Chemistry and Process Optimization of V-Microalloyed N80-Class Seamless Casing Tube

The complex effects of different nitrogen (N) contents and thermal routines on the microstructure and mechanical properties of 33Mn2V steels for N80-Class seamless casing tube application were investigated using Gleeble simulation technique. The results showed that the N additions of 0.014%-0.021% in the steels for the in-line normalization process (ILNP) increased the strength while the toughness remained at a high level as compared with the steel with N content of 0.005%. It was also revealed that the N addition of 0.021% could enhance the performance combination of strength and toughness in the steels by using 700 degrees C as the cooling interrupted temperature (CIT) for the non in-line normalizing process (NILNP). It was further evidenced that the toughness was improved at expense of strength to some degree in all the steels by decreasing reheating temperature for the ILNIP, while an increase of CIT for NILNP severely impaired the toughness and slightly improve the strength in the high-N steel with N content of 0.021%. This can be attributed to the dissolution and precipitation behavior of V(C, N). The optimization of V (C,N) precipitation could he achieved by enhancing N. The precipitation of V(C,N) in austenite was promoted by cooling to a certain temperature lower than the A(rl) for the ILNP. The V(C,N) particles formed in austenite contributed to grain refinement by the VN-induced nucleation of intragranular ferrite, but as a result the effect of precipitation on strengthening would become weaker due to a decrease of the precipitation of V in ferrite.