Studies on the oxide scale in hot-rolling process are reviewed. This paper outlines scale formation and the effects of alloying elements on scale formation, scale behaviour in hot-rolling processes such as rolling and descaling, blistering phenomenon, red scale, whisker generation, scale transformation, and Cu-induced hot shortness.
Steel containers and equipment are used to handle Mg and Mg-alloy melts in industrial processes such as Mg casting and Ti smelting. In this study, the dissolution behavior of SUS430 ferritic stainless steel in liquid Mg was quantitatively evaluated in order to obtain fundamental information on the contamination of Mg with steel materials in these industrial processes. Pure Mg was sealed in a SUS430 crucible and melted at 1073−1273 K for 24−96 h. In addition to Fe and Cr, some minor elements in the SUS430 (Mn, Ni, and Cu) were evaluated as impurity elements dissolved in liquid Mg. The concentrations of Fe and Cr in liquid Mg reached a steady state within 24 h, and the empirical equations describing their temperature dependence were obtained. In contrast, the concentrations of Mn, Ni, and Cu in Mg increased with increase in melt holding time. With the dissolution of these elements, a region with Mn concentration lower than that of the original composition was formed on the inner wall of the SUS430 crucible. The validity of the experimental values of impurity concentration in Mg was discussed based on the thermodynamic data of Mg−i (i = Fe, Cr, Mn, Ni, and Cu) binary systems and SUS430. Furthermore, impurity uptake through liquid Mg during Ti production using the Kroll process was preliminary discussed. The findings of this study provide important and beneficial information for improving impurity control in the melting and casting of Mg and in Ti smelting using Mg as a reductant.
The effect of prior austenite (γ) recrystallization and BN precipitation on B segregation behavior at prior austenite grain boundaries (PAGBs) was investigated by electron backscatter diffraction (EBSD) and time of flight secondary ion mass spectroscopy (TOF-SIMS). We measured changes in the BO2− signal intensity (SI) at PAGBs of 0.09C steel during isothermal holding after deforming at 850°C. The γ recrystallization started within 10 seconds of isothermal holding, and SI at recrystallized PAGBs 10 seconds after deforming was almost the same as prior to the deformation, whereas SI at non-recrystallized PAGBs showed lower values. When subjected to isothermal holding for more than 10 seconds, BN precipitation increased and SI at PAGBs decreased. This decrease in SI was attributable to the decrease in the amount of soluble B caused by BN precipitation. We also confirmed that SI at PAGBs of 70% Ni steel decreased immediately after deforming at 700°C, to levels lower than prior to the deformation. These findings indicate that the amount of segregated B at PAGBs decreases with deformation and the subsequent recovery behavior of SI depends on the recrystallization state. In addition, we measured the change in Vickers hardness (ΔHv) of 0.09C steel associated with B addition to investigate the effect of the amount of segregated B at PAGBs on the hardenability. ΔHv expressed in a linear function and increased as SI at PAGBs increased. These results suggest that the hardenability of B added steel is strongly related to segregated B at PAGBs.
The cyclic fatigue, dwell fatigue and room temperature creep properties were evaluated in three types of Ti-6Al-4V forged bar samples having different micro-texture-regions (MTR) and tensile properties in the loading direction. In the S-N curve where the stress(σnor) was normalized by 0.2%-proof-stress, the fatigue lives of all samples were almost the same, whereas the dwell fatigue lives were not the same. So the ratio of the cyclic fatigue life to dwell fatigue life (dwell debit) changed to 2–60. In cyclic fatigue the initiation site was a facet of 1–2 α grains, and the fracture surface was typical. In dwell fatigue and creep, on the other hand, facet and dimple regions were confirmed. In addition, the facet region consisted of initiation facets of 1–2 α grains and the propagation facets which were the majority of the facet region. Initiation facets in dwell fatigue occurred earlier than 25% of the life ratio, and the angle between the c-axis of the α grains with the initiation facets and loading direction was 15–55°. The propagation facets were the MTR in which the angle between the c-axis of the α grains and loading direction was 30° or less. The lengths of the facet regions were proportional to the MTR size. In dwell fatigue, the larger the σnor or MTR size, the larger was the dwell debit. Therefore, the MTR size was considered the dominant factor determining the dwell fatigue life.
Effect of copper addition on solidification microstructure was investigated for high carbon high speed steel type alloys (Fe-2.0%C-5%Cr-5%Mo-5%V-0~4%Cu). The microstructure of all as-cast specimens with different copper content was composed of dendrite of primary γ, MC-γ eutectic and M2C-γ eutectic but it was found X-ray diffraction peaks of α phase by X-ray diffractometry. The volume fraction of each phase in solidification microstructure was approximately constant regardless of copper content. The shapes of dendrite of primary γ changed significantly in response to amount of copper addition: that of specimen with no copper was equiaxed while that of specimen containing copper was columnar which tends to be marked with increasing amount of copper addition. Furthermore, the secondary dendrite arm spacing λ2 decreased with increasing amount of copper addition. As a result, it concluded that the effect of copper addition on solidification microstructure is columnar crystallization and decreasing λ2 of the dendrite with primary γ.