JPS6023165B2 - Dephosphorization method for high chromium molten metal - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for efficiently and rapidly dephosphorizing high-chromium molten metal, which is difficult to dephosphorize using conventional oxidation refining methods. Oxidative refining is usually used to dephosphorize molten metal.
This is a method in which molten metal is oxidized in the presence of high basicity slag to convert phosphorus into P205, which is then fixed as calcium phosphate in the slag. By this method, hot metal and molten steel with normal components can be dephosphorized relatively easily. However, Cr,
If the above oxidation refining method is applied to a molten metal containing several percent or more of elements that are more easily oxidized than iron, such as Sj, AI, and Mn, the oxidation reaction of these valuable elements will proceed preferentially and the yield will decrease. At the same time, oxidation products migrate into the slag, significantly inhibiting the progress of dephosphorization from both physical (through viscosity, etc.) and chemical (changes in the equilibrium phosphorus content in the falling sun) aspects. For this reason, dephosphorization has conventionally been considered virtually impossible in high-chromium molten metals such as stainless steel and ferrochrome. However, in recent years, as the effects of phosphorus as an impurity in metal materials have been studied, it has become clear that, especially in the case of high chromium steels such as stainless steel, it has a significant negative effect on workability, corrosion resistance, solubility, etc. It has been. Therefore, in order to improve the manufacturability of high-grade stainless steel, which generally has poor workability, and to ensure the quality of materials used in harsh environments, the upper limit of phosphorus content has increasingly been strictly regulated. Ferrochrome is the main chromium source for high chromium steel melting, but as the phosphorus content of raw materials such as chromium ore and coke has increased recently, the phosphorus content of ferrochrome is also increasing. If high chromium molten metal cannot be dephosphorized as mentioned above, the phosphorus content of high chromium steel products will also tend to rise, making it impossible to meet the demand for lower phosphorus from the material standpoint. . Therefore, there is a strong demand for the development of a dephosphorization method for high chromium molten metals, which was previously considered impossible. As a method to meet this demand, the present inventors first developed a method in which a molten metal or alloy with a carbon content below the saturation concentration and a flux containing calcium carbide and calcium oxide as main components were heated in a non-oxidizing atmosphere. He invented a method of contacting the material at the bottom (Tokuken Showa 50-12934 Tokugan Kai 50-99143). In this method, (CaC2)→(Ca)+2[C]...
・Metal calcium is generated in the slag by the reaction of '1), and this is used to refine the falling sun, and for phosphorus, it is 3 (Ca) + 2 [P] 2 (Ca3P2)
．．・．． '2} can dephosphorize. Among the flux components used in this method, fluorides of alkali metals are necessary to stably exist in the slag metallic calcium produced by formula '1' and Ca3P2 produced by formula (2). For this purpose, it is necessary that the flux contains fluoride in an amount of 10% or more, preferably 20 to 40%. If this refining reaction is carried out in a mold, there is no need to worry about the corrosivity of the slag. However, processing in a mold is not suitable for refining melt defects in a mass production process, which is the object of the present invention, in terms of equipment, energy efficiency, and the like. Therefore, the present invention assumes that the reaction is carried out in a reaction vessel lined with a normal oxide refractory (
Carbon or carbide-based refractories cannot be used because they prevent the release of metallic calcium necessary for dephosphorization.)
However, in that case, the biggest problem is how to suppress erosion caused by slag with a high fluoride content. When corrosion of the refractory occurs intensely, the refractory unit consumption of the reaction vessel increases, and the oxidative consumption of metallic calcium occurs due to the oxides that have entered the slag, causing the reaction of equation 2 to proceed to the right. In some cases, there is a risk of causing rephosphorization. Therefore, it is of course necessary to select various conditions such as reaction temperature and flux composition to reduce corrosion of the refractory as much as possible, but it is also necessary to minimize the refining time required to obtain a specified dephosphorization rate. The most important countermeasure is to If the time required for refining is long, it becomes necessary to increase the superheat before treatment to compensate for the temperature of the molten metal, or to heat it during the process, which accelerates refractory corrosion not only temporally but also thermally. In view of the above circumstances, the present invention has been developed by refining carbon-unsaturated high-chromium molten metal using a flux containing calcium carbide and an alkali metal halide, such as calcium fluoride, as the main components.
