Influence of Chromium Content on the Microstructure and Mechanical Properties of Thermomechanically Hot-Rolled Low-Carbon Bainitic Steels Containing Niobium
Abstract
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Abstract
1. Introduction
2. Materials and Methods
2.1. Materials
2.2. CCT Diagrams
2.3. Microstructure Characterization
2.4. Mechanical Properties Characterization
3. Results and Discussion
3.1. CCT and DCCT Diagrams
3.2. Influence of Cr Content on the CCT and DCCT Diagrams and Hardness
3.3. Microstructure of the Hot Rolled and Direct-Quenched Steel Plates
3.4. Mechanical Properties
3.5. Fractography
4. Conclusions
- In the case of non-strained austenite, the microstructure consists mainly of bainitic ferrite at high CRs, a mixture of bainitic ferrite with granular bainite at intermediate CRs, and polygonal ferrite with granular bainite at the lowest CRs. Increasing the Cr content and CRs refined the microstructure significantly. In addition, increasing the Cr content led to an increase in the hardenability, i.e., a decrease in transformation temperatures, as well as the promotion of bainite at the expense of the high-temperature transformation products like polygonal ferrite and granular bainite.
- In the low-Cr steel, deformation of austenite below its recrystallization temperature has a large effect on the final microstructural constituents, changing the microstructure completely to ferrite at all the investigated CRs and causing a large drop in hardness relative to the non-deformed case. However, this effect decreased with increasing the Cr content.
- To get a fully bainitic microstructure in the case of deformed austenite, the Cr content should be in the range of 2.5 wt.%–4 wt.%, and the CR should be higher than 20 °C/s.
- In the case of the 12 mm thick hot-rolled and direct-quenched steel plates, the microstructure consisted mainly of different types of bainite, i.e., bainitic ferrite, granular bainite, and coalesced bainite. A significant amount of polygonal ferrite only appears at the lowest Cr level, i.e., 1 wt.%. Increasing the Cr content led to an increase in the hardness owing to an enhanced hardenability.
- The targeted 700 MPa yield strength with good ductility and toughness were achieved in the hot-rolled and direct-quenched plates with Cr content in the range of 2.5 wt.%–4 wt.%, but not achieved with the lowest Cr content (1 wt.%) due to the presence of a significant fraction of polygonal ferrite, which increased the impact toughness and elongation at the expense of the yield strength.
Author Contributions
Funding
Conflicts of Interest
References
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Steel Code | C | Si | Mn | Cr | Nb |
---|---|---|---|---|---|
Steel I (1 Cr) | 0.041 | 0.211 | 1.02 | 1.0 | 0.065 |
Steel II (2.5 Cr) | 0.041 | 0.168 | 0.95 | 2.5 | 0.066 |
Steel III (4 Cr) | 0.036 | 0.154 | 0.92 | 4.0 | 0.064 |
Steel Code | YS 1, MPa | UTS 1, MPa | El 1, % | Hardness 2, HV | T28J (°C) | USE J/cm2 | YS/UTS | UTS*El (MPa *%) |
---|---|---|---|---|---|---|---|---|
Steel I (1 Cr) | 560 ± 7 | 756 ± 2 | 17 ± 0 | 252 ± 6 | −118 | 318 | 0.74 | 13034 |
Steel II (2.5 Cr) | 699 ± 3 | 909 ± 4 | 14 ± 0 | 292 ± 7 | −84 | 270 | 0.77 | 12852 |
Steel III (4 Cr) | 780 ± 9 | 982 ± 8 | 12 ± 0 | 312 ± 7 | −78 | 265 | 0.79 | 12238 |
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Ali, M.; Nyo, T.; Kaijalainen, A.; Hannula, J.; Porter, D.; Kömi, J. Influence of Chromium Content on the Microstructure and Mechanical Properties of Thermomechanically Hot-Rolled Low-Carbon Bainitic Steels Containing Niobium. Appl. Sci. 2020, 10, 344. https://doi.org/10.3390/app10010344
Ali M, Nyo T, Kaijalainen A, Hannula J, Porter D, Kömi J. Influence of Chromium Content on the Microstructure and Mechanical Properties of Thermomechanically Hot-Rolled Low-Carbon Bainitic Steels Containing Niobium. Applied Sciences. 2020; 10(1):344. https://doi.org/10.3390/app10010344
Chicago/Turabian StyleAli, Mohammed, Tun Nyo, Antti Kaijalainen, Jaakko Hannula, David Porter, and Jukka Kömi. 2020. "Influence of Chromium Content on the Microstructure and Mechanical Properties of Thermomechanically Hot-Rolled Low-Carbon Bainitic Steels Containing Niobium" Applied Sciences 10, no. 1: 344. https://doi.org/10.3390/app10010344
APA StyleAli, M., Nyo, T., Kaijalainen, A., Hannula, J., Porter, D., & Kömi, J. (2020). Influence of Chromium Content on the Microstructure and Mechanical Properties of Thermomechanically Hot-Rolled Low-Carbon Bainitic Steels Containing Niobium. Applied Sciences, 10(1), 344. https://doi.org/10.3390/app10010344