Abstract
Niobium microalloying is the backbone of modern low-carbon high strength low alloy (HSLA) steel metallurgy, providing a favorable combination of strength and toughness by pronounced microstructural refinement. Molybdenum alloying is established in medium-carbon quenching and tempering of steel by delivering high hardenability and good tempering resistance. Recent developments of ultra-high strength steel grades, such as fully martensitic steel, can be optimized by using beneficial metallurgical effects of niobium and molybdenum. The paper details the metallurgical principles of both elements in such steel and the achievable improvement of properties. Particularly, the underlying mechanisms of improving toughness and reducing the sensitivity towards hydrogen embrittlement by a suitable combination of molybdenum and niobium alloying will be discussed.
Highlights
As-quenched martensite is the hardest and strongest microstructure of low-carbon steel, yet is often considered to have a tendency for brittleness
This paper focuses on property improvement of as-quenched martensite utilizing the metallurgical functionality of niobium and molybdenum as alloying elements
Vanadium microalloying has been standardly applied in martensitic steels for a long time as it provides precipitation strengthening during tempering treatment
Summary
As-quenched martensite is the hardest and strongest microstructure of low-carbon steel, yet is often considered to have a tendency for brittleness. Hot-rolled martensitic steel has potential in structural applications where ultra-high strength is required for weight reduction, for instance in mobile hoisting equipment [5] Since in all these applications impact loading must be expected, sufficient upper shelf toughness over the temperature range of operation is required, demanding a correspondingly matching ductile-to-brittle transition temperature. Another significant problem perceived in all ultra-high strength steels is the sensitivity to hydrogen embrittlement [6]. Metals 2018, 8, 234 as-rolled steel, typically having ferritic–pearlitic microstructure, is austenitized at temperatures of 900 to 950 ◦ C and subsequently quenched, generally using water as a cooling medium This process is used for plate products. Successful examples for dedicated application of niobium and molybdenum alloying for property optimization in re-austenitized quenched as well as direct quenched flat rolled martensitic steels will be demonstrated
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