Investigating superplastic deformation mechanisms in HC1000/1470DP advanced high-strength steel through high-temperature tensile and bulge forming tests

This paper systematically investigates the superplastic deformation behavior and mechanisms of HC1000/1470DP dual-phase high-strength steel. Through high-temperature tensile tests and gas pressure bulge forming experiments, combined with microstructural analyses such as electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), its deformation performance and microstructural evolution under conditions of 700–775 °C and 0.0001-0.01 s⁻¹ were comprehensively evaluated. The results show that the maximum elongation of 345% was achieved at 750 °C and 0.0033s⁻¹, with a strain rate sensitivity index (m-value) of 0.32 and a deformation activation energy of approximately 152.6 kJ/mol, where the characteristic parameters are consistent with grain boundary sliding. Based on the dynamic material model, a thermal processing map was constructed and process parameters were optimized. Box-shaped bulge parts with depths of 20 mm and 30 mm were successfully fabricated at 750 °C and 3.3 × 10⁻³ s⁻¹.After bulging, significant dynamic recrystallization occurred at the bottom, accompanied by a decrease in martensite content and dislocation density, leading to a reduction in mechanical properties, with the thinning rate of the 30 mm part reaching nearly 80%. This study clarifies the grain boundary sliding -dominated superplastic deformation mechanism of this steel, providing a basis for its hot forming applications in lightweight automotive components.