JPS6237686B2 - - 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 produced using the electroslag remelting method.
Attempts have been made to dephosphorize using a CaC ₂ -CaF ₂ flux in a ladle. Both of these methods perform dephosphorization with Ca, and the dephosphorization reaction is reductive dephosphorization expressed as 3(Ca) + 2 P → (Ca ₃ P ₂ ), and the latter is CaC ₂ +Ca+ 2C , which is produced by the decomposition reaction of CaC ₂ , is used. 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
By adding a flux consisting of an alkaline earth metal halide such as CaCl ₂ , BaCl ₂ , or BaF ₂ and further adding an oxidizing agent, P in the chromium-containing iron alloy can be effectively removed. ,
Based on the new knowledge that desulfurization progresses well with this highly basic slag with a relatively low oxygen potential, it was discovered that alkaline earth metal oxides and alkaline earth metal halides were added to the molten iron alloy. A method for dephosphorizing and desulfurizing chromium-containing molten iron alloys without generating harmful slag by adding a flux consisting of Gansho 57-
It was proposed as No. 33549 (Japanese Unexamined Patent Publication No. 151416/1983). However, the aforementioned patent application filed in 1983--
The method of No. 33549 (Japanese Unexamined Patent Publication No. 151416/1983) does not require any special slag treatment like the conventional reduction dephosphorization method, and it is possible to dephosphorize and dephosphorize chromium-containing iron alloy molten metal. Although desulfurization can be achieved, depending on the molten metal being treated,
Subsequent studies revealed that dephosphorization and desulfurization efficiency may be extremely reduced. Based on the above-mentioned matters, the present inventors,
First, we investigated the cause of the decrease in dephosphorization and desulfurization efficiency in the proposed method, and found that it is a stable and reliable method that does not generate harmful slag and does not cause changes in dephosphorization and desulfurization efficiency even if the molten metal to be treated changes. As a result of repeated research to find a method for dephosphorizing and desulfurizing chromium-containing molten iron alloy, we found that (a) among the elements that make up the chromium-containing molten iron alloy to be treated, chromium has a stronger affinity for oxygen than chromium;
When there is a large amount of elements such as Si, this consumes oxygen in the flux, and the generated SiO ₂ reduces the activity of alkaline earth metal oxides.
had a greater impact than was initially realized,
As is clear from Figure 1, as the amount of SiO ₂ increases, the dephosphorization efficiency decreases extremely.
In addition, Figure 1 shows the phosphorus distribution ratio between slag and steel:
FIG. 2 is a diagram showing the relationship between (P)/[P] and (BaO)/(SiO ₂ ). (b) In normal molten iron, as long as Si, which has a stronger affinity with oxygen than chromium and is by far the most abundant element, is kept below a certain amount, the reaction efficiency in dephosphorization and desulfurization treatment can be improved. (c) Mass production of high-quality chromium-containing steel is possible by performing melting of iron alloy, desiliconization, and dephosphorization/desulfurization treatment in one electric furnace. We have come to the confirmation as shown in (a) to (c) above that this becomes extremely easy. This invention was made based on the above research results, and the molten iron alloy is preliminarily desiliconized in an electric furnace so that the Si content in the molten iron is 0.20% by weight or less. After slaging, add one or more alkaline earth metal oxides to the molten metal: 30 to 50
Weight %, one or more alkaline earth metal halides:
P and S in the chromium-containing iron alloy can be removed without the need for troublesome operations by adding a flux consisting of the remainder, and adding additives while stirring them, and removing the generated slag. This method is characterized by the fact that it removes the components reliably and with high efficiency. In the method of this invention, the Si content is specifically set at 0.20% by weight or less during the desiliconization treatment because if the Si content exceeds 0.20% by weight, alkaline earth metals in the flux will be removed. This is because the degree of reduction in the activity of the oxide becomes severe and the efficiency of dephosphorization and desulfurization treatment deteriorates, so it is preferable to keep the content to 0.10% by weight or less if possible. For this desiliconization treatment, for example, a method of blowing oxygen gas using a lance, a method of adding an appropriate oxidizing agent such as scale, a method of increasing basicity by increasing the amount of lime charged, etc. are adopted. It can be implemented by After performing such desiliconization treatment, it is necessary to remove slag before dephosphorization and desulfurization treatment, and the reason for this will be explained below. 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 flux) because it oxidizes chromium oxide, but it is also necessary to keep it as low as possible (preferably below 5% by weight of the 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 prevent the above-mentioned disadvantages, a slag removal step is required before adding the dephosphorized flux. The alkaline earth metal oxide used as a basic substance may be BaO, CaO, MgO, or SrO, but BaO is the most suitable from the viewpoint of stabilizing power of P ₂ O ₅ and price. Are suitable. In addition, in order to fix phosphorus oxidized by an oxidizing agent, that is, P ₂ O ₅ , the larger the amount of alkaline earth metal oxide, the more
