Intragranular Acicular Ferrite Research Articles

Inclusions and microstructures in coarse-grained heat-affected zone of Al–Ti–Ca deoxidized shipbuilding steels with different Al contents after high-heat input welding

The inclusions, microstructure evolution and impact toughness of the coarse-grained heat-affected zone (CGHAZ) of Al–Ti–Ca deoxidized shipbuilding steel plates with different Al contents after high-heat input welding of 400 kJ cm−1 are investigated. The characteristics of inclusions and their influence on the refinement of microstructure and impact toughness in CGHAZ was elucidated. Increasing the Al content from 0.016 wt% to 0.022 wt% and 0.043 wt% results in an increase in the average size of prior austenite grain and the brittle phase fractions, while decreasing the content of intragranular acicular ferrites (IAFs). The results of the thermodynamic calculations, high-temperature laser scanning confocal microscopy and differential scanning calorimetry all show that increasing the Al content can diminish the austenitic stable region, elevate the transition temperature from austenite to ferrite and suppress the nucleation of IAF. In the present steels, the IAF formation is successfully induced by the specific inclusions, namely Al2O3–CaO–Ti2O3–MnS(-CaS), in which the calcium aluminate inclusions are embedded with Ti-oxides and CaS–MnS precipitates. The effective inclusions have the number density of 93 mm−2 in 16Al steel and 27 mm−2 in 22Al steel, but they are rarely found in 43Al steel. On average, an effective inclusion can induce 3.4 IAFs in 16Al steel and 2.8 IAFs in 22Al steel. Thus, as the Al content increases from 0.016 wt% to 0.22 wt% and 0.043 wt%, the impact toughness of CGHAZ at −40 °C decreases from 134 J to 78 J and 24 J with the welding heat input of 400 kJ cm−1.

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 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.

Influence of Ti/N Ratio on Inclusions, Microstructures, and Toughness in Heat‐Affected Zone of Shipbuilding Steel Plates with Mg Deoxidation after High Heat Input Welding

The inclusions, microstructures, and toughness in heat‐affected zone (HAZ) of the shipbuilding steel plates with Mg deoxidation and the different Ti/N ratios are studied after the high heat input welding of 400 kJ cm−1. With increasing the Ti/N ratio from 3.00 to 5.67, the average size of inclusions increases, the mass fraction of total TiC inclusions increase from 2.8% to 5.0%, and the average size of MgO–TiN–MnS increases from 2.82 to 5.49 μm, while the number density that can induce intragranular acicular ferrite (IAF) nucleation decreases from 87 to 62 mm−2. These changes in inclusions are detrimental to induce the IAF nucleation. The HAZ microstructures are composed of the IAFs, polygonal ferrites, and upper bainites (UBs). With increasing the Ti/N ratio, the area fraction of the IAFs decreases from 44% to 14%, and the area fraction of UBs increases from 1.8% to 9.5%. The HAZ fracture mode changes from ductile fracture to brittle fracture after the high heat input welding of 400 kJ cm−1, and the HAZ toughness decreases from 183.3 to 48.7 J at −20 °C.

Review of Research on Titanium Oxides in Low‐Carbon Steel

Titanium oxides, which act as nuclei for intragranular ferrite (IGF), are common inclusions in low‐carbon steel. The interlocking structure of IGF greatly improves the mechanical properties of low‐carbon steel. To make the best use of this kind of oxide, the types, sizes, and distribution properties of effective titanium oxides are reviewed that can promote IGF nucleation. The control techniques of titanium oxides are summarized as follows: (1) Ti2O3 is one of the most effective of the common titanium oxides, followed by TiO and TiO2. The type of titanium oxide can be affected by controlling the Ti/O mass ratio in molten steel. (2) Titanium oxide with a size range of 0.4–4 μm can effectively promote the nucleation of IGF or intragranular acicular ferrite (IAF), among them, titanium oxide of about 1 μm is the most beneficial to IAF nucleation. (3) To obtain titanium oxides with effective size, the precipitation of titanium oxides should be controlled during the solidification process, and this process can be achieved with the existing rapid solidification technology. Under the same conditions, the higher solidification rate can obtain finer titanium oxide.

