The 10 most-cited articles in 鐵と鋼 : 日本鐡鋼協會々誌, 2016

In the present study, hardness, elastic modulus and fracture toughness of η-Fe2Al5 (oC24 structure) and θ-FeAl3 (mS102 structure) single phase alloys have been examined by various indentation methods. The average hardness of η and θ phases measured by Vickers indentation are 8.5 GPa and 7.8 GPa, respectively. The hardness variations are approximately 20% in η phase and 15% in θ phase, which are associated with their orientation dependence. A trend was found that the hardness becomes lower close to the [001] indentation direction in both phases. The elastic moduli of η and θ phases measured by nanoindentation are higher than the modulus of pure Fe (about 200 GPa). The fracture toughness of η and θ phases measured by indentation fracture method are 1.38 and 1.27 MPa·m1/2, respectively. The anisotropy of their toughness was not found in the present measurements.

Deep learning by convolution neural network (CNN) was applied to recognize a microstructure of steels. Three typical CNN-models such as LeNet5, AlexNet, and GoogLeNet were examined their accuracy of recognition. In addition to a model, an effect of learning rate, dropout ratio, and mean image subtraction on recognition accuracy were also investigated. Through this study, the potency of deep learning for microstructural classification is demonstrated.

This paper introduces the microstructure, retained austenite characteristics, strain-induced transformation-deformation mechanism and mechanical properties of transformation-induced plasticity (TRIP)-aided martensitic (TM) steels for the automotive applications. Because the microstructure consists of a wide lath-martensite structured matrix and a mixture of narrow lath-martensite and metastable retained austenite (MA-like phase), the TM steel produced a good combination of tensile strength and cold formability. If Cr and/or Mo were added into 0.2%C-1.5%Si-1.5%Mn steel to enhance its hardenability, the resultant TM steel achieved superior notch fatigue strength and impact and fracture toughness to conventional structural steel such as SCM420. These enhanced mechanical properties were found to be mainly caused by: (1) plastic relaxation of the stress concentration, which results from expansion strain on the strain-induced transformation of the metastable retained austenite; and (2) the presence of a large quantity of finely dispersed MA-like phase, which suppresses crack or void…

Recently, 5% Mn steel has been focused on as one of the promising candidates for third generation AHSS by showing an excellent TS-El relationship. The excellent TS-El relationship is brought about by a large volume fraction of retained austenite through the enrichment of austenite stabilizing elements such as C and Mn in reverted austenite. The effect of the microstructure of mother hot band on the changes in microstructure and mechanical properties was compared with the intercritical annealing time in this study. The steel containing about 10% of retained austenite in a mother hot band exhibited a higher volume fraction of retained austenite and higher strength. On the other hand, the steel which did not contain retained austenite in a mother hot band exhibited excellent strength-elongation combination. The difference of work hardening behavior in these steels was analyzed and thought to be brought about by the difference of transformation behavior during deformation determined by the stability of retained austenite affected by Mn concentration.

In this study, hydrogen absorption behavior of steels on a vehicle during driving and parking in deicing-salt-spraying areas was investigated by using temperature-compensating hydrogen absorption monitoring system. During the driving of the vehicle in the areas, hydrogen permeation current of steel was increased due to picking up of salt water from the road when the road was wet. On the other hand, during the parking of the vehicle, the magnitude of the hydrogen permeation current were indicated that the hydrogen permeation current in the automobile driving environments is related to both driving states of a vehicle and environmental conditions such as temperature and relative humidity.

Steelmaking slag is recycled and reused, but its utilization is restricted due to its chemical properties. On the other hand, steelmaking slag is considered to be a promising resource of iron and phosphorus in Japan. In this work, reduction of (FetO) and (P2O5) in steelmaking slag and recovery of iron and phosphorus resources were investigated with the aim of developing a new recycling process for steelmaking slag. In order to investigate the effect of slag basicity (%CaO/%SiO2) and FetO content in slag on phosphorus separation from steelmaking slag, steelmaking slags were reduced by carbonaceous material at high temperature by using an induction furnace. From the results, the effects of slag composition, temperature and oxygen partial pressure on phosphorus separation from steelmaking slag were discussed.

States of hydrogen present in high-strength steels for use as bearing steel SUJ2 and hydrogen embrittlement susceptibility were examined using thermal desorption analysis (TDA) and tensile tests. SUJ2 specimens containing retained austenite phase (γR) in the martensite phase exhibited three hydrogen desorption peaks in the TDA profile. Two of the peaks desorbed at higher temperatures decreased with a decreasing amount of γR, indicating they corresponded to desorption associated with γR. Fracture strength in the presence of hydrogen increased with a decreasing amount of γR and with an increasing strain rate. When the specimens contained γR and hydrogen, a flat facet at the crack initiation site and a quasi-cleavage (QC) fracture in the initial crack propagation area were observed on the fracture surface. Local characterization using electron back-scattered diffraction (EBSD) revealed that the flat facet on the fracture surface corresponded not to γR but to stress-induced martensite. In addition, the facet was {112} plane of martensite, which is the slip plane or deformation twin…

Two kinds of advanced image processing were applied to multi-phase microstructures. One is evolutional image processing where optimized filter set was suggested by genetic programing. Another is trainable WEKA segmentation where features are extracted by many kinds of filters, followed by machine learning for classification. Once an optimized filter set is determined, efficiency of image processing for new data set is improved remarkably in comparison with a case of manual image processing.

Plastic deformation and dislocation accumulation in dispersion hardening alloys are numerically analyzed by a crystal plasticity finite element technique and work hardening characteristics are discussed. The critical resolved shear stress for slip system is given by the extended expression of the Bailey-Hirsch type model which include the Orowan stress as size effect of microstructure. Work hardening of slip system is estimated by statistically stored dislocation (SSD) density. Increment of the SSD density is evaluated by slip strain and the mean free path of dislocations (the Kocks-Mecking model). The mean free path depends on the average spacing of dispersed particles, which is also used to estimate the Orowan stress. The average spacing of dispersed particles is calculated from the volume fraction and average diameter of dispersed particles. As a result, flow stress level at the initial stage of deformation agreed very well with experimental result but work hardening rate was higher than that of experiment. From this fact, it is considered that the mean free path and the average…

The influence of the Mn content of Si-added steel sheets on the Fe-Zn galvannealing reaction was investigated. Three steel sheets, 1.5 mass%Si-1.4, 1.9 and 2.7 mass%Mn, were annealed in a 10vol%H2-90vol%N2 atmosphere. Si and Mn oxides were analyzed by reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectrometry. SiO2 and Mn2SiO4 formed as selective oxides at the steel surface after recrystallization annealing. The ratio of the oxide species changed depending on the Mn content in the steel. When the Mn content was lower, formation of SiO2 was promoted and that of Mn2SiO4 was suppressed. In the selective oxide layer which formed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel sheets, Mn2SiO4 formed at the outer side, and SiO2 formed at the inner side. This can be explained by consideration of the thermodynamic oxygen potential gradient. Furthermore, areas where SiO2 mainly formed and those where Mn2SiO4 mainly formed were distributed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel…