Showing papers on "Microalloyed steel published in 1996"
TL;DR: In this article, the authors used torsion tests to determine the Zener-Hollomon parameter in a selection of 18 steels with compositions appropriate for the study of the influence of each alloying element (C, Mn, Si, Mo, Ti, V, Nb).
316 citations
TL;DR: In this paper, the dynamic recrystallization of a selection of low alloy and micro-alloyed steels was studied and Avrami's equation was modelled based on the experimental determination of flow curves and their subsequent modelling, making it possible to calculate the recrystized fraction (X{sub d}) as a function of all the variables which intervene in hot deformation: temperature, strain rate, austenite grain size and the chemical composition of the steel.
129 citations
TL;DR: In this paper, the effect of Nb supersaturation in austenite, as it applies to the strain-induced precipitation potential of nb(CN), on the suppression of the static recrystallization of austenites during an isothermal holding period following deformation was described.
Abstract: This work describes the effect of Nb supersaturation in austenite, as it applies to the strain-induced precipitation potential of Nb(CN), on the suppression of the static recrystallization of austenite during an isothermal holding period following deformation. Four low carbon steels, microalloyed with Nb, were used in this investigation. Three of the steels had variations in Nb levels at constant C and N concentrations. Two steels had different N levels at constant C and Nb concentrations. The results from the isothermal deformation experiments and the subsequent measurement of the solution behavior of Nb in austenite show that the recrystallization-stop temperature (TRXN) increases with increasing Nb supersaturation in austenite. Quantitative transmission electron microscopy analysis revealed that the volume fraction of Nb(CN) at austenite grain boundaries or subgrain boundaries was 1.5 to 2 times larger than Nb(CN) volume fractions found within the grain interiors. This high, localized volume fraction of Nb(CN) subsequently led to high values for the precipitate pinning force (FPIN). These values forFPIN were much higher than what would have been predicted from equilibrium thermodynamics describing the solution behavior of Nb in austenite.
123 citations
TL;DR: In this article, a model was developed to predict the austenite flow curves of low alloy and microalloyed steels, consisting of two expressions for stress, as a function of strain, temperature, strain rate and the chemical composition of the steel.
111 citations
TL;DR: In this paper, the influence of the most common elements in low alloy steels (C, Mn, Si, Mo) and in microalloyed steels on peak strain has been studied.
76 citations
TL;DR: In this paper, the effect of varying the initial grain size from 12 to 83 μm on the static recrystallization kinetics of a 0.08%C-0.055%Nb steel was investigated at 950°C by means of interrupted hot compression tests; the results obtained in this way were analyzed using three different methods for assessing the fractional softening.
Abstract: The static recrystallization kinetics of a 0.08%C-0.055%Nb steel were investigated at 950°C by means of interrupted hot compression tests; the results obtained in this way were analyzed using three different methods for assessing the fractional softening. In particular, the effect was studied of varying the initial grain size from 12 to 83 μm. The exponent relating initial grain size to the time for 50% recrystallization was found to be about 1.7. This is somewhat less than the exponent of 2.0 conventionally employed for plain C-Mn steels, and which is often quoted in the literature as applicable to Nb grades. In addition, by means of comparison with previous work on grades with lower levels of Nb and Mn, it is shown that higher levels of Mn make significant contributions to the retardation of static recrystallization in Nb steels.
74 citations
TL;DR: In this article, continuous cooling transformation diagrams are determined for microalloyed steels containing Ti, Nb, Mn and Mo in undeformed as well as in thermomechanically processed conditions using dilatometry.
Abstract: Continuous cooling transformation diagrams are determined for microalloyed steels containing Ti, Nb, Mn and Mo in undeformed as well as in thermomechanically processed conditions using dilatometry. Effects of thermomechanical processing, cooling rate and Nb content on γ/α transformation kinetics are presented. Thermomechanical processing is found to accelerate the onset of γ/α transformation (γ transformation start temperature, Ar3, is raised). However, the progress of γ/α transformation is retarded considerably in deformed samples. Increase in cooling rate lowers Ar3 significantly and accelerates the progress of transformation. Nb lowers Ar3 undeformed austenite but raises Ar3 of thermomechanically processed austenite.
