Speaker
Description
The hot deformation and static softening behavior of a commercial medium-carbon Mo–Cr–V–Nb–Ti–B steel used in seamless tube applications was characterized using single-hit and double-hit compression tests on a Gleeble 3500 thermomechanical simulator. Single-hit tests were conducted at deformation temperatures from 900°C to 1200°C, strain rates of 1, 5, and 10 s⁻¹, to a true strain of 0.5, using two reheating temperatures of 1200°C and 1250°C. Mean flow stress (MFS) values were derived and compared against chemistry-based correlations of the Misaka–Yoshimoto, Kang, and Bruna families. While measured MFS fell within the range of existing correlations at high temperatures, it progressively deviated toward higher-stress models below 1000°C, with the deviation increasing at lower strain rates. Reheating temperature had no effect on MFS across the full temperature range investigated.
To interpret this reheating-temperature insensitivity, precipitation evolution during the simulated thermal cycle was modeled using TC-PRISMA. Under a nitrogen-locked scenario, undissolved Nb(C) precipitates at 1200°C reheating were too coarse (~30 nm) to contribute Orowan strengthening, while complete Nb dissolution at 1250°C reheating produced negligible re-precipitation volume fractions above 950°C, yielding effectively zero precipitation hardening at both reheating conditions.
Double-hit tests at 5 s⁻¹ revealed markedly sluggish static recrystallization at 1000°C and 950°C. TC-PRISMA modeling with deformation-enhanced dislocation density confirmed strain-induced Nb(C) precipitation at these temperatures sufficient to suppress recrystallization, while negligible precipitation fractions at 1050°C and 1100°C attributed a transient softening slowdown at ~100 s to experimental scatter. Constitutive flow law parameters were determined from the hyperbolic-sine Arrhenius formulation.
These results define the temperature window where existing MFS correlations cease to apply for this chemistry, provide quantitative flow and softening data for a poorly represented grade family, and offer direct guidance for finishing-mill pass schedule design.