Showing papers on "Microalloyed steel published in 2009"

Precipitation hardening of high-strength low-alloy steels by nanometer-sized carbides

Abstract: The effects of Ti, Ti–Mo, and Ti–Nb microalloy additions on the precipitation strengthening in three experimental high-strength low-alloy steels have been investigated. The objective of this work was to study the carbide precipitation under the conditions of continuous cooling and interrupted cooling. It was found that titanium molybdenum complex carbide, (Ti, Mo)C, can strongly maintain nanometer-scaled sizes and has the largest contribution to the hardness as compared to titanium carbide, TiC, and titanium niobium complex carbide, (Ti, Nb)C. The result emphasizes that (Ti, Mo)C particles possess an excellent behavior of thermal stability.

Influence of martensite morphology on the work-hardening behavior of high strength ferrite–martensite dual-phase steel

Abstract: This study concerns influence of martensite morphology on the work-hardening behavior of high-strength ferrite–martensite dual-phase (DP) steel. A low-carbon microalloyed steel was subjected to intermediate quenching (IQ), step quenching (SQ), and intercritical annealing (IA) to develop different martensite morphologies, i.e., fine and fibrous, blocky and banded, and island types, respectively. Analyses of work-hardening behavior of the DP microstructures by differential Crussard–Jaoul technique have demonstrated three stages of work-hardening for IQ and IA samples, whereas the SQ sample revealed only two stages. Similar analyses by modified Crussard–Jaoul technique showed only two stages of work-hardening for all the samples. Among different treatments, IQ route has yielded the best combination of strength and ductility due to its superior work-hardening behavior. The influence of martensite morphology on nucleation and growth of microvoids/microcracks has been correlated with the observed tensile ductility.

Enhancement of the mechanical properties of a Nb-microalloyed advanced high-strength steel treated by quenching–partitioning–tempering process

Abstract: There is an urgent requirement for better combination of high strength and adequate toughness of steels to meet the demand of industrial applications. The present study shows that an Fe–0.2C–1.5Mn–1.5Si–0.05Nb–0.13Mo steel subjected to quenching–partitioning–tempering (Q–P–T) process based upon modifying the recently developed Q&P treatment by Speer et al. exhibits high-tensile strength (1500 MPa) and relatively high elongation (15%), therefore, Q–P–T steels have become a novel group of advanced high-strength steels. The origin of such excellent mechanical properties was revealed by characterization of microstructure.

Effect of Plastic Hot Deformation on the Hardness and Continuous Cooling Transformations of 22MnB5 Microalloyed Boron Steel

Abstract: The strains, transformation temperatures, microstructure, and microhardness of a microalloyed boron and aluminum precoated steel, which has been isothermally deformed under uniaxial tensile tests, have been investigated at temperatures between 873 and 1223 K, using a fixed strain rate value of 0.08 s−1. The effect of each factor, such as temperature and strain value, has been later valued considering the shift generated on the continuous cooling transformation (CCT) diagram. The experimental results consist of the starting temperatures that occur for each transformation, the microhardness values, and the obtained microstructure at the end of each thermomechanical treatment. All the thermomechanical treatments were performed using the thermomechanical simulator Gleeble 1500. The results showed that increasing hot prestrain (HPS) values generate, at the same cooling rate, lower hardness values; this means that the increasing of HPS generates a shift of the CCT diagram toward a lower starting time for each transformation. Therefore, high values of hot deformations during the hot stamping process require a strict control of the cooling process in order to ensure cooling rate values that allow maintaining good mechanical component characteristics. This phenomenon is amplified when the prestrain occurs at lower temperatures, and thus, it is very sensitive to the temperature level.

