Effects of Nb, V, Ti and Al on Recrystallisation/Precipitation Interaction in Microalloyed Steels

Recrystallisation/precipitation interaction in four steels having Nb, V, Ti, and Al, respectively, as microalloying elements has been studied by means of hot torsion tests. Remarkable differences were found in the results obtained for each steel. Nb and V-microalloyed steels presented long inhibition plateaus, but the steel with Al displayed a very short plateau. Finally the steel with Ti did not show plateau. This means that Nb and V precipitates (nitrides and carbides) can inhibit the static recrystallization but this does not happen for Al and Ti (which form nitrides). The difference between activation energies allows to predict the efficiency of different precipitates to strengthen the austenite during hot rolling. RPTT diagrams showed the interaction between both phenomena, along with the strain induced precipitation kinetics and precipitate coarsening. It is found that AlN particles nucleate and grow faster than NbCN or VN.Peer reviewe

Influence of chemical composition and tempering treatment on toughness of bainitic 38MnV7 steel

This work studies the influence of Mn and the V/N ratio on the toughness of 38MnV7 steel. Four steels with different Mn, N and Ti contents were studied. Lowering the Mn content to approximately 1.5% considerably enhanced toughness by reducing Mn segregation and consequently achieved a more uniform bainitic microstructure. Rais ing the N content to reach a V/N ratio of close to 5 enhanced the intragranular nucleation of bainite and thus also improved toughness. Isothermal transformation prevented the presence of retained austenite and allowed a complete bainitic transformation. O ne of the steels contained Ti to control austenite grain growth by the precipitation of TiN particles. However, the results showed that the presence of nanoparticles TiN did not improve the bainitic microstructure. In sum, the work has allowed the obtainme nt of a 38MnV7 steel with a toughness value (obtained in a Charpy V - Notch impact test) of close to 40 J, and higher than 50 J after tempering at 600 ºC. The steel shows a toughness of close to 20 J at - 40 ºC. The optimal composition of the steel (in wt. %) was approximately: C=0.38; Mn=1.53; V=0.11; N=0.0217Peer ReviewedPostprint (published version

Survey of the analysis of continuous conformational variability of biological macromolecules by electron microscopy

Single-particle analysis by electron microscopy is a well established technique for analyzing the three-dimensional structures of biological macromolecules. Besides its ability to produce high-resolution structures, it also provides insights into the dynamic behavior of the structures by elucidating their conformational variability. Here, the different image-processing methods currently available to study continuous conformational changes are reviewedThe authors would like to acknowledge support from the Spanish Ministry of Economy and Competitiveness through grants BIO2013-44647-R and BIO2016-76400-R (AEI/ FEDER, UE), Comunidad Autonoma de Madrid through grant S2017/BMD-3817, Instituto de Salud Carlos III through grants PT13 /0001/0009 and PT17/0009/0010,the European Union (EU) and Horizon 2020 through West-Life (EINFRA- 2015-1, Proposal 675858), CORBEL (INFRADEV-1-2014-1, Proposal 654248), ELIXIR–EXCELERATE (INFRADEV-3- 2015, Proposal 676559), iNEXT (INFRAIA-1-2014-2015, Proposal 653706), EOSCpilot (INFRADEV-04-2016, Proposal 739563) and the National Institutes of Health (P41 GM 103712) (IB

Assessment of austenite static recrystallization and grain size evolution during multipass hot rolling of a niobium-microalloyed steel

Double-deformation isothermal tests and multipass continuous-cooling hot torsion tests were used to study the evolution of austenite microstructures during isothermal and non-isothermal hot deformation of an Nb microalloyed steel. These tests, coupled with microstructural characterization, have verified that the no-recrystallization temperature (Tnr) corresponds roughly to the temperature where recrystallization starts to be incomplete during rolling. An accurate method to estimate the recrystallized fraction during hot rolling based on stress-strain data, and which does not require metallographic studies, is proposed. The results of this method have been successfully compared to metallographic measurements, the values of non-isothermal fractional softening and the accumulated stress measured in the plots of mean flow stress (MFS) versus the inverse of temperature. A remarkable austenite grain refinement occurs in the first hot rolling passes after reheating. The correlation of isothermal and continuous cooling tests is better understood if the effect of grain size on recrystallization and precipitation is taken into account.Peer reviewe

Effects of aluminium and nitrogen on static recrystallisation in V-microalloyed steels

