Deformation response such as plastic deformation in galvannealed coating layer under Vickers indentation, uniaxial compression and rolling deformation was investigated by scanning electron microscopy and X-ray diffraction analysis for measuring half width value of diffraction peaks. In the region around Vickers indentation hole, initiation and subsequent propagation of the cracks were not observed but extrusively developed protuberance was observed. This result suggests that plastic deformation caused by the Vickers indentation takes place in the coating layer. In uniaxial compression and rolling deformation, thickness reduction of the coating layer paraborically increased with increasing specimen (coating+Fe-substrate) reduction, suggesting that macroscopic plastic deformation takes place in the coating layer. Also, it was found in these deformation modes that the interface between coating layer and Fe-substrate changes to irregularly shaped plane as the deformation increases, accompanied with many cracks in the coating layer. By the X-ray analysis for the specimens subjected to uniaxial compression and rolling deformation, it was shown that the apparent plastic deformation takes place not only in δ1-phase layer but also in Γ-phase layer.
The investigation on GA coating properties and in-situ SEM observation in three-point bend test has been carried out. The intention of this study is to characterize the fracture mechanism in galvannealed coatings on interstitial free steel sheet. Several cracks in coating are induced during heating and cooling process. The segment of the pre-existing cracks is related to by an outburst reaction. The crack spaces are dominated by grain size and divided into segments of approximately 20 μm. Ductility in GA coating is inhomogeneous by the difference of diffusion rate between Fe and Zn. Ductility in GA coating decreases with increasing Fe content in the coating. Therefore, the cracks take place easily in the coating as the product of an outburst reaction. These cracks propagate in a direction perpendicular to the coating phase under tensile loading condition. Generated crack relaxes applied tensile loading and then deformation of steel substrate takes place. Therefore, decohesion at steel-coating interfaces hardly takes place at the tensile loading side. The generation of the crack at the compression loading side is slower than the tensile side because the tensile residual stress in the coating is released by compressive loading. Elastic energy of the compression is then accumulated in the coating and when the stress at steel-coating interface reaches the critical decohesion shear stress, the coating divides the steel substrate. Based on the above results, the optimum coating structure is discussed.
Tensile fracture and peeling behavior of the Fe-Zn intermetallic coating of galvannealed steel, together with that of the alumina coating of anodic-oxidized aluminum as a reference were studied. The Fe-Zn coating exhibited first multiple-fracture perpendicular to the tensile axis. Then the coating exhibited compressive fracture in the width-direction, followed by powdering and the mode ll-type crack extension in the δ1 layer and in the δ1 layer near the δ1-Γinterface. As a result, the upper ζ+ δ1and ζ+δ1+ Γ1 layers were peeled off and the thin δ1+ Γ1 + Γ and Γ1+ Γ layers remained on the steel. Under further applied strain, the remaining δ1+ Γ1+ Γ and Γ1+ Γ layers were fractured in compression in the width-direction, followed by powdering and debonding at the Γ-ubstrate interface. In such a two times-compressive fracture process, coating layers were peeled of The alumina coating on the aluminum substrate exhibited multiple-fracture perpendicular to the tensile stress as similarly as the Fe-Zn intermetallic coating. Then, when the coating was thin, the multiply-cracked layer fractured in compression perpendicularly to the tensile axis, accompanied by buckling and wedging, resulting in peeling as similarly as the galvannealed coating.
Residual stress of the Fe-Zn coating layer on GA (galvannealed) steel was analyzed by means of the finite element analysis. The experimentally observed result that the coating layer has been fractured in the as supplied samples was accounted for by the analyzed high tensile residual stress introduced in the Fe-Zn coating layer due to the difference in coefficient of thermal expansion between the coating layer and the substrate steel during cooling from the galvannealing temperature. The multiple fracture phenomenon of the coating layer under the thermally induced residual stress was explained as the stress-relaxation process. From the stress analysis for the measured average crack spacing in the coating layer on the IF (Interstitial Free) and SPCC (Steel Plate Cold Commercial) steel substrates, the tensile strength of the coating layer was estimated to be 260-270 MPa. Concerning the influence of the coating layer thickness on the multiple cracking, it was revealed that the crack spacing increases with increasing thickness of the coating layer and it is proportional to the thickness of the coating layer if the strength of the coating layer is insensitive to length and thickness. Concerning the influence of the stress-strain behavior of the substrate on the multiple cracking, it was shown that the multiple cracking progresses more when the substrate steel has higher yield- and plastic flowstress.
