Tensile Work Hardening Modeling of Precipitation Strengthened Nb-Microalloyed Steels

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2017

In order to study the influence of composition and thermo-mechanical processing schedule on the kinetics of austenite recrystallization, strain induced precipitation and final microstructural evolution in Nb-microalloyed steels, thermo-mechanical processing simulations have been carried out inside Gleeble® by varying the number of deformation passes (2-pass vs. 6-pass), deformation temperatures (1000-800 °C) and inter-pass times. Low-C high-Mn steel (LCHMn) has been found to offer finer ferrite grain size and finer Nb-precipitation which contributed to superior hardness to that steel, compared to high-C low-Mn steel (HCLMn). Among the deformation schedules applied, 6-pass schedule has been found to be superior over 2-pass schedule in terms of precipitation strengthening and hardness. This study also proposes a mathematical framework to explain the effect of composition and processing schedule in Nb-microalloyed steels following Dutta and Sellars approach on precipitation-recrystallization interaction.

Materials Science Forum

Three novel low carbon microalloyed steels with various additions of Mo, Nb and V were investigated after thermomechanical processing simulations designed to obtain ferrite-bainite microstructure. With the increase in microalloying element additions from the High V- to NbV- to MoNbV-microalloyed steel, the high temperature flow stresses increased. The MoNbV and NbV steels have shown a slightly higher non-recrystallization temperature (1000 °C) than the High V steel (975 °C) due to the solute drag from Nb and Mo atoms and austenite precipitation of Nb-rich particles. The ambient temperature microstructures of all steels consisted predominantly of polygonal ferrite with a small amount of granular bainite. Precipitation of Nb-and Mo-containing carbonitrides (>20 nm size) was observed in the MoNbV and NbV steels, whereas only coarser (~40 nm) iron carbides were present in the High V steel. Finer grain size and larger granular bainite fraction resulted in a higher hardness of MoNbV st...

Metallurgical and Materials Transactions A, 2002

This work presents an austenite decomposition model, based on the thermodynamics of the system and diffusion-controlled nucleation theory, to predict the evolution of microstructure during hot working of niobium-microalloyed steels. The differences in microstructural development of hotdeformed microalloyed steel in the single-phase austenite and two-phase (austenite ϩ ferrite) regions have been effectively described using an integrated computer modeling process. The complete model presented here takes into account the kinetics of recrystallization, recrystallized austenite grain size, precipitation, phase transformation, and the resulting ferrite structure. After considering existing austenite decomposition models, we decided that the method adopted in the present work relies on isothermal transformation kinetics and the principle-of-additivity rule. The thermomechanical part of the modeling process was carried out using the finite-element method. Experimental results at different temperatures, strain rates, and strain levels were obtained using a Gleeble thermomechanical simulator. A comparison of results of the model with experiments shows good agreement.

Metallurgical and Materials Transactions A, 2015

The effect of deformation temperature on microstructure and mechanical properties was investigated for thermomechanically processed NbTi-microalloyed steel with ferrite-pearlite microstructure. With a decrease in the finish deformation temperature at 1348 K to 1098 K (1075 °C to 825 °C) temperature range, the ambient temperature yield stress did not vary significantly, work hardening rate decreased, ultimate tensile strength decreased, and elongation to failure increased. These variations in mechanical properties were correlated to the variations in microstructural parameters (such as ferrite grain size, solid solution concentrations, precipitate number density and dislocation density). Calculations based on the measured microstructural parameters suggested the grain refinement, solid solution strengthening, precipitation strengthening, and work hardening contributed up to 32 pct, up to 48 pct, up to 25 pct, and less than 3 pct to the yield stress, respectively. With a decrease in the finish deformation temperature, both the grain size strengthening and solid solution strengthening increased, the precipitation strengthening decreased, and the work hardening contribution did not vary significantly.

Materials Characterization

The temperature at which thermomechanical controlled processing is undertaken strongly influences strain-induced precipitation (SIP) in microalloyed steels. In this study, the recrystallisation-precipitation-time-temperature curve was simulated to determine the full recrystallisation temperature, recrystallisation-stop temperature and the temperature where precipitation would occur at the shortest time. The calculated temperatures were verified by experimental testing for rolling between 1100¡C and 850¡C. On the basis of this a finishing deformation of 850¡C was chosen in order to maximise the precipitate number density formed in a fully unrecrystallised austenite. The orientation relationship between the SIP in austenite, and subsequent transformation to ferrite was identified by calculation from the coordinate transformation matrix, and by electron diffraction in the transmission electron microscope. The NbC formed as coherent/semi-coherent precipitates in the austenite, and remained coherent/semi-coherent in the ferrite, indicating a Kurdjumov-Sachs orientation relationship between the austenite and ferrite on transformation.

