Microstructure Control,Strengthening And Toughening Mechanism Of Cu-Rich Nanoprecipitate-Strengthened Steel

Precipitation strengthening is one of the most important strengthening methods in steel.Cu-rich nanoprecipitate-strengthened steel replaces the solid solution strengthening of C element with precipitation strengthening of Cu element,achieving high strength,good ductility and weldability.It is widely used in key structures such as oil pipelines,engineering machinery,ships,marine engineering and bridges.The structure,size and number density of Cu-rich nanoprecipitates have significant impact on the strengthening.Therefore,it is of great significance to investigate the evolution of Cu-rich nanoprecipitates for improving the precipitation strengthening.At the same time,the optimal aging treatment temperature for Cu-rich nanoprecipitate-strengthened steel is between 450 and 600℃,which is in the temperature range of temper embrittlement.In order to solve the above problems,this paper takes Cu-rich nanoprecipitate-strengthened steel as the research object,and systematically studies the evolution of matrix phase and Cu-rich nanoprecipitates under different heat treatment by means of electron backscatter diffraction（EBSD）,transmission electron microscope（TEM）,three-dimensional atom probe（APT）and other characterization methods,and clarifies the change of structure,size and quantity density of Cu-rich nanoprecipitates during aging,and combined with the changes in mechanical properties under different microstructures,the strengthening mechanism of Cu-rich nanoprecipitate-strengthened steel in different states was studied.Through an overall analysis of the evolution of microstructure and changes in mechanical properties,the toughening mechanism of Cu-rich nanoprecipitate-strengthened steel under different heat treatment states was elucidated,the main factors affecting low-temperature toughness were revealed,and solutions were proposed,providing a new strengthening and toughening approach for the development of high-strength and high toughness Cu-rich nanoprecipitate-strengthened steel.The main research findings are as follows:The effects of solid solution and aging treatment on the microstructure of the matrix phase were investigated.The matrix structure of Cu-rich nanoprecipitate-strengthened steel quenched after solution treatment above A_C3 temperature is lath martensite and a small amount of quasi polygonal ferrite.With the increase of solution treatment temperature,the ferrite content after quenching increases,and the yield strength and tensile strength decrease.Solution treatment below 940℃has little effect on grain size and impact toughness.After exceeding 940℃,grain size will significantly grow,and impact toughness will rapidly deteriorate.The grain size of Cu-rich nanoprecipitate-strengthened steel will grow slightly after aging treatment at different temperatures,and the matrix structure still maintains the quenched lath martensite structure and a small amount of quasi polygonal ferrite.Some martensite laths are polygonized after aging at550℃or above for a long time.After aging treatment,the dislocation density decreases,and the number of low angle grain boundaries is significantly reduced compared to the solid solution state.Aging treatment has no effect on the content of high angle grain boundaries.The changes in size,quantity density,structure and morphology of Cu-rich nanoprecipitates during aging treatment were revealed.All elements in the solid solution sample are evenly distributed,and there is no precipitation of Cu-rich nanoprecipitates.In the early stage of aging treatment,high-density Cu-rich nanoprecipitates precipitate in the matrix.These precipitates are small in size and dispersed in the matrix,forming a coherent relationship with the matrix.They are mainly spherical precipitates with a core-shell structure,with a BCC copper rich core at the center and a B2-ordered Ni Mn shell at the surface.As the aging time prolongs,the Cu-rich nanoprecipitates precipitate undergoes coarsening,and the quantity density gradually decreases.When the radius is greater than 5nm,it transforms into an FCC structure,and the morphology gradually changes from spherical to rod-shaped and disc-shaped precipitates.The strengthening mechanism of Cu-rich nanoprecipitate-strengthened steel in different states was studied.The main strengthening mechanisms of solid solution samples are grain boundary strengthening,dislocation strengthening and solid solution strengthening.After aging treatment,the increment of dislocation strengthening and solid solution strengthening decreased,while the effect of precipitation strengthening increased.When the radius of Cu-rich nanoprecipitates is less than 5nm,the shear mechanism plays a dominant role,and the ordered strengthening and modulus strengthening in the shear mechanism can provide strength increments of 293MPa and 161MPa,respectively.When the radius is greater than 5nm,it is mainly dominated by the bypass mechanism,which can provide a strength increase of 164MPa through the Orowan mechanism.Precipitation strengthening has a significant effect on the improvement of yield strength,but a small effect on the improvement of tensile strength.After aging treatment,the yield strength ratio generally increases.The introduction of movable dislocations through micro cold rolling can significantly reduce the yield strength ratio.The toughening mechanisms of Cu-rich nanoprecipitate-strengthened steel in different states were clarified.The crack propagation energy is the main factor affecting the ductile brittle transition temperature.Fine Lath martensite structure can improve the crack resistance at the initial stage of crack propagation and delay the unstable propagation.At the beginning of aging,Mn first segregated at the lath boundary,resulting in a decrease in impact toughness.With the aging time increasing,Mo segregates at the lath boundary and the impact toughness increased.The segregation of Mn at the martensite lath boundary reduces the binding force at the lath boundary,while the segregation of Mo at the lath boundary can significantly improve the binding force.The precipitation of carbides containing Mo precipitated after continuous aging treatment,resulting in the decrease of Mo content at the lath boundary.At the same time,the content of Mn and Ni segregating on the lath boundary increased,leading to the toughness decline again.Mn and Ni segregating at the grain boundary after long-term aging treatment can reduce the local A_C1 temperature,resulting in the formation of film like reverted austenite at the lath boundary.A small amount of reversed austenite can significantly improve impact toughness and lower the ductile brittle transition temperature below-196℃.