Showing papers on "Microalloyed steel published in 2020"

The Interdependence of the Degree of Precipitation and Dislocation Density during the Thermomechanical Treatment of Microalloyed Niobium Steel

Abstract: In this paper, thermomechanical processing of niobium microalloyed steel was performed with the purpose of determining the interaction between niobium precipitates and dislocations, as well as determining the influence of the temperature of final deformation on the degree of precipitation and dislocation density. Two variants of thermomechanical processing with different final rolling temperatures were carried out. Samples were studied using electrochemical isolation with an atomic absorption spectrometer, transmission electron microscopy, X-ray diffraction analysis, and universal tensile testing with a thermographic camera. The results show that the increase in the density of dislocations before the onset of intense precipitation is insignificant because the recrystallization process takes place simultaneously. It increases with the onset of strain-induced precipitation. In this paper, it is shown that niobium precipitates determine the density of dislocations. The appearance of Luders bands was noticed as a consequence of the interaction between niobium precipitates and dislocations during the subsequent cold deformation. In both variants of the industrial process performed on the cold deformed strip, Luders bands appeared.

Strain-induced precipitation in Ti microalloyed steel by two-stage controlled rolling process

Abstract: Strain-induced precipitation kinetics and precipitates' characteristics in a Ti microalloyed steel subjected to a two-stage controlled rolling process, simulating rough and finish rolling process of industrial production, were quantitatively investigated by stress relaxation experiments and transmission electron microscopy (TEM), as well as inductively coupled plasma-atomic emission spectroscopy (ICP-AES). In the present work, precipitation during the cooling stage after rough rolling was taken into consideration for the first time. It was found that a small amount of Ti-bearing precipitates, consuming about 10% of the elemental Ti, were formed on dislocations after rough rolling and cooling to 900 °C. The obtained precipitation-time-temperature (PTT) curves exhibited a classic C shape with a nose temperature of 900 °C and the shortest incubation time of 60 s. The PTT curves moved to the lower left by the introduction of 20% deformation at 1050 °C, which was attributed to more nucleation sites for strain-induced precipitation and the decrease of Ti concentration in deformed austenite. During the stress relaxation stage, strain-induced precipitates preferentially nucleated on dislocations and sub-boundaries and were identified as TiC particles. The mean size of precipitated TiC particles increased from 10.2 ± 2.1 to 25.2 ± 2.8 nm, as the holding time was increased from 100 to 1800 s at 900 °C. When the holding time exceeded 600 s the migration of austenite grain boundaries could not be completely inhibited due to the coarsening of precipitates. Isothermal treatment at 900 °C with a holding time of 60–100 s is suggested here as a viable combination for the processing of the 0.05C-0.21Si-1.05Mn-0.13Ti (wt. %) steel due to the shortest incubation time period and effective pinning of grain boundaries by strain-induced precipitates.

Quantitative Analysis of Microstructures and Strength of Nb-Ti Microalloyed Steel with Different Ti Additions

Abstract: A quantitative analysis was carried out in the present study to determine the effects of titanium (Ti) addition on microstructures and strength of Nb-Ti microalloyed steel. The obtained results revealed that strength was significantly improved with an increase in Ti content from 0.041 to 0.079 wt pct. The difference in the yield strength between the two steel samples occurred due to the different strengthening effects of grain refinement, precipitation, and dislocation strengthening, among which the grain refinement and precipitation strengthening contributions were dominating. With a further increase in the Ti content, ferrite grains became refined. Consequently, a homogeneous ferrite microstructure was attained for high Ti contents. Moreover, large-sized (Ti, Nb)C particles manifested the Kurdjumov–Sachs (KS) relationship, whereas fine (Ti, Nb)C particles held the Baker–Nutting (BN) relationship; thus, abundant fine nanoscale (Ti, Nb)C particles formed after coiling. Furthermore, the high dislocation density facilitated the precipitation of (Ti, Nb)C particles along dislocation lines.

