Relationship of Microstructure and Mechanical Properties of Dual Phase Steel after Various Annealing Conditions

Abstract

The development of ultrafine ferrite grain size has become one of attractive way how to improve the behavior of dual phase (DP) steels. The other possible way how to enhance mechanical properties of DP steels is to modify the chemical composition. Therefore object of our investigation was the dual phase steel with modified alloying (three times higher Cr content with addition of phosphorus). The dual phase steel was annealed in laboratory conditions in accordance with three specified annealing cycles: into intercritical region (780°C), into austenite region (920°C) and into austenite region (920°C) by subsequently cooling into intercritical region (780°C) with the hold at the temperature of 495°C. The obtained microstructure after selected annealing regimes consists of three phases (ferritic matrix, martensite and martensite/bainite grains) with different size and distribution. For studied annealing regimes were clearly defined mechanical properties such as: YS, UTS, elongation, n-parameter and ratio YS/UTS. It was defined the scheme of microstructure evolution on base of austenite grain size during the continual cooling process with defined three phases: 1) the hard martensite formed on the grain boundary; 2) the soft interior bainite and 3) the hard isolated martensite.