In the hard cooling process of steel, heat flux and temperature fluctuations are so large due to the wetting that it is difficult to measure unsteady cooling phenomena. In this study, we experimentally analyzed the detailed heat transfer behavior of continuous casting secondary cooling of a moving system using the improved IHCP (Inverse heat conduction problem) analysis with the Laplace transform technique developed in the previous paper. The test piece was a SUS304 rotor with a thickness of 10 mm and an outer diameter of 136 mm, which was heated to 880°C, rotated at a peripheral speed of 0.9 to 8.0 m/min, and cooled from above with a flat air mist spray. Thermocouples were installed at two points 1.5 mm and 3.5 mm from the surface. As a result of the analysis, under the conditions of the film boiling region, the surface heat flux q_w could be expressed as q_w / q_w,peak = (W / W_peak)^0.57 using the spray water flux W. However, after the start of wetting, cooling continued even on the downstream side in the moving direction where the water droplets did not collide directly. In the nucleate boiling region and the film boiling region, the average heat flux when passing through the spray did not change due to the change in casting speed. However, the wetting start temperature became higher as the casting speed becomes slower.

The behavior of magnesium vapor injected in the molten steel and the potential of magnesium vapor for the miniaturization of TiN were evaluated via continuous casting experiments. The position where magnesium vapor should be injected was determined through a non-steady state three-dimensional numerical simulation for molten steel flow in a tundish. The yield of magnesium in the molten steel was changed by the injection position. A high additive yield was achieved when magnesium vapor was injected between the immersion nozzle and the narrow wall of the tundish. The concentration of the soluble magnesium was maintained using a novel method that continuously added magnesium vapor into the molten steel in the tundish, which also ensured stable miniaturization of TiN.

In a process of manufacturing thin steel sheets such as for automobiles, it is common to install and use optical surface inspection system using a light source and cameras that receives regular reflection and non-regular reflection with properly settled incident and observation angles. However, surface inspection is getting more difficult for automobile steel sheets which are being promoted to high tensile strength steel sheets for safety. For example, harmless glittering glossy parts existing on the surface of the steel sheet are detected and mistakenly recognized as a harmful defect, which is called over-detection, or a large amount of over-detection interferes with proper inspection. That is the reason why conventional technologies could not be applied to high tensile strength steel sheets with harmless glittering glossy parts.

This paper describes the cause of harmless glittering glossy parts and essential measures to completely prevent the over-detections by optical means.

A numerical parametric study on friction in cross-roll tube piercing is reported in this paper, in order to assess the role of the different friction coefficients on the different parts of the complex tooling of this process (friction on cross-rolls, Diescher disks, piercing plug). Their effects on entrainment speed, state of strain and stress are quantitatively evaluated using the 3D Finite Element Method (FEM). This knowledge allows measures to be taken in case of friction-dependent defects occurring on the piercing mill. Simple regression formulae are proposed which highlight which friction coefficient(s) most impact feed efficiency, twist angle, piercing plug force and torques on the different tools. Based on these relations, a strategy is developed, involving measurements to be performed and equations to be used for an unambiguous friction coefficients identification procedure.

The partitioning of solute elements during intercritical annealing and the effects of partitioning on ferrite transformation during slow cooling after intercritical annealing in a 0.17% C-1.5% Si-1.7% Mn (mass%) steel were investigated by a new FE-EPMA (field emission electron probe microanalysis) technique. This new technique enables highly accurate measurement of the C distribution. During the intercritical annealing, C and Mn concentrated into austenite, while Si concentrated into ferrite. The distribution of Mn in austenite was inhomogeneous, and austenite with small Mn content was transformed into ferrite during slow cooling. This ferrite transformation proceeded in the NPLE (negligible partitioning local equilibrium) mode. Two kinds of ferrite were produced due to slow cooling, one being intercritically-annealed ferrite, and the other transformed ferrite. The transformed ferrite had larger Mn content than the intercritically-annealed ferrite. Furthermore, the transformed ferrite was classified into the ferrite grown epitaxially from the intercritically-annealed ferrite and that nucleated in the austenite with relatively small Mn content. Prior microstructure and distribution of solute elements before cooling are determined by the intercritical annealing conditions, and then control the ferrite transformation. Precise control of the ferrite transformation is effective for stable production of cold-rolled high strength steel with composite microstructure.

Dislocations are often introduced in Ni-based superalloys to impart sufficient strength at both room temperature and high temperatures prior to their use in automobile exhaust gaskets. However, the interaction between the representative γ′ (Ni₃(Al, Ti))-phase precipitates and dislocations in high temperature remains unclear. Therefore, this study examined the effect of cold rolling on age-hardening behavior and microstructure evolution, focusing on the formation of γ′-phase Ni₃Ti during aging at 700ºC for up to 400 h after 60% cold rolling of solution-treated specimens. During the early stage of aging, at 0.03 h, the hardness rapidly increased from 401 HV to 496 HV. Age-hardening continued until 3 h and reached its peak of 536 HV, followed by gradual decrease with aging time. 3D atom probe investigation revealed that the γ′-phase was confirmed after 0.3 h of aging. However, the composition-modulated structure speculated to be caused by spinodal decomposition was observed in the 0.03 h aged specimen. The change in strength with aging time was considered by calculating the contribution of each strengthening mechanism. In the initial stage of aging (0-3 h), dislocation and solid-solution strengthening dominated along with spinodal strengthening. Strengthening by spinodal decomposition in the 0.03 h aged specimen is presumptively accelerated by the introduced dislocations, which is followed by further precipitation strengthening caused by γ′-phase precipitates. In the later stage of aging (3-400 h), precipitation strengthening became dominant and reached its peak at 20 h aging, while dislocation strengthening decreased with aging time.