Microstructure of intercritical heat affected zone and toughness of microalloyed steel laser welds

Abstract: Microstructure of laser welds of the X70 low-carbon pipe steel was studied. High cooling rates after laser welding and non-uniform distribution of carbon in the ferrite-pearlite base metal caused formation of regions with increased microhardness (up to 650 НV) in inter-critical heat affected zone (ICHAZ). These regions consisted of finely dispersed degenerate upper bainite and martensite-austenite constituents of a slender shape and small fraction of a massive shape along the boundaries of bainite laths, as well as twinned martensite. High concentration of martensite-austenite constituents (10–16%) and residual stresses in ICHAZ, as well as a dendritic martensitic structure with carbide interlayers along the boundaries of martensite laths in fusion zone were the main reasons of sharp decrease in charpy impact energy of the welded samples. High microhardness of the laser welds was decreased down to 320 HV and their brittleness was improved by annealing. Also, in ICHAZ, degenerate upper bainite and the regions of martensite-austenite constituents decayed forming tempered sorbite and Fe2C and Fe3C carbides, respectively. Charpy impact energy of the welds doubled after annealing compared to the welds without annealing, and ductile-brittle transition temperature decreased down to –60°С.

In Situ Observation of the MnS Precipitation Behavior in High-Sulfur Microalloyed Steel Under Different Cooling Rates

Abstract: In situ observation of the precipitation behavior of MnS in a high-sulfur microalloyed steel has been conducted by means of a confocal laser scanning microscopy (CLSM) with cooling rates increased from 50 to 400 °C/min. Differential scanning calorimetry analysis and thermodynamic calculation were carried out prior to and following the CLSM experiments using scanning electron microscopy (SEM) and X-ray energy-dispersive spectrometer. The results suggested that the initial MnS precipitate temperature decreased from 1440.0 °C to 1429.6 °C and the final temperature reduced from 1413.7 °C to 1381.6 °C as the cooling rate increased from 50 to 400 °C/min in the CLSM test. Three typical MnS morphologies of small angular, globular and larger dendrite cluster MnS were observed in the melting surface. Furthermore, with an increase in the supersaturation of MnS and a significant reduction of the local solidification time when the cooling rate increased from 50 to 400 °C/min, the large clusters MnS at interdendritic regions became thinner and more small angular MnS precipitates formed at dendrite crystals.

Enhanced mechanical properties by retained austenite in medium–carbon Si-rich microalloyed steel treated by quenching–tempering, austempering and austempering–tempering processes

Abstract: The microstructure evolution and mechanical properties of a designed 0.56C-1.48Si-0.70Mn-0.71Cr-0.148V-0.011Nb (wt.%) steel subjected to three heat treatments including quenching–tempering, austempering, and austempering–tempering have been investigated. In the quenching–tempering sample, the microstructure of the steel consisted of tempered martensite and a small amount of retained austenite (~8 vol%) with 1.3 wt% C. In the austempering sample, the microstructure contained martensite/bainite laths and adequate amount of retained austenite (~15 vol%) with 1.33 wt% C. Subsequent tempering treatment promoted MC carbides to form and retained austenite to decompose, respectively. Resulting in decrease in the retained austenite amount (~10 vol%) and C concentration (~1.24 wt%). The carbon distribution indicated the stability of the C-rich area along prior austenite grain boundary is higher than that between the martensite/bainite laths. As a result, the present steel shown an ultrahigh ultimate tensile strength (>2200 MPa) and an excellent total elongation (~15%) after austempering treatment. Also, the impact toughness increases from ~12.5 J (quenching–tempering) to ~16 J (austempering) and ~17.5 J (austempering–tempering). As a result of transformation-induced plasticity (TRIP) effect, the austempering sample had the highest values of strain hardening rate and strain hardening exponent in the uniform strain stage and necking stage, thus it obtained a maximum uniform true strain of ~0.087. In addition, the crack source analysis of the three samples all shown ductile dimpled fracture while crack propagation presented as a quasi-cleavage fracture.

The Effect of Nickel on the Microstructure, Mechanical Properties and Corrosion Properties of Niobium-Vanadium Microalloyed Powder Metallurgy Steels.

