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Comparison between the rapid growth starting temperature T rg CGM and the phase transformation temperature T LEM. T rg CGM was calculated by CGM with Eq. (7), using the experimentally evaluated d g 2 S 1 2 and a cooling rate of 1.0 K/s. T LEM was evaluated for various f g values from 0.8 to 1.0 by the LEM method.
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Based on a grain growth model, this paper discusses the gamma grain refinement by phosphorus (P) in as cast 0.1 mass% C slabs. Two important factors, the starting temperature of the rapid grain growth (T-rg) and the grain growth rate, are especially focused in the model. Local Equilibrium Mapping (LEM), an analytical method with local equilibrium c...
Citations
... In addition, austenite grain coarsening is a huge problem in the continuous casting of hypo-peritectic steels [151]. Scientists also proposed the idea of grain refinement by decreasing the depth of oscillation marks [152], increasing the cooling intensity [153][154], dynamic recrystallization [155], element alloying [156][157], and surface-structure control cooling [158][159], but they have not been successfully utilized yet. ...
Hypo-peritectic steels are widely used in various industrial fields because of their high strength, high toughness, high processability, high weldability, and low material cost. However, surface defects are liable to occur during continuous casting, which includes depression, longitudinal cracks, deep oscillation marks, and severe level fluctuation with slag entrapment. The high-efficiency production of hypo-peritectic steels by continuous casting is still a great challenge due to the limited understanding of the mechanism of peritectic solidification. This work reviews the definition and classification of hypo-peritectic steels and introduces the formation tendency of common surface defects related to peritectic solidification. New achievements in the mechanism of peritectic reaction and transformation have been listed. Finally, countermeasures to avoiding surface defects of hypo-peritectic steels duiring continuous casting are summarized. Enlightening certain points in the continuous casting of hypo-peritectic steels and the development of new techniques to overcome the present problems will be a great aid to researchers.
... Many works have studied the mechanisms for the formation and transformation of FCGs. Some studies indicated that FCGs grow continuously and eventually convert to CCGs when the temperature falls below T γ , as shown in Fig. 1(a) [6][7][8]. Recent studies using solidification equipment and phase-field simulations suggested that the transformation from FCGs to CCGs is discontinuous, and the formation of CCGs is accompanied by the rapid movement of FCG/CCG region boundaries (FCRB) along the unidirectional temperature gradient, as shown in Fig. 1(b) [9][10]. ...
The effect of titanium content on the refinement of austenite grain size in as-cast peritectic carbon steel was investigated by fast directional solidification experiments with simulating the solidification and growth of surface and subsurface austenite in continuously cast slabs. Transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to analyze the size and distribution of Ti(C,N) precipitates during solidification. Based on these results, the pinning pressure of Ti(C,N) precipitates on the growth of coarse columnar grains (CCGs) was studied. The results show that the austenite microstructure of as-cast peritectic carbon steel is mainly composed of the regions of CCGs and fine columnar grains (FCGs). Increasing the content of titanium reduces the region and the short axis of the CCGs. When the content of titanium is 0.09wt%, there is no CCG region. Dispersed microscale particles will firstly form in the liquid, which will decrease the transition temperature from FCGs to CCGs. The chain-like nanoscale Ti(C,N) will precipitate with the decrease of the transition temperature. Furthermore, calculations shows that the refinement of the CCGs is caused by the pinning effect of Ti(C,N) precipitates.
... The 0.05%P added in the steel was recommended to decrease the anomalous loss, but phosphorus added in the steel results in a more complicated domain structure and more hysteresis loss generated in return. Yoshida et al. [13,14] disclosed that phosphorus added to steel reduced the grain boundary energy, and then, the grain growth rate decreased. The addition of phosphorus can promote the distribution of Mn among the dendrites and thus affect the segregation of Mn at the grain boundaries. ...
The effect of target phosphorus (P) content on the precipitates, microstructure, texture, magnetic properties, and mechanical properties of low-carbon (C) and low-silicon (Si) non-oriented electrical steel (NOES) was investigated and the influence mechanism was clarified. The results indicate that the precipitates in the steels are mainly aluminum (Al)-manganese (Mn)-Si-bearing complex nitrides ((Al,Si,Mn)xNy) and P-bearing complex nitrides ((Al,Si,Mn)xNy-P). Increasing target phosphorus content in the steels decreases (Al,Si,Mn)xNy, and increases (Al,Si,Mn)xNy-P. The number density of the precipitates is the lowest, and the average size of the precipitates and grain size of the finished steel is the largest in the samples with target P content at the 0.14% level (0.14%P-targeted). The average grain size and microstructure homogeneity of the steels are influenced by the addition of phosphorus. The content of the {111}<112> component decreases, and the favorable texture increases after phosphorus is added to the steel. The magnetic induction of the steel is improved. Grain refinement and microstructure inhomogeneity lead to an iron loss increase after target phosphorus content increases in the steel. The best magnetic induction B50 is 1.765 T in the 0.14%P-targeted samples. The tensile strength and yield strength are improved owing to solid solution strengthening and the grain refinement effect of phosphorus added to the steels.
