Study On High Temperature Flow Behavior And Quenching Process Of Hot Stamping Steel BR1500HS

With the advantages on improving automobile collision safety performance andweight reduction, Ultra high strength steel became the ideal material of auto body’s vitalstructures. But the difficulty of plastic deformation caused by its ultra hight strength,makes it restricted in automotive applications. As a totally new technology, hot forminghas perfectly solved the problem of deformation difficulties at usual temperature. Hotforming combined the traditional stamping and heat treatment technology into oneprocess, not only achieve the purpose of weight reduction, but also improve collisionsafety performance, and became the hot research topic in automotive industrynowadays.This paper studies the hot stamping used ultra high strength steel BR1500HS. Bydoing physical experiments of researches of high temperature mechanical behavior andquenching process of BR1500HS, some referrible application conclusions of aautomotive part’s hot forming practical production has been obtained at last.Firstly, by high temperature tensile test which had simulated thethermal-mechanical process, it obtained the true stress-strain curves with thetemperature600℃,700℃,800℃and strain rates1s-1,0.1s-1,0.01s-1respectively. Byanalyzing these curves, to know the relationships between stress, strain, deformationtemperature and strain rate: flow stress increases rapidly as strain increased. Tensilestrength of BR1500HS increases as temperature decreased and increases as strain rateincreased, the sensitive of strain rate’s effect is less than temperature’s effect. Dynamicrecovery of the material was happened only above700℃, the caused soften effect canpartially counteracts the strain hardening effect, makes the slope of true stress-straincurves lower and the end of curves stabilized.With the results of high temperature tensile test, BR1500HS’s mathematical modelof high temperature flow stress based on Norton-Hoff model has been established. Bycontrasting the curves drawing by the theoretical model and experimental curves, itshows that the calculated values and the experimental values can consistent well expectthe situations of873K(600℃),strain rate at0.1s-1and0.01s-1. That means that theestablished material model can truly describe the high temperature flow stress.In order to get the process parameters be able to guide the hot forming practicalprocess, it designed and implemented a quenching orthogonal experiment with two factors and four levels. After the tests of mechanical properties and microstructureobservation of samples with different heat treatment conditions, it is found that:BR1500HS’s austenitizing temperature is above850℃. When heating material above850℃, it must be experience heat preservation for some necessary time to obtainmicrostructure of full and uniform austenite. But under higher heating temperatureincreases and longer holding time, it results coarse grains in material and decrease ofmechanical properties.With comprehensive consideration of material’s mechanical properties afterquenching and production efficiency, BR1500HS’s best quenching process parameterswere determined as: heating temperature of900~950℃, holding time of4~8min.Finally, the conclusions of quenching experiment was used in hot forming trialprocess of a lower reinforcement of upright post. By some necessary mechanical properties testsand microstructure observation, the testing results showed: strength, hardness and microstructure ofthe part after hot forming has achieved the quality requirements. So it can deduced that the trialprogram and used process parameters were reasonable and applicable.