 30ppm?are selected as added elements.For steel plate of the high strength ocean platform,the effects of trace rare earth elements on the microstructure?inclusions?the mechanism of corrosion resistance and mechanical properties in steel were studied systematically.The existence form of trace rare earth elements in steel was analyzed by non-aqueous electrolyte at low temperature.By the experiment of differential scanning calorimeter?DSC meter?and the thermal dilatometer experiment analyze the influence of trace elements on the phase transformationtem point and continuous cooling transformation curve?CCT?of the high strength ocean platform.Through the“Jominy test”,the influence rule of trace rare earth elements on the steel hardenability was analyzed.Through different heat treatment process tests,such as normalizing?quenching and tempering,the influence of trace rare earth elements on the mechanical properties of steel under different process state wasstudied.The effect of trace rare earth elements on seawater corrosion resistance of high strength ocean platform was studied by means of periods of immersion corrosion and electrochemical test.The results show:?1?When heating rate is 10?/min,trace lanthanum and cerium increases the Ac₃ point about 20?.At different cooling rate,it make the CCT curve significantly shift to the right.?2?Mechanical properties of experimental steel through different heat treatments were investigatied.The results show that tensile strength by hardening and tempering process improve about 30MPa by adding a trace of lanthanum and cerium.At-40?,the effect of trace lanthanum and cerium impact toughness is that the impact energy can be enhanced about 50J,but normalizing reduce about 30J.Trace lanthanum and cerium makes the phosphorus more solid solution and purified the crystal boundary to enhances the strength and improve the impact toughness.?3?Trace lanthanum and cerium has no obvious effect on the structure of steel.It can form composite inclusion with titanium and niobium,the appearance of inclusions is globoid.?4?This work investigated the trace lanthanum and cerium effect on immersion corrosion test model accelerately in room by GB/T10124-88.The effect of trace lanthanum and cerium on corrosion resistance of steel in simulated seawater was studied.The surface corrosion products of the samples were observed and the samples analyze the rust layer by XRD,respectively?—FeOOH?a—FeOOH?Fe₂O₃?Fe₃O₄,the rust of?—FeOOH of the specimen A without rare earth lanthanum cerium is higher than the specimen B and C.The corrosion rate was obtained by the method of weight loss,and the corrosion kinetics of the experimental steel was analyzed.With increasing corrosion time,the corrosion weight loss and corrosion rate of the specimen A is minimal.The dynamic potential polarization curves and electrochemical impedance spectra of rust samples were tested by electrochemical workstation,and the electrochemical behavior and regularity of the experimental steel were analyzed.The experimental results show that the corrosion rate of the steel with lanthanu-m and cerium increases with the corrosion time and its self-corrosion current densit-y was not decreased with the corrosion time and the impedance of the rust layer is reduced.