The Influence of La and Ce Addition on Inclusion Modification in Cast Niobium Microalloyed Steels

The main role of Rare Earth (RE) elements in the steelmaking industry is to affect the nature of inclusions (composition, geometry, size and volume fraction), which can potentially lead to the improvement of some mechanical properties such as the toughness in steels. In this study, different amounts of RE were added to a niobium microalloyed steel in as-cast condition to investigate its influence on: (i) type of inclusions and (ii) precipitation of niobium carbides. The characterization of the microstructure by optical, scanning and transmission electron microscopy shows that: (1) the addition of RE elements change the inclusion formation route during solidification; RE > 200 ppm promote formation of complex inclusions with a (La,Ce)(S,O) matrix instead of Al2O3-MnS inclusions; (2) the roundness of inclusions increases with RE, whereas more than 200 ppm addition would increase the area fraction and size of the inclusions; (3) it was found that the presence of MnS in the base and low RE-added steel provide nucleation sites for the precipitation of coarse niobium carbides and/or carbonitrides at the matrix–MnS interface. Thermodynamic calculations show that temperatures of the order of 1200 °C would be necessary to dissolve these coarse Nb-rich carbides so as to reprecipitate them as nanoparticles in the matrix.We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).The authors from the University of Tehran gratefully acknowledge the financial support provided by the Office of International Affairs and the Office for Research Affairs, College of Engineering, for the project number 8107009.6.34. The authors from Centro Nacional de Investigaciones Metalúrgicas (CENIM) that belong to the Consejo Superior de Investigaciones Científicas (CSIC) would like to acknowledge the financial support from Comunidad de Madrid through the project Diseño Multiescala de Materiales Avanzados (DIMMAT-CM_S2013/MIT-2775). Javier Vivas acknowledges financial support in the form of a FPI (Formación de Personal Investigador) Grant BES-2014-069863. Authors are grateful to the Phase Transformations and Microscopy labs from CENIM-CSIC and to the Centro Nacional de Microscopia Electronica (CNME), located at Complutense Metals 2017, 7, 377 16 of 17 University of Madrid (UCM), for the provision of laboratory facilities. Mr. Javier Vara Miñambres from the Phase Transformations lab (CENIM-CSIC) is gratefully acknowledged for their continuous experimental suppor

Low-carbon cast microalloyed steel intercritically heat-treated at different temperatures: microstructure and mechanical properties

In this study, dual-phase (DP, ferrite + martensite) microstructures were obtained by performing intercritical heat treatments (IHT) at 750 and 800 °C followed by quenching. Decreasing the IHT temperature from 800 to 750 °C leads to: (i) a decrease in the volume fraction of austenite (martensite after quenching) from 0.68 to 0.36; (ii) ~ 100 °C decrease in martensite start temperature (Ms), mainly due to the higher carbon content of austenite and its smaller grains at 750 °C; (iii) a reduction in the block size of martensite from 1.9 to 1.2 μm as measured by EBSD. Having a higher carbon content and a finer block size, the localized microhardness of martensite islands increases from 380 HV (800 °C) to 504 HV (750 °C). Moreover, despite the different volume fractions of martensite obtained in DP microstructures, the hardness of the steels remained unchanged by changing the IHT temperature (~ 234 to 238 HV). Applying lower IHT temperature (lower fraction of martensite), the impact energy even decreased from 12 to 9 J due to the brittleness of the martensite phase. The results of the tensile tests indicate that by increasing the IHT temperature, the yield and ultimate tensile strengths of the DP steel increase from 493 to 770 MPa, and from 908 to 1080 MPa, respectively, while the total elongation decreases from 9.8 to 4.5%. In contrast to the normalized sample, formation of martensite in the DP steels could eliminate the yield point phenomenon in the tensile curves, as it generates free dislocations in adjacent ferrite.The authors are grateful to the Phase Transformations and Microscopy labs from CENIM-CSIC. Mr. Javier Vara Miñambres from the Phase Transformations lab (CENIM-CSIC) is gratefully acknowledged for his continuous experimental support. J. Vivas acknowledges financial support in the form of a FPI Grant BES-2014-069863 from the Ministerio de Economia y Competitividad (MINECO). Open access funding provided by Lulea University of Technology

Contributions of Rare Earth Element (La,Ce) Addition to the Impact Toughness of Low Carbon Cast Niobium Microalloyed Steels

In this research Rare Earth elements (RE), La and Ce (200 ppm), were added to a low carbon cast microalloyed steel to disclose their influence on the microstructure and impact toughness. It is suggested that RE are able to change the interaction between the inclusions and matrix during the solidification process (comprising peritectic transformation), which could affect the microstructural features and consequently the impact property; compared to the base steel a clear evolution was observed in nature and morphology of the inclusions present in the RE-added steel i.e. (1) they changed from MnS-based to (RE,Al)(S,O) and RE(S)-based; (2) they obtained an aspect ratio closer to 1 with a lower area fraction as well as a smaller average size. Besides, the microstructural examination of the matrix phases showed that a bimodal type of ferrite grain size distribution exists in both base and RE-added steels, while the mean ferrite grain size was reduced from 12 to 7 μm and the bimodality was redressed in the RE-added steel. It was found that pearlite nodule size decreases from 9 to 6 μm in the RE-added steel; however, microalloying with RE caused only a slight decrease in pearlite volume fraction. After detailed fractography analyses, it was found that, compared to the based steel, the significant enhancement of the impact toughness in RE-added steel (from 63 to 100 J) can be mainly attributed to the differences observed in the nature of the inclusions, the ferrite grain size distribution, and the pearlite nodule size. The presence of carbides (cementite) at ferrite grain boundaries and probable change in distribution of Nb-nanoprecipitation (promoted by RE addition) can be considered as other reasons affecting the impact toughness of steels under investigation.The authors from University of Tehran gratefully acknowledge the financial support provided by the office of international affairs and the office for research affairs, college of engineering, for the Project Number 8107009.6.34. The authors from CENIM-CSIC would like to acknowledge the financial support from Comunidad de Madrid through DIMMAT-CM_S2013/MIT-2775 Project. Authors are grateful to the Phase Transformations and Microscopy labs from CENIM-CSIC. Mr. Javier Vara Miñambres from the Phase Transformations lab (CENIM-CSIC) is gratefully acknowledged for his continuous experimental support.Peer Reviewe