Grain growth behavior of coarse-grained austenite in a Nb-V-Ti microalloyed steel

Abstract: Grain growth of coarse-grained austenite during equalization in a simulated thin slab casting and direct rolling process (TSDR) has been studied on a Nb–V–Ti microalloyed steel. Coarse-grained austenite produced by reheating at 1350 °C for 90 min were reheated to four equalization temperatures (1100, 1150, 1200 and 1250 °C) for 30 min, then the austenite grain sizes and precipitates were examined. The experimental results show that coarse-grained austenite keep its wide range of grain sizes at elevated equalization temperatures. Ti-rich carbonitrides retain its strong pinning effect on grain boundary of coarse-grained austenite at temperatures below 1250 °C. The occurrence of abnormal grain growth in coarse-grained austenite may be difficult when particle coarsening occurs as it is hard for the smaller grains which has the larger driving forces to grow into neighboring larger grain matrix.

Austenite Recrystallization-Precipitation Interaction in Niobium Microalloyed Steels

Abstract: A good combination of strength and toughness in HSLA steels can be achieved by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at lower temperatures by solute drag or by precipitation pinning. In this study, the recrystallization behavior of four Nb-microalloyed model alloys which were designed to show either extensive or almost no precipitation, was compared by multi-hit torsion tests and double hit compression tests. A good consistency between the different types of tests was found and the results were verified by optical micrographs. Further, by construction of softening‐ time‐temperature diagrams the recrystallization behavior was linked to the precipitation state of the material which was investigated by thermodynamical equilibrium calculations and by experimental observations from TEM-EDX, Inductively Coupled Plasma Mass Spectroscopy and X-ray Diffraction. Quantitative agreement between the experimental measurements and the calculations for precipitated mass fraction and precipitate composition as a function of temperature and steel composition is demonstrated.

Effect of the bainitic transformation temperature on retained austenite fraction and stability in Ti microalloyed TRIP steels

Abstract: TRIP properties in four hot-rolled microalloyed steels were studied as a function of bainitic domain entrance temperature. It was found that this temperature affects the size and morphology of the retained austenite grains. Size and morphology was shown to play a role on the stability of the retained austenite. This stability was shown to be the major factor affecting the mechanical properties of these steels. In the case of the lowest bainitic temperature, the TRIP effect is hindered due to the premature transformation of austenite into martensite probably due to an insufficient carbon enrichment of the small austenite grains. Moreover the bainitic treatment performed at the highest temperature contained a very low volume fraction of retained austenite that does not transform even under deformation. Using TEM and the Kikuchi-line method, it was shown that the sample that did not undergo a phase transformation during FIB milling had a carbon concentration in its retained austenite grains of approximately 1.7%.

Influence of the chemical composition on transformation behaviour of low carbon microalloyed steels

Abstract: In order to design thermomechanical schedules for processing low carbon microalloyed steels, the various critical transformation temperatures, i.e. the start and finish of the austenite transformation (Ar3, Ar1) and the non-recrystallization temperature (Tnr), must be determined. Continuous cooling torsion and compression testing are useful ways to measure these values. In this study six low carbon microalloyed steels with different additions (Nb, Cu, Si and Mo) were examined using these techniques. Moreover, the equilibrium phase diagrams for each alloy were calculated using FactSage. The comparison of the thermomechanical testing results with the thermodynamic calculations leads to a better understanding of the effect of the different elements on the transformation behaviour of pipeline steels. Regarding transformation temperatures, Cu in residual contents showed a strong effect on decreasing both Ar3 and Ar1, which indicates a hardenability effect of this element. On the other hand, increasing Nb contents increased Tnr by accelerating Nb(C,N) precipitation. However, when Si was added to a Nb-microalloyed steel, the Tnr decreased.

Effects of Mo, Cr, and V Additions on Tensile and Charpy Impact Properties of API X80 Pipeline Steels

Abstract: In this study, four API X80 pipeline steels were fabricated by varying Mo, Cr, and V additions, and their microstructures and crystallographic orientations were analyzed to investigate the effects of their alloying compositions on tensile properties and Charpy impact properties. Because additions of Mo and V promoted the formation of fine acicular ferrite (AF) and granular bainite (GB) while prohibiting the formation of coarse GB, they increased the strength and upper-shelf energy (USE) and decreased the energy transition temperature (ETT). The addition of Cr promoted the formation of coarse GB and hard secondary phases, thereby leading to an increased effective grain size, ETT, and strength, and a decreased USE. The addition of V resulted in a higher strength, a higher USE, a smaller effective grain size, and a lower ETT, because it promoted the formation of fine and homogeneous of AF and GB. The steel that contains 0.3 wt pct Mo and 0.06 wt pct V without Cr had the highest USE and the lowest ETT, because its microstructure was composed of fine AF and GB while its maintained excellent tensile properties.

