高炭素高マンガン鋼鋳鋼の炭化物組織に及ぼす高温加熱の影響

抄録

  The effect of austenization heat treatment was investigated on the amount and shape of carbide in high carbon high manganese cast steels containing 11% manganese and 1.5∼2.0% carbon. The alloys, solidified at the cooling velocity of about 50°C/min in CO₂ mold, were quenched into water after heating at 1,000°C, 1,050°C and 1,100°C for 1∼10 hours. The solidification structure of 1.5%C steel was composed of 1.1vol.% of eutectic cementite and 5.3vol.% of very fine proeutectoid cementite precipitated at grain boundaries of austenite. In higher carbon steel more eutectic cementite crystallized in intercellular region of primary dendrite and made up a continuous network together with proeutectoid cementite at austenite grain boundaries. Many plate-like cementite also developed from the periphery of the net-work type cementite, forming widmanstatten structure. With higher austenitizing temperature and longer heating time, more cementite dissolved into austenite, retained cementite was distributed more discontinuously and its shape was changed from complex to globular. Morphology of eutectic can be quantitatively represented by the mean shape factor F which is defined as F=∑A_if_i/∑A_i, where A_i is the area of indivisual carbide and f_i is its shape factor defined as f_i=(peripheral length of eutectic carbide) / (peripheral length of a circle having the same area as the carbide). The mean shape factor was found to be a function of carbon contant of the steel (C, %), heating temperature (T,°K) and hearting time (θ, s) : F=5.79×10¹⁶C^1.44T^−4.44θ^−0.024.