| Hot stamping of ultra-high strength steel has so many advantages as a simplified process,sheet metal deformation resistance reducing,the parts precision and strength significantly increased and so on that the new process is becoming research and application hot spots of industry especially automobile industry.All in one process involves more complicated principles of thermodynamics,mechanics and material science.Research on the behavior of austenite grain refinement and martensite phase transformation strengthening of ultra-high strength steel can further improve the elastic plasticity deformation theory and martensite phase transformation theory with extreme conditions,which had a strong guiding significance for the fine control of the hot stamping process and mold design of ultra-high strength parts.Based on boron alloy cold rolled sheet(B1500HS)and hot rolled steel plate(BR1500HS)as the research object,on the basis of orthogonal experiment analysis,study of the characteristics of material forming and process properties is developed in this paper.Samples under different conditions by the expansion experiments,quenching experiments,hot stamping experiments and thermal simulation experiments,etc.would be done to achieve gain martensite with grain refinement,and the analytical methods of optical microscope(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),electron backscatter diffraction(EBSD)and X-ray diffraction(XRD)would be used to verify the ferrite grain refinement mechanism and the martensitic transformation strengthen behavior during ultra-high strength steel hot stamping process.The main researching results of this dissertation are in the following:(1)Research on hot stamping technological parametersinfluence on ultra-highstrengthsteel martensitic transformation.During ultra-high strength steel hot stamping process,the different processing parameters in hot stamping were interrelated,and their influence on the quality of hot-stamped parts depended on a number of factors.The hardness and strength of hot stamped parts increased significantly,when the initial forming temperature was above 800 ?,the hardness and strength of hot stamped parts with good liquidity and formability changed small.(2)Systematically analysisof the ultra-highstrengthsteelmicrostructure with different cooling speed,different deformation or different strain rate.When the cooling rate is under 30?/s,ferrite phase appears more easily than martensite phase because of deformation.When the cooling rate is higher than 30?/s,material organization is mainly martensite,with the increase of cooling rate,more finemartensite structure comes into being,and it is easy to induce martensitic phase transformation on account of deformation.(3)The fine-grain strengthening rule and the Hall-Patch relationship between micro hardness and grain size of B1500HS would be studied.It shows that the mechanical properties increased and the ferrite grain refined with deformation temperature decreased,cooling rate increased or strain rate increase and the H-P relationship has favorable applicability and dependability on analysis and calculation of process parameter.(4)The rheological properties of ultra-high strength steel on high temperature would be studied through the thermal simulation experiment,which showed thatthe lower the temperature of the sheet metal,the greater the stress,and the higher the cooling rate,the faster the sheet metal stress increase.when the strain rate is larger(greater than 0.05s-1),they have little influence on strain hardening index for factors such as deformation temperature,deformation velocity,cooling rate,and so on.(5)Research onmartensite phase transformation strengtheninginfluence on factors of alloy composition and deformation.In the process of phase change,of trace boron has an effect of solid solution strengthening that boron slant to the grain boundary caused a certain degree of coherent reinforcement,can reduce the distortion(grain boundary free energy)at the same time,extend the austenite transformation incubation period,so as to improve the hardenability of the ultra-high strength steel.Deformation process of austenitic grain fragmentation,promote transformation,dynamic recrystallization and help get more fine martensite structure,fine-grain strengthening effect,enhance mechanical properties of hot stamping parts. |
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