Showing papers on "Microalloyed steel published in 2012"

Modeling and Prediction of Hot Deformation Flow Curves

Abstract: The modeling of hot flow stress and prediction of flow curves for unseen deformation conditions are important in metal-forming processes because any feasible mathematical simulation needs accurate flow description. In the current work, in an attempt to summarize, generalize, and introduce efficient methods, the dynamic recrystallization (DRX) flow curves of a 17-4 PH martensitic precipitation hardening stainless steel, a medium carbon microalloyed steel, and a 304 H austenitic stainless steel were modeled and predicted using (1) a hyperbolic sine equation with strain dependent constants, (2) a developed constitutive equation in a simple normalized stress-normalized strain form and its modified version, and (3) a feed-forward artificial neural network (ANN). These methods were critically discussed, and the ANN technique was found to be the best for the modeling available flow curves; however, the developed constitutive equation showed slightly better performance than that of ANN and significantly better predicted values than those of the hyperbolic sine equation in prediction of flow curves for unseen deformation conditions.

Precipitation of Nb in Ferrite After Austenite Conditioning. Part II: Strengthening Contribution in High-Strength Low-Alloy (HSLA) Steels

Abstract: Often, Nb contributes to the strength of a microalloyed steel beyond the expected level because of the grain size strengthening resulting from thermomechanical processing Two different mechanisms are behind this phenomenon, and both of them have to do with the amount of Nb remaining in solution after hot rolling The first of them is the increase of the hardenability of the steel as a result of Nb, and the second one is the fine precipitation of NbC in ferrite Three Nb microalloyed steels were thermomechanically processed in the laboratory and coiled at different temperatures to investigate the effect of Nb content on the tensile properties The extra strength was linearly related to the Nb remaining in solution after the hot working The maximum contribution from Nb was reached for a coiling temperature of 873 K (600 °C)

An Ultra-low Carbon, Thermomechanically Controlled Processed Microalloyed Steel: Microstructure and Mechanical Properties

Abstract: In the current study, a novel ultra-low carbon, high-molybdenum-bearing microalloyed steel has been thermomechanically processed. Transformation of this steel during continuous cooling has been assessed. Variation in the microstructure and mechanical properties at different finish rolling temperatures has been studied. The average grain size, misorientation of grain boundary, and distribution of ferrite grains have been analyzed by using electron backscatter diffraction. The lower yield strength (251 to 377 MPa) with moderate tensile strength (406 to 506 MPa) along with high ductility (30 to 47 pct) has been achieved in the selected range of finish rolling temperatures. Superior impact toughness value in the range of 153 to 162 J is obtained in the subsize specimen even at subzero temperatures (233 K [−40 °C]), which is attributed to fine average ferrite grain size. The acicular ferrite dominated microstructure obtained at the 1023 K (750 °C) finish rolling temperature is the most attractive microstructure for pipeline applications due to its excellent combination of strength and toughness.

The Microstructure and Mechanical Properties of Hot Forged Vanadium Microalloyed Steel

Abstract: In this article, the effect of hot forging parameters (deformation temperature, strain, and cooling rate) on the microstructure and mechanical properties of commercial vanadium microalloyed forging steel (30MSV6) was investigated. Final, microstructures and mechanical properties were evaluated by optical microscopy, Charpy impact, Brinell hardness, yield, and tensile strength tests. The results show that increasing the cooling rate changes the ferritic-pearlitic microstructure to the acicular ferrite-bainite. It is shown that by increasing post forging cooling rate, both yield and ultimate tensile strength increase, while the impact energy decreases significantly. The specimens with the same cooling rate differ not only in deformation temperature, and the deformation ratio is immaterial in establishing a grain size.

