Numerical Modeling of Concentration Behaviors and Effects of Process Variables on Ruhrstahl-Heraeus Process = RH 공정에서 농도 거동과 공정 변수의 영향에 대한 수치해석 연구

다국어 초록 (Multilingual Abstract)

This study presents a numerical modeling approach to analyze the Ruhrstahl-Heraeus (RH) degassing process used in the production of ultralow-carbon steel. The RH degassing process is a leading technique for decarburization, where inert argon (Ar) gas is injected into the melt and vacuum state is maintained, driving circulation between the ladle and the vacuum vessel.
While previous empirical models offer some understanding, the intricate nature of RH operations requires comprehensive numerical modeling to consider the numerous factors that affect melt flow circulation and chemical reactions. The comprehensive numerical model in this study simulates the fluid flow behavior of the melt and the chemical reactions that occur during the RH process, incorporating key process variables such as vacuum vessel pressure (Pvac), gas flow rate (Q), oxygen influx from slag, and the effects of oxygen blowing (OB) and carbon addition. The numerical model accounts for melt circulation, plume formation, bubble expansion, and buoyancy forces to simulate the variations in carbon, oxygen, and aluminum concentrations during the process. The numerical simulations provide detailed insights into the decarburization considering surface, internal, and bubble reaction mechanisms and deoxidation reactions within the RH system.
By separating flow calculations from concentration calculations and utilizing a velocity database, the model achieves faster computation speeds, enabling real-time process control applicability.
The model’s predictions were validated against plant data, demonstrating strong agreement for final carbon and oxygen concentrations, with potential for further refinement. A carbon concentration difference of 0.57%p was observed. For calculations involving OB and carbon addition, the differences were 0.79%p and 0.35%p, respectively. The model validation has been expanded to include dozens of RH processes using supplied oxygen as a validation indicator.
The study concludes that controlling process variables, including depressurizing rates, minimum pressure, gas flow rates, and initial concentrations, significantly affects carbon concentration and process efficiency. This work contributes to the advanced development of RH operations and offers potential applications for future steelmaking processes.

국문 초록 (Abstract)

이 연구에서는 극저탄소강을 생산하는 공정인 Ruhrstahl-Heraeus (RH) 탈가스 공정을 수치해석을 통한 모델링을 수행하였고 이에 대한 분석을 하였다. RH 탈가스 공정은 탈탄(decarburization) 공정을 하기에 적합한 공정으로, 비활성 기체인 아르곤 가스를 멜트 내에 주입하고, 진공조의 진공상태를 유지하여 래들과 진공조 사이를 멜트가 순환할 수 있도록 한다.
RH 공정에 관한 이전의 실험적인 연구들은 공정에 대한 이해를 제공하였지만, 복잡한 성질을 가진 RH 공정을 이해하기 위해서는 멜트 유동과 화학 반응에 영향을 미치는 많은 요인들을 고려할 수 있는 통합적인 수치해석 모델링 방법이 필요하다. 이 연구에서는 진공조 압력 (Pvac), 가스환류량 (Q), 슬래그 계면 산소 유입, 산소 블로잉 (OB)과 가탄 공정과 같은 RH 공정에서의 중요한 공정 변수를 통합하여 멜트 유동과 화학반응의 거동을 모사하였다. 수치해석 모델은 멜트 유동, plume 생성, bubble 팽창, 부력 등을 고려하여 탄소, 산소, 알루미늄 농도의 변화를 계산한다. 이는 RH 공정에서의 표면, 내부, bubble 매커니즘을 고려한 탈탄 (decarburization) 반응과 탈산 (deoxidation) 반응에 대한 자세한 분석을 제공하였다.
유동 계산과 농도 계산을 분리하고, 속도에 대한 데이터베이스를 이용함으로써 이 수치해석 모델은 계산을 빠르게 수행할 수 있어 실시간 공정 제어의 적용을 가능하게 하였다.
또한 이 모델의 결과값을 실제 공정에서의 측정값과의 비교 및 분석을 하여 다소 오차는 존재하지만 탄소와 산소 농도값이 잘 일치하는 것을 확인하였다. 기본적인 공정에서 탄소의 차이값은 0.57%p이었고, 산소 블로잉이 포함된 공정에서는 0.79%p, 가탄이 포함된 공정에서는 0.35 %p의 차이값을 보였다.
이 연구는 또한 감압 속도, 최저압력, 환류가스량, 초기 농도값과 같은 공정 변수를 제어하고 모델링하여 탄소 농도와 공정 효율에 미치는 영항을 분석하였다. 이 연구는 RH 공정의 고도화에 기여하고, 미래 철강 산업에 적용할 수 있는 가능성을 제시하였다.

