Showing papers on "Microalloyed steel published in 1995"

The effect of niobium on the hardenability of microalloyed austenite

Abstract: The powerful effect that varying the extent of niobium-carbide dissolution has on the “hardenability” of microalloyed austenite is demonstrated using dilatometric measurement of the critical cooling rate required to from microstructures containing >95 Pct martensite. The results can be rationalized on the hypothesis that the hardenability of austenite is enhanced by niobium in solid solution, possibly by its segregation to austenite grain boundaries, but is decreased by precipitation of niobium-carbide particles. This effect appears analogous to that of boron in steels and is found to be independent of variations in the austenite grain size.

Softening and Flow Stress Behaviour of Nb Microalloyed Steels during Hot Rolling Simulation

Abstract: The roles of softening and precipitation were investigated by means of hot torsion experiments under conditions simulating either plate or sheet rolling. Six microalloyed steels containing Nb were studied. During the first few finishing passes in the sheet rolling simulations, the mean flow stress (MFS) increased as the interpass time was decreased. Due to strain accumulation, the rate of static recrystallization (SRX) increased significantly after each pass. By taking both strain accumulation and grain refinement into account, it is shown that SRX plays a marked role under sheet rolling conditions, even at temperatures below the no-recrystallization temperature for plate rolling conditions. The accumulated or retained strain reaches the critical value required to initiate dynamic recrystallization only at the lowest entry and rolling temperatures and shortest interpass times. The kinetics of the strain-induced precipitation of NbCN under continuous cooling conditions, taking partial SRX into account, indicate that precipitation begins after 2 to 5 passes when 3 s interpass times are employed, thus reducing further softening. But when 1 s interpass times are used, most of the passes take place before copious precipitation, so that static and post-dynamic (i.e. metadynamic) softening may continue to take place. As a result, the MFS level decreases as the interpass time is shortened during the final passes. The extent of grain refinement was similar in both the sheet rolling and plate rolling simulations. The ferrite grain size is shown to depend on MFS of the final pass, and is independent of the chemical composition of the microalloyed steel.

The Influence of Bainite on Retained Austenite Characteristics in Si-Mn TRIP Steels

Abstract: Controlled rolling processes are designed to produce a desired microstructure via the control of hot rolling without subsequent heat treatment. Critical to the success of controlled rolling are the stages that occur after hot deformation, when the steel is cooled to room temperature. This can be divided into two parts: the run out table, where the material is allowed to cool relatively rapidly to a pre-determined temperature, and "coiling" at which point the rolled material is coiled, thus showing down the cooling rate considerably. Controlled rolling schedules generally finish by coiling the steel at temperatures below the bainite transformation start temperature (Bs). Any changes in coiling conditions (temperature and time) in this region can result in variations in bainite characteristics (morphology, size, carbide precipitation, etc.). This in turn, may affect the state of the retained austenite and, consequently, the mechanical properties of Si-Mn TRIP steels, which have bainite as the dominant microconstituent. The effect of changes in the bainite transformation conditions were investigated using two grades of Si-Mn TRIP steels, including one a containing Nb as a microalloy addition. The results reveal that the retained austenite volume fraction was strongly influenced by both bainite formation temperature and hold time, The highest values of total elongation (46 and 33%) and formability index (61180 and 40260 MPa·%) were observed for an intermediate hold time (5 min) and temperature (400°C), respectively. These findings are explained by considering the effect of the bainite transformation on the state of the retained austenite.

Effect of alloying elements on metadynamic recrystallization in HSLA steels

Abstract: By means of interrupted torsion tests, the kinetics of metadynamic recrystallization (MDRX) were studied in a Mo, a Nb, and a Ti microalloyed steel at temperatures ranging from 850 °C to 1000 °C and strain rates from 0.02 to 2 s1. Quenches were also performed after full MDRX. In contrast to the case of static recrystallization (SRX), the kinetics of MDRX are shown to be highly sensitive to a change of an order of magnitude in strain rate and are relatively insensitive to temperature changes within the range of values applicable to industrial hot-rolling practice. A similar algebraic dependence of the MDRX grain size on strain rate and temperature was found in the three steels. The kinetics of MDRX were slower in the Nb than in the Mo steel, and those of the Ti steel were slower than in the Nb and Mo steels. Above 900 °C and 950 °C, the retardation of MDRX in the Nb and Ti steels, respectively, is due to solute drag. Models predicting the start time for Nb and Ti carbonitride precipitation showed that MDRX is delayed below these temperatures by this mechanism. Comparison of the MDRX and precipitation start times in the Nb steel indicated that a temperature of “no-MDRX” could not be defined, in contrast to the well-definedT nr (no recrystallization temperature) of SRX. By means of torsion simulations composed of multiple interruptions, it is shown that MDRX is retarded decreasingly as the accumulated strain is increased. This appears to be due to the promotion of precipitate coarsening by the continuing deformation.

