Showing papers on "Microalloyed steel published in 2022"

Improving mechanical properties in high-carbon pearlitic steels by replacing partial V with Nb

Abstract: The effects of combined addition of niobium (Nb) and vanadium (V) on the transformation, microstructure and mechanical properties of high-carbon pearlitic steels were investigated. In-situ observations on a high-temperature laser scanning confocal microscope (LSCM) were conducted to study the pearlitic transformation, the results from which indicate that single V or combined Nb and V addition led to higher transformation temperature and more pearlitic nucleation sites. But the growth rate of pearlite was significantly impeded due to the smaller diffusion coefficient of carbon, resulting in the refinement of the interlamellar spacing and nodule size of pearlite. In addition, compared to the base steel, single V addition in high carbon pearlitic steels was beneficial to the increase of strength, but it deteriorated the toughness, whereas the composited addition of Nb and V resulted in the increase of strength without the expense of the toughness. It is concluded that replacing part V with similar content of Nb is very promising since the impact toughness of the Nb + V steel is increased by 21% without the expense of the strength compared to the V-microalloyed steel. Nb and V strengthened pearlitic steels primarily due to the precipitation of M(C,N), thinner pearlite interlamellar spacing and smaller pearlite nodule size. Compared to the V steel, the improvement of toughness in Nb + V steel was mainly attributed to the smaller nodule size and more uniform grain size distribution. In summary, under the premise of similar microalloying element amount in high-carbon (∼0.77 wt%) pearlitic steels, replacing part V with Nb is beneficial to obtain the finest microstructure and the best comprehensive mechanical properties.

Microstructure evolution and carbide precipitation behavior of microalloyed TS800TB steel during hot rolling and coiling processes

Abstract: A Nb–Ti–Mo–V–Cr complex microalloying has been applied to improve the strength and formability of TS800TB microalloyed low carbon steel for automobile torsion beam applications. The effects of microalloying element and coiling temperature on microstructure evolution, carbide precipitation behavior, mechanical property and formability have been studied. Meanwhile, the influence of microalloying element and coiling temperature on carbide precipitation behavior has been analyzed from the aspects of thermodynamic and kinetic. The results suggest that the coarse ferrite grains and the low temperature transformation structures of lath ferrite and martensite are the main causes for the cold bending cracking of Nb–Ti–Mo–V–Cr bearing hot rolled plate. Microstructure characterization for carbide precipitation behavior in Nb–Ti–Mo–V–Cr bearing steel indicates that the precipitation of Nb was inhibited during hot rolling with addition of Mo, V, and Cr in steel. The precipitation kinetics shows that the fastest precipitation temperatures of Ti(C,N) and (Nb,Ti)C in austenite are 1150 °C and 920 °C, respectively. Hence the (Nb,Ti)C carbides are not fully precipitated in hot rolled plate due to the lower precipitation temperature and the fast laminar cooling after finish rolling. The fastest precipitation temperatures of (Nb,Ti)C and VC type carbides in ferrite are 640 °C and 740 °C, respectively. Therefore, the carbides containing Nb, Ti, V, Mo, and Cr elements precipitated during tempering process at the temperature between 650 °C and 730 °C. Meanwhile, the number of fine carbides with size less than 10 nm increases significantly with the onset of precipitation of carbides containing V and Mo. Since the Mo significantly enhances the nucleation rate of carbides by reducing the interfacial energy and strongly inhibits their coarsening. Carbides containing Ti, Nb, V, Mo, and Cr elements in hot rolled plate not only nucleate and precipitate on the previously precipitated Ti(C,N) and (Nb,Ti)C particles, but also nucleate and precipitate independently in the matrix during tempering. The results of this study indicate that precipitation strengthening and dislocation strengthening are two major strengthening mechanisms of microalloyed TS800TB steels. The precipitation strengthening increases with increasing tempering temperature due to the precipitation of small size carbides. However, due to the reduction of dislocation density, the overall strength decreases.

