Research Situation of Fe-Mn-Al-C System Low-density High-strength Steel
LIU Chunquan1,2, PENG Qichun1,2,XUE Zhengliang1,2, WU Teng1,2
1.The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081 2.Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081
Abstract: The lightweight of automobile has become more and more widely concerned with the needs of energy conservation, environmental protection and economy. The low density and high strength steel of the Fe-Mn-Al-C system combines the low density and excellent mechanical properties, which complies with this topic.The earliest information on low-density steels dates back to 1933 which was related to the first development of Fe-Mn-Al-C system.Until 1958, the Fe-Mn-Al-C system of low-density steel was developed to replace the Fe-Cr-Ni system of stainless steels (added too many expensive Ni and Cr elements).At present, the Fe-Mn-Al-C system low-density steel is a kind of steel with high lightweight potential in the automotive industry, in which the addition of Al element leads to a decrease in density and Young’s modulus. Adding 1wt% Al, the steel density is reduced by 1.3%, and the Young’s modulus is reduced by 2%. Simultaneously,the addition of a large amount of Al,Mn and C elements resulted in the smelting, continuous casting, formability, weldability, microstructure evolution and deformation mechanism of Fe-Mn-Al-C system steels, which are quite different from those of traditional steels. The lightweight Fe-Mn-Al-C system steel can be classified into four categories: single ferritic steels,ferrite based duplex steels, austenite based duplex steels and austenitic steels, according to the composition of the alloy and the main composition phase of room temperature.The single ferritic steel has similar tensile properties of 200—600 MPa as the conventional high-strength low-alloy steel (HSLA) and belongs to the first generation of advanced high-strength steel (1G-AHSS).Ferrite-based Fe-Mn-Al-C system duplex steels are another promising lightweighting scheme with a lower alloy content that can be produced using ferrite plastic deformation and retained austenite TRIP and TWIP effect to increase steel strength and plasticity.The ferrite based Fe-Mn-Al-C double phase steel has superior strength and ductility compared with the first advanced high strength steel, and the middle and upper level of their performance belongs to the category of the third generation advanced high-strength steel (3G-AHSS).The austenitic-based duplex steel is similar to ferritic-based duplex steel, but it has higher alloy content than ferritic-based dual-phase steel, and its lower limit of performance belongs to the 3G-AHSS category.The austenitic steels are the most promising in terms of properties and processing.The main constituent phases of austenitic steel are austenite, a small amount of ferrite and κ-carbide.The mechanical properties of austenitic steels are determined by the deformation of austenite and the interaction of carbide-austenite.The tensile properties of austenitic light steel are similar to those of high manganese TWIP steel, the strength of 600—1 500 MPa and the plasticity can reach of 30%—80% (even up to ~100%), it belongs to the category of the second generation advanced high strength steel (2G-AHSS). The stacking fault energy (SFE) of Fe-Mn-Al-C system low-density high-strength steel increases and short-range ordered (SRO) phase and κ-type carbide are precipitated with the addition of Al content in steel.High-SFE low-density Fe-Mn-Al-C system steel with various deformation mechanisms such as novel microband induced plasticity (MBIP), dynamic slip band refinement (DSBR), shear band induced plasticity (SIP) deformation mechanism, transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) deformation mechanisms.These deformation mechanisms are consistent with the B2 and DO3 type of ordered phases, uniformly arrange of the intragranular nano-sized κ-carbides, dislocation slips, twins and phase transitions.The precipitation of intragranular κ-carbide is a unique strengthening mechanism of austenitic Fe-Mn-Al-C steel containing a large amount of Al and C elements. The applications of the Fe-Mn-Al-C system steels in the automobiles are still not prevalent due to the lack of knowledge related to application properties so far.The most important reason is that high Al content leads to high Young’s modulus reduction and high Mn content leads to problems such as smelting, continuous casting, and machining.The future developments will therefore have to concentrate on the alloying and processing strategies and also on the methods to increase the Young’s modulus. An improved processing strategy and a high value for the Young’s modulus will go a long way towards upscaling these steels to real automotive applications. The fundamental research situation and devoment of Fe-Mn-Al-C low-density high-strength steel were summarized.The composition design and the role of alloying elementsof Fe-Mn-Al-C low-density high-strength steel were introduced.The microstructures of Fe-Mn-Al-C low density high strength steel were analyzed.The mechanism of formation of toughness and toughness, stacking fault energy, physical and mechanical properties of Fe-Mn-Al-C series low density and high strength steels were revealed, and the application properties of Fe-Mn-Al-C alloys were discussed.Finally, some future directions of research on Fe-Mn-Al-C system low density steels have been proposed.
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2019, Vol. 33 
