Microstructural evolution during continuous annealing of a 980 MPa cold rolled steel grade

Abstract

The development of advanced high strength steels (AHSS) is motivated by automotive industry demands for higher strength steels with better formability and specific stretch-flange formability to avoid fractures during parts forming. In response to this, the new steel grades must have special microstructures in terms of constitution, morphology, distribution and hardness. Thus, a better understanding of formation mechanisms and the characterization of these microstructures has shown to be of fundamental importance for the development of new steels. In this study the influence of heating rate and intercritical temperature on microstructural evolution and hole expansion ratio of a cold rolled 980 MPa AHSS grade was evaluated, using a Dilatometer and a Gleeble machine. The stretch-flange formability was evaluated by hole expansion tests and the microstructures were characterized using scanning electron microscopy, electron backscatter diffraction (EBSD) and nanohardness measurements. The increase of heating rate resulted in an overlap between ferrite recrystallization and austenite formation, changing the morphology of austenite from random to fibrous distribution. On the other hand, higher intercritical annealing temperature resulted in more homogeneous microstructures and higher hole expansion ratios. This microestructural homogeneity, evaluated by nanohardness, suggested that the difference in hardness between ferrite and second-phase affects micro-void formation and crack propagation during hole expansion test.