Abstract: Rapid heat treatment method of rapid heating, short time holding and rapid cooling was applied to hot forming process of 22MnB5 steel. The simulation test of material thermal process was carried out by using continuous annealing tester to study the change rule of phase transformation point temperatures and the evolution rule of microstructure and properties of the 22MnB5 steel under different process parameters, and the corresponding mechanisms were revealed. The results show that the increase of heating rate increases the transformation point temperature, and the heating temperature needs to be higher than 950 ℃ to ensure complete austenitization. The final structure resulted by traditional hot forming process is full martensite structure, while that by the rapid heat treatment process is composed of martensite, bainite and undissolved carbide, and the grain size is refined from 11.50 μm to 7.60 μm. The mechanical properties are better when heating at 1000 ℃ for 10 s, the tensile strength is slightly improved compared with that by the traditional hot forming process, and the elongation is increased from 6.39% to 8.44%. The reason for the resulted microstructure with such excellent properties is related to the uneven composition caused by rapid heating and short time heat preservation.

Key words: 22MnB5 steel, rapid heat treatment, microstructure and properties, bainite, martensite