General
Transformation Induced Plasticity (TRIP) and Twinning-Induced Plasticity (TWIP)-grades of Advanced High-Strength Steels (AHSS) are excellent candidates for automotive light weighting, their complex compositions and microstructures (compared to currently used, low-carbon steels) are slowing their integration into structural components.
Typically, AHSS contain martensite, bainite, austenite and/or retained austenite phases, leading to unique mechanical properties. The relationship between tensile strength and elongation for various grades of steels are presented in the figure below.
Even though TWIP and TRIP steels offer enhanced properties, they come with a substantial amount of additional cost. This is mainly due to increased alloying and other processing challenges. The third generation of AHSS, the so-called quenching and partitioning (Q&P) steels now being developed actively, will be significantly more cost effective. High strength is achieved by manipulating the components of the microstructure.
The Q&P steels typically have a microstructure consisting of polygonal ferrite, lath martensite, retained austenite and some bainite. The carbon-rich retained austenite is present as both islands and thin films. It has been shown that quenching temperature has very little influence on the microstructure, while the final microstructure from the Q&P process is dependent on both the partitioning temperature (PT) and the partitioning time (the time for which the material is held at PT).