Crack generation, propagation mechanism and thermal property of Zn-coated hot stamping steel

Abstract

In this study, the Gleeble tests and hot stamping of practical part of Zn-coated hot stamping steel were conducted. Based on the analysis of thermal properties, a material model was employed to fit the relationship between true stress and true strain with high accuracy. The effect of forming temperature, ferrite formation and bending on the liquid metal induce embrittlement (LMIE) was researched. The results show that the true stress increases as forming temperature decreases. LMIE occurs, leading to a low true strain of about 0.13, as the forming temperature reaches 820 °C. According to the Gleeble simulation test and the actual test results, the forming temperature is suggested to be 720 °C. So, LMIE is avoided and the mechanical properties are guaranteed. In practical application, the tensile stress is easy to produce microcrack while the compressive stress constrains it. With the decrease of stamping temperature, the number and width of the microcrack in the coating layer decrease, and the thickness of the coating layer increases. The coating layer is composed of solid α-Fe(Zn) phase. Decreasing the liquid phase in heating, soaking and forming period tends to reduce and even avoid the LMIE cracks.

Keywords

hot stamping steel / Zn-coated layer / crack / direct hot stamping / microstructure

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