Structure modification and constant remelting speed control of a 120-t three-phase electroslag furnace

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1 CHINA FOUNDRY Vol.9 No.4 Structure modification and constant remelting speed control of a 12-t three-phase electroslag furnace *Wang Changzhou and Song Jinchun (Northeastern University, Shenyang 11819, China) Abstract: The traditional large electroslag remelting furnaces have many shortages, such as high short-network impedance and inductance, long maintenance time for electrode replacement, low stiffness of driveline, and low control accuracy of remelting speed. The present research was aimed to solve these problems through structure modification and constant remelting speed control for a 12-t electroslag remelting (ESR) furnace. Based on the technique of three-phase double electrodes in series, the short-network system and the structure of the 12-t ESR furnace were improved; and a continuous feeding system for the self-consumption electrode was proposed. A selfdesigned fully hydraulic driveline system with three degrees of freedom was successfully applied to the 12-t ESR furnace. An electrode auto-replacement system and the S-style speed-control curve of electrode-feeding system were designed on the basis of the soft measurement/sensing model on the remaining electrode length so as to obtain a high accuracy control system for constant remelting speed. The experiment products showed good surface quality and cross-sectional results, indicating good system control, and verifying the effectiveness of the structure modification of the furnace. Key words: electroslag remelting; furnace structure; improvement design; constant remelting speed control CLC numbers: TG146.2 Document code: A Article ID: (212) The main function/application of electroslag remelting (ESR) is to purify metal and to obtain ingots of uniform and dense solidification structures. With the development of modern industry, the ESR ingots with larger tonnage and higher quality are urgently needed. The traditional ESR furnaces usually have the problems of high impedance, high inductance and high power consumption, lack of movement freedom, low stiffness of mechanical system, and unstable remelting speed, which has become a key issue to limit the development of 1-t level ESR furnaces. Furnace structure design and remelting speed control are the critical issues in the design of 1-t level ESR furnaces. To keep the temperature field in the electroslag remelting bath stable and uniform, and to balance the shortnetwork system [1], it is necessary to improve the design on furnace structure, electrode feeding system, and driveline system of the 1-t level ESR furnace. In electroslag remelting process, the constant remelting speed control technique is a critical factor determining the ingot quality, while the electrode replacement speed and the positioning precision are the key factors for the constant remelting speed control system [2]. In this paper, based on the furnace structure of three-phase double- *Wang Changzhou Male, born in 1981, lecturer, doctoral candidate. His research interests mainly focus on mechanical engineering and electroslag remelting technique. chzhwang@mail.neu.edu.cn Received: ; Accepted: electrode in series, the experimental research was conducted on the structure improvement design and the constant remelting speed control system of a 12-t level ESR furnace. 1 Improvement design of furnace structure 1.1 Three-phase double-electrode in-series furnace structure The short-network resistance of the ESR furnace impacts not only on the electric efficiency, power factor and thermal efficiency, but also on the output power and current consumption. Moreover, the distribution mode of the shortnetwork has great influence on the impedance balance, remelting speed, and metallurgical quality of the ingot. To obtain a homogeneous composition distribution and stable thermal source in the remelting bath, and to balance the shortnetwork system, the three-phase conductive cross-arms are usually adopted for the 1-t level ESR furnaces. Smaller ESR furnaces typically use single-electrode conductive cross-arm, leading to problems such as high inductance, high power consumption, poor process stability and control. The bigger the furnace tonnage is, the higher the inductance of the single cross-arm and the current consumption. This can be solved by adopting the double electrodes in series, i.e., two electrodes are clamped on a single cross-arm. 37

