Showing papers on "Microalloyed steel published in 2023"

Microstructure Evolution and Precipitation Behavior in Nb and Nb-Mo Microalloyed Fire-Resistant Steels

Abstract: Microstructure evolution and precipitation behaviors before and after tension at elevated temperature of Nb and Nb-Mo microalloyed fire-resistant steels were investigated by scanning electron microscopy and transmission electron microscopy, physical–chemical phase analysis and small-angle X-ray scattering. Results showed that the martensite and austenite (M/A) islands in a rolled state disappeared after tempering, and cementite and a large number of nanometer-sized carbide precipitated. Those nanosized particles were identified as NbC in Nb steel and (Nb, Mo)C in Nb-Mo steel. The precipitation amount of Nb is nearly equal for the two steels in a rolled state. However, after tempering treatment at 600 °C, the precipitation of Nb increased in Nb-Mo steel more obviously than that in Nb steel, indicating that Mo promotes the precipitation of Nb. Correspondingly, the amount of dissolved Nb was reduced, which results in decreased coarsening kinetics of (Nb, Mo)C in Nb-Mo steel as compared with that of NbC in Nb steel.

Effect of Mg on Inclusion and High Cycle Fatigue Behavior in Titanium Microalloyed Beam Steel

Abstract: In this paper, the fatigue behavior of titanium microalloyed beam steels were studied by high cycle fatigue test and fatigue crack growth rate test. The effect of Mg addition on the fatigue behavior in titanium microalloyed beam steel was systematically analyzed. According to the experimental results, the addition of magnesium can effectively modify the inclusions by reducing the size of Al2O3 and TiN and promoting the formation of finer complex inclusions with a MgO·Al2O3 core in titanium microalloyed high-strength beam steel. Inclusion number of the experimental steels had far less of an impact on the fatigue characteristic than inclusion size. With the heterogeneous nucleation effect of MgO·Al2O3, the inclusions are refined after the Mg addition. Beam-2 steel’s tensile strength decreased by approximately 54 MPa, while its fatigue strength increased by about 33 MPa, showing favorable fatigue resistance. These findings are essential for optimize the fatigue properties of titanium microalloy steel and promote the development of automobile beam steel with excellent fatigue properties.

Effect of Nb-V microalloying on low-cycle fatigue property of Fe-Mn-Al-C austenitic steel

Abstract: The tensile and low-cycle fatigue properties of Fe–Mn–Al–C austenitic steels without and with Nb–V microalloying are analyzed comparatively. The Nb–V microalloyed Fe–Mn–Al–C steel shows higher strength and lower elongation. The cyclic deformation behavior is noted as continuous cyclic softening. The fatigue life of the Nb–V microalloyed steel is higher at total strain amplitudes of 0.4% and 0.6%, but lower at 0.8% than that of the non-microalloyed one. The microstructures of the steels after fatigue test show parallel deformation bands, which are composed of planar dislocations. The precipitation of (Nb,V)C has a positive influence on improving the fatigue life compared with the κ-carbides.

Correlating fractography with mechanical properties of microalloyed 2219Al alloys under different thermo-mechanical conditions

Abstract: 2219Al alloys microalloyed with varying trace contents (0–0.1 wt.%) of Cd were cast, and subjected to standard sequential thermo-mechanical processes of rolling and age-hardening treatments. Uniaxial tensile and Charpy impact tests were performed, and fractured surfaces were studied under scanning electron microscope (SEM). Variations in tensile ductility and toughness with trace additions of Cd were reported under given processing conditions. Independent influences of Cd compositions, peak-ageing treatment and rolling were investigated to identify predominant fracture modes (ductile/fibrous, brittle/cleavage or mixed) of 2219Al alloys for the first time. Salient features and fracture mechanisms as exhibited from the fractographs were further correlated with microstructural evolution, mechanical ductility and toughness. Cast 2219Al alloy exhibited typically ductile fractographs, compared to moderate level of ductility after microalloying. Lower fracture strain and toughness of peak-aged alloys were accompanied with mixed mode of fracture surfaces. Rolling resulted in fractographs of mixed mode, however with higher ductility compared to peak-aged alloys. Impact testing resulted in failure at higher strain rate, exhibiting typically brittle fractographs, correlated with the impact toughness values. Present fractographic analysis provided a structure-property correlation, to validate the susceptibility of these alloys, to different modes of failure, subjected under various loading and thermo-mechanical treatments.

