Key Takeaways & Executive Findings
- •• A novel decarburization treatment effectively controls carbon content and prevents δ-ferrite formation in low-density steels. • Decreasing carbon content transforms full austenite to a dual-phase austenite/ferrite structure, while (Ti,V)C carbides remain stable. • Carbon loss reduces strength due to weakened solid solution strengthening, but alters work hardening behavior. • Deformation mechanisms shift from dislocation planar glide in austenite to cross-slip and dislocation cell formation in ferrite at higher strains.
Abstract
Carbon can change the phase components of low-density steels and influence the mechanical properties. In this study, a new method to control the carbon content and avoid the formation of δ-ferrite by decarburization treatment was proposed. The microstructural changes and mechanical characteristics with carbon content induced by decarburization were systematically examined. Crussard–Jaoul (C–J) analysis was employed to examine the work hardening characteristics during the tensile test. During decarburization by heat treatments, the carbon content within the austenite phase decreased, while Mn and Al were almost unchanged; this made the steel with full austenite transform into the austenite and ferrite dual phase. Meanwhile, (Ti,V)C carbides existed in both matrix phase and the mole fraction almost the same. In addition, the formation of other carbides restrained. Carbon loss induced a decrease in strength due to the weakening of the carbon solid solution. For the steel with the single austinite, the deformation mode of austenite was the dislocation planar glide, resulting in the formation of microbands. For the dual-phase steel, the deformation occurred by the dislocation planar glide of austenite first, with the increase in strain, the cross slip of ferrite took place, forming dislocation cells in ferrite. At the late stage of deformation, the work hardening of austinite increased rapidly, while that of ferrite increased slightly.
1. Introduction
With the rapid advancements in the domain of automotive structural steels, there has been an escalating demand for enhanced steel properties. Consequently, Fe–Mn–Al–C steels have emerged as a prominent research area within the realm of high-performance steels, primarily owing to their favorable attributes, including low density, high strength, elevated ductility, and commendable work hardening characteristics [1–7].
The low-density Fe–Mn–Al–C steels that feature higher concentrations of manganese (Mn) and aluminum (Al) result in the expansion of the iron lattice and subsequently reduce the density of the steel. These steels can be divided into four classes according to phase composition: (1) austenite-based steels, (2) austenitic steels, (3) ferritic steels, and (4) ferrite-based steels [1]. Among them, austenite-based and austenitic steels demonstrate a desirable work-hardening capability [8–13].
It has been reported that carbon content has an obvious solid-solution strengthening effect and can provide 187–300 MPa of yield strength by adding 1wt% carbon to austenite [14]. In addition, the supersaturated austenite rich in Mn, Al, and C can be decomposed into κ carbides in the process of water cooling or aging under certain composition conditions [1]. In general, the favorable work hardening characteristics can be linked to the planar slip of the austenite. The main mechanism is shear band-induced plasticity (SIP) [15–16], microband-induced plasticity (MBIP) [11,17], and dynamic slip band refinement (DSBR) [18]. The κ carbides and the matrix composition together determine this deformation mechanism. Therefore, carbon exerted a substantial influence on the microstructure, strength, and ductility of low-density steel. Austenitic low-density steels are obtained by adjusting the carbon content [19]. The results show that steel with a higher carbon content has a higher yield strength, and the work hardening behavior is obviously different. Zhang et al. [20] obtained banded ferrites by cold rolling. After tempering, the ferrites broke up and became dispersed chunks, and these dispersed ferrites improved the coordination of the austenite and ferrite deformation mechanism. Han et al. [21] achieved a different volume fraction of ferrite within 13Mn–8Al–xC (x = 0.7, 1.2) steel, ranging from 12.0vol% to 53.4vol%, through carbon content adjustments. They reported that an increase in carbon content led to high proportions of intergranular κ-carbides and austenite while simultaneously retarding the recrystallization of austenite and led in an increase in strength and work hardening rates, but diminishing both toughness and impairing machinability and welding performance [22].
Although full austenite low-density steel could achieve higher strength, its high carbon content is not conducive to welding performance [23]. Within the composition range (Mn of 5wt%–30wt% and Al of 5wt%–10wt%), when the carbon content is higher than 0.7wt%, single-phase aus...
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Yongxuan Shang, Mingyu Fan, Shuyong Jiang, Zhongwu Zhang (2025). Effects of carbon content on the microstructure and tensile properties of a low-density steel. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-2937-1
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Frequently Asked Questions
What is the main objective of this study?
The study proposes a new method to control carbon content in low-density steels via decarburization treatment, aiming to avoid δ-ferrite formation and systematically examine the effects of carbon content on microstructure and tensile properties.
How does carbon content affect the microstructure of low-density steels?
Decreasing carbon content through decarburization transforms full austenite into a dual-phase austenite/ferrite structure, while (Ti,V)C carbides remain stable and other carbide formation is restrained.
What are the key mechanical property changes due to carbon loss?
Carbon loss reduces strength due to weakened solid solution strengthening, but alters work hardening behavior, with austenite deforming by planar glide and ferrite by cross-slip and dislocation cell formation at higher strains.
What is the significance of this research for engineering applications?
The findings provide insights into optimizing carbon content to balance strength and weldability in low-density steels, which are promising for automotive structural applications requiring high strength and low weight.
What methods were used to analyze the deformation mechanisms?
Crussard–Jaoul (C–J) analysis was employed to examine work hardening characteristics during tensile testing, revealing distinct deformation modes in austenite and ferrite.
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