**5.4 NanoSteel**

*Engineering Steels and High Entropy-Alloys*

achieved.

room temperature. The complex multiphase fine-grained microstructure together with the TRIP effect arising from the progressive transformation of the retained austenite during deformation provides the excellent mechanical behavior. By these processes, the UTS above 1200 MPa and uniform elongation larger than 12% can be

The concept of Q&P process for automotive materials was first published by Speer et al. [44]. In Q&P process, the material is quenched down below the Ms temperature, where austenite is not fully transformed. Due to the alloying concept of Q&P steels, this temperature usually is in the range of 200–350°C. It means that the microstructure is a mixture of martensite and austenite. Steel is then reheated and aging is done between 300 and 500°C; this is termed as the "partitioning step." During this treatment, carbon diffuses from the supersaturated martensite, providing the carbon enrichment of austenite, which increases its stability at room temperature; furthermore, it supports further TRIP effect during deformation. Besides these, tempering of martensite occurs, which improves its damage resis-

This simplified scheme does not reveal all the complex evolution of the microstructure during partitioning, and the detailed mechanisms of Q&P evolution are still a matter of debate and not fully elucidated. For instance, the formation of bainite during partitioning cannot be completely excluded; it could explain the measured carbon enrichment in the retained austenite, as the partitioning tempera-

Even if the detailed mechanisms are not fully revealed, the benefits of Q&P treatment by the improved mechanical properties have been clearly shown. The current range of strength that can be achieved with this new concept is between 1000 and 1500 MPa, with a total elongation of 20%. Moreover, as the matrix is a kind of tempered martensite, damage resistance is improved compared to DP or

The development of such grades requires an important modification of the annealing line; quenching and reheating step was not possible until the recent years. The strong request from the automotive market toward third-generation Advanced High Strength Steels has led steel making companies to invest in the upgrading of

TBF, a low-alloy grade like Q&P steels, can be produced by the existing heat

treatment facilities. Stable retained austenite is its key component. Bainitic ferrite matrix with retained austenite inclusions may be regarded as the most common microstructure for TBF steels. It is produced by isothermal holding in the bainitic regions after fast cooling from fully austenitic microstructures. Typical chemical compositions of TBF steels contain C, Si, and Mn as major alloying elements. Alloy modifications include variations of the Al, Nb, and Cr content [47]. The cementite formation during bainitic transformation is suppressed by the Si constituent. The added Si enhances the C content in retained austenite, and it stabilizes the austenite. High Si contents of 1.5 wt% are used in these types of steels. Consequently, the transformation of retained austenite into martensite produced by either deformation or thermal processes during final cooling is prevented. Although Si has major importance to prevent carbide precipitation during annealing of the cold rolled material, it causes problems

their annealing lines to ensure processing of Q&P steel products.

*5.2.2 Microstructural properties and formability of Q&P steels*

tance properties, while keeping high strength.

tures are consistent with those for bainite formation.

TRIP steels with the same strength level.

**5.3 TRIP assisted bainitic-ferritic (TBF) steels**

**116**

NanoSteel®, a third class of third-generation AHSS, is still under development and not commercially available. In 2002 (following 6 years of research at Idaho National Laboratory), a NanoSteel Co. was established in the United States [48]. Trial production of NanoSteel sheets was started in 2012. The nanocrystalline structure was produced by special chemistry and heat treatment. After casting, the steel is mainly austenitic. Applying special heat treatment, the grain size of austenite is refined to nanometer scale. During plastic deformation, stress-induced nanoscale phase transformation increases strain hardening.

German company Engineering+Design AG (EDAG) recently published a design study in which the steel used in a 2011 Honda Accord® was replaced with NanoSteel products. The National Highway Traffic Safety Administration (NHTSA) sponsored a research study to compare the results to formerly applied conventional AHSS. The results showed further 8% weight reduction to conventional AHSS and 30% overall weight reduction to former model Honda Accord 2011 [49].
