The tasks and objectives of converter smelting
The main production task of the converter plant is to further smelt the pig iron smelted in the blast furnace into qualified molten steel, and cast it into qualified ingots and send them to various rolling mills for rolling.
The purpose of converter smelting is to oxidize the carbon and other impurities (such as silicon, manganese) in the pig iron to produce steel with better physical, chemical and mechanical properties than iron.
1. Converter smelting production
1. Hot metal pretreatment
The process of removing harmful elements such as S and P from molten iron or recovering certain rare elements such as niobium and vanadium before molten iron enters the converter is called molten iron pretreatment.
(1) Spraying method (spraying granular magnesium from the top of the molten iron tank)
The inert gas nitrogen is used to blow the granular magnesium into the molten iron, which also stirs the molten iron and can be handled in a mixing car or ladle, see the figure below.
(2) KR method (mechanical stirring method)
The cross-shaped impeller stirring head (see Figure 1) is sunk into the molten iron and rotated to form a conical vortex in the center of the molten iron, so that the desulfurizer and the molten iron are fully mixed and reacted (see Figure 2).
2. Steelmaking
Raw materials for converter steelmaking
【Metal Material】
(1) Molten iron: The main metal material in converter steelmaking. The amount of molten iron in converter steelmaking generally accounts for 70-100% of the metal material charge.
(2) Scrap steel: The amount of scrap steel in converter steelmaking generally accounts for 0-30% of the metal material charge.
(3) Ferroalloy: The ferroalloy used for deoxidation is called deoxidizer. Commonly used deoxidizers include ferromanganese, ferrosilicon, aluminum, silicon manganese, silicon calcium, silicon manganese aluminum, etc. Ferroalloys used to adjust the composition of molten steel are called alloy materials. Commonly used alloy materials include ferromanganese, ferrosilicon, ferrovanadium, ferrotitanium, ferrochrome, etc.
【Non-metallic materials】
1. Slag-making materials
(1) Lime: Lime is the basic slag-making material for converter steelmaking. By adding lime to the furnace to make slag, it can remove phosphorus and sulfur, protect the furnace lining, and also act as a coolant.
(2) Fluorite: The main function of fluorite in converter steelmaking is to act as a flux.
(3) Dolomite: Adding dolomite to converter steelmaking can replace part of the lime, reduce the amount of lime used, and promote early slag formation, thicken the final slag, and extend the life of the furnace lining.
2. Oxidant: The oxidant used in converter steelmaking is mainly oxygen. In addition, iron ore and iron oxide can also act as oxidants.
3. Coolant: The coolants used in converter steelmaking include scrap steel, iron ore, iron oxide, lime, dolomite, etc.
4. Carbonizer: Carbonizer is used to increase the carbon content in steel during converter steelmaking, such as carbon powder and carbon wire. e
Main equipment of oxygen top-blown converter
The equipment of the oxygen top-blown converter includes a furnace body, a tilting device and an oxygen supply device, as shown in FIG3 .
1. Furnace body: The outermost layer of the furnace body is the furnace shell, which is used to bear the entire weight of refractory materials, molten steel, and slag, and to maintain the fixed shape of the furnace. It is welded from ordinary boiler steel plates or alloy steel plates. A certain thickness of alkaline refractory materials are built inside the furnace shell.
2. Tilting equipment: It consists of a motor brake device, a speed reducer, and a support bearing. The function of the tilting equipment is to tilt the furnace body to meet the requirements of process operations such as adding molten iron, adding scrap steel, sampling, tapping, and pouring slag, and to ensure that the furnace body rotates 360° forward and reverse.
3. Oxygen supply equipment: The oxygen supply equipment includes an oxygen gun and an oxygen supply system. The oxygen gun (also known as a spray gun or an oxygen spray pipe) is responsible for blowing oxygen into the molten pool.
Blowing
[Oxygen top-blown converter steelmaking method] is to use a water-cooled oxygen gun that extends vertically from the furnace mouth to blow high-pressure oxygen directly into the molten pool from the top, oxidize and remove carbon, silicon, manganese, phosphorus and other elements in the molten iron, and use the physical heat of the molten iron and the chemical heat released by the oxidation of the elements to obtain the high temperature required for smelting. It is a steelmaking method that does not require an external heat source.
[Top and bottom composite blowing] is to blow an appropriate amount of gas from the bottom while blowing the top to increase the stirring of the metal molten pool and slag and control the partial pressure of CO in the gas phase in the molten pool, thereby overcoming the weakness of insufficient stirring capacity of the top-blown oxygen flow, making the reaction in the furnace close to equilibrium, reducing iron loss, and retaining the advantage of the top-blowing method that is easy to control the slag-making process.
Refining outside the furnace
External refining is to move some of the tasks that need to be completed in a general steelmaking furnace to a ladle or other special container. In this way, the traditional steelmaking process is divided into two steps: primary refining and refining. Therefore, external refining is also called secondary steelmaking. Through external refining, molten steel can be deeply decarburized, desulfurized, deoxidized, degassed, the composition can be adjusted (microalloying), the temperature can be adjusted and homogenized, inclusions can be removed, and the morphology and composition of inclusions can be changed under vacuum, inert atmosphere or controlled atmosphere.
Common methods:
[Argon Blowing in Ladle] A certain amount of argon gas with pressure is blown into the molten steel through the air-permeable bricks at the bottom of the ladle to remove harmful gases and non-metallic inclusions in the molten steel, and at the same time, it plays a role in uniformizing the temperature and composition of the molten steel and improving the fluidity of the molten steel.
[RH method (vacuum cycle degassing method)] When treating molten steel with the RH method, the riser and downcomer at the lower end of the vacuum chamber are first inserted into the molten steel in the ladle, and the molten steel is raised to the pressure difference height in the vacuum chamber by vacuuming. At this time, argon gas is blown into the riser, and the molten steel in the riser continues to rise due to the argon bubbles. At the same time, the liquid level in the vacuum chamber rises, and the molten steel in the downcomer flows down and back into the ladle due to its relatively large density. In this way, through continuous and repeated circulation, the molten steel continuously enters the vacuum chamber and is fully degassed under vacuum conditions. Vacuum treatment can also add various alloys according to the needs of the steel type.
[LF method (submerged arc heating argon blowing ladle refining method)] The LF method uses graphite electrodes for heating. During operation, the electric arc is buried in the slag layer above the surface of the molten steel. Synthetic slag-making material is used to make slag, argon is blown at the bottom of the ladle for stirring, and refining is performed while maintaining a strong reducing atmosphere in the ladle.
2. Continuous Casting Process
The final product of steelmaking is qualified steel ingots or billets. Steel pouring is pouring the refined molten steel into the ingot mold or the crystallizer of the continuous casting machine to solidify it into a steel ingot or billet. The quality of the steel ingot or billet depends not only on the smelting process but also on the pouring process. The task of pouring is to pour the molten steel into a solid steel ingot or billet with a good surface, pure interior, uniform and dense. The most commonly used method of pouring is continuous casting.
1. Main equipment for continuous casting
2. Continuous casting production process
3. Billet
