Mechanical and Microstructure Properties of Ultra-High Strength Boron Steel Using Rapid Resistance Heating Without Soaking

Resistance heating (RH) has broad prospects in hot stamping due to its rapid heating rate and high energy efficiency. In this work, the ultra-high strength boron steel was heated by high current and then water quenched (without soaking). The effects of heating temperature and current density on mechanical properties and microstructure were explored. The results indicated that resistance heating method can significantly shorten the heating time, and even the soaking stage can be ignored when overheating is adopted; thus, the whole heating period can be reduced from 5 min to 15 s. Besides, the oxide scale is inapparent on the uncoated sheet with rapid heating, which improves the surface quality evidently. Furthermore, the mechanical properties of the boron steel after resistance heating are better than those after furnace heating (FH), including both strength and elongation. The microscopic characterization confirms that the grain size of the resistance heating sample is obviously finer than that of the furnace heating, showing a mixed microstructure of coarse and fine grains, and the final structure includes a certain amount of retained austenite. These characteristics ensure that the boron steel has excellent properties after resistance heating and quenching.