Ferrite Nucleation Research Articles

Optimization of Inclusion and Microstructure of As‐Cast Nonquenched and Tempered Steel F38MnVS with Trace Ce

Various techniques, such as optical microscopy, high‐temperature confocal laser scanning microscopy, scanning electron microscopy, electron backscatter diffraction, and thermodynamic calculations, are employed to analyze the optimization mechanism of Ce on the inclusions and microstructures in F38MnVS as‐cast samples. The results show that the addition of Ce inhibits MnS precipitation at the grain boundaries and significantly reduces the diameters and aspect ratios of the inclusions. After Ce treatment, a new type of composite inclusion appears in the steel. The modification of the inclusions by Ce promotes the optimization of the as‐cast microstructure during solidification, which mainly exhibits austenite grain refinement and polygonal ferrite formation in the grains. Based on the misfit degree theory, the refinement of the austenite grains is because of the nucleation of austenite grains facilitated by Ce2O2S and CeAlO3. The increased nucleation of the intragranular polygonal ferrite is mainly attributed to Ce2O2S, Ce2S3, CeAlO3, and VC, which can promote the nucleation of α‐Fe. Simultaneously, the addition of Ce reduces the phase transition temperature, increases the driving force of the phase transition, and increases the nucleation rate. This study's results can provide a reference for the application of Ce treatment in the preparation of nonquenched tempered steel for automobiles.

Unravelling the impact of Al-Ti content on the microstructure-toughness relationship in the coarse-grained heated-affected zone of super-high-strength pipeline steels

This study aims to investigate the role of Al-Ti addition on particle modification in super-high-strength pipeline steels, and its following effect on microstructural evolution and impact toughness in the coarse-grained heated-affected zone (CGHAZ) is also explored. The steel plates with low and high Al-Ti content (0.009 wt-% Al + 0.006 wt-% Ti vs. 0.033 wt-% Al + 0.025 wt-% Ti), respectively, are subjected to gas metal arc welding with 6–7 kJ/cm heat input. The results reveal that the second-phase particles including micron-scale inclusion of Al-Mg oxide covered with CaS and nano-scale precipitate of titanium nitride are obtained in low Al-Ti steel, which are modified to the Al-Mg-Ti oxide core surrounded by CaS and TiN precipitate in high Al-Ti steel. The nano-scale TiN precipitate and micron-scale inclusion can restrict the austenite grain growth at high temperatures and induce acicular ferrite nucleation at medium temperatures, respectively, leading to the fine-grained mixed microstructure of bainite and acicular ferrite in CGHAZ of both steels. Because of the variant selection during the decomposition of austenite to bainite and acicular ferrite, those fine-grain structures with amounts of high-angle grain boundaries exhibit excellent CGHAZ toughness in both specimens. Compared with low Al-Ti steel, more precipitates induce smaller austenite grains, and the inclusions containing titanium increase the ability of inclusion to promote acicular ferrite nucleation in CGHAZ of high Al-Ti steel. Both of them resulted in smaller crystallographic grain and toughness improvement in CGHAZ of high Al-Ti steel than that in low Al-Ti steel.

Effect of Initial Intergranular Ferrite Size on Induction Hardening Microstructure of Microalloyed Steel 38MnVS6

In this study, we investigated the effect of grain size of an initial microstructure (pearlite + ferrite) on a resulting microstructure of induction-hardened microalloyed steel 38MnVS6, which is one topical medium carbon vanadium microalloyed non-quenched and tempered steel used in manufacturing crankshafts for high-power engines. The results show that a coarse initial microstructure could contribute to the incomplete transformation of pearlite + ferrite into austenite in reaustenitization transformation by rapid heating, and the undissolved ferrite remains and locates between the neighboring prior austenite grains after the induction-hardening process. As the coarseness level of the initial microstructure increases from 102 μm to 156 μm, the morphology of undissolved ferrite varies as granule, film, semi-network, and network, in sequence. The undissolved ferrite structures have a thickness of 250–500 nm and appear dark under an optical metallographic view field. To achieve better engineering applications, it is not recommended to eliminate the undissolved ferrite by increasing much heating time for samples with coarser initial microstructures. It is better to achieve a fine original microstructure before the induction-hardening process. For example, microalloying addition of vanadium and titanium plays a role of metallurgical grain refinement via intragranular ferrite nucleation on more sites, and the heating temperature and time of the forging process should be strictly controlled to ensure the existence of fine prior austenite grains before subsequent isothermal phase transformation to pearlite + ferrite.