... When the temperature is lowered to that needed for complete austenite transformation (T c ), FCGs continuously grow and eventually convert to CCGs. [9,10] However, recent studies have suggested that the transformation from FGGs to CCGs is discontinuous and that the growth of the short-axis diameter of the CCGs only takes place at T c by rapid unidirectional solidification equipment and phase-field simulation. [11][12][13] Subsequently, Kencana et al. [14] and Tsuchiya et al. [15] studied the refinement of CCGs in 0.2 wt pct carbon steel by adding ferrite-stabilizing elements such as P, Al, or Cr, which could decrease the T c to slow the migration velocity of CCG growth according to the discontinuous growth mechanism. ...
The role of Ti on the growth behavior of the as-cast austenite grains of the Nb-bearing peritectic steel with different Nb contents during solidification was investigated by fast-directional solidification experiments. The results show that the austenite grains of the Nb-bearing peritectic steel were almost of coarse columnar austenite grains (CCGs) with different Nb contents. As the content of Nb increased from 0.02 to 0.08 wt pct, the number of Nb(C, N) particles increased from 876 to 3140/mm2 at a distance of 10 mm from the surface. Accordingly, the short-axis diameters of the CCGs decreased from 1.71 to 1.14 mm as the pinning pressure provided by carbonitride increased from 0.36 to 0.62 kJ·m−3. The addition of Ti to the Nb-bearing peritectic steel greatly promoted the precipitation of carbonitride in the form of (Nb, Ti)(C, N) composite particles during solidification. As a result, the number of (Ti, Nb)(C, N) particles at a 10 mm distance from the surface increased significantly from 12,210 to 14,324/mm2 as the content of Nb increased from 0.02 to 0.08 wt pct with 0.02 wt pct Ti addition compared to that without Ti addition. The substantial increase in the pinning pressure provided by the (Nb, Ti)(C, N) composite particles from 5.1 to 8.4 kJ·m−3 reduced the short-axis diameters of the γ grains from 1.32 mm to less than 0.2 mm. Meanwhile, the growth of CCGs was gradually inhibited. When the Nb content reached 0.08 with 0.02 wt pct Ti addition, the growth of CCGs was completely inhibited.
... Many previous works have studied the mechanism of the transformation process. The results indicated that the FCG grows continuously and converts eventually to the CCGs when the temperature falls below T γ [8,9]. However, the recent studies showed that the transformation from FGG to CCGs was discontinuous and the growth of the shortaxis diameter of CCGs just occurred at the temperature of T γ [10][11][12]. ...
... The H CCG and V CCG are 0 in 0.09Ti sample, which means that the growth of CCG is completely suppressed during solidification process. Studies have shown that the formation of CCG in discontinuous growth mechanism accompany with the motion of FCRB [9,10]. The migration rate of FCRB was the growth rate of CCG. ...
The effect of titanium on the discontinuous growth of coarse columnar γ grain (CCG) of peritectic steel during solidification was investigated by a fast-directional solidification device. Based on the experiments, the growth characteristic of CCG and Ti(C, N) precipitates were detected by optical microscope, transmission electron microscope (TEM) and scanning electron microscope (SEM), respectively. The results showed that the averages height of CCG region and the growth rate of CCG (VCCG) decrease with the increase of titanium content. When the titanium content increases to 0.09 wt-%, the CCG completely disappears during the whole solidification process. The microscale Ti(C, N) particles formed in liquid would decrease the growth rate of CCG which provides the convenient condition for the growth of nanoscale Ti(C, N) in the γ phase. The pinning pressure provided by Ti(C, N) precipitates increases with the titanium content, which is the main reason of the decrease of VCCG.
... [215,216] Adding phosphorus to 0.1 pct C steel has been shown to decrease the primary austenite grain size. [217,218] This was attributed to delaying the d/c transformation to a lower temperature range. ...
Surface quality and castability of steels are controlled greatly by initial solidification. Peritectic steels suffer more from surface quality problems, including deep oscillation marks and depressions, crack formation, and breakouts than other steels. This paper reviews current understanding of the fundamental mechanisms of initial solidification of peritectic steels that lead to these problems. First, different empirical relations to identify peritectic steel grades from their alloy compositions are summarized. Peritectic steels have equivalent carbon content that takes their solidification and cooling path between the point of maximum solubility in δ-ferrite and the triple point at the peritectic temperature. Surface defects are related more to the solid-state peritectic transformation (δ-ferrite → γ-austenite) which occurs after the peritectic reaction (L + δ → γ) during initial solidification. Some researchers believe that the peritectic reaction is controlled by diffusion of solute atoms from γ phase, through the liquid, to the δ phase while others believe that γ growth along the L/δ interface involves microscale heat transfer and solute mixing due to local re-melting of δ-ferrite. There is also disagreement regarding the peritectic transformation. Some believe that peritectic transformation is diffusion controlled while others believe that massive transformation is responsible for this phenomenon. Alloying elements and cooling rate greatly affect these mechanisms.
