Hot ductility of a Nb-microalloyed 1 800 MPa grade hot-stamping steel

Under the background of carbon neutrality,the use of hot-stamping steels representing one of the ultra-high strength steels in automotive body-in-white is gradually increasing. Microalloying technology is helpful to improve the mechanical properties of hot-stamping steels. However,the effect of microalloying elements on the hot ductility of hot-stamping steels remains unknown. The hot ductility of a Nb-microalloyed 1 800 MPa grade hot-stamping steel (0.04% Nb steel,designated as 0.04Nb steel) was evaluated using Gleeble-3500 thermal simulation testing machine,and the fracture mechanisms at various test temperatures were analyzed. The results indicated there were two hot brittle ranges and a temperature range with high ductility for the 0.04Nb steel. The first hot brittle range,spanning from 1 250 ℃ to 1 300 ℃,showed intergranular brittle fracture due to the reduction in grain boundary adhesion caused by high temperatures. Meanwhile,the inclusions formed by S and P with a low melting point may exist,which can further weaken the grain boundary adhesion. The temperature range spanning from 850 ℃ to 1 250 ℃ is featured by high hot ductility with the reduction of area greater than 60%. The high ductility was attributed to the occurrence of dynamic recrystallization and the intensive movement of grain boundaries and dislocations,despite that Nb(C,N) starts to precipitate at the temperature range. The third hot brittle range spans from 650 ℃ to 800 ℃,where the hot ductility of the 0.04Nb steel generally decreases with decreasing temperatures. The poor ductility was caused by intergranular proeutectoid ferrite and fine Nb(C,N),which leads to stress concentration at grain boundaries during straining and results in brittle fracture. During the actual continuous casting process of 0.04Nb steel,the hot brittle range of 650-800 ℃ should be avoided,while the straightening operation should be carried out in the temperature range above 900 ℃.