Takako Yamashita

Novel technique to suppress hydrocarbon contamination for high accuracy determination of carbon content in steel by FE-EPMA

In multiphase steels, control of the carbon contents in the respective phases is the most important factor in alloy design for achieving high strength and high ductility. However, it is unusually difficult to determine the carbon contents in multiphase structures with high accuracy by electron probe microanalysis (EPMA) due to the unavoidable effect of hydrocarbon contamination during measurements. We have investigated new methods for suppressing hydrocarbon contamination during field emission (FE) EPMA measurements as well as a conventional liquid nitrogen trap. Plasma cleaner inside the specimen chamber results in a improvement of carbon-content determination by point analysis, increasing precision tenfold from the previous 0.1 mass%C to 0.01 mass%C. Stage heating at about 100 °C dramatically suppresses contamination growth during continuous point measurement and mapping. By the combination of above two techniques, we successfully visualized the two-dimensional carbon distribution in a dual-phase steel. It was also noted that the carbon concentrations at the ferrite/martensite interfaces were not the same across all interfaces, and local variation was observed. The developed technique is expected to be a powerful tool for understanding the mechanisms of mechanical properties and microstructural evolution, thereby contributing to the design of new steel products with superior properties.

Advantage of Specimen Heating in FE-EPMA for Performing Quantitative Trace Carbon Analysis in Steel Materials

Controlling the microscopic concentration of trace carbon in steel is critically important since it determines the microstructural development in the phase transformation process. However, precise measurement by electron beam techniques is difficult due to unavoidable hydrocarbon contamination. We developed a custom-made field emission gun electron microprobe analyzer (FE-EPMA) to enable microscopic trace carbon analysis (C < 1 mass%). A multiple C-K X-ray signal detection system is used to maximize the weak carbon signal. Conventional and newly-developed anti-contamination devices (ACDs) are installed in parallel to eliminate contamination on the specimen surface. In this study, we evaluate the effect of each ACD on contamination inhibition during EPMA measurements. The effects of a liquid nitrogen (LN2) trap, plasma cleaner, and specimen heating as ACDs are investigated.

Quantitative FE-EPMA measurement of formation and inhibition of carbon contamination on Fe for trace carbon analysis

Hydrocarbon contamination introduced during point, line and map analyses in a field emission electron probe microanalysis (FE-EPMA) was investigated to enable reliable quantitative analysis of trace amounts of carbon in steels. The increment of contamination on pure iron in point analysis is proportional to the number of iterations of beam irradiation, but not to the accumulated irradiation time. A combination of a longer dwell time and single measurement with a liquid nitrogen (LN2) trap as an anti-contamination device (ACD) is sufficient for a quantitative point analysis. However, in line and map analyses, contamination increases with irradiation time in addition to the number of iterations, even though the LN2 trap and a plasma cleaner are used as ACDs. Thus, a shorter dwell time and single measurement are preferred for line and map analyses, although it is difficult to eliminate the influence of contamination. While ring-like contamination around the irradiation point grows during electron-beam irradiation, contamination at the irradiation point increases during blanking time after irradiation. This can explain the increment of contamination in iterative point analysis as well as in line and map analyses. Among the ACDs, which are tested in this study, specimen heating at 373 K has a significant contamination inhibition effect. This technique makes it possible to obtain line and map analysis data with minimum influence of contamination. The above-mentioned FE-EPMA data are presented and discussed in terms of the contamination-formation mechanisms and the preferable experimental conditions for the quantification of trace carbon in steels.

Excellent plasticity revealed in high purity Si steels by the addition of Cr

Abstract The plasticity of high Si steels is improved remarkably by adding Cr in high purity steels. The factors which contribute to this phenomenon were investigated. At the normal level of purity, the plasticity of Fe–Si alloys is deteriorated by Cr addition. However, in high purity steels, the plasticity drastically improves when Cr is added. When the influence of microstructure was removed, the improvement of plasticity by adding Cr to high purity Fe–Si alloys was attributed to the suppression of deformation twinning formation. The microstructural improvement was explained by grain refinement and by formation of subgrains during hot rolling. The properties of these steels are expected to be applied to new functional steels.