Influence of microstructure on the fracture toughness of hot stamped boron steel

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 743; pp. 529 - 539
• Main Authors: Golling, Stefan, Frómeta, David, Casellas, Daniel, Jonsén, Pär
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 16.01.2019; Elsevier BV
• Subjects: 22MnB5; Automobile industry; Automotive bodies; Automotive engineering; Bainite; Boron steels; Cooling; Cooling rate; Crack initiation; Crack propagation; Crashworthiness; Essential work of fracture; Fracture mechanics; Fracture toughness; Fuel consumption; Heat treating; Heat treatment; Hot stamping; Hållfasthetslära; Impact strength; Low alloy steels; Martensite; Material properties; Mechanical properties; Microstructure; Passenger safety; Quenching; Solid Mechanics; Structural steels; Tooling; Weight reduction; Essential work of fracture; Heat treatment; 22MnB5; Hot stamping; Fracture toughness
• ISSN: 0921-5093; 1873-4936; 1873-4936
• DOI: 10.1016/j.msea.2018.11.080

The automotive industry's desire for weight reduction while maintaining crashworthiness demands development of materials and material properties within the economic framework of consumers. The industrial process of hot stamping provides a technique to utilize steel in an efficient way. In hot stamping, microstructural characteristics of a steel blank are influenced by controlling the cooling rate. Hot stamping has become a prevalent method for lightweight solutions in car bodies without sacrificing passenger safety. The process of hot stamping applies sequential forming and quenching in a single production step. During the cooling of the blank, various microstructures can be formed depending on the cooling rate or holding temperature. Special tooling allows the application of different cooling rates within the same blank. Thus, the microstructure and mechanical properties can be influenced in designated areas of a blank. Fracture toughness properties of sheet metal are necessary to better understand fracture initiation and crack propagation during crash loading as well as improve crashworthiness predictions. This paper focus on fracture toughness of low-alloyed boron steel sheet common in the automotive industry. A heat treatment process is used to form different microstructures, predominately consisting of one single phase or mixed microstructures with two distinct phases. The fracture toughness of the present microstructures is evaluated using the Essential Work of Fracture methodology. Results are discussed in terms of the different microstructures obtained and the consequent part performance. Results show a strong connection between microstructure and fracture toughness. The bainitic grade shows favorable fracture toughness while a mixed microstructure of bainite and martensite shows a very brittle fracture behavior. A post heat treatment in the form of paint bake curing shows a negligible effect on fracture toughness of martensite.