Formation Of Intragranular Ferrite Research Articles

Enhancement of Heat-Affected Zone Toughness of a Low Carbon Steel by TiN Particle

Enhancement of heat-affected zone toughness of a weight percentage of 0.014 pct Ti-bearing low carbon steel by TiN particle was investigated. An increase in nitrogen weight percentage from 0.0031 to 0.0083 pct results in increasing of number density of TiN precipitates from 4 × 103 to 3 × 105/mm2, and reduces prior austenite grain size from 850 to 350 μm with a soaking of 1673 K (1400 °C) for 2000 seconds. Effective refinement of austenite grain prohibits formation of ferrite side plate and/or upper bainite, and densely distributed TiN particles promote intra-granular ferrite formation, which is accompanied by an increase of 40 K to 60 K (40 °C to 60 °C) in austenite decomposition temperature during continuous cooling process. The changes in transformed products improved impact toughness of heat-affected zone efficiently, ex., increase absorbed energy of less than 42 J to more than 320 J with a simulated t8/5 of 550 seconds.

Effect of Carbon Content on the Potency of the Intragranular Ferrite Formation

The effect of the carbon content on the potency of the intragranular ferrite (IGF) formation for each inclusion size is investigated in the present work. The TiN inclusion was detected to be the ef ...

Quantitative Research on the Role of Large Precipitates in V–Ti Micro-Alloyed Steel during Dynamic Transformation

To research the effect of large precipitates (size > 0.2 μm) on strain-induced dynamic transformation, the variation of V contents in large precipitates has been investigated quantitatively in two V–Ti micro-alloyed steels. The results showed that high N content promoted V precipitation on the surface of Ti large precipitates rapidly. Subsequently, large precipitates containing V induced the formation of intragranular ferrite, which accelerated the dynamic transformation process remarkably, promoted the occurrence of continuous dynamic recrystallization of ferrite and improved the refinement effect.

Crystallographic analysis for acicular ferrite formation in low carbon steel weld metals

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view in low carbon low alloy steelweld metals. The samples from electro slag welding (ESW) and submerged arc welding (SAW) deposits with various cooling rates were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from inclusions. The inclusions contributing to acicular ferrite formation were of multi-phase type consisting of amorphous phase, spinel type and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and energy-dispersive X-ray spectrometer analyses that the Ti-enriched layer was TiO. The acicular ferrite had a Baker–Nutting orientation relationship with the TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO. Therefore, it is considered that TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit. However, themorphologies of ferrite growth which nucleated from inclusions were different in both samples. Whereas the growth of ferrites nucleated from TiO was enough in ESW, the size of nucleated ferrite in SAW was a few hundred millimetres in size. In the early stage of nucleation from TiO, ferrite had small deviation from Kurdjumov–Sachs orientation relationship (K–S relationship) in both ESW and SAW. However, there was a difference in the growth stage of ferrite. The ferrite orientations were gradually changed to fit to the K–S relationship in ESW. On the other hand, the nucleated ferrite in SAW stopped growing and the newly nucleated ferrite which had K–S relationship prior to austenite was formed adjacently because of large super cooling due to small heat input.

Research and Development of a Yield Strength 400 MPa Class Structural Steel Plate with Enhanced Weldability

A 400 MPa yield strength structural steel plate with enhanced weldability was produced by using advanced steel making technology and thermo-mechanical controlled processing technique. A microstructure consisting of acicular ferrite (3~8 μm) and polygonal ferrite was observed in the rolled plate, which exhibits a yield strength ≥ 420 MPa, tensile strength ≥ 560 MPa, elongation ≥ 26 % and charpy impact toughness ≥ 300 J at-40 °C. Three-wire flux copper backing submerged arc welding with heat input of 230 kJ/cm was applied to butt weld the 36 mm thick plate, and defect-free joint with satisfactory mechanical properties were produced. The coarse grain heat affected zone (CGHAZ) contains mostly intra-granular nucleated ferrite plus a few grain boundary ferrite and ferrite side plate, and shows charpy impact toughness ≥ 90 J at-40 °C. The enhancement impact toughness of CGHAZ resultant from high heat input welding is due to improvement of intra-granular ferrite formation induced by Ca and Ti containing oxides and sulphides.