71 citations
TL;DR: Using torsion tests, a model has been constructed to predict the recrystallized fraction of deformed austenite in low alloy and microalloyed steels as mentioned in this paper.
Abstract: Using torsion tests a model has been constructed to predict the recrystallized fraction of deformed austenite in low alloy and microalloyed steels. The model quantifies the influence of the most common elements (C, Si, Mn, Mo) in low alloy steels and the typical elements (Ti, V, Nb) in microalloyed steels in the recrystallized fraction, when they are in solution. Static recrystallization kinetics follow Avrami's law and expressions for the parameter t0.5 and for the exponent n are not given. The values predicted by the model are compared with the experimental values, good concordance being obtained between both.
62 citations
TL;DR: In this article, the impact toughness of microalloyed steels has been investigated and the effect of temperature on splitting has been discussed, and the impact and fracture toughness of the steel has been calculated for all temperatures and correlated with impact toughness.
61 citations
TL;DR: In this article, the precipitation behavior of a medium carbon steel microalloyed with Ti, V, and N has been studied by analytical transmission electron microscopy in the as-cast and isothermally heat-treated states, as well as at different stages in the thermomechanical processing of the steel.
Abstract: The precipitation behavior of a medium carbon steel microalloyed with Ti, V, and N has been studied by analytical transmission electron microscopy in the as-cast and isothermally heat-treated states, as well as at different stages in the thermomechanical processing of the steel. Mixed (Ti,V) nitrides were found in all the structures, but there was no evidence for mixed carbonitride formation. The Hillert-Staffansson model was used to predict the composition of the nitrides as a function of tem-perature. Upon prolonged aging many of the precipitates became “fragmented” and were no longer single crystals. At the same time, the volume fraction of precipitates dropped, while their average Ti content increased. Possible explanations for this unexpected behavior are discussed in this article.
61 citations
TL;DR: At high temperatures, materials display rate dependent plasticity, which in global terms is controlled by the glide and climb of dislocations as discussed by the authors, which is a common phenomenon in flow behavior under hot temperatures.
Abstract: At high temperatures, materials display rate dependent plasticity, which in global terms is controlled by the glide and climb of dislocations. Classical descriptions of the flow behaviour under hot...
TL;DR: In this paper, the number per unit volume of irreversible traps was correlated with calculated volume fraction of fine and coherent Ti(C,N) precipitates, and the reversible traps were associated with free Ti atoms, dislocations, and ferritic grain boundaries.
Abstract: The presence of traps for hydrogen atoms influences the diffusivity and solubility of hydrogen itself in steels. In the present work, permeation measurements were carried out on hot-rolled microalloyed steels with different C and Ti contents in order to evaluate the number of irreversible and reversible traps. The number per unit volume of irreversible traps was correlated to calculated volume fraction of Ti(C,N) precipitates. These results, combined with microstructural investigations by transmission electron microscopy (TEM), showed that the largest number of irreversible traps was associated with steels having the largest volume fraction of fine and coherent Ti(C,N) precipitates. The reversible traps were associated with free Ti atoms, dislocations, and ferritic grain boundaries. Theoretical calculations confirmed the hydrogen binding energy of Ti free atoms (−27.1 kJ/mol).
TL;DR: In this article, a metallographic technique is presented that enables a quick and accurate identification of the martensite-austenite island by both light optical and scanning electron microscopy.
TL;DR: In this paper, the recrystallizatian of hat-twisted austenite has been investigated in a cylindrical specimen machined from hat rolled plates of 0,052 wt % Niobium microalloyed steel.
TL;DR: In this paper, the evolution of the austenitic grain size in medium carbon steels microalloyed with vanadium and titanium was studied as a function of reheating temperature, heating rate, and titanium content.
