Evolution of Austenite Microstructure and Precipitation State during Hot Rolling of a Nb-Microalloyed Steel

Hot torsion tests were used to simulate hot rolling of a Nb-microalloyed steel. Subsequent graphic representation of Mean Flow Stress (MFS) versus the inverse of absolute temperature for each pass allowed to know the critical rolling temperatures (Tnr Ar3, Ar1,)) and residual stress accumulated in austenite just before austenite to ferrite phase transformation. It has been found that, as successive rolling passes are applied at temperatures below Tnr, mean precipitate size decreases as a result of deformation applied and hardening by incomplete recrystallization of austenite.Peer reviewe

Influence mutual of the deformation and chemical composition on induced precipitation in microalloyed steels

[ES] Mediante ensayos de torsión y usando el método hack extrapolation se ha determinado la cinética de recristalización estática de aceros microaleados con vanadio (V), niobio (Nb) y titanio (Ti) y, a partir de las mismas, ha sido posible dibujar los diagramas recristalizaciónprecipitación- tiempo'temperatura (RPTT), que muestran gráficamente la interacción recristalización-precipitación. Estos diagramas muestran que el efecto de la deformación en la cinética de precipitación depende del contenido de microaleante. En este sentido, se propone una nueva expresión para relacionar la influencia de la deformación y del contenido de microaleante sobre el periodo mínimo de incubación de la precipitación inducida.[EN] By means of torsion tests and applying the "back extrapolation" method, the static recrystallization kinetics in microalloyed steels with vanadium (V), niobium (Nb) and titanium (Ti) has been determined and, recrystallization-precipitation-time-temperature(RPTT) diagrams have been plotted also graphically, which show the Recrystallization-Precipitation interaction. These diagrams show that the effect of the deformation on the precipitation kinetics depends of the microalloy content. In this sense, a new expression is proposed to relate the influence of the deformation and the chemical composition on the minimum incubation of the precipitation kinetics.Peer reviewe

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

Static Recrystallization of Austenite in a Medium-Carbon Vanadium Microalloyed Steel and Inhibition by Strain-Induced Precipitates

The austenite static recrystallization kinetics at several temperatures and the recrystallization-precipitation-time- temperature (RPTT) diagrams of a medium-carbon vanadium microalloyed steel have been determined for a strain ε = 0.35. Unlike many other studies carried out previously on V microalloyed steels, the recrystallized fraction against time curves showed the formation of a double plateau that indicates two stages of inhibition of recrystallization due to the formation of different types of strain induced precipitates. This work makes use of transmission electron microscopy to study the nature and size distribution of these precipitates capable of inhibiting recrystallization. The values of driving and pinning forces for static recrystallization are calculated and an analysis of the relationship between the net balance of these forces, the precipitation state and the progress or inhibition of the recrystallization is accomplished. A value of driving force that decreases as recrystallized fraction grows during isothermal holding time is estimated and helps to interpret the behavior of austenite after deformation.Peer reviewe

Characterization by Electron Diffraction of Two Thermodynamical Phases of Precipitation in Nb-Microalloyed Steels

Excellent mechanical properties (high strength and toughness) of microalloyed steels are mainly caused by induced precipitation during thermomechanical treatment (TMT) and grain refinement. It has been recently found that TMT of Nb-microalloyed steels can give rise to two different kinds of precipitates, manifested by the double plateau in the statically recrystallised fraction (Xa) against time curves. This work presents an electron diffraction study performed in a transmission electron microscope, equipped with an EDS analytical system. Lattice parameters of a great deal of particles, smaller than 200 nm and with face cubic centred structure, have been measured. Frequency distribution of the values of lattice parameters shows that these are grouped in two sets whose mean values are close. Comparison of these values with those found in the literature for carbides, nitrides and carbonitrides usually present in microalloyed steels demonstrates that they are Nb carbonitrides with slight stoichiometric differences (NbCxNy).Peer reviewe

MICROSTRUCTURAL STUDIES FOR OPTIMIZATION OF HEAT TREATMENT IN COMPONENTS OF STEEL X38CrMoV5-1 SUBJECTED TO HIGH STRESSES

This material X38CrMoV5-1 is an alloyed steel used for hot working, with good toughness and high resistance to thermal shock. The presence of Cr, Mo and V gives this steel a high resistance to wear, keeping its hardness properties at high temperature. Cr and Mo delay softening annealing and inhibit the grain growth. The great resistance to high temperatures of this type of steels is related with an easy martensitic transformation. This transformation happens even at low cooling speeds. The properties of these types of martensitic steels result as a consequence of their complex microstructure that is obtained by an extremely controlled thermal treatment. Dilatometric testing was performed on continuous cooling from austenization temperature (1050ºC). This testing shows the high hardenability of this type of steels. ATD studies have been done to complement the dilatometric testing. After the previous results, it has been considered that the optimal treatment to get tough and tenacious structure, consists in submitting material to an annealing processing at 780ºC/1hour, followed by a quenching treatment at 1020ºC/1hour and finally cooling in oil with a double tempering at 580ºC/2 hours. This treatment provides the best properties that guarantee service with safety parts.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