2 November 212 As shown in Fig. 1, to reduce the influence of the short-network impedance and inductance, the furnace structure design of the 12-t ESR furnace in this study selects the electrical supply system with a three-phase double-electrode in-series structure. (a) C phase Crystallizer Fig. 1: Diagram of the three-phase double-electrode in-series furnace The three-phase conductive cross-arms and short-network system form three groups (six points) of heat sources. The six electrodes remelt simultaneously, and thus result in a uniform temperature field in the molten bath [3], and no big disturbance should be caused if any one of the electrodes is lifted. Compared with that of the traditional single-phase or double-phase ESR furnaces [4], the as-designed structure enhances the uniformity of the remelting temperature field, improves the productivity, and reduces the energy consumption. Furthermore, the three-phase power system avoids the imbalance of short-network. The schematic structure of the 12-t ESR furnace is shown in Fig. 2. The conductive cross-arm consists of two clamping devices, each of which clamps one different electrode, thus forming the in-series conductive Operating platform A phase Electrode B B phase Central balanced line 1 - Pedestal; 2 - Crystallizer platform cylinder; 3 - Guidepost; 4 - Rotation pillar; 5 - Corss-arm cylinder; 6 - Crystallizer platform; 7 - Bottom water box; 8 - Ingot leading plate; 9 - Operating platform; 1 - ESR ingot; 11 - Metal bath; 12 - Slag bath; 13 - Water-cold crystallizer; 14 - Air protection cover; 15 - Exhauster; 16 - Electrode; 17 - Transformer (b) Fig. 2: Schematic structure of the 12-t ESR furnace A C Electrode Crystallizer inner wall Research & Development circuit. The short-network conductive copper bars are symmetrically assembled between the two clamping devices in order to directly power the electrodes and to save energy. In addition, the rotating pillar of the corresponding lifting mechanism of the cross-arm should be consisted of two parts instead of an integrated one. 1.2 Continuous electrode feeding system The electrode feeding system is a very important component of ESR furnaces, and the basis to realize the control of the constant remelting speed. The traditional electrode feeding system mainly consists of variable frequency motor, ball screw, and conductive cross-arm. The drawbacks of such system are as follows [5] : (1) Some components such as ball screw, etc. are large in size and require high precision, which often demands imported supplies; (2) High cost of equipment manufacture, assembly and maintenance; (3) Low-precision in control caused by the mechanical abrasion and inefficiency during the transformation from the rotary motion of the ball screw to the rectilinear motion of the cross arm. In this study, the of the 12-t ESR furnace is driven by a relatively large hydraulic cylinder, avoiding the motion conversion and providing the advantages of high power/weight ratio, high stiffness and high frequency of the electrode feeding system, which, therefore, ensures a quick and accurate control of the electrode feeding system. The 3D solid model of the modified electrode feeding system is shown in Fig. 3. A hydraulic cylinder with magnetic ruler is used to control the lifting of the. The cylinder motion is controlled using an electro-hydraulic proportional speed control valve. The piston-rod position is measured and fed back using the high precision magnetic ruler, and thus the closedloop control of the electrode feeding system is realized [6]. Figure 4 shows the schematic diagram of the hydraulic system. 1.3 Fully-hydraulic and three degrees of freedom transmission design All mechanisms of the 12-t ESR furnace are driven by the hydraulic systems, including the electrode feeding system, the water-cooled crystallizer platform lifting system, the pillar driving system, and the clamping device, etc. 371

3 CHINA FOUNDRY Vol.9 No.4 level of large ESR furnaces, dozens of electrodes are usually needed in order to produce a single ESR ingot. The electrodes need to be replaced frequently and continuously, so an electrode auto-replacement system was designed in this study. As mentioned above, the electrode auto-replacement system is also driven by hydraulic cylinders. The position of the crossarm and the crystallizer platform, the rotating angle of the pillar, and the electrode remaining length are measured using the magnetic ruler, encoder, and the soft-sensing technique, respectively. The self-optimizing fuzzy control method was adopted to accurately control the electrode auto-replacement process. The work flow chart of the system is shown in Fig Support; 2 - Rotating pillar; 3 - Cross arm; 4 - Hydraulic cylinder with magnetic ruler; 5 - Electrode Electroslag remelting Soft detection Lifting Rotating pillar out Loosening Fig. 3: 3D solid model of the continuous electrode feeding system Descending Rotating pillar back Clamping Replacing electrode Fig. 5: Flow chart of the electrode auto-replace system Based on the dynamic research results of the split-pillar rotation process [7], to realize fast and accurate electrode autoreplacement, and to reduce the fluctuation of the system, a scheme for electrode feeding-speed control was proposed. The S-style speed control curve of the scheme is set as in Fig Electro-hydraulic directional control valve; 2, 3 - Throttle check valve; 4 - Pilot operated check valve; 5 - Electro-hydraulic proportional speed control valve; 6 - Reversing valve; 7 - Hydraulic cylinder; 8 - Accumulator Fig. 4: Schematic diagram of the hydraulic-drive lifting system The 