Multiscale microstructure engineering of steels

Abstract

The development of modern steels is based on the tailoring of the microstructure to achieve the required properties. While historically this was performed at the micrometre scale length, there is now the scope to undertake this at the nanoscale or atom scale. The present paper reviews recent work related to the development of ultrafine and nanoscale microstructures in steel as well as changes at shorter scale lengths, such as cluster formation and solute effects. This includes the development of ultrafine ferrite through phase transformation, nanoscale and ultrafine bainite, precipitation and cluster strengthening and bake hardening of steels. A key element of the present work has been the use of atom probe tomography to unlock the nature of these structures.

Microstructure–property relationships in TRIP aided medium-C bainitic steel with lamellar retained austenite

“…The untransformed retained austenite films are often observed in many different bainitic–austenitic alloys. 30,38–42…”

“…A complex Si þ Al concept was selected to suppress carbide precipitation during austempering. 1,18,30 The ingot was produced using vacuum induction melting and hot forging to a thickness of 22 mm. Subsequently, the flat samples were hot rolled to a thickness of 4.5 mm.…”

“…This austenite morphology is often revealed in many similar carbide-free bainitic steels. 30,[40][41][42] The bainitic-ferritic steel contains both retained austenite layers and blocky retained austenite of various sizes (Fig. 17b).…”

“…The thermomechanical processing effect shows an increased dislocation density in the bainitic ferrite laths because these transformation products inherit the advanced dislocation density of the austenite deformed in the non-recrystallisation region. 30,38 Amount of retained austenite…”

“…The untransformed retained austenite films are often observed in many different bainitic-austenitic alloys. 30,[38][39][40][41][42] Considering that the carbon concentration in the c phase of the bainitic-ferritic steel is the averaged value of the lath regions and blocky austenite grains (because there is less C enrichment of grains in ferrite), 9,16,42,47 it should be assumed that the c phase films has a higher C content than the determined value of 1.52% (Table 2). This result confirms the substantial importance of the small grain size and compressive stresses that are introduced by bainitic ferrite laths to stabilise the c phase 6,8,18,42 of the bainitic steel with retained austenite and homogeneously enriched carbon at a concentration of approximately 1.35%.…”