Nucleation Mechanism of IGF in the HAZs of Ti-Killed HSLA Steel

The weldability of the steel can be improved by formation of intra-granular ferrite (IGF) in heat affected zones (HAZs) on the edge of weld bead. The nucleation mechanism of IGF of Ti-killed high strength low alloyed (HSLA) steel has already been investigated with the aid of transmission electron microscope. Titanium oxides (Ti2O3) particles with the diameter of 0.4μm and Si-rich complex inclusions (Ti3O5+MnS) with that of 0.5μm can serve as the nuclei of IGF. The nucleation mechanism of IGF is proposed as follows: (1) inclusions are inert substrate. (2) The depletion of the austenite former Mn local to the inclusion increases the thermodynamic driving force of γα for transformation. (3) Lattice matching between inclusion and ferrite reduces the interfacial energy of opposing nucleation.

Effect of Sulfur Content on Inclusion and Microstructure Characteristics in Steels with Ti2O3 and TiO2 Additions

Non-metallic inclusions in steels are generally considered to be detrimental for mechanical properties. However, it has been recognized that certain inclusions, such as Ti-oxide and TiN, can serve as potent nucleation sites for the formation of intragranular ferrite (IGF) in low-alloy steels. The formation of IGF could improve the toughness of the coarse grained heat affected zone (CGHAZ) of weld metals. Thus, the present thesis mainly focuses on the effect of size of nucleation sites on the IGF formation. Quantitative studies on the composition, size distribution and nucleation probability for each size of the inclusions as well as the area fraction, starting temperature and morphology of an IGF have been carried out.In the present work, the Ti-oxide and TiN powders were mixed with metallic powders. The mixed powders were heated up to the liquid state and cooled with a slow cooling rate of 3.6 oC/min. These as-cast steels with Ti-oxide and TiN additions were used to simulate the IGF formation in the CGHAZ of weld metals. Specifically, the inclusion and microstructure characteristics in as-cast steels have been investigated. The results show that the nucleant inclusion was identified as a TiOx+MnS phase in steels with Ti2O3 additions and as a TiN+Mn-Al-Si-Ti-O+MnS phase in steels with TiN additions. In addition, the TiOx and TiN phases are detected to be the effective nucleation sites for IGF formation. It is clearly shown that an increased inclusion size leads to an increased probability of IGF nucleation. This probability of IGF nucleation for each inclusion size of the TiOx+MnS inclusions is clearly higher than that of the complex TiN+Mn-Al-Si-Ti-O+MnS inclusions. In addition, the area fraction of IGF in the steels with Ti2O3 additions is larger than that of the steels with TiN additions. This result agrees with the predicted tendency of the probability of IGF nucleation for each inclusion size in the steels with Ti2O3 and TiN additions.In order to predict the effective inclusion size for IGF formation, the critical diameters of the TiO, TiN and VN inclusions, which acted as the nucleation sites of IGF formation, were also calculated based on the classical nucleation theory. The critical diameters of TiO, TiN and VN inclusions for IGF formation were found to be 0.192, 0.355 and 0.810 μm in the present steels. The calculation results were found to be in agreement with the experiment data of an effective inclusion size. Moreover, the effects of the S, Mn and C contents on the critical diameters of inclusions were also calculated. It was found that the critical diameter of the TiO, TiN and VN inclusions increases with an increased content of Mn or C. However, the S content doesn’t have a direct effect on the critical diameter of the inclusions for IGF formation. The probability of IGF nucleation for each inclusion size slightly decreases in the steel containing a higher S content. This fact is due to that an increased amount of MnS precipitation covers the nucleant inclusion surface.In the as-cast experiment, it was noted that an IGF can be formed in steels with Ti2O3 and TiN additions with a cooling rate of 3.6 oC/min. In order to control the microstructure characteristics, such as the area fraction and the morphology of an IGF, and to investigate the starting temperature of IGF and grain boundary ferrite (GBF) formation, the dynamic transformation behavior of IGF and GBF was studied in-situ by a high temperature confocal laser scanning microscope (CLSM). Furthermore, the chemical compositions of the inclusions and the morphology of IGF after the in-situ observations were investigated by using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) which equipped wavelength dispersive spectrometer (WDS). The results show that the area fraction of IGF is larger in the steels with Ti2O3 additions compared to the steels with TiN additions, after the same thermal cycle has been imposed. This is due to that the TiOx phase provides more potent nucleation sites for IGF than the TiN phase does. Also, the area fraction of IGF in the steels is highest after at an intermediate cooling rate of 70 oC/min, since the competing phase transformations are avoided. This fact has been detected by using a hybrid methodology in combination with CLSM and differential scanning calorimetry (DSC). In addition, it is noted that the morphology of an IGF is refined with an increased cooling rate.

