Effect of V and Nb on the microstructure and properties of high strength and toughness steel for petroleum casing

Abstract

The utilization of high-strength and tough petroleum casing steel has become the focus of various research groups worldwide, particularly in the past decade. However, defining its forming behavior presents several challenges, stemming from the complexity of enhancing strength and toughness on the steel's characteristics. To address this issue, this study employed a low-carbon microalloyed steel and introduced V and Nb elements sequentially, resulting in three groups of steels under investigation. Subsequently, mechanical properties and microstructure characteristics were examined. Interestingly, the addition of V and Nb elements resulted in both tensile and yield strengths above 1200 MPa and 1173 MPa, respectively, satisfying the criteria for the V170 oil casing. Furthermore, after adding V, V(C,N) precipitates uniformly in the steel. Still, the Nb element is not completely precipitated as there is a significant amount of Nb in solid solution. The effective grain size of the investigated steel increases from 0.92 μm to 0.98 μm after adding V and Nb elements, accompanied by a reduction in the fraction of HAGB from 47.4 % to 42.4 %. Therefore, V element plays a vital and positive role in improving the mechanical properties of the experimental steel, while Nb element has only a minor effect.

Keywords

• High strength and toughness
• Original austenite
• Strengthening mechanism
• Effective grain size
• The secondary crack