Numerical simulation of NbC precipitation in microalloyed steel

Abstract: This work describes with the investigation of the precipitation kinetics of NbC in microalloyed steel. Using the thermo-kinetic software MatCalc, computer simulations of NbC precipitation are carried out and compared with several independent experimental results, measured in austenite and ferrite. The selected experiments involve a variety of different dislocation densities originating from distinct thermo-mechanical treatments. Two separate populations of NbC precipitates are accounted for in the simulations, representing precipitates on grain boundaries as well as dislocations. Furthermore, three different diffusion mechanisms are taken into account, which are bulk diffusion in the undisturbed crystal, accelerated diffusion along the dislocation core and fast diffusion along grain boundaries. It is demonstrated that deformation-induced dislocation densities higher than 1013 m−2 lead to prominent diffusion along dislocation networks. Therefore, in such cases, the precipitation kinetics of NbC is dominated by the pipe diffusion mechanism, and the precipitation process is several orders of magnitude faster compared with NbC precipitation in unstrained, well-annealed microstructures.

Ultra Grain Refinement During the Simulated Thermomechanical-processing of Low Carbon Steel

Abstract: Grain re Þ nement is a useful method to improve the strength and toughness of steels without changing their chemical composition. In this study, critical temperatures for the thermomechanical treatment of niobium microalloyed steel were determined experimentally and through thermodynamic data. Simulations of conventional and controlled thermomechanical processing and a thermomechanical treatment to obtain ultraÞ ne-grained microstructures were conducted using torsion tests. The Þ nal microstructures displayed signiÞ cant grain size reÞ nement. Conventional processing produced grains with an average size of 12 .m, while controlled processing led to an average grain size of 4.9 .m and severe plastic deformation at warm temperatures resulted in a grain size of 1.3 .m. The ultra reÞ nement of ferrite grains was associated with strain-induced dynamic phase transformation and dynamic recrystallization of as-transformed ferrite.

Austenite Grain Growth in Heat Affected Zone of Zr-Ti Bearing Microalloyed Steel

Abstract: Austenite grain sizes in the heat affected zone (HAZ) of a high heat input welded Zr-Ti bearing microalloyed steel are measured under different welding conditions simulated by a Gleeble-1500 thermal-mechanical simulator. The austenite grain growth is divided into two regimes in terms of temperature. When the temperature is lower than 1250 °C where the pinning effect of precipitates is strong, the austenite grain size increases slowly with increasing peak temperature, but it increases drastically when the temperature is higher than 1250 °C where the pinning effect of precipitates is weak. Based on the experimental measurements, an analytical model for predicting the austenite grain size in the heat affected zone is derived. Model predictions indicate that the initial grain size has little effect on the final one, and the grain growth depends mainly on heat input and peak temperature as well as growth activation energy and exponent. With the use of the model, the width of coarse grained heat affected zone (CGHAZ) for a thick plate is predicted.

Influence of Strain Rate on Hot Ductility of a V-Microalloyed Steel Slab

Abstract: The hot ductility and malleability of a vanadium-microalloyed steel is investigated by means of tensile and compression tests at temperatures ranging from 700 to 850°C and strain rates of 3 × 10−4 to 0.3 s−1. The deformation tests are performed after austenitization and cooling to test temperature. The so-called second ductility minimum is located around 750°C for all strain rates except for the highest one, where no ductility trough is observed. Ductility steadily increases with strain rate at a given temperature, and the fracture mode progressively changes from intergranular to transgranular. In the region of minimum ductility, intergranular cracking occurs at low strain rates by void nucleation, growth and coalescence within thin layers of deformation induced ferrite covering the austenite grain boundaries. Cracking is favoured by V(C,N) precipitation associated with the γ/α phase transformation. Ductility remains low above the temperature of minimum ductility, where no apparent ferrite formation is observed (790 °C). Void formation takes place as a result of grain boundary sliding in combination with matrix and grain boundary precipitation. These voids are able to grow and link up forming intergranular cracks. Ductility increases with strain rate mainly due to the short time available for precipitation as well as for intergranular void growth and coalescence.

