Liquid nitrogen jet impingement cooling of ultra-high strength steel in hot stamping process

Abstract

Abstract. In the hot stamping process, elevated sheet temperatures and uneven die cooling contribute to insufficient thickness uniformity in the formed parts, leading to reduced forming efficiency and inducing severe thermo-mechanical fatigue in the die. To address these issues, a novel in-die liquid nitrogen (LN2) jet impingement cooling method is introduced to achieve a low and uniform initial holding temperature. The high-temperature steel sheet is initially cooled to slightly above the martensitic transition temperature through the application of an LN2 jet during the forming stage, followed by pressure-holding quenching. This method demonstrates remarkable outcomes, including a notable 74.19% reduction in holding time and a substantial 24.72% decrease in the maximum thinning rate. Furthermore, the maximum temperature difference between the upper and lower dies sees significant reductions of 34.57% and 25.84%, respectively.