Study of a Thermo-Mechanically Rolled 30MnB4 Steel for the Cold Heading of a Complex Geometry Fixing Element

Hot-rolled steels often underperform during the cold heading process of fasteners with complex geometries, which requires large deformations. Cracks in the head of the fastener are often found due to the steel’s poor mechanical properties. This study evaluates the feasibility of replacing standard hot-rolled steel with thermo-mechanically rolled (TMR) EN 10263-2001 30MnB4 steel for producing fixing elements in the automotive sector. The analysis will be focused on a fastener that fixes the steering wheel to the steering column. The microstructure and properties of TMR steel were compared to the standard material at each production stage, from the rolled state to the final cold heading. Analyses included microstructural characterization, tensile tests, and Vickers hardness measurements. The as-rolled TMR steel presented a mixed bainitic, pearlitic, and ferritic microstructure, while the hot-rolled steel had a pearlitic-ferritic microstructure. This resulted in higher spheroidization rates and a more homogeneous and isotropic final microstructure when using the TMR steel. Its higher reactivity to spheroidization allowed for the elimination of the cold drawing stage before the annealing, typically required for standard hot-rolled steel to enhance this heat treatment. However, the TMR steel's higher strength after spheroidization annealing led to increased wear on the cold heading machinery. Nevertheless, the improved microstructure of the TMR steel resulted in a lower number of fasteners presenting cracks at the end of the line. We conclude that there is a trade-off between the cost savings from reducing the discard rate due to cracks and eliminating the cold drawing stage, against the higher machine wear in the cold heading. Further financial studies are therefore necessary to determine the overall profitability of using TMR steel in this specific production process, also considering material costs.