Investigation on grain refinement and precipitation strengthening applied in high speed wire rod containing vanadium

To obtain necessary information for the simulation of high speed wire production process, the effect of grain refinement and precipitation strengthening on two high speed wire rod steels with different vanadium and nitrogen contents was investigated by continuous cooling transformation (CCT) characteristics. CCT curves were constructed by the dilatometer test and microscopic observation. Results showed that the formation of intra-granular ferrite (IGF) could refine grain remarkably and accelerate the ferrite transformation. Schedules for high speed wire production process focused on the effect of cooling rate. Ferrite grain was refined by increasing cooling rate and the formation of IGF. The microhardness calculation revealed that the steels were strengthened mostly by a combined effect of grain refinement and precipitation hardening. Degenerated pearlite was observed at lower transformation temperature and the fracture morphology changed from cementite lamellar to nanoscale cementite particle with increasing cooling rate. Based on the analysis above, an optimal schedule was applied and the microstructure and microhardness were improved.

The Effect of Inclusions on the Formation of Intragranular Ferrite

In order to make a more intuitive and easier analysis the influence of different inclusions on the formation of ferrite, a layer of high purity oxide powder was added in the two parts of the Q235 steel artificially. The effect of the inclusions Ti2O3MgOZrO2 and Al2O3 on the nucleation of intragranular ferrite were studied by means of the physical simulation method. The microstructure of the micro-zone adjacent to the inclusions was observed and the elements of the micro-zone adjacent were analyzed. The results showed that the inclusions Ti2O3 and Al2O3 can form the ferrite layers at the oxides-steels interface; the inclusions Ti2O3 has an ability to induce the nucleation of intragranular ferrite; the inclusions Ti2O3 can change the chemical composition of the metal micro-zone adjacent to the inclusions, where the Mn depletion area was observed.

Mechanisms of inclusion evolution and intra-granular acicular ferrite formation in steels containing rare earth elements

Inclusion characteristic and microstructure of rare earth (RE) elements containing steel were evaluated with scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), element-mapping, optical microscopy (OM), and automated feature analysis (AFA) option equipped with ASPEX PSEM. Factsage was used to calculate the equilibrium inclusion composition. Based on the calculation, an inclusion evolution mechanism was proposed. Furthermore, line scanning analysis was used to elucidate the intra-granular acicular ferrite (IAF) nucleation mechanism. The result showed that two difierent inclusions exist in sample steel: (Mn-Al-Si-Ti-La-Ce-O) +MnS complex inclusion and isolated MnS inclusion. Almost all nucleation sites for IAF are complex inclusions, while single MnS inclusion cannot induce IAF. A possible formation mechanism of complex inclusion is proposed based on calculated results using Factsage, which agrees well with experimental results. A Mn-depletion zone (MDZ) which exists adjacent to the (Mn-Al-Si-Ti-La-Ce-O) +MnS complex inclusion can account for the IAF formation. However, the low volume fraction (1.49£10 i7 )

Effect of Heat Input on Toughness of Coarse-Grained Heat-Affected Zone of an Ultra Low Carbon Acicular Ferrite Steel

The effect of heat input on toughness of coarse-grained heat-affected zone of an ultra low carbon acicular ferrite steel were investigated when the welding was conducted with high heat input. Microstructural observations, energy dispersive X-ray spectroscopy analyses were conducted using optical microscopy, scanning electron microscopy and transmission electron microscopy, respectively. The microstructures of coarse-grained heat-affected zone consist of predominantly bainitic microstructure and a small proportion of acicular ferrite grains. The bainitic microstructures become coarsened with increasing heat input. The impact toughness of coarse-grained heat-affected zone remained at a higher level when the heat input ranged from 42 to 55 kJ/cm. It became not stable and dropped to a lower level when the heat input increased to 110150 kJ/cm. The enhancement in impact toughness was attributable to the MnS precipitation on the pre-formed Ti oxides as well as the formation of intragranular ferrite. When specimens were welded with higher heat input, the deterioration of impact toughness was caused by the coarsening of austenite grains.

