Investigation On Spangle Size,microstructure And Process Of Hot-dip Al-Zn-Si Coated Steel Sheet

Hot dip Aluminum-zinc-silicon（55%Al-43.4%Zn-1.6%Si）alloy coated steel plate,hereinafter referred to as hot-dip aluminum-zinc steel plate.Due to its high corrosion resistance,high reflectivity,and excellent surface properties with spangles,the product has been widely used in the home appliance,construction,and electrical industries.According to the requirements for appearance and corrosion resistance of high-end hot-dip aluminum-zinc products,the product clearly requires the number of spangles≥55/100 mm.The existing production lines in China cannot meet the spangle requirements,and the product relies on imports.Therefore,it is very necessary to study the control technology of spangles,quantify the relationship between the size of spangles and the process,solve the technical problems of controlling the size of spangles in industrial production lines,explore the influence and nucleation mechanism of spangles,microstructure,intermetallic compounds on the surface of hot-dip aluminum-zinc coating,and guide the industrial application of spangle control technology.This article compares the effects of different cooling rates,composition of aluminum-zinc coating solution,and surface state of cold-rolled steel on the variation of spangles on the surface of hot-dip aluminum-zinc coating through hot dip coating simulation experiments and industrial production line experiments.The microstructure characteristics of the coating surface,cross-section,and intermetallic compound layer were studied through analysis techniques such as scanning electron microscopy（SEM）,transmission microscopy（TEM）,and glow discharge spectroscopy（GDS）.Combined with solidification theory,the nucleation mechanism of hot-dip aluminum spangles has been analyzed,and a control technology scheme for hot-dip aluminum-zinc spangles is formed ultimately.The conclusion is as follows:The increase in cooling rate after coating can refine the dendrite structure of the coating and reduce the size of spangles.It was found that during the final solidification stage,the rich Zn solid solution phase produced shrinkage pores.When the cooling rate after coating was5℃/s,the solidification shrinkage porosity was 3.716μm-6.639μm.When the cooling rate after coating is 30℃/s,the solidification shrinkage is 2.798μm-3.349μm.As the distance between dendrites is decrease,the size of the surface solidification shrinkage pores is decrease,and the surface density of the aluminum-zinc coating has been improved.The nucleation mechanism under cooling conditions after coating is that the Al₁₃Fe₄and Fe-Al intermetallic compound layers first precipitate during the cooling process of the coating solution,providing heterogeneous nucleation points for the aluminum rich solid solution phase.An increase in undercooling will lead to an increase in nucleation at the interface between the coating surface and the compound layer,and multi interface nucleation promotes the formation of small spangles.The study of two-stage cooling uses linear regression to obtain a regression equation between the size of spangles and the two-stage cooling speed,quantifying the relationship between spangles and cooling process.L=2.77-0.0426Δt_a-0.0287Δt_b,mm（applicable cooling rate range 0-25℃/s）.The cooling rate of 600℃～520℃in cooling stage a has a greater impact on spangles compared to the cooling rate of 520℃～380℃in cooling stage b.The main reason is that the cooling rate in the range of 600℃～500℃has a greater impact on nucleation rate.After adding 0.06%Ti element to the coating solution,the size of spangles on the surface of the steel plate significantly decreased after immersion.The number of spangles in the two groups of experiments increased from 45 to 155 per 100 mm and from 75 to 215 per 100 mm,respectively.The intermetallic compound layer was significantly refined,and the particle size increased from 2μm to 0.5μm.Confirm that the main phase of the intermetallic compound layer is Al₈Fe₂Si（τ₅）phase,and spangles nucleate in the intermetallic compound layer.The refined intermetallic compound layer phase Al₈Fe₂Si（τ₅）provides more nucleation points for the solidification of aluminum rich solid solution phase.After adding 2%Mg element,the size of spangles on the surface of the steel plate did not change significantly,but the corrosion resistance of the coating plane increased by 56%-66%,the corrosion resistance of the incision increased by 20%-40%,and the bending cracks of the coating became fine.It was found that the Fe content in the coating of the abnormal small spangle is 1.2-2times that of the normal spangle.The uneven distribution of spangles in the rolled hard plate is caused by the adhesion of R-C-OO-Fe to the surface of the steel plate.The nucleation mechanism is that after annealing,Fe powder with strong surface activity will undergo a violent Fe-Al reaction.Excessive diffusion of Fe into the Al₁₃Fe₄phase precipitated in the coating enhances the surface wettability of the steel plate,reduces the wetting angleθ,increases the nucleation rate,and promotes the formation of small spangles.Develop a control technology plan for aluminum-zinc small spangles,obtain a regression equation between the size of spangles and Ti element content through linear regression,a_-nd quantify the relationship between spangles and Ti content（within the range of 0-75ppm）.N=46.0+0.613M_Ti.It is set to directly guide the production process.The Ti content control range in the main zinc pot is 20-40 ppm,and the technical scheme of cooling after coating and adding Ti element is applied to achieve the requirement of hot dip aluminum-spangles≥55/100 mm.