Influence of mechanical properties and alloying elements of low carbon steels on fine blanking and heat treatment processes response

The effect of the negative clearances between punch and die of a flange with different fine blanking half-cutting depths was investigated for two low carbon steels with various elongation rates, C18E (0.18%C) and 20MnB5 steels (0.2%C, with upper mechanical properties than the carbon steel), industrially used for forming car seat parts. A specific tool adaptable on a tensile machine was built to determine the force needed to fine blank a simple geometry, in the objective to further transpose the obtained results to the complex geometry of industrial parts. Heat treatment was carried out to optimize the microstructure, hardness and dimensional modifications for the combination material/clearance/half-cutting height tested. Increasing the elongation rate of the raw material allows to reach a better formability and reduces the force needed to create a given geometry, but cracks were detected for the extreme deformation rate. Moreover, after fine blanking and heat treatment operations, different microstructures were observed according to the material grade, stable for low alloy 20MnB5 steels (martensitic microstructure) while non reproducible for the low carbon steel (mix of ferrite, bainite and martensite).