Study of the correlation between material flangeability and other parameters and fracture toughness in carbon steels

Marcegaglia is a prominent player in Europe’s steel industry, specializing in carbon and stainless steel products. It holds a leading position in the market for carbon and stainless steel welded tubes and operates its own manufacturing plants.

Marcegaglia Ravenna serves as the main carbon steel plant for the group and boasts the largest service center in Italy for automated finishing, slitting, and packaging of steel strips and sheets. The plant covers a vast area with modern equipment and cutting-edge production technologies for various steel processing methods, including chemical pickling, cold rolling, annealing, galvanizing, and pre-painting of steel coils.

The company’s R&D efforts are focused on developing high-performance products with a low carbon footprint, such as AHSS, High Carbon steel, and Stainless steels, as well as optimizing metallurgical processes to enhance energy efficiency, plant productivity, and minimize direct and indirect CO2 emissions.

Case study definition

This case study aimed to find a correlation between fracture toughness properties of carbon steels and different parameters like stretch flangeability, anisotropy and microstructure. For the study, different high-carbon steels with different anisotropies and thicknesses were used.

The final goal of the case study consisted in the clarification of the effect of microstructural constituents (grain size, carbide size, distribution) on EWF and hole expansion tests. The generated data will be helpful to design the metallurgical cycle to achieve optimal product microstructure.

ToughSteel was an excellent opportunity to share technical experience from different companies that increased our metallurgical understanding on fundamental mechanisms behind steel stretching ability. The investigations carried out on our high carbon steel samples clarified several aspects relevant to hole expansion ability and confirmed that further scientific activity must be done on this topic.

Case study results

The fracture toughness of six Carbon steels has been characterized by means of the EWF methodology. Two chemical compositions (C45 and C67) with different anisotropies were analysed.

The comparison of fracture toughness and tensile properties revealed a correlation between we and elongation at fracture; i.e. the higher the elongation the higher the we.  A similar trend is observed between we and the strain hardening exponent, n.

No apparent relationship was established with other tensile parameters like yield strength or ultimate tensile strength. Regarding material anisotropy, it was observed that for C67 steels the fracture toughness tends to increase with increasing the plastic anisotropy (r value).

Further investigations must be carried out to discern the effect of microstructure on fracture behavior and analyse the relationship between we and hole expansion ratio in medium and high carbon steels.

a) Typical microstructure of C45 steel grade after annealing, b) Example of C45 component with an expanded hole, c) Cracks on the collar (C45 steel), d) Crack in the cross section, e) EWF results for Marcegaglia case study.