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Based on thermodynamic calculations and dilatometry experiments performed over a wide range of cooling rates with on two continuously cast steels, an empirical model was developed to describe the relationship between the critical temperatures of austenite transformation, the cooling rates, and the equilibrium temperatures of phase transformation, w...
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... Figure 5 shows the variation of bulk thermal expansion coefficient of Steel-1 at different cooling rates. The change pattern of the bulk thermal expansion coefficient was similar to that shown in Fig. 4. The critical values of bulk thermal expansion coefficient are summarised in Table 3. ...
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... . Currently, the average cooling rate in the secondary cooling zone is less than 30 K/min during the continuous casting [28,29]. Meanwhile, it can be found that the austenite decomposition temperatures (Ar3 and Ar1) are inversely proportional to the cooling rate, but the linear thermal expansion coefficient of the single-phase region at different cooling rates does not seem to be significantly different according to the published literatures [30,31]. ...
Elucidating the evolution law of the elastic properties of the matrix phase is of great significance for the control of steel properties and quality during continuous casting and subsequent heat treatment. In this paper, thermal expansion experiments and ab initio calculations are used to study the elastic properties of the interstitial free (IF) steel matrix phase in different magnetic states and crystal structures. The results show that the bulk modulus B and the tetragonal shear elastic constant C' for the entire temperature range decrease with increasing temperature, but C44 is the opposite. While from paramagnetic (PM) to ferromagnetic (FM) state, C'(C44) have changed ~188% (~27%), B increases by ~55% during the crystal structure change (fcc→bcc). With the FM to PM state, the Zener anisotropy parameter increases sharply, and Young’s modulus decreases significantly in the [001] direction; the maximum difference is ~76 GPa. The evolution rate of average Young's modulus in single bcc-phase FM (fcc-phase PM) range reaches ~5.5(~5.6) × 10−2 GPa K−1. The research provides an effective method for ab initio calculation of the elastic properties of interstitial free and ultra-low carbon steels at high temperature, also furnishing a basis for the application of ab initio calculations to the high temperature performance of steel materials.
... In order to achieve the required strength properties and critical brittleness temperature for metal of a large forging a bainitic structure is required throughout the whole volume, which is achieved by providing a certain range of cooling rate during hardening [9,10], with which in accordance with the thermokinetic diagram for austenite decomposition of this steel there is formation of bainite of the required morphology (for example lower or granular). In those cases within the structure presence structurally free ferrite is undesirable since its development leads to an increase in the original critical brittleness temperature and a reduction in yield strength, and with a significant ferrite content for the ultimate strength. ...
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... The average cooling rate of the casting slab in the secondary cooling zone is less than 30°C/min during continuous casting. 25 To simulate a same thermal history as that in continuous casting, the test of precipitation caused by oversaturated vanadium was performed in a high-resolution thermal analyzer of DSC 404C (NETZSCH-Gerätebau GmbH, Selb, Germany) with a slow cooling rate of 20°C/min. Similarly, in subsequent dissolution process of the precipitates, corresponding DSC test was carried out with the same heating rate of 20°C/min. ...
In this work, differential scanning calorimetry (DSC) was used to characterize and analyze the precipitation/dissolution kinetics of second phase particles during the cooling/reheating process in a vanadium microalloyed steel. The results indicated that three obvious exothermic peaks were detected on the cooling DSC curve. Furthermore, three corresponding endothermic peaks were also detected on the heating DSC curve. Combined with thermodynamic calculation and transmission electron microscopy analysis, these three exothermic peaks along cooling DSC curve were defined as the precipitation reaction of V(CN), the reaction of austenite transformation into ferrite and the precipitation reaction of VC, respectively. Meanwhile, three corresponding reverse reactions for cooling were also defined along the reheating DSC curve. The linear regression result revealed that the precipitation activation energies for V(CN) and VC were identified as 311.2 kJ/mol and 167.6 kJ/mol, respectively. The dissolution activation energies for VC and V(CN) were identified as 255.4 kJ/mol and 592.6 kJ/mol, respectively.
... Therefore, the property and quality of casting slabs play a fundamental role for subsequent control rolling and control cooling schemes, as well as the quality of final steel products. 1,2 Currently, microalloyed hypoeutectoid steels with a superior combination in mechanical properties, such as high strength, high toughness, and weldability, have been considered as promising engineering materials. [3][4][5] The solidification of hypo-eutectoid steel undergoes generally a series of phase transformation process during continuous casting, such as L ! d ! ...
... As from the previous research of Chen and Long et al., 2,35 the cooling of casting slab during continuous casting is generally divided into two stages: mold cooling and secondary cooling. The surface temperature of the slab outside the mold is usually above 1100°C, which is higher than A r3 . ...
