Effect of microstructural features on the hot ductility of 2.25Cr–1Mo steel

Abstract

Some factors contributing to the hot ductility losses of a 2.25Cr–1Mo steel were identified over the temperature range 750–950 °C, after the specimens were austenitized at 1000 °C, furnace cooled to different temperatures, and held there for sufficient periods of time, followed by tensile testing. There were two types of ferrite present in the microstructure, namely, pro-eutectoid ferrite and deformation-induced ferrite. The pro-eutectoid ferrite was only formed below Ar₃ (∼825 °C), which was nucleated on the inclusions and distributed uniformly. Nevertheless, the deformation-induced ferrite was formed in a much wider temperature range. It was distributed mainly along austenite grain boundaries above Ar₃, and around the pro-eutectoid ferrite below Ar₃. The deformation-induced ferrite had a primary effect on the hot ductility, which was mainly responsible for a hot ductility trough. There was a peak in the quantity of deformation-induced ferrite between 800 and 900 °C, which was just corresponding to the hot ductility trough. The morphology of ferrite was also essential. The net-like structure of ferrite formed along austenite grain boundaries was the most deleterious to the hot ductility.

Introduction

Transverse cracking on the surface of continuously cast products is still a serious problem which causes ductility losses in the austenite–ferrite region for low alloy low carbon steels. Cracks can nucleate and propagate during the straightening operation which is carried out normally in the range 700–950 °C. Previous studies [1], [2], [3], [4], [5], [6] have indicated that hot ductility deterioration in the austenite–ferrite region is caused by the following factors: (1) formation of a thin pro-eutectoid ferrite layer along austenite grain boundaries; (2) segregation of undesirable elements, such as S, Sb, Sn, As, P, Cu and others, at ferrite/austenite interfaces or at austenite grain boundaries; and (3) intergranular precipitation of carbides, carbonitrides, or nitrides.

Mintz et al. [1] suggested that the hot ductility trough is controlled by the austenite–ferrite transformation. It was confirmed [2] that the hot ductility trough in austenite–ferrite region depends primarily on the thickness of pro-eutectoid ferrite formed along austenite grain boundaries. Nachtrab and Chou [3] put forward that grain boundary segregation of Sn, Cu and Sb can seriously reduce the hot ductility of C–Mn steel and deformation at high temperatures can enhance the grain boundary segregation. Harada et al. [4] pointed out that the origin of surface transverse cracks is the micro-segregation, particularly of phosphorus. A study by Guo et al. [5] demonstrated that a 2.25Cr–1Mo steel containing 0.06 wt.%P has a lower hot ductility than the steel containing 0.01 wt.%P at temperatures between 750 and 1000 °C. However, as reviewed by Mintz [6], phosphorus is likely to improve the hot ductility and reduce transverse cracking during straightening if the segregation to grain boundary is controlled.

Until the present time, there have been few studies that have distinguished the effects of deformation-induced ferrite and pro-eutectoid ferrite on the hot ductility. It was the aim of the present work to investigate the effects of the morphology and magnitude of these two different ferrites on the hot ductility of a 2.25Cr–1Mo steel. Dynamic recrystallization and non-equilibrium segregation of phosphorus during high temperature deformation were also addressed.