Abstract: The evolution of the austenitic grain size in medium carbon steels microalloyed with vanadium and titanium was studied as a function of reheating temperature, heating rate, and titanium content. High resolution dilatometric techniques were used to determine the continuous cooling transformation (CCT) diagrams for two different austenitization temperatures. The microstructure and hardness were determined for different cooling rates. The results revealed a significant effect of titanium concentration on the austenitic grain growth control. The smallest grain size was found in the steel with a Ti concentration = 0.019 wt%. Low heating rates produced smaller grain sizes than high heating rates although an abnormal grain growth took place. In these steels, at temperatures above 1050 °C the influence of the reheating temperature on their hardness for cooling rates around 2 °C · s−1 was negligible. The higher reheating temperatures caused a slight increase in their hardenability. Finally, it was found that the greater the titanium content, the greater the hardness of these steels, but only when the titanium percentages were higher than 0.020 wt%.
TL;DR: In this article, an experimental study on the grain growth of a medium-carbon V-Ti microalloyed steel with two levels of AIN has been carried out and a system to study grain-size distributions in order to detect the abnormal grain growth has been proposed.
Abstract: An experimental study on the grain growth of a medium-carbon V-Ti microalloyed steel with two levels of AIN has been carried out. A system to study grain-size distributions in order to detect the abnormal grain growth has been proposed. Log-normal distributions were verified and then properties of normal distributions were applied to distinguish normal and abnormal grains. The benefits of working with the relative difference (RD) of grain size in order to compare the grain-growth behaviour have been discussed. It was experimentally concluded that abnormal growth appears when RD is larger than 2.5. From the results a map of abnormal grain growth against time and temperature could be plotted. It was concluded that abnormal grain growth is due to the AIN dissolution when the above maps are correlated with the theoretical volume fraction of precipitates. The importance and effect of heating rate have also been shown: high heating rates can produce abnormal growth at higher temperatures than those of the equilibrium dissolution.
15 Jun 1996-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the microstructural evolution during simulated on-line accelerated cooling (OLAC) of a commercial Grade 80 pipe steel was studied using a quench deformation dilatometer.
Abstract: The microstructural evolution during simulated on-line accelerated cooling (OLAC) of a commercial Grade 80 pipe steel was studied using a quench deformation dilatometer. The transformed matrix microstructure contains various amounts of polygonal ferrite, granular bainite and acicular ferrite, depending mainly on the accelerated-cooling interrupt temperature. The final microstructure is predicted well by drawing the OLAC schedule on the appropriate CCT diagram. Three distinct groups of precipitates are found in the final microstructure, which form during reheat, austenite deformation, and cooling, respectively. The distribution and composition of the precipitates varies widely with steel composition and processing schedule. The microstructure of industrially processed plate agrees well with that of corresponding laboratory simulations.
TL;DR: In this paper, a statically recrystallized fraction of low carbon microalloyed steels containing vanadium, niobium and titanium, respectively, has been determined for different temperatures and strains.
Abstract: Using torsion tests and applying the back extrapolation method, the statically recrystallized fraction of low carbon microalloyed steels containing vanadium, niobium and titanium, respectively, has been determined for different temperatures and strains. From the recrystallized fraction against time curves it is possible to draw precipitation-time-temperature (PTT) diagrams. These diagrams show that the strain does not affect precipitation kinetics in an independent manner, but that this influence is related with the microalloy content. The greater the strain applied, the shorter the incubation time of induced precipitation, but this influence diminishes as the microalloy content increases. It is also demonstrated that the incubation time is practically independent of the nature of the metal microalloy (V, Ti, Nb). In this sense, new expressions are proposed to relate the incubation and precipitation end times with the strain and the microalloy content. A model is also established for precipitation kinetics at the curve nose temperature.
TL;DR: The atomic force microscopy (AFM) is now a well-established technique for imaging surface topography with high resolution and can be used to study the microstructure of structural materials in a nanometer range.
TL;DR: In this paper, the hot workability of a microalloyed medium carbon steel is optimized using the power dissipation maps developed on the basis of the Dynamic Materials Model, and the selected steel undergoes single peak dynamic recrystallization in the domain centered about 1,150 C and 10 s.