Dissolution and Precipitation Kinetics of Nb(C,N) in Austenite of a Low-Carbon Nb-Microalloyed Steel

Abstract: The dissolution and the precipitation kinetics of Nb(C,N) particles in both strain-free and strained austenite of a low-carbon Nb-microalloyed steel were investigated using electrical resistivity, interrupted compression, and electrolytic dissolution methods as well as transmission electron microscopy (TEM). The dissolution temperature of the 0.08C-0.038Nb steel was determined to be about 1230 °C for 600 seconds using electrical resistivity method and was confirmed by TEM. With increasing strain, the isothermal precipitation kinetics of Nb(C,N) was accelerated and the particle distribution became more random. The precipitation-time-temperature diagrams of the Nb(C,N) precipitates in strain-free and strained austenite were generated between 850 °C and 950 °C. The nose temperatures for both diagrams were about 900 °C. Compared to the electrolytic dissolution method, the interrupted compression and electrical resistivity methods show more reasonable strain-induced precipitation kinetics. The electrical resistivity method has wider applications to the dissolution and precipitation kinetics in microalloyed steels.

Evolution of austenite static recrystallization and grain size during hot rolling of a V-microalloyed steel

Abstract: Laboratory double-deformation isothermal tests and multipass continuous cooling hot torsion tests were used to study the static recrystallization of austenite under isothermal and anisothermal conditions as well as to simulate the hot rolling of a 0.13% V-microalloyed steel. Characterization of the evolution of austenite microstructure was carried out. It has been verified that no-recrystallization temperature ( T nr ) approximately corresponds to the temperature where recrystallization starts to be incomplete during rolling. However, incomplete recrystallization is visually evident at temperatures 25–50 °C below T nr , where grain elongation and increase in aspect ratio with temperature drop start to be significant. An accurate method to estimate the recrystallized fraction during hot rolling from stress–strain data and with no need of metallographic studies has been designed. The results of this method have been compared to metallographic measurements, the values of anisothermal fractional softening and the accumulated stress measured in the MFS plots at T T nr . A pronounced austenite grain refinement has been detected in the first hot rolling passes after reheating, as grain size decreases from 155 μm to 27 μm in six passes. If the effect of grain size on recrystallization and precipitation is taken into account, the correlation of isothermal and continuous cooling tests as well as the relationship between SRCT and T nr or RLT temperatures can be better understood.

Hot Ductility of Nb- and Ti-Bearing Microalloyed Steels and the Influence of Thermal History

Abstract: The hot ductility of Nb, Ti, and Nb-Ti containing steels has been studied under direct-cast conditions. A Gleeble 3500 thermomechanical simulator was used to determine hot ductility over the temperature range 1100 °C to 700 °C at a low strain rate of 7.5 × 10−4 s−1. Tensile samples were cooled at two different cooling rates, 100 °C/min and 200 °C/min, simulating, respectively, thick and thin slab casting processes. Complex thermal patterns designed to simulate the cooling conditions experienced near the surface of a slab during continuous casting were carried out for the Nb-Ti steel. The Nb-Ti steel had lower ductility than both the Nb and Ti steels. Increasing the cooling rate generally deteriorated ductility. The low recovery of ductility at higher temperatures is explained in terms of a low strain rate and fine precipitation delaying the onset of dynamic recrystallization. This can promote intergranular cracking as a result of grain boundary sliding in the austenite. At lower temperatures, ductility was further reduced due to the formation of thin ferrite films at the prior austenite grain boundaries. Simulating the thermal history experienced near the surface of thin (90 mm) cast slab improved ductility of the Nb-Ti steel by promoting coarser NbTi(C,N). This exposes a potential flaw in a simplified hot-ductility test: a failure to accurately represent the influence of the thermomechanical schedule on precipitation and, hence, hot ductility.

