JPS6212301B2 - - Google Patents

Description

[Detailed description of the invention]

This invention aims to dephosphorize and dephosphorize molten iron alloys containing chromium.
It relates to a desulfurization method. In general, phosphorus (P) in high chromium steel or stainless steel is known to be a harmful impurity that adversely affects the mechanical properties and stress corrosion cracking of steel. The number of steel types that have strict regulations regarding P, including P, is increasing. However, dephosphorization of such chromium-containing molten iron alloys requires the use of the method used for ordinary iron alloys, i.e., strong oxidation refining in which CaO-FeO fluxes, quicklime, etc. are added and oxygen blowing is carried out. However, it is considered almost impossible to produce low-phosphorus stainless steel because only a large amount of chromium is preferentially oxidized, hardening the slag, and making it difficult for oxidation of P to progress. The usual solution was to use ferrophosphorous alloys. For this reason, conventionally, with the aim of efficiently dephosphorizing molten iron alloys containing chromium, Ca-CaF ₂ was used using the electroslag remelting method.
Attempts have been made to dephosphorize using CaC ₂ - CaF ₂ flux in a ladle, etc. Both of these methods perform dephosphorization with Ca, and the dephosphorization reaction is a reductive dephosphorization represented by 3(Ca) + 2P → (Ca ₃ P ₂ ), and the latter is CaC ₂ → It utilizes Ca, which is produced by the decomposition reaction of CaC ₂ called Ca+2C. However, no matter which of these methods is adopted, Ca ₃ P ₂ will be present in the slag after dephosphorization treatment, and as shown by Ca ₃ P ₂ + 3H ₂ O → 3CaO + 2PH ₃ , There is a problem in that it reacts with H ₂ O in the atmosphere and generates toxic phosphin (PH ₃ ) with a strong garlic odor, which poses a major problem in slag treatment after dephosphorization. Therefore, the present applicant previously discovered from the above-mentioned point of view that ``a molten iron alloy containing about 3 to 30% by weight of chromium before decarburization contains P oxidized by an oxidizing agent.'' ₂ Alkaline earth metal oxides such as CaO and BaO to fix O ₅ and as a solvent
When a flux consisting of an alkaline earth metal halide such as CaCl ₂ , BaCl ₂ , or BaF ₂ is added, and a certain amount of an oxidizing agent is added,
P in the chromium-containing iron alloy is effectively removed,
At the same time, this highly basic and relatively low oxygen potential slag facilitates desulfurization. A method for dephosphorizing and desulfurizing chromium-containing iron alloy molten metal without generating harmful slag by adding flux composed of halides and a predetermined amount of oxidizing agent. Special request 1987
Proposed as No. -33549. However, the aforementioned patent application filed in 1983-
Although the method of No. 33549 can certainly achieve dephosphorization and desulfurization of chromium-containing iron alloy molten metal without requiring special slag treatment like the reductive dephosphorization method tried in the past, It has become clear through subsequent studies in actual operation by the present inventors that the dephosphorization and desulfurization efficiency may be extremely reduced depending on the target molten metal. For these reasons, the present inventors investigated the cause of the decrease in dephosphorization and desulfurization efficiency in the method of the above-mentioned Japanese Patent Application No. 57-33549 (hereinafter referred to as the previously proposed method), and discovered that harmful slag As a result of repeated research to find a safe and reliable method for dephosphorizing and desulfurizing chromium-containing iron alloy molten metal that does not involve the generation of chromium and does not cause any change in dephosphorization and desulfurization efficiency even if the molten metal to be treated changes. (a) Among the elements constituting the chromium-containing molten iron alloy to be dephosphorized and desulfurized, if there are many elements such as Si, which have a stronger affinity for oxygen than chromium, this will consume the oxygen in the flux. In addition, since the generated SiO ₂ is an acidic oxide,
This reduces the activity of alkaline earth metal oxides such as BaO, and as the amount of SiO ₂ increases,
As is clear from Figure 1, the dephosphorization efficiency is lower than expected. In addition, Figure 1 shows the phosphorus distribution ratio between slag and steel: (P)/[P],
FIG. 2 is a diagram showing the relationship between (BaO)/(SiO ₂ ). (b) Normally, in molten iron alloys containing chromium, Si has a stronger affinity for oxygen than chromium, and the content is conspicuous, so the molten metal is desiliconized before dephosphorization and desulfurization. If applied, dephosphorization and
It has been confirmed as shown in (a) and (b) above that it is possible to reliably prevent a decrease in reaction efficiency during desulfurization treatment. This invention was made based on the above research results, and the molten iron alloy is desiliconized in advance so that the Si content in the molten iron is 0.20% by weight or less, and after removing slag, AOD Adding to the molten iron alloy in a furnace a flux consisting of 30 to 50% by weight of one or more oxides of alkaline earth metals, the remainder of one or more halides of alkaline earth metals, and By adding an oxidizing agent while stirring with inert gas and removing the generated slag, the P and S components in the chromium-containing iron alloy are reliably removed with high efficiency without the need for troublesome operations. It is characterized by the fact that it is made as follows. In addition, in the method of this invention, the reason why the Si content is set to 0.20% by weight or less during the desiliconization treatment is because if the Si content exceeds 0.20% by weight, oxygen in the flux will be consumed as described above. This is because the amount increases, and the activity of alkaline earth metal oxides in the flux decreases more severely due to the generated SiO ₂ , which impairs the efficiency of dephosphorization and desulfurization treatment. This desiliconization treatment is preferably carried out in an electric furnace or an AOD furnace, for example, (i) by blowing oxygen gas using a lance, (ii) by adding an appropriate oxidizing agent such as scale. (iii) increasing basicity by increasing the amount of lime charged, etc. Next, the reason why the slag removal step after the desiliconization treatment is made essential will be explained. Normally, flux contains various impurities, but in the case of the flux used in the method of this invention, it is said that the flux used in the method of this invention contains substances such as iron oxide, which have a stronger oxidizing power than chromium oxide. It goes without saying that it is necessary to keep the flux as low as possible (preferably below 5% by weight of the total flux) as it oxidizes chromium oxide, but it is also necessary to keep it as low as possible (preferably less than 5% by weight of the total flux), but it is more important than chromium oxide, which should basically have no problems. Even if the oxide has weak oxidizing power, acidic or neutral oxides such as SiO ₂ and Al ₂ O ₃ reduce the basicity of the slag, as is clear from Figure 1 above.