목차 (Table of Contents)

• Abstract i
• Table of Contents iv
• List of figures vii
• List of tables xii
• Chapter 1. Introduction 1
• 1.1 RH process in steel industry 1
• 1.2 Literature review 2
• 1.2.1 Melt flow of RH 2
• 1.2.2 Concentration calculation 4
• 1.3 Objectives of this study 6
• Chapter 2. Methodology 8
• 2.1 System of RH equipment and calculation domain 8
• 2.2 Fluid flow calculation 12
• 2.2.1 Governing equations of fluid flow 12
• 2.2.2 Flow conditions of RH 15
• 2.3 Concentration calculation 17
• 2.3.1 Governing equations of concentration 17
• 2.3.2 Decarburization reaction 19
• 2.3.3 Deoxidation reaction 24
• 2.4 Other factors for calculating concentrations 26
• 2.4.1 Initial concentrations of carbon and oxygen 26
• 2.4.2 Oxygen inflow from slag and OB 27
• 2.4.3 Addition of mass during RH operations 39
• 2.5 Calculation system 41
• 2.5.1 Grid generation 41
• 2.5.2 Calculating procedure 44
• 2.6 Control of process variables 50
• 2.6.1 Changes of pressure 50
• 2.6.1.1 Changes of depressurizing rate 50
• 2.6.1.2 Changes of minimum pressure 53
• 2.6.2 Changes of gas flow rate 55
• 2.6.3 Changes of initial concentration 56
• Chapter 3. Fluid flow of RH 57
• 3.1 Relation between variables and circulation rate 57
• 3.2 Velocity profiles of RH flow 60
• Chapter 4. Concentration behaviors during RH 62
• 4.1 Decarburization reaction 62
• 4.1.1 Result of concentrations 62
• 4.1.2 Result of decarburization reaction rate 72
• 4.1.3 Result of reaction contribution 76
• 4.2 Deoxidation reaction 78
• 4.3 Model verification of RH 81
• Chapter 5. Analysis of OB and carbon addition 94
• 5.1 Effects of OB on RH 94
• 5.1.1 Result of concentrations under OB 94
• 5.1.2 Result of decarburization reaction rate under OB 97
• 5.1.3 Result of reaction contribution under OB 99
• 5.2 Effects of OB as an operating parameter 104
• 5.2.1 Result of concentrations with different OB start 104
• 5.2.2 Result of reaction rate with different OB start 108
• 5.2.3 Result of reaction contribution with different OB start 110
• 5.2.4 Discussion of OB parameter 112
• 5.3 Effects of carbon addition on RH 114
• 5.3.1 Result of concentrations under C addition 114
• 5.3.2 Result of reaction rate under C addition 117
• 5.3.3 Discussion of C addition parameter 119
• 5.4 Model verification under OB and C addition 121
• Chapter 6. Results of controlling process variables 135
• 6.1 Effects of pressure changes on RH 135
• 6.1.1 Effects of depressurizing rate on RH 135
• 6.1.2 Effects of minimum pressure on RH 143
• 6.2 Effects of gas flow rate on RH 148
• 6.3 Effects of initial concentration on RH 152
• Chapter 7. Conclusion 158
• Bibliography 161
• Abstract in Korean 165