Effect of Coiling Temperature on Microstructure and Properties of a Ti‐Nb Microalloyed High Speed Guardrail Steel

Abstract: The effects of coiling temperature on microstructure and properties of a Ti-Nb microalloyed high speed guardrail steel were investigated. The results show that when the coiling temperature varied from 570 °C to 670 °C, the optimum comprehensive mechanical properties were obtained at the coiling temperature of 620 °C. The yield strength, tensile strength and total elongation were 729 MPa, 813 MPa and 11.8%, respectively. The difference in yield strength between the samples at varying coiling temperature is attributed to different strengthening effects of grain refinement, precipitation and dislocation strengthening, among them the grain refinement and precipitation strengthening contributions are dominating, with contributions of 290 MPa and 186 MPa, respectively, at 620 °C. Moreover, the fine nano-scale (Ti, Nb)C particles formed during and after the coiling stage. This article is protected by copyright. All rights reserved.

Subcritical Spheroidization Annealing of Cold‐Rolled Cr–Mo Microalloyed Medium‐Carbon Steel with Different Initial Microstructure

Abstract: According to the industrial production process of fine‐blanking steel (including hot rolling, cold rolling, and spheroidization annealing), a Cr–Mo‐microalloyed medium‐carbon steel is taken as the research object for laboratory simulation in this paper. The ferrite+pearlite (F + P) and ferrite+bainite (F + B) are obtained by changing coiling temperature during hot rolling conduct cold rolling with 25%, 50%, and 75% total reduction and subcritical annealing at 740 °C for 1–8 h. Microstructure evolution during spheroidization annealing is studied and analyzed by an electron probe microanalyzer, electron back‐scattered diffraction, DICTRA simulation, transmission electron microscopy, and microhardness tester. Compared with F + P, the dislocation density distributed to the ferrite of F + B during cold rolling is higher. The cold‐rolled F + B microstructure shows the rapid spheroidization annealing behavior, which leads to the uniform distribution of finer spherical carbides and the full recrystallization of the ferrite matrix. In addition, properly increasing the reduction of cold rolling can also speed up the spheroidization process of carbide. Herein, a 50% reduction is determined to be the most suitable cold‐rolling process parameter for carbide spheroidization. Moreover, the recrystallization of ferrite is enhanced with the increase of cold‐rolling reduction.

Structure forming processes in economically alloyed shipbuilding steel of 890 MPa yield strength level with a bainite-martensite structure when microalloyed with vanadium

Abstract: The kinetics of growth of austenite grains during heating, the features of the processes of dynamic and static recrystallization occurring under various temperature-deformation regimes of hot plastic deformation have been studied. Phase transformations have been studied during continuous cooling of hot-worked austenite in a low-carbon low-alloy steel with a guaranteed yield strength of 890 MPa. As a result, the boundary temperature-deformation conditions for the formation of a finely dispersed bainite-martensite structure were established, on the basis of which technological modes for the production of thick-plate rolled products in industrial conditions were developed. The structure and properties of rolled sheets 35 mm thick from shipbuilding sparingly alloyed steel of strength level 890 are presented.

Hot deformation behavior, dynamic recrystallization mechanism and processing maps of Ti-V microalloyed high strength steel

Abstract: The hot deformation behavior, dynamic recrystallization mechanism and processing maps of Ti-V complex microalloyed high strength steel under temperatures ranging from 840 to 1040 °C and strain rates varying from 0.01 to 10 s-1 were studied by using thermal-mechanical simulation (Gleeble-3800), optical microscopy (OM), and electron backscatter diffraction (EBSD). The true stress-strain curves of the investigated steel were obtained under the different deformation conditions, the thermal deformation activation energy was calculated, the thermal constitutive equations based on the true stress-strain curves were established, and the 3D power dissipation diagrams, the 3D plastic instability diagrams, and the thermal processing maps under different true strains were drawn. The results show that the flow stress curves are of dynamic recrystallization type at 960∼1040 °C and strain rate of 0.01 s-1 and at 1040 °C and strain rate of 0.1 s-1, and the peak stress decreases gradually with the increase of the deformation temperature and the decrease of the strain rate. The hot deformation activation energy of Ti-V microalloyed steel was calculated to be 349.681 kJ/mol. The average grain size of austenite first decreases and then increases with increasing the strain rate under the deformation temperatures of 1000 °C and 1040 °C, and has the smallest value of 15.0 μm and 16.1 μm, respectively, after deformation at the strain rate of 1 s-1. With the true strain increasing from 0.2 to 0.6, the area occupied by the destabilized region increases and the machinable region area decreases. It reveals that the most available hot processing parameter ranges of Ti-V complex microalloyed steel are at the temperature range of 1000∼1040 °C with the strain rate of 0.01∼1 s-1.

