Study On Warm Formability Of DP780 High Strength Steel Sheets

With the rapid development of automobile industry, the requirements of energy conservation and environmental protection are more and more obvious in the field of automobile manufacturing, at the same time the concept of lightweight automotive body is also caused widespread concerns. In the field of automobile manufacturing,the use of high strength steel has become one of the main ways to ensure the strength of the car and reduce body weight. However, with the strength of high strength steel,In the process of cold stamping the high strength steel plate has high pressing force,workpiece easily broken, serious shape distortion and other issues. In the process of hot stamping the workpiece surface is easily oxidized and the surface quality is not up to the requirements. Energy consumption is relatively high. Warm stamping is expected to overcome the limitations of hot stamping and cold stamping in the forming of high strength steel sheet and inherit some of their advantages. In this paper,the warm forming properties of DP780 high strength steel sheet are investigated by the method of experiment and microscopic, which will lay the foundations for further understanding the warm forming process of DP780 high strength steel sheet.The hot sheet formability test machine(BCS-50AR) developed by BUAA was used to determine the FLD of DP780 high strength steel sheet at the forming temperature range of 25℃～500℃. The study shows that the forming temperature has great influence on the forming limit diagram of DP780 high strength steel sheet. With increase of forming temperature, the position of the FLD curve rises. But the position of FLD curve is lower than 25 ℃ at 300 ℃. The increase amplitude of the forming limit curve at 25℃～200℃ is small and at 200℃～400℃ obvious, while the increase amplitude is small at 400 ℃～500℃. With the increase of forming temperature, the plasticity of high strength steel sheet has been improved. The plasticity of the material significantly improves at 400 ℃, but the material develops blue brittle at 300 ℃. At the same time, the influence of punch velocity and stress state on the forming limit of DP780 high strength steel sheet was studied and analyzed. Based on the DP780 high strength steel plate forming limit test, the forming limit curve calculation model at different temperature was established.The deformation characteristics of DP780 high strength steel plate werecharacterized by hardness analysis, optical microscope(OM), scanning electron microscopy(SEM) and transmission electron microscopy(TEM). Hardness analysis shows that DP780 high strength steel appears work hardening phenomenon in the forming process of 25 ℃. With the forming temperature increasing, the hardness of the sample after the forming limit decreases gradually. The material develops blue brittle at 300 ℃ so that DP780 high strength steel plate becomes brittle and hard.Microscopic analysis shows that with the increase of temperature the number of martensite decreases gradually. Some black spots can be observed at 400 ℃ and 500 ℃,which are the carbide particles from martensite. Fracture surfaces form sample of DP780 high strength steel sheet forming limit at 25 ℃ appear small dimple and the number of dimple is small. With the increase of temperature, the number, size and depth of the dimple are significantly improved, which shows that the plasticity of the material is improved. Fracture morphology is cleavage fracture at 300 ℃, which shows that material brittleness is higher and plasticity is lower. Carbon atoms of DP780 high strength steel gather in martensite at 100 ℃. Martensite translates to tempered martensite structure at 200 ℃. The martensite continues to decompose and retained austenite completely translates to bainite at 300 ℃. Martensite decomposes into tempered troostite structure at 400℃～500℃. The dislocations movement of DP780 high strength steel plate in the plastic deformation process is hindered by the friction between crystals at 25 ℃ so that the dislocations entangle and pile up. The rise of forming temperature strengthens the mechanism of dislocation annihilation and proliferation mechanism, but the dislocation annihilation mechanism plays a leading role. Material dislocation density decreases.