低合金鋼の過熱後冷却時における硫化物の粒界析出挙動と熱間延性

Abstract

Extraction carbon replica technique was used to investigate the precipitation behaviour of sulphides at grain boundaries in 0.6%C-1%Ni-3%Cr steels on cooling after over-heating. The over-heating embrittlement (the loss in ductility on cooling) was also investigated by the Gleeble tensile test.The number of precipitates per unit area and their growth rate were decreased by the addition of manganese or calcium, and the over-heating embrittlement was restricted. Especially, the addition of a small amount of calcium has a great effect. The addition of phosphorus less than its solbility limit in γ-Fe suppressed the over-heating embrittlement in some degree.The sulphide precipitated in the manganese bearing steel has a spinel structure (a=1.00 nm) in the early stage of precipitation, and then transforms to α-MnS (NaCl type). The precipitation of FeS was observed in the calcium treated steels.The change in hot workability on cooling after over-heating was qualitatively interpreted by the grain boundary precipitation of sulphides, the dynamic recrystallization, and the change in fracture mode.