This is preferable because P ₂ O ₅ is easily stabilized. If the amount is less than 30% by weight of the total flux, stabilization of P ₂ O ₅ becomes extremely poor, while if it exceeds 50% by weight, the flux will not become slag. Therefore, in order for the flux to easily sludge and to perform effective dephosphorization with low viscosity, the amount of alkaline earth metal oxide must be 30 to 50% by weight of the total flux. Since alkaline earth metal oxides have high melting points, in order to make them effectively contribute to the reaction, it is necessary to use a solvent to melt them without reducing their basic ability. As this solvent, halides of alkaline earth metals in the same group as the basic substance are suitable, and among them, chlorides and fluorides such as BaCl ₂ , BaF ₂ , CaCl ₂ , CaF ₂ are common; Good results are 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 more effective it is for dephosphorization and desulfurization, but due to workability issues during processing, it is best to use less than 300 kg per ton of molten iron, and it is recommended to use a larger amount of flux. In that case, it is necessary to increase the temperature and perform the treatment two or three times. The oxidizing agent used includes one or more of chromium oxide, chromium ore, iron oxide, molybdenum oxide, nickel oxide, manganese oxide, or oxidizing gases such as oxygen and air. When using chromium, the oxidizing power is not so great that it does not harden the slag excessively, and the chromium oxide that acts as an oxidizing agent is reduced to chromium, resulting in substantial loss of chromium. Therefore, chromium oxide can be recommended as the most preferable oxidizing agent. 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 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. 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 in small amounts gradually, continuously, semi-continuously, or intermittently. Any stirring method can be used in an electric furnace, such as an arc, a stirrer, or inert gas injection, and a good reaction rate can be obtained by such stirring. In the method of this invention, BaCO ₃ is used instead of the alkaline earth metal oxidizing agent as a flux component.
Alkaline earth metal carbonates 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/desulfurization treatment can be achieved with just stirring. can be terminated. When using alkaline earth metal carbonates, CO ₂
If the amount of alkaline earth metal oxides produced is 30 to 50% by weight of the total flux, considering the amount of decomposition as a gas, the same effect as using alkaline earth metal oxides 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. If the chromium-containing iron alloy molten metal that has been dephosphorized and desulfurized by the method of the present invention as described above is then directly subjected to ordinary electric furnace refining for decarburization and other treatments, the desired high-quality steel can be obtained. can be obtained. Next, the present invention will be specifically explained with reference to Examples. Example 1 A chromium-containing iron alloy having a composition as shown in Table 1 is melted in an electric furnace, and then desiliconized by a conventional method to obtain molten iron having a composition as shown in Table 1. Obtained. After removing the generated slag,
Continuing in the same electric furnace at a temperature of 1500℃,
A sintered flux consisting of BaO: 40% by weight and BaCl ₂ : 60% by weight was added in an amount of 60Kg/ton, and while stirring with an arc, 40Kg/ton of Cr ₂ O ₃ was dispensed in 1Kg/ton portions. This treatment continued for 30 minutes. The molten iron components at this time are also shown in Table 1. From the results shown in Table 1, it is clear that 32% dephosphorization and 92% 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 73% of vanadium removal has progressed.

[Table] Example 2 First, an iron alloy containing chromium is prepared, melted in an electric furnace, and then desiliconized in the same furnace by a normal method to obtain the components shown in Table 2. A molten iron alloy of the composition was obtained. After removing the generated slag, BaCO ₃ :
A flux consisting of 50% by weight and 50% by weight of BaCl ₂ was added at 85 kg/ton, and stirring was continued for 10 minutes by blowing Ar gas through a lance. The subsequent molten iron component composition is also shown in Table 2.

[Table] From the results shown in Table 2, 42% 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. This brings about industrially useful effects, 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. In an electric furnace, Si content in molten iron is 0.20
After the molten iron alloy has been desiliconized in advance and the slag removed so that the amount is less than 30 to 50% by weight, one or more types of alkaline earth metal oxides are added to the molten metal.
Weight %, one or more alkaline earth metal halides:
A method for dephosphorizing and desulfurizing a molten iron alloy containing chromium, the method comprising: adding a flux consisting of the remainder, and further adding an oxidizing agent while stirring the flux, and removing the generated slag.