Showing papers on "Microalloyed steel published in 2018"
07 Mar 2018-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the isothermal evolution of nanometre-sized precipitates formed in a Ti-Mo microalloyed steel through interphase precipitation has been investigated using atom probe tomography and small-angle neutron scattering.
Abstract: The isothermal evolution of nanometre-sized precipitates formed in a Ti-Mo microalloyed steel through interphase precipitation has been investigated using atom probe tomography and small-angle neutron scattering. The coiling time and applied strain have been varied to observe the precipitate evolution at a constant coiling temperature of 650 °C, where various evolution parameters such as particle radius, number density, volume fraction and chemical composition have been evaluated and compared. The possibility of early stage solute clustering and its effect on precipitate formation have also been investigated. Clustering of Ti, Mo and C atoms as Ti-C and Mo-C has been observed at the shortest coiling time of 5 min. These clusters are assumed to be precursors to the carbide precipitates observed in the system, which exhibit a metastable composition, containing a carbon fraction (C/Ti+Mo ratio) in the range of 0.2–1. In particles having a Guinier radius > 3 nm, however, the average chemical composition approached the stable MC carbide stoichiometry with Ti/Mo ratio ~2.5 and C/(Ti+Mo) ratio ~0.55. This study reveals that the precipitate coarsening kinetics are very slow, with average particle diameter 10 h) in both the undeformed and deformed conditions. This is believed to be due to the reduction in equilibrium Ti content in the matrix as a result of partial replacement of Ti by Mo (Ti/Mo ratio > 2) in the precipitate lattice, in the presence of excess C in the system.
45 citations
TL;DR: In this paper, a vanadium-containing high-strength low-alloy steel has been investigated for interphase precipitation of nanoscale carbides and the results showed that the major axis diameter increased from 18nm after 3min to 35nm after 600min.
39 citations
TL;DR: In this article, rare earth elements (RE), La and Ce (200 ppm), were added to a low carbon cast microalloyed steel to disclose their influence on the microstructure and impact toughness.
Abstract: In this research Rare Earth elements (RE), La and Ce (200 ppm), were added to a low carbon cast microalloyed steel to disclose their influence on the microstructure and impact toughness. It is suggested that RE are able to change the interaction between the inclusions and matrix during the solidification process (comprising peritectic transformation), which could affect the microstructural features and consequently the impact property; compared to the base steel a clear evolution was observed in nature and morphology of the inclusions present in the RE-added steel i.e. (1) they changed from MnS-based to (RE,Al)(S,O) and RE(S)-based; (2) they obtained an aspect ratio closer to 1 with a lower area fraction as well as a smaller average size. Besides, the microstructural examination of the matrix phases showed that a bimodal type of ferrite grain size distribution exists in both base and RE-added steels, while the mean ferrite grain size was reduced from 12 to 7 μm and the bimodality was redressed in the RE-added steel. It was found that pearlite nodule size decreases from 9 to 6 μm in the RE-added steel; however, microalloying with RE caused only a slight decrease in pearlite volume fraction. After detailed fractography analyses, it was found that, compared to the based steel, the significant enhancement of the impact toughness in RE-added steel (from 63 to 100 J) can be mainly attributed to the differences observed in the nature of the inclusions, the ferrite grain size distribution, and the pearlite nodule size. The presence of carbides (cementite) at ferrite grain boundaries and probable change in distribution of Nb-nanoprecipitation (promoted by RE addition) can be considered as other reasons affecting the impact toughness of steels under investigation.
35 citations
TL;DR: In this article, a series of two-stage hot compression tests were performed on Gleeble 1500 Nb-V microalloyed steel and a modified Avrami model was proposed.
21 citations
TL;DR: In this paper, the effect of carbon contents on intragranular ferrite (IGF) formation was studied and the critical size of IGF nuclei was calculated on the basis of classic heterogeneous nucleation theory.
18 citations
TL;DR: In this paper, the effects of rare earth elements (RE) addition on the pearlite microstructure in low carbon microalloyed steels have been investigated under two heat treatment conditions: (1) a normalizing treatment (as a conventional heat treatment used industrially to obtain the final mechanical properties of such steels), and (2) an isothermal treatment at 650 °C.
