Research On Microstructure Evolution And Fracture Strain Property Of Al-Si Coatings On Hot Stamped Steel

Lightweight,electric,intelligent is the main direction of the development of automotive technology.Improving the strength of materials can achieve lightweight by reducing the thickness of materials on the premise of ensuring safety.Hot stamped steel with strength over 1500 MPa is the highest strength and most widely used high-strength steel for the car body.AlSi coating technology can avoid high-temperature oxidation and decarburization of sheet metal in the hot stamping process,and ensure the parts have excellent corrosion resistance after forming.At present,the application of Al-Si coating plate in hot-stamped steel accounts for more than 60%.However,the bending fracture strain of the existing Al-Si coated hot-stamped steel plate needs to be improved to meet the needs of a further lightweight,and the risk of delayed cracking resistance needs to be reduced to improve its manufacturing reliability further.In this paper,the physical nature of the reduction of the toughness of hot stamped steel caused by Al-Si coating is proposed.Taking the hot stamped steel plates with different thickness of Al-Si coating as the research object,the structure of Al-Si coating after hot-dip,the transformation rule of coating structure during hot stamping and the cold bending properties after hot stamping are systematically studied utilizing industrial production process simulation and laser confocal in-situ observation in the laboratory.The high carbon embrittlement mechanism between Al-Si coating and boron steel substrate was revealed.The main work and research results are as follows:(1)The microstructure of Al-Si coating after the hot-dip was studied.The hot-dipped AlSi coating with different thickness is mainly divided into two layers,the outer aluminum layer and the inner intermetallic layer.The aluminum layer contains acicular Al-Si phase,and the intermetallic compound is mainly Fe2SiAl7 phase.In order to maintain the thermodynamic equilibrium between Al-Si coating and boron steel matrix,there is a fragile band structure at the junction of Fe2SiAl7 layer and boron steel matrix,which is mainly composed of Fe2Al5 phase and FeAl3 phase.(2)The microstructure evolution of Al-Si coating during hot stamping was studied,and the microstructure of Al-Si coating after hot stamping was analyzed.The hot stamped steel plates with different thickness of Al-Si coating were heated to 920? for 4 minutes to realize complete austenitizing,and then the die was quenched.The structure of Al-Si coating was mainly divided into four layers,from the outside to the inside,there are surface layer,middle layer,intermetallic compound layer and mutual diffusion layer.The surface layer is mainly composed of alumina;the middle layer is mainly FeAl2 phase;the intermetallic compound layer is mainly Fe2SiAl2;and the interdiffusion layer is mainly ?-Fe phase rich in Al and Si.(3)The microstructure evolution behavior of Al-Si coating with standard thickness and thin thickness during heating was studied.The influence mechanism of austenitizing temperature,holding time and coating thickness on the microstructure evolution of Al-Si coating was discussed.The diffusion of Al,Si and Fe elements in Al-Si coating and boron steel matrix can be promoted by prolonging the holding time,increasing the austenitizing temperature and reducing the thickness of the coating.The increase of diffusion degree leads to the evolution of the Al rich FeAl2 phase to the Fe rich Fe2SiAl2 phase and the increase of the thickness of the mutual diffusion layer(?-Fe).(4)The in-situ growth of ?-Fe phase in the diffusion layer of standard thickness and thin thickness Al-Si coating during heating was studied.The growth of ?-Fe phase in the diffusion layer of different thickness Al-Si coating during austenitizing heating and heat preservation was dependent on the matrix.With the increase of heating temperature and holding time,the thickness of ?-Fe phase in the diffusion layer increases gradually and the increasing trend decreases.(5)The physical nature of toughness deficiency of Al-Si coated on hot stamped steel was revealed.During the austenitizing process of Al-Si coating,the interface between the coating and the substrate will move to the substrate side.The movement of the interface between the coating and the substrate will lead to the diffusion of carbon elements in the matrix to the substrate side,forming a significant carbon enrichment zone at the interface between the coating and the substrate,and forming a high carbon martensite during the subsequent cooling process.The high carbon martensite significantly reduces the bending fracture strain of Al-Si coating on hot stamped steel.(6)In this paper,the toughening mechanism of the interface between coating and substrate was described.Without changing the composition of the coating and substrate,the alloying process and interface movement behaviour of Al-Si coating during heating can be controlled by reducing the thickness of the coating.This design reduces the thickness and carbon content of high carbon martensite region and improves the fracture brittleness of Al-Si coated hot stamped steel.