Abstract: In this study, the effects of adding Ni in different ratios to Fe-matrix material containing C-Nb-V produced by powder metallurgy on microstructure, tensile strength, hardness and corrosion behaviors were investigated. Fe-C and Fe-C-Nb-V powders containing 5%, 10%, 13%, 15%, 20%, 30% and 40% nickel were pressed at 700 MPa and then sintered in an Ar atmosphere at 1400 °C. Microstructures of the samples were characterized with optical microscope, scanning electron microscope (SEM) and XRD. Corrosion behaviors were investigated by obtaining Tafel curves in an aqueous solution containing 3.5% NaCl. Mechanical properties were determined by hardness and tensile testing. While Fe-C alloy and Fe-C-Nb-V microalloyed steel without Ni typically have a ferrite-pearlite microstructure, the austenite phase has been observed in the microstructures of the alloys with 10% nickel and further. Yield and tensile strength increased with nickel content and reached the highest strength values with 13% Ni content. The addition of more nickel led to decrease the strength. Analysis of Tafel curves showed that corrosion resistance of alloys increased with increasing nickel concentration.

Microstructural characteristics and impact fracture behaviors of low-carbon vanadium-microalloyed steel with different nitrogen contents

Abstract: The influence of nitrogen content between 0.0032 and 0.0081 wt % on the microstructures and the impact toughness of low-C V-microalloyed steel was investigated. The isothermal transformation experiments in the medium temperature were performed at Gleeble-3800. The microstructures and impact fracture behaviours were obtained through a series of characterization means and the mechanism were investigated. Results indicated that increasing the nitrogen content created a larger number of finer precipitated (Ti,V) (C,N) particles as well as micro-sized particles. The former decreased the average diameter of prior austenite grains (PAGs) and the latter promoted the nucleation of intragranular acicular ferrite (AF), which led to an increased amount of AF and a reduced mean equivalent diameter (MED) of ferrite grains. Moreover, the increased nitrogen content increased the amount of fine martensite-austenite (M/A) constituents and tended to change the internal substructure of M/A constituent from twin-type to lath-type. Thereby these variations in microstructure resulted in the decrease in 50% fracture appearance transition temperature (50% FATT), and alterations in the fracture behavior in the upper shelf region and lower shelf region. Moreover, the increasing N content led to changes in the mechanism of crack initiation from micro-cleavage to microvoid, and the mechanism of crack propagation from predominantly transgranular cleavage to predominantly intragranular microvoid coalescence at the test temperature of -20 °C. As a result, the initiation and propagation energy increased significantly with the increased nitrogen content.

Improvement of fracture toughness of Ti+Nb stabilized microalloyed and interstitial free steels processed through single phase regime control multiaxial forging

Abstract: Objective of the current study is to enhance the mechanical properties, with a special emphasis on fracture toughness, of Ti + Nb stabilized interstitial free and microalloyed steels through microstructural modification by single-phase controlled multiaxial forging at large cumulative strains. Analysis of fracture toughness was executed through calculating KQ (conditional fracture toughness), Kee (equivalent energy fracture toughness) and J-integral (crack initiation energy) values from single-edge bend test data of the forged specimens. The effect of strain hardening rate and strain hardening exponent on deformation behavior were examined to correlate the yield strength (YS) and uniform elongation. Also, theoretically calculated YS (obtained from analysis of strengthening mechanisms) was correlated well with the experimentally obtained results. The quantitative measurement of grain size, low- and high-angle grain boundaries and their distribution in the deformed state were investigated through EBSD/TEM analysis. Superior combinations of the YS, ductility (%El.) and fracture toughness were obtained through intercritical (α+γ) phase regime (~Ar1) control 15 cycles multiaxially forged (MAFed) microalloyed steel (YS = 1027 MPa, %El. = 8.3% and Kee = 90 MPa√m) and pure α-ferritic region (

Study on the precipitation and coarsening of TiN inclusions in Ti-microalloyed steel by a modified coupling model

Abstract: A modified coupling model, considering microsegregation, precipitation and growth of inclusion, and variable solute partition coefficient (ki), was established to investigate the precipitation and coarsening of TiN inclusion for the 0.053 wt% Ti-microalloyed steel. The influence of TiN inclusion on the steel impact toughness was revealed. Results show that the effect of phase composition on both the kTi and kN is greater than the effect of temperature, and the kTi and kN are in the range from 0.20 to 0.30 and from 0.28 to 0.45 during solidification, respectively. The change of the ki in different crystalline phases significantly affects the microsegregation and TiN precipitation. The TiN precipitation is promoted by segregation and the TiN starts to precipitate around 1754 K (corresponding to fs = 0.82) in the mushy zone. The predicted size of TiN inclusion is between 7.57 and 16.68 μm at a conventional cooling rate (1–5 K/s), which is consistent with the measured results in the slab. The square or triangle TiN inclusions with large size are harmful to the impact toughness. Many TiN inclusions can be found at the junction of the cleavage cracks. To control the TiN size and ensure the impact toughness, the cooling rate for the Ti-0.053 wt% steel solidification should be enhanced and greater than 10 K/s. Furthermore, reducing the Ti content to 0.03 wt% is a more effective way to control the TiN size.

