- The task and purpose of converter smelting
The main production task of the converter plant is to further smelt the pig iron smelted from the blast furnace into qualified molten steel, and cast it into qualified slabs and send them to various rolling mills for rolling.
The purpose of converter smelting: to oxidize carbon and other impurities (such as silicon, manganese) in pig iron to produce steel with better physical, chemical and mechanical properties than iron.
- Converter smelting production
1. Hot metal pretreatment
Before the molten iron enters the converter, the process of removing harmful elements such as S and P in the molten iron or recovering some rare elements such as niobium and vanadium is called molten iron pretreatment.
（1）Blowing method (spraying granular magnesium on the top of molten iron tank)
The granular magnesium is blown into the molten iron using the inert gas nitrogen, which also agitates the molten iron, which can be processed in a mixing car or in a ladle, as shown in 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, forming a conical vortex in the central part of the molten iron, so that the desulfurizer and the molten iron can be fully mixed and acted (see Figure 2).
2.1 Raw materials for converter steelmaking
（1）Hot metal: The main metal material for converter steelmaking, the amount of molten iron used in converter steelmaking generally accounts for 70 to 100% of the amount of metal material charged.
（2） Scrap steel: In converter steelmaking production, the amount of scrap steel generally accounts for 0 to 30% of the metal charge.
（3）Ferroalloys: Ferroalloys used for deoxidation are called deoxidizers. Commonly used deoxidizers are ferromanganese, ferrosilicon, aluminum, silicon manganese, silicon calcium, silicon manganese aluminum, etc. The iron alloy used to adjust the composition of molten steel is called alloy material, and the commonly used alloy materials are ferromanganese, ferrosilicon, ferrovanadium, ferrotitanium, ferrochromium, etc.
Slag making material
(1) Lime: Lime is the basic slag-making material for converter steelmaking. By adding lime to the furnace to make slag, it can dephosphorize and desulfurize, 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 in converter steelmaking production can replace part of lime, reduce the amount of lime, and can also promote the early slag aging, thicken the final slag, and prolong the life of the furnace lining.
2. Oxidant: The oxidant used in converter steelmaking is mainly oxygen, and iron ore and iron oxide scale can also act as oxidants.
3. Coolant: The coolants used in converter steelmaking production include scrap steel, iron ore, iron oxide scale, lime, dolomite, etc.
4. Carburizer: Carburizer is used to increase the carbon content in steel during converter steelmaking production, such as carbon powder and carbon wire.
2.2 Main equipment of oxygen top blowing converter
The equipment of the oxygen top-blown converter includes the furnace body, tilting equipment and oxygen supply equipment, as shown in Figure 3.
1. Furnace body: The outermost layer of the furnace body is the furnace shell, whose function is to bear the full weight of refractory materials, molten steel and slag, and maintain the fixed shape of the furnace, which is welded by ordinary boiler steel plates or alloy steel plates. A certain thickness of basic refractory material is built in the furnace shell.
2. Tilting equipment: It is composed of motor braking device, deceleration device and 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, so as to ensure that the furnace body is reversed 360°.
3. Oxygen supply equipment: oxygen supply equipment, oxygen lance and oxygen supply system. Oxygen lance (also known as spray gun or oxygen spray tube) is responsible for blowing oxygen into the molten pool.
[Oxygen top-blowing converter steelmaking method] is a water-cooled oxygen lance extending vertically into the furnace from the furnace mouth, and high-pressure oxygen is directly blown into the molten pool from the top to oxidize and remove carbon, silicon, manganese, phosphorus and other elements in the molten iron. , and use the physical heat of molten iron and the chemical heat released by element oxidation to obtain the high temperature required for smelting, which is a steelmaking method that does not require an external heat source.
[Top-bottom composite blowing] is to blow an appropriate amount of gas from the bottom while top blowing 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, thus overcoming the top-blowing oxygen flow stirring ability The weak point of the furnace makes the reaction in the furnace close to equilibrium, reducing the loss of iron, and at the same time retaining the advantages of the top blowing method to easily control the slagging process.
4. Out-of-furnace refining
Out-of-furnace refining is to move some of the tasks to be completed in a general steel-making furnace to a ladle or other special containers. This divides the traditional steelmaking process into two steps: primary refining and refining. Therefore, out-of-furnace refining is also called secondary steelmaking. Through out-of-furnace refining, the molten steel can be deeply decarburized, desulfurized, deoxidized, degassed, adjusted for composition (micro-alloying) and temperature adjusted and homogenized to remove inclusions under vacuum, inert atmosphere or controlled atmosphere. change the morphology and composition of inclusions.
[Ladle Blowing Argon]
A certain amount and pressure of argon gas is blown into the molten steel through the breathable bricks at the bottom of the ladle to remove harmful gases and non-metallic inclusions in the molten steel, and at the same time to uniform the temperature and composition of the molten steel, improve the steel The role of fluid mobility.
[RH method (vacuum cycle degassing method)]
When treating molten steel by RH method, firstly insert the rising pipe and descending pipe at the lower end of the vacuum chamber into the molten steel in the ladle, and then evacuate the molten steel in the vacuum chamber to the height of the pressure difference (Figure 4).
At this time, when argon gas is blown into the riser, the molten steel in the riser contains argon bubbles, and the density decreases and continues to rise. At the same time, the liquid level in the vacuum chamber rises, and the molten steel in the descending pipe descends and flows back into the ladle due to its relatively high 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.
【LF method (submerged arc heating ladle refining method with argon blowing)】
The LF method uses graphite electrodes for heating. During operation, the arc is buried in the molten slag layer on the molten steel surface. Synthetic slag-forming materials are used to form slag, and argon is blown at the bottom of the ladle to stir while maintaining a strong reducing atmosphere in the ladle. under refining (Figure 5). 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.
- Continuous casting process
The final product of steelmaking is a qualified ingot or billet. The casting of steel is to pour the smelted molten steel into the ingot mold or the mold of the continuous casting machine to solidify it into a steel ingot or billet. The quality of the 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 ingot or billet with a good surface, a pure interior, and a uniform and dense. The most common method of casting is continuous casting.
- Main equipment of continuous casting
2. Continuous casting production process