Effect of Carbon Content and Intercritical Annealing on Microstructure and Mechanical Tensile Properties in FeCMnSiCr TRIP-Assisted Steels
Abstract
:1. Introduction
2. Materials and Methods
2.1. Base Material Manufacturing
2.2. Thermal Study and Heat Treatments
2.3. Microstructural Study
2.4. Tensile Mechanical Properties
3. Results
3.1. Base Material
3.2. Microstructural Evolution
3.2.1. Optical Microscopy
3.2.2. Scanning Electron Microscopy (SEM)
3.2.3. Atomic Force Microscopy (AFM)
3.3. Tensile Test
3.3.1. Mechanical Properties
3.3.2. Fracture Surfaces
4. Discussion
5. Conclusions
- The effect of carbon was decisive in the evolution of the microstructure achieved in steels A, B and C. These were subjected to the same heat treatment to achieve TBF steels, developing between them a difference in terms of vol.% of RA, and morphology of its phases and microconstituents, which were observed by OM, SEM and AFM. As the wt.% C increased, the amount of BF decreased and the M/RA evolved from sheets to increasingly thicker blocks, perceiving an increase in its vol.% relative to BF.
- Steel D showed a granularization process of the BF and M/RA, atypical in the bainitic transformation with isothermal treatments. A possible explanation could be that with a high wt.% C in austenite, the Bs is lower than in the rest of the steels, implying that at 400 °C, IT temperature, a higher BF is obtained, less fine than the one formed at lower temperatures, which was subsequently aged after the 1500 s of bainitic transformation, evolving from plates to granules, also affecting the morphology of the M/RA, like what could occur in continuous cooling.
- Given the potential to evaluate surfaces with atomic resolution, AFM is a tool for future studies in the observation and determination of morphology for nanostructured polyphase steels, achieved with modern techniques of heat treatments for fine grain size, such as ultrafast annealing.
- In steels A, B and C, as the percentage of carbon increases, the YS and UTS rise, and the TE decreases, due to the increase in volume and thickening of the M/RA microconstituent, which is hard and brittle, confirmed with the fracture surfaces, showing an evolution from a ductile to a brittle morphology.
- Steel C, despite being the strongest and most brittle, with a large amount of M/RA in blocks, has a similar and even higher UE than steels A and B, which have less carbon and less strength. The above indicates a greater TRIP effect, due to the large amount of RA, which is evidenced by its higher “n” index.
- Steel D is a TPF steel achieved with an IA. The initial and final behavior is ductile, hardening as it progresses in homogeneous plastic strain, reaching a UTS like of steel A, but with a widely higher UE, which is confirmed too by analyzing the surface of fracture, which presents a ductile morphology.
- From the point of view of tensile toughness, the steel with the best characteristics to be used in the high-thickness structural field is D. Despite having the lowest YS, it has the best YS/UTS ratio, the highest UE, the largest “n”, and its fracture mode is mainly ductile. The higher toughness is attributed to the soft and ductile PF matrix with fine aggregates of M/RA dispersed throughout the matrix in the form of granules, which concentrate less stress than acicular morphology.