Particle-Size-Grouping Model of Precipitation Kinetics in Microalloyed Steels

Abstract: The formation, growth, and size distribution of precipitates greatly affects the microstructure and properties of microalloyed steels. Computational particle-size-grouping (PSG) kinetic models based on population balances are developed to simulate precipitate particle growth resulting from collision and diffusion mechanisms. First, the generalized PSG method for collision is explained clearly and verified. Then, a new PSG method is proposed to model diffusion-controlled precipitate nucleation, growth, and coarsening with complete mass conservation and no fitting parameters. Compared with the original population-balance models, this PSG method saves significant computation and preserves enough accuracy to model a realistic range of particle sizes. Finally, the new PSG method is combined with an equilibrium phase fraction model for plain carbon steels and is applied to simulate the precipitated fraction of aluminum nitride and the size distribution of niobium carbide during isothermal aging processes. Good matches are found with experimental measurements, suggesting that the new PSG method offers a promising framework for the future development of realistic models of precipitation.

Effect of cooling rate after controlled forging on properties of low carbon multi-microalloyed steels

Abstract: Two low carbon steel grades were used in the present investigation. One of them was microalloyed with Ti, V, and Nb. Both steel grades were subjected to a controlled hot forging followed by either cooling in air or quenching water. The microstructures of all TMT conditions are dominated by ferrite phase with different morphologies and grain sizes according to both chemical composition and cooling rate. Polygonal ferrite is considered to be a dominated phase of air cooled microstructures for both steel grades that is responsible for decreasing the hardness, yield, and tensile strength with the attendant increase in ductility. Water quenching leads to a formation of relatively fine polygonal ferrite in low carbon steel or transformation into acicular ferrite in low carbon microalloyed steel. Relatively fine polygonal ferrite and acicular ferrite increase strength but decrease ductility. The cooling rate has a negligible effect on the impact toughness at room temperature.

Vanadium and titanium compound microalloyed steel bar and production method thereof

Abstract: The invention discloses a vanadium and titanium compound microalloyed steel bar and a production method thereof. The steel in the steel bar comprises the following chemical components in percentage by weight: 0.16 to 0.25 percent of C, 0.20 to 0.80 percent of Si, 1.20 to 1.50 percent of Mn, 0.001 to 0.12 percent of Ti, 0.001 to 0.10 percent of V, less than or equal to 0.01 percent of N, less than or equal to 0.045 percent of S, less than or equal to 0.045 percent of P, and the balance of Fe and inevitable impurities. The production method for the steel bar comprises the following steps of: a, smelting crude molten steel, tapping, deoxidizing and alloying; b, refining, trimming the components to make the components meet the component requirement of the steel bar, and casting; and c, performing hot rolling, wherein alloying vanadium and titanium after deoxidizing in the process of tapping, or in the process of refining. According to the vanadium and titanium compound microalloyed steel bar, the yield strength and the tensile strength of the steel are improved under the condition of not changing the ductility of the steel, and the performance requirements of the steel bars in 400 MPa or 500 MPa of grades are met.

Influence of plastic deformation on CCT-diagrams of new-developed microalloyed steel

Abstract: Purpose: The aim of the paper is to investigate the influence of plastic deformation and cooling conditions on a structure and a shape of CCT-diagrams of new-developed Nb-Ti-V microalloyed steel. Design/methodology/approach: The diagrams of undeformed and plastically-deformed supercooled austenite transformations for Nb-Ti-V microalloyed steel were determined. A part of the specimens were austenitized at a temperature of 885°C and next cooled to ambient temperature with a various rate from 234°C/s to 1°C/min. To investigate the influence of plastic deformation on a shape of CCT (Continuous Cooling Transformations) diagrams, another part of the specimens were 50% deformed at 885°C or 1100°C and cooled to ambient temperature with a rate from 95°C/s to 1°C/min. The DIL 805A/D dilatometer, with a LVDT-type measuring head, was used to carry out dilatometric test. Findings: Performed dilatometric research revealed that the steel is characterized with Ac3=843°C, Ac1=707°C and a relatively low Ms temperature equal 370°C. Plastic deformation of steel at the temperature of 885°C prior to the start of phase transformations results in distinct acceleration of pearlitic transformation and slight translation of bainitic transformation towards shorter times. Research limitations/implications: Elaborated curves of supercooled austenite transformations of studied steel fully predispose it to production of forgings quenched directly from forging finish temperature and successively subjected to high temperature tempering. Practical implications: The obtained CCT diagrams of supercooled plastically-deformed austenite transformations can be useful in determination of cooling condition of the thermo-mechanical processing for high strength forged machine parts obtained from microalloyed steels. Originality/value: The diagrams of the plastically-deformed supercooled austenite for a new-developed microalloyed steel were obtained.