Very high cycle fatigue mechanism of carbide-free bainite/martensite steel micro-alloyed with Nb

Abstract: In this study, the carbide-free bainite/martensite (CFB/M) steel was micro-alloyed with Nb (CFB/M-Nb). The very high cycle fatigue (VHCF) behaviors of this CFB/M-Nb steel were investigated by ultrasonic fatigue test. Especially, different microstructures of the CFB/M-Nb steel were deliberately designed to disclose the VHCF failure mechanism. In addition, the effect of hydrogen in the formation of the optical dark area (ODA) was analyzed, which was thought to control the formation of ODA by concentrating around interior inclusions. Results show the hydrogen cannot be considered to dominate the formation of ODA. Mostly important, “soft or coarsely soft structure induced fatigue crack” should be responsible for the primary VHCF failure mechanism of CFB/M-Nb steel.

Fracture-Toughness Analysis in Transition-Temperature Region of Three American Petroleum Institute X70 and X80 Pipeline Steels

Abstract: The fracture toughness in the transition-temperature region of three American Petroleum Institute (API) X70 and X80 pipeline steels was analyzed in accordance with the American Society for Testing and Materials (ASTM) E1921-05 standard test method. The elastic-plastic cleavage fracture toughness (K Jc ) was determined by three-point bend tests, using precracked Charpy V-notch (PCVN) specimens; the measured K Jc values were then interpreted by the three-parameter Weibull distribution. The fracture-toughness test results indicated that the master curve and the 98 pct confidence curves explained the variation in the measured fracture toughness well. The reference temperatures obtained from the fracture-toughness test and index temperatures obtained from the Charpy impact test were lowest in the X70 steel rolled in the two-phase region, because this steel had smaller effective grains and the lowest volume fraction of hard phases. In this steel, few hard phases led to a higher resistance to cleavage crack initiation, and the smaller effective grain size led to a higher possibility of crack arrest, thereby resulting in the best overall fracture properties. Measured reference temperatures were then comparatively analyzed with the index temperatures obtained from the Charpy impact test, and the effects of microstructures on these temperatures were discussed.

Effects of aluminium and nitrogen on static recrystallisation in V-microalloyed steels

Abstract: Static recrystallisation of three steels having different Al, V and N contents (one of them without V) has been studied by means of hot torsion tests. It has been found that strain-induced precipitates of AlN in the austenite have a mean size of approximately 86 nm. These particles barely inhibit the static recrystallisation, as associated pinning forces are very weak. However, mean size of VCN particles is equal to 11 nm. This fine size leads to a temporary inhibition of recrystallisation, revealed by the characteristic “plateau” of the plots of recrystallised fraction versus holding time after deformation. Besides, activation energy for recrystallisation considerably augments when VCN precipitation occurs, but it hardly increases when AlN particles precipitate. On the other hand, diffusion coefficient of Al in austenite is two orders of magnitude higher than for V. Furthermore, according to thermodynamic calculations based on Hillert and Staffanson method, precipitation of AlN particles starts at much higher temperatures than VCN. Aforesaid reasons make AlN particles to be much coarser than VCN precipitates. From the results it can be concluded that low Al contents would lead to more intense precipitation of VCN that is beneficial for V-microalloyed steels.

Recrystallization Behavior of Deformed Austenite in High Strength Microalloyed Pipeline Steel

Abstract: Using methods of single-hit hot compression and stress relaxation after deformation on a Gleeble 1500D thermomechanical simulator, the curves of flow stress and stress relaxation, the microstructure and the recrystallization behavior of Nb-V-Ti high strength microalloyed low carbon pipeline steel were studied, and the influence of the thermomechanical treatment parameters on dynamic and static recrystallization of the steel was investigated. It was found that microalloying elements improved the deformation activation energy and produced a retardation of the recrystallization due to the solid solution and precipitation pinning. The deformation conditions such as deformation temperature, strain, and strain rate influenced the recrystallization kinetics and the microstructure respectively. Equations obtained can be used to valuate and predict the dynamic and static recrystallizations.

