On the Cause of Banded Structure (Role of P, Mn, Cr and Ni)

Abstract

It has been suggested that the cause of banded structure in steel is the segregation of phosphorous. But no direct evidence has been offered nor the effect of the other elements cleared out. We prepared the thin segregated region by means of stump welding, and after various heat-treatments examined the change of microstructure. The results obtained were as follows: (1) By slow cooling, the ferrite band appeared in the region where phosphorous was rich and the pearlite band in the region rich in manganese, chromium and nickel. (2) By air cooling, these bands did not appear, or appeared only indistinctly. (3) These were the same as the actual banded structure of steel. The mechanism of these phenomena may be described as follows: If cooling takes place slowly, a small number of ferrites are first deposited in the region with a high phosphorous content, because Ar₃ is higher in the region rich in phosphorous content than in P-poor one. This results in the concentration of carbon in the remaining austenite, and in the migration of carbon atoms away from the P-rich region into P-poor one. Thus ferrite band appears in the region with high phosphorous. If rapid cooling (as air cooling) is applied, a large number of ferrites nucleate, because of the large value of N/G at low temperature, in all regions, and the migration of carbon above mentioned does not take place. On account of these, the ferrite band does not appear or appears indistinctly. In the case of pearlite band caused by manganese, chromium or nickel, the same reasoning may be adduced.