It is known that the Al added to the Zn coating layer of hot-dip galvanized steel sheets (HDG) segregates on the surface of temper-rolled HDG as Al-based oxides with increasing aging time in air at room temperature. In this study, the surfaces of Zn-0.2mass%Al HDG with and without temper rolling were investigated to clarify the segregation mechanism. Specimens with a Zn coating weight of 55-57 g/m 2 including 0.19-0.20 mass% of Al were used. The specimens were aged in air at 20°C or held in liquid nitrogen, and the surface and cross sections of the specimens were then observed and analyzed by XRF, SEM-EDX and EBSD. As a result, it was found that the velocity of Al-based oxide segregation on the surface of the temper-rolled HDG was much higher than that of the HDG without temper rolling. This was attributed to the difference in the area where formation of Al-based oxides was possible. It was also found that the Zn crystal grains in the coating layer were refined by recrystallization due to contact with the temper roll, resulting in an increased number of grain boundaries that can serve as Al diffusion paths. Some unrecrystallized grains also remained after temper rolling and could increase the number of formation sites for Al-based oxides, as they contain numerous dislocations that can serve as Al diffusion paths. These two different formation sites could lead to difference in the segregation rates observed in this study.

No abstract

The effect of the Al-based oxide layer which segregates on the surface of hot-dip galvanized steel sheets (GI) with aging on the frictional properties of the GI was investigated.Conventional GI with the Zn coating weight of 67 g/m 2 including 0.36 mass% Al were used as test specimens. It was found that an Al-based oxide layer grew on the GI surface with aging after production, and the friction coefficient tended to decrease due to the existence of these Al-based oxides. However, this tendency was clearer under the sliding conditions of shorter tool length and higher contact pressure than of longer tool length and lower contact pressure.In order to understand this behavior, surface observation and analysis of both the test specimens and the tools after sliding were carried out by SEM, EDX and EPMA. Both Al-based oxides and metallic Zn were detected as adhered materials on the tool surface after sliding, and the surfaces of adhered materials were covered with Al-based oxides. This suggests that the adhered materials on the tool have the effects of reducing the adhesion force between the metallic Zn of the GI and the tool and reducing the tool roughness. These effects led to a lower friction coefficient because both shearing and plowing resistance were decreased. In addition, the area on the tool which were covered by the adhered materials depended on the tool length. This is thought to be the reason why the effect of the Al-based oxide layer depended on the sliding conditions.

The effect of the Al oxide layer which segregated on the surface of hot dip galvanized steel sheet (GI) with aging on the frictional properties of the GI was investigated. Conventional GI with the Zn coating weight of 67 g/m 2 including 0.36 mass% Al were used as test specimens. It was found that an Al oxide layer grew on the GI surface with aging after production, and the friction coefficient tended to decrease due to the existence of this Al oxide. However, the tendency was clearer under the sliding conditions with shorter tool length and higher contact pressure than with longer tool length and lower contact pressure. In order to clarify the mechanism, surface observation and analysis of both the test specimens and the tools after sliding were carried out by SEM, EDX and EPMA. Both Al oxide and metallic Zn were detected as adhesions on the tool surface after sliding, and the surface of the adhesions was covered with Al oxide. It is thought that the adhesions on the tool had the effects of reducing the stickiness between metallic Zn of GI and the tool, and reducing the tool roughness. These effects led to a lower friction coefficient because both shearing and plowing resistance were decreased. In addition, the area on the tool which was covered by the adhesions depended on the tool length. This is thought to be the reason why the effect of the Al oxide layer on the friction coefficient depended on the sliding conditions.

No abstract

The surfaces of dies used for flat sliding tests of galvannealed steel sheet (GA) have been investigated using several electron microscopic techniques in order to clarify the adhesion mechanism. Two kinds of adhesive materials were identified on the die surface. One consists of an Al oxide -Fe-Zn alloy composite and tightly bonds to ridges of the die surface. This resembles the built-up edges formed on tool surfaces during metal-cutting operations. Another adhesive material is composed of Fe-Zn alloys located in the hollows of the die surface which do not bond or only loosely bond to the die surface. A new adhesion model consisting three steps is proposed; (1) the Al oxide -Fe-Zn alloy composite layers are formed on the ridges of the die surface. (2) The Fe-Zn intermetallics on the GA surfaces are cut by this layer and (3) accumulate in the hollow of the die surface.

The frictional behavior of galvannealed steel sheets (GA) depending on tool materials was investigated. GA having different ζ/δ 1 phase intensity ratios was prepared as specimens. In addition, lubrication-treated GA was also prepared using these specimens as the base material. Four kinds of tools composed of zinc alloy die casting (ZAS), ductile cast iron (FCD), alloy tool steel (SKD) and Cr-coated FCD (CR) were used as sliding tools in order to simulate actual press tools used in trials and mass production. When the tool material was harder than the test specimens, the friction coef cient of the GA increased as the amount of the ζ phase increased, and the effect of the lubrication treatment was clearly observed. However when the tool material was softer than the test specimens, the friction coef cient was constant and independent of the existence of the ζ phase and lubrication treatment. The change in the frictional behavior was discussed from the viewpoint of the change in the friction mechanism depending on the relative hardness of the surfaces of the specimens to the tool materials.

Synopsis :The influence of the Mn content of Si-added steel sheets on the Fe-Zn galvannealing reaction was investigated. Three steel sheets, 1.5 mass%-Si-1.4, 1.9 and 2.7 mass%Mn, were annealed in a 10vol%H 2 -90vol%N 2 atmosphere. Si and Mn oxides were analyzed by reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectrometry. SiO 2 and Mn 2 SiO 4 formed as selective oxides at the steel surface after recrystallization annealing. The ratio of the oxide species changed depending on the Mn content in the steel. When the Mn content was lower, formation of SiO 2 was promoted and that of Mn 2 SiO 4 was suppressed. In the selective oxide layer which formed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel sheets, Mn 2 SiO 4 formed at the outer side, and SiO 2 formed at the inner side. This can be explained by consideration of the thermodynamic oxygen potential gradient. Furthermore, areas where SiO 2 mainly formed and those where Mn 2 SiO 4 mainly formed were distributed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel sheets. In this case, the Fe-Zn intermetallic compound (IMC) formed preferentially on the Mn 2 SiO 4 between the zinc coating and the substrate steel after galvanizing, and the Fe-Zn galvannealing reaction was suppressed on the SiO 2 layer. It is considered that a dense and continuous protective SiO 2 layer acted as a barrier to the Fe-Zn galvannealing reaction.