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In this study, the crack propagation process in Al-Si coating on boron steel was investigated by performing isothermal hot tensile tests on Gleeble 3500. A scanning electro microscope was used to observe the crack morphologies under different strain conditions. Furthermore, surface morphologies of the segment coating under different deformation temperatures or strain rates were observed with a 3-D ultra-depth microscope. The results showed that coating cracks could be classified into two patterns with the increasing applied tensile strain. Under lower strain conditions, only cracks which were almost perpendicular to the coating/substrate interface were observed. As the tensile strain was large enough, interfacial cracks occurred and resulted in crack debonding. By comparing the surface morphologies and crack densities under different deformation conditions, it suggested that a higher deformation temperature or a lower strain rate could reduce the crack number.

This paper introduces the research work of “High Performance Sheet Metal Forming Team” from Huazhong University of Science & Technology, in hot stamping technology and application. Aiming at the basic theory and engineering application of hot stamping, the team has begun comprehensive study in materials, process and equipment for hot stamping of high strength steel since 2008. The representative achievements in many aspects of the team are illustrated, including microstructure control, heating, simulation of phase transition, frictional behavior between materials and die, and simulation and experiments of tailored tempering process. In addition, the CAD/CAE software and non-destructive testing instrument for hot stamping were developed to meet the industrial requirements.

In order to explore the crack propagation of aluminum-silicon coated boron steel, biaxial tensile tests under complex loads were performed. The biaxial tensile device is installed in the AG-100kN machine at high-temperature, which provides plane-stress conditions. Strain rates of 0.1, 1s^-1 were selected as the forming process parameters. Real data obtained from the biaxial tensile tests produced stress-strain curves of the coated boron steels, which could then be used to simulate the crack propagation path more accurately. When the fatigue crack kept growing in tensile tests, the path could be observed by Photron FASTCAM Viewer, which is in good correspondence with the result of the tests. Fracture surfaces of the coated steels were inspected under SEM. This paper reports the significance to study the microscopic mechanism of failure and the great importance in practical application of aluminum-silicon coated boron steel in the forming process.

Hot forming technology can not only improve the impact resistance and safety performance of the car, but also reduce its weight, so it has become a popular research topic in China and abroad. Hot forming steel is divided into uncoated hot forming steel and coated hot forming steel. Hot stamping parts of coated hot forming steel do not need shot blasting treatment and have better corrosion resistance compared with uncoated hot forming steel, so coated hot forming steel has been used more widely. Al-Si coating is the most mature coating for hot forming steel, and the Zn coating hot forming steel is still in the research stage because of the narrow process window. This paper focuses on Masteel Al-Si coating hot forming steel, whose coating composition, matrix microstructure and mechanical properties before and after hot stamping is studied. The research shows that Masteel Al-Si coating hot forming steel has good performance, and it can meet the requirements of mainstream car manufacturers.

Properties and corrosion resistance of the electrophoretic paint films on two types of uncoated press-hardened 22MnB5 steels (cold-rolled and CSP hot-rolled) and an Al-Si steel based on 22MnB5 were compared in the present contribution. The physicochemical properties of the electrophoretic paint films on these three types of press-hardened steels were found to be proximal, and all meet the requirements of related standards of automobile manufacturers. The corrosion resistance of unpainted press-hardened samples of the three kinds of materials was found to be poor. The poor corrosion resistance of the Al-Si sample is related to the high iron concentration and the presence of microcracks in the coating after hot-stamping process.