Katsumi Yamada

TRIP steel microstructure visualized by slow and very slow electrons

The aim of the present paper is to demonstrate the ability of the scanning low-energy electron microscopy to visualize the transformed induced plasticity steel microstructure with extremely high sensitivity. Using the retarding mode in the scanning electron microscope, the high contrast between the individual phases has been obtained, which enables us to differentiate the retained austenite and the other phases. The sets of the micrographs have been collected from the sample at a wide range of landing energies of primary electrons from 50 eV to 10 keV and the dependence of the contrast between the phases on the landing energy has been calculated. Upon a comparison of these contrast curves, the optimal conditions for achieving of maximum contrast have been established.

Characterization of complex phase steel using backscattered electron images with controlled collection angles

For optimizing the microstructure of complex phase (CP) steels, characterization using scanning electron microscopy (SEM) is powerful because it allows observations from very low to high magnification. SEM specimens of steels are often etched in order to distinguish between the different phases by producing topographic information. This is however an indirect method of characterization, which does not give precise structural information. We have developed a new technique for the selective imaging of the martensite (M) phase in a ferritic (F)-M complex phase steel. Backscattered electron (BSE) images at 15-20 kV were recorded by systematically changing the collection angle θ, where θ is measured from the specimen surface. When θ was 30-45°, strong channeling contrast was observed. For lower values of θ, it is the low energy loss electrons that mainly contribute to the contrast. As θ increases, the M phase exhibits brighter contrast. When θ exceeds 60°, a selective imaging of the M phase is achieved. This is not because martensite has a larger mean atomic number than ferrite, but is due to the fact that martensite has a high crystallographic defect density. Anomalously bright M contrast is due to multiple scattering of BSE due to the high density of planar defects and dislocations. Low angle BSE allows high resolution characterization of complex microstructures, while high angle BSE gives quantitative assessment of the distribution and the volume fraction of the martensite phase.

Dual-phase steel structure visualized by extremely slow electrons

Mechanical properties of complex steels are affected by their multi-phase structure. Scanning electron microscopy (SEM) is routinely used for characterizing dual-phase (DP) steels, although the identification of steel constituents is not straightforward. In fact, there are several ways of enabling the ferrite-martensite segmentation by SEM, and a wide range of electron energies can be utilized. This study demonstrates the phase identification of DP steels at high, low and extremely low landing energies of the primary electrons from tens of keV to tens of eV. Visualization of the specimen surface at very low landing energies has been achieved by inserting an earthed detector between the pole piece and the negatively biased specimen. This cathode lens mode enables the use of the full energy range up to the primary electron energies. It has been found that extremely slow electrons (<100 eV) are exceptionally suitable for separation of the martensite from the ferrite matrix due to high surface sensitivity, enabling visualization of very fine features. Moreover, the channelling contrast is significantly suppressed at the landing energy of tens of eV of the primary electrons, which enables separation of the phases clearly even in the images acquired at low magnification. The contrast between the phases at tens of eV can be explained by the different thickness of native oxide covering the martensite and the ferrite phase.

Effect of scale microstructure on scale adhesion of low carbon sheet steel

For industrial purposes, the adhesion control of secondary scale on hot rolled steel sheet is important. A basic study was carried out to clarify the effect of scale microstructure on the scale adhesion of low carbon steel (0.03%C-0.2%Mn). When scale of FeO (about 8μm thickness) was generated at 800°C and transformed by continuous cooling from 250~600°C to 200°C, the scale transformed from 400°C showed good adhesion. The scale consisted of magnetite seam from the steel substrate, lamellar structure of magnetite and α-Fe, and magnetite layer from the scale surface. The orientation analysis by TEM showed the relationship {110}Fe // {100}Fe3O4, <110>Fe // <100>Fe3O4, and the lattice strain was calculated as 4%. On the other hand, FeO/Fe substrate showed the relationship {100}Fe // {110}FeO, <110>Fe // <110>FeO, and 25% lattice strain was calculated. It is considered that the adhesion of scale should be affected by the lattice strain, thus Fe3O4/Fe substrate showed better adhesion than FeO/Fe substrate. The temperature of FeO formation also affects the scale adhesion through the extent of Fe super saturation in FeO.