Corrosion Behavior and Mechanical Properties of AISI 316 Stainless Steel Clad Q235 Plate

Abstract: This paper deals with carbon steel and stainless steel clad-plate properties. Cladding is performed by the submerged-arc welding (SAW) overlay process. Due to element diffusion (Fe, Cr, Ni, and Mn), a 1.5 mm wide diffusion layer is formed between the stainless steel and carbon steel interface of the cladded plate affecting corrosion resistance. Pitting resistance is evaluated by measuring the critical-pitting temperature (CPT), as described in the American Society for Testing and Materials (ASTM) G-48 standard test. Additionally, Huey immersion tests, in accordance with ASTM A262, Type C, are carried out to evaluate the intergranular corrosion resistance. Some hardness peaks are detected in microalloyed steel close to the molten interface line in the coarse-grained heat-affected zone (CGHAZ). Results show that stress-relieving treatments are not sufficient to avoid hardness peaks. The hardness peaks in the CGHAZ of the microalloyed steel disappear after quenching and tempering (Q and T).

Microstructure and mechanical properties of laser surface treated 44MnSiVS6 microalloyed steel

Abstract: Fatigue property improvement for automotive components such as crankshafts can be achieved through material selection and tailored surface design. Microalloyed steels are of high interest for automotive applications due to their balanced properties, excellent hardenability and good machinability. Lasers facilitate efficient and precise surface processing and understanding the laser-material-property interrelationships is the key to process optimisation. This work examines microstructural development during laser surface treatment of 44MnSiVS6 microalloyed steel and the r esulting mechanical properties. Laser beam shaping techniques are employed to evaluate the impact of beam shaping on the process. It revealed that ferrite structures remain in the treated area surrounded by martensite due to insufficient heating and dwell time of carbon diffusion.

The effect of the initial microstructure of the X70 low-carbon microalloyed steel on the heat affected zone formation and the mechanical properties of laser welded joints

Abstract: In this paper, the heat affected zone (HAZ) of laser welded joints of the X70 steel were studied by the transmission electron microscopy method. The effect of the initial microstructure (coarse-grained hot-rolled and fine-grained after cross-helical rolling) on the HAZ formation and the mechanical characteristics of the welded joints were shown. It was found that the microstructure in the inter-critical HAZ of the steel after cross-helical rolling was more dispersed, homogeneous, and uniform compared to that of the coarse-grained hot-rolled one due to the initial fine-grained ferrite-bainitic-pearlite microstructure and the absence of pronounced ferrite-pearlite banding in the base metal. The character of the microhardness value distribution in the HAZ of the steel after cross-helical rolling was smooth with the gradual decrease from 370 down to 185 HV as shifted towards the base metal. In the HAZ of the coarse-grained hot-rolled steel, the heterogeneous microhardness value (up to 640–670 НV) distribution was revealed. The reason was the upper degenerate bainite microstructure with high residual stresses, characterized by laths up to 2.0–2.5 μm long and a high martensitic-austenitic constituent fraction (10–16%) of a slender shape along the boundaries of bainite laths. The conclusion was drawn that one of the ways to reduce the brittleness of the laser welded joints could be using the initially fine-grained steels possessing the homogeneous (mainly bainitic) microstructure.

A comparative study on hot deformation behaviours of low-carbon and medium-carbon vanadium microalloyed steels