Abstract: The effects of rare earth elements (RE) addition on the pearlite microstructure in low-carbon microalloyed steels have been investigated under two heat treatment conditions: (1) a normalizing treatment (as a conventional heat treatment used industrially to obtain the final mechanical properties of such steels), and (2) an isothermal treatment at 650 °C. This research reports the following effects due to the addition of RE: (i) refinement of the nodule and colony size of pearlite along with the ferrite grain size in the normalized condition, without a significant change in the volume fraction of pearlite. This microstructural refinement observed at room temperature is a consequence of the refinement of cast and austenitic microstructures formed during cooling in the presence of RE; (ii) the interlamellar spacing of pearlite isothermally transformed at 650 °C, as observed by SEM and TEM, is effectively reduced in the RE-added steel. This is likely due to two different effects combined: (i) direct influence of RE on atom carbon diffusion; and (ii) pearlite growth being boundary diffusion controlled by RE partitioning.
16 citations
18 Apr 2018-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the size of a Nb-Ti microalloyed steel was investigated at various strain rates and temperatures, and it was shown that Nb preferably precipitates on TiN particles at temperature range of T
Abstract: Dynamic recrystallization (DRX) grain size of a Nb-Ti microalloyed steel is investigated at various strain rates and temperatures. Electron microscopy reveals that Nb preferably precipitates on TiN particles at temperature range of T
16 citations
TL;DR: In this article, an in-situ grown composite case on the surface of micro-alloyed steel has been developed for improved hardness through surface melting by employing Tungsten Inert Gas (TIG) arcing.
Abstract: Surface of microalloyed steel, hereafter referred as steel, has been modified by developing an in-situ grown composite case on its surface for improved hardness. It is done through surface melting by employing Tungsten Inert Gas (TIG) arcing. The hard reinforcements were made to grow in the surface matrix of steel through chemical reactions of the inorganic powders present in the applied coating and the molten base. The distribution and incorporation of these reinforcements were taken care by addition of Al and TiO2 in the coating. Three different mixtures, comprising different proportions of Al and TiO2, were prepared to develop a hybrid composite primarily containing Al2O3 and a small fraction of TiC along with other oxides as reinforcements. The modified particulate composite surface was analyzed under Vickers' micro-hardness tester confirming its significant improvement in hardness of the order of 1.88–2.24 times in comparison to that of the base metal, depending upon different chemistry of the powder mixture of the coating.
15 citations
TL;DR: In this paper, the effect of microstructure on dynamic behavior of structural steels was investigated through Taylor impact experiments. But, the authors focused on understanding the effects of micro-structural properties on dynamic behaviour of investigated materials.
13 citations
TL;DR: In this article, an ultrafine grain with the effective grain size of 1.10μm was fabricated by intercritical deformation on low-carbon microalloyed steel, which was finer than the deformation induced ferrite transformation (DIFT) at 1.55μm.
13 citations
04 Apr 2018-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, two algorithms, Simulated Annealing and Iterated Local Search, were used to evaluate their effectiveness and maximize the mechanical strength of microalloyed steel, and the experimental results showed that the yield strength obtained in the laboratory is comparable to that obtained in computational tests.
Abstract: This paper presents two algorithms, Simulated Annealing and Iterated Local Search. Both metaheuristics use a neighborhood hybrid structure to evaluate their effectiveness and maximize the mechanical strength of microalloyed steel. Tests show that the best metaheuristic for this type of problem, which makes use of a neighborhood structure and a chemical composition, is Iterated Local Search because it gives a better mechanical strength than Simulated Annealing. Acicular Ferrite was developed in the laboratory using the best mechanical properties obtained by the heuristics in computational tests. Then the mechanical strength of the created steel was evaluated. The experimental results show that the yield strength obtained in the laboratory is comparable to that obtained in computational tests.
TL;DR: In this article, the authors investigated the changes of volume fraction and particle size of (Ti, V, Mo) precipitates as the coiling temperature of the ferritic Ti-V-Mo complex microalloyed steel increased.