Grain boundary segregation of minor arsenic and nitrogen at elevated temperatures in a microalloyed steel

Abstract: Auger electron spectroscopy (AES) was used to investigate the grain boundary segregation of arsenic and nitrogen in a kind of microalloyed steel produced by a compact strip production (CSP) technology at 950 to 1100°C, which are similar to the hot working temperature of the steel on a CSP production line. It was discovered that arsenic segregated on grain boundaries when the steel was annealed at 950°C for 2 h. When the annealing temperature increased to 1100°C, arsenic was also found to have segregated on grain boundaries in the early annealing stage, for instance, within the first 5 min annealing time. However, if the holding time of the steel at this temperature increased to 2 h, arsenic diffused away from grain boundaries into the matrix again. Nitrogen was not found to have segregated on grain boundaries when the steel was annealed at a relatively low temperature, such as 950°C. But when the annealing temperature increased to 1100°C, nitrogen was detected to have segregated at grain boundaries in the steel.

Modelling microstructure evolution and work hardening in conventional and ultrafine-grained microalloyed steels

Abstract: Aspects of the mechanical behaviour of microalloyed steels characterised by grain refinement due to large plastic deformation are discussed. The studies presented in this chapter are intended to promote understanding of the mechanical behaviour of multiphase and ultrafine-grained materials related to the refinement of the microstructure, particularly taking into account the effects of microalloying. Several levels of grain refinement are identified with respect to changes in the strengthening mechanisms. Experimental results have been used to calibrate constitutive equations describing plastic flow mechanics. These equations have been modified and systematically ordered for ultrafine-grained microalloyed steels. Experimental results supported by multiscale numerical analyses have allowed the development of a rheological model for ultrafine-grained microalloyed steels.

The Bauschinger effect and the formation of microalloyed-steel properties in pipe manufacture

Abstract: The Bauschinger effect is investigated on samples of microalloyed pipe steel. Tests are conducted with one and several extension-compression cycles on a Gleeble-3800 system, at room temperature and elevated temperatures. The influence of temperature on the final sheet properties is considered in conditions of small alternating deformation, so as to simulate the straightening of hot-rolled sheet. The successive influence of each cycle on the final properties of the metal is demonstrated. The strength of the final strip may be modified by 150 MPa, depending on the selected straightening temperature.

EFFECT OF CONTROLLED ROLLING PROCESSING ON NANOMETER–SIZED CARBONITRIDE OF Ti–Mo FERRITE MATRIX MICROALLOYED STEEL

Abstract: Single nanometer-sized particles, which are smaller than 10 nm, can significantly en- hance the precipitation strengthening in microalloyed steels, thus causing their strength to be promoted greatly. In order to improve the strength of the steel, it is quite necessary to get a large amount of single nanometer-sized particles through optimizing rolling technology. In this work, the effects of two different kinds of controlled rolling technologies on the size and distribution of precipitated particles in the Ti-Mo ferritie matrix microalloyed steel have been researched using SEM, TEM and small-angle X-ray scattering. The results show that with the same total rolling reduction, the steel rolled only in phase crystallization zone can obtain a higher portion of single nanometer-sized particles than that rolled respectively in phase recrystallization and nonrecrystallization zones, in which those single nanometer-sized particles account for about 75% (mass fraction) of whole precipitated particles. In order to study the effect of deformation potency in phase zone on the amount of precipitates in phase and the micro-crystal size, nucleation rate and incubation time of following precipitates in ! � transformation and ferritie matrix after ! � transformation, some thermodynamics and kinetics calculations and analysis on precipitation are also conducted.