A new theory and kinetic modeling of strain-induced precipitation of Nb(CN) in microalloyed austenite

Abstract: A new theory of strain-induced precipitation of Nb(CN) in microalloyed austenite has been developed by considering the effect of nonequilibrium segregation of Nb on the nucleation and growth of the precipitates. With the aid of the theory, the discrepancy existing between the conventional theory for strain-induced precipitation and experimental data can be reconciled. A kinetic model has been developed based on the new theory. The model is capable of predicting both the Nb(CN) precipitation kinetics and the resulting microstructure in microalloyed austenite. Excellent agreement has been obtained between the model predictions and the experimental observations reported by different investigators.

Cleavage fracture of microalloyed forging steels

Abstract: Microalloying in medium C forging steels together with new processes such as thermomechanical treatments or direct quenching from the finishing forging temperature are being used to avoid expensive heat treatments and to obtain required optimum mechanical properties directly after forging. Good strength levels have been obtained without difficulty but for the same strength level, toughness is improved in quenched + tempered steels. In recent years, efforts have been concentrated on improving the toughness by refining the microstructure, and for that, Ti addition is becoming a common method. Several recommendations are made to obtain the maximum yield of fine TiN precipitates, but a certain proportion of TiN precipitates is usually in a coarse form. These precipitates act as cleavage nucleation sites. The present work reports the influence of TiN particles on cleavage fracture stress in four microalloyed forging steels with ferrite-pearlite microstructures and one with lath martensite structure. The results show that cleavage fracture propagation at liquid nitrogen temperature is controlled by the energy required for the microcrack to trespass the TiN particle-matrix interface.

Microstructure and properties of ferritic steel welds containing Al and Ti

Abstract: The combined effect of Al and Ti, in the range 5 to 500 ppm, on the microstructure and properties of C-Mn shielded metal arc welds has been studied. It was found that Ti, in contrast to Al, dramatically enhanced the formation of acicular ferrite and improved notch toughness. A strong interactive effect was encountered, with Al at low concentrations tending to diminish the influence of Ti. Unless a critical balance is achieved, with regard to oxygen content, it is concluded that Ti be optimized at 30 to 40 ppm and that Al be kept as low as possible.

Hot deformation characteristics of Si-Mn TRIP steels with and without Nb microalloy additions

Abstract: It has been long recognized that steels with high formability and strength can be produced with the aid of the deformation induced transformation of retained austenite to martensite. Selecting the steel composition and microstructure to obtain significant amounts of retained austenite has been the first step in designing TRIP steels. Thus, it is now well established that a significant amount of retained austenite can be obtained in steels containing high concentrations (>1%) of Si, where bainite is one of the main microconstituents. The effects of different intercritical annealing conditions on various aspects of Si-Mn TRIP steels have been extensively studied in other investigations. The present work was, however, conducted to investigate the influence of thermomechanical processing on the general behaviour of two Si-Mn TRIP steels, including a microalloyed Nb-bearing grade. The effects of microstructural features produced by dynamic recrystallization and pancaking of austenite, along with that of finishing temperature below tha Tnr (the austenite no-recrystallization temperature) on the subsequent transformations of the parent austenite, and the state of retained austenite at room temperature, were examined. It was found that, for microstructures comprised of polygonal ferrite, bainite and retained austenite, the variation of the retained austenite volume fraction with strain in the no-recrystallization region exhibits a maximum. Furthermore, the dynamically recrystallized austenite substructure, when retained to transformation temperatures, tends to retain more austenite.

