Thermodynamic calculation and experimental analysis on effects of rare earth Ce on TiN phase precipitation in 20CrMnTi steel
The size and morphology of TiN inclusions have an effect on the quality and service performance of gear steel 20CrMnTi.Controlling the size and morphology of TiN precipitation is the key to improving the service stabil-ity of 20CrMnTi.The nucleation behaviors of TiN inclusions in 20CrMnTi before and after the addition of rare earth Ce element was studied by the calculations and experiments.The morphologies,types and sizes of the inclusions were characterized and analyzed statistically.The behavior mechanisms of rare earth Ce on the modifications of TiN inclusions in 20CrMnTi were studied.The results show that TiN precipitation temperature calculated by thermody-namics is 1 744 K.TiN inclusions cannot precipitate in the molten steel at 1 873 K.TiN can only precipitate and grow during the solidification of the steel.After the additions of rare earth Ce,a composite phase enriched with Ti,O,Ce,Al and N elements will be formed in the steel.The composite phase is angular contracted,and the shape is spherical or ellipsoid,smaller than TiN and Al2O3-TiN inclusions.Through phase identification analysis,it can be seen that the composite phase is CeAlO3 produced by the reaction of rare earth Ce and Al2O3 in steel,which can be used as the nucleation cores of TiN inclusions to form the ellipsoidal CeAlO3-TiN,and then achieve the purpose of modifying titanium-containing inclusions.After the addition of Ce element,the total areas of TiN-containing inclu-sions in steel does not decrease,but a smaller CeAlO3-TiN is formed.Due to the formations of the smaller compos-ite inclusions of CeAlO3-TiN,the number of inclusions with a size larger than 5 μm reduces by 17.2%,and refines TiN containing inclusions in steel.The addition of Ce effectively reduces the size of TiN inclusions in steel,and achieves the effect of modifying TiN in terms of morphology and size.
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