Research On Preparation And Deformation Behavior Of Steel Sheet With Multi-structural And Corrosion-resistant Aluminum Coating

Due to its excellent mechanical properties and formabilities,Steel sheet has been widely used in automobiles,ships,bridges,and constructions.However,it is extremely susceptible to be corroded by oxidizing gases and humid environment,which would shorten the service life of steel sheet and bring severe economic loss.Hot-dip aluminizing is one of the most effective methods of surface protection for steel sheet to improve its corrosion resistance.Due to brittle intermetallic compounds forming in hot-dip aluminizing,the cracks are easily initiated in Fe-Al alloy layer.The defects will seriously decrease the corrosion resistance and formability of aluminum coated steel sheet.By thickness control and structure optimization of alloy layer,the overall performance of aluminum coated steel sheet can be improved.In this research,multi-structure aluminum coatings were prepared on Q235 steel surface(Al_H/Q235,Al_H+M+HT/Q235)by hot-dip aluminizing and combining magnetron sputtering?companying heat treatment?with hot dipping process,respectively.The barrier effect of magnetron sputtering aluminum coatings on the diffusion of iron and aluminum atoms in hot-dip aluminizing process was investigated.Also,the formation and growth mechanism of aluminum coatings was proposed.The corrosion resistance and formability of aluminum coated steel sheet were evaluated by full immersion test and deep drawing test,respectively.The mechanism of corrosion protection and fracture failure to alumimun coatings were discussed.The specific research contents and results are as follows:?1?Aluminum coatings were prepared on steel surface by hot-dip alumini-zing.The effects of process parameters on the quality,thickness,morphology,composition and properties of aluminum coatings were investigated.Aluminum coating is mainly composed of an outer Al layer and an inner Fe-Al alloy layer,which is metallurgically bonded to substrate.A needle-like FeAl₃ phase is distributed in Al layer.In Fe-Al alloy layer,there are a granular-like FeAl₃ layer adjacent to Al layer and a serration-like Fe₂Al₅ layer close to substrate.With the increase of hot dipping temperature,Al layer becomes denser and its thickness gradually decreases.Adversely,the thickness of Fe-Al alloy layer gradually increases.As hot dipping time increases,the thicknesses of Al layer and Fe-Al alloy layer both increase.Moreover,the relationship between thickness of alloy layer and hot dipping time is in accordance with x=15.6t^1/2-62.9.Hot dipping aluminum coatings have excellent bonding strength to the subtrate.The hardness distribution of aluminum coating abruptly increases and then decreases sharply from Al layer to substrate.The aluminum coating which immersed at720°C for 20 s possesses better morphology and a thinner alloy layer?16.86?m?.Aluminum coatings were prepared by combining magnetron sputtering?companying heat treatment?and hot dipping process.The effects of barrier layer on coating thickness,morphology and structure of magnetron sputtering aluminum coating?Al_M?were studied.After Al_M being treated 30 min at 600°C,the diffusion of iron and aluminum atoms is effectively prevented.The structure of alloy layer of Al_H+M+HT/Q235 transforms to comb-tooth structure,while its thickness decreases by 11%,compared to Al_H/Q235.?2?The formability of aluminum-coated steel sheet and its morphology after deformation were investigated by deep drawing test.The mechanism of cracks initiation and propagation in different regions was revealed.It is found that I-cracks which are perpendicular to the substrate are generated in Fe-Al alloy layer owing to tensile stress.II-cracks parallel to the interface between alloy layer and substrate are formed by shear stress.Gernerally,they are generated at the interface between FeAl₃ phase layer and Fe₂Al₅ phase layer.When the blanking force is 2 kN and loading speed is 0.5 mm/s,as coating thickness increases,scale-like cracks appear on coating surface at the punch shoulder.Furthermore,the fracture formed in Al layer and cracks formed in Fe-Al alloy layer gradually widen and deepen.Rod-like Fe₂Al₅ phase embedded in cracks is further shattered as well.As the deformation rate and drawing degree decrease,the width and density of cracks in aluminum coatings become lower.Moreover,the formability of aluminum-coated steel sheet is improved,owing to a relatively thin alloy layer and loose structure of Fe₂Al₅ phase layer.?3?The corrosion resistance of aluminum-coated steel sheet was investigated by full immersion test.The effects of hot dipping process parameters and different coatings on the weight loss and corroded morphologies of aluminum coated steel sheet were investigated.It is showed that,the weight loss of aluminum coated steel sheet is proportional to corrosion time.With the hot dipping temperature and time increase,the corrosion resistance of aluminum coating is improved.The corrosion rate of Al_H/Q235 is less than 1/5 of uncoated steel.Furthermore,the corrosion rate of Al_H+M+HT/Q235 decreases by 3%,compared to Al_H/Q235.After full immersion test,corrosion pits appear on and inside aluminum coating.Meanwhile,corrosion products and cracks exist around and inside corrosion pits.The corrosion products mainly include Al₂O₃and AlO?OH?,which deposit in the defects and corrosion pits.The?self-sealing?behavior of aluminum coating alleviates the corrosion process of aluminum coated steel sheet.In this research,aluminum coatings were prepared on the surface of steel sheet by combining magnetron sputtering?companying heat treatment?and hot dipping proccess.The thickness and structure of Fe-Al alloy layer are effectively controlled.Moreover,it has laid a foundation for preparing aluminum coated steel sheet with excellent corrosion resistance and formability.