The effect of B on the surface oxidation behavior and phosphatability of cold-rolled steel sheets was investigated using 0.001 wt% B-added and B-free steels containing 0.6 wt% Si and 2.0 wt% Mn. The specimens were annealed at 800°C in a 5 vol% H₂–N₂ atmosphere with a dew point of −50°C. The surface oxides of the annealed samples were analyzed by GD-OES, FT-IR, SEM-EDX and TEM. The annealed steel sheets were then subjected to zinc phosphate treatment, and the effect of the surface oxides on phosphatability was evaluated by SEM-EDX. In the initial stage of annealing, fine granular Mn₂SiO₄ mainly formed and film-like SiO₂ partly formed on both steels. As the soaking time at 800°C increased, the granular Mn₂SiO₄ increased in the B-free steel. In contrast, in the B-added steel, the granular Mn₂SiO₄ coarsened, MnSiO₃, MnO and B₂O₃ formed, and the film-like SiO₂ formation area expanded. Addition of B reduced the melting point, causing coarsening of Mn₂SiO₄, exposing the base steel. This results in a difference in the oxygen potential between the exposed area of the steel and the oxide covered area. This local inhomogeneity of the oxygen potential changes the surface oxide species of the B-added steel. To elucidate the reason for the poor phosphatability of the B-added steel, a SEM-EDS analysis of the steel surface in the initial stage of zinc phosphate treatment was conducted, revealing that the coarse Si–Mn complex oxides and large film-like SiO₂ inhibited the zinc phosphate reaction.

Boron (B) is frequently used as additives to improve the hardenability of advanced high strength steel. It has been reported that B in steel reacts with atmospheric N₂ during annealing at low oxygen potential (low dew point) to form boron nitride (BN) by the thermodynamical calculation. In this study, the effect of BN formation on the steel surface on the coatability during hot-dip galvanizing was investigated, experimentally. B-free specimens and specimens containing 15 or 30 ppm B were annealed at various temperature and dew point, and then hot-dip galvanized. The annealed specimens were also prepared and analyzed with GD-OES, XPS, SEM-EDX and TEM-EELS to investigate the oxide and nitride formation on the steel surface during annealing. As results, coatability deteriorated as the amount of B in steel and the annealing temperature increase, and as the dew point decrease. These trends were not correlated with the amount of oxide but the amount of BN formation, suggesting that BN formation deteriorated the coatability. The surface and cross-sectional analysis revealed that BN formed around the oxide to cover the steel surface. This would lead the deterioration of the coatability because most of the steel surface was covered with BN as well as oxide, which are known to have low wettability with molten Zn.

The effect of Mn on the alloying reaction during hot-dip galvanization was investigated. The microstructure of the Fe–Zn intermetallic layers consisted of ζ, δ, and Γ phases for both pure Fe and Fe–2Mn (wt.%) alloy. The intermetallic layers grew thicker with increasing dipping time, and the growth rate of each layer was similar for both substrates. In the case of Fe–2Mn, the formation of the δ_1p phase was observed after dipping for 2 s. However, δ_1p formation was delayed for pure Fe, indicating that Mn may promote nucleation of the δ_1p phase. It is known that the δ_1p phase nucleates in the Fe-saturated ζ phase. The Fe content at the ζ/δ_1p interface was found to be lower for the Fe–2Mn alloy by electron probe microanalysis, suggesting that the supersaturation of Fe for the nucleation of δ_1p is decreased by Mn addition and Mn may stabilize the δ_1p phase. Once δ_1p became a continuous layer, the growth rates of the δ_1p layer on pure Fe and Fe–2Mn were similar. Mn could affect only the nucleation of δ_1p during the initial stage of the alloying reaction.

The stability of η-Fe₂Al₅ phase at α-Fe/molten Zn–0.1Al (wt.%) (L) interface at 723 K in Fe–Zn–Al ternary system was investigated by phase-field simulations. Thin layers of intermetallic compound (IMC) phases (η, Г-Fe₃Zn₁₀, Г₁-Fe₅Zn₂₁ and δ₁-FeZn₇) were placed between the α and L phases, and the growth of the IMC layers and the atomic diffusion of constituent elements along the direction perpendicular to the α/L interface were calculated by one-dimensional phase-field simulation. The simulation result showed that Г and Г₁ phases dissolved, and thin η phase and thick δ₁ phase remained stable at the α/L interface. Moreover, several phase-field simulations were performed by varying the values of interdiffusion coefficients in each phase. The simulation results showed that the diffusion and partitioning behaviors of Al have a significant effect on the stability of IMC layers at the α/L interface. It was found that the partitioning of Al to the α phase was suppressed due to the fact that the value of interdiffusion coefficient in the α phase was several orders of magnitude smaller than those in the IMC phases. The resultant Al partitioning to the IMC phases was the direct cause of the stabilization of the η phase and the destabilization of the Г and Г₁ phases.