The brittle Fe-Zn intermetallic compound coating layer on GA steel showed multiple cracking perpendicular to the tensile axis. The average crack spacing of the coating layer decreased with increasing applied strain. The crack spacing was dependent on the coating thickness and species of the steel substrate. The calculation of the exerted stress of the coating layer with the finite element method revealed that, when the steel substrate deforms plastically and the crack spacing is narrower than that of critical length, the maximum tensile stress of the coating layer is approximately proportional to the crack spacing and to the tensile stress of substrate, and inversely proportional to the coating thickness. From the comparison of the measured crack spacing on the IF (Interstitial Free) and SPCC (Steel Plate Cold Commercial) steels with the calculated one, the strength of the coating layer was estimated to be around 260 MPa. From these results, an empirical equation was proposed, which can be used for rough prediction of the crack spacing as a function of applied strain for any substrate steels and thickness of the coating layer.
Multiple cracking behavior and its relation to the grain boundary of the substrate steel were investigated. The crack spacing and its distribution of the specimens strained to prescribed strains were observed with a scanning electron microscope. The crystallographic orientation of the substrate grains was estimated from the electron back scattering pattern. The images of the cracks of the coating layer and grain boundaries of the substrate steel were combined together with image processing software to know whether the cracking of coating layer initiates above the substrate grain boundaries or not. The main results are summarized as follows. (1) The cracking of the coating layer initiates in the regions just above the substrate grain boundaries. (2) The cracks that initiate above the substrate grain boundaries propagate transversely; namely they propagate into the region, below which no substrate grain boundary exists. (3) It was suggested the cracking of the coating layer initiates above the small angle grain boundaries or above the grain boundaries intact to the grains with large Schmid factors. (4) In the multiple cracking process of the coating layer, the similarity of the crack spacing distribution to the grain boundary spacing distribution is kept.
The microstructural evolution of intermetallic compound layers formed in Fe/Zn diffusion couples (DCs) was examined by optical and scanning electron microscopies and electron-probe microanalysis. In the solid-Fe/liquid-Zn DCs at 450°C, the δp phase nucleates between the δk, and ζ phases after dipping for about 100 s and every intermediate phase seems to obey the square root law individually before and after the appearance of the δpphase at 100 s. In the concentration-penetration profiles, a composition gap is observed in the δp phase of the solid-Fe/liquid-Zn DCs dipped for a long time at temperatures ranging from 450 to 550°C, while only a singular point, but no gap, is obtained in the DCs dipped at elevated temperatures over 575°C. On the other hand, in the case of the solid-Fe/solid-Zn DCs at 400°C, the d phase instead of the δpphase appeares later between the δpand Γphases and finally covers the whole δ region.
Interstitial-free high strengthened steels with the tensile strength of 340-440 MPa produced by the addition of Ti, Nb, Mn and P are galvannealed for automobile panels to increase their corrosion resistance. It has been well known that the substrate characteristics such as chemical composition, roughness, grain size and texture significantly affects the visual appearance of the galvannealed coating. An understanding of the micro-structural phase evolution that takes place at the interface between zinc and substrate steel is essential to obtain good surface quality of coating. In the present study, both conventional interstitial-free scavenged by Ti and Nb and interstitial-free high strengthened sheet steels containing Mn and P were galvannealed in a Rhesca hot-dip simulator. The effect of chemical composition and substrate properties on the surface quality of galvannealed steel sheets has been investigated by glow discharge spectrometer, scanning electron microscopy, transmission electron microscopy with Gatan image filter and focused ion beam techniques.