Hot Deformation Behavior and Hot Processing Map Analysis of Q420DR Low-Temperature Pressure Vessel Steel

Nominally isothermal compression tests were performed on Q420DR low-temperature pressure-vessel steel at 900–1150 °C and strain rates of 0.05–10 s−1, with a target true strain of 0.7. The effects of temperature and strain rate on flow behavior were investigated. Flow stress increased with decreasing temperature and increasing strain rate, while higher temperatures and lower strain rates favored more pronounced flow softening. A peak-stress-based Arrhenius-type constitutive relationship was established, yielding an apparent hot-deformation activation energy of 385.987 kJ·mol−1. Processing maps based on the dynamic materials model were constructed using experimental flow stresses at fixed true strains of 0.1–0.6. At a true strain of 0.6, relatively high power-dissipation efficiencies combined with positive instability parameters indicated a potentially favorable processing domain at approximately 1050–1100 °C under low strain rate conditions. Predicted flow instability was mainly concentrated at low temperatures and relatively low strain rates. The distributions of power-dissipation efficiency and instability boundaries varied with strain, indicating that processing conditions should be assessed over the deformation path. These findings provide preliminary guidance for hot working under the investigated single-pass compression conditions.