Relationship between grain size and ductile-to-brittle transition at room temperature in Fe–18Mn–0.6C–1.5Si twinning-induced plasticity steel

Abstract

The effect of grain size on tensile properties, particularly elongation (El), was systematically investigated using cold-rolled and annealed Fe-18Mn-0.6C-1.5Si (wt.%) twinning-induced plasticity (TWIP) steels. Whereas both the yield (YS) and ultimate tensile strengths (UTS) decreased with grain coarsening, the El increased up to a grain size of approximately 18 mu m, and then sharply decreased beyond the critical grain size. This unusual reduction in El in coarse-grained specimens was caused by the transition of the fracture mode from ductile to brittle. The transition in fracture stemmed from a change in the deformation mechanism from mechanical twinning to strain-induced epsilon-martensitic transformation. The epsilon plates provided nucleation sites and propagation paths for cracks, causing quasi-cleavage brittle fracture in coarse-grained specimens. The cracks growing along the {10 (1) over bar0} or (0001) plane were bent by epsilon plates with different plane orientations and were blocked by gamma-austenite plates. When the local stress in the gamma plate near the crack tip was high, new epsilon-martensitic plates formed within the gamma plate, enabling the crack to pass through the c plate, and changed the crack path. The quasi-cleavage fractured surface consisted of (10 (1) over bar1), (10 (1) over bar2), and (2 (1) over bar(1) over bar1) planes. Ridges of bamboo-like structures observed on the fractured surface formed by the variation in crack path occurring at the epsilon plates with different orientations. The Si-added TWIP steel exhibited a superior combination of UTS and El of more than 70,000 MPa %.