This will inhibit the stabilization of P ₂ O ₅ . Therefore, in order to suppress these impurities to about 20% by weight of the total flux and avoid the above-mentioned disadvantages,
Before adding the dephosphorization flux, the slag removal process may be performed in an electric furnace or an AOD furnace used in the next process, for example, if desiliconization is performed in an electric furnace.
If desiliconization is performed in an AOD furnace, it is sufficient to carry out the desiliconization in that AOD furnace. In the method of the present invention, the dephosphorization and desulfurization steps are carried out in an AOD furnace; however, if an AOD furnace is used, (i) slag removal is easy; (ii) Strong stirring power can be obtained. (iii) There is almost no damage to refractory materials. Good results can be obtained that cannot be expected with other furnaces. The alkaline earth metal oxide as a basic substance may be BaO, CaO, MgO, or SrO, but the stabilizing power of P ₂ O ₅ , the corrosiveness of the furnace refractory brick, and the price From this point of view, BaO is the most suitable. In addition, in order to fix phosphorus oxidized by an oxidizing agent, that is, P ₂ O ₅ , a larger amount of alkaline earth metal oxide is preferable because P ₂ O ₅ is more easily stabilized. amount of total flux
If it is less than 30% by weight, the stabilization of P ₂ O ₅ becomes extremely poor, while if it exceeds 50% by weight, the flux will not become a slag. Therefore, in order for the flux to easily sludge and perform effective dephosphorization with low viscosity, the amount of alkaline earth metal oxide should be 30 to 50% of the total flux.
Must be expressed as % by weight. In addition, since these alkaline earth metal oxides have high melting points, in order to effectively contribute to the reaction, it is necessary to use a solvent to melt them without reducing their basic ability. be.
As this solvent, alkaline earth metal halides that are homologous to the basic substance are suitable, and among them, chlorides and fluorides such as BaCl ₂ , BaF ₂ , CaCl ₂ , and CaF ₂ are common. Good results can be obtained.
When BaO is selected as the oxide, the solvent is preferably a compound of the same alkaline earth metal such as BaCl ₂ or BaF ₂ . The greater the amount of flux added, the higher the dephosphorization and desulfurization effect, but due to workability issues during processing, it is best to use less than 300 kg per ton of molten iron. Ba,
It is necessary to heat up and perform the treatment two or three times. The oxidizing agents used include chromium oxide, chromium ore, iron oxide, molybdenum oxide, nickel oxide,
Examples include manganese oxide, or a gas containing one or more types of oxidizing gases such as oxygen and air. It is important for effective dephosphorization and desulfurization to suppress the amount of oxidizing agent added to an amount that does not harden the generated slag. because,
The added oxidizing agent preferentially oxidizes Cr over P in the molten iron, and also partially oxidizes Fe and C, but
This Cr oxide has a relatively high melting point (for example, Cr ₂ O ₃₎ .
1990℃), so if the amount exceeds the amount that can be dissolved in the slag produced by the added flux (solubility), the slag will harden and the dephosphorization reaction will not physically proceed. It is from.
In order to perform dephosphorization and desulfurization even more effectively, instead of adding the oxidizing agent together with the flux all at once, first add a mixed or synthetic flux containing BaO, _BaCl2, etc. as the main components, and then add the flux while stirring. It is preferable to obtain a slag with as low a viscosity as possible and to add the oxidizing agent little by little, continuously, semi-continuously, or intermittently. As a stirring method, gas stirring in which an inert gas such as Ar gas or N ₂ gas is blown is effective, and a good reaction rate can be obtained depending on the strength of the stirring force. In the method of this invention, BaCO ₃ is used instead of alkaline earth metal oxide as a flux component.
Carbonates of alkaline earth metals such as alkaline earth metal carbonates can also be used as an alternative. This is because when alkaline earth metal carbonates are added to the target molten iron alloy, they decompose as BaCO ₃ →BaO+CO ₂ and become alkaline earth metal oxides. In this case, the generated CO ₂ gas plays an important role as an oxidizing agent, so the dephosphorization/desulfurization reaction proceeds without using an oxidizing agent, and the desired dephosphorization can be achieved with only inert gas stirring.・Desulfurization treatment can be completed. When using alkaline earth metal carbonates, CO ₂
Considering the amount of decomposition as a gas, if the alkaline earth metal oxide is added so that the amount of alkaline earth metal oxide generated is 30 to 50% by weight of the total flux, the same effect as using alkaline earth metal oxide can be obtained. is obtained. In addition, if slag remains after dephosphorization and desulfurization treatments, it will cause rephosphorization, so a slag removal step after these treatments is also indispensable in the method of the present invention. Dephosphorization and
The desulfurized chromium-containing iron alloy molten metal is then subjected to the normal AOD refining process, but the temperature of the molten iron after dephosphorization and desulfurization treatment slag removal is considerably lower, reaching a temperature of over 1500℃. It is preferable to raise the temperature of Al in this way because it leads to sufficiently improving the decarburization efficiency in the AOD refining process. Next, the present invention will be specifically explained with reference to Examples. Example 1 A chromium-containing iron alloy having the composition shown in Table 1 was melted in an electric furnace, and then the amount of lime charged was increased to increase basicity, thereby removing Si in the molten iron. A desilicon treatment was performed to reduce the amount of silicon. After that, the molten iron was transferred to an AOD furnace together with the slag, and after removing the slag, BaO: 40% by weight was heated at a temperature of 1500℃.
_BaCl2 : 60% sintered flux consisting of 60% by weight
Kg/ton while stirring with Ar gas.
6Kg/ton of Cr ₂ O ₃ was added in 1Kg/ton portions, and this treatment was continued for 15 minutes. The molten iron components at this time are also shown in Table 1.