... The negative value indicates the contraction behavior of the sample during continuous cooling, while a positive value means the expansion. 2 Moreover, two main change stages of a line can be identified from the LTEC versus the temperature curve, including a weak peak at lower temperature region and a strong peak at higher temperature region, as shown in Fig. 1(b). It means that two different transformations took place in nonlinear thermal expansion characteristic zone during austenite decomposition. ...
This work was aimed to use the peak separation method to directly measure the critical temperatures and phase transition fractions of austenite decomposition products based on experimental dilatometric curves in hypo-eutectoid steels. The results indicated that pearlite transformation start temperature and ferrite transformation finish temperature could be clearly obtained through peak separation processing, which were generally hidden in the overlapped peaks of the linear thermal expansion coefficient curve. Moreover, four critical temperatures of austenite decomposition were retarded to lower temperature with cooling rate increasing. The phase transition fraction for austenite decomposition was quantitated by measuring the area of the corresponding phase transformation peak. The final ferrite phase fraction after austenite decomposition decreased with cooling rate increasing. On the contrary, the final pearlite phase fraction increased with cooling rate increasing. Compared with the lever rule, the calculation result using peak area method can accurately reflect the actual phase fraction change versus the temperature during austenite decomposition.
... Then the specimens were cooled fast to room temperature as a compared specimen via quenching with help of helium gas. As presented in previous research [17], the average cooling rate in the secondary cooling zone is less than 0.5°C/s. Meanwhile, it is considered that the crack in the slab usually occurs in 1/4 location near the inner arc side. ...
The coarse TiN inclusions can act as potential fracture initiation sites and deteriorate the impact toughness of steels. The experimental observation results indicated that CaS and TiN inclusions precipitated in solidification mushy zone. Meanwhile, it was found that CaS inclusions could act effectively a heterogeneous nucleation substrate for the precipitation of TiN inclusions, and the size of TiN inclusions with CaS core was obviously larger than those TiN inclusions without CaS core. Additionally, the thermodynamic calculation result showed that TiN inclusions began to precipitate at the end of solidification from the dendrites front; moreover, the preexisted CaS inclusions promoted the formation of TiN inclusions. Decreasing sulfur and nitrogen content could significantly reduce precipitation temperature and growth time of CaS and TiN inclusions in mushy zone. Furthermore, TiN inclusions could be prevented from precipitation at the surface of CaS inclusions when the S content of molten steel was below 0.0008 wt%.
... (4) and (5) is that they include the effect of relatively high cooling rates 2-8 • C s −1 , which are closely related to real operating conditions. Most experiments by other authors were performed at cooling rates of a lower order (see, for example, 5 • C min −1 [21], 5-20 • C min −1 [18,22], or 20 • C min −1 [23]). This, on the other hand, allowed a much more stable drawing of the dilatometric curves and their more reliable analysis, leading to a more exact determination of the individual phase transformation temperatures. ...
... (4) and (5) for significantly lower or higher cooling rates. Long et al. [22] developed an empirical model to describe the relationships between the critical temperatures of the austenite transformation (A r ) and the cooling rate CR, written as A r = a − exp(b + c/CR), where a, b and c are material constants. The combination of reciprocal and exponential function in A r = f (CR) dependence is more physically justified than a linear dependence used in Eqs. ...
... The temperatures at the beginning were around 820-860 °C. The cooling is slower in the middle of the stack, i.e. between slab five and six, and after approximately 13 h the temperature is just a little bit above 700 °C, which coincides with the temperature where phase transitions takes place [12]. A small increase in temperature accompanies the phase transformation due to the release of latent heat. ...
Subsequent cooling of slabs after continuous casting is an important step to control before further processing such as hot-rolling. In this work the stresses that occur in slabs during the cooling process were investigated by using a finite element model. In the specific situation that we model a stack of slabs are left to cool under a hood, insulated by a ceramic material, which makes the cooling slower and the temperature distribution more even during the cooling process. The accuracy of the numerical model was validated by a comparison with temperature measurements, and also by a comparison with hot tensile tests for the specific cooling situation under investigation. The results show that the highest stress levels achieved during cooling did not exceed the mechanical strength of the material at the temperatures investigated.
... Moreover, the thermal expansion coefficient can also be accurately determined based on the measured dilatation curve. 5,18 The current study thus examined the effects of the heating or cooling rate on the phase transformation and thermal expansion coefficient of C-Mn as-cast steel at elevated temperatures, by using dilatometric analysis. The evolution of the C, Si, and Mn contents in austenite during heating and cooling was also obtained using a quantitative analysis model. ...