In-depth understanding of the relationship between dislocation substructure and tensile properties in a low-carbon microalloyed steel

Abstract: At present, the microstructure-tensile property relationship of low carbon microalloyed steel has been widely investigated, mainly with the effects of microstructure type and its simple morphology including size and shape etc. The correlation between dislocation substructure and tensile properties, however, seems more essential and needs in-depth analysis. In this study, four typical Mn containing (from 1.08 to 1.77 wt%) low-carbon microalloyed steels were prepared by Thermo Mechanical Control Process, and the correlations among Mn content, dislocation substructure and tensile properties were investigated. A mixed microstructure consisting of polygonal ferrite, degenerated pearlite, granular bainitic ferrite and martensite/austenite (M/A) constituents formed in each steel, with highly complex dislocation configuration and distribution. For the 1.08% Mn steel, the dislocation substructures appeared mainly in dislocation wall distributing in the ferrite matrix of degenerated pearlite, and dislocation tangle in the granular bainitic ferrite around the M/A constituents and the bainitic ferrite lath boundary. As the Mn content increased from 1.45% to 1.77%, the dislocation cells appeared and increased. Simultaneously with the rising of Mn content, the mean equivalent diameter (MED) of all the ferrite matrix decreased. The ferrite grains whose MED was defined by the misorientation tolerance angles (MTAs) from 2 to 8°, featured a variety of dislocation substructures including dislocation cell boundaries, dislocation walls, bainitic ferrite lath boundaries, and dislocation tangles. They effectively governed the yield strength due to their determined close Hall–Petch relationship. Moreover, the significant increase in the amount of M/A constituents enhanced the strain hardening capacity, leading to an elevated tensile strength. Dislocation tangles exhibiting around the M/A caused a microstrain concentration, promoted additional microvoids and deteriorated the elongation.

Flow stress modeling and microstructural characteristics of a low carbon Nb-V microalloyed steel

Abstract: Hot and warm deformation behaviors of a Nb-V added low carbon microalloyed steel have been examined in the present study. The deformation was performed in 700–1100 °C temperature range with 100 °C interval in 0.01–10 s -1 strain-rate range. The total deformation was subjected to a0.7 true strain in compression by employing a Gleeble-3800® thermomechanical simulator. The plastic flow behavior during hot and warm deformation was characterized from the analysis of generated flow curves. The flow stress decreased when the strain-rate was reduced or increased in temperature. The plastic deformation was majorly governed by the strain hardening and dynamic recovery (DRV) behavior over dynamic recrystallization (DRX). To predict the flow stress, the constitutive models equations were developed using activation energy ( Q ) and various material constants to anticipate the impact of strain-rate and deformation temperature on flow stress in ferrite+austenite and austenite phases, separately. The Q was estimated to be 367.3 kJ/mol and 411.5 kJ/mol for ferrite+austenite and austenite phases, respectively, with a respective stress exponent ( n ) value of 14.2 and 5.9. The model’s flow stress was correlated with the experimental value for both ferrite+austenite and austenite phase regions with a worthy fitting value ( R ) of 0.988 and 0.969, respectively. The micromechanical behavior of the deformed samples has been demonstrated through the correlation of the flow stress with microstructural validity. Texture analysis of the deformed samples shows that the formation of the cube component was weak in the analyzed samples indicating the formation of grains through DRV over DRX. A detailed analysis of activation energy, stress exponent and flow stress for DRV and DRX using the constitutive models suggested that the warm and hot deformation processes were governed by dislocation climb and glide mechanisms. • Strain hardening and dynamic recovery (DRV) behavior dominate over DRX. • Constitutive model equations derived using activation energy & material constants. • Serrated metal flow is attributed to cracks & deformation bands. • Formation of the cube component is weak in the analyzed samples.

