| Steel materials are undoubtedly the most widely used metal materials in the world.China is a big producer and consumer of steel,and steel materials are closely related to our national economy.In the fast-developing modern society,with the rapid development of automobile industry,machinery manufacturing and other industries,people have put forward higher requirements for the process and performance of steel materials.Traditional steel can not meet the growing demand for higher strength,toughness and other comprehensive mechanical properties.The goal of lightweight has also brought us new challenges.To enhance the comprehensive mechanical properties of steel,traditional methods include adding other metal elements into the steel matrix for alloying and composition optimization,developing new heat treatment process and new manufacturing process to improve the strength and toughness of steel.The above methods can significantly enhance the performance of steel,but these methods have met the bottleneck in improving the strength and toughness of traditional steel,and it is difficult to further improve the strength and toughness of steel significantly.Therefore,the urgent need for lightweight,high strength and high toughness steel requires the development of a new,more convenient,fast,flexible and practical,low-cost method to significantly improve the mechanical properties of steel.In this paper,the method of combining master alloy with traditional casting is creatively put forward,and nano-ceramic particle reinforced steel is successfully produced,which significantly enhances the strength,toughness and other mechanical properties of steel.It solves the problems of difficult addition,poor dispersion of nano-ceramic particles and poor bonding with steel matrix materials,and has the advantages of simple process,low cost,low environmental requirements,no need to change the existing equipment,unlimited size and shape of products,and can be combined with traditional steel strengthening methods and synergistic strengthening,obvious strength effect.This study provides a new way and experimental and theoretical basis for strengthening steel materials.The effects of nanoceramic particles on the solidification and heat treatment structure of low alloy steel and the strengthening and toughening of mechanical properties of steel at room temperature and high temperature were studied.The mechanism of nano-ceramic particles manipulating the structure and strengthening and toughening of low alloy steel was analyzed.Four main innovations are as follows:1.The effects of nano-ceramic particles on the solidification structure of low alloy steel and its control mechanism were revealed as follows:(1)Single-phase nano-Ti C particles can refine and homogenize the as-cast structure of 45# steel and 40 Cr steel.The ferrite structure in the tissues is increased,and the long ferrite is refined into granular and homogenized.The pearlite was refined and the pearlite clusters disappeared.(2)Dual-phase nanoparticles could also refine the as-cast structure of 40 Cr,and the ferrite structure increased,refined and homogenized.Pearlite clusters are dispersed and small pearlite particles are mixed with granular ferrite evenly;(3)ferrite structure in 40 Cr steel reinforced by single-phase Ti C nanoparticles is finer and more uniform than that reinforced by dual-phase nanoparticles,and pearlite clusters are smaller;(4)During solidification,the number of nanoparticles is huge,and some nano-Ti C particles as heterogeneous core increase nucleation rate.The remaining nanoparticles are adsorbed at the front of the solid-liquid interface,which hinders the progress of the solid-liquid interface and inhibits the growth of austenite dendrites.The bigger Ti B2 particles in the biphase nanoparticles take precedence over the smaller Ti C particles as nucleation core,which improves the nucleation rate of austenite.The smaller Ti C particles mainly adsorb on the solid-liquid front,inhibit the growth of austenite dendrites and refine the primary austenite.Because of the refinement of primary austenite,pearlite and ferrite in as-cast steel are refined.2.The effects of nano-ceramic particles on the heat treatment structure of low alloy steel and its control mechanism are revealed as follows:(1)single-phase nano-Ti C particles refine the structure of 45# steel after heat treatment,and the grain size is greatly refined,which significantly reduces the thickness of pearlite layer;bainite structure of 40 Cr steel is significantly refined;(2)dual-phase Ti C+Ti B2 nano ceramic particles refine the structure of 40 Cr steel after heat treatment and reduce the grain size;The martensite lath increased significantly,and a large number of carbide nanoparticles were dispersed in the ferrite matrix.