Alligatoring phenomenon occurring during hot rolling of free-machining steel wire rods

An investigation was conducted into effects of MnS inclusions on the alligatoring phenomenon which often occurred in hot-rolled free-machining steel wire rods fabricated by varying carbon content and cooling condition. In the wire rod containing high carbon content, MnS inclusions were continuously formed inside thin ferrite bands, and caused the cracking and the subsequent alligatoring. The wire rod containing low carbon content was often subjected to the alligatoring when a considerable amount of pearlite was formed by the slow cooling and thus ferrite bands were thinly formed between populated pearlite bands. On the other hand, the wire rods having thick ferrite bands were not cracked as MnS inclusions were homogeneously and discontinuously distributed inside thickened ferrite bands, even though a considerable amount of MnS inclusions were formed. These results were confirmed by the in situ observation of crack initiation and propagation occurring near a sharp notch tip and the R-curve behavior. In order to prevent the alligatoring, thus, it was recommended that the carbon content should be maintained below 0.1% at least, and that the fast cooling rate should be achieved above a certain cooling rate level to form MnS inclusions discontinuously inside thick ferrite bands.