The changes in the state of carbon in tempered martensite and retained austenite in carburized SAE4320 steel under the rolling contact fatigue (RCF) were investigated using atom probe tomography (APT). In the tempered martensite, the carbons in solid solution and in carbon cluster were readily transferred to the preexisting metastable (ε) carbide due to rolling contact, resulting in a localized change from tempered martensite to ferrite accompanied by the growth of carbides. This supports the recently proposed dislocation assisted carbon migration theory. On the other hand, retained austenite with uniformly distributed enriched solute carbon was partially transformed into the very fine deformation-induced martensite due to rolling contact. Furthermore, carbon seemed to be partitioned into retained austenite from the deformation-induced martensite during further rolling contact cycles. This is a new insight into the characteristics of deformation-induced martensite and retained austenite generated by rolling contact. The present study provides a plausible explanation to the phenomenon that the deformation-induced martensitic transformation improves the RCF life.

It is important to improve fatigue life for pitting of gear in order to attempt down-sizing of gear unit and low fuel consumption. Rolling and sliding contact fatigue under high pressure in carburized SCM420 steel is studied. The ultimate aim is to understand the effect of material properties on the fatigue life. In this research, there was considerable interest in change of the material in the contact surface of specimens depending on the shift in tangential force acting on a moving roller in the direction of a tangent to the path of the roller during the fatigue test. Three lubricants were used in these fatigue tests for the purpose of changing the degree of tangential force. A roller tested under a lubricant that the tangential force is shifted higher compared with that of the other lubricant produced a pitting failure earlier. The roller also promoted to decrease the hardness and to narrow the half width of the (211) peak of alpha iron at the contact surface. The transition of these characteristics related to the creation of fine grain layer. That is regarded as worked structure formed during fatigue process. The depth of fine grain layer expands according to repeated loading. Fatigue crack propagation depth was almost equal to the depth of fine grain layer. Not only well-known softening caused by increase in temperature but also microstructural change caused by repeated stress loading have an effect on the life through the involvement in fatigue crack initiation and propagation behavior.

In order to investigate the adaptability of small punch (SP) creep testing technique to the remaining-life assessment, the SP creep test was carried out with 2.25Cr-1Mo steel hot rehear steam piping, which had been actually used in a fossil power plant for long periods of time. The SP load (F) was converted to the stress (σ) by three different equations, which were derived based on the displacement to maximum load in the SP test (u_m) and the deflection to minimum deflection rate in the SP creep test (u_min) for correlating the SP creep rupture data with the uniaxial ones. The experimental results showed that the SP creep rupture time of specimen removed at around outer surface of piping tended to be slightly shorter than those taken at around inner surface and center. It was also found that the SP creep rupture data were relatively in good agreement with the uniaxial ones by converting F to σ with the equations, and the creep remaining-life was well predicted by the extrapolation of short-term SP creep rupture data. The highest prediction accuracy was obtained by using the equation derived from the SP test result, that is, u_m. Consequently, it was confirmed that the SP testing technique could be a strong tool for creep remaining-life assessment of boiler piping.

Micro-tensile behaviour and the corresponding microstructural evolution under hydrogen pre-charging conditions were examined on single-crystalline and twinned bi-crystalline specimens with the same [111] loading axis to elucidate the hydrogen-induced twin boundary separation in type 304 stainless steel. A hydrogen pre-charge increased the flow stress during tensile testing but decreased the elongation-to-failure in both single-crystalline and twinned specimens. Although the hydrogen-charged single-crystalline specimen exhibited a quasi-cleavage, the presence of a twin boundary induced a premature failure at the twin boundary interface. Flat-facetted features due to the twin boundary separation had linear steps in the three <110> directions, which corresponded to the intersections between the twin plane and the other {111} close-packed planes of austenite. Matching halves of the fracture surface along the three directions perpendicular to the linear steps, i.e. <112> on the (111) twin plane, revealed two sets of concavity–flat surface and a peak-and-valley correspondence. In addition, electron backscatter diffraction analysis of the substructures below the fracture surfaces revealed that martensite variants developed mainly with their habit planes parallel to the most favourably shear-stressed plane in each crystal, and they grew towards the concavities on the fracture surfaces. These findings suggest that the hydrogen-induced twin boundary separation is triggered by cracks generated by the high hydrogen concentration at the twin boundary due to deformation-induced martensitic transformation, and this is followed by coalescence of cracks through hydrogen-enhanced alternating shear on the slip planes situated symmetrically with respect to the twin boundary.