The 10 most-cited articles in 鐵と鋼 : 日本鐡鋼協會々誌, 2019

Global warming has been regarded as a crucial issue in every industry. Since long term global goal was set on the basis of Paris Agreement, a considerable evolution toward CO2 mitigation in 2050 is desired even in steel industry. Until now, many various technology developments were carried out in the ironmaking area; however, the innovative progress beyond the past progressive developments is required to attain the long-term goal in 2050. This review focuses on the current technology development on CO2 mitigation to date and the design of an ambitious ironmaking process for the future. In particular, the directions for low carbon and decarbonization are discussed from the viewpoints of technological aspects and the comprehensive consistency with sustainability in steel industry. Moreover, the perspectives on CCU (CO2 Capture and Utilization) and hydrogen ironmaking process based on the renewable energy aiming for carbon direct avoidance are described.

Severe plastic deformation has attracted interests as one of the breakthrough procedures to improve various properties of metals and alloys. Recently, it has been revealed that heavy cold rolling of some kinds of austenitic stainless steels can cause ultrafine-grained structure comparable with those achieved by severe plastic deformation. Coarse initial grains were fragmented by deformation induced microstructure to develop heterogeneous nanostructure. Tensile strength of heterogeneous-nanostructured stainless steel exceeds 2 GPa. It is considered that high strength of heterogeneous-nanostructured metals is attributed to such peculiar microstructure with dispersed “eye-shaped twin domains”. In this study, microstructural mechanisms and factors which contribute to macroscopic strength of heterogeneous-nanostructured austenitic stainless steel were evaluated on the basis of multiscale crystal plasticity simulation. Microstructure of heavily cold-rolled SUS316LN austenitic stainless steel was investigated by transmission electron microscopy, and stress-strain curves were attained by…

We measured local grain orientation gradient and dislocation density by electron backscatter diffraction (EBSD) measurement and electron channeling contrast imaging (ECCI) to obtain strain maps near a stress concentration source in a pure nickel as a FCC model specimen. In particular, we obtained relationship among grain orientation spread (GOS), dislocation density, and equivalent plastic strain on the specimen surface, which were obtained by EBSD, ECCI, and digital image correlation (DIC), respectively. After obtaining GOS-strain and dislocation density-strain relations, the strain distribution in the specimen interior was also determined by measuring the GOS and dislocation density. Both of the GOS and dislocation density showed a linear correlation, and the dislocation density-strain relation showed a relatively small deviation.

SUS316L is generally believed as a stable austenitic stainless steel, but strain-induced martensitic transformation can occur when large deformation is given. The predominant mechanisms, especially, in terms of the effect of grain size have been reported, however, it is not still clear. In this study, strain-induced transformation behavior of SUS316L steel was investigated from the points of grain size and dislocation density. Microstructures with various grain sizes and dislocation densities were fabricated by warm multi-pass multi-directional caliber rolling and annealing. Fully recrystallize microstructures with austenite grain sizes of 12, 18 and 27 µm were fabricated. Bimodal structures with ultrafine grains and coarse grains including high density dislocations were also fabricated. These specimens were rolled at a reduction strain from 0.12 to 2.7 at liquid nitrogen temperature to occur enough amount of strain-induced martensitic transformation. To evaluate martensite volume fraction in these cryogenic rolled materials, transmission type X-ray diffraction in synchrotron…

Harmonic Structure (HS) has a heterogeneous microstructure consisting of bimodal grain size together with a controlled and specific topological distribution of fine and coarse grains. In other words, the HS is heterogeneous on micro- but homogeneous on macro-scales. The most unique feature of HS is its continuously connected UFG Shell structure. In the present work, the HS design has been applied to pure metals and alloys via SPD powder metallurgy process. At a macro-scale, the harmonic structure materials exhibited significantly better combination of strength and ductility, as compared to their homogeneous microstructure counterparts. This behavior is essentially related to the ability of the HS to promote a large strain hardening and uniform distribution of strain during plastic deformation, leading to improved mechanical properties by avoiding or delaying localized plastic instability. Such outstanding mechanical properties of HS materials are attributed to the continuously connected UFG-Shell structure. This unique microstructure provides stress partitioning in the micro scale…

