Redistribution of Bainitic Transformation by an Intermediate-Cooling Schedule in an 800 MPa Steel

This study examines how a fast–slow–fast intermediate-cooling schedule redistributes bainitic transformation and hierarchical boundaries in an 800 MPa steel. Among intermediate temperatures of 630, 650, and 710 °C, 650 °C produced the highest tangent-derived first-interval contribution (79.44%). At Th = 750 °C, a normalized bainite rate index reproduced the schedule ranking (Qb,eff = 68.58 kJ mol−1; root-mean-square error (RMSE) = 0.0043). Relative to continuous cooling, intermediate cooling increased the first-interval dilation share from 40.6% to 58.7% (+18.1%) and the processed hierarchical high-angle share from 46.9% to 52.2%. A common kernel average misorientation (KAM)–geometrically necessary dislocations (GND) conversion factor reproduced the reported medians within ±0.07%. Mean hole expansion ratio (HER) increased from 53% to 69% (+30.2%), whereas elongation changed by −0.5%. These results establish a process–kinetics–boundary–property pathway in which intermediate cooling shifts the transformation response toward the early interval, enriches high-angle boundary character, and improves local formability while maintaining the tensile-property balance.