Physical Metallurgy of Aluminized 22MnB5 Hot Press Forming Steel

Abstract

The physical metallurgical aspects of 22MnB5 steel and its aluminized coating were systematically investigated through the Hot Press forming (HPF) thermo-mechanical cycle in the present study. The CCT diagram of 22MnB5 steel was studied experimentally. The effect of compressive deformation on the CCT diagram was found that all the phase transformations were enhanced by deformation. Further study showed that both the amount of plastic strain and strain rate had influence on the martensitic transformation kinetics. It was found that plastic deformation decreased the Ms temperature. Increasing the strain rate of the deformation suppressed the martensitic transformation. Low strain rate plastic deformation enhanced the martensitic transformation. The influence of isothermal deformation on the microstructure evolution and mechanical properties was also investigated. At high temperature, the deformation refined the final martensitic microstructure and increased its final strength. At intermediate temperature, the deformation enhanced the ferrite and bainite formation and decreased its final strength. At low temperature the deformation had no effect on the microstructure evolution. The fracture morphology was found to be dependent on the ferrite morphology and volume fraction.The hot workability of the 22MnB5 steel was characterized by the high temperature tensile tests. The relation among mean flow stress, deformation temperature and strain rate could be fitted to the Sellars-Tegart equation. The paint baking treatment played as a low temperature tempering to the martensitic microstructure of 22MnB5 steel. Both the strength and total elongation were increased by the baking process.The degradation of type 1 aluminized coating on 22MnB5 steel during HPF was caused by the formation of complex intermetallic phases. These Fe-Al and Fe-Si-Al intermetallic phases evolved to the phases with a higher Fe content as the holding time increased. Meanwhile, the coating thickness increased, voids were formed at the coating surface and the surface roughness increased. Kirkendall voids appeared in the diffusion zone and cracks were formed in the coating. Plastic deformation caused the segmentation of the coating. The intermetallic coating was brittle at room temperature as well. The present research found that the formation of disordered α-Fe phase with Al in solid solution could be an alternative coating method for HPF. This high temperature disordered α-Fe phase transformed to the ordered D03 Fe3Al phase during cooling to room temperature. A 10μm aluminized coating could form the desired coating after heating at 1050°C for only 4min. The decreased steel strength caused by higher pre-heating temperature was also studied.In the present study, an N enriched layer was formed at the coating/steel interface after gas nitriding and followed hot dip aluminizing. This N enriched layer could suppress the Fe diffusion and retard intermetallic phase formation at high temperature. But it was not sufficient for the application of HPF. However, the nitro-aluminized steel shown much superior performance than common type 1 aluminized steel at intermediate temperature. The N enriched layer effectively suppressed the formation of Fe2Al5 phase, and increased the service life of the aluminized steel at intermediate temperature.