Unraveling the effects of strain‒induced precipitation on continuous cooling ferrite transformation in titanium‒molybdenum microalloyed steel

Strain‒induced precipitation is a characteristic physical‒metallurgical phenomenon during hot‒rolling in microalloyed steel production that strongly affects the overallthermomechanical control process. In this study, the strain‒induced precipitation behavior in titanium‒molybdenum microalloyed steel was comprehensively investigated, and its complex effects on the austenite/ferrite transformation during continuous cooling were analyzed for the first time, based on stress relaxation and multi‒aspect characterization methods. The stress relaxation results revealed that the fastest strain‒induced precipitation occurred at 900 °C. The precipitates were identified as FCC structured (Ti, Mo)C particles with a coherent or semi‒coherent cubic‒cubic orientation relationship to the austenite matrix. The strain‒induced precipitation proved to increase the ferrite transformation temperature and proportion, significantly refine and homogenize the transformed grains. The intermittent quenching at 0.5 C/s further revealed that the (Ti, Mo)C particles with cubic‒cubic orientation relationship to austenite matrix exerted a dual pinning effect: by pinning dislocations, these particles facilitated diffusion‒controlled ferrite nucleation and growth both at austenite grain boundaries and within grains; by pinning migrating phase interfaces, the particles inhibited the coarsening of ferrite grains. Coupled with compressive testing and strengthening contribution analysis, the strain‒induced precipitation was shown to weaken precipitation strengthening but enhance grain refinement strengthening, thereby providing a novel approach to achieving an optimal balance between microstructural homogeneity and mechanical properties.

Effect of initial microstructure on microstructure and properties of bainitic steels achieved by rapid heat treatment

Rapid heat treatment is essential in manufacturing advanced high-strength steels. The initial microstructure significantly influences the steel's microstructure and properties after rapid heat treatment. The present study investigated how the initial microstructure affected phase transformation, microstructure, and properties of bainite in high-carbon low-alloy steel during rapid heat treatment. Under rapid heating, ultrafine bainite (UB) sample with homogeneous composition exhibits the lowest austenitization temperature, while globular pearlite (GP) sample with poor composition homogeneity has a higher austenitization temperature. After rapid heat treatment, the compositional distribution of the initial microstructure was partially retained in the final sample. The increase of alloying elements inhibits the nucleation of bainitic ferrite, making it easier for bainitic ferrite to grow at low-alloy positions. Finally, a large amount of retained austenite is obtained in the alloying element-rich area. The difference in microstructure results in samples with different initial microstructures having similar strengths but differing plasticities. In samples with an initial lamellar pearlite (LP) microstructure, there was a substantial amount of thicker, filmy retained austenite, with an average thickness of 96.8 nm. This led to a small increase in Kernel Average Misorientation (KAM) with strain during the tensile process. This improvement in coordinated deformation during straining indicates that the sample has achieved excellent performance, with a tensile strength of 1483 MPa and a uniform elongation of 23 %. When the initial microstructure was GP, the sample contained a large amount of crumbly retained austenite with a low aspect ratio. This structure poorly coordinated deformation with higher strain, resulting in numerous microcracks.

Effect of high welding heat input on the microstructure and low-temperature toughness of heat affected zone in magnesium-treated EH36 steel

In the present study, the welding thermal cycle of magnesium-treated EH36 steel was simulated by a thermal simulation experiment machine. The effects of welding heat input on the microstructure and low temperature toughness of the welded joint were studied, and the mechanism of acicular ferrite nucleation induced by MgAl2O4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm { MgAl_2O_4}$$\\end{document} inclusion was revealed. The results showed that, when the welding heat input of 150 kJ/cm, 250 kJ/cm and 350 kJ/cm was utilized, the microstructure of heat affected zone in the experimental EH36 steel consisted of grain boundary ferrite, acicular ferrite, granular bainite, and a small amount of pearlite, but the volume fraction and grain size of each independent microstructure were different. With the increase of welding heat input, the content of acicular ferrite and granular bainite decreased, the content of grain boundary ferrite increased, and the average grain size increased, whereas the low temperature toughness of the experimental EH36 steel welded joint reduced from 174 to 67 J. Furthermore, MgAl2O4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {MgAl_2O_4}$$\\end{document} inclusions can induce acicular ferrite nucleation, whereas the nucleation mechanism is a combination of low lattice mismatch and low interfacial energy.

