Thanks Ramesh for your answer.

Castability of any new steel is one of the major constraints in modern steel industry to realize the high alloy steel production. In that point of view, your expectation is true and high Mn steel may fade out from the list. Already it had been replaced by by medium Mn steels. which are simpler in composition than the earlier one, but still have castability and weldability issues, particularly when you add bit of Al into medium Mn to increase the strength to weight ratio. Innovations in liquid steel processing and continuous casting process is actually required to overcome the issues due to high alloy content in steels.

I agree with you Ashok Kamaraj. The weldability of high-Mn TWIP steel is significantly poor. The third generation of advanced high strength steel AHSS, Type medium Mn steel, will replace the second generation of AHSS in automotive applications because of several reasons.

Never been in use at scale in automotive (only one time use in Europe by one vehicle program, if I remembered it correctly). So it appears there is no wide adoption by automotive OEMs.

To my opinion, one of the original ideas was to replace Ni stabilized austenites with Mn stabilized austenites. The Ni price has risen massively in recent weeks, which could again strengthen interest in CrMn-steels. I think theyre currently produced in Korea and used for LNG-vessels.

I see a huge potential to develop either Med-Mn or High-Mn steel for a given application. Stabilizing the austenite, retaining the properties after seconday manufacturing processes is quite challenging which opens the new paradigm for the development of TWIP assisted next gen steel for auto industries !!!

like, Sebastian Wesselmecking mentioned, Mn was used to replace Ni, what else could be done as Ashok Kamaraj highlighted how to be being innovative drives the research; when it dies, everything will die eventually !!!

Twin induced plastic steel (TWIP) is a full austenitic high manganese steel with excellent strong plasticity, which is one of the hot research areas of iron and steel materials. In the past 20 years, it has been considered that the excellent strong plasticity of TWIP steel is mainly caused by deformation twins.

The research results of Professor Ming-xin Huang and his team from the University of Hong Kong challenge the traditional view and find that the role of twins in TWIP is very limited, and the high dislocation density caused by interstitial carbon atoms is the most critical factor for the excellent strong plasticity of TWIP steel. The related paper is entitled "Therole of interstitial carbon atoms on the Strain -hardening rate of Twinning -induced plasticity "Steels" was published on Scripta Materialia. As early as 2016, Professor Ming Xin of Hong Kong Dahuang made a quantitative analysis of the strengthening mechanism of TWIP steel based on the measurement technology of dislocation density, and believed that 90% of the work hardening of TWIP steel was contributed by forest dislocation strengthening, and the twins themselves had no significant effect on stress [1].

In 2018, Fe-18Mn-0.75C-1.7Al-0.5Si (wt.%) was used as the research object, and tensile specimens were carried out at room temperature to 300℃. Inlens mode was used to calculate twin fraction, and synchrotron radiation was used to quantitatively analyze dislocation density. Figure 1 shows the tensile properties and work-hardening curves of the TWIP steel at different temperatures. At 200℃, it is found that the elongation of TWIP steel is the highest and the work-hardening rate is still high, but the volume fraction of twins generated by deformation is very small, while the dislocation density is quite high. It can be seen that TWIP is not a necessary condition for high dislocation density and high work hardening of TWIP steel [2].

In order to further determine the work hardening mechanism of TWIP steel, Professor Huang continued to study and compare the mechanical properties and microstructure evolution of FE-30Mn-3Al-3Si and FE-30Mn-0.6C (wt%) TWIP steel. It is found that the work hardening properties of the two TWIP steels are very different, but a large number of deformation twins are generated during the tensile process of both steels. After the same amount of plastic deformation, the dislocation density of Fe30Mn0.6C is significantly higher than that of Fe30Mn3Al3Si.

It is found that the work hardening property of TWIP steel is not directly related to the stacking fault energy, and the carbon content is the key to the work hardening property of TWIP steel. The work hardening property of TWIP steel without carbon is very close to that of 316 stainless steel. Further analysis shows that the gap carbon atoms affect the dislocation density evolution of TWIP steel, and then affect the work hardening. In other words, in TWIP steel, the gap solid solution carbon atoms are the key reason for the high dislocation density and high work hardening rate of TWIP steel (Luoand Huang 2020). The TWIP steel without carbon is like 316L stainless steel.

The above part is about the strengthening mechanism of TWIP steel. The following is the development of high strength steel.

AHSS (Advanced High Strength Steel) has been the focus of material research and development in order to meet the needs of automotive industry for safer, lighter, more environmentally friendly and more economical fuel consumption. It is predicted that by 2015, AHSS steel will increase to 35% of the world light vehicle body and its enclosure, while mild steel will decrease to 29% from 55% in 2007.

In recent years, countries have been committed to the research and development of the third generation AHSS steel. American scientists first proposed the concept of the third generation AHSS steel. The Automotive/Steel Alliance, with support from the U.S. Department of Energy and the National Foundation for Health, launched a three-year effort in October 2007 to develop strong plastic and generation 3 AHSS steel, which is between generation 1 and Generation 2 AHSS steel in cost. At the same time, China and South Korea have also started the research and development of high strength and high strength plastic steel. So far, the research and development of the third generation AHSS steel is still under active exploration. In November 2010, China Iron and Steel Research Institute and Taiyuan Iron and Steel Group co., LTD successfully developed the third generation OF AHSS steel hot rolled coil and cold rolled sheet in the industrial production process, with strong plastic volume exceeding 30GPa·%, and developed China's 3AHSS steel and its industrial production technology.