... This article considers processing of liquid steel and novel advancements in understanding the reaction pathways undertaken for key refining reactions, occurring across molten metals and oxides, such as carbon, phosphorus, and sulfur removal, which is required for the formation of highly engineered high-value steel grades. [1][2][3][4][5][6] It also touches upon unwanted reactions involving aluminum in the steel melt. ...
The transient trajectory taken for a system striving toward equilibration has consequences on the rate of processes and on the chemical and physical state of products in metallurgical processes. A case study approach to recent advancements in liquid steel processing is given. A combination of techniques and knowledge developed is given as a targeted showcase of the authors’ contributions to the understanding of liquid metal droplet reactions and their contribution to the large-scale production processes within the steel industry. Examples relevant to novel ironmaking technologies, oxygen steelmaking, ladle metallurgy, and continuous casting are discussed, showing the range of processes that benefit from greater understanding in this area. This article considers specifically the reaction of liquid ferrous droplets, immersed in molten oxides, involving key alloying components, including phosphorus, aluminum, and carbon. The studies use high-temperature–confocal scanning laser microscopy (HT-CSLM), X-ray computed tomography (XCT), phase-field modeling, and in situ limited angle X-ray imaging. These techniques have seen significant development over recent years, and the combination of these powerful tools reveals the occurrence of spontaneous emulsification driven by chemical reaction (in the case of oxygen/phosphorus/aluminum reactions) and gas-phase formation (in the case of decarburization) both internally and externally to a steel droplet. A key finding is that the interfacial area pertinent for the heterogenous reactions to occur changes considerably (by up to an order of magnitude) depending on the chemical driving force. Additional key findings include the shift between preferential internal and external gas nucleation during decarburization, an inflection point of behavior as to whether or not spontaneous emulsification will occur (within the study discussed, this is between 3 and 4 wt pct Al) and the pathway of perturbation growth through which spontaneous emulsification occurs, including the physical maxima a perturbation will grow to before breaking away from the parent droplet.
... Other authors have characterized microsegregation profiles using EPMA technique in steels. Similar tendency in the results have been reported by other authors [52][53][54][55]. ...
In this work, the macro and microsegregation of Cr, Si and Mn have been investigated in a high-carbon high-silicon cast steel using X-ray fluorescence (XRF) and electron probe microanalysis (EPMA), respectively. Two different keel block sizes with leg thicknesses of 12.5 and 75 mm have been compared. In each of the keel blocks, three different locations along the leg thickness have been analyzed: A) zone near surface (≈1 mm); C) the central zone of the leg thickness and B) an equidistant zone from A and C. After comparing the analysis performed by XRF in these three zones no macrosegregation of the Cr, Mn and Si has been observed in any of the two keel blocks. However, clear microsegregation patterns have been obtained by EPMA for these three elements; interdendritic zones are enriched while dendrites are impoverished in these elements implying that their partition coefficient is lower that the unity (k < 1). This coefficient has been estimated using the EPMA measurements and Thermo-Calc calculations, finding good agreement between both approaches for Si and Mn but not for Cr. Finally, for both keel block leg thicknesses, similar microsegregation intensity, measured in terms of alloying element concentration has been observed. This result suggests that the cast part size (or dimension) do not have a strong influence on the microsegregation profiles. This it attributed to the back diffusion phenomena involving redistribution of solute during the solidification process.
... In this range, the δ or liquid phase generally surrounds the γ phase and the γ grain growth is thereby suppressed by the pinning force due to the presence of δ or liquid phase. [2][3][4][5][6] However, a rapid γ grain growth takes place immediately after the δ or liquid phase disappears below T γ which is a temperature of completion of the transformation to γ single phase. This behavior is well substantiated for the formation of equiaxed γ grain structures in slow cooling processes with small temperature gradients. ...
Formation processes of as-cast austenite grain structures in hypoperitectic carbon steels have been investigated by means of a rapid directional solidification method, the cooling conditions of which are similar to those in the vicinity of slab surfaces in continuous casting processes. Coarse Columnar austenite Grain (CCG) structure was observed in all the hypoperitectic carbon steels employed in this study. It was demonstrated that its formation mechanism is ascribable to the discontinuous grain growth from Fine Columnar austenite Grain (FCG) formed in the delta phase, in which both the delta phase and residual liquid phase act as the pinning phase in the grain growth process. Then, a summary of the findings was provided regarding the microstructural features and the formation mechanisms of as-cast austenite grain structures formed in the rapid directional solidification in carbon steels with the carbon composition ranging from 0.05 to 0.45 mass%.
... In addition, at present properly controlling the solidification and post-solidification steps in order to significantly refine the as-cast austenite grain size does not appear to be an easy matter. In this context, the work done by Yoshida et al. and Kobayasi et al. indicate that adding bcc stabilising elements (that is, by shifting the δ/γ transformation to lower temperatures) in addition to higher cooling rates can significantly favour austenite refinement [5][6][7]. The refinement of this microstructure must be achieved in the initial stages of rolling with the help of both dynamic and static recrystallisation softening mechanisms. ...