The effect of size of Cu precipitation on the mechanical properties of microalloyed steel

Abstract: The effect of size of nanoscale Cu precipitate on the mechanical response of microalloyed steel was investigated computationally and experimentally. A phenomenological constitutive description is adopted to build the computational crystal model. The material is envisaged as a composite; the Cu precipitate is modeled as a monocrystalline core surrounded with a lower yield stress and higher work hardening rate response. Both a quasi-isotropic and crystal plasticity approaches are used to simulate the matrix. The nanoscale precipitate is modeled as ellipsoidal inclusion with different Yound's modulus to matrix. Elastic and plastic anisotropy are incorporated into this simulation. An implicit Lagrangian finite element formulation with von Mises plasticity or rate dependent crystal plasticity is used to study the nonuniform deformation and localized plastic flow. The computational predictions are compared with the experimentally determined mechanical response of HSLA-100 steel with average size of nanoscale precipitates of 2.02±1.89 nm. The tendency of the calculated yield strength attributed to Cu precipitates is in good agreement with experimental result.

80Kg grade steel plate with ultrahigh toughness and extreme thickness and manufacturing method of the steel plate

Abstract: The invention discloses a 80Kg grade steel plate with ultrahigh toughness and extreme thickness and a manufacturing method of the steel plate. The steel plate comprises the following components in percentage by weight: 0.08-0.13% of C, Si smaller than or equal to 0.10%, 0.80-1.20% of Mn, P smaller than or equal to 0.013%, S smaller than or equal to 0.0030%, 0.20-0.45% of Cu, 1.00-1.60% of Ni, 0.35-0.65% of Cr, 0.30-0.60% of Mo, 0.040-0.070% of Als, 0.010-0.030% of Nb, 0.030-0.060% of V, 0.004-0.010% of Ti, N smaller than or equal to 0.0050%, 0.001-0.004% of Ca, 0.0008-0.0016% of B, and balance of Fe. According to the manufacturing method, a component system of low C-ultralow Si-medium Mn-(Ti+V+Nb+B) microalloyed steel is used as a basis, special rolling control and offline gradient tempering technology (QQ+T) are optimized, so that microstructures of the produced steel plate are small tempered bainite+ tempered martensite, the average crystal colony size is less than 20 microns, excellent obdurability and plasticity match is obtained, and mechanical performance in the direction of the thickness of the steel plate is uniform.

Effect of particles on the dynamic recrystallization behavior of Al–V–N micro-alloyed medium carbon steel

Abstract: Using a Gleeble hot-deformation simulator, we have studied the dynamic recrystallization (DRX) behavior of Al–V–N micro-alloyed steel subjected to two different cooling conditions, i.e., air-cooling (AC) and water-cooling (WC), after solution treatment. The different prior-thermal treatment resulted in distinctively different DRX behavior of the steel. The DRX at a slow strain rate (0.01 s −1 ) was significantly retarded by dynamic precipitation of fine VCN particles; in contrast, the DRX was accelerated at a fast strain rate (5 s −1 ) due to coarse AlN particles, which were formed by rapid coarsening of static AlN particles during fast deformation. The activation energy for DRX was 420–480 kJ/mol, which was significantly larger than that of V-microalloyed steel. The peak and critical strains significantly deviated from the conventional exponential function of the temperature corrected strain rate. The grain size was refined from 130 μm to 5–60 μm by DRX and the dynamic precipitation appeared to be effective for grain size refinement.

