Science Advances reports the recent progress of Nano and Heterogeneous Materials Centerabout the ultra-strong low carbon steel with nano-heterostructure

Recently, the research group of Nano and Heterogeneous Materials Center in Nanjing University of Science and Technology (NJUST) get a breakthrough on the study of ultra-strong heterostructured steel. The results were published in Science Advances,titled “Ultra-strong low carbon nano-steel produced by heterostructure and interstitial mediated warm rolling”. Ph. D student B. Gao and A/Prof. Q. Lai are the first authors. A/Prof. H. Zhou, Prof. X.L. Wu, et. al. are the corresponding authors of this paper. It has been available for readers to get the full text by clicking this link:

It is always a challenge for researchers to get out of the troubles of nanostructured metals about limited scale-up production and microstructural instability in the last four decades. On one hand, the high strength of nanostructured metals usually sacrifices its ductility, which affects the safety and service life of the products. On the other hand, the traditional preparation methods of nanostructured metals, severe plastic deformation (SPD), have some fatal issues including the feasibility of large-scale production and preparation cost. Recently, heterostructured materials have been achieved wide attention, because it provides a feasible route to alleviate or even avoid the trade-off in strength and ductility. More importantly, these heterostructured materials can be prepared by conventional industrial processing methods, which has significant perspective in scale-up industry applications.

It is found that heterostructured materials can not only realize the superior strength-ductility synergy, but also promote breaking the record of grain refinement in metals. The researchers of NJUST have produced the bulk steel with average grain size of ~18 nm by tuning the microstructural heterogeneity of the commercial plain steel, and controlling the diffusion and distribution of interstitial atoms through conventional warm rolling. Meanwhile, this nanostructured steel possesses an ultra-high strength of ~2.15GPa. The result breaks the record of the grain refinement and strength of low carbon-low alloy steels.

Left: The relationship between yield strength and equivalent processing strain of low carbon steels; Right: Comparison of yield strength and raw materials cost for carbon steels.

It is well known that deformation-induced structural refinement is governed by the competition among dislocation generation, dynamic recovery, recrystallization, and grain boundary migration in metallic materials. In this study, the deformation incompability of heterostrcture can boost the dislocation densities during plastic deformation. In addition, the pinning effect of interstitial atoms on dislocations and interfaces help with obtaining stable nanostructures by impeding dynamic recovery and recrystallization.

In this study, the provided two-steps method has wide potential in scale-up production. The first step is to obtain ultrafine ferrite-martensite dual-phase microstructure by simple intercritical annealing. The second step is refining the microstructure of the low-carbon steel by rolling at designated temperatures. This novel approach can produce bulk nanostructured steels with higher efficiency and overcome the limitation of SPD method in industrial production. It will be realized through simple process adjustment in the existing production line of steel plant. Meanwhile, this novel method is applicable to most carbon steels, which is promising for industrial production of ultra-strong steels at low cost.

TopThe nano-lamellae structure and statistics of the grain sizeBottomSolute diffusion and segregation in the nanostructured steel.

The research is supported by the the National Key R&D Program of China and the National Natural Science Foundation of China.