Effect of Ti Addition on the Precipitation Mechanism and Precipitate Size in Nb-Microalloyed Steels

Abstract: The effect of Ti microalloying on the precipitation of NbC particles in Nb-microalloyed and Nb-Ti-microalloyed steels was investigated by scanning transmission electron microscopy. The experimental results illustrate that NbC precipitates tend to be formed via a conventional “nucleation and growth” mechanism in Ti-free steel, while the particles would be precipitated as a complex form of TiN cuboid core and NbC hemispherical cap in Ti-Nb-microalloyed steel with 0.009 wt.% Ti and 0.0046 wt.% N. Ti microalloying contributed to the refinement of the precipitate size, and an enhancement of the volume fraction of NbC particles was also found based on the experimental observations.

Effect of Deformation Sequence and Coiling Conditions on Precipitation Strengthening in High Ti–Nb-Microalloyed Steels

Abstract: Abstract In this work, multipass torsion tests followed by coiling simulations under different conditions have been performed with a reference Nb (0.03 pct) and a high Ti (0.1 pct)–Nb-microalloyed (0.03 pct) steel. In the case of the high Ti steel, estimated yield strengths close to or over 700 MPa were obtained for some of the conditions researched. However, a very significant effect of previous austenite grain size and strain accumulation on precipitation strengthening has also been observed. As a result, depending on deformation sequence and final cooling conditions, the coiling simulation temperatures that lead to the highest mechanical strength varied from 600 °C to 500 °C. The effect of increasing strain accumulation was mainly related to higher phase transformation temperatures, which led to a lower driving force for precipitation and higher microalloying element diffusivity, resulting in the formation of less and coarser precipitates.

Effect of Nanosized Precipitates on Corrosion Resistance of Nb-Microalloyed Steels

Abstract: High-strength cold-rolled low-carbon microalloyed steels are widely used in the automotive industry. Preference is generally given to microalloying with niobium, since its effect on the mechanical properties of steel is most pronounced due to both precipitation hardening and a reduction in the ferrite grain size. For the operation of a car, the corrosion resistance of metal parts is an important factor, since, along with other properties of the material, it determines its service life. The study of the effect of the structural state of cold-rolled sheet low-carbon Nb-microalloyed steels, processed in continuous annealing units, on their corrosion resistance has been carried out. Methods of optical, scanning and transmission electron microscopy, mechanical and corrosion tests were used. It is shown that one of the main structural factors that determine the corrosion resistance of rolled products is the size of nanosized NbC precipitates. The influence of the temperature parameters of hot rolling and annealing on their formation has been established. An increase in the temperatures of the hot rolling end and coiling, as well as annealing, leads to an increase in their average size in the rolled stock after annealing, which increases the corrosion resistance of the steels under consideration.

Effect of V on the Precipitation Behavior of Ti−Mo Microalloyed High-Strength Steel

Abstract: In this work, the precipitates in Ti−Mo−V steel were systematically characterized by high-resolution transmission electron microscopy (HRTEM). The thermodynamics and kinetics of precipitates in Ti−Mo and Ti−Mo−V steels were theoretically analyzed, and the effect of vanadium on the precipitation behavior was clarified. The results showed that the precipitation volume fraction of the Ti−Mo−V steel was significantly higher than that of Ti−Mo steel. The randomly dispersed precipitation and interphase precipitation (Ti, Mo, V)C particles coexisted in the Ti−Mo−V steel. When the temperature was higher than 872 °C, the addition of vanadium could increase the driving force for (Ti, Mo, V)C precipitation in austenite, resulting in an increased nucleation rate and shortened incubation period, promoting the (Ti, Mo, V)C precipitation. When the temperature was lower than 872 °C, the driving force for (Ti, Mo, V)C precipitation in austenite was lower than that for (Ti, Mo)C precipitation, and the incubation period of (Ti, Mo, V)C precipitation was increased. Moreover, it was also found that the precipitated-time-temperature curve of (Ti, Mo, V)C precipitated in the ferrite region was “C” shaped, but that of (Ti, Mo)C was “ε” shaped, and the incubation period of (Ti, Mo, V)C was significantly shorter than that of (Ti, Mo)C.

