フェライトバンドの生因について

抄録

The author studied the formation of ferrite band in vacuum-melted steel and sandwich steel plate. The results were as follows: (1) Ferrite band did not form in pure carbon steel but formed in Ni-steel and Sn-steel. The microstructure of Ni-steel was studied with X-ray probe micro-analyser and it was found that Ni was higher in pearlite region and was lower in ferrite region. (2) Ferrite band formed only when steel was slowly cooled between Ar₃ and Ar₁, and the formation of ferrite band was not affected by the cooling rate over Ar₃ and under Ar₁. (3) The thickness of ferrite region was affected by the degree of segregation and cooling rate. Hence, it could be explained why ferrite band was eliminated by soaking and formed again by forging. (4) The results showed that the primary cause of formation of ferrite band was the heterogeneity of A₃ point. When steel was slowly cooled, the preferential formation of ferrite occured in the A₃-higher region and carbon was concentrated in the A₃-lower region.