Characterization of the aluminium coating layer in the hot press forming of boron steel

Abstract

Hot press forming allows geometrically complex parts to be easily formed from boron steel blanks. The rapid cooling after forming produces a product with extremely high strength. To prevent the blanks from oxidizing and decarburizing during heating, forming, and subsequent press cooling, the boron steel is supplied with an aluminium-based coating. This surface coating influences the formability of the component and the quality of the final product. The main purpose of the present research is to characterize the changes in the aluminium-coated layer on a boron steel during hot press forming. To characterize the evolution of the coated layer, experiments for hot press forming were conducted under various conditions that simulated a production process. Test specimens were heated to temperatures between 810 and 930 °C and were then press hardened. The aluminium-coated layer develops four distinct microstructural regions: (a) a diffusion zone; (b) an aluminium—iron (Al—Fe) zone I; (c) a low-aluminium zone (LAZ); and (d) an Al—Fe zone II. The band-like LAZ is clearly observable in the temperature range of 810 to 870 °C and becomes sparsely dispersed at temperatures above 900 °C. The microcracking behaviour of the aluminium-coated layer during forming was also analysed with both bending and deep-drawing tests. The strain concentration in the softer LAZ is found to be closely connected to both microcracking and exfoliation of the coated layer during the press forming.