(3)The effect of dual-phase nanoparticles on the refinement of 40 Cr steel was not as significant as that of singlephase Ti C particles,and the content of bainite and martensite in the steel reinforced by dual-phase nanoparticles was more than that of single-phase Ti C particles.(4)The nano-ceramic particles significantly refined the as-cast structure of steel and obtained more during austenitizing.Fine austenite grain,after solid phase transformation,finer and uniform sorbite(45# steel)and bainite structure(40Cr steel)can be obtained.Dual-phase and single-phase nanoparticles have different effects on the solid-state transformation process of steel.Ti B2 with hexagonal prism and larger size tends to induce the appearance of coarse martensite and bainite,which improves the hardenability of steel.3.The strengthening and toughening regularity and mechanism of nano-ceramic particles on mechanical properties of low alloy steel at room temperature are revealed:(1)It is found that the strength,plasticity and impact toughness of steel can be significantly improved by adding single-phase nano-Ti C particles.The yield strength,tensile strength and fracture strain of 45# steel were increased by 19.3%,13.0% and 8.8% respectively by 0.054 wt.% nano-Ti C particles,and the yield strength,tensile strength and fracture strain of 40 Cr steel were increased by 19.7%,30.4% and 15.6%,respectively.0.018 wt.% nano-Ti C particles increased the impact toughness of 45# steel by 46.8% and 40 Cr by 43.4%.(2)It is found that the strength of steel can be significantly increased but the plasticity can be reduced by adding biphasic nanoparticles.The yield strength and tensile strength of 40 Cr steel increased by 31.0% and 38.8% respectively by 0.054 wt.%.The fracture strain and impact toughness of 40 Cr are reduced by high content of biphasic nanoparticles.(3)Single-phase Ti C nanoparticles can improve the yield strength,tensile strength,elongation and impact properties of steel at the same time,while dual-phase nanoparticles can significantly increase the strength of steel but reduce the elongation and impact toughness of steel;(4)The main mechanisms of single-phase Ti C nanoparticles strengthening and toughening steel are fine grain strengthening,Aurowan strengthening and nanoparticles pinning grain boundary dispersion stress.Fine grain strengthening: Fine grains and pearlite lamellae hinder dislocations and enhance the interaction between dislocations and dislocations and grain boundaries.Aurowan hardening: Fine and uniformly distributed nano-hardening phase hinders dislocation movement,generates dislocation multiplication and dislocation accumulation,which increases material deformation resistance and strengthens material.Toughening mechanism: Ti C nanoparticles on fine grain boundaries strengthen pinning grain boundaries and disperse stress.The main crack propagates along grain boundaries and encounters grain boundaries strengthened by nanoparticles.The cracks are scattered into many microcracks,which consume the energy of crack propagation,reduce stress concentration,increase the path of crack propagation and delay the propagation.Microvoids before fracture usually appear near grain boundaries and second-phase particles.Increased grain boundaries and nano-Ti C particles can increase the number of microvoids.The formation and growth of microvoids require more energy than cleavage fracture.Ti B2 is a hexagonal prism with many edges and corners,large size and easy to induce cracks.On the other hand,the lath martensite in 40 Cr steel containing biphasic nanoparticles increases,and the nano-carbide particles in the micro-structure also increase significantly,which greatly improves the strength of 40 Cr steel.The lath martensite with higher hardness is unfavorable to plasticity and toughness.4.The strengthening and toughening rules and mechanisms of nano-ceramic particles on the high temperature mechanical properties of 40 Cr steel were revealed:(1)It was found that single-phase nano-Ti C particles with different contents could significantly improve the high temperature tensile yield strength,tensile strength and fracture strain of steel at the same time;(2)Dual-phase nano-particles could significantly improve the high temperature tensile strength and plasticity of 40 Cr steel.(3)Dual-phase nanoparticles improve the tensile strength of 40 Cr steel at high temperature slightly less than single-phase particles,but increase the plasticity more;(4)Hightemperature strengthening mechanism: nanoparticles pin grain boundaries and hinder dislocation climbing,when single-phase nano-Ti C is finer and more uniform,the structure of 40 Cr steel is uniform,making the strength of single-phase Ti C-reinforced 40 Cr steel increase more obvious.At high temperature,phase transformation occurs in the structure,and martensite dissolution transforms into carbide and tempered martensite.Therefore,the enhancement effect of dual-phase nanoparticles is inferior to that of single-phase Ti C particles. |
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