TL;DR: In this article, a mathematical model for the prediction of the evolution of the microstructure during hot forming of microalloyed steels is presented, combined with a finite-element model of the deformation.
Abstract: 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. Multi-stage, isothermal compression tests are used for verification. The effect of the interruption between stages of compression on the restoration mechanisms is studied in three-stage tests. The influence of the deformation history on the microstuctural 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.
TL;DR: In this article, a model has been constructed to predict the recrystallized fraction of deformed austenite in Nb, V and Ti microalloyed steels at temperatures below the temperature at which the inhibition of recurstallization commences due to induced precipitation, which is modelled as a function of grain size, strain and solubility temperature.
Abstract: Using torsion tests and applying the back extrapolation method a model has been constructed to predict the recrystallized fraction of deformed austenite in Nb, V and Ti microalloyed steels at temperatures below the temperature at which the inhibition of recrystallization commences due to induced precipitation, This temperature, named static recrystallization critical temperature, is modelled as a function of grain size, strain and the solubility temperature. A discussion is made of the importance of being able to predict SRCT in order to effectively apply the model of static recrystallization at temperatures both above and below it. It is demonstrated that Niobium precipitates delay the recrystallization most.
TL;DR: In this article, the fatigue characteristics of a Cu-bearing high-strength low-alloy (HSLA) steel were investigated in air, relative humidity ≈50 pct, as a function of microstructure, which was altered by heat treatments and welding.
Abstract: The fatigue characteristics of a Cu-bearing high-strength low-alloy (HSLA) steel were investigated in air, relative humidity ≈50 pct, as a function of microstructure, which was altered by heat treatments and welding. Small fatigue cracks (≈30-Μm long) were naturally initiated from smooth specimens and grown past the transition length (≈200 Μm), where they exhibited the characteristics of large fatigue cracks. The number of cycles to crack initiation depended on stress magnitude but not on microstructure, although the site of initiation was microstructurally dependent. Small cracks in all microstructures grew at δK values below the large crack threshold. The as-received (polygonal ferrite) microstructure and one of the lath microstructures that resulted from heat treatment exhibited the same growth rate correlation as large cracks in the linear (Paris) region, and could be considered as an extension of the large crack growth region down to the point of initiation. Small cracks grew at rates faster than expected through one of the heat-treated and the weld microstructures; therefore, the number of cycles required for growth from initiation to the transition to large crack growth decreased about threefold, which is a potentially important factor in predicting lifetimes of structures made from this steel.
TL;DR: In this paper, the microstructures of as-quenched and aged HSLA100 steel were investigated using transmission electron microscopy, and it was shown that the mixed microstructure of 50% lath martensite + 40% acicular ferrite + 9% retained austenite + 1% twinned martensites.
TL;DR: In this paper, the effects of three on-line accelerated cooling parameters (ACcelerated cooling start temperature TA, cooling rate Ṫ, and cooling interrupt temperature T1) on transformation temperatures and microstructure in a low carbon microalloyed plate steel were studied by laboratory simulations in a quench deformation dilatometer.
Abstract: The effects of three on line accelerated cooling parameters (accelerated cooling start temperature TA, cooling rate Ṫ, and cooling interrupt temperature T1) on transformation temperatures and microstructure in a low carbon microalloyed plate steel were studied by laboratory simulations in a quench deformation dilatometer. Varying the on line accelerated cooling parameters changes the austenite condition and transformation path. In general, the transformation path shifts from polygonal ferrite towards bainite with increasing TA, increasing Ṫ, and decreasing T1. There is also a corresponding refinement in the microstructure and increase in hardness. In comparison with the laboratory thermomechanical processing treatments, the multipass industrial rolling schedule produces a much more heavily deformed austenite structure than laboratory thermomechanical processing treatments, which would favour high transformation temperatures, fine polygonal ferrite microstructure, and lower hardness.MST/3425
TL;DR: In this article, the impact resistance and fracture toughness of vanadium-based microalloyed (MA) steels are investigated and compared with the traditional quenched and tempered (Q and T) steel.