Investigation on Ti-Deoxidized Products Inducing Formation of Intragranular Ferrite

Steel samples were prepared using a vacuum carbon deoxidization process combined with a final Ti-deoxidation process. Ti-deoxidized products inducing formation of intragranular ferrite (IGF) was investigated by using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) through various specimens by different methods, as follows: metallographic specimens, electrolyzation, ion thinning technology, RTO method and so on. Mn is found to be more extensively distributed than S in the core of inclusion, which shows that Ti2O3 has character of absorbing Mn. The Mn-depleted zone (MDZ) exists in the peripheral steel of inclusion, the more near the inclusion, the more obvious depletion of Mn. It is also found that outer layer of inclusion exists S-rich and Mn depletion area in specimens prepared using RTO method under TEM. Mn- depleted zone in steel and Mn-depleted but S-rich layer in the periphery of the inclusion indicate that Mn diffuses and diverts is step by step in steel and inclusion, and the extent of S-rich and Mn depletion area should be related with degree of Mn absorbed by Ti2O3 and original precipitation amount of MnS.

Influence of Ce on Characteristics of Inclusions and Microstructure of Pure Iron

The experiments on adding Ce, Mn and S to molten iron have been carried out by means of the electric furnace, and the concentration of oxygen, the evolution of inclusions and microstructures of samples are analyzed, indicating that the concentration of dissolved oxygen decreases greatly after adding Ce. The calculated thermodynamics results at 1273 K performed on Factsage show that Ce 2O 2S and CeAlO 2 are formed with the increase of Ce content for w(Ce) > 0.0002 and w(S) < 0.0001, while three cerium sulphides (CeS, Ce 3S 4 and Ce 2S 3, in sequence), MnS and Ce 2O 2S are formed with the increase of S content and the decrease of Ce content for w(Ce) > 0. 0002 and w(S) > 0.0001. These results are consistent with the SEM observations. Finally, some possible reasons are given to explain the formation of intragranular ferrite in the pure iron.

Effects of Soluble Ti and Zr Content and Austenite Grain Size on Microstructure of the Simulated Heat Affected Zone in Fe–C–Mn–Si Alloy

The effect of soluble Ti and Zr contents on the microstructure in a simulated heat affected zone of a Fe-0.05mass%C-1.5mass%Mn-0.20mass%Si alloy was studied. This was done as a function of the austenite grain size as well as the number and size of TiN, Ti2O3, MgO(+Ti2O3), TiN(+MgO) and ZrO2(+Ti2O3) particles under different peak temperature (1300 and 1400°C), peak holding time (0, 60 and 600 s) and a constant time of cooling (70 s) from 800 to 500°C. The effectiveness of inclusion phases for intragranular ferrite (IGF) formation was studied from the area fraction of IGF for a given austenite grain size and soluble Ti content. For a given austenite grain size, the primary ferrite decreases and the IGF increases with an increased soluble Ti content up to about 100 ppm. After going through a maximum the replacement of IGF by lath bainitic ferrite occurs with an increased soluble Ti content. It is found that the effect of soluble Zr content on the microstructure is much stronger than that of a soluble Ti content. The area fraction of IGF decreases gradually with an increase in the austenite grain size (50–1000 μm) for a given soluble Ti content (50–400 ppm). By using the hardenability parameter based on the non-equilibrium grain-boundary segregation model, it is observed that the IGF formation is encouraged in the following order: TiN>{Ti2O3, MgO(+Ti2O3)}>ZrO2(+Ti2O3).

Ferrite Formation Behaviors from B1 Compounds in Steels

The effect of chemical composition of B1 compounds on the formation of intragranular ferrite during isothermal austenite-to-ferrite transformation was investigated by embedding single crystalline grains of chemically pure B1 compounds in low-alloy steel. Two kinds of B1 compounds having similar misfit against ferrite, TiO and TiN, were adopted in this study. Isothermal transformation experiments were carried out both above and below the bainitic transformation temperature. The orientation relationships between compound and ferrite and between ferrite and prior austenite were characterized by scanning electron microscope equipped with electron backscatter diffraction analyzer.It was found that the Baker-Nutting (B-N) orientation relationship played a dominant role in ferrite formation from B1 compounds at higher transformation temperature, but some low-index orientation relationships other than the Baker-Nutting orientation relationship were also observed as the transformation temperature decreased, which led to an increase in ferrite formation having the Kurdjumov-Sachs (K-S) orientation relationship with prior austenite. Ferrites from TiO had larger misorientation angles from the exact B-N orientation relationship and thus tended to have the K-S orientation relationship with prior austenite at lower transformation temperatures.

