Achieving 2.6 GPa tensile strength with outstanding ductility in high-carbon low-alloy steel

Abstract

Increasing the carbon content in low-alloy steels is one of the most cost-effective and efficient methods for enhancing strength, often resulting in a significant reduction in ductility. In this study, a high-carbon low-alloy steel with a tensile strength of about 2.6 GPa and a total elongation of 12% was developed, through the synergistic applications of two key strategies: i) refine prior austenite grains (PAGs) leading to the transition of quenched microstructure from brittle twinned martensite to dislocation martensite; ii) suppress the martensitic transformation finish temperature to sub-room temperature by the combined effect of high content of carbon and alloying elements, i.e., Ni, Mn, Si, Cr, and Mo. After quenching and tempering, the steel retains approximately 15vol% stable retained austenite (RA), which enhances ductility through the transformation-induced plasticity (TRIP) effect. These strategies collectively contribute to both high strength and excellent ductility, enhancing the strength–ductility synergy in ultra-high strength steels.

1. Introduction

The remarkable mechanical properties of high-strength low-alloy steel have positioned it as a key material in industries, such as automotive, marine, and aerospace [1–3]. However, as the pursuit of higher strength intensifies, a critical challenge emerges that the ductility tends to diminish with increasing strength [4–5]. This paradox complicates manufacturing processes and limits the potential applications of high-strength steel, highlighting the need for innovative solutions to balance these conflicting properties.

It has been demonstrated in many studies that retained austenite (RA) plays a crucial role in enhancing the ductility and toughness of high-strength steels [6–8], offering insights for the development of high-strength steels with improved ductility. Especially, several recent studies on medium- and high-carbon low-alloy steels have demonstrated that the introduction of RA can enable a satisfactory level of ductility while achieving tensile strengths exceeding 2200 MPa, highlighting the potential of RA-assisted design for developing ultrahigh-strength steels with excellent strength–ductility synergy [9–10]. To effectively introduce and stabilize RA, Speer et al. [11] proposed the quenching and partitioning (Q&P) process, which increases the volume fraction of RA at room temperature (RT). The process involves quenching the steel to a temperature between the martensitic start (Ms) and finish (Mf) temperatures, followed by holding or further heating to partition carbon from martensite to austenite. The austenite is then stabilized and retained upon cooling, forming a certain volume of RA after the Q&P process [12].

Although Q&P is effective in improving ductility and toughness compared to traditional quenching and tempering (Q-T), its process is difficult to control. Precise management is required, as quenching must occur within a narrow temperature range between Ms and Mf, both typically above RT [13]. To address this issue, Al was added to medium-carbon steel to promote the formation of carbon-depleted δ-ferrite and carbon-enriched austenite at elevated temperatures [14]. The carbon-enriched austenite, with enhanced stability and Mf temperature lower than RT, facilitates the retention of a high volume of RA after quenching to RT. This approach simplifies the Q&P process by eliminating the requirement for quenching at a temperature above RT. However, the introduction of a high proportion of δ-ferrite limits its strength normally lower than 1200 MPa [14–15]. Recently, He et al. [16] proposed a medium-manganese (approx. 10wt%) room-temperature quenching and partitioning (RT-Q&P) steel based on a similar strategy. This process leverages the positive effect of Mn on austenite stability and lowers the Mf below RT, achieving 10vol% to 30vol% RA after quenching [17]. However, adding approximately 10wt% Mn to stabilize austenite creates a new challenge in the steel smelting process, i.e., Mn segregation [18]. Further effort to reduce Mn content to 4wt% in the implementation of the RT-Q&P process have then been explored, relying on the non-uniform distribution of Mn to achieve the desired effects [19]. Nonetheless, controlling the segregation of Mn element remains a significant challenge in industrial production with medium to high contents of Mn [20].