Influence of deformation and N content on the microstructure of medium-carbon V-Ti-N microalloyed steels for oil-well tubes

The influence of deformation during stretch reducing, C and N contents on the microstructure of medium-carbon V-Ti-N microalloyed steels for oil-well tubes was investigated with a Gleeble-1500 simulator. The results show that the volume fractions of ferrite in HCLN steel with deformations of 20%, 40% and 60% at 800℃ are 17.2%, 19.7% and 29.9%, respectively. The amount of ferrite in HCHN steel with an N content of 2.3 × 10^-4 is almost equal to that in LCLN steel with a C content of 0.26%, and 1.7 times that in HCLN steel with an N content of 1.1 × 10^-4. However, the size of ferrite grains in HCHN steel is about 3μm, the smallest among the experimental steels. It is obvious that increasing deformation and N content is beneficial to the amount of ferrite, and it is more effective that deformation and N are used simultaneously. According to analyses, when the degree of deformation enlarges, defects in non-recrystallized austenite grains increase, which is helpful to provide plenty of nucleation sites for intragranular ferrite. While increasing the content of N to 2.3 ×10^-4, second-phase precipitation is promoted, the amount of ferrite increases, and ferrite grains are refined simultaneously.