Influence of Microalloying on the Microstructures and Properties of Spalling-Resistant Wheel Steel

Abstract: Microalloyed steels have emerged to replace conventional plain-carbon steels to achieve longer wheel life on Chinese railroads. In this work, with the aim of preventing spalling, a mechanism that consists of ratcheting and shakedown theory correlated with steel properties is systematically investigated. Mechanical and ratcheting tests were carried out for microalloyed wheel steel to which vanadium was added in the range of 0–0.15 wt.% and the results were compared with that obtained for conventional plain-carbon wheel steel. The microstructure and precipitation were characterized via microscopy. As a result, the grain size was not obviously refined, and the pearlite lamellar spacing decreased from 148 nm to 131 nm in microalloyed wheel steel. Moreover, an increase in the number of vanadium carbide precipitates was observed, which were mainly dispersed and uneven, and precipitated in the pro-eutectoid ferrite region, in contrast to the observation of lower precipitation in the pearlite. It has been found that vanadium addition can lead to an increase in yield strength by precipitation strengthening, with no reduction or increase in tensile strength, elongation or hardness. The ratcheting strain rate for microalloyed wheel steel was determined to be lower than that for plain-carbon wheel steel via asymmetrical cyclic stressing tests. An increase in the pro-eutectoid ferrite content leads to beneficial wear, which can diminish spalling and surface-initiated RCF.

Effect of Nb/V/Ti Microalloy on Microstructure and Properties of 16Mn Seamless Steel Pipe

Abstract: In order to study the effect of Nb, V and Ti microalloys on the microstructure and properties of 16Mn seamless steel tube, the static CCT diagram of tested steel was made on Glecble-3800 thermo-mechanical simulator. Based on the observation of microstructures and precipitates by optical microscopy, SEM and TEM, the effect of different microalloy on mechanical properties of 16Mn steel was analyzed. The results show that Ti treated 16Mn steel is advantageous to both grain refinement and the low temperature impact toughness. The impact toughness of Nb or V microalloyed steel is decreased, which appeared bainite in hot rolling. Normalizing can improve the impact toughness of Nb microalloyed steel, while the microstructure and properties of V microalloyed steel are not improved. 16MnTi is taken to produce hot-rolled pipes with wall thickness between 8 mm to 16 mm, that impact value is more than 80 J at - 40 °C; while 16MnTiNb is taken to produce normalized pipes with wall thickness greater than 16 mm, that impact value reaches 120 J at -40°C.

Evolution of the microstructure and mechanical properties of a V-containing microalloyed steel during coiling

Abstract: The microstructural evolution during coiling and its effects on the mechanical properties of a vanadium microalloyed steel were investigated. Samples were held at 500 °C for 80 s (S1), 600 s (S2), 3600 s (S3), and 18000 s (S4) to capture the various stages of microstructure evolution. Experimental results demonstrated that nano precipitates (<10 nm) nucleated during holding at 500 °C were heterogeneously distributed. These precipitates mainly contained V and N were more commonly found in areas with a high value of the Kernel Average Misorientation (KAM) because these areas have a larger number of dislocations which act as nucleation sites for the precipitates. The tensile flow curves of the samples show extended elastoplastic regions that are related to the heterogeneity of the microstructure as well as the presence of microscopic residual stresses. The precipitation strengthening effect is not very large and is offset by softening which is caused by the aging of bainite and the associated recovery of dislocations.

