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New technology and process operation points of ultra-high power electric arc furnace steelmaking

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New technology and process operation points of ultra-high power electric arc furnace steelmaking

[/vc_column_text][vc_separator border_width=”4″ css=”.vc_custom_1634717105239{padding-top: 30px !important;padding-bottom: 30px !important;}”][vc_column_text]Description:This paper mainly introduces the ultra-high power electric arc furnace steelmaking technology and process operation from six aspects

Keyword:UHP EAF Steelmaking

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  1. Rapid melting and heating operation

Rapid melting and heating are the most important functions of the UHP EAF, and this process begins when the first basket of preheated scrap is added to the furnace. Ultra-high-power electric arc furnace is powered with maximum power, oxygen-burner flux, oxygen blowing flux and stirring, bottom blowing stirring, foamed slag and other technologies such as strengthening smelting and heating, providing ingredients and temperature for secondary refining. The required primary molten steel.

 

  1. Dephosphorization operation

The three elements of dephosphorization operation, that is, the key factors of phosphorus distribution between slag and steel are: slag oxidizability (%TFeO), lime content (%CaO) and temperature. With the increase of %TFeO. %CaO in the slag and the decrease of the temperature, the distribution ratio of phosphorus in the slag steel (% (P) /% [P]) increased significantly. The main processes adopted are: strengthening oxygen blowing and oxygen-combustion to assist melting to improve the oxidizing property of the primary slag; creating foamy slag with strong oxidizing property and high calcium oxide content in advance, and making full use of the favorable conditions of low temperature in the melting period, Improve the ability of slag dephosphorization, release the primary slag with high phosphorus content in time, and replenish new slag to prevent the slag from returning to phosphorus after the temperature rises and during tapping, and use oxygen to directly blow lime and fluorite powder into the molten pool to dephosphorize Generally, the slag rate can reach 80%, and the desulfurization rate is less than 50%. The slag-free (or less slag) tapping technology is adopted, and the amount of slag is strictly controlled to minimize the phosphorus after tapping. Generally, the amount of slag can be controlled at 2kg.

The control of phosphorus content in tapping should be comprehensively considered according to product specifications, alloying, etc., generally P<0.01%.

 

  1. Decarbonization operation

Carbon distribution can be made of high-carbon scrap steel and pig iron, or carbonaceous materials such as coke or coal. The latter can be added to the furnace at the same time as the scrap, or injected in powder form. The amount of carbon allocated has a great relationship with the form of carbon addition, the method of oxygen blowing, the intensity of oxygen supply and the power of the furnace (determining the cycle time), which should be determined according to the actual situation.

There is a certain carbon content in the charge and the role of decarburization reaction: when oxygen is blown in the melting period, carbon is oxidized before iron, thereby reducing the burning loss of iron; carburizing can reduce the melting point of scrap steel and accelerate melting; carbon-oxygen The reaction causes the molten pool to stir, which promotes the steel-slag reaction and is conducive to early dephosphorization: in the refining heating period, the active carbon-oxygen reaction expands the steel-slag interface, which is conducive to further dephosphorization, and is conducive to the composition of molten steel and The homogenization of temperature and the floating of gas and inclusions; the active carbon-oxygen reaction helps the formation of foamy slag, improves heat transfer efficiency, and accelerates the heating process. The decarburization rate is generally > 0.1% min.

  1. Temperature control

Good temperature control is the guarantee for the successful completion of the metallurgical process. For example, dephosphorization requires not only slag with high oxidation and high basicity, but also good temperature coordination. This is why it is emphasized that dephosphorization should be done at an early stage. Because the temperature is lower at that time, which is conducive to dephosphorization; and in the oxidation refining period, in order to cause active carbon and oxygen boiling, a higher temperature (greater than 1550 ° C) is required. Different processes require a certain degree of superheat in the initial molten steel of the electric arc furnace to compensate for the temperature loss in the process of tapping, ladle refining and molten steel transportation. The tapping temperature should be determined according to different steel grades and after fully considering the above factors. If the tapping temperature is too low, the fluidity of molten steel is poor, resulting in short length or solidified steel after pouring; if the tapping temperature is too high, the cleanliness of the steel will deteriorate, the defects of the casting billet (or ingot) will increase, and the consumption will increase. The steel temperature should be controlled as low as possible under the premise that the pouring can be successfully completed.