In particular, this method was obtained through various studies to determine the conditions for shortening the refining time, reducing corrosion of refractories, and obtaining a stable refining effect for dephosphorization (dephosphorization). This method is characterized by adding flux after adjusting the temperature. Hereinafter, the configuration of the invention will be explained in detail. The targets of smelting are high-chromium molten metals that are difficult to dephosphorize using normal oxidation refining methods, that is, ferrochrome, stainless steel, and heat-resistant steel that contain several percent or more of chromium. Prior to refining these fallen suns for dephosphorization, it is necessary to carbon unsaturate them so that they can absorb carbon from calcium carbide by the reaction of formula '1}. Therefore, in the case of high carbon ferrochrome, an appropriate amount of carbon is removed by methods such as oxidation, and in the case of high chromium steel, appropriate raw materials are melted and mixed, and if necessary, a portion of the carbon is further decarburized. . Note that conditions related to the oxygen content of the setting sun, which is the key point of the present invention, will be described later. The flux used for dephosphorization contains calcium carbide and alkali metal halides as main components. Calcium carbide in the flux is required to react with carbon-unsaturated port water according to formula m to produce metallic calcium in the flux, which is essential for the dephosphorization reaction. In addition, when the degree of carbon unsaturation of the molten metal is low, it becomes advantageous for C dephosphorization in which a part of calcium carbide is replaced with metallic calcium or its alloy. As mentioned above, the alkali metal halide is necessary for stably existing calcium metal and Ca3P2 in the slag. It also has the effect of making the refining reaction proceed smoothly by adjusting the melting point and viscosity of the slag. When the object of refining is high chromium and molten metal, the refining temperature is 145,000 or higher, so calcium fluoride is optimal as the alkali metal halide in terms of stability at high temperatures. An example using calcium fluoride will be explained below. As mentioned above, the amount of fluoride is required to be 10% or more, and CaC2
/CaF2=3 to 5 is optimal. The remaining components in the flux are mixed as impurities, including Ca○,
It is desirable that the amount of oxides that are easily reduced (that is, easily oxidize metal calcium), such as SiO2 and MZO, be as small as possible. Therefore, when using slagite as a fluoride source, it is desirable to use ordinary crabite refined by a mineral beneficiation method. Various forms of contact between the molten metal and flux are possible as in the case of normal slag refining, but as mentioned above, in the present invention, the slag has a high fluoride content and tends to corrode refractories, so it is possible to In order to shorten the reaction time, it is desirable to be able to damage slag and metal; for example, by using the slag effect when moving from the setting sun, by blowing gas into the melted damage, or by using induced damage. It is desirable to adopt a method such as using a device that can do this. The reaction vessel for carrying out the dephosphorization treatment of the present invention is lined with a refractory, but the refractory must be stable and not easily interacted with the slag both physically and chemically. For example, a magnesia-based material with a low vaporization rate is suitable. The refining atmosphere needs to have a low partial pressure of oxidizing gases, such as oxygen, carbon dioxide, and water vapor, in order to prevent oxidative consumption of metallic calcium produced in the flux. To this end, t) Seal the atmosphere to prevent it from entering the refining atmosphere. {o} Reduce the content of water and carbonates in the flux so as not to generate oxidizing gases such as carbon dioxide and water vapor in the reaction vessel. It is desirable to take measures such as: In addition, in order to prevent the oxidizing slag generated during the preparation of the carbon-unsaturated molten metal from mixing with the dephosphorization slag and oxidizing and depleting the metallic calcium, the slag must be evacuated as much as possible in advance, and the slag mixed into the reaction vessel should be It is desirable to add the flux to the molten metal in a state in which the flux is separated by being pushed away by a partition wall or the like. In addition to the above-mentioned conditions, in the present invention, it is important to adjust the oxygen content of the molten metal to 180 or less before adding flux. This is based on the experimental results described below. That is, 18%Cr-8% in a 100k9 high frequency induction furnace.
After making Ni-approximately 0.7% C stainless crude molten steel and scraping out the slag generated during melting, the space above the molten metal is closed with a lid to prevent air from entering, and then 70% calcium carbide and purified crab are added as a flux. stone 30%
The amount of addition of the powder/grain mixture is 10k9/t and 30
It's a 2 level k9/t. Various amounts of A are added to the molten metal before addition.
I was added to vary the oxygen content over a wide range from 3 Gong Fan to 250 Blood. Figure 1 shows 2 minutes, 5 minutes after adding flux,
The relationship between the dephosphorization rate and the oxygen content before adding flux at 1 and 2 days is shown. From this figure, the oxygen content is 18
It can be seen that when 0 legs is exceeded, the dephosphorization rate decreases rapidly in all cases, and especially the initial dephosphorization rate decreases. This is due to the following reason. In other words, when a fresh, powdery flux with a large specific surface area comes into contact with molten metal with a low oxygen content, the flux becomes molten and becomes '1'.
, (The reaction of formula 2 proceeds quickly. This high reaction rate under appropriate conditions makes it possible to perform highly corrosive slag smelting in a refractory-lined container as in the present invention. As time passes, the adverse effect of refractory corrosion acts as a factor that inhibits the progress of the reaction, so the reaction appears to stop.On the other hand, if the oxygen content of the molten metal is high, '11 ，
At the same time as the reaction of formula '21, (CaC2)

