The insights to the phase transformation kinetics of middle-carbon Mn-Si high-strength steels: Effect of manganese content

Abstract

This study investigates the influence of manganese additions ranging from 2.6 to 8.2 wt.% on the transformation behavior of undercooled austenite in 0.5 wt.% C – (1.6-2.8) wt.% Si steels. Experimental results reveal a systematic decrease in critical transformation temperatures Ac₁ and Ac₃ with increasing Mn, which is accompanied by a shift in transformation mechanisms. At lower Mn levels (2.6-3.1 wt.%), heating induces a two-stage transformation characterized by abnormal expansion and the separation of pearlitic and ferritic reactions, whereas higher Mn contents (6.3-8.2 wt.%) promote a continuous transformation with diminished volumetric and thermal effects. Dilatometric cooling experiments demonstrate that pearlite/bainite formation is fully suppressed at cooling rates of 0.1 < V ≤ 0.5 °C/s for the Mn2.6 and Mn3.1 steels, and V ≤ 0.1 °C/s for the Mn6.3 and Mn8.2 grades, giving way to a martensitic transformation. Consecutively, the M_s temperatures decrease from 218 ^oC to ambient temperature as Mn increases from 2.6 to 8.2 wt.%. Finite-difference thermal simulations confirm that the 2.6-3.1 wt.% Mn steels can undergo effective air hardening in sections up to 20 mm, whereas the 6.3–8.2 wt.% Mn steels achieve through-thickness air hardening in sections up to 50 mm.  Thermodynamic modeling highlights the pronounced retardation of bainitic transformation with increasing Mn, culminating in near-complete suppression at 8.2 wt.%. Finally, pre-quenching below M_s significantly accelerates the subsequent bainitic transformation by reducing incubation times and enhancing transformation rates, as demonstrated for the 3.1 wt.% Mn steel grade.