The influence of microstructural characteristics on the properties of high carbon Nb microalloyed steel (original) (raw)
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IASET, 2013
Rebar steels are usually processed from carbon steel. However, carbon limits the deformability and weldability of steel rebars. Microalloying with V and Nb is the optimum solution when high strength in combination with good formability and weldability are essential. Nb replaces V partially for better ductility purposes. Nb plays a bilateral role where, it forms carbonitride which acts as precipitation strengthener. Further active role is played by Nb in grain refinement which would improve appreciably the yield strength and ductility. However, to get full benefit of Nb microalloying, the soaking temperature, deformation amount, finish rolling temperature and cooling after rolling should be taken into consideration.
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...
Microstructural evolution in a new 770MPa hot rolled Nb–Ti microalloyed steel
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.
Microstructure and mechanical properties of a microalloyed steel after thermal treatments
Mater Res Ibero Am J Mater, 2003
The properties of a microalloyed steel, with Nb and V in its composition, were studied, after different intercritical thermal treatments and at different austenitizing and tempering temperatures. The mechanical properties of the specimens were measured in a Vickers hardness tester, and their microstructures were analyzed by optical microscopy, with the aid of a digital image processor. After austenitizing at 1100 °C and tempering at 625 °C, the samples showed significantly higher tempering resistance, reflected by their retention of high hardness, which may be associated with a secondary hardening precipitation of Nb carbon nitrides. In the sample with dualphase microstructure, the martensite volume fraction varied from 18.2 to 26.3% and the ferrite grain size remained unchanged, upon the variation of the time length of the intercritical treatments. Tempered samples showed Vickers hardness (HVN) varying from 327 to 399, and dualphase samples showed HVN from 362 to 429.
Effect of Deformation and Cooling Rate on the Microstructures of Low Carbon Nb-B Steels
ISIJ International, 1998
The transformation behaviours and microstructural characteristics of three B-containing steels were investigated. [n particu lar, the effects of deformation in the no-recrystallization temperature range and cool ing rate were studied by means of compression tests. It was found that over a large cooling rate range (from 1 to 50'Cls). Mo-Nb-B steel exhibits microstructures consisting of a mixture of plate-like or lath-like ferrite with retained austenite or martensite (i.e. M/A) islands. This is basically a low carbon bainitic microstructure. and can be identified as B, in the Bramfitt and Speer classification system. The lengths of the ferrite laths increase and the widths decrease as the cooling rate is increased. The shapes and distributions of the M/A islands change from being blocky and randomly distributed to fine and more aligned, as the cooling rate is increased. Also, the lengths of the bainitic ferrite laths are shortened by heavy deformation in the norecrystallization temperature range. The microstructures of the Nb-1 5B and B-only steels are basically polygonal ferrite at low coo]ing rates, however, the fractions of bainite in these two grades increase with cooling rate. The minimum cooling rate required for avoiding polygonal ferrite formation during continuous cooling are much higher in these two grades than in the Mo-Nb-B steel.
Modelling the Evolution of the Microstructure of a Nb Steel
ISIJ International, 1996
A mathematical model, for the prediction of the evolution of the microstructure during hot forming of microalloyed steels, is presented. The material behavior is combined with a finite-element model of the deformation. M ulti-stage, isothermal compression tests are used for verification. The effect of the interruption betweenstages of compression on the restoration mechanisms is studied in three-stage tests. The influence of the deformation history on the microstructural development and on the softening mechanisms is also analyzed. The distribution of austenite grain sizes is predicted. The inhomogeneity of the resulting structure is connected to that of the mechanical attributes of the deformed material. The studies show that additional grain refinement can be expected by controlling the recrystallization kinetics and the retained strain,
Materials Science and Engineering: A, 2006
We describe here the relationship between microstructure and impact toughness behavior as a function of cooling rate for industrially processed Nb-and V-microalloyed steels of almost similar yield strength (∼60 ksi). Both Nb-and V-microalloyed steels exhibited increase in toughness with increase in cooling rates during processing. However, Nb-microalloyed steels were characterized by relatively higher toughness than the V-microalloyed steels under identical processing conditions. The microstructure of Nb-and V-microalloyed steels processed at conventional cooling rate, primarily consisted of polygonal ferrite-pearlite microconstituents, while Nb-microalloyed steels besides polygonal ferrite and pearlite contained significant fraction of degenerated pearlite. The microstructure of Nb-and V-microalloyed steels processed at relatively higher cooling rate contained degenerated pearlite and lath-type (acicular) ferrite in addition to the primary ferrite-pearlite constituents. The fraction of degenerated pearlite was higher in Nb-microalloyed steels than in the V-microalloyed steels. In both Nb-and V-microalloyed steels the precipitation characteristics were similar with precipitation occurring at grain boundaries, dislocations, and in the ferrite matrix. Fine-scale (∼5-10 nm) precipitation was observed in the ferrite matrix of both the steels. The selected area diffraction (SAD) pattern analysis revealed that these fine precipitates were MC type of niobium and vanadium carbides in the respective steels and followed Baker-Nutting orientation relationship with the ferrite matrix. The microstructural studies suggest that the increase in toughness of Nb-microalloyed steels is attributed to higher fraction of degenerated pearlite in the steel.
Materials Characterization, 2018
Keywords: High carbon steel Micro-alloying Microstructure Mechanical properties A B S T R A C T Two C-Mn-Si steels with and without Nb micro-alloying are selected in the present study. Both these high carbon steels have been thermo-mechanically processed using Gleeble 3800 simulator. Optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscope (AFM) were utilised for microstructural characterisation. The austenitising temperature was varied from 1150°C to 1200°C followed by hot compression and subjected to free cooling to evaluate the influence of austenite grain size on transformed microstructure and mechanical properties. It is evident that higher austenite grain size is obtained for both types of steels subjected to higher austenitising temperature. The refinement of pearlite in-terlamellar spacing (< 0.20 μm) with degenerated morphology as well as finer prior austenite grain size (< 40 μm) is found to be more prominent for Nb micro-alloyed sample. The average hardness values of both the steels are higher for the specimens, treated at a higher austenitising temperature which is attributed to finer interlamellar spacing. Finally, the correlation between the evolving microstructure and resulting mechanical properties (hardness and yield strength) has been made.
Materials, 2022
The dynamic recrystallization behavior of ultra-high strength boron-microalloyed steels optionally alloyed with niobium and molybdenum is analyzed in this paper. Multipass torsion tests were performed to simulate plate rolling conditions followed by direct quenching. The influence of alloy composition on the transformed microstructure was evaluated by means of EBSD, thereby characterizing the morphology of the austenite grain morphology after roughing and finishing passes. The results indicated that for Nb-microalloyed steel, partial dynamic recrystallization occurred and resulted in local clusters of fine-sized equiaxed grains dispersed within the pancaked austenitic structure. A recrystallized austenite fraction appeared and transformed into softer phase constituents after direct quenching. The addition of Mo was shown to be an effective means of suppressing dynamic recrystallization. This effect of molybdenum in addition to its established hardenability effects hence safeguards t...
Strengthening Mechanisms in Thermomechanically Processed NbTi-Microalloyed Steel
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.