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Journal of materials research and technology 17, 2601 - 2613 (2022). doi:10.1016/j.jmrt.2022.02.008, Breaking the strength-ductility paradox of very-low (0.05 wt.%)-C medium (3e12 wt.%)- Mn steels (MMnS) has been a hard-wired topic, since in these steels multi-step heattreatments are usually required to obtain austenite for improved ductility and precipitates for higher strength. In this study, a compact two-step heat treatment comprising short(2 min) annealing and tempering was developed to investigate the synergetic effect of austenite reversion and nano-precipitation on the tensile behavior of a very-low-C MMnScontaining 1.5 wt.% Cu and 1.5 wt.% Ni. The annealing step promoted considerable amount of reverted austenite (33 vol.%), and the annealing was short to prevent Cu and Ni from partitioning into austenite, since they were supposed to maintain in the ferrite phase and then promote the nano-precipitation in the subsequent tempering stage. During the subsequent tempering step, the nano-precipitates with Cu concentration of 20e50 at.% in the precipitation core and enriched with Cu, Ni, Al and Mn were observed in the ferrite phase. The volume fraction of reverted austenite reached 38.5 vol.% after tempering, which led to the ultimate tensile strength of 1222 MPa and total elongation of 29% by the transformation induced plasticity during plastic deformation. The current study demonstrates the beneficial influence of the compact two-step heat treatment on the austenite reversion and nano-precipitation behavior of very-low-C MMnS with the addition of Cu and Ni, which subsequently enables an enhanced strain hardening behavior, thereby improving the mechanical property profile of the MMnS., Published by Elsevier, Rio de Janeiro

By adopting the variable gauge rolling (VGR) technology and the annealing process, the tailor rolled blanks (TRBs) with both continuously varying thickness and inhomogeneous material properties could be obtained. Through the procedure of subsequent forming and laser welding technology on TRBs, the tailor rolled tubes (TRTs) with axially varying thickness and material properties were successfully produced, and performed by the quasi-static axial crushing afterwards. On the basis of the microstructural transformation at different thickness locations, the relationships between the material properties and thicknesses were analyzed. Moreover, the failure mode and the deformation mechanism of TRTs have been discussed. It is noted that the progressively increased distances between the top and bottom of the plastic hinges is the essential characteristic. Combined with the classical crushing theories and models, a novel predictive mathematical model considering the thickness variation, changing material properties and variety heights of folding elements has been established. Compared with the series conventional computing models, this novel model performed better. On this foundation, the effects of different thickness transition form and distribution of material properties on the crushing performances were investigated. Besides, detailed effects of the tube structure and with or without changing material properties on the crushing capacity were also studied by introducing the concept of equivalent strength and equivalent thickness.

In this study, the austenite transformation in a cold-rolled medium-Mn steel (MMnS; 7 wt% Mn) was adjusted by inter-critical annealing (IA) at 680 °C (above the Ac1 temperature) and by partition annealing (PA) at 650 °C (below the Ac1 temperature). The deformation behavior associated with the microstructural evolution, and crystallographic changes were investigated using in situ synchrotron X-ray diffraction during tensile deformation. Electron backscatter diffraction was used to characterize the microstructure. A considerable amount of austenite (approximately 30 and 20 vol%) was promoted by reversion transformation during the IA and PA treatments, respectively. The difference in deformation behaviors between the IA and PA specimens was attributed to the different mechanical stabilities of the reverted austenite. The relatively low mechanical stability of retained austenite (RA), due to less Mn enrichment during IA, led to a pronounced activation of the transformation-induced plasticity effect, which improved the strain hardening capacity, the ultimate tensile strength, and total elongation. However, the low recovered/recrystallized fraction of the ferrite phase resulting from PA contributed to a significant increase in the yield strength. The current understanding of the characteristics and mechanical stability of RA induced by annealing at different temperatures below and above the Ac1 temperature will help in further optimizing annealing parameters to achieve better mechanical properties for MMnS.

By properly controlling the parameters of the rolling mill, multiple types of tailor rolled blanks (TRBs) are obtained, which in turn are used to form the tailor rolled tubes (TRTs) with axially varied thickness. Axial-crushing tests have been performed, and the results show that the reaction forces of TRTs show a rising trend with the integrative effect of crushing and repetitive load fluctuations after reaching the relatively low initial loads. Furthermore, investigations of different thickness and lengths are introduced into the TRT structures. It can be concluded that more energy can be absorbed for TRTs when choosing more edge numbers of the polygonal cross-sections. By contrast, the biggest difference in energy absorption is between the triangle and circular section shape, which is about 43.4%. In addition, the energy absorption capacities of TRTs with longer tube or side lengths are improved, but the energy absorption efficiencies are decreased by 21.76% and 8.7%, respectively. Meanwhile, the various distributions of transition zone and layout mode not only alter the characteristics of load curves but also affect the energy absorption capacity and initial resistance. Furthermore, multi-tube structures including TRTs, tailor welded tubes (TWTs) and traditional uniform tubes (UTs) are introduced to carry out the contrast crushing experiments. The initial peak loads of TRTs are the lowest, which are in the range of 43.05–58.28 kN. For the energy absorption, the UT structures can absorb more energy before the moving head reaches about 2/3 of the overall collapse stroke. However, the ability of TRTs to absorb energy is improved significantly in the later stage, making the overall energy absorption greater than in UTs. The reasons for the improved energy absorption efficiency of TRTs are discussed in detail. The corresponding FE models considering the variation of thickness and material properties of TRTs are established and verified with the experiment results. In order to conveniently predict the energy absorption performance of crushing TRTs, mathematical models are adopted.