Microstructure Control And Dynamic Mechanical Properties Of Medium-carbon Micro/nano-structured Bainitic Steels

There is a continuing demand for advanced steels with both high strength and good ductility to ensure service safety by reducing the weight of steel components and to improve fuel efficiency by reducing the steel amount used on components.In recent years,a high-carbon micro/nano-structured bainitic steel composed of very fine bainitic ferrite laths and thin carbon-enriched retained austenite films between the laths attract much people's attention because of its high strength,high plasticity and toughness.However,high carbon content leads to much prolonged bainitic transformation duration and poor molding,which restricts significantly the industrial application of such steel.Therefore,one medium-carbon micro/nano-structured bainitic steel and proper heat treatment processes were designed in this work.Optical microscopy,scanning electron microscopy,electron back-scattering diffraction,transmissionelectron microscopy,X-ray diffraction,atom probe tomography and electron probe micro-analysis were employed to study the effect of composition design,heat treatment processes and strain rates on mechanical properties and microstructures.The main achievements are expressed below.?1?The effect of different contents of Mn and Si on the tempering process of medium-carbon bainitic steel were investigated.Adding high Mn and Si element can improve the volume fraction of residual austenite and the strength and hardness of microstructure.With the increasing of tempering temperature,the yield strength,tensile strength and low-temperature toughness all increased first and then decreased,and the elongation gradually increased.High Mn and High Si steels have the best strength and toughness after 300°C tempering.The main reason for the temper brittleness of low Si steel is the large number of large and complex strip/needle carbides.However,the dispersion precipitates carbides of M₆C,M₃C and MC after tempering at 300°C for medium and high Si steels can improve the strength and toughness.In addition,the TRIP effect occurred by the residual austenite also can improve the strength and toughness.?2?The effect of holding time and austempering temperatures on microstructure and mechanical properties of medium-carbon bainitic steel was investigated.The microstructure obtained by long time austempering mainly includes bainite ferrite+film residual austenite.With increasing in the holding time,the amount of retained austenite and carbon content increases at first and then keeps unchanged,the tensile strength tends to decrease and the total elongation tends to increase.With increasing the austempering temperature,the size of block retained austenite,and the thicknesses of bainitic ferrite laths all exhibit increasing tendency and accordingly,tensile strength decreases and the total elongation increases.?3?The effect of magnetic field on the austempering process of medium-carbon bainitic steel was investigated.Compared with the traditional heat treatment,the application of 12 Tesla magnetic field increased the volume fraction of bainite,refined the bainite structure and reduced the hardness of each phase.This is because the magnetic field can reduce the free energy of bainite phase transformation and increase the phase transformation driving force.In addition,the magnetic field can promote carbon diffusion into residual austenite.?4?The effects of different partitioning time on the microstructures and mechanical properties of the Q&P steels were investigated.The results show that experimental steel gets the best mechanical properties in the partition time 60 s after Q&P processing,tensile strength is 1460 MPa,and strong plastic product reaches the maximum 22.78 GPa%.With the increase of the partition time,tensile strength decrease gradually and elongation increase gradually.However,For the Q&T steel,the tensile strength is 1496 MPa,elongation is 6.9%and strong plastic product is 10.3GPa%,which are lower than the Q&P steel.?5?The dynamic and quasi-static mechanical properties of tempered steel,hot rolled steel,Q&P steel and Q&T steel at various strain rates were measured.With increasing strain rate,the tensile strength and ductility of steels are both enhanced and the volume fraction of retained austenite decrease gradually,which prove the existence of TRIP effect.The strain sensitivity of Q&P martensitic steels is better than that of tempered and hot rolled bainite steels.?6?The effects of different strain rate on fracture morphology and microstructure of tempered steel,hot rolled steel,Q&P steel and Q&T steel were measured.Large and deep dimples appeared in the tensile fracture center of tempered steel under high strain rate.The dimples were elongated and the elongation amplitude was significantly greater than that of the quasi-static process.It has cracks in the tensile fracture center of Q&P steel under the high strain rate.No dimples were found under high strain for Q&T steel.The bainitic laths showed obvious deformation for tempered steel under high strain rate and accompanied a large number of dislocations,which proved the best strength and plasticity of tempered steel under high strain rate.