Staticrecrystallisation of three steels having different Al, V and N contents (one of them without V) has been studied by means of hot torsion tests. It has been found that strain-induced precipitates of AlN in the austenite have a mean size of approximately 86 nm. These particles barely inhibit the staticrecrystallisation, as associated pinning forces are very weak. However, mean size of VCN particles is equal to 11 nm. This fine size leads to a temporary inhibition of recrystallisation, revealed by the characteristic “plateau” of the plots of recrystallised fraction versus holding time after deformation. Besides, activation energy for recrystallisation considerably augments when VCN precipitation occurs, but it hardly increases when AlN particles precipitate. On the other hand, diffusion coefficient of Al in austenite is two orders of magnitude higher than for V. Furthermore, according to thermodynamic calculations based on Hillert and Staffanson method, precipitation of AlN particles starts at much higher temperatures than VCN. Aforesaid reasons make AlN particles to be much coarser than VCN precipitates. From the results it can be concluded that low Al contents would lead to more intense precipitation of VCN that is beneficial for V-microalloyedsteels.Financial support of this work by the ECSC Project 7210-PR/289 is gratefully acknowledged.Peer reviewe

Evolution of austenite static recrystallization and grain size during hot rolling of a V-microalloyed steel

Laboratory double-deformation isothermal tests and multipass continuous cooling hot torsion tests were used to study the static recrystallization of austenite under isothermal and anisothermal conditions as well as to simulate the hot rolling of a 0.13% V-microalloyed steel. Characterization of the evolution of austenite microstructure was carried out. It has been verified that no-recrystallization temperature (Tnr) approximately corresponds to the temperature where recrystallization starts to be incomplete during rolling. However, incomplete recrystallization is visually evident at temperatures 25–50 °C below Tnr, where grain elongation and increase in aspect ratio with temperature drop start to be significant. An accurate method to estimate the recrystallized fraction during hot rolling from stress–strain data and with no need of metallographic studies has been designed. The results of this method have been compared to metallographic measurements, the values of anisothermal fractional softening and the accumulated stress measured in the MFS plots at T < Tnr. A pronounced austenite grain refinement has been detected in the first hot rolling passes after reheating, as grain size decreases from 155 μm to 27 μm in six passes. If the effect of grain size on recrystallization and precipitation is taken into account, the correlation of isothermal and continuous cooling tests as well as the relationship between SRCT and Tnr or RLT temperatures can be better understood.Peer reviewe

Microstructural Unit Controlling Cleavage Crack Propagation in High Strength Bainitic Steels

The strengthening mechanisms which are operative in bainite are very well known: small bainite packet, small width of the laths, dislocation density and size and number of carbide particles (Fe3C), among others. Bainite packet size has been traditionally considered as the value measured by optical microscopy (OM), as electron back scattered diffraction (EBSD) technique is relatively recent. In a V-microalloyed steel with bainitic microstructure of C=0.38%, V=0.12% and N= 0.0214% the average length and width of ferrite laths and of cementite carbides were measured. On the other hand, the bainite packet size was measured by OM and EBSD with a misorientation of 15º. These values of the microstructural units have been taken in account to calculate the effective surface energy ¿p given by Griffith¿s model for cleavage fracture. It was concluded that bainite packet size determined by EBSD with a misorientation angle criterion of 15º was the microstructural parameter that controls cleavage crack propagation. Given the relationship between the average unit crack path (UCP) and the bainite packet size, it was concluded that the effective surface energy of cleavage fracture (¿p) would be between 71.6 and 82.6 J m-2.Peer Reviewe

Grain refinement by intragranular nucleation of ferrite in high nitrogen content vanadium microalloyed steel

The intragranularnucleation of ferrite was studied in a hot strained V-microalloyed steel. By means of torsion tests, the recrystallized fraction was determined for a strain of 0.35 and temperature of 950 °C. Tests at the same strain and holding times corresponding to the start and end of precipitation with subsequent cooling of the specimen showed that the intragranularnucleation of ferrite on VN precipitates leads to a significant decrease in the grain size, close to 50%.Peer reviewe

DEVELOPMENT OF A HIGH TOUGHNESS BAINITE IN A 38MnV7 STEEL

Previous works have shown that high toughness can be obtained in medium carbon bainitic steel (38MnV7) after a careful control of the chemistry and heat thermal treatment. Charpy-V tests at room temperature provided 40J for the steel after an isothermal bainitic treatment. Further improvement in toughness was obtained by subsequent tempering reaching an absorbed energy value of 50J with a FATT of -40ºC. Present work is oriented to an optimization of the above chemical composition. To attain this purpose six different casting were prepared, ranging the chemical composition as follows %C: 0.35-0.46, %Mn: 1.33-1.84, %V: 0.066 0.14% and %Ti: 0.010-0.025. To appropriate design the thermomechanical cycle, precipitation curves, TTT curves (including the effect of the initial grain size, i.e. austenitization temperature , were determined for each steelPeer Reviewe