In the steelmaking process, it is necessary to decrease impurities in steel to meet the increasing demand for high-grade products. Top blowing and blasting of powder reagent are desirable for the purpose and the deeper penetration of particles into the bath is important for efficient refining. In the present work, CFD calculation with VOF (Volume of Fluid) method and dynamic mesh was executed to study the reported penetration and residual bubble behavior of polypropylene sphere (diameter of 9.6 mm) with a static contact angle of 87° and 143° and an entry solid sphere velocity of 0.63, 0.89, and 1.53 m/s in the water model experiments. Calculated results showed the numerical analysis could evaluate the formation and breakup of air column behind the sphere and the generation of consequent residual bubble on the sphere. Good wettability and high entry speed promoted the deeper penetration of the sphere. Calculated dynamic contact angle on the basis of Kistler’s model indicated that the difference between static and dynamic contact angles was within 13.7° in the present conditions and the discrepancy could not wield a substantial influence on the result of CFD calculation. The adoption of base and refined mesh without parallel zone around the sphere could not give a good agreement with the experimental results. On the other hand, the use of layer mesh was appropriate for reproducing the penetration depth and residual bubble volume observed in the experiments.

Magnetic materials are used for the iron cores of transformers, reactors, and rotating machines. The reduction of the iron loss of the iron core can reduce the loss of these electrical equipment and contribute to energy saving. Therefore, it is important to quantitatively evaluate the magnetic properties of magnetic materials. On the other hand, the eddy current test is widely used to detect changes in electromagnetic characteristics such as conductivity and magnetic permeability without the specimen contacting the sensor. In this paper, we image and quantitatively evaluate changes in the magnetic properties of steel sheets using rectangular wave eddy current testing (RECT). First, we basically verify whether RECT can detect the spatial magnetic property change of the steel sheet due to the magnetization of the permanent magnet. The result demonstrates that RECT is useful image and quantify the spatial magnetic property change. Second, the effect of magnetization on the signal obtained by RECT is examined by changing the intensity of DC magnetic field, and electromagnetic field simulation based on the finite element method is also conducted to calculate the magnetic permeability distribution inside the steel sheet when a DC magnetic field is applied and validate the experimental results. The experimental result demonstrates that the signal depends on the intensity of the DC magnetic field, and the simulation result supports the experimental result.

Morphologies of retained γ grains in TRIP steels are a very important factor for understanding the relationship between the microstructure and mechanical properties of TRIP steels. We have investigated a serial sectioning technique using a scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) and a focused ion beam (FIB) to reconstruct 3D microstructure of TRIP steels. In this paper, FIB fabrication condition dependences of reconstructed 3D structures are presented in TRIP steels and γ-grains stabilities are discussed in terms of ion beam effects. Retained γ-grains are not recognized by EBSD map from a FIB fabricated surface using normal ion accelerating voltage of 30 kV, because γ grains are easily transformed into martensite due to ion irradiation. It was found that the transformation was suppressed by using a low acceleration voltage of 7 kV or less, and the three-dimensional morphology could be observed using low acceleration voltage serial sectioning technique. It becomes possible to evaluate morphology of each phase and we proposed evaluation method of γ stabilities by ion irradiation transformation.

Changing mold width of continuous casting takes long shut down time. Therefore in order to produce various width slabs in roughing mills, sizing press has been installed in most hot strip mills. Installation of sizing press can improve total productivity of continuous casting and hot strip mills.

After sizing press and horizontal rolling, it is known that width around head and tail portions of slab are narrower than that of middle. These narrow portions, which are called “width drop”, cause yield loss. Therefore some technologies have been developed to prevent width drop.

It is said that width drop results from difference of width spread in horizontal rolling depending on distribution of dog bone profile along rolling direction of slab. However some experimental and simulated data in some papers indicated width shrinkage with horizontal rolling of slabs whose cross sectional shape was dog bone profile. Although it is presumed that width drop is influenced by not only width spread but also width shrinkage, there are no papers which describe mechanism of width shrinkage clearly.

In this paper, experiment and FE analysis of horizontal rolling with slabs which have dog bone profile were executed in order to investigate mechanism of width shrinkage. As the result of FE analysis, width shrinkage is influenced by area reduction ratio between width edge part and width center part. Furthermore longitudinal velocity distribution along width direction around head and tail portions cause width shrinkage during horizontal rolling.

Friction stir welding (FSW) was performed under the two welding conditions (rotation speed - traveling speed) of 150 rpm - 100 mm/min and 200 rpm - 400 mm/min using 24%Ni - 0.1%C (mass%) steel. The rapid cooling utilizing liquid CO₂ was exploited to strictly evaluate the microstructural evolution of austenite in stir zone after FSW. In addition, the effect of the microstructural characteristics of austenite on martensitic transformation behavior during cooling were compared with that of 6%Ni - 0.63%C steel. Irrespective of the welding conditions, fine equiaxial grains with simple shear texture of {111}<110> orientation formed. At 200 rpm - 400 mm/min, finer grain formed without grain growth during cooling. On the other hand, at 150 rpm - 100 mm/min, recovery and grain growth occurred during relatively slow cooling after FSW. The grain growth preferentially proceeded presumably due to the strain-induced grain boundary migration of grain with lower dislocation density having {110}<110> orientation. Larger amount of retained austenite remained in the stir zone of 6%Ni - 0.63%C steel FSWed at 200 rpm - 400 mm/min due to both finer austenite grain size and higher dislocation density. Moreover, the influence of austenite grain orientation on the thermally induced martensitic transformation during cooling was revealed to be negligible, which is different from that of deformation induced martensitic transformation.

