Challenges of Nb Application in Thermomechanical Processes of Steels for Long Products

Abstract:

Nb is a classical microalloying element in the design of thermomechanical treatments in low carbon steels for flat products applications. However, its use in medium-high carbon grades, as occurs in hot rolling of bars, is less common. This is, in part, because of the diversity of characteristics required to those grades of steels and the less knowledge about the function of Nb in these cases. Consequently, less information is reported concerning thermo-mechanical processing of Nb microalloyed steels in long products applications. In this case, it is necessary to consider the singularities related to these processes, such as the short interpass times and the wide range of chemical compositions usually applied on these products. Short interpass times result in high strain rate values that can lead to metallurgical changes on the mechanisms occurring during the hot rolling must be considered. Moreover, the high Carbon contents applied in long products, usually between 0.20–0.40%, can influence the Nb solubility and precipitation in each stage of the process: prior to hot rolling on the reheating furnace, during the process and after hot rolling, depending on the cooling strategy adopted and on the post-rolling heat treatments that can be applied. This paper analyses different singularities associated with the use of Nb microalloying for long products. Several aspects, such as the partial or complete dissolution of the Nb prior to hot rolling, its role in the control of austenite microstructure and its incidence in the final microstructure and mechanical properties, will be considered.

• Periodical: Materials Science Forum (Volume 941)
• Pages: 386-393
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.941.386
• Online since: December 2018
• Authors: Beatriz López*, Beatriz Pereda, Felipe Bastos, Marcelo A. Rebellato, J.M. Rodriguez-Ibabe
• Keywords: Long Product, Nb-Microalloying, Thermomechanical Treatment

References

3%C-Mn HV1: 269 ± 8.

3%C-0.18%V HV1: 264 ± 7.

3%C-Mn Dg =4.4±0.2 mm HV=231.

3%C-0.18%V Dg =3.5±0.1 mm HV=232.

3%C-0.11%V-0.017%Nb Figure 6 Stress-strain curves obtained after multipass hot torsion tests simulating Æ6mm rebar rolling and the corresponding austenite microstructure given by quenching after the last pass and transformed microstructure after cooling. Although the stress is continuously increasing due to temperature decrease, the shape of the curves clearly denotes the occurrence of dynamic recrystallization as it is confirmed by the equiaxed shaped of the austenite microstructure. Very fine austenite grain size is obtained, especially when Nb is added. As a result after cooling down to 560ºC at 2ºC/s, very fine ferrite+pearlite microstructure is obtained, with a significant increase in hardness compared to plain C-Mn. Note that the partial substitution of V by a small addition of Nb leads to similar hardness values. The fact that Nb favors grain refinement through the activation of dynamic recrystallization by means of a delay of the static recrystallization, opens the possibility of proposing thermomechanical processes in which it is not a requisite this element to be completely dissolved before rolling. Suitable precipitates formed prior to rolling can, simultaneously, avoid excessive grain growth in the first stages of the rolling (similar to recrystallization controlled rolling) and in the case of a high number of passes, favor a fine final grain through the activation of dynamic recrystallization. In both cases, the situation is completely different from conventional controlled rolling of low carbon steels, where austenite refinement is mainly based on pancaking. Conclusions The use of Nb in long products with medium-high carbon contents offers new opportunities for the achievement of steels with higher requirements in final mechanical properties. However, this technology requires the definition of new process parameters, since it cannot be considered as a mere extrapolation of the technology developed for low carbon steels. In addition, Nb in solution or precipitated at different stages of hot rolling, offers very different possibilities that must be taken into account depending on the final application of the product and whether post rolling thermal treatments will be applied or not. Acknowledgments Part of the results present in this work have been obtained under the frame of a Project funded by CBMM. References.