Md30 is defined as the temperature at which 50 vol.% of α’-martensite is formed at a true tensile strain of 0.3 in metastable austenitic steels. It has been generally believed that the effect of carbon content on Md30 was estimated to be identical to that of nitrogen as shown by Nohara’s equation. However, we found in this study that Md30 in carbon-added steel is lower than that in nitrogen-added steel, which indicates that the effect of carbon content on the mechanical stability of austenite is more significant than that of nitrogen. In addition, the relationship between Md30 and carbon and nitrogen content is not linear. The effect of carbon and nitrogen content on Md30 is higher at lower carbon and nitrogen content region (<0.1%). As this effect was not considered in the previous study, the austenite-stabilizing effects of both the elements were underestimated. Therefore, in this study, new equations are proposed to accurately estimate Md30 of a Fe-Cr-Ni alloy system. As a result, modified Md30 equation is suggested as below:

To recover the degraded paddy field, the efficient supply of Fe, Ca, and Si to the soil water is important. Steelmaking slag was confirmed as a useful fertilizer for paddy growth to supply Ca and Si. In addition, to suppress the H2S generation in paddy soil, supplement of Fe from fertilizer made of slag has been expected.

An evaluate method for hydrogen embrittlement property of high strength steel sheet has been proposed in this study. To take into consideration of the effect of plastic strain in addition to the effects of applied/residual stress and diffusible hydrogen, U-bend specimens have been adopted because steel sheets for automobiles are usually used after press forming into various parts. After U-shape bending, the specimen was loaded using a bolt. The proposed evaluation method is based on the measurement of critical hydrogen content or critical hydrogen charging condition for hydrogen embrittlement fracture at given stress and strain conditions. The hydrogen charging current density was increased in step-wise manner until cracking was observed, and cracking was detected by optical observation and by monitoring voltage between the sample and a counter electrode. The critical hydrogen contents for specimens with varied applied stress were obtained by means of thermal desorption spectroscopy. For the critical hydrogen content, both the hydrogen contents in strained portion of the specimen…

The effects of hydrogen charge on the defect formation behavior in austenitic stainless steels and nickel-chromium alloys during slow strain rate tensile tests (SSRT) were investigated by positron lifetime spectroscopy. Vacancy clustering was significantly promoted by hydrogen charge, although there was no remarkable difference in the dislocation density and mono-vacancy equivalent vacancy density between hydrogen-charged and uncharged samples under the same tensile strains. In various steels and nickel-chromium alloys, vacancy cluster sizes showed a good correlation with breaking elongations of hydrogen-charged samples, implying the vacancy clustering contributes to hydrogen embrittlement by affecting local plasticity.

In steelmaking industry of Japan, one-third steelmaking slag is reused for road constructions. Steelmaking slag contains a few weight percent of calcium oxide, which is termed as free-lime. Free-lime expands its volume by the reactions with water and carbon dioxide, leading to collapse of road substrates. To prevent the road collapse, free-lime in steelmaking slag is now treated to be transformed to Ca(OH)2 and CaCO3 by an aging process; however, the residual free-lime still causes the road collapse. Therefore, it is necessary to detect free-lime in steelmaking slag to safely reutilize it for road constructions. In the present study, we carried out a cathodoluminescence (CL) analysis for identification of free-lime in steelmaking slag. For this purpose, we prepared a synthesized slag sample including free-lime by heating mixtures of reagents. A part of the sample was soaked in water at 70°C for 3 h (aging process). Crystallized free-lime in the sample illuminated orange due to an emission peak of Mn2+ at 600 nm regardless of the precipitation forms of free-lime. Undissolved…