Hole Expansion Test and Characterization of High-Strength Hot-Rolled Steel Strip

The hole expansion characteristics of two steel grades with equivalent tensile strengths: a microalloyed steel, HS-700, and a dual-phase steel, DP-600, was examined. The hole expansion ratio (HER) was 63% for HS-700 and 41% for DP-600. The HS-700 steel gave superior HER due to more homogeneity in microstructure and hardening phases. The DP-600 had inferior HER attributed to the microcrack generation & propagation at the interface ferrite/martensite interface. While the microstructure of DP-600 shows 22% hard martensite in a very soft ferrite, the HS-700 steel hardening is mainly due to thermomechanical processing, which gave fine grain structure along with finer carbides and nitrides associated with Nb and Ti. The YS/UTS ratio of HS-700 was 0.92 against 0.61 for DP-600, which favoured a more homogeneous strain distribution in ferrite that enhanced its HER. Although the DP-600 had higher elongation (26% against 22% for HS-700) and higher n value (0.21 for DP-600 against 0.12 for HS-700), the stretch formability was lower.