Development of carbide-free bainite/martensite/austenite triplex steels through short-term austenitizing and isothermal bainitic treatments

It has been recently reported that carbide-free bainite/martensite/austenite triplex steels can be obtained through long process times (30 >= min), which makes their development at industrial level very difficult. In the present work, triplex steels were obtained through short-term austenitizing (1-3 min) and isothermal bainitic treatments (0-7 min) at temperatures between 430 and 450 degrees C, which allows the fabrication of these steels under thermal cycles like those used in continuous annealing and galvanizing lines. Microstructural analysis by microscopy and X-ray diffraction revealed that the amounts of martensite and carbide-free bainite (CFB) vary with the austenitization time; longer times result in higher amounts of martensite, while shorter times lead to higher amounts of CFB and lower martensite amounts in the final product. Longer times and lower temperatures of isothermal bainitic treatment lead to larger amounts of CFB. The increase in the amount of bainite and the reduction of martensite cause a reduction in microhardness.

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