... This a T of single austenite measured in this study agrees well with those reported in the previous works, such as 2.09 Â 10 À5 , 25 2.31 Â 10 À5 , 26 2.065 Â 10 À5 , 27 and 2.1 Â 10 À5°CÀ1 . 18 As can be seen in Fig. 8, when A c1 (A rp ) , T , A c3 (A r3 ), the relationship between the thermal expansion coefficient and temperature is significantly nonlinear, and the value of a T is between À6.31-2.25 Â 10 À5°CÀ1 for the different rates of temperature change. More specifically, the a T of the pearlite dissolution process (stage I) is between À0.90-1.61 ...
Dilatometric studies of C–Mn hypoeutectoid steel with an as-cast structure were carried out to study the effects of the heating or cooling rate, heating and cooling process on phase transformation, and the thermal expansion coefficient. As the heating or cooling rate (Vc) increased, the characteristic temperatures of Ac1, Acp, and Ac3 also rose, while Ar3, Ar1, and Arp fell. In addition, the phase transformation temperature range (Ac3–Ac1) rose, while (Ar3–Arp) fell as the heating or cooling rate increased. At the same time, the maximum thermal expansion coefficients│αT│ between the heating and cooling processes during phase transformation showed significant differences, and the difference (│ΔαT│) in the maximum │αT│ between these processes increased along with the heating or cooling rate, and this is because of the different phase transformation rates, with regard to the change from austenite to ferrite on cooling and ferrite to austenite on heating. During the heating process, the phase transformation rate of ferrite to austenite first decreases and then increases as the temperature rises, and the phase transformation rate of austenite to ferrite first increases and then decreases during the cooling process. The evolution of carbon and substitutional alloying elements (Si and Mn) in austenite during heating and cooling is also analyzed in this work.
Accurate prediction of the crack sensitive temperature region and phase fractions variation of slabs during continuous cooling is an important guide to avoid cracks and effectively control the quality. Based on finite number of measurements, at different cooling rates of the continuous casting process, a prediction model for characteristic temperatures of austenite decomposition, the variation of phase fractions with temperature, the crack sensitive temperature regions, and the final microstructural compositions of casting slabs at different cooling rates has been established and evaluated the accuracy. The results show that austenite decomposition temperature range moves toward the low temperature region as cooling rate increases, and the independent peak of ferrite transition become weaker. The characteristic temperatures of austenite decomposition can be quantitatively calculated by TC(CR)=A-exp(B+C/CR) at different cooling rates, which the maximum relative error for experimental steels is -2.2%. The ferrite and pearlite phase fractions increases with decreasing temperature during continuous casting cooling, which means that the ability of the billet to resist deformation and external force changes. Meanwhile, the final ferrite content of slabs for Steel B and Steel C at different cooling rates are 83.24620-exp(2.59364-13.72283/CR) and 85.07143-exp(1.71320-15.82244/CR), respectively. The crack sensitive temperature region Ae3 ∼ Tα40%(CR) calculated by the prediction model is in good agreement with the low ductility zone measured by experiment. Moreover, the critical temperatures Tα40%(CR) of the crack sensitive temperature regions are 890.35731-exp(2.99719-20.67781/CR), 745.87462-exp(4.83056-44.18511/CR) and 729.46168-exp(2.96621-12.21949/CR) for three experimental steels under different cooling rates.
This paper considers the issues related to plotting of continuous cooling transformation (CCT) phase diagram based on the dilatometric test results. The numerical data processing algorithm for uniformly analysis of the dilatometric curves is developed and implemented. Computing of the phase transformation ranges and coefficient of thermal expansion (CTE) were made in Microsoft Excel. Numerical algorithm contains determination of the critical points as the deviation dots from linear approximation function of the monotonous segment of experimental curve which is extrapolated to the start (or finish) of the phase transformation. Method of CCT diagram plotting based on the quantitative analysis of CTE during austenite continuous cooling is proposed. CTE quantitative evaluation of austenite and its decomposition products were accomplished by the analysis of linear approximation function of monotonous segment of dilatometric curve. Dependence of the CTE values on the range of cooling rates were used to define the phase transformation ranges on the CCT diagram. Obtained CTE dependence clearly defines critical cooling rate (ССR) as an intersection point of functions, describing austenite CTE changes with cooling rates and austenite- ferrite mixture. Point of intersection was found by solving the system of functions equations. Confirmation and clarification of results, based on the numerical analysis of the dilatometric curves, could be done as usual, using metallographic analysis and microhardness testing. The application of the developed numerical algorithm for dilatometric curves provides opportunity to unify dilatometric test analysis and to improve the accuracy of the CCT diagram plotting. © 2018, Institute for Metals Superplasticity Problems of Russian Academy of Sciences. All rights reserved.
