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Ms-Mf 온도 영역에서 일어나는 저탄소강의 등온 상변이 연구

Ms-Mf 온도 영역에서 일어나는 저탄소강의 등온 상변이 연구
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An isothermal transformation below the MS temperature was observed during the ‘Quenching and Partitioning’ (Q&P) processing of low-carbon, lean-alloyed steels. The main concept of the Q&P processing is to stabilize a optimal amount of austenite at room temperature by carbon partitioning after quenching to a temperature between the MS and Mf temperature to produce a controlled amount of athermal martensite. The observation of an isothermal transformation during Q&P processing is an interesting phenomenon because the transformation is generally expected to stop when the quenching for the athermal martensitic transformation is interrupted at a temperature in the MS to Mf temperature range. The transformation is not, however, halted when the cooling is interrupted at the quenching temperature. Instead, the transformation continues isothermally for all quenching temperatures in the MS - Mf temperature range. In the present work, the comparison of the microstructure produced by this isothermal decomposition of austenite with that of athermal martensite and lower bainite was done using qualitative and quantitative analysis methods which are able to clarify the nature of the isothermal transformation products formed in the MS–Mf temperature range. The kinetics of both the isothermal transformation below the MS temperature and the bainitic transformation was examined by analysis of dilatometry data. The thermal cycle for the isothermal transformation and the bainitic transformation was simulated in a dilatometer and the thermal dilatation data was fit to a Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation to obtain the kinetics parameters. It was found that the activation energy for the isothermal transformation was less than 50 kJ/mol, which is lower than for the bainitic transformation, 80 kJ/mol. Also, the difference in the kinetics exponent implies that there is a difference in the dimension of the basic unit growing during the transformation. The morphology and orientation relationship of the athermal martensite, the isothermal product, and the lower bainite were also investigated systematically by light microscopy and the electron-backscattered diffraction (EBSD) technique. Characteristic packet-block morphologies of the three transformation products and their different distribution of carbide and retained austenite were studied by optical microscopy and scanning electron microscopy (SEM). Pole figure maps obtained by EBSD analysis showed that all three transformation products follow the Nishiyama-Wasserman (N-W) orientation relationship between the parent austenite and the product phase. Moreover, a considerable difference in structure of the constituent units was observed in transmission electron microscopy (TEM). The athermal martensite and lower bainite consisted of the well-known long and narrow lath-like units with straight boundaries. Shorter units accumulated along the longitudinal direction in the case of lower bainite. In the isothermal product the units were as long as those in the athermal martensite but thicker in width. The unit boundaries were curved, and displayed clear ledged in some cases. The orientation angle between adjacent units was larger than in the case of athermal martensite or lower bainite. Mechanical spectroscopy measurement was used to examine the microstructural differences of three transformation products. The internal friction (IF) spectrum gave valuable information about the interaction between solute atoms and dislocations. All three transformation products showed two clear IF peaks, a dislocation-enhanced Snoek (DES) peak for carbon diffusion in the distorted lattice around 373 K (100 °C) and a Snoek-Köster (S-K) peak related to the interaction of solute carbon atoms with the kink pair formation on screw dislocations around 573 K (300 °C) for a test frequency of about 1000 Hz. By fitting the IF peaks to broadened Debye peaks, it was found that the dislocation density, the number of pre-existing kinks on edge dislocations, and the amount of solute carbon atoms segregated to dislocations was highest in athermal martensite due to the easier carbon diffusion in the severely dislocated lattice. In contrast, the lower bainite had the smallest number of kinks, dislocations, and solute carbon content due to the recovery and the partitioning of carbon occurring during the bainitic transformation. The features of the isothermal transformation product formed in the MS–Mf temperature range were between those of the athermal martensite and the lower bainite. X-ray diffraction (XRD) analysis supported the findings obtained by the IF technique. XRD results of athermal martensite, the isothermal product, and lower bainite were compared by means of the modified Williamson-Hall (W-H) method to compare the dislocation density in the three microstructures. It was shown that the dislocation density was the highest in the case of the athermal martensite and the lowest in the case of lower bainite. The lattice parameter of the athermal martensite was also largest due to the presence of large amount of solute carbon atoms trapped in the octahedral sites of the body-centered cubic (bcc) lattice. The findings of the XRD analysis agreed exactly with those of the IF spectra analysis. It was also found that 3 vol-% of retained austenite was present in the case of the isothermal product and lower bainite. This implies that carbon atoms diffuse to the untransformed adjacent austenite. This results in austenite stabilization during both transformations whereas a negligible amount of retained austenite was found in athermal martensite. In the present work, athermal martensite and lower bainite were examined systematically based on the general description of those displacive transformation products to clarify the nature of the isothermal transformation below the MS temperature observed during Q&P processing. The quantitative analysis of the isothermal transformation revealed that the properties of this microstructure were in between that of athermal martensite and lower bainite. This can be attributed mainly to the transformation temperature, as the transformation temperature 390 °C is high enough for a considerable degree of recovery and carbon diffusion. As a result, the properties of the microstructural features such as dislocations and retained austenite are in between those of athermal martensite (negligible recovery and carbon partitioning) and lower bainite (considerable amount of recovery and carbon diffusion). The qualitative analysis result, however, showed characteristic features of the isothermal product even though EBSD pole figure analysis and OM/SEM micrographs showed similarity in orientation relationship and general morphology, respectively. Detailed TEM micrographs analysis revealed that the constituent unit of the isothermal product was stretched from one end to another end in a packet like athermal martensite, which was about 1 μm in thickness with wavy and ledged-type boundaries. The multi-variant carbide particles were also found in the isothermal product comparable to what is seen for athermal martensite. In practice, the athermal martensite and the lower bainite are difficult to distinguish in a low-carbon, lean-alloyed steel because the constituent unit is lath-like in both cases and the carbide distribution changes according to the carbon and silicon contents. It would appear, therefore, that it is difficult to conclude whether the isothermal transformation below the MS temperature is martensitic of bainitic. Nevertheless, the approach used in the present work based on the general definition of athermal martensite and lower bainite, and a systematic investigation to compare the microstructural difference showed that the isothermal transformation below the MS temperature observed during the Q&P processing is more martensitic than bainitic in spite of the carbon partitioning during the gradual isothermal reduction of the austenite phase fraction at relatively high temperature.
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