Effect of Heat Treatment on the Formability of Zn‐Coated Hot Stamping Steel

Abstract

Herein, the effect of heat treatment with die quenching on Zn‐coated hot stamping steel is analyzed. Heating temperatures are 850, 880, 900, and 930 °C, and soaking times are 4 and 8 min. Then, microstructure, hardness, coating layer morphology, element diffusion and distribution, formation of microcrack and microvoid, and oxidation are analyzed. Results reveal that as heating temperature increases, the Vickers hardness of Zn‐coated hot stamping steel increases when soaking time is 4 min, while it decreases when soaking time is 8 min. The least average hardness value is still larger than 460, resulting from the formation of the martensitic phase. The diffusion tends to be significant as heating temperature and soaking time increase. Both the diffusion and the evaporation affect the thickness of the coating layer remarkably. The average layer thickness is at least 18.2 μm. Microcrack generates in the coating layer due to different thermal expansions between substrate and coating layer and the melting of Zn or Zn–Fe binary phase with high content of Zn. Microvoid forms in the coating layer, resulting from the Kirkendall effect and oxidation of the Zn element. Moreover, with the increase in heating temperature and soaking time, Fe2O3 is also generated on the coating surface along with ZnO.

Experimental Investigation of Hot Forming of Zn‐Coated Hot‐Stamping Steel

“…As the diffusion speed of different elements varies, the vacancies can't be replenished immediately, leading to the formation of the Kirkendall voids. [ 8 ]…”

“…As the diffusion speed of different elements varies, the vacancies can't be replenished immediately, leading to the formation of the Kirkendall voids. [8] The coating layer morphology for case 3 is given in Figure 6b. The forming temperature is 760 °C and the specimen is stretched to break under case 3.…”

“…It is also harmful for the shape precision of the part, especially as the thickness of part is small. [8,9] To avoid the oxidation and decarburization, the AlSi coating was developed for the hot-stamping steel. The AlSi-coating layer was hot dipped on the surface of substrate.…”

“…However, the AlSi-coating layer shows limited cathodic protection, and then the Zn-coated hot-stamping steel was developed. [8,9] Many researchers analyzed the hot-stamping steel with Zn-coating layer. [15,16] Ma et al [9] studied the thermal formability of Zn-coated hot-stamping steel at the vacuum condition.…”

“…Li et al [ 8 ] analyzed the effect of the heat treatment on the morphology, microcrack, microvoid, and surface of Zn‐coating layer. They found that the oxidation of Zn element promotes the formation of microvoid and microcrack.…”