Abstract: Precipitation behavior of (Ti, V, Mo)C and microstructural evolution of the ferritic Ti–V–Mo complex microalloyed steel were investigated through changing coiling temperature (CT). It is demonstrated that the strength of the Ti–V–Mo microalloyed steel can be ascribed to the combination of grain refinement hardening and precipitation hardening. The variation of hardness (from 318 to 415 HV, then to 327 HV) with CT (from 500 to 600–625 °C, then to 700 °C) was attributed to the changes of volume fraction and particle size of (Ti, V, Mo)C precipitates. The optimum CT was considered as 600–625 °C, at which the maximum hardness value (415 HV) can be obtained. It was found that the atomic ratios of Ti, V and Mo in (Ti, V, Mo)C carbides were changed as the CT increased. The precipitates with the size of < 10 nm were the V-rich particles at higher CT of 600 and 650 °C, while the Ti-rich particles were observed at lower CT of 500 and 550 °C. Theoretical calculations indicated that the maximum nucleation rate of (Ti, V, Mo)C in ferrite matrix occurred around 630 °C, which was consistent with the 625 °C obtained from experiment results.
TL;DR: In this paper, the hot deformation behavior of X80 steel has been investigated under different temperatures and strain rates, and the results show that the optimum processing window is at the temperature of 1300-1473 K and the strain rate of 0.01-10 s−1.
Abstract: X80 steel, as typical low-carbon Nb-V-Ti microalloyed steel, is getting more and more attention in oil and gas transmission pipeline manufacturing. In this paper, the hot deformation behavior of X80 steel has been investigated. Hot compression tests of the steel were conducted under different temperatures and strain rates. Based on the experimental data, the flow stress constitutive equations were established. It is found that the hot deformation activation energy of this steel is higher than C-Mn and low-carbon steel. Then, the kinetics model and grain size model of dynamic recrystallization were developed according to the flow curves and the optical microstructures. In addition, the processing maps were developed to analyze the workability of X80 steel at elevated temperature. The analysis results show that the optimum processing window is at the temperature of 1300-1473 K and the strain rate of 0.01-10 s−1. The microstructure observation indicates that the optimum processing parameters are applicable to the tested steel.
TL;DR: In this article, the influence of hot deformation conditions on the flow behavior and the effect of alloying elements, Nb in particular, on austenite recrystallization kinetics is highlighted.
Abstract: Recrystallization behavior of a Nb-Mo microalloyed steel has been investigated using double-hit compression tests. A suitable reheating temperature and soaking time were established for the complete dissolution of the microalloying precipitates prior to hot deformation. The influence of hot deformation conditions on the flow behavior and the effect of alloying elements, Nb in particular, on austenite recrystallization kinetics are highlighted. The strain-induced precipitation has been found to play an important role in hindering recrystallization kinetics. Post-deformation microstructural analysis indicates that strain-induced grain boundary migration (SIMB) is one of the mechanisms for the formation of recrystallized grains. Retardation of recrystallization has been explained by estimating pinning force of Nb(C, N) precipitates and recrystallization driving force at 1000 °C, which is in good agreement with the experimental observations in the present study as well as other reported data. The study brings out a better understanding on the influence of strain-induced precipitates on recrystallization behavior.
06 Dec 2018
TL;DR: In this article, the effects of hot deformation and cooling paths on the phase transformation kinetics in a precipitation-strengthened automotive 0.2C-1.5Mn-0.5Si steel with Nb and Ti microadditions were assessed.
Abstract: The aim of the present study is to assess the effects of hot deformation and cooling paths on the phase transformation kinetics in a precipitation-strengthened automotive 0.2C–1.5Mn–0.5Si steel with Nb and Ti microadditions. The analysis of the precipitation processes was performed while taking into account equilibrium calculations and phase transitions resulting from calculated time–temperature–transformation (TTT) and continuous cooling transformation (CCT) diagrams. The austenite decomposition was monitored based on thermodynamic calculations of the volume fraction evolution of individual phases as a function of temperature. The calculations were compared to real CCT and DCCT (deformation continuous cooling transformation) diagrams produced using dilatometric tests. The research included the identification of the microstructure of the nondeformed and thermomechanically processed supercooled austenite products formed at various cooling rates. The complex interactions between the precipitation process, hot deformation, and cooling schedules are linked.
TL;DR: In this article, a phenomenological constitutive model is developed to derive the flow stress at the onset of DRX and saturation stress of DRV as functions of the Zener-Hollomon parameter by the classical hyperbolic sine equation.
