Effect of Strain-Induced Precipitation on the Austenite Non-recrystallization (T-nr) Behavior of a High Niobium Microalloyed Steel

The non-recrystallization temperature (T-nr) of high niobium microalloyed steel was determined from both multihit and double-hit compression tests obtained under plane strain condition. The T-nr was determined to be in the range of 985 degrees C to 1010 degrees C for multihit conditions. The double-hit tests carried out at an interpass time of 5 seconds gave very low T-nr in the range of 860 degrees C to 900 degrees C. In order to understand this, double-hit experiments were carried out for different interpass times (2, 15, 100 seconds) at three different temperatures (950 degrees C, 1050 degrees C, and 1150 degrees C). The negative softening behavior was observed at 950 degrees C for higher interpass times of 15 and 100 seconds. This implies that the T-nr of this steel is well above 950 degrees C. This was due to the high amount of strain-induced precipitation of niobium carbonitrides at higher interpass times as revealed from TEM investigation. The precipitate size evolution considering the Dutta and Sellars nucleation condition in TC-PRISMA agrees well with the experimentally observed precipitate sizes. However, the evolution of Zener pinning forces considering TC-PRISMA nucleation condition is similar to both reported and experimentally determined values. The evolution of tensile properties also correlates well with the observed austenite recrystallization softening behavior. Therefore, a minimum interpass time of 15 seconds is required during double-hit compression tests to effectively precipitate Nb(C, N) and delay the static recrystallization softening behavior of this steel. This lead to the determination of comparable T-nr values between double-hit and multihit methods