Stress-state dependence of dynamic strain aging:Thermal hardening and blue brittleness

This study aims to discover the stress-state dependence of the dynamic strain aging(DSA)effect on the deformation and fracture behavior of high-strength dual-phase(DP)steel at different deformation temperatures(25-400°C)and reveal the damage mechanisms under these various configurations.To achieve different stress states,predesigned specimens with different geometric features were used.Scanning electron microscopy was applied to analyze the fracture modes(e.g.,dimple or shear mode)and underlying damage mechanism of the investigated material.DSA is present in this DP steel,showing the Portevin-Le Chatelier(PLC)effect with serrated flow behavior,thermal hardening,and blue brittleness phenomena.Results show that the stress state contributes distinctly to the DSA effect in terms of the magnitude of thermal hardening and the pattern of blue brittleness.Either low stress triaxiality or Lode angle parameter promotes DSA-induced blue brittleness.Accordingly,the damage mechanisms also show dependence on the stress states in conjunction with the DSA effect.

Punchability and Punching Fracture Behavior of High Silicon Steel Sheets

The 0.2-0.3mm thick cold-rolled sheets and annealed sheets of high silicon steel were produced by rolling process,then punching tests were carried out at room temperature to 150℃.The punchability and punching fracture behavior were investigated by means of scanning electron microscope（SEM）and electron back-scattered diffraction（EBSD）.It is found that cold-rolled sheets of high silicon steel show good punchability under the condition of 0.01 mm punch-die clearance and at the temperature of 100℃;the punchability of annealed sheets with fine grain size is better than that of annealed sheets with coarse grain size;the punching temperature of 150℃is suitable for annealed high silicon steel sheets especially with coarse grain size on account of punching edge quality.The major cleavage planes of annealed high silicon steel sheets are{100}crystallographic planes on which brittle fracture occurs at low temperatures.The cleavage systems include{100}〈110〉and{100}〈100〉,and cleavage fracture is completed by one or both of them.The punchability of high silicon steel sheets is related with punching temperature,grain size and ordering degree.Reducing ordering degree,increasing punching temperature,as well as refining grain size can improve the ductility and punchability of high silicon steel sheets.