JPH0260723B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for decarburizing chromium-containing molten steel. In decarburizing chromium-containing molten steel, the higher the steel bath temperature and the lower the CO gas partial pressure in the steel bath, the lower the equilibrium value of the [C] concentration in the molten steel. It is clear that the temperature of the bath steel should be increased and the CO partial pressure should be reduced. However, too high a steel melting temperature promotes melting and loss of refractories, which is undesirable from the viewpoint of refractory cost, and it is common practice to decarburize at temperatures below 1700°C. or
Widely used methods for efficiently decarburizing by lowering the CO partial pressure include decarburizing in a vacuum or diluting decarburization, in which a diluent gas that lowers the CO partial pressure is simultaneously injected with oxygen gas. The former is generally referred to as VOD, and the latter as AOD. In the AOD method, the ratio of diluent gas and oxygen gas is generally changed in stages so that the diluent gas becomes enriched according to the [C] concentration in the steel as decarburization progresses. This is because as the [C] concentration in steel decreases, the decarburization oxygen efficiency (ratio of oxygen consumed for decarburization of blown oxygen gas) decreases, so it is necessary to enrich the diluent gas and lower the CO partial pressure. The decarburization oxygen efficiency is maintained at a high value, the amount of oxidation of [Cr] in the steel is reached, and the use of expensive reducing agents is reduced. Figure 1 shows the decarburization mechanism of the AOD method described above when Ar gas is used as the diluent gas.
That is, among the mixed gas of diluent gas and oxygen gas injected into the molten steel 2 from the tuyere 1, the oxygen gas immediately reacts with [Cr] in the molten steel to generate [Cr ₂ O ₃ ] and the surrounding area of the dilution gas bubble. This [Cr ₂ O ₃ ] and [C] in the molten steel react to produce CO gas. The generated CO gas is absorbed by an argon dilution gas bubble with a low CO partial pressure and is discharged outside the furnace as a reaction gas. This can be expressed as chemical reaction equations (1) and (2). 3/20 ₂ +2 [Cr] → [Cr ₂ O ₃ ] ………(1) [Cr ₂ O ₃ ] + 3 [C] → 3CO + 2 [Cr) ……… (2) The principle of the AOD method is that in molten steel [ C] As the concentration decreases, the reaction of equation (2) slows down, so the CO partial pressure is lowered by enriching the diluent gas and the reaction of equation (2) is promoted in the right direction. Considering this decarburization mechanism, the oxygen gas injected after the middle stage of decarburization in the AOD method hardly reacts directly with [C] in molten steel, and most of the oxygen gas forms [Cr ₂ O ₃ ]. However, it is thought that it then reacts with [C] in the bath steel. and
Since the reaction rate of equation (2) is slow, a part of [Cr ₂ O ₃ ] generated in equation (1) migrates into the slag. This can be expressed as a chemical formula as shown in equation (3). 3/20 ₂ +2 [Cr] → α [Cr ₂ O ₃ ] + β [Cr ₂ O ₃ ]
......(3) α+β=1, [ ] indicates in molten steel, [ ] indicates in slag. Since the temperature of the molten steel rises in the process of generating this [Cr ₂ O ₃ ], a large amount of coolant is required to prevent the temperature from becoming higher than necessary. Common steel and similar steel scraps are usually used as a coolant, but ideally small pieces are available in large quantities and are added continuously while decarburizing with the furnace upright. It is. However, currently, in order to obtain small pieces of coolant with a good shape, it is necessary to process it, which is extremely expensive, so decarburization has to be interrupted and large-sized coolant is charged all at once from the furnace mouth. are doing. As a result, there is a loss of time due to interruptions, a loss of gas during the tilting furnace, and the temperature of the molten steel decreases all at once, resulting in a significant decrease in decarburization oxygen efficiency and an increase in Si consumption for reduction. This has become a major operational problem. The present invention makes it possible to use small pieces of coolant with a good shape at low cost, eliminates these drawbacks, reduces the amount of blown oxygen gas required for decarburization, and makes effective use of inexpensive sources of iron, chromium, and Ni. This provides a decarburization method that makes it possible to That is, the present invention adds metal oxides such as chromium ore, NiO, scale, or iron ore after the middle stage of decarburization, and converts the oxygen source necessary for decarburization into Cr ₂ O ₃ , NiO, FeO,
It is characterized by being given in the form of Fe ₂ O ₃ and Fe ₃ O ₄ .
In the present invention, after the middle stage of decarburization, the [C]% in molten steel is 0.25
% or less. At this time, each metal oxide is reduced by [Cr] in the molten steel according to equations (4) to (6), and the generated [Cr ₂ O ₃ ] contributes to decarburization according to equation (2). It is estimated that 3 (NiO) + 2 [Cr] → 3 [Ni] + [Cr ₂ O ₃ ] ………(4) 3 (FeO) + 2 [Cr] → 3 [Fe] + [Cr ₂ O ₃ ] ………( 5) (Fe ₂ O ₃ ) + 2 [Cr] → 2 [Fe] + [Cr ₂ O ₃ ]
