A scale up study on chemical segregation and the effects on tensile properties in two medium Mn steel castings

TL;DR

This study investigates how casting size and processing affect chemical segregation, microstructure, and tensile properties in medium Mn steel ingots, providing insights for scalable production.

Contribution

It demonstrates the impact of casting scale on segregation, microstructure, and mechanical properties, and validates micress modelling for predicting segregation in medium Mn steels.

Findings

Segregation decreases significantly after homogenisation.

Large ingots show more residual δ-ferrite after homogenisation.

Mechanical properties remain high and consistent despite segregation differences.

Abstract

Two ingots weighing 400 g and 5 kg with nominal compositions of Fe-8Mn-4Al-2Si-0.5C-0.07V-0.05Sn were produced to investigate the effect of processing variables on microstructure development. The larger casting has a cooling rate more representative of commercial production and provides an understanding of the potential challenges arising from casting-related segregation during efforts to scale up medium Mn steels, whilst the smaller casting has a high cooling rate and different segregation pattern. Sections from both ingots were homogenised at 1250 \degree C for various times to study the degree of chemical homogeneity and $\delta$-ferrite dissolution. Within 2 h, the Mn segregation range (max $-$ min) decreased from 8.0 to 1.7 wt\% in the 400 g ingot and from 6.2 to 1.5 wt\% in the 5 kg ingot. Some $\delta$-ferrite also remained untransformed after 2 h in both ingots but with the 5 kg ingot showing nearly three times more than the 400 g ingot. Micress modelling was carried out and good agreement was seen between predicted and measured segregation levels and distribution. After thermomechanical processing, it was found that the coarse untransformed $\delta$-ferrite in the 5 kg ingot turned into coarse $\delta$-ferrite stringers in the finished product, resulting in a slight decrease in yield strength. Nevertheless, rolled strips from both ingots showed $>$900 MPa yield strength, $>$1100 MPa tensile strength and $>$40\% elongation with $<$10\% difference in strength and no change in ductility when compared to a fully homogenised sample.