In-situ observation of phase transformation during rapid cooling processes

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view to understand the formation mechanism for acicular ferrite microstructure in low alloy steel laser weld metals. The sample was low carbon Ti-B weld metals with an oxygen content of 480 ppm. In this sample, intragranular acicular ferrite formation was observed from some inclusions and acicular ferrite had Kurdjumov-Sachs orientation relationship with austenite matrix. The multi-phase inclusions contributed to nucleation of acicular ferrite. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction and EDS analysis that the Ti-enriched layer was TiO. The acicular ferrite had Baker-Nutting orientation relationship with TiO layer on the inclusion surface. The lattice misfit was 3.0 % at the interface between the acicular ferrite and TiO.Therefore, it is considered that the TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit.

Phase Equilibrium between CaO·Al2O3 Saturated Molten CaO–Al2O3–MnO and (Ca, Mn)S Solid Solution

Refinement of austenite grains by generating intra granular ferrite (IGF) in austenite grains is one of the most effective methods for improving the toughness of the heat-affected zone (HAZ). It is known that MnS precipitated on fine oxides enhance formation of IGF in austenite grains. Flux composed of CaO–Al2O3 is often used in secondary refining and Ca–Mn–Al complex oxide may form, and it is important to clarify the mechanism of MnS precipitation on oxide inclusions. Therefore, molten CaO–MnO–Al2O3 oxide was equilibrated with (Ca, Mn)S solid solution in Al2O3 crucible in an electric resistance furnace at 1648, 1698 and 1748 K in the present work. Phase relation between CaO·Al2O3 saturated molten CaO–MnO–Al2O3 oxide and (Ca, Mn)S solid solution was clarified.

Ti-Deoxidized Products and Formation Mechanism of Intragranular Ferrite in High Grade Pipeline Steels

High grade pipeline steels were prepared using vacuum carbon deoxidization process combined with a final Ti-deoxidation process. The microstructure of the as-cast steels was investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM observation shows that the formation of intragranular ferrite (IGF) structure is induced by fine inclusions. TEM selected area diffraction (SAD) patterns and elemental distribution analysis indicate that these inclusions are mainly Ti 2O 3 and MnS. It is also found that Ti 2O 3 may act as nucleus in the formation of MnS during solidification process. Raman spectroscopic analysis demonstrates the presence of another phase, MnTiO 3, which could be formed through entrapment of Mn by Ti 2O 3. It is believed that the formation of Mn-depleted region in the inclusions and thus the formation of MnTiO 3 phase will increase the Mn pickup from matrix and promote the formation of IGF during solidification of molten steel.

Thermodynamic Calculation and MnS Solubility of Mn-Ti Oxide Formation in Si-Mn-Ti Deoxidized Steel

Mn-Ti oxides in Si-Mn-Ti deoxidized steels after cooling in the furnace were investigated. The composition and morphology of inclusions were analyzed by using FE-SEM with EDS. Mn-Ti oxides were found to be effective sites to induce intragranular ferrite formation. The thermodynamic calculation was employed to interpret the critical condition for Mn-Ti oxide formation. Mn-Ti oxide formation was controlled not only by Mn and Ti content, but also by total oxygen content in steel. When the Mn and Ti contents were around 1.5% and 0.005%–0.01%, respectively, Mn-Ti oxide could form as the total oxygen content was 0.001%–0.002%. The experimental results were in good agreement with thermodynamic calculation results. Also, MnS solubility was examined in Mn-Ti oxide inclusion system. With an increase of MnO content in Mn-Ti oxide, MnS solubility in the oxides increased. MnS precipitation benefited from high MnO content in Mn-Ti oxide.

Microstructure evolution of medium carbon steel during manufacture process for non-quenched and tempered oil well tubes

The microstructure evolution of medium carbon microalloyed steel during the manufacture sub-processes of N80 grade hot rolling seamless oil well tubes was investigated by compression simulation on Gleblee 1500 thermal simulator and industrial experiments of seamless tubes production. The results show that the tested steel has good anti-coarsening ability at equalization sub-procedure. The quantitative analysis of austenite grain size has illustrated that the average austenite grain size in the steel would almost always continuously increase in the tube manufacture process till just before the stretch-reduction-diameter deformation. The most effective microstructure refinement in industrial experiment did not occur in austenite state, but mainly in austenite decomposition following the stretch-reduction-diameter process. The intra-granular ferrite formation should be very useful for effective refinement of the whole microstructure. The variation tendencies of austenite grain size in simulated and industrial experiments are nearly the same, while the optical microstructures and precipitation characterization are quite different.