Replacement of Si by Al in Q&P-steels and its effect on the tempering behavior of martensite

With the quenching and partitioning (Q&P) process, high-strength and crash-resistant steels for body in white applications with a good combination of local and global formability can be produced in continuous heat treatment lines. This is attributed to a fine-grained and homogeneous microstructure of tempered martensite and substantial amounts of stabilized retained austenite. This retained austenite transforms into martensite during plastic deformation, thereby increasing the work hardening behavior. To adjust these required fractions of stable retained austenite, carbide precipitation during heat treatment needs to be suppressed. This is primarily ensured with the alloying element Si. Since higher contents of Si cause problems with regard to hot-dip galvanizing and subsequent welding, this element is increasingly being replaced by the element Al. However, it is known that Al exhibits a different, less favorable behavior with regard to carbide precipitation, especially in tempered martensite. Therefore, the tempering behavior of pure Si- and Al-steels, but also combined Si-Al-steel, were investigated by state-of-the-art investigation methods. Using a combination of dilatometry and differential scanning calorimetry, the tempering behavior in martensite during continuous heating was investigated on lab-scaled produced steels with different Si:Al ratios. Additional investigation methods such as hardness measurments, retained austenite measurments and high-resolution scanning electron microscope were used to characterize the microstructure after tempering in more detail. With these findings, it was possible to describe the differences in the tempering and retained austenite-stabilizing effect of Si and Al during the Q&P process. The investigations show a clear influence of the alloying elements Si and Al on the tempering behavior in martensite. As Al alloyed steels show a more pronounced carbide precipitation than Si alloyed grades at the early stage of tempering, lower amounts of retained austenite could be stabilized during Q&P. However, a significant reduction in Si is achieved by adding balanced amounts of Al to stabilize high contents of stable retained austenite during successive Q&P.