Study On Mechanical Properties And Wear Resistance Of Microalloyed Ultra-high Strength Steel

Microalloyed ultra-high-strength steel has good strong plasticity and high wear resistance,and is widely used in military and civilian fields.As the performance requirements for microalloyed ultra-high-strength steels increase,more stringent requirements are put forward for the uniformity of mechanical properties and wear resistance.In this paper,through hardness test,quasi-static tensile test and high-speed tensile test,the mechanical properties and fracture mechanism of NM550 wear-resistant steel were studied.Optical microscopy(OM),scanning electron microscopy SEM,and Energy Dispersive Spectrometer(EDS)were used to observe the microstructure and fracture morphology of wear-resistant steel.X-ray diffraction(XRD)analyzes the structure of steel.Through the different heat treatment process treatments of NM450 wear-resistant steel,the mechanical performance test and microstructure observation were carried out,and the influence of different heat treatment processes on the microstructure,mechanical properties and wear resistance of the steel were compared and analyzed.In order to obtain ultra-high-strength steel plates with good mechanical properties and excellent wear resistance.The main results are as follows:(1)NM550 wear-resistant steel has excellent mechanical properties.There are large-sized micron TiN and TiC inclusions in steel.Micron TiN and TiC inclusions act as crack sources to initiate cleavage fracture during the tensile process of the experimental steel.The fracture process of micron TiN inclusions can be divided into4 steps:1)Micro-cracks are generated on TiN inclusions;2)Multiple cracks are initiated and widened;3)TiN inclusions are broken;4)TiN inclusions are further separated at the broken places to form holes.In contrast,micron TiC inclusions break in the steel and separate from the matrix to produce voids.As the stress increases,the matrix deforms,and the voids elongate to form gaps.In order to reduce the size of TiN and TiC,the precipitation thermodynamic and kinetic behaviors of TiN and TiC in the experimental steel were further studied.The results showed that the size of TiN nucleated and precipitated in the liquid phase and TiC precipitated in austenite are relatively large Coarse;the size of TiN and TiC is related to the cooling rate,Ti and N content,that is,the larger the cooling rate,the smaller the Ti and N content,the smaller the size.Therefore,increasing the cooling rate and reducing the content of Ti and N are effective means to reduce the size of TiN and TiC and improve material properties.This research lays the foundation for the application of NM550 steel in large construction machinery.(2)NM550 wear-resistant steel has different tensile strengths at different strain rates and shows higher strain sensitivity.Within 0.001 s^-1?100 s^-1,the tensile strength gradually increases with the increase of the strain rate,while the tensile strength decreases slightly when the strain rate is 500 s^-1.In the range of 0.001 s^-1?10 s^-1,the elongation gradually decreases,and the elongation is the lowest when the strain rate is10 s^-1,and the elongation is the highest when the strain rate is 100 s^-1,and the strain rate is 500 s^-1 when the elongation is slightly reduced.As the strain rate increases,the TiN on the fracture surface increases,and the chain-like TiN distributed in the steel easily forms a weak zone,causing quasi-cleavage fracture.(3)The microstructure of NM450 wear-resistant steel varies greatly under different heat treatment processes.As the tempering temperature increases,the spheroidization of the tissue is obvious.The quenched and tempered samples are lath martensite,the one-step and two-step heat treatment process samples are both martensite+retained austenite,and the two-step heat treatment samples have the most obvious spheroidization.And the higher the carbon distribution temperature,the higher the retained austenite content in the steel.Different heat treatments have a greater impact on the mechanical properties of steel.The hardness and strength of the quenched and tempered specimens are the highest,but the plasticity and elongation after fracture are the lowest;the strength of the specimens heat-treated by the one-step and two-step methods is lower,but the plasticity is significantly higher than that of the quenched and tempered specimens.Different heat treatments have a greater impact on wear resistance.The wear surface of the sample is mainly composed of furrows,micro-cutting and fatigue zones.The quenched and tempered sample has the best wear resistance.The hardness and strength are the main factors affecting the wear resistance.The higher the hardness and strength,the greater the wear resistance.it is good.