Elucidating the heat input on CGHAZ microstructure and its irregular effect on impact toughness for a novel V–N microalloying weathering steel

This study investigated the effect of heat inputs on coarse-grained heat affected zone (CGHAZ) microstructure and impact properties of a novel vanadium (V) and nitrogen (N) microalloyed weathering steel via a Gleeble™ system, revealing the metallurgical essence of the steel suitable for large heat input (Ej) welding. The results indicate that in CGHAZ, the dominant microstructure was composed of lath bainitic ferrite (LBF) at Ej = 15 kJ/cm, granular bainitic ferrite (GBF) at Ej = 35 kJ/cm, and a mixture of intragranular acicular ferrite (IGAF), intragranular polygon ferrite (IGPF) and grain boundary polygonal ferrite (GBPF) at Ej = 55 and 75 kJ/cm, respectively, apart from martensite/austenite (M/A) constituents in each one. The area fraction, fM/A, and mean size, dM/A, of M/A constituents increased monotonously with the Ej. In addition, with increasing Ej from 15 to 35 kJ/cm, the fraction, fMTAs＞15°, of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) greater than 15° reduced, while the mean equivalent diameter, MEDMTA≥15°, of grains with HAGBs increased. However, as Ej further increased to 55 and 75 kJ/cm as the two large heat inputs, the precipitation of (Ti, V) (C, N) and thereby the heterogeneous nucleation of polygonal and acicular ferrite were promoted by a lowered degree of undercooling, resulting in the decreased MEDMTA≥15° and the increased fraction fMTAs＞15°. Accordingly, the impact toughness of CGHAZ was degraded with the increase of Ej from 15 to 35 kJ/cm, but enhanced with the increase of Ej from 35 to 75 kJ/cm.

Effect of Ca Deoxidation on Toughening of Heat-Affected Zone in High-Strength Low-Alloy Steels after Large-Heat-Input Welding

Large-heat-input welding can effectively increase the efficiency and reduce the cost of manufacturing a super-large container ship for marine trade worldwide with thick, high-strength low-alloy (HSLA) steel plates; however, it significantly degrades the toughness of the welding heat-affected zone (HAZ). This paper describes the effect of Ca deoxidation on the impact toughness of simulated coarse-grained HAZs (CGHAZs) in HSLA steels after large-heat-input welding at 400 kJ cm−1. The average impact energy of the CGHAZ increases with an increase in Ca content; in particular, the energy of the steel with 25 ppm Ca content is satisfactorily high, owing to the uniform and fine prior austenite grains. In contrast, the grains in the CGHAZs of the steels with relatively low Ca contents are not uniform, leading to large test variabilities at −20 °C. Failure analyses reveal that the major and secondary cracks preferentially propagate along the coarse and brittle grain boundary ferrite (GBF), but their propagation is retarded by the fine and interlocking intragranular acicular ferrite (IAF) as the grain size decreases. It is therefore recommended to increase the Ca content to about 25 ppm during the deoxidation of steelmaking to improve HAZ toughness by forming fine and uniform prior austenite grains and IAF within grains.

Evaluation of Mechanical Properties and Microstructure of X70 Pipeline Steel with Strain-Based Design

The microstructure and mechanical properties of X70 pipeline steel with a ferrite/martensite dual-phase microstructure produced by thermo-mechanical controlled processing were investigated by tensile tests, Charpy V-notched (CVN) impact tests, drop-weight tear tests, guided-bend tests, scanning electron microscopy and transmission electron microscopy combined with thermodynamic simulation analysis. All the mechanical properties met the strength, ductility, toughness and deformability properties requirements of X70 grade pipeline steel with strain-based design. The shear fracture area and absorbed energy of CVN at −10 °C were >97% and >205 J in base metal (BM), weld metal (WM) and heat affected zone (HAZ) with low transition temperature, indicating adequate resistance to propagating fracture. The microstructure of WM was mainly intragranular acicular ferrite that can guarantee high strength, toughness and over matching requirements of the welded joint. Because of being exposed to successive heat inputs, the ferrite plus martensite/bainite microstructure of BM was heated between Ac1 and Ts forming the HAZ. However, a high CVN impact toughness of 345 J at −10 °C in HAZ was obtained, which indicated that the excellent mechanical properties of BM would not be seriously deteriorated during the welding thermal cycles with the reasonable addition of Ti and Nb.