The investigation of the effect of cu addition on the Nb-V microalloyed steel produced by powder metallurgy

Abstract: In this work, the effect of Cu content on the microstructures, mechanical properties and electrical conductivity of Nb-V added microalloyed powder metallurgy (PM) steels were investigated. Microalloyed steel samples were pressed at 750 MPa and sintered at 1400oC in argon atmosphere for 1 h. The grain size and phase distribution of the microalloy steels were determined by optical microscope. The precipitates and fracture surface of samples were analyzed with the help of SEM and EDS analyses. Tensile test, hardness test and electrical conductivity measurement were carried out for the Nb-V added microalloyed steel with different Cu content. Results indicated that 10 wt.% Cu added PM microalloyed steel showed the highest values in yield strength (YS) and ultimate tensile strength (UTS). However, when the amount of Cu content increased from 10 to 15 wt.%, YS and UTS decreased. Elongation also tends to decrease with increasing Cu content. Although the electrical conductivity in general increased with the addition of Cu, a decrease in some conductivity was observed in the addition of 15 wt.% Cu.

Thermomechanical and Microstructural Analysis of the Influence of B- and Ti-Content on the Hot Ductility Behavior of Microalloyed Steels

Abstract: The effects of the combined addition of B and Ti, as well as the influence of different strain rates on the hot ductility behavior of low carbon, continuously cast, microalloyed steels were investigated in this work. Tensile tests, microstructure analyses, and thermokinetic simulations were performed with in situ melted samples. Furthermore, prior austenite grain evaluations were carried out for the two different microalloyed steels. Increasing the strain rate brought improvements to the ductility, which was more significant in the steel with the leanest composition. The steel containing more B and Ti presented a better hot ductility behavior under all conditions tested. The main causes for the improvements rely on the precipitation behavior and the austenite–ferrite phase transformation. The preferential formation of TiN instead of fine BN and AlN was seen to be beneficial to the ductility, as well as the absence of MnS. Grain boundary segregation of free B that did not form BN retarded the ferrite formation, avoiding the brittleness brought by the thin ferrite films at the austenite grain boundaries. Furthermore, it was revealed that for the steels in question, the prior austenite grains have less influence on the hot ductility behavior than the precipitates and ferrite formation.

The Role of Splitting Phenomenon under Fracture of Low-Carbon Microalloyed X80 Pipeline Steels during Multiple Charpy Impact Tests

Abstract: The ambiguity of the splitting effect on X80 low-carbon microalloyed pipeline steels’ tendency towards brittle fracture prompted an experimental study of impact toughness scattering based on multiple Charpy impact tests in a temperature range from 20 °C to −100 °C. A fractographic analysis of a large number of fractured samples was carried out. The relationships between impact toughness, deformability and splitting characteristics were studied. A number of common features of three X80 low-carbon microalloyed pipeline steel fractures were revealed. It was experimentally established that the reason for the scattering of the impact toughness values during completely ductile fracture of specimens, as well as during fracture accompanied by the splitting formation, is the local inhomogeneity of plastic properties. The higher the susceptibility to the formation of splits for a particular steel, the lower the impact toughness. Using the electron backscatter diffraction (EBSD) technique, an uneven distribution of local plasticity in the plastic zone of impact-fractured specimens was established. A comparative analysis of specimens with equal impact toughness values at different test temperatures makes it possible to identify the mechanism of negative splitting influence compensation by the increased plasticity of certain specimen.

