||Microstructure Modification of Low Carbon Steel (SS400) by Adding Rare-Earth Metals (Cerium) and its Mechanism
||Department of Materials Science and Engineering
Intra-granular Acicular Ferrite
Stored strain energy
austenite grain growth
Intra-granular Acicular Ferrite widely known as IAF is one of the most famously favorable ferrite microstructure which is recognized by its special pattern, needle-shaped crystallites or grains with a disordered crystallographic orientation. Reaching to great mechanical properties, high strength and good toughness at the same time, is a goal in all industries from the point of view of mechanical properties. The most influence of having this popular microstructure, IAF, is the fact that it can result in both refining the microstructure and postponing the propagation of cleavage crack. Besides, it can provide great mechanical properties. The present research was performed on the cerium influence on final microstructure of steel and its role on controlling proper cerium-based inclusions in low-carbon commercial steel (SS400) in order to improve the steel properties. Results show that for samples without Ce, MnS inclusions are widely found in this sample. For samples with very low amount of Ce, a very small amount of MnS inclusion can be found but by adding more Ce MnS inclusion cannot be found. Instead of MnS inclusion, different inclusions with cerium would be formed. With increasing the amount of cerium, the type of inclusions changes from AlCeO3 to Ce2O2S, Ce2O3 and complex inclusion. Furthermore, it is found that the proper grain refinement can be achieved by adding the optimum amount of cerium, 0.0235 wt. %, which results in the formation of cerium oxide, oxy-sulfide and sulfide inclusions. Further investigations show that samples with high cerium would result in enhancing the inclusions number notably so it would not be useful for refining the grains; therefore, the inclusions not only would be proper but also act as obstacles for other inclusions. Based on our results, the proper inclusion size would be nearly 47m which can serve as proper sites for acicular ferrite nucleation. Moreover, for prediction of the inclusion formation in this molten steel, thermodynamic calculations have been performed, which show a great consistency with the founding of the experiments.
For the next section of this research, we worked on the heat treatment influences on a well-established SS400 steel which was modified by cerium for obtaining the maximum possibility of acicular ferrite formation. The influences of various heat treatment conditions, like holding temperature and cooling rate, are briefly discussed. With a cooling rate of 10℃/s and holding temperature of 500℃, the typical interlocked Acicular Ferrite (AF) structure was well developed. By examining the so called in-situ morphological evolution of the phase transformation, three different solid-to-solid nucleation types were detected inside the initial austenite grains, namely primary ferrite nucleation on the grain boundaries of austenite, sympathetic nucleation and nucleation on inclusions. Finally, the stored strain energy is calculated at the inclusions-steel interface. The results show that for a certain holding temperature, the maximum stored strain energy is equal for different cooling rates, and the time needed to reach to a critical strain energy level plays a critical role in the nucleation of AF. For different inclusions with different size and shape, the stored strain energy is calculated. According to the results, the maximum stored strain energy is located at inclusion-steel interface on the sharp corners. In fact, the concaved interface between inclusion-steel is responsible for inducing the stored strain energy inside the steel. At the same condition, inclusions with more irregular shapes can form more AF rather than circular inclusions. Also it proved that in the micro scale, the inclusion size is not an effective parameter on stored strain energy, at least not that much critical for small differences.
Moreover, a semi-empirical-simulation is used to investigate the pining effect of cerium inclusions in austenite grain growth of SS400 steel at 1300℃. First of all, the samples modified with cerium inclusions are analyzed for SEM characterization for determination of the properties of inclusions. Then, laser scanning confocal microscopy known as LSCM is applied to perform the so called in-situ observation of austenite grain growth, the goal is determination of the fitting parameters of the model such as the grain mobility and the pinning parameter. Then, these fitting parameters directly inserted into our phase filed simulation. In our phase field model, the Time-Dependent Ginzburg-Landau (TDGL) equation is implemented, where the effect of inclusion and grain boundary interaction is inserted as a potential term in the local free energy. Based on our results it is proved that in order to reach to the optimal size of austenite grains, it is essential to change the volume fraction of inclusions. In fact, the austenite grain growth can be reduced by enhancing the volume fraction of inclusions from 0 to 0.1, where the boundary mobility decreases from 2.3 (×10-12) m4/Js to 1.0 (×10-12) m4/Js. The results also show that more energy for grain can be provided by increasing temperature in order to overcome pining force of inclusions. Moreover, it was shown that the classical Zener model R_c=0.45r_p f_i^(-1) describes the pining effect of cerium inclusions.
List of Figures…………………………………………………………...………………IX
List of Tables………………………………………………..........................................XIV
List of symbols………….…….………………………...…………………………...…XV
Chapter 1 Introduction 1
1.1 Overview 1
1.1.1 Basic Concept of Oxide Metallurgy 13
1.2 Acicular Ferrite 15
1.2.1 General Characteristics and Morphology 16
1.2.2 Mechanism of Growth and Nucleation of AF 17
1.3 Nucleation and Role of Inclusions 19
1.4 Mechanical Properties affected by Acicular Ferrite 21
1.5 Important Factors for AF nucleation 23
1.5.1 The Effect of Various Alloying Elements 23
126.96.36.199 Carbon 24
188.8.131.52 Manganese 26
184.108.40.206 Titanium 28
220.127.116.11 Cerium 29
1.5.2 Cooling Rate of Steel 30
1.5.3 Austenite Grain Size 31
1.5.4 Inclusion Size 31
1.6 Mechanisms of Acicular Ferrite Nucleation 32
1.6.1 Reduction of Interfacial Energy 32
1.6.2 Strain of Lattice Mismatch 33
1.6.3 Thermal Strains around the Inclusions 35
1.6.4 Formation of Mn-depleted Zone near Inclusion 37
1.7 Our goal 39
Chapter 2 Experimental Procedures 41
2.1 Materials and Processing Conditions 41
2.2 Laser Scanning Confocal Microscope (LSCM) 43
2.3 Scanning Electron Microscopy (SEM) 43
2.4 Microstructure and Morphology Evolution 45
2.5 Thermodynamic Analysis of Rare Earth Inclusions Formed in SS400 46
Heat treatment 49
2.6 Mechanism investigation 50
2.7 Grain Growth Simulation by means of Phase Field Modeling 54
2.8 Simulation procedure of pinning effect of cerium inclusions on Austenite grain growth 59
Chapter 3 Experimental Results and Discussion 61
3.1 Thermodynamic Analysis of Rare Earth Inclusions Formed in SS400 61
3.2 In situ Observation of Austenite Grain Refinement by means of Confocal Microscopy before and after REM Modification 64
3.3 Controlling the Microstructure, Morphology and Type of Formed Ce-based Inclusions 66
3.4 Microstructure and Morphology Detection by means of SEM-EDS before and after REM Modification 67
3.5 The Results of Optical Microscopy after Etching 80
3.6 Heat treatment procedure 89
3.6.1 Analysis of SS400 Modified by cerium before heat treatment 89
3.6.2 The microstructure evolution observation during solid phase transformation 92
3.6.3 Microstructure evolution after heat treatment histories 97
3.7 Mechanism of nucleation of Acicular Ferrite 100
3.8 Grain Growth Simulation by means of Phase Field Modeling 105
3.8.1 The Boundary Mobility 105
3.8.2 Adjusting pinning parameter φ 108
3.8.3 The effect of Inclusion volume fraction, fi 110
3.8.4 The Effect of Temperature on Grain Growth 112
3.8.5 Zener Pinning Model 114
Chapter 4 Conclusions 115
Chapter 5 Future work 118
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