Effect of B Segregation at Prior Austenite Grain Boundaries on Microstructures and Toughness in the Heat‐Affected Zone of Mg‐Deoxidized Shipbuilding Steel Plates

The effect of B segregation at prior austenite grain boundary (PAGB) on the microstructure and toughness in the heat‐affected zone (HAZ) after high heat input welding at kJ cm−1 for Mg‐deoxidized shipbuilding steel plates is clarified. With 0.0022 wt.% B addition, B segregation occurs at PAGBs in two forms: solute B and fine Fe23(BC)6 precipitates. B segregation with a concentration of about 4 at.% decreases boundary energy by 0.13 J m−2, which reduces the nucleation density of boundary ferrites by increasing the critical activation energy of ferrite nucleation. Thus, coarse grain boundary ferrites (GBFs) and ferrite side plates (FSPs) are suppressed with film‐like allotriomorphic ferrites formed at PAGBs, and ferrite transformation temperature reduces from 712–543 to 679–541 °C. In B‐free steels, (Fe, Mn)3C particles form at PAGBs with the Mn‐depleted zone (MDZ) at around (Fe, Mn)3C, promoting GBFs and FSPs. However, (Fe, Mn)3C precipitates and MDZ are not observed at PAGBs with B addition, because B segregation delays Mn diffusion to PAGBs. Thus, GBFs and FSPs reduce, and intragranular acicular ferrites (IAFs) increase with B addition, making the dislocation density and high‐angle grain boundary density increase, which improve HAZ toughness at −40 °C from 40 to 141 J.

The Formation Mechanisms and Evolution of Multi-Phase Inclusions in Ti-Ca Deoxidized Offshore Structural Steel

To understand and clarify the formation mechanisms and evolution of complex inclusions in Ti-Ca deoxidized offshore structural steel, inclusions in industrial steel were systematically investigated. The number density of total inclusions generally decreased from Ladle Furnace (LF), Vacuum Degassing (VD), Tundish to the final product except for Ti and Ca addition. The major inclusions during the refining process were CaO-Al2O3-SiO2-(MgO)-TiOx and CaO-Al2O3-SiO2. CaO-Al2O3-SiO2-(MgO)-TiOx inclusion initially originated from the combination of CaO-SiO2-(MgO) in refining slag or refractory and deoxidization product Al2O3 and TiO2. With the refining process proceeding and Ca addition, the Al2O3 concentration in the CaO-Al2O3-SiO2-(MgO)-TiOx inclusions gradually dropped while the CaO and TiO2 concentrations gradually increased. The CaO-Al2O3-SiO2 inclusions originally came from refining slag, existing as 2CaO∙ Al2O3∙ SiO2, and maintained a liquid state during the early stage of LF. After Ca treatment, it was gradually transferred to 2CaO∙ SiO2 due to Al2O3 continuously being reduced by Ca. The liquidus of 2CaO∙ SiO2 inclusion was higher than that of molten steel, so they presented as a solid-state during the refining process. After welding thermal simulation, CaO-Al2O3-SiO2-(MgO)-TiOx inclusions were proven effective for inducing intragranular acicular ferrite (IAF) while CaO-Al2O3-SiO2 was inert for IAF promotion. Additionally, Al2O3-MgO spinel in multiphase CaO-Al2O3-SiO2-(MgO)-TiOx inclusion has different formation mechanisms: (1) initial formation as individual Al2O3-MgO spinel as a solid-state in molten steel; (2) and it presented as a part of liquid inclusion CaO-Al2O3-SiO2-(MgO)-TiOx and firstly precipitated due to its low solubility.

