Study On Exfoliating Mechanisms And Evaluation Methods For Coatings On Galvannealed Sheet Steel

Auto-body Corrosion is one of the main reasons for automobile damage. It not only shortens service life, but also wastes social public resources with severe environment pollution. What’s more, it results in fatal traffic accidents.Galvannealed steel sheets can provide auto-body protection from road deicing salt and acid rain under harsh environment problem, as well as offering advantages of high productivity and low cost and excellent properties. So they are widely used in auto-body manufacture, especially in the moderate and top-class cars.Unfortunately, Galvannealed coatings occur exfoliating during press-forming process and result in loss of protective capability. Furthermore, it will bring about other harmful mechanical or aesthetic problems caused by corrosion, which greatly limiting the market expand of galvannealed steel sheets in automobile fields.In this reason, coating exfoliating becomes an urgent problem of automotive industry. Both domestic and overseas develop many studies, including the effects of galvanneal conditions, chemical composition, coatings microstructure and press-forming processing parameters on exfoliating resistance of galvannealed steel sheets. However, these researches concern only effects from chemical composition and forming processing parameters, which do not take into account the properties of exfoliating resistance during different stress and strain states. Furthermore, they are all based on a certain forming process, which are difficult to be applicable to the whole situation. Correlation between fracture energy releases rate criterion and exfoliating quantification of galvannealed coatings should be researched detailedly and systematically based on the critical adhesion failure rule of galvannealed coating along different strain pathsFocusing on improvement of exfoliating resistance and quantifying galvannealed coatings adhesions, analysis of galvannealed coating adhesion failure during different strain paths has been performed. A rule of coating adhesion failure is presented on basis of strain energy density factor theory. Further, galvannealed coatings’adhesions mechanical properties are studied. A new exfoliating rating method for galvannealed coatings is developed to evaluate the correlation between exfoliating resistance and adhesions mechanical properties. Work pieces in different forming process are analyzed with FEM attempting to predicate their coatings actual exfoliating location. To fulfill the above research objectives, the following four aspects are performed:1) Research on exfoliating mechanisms in galvannealed coatingsAiming at brittle and multilayer microstructure of galvannealed coatings, the exfoliating mechanisms in them are investigated based on laminate composite failure criterion and strain energy density S factor theory. Major factors in inducing coating exfoliation are monolayer occur failure and crack propagates in coating along with different deformation. It is found that coatings exfoliate on compress strain zone because mixed mode brittle fracture of sliding mode fracture exists in the zone. A Y -factor model of coating failure is built on the basis of strain energy density S factor theory. The Y -factor model clarifies correlation between the bond peel energy release and bond shear energy release. It means that galvannealed coatings exfoliating resistance capability are depended on their bond mechanical property. Consequently, it presents that exfoliating occurred only once the compress strain reached a certain point.2) Research on bond mechanical properties of galvannealed coatingsGeneral peeling experiment can not peel hard and brittle coatings from the substrate and there are no open publications on it. An asymmetry T type cantilever beam is developed to perform a novel kind of peeling test of galvannealed coatings. The results of EDS and XRD prove galvannealed coatings to be peeled off. Single lap joint test is used to tear the coatings from the substrate.Theoretical calculation of bond peel and shear energy release is deduced from the relation between displacement and load. Effect of microstructures of the coatings on their bond mechanical properties is also studied.3) Development of an automatic coating exfoliating rating apparatusIn order to correctly assess the correlation ship between bond mechanical properties and exfoliating resistance capability of galvannealed coatings, a justice evaluate galvannealed coatings exfoliating method is investigated. Based on traditional evaluate methods and some operating needs of manufacturers, a novel evaluation technology of exfoliating performance of galvanneal sheets products, using a CCD camera and a pc-based image analysis system, is developed. A newly illumination structure style is designed to eliminate lights reflection. Two methods of locating sample region are performed, which reducing requirement of locate precision and calculation time. The apparatus can exactly rate automatically coatings exfoliating even in the case that there are dirties and mark on samples surface. The reasonable rating criterion and scientific classification standard are built on the basic of traditional rating criterion.5) Research and verification about adhesion failure rule of galvannealed coatingGalvanneal coatings exfoliation is quantified by the devised apparatus. Experimental results reveal that there is curve relationship between the bond mechanical properties and exfoliation quantified of Galvanneal coatings, as Y -factor model studied before, wihich proves that the Y -factor model is correct.The whole exfoliating process of galvanneal coatings during forming processes is shot, and the change of strain state of the deforming coatings is analyzed with the help of numerical simulation of multi-steps forming and strain calculated by the measurement of formed circle grids. It proves that exfoliating is influenced by compressing strain and exfoliating only occurs when the compress strain exceeds above a certain limit. The coated exfoliating locations are predicated and verified based on the former strain critical value.Based on gradient information and some necessary experiments validation, this dissertation proposed a systematic and detailed study on exfoliating resistance quantification for galvannealed coatings based on their bond energy release rate. It will contribute to not only revealing the exfoliating mechanisms on galvannealed coatings, but also reasonable evaluating the exfoliating resistance galvannealed steel sheets. All in all, the research results presenting in the dissertation have theoretical and practical significance.