The effect of Ti–Mo–Nb on the microstructures and tensile properties of a Fe–Mn–Al–C austenitic steel

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Abstract

An investigation was performed to evaluate the effect of the Ti–Mo–Nb co-addition on the microstructure and the tensile behavior of a Fe–26Mn–8Al-1.5C austenitic steel after aging at temperatures from 500 to 600 °C. The co-addition of Ti–Mo–Nb in the steel refines grains, increases the sizes and volume fractions of κ-carbides, and forms inter- and intra-granular (Ti,Mo,Nb)C particles. The promotion effect of Ti–Mo–Nb on the κ-carbides precipitation varies with different aging temperature. The steel with co-addition of Ti–Mo–Nb possesses higher yield strength due to the combination of grain refinement, more κ-carbides precipitation, and the formation of fine (Ti,Mo,Nb)C particles distributed uniformly within the grains, but has lower ductility ascribed to the coarseness of κ-carbides with larger volume fraction and the existence of micron-sized inter-granular (Ti,Mo,Nb)C particles along the grain boundaries. With aging temperature increasing, the strength increases but the ductility decreases for both steels with and without microalloying. In addition, the strengthening mechanisms were analyzed quantitatively. The results show that the precipitation hardening is the dominant strengthening mechanisms for both steels.

Introduction

Weight reduction has become the development trend in the automotive industry in order to achieve energy saving and emission reduction. For this reason, Fe–Mn–Al–C lightweight steels have garnered widespread attention as promising candidate materials for reducing vehicle weights, due to their excellent combination of strength and ductility as well as the significant density reduction [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]. Of particular interest are the austenite-based Fe–Mn–Al–C steels having ~20–30 wt% Mn, 7–12 wt% Al and 0.8–2.0 wt% C, which exhibit outstanding mechanical properties after quenching or aging treatments, such as high ultimate tensile strength ranging from ~800 to 1500 MPa and high total elongation ranging from ~25 to 100% [[1], [2], [3], [4], [5], [6],[10], [11], [12], [13], [14]]. Such superb strength-ductility matching is mainly attributed to the nano-sized κ-carbide (nominal stoichiometry (Fe,Mn)3AlC, L′12 type perovskite structure [4]): precipitates and the planar-glide-dominated deformation mechanism [[16], [17], [18], [19], [20]]. The nano-sized κ-carbides precipitated in the austenite grains can strongly increase the strength but making a certain loss of the ductility [2,3]. However, the growth of intra-granular κ-carbides and the formation of κ-carbides along grain boundaries during aging cause a sharp decline in ductility [1,21,22].

Micro-alloying is an effective way to enhance the mechanical properties of steels. It has been reported that the co-addition of 0.05Nb-0.22Mo (wt%) increased the yield strength by ~180MPa without sacrificing the ductility in a low-carbon steel [23]. Researchers also found that the Nb addition to a Ti–Mo micro-alloying low-carbon steel accelerated the precipitation of MC-type carbides and refined the carbides which led to an increase in strength [24]. Grain refinement can be resulted from the formation of carbides such as TiC, NbC, VC, etc., which played an important role in improving the yield strength of the steels [[25], [26], [27]].

For the Fe–Mn–Al–C lightweight steels, a few investigations on the effect of alloying elements with content over 1 wt % on microstructure and mechanical properties are available [[28], [29], [30], [31], [32], [33]]. It showed that different elements had various influences on the precipitation of κ-carbides. For example, It was found that Si promoted the precipitation of κ-carbides by facilitating the C partitioning from the matrix to κ [[28], [29], [30]]. But Mo acted as a suppressor of κ-carbides due to the increase in formation energy of κ [31]. The B2 ordered phase was introduced to a Fe–16Mn–10Al–5Ni-0.9C-0.04Ti steel because of the addition of Ni which leaded to an enhancement in tensile strength [32]. Another way to introduce B2 phase was adding Cu. By the addition of 3 wt % Cu to a Fe-0.5C–12Mn–7Al steel, the B2 particles precipitated inside the austenite matrix, which delayed the occurrence of recrystallization [33].

To date, no reports on the micro-alloyed Fe–Mn–Al–C lightweight steels are available, even though these alloy elements have remarkable influences on the microstructure and mechanical properties. Consequently, in this work, aging treatment at temperatures in the range of 500–600 °C after solid solution was conducted to clarify the effect of co-addition of Ti–Mo–Nb in a Fe–26Mn–8Al-1.5C austenitic steel on the microstructural characterization, such as grain size, precipitates, especially the precipitation behavior of the κ-carbides. Meanwhile, the corresponding influence on the tensile properties and strain hardening behavior was systematically investigated.

Section snippets

Experimental material and procedures

The chemical compositions of investigated steels are Fe-25.89Mn-7.78Al-1.51C and Fe-26.07Mn-8.16Al-1.54C-0.51Ti-0.34Mo-0.075Nb, respectively. The steels were fabricated by a vacuum induction melting furnace under an Ar atmosphere, and forged into rods with ~20 mm in diameter at 1100–900 °C followed by air cooling. After that, the rods were solution-treated at 950 °C for 1 h followed by water quenching and subsequent aging at 500, 550 and 600 °C for 2 h, respectively, and then air cooled to room

Characterization of microstructure

Fig. 1 illustrates the SEM micrographs of the steels aged at different temperatures. The microstructures of M steel show equiaxial single-phase grains having average grain sizes of 35.91, 36.03 and 35.22 μm after aging at 500, 550 and 600 °C, respectively, with annealing twins as displayed in Figs. 1(a−c). While finer grains with average sizes of 16.47, 16.77 and 16.08 μm are observed in TMN steel aged at 500, 550 and 600 °C, respectively, shown in Figs. 1(d−f). It indicates that the aging

Effect of the co-addition of Ti–Mo–Nb on the microstructures

After aging treatment, a smaller grain size is attained in TMN steel compared with M steel shown in Figs. 1(a−f) and Fig. 3. This is because the (Ti,Mo,Nb)C particles, especially the inter-granular particles, formed in TMN steel inhibit grain boundaries migration during heat treatment. Besides, Nb is prone to segregate at γ grain boundaries hindering grain coarsening due to the large lattice mismatch between Nb and Fe [24]. The sizes and distribution of both inter- and intra-granular

Conclusions

In this work, the effects of Ti–Mo–Nb on the microstructure and mechanical properties of a Fe–26Mn–8Al–1.5C austenitic steel after solid solution at 950 °C and aged at temperatures from 500 to 600 °C were investigated. The main conclusions were drawn as follows:

  • 1.

    After aging from 500 to 600 °C, equiaxial grains with annealing twins and nano-sized κ-carbides distributed homogeneously within the matrix are found for both M and TMN steels, while additional inter- and intra-granular (Ti,Mo,Nb)C

CRediT authorship contribution statement

Zhuang Li: Conceptualization, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Yingchun Wang: Conceptualization, Writing - review & editing, Supervision. Xingwang Cheng: Project administration, Funding acquisition, Supervision. Zongyuan Li: Investigation, Writing - review & editing. Jinke Du: Investigation. Shukui Li: Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China under Grant no. 51671030.

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