Determination of yield behaviour of boron alloy steel at high temperature

บทคัดย่อ

The hot tensile test is a basic material testing method for investigation of the mechanical properties of steel at high temperature conditions. In industrial hot sheet forming processes such as hot stamping and hot gas metal forming, the influences of temperature and strain rate on microstructure and material behaviour play a significant role. In this work, sheet samples of steel grade 22MnB5 were examined using a hot deformation simulator. Different temperatures between 900 and 1200°C, which are above Ac3, and strain rates between 0.001 and 0.1 s^-1 were considered. During the tensile test, a laser speckle extensometer (LSE) was deployed for measuring the local strain values on the deformed sample. By means of this LSE, more accurate stress-strain responses were obtained. Reliable flow curves are indispensable data for simulations of the hot forming processes. The influences of temperature and strain rate on the plastic behaviour of the boron steel were investigated. From temperature higher than 900°C, the flow stresses explicitly decrease and the formability enhances. Due to the martensitic transformation during a fast cooling the strength of this steel is improved. A fully martensitic microstructure with a small amount of retained austenite was observed in the samples after the hot tensile test and a typical ductile fracture was detected.