Effect of tempering temperature on the microstructure and mechanical properties of 30MnB5 hot stamping steel after quenching

30MnB5 hot stamping steel was quenched and tempered at different temperatures. The effects of tem-pering temperature on the microstructure and mechanical properties were investigated by scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, and tensile testing. The best mechanical properties were obtained after the steel being tempered at 200℃ for 2 min, which are the tensile strength of 1774 MPa, the total elongation of 8%, and the product of tensile strength and elongation more than 14 GPa.%. It can meet the require-ment for automotive structural parts after hot stamping. The mechanical properties non-monotonically change with tempering temperature. The microstructure of the steel tempered at 200℃ is lath martensite and ε-carbides, and the dislocation density decreases slightly. The ε-carbide particles with the size of about 100 nm in length are needle-like distributed in martensite laths and pinned with dislocations. When the tempering temperature increases, the recovery and recrystallization of lath martensite occur, the lath boundary becomes more and more obscure, and the dislocation density decreases sharply. Furthermore, some of the lath martensite transforms into equiaxed ferrite, and the ε-carbides gradually convert into low-boundary-layer-energy spheroidal cementite, which is coarsened to about 200 nm. Owing to these, the interaction between carbides and dislocations largely weakens.