Abstract: Carbon is an essential element in steel, but there are still discrepancies regarding its effect on steel hot deformation behaviours. In this research, isothermal compression tests were carried out for a low-carbon (0.05 C) and a medium-carbon (0.38 C) vanadium microalloyed steel with deformation temperatures of 900−1050 °C and strain rates of 0.01−30 s−1. It was found that carbon causes a softening effect at low strain rates (0.01−1.0 s−1), while a hardening effect at high strain rates (10.0−30 s−1). Through constitutive analysis, the hot deformation activation energy for 0.05 C steel is 305.9 kJ/mol in the whole strain rate range, while for 0.38 C steel, the activation energy is 292.3 kJ/mol in the low strain rate range (0.01−1 s−1) and 475.0 kJ/mol in the high strain rate range (10−30 s−1). It was proposed that the addition of carbon decreases the deformation activation energy at low strain rates (0.01−1 s−1), due to its positive influence on the self-diffusion coefficient of iron, which increases the rates of dislocation climb and recovery. On the other hand, carbon lowers the stacking fault energy of austenite, which could make partial dislocation collapse more difficult than dislocation climb and thus becomes the rate-controlling mechanism for 0.38 C steel at high strain rates (10−30 s−1). This gives rise to the higher activation energy and work hardening rate of 0.38C steel at high strain rates. Comparing the power-dissipation-efficiency maps, two peak domains were found in those of 0.38 C steel, while only one peak domain exists in those of 0.05 C steel.

Austenitizing Temperature and Cooling Rate Effects on the Martensitic Transformation in a Microalloyed-Steel

Abstract: The effects of the austenitizing temperature and the cooling rate upon the kinetic of athermal martensitic transformation in a microalloyed steel were evaluated. Considering the studied steel, the knowledge about these effects on the martensitic transformation has a great relevance for naval manufacturers and steel researchers. In this study, computational simulation was performed aiming to evaluate the phase’s stability. Specimens were submitted to quenching simulations in a dilatometer, considering four different austenitizing temperatures and four cooling rates. The results shown that the austenite chemical composition was not significantly affected by the austenitizing temperatures. Both the austenitic grain size and the cooling rate affected the martensitic transformation kinetics. The larger the austenitic grain size, the higher the Ms. The austenitic grain growth promoted a decrease in the required chemical energy to compensate the free energy increase associated with the lattice strain and the creation of new interfaces, leading to a lower austenite undercooling. An extrinsic effect of the cooling rate on the Ms was observed. For lower cooling rates, the carbide precipitation modified que austenite chemical composition, changing its stability and increasing Ms. A predictability equation, correlating the MS with the austenite grain size and the steel cooling rate, was proposed.

Effect of Alloy Elements in Time Temperature Transformation Diagrams of Railway Wheels

Abstract: The Heavy-Haul railroad wheels started to use higher wear resistance steels microalloyed with niobium, vanadium, and molybdenum [1]. During continuous cooling, these elements depress the temperature of the pearlite formation, producing smaller interlamellar spacing that increases the hardness of the steel, besides to favor the precipitation hardening through the formation of carbides [2, 3]. Also, they delay the formation of difusional components like pearlite and bainite during isothermal transformation. The effects of these alloy elements on microstructure during isothermal transformation were studied in this work using a Bähr 805A/D dilatometer. Three different compositions of class C railway wheels steels (two microalloyed and one, non microalloyed) were analyzed in temperatures between 200 and 700 °C. The microstructure and hardness for each isothermal treatment were obtained after the experiments. Comparing with non microalloyed steel (7C), the vanadium addition (7V steel) did not affect the beginning of diffusion-controlled reactions (pearlite and bainite), but delayed the end of these reactions, and showed separated bays for pearlite and bainite. The Nb + Mo addition delayed the beginning and the ending of pearlite and bainite formation and also showed distinct bays for them. The delays in diffusion-controlled reactions were more intense in the 7NbMo steel than in 7V steel. The V or Nb + Mo additions decreased the start temperature for martensite formation and increased the start temperature for austenite formation.

Heterogeneous Microstructure of Low-Carbon Microalloyed Steel and Mechanical Properties

Abstract: The microstructure plays a major role in the performance of metallic materials, which can be tailored through the composition and/or processing technique. In this study, a heterogeneous microstructure was developed for low-carbon microalloyed API X65 steel, the most commonly used pipeline steel for oil and gas transportation, using a heat treatment process. The heat treatment process involved intercritical heating of the steel followed by high-temperature isotheral cooling, allowing for phase transformation, as well as alloying element partitioning. The heat treatment transformed the banded ferrite–pearlite microstructure of rolled steel to a quasi-polygonal ferrite microstructure, with the sporadic presence of austenite at the grain boundaries. The quasi-polygonal ferrite was distributed in a heterogeneous form with a fine-grain shell surrounding the coarse-grained core. The heterogeneity in the microstructure, despite being single phase, led to a significant improvement in the tensile yield strength, ultimate tensile strength, ductility and toughness of the steel, with a marginal reduction in microhardness values.

