SECURE 400 steel is used for basic ballistic protection of armoured vehicle chassis against the pressure energy of exploding mines, IEDs and grenades. It is a low-alloy high-strength steel with a manufacturer's declared hardness of 380 to 430 HB, which is characterized by a good combination of high strength and toughness. As standard, this steel is processed by quenching followed by tempering, but new possibilities for improving its mechanical properties are constantly being sought. A modern heat treatment process, the so-called Q-P process, leads to the formation of a multiphase structure, which positively influences the mechanical properties. Therefore, this paper focuses on describing the influence of Q-P process parameters on the development of the microstructure and mechanical properties. During the experimental program, it was found that the Q-P process increased the ultimate tensile strength by up to 300 MPa and increased the hardness by up to 68 HV10 compared to the post-delivery condition where tensile strength and hardness values of 1354 MPa and 438 HV10, corresponding to 414 HB, were measured. Redistribution of the carbon from the supersaturated martensite stabilised the austenite and a finer grained microstructure was achieved.
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