………(6) When Cr ₂ O ₃ is added, Cr ₂ is distributed in the molten steel as shown in (Cr ₂ O ₃ ) → [Cr ₂ O ₃ ] ………(7) due to the distribution among the slag metals. It is estimated that O ₃ is transferred. As mentioned above, the added metal oxide maintains the oxygen temperature ([Cr ₂ O ₃ ] concentration) in the molten steel at a high level, so stirring that only oxygen gas is injected without injecting oxygen gas can be used instead of the conventional method. Decarburization is possible at the same decarburization rate. This situation is illustrated in FIG. 6 as described below. However, since equations (4), (5), and (6) are all endothermic reactions, the temperature of the molten steel decreases. Therefore, in places where there is no temperature margin in the concentration range of [C] in molten steel in which decarburization is performed while adding metal oxides, it is necessary to inject a small amount of oxygen gas together with diluent gas. In addition, in order to completely utilize the added metal oxides for decarburization, and to effectively utilize (Cr ₂ O ₃ ) generated in the early to middle stages of decarburization, it is necessary to Stirring decarburization is performed by injecting only inert gas while adding a reducing agent to limit the residual oxygen source necessary for decarburization in the slag. This is an effective decarburization method to ensure a reduction in carburization, and is used following decarburization with the addition of metal oxides. The final stage of decarburization refers to the stage where the amount of [C] in molten steel is approximately 0.1% or less. The fluidity of the slag is improved by introducing the reducing agent at the same time as the inert gas is blown into the slag. This increases the chance of contact between the slag and molten steel,
Since the oxygen in the metal oxide actively migrates into the molten steel, the oxygen in the molten steel reacts efficiently with [C] in the molten steel, and reduction and decarburization progress significantly. At the end of decarburization, the reduction reaction is already in progress, and even if the reduction stage begins, it is sufficient to add the reducing agent and slag-forming agent to compensate for the insufficient amount of reduction, or as necessary. Compared to methods in which the entire amount of reducing agent or the reducing agent and slag-forming agent are added during the reduction period, the reducing agent can be dissolved easily, making it possible to significantly shorten the reduction time, preventing corrosion of refractories, and refining. Efficiency improvement effect can be obtained. The amount of reducing agent to be added is necessary to leave metal oxides as an oxygen source necessary for decarburization in the slag, and if the amount of reducing agent to be added is too large, decarburization will not progress. In addition, the time to stop blowing oxygen gas is set to 0.25% or less [C] in the molten steel in order to obtain the above-mentioned effects sufficiently. Also, the lower limit needs to be above the final [C] value. As described above, in the decarburization method of chromium-containing molten steel of the present invention, an oxygen source necessary for decarburization is added in the form of metal oxide in the [C] concentration range in the molten steel after the decarburization, and the oxygen source is added in the form of a metal oxide. By adding a reducing agent at the final stage of coal and injecting only inert gas for stirring and decarburizing, it is possible to use inexpensive cold material with a good shape.
AOD because there is no need to tilt the furnace when charging
Operation is stable. By using inexpensive solid oxides, blown oxygen gas can be reduced. Inexpensive iron, nickel, and chromium sources can be used as active ingredients. Oxygen sources in added metal oxides can be used in molten steel. In order to achieve efficient reduction in [C], the amount of reducing agent added during the reduction period can be reduced, and the time required for the reduction period can be shortened. be. A more detailed explanation will be given below using a specific example in which the present invention is applied to the AOD method. Figure 2 shows the conventional AOD for refining SUS-340.
Show the law. Argon gas is gradually enriched according to the [C] concentration in the molten steel, and the [C] in the molten steel is decarburized to 0.06%. During this time, the reaction heat generated by the oxidation of [Cr] in the molten steel is cooled by adding ordinary steel scraps and SUS304 scraps as a coolant, and CaO, which is necessary to protect the resistant materials, is added to reduce the slag basicity. After proper maintenance and completion of decarburization, CaO and CaF ₂ are added using Fe-Si as a reducing agent and a solvent to reduce the oxidized chromium, and then the generated slag is removed.
Refining has been completed after desulfurization and component temperature adjustment. Figure 3 shows the case where the period after the repayment period is significantly shortened.
