鋼における固溶型硫化物の挙動

抄録

In order to make a systematic survey on the behavior of sulfur in solid steel, the precipitated sulfide inclusions in solid steel were studied microscopically through their heat treatment and the electrolytically isolated sulfide inclusions were investigated by the electron diffraction method and the electron probe method.
It was shown that sulfide inclusions were precipitated in solid steel below about 1200°C, when the steel was cooled from the temperature above the solution temperature of the preprecipitated cipitated sulfide inclusion at a slower rate than 60°C/min.The precipitated sulfide inclusions had two forms: a plate-like inclusion which had a widmastätten structure and was preci-pitated on plane {100}γ of austenitic grain, and a very fine particle inclusion which was precipitated at the grain boundary of austenite.As the result of impact test over the temperature range of 800°C to 1350°C of steels which were slow-cooled from 1405°C, it was found that the brittleness of the steel below 1200°C was induced by precipitation hardening of austenitic grain and reduction of grain boundary cohesion, and it did not vanish even at the temperature below Ar₃.
When the steel in which the precipitation of sulfide inclusions had been suppressed by rapid cooling from the high temperature was reheated at austenitizing temperature, the uncompleted plate-like inclusion at grain and the very fine particle inclusion at grain boundary were precipitated.
When the steel in which the precipitation of sulfide inclusion had occurred through slow cooling from a high temperature to the room temperature was reheated at austenitizing temperature, the plate-like inclusion in it was decomposed rapidly to a very fine particle inclusion. But when the steel which had been slow-cooled to the temperature of lower austenitic region from the high temperature was reheated, the plate-like inclusion in it was not decomposed and kept its shape up to the solution temperature.
The results of electron diffraction analysis and electron probe analysis on the electrolytically isolated plate-like sulfide inclusion show that the plate-like inclusion was composed of single crystal of α-MnS solid solution containing 23 wt% FeS and matched the lattice of austenite on plane {100}γ, and the phenomena of decomposition of the plate-like inclusion in reheated steel occurred due to lack of cohesion between lattices of the plate-like inclusion and refreshed austenite.
In the steel containing up to about 0.7% manganese, precipitated sulfide inclusions were observed, but in the steel with 0.03% manganese they were in molten state at the temperature and migrated into grain boundary of austenite.
The solution temperature of the precipitated sulfide inclusion was affected by contents of manganese and carbon in steel.In low manganese steel (0'1%), the solution temperature was 1290°C independently of content of carbon and in steel containing 0.2-0.4% manganese, the higher the carbon content of (0.1-0.8%C) steel the lower the solution temperature was (1400-1320°C).But, the precipitation temperature of sulfide inclusion was almost constant (1180-1200°C), independently of contents of manganese and carbon in steel.