The Relationship Between Fracture Toughness and Blanking Performance of 850MPa Hot-Rolled Steels

Hot-rolled steel grades can serve as cost-effective and high-performance solutions for automotive and heavy transportation chassis and seat parts. These applications generate significant demand for cut-edge quality, stretch flangeability, and fracture toughness. This study investigated two thermomechanically hot-rolled steel grades, 850F and 850CP, both of which exhibited a tensile strength of 850 MPa. To assess the performance in chassis and seat applications, materials were examined using ISO 16630 hole-expansion, mechanical punching, shearing, and fine blanking tests. Thin sheet fracture toughness was determined by essential work of fracture (EWF) tests, with fatigue pre-cracks. The test results indicate that the 850CP material demonstrates only a marginally better hole-expansion ratio (HER), but significantly enhanced cut edge quality in punching, shearing, and fine blanking processes. Another substantial difference is the fracture performance of the materials, with 850CP exhibiting a superior essential work of fracture (We) compared to 850F: 545 kJ/m2 vs. 203 kJ/m2, respectively. The results indicate that 850CP is a highly suitable material for applications requiring superior cut-edge quality across diverse cutting processes, in conjunction with exceptionally high fracture toughness. The results also demonstrate that the fracture toughness can be a good indicator of the sheared-edge crack propagation behavior.