New methods are sought today for improving the mechanical properties of heavy-duty parts of high-strength steels, particularly in the automotive industry. AHS steels tend to be associated with high strength and hardness, which are achieved at the cost of poor ductility. Nevertheless, ductility can be improved: by alloying or by creating an additional phase through heat treatment. In the present study, this phase was retained austenite which, if present in a stable condition in a martensitic matrix, improves the material’s ductility. The experimental steel was specially alloyed in order to depress the Ms and Mf temperatures. It contained 2.5% manganese, 2.09% silicon and 1.34% chromium, with additions of nickel and molybdenum. An actual closed-die forged part was made of this steel. This forged part was then fitted with thermocouples attached to its surface and in its interior and treated using the Q&P process. The Q&P process is an innovative heat treatment procedure. It is characterized by rapid cooling from the soaking temperature to a quenching temperature between the Ms and Mf. Subsequent reheating and holding at a partitioning temperature stabilizes retained austenite in martensitic matrix. Cooling was performed in boiling water and then in still air without tempering.