Resistance to Sulfide Stress Cracking (SSC) caused by local hard zones of pipe inner surface has been required in low alloy linepipe steel. In this study, using two samples with different surface hardness, the detailed SSC initiation behavior was clarified by four-point bend (4PB) SSC tests in which immersion time and applied stress were changed in a sour environment containing 0.15 bar hydrogen sulfide (H₂S) gas. SSC cracks occurred when the applied stress was higher than 90% actual yield strength (AYS) in higher surface hardness samples over 270 HV0.1. From the fracture surface observation of SSC crack sample, it was found that the mechanism gradually shifted from active path corrosion (APC) to hydrogen embrittlement (HE), and that the influence of APC mechanism remained partially in the process of SSC initiation at the tip of corrosion pit or groove. The polarization measurement in the 4PB SSC test showed that the anodic and cathodic reactions (especially cathodic reactions) were activated when the applied stress was 90% AYS or higher. The FEM coupled analysis simulating the stress and strain concentration at the bottom tip of the corrosion groove and the hydrogen diffusion and accumulation was carried out. The principal stress in the tensile direction showed the maximum value at 0.04-0.06 mm away from the tip of the corrosion groove, and the hydrogen accumulation became the maximum. It was analytically found that the SSC crack initiated and propagated with HE mechanism dominated type when the threshold value of about 0.82 ppm is exceeded.

The hydrogen trapping of V and Mo carbide precipitates were investigated using tempered martensitic steels (0.3%C-1.2%Cr and 0.2%V or 1.0%Mo), for the purpose of improving hydrogen embrittlement strength. The effect of tempering conditions on hydrogen trapping energy and trapped hydrogen content was studied. The trapped hydrogen content of V or Mo carbide was maximum value at the tempering temperature of 600°C for 1 hour and the trapped hydrogen content in Mo carbide was more dependent on tempering temperature than V carbide. In cases where the tempering time exceeded 1 hour at a tempering temperature 600°C, the trapped hydrogen content in Mo carbide decreased remarkably. At the tempering temperature of 600°C for 1 hour, the hydrogen trapping energy in V carbide was about 57 kJ/mol, which was a little higher than in Mo carbide. The V and Mo carbide at the tempering temperature of 600°C for 1 hour were identified cubic VC and MoC by TEM observation, respectively. The both form were plate-shaped with a width of about 1nm and a length of about 20 nm or less. At a tempering temperature of 650°C for 1 hour or 600°C for 24 hours, MoC was significantly reduced and precipitated rod-shaped hexagonal Mo₂C. It was presumed that this was the cause of the decrease in the hydrogen trapped content of Mo carbide under these tempering conditions.

The aim of this paper is to predict the deformation-induced martensitic transformation of the retained austenite in steels under various deformations, including loading-path changes, by using mesoscopic finite element analyses (FEAs). First, a TRIP steel was subjected to monotonic uniaxial tension and compression, as well as a couple of two-stage loadings to investigate the effect of loading direction and loading-path on transformation behavior experimentally. In monotonic loading, tension induced transformation at higher rate than compression did. Whereas, in two-stage loadings, the transformation progress was suspended immediately after the start of secondary loading. As the secondary tension proceeded, the transformation resumed and gradually accelerated toward the transformation rate for monotonic tension. These experimental results were analyzed by FEAs with a two-dimensional image of microstructure. The transformation rates under monotonic loading are well predicted by the simulation. It is also suggested that the difference in the transformation rate between tension and compression is mainly due to the volumetric expansion associated with martensitic transformation, and that the transformation behavior of the untransformed austenite is dominated by the distribution of the hard transformed martensite. In addition, the prediction of the transformation rate in secondary tension after pre-compression required the consideration of back stress in the austenite. The reproducibility of the transformation behavior just after the onset of secondary deformation was improved by the hypothesis that the equivalent value of back stress tensor at the transformation needs to exceed its maximum value in the past.

Long-term creep rupture data of 0.5Cr-0.5Mo (STBA20), 1Cr-0.3Mo (KA-STBA21) and 1Cr-0.5Mo (STBA22) steels were analyzed to estimate 100,000 h creep rupture strength.

The creep rupture data were fitted to the regression equation of logarithmic stress using the time-temperature parameters of Larson–Miller (LM), Orr–Sherby–Dorn (OSD) and Manson–Haferd (MH) to estimate the 100,000 h creep rupture strength. The appropriate parameter and degree of regression equation was MH-2nd degree and LM-4th degree for 1Cr-0.3Mo and 1Cr-0.5Mo steels, respectively. For 0.5Cr-0.5Mo steel, the creep rupture data under stresses lower than 80% of tensile strength was analyzed because the scatter band of the creep rupture data was very large under high stress regimes. The appropriate parameter and degree of regression equation was LM-3rd degree for the 0.5Cr-0.5Mo steel. It was confirmed that the creep rupture data up to 550°C should be used for evaluation of 100,000 h creep rupture strength because oxidation scale decreased creep rupture strength at 600°C or above. The allowable stress of the steels studied was estimated based on average and minimum values of 100,000 h creep rupture strength. The estimated allowable stresses were equal to or greater than current allowable stress of ASME boiler and pressure vessel code and Technical Standard for Thermal Power Plant in Japan.

A certain relation was found between melting temperature of multi-component fluorides and their bond strength index. It can be used as a convenient method to know the melting temperature of multi-component fluorides for which thermodynamic information is lacking. These results are similar to those of the previously reported multi-component oxides.