Metallurgical Effects of Niobium in Dual Phase Steel
Abstract
:1. Introduction
2. Materials and Methods
3. Results
3.1. Characterization of As-Hot Rolled Condition
3.2. Characterization of Batch Annealed Condition
3.3. Characterization of Final Condition
3.4. Evolution of Niobium Precipitation
4. Discussion
4.1. Solubility Considerations and Precipitation Kinetics
4.2. Precipitation Strengthening
4.3. Microstructural Refinement and Homogenization
4.4. Microstructural Implications on Mechanical Properties
- Grain refinement is known to raise strength and toughness simultaneously. In conventional high strength steels with ferritic, ferritic-pearlitic or bainitic matrix grain refinement has a stronger effect on yield than on tensile strength. Thus, the cold working potential is diminished, and uniform elongation is reduced. In DP steel the influence of grain refinement however is opposite [24,25]. Consequently, the work hardening potential will be enhanced by refining the microstructure. The Hall–Petch coefficients for yield and tensile strength in DP steels related to ferrite grain size were determined by Ramazani et al. [26] as being 3.98 and 8.39 MPa∙mm−1/2, respectively. Accordingly, the strengthening contribution resulting from ferrite grain size in the present steel is estimated to be in the order of 70 and 150 MPa for yield and tensile strength, respectively. In comparison to typically coarser grained conventional DP steel with similar martensite volume fraction, the tensile strength increment by grain refinement is expected to reach up to 100 MPa [24,25].
- Precipitation strengthening in the ferrite phase [27] in combination with grain refinement results in more homogeneous strain distribution within the microstructure with the potential of postponing plastic instability or necking of the material towards higher strains, i.e., an improved strength-ductility balance can be expected.
- The very fine martensite size and the good separation of martensite islands from each other in the present steel is expected reducing void nucleation and growth along ferrite/martensite interfaces as well as obstructing cleavage fracture in martensite islands.
- The nano-sized niobium carbides dispersed within ferrite phase are expected to significantly increase dislocation density in the very beginning of deformation leading to large instant work-hardening. The dislocation loops formed around the precipitates reduce the effective mean-free path length (parameter L in Equation (5)) between particles with increasing Ashby-Orowan strengthening, i.e., work hardening as a consequence. With the progress of deformation, cross-slip on secondary slip planes can be activated in the vicinity of particles resulting in dynamic recovery [28]. Differential strain hardening analysis by Suppan et al. [6] on a similar high-ductility DP steel indeed observed such recovery behavior after initial straining. The work hardening behavior observed in the present steel (Figure 12b) reveals the same phenomenon after the initial very high strain hardening rate.
- The sequence of instant strain hardening by precipitates, followed by increasing dislocation pile-up against grain or phase boundaries, and finally retained austenite transformation provides a continuously high strain hardening rate over a wide deformation range. In this steel the strain hardening coefficient varies between 0.12 and 0.18 until homogeneous elongation is reached. It guarantees high strength in a formed component irrespective of the actual degree of strain in local areas of the part.
5. Conclusions
Author Contributions
Funding
Conflicts of Interest
References
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Alloy (Mass%) | C | Mn | Si | Cr | Nb | Ti | B |
---|---|---|---|---|---|---|---|
DP980 HD: | ≈0.2 | 2.2–2.6 | 0.5–1.0 | <0.7 | 0.03 | <0.02 | <0.003 |
Processing Stage | Slab Soaking | Finish Rolling | Coiling | Batch Annealing | Continuous Annealing | ||
Temperature: | 1240 °C | >900 °C | ≈600 °C | 580 °C/10 h | 850 °C |
Position in Strip | Phase Share (%) | ||
---|---|---|---|
Ferrite | Martensite | Austenite | |
Head-center | 44 | 48 | 8 |
Head-edge | 46 | 46 | 8 |
Tail-center | 45 | 48 | 7 |
Tail-edge | 42 | 52 | 6 |
Processing Stage | Particle Size (nm) | Particle Count (µm−3) | Particle Volume Fraction (%) | Amount (ppm)/Share (%) of Precipitated Nb |
---|---|---|---|---|
Hot rolled | 9.6 | 188 | 0.004 | 35/10% |
Batch annealed | 11.6 | 1072 | 0.023 | ~200/~70%1 |
Final conditioning | 6.5 | 2480 | 0.034 | 303/100% |
Steel Grade | Yield Strength Rp0.2 (MPa) | Tensile Strength Rm (MPa) | Elongation at Fracture A80 (%) | n-value | |
---|---|---|---|---|---|
n4-6 | n10-20 / Ag | ||||
CR440Y780T-DH | 440–550 | 780–900 | ≥18 | ≥0.18 | ≥0.13 |
CR700Y980T-DH | 700–850 | 980–1180 | ≥13 | - | - |
Current production (typical values) | 740 | 1030 | 15 | ≥0.14 | ≥0.12 |
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Mohrbacher, H.; Yang, J.-R.; Chen, Y.-W.; Rehrl, J.; Hebesberger, T. Metallurgical Effects of Niobium in Dual Phase Steel. Metals 2020, 10, 504. https://doi.org/10.3390/met10040504
Mohrbacher H, Yang J-R, Chen Y-W, Rehrl J, Hebesberger T. Metallurgical Effects of Niobium in Dual Phase Steel. Metals. 2020; 10(4):504. https://doi.org/10.3390/met10040504
Chicago/Turabian StyleMohrbacher, Hardy, Jer-Ren Yang, Yu-Wen Chen, Johannes Rehrl, and Thomas Hebesberger. 2020. "Metallurgical Effects of Niobium in Dual Phase Steel" Metals 10, no. 4: 504. https://doi.org/10.3390/met10040504