Improvement of impact toughness by microstructure refinement of simulated CGHAZ through enhancing welding heat input of low carbon Mo-V-Ti-N-B steel

Welding heat input greatly influences the microstructure and impacts the affected zone’s toughness. To interpret the relationships between the welding heat input, microstructure, and low-temperature toughness of the coarse-grained heat-affected zone (CGHAZ) of Mo-V-Ti-N-B steels, welding heat cycles with different heat inputs (25–75 kJ cm−1) were performed on a Gleeble 3500 simulator. Intragranular ferrite in the simulated samples subjected to different thermal cycles was characterized and quantified, and the impact energies of simulated samples were evaluated at −20 °C. Upon increasing the heat input, the intragranular ferrite content rose sharply from 4.3% to 76.0%. The V(C, N) enrichment on the precipitate surface increased the size of precipitates, providing favourable nucleation conditions for intragranular ferrite. The prior austenite grain (PAG) and martensite/austenite (M/A) constituents became rough, and the content of the M/A constituent increased while the impact energy of the CGHAZ increased. This behaviour occurred due to the formation of intragranular acicular ferrite (IGAF), which refined the microstructure of the CGHAZ. Grain refinement eliminated the negative influence of higher M/A content on the impact toughness of the CGHAZ.

Extremely high temperature carbide precipitation induced intragranular acicular ferrite transformation of M2 steel during semi-solid cooling

Intragranular acicular ferrite (IAF) transformation of M2 steel occurred at extremely high temperature (1180 ∼ 960 °C) during semi-solid cooling. The uncommon IAF transformation was observed in situ via confocal laser scanning microscope. Small VC carbide particles precipitated explosively during semi-solid cooling and they acted as the preferential site for the nucleation of IAF due to the low lattice misfit with ferrite. The length of ferrite lath showed a bimodal behavior that long primary ferrite laths and short secondary ferrite laths both existed. The acicular laths intersected with each other and terminated at the solid boundary or interface. The prior solid austenite grain was filled with acicular ferrite. The IAF might help refine the effective grain size and improve the strength of semi-solid M2 steel.

Effect of Ferritic Morphology on Yield Strength of CGHAZ in a Low Carbon Mo-V-N-Ti-B Steel

For investigating the impact of ferritic morphology on yield strength (YS) of the high-heat-input welding induced coarse-grained heat-affected zone (CGHAZ) of a low carbon Mo-V-N-Ti-B steel, a group of particular welding heat inputs were designed to obtain different ferritic microstructures in CGHAZ. The tensile properties were estimated from typical samples with ferritic microstructures. The mixed microstructures dominated by the intragranular polygonal ferrite (IGPF), the intragranular acicular ferrite (IGAF), and the granular bainite (GB) were obtained at the heat inputs of 35, 65, 85 and 120 kJ/cm, respectively. When the main microstructure changed from IGPF to IGAF and GB, YS increased first and then decreased. The microstructure consisting mainly of IGAF possessed the maximum YS. As the main microstructure changed from IGPF to IGAF and GB, the contribution of grain refinement strengthening to YS was estimated to be elevated remarkably. This means the strength of CGHAZ in a low-carbon steel subjected to the high-heat-input welding could be enhanced by promoting the fine-grained AF and GB formation.

