A strain rate sensitive constitutive model for quenched boron steel with tailored properties

• Published in: International journal of impact engineering 2012-12, Vol.50, p.49-62
• Main Authors: Bardelcik, A., Worswick, M.J., Winkler, S., Wells, M.A.
• Format: Article
• Language: English
• Subjects: Applied sciences; Bainite; Constitutive model; Exact sciences and technology; Hardening. Tempering; Hardness; Heat treatment; Hot stamping; Martensite; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Production techniques; Split Hopkinson tensile bar
• ISSN: 0734-743X; 1879-3509
• DOI: 10.1016/j.ijimpeng.2012.06.007

In this work, boron steel sheet metal blanks were austenized and quenched at five different cooling rates ranging from 14 °C/s to 50 °C/s, which resulted in as-quenched microstructures that ranged from bainitic to martensitic respectively. Micro-hardness tests revealed a linear relationship between the Vickers hardness and percent area fraction of martensite and bainite present in the quenched specimens. Miniature tensile specimens were machined from the quenched blanks and tested in tension at four strain rates from 0.003 s−1 to 1075 s−1. For the 0.003 s−1 tests, the ultimate tensile strength (UTS) increased from 816 MPa to 1447 MPa for the 14 °C/s and 50 °C/s quench condition respectively. By elevating the strain rate from 0.003 s−1 to 1075 s−1, the UTS of the 14 °C/s specimens increased by 134 MPa, while the increase in UTS of the 50 °C/s specimens was measured to be 170 MPa for an equivalent change in loading rate. The percent area reduction of the fracture surface was 70% for the 14 °C/s specimens and decreased to 58% for the 50 °C/s specimens. The true stress versus effective plastic strain (flow stress) curves were used to develop the “Tailored Crash Model” (TCM) which is a constitutive model that is a function of strain, strain rate, and as-quenched Vickers hardness (or area fraction martensite/bainite). Scanning electron microscope and optical microscope images of the fracture surfaces revealed a transition from ductile to shear fracture behaviour as the quench condition increased from 14 °C/s to 50 °C/s. ► Boron steel sheet metal was quenched at sub-critical (