Effect of the Air-Jet Quenching Process on the Microstructure and Properties of Ultra-High Strength Steel Sheets

This paper presents a roller-constrained air-jet quenching process that utilizes high-pressure air as a quenching medium for more controllable and uniform heat transfer. A coupled CFD-FEM numerical model was developed to investigate the effects of three quenching parameters—soaking temperature (ST), sheet velocity (SV) and flow distribution (FD)—on the microstructure and properties of ultra-high strength steel sheets. Industrial test verification was subsequently performed. The results indicated that among the main effects, FD, SV and ST progressively had weaker effects on steel hardness, with partial effect sizes of 0.854, 0.764 and 0.030, respectively, as determined by analysis of variance (ANOVA). Among the two-factor interactions, the interaction between SV and FD exhibited the most significant influence on hardness, with a partial effect size of 0.907. In this study, 9 applicable combinations were identified from 90 candidate schemes, and NM400 quenched steel sheets measuring 6000 × 1500 × 2 mm with hardness ≥ 410 HV and global flatness ≤ 6 mm/m were produced through industrial testing. These results validate the effectiveness of the numerical model and demonstrate the superiority of the roll-constrained air-jet quenching process in shape control, providing a successful case for the industrial production of ultra-high strength steel sheets with high flatness.