Effects of Heating Rate on Formation of Globular and Acicular Austenite during Reversion from Martensite

Xianguang Zhang,Tadashi Furuhara,Yuki Toji,Goro Miyamoto

doi:10.3390/met9020266

Xianguang Zhang, Tadashi Furuhara + Show 2 more

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https://doi.org/10.3390/met9020266

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Abstract

The effects of heating rate on the formation of acicular and globular austenite during reversion from martensite in Fe–2Mn–1.5Si–0.3C alloy have been investigated. It was found that a low heating rate enhanced the formation of acicular austenite, while a high heating rate favored the formation of globular austenite. The growth of acicular γ was accompanied by the partitioning of Mn and Si, while the growth of globular γ was partitionless. DICTRA simulation revealed that there was a transition in growth mode from partitioning to partitionless for the globular austenite with an increase in temperature at high heating rate. High heating rates promoted a reversion that occurred at high temperatures, which made the partitionless growth of globular austenite occur more easily. On the other hand, the severer Mn enrichment into austenite at low heating rate caused Mn depletion in the martensite matrix, which decelerated the reversion kinetics in the later stage and suppressed the formation of globular austenite.

Highlights

Coarse polygonal ferrite and retained austenite (γ) in conventional transformation-induced plasticity (TRIP)-assisted steels were replaced by fine lath-type morphologies of the annealed martensite matrix and retained γ to improve the stretch-flangeability [1]
The current results clearly showed that a change from acicular to globular morphology was observed, experimentally, from low to high heating rates, accompanied by a change from a partitioning to partitionless mechanism of the substitutional elements
The measured Ac1 and Ac3 temperatures (α→γ starting and finishing temperatures) against the heating rate is shown in Figure 1a, in which the equilibrium Ae1 and Ae3 temperatures were plotted for comparison

Summary

Introduction

Coarse polygonal ferrite and retained austenite (γ) in conventional transformation-induced plasticity (TRIP)-assisted steels were replaced by fine lath-type morphologies of the annealed martensite matrix and retained γ to improve the stretch-flangeability [1]. These fine lath-type morphologies can be generated by starting with martensite as the initial structure for the reversion treatment and subsequent austempering process [1]. The acicular γ grains hold near-Kurdjumov–Sachs (K–S) orientation relationships (OR)

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Conclusion