Influence of Thermal History on Microstructure and Mechanical Properties of Steels for Hot Stamping

Hot stamping is an attractive method to produce extra high strength automotive components. in the conventional hot stamping, the furnace heating is employed and the heating rate is quite low. To improve the productivity of the hot stamping technology, the reduction of time for the heating process is required. In this study, the influence of the heating rate in a range up to 200 degrees C/s, heating temperatures between 650 degrees C and 950 degrees C and cooling condition on microstructure and mechanical properties of 0.22% C -3%Mn steel has been investigated. The steel is a promising material for the highly productive new hot stamping technology because this steel transformed into martensite from austenite even at cooling in free air. The specimens heat-treated at a high heating rate and for short holding time at the heating temperature just above Ac3 show significantly fine martensite microstructure and a good strength-toughness balance. In this paper, the alpha --> gamma transformation behavior and the gamma --> alpha transformation behavior after inter-critical annealing are discussed to explain the evolution of the microstructures and mechanical properties.