Flux enhancement with titanium or vanadium oxides addition for superior submerged arc welding of HSLA steel plates

A high-strength low-alloy (HSLA) steel plate of 10 mm thickness underwent submerged arc welding with enhanced fluxes containing additional titanium oxide (TiO2) or vanadium oxide (V2O5). The addition of TiO2 led to the development of a finer acicular ferrite structure but coarsening the carbide and martensite/austenite (M/A) constituents, which marginally improved the hardness, tensile strength, and ductility of weld metal. Conversely, incorporating V2O5 facilitated a substantial vanadium absorption (0.7 wt. %) in the weld metal, giving rise to a distinctive acicular microstructure less reliant on ferrite nucleation at non-metallic inclusions than conventional acicular ferrite. The distinctive microstructure, unique to vanadium steels, combined lath bainite with irregularly shaped granular bainite. The resultant dual-mode bainitic structure, coupled with a more uniform distribution of refined microphase constituents, outperformed the conventional acicular ferrite, delivering more than 20% and 13% improvements in yield and tensile strengths respectively, as evidenced by transverse tensile tests on the weld metals.

Inclusions Evolution of Ca‐Treated Steel During High‐Heat Input Welding

This article has investigated a 60 mm Ca‐treated Q550 steel used for high‐heat input welding. The results have shown that the Ca‐type inclusions account for 70% of the inclusions in the steel which are mainly CaAlMg type, Ca–Al type, and a small amount of CaS type while the non‐Ca‐type inclusions are mainly FeCO. After undergoing the welding thermal cycle, the number of Ca‐containing inclusions above 2 μm in size is decreased while the number of non‐Ca‐containing inclusions below 1 μm (mainly FeO) is increased significantly. However, the total area of inclusions is decreased. It has been found out that the CaAlSO‐type inclusions undergo “diffusion” of O element to the surroundings during the thermal cycle, forming a ring‐shaped oxygen‐rich zone and carbon‐rich zone which promotes the nucleation of acicular ferrite. In contrast, the CaAlMgSO‐type inclusions undergo “decomposition” during the thermal cycle which forms a group of inclusions with different sizes and types, promoting nucleation of multioriented acicular ferrite.

The comparative study of Ti-bearing oxides introduced by different methods

Abstract The Ti-bearing oxides are introduced into HRB400 steel melt through external addition method (EAM, TiO2) and internal precipitation method (IPM, Ti alloy) using a vacuum induction furnace, respectively. For each experiment, the instantaneous evolution of oxide properties was investigated through the analysis of intermediate samples. For the EAM method, it was found that the titanium content gradually increased with increase in the treatment time. Similar increasing trend is also observed for Ti-bearing oxide properties, such as the number density and normalized titanium content. These phenomena are totally different from those in the IPM intermediate samples, in which both Ti content and Ti-bearing oxide features show rapid increase, reaching saturation at 1 min after the addition of Ti alloy, and remain relatively stable thereafter. This comparison hints that after TiO2 addition, they first dissolve in molten steel, then decompose, and finally again form Ti-bearing oxides. Furthermore, in the case of similar titanium content, EAM intermediate samples exhibit higher Ti-bearing oxide number density and higher normalized titanium content compared to IPM samples. After heat treatment, the intragranular ferrite nucleation was observed in cast sample of EAM, which was induced by the introduced Ti-bearing oxides.

Grain refinement of flux-cored wire arc additively manufactured duplex stainless steel through in-situ alloying of Ti

One of the major problems in the wire arc additive manufacturing (WAAM) of duplex stainless steel (DSS) is the epitaxial growth of coarsen columnar ferrite grains crossing the deposition layers, which leads to the mechanical property anisotropy and increases the risk of hot tearing. In this investigation, Ti was in-situ alloyed to the WAAM DSS to promote grain refinement of ferrite. The results revealed that the ferrite transformed from columnar to equiaxed morphology with the increase of Ti content. In the sample with 0.5%Ti addition, some fine equiaxed ferrite grains with an average grain size of 39 μm formed at the fusion line, which inhibited the epitaxial growth of columnar grains. With further increase of Ti addition to 1.0%, the ferrite grains in the as-deposited WAAM DSS completely transformed from columnar to equiaxed structure, with an average grain size of 12 μm at the fusion line and 19 μm at the interior of deposition layer. Ti2N-Al2O3 particles with an average diameter of 400 nm were found in the equiaxed grains at room temperature, which existed in the form of TiN-Al2O3 in the molten pool and promoted the heterogeneous nucleation of ferrite. The analyzation based on Interdependence Theory demonstrated that Ti refined the ferrite in WAAM DSS by promoting the formation rate of the constitutional supercooling zone ahead of the solid/liquid interface and by introducing effective nucleat particles into it.

