A new approach to quantifying free magnesium oxide (f-MgO) in steelmaking slag has been proposed by using solid-state25Mg nuclear magnetic resonance (NMR) spectroscopy. Although 25Mg (I= –5/2) signals in solid materials often suffer from the second-order quadrupolar broadening, a quite narrow and symmetric signal of MgO is obtained in the 25Mg NMR spectrum thanks to its highly symmetric Mg sites. By contrast, other Mg-containing compounds (e.g. magnesium hydroxide, diopside and ¬kermanite) are more or less affected by the second-order quadrupolar interaction, resulting in severe broad signals buried in the spectral noise. Therefore, the MgO peak can be only detected for steelmaking slag in the 25Mg NMR spectrum without any hindrance. It is significant to select the optimum internal standard (e.g. Mg2Si and MgS) showing the narrow 25Mg signal being free from the second-order quadrupolar broadening like MgO to obtain reliable data. Accuracy of f-MgO concentration obtained by the present method has been demonstrated by using the standard samples in which f-MgO concentration is known. The advantages of the present method to quantify f-MgO in steelmaking slag, no need to dissolve slag in any solvent and detection of surface f-MgO as well as inner f-MgO of slag particles, should outweigh the disadvantages of poor signal-to-noise ratio which is surmounted by using a larger sample rotor and/or a higher magnetic field. These ideas for quantification of f-MgO by NMR should be explicated to that of free calcium oxide (f-CaO) which is responsible for hydrated expansion of steelmaking slag as well.
X-ray diffraction analysis of converter slag was performed by focusing on the lime-phase solid solution. Solid solutions of Ca1-xFexO and Ca1-xMnxO (0 < x < 1) were prepared by mechanochemical processing or high-temperature solid-state reaction, and the relationships between solid solubility x and lattice parameters were determined. Crystallized lime could be distinguished from undissolved lime by the shift of the diffraction angle associated with the formation of the solid solution. The effect of slag aging treatment with vapor was examined by comparing the solid solubility x and the amount of lime phase in the slag subjected to aging treatment with vapor and in the slag that is not subjected to aging treatment. It was confirmed that x affects the hydration behavior of the lime phase and that crystallized lime with high x tends to remain unchanged even after aging. Moreover, about two-thirds of the lime phase was confirmed to have been removed by hydration as a result of the aging process.
Blistering occurs when oxide scale swells during oxidation at high temperatures. Blistered scale causes surface defects when rolled. The present study investigated the effect of nitrogen on blister growth when steel is oxidized at high temperatures, and drew the following conclusions. Atmospheric conditions before oxidation affect blister growth. Blisters nucleate but do not grow, when a steel sample is held in Ar gas or in vacuum before oxidation. Blisters inflate when a steel sample is held in N2 gas before oxidation. The gas inside the grown blisters is mainly N2 gas. The steel surface is nitrided in N2 gas at high temperatures. It is deduced that the steel surface is nitrided before oxidation, and the nitrogen component causes blister growth upon its release as N2 gas at the scale/steel interface.
In our previous paper, we reported that the surface carbon concentration (Cs) in carburized Nb-bearing steel (SCM420Nb) decreases substantially with increasing machining speed before carburizing. In the present study, a systematic study was made to clarify the effect of the hardness of starting material and the effect of machining condition (cutting depth, cutting speed, feed per revolution) on the Cs. It was found that the Cs in eleceropolished Nb-bearing steel was reduced from 1.0% to 0.7% with the change with microstructure from Ferrite + Pearlite to Martensite (in other words, with the increase in hardness). It was also found that the Cs in Ferrite + Pearlite Nb-bearing steel decreases almost linearly with the increase in logP (P:cutting power). To obtain normal Cs in high speed machined Nb-bearing steel, two methods were shown to be effective,① removal of severely deformed surface layer by polishing and ② oxidation treatment at about 600ºC in air prior to gas-carburizing. The formation of Cr oxide layer was observed only on the surface of specimens exhibited abnormally low Cs. Thus, it was concluded that abnormally low Cs observed in Nb-bearing steel was due to the formation of Cr oxide layer during gas-carburizing which prevent the carbon atoms enter into steels.
The effort of this study is to establish simulation method for predicting the effect of mechanical and morphological characteristics of each constituent phase of two-phase steel on two types of ductile properties that has been found to be ductile crack growth resistance controlling mechanical properties; one is critical local strain that controls shear mode ductile cracking, and the other is stress triaxiality dependent ductility. Notched micro-tensile tests for ferrite-pearlite two-phase steel exhibits that ductile failures from specimen center with dimple mode and notch-root surface with shear mode are both controlled by micro-void nucleation at softer ferrite phase near ferrite/pearlite boundary. 3D micro-structural FE-model is developed for analyzing the stress/strain localization behaviors associated with heterogeneous microstructure in strength, and ductile damage model for reproducing damage evolution up to micro-void/micro-crack formation is proposed. The 3D meso-scopic damage simulation method is applied for simulating ductile cracking behaviors of notched tensile specimens of two-phase steel. The damage evolution associated with micro-structural heterogeneity up to ductile cracking from specimen center as well as notch-root surface is well predicted.
It is known that a cracking of brittle phase such as cementite works as a trigger of cleavage fracture initiation. This study shows a microscopic observation of cracked cementite and its quantitation of the cracking nucleation in ferrite-cementite steels. Seven steels with various sizes of microstructures are produced by laboratory scale vacuum melting and rolling. The cementite particle thickness was measured by SEM observation and image analysis. Tensile tests using circumferential notched round bar specimens were conducted. A trace analysis was carried out to identify crystal plane of cleavage surface of cementite using EBSD analysis. The results indicated a possibility that the cleavage surface is formed on (010)-plane. Distributions of length of cementite particle crack were measured for various strain and stress conditions. In order to understand a microscopic internal stress of cementite particle, a finite element analysis was carried out. An estimation formula of internal stress of the cementite particle from macroscopic stress and strain was developed based on the numerical results. A nucleation of cementite cracking should be quantitated based on a stochastic framework because of its uncertainties such as distribution, shape, orientation and so on. The measured distributions of cementite particle thickness and crack length were approximated by introducing a distribution function considering upper limit. Probability of nucleation of cementite cracking was formulated as a function of cementite particle thickness and macroscopic stress and strain, based on the approximated distribution function and the estimation formula of internal stress of the cementite particle.