Nucleation and Growth of Precipitates in a V-Microalloyed Steel According to Physical Theory and Experimental Results

Abstract: Using a theoretical model, the nucleus number and nucleation time were determined for a V‑microalloyed steel. The calculated data has made it possible to plot the nucleus number vs. temperature, nucleation critical time vs. temperature, and precipitate critical radius vs. temperature. The nucleus number was calculated by integration of the nucleation rate expression. On the other hand, an experimental study was performed and the nucleation time vs. temperature was plotted (PTT diagram), thus allowing a comparison between the theoretical values and experimental results. It has been found that the growth of precipitates during precipitation obeys a quadratic growth equation and not a cubic coalescence equation. The experimentally determined growth rate coincides with the theoretically predicted growth rate. The experimental nucleation time is longer than the calculated time due to conceptual differences.

Correlative analysis of interaction between recrystallization and precipitation during sub-critical annealing of cold-rolled low-carbon V and Ti–V bearing microalloyed steels

Abstract: In this paper a new insight into fundamentals of static recrystallization, precipitation and their interaction during sub-critical annealing of three cold-rolled low-carbon microalloyed steel grades is presented. The grades under investigation are a base grade containing V as a microalloying element, a Ti + grade containing Ti as microalloying element added into the base grade, and a Ti + Mn + grade containing additional Mn added into the Ti + grade. The cold-rolled steels are sub-critically annealed inside a muffle furnace to simulate industrial continuous annealing parameters in order to investigate the interaction between recrystallization and precipitation across transient stages of the annealing process as a function of temperature and time. The Zener pinning of precipitates and solute drag force of Mn on the recrystallization process are calculated and compared with measured values obtained from experimental studies on the recrystallization kinetics. Results suggest that the recrystallization kinetics is fastest in the base grade. For the Ti + grade, fine (

Hot Strip Mill Processing Simulations on a Ti-Mo Microalloyed Steel Using Hot Torsion Testing

Abstract: Precipitation strengthened, fully ferritic microstructures in low-carbon, microalloyed steels are used in applications requiring enhanced stretch-flange formability. This work assesses the influence of thermomechanical processing on the evolution of austenite and the associated final ferritic microstructures. Hot strip mill processing simulations were performed on a low-carbon, titanium-molybdenum microalloyed steel using hot torsion testing to investigate the effects of extensive differences in austenite strain accumulation on austenite morphology and microstructural development after isothermal transformation. The gradient of imposed shear strain with respect to radial position inherent to torsion testing was utilized to explore the influence of strain on microstructural development for a given simulation, and a tangential cross-section technique was employed to quantify the amount of shear strain that accumulated within the austenite during testing. Greater austenite shear strain accumulation resulted in greater refinement of both the prior austenite and polygonal ferrite grain sizes. Further, polygonal ferrite grain diameter distributions were narrowed, and the presence of hard, secondary phase constituents was minimized, with greater amounts of austenite strain accumulation. The results indicate that extensive austenite strain accumulation before decomposition is required to achieve desirable, ferritic microstructures.

On the Influence of Surface Hardening Treatments on Microstructure Evolution and Residual Stress in Microalloyed Medium Carbon Steel

Abstract: Tailoring surface properties is a key to superior performance of components subjected to fatigue loadings in application. Process–microstructure–property relationships have to be established to allow for optimization of techniques employed for surface treatments such as deep rolling and induction hardening. Although both techniques are employed widely in industrial application, studies examining microstructure evolution and residual stress states for a single material in a comparative manner are missing. Amongst others, this is related to the labor-intensive characterization techniques to be employed for this purpose. In order to establish pathways toward more efficient characterization approaches, the present work evaluates relations between microstructure evolution, hardness and results obtained by x-ray diffraction for a medium carbon steel treated by established surface hardening techniques. In this context, a strong correlation between hardness values and integral width distributions obtained by x-ray diffraction can be seen, while only weak correlations between hardness and residual stress measurements are existing. For in-depth microstructure analysis, high-resolution electron optical microscopy has proven to be very effective in resolving microstructural features down to the nanoscale substantiating elementary relationships. The study focuses on highly stressed fillet regions of real components, i.e., crankshaft sections. A 44MnSiVS6 microalloyed steel grade was used for measurements, representing a current standard for the crankshaft production in the automotive sector.

