Reheat treatment process of mold is necessary when partial machining of the mold is required, such as shape correction for an existing mold. If defects such as cracks or significant deterioration of mechanical properties of the mold occur during reheat treatment, it is impossible to reuse the mold. In this study, reheat treatment was performed up to two times for STD11 tool steel, and microstructure and mechanical properties according to the reheat treatment were evaluated. Carbide fraction and grain size of prior austenite were almost unchanged after the reheat treatment. Hardness and impact toughness increased significantly after QT treatment, and these properties were maintained without significant change even after the reheat treatment. It is concluded that up to two iterations of reheat treatment does not cause deterioration of properties of STD11 tool steel. Based on these results, a mold for a face-lifted front bumper was manufactured through machining and reheat-treating of an existing mold.

In this study, we were turning STD11 and was carried out to presume for mutual relation of turning condition to get optimum cutting force(principal, radial, feed force) and experimental equation by variance, correlation coefficient and multi regression analysis with whisker reinforced ceramic tool. To predict cutting force, analyze principal, radial, feed force with multi-regression analysis. Results are follows: From the analysis of variance, affected factor to cutting force feed rate, depth of cut, cutting speed in order and cutting speed was very small affect to cutting force. From multi-regression analysis, we extracted regression equation and the coefficient of determination(R2) was 0.84, 0.88, 0.7 at principal, radial and tangential force. It means regression equation is significant. From the experimental verification, it was confirmed that princial, radial and tangential force was predictable by regression equation. Through the analysis of the correlation coefficient between each component forces and surface roughness, the principal force was found to have the greatest impact

In General, the surface roughness of turning is depends on cutting condition which are cutting speed, depth of cut, and feed, etc. This study was carried out to presume for mutual relation of turning condition to get optimum surface roughness and experimental equation by variance and multi regression analysis As the result, multiple correlation coefficient was calculated 97%, the coefficient of determination (R2) was calculated 95% and the adjusted coefficient was calculated 97% by multiple regression analysis So, the formula was made by this multiple regression analysis is reliable.