Effect of the Indirect Hot-Forming Process on Al-Si Coating Tailor-Rolled Blanks

Al–Si-coated 22MnB5 tailor-rolled blanks (TRBs) are promising for lightweight complex components, but the effect of rolling reduction on coating evolution during indirect hot forming remains unclear. Al–Si-coated TRBs with 10–50% rolling reductions were annealed at 580–620 °C and subjected to laboratory-scale indirect hot forming. Coating evolution was evaluated using SEM/EDS and XRD, together with tensile and three-point bending tests. Increasing reduction decreased the mean coating thickness from 25.1 μm in the unrolled sheet to 8.1 μm at 50% reduction, while the approximately 5 μm alloy diffusion layer became increasingly fragmented. Annealing restored fracture elongation to 15.3–22.5%. XRD of the 30% reduction specimen annealed at 580 °C for 1 h indicated the presence of τ5, τ1, and η-Fe2Al5 within the overall coating, while EDS analyses revealed distinct Al–Fe–Si compositional gradients across the coating; at 620 °C, a heterogeneous Fe–Al–Si intermetallic band and substrate-side interdiffusion-layer developed. After indirect hot forming, tensile strength, yield strength, and elongation were 1562–1664 MPa, 1117–1241 MPa, and 6.2–8.6%, respectively. Total coating and interdiffusion-layer thicknesses were 23.2–35.3 and 6.5–11.8 μm, and maximum bending angles were 52.4–61.2°. For the comparable 10% and 50% reduction specimens, the image-based cross-sectional area fraction of the Fe-enriched substrate-side regions increased from approximately 68% to 77%. Neutral salt-spray testing verified that the surface coating after austenitizing treatment maintained good corrosion performance, and the coating exhibited good adhesion to the boron steel substrate. The above analyses also confirm that the indirect hot-forming process for Al-Si-coated 22MnB5 steel TRB parts is feasible and can be promoted for subsequent industrial application.