As a countermeasure for deterioration of raw materials’ quality, improvement of sinter productivity is needed. To increase sinter productivity, steam heating granulation technologies have been developed. Heated granules reduce water condensation at wetting zone of sintering bed. It improves permeability of sinter bed and sinter productivity. At first, sintering properties and heat conductive efficiency were investigated at laboratory steam injection apparatus. As a result of the laboratory test, it was confirmed that the moisture condensation at the raw material bed was suppressed by steam injection, permeability was improved, and the sinter productivity was improved. Second, on the actual plant test, the rise of raw material’s temperature by steam heating was verified. On that test, granules were heated by 20°C. In terms of difference of heat conductive efficiency between laboratory and actual plant test, heat and material balance were examined to establish the heat transfer model at actual process. According to the heat analysis of actual plant test, indirect heat transfer via lining of drum mixer also contributed to heat granules.

This study reports a case of cold cracking along welds, which arises from solidification cracking within the crater during the laser welding of high-strength steel sheets. In this investigation, we aimed to delineate the factors influencing cold cracking that originates from solidification cracking in the crater. This was achieved by using steel sheets whose mechanical properties (tensile strength: 0.6 to 1.5 GPa) and chemical composition (carbon content: 0.20 to 0.55%) were individually adjusted. The evaluation method involved performing laser welding in a stitch pattern on an oiled steel sheet, with variations in welding length. The evaluation focused on the maximum welding length at which cold cracking occurred (L_MAX). The results indicated that while a high tensile strength of the steel sheet marginally increased the L_MAX, the impact remained limited. Conversely, the carbon content of the steel sheet significantly influenced cold cracking; the L_MAX for carbon contents of 0.30% and 0.45% was substantially greater than that for 0.20%. However, an unusual behavior was observed at a carbon content of 0.55%, where the L_MAX was smaller than that for 0.45%, despite the significant hardening of the weld metal. This phenomenon was hypothesized to occur because the tensile residual stresses in the welds decreased as martensitic transformation starting temperature lowered and the expansion strain during the transformation increased with higher carbon content.

In order to reduce the construction cost of hydrogen infrastructure facilities, application of inexpensive materials such as low alloy steel is required. However, when hydrogen embrittlement is a concern, as in the case of low alloy steels, acquisition of material data in high-pressure hydrogen gas and proof of safety are necessary for application. Since the testing in high-pressure hydrogen gas incurs a high cost as a simple evaluation technique for the mechanical properties of materials in high-pressure hydrogen gas. While the comparison of mechanical properties such as total elongation in these two test environments has been an object of research, the difference in fracture processes is still not clear. In this study, slow strain rate tensile (SSRT) tests in high-pressure hydrogen gas and cathodic charge were conducted with two materials that had been controlled to different strengths by heat treatment, and their fracture behavior was investigated. Regardless the fracture mechanisms attributed to material strength, such as quasi-cleavage and intergranular-like fracture, the cathodic hydrogen charging SSRT test showed fracture displacement and a reduction of area equivalent to those in high-pressure hydrogen gas. However, the nominal stress-displacement curves showed different behavior immediately before final fracture. A detailed examination of the fracture processes of the specimens revealed that the difference in the fracture behavior of the specimens in the high-pressure gas test and in the cathodic hydrogen charging test was caused by the difference in hydrogen-induced crack growth behavior.

Our previous study clarified the effect of MnS on gigacycle fatigue properties of SCM440 steel by conducting gigacycle fatigue tests on High-S steels whose S-content was so high as to be close to an upper limit of the JIS standard. On the other hand, this study discusses methods to avoid fatigue-failure from MnS by conducting the gigacycle fatigue tests on Low-S steel whose S-content is as low as possible in commercially available processes. Three types of materials were prepared with different working ratios, and the fatigue tests were conducted in transverse directions. The Low-S revealed higher fatigue strengths than the High-S. The fatigue strengths of Low-S with high working ratios were as high as those of a hot-rolled round bar. The Low-S with the lowest working ratio was fractured from shrinkages, resulting in low fatigue strength. The Low-S with the highest working ratio was fractured from oxides and matrix. As the results, MnS caused fatigue failure in a few specimens of the Low-S only with the middle working ratio. These results meant that the MnS-induced fatigue-failure was avoidable by minimizing the S-content. The fatigue test results were analyzed by using a previously derived prediction. The analysis results suggested that the MnS of High-S and the shrinkages of Low-S were more harmful than others. This was probably attributed to the large number of the MnS of High-S and of the shrinkage of Low-S, which accelerated crack propagations by interaction between the MnS inclusions and between the shrinkages.

Plasma nitriding of JIS SKD61 tool steel was performed by open-air type atmospheric-pressure plasma jet. The results of our experiments show that the surface hardness of tool steel work pieces was increased by more than two times that of the core material after within 30 min of treatment time.