Segregation Neutralised Steels: Microstructural Banding Elimination from Dual Phase Steel Through Alloy Design Accounting for Inherent Segregation

Abstract

Banding in commercial dual phase steels, such as banded ferrite and pearlite or ferrite and martensite microstructures, is inherited from segregation during solidification in continuously cast material, predominantly from Mn segregation, and subsequent rolling. The banded microstructures lead to anisotropic mechanical properties which is generally undesirable. This paper presents an alloy design approach (termed “segregation neutralised” steels) to remove banding of the second phase by utilising co-segregation of both austenite and ferrite stabilisers to reduce local variability in second phase stability. The new composition proposed also considers achieving the same strength levels through maintaining the same second phase fraction, grain size and solid solution strengthening increments. Phase field modelling has been used to predict the segregation and phase transformation behaviours for commercial dual phase steels and the new composition. A 5 kg laboratory alloy production route (casting, hot rolling and coiling simulation, cold rolling and annealing) has shown that the banded structure seen in commercial dual phase steels is accurately reproduced and that banding has been reduced dramatically in both the hot rolled condition as well as after cold rolling and annealing in the new composition steel. Chemical analysis has shown that in the modified alloy the second phase distribution shows no correlation to the segregation bands, due to the achieved balance in austenite and ferrite stabilisers.