Characterization of Nanosized Carbide Precipitates in Multiple Microalloyed Press Hardening Steels

Abstract: Press hardening steel standardly relies on titanium microalloying for protecting boron from being tied up by residual nitrogen. This practice safeguards the hardenability effect of boron during die quenching. More recently, additional microalloying elements were added to press hardening steel to further improve properties and service performance. Niobium was found to induce microstructural refinement, leading to better toughness, bendability, and hydrogen embrittlement resistance. In that respect, niobium also extends the operating window of the press hardening process. Vanadium microalloying has been proposed to provide hydrogen trapping by its carbide precipitates. A recently developed press hardening steel employs all three microalloying elements in an attempt to further enhance performance. The current study analyses the microstructure of such multiple microalloyed press hardening steel, and compares it to the standard grade. Particularly, the effect of various heat treatments is investigated, indicating that the multiple microalloyed steel is more resistant against grain coarsening. TEM analysis is used to identify the various particle species formed in the steels, to track their formation, and to determine their size distributions. Nanosized microalloy carbide particles typically comprise a mixed composition involving niobium, titanium, and vanadium. Furthermore, these precipitates are incoherent to the matrix. Regarding tensile properties, it is found that the multiple microalloyed press hardening steel is superior to the standard grade.

Effect of electropulsing treatment on the precipitation behavior of MnS in sulfur-containing microalloyed 49MnVS3 steel

Abstract: An experimental investigation has been conducted with respect to the influence of electropulsing treatment (EPT) on the 2D and 3D morphologies, distribution, number and size of MnS precipitates in the high-sulfur micro-alloyed 49MnVS3 steel. After the application of electric current pulse with a peak pulsed voltage of 30 V, the microstructure was refined due to the increasing nucleation rate of particles in liquid steel. Moreover, compared with the untreated steel with spherical MnS randomly distributed in the matrix, the MnS precipitates moved directionally to aggregate on the bottom and top surfaces of the ingot in order to minimize the free energy of the system in case of EPT. For untreated steel samples, the number of MnS was 409 (bottom), 579 (middle) and 437 (top), while that of the treated samples were 226 (bottom), 38 (middle) and 224 (top) in the scanning area of 2.65 × 107 µm2 with ASPEX. The number of MnS with small size decreased while the large-size MnS increased with the application of EPT.

Effect of Nitrogen Content on the High Cycle Fatigue Properties of Titanium Microalloyed Steel

Abstract: The high cycle fatigue properties of three industrial 0.12% Ti microalloyed steels with different nitrogen contents (56, 40, and 30 ppm in molten steel of tundish) are investigated. The results show that <20% of the fatigue crack initiation sites are oxide inclusions of size in 16.8 μm ≈ 55.9 μm, while the rest 80% are surface defects. No TiN inclusions cause fatigue failure and the fatigue limit strength slightly decreases with increasing N contents as the yield strength decreases with a coarser ferrite grain. Inclusions characterization in the section near the fracture shows that the average sizes of TiN inclusions in H56, M40, and L30 steels are 3.50, 3.22, and 2.89 μm, and the maximum sizes are 6.92, 6.67, and 6.23 μm, respectively. Calculating for a cooling rate of 0.2 K s−1 using ChemAppPy precipitation model, the size of TiN inclusions will increase from 6.1 to 7.1 μm, when increasing N content from 30 to 60 ppm. The relationships between nitrogen content, TiN inclusions, and fatigue failure quantified by experimental test and modeling show that the nitrogen content in steel can be relaxed up to 60 ppm when considering economical denitrogenization and fatigue safety.

Hot Ductility, Homogeneity of the Composition, Structure, and Properties of High-Strength Microalloyed Steels: A Critical Review

Abstract: High-strength micro-alloyed steels are widely used in various branches of technology and industry due to the simultaneous combination of high indicators of strength, ductility, fatigue, corrosion resistance, and other service properties. This is achieved due to the reasonable choice of the optimal chemical composition and parameters of temperature deformation treatment of steel that provide a synergistic effect on the dispersed microstructure and characteristics of excess phase precipitates, which control the achievement of these difficult-to-combine properties of rolled products. Additionally, the improvement of the level and stability of these properties, as well as the prevention of the occurrence of defects, is largely determined by the indicators of the homogeneity of the composition, structure by volume and manufacturability of the metal, and primarily hot ductility, which are controlled by the presence of precipitation of excess phases, including micro-alloying elements. In accordance with the circumstances noted, in the present review, a generalization, systematization, and analysis of the results of the studies are conducted on the effect of phase precipitates on the hot ductility and homogeneity of composition and structure, depending on the chemical composition and parameters of the temperature-deformation treatment of steel.