Yasuhiro Yogo

Formability Improvement Technique for Heated Sheet Metal Forming by Partial Cooling

A forming process for heated sheet metal, such as hot-stamping, has limited use in deformable shapes. Higher temperature areas which have not yet come into contact with dies are more easily deformed; therefore, local deformation occurs at these areas which leads to breakage. To improve the formability of heated sheet metal, a deformation control technique utilizing the temperature dependence of flow stress is proposed. This technique can suppress local deformation by partial cooling around potential cracking areas to harden them before forming. In order to apply this technique to a variety of product shapes, a procedure to determine a suitable initial temperature distribution for deep drawing and biaxial stretching was developed with a coupled thermal structural simulation. In this procedure, finite elements exceeding forming limit strain are highlighted, and an initial temperature distribution is defined with areas of decreased temperature around the elements to increase their resistance to deformation. Subsequently, the partial cooling technique was applied to a deep drawing test with a heated steel sheet. The results of the experiment showed that the proposed technique improved 71% in the forming limit depth compared with results obtained using a uniform initial temperature distribution.

MRT letter: In situ observation method for microstructural changes of steel during hot deformation.

We report on the result of an in situ method for observing microstructural changes during hot deformation. The observation of microstructural changes of steel at 1,473 K under tensile strain is demonstrated using the reported method. The development of deformed structures and the formation of a new grain boundary, which subsequently moved with increased strain, were clearly observed. The effectiveness of this method was confirmed by the results of several examples.

Calculation for grain growth rate of carbon steels by solute drag model considering segregation effect of each substitutional element

Abstract Based on the solute drag model, a practical model incorporating the segregation effect is proposed to calculate grain growth rates in carbon steels. The segregation effect is modelled using two factors: the difference in atomic diameter between a solvent and a substitutional element, and the solubility of a substitutional element. By including the segregation energy, the proposed model enables the simulated retardation of grain growth by the addition of microalloying elements. The calculated grain growth rate by the proposed model shows reasonable correspondence between grain growth rates for experimental and calculated results. The temperature dependence of the grain growth rate is also well simulated.

In Situ Observation of Grain Growth and Recrystallization of Steel at High Temperature

An in-situ observation method for structures at high temperature is developed. The new observation device can reveal grain boundaries at high temperature and enables dynamic observation of these boundaries. Grain growth while maintaining microstructure at high temperature is observed by the new observation device with only one specimen for the entire observation, and grain sizes are quantified. The quantifying process reveals two advantages particular to the use of the new observation device: (1) the ability to quantify grain sizes of specified sizes and (2) the results of average grain size for many grains have significantly less errors because the initial structure is the same for the entire observation and the quantifying process. The new observation device has the function to deform a specimen while observing structures at high temperature, so that enables it to observe dynamic recrystallization of steel. The possibility to observe recrystallization is also shown.

Numerical Prediction of Springback Shape of Severely Bent Sheet Metal

In the sheet metal forming simulation, the shell element widely used is assumed as a plane stress state based on the Mindlin‐Reissner theory. Numerical prediction with the conventional shell element is not accurate when the bending radius is small compared to the sheet thickness. The main reason is because the strain and stress formulation of the conventional shell element does not fit the actual phenomenon. In order to predict precisely the springback of a bent sheet with a severe bend, a measurement method for through‐thickness strain has been proposed. The strain was formulated based on measurement results and calculation results from solid element. Through‐thickness stress distribution was formulated based on the equilibrium. The proposed shell element based on the formulations was newly introduced into the FEM code. The accuracy of this method’s prediction of the springback shape of two bent processes has been confirmed. As a result, it was found that the springback shape even in severe bending can b...

Surface Deteriorations During Scuffing Process of Steel and Analysis of Their Contribution to Wear Using In Situ Synchrotron X-Ray Diffraction and Optical Observations

In a previous study, we developed a novel in situ analysis and observation system that allows for simultaneous synchrotron X-ray diffraction (XRD) and optical observations of a frictional surface. This in situ system was used to investigate the scuffing phenomena of SUJ2 bearing steel (AISI 52100); characteristic surface deteriorations occurred during the scuffing process, including plastic flow, heat-spot formation, austenite transformation, and a decrease in the width of the XRD peaks (indicating a decrease of dislocations and strain). These surface deteriorations are not observed during normal wear, hence it is possible that they cause catastrophic wear during the scuffing of steel. In this study, to elucidate the scuffing mechanism of steel, we focused on the following two points: (1) whether the above surface deteriorations are unique to SUJ2 steel or whether they occur in general steels as well, and (2) the extent to which these surface deteriorations contribute to the wear amount. To achieve these objectives, we performed scuffing tests on four types of steel using the previously developed in situ system. In particular, we focused on the first stage of the scuffing process. The present test results suggest that these surface deteriorations also occur in general steels, and that plastic flow and heat-spot formation, which originate from the same phenomenon, are the dominant contributors to the wear amount during the scuffing of steel. Furthermore, the wear amount per unit plastic flow appears to be independent of steel composition.

Investigation of Hardness Change for Spot Welded Tailored Blank in Hot Stamping Using CCT and Deformation-CCT Diagrams

When an outer panel of a B-pillar is manufactured with the hot stamping process, reinforcements are spot welded on its inner side. Before reinforcements are added, the microstructure of the outer panel is martensite. However, reheating during spot welding changes the martensite to ferrite, which has a lower hardness in the heat-affected zone than in other areas. If spot welding is conducted before hot stamping for making a spot welded tailored blank, the microstructure in the spot welded tailored blank after hot stamping is martensite. This sequence of processes avoids hardness reduction due to spot welding. In this study, the hardness and microstructure around spot welded parts of the tailored blank were investigated. The results clearly showed that areas close to the spot welded parts are severely stretched during hot stamping. In addition, stretching suppresses the martensitic phase transformation and reduces the hardness. To characterize this phenomenon, a simulation was conducted that considered the effects of pre-strain on the phase transformation. A continuous cooling transformation (CCT) diagram and a deformation continuous cooling transformation (DCCT) diagram were made in order to quantify the effect of the cooling rate and pre-strain on the phase transformation and hardness. The hardness was then calculated using the experimentally measured CCT and DCCT diagrams and the finite element analysis results. The calculated hardness was compared with the experimental hardness. Good agreement was found between the calculated and experimental results.

Development of the Fast FE Method for Welding Deformation

We propose a fast finite element method for the prediction of welding deformation. To decrease the calculation time, this method allows the shell element to be used with the modeling for welding phenomena such as the heat source and joining. For the sake of further time savings, this method includes discrete thermal mechanically coupled analysis, which controls the frequency of mechanical calculations. Reasonable agreement between experimental results and calculation results has been obtained using simulated automotive parts with a short calculation time.