Effect of Nb Microalloying on Mechanical Properties and Microstructure of NM550 Wear-resistant Steel

Wear-resistant steels have been developed in recent decades for various applications including components used in machinery and work tools, agricultural machinery, pipelines and military equipment. One of the most popular wear-resistant steel grades is NM550, which contains high levels of carbon, vanadium and molybdenum. The addition of niobium (Nb) microalloying to the NM550 steel can further enhance the mechanical properties and toughness of the steel, and hence improve its wear resistance.
    
The microstructure of NM550 steel is composed of acicular ferrite (AF), bainite and martensite as shown in Figure 1. By adding Nb as an alloying element, the formation of acicular ferrite is promoted due to the added Nb nanoparticles being able to uniformly distribute within the steel matrix and promote the nucleation of ferrite. Nb also improves the dynamics of recrystallization and grain refinement, leading to finer grain structures with small granular size and higher microhardness. The improved microstructure further enhances the fatigue and wear behaviors of the steel.
    
The effects of Nb microalloying on the mechanical properties of NM550 steel are also significant. Studies have shown that adding Nb microalloying to NM550 steel resulted in a significant increase in the yield strength and tensile strength, up to ~590MPa (Figure 2). The addition of Nb also resulted in a reduction in the ductility of the steel, with a reduction in the percentage elongation drop of ~22%. This indicates that the use of Nb microalloying improves the toughness of the steel while still retaining its high yield strength.
    
Overall, Nb microalloying is an effective way to enhance the mechanical properties and wear-resistance of NM550 steel. The addition of Nb nanoparticles during steelmaking can promote the formation of acicular ferrite, thus resulting in finer grain structures and higher microhardness. This further improves the fatigue and wear behaviors of the steel, while still maintaining its high yield strength.

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