The Influence of the Second Phase on the Microstructure Evolution of the Welding Heat-Affected Zone of Q690 Steel with High Heat Input.

Q690 steel is widely used as building steel due to its excellent performance. In this paper, the microstructure evolution of the heat-affected zone of Q690 steel under simulated high heat input welding conditions was investigated. The results show that under the heat input of 150-300 kJ/cm, the microstructures of the heat-affected zone are lath bainite and granular bainite. The content of lath bainite gradually decreased with the increase in heat input, while the content of granular bainite steadily increased. The proportion of large-angle grain boundaries decreased from 51.1% to 40.3%. Overall, the average size of original austenite increased, and the precipitates changed from Ti (C, N) to Cr carbides. During the cooling process, the nucleation position of bainitic ferrite was from high to low according to the nucleation temperature, and in order of inclusions at grain boundaries, triple junctions, intragranular inclusions, bainitic ferrite/austenite phase boundaries, twin boundaries, grain boundaries, and intragranular inclusions at the bainitic ferrite/austenite phase interface. The growth rate of bainitic ferrite nucleated at the phase interface, grain boundary, and other plane defects was faster, while it was slow at the inclusions. Moreover, it was noted that the Mg-Al-Ti-O composite inclusions promote the nucleation of lath bainitic ferrite, while the Al-Ca-O inclusions do not facilitate the nucleation of bainitic ferrite.

An improved toughness process for high-temperature hot-rolled HSLA steel via inclusion-induced acicular ferrite nucleation

A feasible method of enhancing the impact toughness (−20 °C) of high-temperature hot-rolled HSLA steels was proposed. The results show that the toughness of the improved hot-rolled steel containing TiOx-CaO-ZrO2 oxide particles was significantly enhanced by ∼132 J compared with conventional steel containing Al-Ca oxide particles. The fine (Ti, Ca, Zr) Ox particles with high-density of ∼937 counts/mm2 significantly promotes anti-cracking acicular ferrite (AF) nucleation. Moreover, the fractional volume of AF increased from ∼79 % to ∼85 % when the rolling temperature was increased from ∼1055 °C to ∼1085 °C.

Evolution of Complex Oxide Inclusions During the Smelting Process of Oxide Metallurgical Steel and Their Effect on Acicular Ferrite Nucleation

Evolution of Complex Oxide Inclusions During the Smelting Process of Oxide Metallurgical Steel and Their Effect on Acicular Ferrite Nucleation

Role of Cerium on the Intricacies of Deformed State and Softening Mechanisms of Low‐Carbon Steels

The present study investigates the softening kinetics of two cerium (Ce)‐modified steels after 60% cold rolling and annealing at 600 °C for 2–16 h. Cold rolling accumulates substantial strain in the ferritic matrix of low Ce (LCe) steel (0.03 wt% Ce) compared to high Ce (HCe) steel (0.6 wt% Ce). The acicular ferrite and Fe3C partition the imposed strain preferentially inside the ferrite matrix of LCe sample. Contrarily, a homogenous strain distribution in HCe sample is promoted by soft Ce2O3 particles embedded in ferrite. Both the steels achieve partial recovery and recrystallization even after 16 h of annealing. LCe steel experiences a softening fraction of ≈28 vol% after 16 h, inferior to HCe steel (≈34 vol%). During initial stage of annealing, nucleation of strain‐free grains are observed in LCe sample due to availability of grain and interphase boundaries. Subsequently, recrystallization kinetics get delayed because of the pinning effect exerted by fine CeO2 and Fe3C particles. In HCe samples, the early stage of ferrite nucleation is hindered by the segregation of Ce at grain boundaries. However, at a later stage, the recrystallization kinetics are accelerated owing to the ineffective pinning of dislocations and boundaries by coarse Ce2Fe17 and Ce2C3 particles.

