JPH02179812A - Method for refining stainless steel - Google Patents

Abstract

PURPOSE:To improve adding yield of Al while controlling N content to low by making the composition of slag on molten steel the specific composition having high Al2O3 content at the time of tapping the molten steel in a furnace while covering the slag on the upper face thereof and adding Al-containing agent in this molten steel. CONSTITUTION:Slag making agent is added in the molten stainless steel stored in the furnace under high temp. condition and the slag composed of 35-45wt.% CaO, 1-5% SiO2, 40-50% Al2O3 and 5-15% MgO is produced and the molten steel in the furnace is refined with this slag. Successively, the furnace is tilted as remaining the above slag and the molten steel in the furnace is tapped and charged into the ladle together with a part of the slag. After that, the Al- containing agent is added under floating condition of the slag on the upper face part of the molten steel in the ladle to adjust the Al content in the molten steel. By this method, reaction between the slag and Al can be controlled and the Al yield can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for refining stainless steel. This refining method can be used to produce soft magnetic stainless steel containing aluminum.

[Prior Art] Conventionally, in the field of stainless steel, stainless steel containing a required amount of aluminum has been provided in order to guarantee the magnetic properties of stainless steel.

In refining this stainless steel, oxygen is injected into the molten steel stored in an AOD furnace to decarburize it to reduce carbon in the molten steel, and the molten steel is refined using slag floating on the top of the molten steel. After refining, slag removal work is carried out to remove the slag floating in the molten steel in the furnace. Once the slab is removed, the furnace is tilted and the molten steel from the furnace is poured into a ladle and transferred. Furthermore, metallic aluminum and sulfur are added to the molten steel in the ladle, thereby ensuring the aluminum content and sulfur content in the molten steel. In this case, the composition of the slag is substantially 50% CaO and 20% 5hot by weight as shown in Table 2.
, Ait 03 was 8%, and MgO was 10%. Furthermore, the addition yield rate of metallic aluminum added to the molten steel in the ladle was 88%, and the addition yield rate of sulfur added to the molten steel in the ladle was 67%.

[Problems to be Solved by the Invention] Furthermore, in the industrial world, there is a demand for stainless steel products to not only contain a required amount of aluminum, but also to maintain a relatively low nitrogen content in stainless steel products.

Therefore, in order to suppress the nitrogen content in stainless steel products, the present inventor tilted the furnace while leaving the slag floating in the molten steel in the furnace without performing slag removal work, and removed the slag from the furnace to the ladle. While covering the upper surface of the molten steel as much as possible with slag,
A method of transferring molten steel from the furnace to the ladle was tested. However, with this method, when the molten steel is transferred from the furnace to the ladle, the area covered by the slag increases, which prevents the molten steel from picking up nitrogen. becomes lower. That is, since the slag is also transferred into the ladle, the slag floats on the upper surface of the molten steel in the ladle. Even if metallic aluminum is added to the molten steel with slag floating on the upper surface of the molten steel in the ladle, the slag and metallic aluminum tend to react and metallic aluminum is easily formed into a slab. Therefore, the addition yield rate of metallic aluminum added to the ladle was low.

The present invention was developed in view of the above-mentioned circumstances, and its purpose is to refining stainless steel that can maintain a high addition yield rate of aluminum added to molten steel in a ladle while keeping the nitrogen content low. The purpose is to provide a method.

[Means for Solving the Problems] As a result of intensive research on stainless steel refining methods based on the above-mentioned objectives, the present inventor found that CaO is 35 to 45% and 5iOz is 1 to 5% by weight. , A1203 is 40
A molten slag rich in 8λ203 with a composition of ~50% MgO, 5~15% MgO, and unavoidable impurities is produced in a furnace, and the molten slag is transferred together with the molten steel into a ladle. It has been discovered that if an aluminum-containing agent such as metallic aluminum is added to molten steel while suspended in the molten steel, the nitrogen content can be kept low and the aluminum addition yield rate can be maintained at a high level.Based on this, the present invention has been developed. It has been completed.

That is, in the stainless steel refining method according to the present invention, a slag forming agent is added to high-temperature molten stainless steel stored in a furnace, and the content of CaO is 35 to 45% and 5iOz is 1 to 5% by weight.
%, Alto3 40-50%, MqO 5-15%,
A process of generating slag with a composition of unavoidable impurities and refining the molten steel in the furnace with the slag, and a process of tilting the furnace while leaving the slag in the furnace and tapping the molten steel in the furnace together with the slag into a ladle. The slag is suspended on the upper surface of the molten steel in the ladle, and an aluminum-containing agent is added to adjust the aluminum content of the molten steel. That is.

The stainless steel stored in the furnace varies depending on the type of steel being manufactured, but for example, stainless steel with a chromium content of 11 to 13%
It can be a 1-13 chromium type, or an 18 chromium type with a chromium content of 18%. As an example, silicon is less than 0.30% and manganese is less than 0.4% by weight.
% or less, phosphorus is 0.04% or less, sulfur is 0.03-0
．． 017%, copper 0.2% or less, chromium 11-1L%
, nickel 0.2% or less, aluminum 0.2-0
．． 6%, balance iron. Here, chrome,
Nickel is an element necessary for stainless steel, sulfur improves free machinability, silicon ensures magnetic properties,
Aluminum is added for the purpose of ensuring magnetic properties. Refining can usually be carried out by blowing oxygen or a mixed gas containing oxygen and argon gas through tuyeres installed at the bottom of the furnace. In the mixed gas, the flow rate of oxygen can be set to 0 to 150 ONm3/H, and the flow rate of argon gas can be set to 0 to 150 ONm3/H.