Strain-rate dependent workability and plastic flow instability of a (Nb+V) stabilized microalloyed steel

Abstract: In the current work, the strain-rate dependent workability and flow instability has been investigated in a (Nb+V) stabilized microalloyed steel. Uniaxial compression tests were conducted in intercritical and single phase austenitic temperature domain (700–1100 °C) at several strain-rates (0.01–10 s −1 ), using a thermo-mechanical simulator (Gleeble®−3800). The results show that the flow stress increases at higher strain-rates. A good sample processing window arises at medium strain-rates (0.1–1 s −1 ); whereas, flow instability (serrations) at high (10 s −1 ) and low (0.01 s −1 ) strain-rate plastic deformations. After a detailed sample characterization, it appears that the reasons for flow instability during the hot and warm deformations are the formation of micro-cracks or void nucleation at low (0.01 s −1 ) strain-rate; whereas, flow localization and shear banding by adiabatic heating at a higher strain-rate (10 s −1 ). In both cases (0.01 and 10 s −1 ), the serration arises at periodic intervals with negative strain-rate sensitivity. At a higher strain-rate (10 s −1 ), the flow instability dominates till 1100 °C, because of relieving the deformation-induced stored energy mainly by dynamic recovery. At lower deformation temperatures (i.e., 700–800 °C), fine ferrite grains nucleate around shear bands by the diffusional transformation of austenite. The dynamic recrystallization is either absent or incomplete in agreement with the experimental determination of T nr (non-recrystallization temperature) and texture analysis. The overall flow instability characteristics in terms of temperature, strain and strain-rate substantiate well with the dynamic materials model, by the superposition of flow instability and power dissipation efficiency, on revisiting the processing map. • Strain-rate effect on workability and flow instability investigated. • Outcome suggested defects generation (e.g., flow localization) governing serrations. • Observe martensite despite that crystal defect retards martensite formation. • Instability and processing domain verified simultaneously by DMM, MDMM etc. • DRV & DRX correlated with texture analysis.

Cleavage Fracture of the Air Cooled Medium Carbon Microalloyed Forging Steels with Heterogeneous Microstructures

Abstract: Cleavage fracture of the V and Ti-V microalloyed forging steels was investigated by the four-point bending testing of the notched specimens of Griffith-Owen’s type at −196 °C, in conjunction with the finite element analysis and the fractographic examination by scanning electron microscopy. To assess the mixed microstructure consisting mostly of the acicular ferrite, alongside proeutectoid ferrite grains and pearlite, the samples were held at 1250 °C for 30 min and subsequently cooled instill air. Cleavage fracture was initiated in the matrix under the high plastic strains near the notch root of the four-point bending specimens without the participation of the second phase particles in the process. Estimated values of the effective surface energy for the V and the Ti-V microalloyed steel of 37 Jm−2 and 74 Jm−2, respectively, and the related increase of local critical fracture stress were attributed to the increased content of the acicular ferrite. It was concluded that the observed increase of the local stress for cleavage crack propagation through the matrix was due to the increased number of the high angle boundaries, but also that the acicular ferrite affects the cleavage fracture mechanism by its characteristic stress–strain response with relatively low yield strength and considerable ductility at −196 °C.

Lath martensite substructure evolution in low-carbon microalloyed steels

Abstract: Abstract Lath martensite substructures in as-quenched plain carbon steels exhibit dislocation-like contrast in the transmission electron microscope. More recent observations reported internal twins and nanoscale auto-tempered intra-lath carbides as additional lath substructures in ultra-low-C binary Fe–C steels. Modern microalloyed steels often have similar ultra-low C contents besides microalloying elements like Ti, Nb or V and, more recently, Mo, to achieve high strength, toughness and weldability. Nonetheless, little is known about the lath substructure evolution in the as-quenched state of microalloyed steels. This study investigates the hierarchical martensite substructure evolution post-quenching of microalloyed Nb and NbMo steels with 0.1 wt% C. Hierarchical microstructure characterization was done using scanning and transmission electron microscopy, and electron backscatter diffraction methods including parent grain reconstructions with MTEX. Thermokinetic simulations using MatCalc to determine the carbide evolution during auto-tempering were corroborated with site-specific transmission electron microscopy. Mo addition led to lowering of the martensite start temperature, yet the Nb steel showed a finer hierarchical microstructure. Finer laths with in-lath dislocations, short and long twins, and lath boundary decoration of carbides were found in the Nb steel. Conversely, laths in the NbMo were wider, with frequent intra-lath auto-tempered precipitates in the vicinity of dislocations, without twins.