Plasma nitriding of microalloyed steel

Abstract: 3icroalloyed or high strength low alloy (HSLA) steels are carbon-manganese steels containing small amounts of Nb, V or Ti. The excellent mechanical properties of these alloys, particularly high yield strength, usually obviate the need for expensive quench and tempering operations. Furthermore, the presence of a significant amount of nitride-forming elements in some microalloyed steels has generated interest in the applicability of these alloys as a new generation of nitriding steels. In this paper, a study of the plasma nitriding behaviour of a commercially available microalloyed steel MAXIMATM is reported. A comparison is made with a traditional quenched and tempered nitriding steel (En19), plasma nitrided under similar conditions. Optical and scanning electron microscopy in conjunction with microhardness measurements and X-ray diffraction were utilized to characterize the nitrided surfaces. The observed differences in the thickness and structure of the compound layer and the diffusion zone are discussed in terms of chemical composition and microstructure of these steels.

Microstructure and Precipitation Behavior in Heat Affected Zone of C-Mn Microalloyed Steel Containing Nb, V and Ti

Abstract: Effects of total N content on HAZ toughness, microstructure and precipitate behavior in Nb-V-Ti bearing C-Mn microalloyed steels has been investigated under non-equilibrium state such as welding thermal cycle.The HAZ toughness is improved with increasing total N content and the lowest 100 J transition temperaturre concerning HAZ toughness was obtained at a steel with high total N content about 80 ppm. Improbement of HAZ toughness with increasing total N content is caused by refinement of microstructure and decreasing souluble Nb and Ti with increasing total N content. Complex precipitates that contain both Nb and Ti are observed in Nb-V-Ti bearing steels after a weld thermal cycle, and they are refined and their density tend to increase with increasing total N content. The amount of Nb and Ti as precipitates increase with increasing total N content. And then the complex precipitation behavior in HAZ could be quantitatively predicted by a thermodynamic model.In Nb-V-Ti steels refinement of prior austenite grains occurs at higher total N content than in Ti bearing steels. It is caused by increasing the stability of precipitates through the formation of complex precipitates in Nb-V-Ti bearing steels.

Effect of silicon on CGHAZ toughness and microstructure of microalloyed steels

Abstract: The aim of this article is to present the beneficial effect of a reduction of silicon content on coarse-grained heat-affected zone (CGHAZ) toughness. This study was achieved with experi-mental and industrial E355 structural steels. These 0.09 wt pct C steels were Ti-microalloyed with silicon contents ranging from 0.05 to 0.5 wt pct. First, we demonstrate that the CGHAZ toughness is predominantly affected by the volume fraction of retained austenite (γr). Second, we show that the existence of retained austenite pertains only to its carbon enrichment. This enrichment is promoted essentially by an increase of the silicon level due to the retarding action of silicon on the formation of carbides in ferrite as well as in austenite. In the same way, the increase of silicon content slows down the decomposition of retained austenite into pearlite. The reduction of the silicon content of the steel greatly increases the ductility of the CGHAZ through the decrease of the volume fraction of retained austenite.

Modelling the thermomechanical and microstructural evolution during rolling of a Nb HSLA steel

Abstract: A simple approach to calculate rolling loads based on Sims' model was compared to a finite element model (FEM) using the same constitutive law and recrystallization equations. The comparison was done with a Nb microalloyed steel using a laboratory rolling simulation of plate and strip mill rolling schedules. The FEM model accurately predicted the temperature, the change in rolling loads, and the evolution of microstructure before and after transformation. These results clarified the accuracy of carious microstructure models, particularly the model for recrystallization. The FEM model gave an improved prediction of the rolling loads slightly compared with the slab method. For on-line modelling of the rolling loads a simpler approach based on Sims model gave acceptable prediction when using an accurate temperature model to predict the average temperature throughout the rolling process. The combination of the constitutive law and recrystallization kinetics in both cases enabled the prediction of the increase in rolling loads over the entire temperature range. The results also suggest that solute drag, rather than strain induced precipitation, has the primary effect on retarding recrystallization under the conditions examined.

The grain size distribution and the detection of abnormal grain growth of austenite in an eutectoid steel containing niobium

Abstract: The grain size distribution shape did not change during normal grain growth but this distribution widened and flattened during the abnormal grain growth. The initial smaller mean size of carbonitrides and/or the highest homogeneity of niobium carbonitride size distribution of the samples submitted to thermal cycles (“sc”) in comparison with the normalized samples (“sn”) increased the abnormal grain growth temperature from 1373 K at “sn” series to 1473 K at “sc” series.

