CHALLENGES OF MAKING LOW DENSITY FE-MN-AL-C STEELS

The topic of low density steels, also called light weight steels, has gained new interest in the last decade. These steels have remarkably low densities around 6.7 kg/l. They are based on the Fe-Mn-Al-C alloy system and contain around 15 % Mn and around 10 % Al, with high contents of C, around 0.8 %. Due to high amounts of alloying elements, their microstructures can be ferritic, austenitic to duplex (also sometimes triplex). Besides the low density, the tensile tests can show elongations up to 90 % and strengths above 1000 MPa. The promising steel properties are, however, accompanied by a complicated microstructural evolution, with kappa carbide precipitation and ordering of ferrite. The solidification intervals are about 100 degrees C, and the slow cooling leads to B2 ordered phase precipitation, and the formation of different morphologies of kappa carbides, that cause embrittlement. Fast cooling prevents secondary precipitation, and avoids phase transformations but the internal and thermal stresses cause cracking. The melting itself is not trivial due to the complex chemistries. The solidification, microstructural evolution and practical cases of heat treatments and hot working, with examples of material failure are monitored.