Effect of Nb Content on the Microstructure and Impact Toughness of High-Strength Pipeline Steel

In this study, X80 pipeline steel is prepared with different Nb contents through the thermo-mechanically controlled rolling process. The effects of using two different Nb contents on the impact toughness and microstructure of the pipeline steel are examined using various experimental techniques. The results show that with the increase in Nb content, the transformation temperature Ar3 decreases, and the nucleation and growth of bainitic ferrite with lath features (LB) are promoted, while those of granular bainite (GB) are inhibited. In addition, the stability of the austenite phase increases with the increase in Nb content. Therefore, the volume fractions of LB and martensite–austenite (M/A) constituents increase, while the proportion of high-angle grain boundaries (HAGBs) decreases. The impact energy of pipeline steel at −45 °C is closely related to the Nb content. Specifically, the impact energy decreases from 217 J at 0.05 wt.% Nb to 88 J at 0.08 wt.% Nb. The cracks are preferentially formed near the M/A constituents, and the HAGBs significantly inhibit the crack propagation. The steel with 0.05 wt.% Nb has a lower content of M/A constituents and a higher proportion of HAGBs than the one with 0.08 wt.% Nb. In addition, as the Nb content increases, the crack initiation energy and the crack propagation energy decrease. Thus, the 0.05 wt.% Nb steel has a higher low-temperature impact energy.

Refinement mechanism of large heat-input welding CGHAZ microstructure by N addition and its effect on toughness of a V-Ti-N microalloying weathering steel

The large heat input welding performance of uncoated weathering steel remains a great challenge since their coarse grain heat-affected zones (CGHAZs) are usually occupied by coarse and brittle microstructure consisting of granular bainitic ferrite (GBF) and martensite/austenite (M/A) constituents. In this article, a V-Ti-N microalloying weathering steel suitable for large heat input welding was achieved through the N addition, and the mechanism dominating the modified microstructure and the enhanced impact toughness of CGHAZ was deeply elucidated mainly by HRTEM. The results indicated that in the simulated CGHAZ with large heat input of 80 kJ/cm, the increase of N content from 0.0030 wt% to 0.0075 wt%, and 0.0115 wt% elevated the density of nano- and submicron-sized particles of (Ti, V)(C, N) containing VN, which in turn resulted in the refinement of prior austenite grains (PAGs) by the improved grain boundary pinning effect and the enhancement in heterogeneous nucleation of intragranular ferrite (IGF) by the decreased lattice disregistry (δ) between (Ti, V)(C, N) and IGF. The fraction of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) higher than 15° was thereby increased while the mean equivalent diameter (MED) of ferritic grains with HAGBs was decreased. Simultaneously, the N addition scattered the M/A constituents, promoted the internal substructure of dislocation-type M/A at the expense of twin-type M/A, and accordingly reduced the hardness difference between the M/A constituents and the matrix. All these factors retarded brittle crack initiation and propagation, and low-temperature impact toughness of simulated CGHAZ increased from 62.8 J to 141.2 J.

Influence of Hot Rolling Practice and Furnace Residence Time on the Strength and Toughness of Normalised Nb-Ti-V Structural Steel Plate

The influence of moderate thermomechanical rolling and heat treatment time on the microstructure and mechanical properties of normalised Ni-containing Nb-Ti-V microalloyed plate steels was investigated. Conventional rolling (CR) and low austenitic temperature rolling (TM) schedules were applied to 25 and 55mm thick plates. Predicted plate thermal profiles were used to simulate normalising of 25 and 55mm thick plates at the mid-thickness. As expected, the microstructure after TM rolling was finer due to austenite sub-structure development below the no-recrystallisation temperature which increases the amount of ferrite nucleation sites. The tensile strength of the CR plates was generally higher due to a larger pearlite volume fraction resulting from slow cooling from a relatively coarse austenite. In 25mm plate, the presence of acicular ferrite after CR resulted in a higher yield strength than that obtained after TM rolling. Furnace residence times of 0.75min/mm and longer allowed for plates to be completely normalised – characterised by continuous, well-defined polygonal ferrite grain boundaries. Short furnace residence times restricted the plate temperature to below the end of the ferrite-to-austenite transformation (Ac3), where untransformed ferrite is retained from the as-rolled microstructure and is characterized by poorly defined grain boundaries. Rolling practice and normalising residence time had little to no effect on impact toughness, but significantly influenced the normalised yield strength of thicker plate.

