2012 Volume 52 Issue 4 Pages 688-696
Hot stamping is a very promising method for producing ultrahigh-strength automotive components. The heating and cooling conditions in the conventional hot stamping process are quite limited. To improve the productivity of the process and/or the mechanical properties of the product, optimizing the heating and cooling conditions is an important and urgent research subject.
In this study, the influence of the heating rate, heating temperature, and cooling conditions on the microstructure and mechanical properties of 0.22%C–2.5%Mn steel was investigated. Samples were heated at two different heating rates (10°C/s and 200°C/s) to various temperatures and then either cooled in air or quenched using water sprays. The sample heated at 200°C/s to 800°C and then immediately quenched in water exhibited a very fine martensite microstructure having a grain size with an average diameter of 4 μm and a hardness value around 550 Hv. The sample heated above Ac3 and subsequently cooled in air exhibited a full martensite microstructure and a hardness of 450 Hv. A model that predicted the precipitation of cementite during cooling was developed to clarify the cause of the difference in hardness of the martensite in the samples that were quenched in water and cooled in air. The calculations showed that the hardness of samples cooled in air was lower mainly because of the decrease in the solid solution hardening of C owing to the precipitation of cementite below the Ms temperature. Drawing on the results obtained, hot stamping technology is discussed and favorable operational conditions proposed.