Theoretical and experimental nucleation and growth of precipitates in a medium carbon–vanadium steel

Using the general theory of nucleation, the nucleation period, critical radius, and growth of particles were determined for a medium carbon V-steel. Several parameters were calculated, which have allowed the plotting of nucleation critical time vs. temperature and precipitate critical radius vs. temperature. Meanwhile, an experimental study was performed and it was found that the growth of precipitates during precipitation obeys a quadratic growth equation and not a cubic coalescence equation. The experimentally determined growth rate coincides with the theoretically predicted growth ratePeer ReviewedPostprint (author's final draft

Designing the physical metallurgy of a bainitic microalloyed steel with a dilatometer

During the past decade, medium carbon microalloyed steels have become increasingly important in the automotive sector. These steels are especially suitable for automobile components such as connecting rods, crankshafts and wheel hubs. Their mechanical properties are generally adequate in most cases although their toughnesses are consistently low. High toughness can be obtained in medium carbon microalloyed bainitic steel (38MnV7) after a careful control of the chemistry and heat thermal treatment. An specific chemical composition of a 38MnV7 steel has been developed, providing impact energies after Charpy-V tests at room temperature as high as 40J (the steel in bainitic state). Present work is oriented to an optimization of the above chemical composition by control of the Transformation Time Temperature (TTT) curves as well as the Precipitation Time Temperatures (PTT) curves of the present microallying elements. 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 thermal cycle, TTT curves were determined for each steel at two austenitization conditions, in order to promote fine and large initial grain sizes.. PTT curves were determined by the stress relaxation technique, a method which can be also be used to derive recrystrallization kinetics. All curves, TTT and PTT curves where derived by using a quenching dilatometer Bahr DIL805A/D. While the TTT curves were obtained in a classical way, the relaxation test consist in sample austenization followed by cooling down to the testing temperature. After a short stabilization period of 10s, samples are deformed to different strain levels and then relaxed, i.e. deformation is kept constant and the variation of the stress with the time is recorded. The different deformation levels are selected to evaluate the effect of deformation on precipitation characteristics. The relaxation curves under these conditions gave information about the kinetics of precipitation when there is no plastic deformation and, thus, generation of dislocations involved. Results are finally discussed in terms of the chemical composition, initial microstructure and precipitates interaction.Peer Reviewe

Determination of Residual Stress and Critical Rolling Tempera- tures in a Microalloyed Steel with Low Carbon and Niobium Con- tents

Using torsion tests, residual stress (Δσ) and critical rolling temperatures (T nr , A r3 , A r1 ) have been determined for a low Nb content microalloyed steel by means of simulation of rolling cycles and subsequent representation of mean flow stress versus the inverse of the temperature. The above magnitudes were determined as a function of interpass time for two strains applied in each pass (0.20, 0.35), respectively. Among the results found, it is notable that Δσ decreases with longer interpass times until it reaches zero, and is greater the smaller the strain applied. With regard to the cooling transformation temperatures A r3 and A r1 , these were found to be practically independent of the interpass time and were higher for smaller applied strains. Cooling transformation temperatures A r3 and A r1 are, together with the no-recrystallisation temperature T nr , considered to be critical in hot rolling. The parameter T nr , defined as the temperature at which static recrystallisation starts to be inhibited in hot rolling, is of great importance as it influences the austenite microstructure at the end of rolling, completely or partially recrystallised or completely strengthened. The phase transformation temperatures A r3 and A r1 are also important since their values indicate the temperature limits between which rolling will be performed, either in the austenitic region or in the intercritical region. The parameter T nr can be determined by simulation of several rolling passes and subsequent graphic representation of mean flow stress (MFS) versus the inverse of the temperature for each pass [1…4]. A r3 and A r1 can be determined simultaneously with T nr using the same method, i.e. from the graphs that represent MFS versus 1/T, [5…9] or by other methods such as the dilatometry technique. The latter is a classic method which offers very good precision. On the other hand, it is well known that the value of T nr depends on the chemical composition of the steel, the equivalent strain applied in each pass, the strain rate and the interpass time (Δt). [3; 5] For their part, temperatures A r3 and A r1 are dependent on the chemical composition of the steel and the austenite microstructure (grain size, strengthening) before the start of the γ→α transformation, and are easier to determine the lower the carbon content, especially when the technique used is representation of MFS versus 1/T. Finally both the accumulated strain and accumulated stress, referred to in this work as residual stress (Δσ) by allusion to the stress accumulated just before the temperature A r3 is reached, represent the appropriate magnitude for relating the state of the austenite microstructure before the start of the γ→α transformation with the final fer