Ultra-high strength attributed to retardation of recrystallization during intercritical annealing in cold-rolled (V,Nb) microalloyed 5Mn steel

Abstract: The effects of V and Nb addition on the microstructure and mechanical properties of a cold-rolled and intercriticallly annealed 5Mn steel were investigated. Compared to a steel without V or Nb the recrystallization in microalloyed steel was retarded significantly from the beginning of annealing, which resulted in a 100–200 MPa increase in both yield and tensile strength while the degradation of ductility was not observed. Negligibly small change in elongation was attributed to a larger degree of carbon enrichment due to the smaller volume fraction of retained austenite and accelerated Mn transport through high density dislocations, which may well have operated to increase the austenite stability.

MnS Precipitation Behavior of High-Sulfur Microalloyed Steel Under Sub-rapid Solidification Process

Abstract: A typical high-sulfur microalloyed steel was investigated by a sub-rapid solidification process for grain refinement of the as-cast microstructure. The size and distribution characteristics of the MnS precipitates were analyzed. The variations in the dendrite morphology and secondary dendrite arm spacing (SDAS) under different cooling rates have been studied, which strongly influence the precipitation behavior of MnS. The 3D-morphology of MnS precipitates was revealed by a novel saturated picric acid deep-etching method. Most MnS precipitates with a length smaller than 5 μm were columnar or equiaxed in the corresponding dendrite zones under sub-rapid solidification conditions at cooling rates of 261 to 2484 K/s. Furthermore, an area scan analysis of the precipitates showed the number of small MnS per square millimeter with lengths lower than 3 μm decrease from 200,537 to 110,067. The percentage of large MnS with a length over 5 μm increased from 2.6 to 6.2 pct as the solidification condition changed from sub-rapid to air cooling. In addition, the size of MnS precipitate was found to depend linearly on the SDAS.

Fracture toughness, fatigue crack resistance and wear resistance of two railroad steels

Abstract: This research evaluated the microstructure and compared important mechanical properties of two steels for use in the railway sector. The main objective of the work was to verify the possibility of replacing a traditional C-Mn-Si pearlitic steel widely used in the world for application on rails, here called CS (common steel), by an also pearlitic steel with Nb and V micro-additions, rarely applied on rails, here called MS (microalloyed steel). The microstructures were characterized by means of pearlite colony size and pearlitic interlamellar spacing measurements, using light optical microscopy (LOM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The mechanical properties were evaluated by tensile tests, hardness tests, fracture toughness tests, force controlled axial fatigue tests, fatigue crack growth rate tests and microabrasion wear tests. CS presented a more refined microstructure than MS, due to differences in the thermomechanical industrial procedures. However, hardness, yield and tensile strength, and fracture toughness were similar for both steels. The main differences in the mechanical behavior were verified in the tensile ductility, fatigue crack growth resistance and wear resistance; the value for these three properties was higher for MS. Considering the main metallurgical requirements for an adequate selection of materials to be applied in the railway sector, these results show that the use of a Nb-V microalloyed steel is therefore a good option to ensure the best performance in service of the rail.

Microstructure evolution of ferrite during intercritical deformation in low carbon microalloyed steels

Abstract: Effects of strain, deformation temperature and strain rate on microstructure evolution of ferrite during intercritical deformation in a low carbon microalloyed steel have been studied via electron ...

Residual Stress Characterization by X-Ray Diffraction and Correlation with Hardness in a Class D Railroad Wheel

Abstract: This article focused on the microstructure characterization and residual stress measurements of the flange from classes D and C railway wheels (called 7D and 7C steel, respectively) to contribute with the residual stress level on new forged wheels flange area. A correlation with the hardness was conducted. The residual stress was measured in three points of the flange using the x-ray diffraction technique, and the microstructure characterization on SEM microscopy. We found the 7C steel has fine pearlite and ferrite microstructures, and 7D steel has degenerated pearlite and bainite microstructures. In the 7D steel, the compressive residual stress in the flange region was higher than in the 7C steel, which is related to the presence of bainite on the microstructure. There was a correlation between the hardness and residual stress value. The knowledge of the residual compression stress level is important for safety train wheels operation. The traction stress generated by the brake system on the wheel is attenuated by residual compression stress.