Advanced high strength steels (AHSS) can be optimized regarding strength and ductility by different heat treatments procedures. These types of steels are suitable for tooling applications as they can achieve high wear resistance. There are also other industrial applications such as in automotive and aerospace industry. By alloying design, the microstructure can be tailored for increased mechanical stability during operation.

In this work, the alloy design strategy has been considered for four different AHSS alloys, two of them with an addition of rare earth elements (RE). Furthermore, a special heat treatment denoted Quenching and Partitioning (Q&P) was used to find the optimum between impact toughness and hardness. Three different thermal patterns were used to study the effect of partitioning temperature (PT) and partitioning time (Pt). The Q&P heat treatment process can produce a complex fine-grained microstructure consisting of martensite, bainite and retained austenite (also called stabilized austenite) leading to exceptional mechanical properties. Mechanical properties were evaluated by impact toughness tests, (Charpy-V) and microhardness tests. The microstructural characteristics were investigated by optical microscopy, scanning electron microscopy and X-ray diffraction.

The results showed that heat treatment with higher PT and lower Pt showed the best performance due to the high diffusivity of carbon. The amount of retained austenite increased and as a consequence the impact toughness improved. Besides the Q&P heat treatment pattern, also the addition of RE element helped to increase the amount of retained austenite. Furthermore, the grains were finer with RE addition.

With these Q&P thermal patterns and the addition of RE elements it was possible to find a significant amount of stabilized austenite and an optimum value of hardness and impact toughness.