Effect Of Hot Dip Aluminizing-Silicon-Lanthanum,Diffusion Annealing And High Temperature Oxidation On Surface Microstructure Of Q355 Steel

Low alloy high strength steel（Q355）has been more and more used in the automotive industry because of its high strength,good welding performance and good corrosion resistance.However,after long-term use of low-alloy high-strength steel in high-temperature environment containing water vapor,iron oxide will form on the surface of steel,which greatly reduces the surface quality of high-strength steel and limits its use in high-temperature environment.Therefore,it is necessary to improve the high-temperature oxidation resistance of Q355 steel.Hot-dip aluminizing is widely used as an effective and cheap metal surface modification method.Adding a certain amount of alloy elements to the hot dip aluminizing bath can also effectively improve the fluidity of the bath and improve the coating quality.Relevant studies also show that diffusion annealing treatment of hot-dip aluminized steel can significantly improve its high-temperature oxidation resistance,however,there are few studies on the effects of hot-dip aluminizing and diffusion annealing on the surface microstructure and high temperature oxidation resistance of Q355 steel.The microstructure evolution of intermetallic compound layer formed by ho-dip of Q355 high strength steel in pure aluminum bath with Si content of 0～10 wt.%and La content of 0～1wt.%is studied in this work.From the type and thickness of intermetallic compound layer,the influence of alloy elements Si and La was discussed,and the influence of diffusion annealing process on microstructure evolution was studied.The formation of Al₂O₃ film under different oxidation processes was also compared in this paper.The research results can provide a theoretical basis for the development of surface corrosion protection technology of Q355steel.The main contents are as follows:（1）The effects of hot-dip temperature,time and Si content in molten pool on the microstructure of hot-dip aluminum coating of Q355 steel were studied by scanning electron microscope and energy dispersive spectrometer;The effect of diffusion annealing on the microstructure evolution of the coating was studied.By controlling the oxidation process,the formation and high temperature oxidation resistance of Al₂O₃ oxide film were compared.The results show that the alloy layer was mainly composed of Fe₂Al₅ phase when the hot-dip time of Q355 steel was extended from 30 s to 3 min;When the hot-dip temperature is 700℃,the thickness of the surface layer is thicker,and the thickness of the alloy layer is only increased by about 3μm.When the hot-dip temperature was 750℃,the thickness of the alloy layer increased about 25μm.When the hot-dip temperature was 800℃,the thickness of the alloy layer decreased inversely;When the hot-dip temperature was 750℃and the hot-dip time is 30 s,the thickness of the alloy layer was moderate and the interface between the alloy layer and the substrate was relatively flat.With the increase of Si content in pure aluminum molten pool,the thickness of alloy layer decreasedcontinuously,and the interface between alloy layer and steel matrix changed from irregular shape to flat shape.When the Si content in molten pool exceeded 2.5 wt.%,Fe-Al--Si ternary phase begain to appear in alloy layer.The annealing temperature affects the transformation of mesophase in the coating.The longer the time,the more favorable the transformation from brittle Fe₂Al₅ phase to Fe Al phase with good toughness.When the hot-dip temperature was 750℃with hot-dip time of 30s and annealed at 900℃for 3h,intermetallics in samples that hot-dipped in pure aluminum and Al-2.5Si alloys had both completely transformed into the Fe Al phase,and the phase transformation speed of hot-dipped in Al-2.5Si was faster.The Al₂O₃film formed by high-temperature oxidation had a good high-temperature protection effect on Q355 steel matrix.The Al₂O₃ film formed by the sample annealed at 900℃for 3 hours in air had the best high-temperature oxidation resistance.（2）The effect of La on hot-dip aluminum coating of Q355 steel was studied by controlling the content of La in the molten pool.The effect of diffusion annealing on hot-dip Al-0.4La coating of Q355 steel was studied by controlling the process parameters of diffusion annealing.Scanning electron microscopewas used to observe the cross-section structure of the sample,and energy dispersive spectrometer was used to analyze the alloy composition and the distribution of various elements.The results show that with the increase of La element in the molten pool,the thickness of the alloy layer increased first and then decreased.When the amount of La element was 0.4 wt.%,the thickness of the alloy layer reached the peak,and the thickness of the alloy layer was about 46μm.The higher the annealing temperature,the faster the phase transformation speed.When the hot-dip Al-0.4La sample was annealed at 900℃for 3h,the alloy layer was not completely transformed into the Fe Al phase,which indicated that under the same diffusion annealing conditions,the phase transformation speed of hot-dip Al-0.4La sample was slower than that of hot-dip aluminum and hot-dip Al-2.5Si,and the thickness of hot-dip Al-0.4La alloy layer was the thickest of the three samples,which indicated that while annealing the thicker the alloy layer was,the slower the phase transition rate was.（3）By controlling the content of Si and La in the molten pool,the synergistic effect of Si and La on the hot-dip aluminum coating of Q355 steel was studied.Scanning electron microscopewas used to observe the cross-section structure of the sample,and energy dispersive spectrometer was used to analyze the alloy composition and the distribution of various elements.The results show that the alloy layer of hot-dip Al-2.5Si-x La sample was mainly composed of Fe₂Al₅ phase,Fe Al₃ phase,Al₂Fe₃Si₃ phase and Al₄Fe_1.7Si composition.With the addition of Si in the molten pool and the continuous increase of La,the thickness of the alloy layer increased first and then decreasesd,and the thickness of Al-2.5Ai-0.1La was 19μm.It was thinner than the hot-dip aluminum under the same hot-dip conditions and thicker than the hot-dip Al-2.5Si under the same hot-dip conditions.When Al-2.5Si-0.4La was hot-dipped,the thickness of the alloy layer reached the maximum,about31μm.In this work,the effects of alloying elements and diffusion annealing process on the microstructure evolution of Q355 hot-dip aluminum coating were studied,and the high-temperature oxidation resistance of Al₂O₃ film under different conditions was compared and analyzed.The research results are of great significance to improve the high-temperature corrosion resistance of Q355 steel.