Effect of Microstructure and Mechanical Properties on Hole Expansion Property of High Manganese TWIP Steel

Hole expansion property is an important property for ultra-high strength steel (UHSS) sheets for press forming. To evaluate the hole expansion property of 980MPa TWIP steel, A fully ferritic IF steel and 980MPa DP steel were used as reference materials in the present study. For three kinds of steels the microstructures were observed by scanning electron microscopy (SEM), and tensile tests and hole expansion tests were carried out. And the microstructures after tensile tests and hole expansion tests were assessed by electron back-scatter diffraction (EBSD) analysis. The results indicate although TWIP steel showed a much larger uniform elongation, the hole expansion ratio was less than for the IF steel. The reason for the increase in the hole expansion rate of TWIP steel comes from the uniform deformation caused by early twinning induced plasticity (TWIP effect). At the same time, this deformation mechanism leads to the hard austenite with deformation twins. The hard austenite with deformation twins and soft austenite match will deteriorate the local deformation to hinder the hole expansion to further enhance. It was found that the cracking position of TWIP steel after hole expansion tests was concentrated at the interface boundary.