Effect Of Trace Ce On Inclusions,Microstructure And Corrosion Resistance Of EH36 Shipbuilding Steel

With the development of ship construction to the large,light,environmental protection and deep-sea in recent years,higher demand on shipbuilding steel with ultra-high strength,enhanced toughness,excellent welding performance and corrosion resistance has been put forward.High strength shipbuilding steel has been widely used in shipbuilding industry due to the normal strength of the steel has been unable to meet the requirements of the hull structure.Meanwhile,affected by the global financial crisis,the ship market has shrunk dramatically and shipbuilding steel market has been tough with the increasingly fierce homogeneous competition among steel production enterprises.Because of the steel product quality still lags far behind the advanced world standard,developing high strength and ultra high strength steel,improving the steel toughness,weldability and corrosion resistance can not only improve the economic efficiency of enterprises,reduce the pressure of homogeneous competition,but also meet the requirements of the national marine development strategy direction.In order to develop shipbuilding steel with low cost and high performance,EH36 shipbuilding steel which is widely used in shipbuilding industry is taken as the experimental object in this research.The effect of different content of Ce addition on the inclusions,microstructure and corrosion resistance of EH36 shipbuilding steel is investigated by SEM-EDS,OM and electrochemical workstation.Meanwhile,the evolution mechanisms of inclusions after Ce addition and the nucleation mechanism of intra-granular acicular ferrite which is induced by the Ce-containing inclusions are also explored.It would be contribute to provide technical support in practical application of rare earth elements through the experimental results and analysis.Based on the experiment results and analysis,the following conclusions are obtained:（1）Inclusions existing in EH36 shipbuilding steel are mainly Al₂O₃ and MnS in this experiment.The shape of most of the inclusions is irregular and the size of inclusions is larger than 5μm.The inclusions are transformed into Ce-containing complex inclusions after Ce addition.When the content of Ce element in steel is up to 0.026%,the inclusions are totally changed into Ce₂O₂ S,which are finest and distribute dispersedly.（2）With the increasing content of Ce,The percentage of inclusions less than 2μm firstly increased to 87% when the content of Ce element was up to 0.026%,However,when the content of Ce element was up to 0.03%,it shows an decreasing trend.（3）The quantity of intra-granular acicular ferrite increases and then decreases with increasing content of Ce element and the optimum content of Ce is about 0.026%.（4）The orrosion resistance of EH36 shipbuilding steel can be improved by reducing the corrosion potential and increasing the polarization resistance after Ce treatment.When the content of Ce element was up to 0.034% it shows the best corrosion resistance performance.（5）Based on the thermodynamic calculation,Ce₂O₂ S has the greatest negative Gibbs free energies of formation.Therefore,Ce₂O₂ S should be priority precipitated preferentially in present work.Model is established to study change mechanisms of inclusions.It is found that the diffusion of Ce³⁺ and Al³⁺ in both of un-transferred core and intermediate layer is the rate limited step.The misfit value between different Ce₂O₂ S and α-Fe is only 1.2%,thus,it can act as very effective nucleus of intra-granular ferrite to promote its formation.