New numerical method for the fit of Garofalo equation and its application for predicting hot workability of a (V–N) microalloyed steel

Abstract: A new method, Rieiro, Carsi, Ruano (RCR), for solving the Garofalo equation is developed. This method is based on an integrated algorithm that allows determination of the equation parameters for any given material and does not need initial values. The RCR method is used to analyse the Garofalo equation best fit applied to torsion data at various temperatures and strain rates from a (V–N) microalloyed steel. The predictive capability of the RCR method on experimental results is ∼6% in stress and the magnitude of the predicted errors is of the same order as the interpolation errors. On the other hand, the n value is slightly lower and the Q value slightly higher than those expected for a slip creep mechanism controlled by lattice diffusion. However, the obtained values agree with those found in the literature for microalloyed steels. These differences can be attributed to microstructure changes during deformation.

Study of the effect of grain size on the dynamic mechanical properties of microalloyed steels

Abstract: In the present paper the effect of grain refinement on the dynamic response of ultra fine-grained (UFG) structures for C–Mn and HSLA steels is investigated. A physically based flow stress model (Khan-Huang-Liang, KHL) was used to predict the mechanical response of steel structures over a wide range of strain rates and grain sizes. However, the comparison was restricted to the bcc ferrite structures. In previous work [K. Muszka, P.D. Hodgson, J. Majta, A physical based modeling approach for the dynamic behavior of ultra fine-grained structures, J. Mater. Process. Technol. 177 (2006) 456–460] it was shown that the KHL model has better accuracy for structures with a higher level of refinement (below 1 μm) compared to other flow stress models (e.g. Zerrili-Armstrong model). In the present paper, simulation results using the KHL model were compared with experiments. To provide a wide range of the experimental data, a complex thermomechanical processing was applied. The mechanical behavior of the steels was examined utilizing quasi-static tension and dynamic compression tests. The application of the different deformation histories enabled to obtain complex microstructure evolution that was reflected in the level of ferrite refinement.

Surface characteristics analysis of nitrocarburized (Tenifer) and carbonitrided industrial steel AISI 02 types

Abstract: Generally, tool steels for cold work are obtained by rolling and forging processes. They are treated to have a structure conferring to the material a high toughness limit in terms of wear resistance and endurance. The objective of this study is the thermochemical heat treatment of industrial steel blades made of AISI 02 types, intended for polymer crushing. The effects of nitrocarburizing (Tenifer) and gaseous carbonitriding processes on surface characteristics are considered. These surface treatments increase the usefulness of properties, that is, fatigue strength, wear and corrosion resistance of this microalloyed steel. The influence of treatment duration and the thickness of the layers on surface properties are investigated. The analysis and characterization are carried out using physical analysis [optical microscopy, scanning electron microscopy, X-ray diffraction and glow discharge optical emission spectroscopy (GDOES) techniques] and mechanical measurements (microhardness, weight loss and residual stresses) of treated material. The results are intended to contribute in defining and optimizing the adequate choice of treatments for this type of steel in industrial conditions.

On the Cu precipitation behavior in thermomechanically processed low carbon microalloyed steels

Abstract: The present study concerns determination of activation energies of copper-bearing low carbon microalloyed steels processed under different thermomechanical schedules. Selected samples were subjected to 50% cold deformation to examine the effect of prior strain on precipitation of copper. Activation energies were determined by the Kissinger method using differential scanning calorimetry plots obtained at various heating rates. The results are suitably compared with the published results. In view of the results of differential scanning calorimetry, isothermal ageing treatments have been carried out along with suitable microstructural investigations. An attempt has also been made to correlate the variation of hardness and microstructural evolution during ageing treatment.

