Molten iron contains impurities such as C, S, and P, which affect the strength and brittleness of iron. It is necessary to re-smelt the molten iron to remove the above impurities and add Si, Mn, etc. to adjust its composition. The process of re-smelting molten iron to adjust its composition is called steelmaking.
The main raw materials for steelmaking are molten iron or pig iron with a high carbon content and scrap steel. In order to remove impurities in molten iron, it is also necessary to add oxidants, deoxidizers, slag-making materials, and ferroalloys and other materials to the molten iron to adjust the composition of steel. After the molten iron or pig iron with a high carbon content is added to the steelmaking furnace, it is oxidized and removed through processes such as oxygen blowing, adding ore, and decarbonization. Finally, alloys are added for alloying to obtain molten steel. There are three types of steelmaking furnaces: open-hearth furnaces, converters, and electric furnaces. The open-hearth steelmaking method has been gradually eliminated due to high energy consumption and poor working environment. The converter and open-hearth steelmaking process is to first load molten iron into a mixing furnace for preheating, add scrap steel into the converter or open-hearth furnace, and then use a mixing car to transfer the high-temperature molten iron in the mixing furnace into the converter or open-hearth furnace for melting and heating. When the temperature is appropriate, it enters the oxidation period. Electric furnace steelmaking is to add all cold scrap steel into the electric furnace steel, and after a long period of melting and heating, it enters the oxidation period.
(1) Melting process. Molten iron and scrap steel contain impurities such as C, Mn, Si, P, and S. During the low-temperature melting process, C, Si, P, and S are oxidized, that is, the impurities in the elemental state are converted into impurities in the compound state, which is conducive to further removal of impurities in the later stage. Oxygen comes from rust (component is Fe2O3·2H2O) in the charge, iron oxide, added iron ore, oxygen in the air, and oxygen blowing. The oxidation process of various impurities takes place at the interface between the slag and the molten steel.
2) Oxidation process. The oxidation process is a decarbonization, dephosphorization, degassing and impurity removal reaction at high temperature.
(3) Deoxidation, desulfurization and steel tapping. At the end of oxidation, the steel contains a large amount of excess oxygen. The excess oxygen in the steel is removed by adding block or powder ferroalloy or multi-element alloy to the molten steel. The harmful gas CO produced is discharged with the furnace gas, and the slag produced can be further desulfurized. That is, in the final steel tapping process, the slag and steel are strongly mixed and washed to increase the desulfurization reaction.
(4) Refining outside the furnace. The molten steel smelted from the steelmaking furnace contains a small amount of gas and impurities. Generally, the molten steel is injected into the refining ladle and subjected to argon blowing, degassing, ladle refining and other processes to obtain a relatively pure steel.
(5) Casting. The pure molten steel coming out of the steelmaking furnace or refining furnace can be tapped when its temperature is appropriate and the chemical composition is adjusted appropriately. The molten steel is tapped into the ingot mold or continuous steel casting machine through the ladle to obtain an ingot or continuous casting billet.
There are two ways of casting: mold casting and continuous casting. Mold casting is further divided into upper casting and lower casting. The upper casting method is to directly inject molten steel from the ladle into the mold through the upper opening of the mold to form an ingot. The lower casting method is to pour the molten steel in the ladle into the middle pouring pipe and flow steel bricks, and the molten steel enters the mold from the lower opening of the ingot mold. The molten steel solidifies in the mold to obtain an ingot. After removing the insulation cap, the ingot is sent to the soaking furnace of the rolling mill for heating, and then the ingot mold is transported back to the steelmaking plant for mold remodeling.
Continuous casting is to pour molten steel from the ladle into the tundish, and then into the cleaner. After the molten steel is quenched, it is pulled out of the crystallizer at a certain speed by the billet drawing machine. After secondary cooling and forced cooling, it is cut into a certain size of continuous casting billet after it is completely cooled, and finally sent to the rolling workshop.
Main safety technologies for steelmaking production
(1) Prevention of explosion accidents caused by elbows or reducers. The oxygen pipe bend or reducer at the top of the oxygen lance has a large flow rate and large local resistance loss. If there is slag in the pipe or the degreasing is not clean, it is easy to induce high-purity, high-pressure, and high-speed oxygen to explode. Accidents should be avoided by improving the design, preventing sharp bends, slowing down the flow rate, blowing the pipe regularly, cleaning the filter, and improving degreasing.
(2) Prevention of flashback explosion accidents. Low-pressure oxygen leads to negative pressure in the oxygen pipe and blockage of the oxygen lance nozzle, which are prone to flashback of the gas generated by the high-temperature molten pool, resulting in explosion accidents. Therefore, the oxygen pressure should be closely monitored. When multiple furnaces use oxygen, do not rush to use oxygen to avoid flashback of the pipeline.
(3) Prevention of steam-blocked explosion accidents. Due to operating errors, the oxygen lance is blocked from returning water, and the accumulated water in the oxygen lance vaporizes in the high temperature of the molten pool, preventing the high-pressure water from entering. When the steam pressure in the oxygen lance exceeds the strength limit of the lance wall, an explosion occurs.
Safety technology for scrap steel and furnace demolition blasting
(1) Possible hazards of blasting: explosion seismic waves; explosion shock waves; hazards of fragments and flying pieces; noise.
(2) Safety countermeasures: First, heavy scrap steel blasting. Scrap steel must be blasted in an underground blasting pit. The blasting pit must be strong and have pressure relief holes. Column retaining walls must be set up around the pressure relief holes. Second, for furnace demolition blasting, the amount of explosives must be limited and the blasting energy must be controlled. Third, necessary prevention and control measures must be taken.
Steel, iron, and slag burn protection technology
The temperature of iron, steel, and slag liquid is very high, the heat radiation is very strong, and it is easy to splash. In addition, the temperature of the equipment and the environment is very high, so burn accidents are very likely to occur.