Effects of Grain Refinement on the Corrosion Behaviour of Microalloyed Steel in Sulphuric Acid Solutions

Abstract: Microalloyed steels have been widely used in many applications, particularly in car bodies and other engineering parts. This is because these steels are characterized by their high strength and high ductility. The combination of strength and ductility for these steels are usually resulting from the very fine grained microstructure. In the current work, the effects of grain refinement on the corrosion resistance property of BSK 46 microalloyed steel in 1N H2SO4 solution under different microstructural conditions were reported. The repeated quenching of the BSK 46 steels was found to increase the refining in their microstructures. Effects of changing temperature on the corrosion properties of the steel were also investigated at three different temperatures namely, 20 0 C, 30 0 C and 40 0 C. The corrosion behaviour of the steel was investigated using galvanostatic polarization. It was found that repeating the quenching of steel, grains become finer and corrosion rate increases suggesting that a compromise has to strike between high mechanical property and corrosion rate.

Effects of Nb, Ti and V on recrystallization kinetics of austenite in microalloyed steels

Abstract: Purpose: The work presents research results of impact of Nb, Ti and V microadditions on flow stress, recrystallization kinetics and microstructure of newly elaborated steels assigned for production of forged machine parts, using the method of thermo-mechanical treatment. Design/methodology/approach: The study was performed with the use of Gleeble 3800 simulator. Stressstrain curves were determined during continuous compression test in a temperature range from 900 to 1100°C and at a strain rate of 1, 10 and 50 s-1. In order to determine recrystallization kinetics of plastically deformed austenite, discontinuous compression tests of specimens were done with a given strain at the rate of 10 s-1, in a temperature range from 900 to 1100°C, with isothermal holding of samples between successive stages of deformation for 2 to 100 s. Recrystallization kinetics of plastically deformed austenite was described using the Johnson-Mehl-Avrami equation. The observations of microstructures of thin foils were done using JEOL JEM 3010 transmission electron microscope. Findings: Basing on the analysis of the form and the course of curves obtained in the compression test, it was found that in the studied range of parameters of hot plastic deformation, the decrease of strain hardening of studied steels is caused by the process of continuous dynamic recrystallization. This is also confirmed by calculation results of activation energy of plastic deformation process. Performed two-stages compression tests revealed that microadditions introduced into steel considerably influence the kinetics of static recrystallization. Research limitations/implications: It was found that the time necessary for a total course of recrystallization of austenite is too long to be accepted in the production process of forgings. Practical implications: Executed hot compression tests will contribute to establishing conditions of forging with the method of thermo-mechanical treatment. Originality/value: Strain-stress curves and recrystallization kinetics curves of newly elaborated microalloyed steels have been determined.

Numerical simulation of the evolution of primary and secondary Nb(CN), Ti(CN) and AlN in Nb-microalloyed steel during continuous casting

Abstract: The precipitation of carbides and nitrides in Nb-microalloyed steel is investigated experimentally and theoretically using light optical, scanning and transmission electron microscopy, and the thermokinetic software MatCalc. The simulations are based on a recently developed two-step methodology for precipitation simulation in primary solidification microstructures, where the compositional inhomogeneities from microsegregation are taken into account. The computed segregation is clearly evidenced in electron probe microanalyses of the as-cast microstructure. Based on the results of a Scheil–Gulliver simulation, precipitation kinetics simulations are performed with the chemical compositions corresponding either to the solute-rich interdendritic zone or the solute-depleted dendrite core zone. The predicted phase fractions, mean radii, number densities, and compositions of the precipitates are in good agreement with the experimental investigations.

An analytic approach to the effect of anisotropic growth on diffusion-controlled transformation kinetics

Abstract: The effects of the anisotropic growth and soft-impingement (SI) on the kinetics of diffusion-controlled (DC) transformation are investigated. An analytic approach to describe the kinetics of DC transformation subjected to anisotropic effect is presented, assuming pre-existing nuclei, one-dimensional growth, and linear approximation of concentration gradient. The anisotropic effect and SI effect are evaluated from the varying Avrami exponent. SI effect only occurs at the last stage of transformation, although anisotropic effect broadens its range. This approach is applied to predict the isothermal thickening of ferrite layer in Fe–0.17 wt% C alloy at 973 K and isothermal transformation of austenite to allotriomorphic ferrite in 0.37C–1.45Mn–0.11V microalloyed steel at 913 K; good agreements are achieved.