This study was set to fundamentally understand the effect of Si addition on the interfacial reaction between Zn–55%Al alloy liquid (corresponding to a nominal composition of Al–25Zn (at%)) and Fe solid in the production process of GALVALUME steel sheets. The pure Fe sheets were hot-dipped in Al–25Zn and Al–25Zn–2Si (at%) alloy melts at 600, 650, and 700°C for 2~3600 s. Significantly thick coatings were formed on Fe sheets hot-dipped in the Al–25Zn binary alloy melt for a longer time than 10 s. The coating thickness became several millimeters after 30 s, resulting in a delamination of the coating. The significant Fe dissolution occurred in the Al–Zn binary alloy melt, accompanied by a significant growth of η phase (Fe₂Al₅) toward the solid Fe. The growth could be promoted by the Zn-rich liquid phase with a lower melting temperature. However, in the case of hot-dipping in the Al–25Zn–2Si ternary alloy melt, uniform coatings were formed on the hot-dipped Fe sheets due to the suppressed interfacial reactions. The Fe dissolution slightly occurred, and a continuous layer of Si-rich T₅ (Fe₂Al_7.4Si) phase was formed at the interface of solid Fe with the Al–25Zn–2Si alloy melt. The continuous T₅ phase layer would play a role in a diffusion barrier at the interface of solid Fe with liquid Al–Zn alloy, resulting in the suppressed interfacial reaction. These interfacial reaction processes are discussed based on thermodynamic calculations of the Fe–Al–Zn ternary and Fe–Al–Zn–Si quaternary systems.

The solid–liquid interface energy anisotropy of Zn alloys remains poorly understood. Recently, characteristic 14-arm dendritic growth has been observed using time-resolved X-ray computed tomography at SPring-8 during the solidification of a Zn–4mass%Al alloy. This study investigates the dependence of the dendrite morphologies of Zn alloys on solid–liquid interface energy anisotropy through systematic phase-field simulations of the growth of an isolated equiaxed dendrite. We also develop a data assimilation system to estimate the anisotropy parameters of solid–liquid interface energy and crystal orientation in Zn alloys and validate the system through twin experiments. This study provides insights into the solidification of Zn alloys and a powerful tool for their investigation.

The friction-type joints using high-strength bolts are frequently employed for the assembly of structural steel components. The drawback of the combination of the friction-type joints and hot-dip galvanized steel plates for highly corrosive environments is the low slip coefficient at the friction interface in the as-coated condition. To increase the slip coefficient, labor-intensive blast processing or phosphate treatment is applied to the surface of the galvanized steel plates before assembly. In this study, we investigated the slip mechanism at the friction interface between as-galvanized steel plates through slip resistance tests on high-strength bolted friction joints, in hope of determining effective methods for overcoming the low slip coefficient in the as-coated condition. In the as-galvanized material, both the outermost Zn- and ζ(FeZn₁₃)-phase layers exhibit c-axis texture. Since the easiest basal (dislocation) slip plane for the Zn phase with the hexagonal close-packed structure is parallel to the friction interface, the Zn phase is geometrically prone to plastic deformation due to the shear stress applied on the friction interface. The evidence that the coarse-grained Zn phase was refined to small crystal grains upon macroscopic slippage at the friction interface indicated that the low slip coefficient was attributed to the readily deformable nature of the outmost Zn phase. Potential strategies for increasing the slip coefficient without pre-surface treatment include strengthening the soft Zn phase through grain refinement or texture modification, or complete removal of the Zn phase during galvanizing.

The study investigated the dwell fatigue characteristics of hot-dip galvanized steel. Cyclic and dwell fatigue tests were conducted, their fatigue life was compared, and fracture surfaces were analyzed. When the cyclic maximum stress (σ_max) was the upper yield stress (σ_UYS), there was hardly a difference in fatigue life between cyclic and dwell fatigues. In σ_max=0.9 × σ_UYS, the fatigue life in dwell fatigue was shorter than that in cyclic fatigue. The cracks under dwell fatigue were generated in σ_max= σ_UYS before N=10 cycles. Their cracks did not grow until N=100,000 cycles. On the other hand, no cracks were observed on the specimen surface under cyclic fatigue before N=100,000 cycles. The formation of cracks on the surface of the galvanized layer under cyclic fatigue was remarkably delayed compared to that under dwell fatigue, regardless of the applied stresses in this study. Therefore, dwell fatigue mode debases the surface of the hot-dip galvanized steel. The applied stress affected the crack morphology on the specimen surface. In σ_max= σ_UYS, the large cracks were observed at the grain boundary triple junctions. In σ_max=0.9 × σ_UYS, not only the cracks at triple junctions of grain boundary but also some cavities along the grain boundaries were detected. Their defects were often reported under creep deformation. The cavities seemed to adjoin each other and coalesce. In the stress relaxation testing, the hot-dip galvanized steel exhibited creep behavior. The decrease in the fatigue life under dwell fatigue would be due to the creep phenomena.