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Title

MICROSTRUCTURAL EVOLUTION DURING CONTINUOUS ANNEALING OF A 980 MPA COLD ROLLED STEEL GRADE WITH HOLE EXPANSION REQUIREMENTS

Authorship

Arruda, Marcus Vinícius Pereira; Melo, Túlio Magno Fuzessy de; Costa, Fernando de Souza; Santos, Dagoberto Brandão

Arruda, Marcus Vinícius Pereira

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Melo, Túlio Magno Fuzessy de

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Costa, Fernando de Souza

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Santos, Dagoberto Brandão

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DOI

10.5151/9785-9785-32501

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Abstract

The development of advanced high strength steels (AHSS) is motivated by the automotive industry demands for higher strength steels with better formability and stretch-flangeability 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, which are strongly dependent on the chemical composition of the steel sheet and its processing, especially in the continuous annealing stage, in the case of cold rolled sheets. In this study the influence of the annealing heating rate and intercritical temperature (IT) on the microstructural evolution and the final hole expansion ratio of a cold rolled 980 MPa AHSS grade was evaluated. The annealing process was simulated using a Dilatometer and a Gleeble machine, and the microstructures were characterized using scanning electron microscopy, electron backscattered diffraction (EBSD) and nanohardness measurements. The increase of heating rate resulted in an overlap between ferrite recrystallization and austenite formation, changing the austenite morphology from a random to a fibrous distribution, while a higher IT resulted in more homogeneous microstructures and higher hole expansion ratios. This microestructural homogeneity, confirmed by nanohardness measurements, suggested that the difference in hardness between ferrite and the second phase affects the micro-void formation and crack propagation during the hole expansion test.

Keywords

AHSS; Microstructure; Stretch-flangeability; Nanohardness.