1999 年 39 巻 1 号 p. 75-83
The hot ductility of a low carbon steel was investigated at various strain rates in austenite temperature range. The embrittlement with intergranular fracture occurs in the specimen deformed in lower temperature range of austenite following solution-treatment at higher temperature range of austenite, especially at 1680K just under the melting point. Sulphides are dissolved and sulphur is segregated to grain boundary by volume diffusion during the solution-treatment. Fine particles of FeS precipitate on grain boundaries during cooling to deformation temperatures. Voids are initiated at such fine precipitates because of incoherence with the matrix. The precipitation of FeS is caused by decrease in sulphur solubility with lowering temperature. The embrittlement occurs when a difference in the solubility of sulphur between at solution-treatment temperature and at deformation temperature reaches higher than 65 ppm. The critical reduction in area, at which brittle fracture was distinguished from ductile fracture, was 60%. The intergranular fracture feature depends on strain rate. At high strain rates, the specimen was fractured after plastic deformation and the dimple pattern is found in the intergranular fracture. At low strain rates, the specimen was fractured with extremely small plastic deformation after a crack is initiated and intergranular decohesion fracture is found. The energy required for crack propagation is calculated to be 3.45J/m2 at low strain rates.