Abstract: Microalloyed (MA) steels are a family of steels which are becoming an increasingly important economic alternative to the traditional quenched and tempered (Q and T) steels. Impact resistance and fracture toughness of vanadium-based MA forging steel, which is the most commonly produced MA steel, are investigated in this study. To compare the behavior with the Q and T steel, both the as-forged and the Q and T conditions are evaluated. Experimental results from Charpy V-notch impact and fracture toughness (K{sub R}-curve and J{sub IC}) tests are presented and discussed. Correlations between fracture resistance properties based on several proposed equations in the literature are also examined.
15 Oct 1996-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the combined of copper and niobium in high strength steel weld metal derives from the combination of two kinds of precipitates with distinct precipitation reaction kinetics and thermal stability.
Abstract: Experimental work to produce high strength steel welds with systematic additions of copper (up to 3.5 wt.pct.) and niobium (up to 0.45 wt.pct.) at 3.6 kJ mm −1 (90 kJ in −1 ) heat input was carried out. Copper additions were found to precipitation strengthen the high strength steel weld metals. An estimated yield strength of 900 MPa (130 ksi) can be achieved in single-pass welds with approximately 2.8 wt.pct. of copper addition. Nevertheless, multipass welding over the copper-enhanced weld deposit resulted in monuniform and weld metal properties across the weld beads. Niobium additions did not provide as powerful a strengthening effect in the single-pass high heat input welds as the copper additions. At approximately 3.2 wt.pct. copper and 0.06 wt.pct. niobium in the weld deposits, the two elements provided the required strenghtening effect with thermally stable precipitates to produce high strength steel weld metals that exhibited acceptable properties in high heat input, single- and multi-pass conditions. The combined of copper and niobium in high strength steel weld metal derives from the combination of two kinds of precipitates with distinct precipitation reaction kinetics and thermal stability.
TL;DR: In this article, Austenite recrystallization and ferrite transformation behavior of low carbon TiO steel was studied by uniaxial hot compression in a temperature range of 780 to 1300°C.
Abstract: Austenite recrystallization and ferrite transformation behaviour of a Ti treated low carbon TiO steel was studied by uniaxial hot compression in a temperature range of 780 to 1300°C. The results have been compared to that of a conventional TiN steel. It was found that large oxide particles present in TiO teel have shown ability to stimulate recrystallization nucleation and transformation during deformation but failed to accelerate recrystallization of bulk material because of an insufficient volume fraction of these large particles in the steel. The influence of Ti: N ratio in TiO steel is critical to TiN precipitation behaviour which affects microstructural development during thermomechanical processing. TiN precipitates in TiO steel are coarser in comparison to conventional Ti microalloyed steel thus reducing grain boundary pinning effects. It was also found that recrystallization of ferrite in cold worked TiO and TiN steels was not affected significantly by large particles and fine precipitates but controlled by pearlite size and distribution.
TL;DR: In this paper, it is recognized that the retardation of austenite recrystallization is due in part to solute drag but also to strain-induced precipitation, the latter exerting usually the stronger effect.
TL;DR: In this article, it was found that Niobium alone or in combination with nitrogen decreased the thickness of cementite network near the surface of the carburized case of the steels.
Abstract: Carbon (0.07%) steel samples containing about 0.04% Nb singly and in combination with nitrogen were carburized in a natural Titas gas atmosphere at a temperature of 1223 K (950 °C) and a pressure of about 0.10 MPa for 1/2 to 4 h, followed by slow cooling in the furnace. Their microstructures were studied by optical microscopy. The austenite grain size of the case and the case depths were determined on baseline samples of low-carbon steels and also on niobium and (Nb + N) microalloyed steel samples. It was found that, when compared to the baseline steel, niobium alone or in combination with nitrogen decreased the thickness of cementite network near the surface of the carburized case of the steels. However, niobium in combination with nitrogen was more effective than niobium in reducing the thickness of cementite network. Niobium with or without nitrogen inhibited the formation of Widmanstatten cementite plates at grain boundaries and within the grains near the surface in the hypereutectoid zone of the case.