Journal of Materials Processing Technology, 2006

Materials Science and Engineering: A, 2005

The microstructural evolution during hot rolling of a commercially developed hot rolled Nb-Ti steel with a yield strength of 770 MPa is described and analyzed in terms of strengthening mechanisms. The objective of the study is to examine the constituents of the microstructure (type of microstructure, nature of precipitates, dislocation density) that contributed to the attractive strength-toughness combination of a new high strength 770 MPa Nb-Ti microalloyed steel. From the transmission electron microscopy observations, the precipitates can be categorized into four classes depending on their size and shape. Type I were intergranular rod-like (Fe,Mn) 3 C precipitates, while type II were TiN precipitates of size range 120-500 nm containing small amounts of niobium. The type III precipitates identified as (Nb,Ti)C were ∼10-200 nm size and randomly distributed in the matrix, and type IV were spherical or needle-shaped (3-5 nm) (Nb,Ti)C precipitates that nucleated preferentially on sub-boundaries and dislocations in ferrite. The dislocation density was high in some grains and less in other grains. The high dislocation density and fine-scale precipitation are the dominant factors responsible for the high strength of 770 MPa microalloyed hot rolled steel.

AIP Conference Proceedings , 2019

In the present investigation, high carbon steel and Nb microalloyed steel have been subjected to hot rolling (~80% hot deformation) by varying the finish rolling temperature (FRT) followed by air cooling to room temperature. The specimens were austenitised at 1200°C for 1 h and then subjected to hot deformation with two FRTs of 800°C and 1000°C. The deformation at a higher temperature results in a continuous grain boundary network structure with apparent evidence of recrystallisation whereas, finer grains at lower deformation temperature has been observed. The volumetric percentage of ferrite has been found to be more in case of Nb microalloyed steel at a lower FRT whereas, more volume percentage of pearlite has been observed when cooled from higher deformation temperature. On the other hand, deformation at a lower temperature (800°C FRT) results in more amount of ferrite formation in the Nb microalloyed steel. The average hardness value has been found to be higher (≈270 HV30/20) for the Nb microalloyed steels at higher deformation temperature (1000°C FRT) which is attributed to the finer interlamellar spacing (≤ 100 nm). Subsequent air cooling to room temperature from a higher deformation temperature increases the ultimate tensile strength (UTS) of high carbon steel marginally but improve the UTS of Nb microalloyed steel significantly. The tensile fracture morphology reveals the abundant presence of dimples at a lower deformation temperature, indicating a ductile fracture. The fracture surface of the Nb microalloyed steel subjected to higher deformation temperature exhibits a typical river-like pattern indicating cleavage fracture. Finally, a correlation between microstructure and properties have been established.

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2011

Recrystallization-precipitation-time-temperature (RPTT) diagrams for strains of 0.20 and O.35 have been determined for two microalloyed steels with niobium percentages of 0.024 and 0,058, respectively, and equal percentages of carbon and nitrogen. The method knownas "back extrapolation" has been used for the determination of static recrystallization kinetics and also for the plotting of the diagrams, While a single plateau was observed on the recrystallized fraction against time curves for the first steel, as a consequence of sttain induced precipitation, these curves for the second steel showed the formation of a double plateau. whose interpretation, confirmed by calorimetric analysis, supposes the formation of two types of preci pitates, The work uses transmission microscopy to show the precipitates which are formed in both steels, as well as the size most probably capable of inhibiting recrystallization. Final]y, an analysis is made of the RPTT diagrams and of the large amount of information which they offer for designing a more appropriate rolling schedule in order to obtain finer precipitates and a better austenitic microstructure before the austenite+ferrite transformation.

Contributed Papers from MS&T17, 2018

As new complex steel compositions for automotive applications and new forming technologies emerge the hot working and forming of microalloyed steels continue to be challenging. Based on the laboratory simulations, the effects of individual and combined additions of Al and Ti + B on deformation resistance at low strain rates and dynamic softening of Nb microalloyed steels are evaluated. The major dynamic softening mechanism under studied conditions is dynamic recrystallization (DRX). In the presence of Nb the additions of Ti + B retard DRX of austenite recrystallization. In contrast, Al facilitates recrystallization of austenite thus reducing strengthening effects of alloying. These data can be employed in process setup of hot working.