Abstract: Under the background of carbon reduction and safety, the development of ultra-high strength steels for automobiles are facing new challenges. By changing the annealing process of the cold-rolled sheet, the annealed microstructure could be transformed into martensite. The efficiency of complete austenitization of the material in the hot forming process can be greatly improved, thereby shortening the total heating time required for hot forming. A short-flow hot forming process was proposed by combining new materials and new processes. Taking coating-free hot forming steel as an example, the microstructure evolution and mechanical properties of steel under short-flow hot forming process are studied. Results shown that the non-plating steel with the initial structure of martensite and carbide can be fully austenitized by heating at 100 ℃/s to 930 ℃ and holding for 30 s, and martensite and 3%-5% retained austenite can be obtained. The final material owns high strength and toughness: with tensile strength of 1 578 MPa, and elongation of 7.8%. Therefore, the heating time required for hot forming can be shortened to less than 1 min. Compared with the 5 min heating time required for traditional hot forming process of boron steel plate, the short process hot forming technology reduces the total heating time by more than 80% while ensuring the strength and toughness of the parts, which can effectively help energy saving and carbon reduction.

Key words: hot forming steel, short process, martensite, rapid heating