Forms and Roles of Bismuth in Steel: A Review

Bismuth (Bi), as a green free‐cutting metal element, has been used for the production of free‐cutting steels in the field of free‐cutting steel for a couple of decades. One of the merits of bismuth is to improve the machinability of steel, while more Bi addition to steel causes cracking of steels during thermal rolling, attributed to the formation of liquid Bi films. Due to a lack of summary, people have limited mastering the Bi‐bearing steels. Herein, the forms and roles of Bi in steel based on the literature survey are summarized. These specific reviews concentrate on the forms, mechanical properties, segregation, solidification, and solubility of Bi in steel, and the effect on grain, machinability, and thermal expansion of steel is also reviewed. This review is of great significance in advancing the understanding of Bi‐bearing steels. Bismuth as a green additive has been used for the production of bismuth‐free‐cutting steels for decades; however, scholars do not well master the bismuth steels. Herein, the forms and roles of bismuth in alloys and steels, for instance, in the liquid, solid state, machinability, and mechanical properties of steel, which provide key references for the scholars, are summarized.