 

  1. Scrap preheating

Exhaust gas dust removal and emission problems Although the electric furnace hot charging process can save energy and improve output and quality, due to the increased decarbonization amount and the accelerated decarbonization rate, a large amount of process gas (mainly CO) is generated. In factories with secondary combustion or scrap preheating equipment, these waste gases can be fully utilized to further play the role of secondary combustion or (and) scrap preheating to strengthen the utilization of waste heat. In a factory without secondary combustion and/or scrap preheating, it is bound to increase environmental pollution. Even if the electric furnace with a dedusting device originally had to use the original device to remove all process gases, it would be insufficient. How to deal with the excess gas generated in the electric furnace hot charging process The continuous strengthening of oxygen in the electric furnace produces a large amount of furnace gas and increases the heat loss. In order to reduce energy consumption and recover heat source, furnace gas is used to preheat scrap steel.

In 1980, Japan took the lead in developing the scrap preheating device, which adopts a long-distance split arrangement. The furnace gas is sent to the scrap preheating furnace through the pipeline. The inlet furnace gas temperature is 700-800 °C, and the scrap is preheated to about 300 °C, which can save electricity. 30-45kwh/t, the smelting time is shortened by 5-8min. However, this method has a large area, high heat loss of furnace gas, low scrap preheating temperature, and the need to add additional burners to deal with the white smoke and odor in the waste gas discharged from the scrap preheating furnace. rarely used.

The new scrap preheating technology is a double-shell electric furnace. In actual operation, two furnace bodies are used. When one furnace is smelting, the exhaust gas discharged from it is sent to the other furnace filled with scrap steel for scrap steel preheating treatment. Immediately after the smelting of one furnace is completed, the preheated scrap steel of the other furnace is smelted. This interactive smelting method, which fully utilizes the heat of the exhaust gas, has attracted widespread attention. The disadvantage of this method is that the design rhythm of switching between the two furnaces is not easy to grasp, the operation is difficult, the requirements are high, the floor space is increased, the investment cost is also large, and the coordination with the subsequent steps is prone to confusion. The scope of application remains to be seen.

Since the 1990s, the shaft furnace electric arc furnace has come out. It is to place the preheated material blue of the scrap steel directly at the exhaust gas discharge port of the top of the electric furnace, so that the exhaust gas at 1200-1500 ℃ is preheated through the scrap steel. The exhaust gas temperature is only 250 ℃, which achieves the purpose of preheating scrap steel and recovering energy.

The main features of DP’s localized Kangsidi electric furnace are continuous feeding, continuous preheating and continuous smelting. DP’s localized Considi electric furnace is an electric furnace steelmaking process, which achieves continuous feeding and preheating of charge through special online conveying equipment, saving electrical energy and chemical energy. The DP localization Consdy electric furnace process is a new type of steelmaking concept in which electric furnaces continuously feed – preheat – smelt and refine scrap steel or direct reduced iron. The special conveying equipment continuously sends the metal material into the furnace. The charge entering the preheating section and the flue gas of the electric furnace meet in opposite directions. The sensible heat of the flue gas and the secondary combustion heat of CO jointly preheat the scrap steel continuously, and the preheated scrap steel is added to the furnace. Furnace smelting. The scrap is melted directly by the molten steel that has been left in the furnace, and the electrical energy is used to heat the molten steel instead of directly melting the scrap with an electric arc in the traditional top-charging process. During the smelting process of the localized Kangsidi electric furnace, the molten pool is stable, the electrical interference is small, the noise in the workshop is low, and the working environment is greatly improved.

 

  1. Preheating and continuous production of other scrap steel

Introduced by Fuchs System in 1922, the shaft furnace has a scrap preheating system, which can be a single shaft furnace with a scrap preheating system, can be a single shaft furnace or a double shaft furnace, and can be DC or AC. It uses the physical heat and chemical heat of the exhaust gas and the oxygen-burner at the bottom of the shaft furnace to heat the scrap steel column installed in the water-cooled shaft furnace. The domestic Zhangjiagang Shagang Runzhong 90t/65MVA shaft furnace is a successful example. The structural feature of the double-shell electric arc furnace is that the two furnace shells share a set of power supply, and its technical feature is the combination of scrap preheating and saving non-power-on time. Reducing the non-power-on time mainly starts from the connection and disconnection of the power supply. Consteel scrap is sent to the preheater of the system by the conveyor belt through dynamic sealing. The scrap is preheated and heated to 550-600 ℃ by the chemical energy and physical heat of the furnace gas, and then enters the molten pool. There is always a certain amount of residual molten steel in the furnace. , which helps to melt the heated scrap. Available charges are: scrap, pig iron and hot briquettes, hot and cold DRI.

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