Dynamic recrystallization characteristics and processing map development of Mn-Ni-Mo steel using constitutive modeling

Abstract

The manufacturing of reactor pressure vessels (RPVs) involves a complex multi-stage forging process with varying strain, temperature, and strain rate, resulting in dynamic recrystallization (DRX), dynamic recovery, and work hardening. These processes continuously alter the steel's microstructure, making it challenging to predict its final properties. This study addresses this challenge by investigating the hot deformation characteristics of Mn-Ni-Mo steel through compression tests using the Gleeble 3500 simulator. Tests were performed across temperatures from 900 °C $-$ 1200 °C and strain rates from 0.001 s⁻¹ $-$ 1 s⁻¹. The effects of temperature, strain rate, and strain on DRX were investigated by examining work hardening rate flow stress curves and microstructural evolution. A hyperbolic sine constitutive equation was employed to model the relationship between peak stress, strain rate, and deformation temperature. Results revealed that higher deformation temperatures or lower strain rates reduce the critical strain for DRX and increase the DRX volume fraction. The strain rate sensitivity (SRS) of the steel varies with strain and temperature, with significant variations at lower strain rates (0.001 s⁻¹ and 0.01 s⁻¹) but decreasing at higher strain rates (1 s⁻¹) due to incomplete DRX. Temperature increases from 900 °C to 1050 °C improve SRS via thermally activated dislocation annihilation, whereas temperatures above 1050 °C cause a sharp decrease in SRS due to abnormal grain coarsening and microstructural heterogeneity. The study identifies an optimal processing window (η > 0.40) for hot deforming Mn-Ni-Mo steel, with strain rates between 0.03 s⁻¹ $-$ 0.3 s⁻¹ and temperatures from 1000 °C $-$ 1150 °C. The highest power dissipation efficiency (η ≈ 0.46) is observed at 1050 °C and 0.1 s⁻¹, resulting in fine, equiaxed grains of 5 ± 2 μm.