Abstract: The hot flow behavior of an Nb-Ti microalloyed steel is investigated through hot compression test at various strain rates and temperatures. By the combination of dynamic recovery (DRV) and dynamic recrystallization (DRX) models, a phenomenological constitutive model is developed to derive the flow stress. The predefined activation energy of Q = 270 kJ/mol and the exponent of n = 5 are successfully set to derive critical stress at the onset of DRX and saturation stress of DRV as functions of the Zener–Hollomon parameter by the classical hyperbolic sine equation. The remaining parameters of the constitutive model are determined by fitting them to the experiments. Through substitution of a normalized strain in the DRV model and considering the interconnections between dependent parameters, a new model is developed. It is shown that, despite its fewer parameters, this model is in good agreement with the experiments. Accurate analyses of flow data along with microstructural analyses indicate that the dissolution of NbC precipitates and its consequent solid solution strengthening and retardation of DRX are responsible for the distinguished behaviors in the two temperature ranges between T < 1100 °C and T ≥ 1100 °C. Nevertheless, it is shown that a single constitutive equation can still be employed for the present steel in the whole tested temperature ranges.
TL;DR: In this paper, the binding energy of Nb atom and vacancy was predicted to be 0.45 eV, indicating that Nb segregation by welding thermal cycle is probably a result of the nonequilibrium mechanism.
Abstract: Coarse-grained, welding heat-affected zone microstructure was simulated in a Nb-bearing microalloyed steel. The granular bainite with a great number of martensite-austenite (M-A) constituents was the predominant phase. Using atom probe tomography (APT), the distributions of niobium at prior austenite grain boundary (PAGB), ferrite/martensite-austenite (M-A) constituent interface (FMAI), and ferrite/ferrite interface (FFI) were investigated. The binding energy of Nb atom and vacancy was predicted to be 0.45 eV, indicating that Nb segregation by welding thermal cycle is probably a result of the nonequilibrium mechanism. The maximum enrichment of Nb was found at FMAI with enrichment factor of 3.50. Intermediate enrichment of Nb was at PAGB with enrichment factor of 3.12. The interfacial excess of Nb solute element ГNb at PAGB determined by APT was 0.27 × 1019 atoms/m2. The segregation energy was calculated to be 22.91 kJ/mol. The minimum enrichment of Nb was at FFI with an enrichment factor of 1.80.
04 Sep 2018
TL;DR: In this article, plate hot rolling conditions were reproduced in low carbon Ti-Mo microalloyed steel through laboratory simulation tests to generate different hot-rolled microstructures, and a rapid heating process was applied in order to simulate induction heat treatment conditions.
Abstract: Cost-effective advanced design concepts are becoming more common in the production of thick plates in order to meet demanding market requirements Accordingly, precipitation strengthening mechanisms are extensively employed in thin strip products, because they enhance the final properties by using a coiling optimization strategy Nevertheless, and specifically for thick plate production, the formation of effective precipitation during continuous cooling after hot rolling is more challenging With the aim of gaining further knowledge about this strengthening mechanism, plate hot rolling conditions were reproduced in low carbon Ti-Mo microalloyed steel through laboratory simulation tests to generate different hot-rolled microstructures Subsequently, a rapid heating process was applied in order to simulate induction heat treatment conditions The results indicated that the nature of the matrix microstructure (ie, ferrite, bainite) affects the achieved precipitation hardening, while the balance between strength and toughness depends on the hot-rolled microstructure
TL;DR: In this article, the influence of coiling temperature on the microalloying and properties of an (Nb, V, Ti)-containing HSLA steel was investigated. And the results showed that the grain size was refined when coiling temperatures dropped from 570°C to 450°C, which improved the yield strength of about 100 MPa.
Abstract: Traditional thermo-mechanical controlled processing (TMCP) and microalloying technology are two major methods for refining microstructure and improving the mechanical properties of HSLA steels.1) The microstructure and mechanical properties of thermo-mechanically processed hot rolled strip steels are significantly influenced by the process parameters such as rolling ratio, rolling temperature, cooling pattern, cooling rate, and coiling temperature. Controlling the coiling temperature is the most economical and efficient way to improve the properties of steels.2,3) Zhang found that the microstructure of an X70 pipeline steel transformed form granular ferrite to bainite ferrite strip as the coiling temperature decreased.4) Xu et al. studied the influence of coiling temperature on the microalloying and properties of an (Nb, V, Ti)-containing HSLA steel, and concluded that the grain size was refined when coiling temperature dropped from 570°C to 450°C, which improved the yield strength of about 100 MPa.5) Microalloying elements such as Nb, Ti, and V have been Effects of Coiling Temperature on Microstructure and Precipitation Behavior in Nb–Ti Microalloyed Steels
24 Oct 2018
TL;DR: In this paper, the effect of size, number density, and location of a coarse microalloying particle on the impact toughness of B-microalloyed steel with various Ti/N ratios was investigated.