N-induced microstructure refinement and toughness improvement in the coarse grain heat-affected zone of a low carbon Mo–V–Ti–B steel subjected to a high heat input welding thermal cycle

In the coarse grain heat-affected zone (CGHAZ) of a low carbon Mo–V–Ti–B steel subjected to a simulative welding thermal cycle with a heat input as high as 75 kJ/cm, the influence of an increase in nitrogen content from 0.0085 to 0.0144 wt% on the microstructures and impact properties of the steel was comparatively estimated using Gleeble-3800 simulation, microstructure characterization and impact testing. With a rise in the nitrogen content, the number of fine and coarse particles also increased. An increase in the number of fine precipitates can effectively pin the prior austenite grain (PAG) boundary and inhibit the grain growth, resulting in the refinement of PAG in the CGHAZ. The intragranular and intergranular V-containing coarse particles effectively promoted the nucleation of intragranular acicular ferrite (IGAF) and grain boundary polygonal ferrite (GBPF). Accordingly, the contents of heterogeneously-nucleated IGAF and polygonal ferrite (PF) increased; however, the bainite content decreased with a rise in N content, leading to a prominent microstructure refinement in this CGHAZ with a high heat input. Furthermore, the nitrogen addition led to the formation of finer and more dispersed martensite-austenite (M/A) constituents in the CGHAZ. Therefore, the impact toughness of this CGHAZ was enhanced. This can be attributed to the microstructure refinement of the CGHAZ, which in turn was ascribed to an increasing nitrogen.

Evolution of Al–Ti Oxide Inclusions in Fe-Based Alloys During Thermal Holding by a Novel Inductive Separation Method

Steelmaking has undergone significant technological advancements, particularly regarding oxide metallurgy. Herein, we employed a novel method of inductive separation for separating Al–Ti oxide from Ti-bearing oxides to trace the evolution of a specific type of oxide in a solid Fe-based alloy during thermal holding. The evolution occurred within the solid alloy; hence surface effects were prevented. The evolution was investigated at a lower temperature than what was used in previous studies; using this information, the impact of temperature was ascertained. The results revealed that, after holding for 0.5 hours at 1273 K, the shape of the Al–Ti oxide changed from spherical to irregular. The homogeneous oxide changed to a heterogeneous oxide containing Al- and Ti-rich parts, and the compositional deviation became distinct after thermal holding. After 12 hours, the composition of the Al-rich parts approached that of Al2O3. A comparison of the variation of the Al–Ti oxide at 1273 K and 1573 K revealed that the ratio of the heterogeneous oxide increased at lower temperatures due to the greater degree of supersaturation of Al2O3. The mechanism involved the precipitation of Al2O3 from Al–Ti oxide during holding. The fundamental data on the time-dependent variation in the morphology and composition of the Al–Ti oxide function as a first step to modify the oxide using thermal holding to promote the induction of an intragranular acicular ferrite (IAF) microstructure.

Effects of coarse particles, prior austenite grains, and microstructures on impact toughness in heat-affected zone of Mg deoxidation steel plates without or with Al addition

ABSTRACT The effects of coarse TiN particles, prior austenite grains, and intragranular microstructures on the impact toughness in heat-affected zone of Mg deoxidation steel plates without Al (0Al) or with 0.02 wt-% Al (200Al) were studied from the cleavage initiation and propagation. The coarse particles are mainly TiN. 0.02 wt-% Al addition increases coarse TiN particles (>0.5 μm) from 124 to 151 mm–3, increases brittle microstructures from 25% to 78%, and increases dislocations between ferrite laths, which impairs the toughness, although the austenite grain size is reduced from 213 to 184 μm. Coarse TiN particles alone can’t govern the brittle fracture with the additional effect of microstructures in the case of intragranular acicular ferrites (IAFs). The importance of intragranular microstructures on preventing crack propagation is greater than that of austenite grain boundaries when the grain size is smaller than a critical size to suppress the IAF formation for ferritic steels.

