| In the recent years hot stamping process has become a research hotspot of automobile industry.It accords with the spirit of modern automobile lightening.The crash safety will be developed at the same time.Boron alloys of steels are the typical steel grades in hot stamping materials.22MnB5 is the widespread steel grade of the boron steels.During the process,the 22MnB5 blank will be austenitized in a furnace at about 950℃.Afterwards,the blank will be formed and quenched by the cooled tool.The microstructure of 22MnB5 transforms form ferritic-pearlitic microstructure to fully martensitic microstructure.This increase the strength and hardness of 22MnB5.At the austenitized temperature,oxidization and decarburization will occur when the blank is in contact with air.In order to avoid the problems,pre-coated methods which can provide protection are developed.Al-Si layer is one of the most widely used coating technologies.Al-Si coating provides high temperature oxidation resistance and high corrosion resistance.But the melting point of Al-Si coating is approximately 600℃,melting phenomenon will occur in the heating part.Some of the Al-Si substances will remain and cover on the rollers in the furnace,a negative impact for production.In this paper the basic properties of hot stamping steel 22MnB5 are studied.The chemical behavior of the Al-Si coating is mainly analyzed.The main research results of this dissertation are as follows.(1)Different initial structure sheets are made by experiment steel sheets and the heat treatment of experiment steel sheets,in order to analyze the influence of properties after hot stamping.The performance of the simulated mask annealing sheets are poor.(2)The structure of Al-Si coating before and after hot stamping process was studied.The composition of Al-Si coating before hot stamping process is surface layer(oxide layer),the outer layer(Al layer),intermetallic compounds(Fe2SiAl7 phase),the inner layer(Fe2Al5 and FeAl3 phases)and boron steel matrix;the composed of Al-Si coating after hot stamping is surface layer(oxide layer),middle layer(FeAl2),alloy compound layer(Fe2SiAl2),diffusion layer(Fe(Al,Si))and boron steel matrix.(3)The microstructure of the Al-Si coating evolution process during heating was studied.The thickness of the coating and the distribution of the coating can be controlled by controlling the heating time and heating temperature,to obtain the coating layout in favor of the subsequent processing after hot stamping.The evolution process of the coating was increased with temperature and heating time,diffusion layer between Al-Si coating and boron steel matrix of continuous improvement;From the beginning of the hot dip coating alloy layer distribution,alloy compound layer continue to promote to the Al layer,until the full range of coatings occupied,and then Si rich phase continuous formed in the middle of the whole coating.Pore also produced at the different layer of the coating.The coating microstructure gradually transform to the situation after the hot stamping Process.(4)The issue of the contamination of the transmission roller by the coating liquefaction of the Al-Si coating plate in the actual production process is analyzed.Ceramic roller consisted of Si and O elements by chemical analysis.The O and Al elements accounted for the most of proportion of contaminated ceramic roller,and the proportion of Si content is very low.It indicated that liquefied Al-Si coating remains residue in the driving rollers,which react with O elements in the air.Ceramic roller will be contaminated constantly.An improved method is proposed.High heating speed within the first 120s and low heating speed of 60s are considered.The method can keep the diffusion and reduce the melt. |
PDF Full Text Request