Adjusting the Thermomechanical Condition to Change the Microstructure of C-Mn Cold Heading Quality Steel for Rapid Cementite Spheroidization at Subcritical Temperature: Effect of Stored Energy

초록

The effect of the amount of stored energy on the cementite spheroidization rate of 0.45 pct C-0.75 pct Mn (wt pct) steel was investigated. This was accomplished by using recently proposed thermomechanical conditions to control the microstructural characteristics of medium carbon cold-heading-quality steel. Wire milling was conducted right above the A(3) temperature, after which the specimens were cooled at rapid cooling (TMRC) and slow cooling (TMSC) rates. The proeutectoid ferrite and pearlite fractions, cementite and grain size, grain boundary characteristics, and the energy stored in the specimens were quantitively measured to determine the effect of the manufacturing conditions. The spheroidization time was defined as a standard of the cementite aspect ratio of 8. The spheroidization times for the three specimens at 923 K (650 degrees C) were 3.8 hours (TMRC), 5.1 hours (TMSC), and 8.0 hours (conventional wire, CW), respectively. The amounts of recovered energy of the CW, TMRC, and TMSC specimens were 3.86, 5.75, and 5.00 J/g. The temperatures at which recovery started were 352 degrees C, 332 degrees C, and 337 degrees C for the CW, TMRC, and TMSC specimens. Thus, accelerated spheroidization times that were 1.5 to 2.1 times faster than those of the conventional CW sample could be obtained by tuning the initial microstructures with their large amounts of stored energy. The results of the microstructural analysis using FE-SEM and HR-EBSD showed that spherical cementites were located at the high- and low-angle grain boundaries. The different amounts of stored energy gave rise to differences in the cementite dissolution rates occurring at the beginning of spheroidization. A relatively large amount of stored energy (TMRC) seemed to promote an increase in the rate of self-diffusion of Fe, which in turn accelerated the diffusion rate of C (cementite growth and shape change), and finally resulted in the rapid spheroidization behavior.

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