The Microstructure and Toughness of Steels Corresponding to the Chemical Composition of the Segregated Zone of the Plate

Abstract

The microstructure and toughness of steels corresponding to the chemical composition of the segregated zone in the plate by continuous casting process or in the ingot by non-continuous process were investigated by means of microstructural observations and notch tensile tests to clarify the degree of embrittlement of the segregated zone.
The nil-ductility transition temperature (NDTT) in notch tensile tests of the martensite-bainite duplex structure due to the high manganese and high phosphorus contents is above +200°C, while that of the ferrite-pearlite structure containing low manganese contents with even high phosphorus contents is below -100°C. Therefore, the embrittlement caused by the independent increase of phosphorus content is comparatively small, whereas the one caused by the formation of low-temperature transformed structure due to the increase of both manganese and phosphorus contents is large. Here the embrittlement becomes small by tempering at 650°C even when the low-temperature transformed structure is formed. The fracture mode is transferred from cleavage to intergranular with the increase of phophorus content, which bring about lower fracture strength.