Abstract: Simultaneously improving the toughness and strength of B-microalloyed steel by adding microalloying elements (Nb, V, Ti) has been an extensively usedmethod for researchers. However, coarse Ti(C, N) particle will precipitate during solidification with inappropriate Ti content addition, resulting in poor impact toughness. The effect of the size, number density, and location of Ti(C, N) particle on the impact toughness of B-microalloyed steel with various Ti/N ratios was investigated. Coarse Ti(C, N) particles were investigated to act as the cleavage fracture initiation sites, by using scanning electron microscopy (SEM) analysis. When more coarse Ti(C, N) inclusions were located in ferrite instead of pearlite, the impact toughness of steel with ferrite–pearlite microstructure was lower. Meanwhile, when the size or the number density of Ti(C, N) inclusions was larger, the impact toughness was adversely affected. Normalizing treatment helps to improve the impact property of B-microalloyed steel, owing to the location of Ti(C, N) particles being partly changed from ferrite to pearlite. The formation mechanism of coarse Ti(C, N) particles was calculated by the thermodynamic software Factsage 7.1 and Thermo-Calc. The Ti(C, N) particles formed during the solidification of molten steel, and the N-rich Ti(C, N) phase precipitated first and, then, followed by the C-rich Ti(C, N) phase. Decreasing the Ti and N content is an effective way to inhibit the formation of coarse Ti(C, N) inclusions.
TL;DR: In this paper, neural network models have been developed to estimate the mechanical properties of microalloyed steels containing Nb + V or Nb+ Ti, which can be used for prediction of properties in a running plant and for development of new alloys.
Abstract: The optimization of process parameters and composition is essential to achieve the desired properties with minimal additions of alloying elements in microalloyed steels. In some cases, it may be possible to substitute such steels for those which are more richly alloyed. However, process control involves a larger number of parameters, making the relationship between structure and properties difficult to assess. In this work, neural network models have been developed to estimate the mechanical properties of steels containing Nb + V or Nb + Ti. The outcomes have been validated by thermodynamic calculations and plant data. It has been shown that subtle thermodynamic trends can be captured by the neural network model. Some experimental rolling data have also been used to support the model, which in addition has been applied to calculate the costs of optimizing microalloyed steel. The generated pareto fronts identify many combinations of strength and elongation, making it possible to select composition and process parameters for a range of applications. The ANN model and the optimization model are being used for prediction of properties in a running plant and for development of new alloys, respectively.
01 Jan 2018
TL;DR: In this paper, three different microstructures, namely ferrite-pearlite, tempered martensite and ferrite bainite-martensite of 38MnSiVS5 microalloyed steel, were produced using controlled thermomechanical process.
Abstract: Three different microstructures, namely ferrite–pearlite, tempered martensite and ferrite–bainite–martensite of 38MnSiVS5 microalloyed steel, were produced using controlled thermomechanical process...
TL;DR: In this paper, the properties changes of an X52 microalloyed steel processed by a full-scale industrial electric resistant welding (ERW) equipment was studied, and it was shown that the low toughness value could only be satisfactorily addressed after a two-stage annealing process.
Abstract: Properties changes of an X52 microalloyed steel processed by a full-scale industrial electric resistant welding (ERW) equipment was studied. Lack of proper toughness level in the weld zone was investigated. Results show that the low toughness value could only be satisfactorily addressed after a two-stage annealing process. {100} pole figures for each processing step revealed a completely random texture for the initial plate. However, after the ERW process, a textured structure with the main components of brass, S and copper was obtained. Texture components were substantially weakened after the second stage of annealing and instead a cube texture was developed which could explain satisfactory level of toughness for the ERW pipe after conducting the two-step annealing process.
TL;DR: In this article, the effectiveness of Nb in retarding dynamic recrystallization (DRX) during warm forging process (700∘C-900 ∘C) and its dependence on the process parameters ( e, e ˙, T ), which are the definers of formation conditions and morphology of the strain induced Nb(C,N), were studied.
