| With the development of marine engineering equipments,the safety requirements of marine engineering equipment are increasing day by day,especially the comprehensive performance of high-strength steel,among which,low-temperature toughness and yield ratio are the main assessment indicators.Copper-rich nano-precipitate strengthened steel is a type of high-strength low-alloy steel,which combines nano-precipitate strengthening and traditional strengthening methods to improve the strength while maintain high plasticity.Formulating reasonable thermomechanical treatment process is very crucial for the microstructure and mechanical properties of the copper-rich nano-precipitate strengthened steel.This paper regards TMCP copper-rich nano-precipitate strengthened steel as the studying material,invesigating the effects of thermomechanical treatment process on the microstructure and mechanical properties,especially the yield ratio.First,the microstructure of TMCP copper-rich nano-precipitate strengthened steel was studied.Research shows that the microstructure mainly composes of a large number of the lath martensites and a small part of polygonal ferrites after TMCP treatment.There is a clear band structure,which is mainly caused by the uneven deformation of the thick plate during the rolling process.A significant stress concentration will occur in the region with large deformation,causing the deflection of grains and the change of the orientation of the grains.At the same time,a large amount of dislocation plug-in will generate in the process of plastic deformation,promoting the subsequent recrystallization process,finally forming fine equiaxed crystals(small grain bands).The small grain band is more susceptible to corrosion during the corrosion process,appearing as a band structure in the metallographic observation.The study of inclusions in the copper-rich nano-precipitate strengthened steel shows that the types of non-metallic inclusions in the rolled samples are mainly bulk oxides,spherical composite inclusions and elongated silicon carbides.The oxides are mainly composite inclusions composed of alumina and calcium oxides,which are deoxidation products in the molten steel smelting process.Spherical composite inclusions consist of sulfide-coated oxides.The long-shaped silicon carbide is the residual product of the deoxidizer which has the characteristics of high melting point and high hardness,so no obvious change occurs during the rolling process.In the subsequent solution treatment,the size,composition,and appearance of oxide and silicon carbide inclusions don't change significantly.Spherical inclusions are partially composed of sulfide-coated oxides.Because the spherical inclusions are partly wrapped by sulfide,some of the sulfide dissolved into the matrix during solution treatment,resulting in the reduction of the size of the spherical inclusions.The study of the effect of aging treatment on the microstructure and yield ratio of TMCP copper-rich nano-precipitate strengthened steel shows that after aging treatment in the temperature range of 525~oC-700~oC for 0.5h-50h,the matrix structure does not change significantly,which is still martensite primarily.At the same time,there is no obvious difference of the microstructure between the specimen surface and the heart.With the increase of the aging temperature and the extension of the aging time,the lath among the lath martensite begins to decompose.When the aging temperature is low,due to the precipitation strengthening effect of nano-precipitates,the yield strength is significantly improved compared with the TMCP state sample,while the yield ratio decreases with the increase of the aging temperature.In the treatments with different aging times of 525 ~oC and 575 ~oC,the yield ratio maintains above 0.95,and the extension of the aging time has no obvious effect on the yield ratio.After aging at 690 ~oC for 1.5 h,the yield strength ratio will be significantly reduced.The decrease of yield ratio after aging at 690 ~oC is due to over-aging leading to the growth of the nano-precipitates.On the other hand,it is due to the formation of small amount of reversed austenite during the high-temperature aging process.The behavior that phase change induces plastic deformation may occur in the later stage of reverse austenite deformation,resulting in the increase of rheological stress and tensile strength and the decrease of yield strength ratio. |
PDF Full Text Request