The role of phosphorus in cold work embrittlement in Ti and Ti+Nb stabilized high strength ultra-low carbon steels

The new generation of ultra low carbon (ULC) steels used in the automotive industry requires both good formability and high strength. Good formability is essential for aerodynamic vehicle bodies while high strength is essential for weight reduction purposes and dent resistance. It is known that the lowering of the interstitial elements, such as C and N, results in higher formability. However, the reduction of the interstitial elements also results in a decrease in their strength. One of the ways of increasing the strength without considerably affecting the formability is by the addition of phosphorus (P) to the ULC steels, which is believed to increase the strength by the solid solution strengthening mechanism. The addition of P increases the strength, but it causes the problem of cold-work embrittlement (CWE) in the steels. Cold-work embrittlement is defined as the susceptibility of the sheet material to intergranular fracture during the secondary working of a deeply drawn part or while in service. Although, the problem of CWE is believed to be due to the segregation of P to the ferrite grain boundaries, it was not clear at what state of processing does the segregation occur. Furthermore, although it has been shown that the Nb-containing ULC steels exhibit better CWE resistance than the Ti-containing ULC steels, the effect of Nb on the segregation behavior of P is not known.; The major objective of this study was to understand the segregation behavior of P during the different stages of thermomechanical processing in the high P-containing Ti and Ti+Nb stabilized ULC steels. Two techniques, the scanning transmission electron microscopy (STEM), and the atom probe field-ion microscopy (APFIM), were used in conjunction to study the segregation behavior of P in the steels. These studies revealed that the segregation of P to the ferrite grain boundaries primarily occurred during the coiling process in the Ti stabilized steel. The Ti+Nb stabilized steels showed lower segregation of P to the ferrite grain boundaries in the as-coiled condition which was believed to be due to the presence of Nb on the grain boundaries. The drop weight tests conducted to determine the CWE resistance confirmed that the high P-containing steels exhibited higher transition temperatures, i.e., poor CWE resistance, than the low P-containing steels. The presence of the Nb on the ferrite grain boundaries improved the CWE resistance which was believed to be due to the increase in the grain boundary cohesion, an effect similar to boron.; This study also revealed that the segregation of P to the ferrite grain boundaries results in an increase in the yield strength of the ULC steels by the grain boundary hardening mechanism. Thus, both high strength and good CWE resistance cannot be concomitantly achieved by the addition of phosphorus. The effect of P on the textures and formability properties was also studied.