Abstract: The toughness and ductility of cast NiCrMoCu HSLA steel are dec-reased due to the existence of fracture defect during fabrication. The fracture appearance, non-metallic inclusions and the segregation of alloy elements have been investigated by means of microprobe analysis and scanning electron microscope.
The results indicate that the fracture defect of cast HSLA steel is the conchoidal fracture. Its macrofracture appearance consists of bright gray, non-metallic lustrous and coarse grain structure. Its microstruc-ture appearance consists of dimples. This is ductile intergranular fracture.
The conchiodal fracture is caused by the I type MnS and the TiN along the austenite grain boundaries. They become fracture origins, reduce intergranular cohesion force and promote the propagation of intergranular cracks.
The modification of MnS inclusions and elimination of the conchoidal fracture are carried out by addition of RE and SiCa into the steel.