TL;DR: In this article, the effects of Nb and Ti on the microstructures, precipitates and properties of nb-Ti microalloyed steel were investigated, and the results showed that an increase in Ti content resulted in the appearance of many fine precipitates leading to a strong precipitation strengthening effect.
Abstract: Four Nb-Ti microalloyed steels were refined and rolled to study the composition optimization of Nb-Ti microalloyed steels. The effects of Nb and Ti on the microstructures, precipitates and properties of Nb-Ti microalloyed steel were investigated. The results showed that an increase in Ti content resulted in the appearance of many fine precipitates leading to a strong precipitation strengthening effect. Hence, the yield strength increased. Besides, the increased strength by the combined increase of Nb and Ti was similar to that observed for the increase in Ti content alone. This increase in strength was attributed widely to the increase in the Ti content alone rather than Nb. Moreover, the increase in Nb content beyond 0.036 wt% exerted no significant effect on the strength of Ti-Nb microalloyed steels, in which more Ti could be added to further improve the strength of steels.
21 Dec 2018
TL;DR: In this article, a layer of composite material has been developed on the surface of microalloyed steel by embedding hard SiC particles into the matrix and reinforcement of particles in a fused matrix was performed by Tungsten Inert Gas arcing on a flux coating containing SiC particle applied on the substrate.
Abstract: A layer of composite material has been developed on the surface of microalloyed steel by embedding hard SiC particles into the matrix. Reinforcement of particles in a fused matrix was performed by Tungsten Inert Gas arcing on a flux coating containing SiC particles applied on the substrate. The modified surface has been characterized by using optical microscopy, field emission scanning electron microscopy, energy dispersive spectroscopy, and x-ray diffraction analysis to confirm the formation of the composite layer. The modified surface was further examined by Vickers' microhardness testing, which confirms a noteworthy improvement in hardness of about 2.9 times when compared to that of the base metal.
TL;DR: In this paper, the solidification path of a high-temperature processed (HTP) X65 sour service steel with 0.039 wt-pct C, 0.09 wt−pct Nb, and 0.54 wt −pct Mn and its effect on the segregation, microstructure, and precipitation distribution of Ti,Nb(C,N) was studied using optical and confocal microscopy, scanning electron microscopy (SEM), and computational simulation.
Abstract: The solidification path of a high-temperature processed (HTP) X65 sour service steel with 0.039 wt pct C, 0.09 wt pct Nb, and 0.54 wt pct Mn and its effect on the segregation, microstructure, and precipitation distribution of Ti,Nb(C,N) was studied using optical and confocal microscopy, scanning electron microscopy (SEM), and computational simulation (Thermo-Calc and DICTRA). The results were compared with those obtained for another commercial microalloyed steel, containing 0.09 wt pct C, 0.04 wt pct Nb, and 0.97 wt pct Mn. The results indicate that the main parameter that influences microsegregation is the C content, which has a large influence on the solidification path. The difference in segregation between different positions in industrial continuous cast slabs of the steels was also observed, as expected. The larger solidification interval (TL-TS) of the commercial microalloyed steel indicates the formation of a solidification front that has higher solute concentration than the X65 HTP sour service steel, which concurs with the higher macro- and microsegregation observed.
TL;DR: In this paper, the structure-property relationship in heat-affected zone (HAZ) of a low-carbon steel bearing V-N subjected to gas-shielded arc welding was explored.
Abstract: The structure–property relationship in heat-affected zone (HAZ) of a low-carbon steel bearing V–N subjected to gas-shielded arc welding was explored. The microstructural characteristics of base metal (BM), coarse-grained HAZ (CGHAZ), fine-grained HAZ, and intercritical HAZ were significantly different. The effect of grain-refinement strengthening and transformation hardening on HAZ contributed to equivalent hardness of 260.8–278.5 HV in comparison with BM hardness of 272.0 HV. Moreover, excellent impact toughness at − 20 °C was obtained because of high resistance to crack propagation by high-misorientation boundaries, leading to impact fracture consisting of dimples. In CGHAZ, free N was partly fixed by V(C, N) precipitates, such that the deterioration effect of N on toughness was considered to be nearly eliminated. In comparison with CGHAZ, weld metal contained higher fraction of acicular ferrite with fine plates, while the impact toughness was inferior because of the detrimental influence of coarse inclusions from the welding wire. The nanoscale V(C, N) precipitates in CGHAZ had weak effect on toughness because of small size.