Beijing University of Science and Technology has made an important breakthrough in the research of ultra-fine grain high-strength steel

Recently, the Beijing university of science and technology, state key laboratory of new metal materials Lv Zhaoping professor team and the university of Sheffield, the U.S. national institute of standards and technology and tice, a research firm, zhengzhou university and other research institutions at home and abroad to cooperate for the first time through regulating the coherency disorderly precipitation timely and sustained pinning recrystallization grain boundary migration, the superfine crystal TWIP steel has high heat stability, strength and strain hardening increase at the same time, on the basis of the development of a kind of only through simple rolling and annealing process can obtain high performance industrial grain refinement technology of ultrafine silicon steel.The breakthrough was published on 10 February 2021 under the title "Facile route to bulk ultrafine-grain steels for high strength and ductility" in the international academic journal Natur.

The development of new high strength steel with excellent performance and suitable for mass production can realize the lightening of traffic equipment, which has a huge market demand.Twin steel has become the preferred material for automotive lightweight design due to its excellent formability, tensile strength (800-1000MPa) and uniform ductility (≥50%). However, its low yield strength seriously restricts its application in engineering.How to improve the material strength significantly while ensuring its high work hardening rate and high plastic toughness is the key problem faced by national economic fields such as transportation equipment manufacturing.

Fine-grain strengthening is an important means of simultaneously improving the toughness and strength of materials.However, because twinning steel does not have solid phase transformation during cooling process, it is not possible to achieve the purpose of ultra-fine grain through rolling and rapid cooling processes like low alloy and high strength steel, so it has to adopt large plastic deformation methods such as equal-channel angular extrusion and high pressure torsion to obtain ultra-fine grain.These methods have the advantages of high production cost, small sample size, and high density dislocation, vacancy and other crystal defects in the fine grain material, which greatly reduce the uniform ductility and make it difficult to achieve large-scale production.

The new grain refinement technology developed in this paper is to realize rapid and large number of coherent disordered precipitation of ultrafine recrystallized grains in the process of recrystallization by micro-Cu alloying, and to suppress the growth of ultrafine recrystallized grains by strong and continuous Zener pin, so as to obtain ultrafine grain TWIP under industrial conditions.The mechanical twins of ultra-fine grain TWIP steel were refined by influencing the local stacking fault energy, and the dislocation movement in the grain was almost non-pin dislocation, which further improved the work hardening ability of TWIP steel while refining the grain.The yield strength and tensile strength of the ultrafine grain steel obtained by this technique are up to 710MPa, up to 2000MPa and the uniform true strain is over 45%.The technology has a certain universality and has a certain guiding significance for grain refinement of other alloy systems.