Abstract: The internal frictions from 290 to 630K in the H180 steel specimens hotrolled or baked for various time were measured under different conditions with and without a magnetic field (magnetic field strength is 300Oe) using an audiofrequency internal friction instrument. It is found that there are two clear peaks of internal friction in all the Q-1T curves， and that the first peak appears at about 362K， while the other one occurs at about 474K (measurement frequencies are 2300~2150Hz). With increasing baking time， the first peak continuously decreases but the second peak increases. Analysis demonstrates that the first peak is the Snoek peak and the second one is the B peak. A comparison of the internal frictions measured with and without a magnetic field shows that the background internal friction measured under a magnetic field is higher than that measured without a magnetic field. Such a background internal friction is enhanced under the present audiofrequency measurements， but it decreases with increasing the measurement temperature.

Key words: automobile steel, bake hardening property, audiofrequency, internal friction, thin Cottrell atmosphere