Deformation and Fatigue Behavior of Vanadium-Based Microalloyed Forging Steel in the As-Forged and Q&T Conditions

Abstract: The use of microalloyed (MA) forging steels in the asforged condition can result in the elimination of the costly post-forging quenching and tempering (Q&T) processes. MA steels can therefore serve as economical alternatives to the traditional Q&T steels if equivalent mechanical properties can be obtained. In this study, monotonic and cyclic behaviors of a vanadium-based MA forging steel, one of the most commonly produced MA steels, were investigated under both the as-forged and Q&T conditions. The mechanical properties evaluated and compared include monotonic tensile behavior, cyclic deformation response, and low and high cycle fatigue resistance. Experimental results are presented and the differences in the behaviors under the two processing conditions are discussed.

Simulation of microstructural evolution in Nb-Ti microalloyed steel during hot direct rolling

Abstract: Experiments on hot direct rolling (HDR) require a freshly made ingot for every experimental variable. To carry out systematic research economically on a laboratory scale, a melting and casting facility was constructed in which 1 kg ingots of controlled composition were cast into a hot, reusable ceramic mould system and cooled slowly enough to simulate the microstructure and constitution of the austenite in continuously cast slabs. The preliminary results for Nb-Ti microalloyed steel showed that carbonitrides were not detected in the virgin, as cast austenite at any temperature down to 1000°C when Ti contents were less than 0.011%, although Nb rich carbonitrides were present in ingots cooled all the way to room temperature in the mould. Carbonitrides were only formed in the as cast austenite after holding isothermally at high temperature. Thus, the as cast Nb-Ti austenite was highly supersaturated with precipitants during HDR. For this reason, recrystallisation was considerably more difficult in as cast austenite than after reheating. The slow recrystallisation of the very coarse austenite during HDR, exacerbated by high supersaturation of microalloying elements of the Nb-Ti steels, accounts for the tendency of HDR to produce mixed ferrite grain sizes. It was also found that allowing the ingot to cool below the transformation temperature before reheating to the rolling temperature refined the austenite grain size and removed the supersaturation. This explained the better results obtained from α hot charge rolling (αHCR).

Injection molding and extrusion barrels and alloy compositions thereof

Abstract: A bimetallic barrel for use in injection molding and extrusion. The barrel is formed of a backing steel and a metal inlay liner. The backing steel is either a microalloyed steel, carbon steel 1045 or carbon steel 1060. Inlay compositions, including copper containing ones, are disclosed.

Thermo-mechanical treatment of a high Nb-high V bearing microalloyed steel

Abstract: The response of a Nb-V microalloyed steel to thermal-mechanical treatment is examined. Constant strain rate, isothermal compression tests of axially symmetrical samples, to a strain of unity, are used to determine the metal's response at high temperatures. Four-stage, constant true strain rate compression of the samples is used to simulate partially the process of strip rolling. The loading is interrupted at various strains for ever shorter intervals. The resulting true stress-true strain curves are used to examine the steel's response to interrupted loading. The ability of the microalloying components to retard to dynamic restoration processes, after an interruption, is observed. Microstructural observations indicate that the most important process parameter is the total strain.

Softening and Flow Stress Behaviour during Hot Rolling Simulation of Nb Microalloyed Steels

Abstract: no-recrystallization temperature for plate rolling conditions. The accumulated or retained strain reaches the critical value required to initiate dynamic recrystallization only at the lowest entry and ro]Iing temperatures and shortest interpass times. The kinetics of the strain-induced precipitation of NbCN under continuous cooling conditions, taking partial SRX into account, indicate that precipitation begins after 2to 5 passes when3s interpass times are employed, thus reducing further softening. But whenI s interpass times are used, most of the passes take place before copious precipitation, so that static and post-dynamic (i.e. metadynamic) softening may continue to take place. As a result, the MFS Ievel decreases as the interpass time is shortened during the final passes. Theextent of grain refinement was similar in both the sheet rolling and plate rolling simulations. The ferrite grain size is shownto dependon MFS of the final pass, and is independent of the chemical composition of the microalloyed steel.

