Introduction of Silicon Chromium Alloy
An iron alloy is mainly composed of silicon, chromium, and iron. Also known as silicon-chromium ferroalloy. It is mainly used as a master alloy for the production of medium, low, and micro carbon iron by electrosilicothermal method. Used as deoxidizer (replacing ferrosilicon) and chromium additive in steelmaking. Generally, it contains Cr>30%, Si>35%, and the balance is iron and a small amount of impurities. Classified according to carbon content, such as carbon content ≤0.06%, ≤0.10%, ≤1.0%, etc.
An alloy of iron, chromium, and silicon-containing not less than 30.0% chromium and not less than 35.0% silicon. Its production process is divided into two types: one-step method (usually called “slag-containing method” or “ore method”) and two-step method (usually called “slag-free method”). The raw materials used in one-step smelting include silica, chromium ore, and coke. The raw materials used in the two-step smelting include silica, carbon ferrochrome, and steel scraps. Both smelting methods use coke as the reducing agent.
The source of silicon in the alloy is mainly SiO in silica, and the sources of chromium are chromium ore and carbon ferrochrome. They are all produced by smelting in ferroalloy submerged arc furnaces. It is mainly used as a reducing agent for the production of medium-low carbon ferrochrome and micro-carbon ferrochrome and as an intermediate alloy agent for steelmaking. According to the content of Si, Cr, and C, silicon-chromium alloys can be divided into many different brands of silicon-chromium alloys. The composition phase of its alloy system is: Si>50% is composed of (Fe·Cr)Si, Si>40% is composed of (Fe·Cr) Si and (Fe·Cr)Si, Si>30% is composed of (Fe· It is composed of Cr)Si and (Fe·Cr)Si. When Si<19%, (Fe·Cr)C and (Fe·Cr)C account for the majority.
Since the phase composition of silicon-chromium alloy determines the carbon reduction effect outside the furnace, in industrial production, the Si content in silicon-chromium alloy is generally controlled to be greater than 40%. The C content of silicon-chromium alloys released from the furnace is generally 0.2-0.8%, which is difficult to use directly. It must undergo carbon reduction treatment outside the furnace. The C content of the alloy can be reduced to less than 0.02%. Different grades of silicon-chromium alloys are refined according to user requirements. In addition, ferrosilicon is added to liquid high-carbon ferrochromium to produce silicon-chromium alloy.
Properties of Silico-chromium Alloy
Chromium and silicon produce two stable compounds at high temperatures: CrSi and CrSi. Because chromium silicide is more stable than its carbide when silicon is present, part of the carbon will be replaced by silicon to form a carbon-silicon composite chromide until silicide is formed. Pavlov (Ю.А.Павлов) studied the phase structure of Cr-Si-Fe-C cast alloy with Cr: Fe=1.
When Si<20% in the alloy, it is composed of one phase (Cr, Fe) (C, Si). It can be considered that part of Cr in CrC is replaced by Fe and part of C is replaced by Si. When the silicon content increases to >20% to 29%, a new composite phase (Cr, Fe) (Si, C) is formed. Excess Cr and Fe form the intermetallic compound FeCr, which is the σ phase. Containing Si between 29% and 34%, a new phase (Cr, Fe)Si is added. When Si exceeds 34%, chromium, iron, and silicon form silicide. CrSi and SiC phases appear due to the increase in silicon content. The affinity between chromium and silicon is greater than that between iron and silicon, so CrSi is formed first. However, CrSi and FeSi have different crystal structures and cannot form solid solutions with each other. When the Si content is 44% to 51%, Cr and Si form CrSi, and part of FeSi and Si form FeSi. When Si is 51% to 60%, the alloy is composed of Cr-Si, FeSi, SiC, and Si. From the above results, it can be seen that the chromium-silicon ferroalloy with high silicon content is composed of chromium and iron silicides, SiC and Si, that is, carbon exists in the SiC phase. The structural analysis of industrially produced silicon-chromium ferroalloys is consistent with this. Carbon exists in the SiC phase and is insoluble in the liquid ferrosilicon.
Production Process of Silicon-chromium Ferroalloy
There are two processes for producing silicon-chromium ferroalloy from chromium ore: the one-step method and the two-step method.
The one-step method is to directly smelt chromium ore, silica, coke, etc. into ferrosilicon alloy in a submerged arc reduction electric furnace. Therefore, it is also called the direct method or the slag method. Due to the large amount of slag in this method, more chromium is lost in the slag. MgO, AlO, and CaO were partially reduced and volatilized, which increased production energy consumption. Therefore, a two-step method was developed, that is, chromium ore, coke, and part of silica were used as fluxes to refine high-carbon chromium in a submerged arc reduction electric furnace. Iron is then granulated or broken into granules; then the high-carbon ferrochromium particles, silica, and coke are smelted into ferrosilicon alloy in another submerged arc reduction electric furnace. Because the product contains high carbon content, carbon reduction treatment is required outside the furnace to obtain silicon-chromium ferroalloy with qualified carbon content. Because the chromium ore is refined into an alloy in two steps, it is called the two-step method. Also known as the indirect method or the slag-free method. After long-term improvement and operation, compared with the two-step method, the one-step method uses one less electric furnace, which reduces investment; low-carbon products can be obtained directly; and the process flow is short. After recovering metal through slag, the recovery rate of chromium is higher and the total smelting power consumption is lower; but the process is difficult to master. The main reason is that the slag is sticky and the problem of smooth discharge of slag from the furnace is difficult to solve. In addition, there are the shaking package decarburization method and the slag washing decarburization method.
