Local variations in microstructure and mechanical behavior of vanadium-added 2.0 GPa hot-stamping u-shaped high strength steel parts

This study explores the differences in microstructure and mechanical properties of different areas (designated as locations A-F) of U-shaped hot-stamping steel parts. Results highlight clear microstructural differences: the bending areas at locations A and B are characterized by lath and massive martensites, which are richer in smaller nanoscale precipitates. However, the flat areas (locations C-F) show a compound mixture of martensite packets, blocks, and laths, indicating a considerable microstructural difference between the bending and flat areas. The hardening effect is greater in the bending areas, reflected by the substantially higher dislocation density. In particular, the dislocation density in the bending areas ranges from (3.6 f 0.2) x 1015 to (4.1 f 0.3) x 1015 m-2 , while the dislocation density in the flat areas ranges from (2.4 f 0.3) x 1015 to (3.4 f 0.2) x 1015 m-2 . Furthermore, the average hardness of the bending areas (563HV) is clearly higher than that of the flat areas (485HV), indicating that the plastic deformation rate and stress concentration differ. Mechanical evaluation shows that the strength of the flat areas always exceeds 1800 MPa while maintaining an elongation of over 6.0 %; thus, the hot-stamping parts possess ultrahigh strength. An important discovery is that the size of nanoparticles is lower in areas with larger deformation, particularly in the bending areas, which improves material strength through the effective Orowan strengthening mechanism. In particular, the maximum Orowan strength increase between the flat and bending areas is 74 MPa.