An example in which the present invention is applied to the AOD method will be shown. A feature of this example is that the period after the reduction period is significantly shortened, so the steel bath temperature at the end of decarburization can be extremely low. Therefore, if a normal coolant is used, a large amount of high-quality coolant with a low carbon content and a good shape is required, whereas with the addition of metal oxides according to the present invention, a relatively small shape is required. Good, low carbon content is readily available and AOD
This is extremely convenient for operation. Figure 3 shows an example using iron ore, which has a carbon content of 0.25% to 0.12% in steel.
The amount of oxygen required to decarburize is approximately equivalent to iron ore.
It becomes 6Kg/T-S. If the amount of temperature drop at this time is calculated, the total heat capacity of the iron ore and the amount of heat absorbed by the reduction reaction of Fe ₂ O ₃ will be approximately 50 to 60° C./TS.
Therefore, even if a range of 0.25% to 0.12% [C] in steel is decarburized by stirring by blowing only diluent gas, the temperature can be balanced. Therefore, in the case of Figure 3, iron ore should be added from the point when [C] in the steel reaches 0.25%.
Decarburization is carried out by stirring only diluent gas while sequentially adding up to 0.12%, completes the addition of iron ore at 0.12% [C] in the steel, and a part of the medium necessary for reduction and part of the medium necessary for decarburization. (Cr ₂ O ₃ ) in the slag improves the fluidity of the slag by limiting the oxygen source left in the slag.
A reducing agent (Fe-Si or Al) is added to promote the distribution reaction that causes the carbon to migrate into the steel, and the diluent gas is continued to be stirred to complete decarburization. FIG. 4 shows another example of the present invention, in which iron ore was added as a metal oxide at the final stage of decarburization and only diluent gas was blown. FIG. 5 shows a comparison of the decarburization speed when a metal oxide is added according to the present invention and stirring decarburization is performed using an inert gas, compared with a conventional oxygen blowing method. In the case of stirring decarburization using diluent gas, if no oxide is supplied, there will be a large difference in the amount of decarburization around 400 Nm ³ of inert gas, but when metal oxide is added according to the present invention, [C] 0.06 %, there is almost no difference from the normal decarburization method, and it can be said that the present invention has no effect on the decarburization time, which was a concern. In addition, when only dilution gas is blown into the steel before blowing [O]
The relationship between the amount and the amount of [C] reduction in steel is shown in FIG. 6 (dilution gas injection amount 200 Nm ³ , no oxide supply). All of the examples explained above are only examples using iron ore, but as mentioned above, there are various metal oxides, and the most suitable one is used depending on the decarburization conditions and availability at the time. You can. Table 1 summarizes the amount of heat absorbed by the reaction of these metal oxides with [C] in steel. As is clear from Table 1, compared to iron ore, chromium ore has a greater cooling effect per kilogram of carbon required for decarburization, which limits the amount of chromium ore used. Also scale (FeO) and oxidation
Compared to iron ore, Ni etc. absorb less heat and have less cooling effect, so they can be used in relatively large quantities and are relatively easy to use if available. In either case,
It is possible to calculate the oxygen balance and temperature balance depending on which metal oxide is added, and it is possible to determine whether decarburization is performed only by stirring the diluted gas or whether blown oxygen gas is required. The above advantages of the present invention are summarized in Table-2.

【table】

【table】 [Brief explanation of drawings]

Figure 1 shows the mechanism of AOD, Figure 2 shows the conventional method, Figures 3 to 4 show examples of the method of the present invention, and Figures 5 to 6 show the effects of the method of the present invention. It is a diagram.

Claims (1)

[Scope of Claims] 1 In a decarburization method in which oxygen gas and diluent gas are injected into molten steel, a metal oxide is added after the middle stage of decarburization and decarburization is performed using the oxide as the main oxygen source, In the final stage of decarburization following the middle stage of decarburization, decarburization is carried out by blowing inert gas while adding a reducing agent to the extent that an oxygen source necessary for decarburization remains in the slag. A method for decarburizing chromium-containing molten steel. 2. The method for decarburizing chromium-containing molten steel according to claim 1, characterized in that a decarburization period by stirring in which only diluent gas is blown is provided at the end of decarburization following the middle period of decarburization. 3. A method for decarburizing chromium-containing molten steel according to claim 1 or 2, characterized in that only diluent gas is blown in after the middle stage of decarburization.