Converter steelmaking is to blow oxygen into molten pig iron to oxidize impurities such as silicon and manganese. A large amount of heat is released during the oxidation process (1% silicon can increase the temperature of pig iron by 200 degrees Celsius), which can make the furnace reach a sufficiently high temperature. Therefore, converter steelmaking does not require the use of additional fuel. The basic tasks of steelmaking are decarbonization, dephosphorization, desulfurization, deoxidation, removal of harmful gases and non-metallic inclusions, raising the temperature and adjusting the composition. It can be summarized as: “four removals” (carbon, oxygen, phosphorus and sulfur), “two removals” (degassing and removal of inclusions), and “two adjustments” (composition and temperature). The main technical means used are: oxygen supply, slag making, temperature increase, addition of deoxidizers and alloying operations.
Purpose of converter smelting:
Oxidize carbon and other impurities (such as silicon, manganese) in pig iron to produce steel with better physical, chemical and mechanical properties than iron.
The difference between steel and pig iron:
First of all, the carbon content. In theory, steel is generally called steel if the carbon content is less than 2.11%. Its melting point is 1450-1500℃, while the melting point of pig iron is 1100-1200℃. In steel, carbon and iron form Fe3C solid solution. As the carbon content increases, its strength and hardness increase, while plasticity and impact toughness decrease. Steel has good physical, chemical and mechanical properties, and can be deep processed by drawing, pressing, rolling, punching and drawing. It has a wide range of uses.
Introduction to converter steelmaking principle
Converter steelmaking is carried out in a converter. The converter looks like a pear, with refractory bricks on the inner wall and many small holes (tuyere) on the side of the furnace. Compressed air blows into the furnace from these small holes, which is also called a side-blown converter.
At the beginning, the converter is horizontal, and 1300 degrees Celsius liquid pig iron is injected into it, and a certain amount of quicklime is added. Then air is blown in and the converter is rotated to make it stand upright. At this time, the surface of the liquid pig iron reacts violently, causing iron, silicon, and manganese to oxidize (FeO, SiO2, MnO,) to form slag. The convection of molten steel and slag is used to make the reaction spread throughout the furnace. After a few minutes, when only a small amount of silicon and manganese are left in the molten steel, carbon begins to oxidize to form carbon monoxide (exothermic), causing the molten steel to boil violently. A huge flame appears at the furnace mouth due to the combustion of the overflowing carbon monoxide. Finally, phosphorus is also oxidized and further forms ferrous phosphate. Ferrous phosphate reacts with quicklime to form stable calcium phosphate and calcium sulfide, which together become slag. When phosphorus and sulfur gradually decrease, the flame fades, and brown steam of ferroferric oxide appears at the furnace mouth, it indicates that the steel has been smelted. At this time, the blast should be stopped immediately, and the converter should be turned to a horizontal position, the molten steel should be poured into the ladle, and deoxidizer should be added for deoxidation. The whole process only takes about 15 minutes. If oxygen is blown in from the bottom of the furnace, it is a bottom-blown converter; if oxygen is blown in from the top, it is a top-blown converter.
Introduction to converter smelting process
The basic smelting process of a batch of steel in a converter. The operation process of smelting a batch of steel in a top-blown converter mainly consists of the following six steps:
(1) Load the furnace, tap steel, pour slag, check the furnace lining and tilting equipment, and make necessary repairs and repairs;
(2) Tilt the furnace, add scrap steel, add molten iron, and shake the furnace body (to a vertical position);
(3) Lower the gun and start blowing, and add the first batch of slag at the same time (the noise in the furnace is loud at first, red smoke comes out from the furnace mouth, and then a dark red flame is sprayed; after 3 to 5 minutes, the silicon, manganese and oxygen are close to the end, the carbon-oxygen reaction gradually becomes intense, the flame at the furnace mouth becomes larger, and the brightness increases accordingly; at the same time, the slag melts and the noise decreases);
(4) After 3 to 5 minutes, add the second batch of slag and continue blowing (the carbon content in the steel gradually decreases as the blowing progresses, and the flame becomes weak after about 12 minutes, so stop blowing);
(5) Tip the furnace, measure temperature, take samples, and determine the time for additional blowing or tapping;
(6) Tap steel, and at the same time (add the calculated alloy into the ladle) perform deoxidation and alloying.
Introduction to the main process equipment of converter steelmaking
Converter
The furnace body can rotate and is used for blowing steel or blowing matte. The converter body is made of steel plate, cylindrical, lined with refractory materials, and heated by chemical reaction heat during blowing, without external heat source. It is the most important steelmaking equipment and can also be used for copper and nickel smelting.
AOD refining furnace
AOD, or argon oxygen decarburization refining furnace, is a proprietary process for stainless steel smelting. AOD furnace types are 3t, 6t, 8t, 10t, 18t, 25t, 30t, etc. according to capacity. The equipment level has also developed from semi-automatic control to intelligent computer control to smelt stainless steel.
VOD refining furnace
VOD refining furnace (vacuumoxygen decarburization) is an off-furnace refining furnace that performs oxygen decarburization under vacuum. It is mainly used to refine chromium-nickel stainless steel, ultra-low carbon steel, ultra-pure ferritic stainless steel and pure iron. The primary molten steel is placed in a refining bag and placed in a sealed vacuum tank for oxygen decarburization, desulfurization, degassing, temperature adjustment, and chemical element adjustment.
LF refining furnace
LF (ladle furnace) is a ladle refining furnace with heating and stirring functions. Heating is generally performed by electrode heating, and stirring is performed through the bottom air-permeable bricks.
Oxygen lance
The oxygen lance is the main equipment for the converter oxygen supply. It consists of a nozzle, a gun body and a tail structure. The nozzle is made of copper with good thermal conductivity through forging and cutting, and can also be made by pressure casting. The shapes of the nozzles include Laval type, straight type and spiral type. Currently, the most commonly used is the multi-porous Laval type nozzle. The Laval type nozzle is a contraction-expansion contraction type nozzle. When the ratio of the outlet oxygen pressure to the inlet oxygen pressure pout/p0<0.528, a supersonic jet is formed.
Converter tilting system
Tilting system: variable frequency speed regulation (inverter + motor + reducer + large gear)
Tilting mechanism: The tilting mechanism consists of rails, tilting cylinders, cradle platforms, horizontal support mechanisms and supports.
Converter primary, secondary and tertiary flue gas
Converter primary flue gas: refers to the brown smoke discharged during the converter blowing process, which is a mixture of furnace gas and smoke, and its main component is CO.
Converter secondary flue gas: In the converter production process, the flue gas generated when adding scrap steel, slag, steel, blowing oxygen, and the flue gas that cannot be captured during the smelting process are generally called secondary flue gas. This part of the smoke enters the bag filter for purification through the pipeline.
Converter tertiary flue gas: The flue gas generated during the transportation of the molten iron ladle and the flue gas generated during the water addition process into the converter that is not captured by the converter secondary dust removal side suction hood are captured by the furnace top dust hood.
