Abstract
The mechanical properties of hot stamped boron steel have been generally predicted by many empirical models based on the Kirkaldy's equation. Although predicting previous models are well-developed for prediction of mechanical properties, use of those models is still limitary in case of partial quenching process for tailored properties of component. Therefore, this study predicted the mechanical property of partially quenched boron steel by heated tools using the FE-simulation coupled with quench factor analysis (QFA). The dilatometry test of boron steel is performed at various ranges of cooling rates from 0.5 to 70℃ /s of which results are used to determine the material constants (K1~K8) of QFA and the time-temperature-property (TTP) diagram of boron steel. And then, FE-simulation of partial quenching is carried out to obtain the cooling curves according to heating temperatures of tool. The extracted results from FE-simulation are combined with the QFA to calculate the hardness distribution of partially quenched parts. Finally, experiment of partial quenching is performed to verify the predicted results and to examine the effect of tool temperatures on the hardness of formed part. The predicted hardnesses of U-channel parts are in good agreement with the measured ones within a maximum error of 7.4%.
Highlights
In the automotive industry, hot stamped components are widely used in order to reduce the weight of vehicle and improve the fuel efficiency
Hardness distribution of partially quenched U-channel parts by tool heating was predicted by the FE-simulation coupled with quench factor analysis (QFA)
Based on the analysis and experiment, the following conclusion can be drawn: 1) Dilatometry test was performed to obtain the cooling curves according to various cooling rates which were used to determine the material constants of QFA and it shows the TTP diagram of boron steel at austenitizing condition of 900oC for 5min
Summary
Hot stamped components are widely used in order to reduce the weight of vehicle and improve the fuel efficiency. Many researchers are recently focusing on the modification of conventional hot stamping called as partial quenching (or tailored tempering) to manufacture the component with tailored mechanical properties [2,3,4]. Partial quenching process is generally classified by following methods: tailored blank tempering; tailored tool tempering; post tempering of hot stamped parts [5]. Tailored tool tempering is most widely used process in the automotive industry because it provides tailored property to component with a lower unit cost of production. In the tailored tool tempering of partial quenching, ductility of component is obtained by heating of tool in which certain regions locally experience a lower cooling rate below 30oC/s. Prediction technique of mechanical properties is vitally required for partial quenching process
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