[0]→(
Cao) + next ○ ...[3} (Ca) ten

[0] → (Ca
○) ......The reaction of ■ takes place, and calcium carbide is oxidized to metallic calcium, increasing calcium oxide. Oxidation of metallic calcium directly inhibits the progress of dephosphorization, and the production of calcium oxide lowers the concentration of calcium carbide in the area in contact with the molten metal.
It also inhibits the production of metallic calcium in equilibrium. In this way, the progress of the initial dephosphorization reaction is directly or indirectly inhibited (
Figure 1, 2-5 minutes). The oxidation of the slag by the molten metal decreases over time, and the delay in the dephosphorization reaction can be recovered to some extent (first
At the same time, the dephosphorization reaction driving force becomes smaller due to the adverse effects of the refractory, and the achieved dephosphorization rate (Fig. 1, 2 minutes) also becomes lower than when the oxygen content is low. Note that the lower the flux consumption rate, the more pronounced the adverse effect of oxygen content (Fig. 1, 10k9/t). It is natural to expect that if the amount of oxidation of slag by oxygen in molten steel is large, the basic unit of flux will be higher in order to obtain the same dephosphorization rate. The reaction rate is an important issue when using this method, and it is a new finding that the oxygen content has a large effect on the reaction rate. As shown in Figure 1, if the oxygen content exceeds 180 yen, the dephosphorization reaction is significantly inhibited, so it is necessary to adjust the oxygen content before adding flux to 180 yen or less. It is desirable to set it to four or less. Furthermore, it has been experimentally confirmed that even if the Cr content in the flaw is varied from a few percent to 70% and the Ni content is varied from 0 to 20%, this oxygen content limit value hardly changes. The conditions for the invention are ferrochrome and stainless steel (
C% is higher than a predetermined value and the feature component is not adjusted). The following method can be used to adjust the oxygen content of the molten metal before addition of flux so as to satisfy the above conditions. m Do not excessively lower the melt C% before adding flux. For example, in the case of 18%Cr-8%Ni steel, C is 0.6~
Optimal is 2%. [2] Forced deoxidation using AI, Si, etc. Note that it is not preferable to add flux with deoxidation products remaining after forced deoxidation because the flux will be oxidized by the deoxidation products. Therefore, after forced deoxidation,
It is desirable to add the flux after performing an operation to separate the deoxidized product as much as possible. [3] When performing dephosphorization treatment when transferring molten metal, it is desirable to take measures such as sealing with inert gas to prevent air oxidation of the molten metal. As described above, by adjusting the oxygen content of the molten metal before addition of flux to 18 or less, the time required for {a} reaction can be shortened and the refractory unit consumption can be lowered. 'b' Flux basic unit can be lowered. The amount of phosphorus reached can be lowered. Effects such as this can be obtained. Therefore, by carrying out the present invention, it becomes possible to dephosphorize high chromium molten metal, which cannot be dephosphorized by normal oxidation refining, in a mass production process, that is, in a refractory-lined container, which has a great industrial effect. Example 13 18%Cr-8%Ni-1.1 in electric furnace
%C stainless crude molten steel is melted, the oxygen content is adjusted by adding various levels of AI before tapping, and the molten metal is tapped via a pony ladle into a ladle whose atmosphere is kept non-oxidizing by Ar. . It takes about 5 minutes to bathe. 15k9/t of calcium carbide and 4 kg/t of refined crabite are added to the tapped water, and the reaction is carried out by utilizing the force of the molten metal flow. The components of the molten metal in the pony ladle and the molten metal in the ladle after dephosphorization treatment are as follows. Example 2 After receiving the stainless assembled molten steel produced in the same manner as in Example 1 into a ladle, a dipper tube was inserted into the molten metal to push away the slag, and calcium carbide 12k was poured onto the exposed surface of the molten metal.
Add 9/t and 5k9/t of kasu-sei crab stone, blow in ~ through a porous plug attached to the bottom of the ladle, and worship the setting sun for 10 minutes. The flaw components before and after addition of flux are as follows.

[Brief explanation of the drawing]

Figure 1 shows 18%Cr-8% melted in a high frequency induction melting furnace.
Effect of oxygen content before addition of flux on dephosphorization behavior when a flux consisting of 70% calcium-strengthening birch bide and 30% cyanite is added to Ni-0.7%C stainless crude molten steel. It is a figure showing an influence. Figure]

Claims (1)

[Claims] 1. When refining a carbon-unsaturated high-chromium molten metal in a non-oxidizing atmosphere using a flux containing calcium carbide and alkaline earth metal halides as main components,
A method for dephosphorizing high chromium molten metal, which comprises adding flux after adjusting the oxygen content of the molten metal to 180 ppm or less.