A biphase ferritic austenitic steel with low manganese (5%-7%) and low carbon (0.1%) composition developed by Merwin based on the earlier work of Grange and Miller. The manganese content of TWIP steel is higher than that of conventional steel, but much lower than that of TWIP steel. Prolonged holding at the peak of the annealing temperature leads to annealing in the dual phase zone, where manganese distribution occurs. Ultrafine crystal microstructure composed of ferrite and high manganese austenite was obtained at room temperature. In 0.10C-5.2Mn steel, 17% of austenite was detected, while in 0.10C-5.8Mn and 0.10C-7.1Mn steel, 28% and 38% of austenite were detected, respectively. In addition to reducing the alloy costs associated with TWIP steel, the process route also enables the production of high-strength steel from lines that do not have continuous annealing or advanced cooling capabilities.

Various composition series have been proposed for the second generation austenitic steel grades, and the current research is mainly focused on how to reduce the alloy content. Frommeyer et al. investigated fe-Mn-3Si-3Al series steels with lower Manganese (15%Mn). The content of Mn, C, Al and other alloying elements in high manganese steel is high, which makes a very demanding requirement for smelting, continuous casting and hot rolling process. The main performance is:

(1) In the smelting process, the Al element in Fe-Mn-Al-Si high manganese system is seriously burned, and the composition is difficult to accurately control; In addition, Mn, C and coating agent are easy to react.

(2) In the continuous casting process, the microstructure segregation of Mn element results in low strength of the solidified billet shell on the meniscus surface and easy steel leakage; As the temperature drops, the strength of high manganese steel increases rapidly, which makes the tension correction process difficult.

(3) In the process of hot rolling, grain boundary is very easy to be oxidized when heated in tunnel furnace, and K phase carbides in steel also bring difficulties in hot rolling. High Mn and C content easily leads to the formation of pit defects on the surface of hot rolling plate.

It can be seen that there are many technical bottlenecks in the production of high manganese steel by using the traditional process of continuous casting and hot rolling. The rapid solidification of thin strip continuous casting technology can effectively reduce the element segregation in high alloy steel and improve the molding properties of the material. Steel groups are trying to use the technology to produce high manganese steel. Thyssen-krupp iron & Steel Co., Ltd. tries to produce fe-Mn-Al-Si high manganese steel thin strip by using strip type thin strip continuous casting technology. Posco is also trying to produce Fe-Mn-C high manganese steel with thin strip continuous casting process.

[1]Liang, Z. Y., Y. Z. Li and M. X. Huang(2016). “The respective hardening contributions of dislocations and twinsto the flow stress of a twinning-induced plasticity steel.” ScriptaMaterialia 112: 28-31. [2]Luo, Z. C. and M. X. Huang (2018).”Revisit the role of deformation twins on the work-hardening behaviour oftwinning-induced plasticity steels.” Scripta Materialia 142: 28-31. [3]Luo, Z. C. and M. X. Huang (2020). “The role of interstitialcarbon atoms on the strain-hardening rate of twinning-induced plasticitysteels.” Scripta Materialia 178:264-268.

At present, the development of the third generation of high strength steel is in full swing, which also stems from the ultimate pursuit of lightweight design in the automotive industry. By means of alloying design of medium manganese carbon steel and reverse austenite phase transformation, about 30% of metastable austenite and ultra-fine grain matrix duplex composite steel has been prepared in the laboratory. The tensile strength of the steel at room temperature is 0.8~1.6GPa, the elongation after fracture is 30%~45%, and the strong plastic product is 30~48GPa·%. At present, the basic research on series of C-Mn steel with different carbon content and different manganese content has been completed in the laboratory. Basic process is designed for the original steel quenching processing first, obtain martensitic structure, and then on the hardened steel duplex zone (Ac1 - Ac3) under different annealing time, austenitic reverse change, including austenitic, ultrafine ferrite crystal and polyphase structure of precipitated phase, the annealing process is called austenite reversed annealing.

It can be seen from the above laboratory research results of medium manganese steels that the austenite and ferrite dual-phase steels containing a large number of austenite phases with ultrafine grain structure can be obtained by reverse transformation and corresponding deformation treatment. When the steel contains about 30% metastable austenite, high strength and high plastic automobile steel with strong plastic volume of 30~40GPa·% can be obtained. During tensile deformation, metastable austenite gradually transforms into martensite, forming a metastable, multiphase and multiscale structure called M3. In November 2011, the third generation of automotive steel jointly developed by China Steel Research institute and TiSCO was trial-produced in TiSCO. The hot rolled coils and cold rolled plates of 0.1C-5MN steel preliminatively achieved the basic objectives of the third generation of automotive steel.

In the field of automobile manufacturing, the competition between steel and aluminum and magnesium alloy has entered a new stage. At the same time, automobile manufacturers also put forward higher requirements on the performance and cost of steel materials, and the development of AHSS steel, especially the third generation OF AHSS automobile steel with good cost performance, is particularly urgent. At present, all countries in the world are committed to the research and development of AHSS, especially the third generation OF AHSS steel. At the same time, the new user processing technology and process has increased the research efforts, so that the strength and toughness of steel has been further improved. The research and development of the third generation AHSS steel, represented by Q&P steel and advanced bainitic steel, is currently close to the target of the third generation AHSS steel. Other process routes and new steel grades are also being explored.