12-t ESR furnace driven by the hydraulic systems can realize three degrees of freedom. Its electrode feeding system and the electrode tilting system are driven by two different hydraulic cylinders, respectively and the pillar rotation system is driven by a low-speed hydraulic motor. The full-hydraulic transmission system is self-designed and applied to the ESR furnace for the first time. 2 Constant ESR speed control 2.1 Electrode auto-replacement system Quick and automatic replacement of the electrode is the basis for accurate control of the constant remelting speed. For 1-t Fig. 6: S-style speed control curve According to the S-style speed control curve, the electrode feeding speed undergoes a control process of "acceleration, constant speed, and deceleration", which defines the change rate of the acceleration. As a result, the control process is smoother than that of the traditional control methods, and the control precision of the speed and the real-time position are much higher. 2.2 Soft sensing model for the electrode remaining length Soft sensing technique for the remaining electrode length is the key point in the electrode auto-replacement system. 372

4 November 212 In the ESR process, the space between the electrode tip and the molten metal surface determines the electric resistance and the input power to the electroslag bath. Along with the continuous self-consumption of the electrode during remelting, the remaining electrode length keeps shortening, and the molten metal level keeps rising. To maintain a certain space between the electrode and the molten steel, and to keep the melting current constant, a real-time position detection is needed to control the electrode [8]. Due to the high melting temperature and vast exhaust gas during the ESR process, direct measuring of the remaining length is very difficult. In this research, a soft sensing method was applied to realize the length measurement of the remelting electrode. As to the soft sensing method, the electrode position is detected using the magnetic ruler, and the electrode weight is confirmed using a weight sensor. Thus, the electrode remaining length can be obtained using the data-integration technique. Figure 7 shows the relative position relationships of the electrode, slag bath, and electroslag ingot. Then the electrode remaining length can be obtained by: Where, l ri is the remaining length, l i is the initial length and l i is consumed length of the electrode; H i (t) is the effective melting position of the electrode, H i () is the electrode s initial Fig. 7: Position relationships among the electrode, slag bath, and electroslag ingot l ri ΔH g(v) H i (t) (1) Research & Development steady arcing position, which should be updated every time, and g(v) is the slag-replenishment model, ΔH = f (l 1, l 2, l 3 ) is the ingot melting-crystallization model. In practice, the uneven electrode density, the crystallizer taper and the uncertain electro-slag replenishment volume will cause errors in the calculation of the electrode remaining length. If a weight sensor is available, the value of the electrode remaining length can be calculated by formula (2), meaning the remaining length is a function of weight: ΔH = f (G i ) (2) where, f(g i ) is the ingot melting-crystallization model based on ingot-weight detection. Considering the influence of the crystallizer taper and slag quantity, formula (3) can be adopted to estimate the electrode remaining length. l ri = f (G i ) ± F i - G (3) where, G is the slag shell weight and F i is the friction and electromagnetic force that has some uncertainties. 2.3 Constant ESR speed control system Compared with smaller furnaces, the height and diameter of water-cooled crystallizer for the 12-t ESR furnace are much larger, resulting considerably longer remelting time. As the remelting process goes on, the cooling intensity along both the transversal and longitudinal directions of the ingot reduces gradually [9]. To keep the shape of the remelting bath, the remelting speed of the electrodes need to be controlled as constant as possible according to the steel type and the remelting stage. The constant remelting speed is realized by controlling the remelting power. The electrode feeding speed is influenced by many factors in the remelting process [1]. According to production data and experimental result [11], if only the double-closed-loop control of remelting speed and current is used, the control system will be overloaded. Moreover, the current will strongly fluctuate and thus influences the product quality. In this work, a PID iterative-learning control method is presented based on the double-closed-loop technique, and its controlling principle is shown in Fig. 8. Pre-set remelting speed v(t) Error e v (t) PID Pre-set curent i(t) Measured current i 1 (t) Error e i (t) PID sensor ESR furnace output Ingot mass output Basic speed model Remelting speed calculation v 1 (t) Memory #2 u k+1 (t) Time delay PID Window average Memory #2 u k (t) Remelting speed calculation Pressure sensor Fig. 8: Schematic diagram of the PID iterative-learning control method 373

