Summary In situ electron backscatter diffraction microstructural analysis of recrystallizing interstitial free steels deformed to strains of 0.75 and 1.6 has been carried out in a FEG‐SEM. The experimental procedures are discussed, and it is shown that there is no degradation of the electron backscatter diffraction patterns at temperatures up to 800°C. Analysis of the surface and interior microstructures of annealed samples shows only minor difference, which suggests that in situ annealing experiments are of value. In addition, it is shown that in situ measurements allow a detailed comparison between the same areas before and after annealing, thereby providing information about the recrystallization mechanisms. Sequential recrystallization phenomena, such as initiation and growth of new grains, are observed at temperatures over 740°C, and depending on the deformation histories, different recrystallization behaviour is observed. It is found that {111}〈123〉 recrystallized grains are preferentially formed in the highly deformed material, whereas no strong recrystallization texture is formed in the lower strained material.

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Based on the strain-based concept of linepipe products, materials with high deformability are desirable and a dual-phase microstructure consisting of harder and softer phases is essential to obtain higher deformability. Martensite-austenite constituents (MA) are very important in the hard phase for achieving good mechanical properties such as deformability and toughness. For understanding the formation process and its effect on mechanical properties, microstructural analysis of the MA formed through two different heat treatments (on-line heat process and off-line heat process) is conducted.Based on SEM and TEM observation, it is found that the MA consisted mainly of martensite and its distributions and shapes are different between on-line and off-line heat process samples. Because of the different formation processes, grain boundary character is also different. The interface of MA and matrix in the on-line heat process sample is divided by short length Kurdjumov-Sacks (K-S) orientation grain boundaries. On the other hand, long length random grain boundaries are formed at the interface of the off-line heat process material. This characteristic morphology results in the difference in the toughness of the materials by causing a different strain accumulation and crack propagations mechanism at the vicinity of MA.KEY WORDS: martensite-austenite constituent; high deformable steel; on-line heat process; grain boundary character.

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緒言Bulk specimen is of big advantage for studying internal microstructures because true strained state of materials is to be easily relaxed in thin specimen and then various micro structural factors that control mechanical properties are subjected to change. In this study, HVEM, 1250kV acceleration voltage, was applied to study microstructure in high strength steels using specimens over several 100nm thicknesses. In addition to improving transparency of incident electron, reducing chromatic and stigmatic aberration using an energy filtering technique was useful to conduct observation of such thick and magnetic materials. Density of dislocation and fine carbides from direct observation were consistent to the values estimated from XRD and quantitative chemical analysis. In-situ loading experiment using a tensile specimen holder was also successfully performed to track mutual interaction between nano-sized carbide and dislocations. This in-situ observation proved that dispersed fine carbides in studied steel was to be strong obstacle against to mobile dislocations.