Prevention of Delamination by Suppressing Secondary Ferrite Formation in Hypereutectoid Pearlitic Steel Wires

Takuya Kochi, Mamoru Nagao, Koichi Makii, Hiroshi Yaguchi and Nobuhiko Ibaraki
Kobe Steel Corporation

It is well known that pearlitic steels can be strengthened significantly with cold working. The strength of eutectoid pearlitic steels drawn to true strain of 4 can exceed 4000MPa. Therefore, pearlitic steels are widely used for structural material in need of high strength, such as pre-stressed concrete wires, tyre cords and saw wires.

Higher strength can be achieved with drawing to higher strain, with using steel containing higher carbon content and with chromium addition. However, if wire strength becomes too high, longitudinal fracture, called delamination, may occur easily under torsional stress. Thus, strength of drawn steel wire is limited for actual application.

In order to prevent delamination, a large number of studies have been carried out. The mechanism of delamination has also been investigated, and strain aging, texture and non-uniform deformation between surface and inside of drawn wire etc., have been proposed as a possible cause of delamination. As another factor, we have studied on a viewpoint of initiation site of fracture and clarified that the secondary ferrite, which is non-lamellar ferrite in pearlite structure in hypereutectoid steels, acts as a crack initiation site.

In this study, the mechanism of secondary ferrite formation, the effects of micro alloying elements on the secondary ferrite formation and on delamination were investigated with three steels, PC (0.98mass%C steel), NB (Nb added steel) and TB (Ti and B added steel).

In order to identify nucleation site of secondary ferrite, microstructure at early stage of transformation was observed in the TB steel. It was observed that the secondary ferrite was nucleated at the side of proeutectoid cementite precipitated at pre-austenite grain boundaries. It was assumed that a formation of locally carbon poor zone around the proeutectoid cementite caused the secondary ferrite formation in hypereutectoid steel.

In order to clarify the effects of alloying elements on secondary ferrite formation, a volume fraction of the secondary ferrite was measured. The fraction of secondary ferrite was different between the steels, the secondary ferrite was increased in niobium added NB steel but remarkably decreased in titanium and boron added TB steel, in comparison with plain carbon steel PC. It was also assumed that in NB steel, niobium carbides caused the formation of locally carbon poor zone and increased the driving force for ferrite transformation. On the other hand, in TB steel free boron segregated at austenite grain boundaries and decreased the driving force. It is understood that free boron could exist even in high carbon steel, because time period going through boride formation temperature region is very short in patenting treatment.

In drawing tests, in TB steel wire delamination did not occur until drawing strain e = 4.2, but in NB steel wire delamination occurred at e = 3.8. It can be concluded that added boron acts a role to prevent the secondary ferrite, and consequently the delamination is suppressed.

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