• One-Step Method
After the chromium ore, silica, coke, and steel chips are evenly mixed, they are added to the submerged arc reduction electric furnace. Choose a reasonable power supply system to insert the electrode deeply into the furnace material layer to keep the slag temperature high, which is beneficial to the reduction reaction and refining of the produced alloy, and destroys SiC to make the slag easier to discharge from the furnace. Only by making the ratio of MgO/AlO in the slag component less than 2 and controlling the SiO content between 40% and 50% can the carbon content in the alloy be less than 0.04% and be beneficial to slag discharge. There is a thick slag layer in the molten pool. The upper layer is similar to the situation of smelting high-carbon ferrochrome. The slag contains SiO44% and SiC 23%. The lower part is silicon-chromium ferroalloy and the final slag. The lower slag contains SiO30% and SiC 2%. The main difficulty in production is the discharge of slag from the furnace. Therefore, a slag-pulling machine should be installed in front of the furnace. That is, a steel rod is extended into the furnace from the tap hole to stick the slag on the iron rod and pulled out of the molten pool. The slag has a high viscosity and contains a large number of alloys, so it needs to be separated by gravity beneficiation methods such as a jig. It can increase the chromium recovery rate by about 5%.
• Two-step Method
The first step is to produce high carbon ferrochrome; (see ferrochrome for the process). The second step is to smelt ferrosilicon alloy using high-carbon ferrochrome, silica, coke, and steel scraps. The process is similar to that of smelting 45% ferrosilicon. The difference is that high-carbon ferrochrome is used. Carbon chromium iron particles replace steel chips. The smelting process is shown in Figure 4. Carbon (carbon of coke and carbide) reduces SiO to Si to promote the transformation of (Cr, Fe)C into CrSi, FeSi, and SiC. The particle size of high carbon ferrochrome has a great influence on the destruction of (Cr, Fe)C. The smaller high-carbon ferrochrome particle size mixed with the uniform charge increases the contact of Si to (Cr, Fe)C, which can be destroyed before entering the molten pool. Production practice has proven that when larger pieces of high-carbon ferrochrome are used to smelt ferrosilicon alloy, the carbon content can reach 0.13%; while when using a particle size of less than 20mm, the carbon content is <0.06%. As the silicon content of ferrosilico alloy increases, the carbon content decreases. When the Si content is >34%, SiC is generated. When the Si content is >43%, the carbon content of the alloy will not decrease significantly.
Therefore, when producing silicon-chromium ferroalloy, it is more appropriate to control the silicon content between 43% and 53%. The solubility of SiC in silicon-chromium ferroalloy is very small, and it exists as a suspended matter. It requires appropriate conditions to be separated from the alloy. The floating of SiC from the alloy requires a higher furnace temperature, such as 1650-1750°C; a longer sedation time in the molten iron ladle, such as more than 60 minutes; and the chromium content of the alloy should be controlled below 34% to reduce the viscosity of the alloy. Through thermal insulation and sedation, the carbon content of the alloy can be reduced from 0.15% to 0.30% to 0.04%. However, there is a large difference in carbon content between the upper and lower layers of the alloy in the package and between the center and edges. The alloy discharged from the furnace contains C0.4%~0.8% and cannot be used as an intermediate alloy for the production of micro-carbon ferrochromium. It needs to undergo decarburization treatment outside the furnace.
Industrial Production Method of Silicon Chromium Alloy
• Shake Package Decarburization Method
Place the molten iron ladle containing silicon chromium alloy liquid and decarburizing agent on the ladle shaking frame to make the ladle move eccentrically. At a rotation speed of 50 to 55 r/min, the melt in the ladle is caused to move like “sea waves”. The particles of the melt move up and down to produce mixing and stirring effects, creating good conditions for the mixing of ferrosilicon alloy and decarburizing agent. SiC precipitates from the alloy and is absorbed by the decarburizing agent. The decarburizing agent is microcarbon ferrochromium slag, or lime and fluorite slag. The dosage is 5% to 8% of ferrosilicon alloy. The shaking time is 5 to 10 minutes. After the shake package decarburization treatment, the carbon content of silicon-chromium ferro alloy can be reduced to 0.02%.
• Slag Washing and Decarbonization Method
The liquid silicon-chromium alloy is directly injected into the liquid micro-carbon ferrochromium slag. The slag liquid is dispersed and mixed with the alloy liquid. As the liquid alloy rises, the slag absorbs most of the silicon carbide. During the cooling process, the alloy continues to precipitate silicon carbide, which floats up to the contact surface between the slag and the alloy and enters the slag. Through slag washing, the carbon content in the slag can reach 4%, and the carbon content in the alloy can be reduced to 0.02%. Slag washing not only has a high carbon reduction rate but also can recover chromium in micro-carbon ferrochrome slag. The chromium content in the slag is reduced to about 0.5%. Through slag washing, the phosphorus content of ferrosilicon alloy can be reduced by 75% to 90%.