Elimination of Primary NbC Carbides in HSLA Steels for Oil Industry Tubular Goods

Abstract

Two high-strength low-alloyed steels alloyed with various Nb contents were prepared using continuous casting (CC) technology. In the round Steel 30Nb CC billet, it was the segregation of C and Nb in the central equiaxed zone that promoted the precipitation of primary NbC carbides at grain boundaries. After processing into seamless tubes, micron-sized primary NbC carbides were located in the middle of the segregation bands, surrounded by many nano-sized NbC carbides formed during the post-solidification cooling and/or hot-piercing rolling. Even when the Nb content was decreased down to 0.006 wt.%, there were still coarse primary NbC carbides in the segregation bands. However, no primary NbC carbide was found in the round Steel 30Nb billet prepared using the electroslag remelting technology. Therefore, this study revealed that a rapid solidification rate was relatively effective in eliminating primary NbC carbides compared with merely decreasing the Nb content.