Bulging Recrystallization During Tempering of Low-Carbon Lath Martensite: Roles of Martensitic Substructure and Carbide Pinning

Although lath martensite has generally been considered resistant to recrystallization during tempering, the present study demonstrates that bulging recrystallization can occur even in low-carbon (LC) lath martensite after prolonged tempering. The recrystallization behavior of ultra-low-carbon (ULC) and LC steels was systematically investigated to clarify the microstructural factors governing this phenomenon. Optical microscopy and electron backscatter diffraction (EBSD) analyses revealed that bulge recrystallization initiated at prior austenite grain boundaries in both steels, although its onset was significantly delayed in LC steel. KAM profiles across the prior austenite grain boundaries showed no pronounced difference between the unrecrystallized matrices on the two sides, suggesting that a pre-existing dislocation-density difference across the boundary is not indispensable for the initiation of bulging. The recrystallized grains inherited the crystallographic orientation of the adjacent martensitic matrix and preferentially grew along the longitudinal direction of martensite blocks, demonstrating that recrystallization proceeds through protrusive migration of pre-existing grain boundaries guided by the anisotropic boundary network of lath martensite. Increasing carbon content refined the martensitic substructure and increased the dislocation density, while simultaneously enhancing carbide precipitation and delaying recrystallization. Phase-field simulations demonstrated that intersections between prior austenite grain boundaries and martensitic substructure boundaries can generate an initial protrusion even without a pre-existing dislocation-density difference, whereas the stored-energy difference associated with dislocations promotes its subsequent growth. Carbide particles suppress recrystallization through boundary pinning. In the present low-alloy LC steel, rapid coarsening of cementite during prolonged tempering progressively reduced the pinning force, allowing recrystallization to occur despite the higher carbide density. These results demonstrate that bulging recrystallization in low-carbon lath martensite is governed by the interplay among martensitic substructure, stored energy, and carbide pinning, providing a unified mechanistic framework for recrystallization during tempering.