[Table] From the results shown in Table 1, it is clear that 44% dephosphorization and 95% desulfurization were achieved without any change in the chromium content before and after the dephosphorization/desulfurization treatment. At the same time, it can be seen that 85% of vanadium removal has progressed. Example 2 First, an iron alloy containing chromium was prepared, melted in an electric furnace, and then desiliconized to obtain a molten iron alloy having the composition shown in Table 2. After this, the molten iron is transferred to the AOD furnace with slag, and after removing the slag, 1550
Add a flux: 70Kg/ton consisting of 50% by weight of BaCO ₃ and 50% by weight of BaCl ₂ at a temperature of ℃,
After continuing Ar gas stirring for 10 minutes, the molten iron component was
It is also shown in the table.

[Table] From the results shown in Table 2, 48% dephosphorization and 92% desulfurization can be achieved with almost no change in the chromium content before and after dephosphorization/desulfurization treatment.
It is clear that 88% vanadium removal has also progressed. As described above, according to the present invention, it is possible to easily and inexpensively dephosphorize and desulfurize a molten iron alloy containing chromium without producing harmful slag, and with high efficiency, resulting in a high-quality product. Industrially useful effects can be obtained, such as the ability to easily produce high chromium steel and stainless steel.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the phosphorus distribution ratio of molten iron and slag and (BaO)/(SiO ₂ ).

Claims (1)

[Claims] 1. After the molten iron alloy is desiliconized in advance so that the Si content in the molten iron is 0.20% by weight or less, and the slag is removed,
Adding to the molten iron alloy in an AOD furnace a flux consisting of 30 to 50% by weight of one or more oxides of alkaline earth metals, the remainder of one or more halides of alkaline earth metals; A method for dephosphorizing and desulfurizing a molten iron alloy containing chromium, which is characterized by adding an oxidizing agent while stirring with an inert gas and removing the generated slag.