Microstructural Evolution and Ductile-to-Brittle Transition in a Low-Carbon MnCrMoNiCu Heavy Plate Steel

Abstract

Low carbon MnCrMoNiCu alloyed steels are typically used to produce highly ductile thick plates for offshore structures and bulk shipbuilding. The current study revealed how microscopic factors affect the toughness and the occurrence of cleavage fracture of the steel. In this regard, a series of thermal treatments was performed on the test steel by employing a thermomechanical simulator. These involved reheating samples at different temperatures (1168 K to 1623 K (895 °C to 1350 °C))  producing different prior austenite grain sizes, followed by a continuous cooling transformation process. The Charpy V notch (CVN) toughness was determined, and the effect of the austenite grain size on the ductile–brittle transition-temperatures of the steel was investigated. The microstructural evolution of the austenite grain sizes was studied, fracture features were characterized, the critical event for cleavage fracture was identified, and the local cleavage fracture stress σ_f was calculated. The CVN toughness and σ_f were maximized in the steel which was reheated at 1273 K (1000 °C) and containing refined lathlike bainite.