Effect of Ca Addition on Inclusions of Welding Heat-Affected Zone in Pressure Vessel Steels

Pressure vessel steels are used in the manufacture of tanks for the storage of gases, chemical materials and oil. To meet the increasing production demands, high-wire-energy welding is widely used in the manufacture of pressure vessel steels. This means that the weldability of pressure vessel steels needs to be improved. Therefore, in order to reveal the microalloying effect of Ca in pressure vessel steel, this study took a commonly used pressure vessel steel as the research object, and three groups of experimental steels with different Ca mass fractions were prepared using vacuum metallurgy, controlled rolling and controlled cooling. Welding heat simulation technology was used to simulate the welding heat of experimental steel and the welding heat-affected zone (HAZ) was investigated. The inclusions of the welding HAZ in the experimental steels were observed by using a metallographic microscope and scanning electron microscope (SEM). The mechanism of intragranular acicular ferrite (IAF) nucleation induced by the inclusions containing Ca elements in the welding HAZ of pressure vessel steels was also discussed. The research results show that the addition of Ca increased the number density of effective inclusions in the welding HAZ of the experimental steel up to 535.60 pieces/mm2. The addition of the Ca element was beneficial for producing more pinning inclusions in the experimental steel welding HAZ under the experimental conditions, and the inclusions were mainly elliptical oxide complex inclusions of Ca-Si with a size of about 2 μm. Meanwhile, Al2O3 and MnS were precipitated. After the addition of Ca elements, Mn-poor regions appeared around the inclusions containing Ca in the welding HAZ. IAF nucleation was mainly induced by the local compositional change mechanism and supplemented by the stress–strain energy mechanism and inert interface energy mechanism. This study provides a valuable reference for optimizing the welding process of pressure vessel steels and is of great importance for understanding the IAF nucleation mechanism of Ca-containing inclusions in the welding HAZ of pressure vessel steels.

Effect of Mg Addition on Inclusions in the Welding Heat-Affected Zone of Pressure Vessel Steels

With the development of the pressure vessel industry, high-energy wire welding has a great future. However, this means higher demands on the weldability of pressure vessel steels. Controlling inclusions via oxidative metallurgy is a reliable method of improving the weldability of pressure vessel steels. Hence, in this paper, experimental steels with different Mg element mass fractions were prepared using vacuum metallurgy. Simulated welding for high-heat input welding was carried out using the Gleeble-2000 welding thermal simulation test machine. The inclusions in the welding heat-affected zone (HAZ) in the experimental steels were observed using an optical microscope (OM) and scanning electron microscope (SEM). The compositions of the inclusions were analyzed using an energy-dispersive spectrometer (EDS). The research results indicated that the addition of Mg could increase the number density of the inclusions in the welding HAZ. With the addition of Mg from 0 to 5 wt.%, the total number density of the inclusions increased from 133 to 687 pieces/mm2, and the number density of the inclusions with a size of 0–5 μm2 increased from 122 to 579 pieces/mm2. The inclusions in the experimental steel welding HAZ with Mg elements were mainly elliptical composite inclusions composed of (Mg-Zr-O) + MnS. Moreover, MnS precipitated on the surface of the Mg-containing inclusions in the welding HAZ. Intragranular acicular ferrite (IAF) nucleation was primarily induced via the minimum lattice mismatch mechanism, supplemented with stress-strain energy and inert interface energy mechanisms.

Effect of N Content on the Microstructure and Impact Properties of Normalized Vanadium Micro-Alloyed P460NL1 Steel

In this work, the effect of nitrogen doping on vanadium micro-alloyed P460NL1 steel is studied in terms of microstructures and impact toughness. As the nitrogen content increased from 0.0036% to 0.0165%, the number of V (C,N) particles increased. The fine precipitates of V (C,N) effectively pin the prior austenite grain boundary, resulting in the refinement of the austenite grain. The intragranular and intergranular V-containing coarse particles enhanced the nucleation of intragranular ferrite and the grain boundaries of polygonal ferrite during air cooling. Accordingly, the proportion of heterogeneously nucleated ferrite increased, and the grain size of ferrite decreased. Notably, the size of the pearlite microstructure decreased, and the bainite microstructure appeared with a high doping of N. With the increase in N content, the impact toughness of vanadium micro-alloyed P460NL1 steel was enhanced. This can be attributed to the refinement of ferrite and the reduction in pearlite, which, in turn, was ascribed to the increase in nitrogen.