5 CHINA FOUNDRY 3 Main technical parameters of the 12-t ESR furnace As shown in Fig. 9, the 12-t ESR furnace, as well as the auxiliary equipment including dust separation unit and gas protection device, has been manufactured. The main parameters of the furnace are listed in Table 1. 4 Experimental research Table 2: Main experimental parameters Vol.9 No.4 Experimental studies were carried out using the 12-t ESR furnace. The main parameters applied are shown in Table 2. Figure 1 is a picture showing the experimental production using the improved ESR furnace. Parameters Values (single phase) Ingot weight (ton) 12 Fake (or apparent) electrode diameter (mm) 3 Fake (or apparent) electrode length (mm) 2,6 Electrode diameter (mm) 48 Effective electrode length (mm) 3,54 Electrode weight (kg) 5, Slag thickness (mm) 4 Electrode remelting speed (kg h -1 ) 3,6-4,8 Fig. 9: Photo of the 12 t ESR furnace Table 1: Main technical parameters of the 12-t ESR furnace Parameters Values Height of the pillar (mm) 9,5 Sectional dimension of the pillar (mm 2 ) Electrode smelting speed (mm min -1 ) 5-1 Electrode displacement (mm) 5,5 Balance (counter) weight (ton) 8 Length of the cross-arm (mm) 2,-2,8 Max displacement of the cross-arm (mm) 8, Max lifting speed of the cross-arm (mm min -1 ) 4,5 Min lifting speed of the cross-arm (mm min -1 ) 5 Horizontal rotation range of the cross-arm ( ) -15 Rotation speed of the cross-arm (r min -1 ).8 Extended displacement of the cross-arm (mm) ± 45 Clamping force (ton) 25 Clamping speed (mm min -1 ) 6-8 Fig. 1: Experimental production using the improved ESR furnace The curves of remelting process are shown in Fig. 11. The electrode feeding speed and remelting current can be adjusted in real-time. It can be seen from Fig. 11 that the remelting speed is basically stable. It means that the constant remelting speed control system works very well. As shown in Fig. 12 and Fig. 13, the ingot has excellent surface quality, high purity and dense structure (A) Voltage (V) Control Feeding Remelting 7:8:1.981 PM 7:27:19.33 PM 7:46: PM 8:5:36.28 PM 8:24: PM gear speed speed (mm min -1 ) (t h -1 ) Fig. 11: Experimental curves of the remelting speed, current, and voltage 374

6 November 212 (a) (b) Fig. 12: Experimental ingot and its sectional structure 5 Conclusions Research & Development (1) The structure and mechanical system of the traditional ESR furnace have been improved, leading to a 12-t threephase double-electrode in-series ESR furnace. A fullhydraulic driveline with three degrees of freedom is realized and successfully applied to the 12-t ESR furnace. (2) An ESR constant remelting speed control system is designed based on the electrode auto-replacement system and the soft sensing model for the electrode remaining length. (3) The experimental results show that the ingot produced by the improved ESR furnace has smooth surface and dense structure, verifying the effectiveness of the improved structure design of the furnace. (a) (b) (a) Ingot by a traditional 15-t single-phase ESR furnace; (b) Ingot by the experimental 12-t ESR furnace Fig. 13: Metallographic structure comparison of the experimental ingot and a single-phase furnace ESR ingot References [1] Li Zhengbang. Electroslag Metallurgy Principle and Application. Beijing: Metallurgical Industry Press, 1996: (in Chinese) [2] Hoyle G. Electroslag Processes Principle and Practice. London & New York: Applied Science Publishers, 1983: [3] Modovar B I, Demchenko V F, Bogachenko A G, et al. Temperature Fields of Large ESR Slab Ingots. Mechanical Engineering Transactions-Institution of Engineers, Australia, 1977: [4] Shan Meilong, Li Baokuan. A finite element analysis of joule heating and temperature distribution of electrode remelting process. In: Proc. China Foundry Week 21, Hangzhou China, 21: [5] Ernet C, Raschke K. Ernet C, Raschke K. Nitrogen alloyed tool steels. In: Proc. International European Conference on Tooling Materials, Schweiz, 1982: [6] Lu Yongxiang, Hu Dahong. Electro-hydarulic Proportional Control Technology. Beijing: China Machine Press, 1998: (in Chinese) [7] Wang Changzhou, et al. Dynamic research of the split-pillar rotation process for electroslag remelting furnace. Applied Mechanics and Materials, 211, 52-54: [8] Stein G. Industrial Manufacture of Massively Nitrogen Alloyed Steels in a Pressure ESR Furnace. Steel Times, 1989, 217(3): [9] Huang Feng, Chen Ruirun, Guo Jingjie, et al. Continuous melting and directional solidification of silicon ingot with an electromagnetic cold crucible. China Foundry, 212, 9(1): [1] Liu Xihai, Wang Junqing, Jia Weiguo, et al. Simulation of electroslag remelting process of 12-t large ingot for nuclear power station and its application. China Foundry, 211, 8(4): [11] Zhao Lili. Intelligent Optimal Controll of Electroslag Remelting Process. Ph.D dissertation of Northeastern University, 28: (in Chinese) This study was financially supported by the National Science and Technology Major Project of the Eleventh Five- Year Plan of China (29ZX46-32). 375