Effect Of Nanoparticles On Microstructure And Properties Of Q440 Steel

Iron and steel materials are the cornerstone of the development of modern industrialization.Iron and steel materials with better comprehensive mechanical properties and longer service life are more needed by the society.In order to improve the comprehensive mechanical properties of iron and steel materials,ultrafine nanoparticles can be used as an external reinforcement phase.This method is an innovative method for strengthening iron and steel materials.In this thesis,nanoparticles reinforced Q440 steel was prepared by high energy ball milling predispersing nanoparticles with different contents to prepare prefabricated blocks and intermediate alloys as carriers of nanoparticles into the vacuum high frequency induction melting furnace and remelting Q440 steel.The effects of addition of single-phase Zr O₂ nanoparticles,biphase Zr O₂+Al₂O₃nanoparticles and biphase Ti C+Ti B₂ nanoparticles with different contents on microstructure,hardness,tensile properties and impact properties of Q440 steel were studied by metallographic microstructure observation,hardness,tensile and impact tests.The tensile and impact fracture morphology and fracture mechanism were observed by scanning electron microscopy.The results show that the microstructure of Q440 steel can be improved obviously by adding different kinds and contents of nanoparticles.The matrix structure of Q440 steel is composed of unevenly distributed ferrite and pearlite with a small amount of carbide precipitation.Adding a certain amount of nanoparticles can refine uniform microstructure,increase pearlite content and reduce carbide precipitation.The grain size decreases first and then increases with the increase of the addition amount of nanoparticles.When the addition amount of Zr O₂ nanoparticles and Zr O₂+Al₂O₃ nanoparticles is 0.7wt.%and Ti C+Ti B₂ nanoparticles is 0.03wt.%,the ferrite grain refinement effect is the most significant,the grain size is the largest and the microstructure is the most uniform.The phenomenon of carbide precipitation is obviously improved.The mechanical properties of Q440 steel can be improved to some extent by adding different kinds and contents of nanoparticles.The hardness,yield strength,tensile strength,elongation and impact absorption work of Q440 steel firstly increased and then decreased with the increase of the amount of nanoparticles added.When the addition of Zr O₂ nanoparticles is 0.7wt.%,Q440 steel has the best hardness,tensile properties and impact properties,and the average Vickers hardness increases from225HV to 265HV.The yield strength,tensile strength and elongation were614.34MPa,785.55MPa and 24.9%,respectively.The impact absorbed energy reached 18.6J.When the addition of Zr O₂+Al₂O₃ nanoparticles is 0.7wt.%,Q440 steel has the best hardness,tensile properties and impact properties,and the average Vickers hardness increases to 273HV.The yield strength,tensile strength and elongation were 645.06MPa,824.29MPa,26.1%,respectively.The impact absorbed energy can reach 19.7J.By comparing the experimental data,the addition of bi-phase Zr O₂+Al₂O₃ nanoparticles has more obvious improvement effect on the comprehensive mechanical properties of Q440 steel.When the addition of Ti C+Ti B₂nanoparticles is 0.03wt.%,Q440 steel has the best hardness,tensile properties and impact properties,and the average Vickers hardness increases to 264HV.The yield strength,tensile strength and elongation were 638.11MPa,804.9MPa and 24.6%,respectively.The impact absorbed energy increased to 20.1J.The morphology of tensile fracture and impact fracture changes gradually from brittle fracture to ductile fracture with the increase of nanoparticle content.