Key Takeaways & Executive Findings
- •• Cobalt addition refines bainitic/martensitic blocks and lowers the ductile-brittle transition temperature, enhancing impact toughness. • Co accelerates transformation kinetics at intermediate stages (bainite fraction 0.1-0.6) by increasing driving force and reducing bainite start temperature. • Improved toughness is attributed to a higher density of block boundaries dominated by the V1/V2 variant pair, as revealed by crystallographic analysis. • The findings provide a theoretical basis for designing high-strength steels with superior toughness via cobalt alloying.
Abstract
This work reveals the significant effects of cobalt (Co) on the microstructure and impact toughness of as-quenched high-strength steels by experimental characterizations and thermo-kinetic analyses. The results show that the Co-bearing steel exhibits finer blocks and a lower ductile–brittle transition temperature than the steel without Co. Moreover, the Co-bearing steel reveals higher transformation rates at the intermediate stage with bainite volume fraction ranging from around 0.1 to 0.6. The improved impact toughness of the Co-bearing steel results from the higher dense block boundaries dominated by the V1/V2 variant pair. Furthermore, the addition of Co induces a larger transformation driving force and a lower bainite start temperature (BS), thereby contributing to the refinement of blocks and the increase of the V1/V2 variant pair. These findings would be instructive for the composition, microstructure design, and property optimization of high-strength steels.
1. Introduction
High-strength low-alloy (HSLA) steels have gained wide global attention during the past half-century due to their balanced strength and toughness for various structural applications, such as shipbuilding, building, bridge, and offshore structures [1]. The strength and toughness of HSLA steels with martensite and/or bainite structures can be optimized via composition design and microstructure tailoring [2–3]. The microstructure of HSLA steels is highly dependent on their chemical composition, as alloying elements can affect the transformation process and final microstructure [4]. As such, ferrous metallurgy emphases have been increasingly placed on understanding the relationship between alloying elements and transformation structures to explore the balancing mechanism of strength and toughness in HSLA steels.
Many efforts have been made to improve the strength and toughness combination by optimizing the alloy design, expecting to develop new HSLA steels with excellent toughness [5–9]. Carbon, one of the most common alloying additions, can increase strength, while this usually leads to a decrease in toughness [10]. However, in certain situations, the addition of trace carbon to low-alloy steels can not only increase strength but also improve toughness [7,11–12]. The improvement of toughness results from the highly dense block boundaries caused by increased carbon content. Nickel (Ni) is typically added to enhance the hardenability and toughness of steel, by which fine martensite/bainite structures can be formed [5–6,13–14]. Niobium (Nb), vanadium (V), and titanium (Ti) can improve strength and toughness by promoting carbonitride precipitation, which hinders the growth of austenite during hot rolling [1,15–16]. Furthermore, excellent toughness in the coarse-grained heat-affected zone (CGHAZ) is achieved with the increase of Nb content, which results from its highly dense high-angle grain boundaries (HAGBs) [8]. These findings indicate that the superior toughness of HSLA steels can be achieved by refining the transformation products via tailoring steel composition. A recent study [9] revealed that cobalt (Co) is a highly appealing alloy element that is used to improve the toughness of high-strength steel. Moreover, the addition of Co can accelerate bainite transformation [17–18] and shorten the duration of pearlite transformation [19] by enhancing the driving force for transformation. However, the effect of Co on transformation products in low-alloy steels is poorly understood. In particular, the microstructure–toughness relationship in terms of crystallography remains unclear.
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Yishuang Yu, Jingxiao Zhao, Xuelin Wang, Hui Guo, Zhenjia Xie, Chengjia Shang (2025). Unraveling the significance of cobalt on transformation kinetics, crystallography and impact toughness in high-strength steels. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-2935-3
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Frequently Asked Questions
What is the effect of cobalt on the transformation kinetics of high-strength steels?
Cobalt accelerates the transformation kinetics, particularly at intermediate stages where bainite volume fraction ranges from 0.1 to 0.6, by increasing the transformation driving force and lowering the bainite start temperature.
How does cobalt improve the impact toughness of high-strength steels?
Cobalt refines the bainitic/martensitic blocks and increases the density of block boundaries, especially those dominated by the V1/V2 variant pair, which enhances impact toughness and lowers the ductile-brittle transition temperature.
What crystallographic features are associated with the improved toughness in cobalt-bearing steels?
The improved toughness is associated with a higher density of block boundaries dominated by the V1/V2 variant pair, which is promoted by cobalt addition.
What is the significance of this study for steel design?
The findings provide a theoretical basis for optimizing composition and microstructure to achieve high-strength steels with superior toughness, particularly through the use of cobalt as an alloying element.
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