The effect of M (M = Ce, Zr, Ce–Zr) on rolling microstructure and mechanical properties of FH40

AbstractCe, Zr and Ce–Zr composite experimentl steel were prepared by vacuum induction furnace and 550 twin-roll reversible rolling mill. Optical microscope (OM), scanning electronic microscopy (SEM) and energy dispersive spectrometer (EDS) were used to observe the rolling microstructure of the experimental steel. The mechanical properties of the experimental steel were tested and analyzed. The effect of cerium zirconium oxide inclusions on nucleation, tensile and impact fracture mechanism of intragranular acicular ferrite (IAF) was investigated. The results show that the rolling microstructure of steel containing 0.0052% Ce and the steel under composite treatment containing 0.0053% Ce and 0.0055% Zr is refined. IAF generation can be induced by Al–Ce–O inclusion of the size of 4 µm or induced by Al–Ce–Zr–O + MnS inclusion of the size of 3 µm. The yield strength and tensile strength of the steel treated by Ce–Zr are 428 and 590 MPa, respectively, the elongation is 23.55%, the longitudinal impact energy at −60°C is 189 J, which are 31, 45, 46 J and 6.25%, respectively, higher than those of the matrix steel. The dimple of the experimental steel at the fracture surface is larger and deeper than that of the matrix steel. The small inclusions in uniform distribution contribute to the high tensile strength of the experimental steel.

In-depth analysis of intragranular acicular ferrite three-dimensional morphology

Precipitation of intragranular acicular ferrite (IAF) is the principle means for increasing the strength of steel. This study investigated the three-dimensional morphology and growth behavior of IAF. Focused ion beam, confocal laser scanning microscopy and electron backscattered diffraction revealed that IAF had a plate-like morphology with numerous pits and defects when precipitated at 750 °C for 1 min and was needle-like with a smooth surface when precipitated at 700 °C for 1 min. Furthermore, sub-grains were observed within the IAF. This study elucidates more on the theoretical data of the IAF 3D morphology and provides novel insights into IAF.

An in-situ microscopy study on nucleation and growth of acicular ferrite in Ti-Ca-Zr deoxidized low-carbon steel

We describe here an in-situ microscopy study on the nucleation and growth of acicular ferrite in Ti-Ca-Zr deoxidized low-carbon steel as a function of cooling rate. The study indicated the starting transformation temperature of acicular ferrite decreased with increasing cooling rate. Moreover, the average growth rate of acicular ferrite was increased from ~2.2 μm/s to ~14.4 μm/s with the increase of cooling rate from 2 °C/s to 15 °C/s. In Ti-Ca-Zr deoxidized steel, (Ti, Ca, Zr, Al)-O-Mn-S oxide inclusions effectively promoted the formation of intragranular acicular ferrite. A significantly high proportion of acicular ferrite was characterized by high angle grain boundaries and reduced grain size. When the cooling rate was low at 5 °C/s, the proportion of acicular ferrite was high at ~85%, with ~48% high angle grain boundaries and small average grain size of ~2.7 μm. Additionally, MnS precipitated in the vicinity of (Ti, Ca, Zr, Al)-O-Mn-S oxide particles, which promoted the nucleation of acicular ferrite.

Mechanism of Improving Heat-Affected Zone Toughness of Steel Plate with Mg Deoxidation after High-Heat-Input Welding

In the present study, the mechanism of improving HAZ toughness of steel plate with Mg deoxidation after the simulated welding with the heat input of 400 kJ/cm was investigated through in situ observation, characterization with SEM-EDS and TEM-EDS, and thermodynamic calculation. It was found that intragranular acicular ferrite (IAF) and polygonal ferrite (PF) contributed to the improvements of HAZ toughness in steels with Mg deoxidation. With the increase of Mg content in steel, the oxide in micron size inclusion was firstly changed to MgO-Ti2O3, then to MgO with the further increase of Mg content in steel. The formation of nanoscale TiN particles was promoted more obviously with the higher Mg content in the steel. The growth rates of austenite grains at the high-temperature stage (1400~1250 °C) during the HAZ thermal cycle of steels with conventional Al deoxidation and Mg deoxidation containing 0.0027 and 0.0099 wt% Mg were 10.55, 0.89, 0.01 μm/s, respectively. It was indicated that nanoscale TiN particles formed in steel with Mg deoxidation were effective to inhibit the growth of austenite grain. The excellent HAZ toughness of steel plates after welding with a heat input of 400 kJ/cm could be obtained by control of the Mg content in steel to selectively promote the formation of IAF or retard the growth of austenite grain.