Influence of N/C ratio on microstructure and properties of new high-strength weathering steels

• Published in: Journal of iron and steel research, international Vol. 26; no. 1; pp. 84 - 90
• Main Authors: Cheng, Jian, Guo, Yue-hua, Liu, Ming, Shen, Hou-fa
• Format: Journal Article
• Language: English
• Published: Singapore Springer Singapore 24.01.2019
• Subjects: Applied and Technical Physics; Engineering; Machines; Manufacturing; Materials Engineering; Materials Science; Metallic Materials; Original Paper; Physical Chemistry; Processes; CALPHAD; Weathering steel; Microstructure; Tensile test; Impact test
• ISSN: 1006-706X; 2210-3988
• DOI: 10.1007/s42243-018-0217-x

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.