Segregation of impurities, temper brittleness, and hydrogen embrittlement of steel 30KhNMA with different concentrations of molybdenum

Abstract: In recent years practical and scientific interest in investigating segregation processes to assess the mechanism of steel embrittlement by impurity elements has grown. This interest was noticeably stimulated by industrial use of new metallurgical technologies that ensure higher purity of steel with respect to impurity elements. It has become possible to decrease the content of expensive molybdenum due to the higher purity of steels without substantial change in their properties. In the present work the structure and properties of steel 30KhNMA with conventional and elevated purities, standard and reduced concentrations of molybdenum, and microadditions of V, Ti, and Nb are investigated.

Rolling procedure and thickness effects on strength and fracture behaviour of microalloyed steels

Abstract: Two microalloyed steel heats have been produced to obtain sheets of different thicknesses by controlled rolling in one instance and conventional rolling followed by a normalising heat treatment in the other. The mechanical properties and microstructures of these materials have been evaluated and their fracture behaviour, from the ductile region to the brittle region, has been assessed by notched impact testing and elastoplastic fracture mechanics testing. The sheets submitted to thermomechanical treatment, having similar optical microstructures, show higher strength and lower ductility and toughness than the normalised sheets. In addition, both heats manifest the influence of the sheet thickness on these properties owing to its effect on the final cooling rate. Finally, although a linear correlation has been obtained between the results of notched impact and crack tip opening displacement or J interval, the dispersion of these data does not enable the prediction of fracture mechanics parameters fr...

Influence of titanium on the static recrystallization of a medium carbon microalloyed steel

Abstract: Static recrystallization kinetics of three medium carbon steels, microalloyed with vanadium and titanium (titanium varied from 0.003 to 0.039% in weight), were studied by hot torsion test simulation. A higher recrystallization time was observed in the steel with 0.019 wt% Ti. This difference in recrystallization time was checked by metallographic observations and mechanical softening. This time shift implies different activation energies, which were calculated by the time needed to obtain a 50% recrystallized structure and also by solving the Zener-Hollomon equation. Evolution of the kinetics of recrystallization as a function of temperature was also studied. In addition, the critical allowable temperature for a fully recrystallized structure was investigated.

Mechanical properties of low carbon-vanadium microalloyed quenched and tempered steels vvith aluminium additions

Abstract: The mechanical properties of Fe–0·1 C–1·5Mn–0·05V steels with varying Al contents (0·04–0·35 wt-%) were examined in the tempered condition. After hot rolling, the steels were solution treated at 1200°C for 2 h, then quenched into water. Tempering was mainly carried out at 600 or 650°C for 1 h. Hardness, tensile strength, and Charpy toughness were studied and compared with the microstructure. The precipitates were measured and identified using transmission electron microscopy and energy dispersive X-ray analysis. It wasfound that the precipitation ofVC was at a minimum in the steel containing 0·13%Al, and it is suggested that the precipitation of VC or V (C, N) was retarded at this level of Al addition. The tensile strength and lower ductile-brittle transition temperature decreased to their minimum values in the samples containing 0·1–0·15%Al, which was also believed to be a result of less VC or V(C,N) precipitation giving benefits of increased toughness and a decrease in the transition temperature...

Hot Ductility of a Microalloyed Steel in the Intermediate Temperature Range

Abstract: In this study hot ductility has been determined from tensile tests for two states of a microalloyed steel : after casting and after rolling processes. Hot deformations were carried out at speeds varying from 10 -4 s -1 to 10 -2 s -1 and temperatures from 750°C to 1100°C. Two heat treatments were chosen before hot deformation. A ferrite precipitation is observed at austenitic grain boundaries in the intercritical temperature range, causing intergranular embrittlement. Ductility trough is deeper in the as-cast samples due to the growth of large grain size. Also, precipitation makes the hot ductility curve wider and deeper around 900°C. The results show a decrease in hot ductility. Minimum values of hot ductility are determined for (ITC) treatment at 900°C and for (DTC) treatment at 800°C. For this second treatment another decrease in hot ductility was observed at 900°C. We can explain hot ductility losses by the presence of precipitates in the austenitic region and the presence of the two-phase structure in the intercritical region.

Influence of niobium on structure/property relationships in C-Mn shipbuilding steel

Abstract: The effect of niobium microaddition in C-Mn steel on microstructure, tensile properties, ductile-brittle transition temperature, and weldability have been investigated. Niobium microaddition adversely affects the transition temperature and toughness of the heat-affected zone, counteracting its favorable effects of refined grain size and low inclusion and pearlite content.