Influence of N/C ratio on microstructure and properties of new high-strength weathering steels
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Influence of the N/C ratio on the microstructure and properties of new-generation high-strength weathering steels was investigated using calculation of phase diagram (CALPHAD) and experiments. The microstructures of weathering steels containing different N/C ratios were predicted by a CALPHAD approach, and only three phases were predicted in these steels within the rolling temperature range of 850–1050 °C. The precipitation fraction of VN/V(C, N) increases with increasing N/C ratio. Microstructures of the four tested steels were all experimentally determined to contain bainite, ferrite, and VN precipitates after air cooling to room temperature. The bainite fraction increases with increasing N/C ratio, and it is 85% in the steel containing 0.038% N and 0.032% C. The results of the tensile tests and impact tests demonstrated that the yield strength and tensile strength of the steel containing 0.038% N and 0.032% C are greater than 550 and 650 MPa, respectively, and the elongation is greater than 24%, which satisfies the design objectives for mechanical properties. The impact toughness values of the four steels at 0, − 20, and − 40 °C are all greater than 24 J. With increasing N/C ratio, the bainite fraction and the precipitation fraction of VN/V(C, N) increase, resulting in increasing yield strength and tensile strength.
KeywordsCALPHAD Weathering steel Tensile test Impact test Microstructure
This research work received financial support from the Green Manufacturing System Integration Project of MIIT China (2016-64).
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