- The most important aspect of this work is that the best mechanical behavior of the high thicknesses TRIP steels studied, is related to a fine morphology of the microstructure and an adequate amount of retained austenite.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Steels | C | Mn | Si | Cr | Al | Cu | Mo | Ni | P | S | Fe |
---|---|---|---|---|---|---|---|---|---|---|---|
S1 | 0.166 | 1.872 | 1.534 | 0.242 | 0.343 | 0.0995 | 0.017 | 0.050 | 0.005 | 0.022 | Bal. |
S2 | 0.285 | 1.829 | 1.445 | 0.242 | 0.068 | 0.0873 | 0.017 | 0.049 | 0.005 | 0.021 | Bal. |
S3 | 0.397 | 1.920 | 1.470 | 0.462 | 0.008 | 0.0873 | 0.013 | 0.481 | 0.016 | 0.019 | Bal. |
Steel | Source | A1 | A3 | Bs | Ms | 50/50 |
---|---|---|---|---|---|---|
S1 | BBL | 728 | 866 | 581 | 389 | - |
DSC | 713 | 855 | - | - | - | |
DTA | 725 | 855 | - | - | - | |
IMG | 712 | 865 | - | - | 760 | |
S2 | BBL | 725 | 835 | 556 | 343 | - |
DSC | 728 | 820 | - | - | - | |
DTA | 717 | 825 | - | - | - | |
IMG | 715 | 830 | - | - | - | |
S3 | BBL | 720 | 796 | 473 | 228 | - |
DSC | 730 | 822 | - | - | - | |
DTA | 727 | 812 | - | - | - | |
IMG | 720 | 815 | - | - | - |
Heat Treatments for TPF Steels (S1) | Heat Treatments for TBF Steels (S1, S2, S3) | ||||||||
---|---|---|---|---|---|---|---|---|---|
N | T (°C) IA | t (s) IA | T (°C) IT | t (s) IT | N | T (°C) AA | t (s) IA | T (°C) IT | t (s) IT |
1 | 760 | 1200 | 350 | 600 | 1 | 910 | 1200 | 350 | 600 |
2 | 760 | 1200 | 350 | 1000 | 2 | 910 | 1200 | 350 | 1000 |
3 | 760 | 1200 | 350 | 1500 | 3 | 910 | 1200 | 350 | 1500 |
4 | 760 | 1200 | 400 | 600 | 4 | 910 | 1200 | 400 | 600 |
5 | 760 | 1200 | 400 | 1000 | 5 | 910 | 1200 | 400 | 1000 |
6 | 760 | 1200 | 400 | 1500 | 6 | 910 | 1200 | 400 | 1500 |
7 | 760 | 1200 | 450 | 600 | 7 | 910 | 1200 | 450 | 600 |
8 | 760 | 1200 | 450 | 1000 | 8 | 910 | 1200 | 450 | 1000 |
9 | 760 | 1200 | 450 | 1500 | 9 | 910 | 1200 | 450 | 1500 |
Steels | Final Heat Treatment | XRD Results | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
T (°C) A | t (s) A | T (°C) IT | t (s) IT | vol.% RA | vol.% α | a0 (Å) RA | wt.% C RA | |||
A | TBF | S1 | 910 | 1200 | 400 | 1500 | 10.4 | 88.3 | 3.609400 | 1.4 |
B | TBF | S2 | 910 | 1200 | 400 | 1500 | 19.2 | 79.2 | 3.613900 | 1.6 |
C | TBF | S3 | 910 | 1200 | 400 | 1500 | 23.7 | 74.2 | 3.613690 | 1.5 |
D | TPF | S1 | 760 | 1200 | 400 | 1500 | 17.9 | 80.3 | 3.606854 | 1.4 |
STEEL | Engineering Tensile Test | True Tensile Test | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
YS | UTS | YS/UTS | FS | UE | TE | Εxz | YS | UTS | YS/UTS | UE | n | K | |
MPa | MPa | - | MPa | % | % | % | MPa | MPa | - | % | - | MPa | |
A | 735 | 913 | 0.81 | 713 | 10.7 | 18.4 | 35.6 | 735 | 1016 | 0.72 | 11.1 | 0.12 | 757 |
B | 1020 | 1442 | 0.71 | 1375 | 11.5 | 14.3 | 12.9 | 1020 | 1615 | 0.63 | 11.6 | 0.17 | 1045 |
C | 1060 | 1527 | 0.69 | 1511 | 12 | 12.1 | 0.9 | 1060 | 1710 | 0.62 | 11.4 | 0.18 | 1077 |
D | 560 | 897 | 0.62 | 777 | 15.8 | 22.3 | 29.2 | 560 | 1047 | 0.53 | 16 | 0.23 | 559 |
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Tesser, E.; Silva, C.; Artigas, A.; Monsalve, A. Effect of Carbon Content and Intercritical Annealing on Microstructure and Mechanical Tensile Properties in FeCMnSiCr TRIP-Assisted Steels. Metals 2021, 11, 1546. https://doi.org/10.3390/met11101546
Tesser E, Silva C, Artigas A, Monsalve A. Effect of Carbon Content and Intercritical Annealing on Microstructure and Mechanical Tensile Properties in FeCMnSiCr TRIP-Assisted Steels. Metals. 2021; 11(10):1546. https://doi.org/10.3390/met11101546
Chicago/Turabian StyleTesser, Enzo, Carlos Silva, Alfredo Artigas, and Alberto Monsalve. 2021. "Effect of Carbon Content and Intercritical Annealing on Microstructure and Mechanical Tensile Properties in FeCMnSiCr TRIP-Assisted Steels" Metals 11, no. 10: 1546. https://doi.org/10.3390/met11101546