Analysis and Modeling of Stress–Strain Curves in Microalloyed Steels Based on a Dislocation Density Evolution Model

Abstract: Microalloyed steels offer a good combination of desirable mechanical properties by fine-tuning grain growth and recrystallization dynamics while keeping the carbon content low for good weldability. In this work, the dislocation density evolution during hot rolling was correlated by materials modeling with flow curves. Single-hit compression tests at different temperatures and strain rates were performed with varying isothermal holding times prior to deformation to achieve different precipitation stages. On the basis of these experimental results, the dislocation density evolution was evaluated using a recently developed semi-empirical state-parameter model implemented in the software MatCalc. The yield stress at the beginning of the deformation σ0, the initial strain hardening rate θ0, and the saturation stress σ∞—as derived from the experimental flow curves and corresponding Kocks plots—were used for the calibration of the model. The applicability for industrial processing of many microalloyed steels was assured by calibration of the model parameters as a function of temperature and strain rate. As a result, it turned out that a single set of empirical equations was sufficient to model all investigated microalloyed steels since the plastic stresses at high temperatures did not depend on the precipitation state.

Ultrahigh strength ductile microalloyed steel with a very low yield ratio developed by quenching and partitioning heat treatment

Abstract: In this study, a microalloyed low carbon steel was subjected to quenching and partitioning (Q&P) heat treatment processes. The primary ferrite-pearlite microstructure of the steel was transformed into a bainitic microstructure containing interlath and sporadic blocks of retained austenite. The applied heat treatment process partitioned the carbon into the retained austenite to a weight percentage of 0.136. The microalloyed low-carbon steel acquired a continuous yield with high yield strength, a gigapascal level of ultimate tensile strength (i.e., ~ 1.1 GPa), and a very low yield ratio (i.e., 0.55) while retaining reasonable ductility and toughness when compared to the preheat-treated values.

Correlation between Precipitation and Recrystallisation during Stress Relaxation in Titanium Microalloyed Steel

Abstract: This study investigated the correlation between strain-induced precipitation (SIP) and static recrystallisation (SRX) in Ti microalloyed steel during stress relaxation after controlled compression. The final compression temperature strongly influenced the order of SIP and SRX and thus the evolution of the austenite structure. Precipitation-time-temperature (PTT) curve obtained for the experimental steel exhibited an inverted “S” shape. A recrystallisation kinetics model revealed that SRX, which occurs preferentially above 940 °C, resulted in delayed subsequent SIP, thus causing deviation in the PTT curve from the typical ‘C’ shape. Below 940 °C, the fastest nose temperature for precipitation was located at 900 °C, and the precipitate was constituted by TiC particles with a NaCl-type FCC structure. The dynamic competition between SIP and SRX processes were evaluated by comparing the relative magnitude of the recrystallisation driving force and precipitation pinning force during stress relaxation, combined with the evolution of precipitate and austenitic structure. The results indicated that the plateau period occurred because of the precipitation pinning effect inhibited recrystallisation-induced austenite softening. However, the non-uniform distribution of SIP restricted the mobility of the boundaries to a portion of the austenite grains, resulting in abnormal grain growth during the plateau period.

Effect of Heat-Input on Microstructure and Toughness of CGHAZ in a High-Nb-Content Microalloyed HSLA Steel

Abstract: The effect of various heat inputs on the microstructure and impact toughness of the simulated coarse-grained heat-affected zone (CGHAS) of a niobium microalloyed (0.14 wt.%) low-carbon steel was studied. The results showed that higher impact toughness was achieved at a low heat input of 20 kJ/cm, which resulted from the formation of acicular ferrite laths/plates. They sectioned large prior austenite grains into many smaller regions, resulting in smaller crystallographic grains and high-angle grain boundaries. Conversely, when specimens were simulated with larger heat-inputs (100, 200 kJ/cm), the microstructure of the CGHAZ was predominantly composed of granular bainite plus massive MA constituents, thus impairing the impact toughness.