The Influence of Hot‐Rolling Conditions on the Content and Morphology of Retained Austenite in Ultra‐High Strength Bainitic Steel and Its Mechanical Properties

Abstract

Bainitic steel with retained austenite belongs to the family of multi‐phase steels, which exhibit high strength combined with high deformability due to their special microstructural composition. In this contribution, the microstructural conditions of the hot‐rolled bainitic steel are presented and characterized as a function of the chromium content and the hot‐rolling conditions. In particular, the aim of this study is to understand the influence of hot rolling conditions and of Cr content on retained austenite morphology for a Cr containing, carbide‐free bainitic steel. The mechanical properties are largely controlled by the multi‐phase microstructure. A high degree of deformability is evaluated unlike to the large amount of retained austenite islands in the microstructure. The morphology of the retained austenite is described as blocky‐type, which questions previous findings on retained austenite in literature. In this work, the best ratio between tensile strength (1340 MPa) and total elongation (22%) is found for blocky retained austenite while the island size do not measure more than 2 μm. These results are found for steel with a Cr‐content of 0.9% and after hot rolling (finish rolling temperature >970 °C) with a coiling temperature of 450 °C.