Segregation behaviour in Nb microalloyed steels

Abstract: Four continuously cast slabs from two sources were characterised by optical microscopy and scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDS) in the as cast state and after normalisation close to the Ae 3 temperature. For all four slabs the composition of the majority of the slab depth (>80%) was the bulk composition and in this region microsegregation, particularly of Nb, was found to agree with predicted partition ratios from equilibrium thermodynamics. For two of the slabs, which gave final solidification as single phase austenite, equilibrium thermodynamics were also able to predict the compositional centreline macrosegregation. In the other two cases, this simple approach was not successful, which has been ascribed to solidification as a more complex phase mixture (austenite and δ-ferrite) or more complex processing during solidification (e.g. soft reduction).

Investigation of dynamic and static recrystallization behavior during thermomechanical processing in a API-X70 microalloyed steel

Abstract: Recovery and recrystallization phenomena and effects of microalloying elements on these phenomena are of great importance in designing thermomechanical processes of microalloyed steels. Control of these phenomena can lead to manufacturing of products with suitable structures and desirable properties. The aim of this study was to investigate the microstructural evolution occurring during thermomechanical processing through hot compression testing. The niobium carbonitride approximate solution temperature was initially determined to guess the optimum reheating temperature. Then continuous and interrupted compression tests were utilized to physically model the dynamic and static recrystallization (SRX) behavior of the steel during thermomechanical processing. The parameters of the flow stress model and the activation energy of dynamic recrystallization (DRX) were calculated and the effects of deformation conditions on peak strain of DRX were evaluated. In addition, a kinetic study of the static softening of austenite indicated that the plateau occurred in recrystallization fraction-time curves below 1025 °C. This was considered in determining the Static Recrystallization Critical Temperature (SRCT) and construction of the Recrystallization Retardation-Temperature Time (RRTT) diagram for this steel.

Characterization of Grain-boundary Precipitates after Hot-ductility Tests of Microalloyed Steels

Abstract: The hot ductility of microalloyed steels was investigated by interrupted tensile tests at the temperatures of 850 and 950°C. Analyses of microstructural damage during plastic straining of the steels were performed using an experimental setup that allowed rapidly quenching the tensile specimens after straining to a predefined level. Microstructural investigations on the materials were carried out on longitudinally sectioned samples. Further analyses on crack surfaces were performed by fracturing the strained specimens in liquid nitrogen and by analyzing the surfaces formed by high-temperature decohesion through conventional and field emission SEM. It was demonstrated that AlN and Nb(C,N) precipitates, in isolated or combined form, affected the prior-austenite grain boundaries. Differences in hot cracking sensitivity among the steels was accounted for by modifications of the precipitate size and volume fraction.

Effect of deformation and Nb segregation on grain size bimodality in HSLA steel

Abstract: Bimodal ferrite grain sizes (mixed coarse and fine grain bands) have been observed in Nb microalloyed thermomechanically controlled rolled (TMCR) steel plates and are undesirable as they can reduce toughness. This paper examines the role of rolling deformation on the formation of bimodal grain structures in reheated continuously cast slab material with initial uniform or bimodal austenitic grain structures. The slab material contains solute rich and solute poor regions, due to interdendritic segregation, which have been shown to cause bimodal austenite grain structures during reheating within a certain temperature range. It is known that deformation in the partial recrystallisation region can result in a mixed coarse and fine grained structure. Theoretical calculations (based on the Dutta–Sellars model) and deformation experiments indicated that the segregated microalloying elements (particularly Nb) can promote bimodality during deformation by affecting the local recrystallisation kinetics over a...

Effects of composition and processing parameters on precipitation and texture formation in microalloyed interstitial free high strength (IFHS) steels

Abstract: Precipitation in IFHS steels affects texture formation during annealing, which in turn determines the formability. The single most important type of precipitates in IFHS steels is FeTiP. Precipitation of FeTiP leads to the decrement in the intensity of the favourable {111} texture, which causes a degradation of the formability properties. The batch annealed steels are particularly susceptible to this. Since formation of FeTiP as well as of the other types of precipitates in IFHS steels is dependent on steel composition and processing parameters, a detailed and systematic investigation is urgently needed to look into the various aspects of precipitation in IFHS steels. Proper scientific investigation is expected to provide necessary processing windows that will not allow formation of unwanted precipitates. This will help in maximising the strengthening effects of P, without any degradation of the deep drawability of the steels.