When refining, you can perform the following operations:

That is, the composition of the molten steel is adjusted by appropriately adding alloys, etc., and a slag-forming agent such as fluorite, lightly calcined dolomite, and lime is appropriately added to the molten steel in the furnace to produce a slab having a desired composition. The composition of the slab is adjusted appropriately depending on the steel type to be manufactured, but the composition range is that the alumina content is higher than normal slag, CaO is 35 to 45%, and 5iOz is 1
~5%, A-text 203 40-50%, MgO 5-15
%. Also, ((NCaO+NMg0)/(NSi
The FA basicity expressed as O2+NA, Il 203)) can be from 1.7 to 2.3, in particular from 1.9 to 2.1. If the slag has the above composition and basicity, it is possible to suppress the reaction between the slag and aluminum, which is advantageous in securing the aluminum yield rate. As such, the reaction between slag and sulfur is difficult to proceed, and desulfurization can be suppressed, which is advantageous in ensuring a high sulfur yield.

Then, the furnace is tilted to transfer the molten steel together with the slag to the ladle from the tapping port of the furnace, while leaving the slab floating in the molten steel stored in the furnace in the furnace. At this time, while covering the upper surface of the molten steel that is tapped from the furnace with a slab as much as possible,
Since the molten steel is tapped, it is possible to prevent nitrogen in the air from being picked up by the molten steel as much as possible.

When the molten steel is transferred into the ladle as described above, slag having the above-mentioned composition floats on the molten steel. Then, while stirring the molten steel, an aluminum-containing agent such as metallic aluminum or an aluminum-based alloy is added from above the molten steel.

Furthermore, in addition to the aluminum-containing agent, sulfur can be added to adjust the sulfur content in addition to the aluminum content in the molten steel. At this time, since the slag having the above composition has a low desulfurization ability and is difficult to react with sulfur, a high sulfur yield can be maintained.

[Examples] Hereinafter, the refining method according to the present invention will be specifically explained based on Examples.

In the method for refining stainless steel according to this embodiment, first, raw materials blended to have the desired composition are charged into an electric furnace, and melted in the electric furnace at a temperature of about 1550 to 1650°C to obtain the desired composition. Obtain molten stainless steel.

Then, using a 20-ton AOD furnace with a storage chamber partitioned by a layer of lining bricks, the molten stainless steel is transferred into the AOD furnace. This molten steel is so-called 13 chromium stainless steel, and has a carbon content and a nitrogen content of 4"51jl.
is extremely low. Specifically, its composition is 4.125% carbon, 148 ppm nitrogen, and 0.4% silicon.
, manganese 1.88%, phosphorus 0.028%, sulfur 0.0
35% copper, 0.06% copper, 12.25% chromium, 0.06% nickel, 0.002% aluminum, unavoidable impurities, the balance being iron.

With the molten steel stored in the AOD furnace as described above, the AO
The P! By supplying a mixed gas of two elements and argon gas, the mixed gas of oxygen and argon gas is blown into the molten steel from the bottom side of the furnace, and due to the boiling phenomenon accompanying the generation of Co gas, i'
1. The decarburization reaction of molten steel is advanced while stirring the steel. At this time, 600 pieces of previously lightly calcined dolomite were prepared at an appropriate time.
300 kg of burnt lime are added from above the molten steel.

Note that the blowing time is about 90 minutes.

In the mixed gas used in this example, the flow rate of oxygen was 120 ONm3/H and the flow rate of argon gas was 30 ONm3/H during the oxidation period, which is the initial stage of injection.

When the carbon content in the molten steel in the AOD furnace reaches about 1 ooppm, the oxygen flow rate increases to 15ONm as shown in Table 1.
3 /H, argon gas flow rate is 120ONm3 /H
The amount of oxygen was reduced. In this case, the flow M ratio between the flow rate of oxygen and the flow rate of argon gas is (1/8).

Table 1 In this example, the molten steel humidity at the start of injection was 1500°C, and as shown by characteristic line A in Fig. The temperature of the molten steel at the final stage of blowing did not substantially change compared to the temperature at the start of blowing, and was maintained at approximately the same temperature. The molten steel temperature was measured using an R-type thermal lightning pair.

'After the injection of M element is completed, as a reduction period, only argon gas is blown into the molten steel in the ΔOD furnace for about 10 minutes for the purpose of reduction (desulfurization, denitrification).

Through the gas injection described above, molten steel of ultra-low carbon stainless steel with extremely low carbon content was obtained.

After gas injection is completed in the AOD furnace as described above, 120 kQ of fluorspar, 400 kQ of light calcined dolomite, and 600 kQ of lime are added to the molten steel in the AOD furnace as slag forming agents, and further, metal Added 450 kg of aluminum,
This produces slag that is rich in alumina and has low desulfurization ability. This slag floats on the upper surface of the molten steel stored in the AOD furnace. As shown in Table 2, the composition of this slag is higher in alumina content than normal slag, less silica, 40% CaO, 3% 5iOz,
3 is 45% and fvlo is 10%.

The basicity of this slag is 2.0. Here, the reason why this slag has a higher alumina content than normal slag is to reduce the alumina and ensure the aluminum content of the molten stainless steel, and the reason why the silica content is low is because of the 3t In order to prevent the reduction of
This is to perform I reduction.

Then, △OD with the slab left without being substantially removed.
The furnace is tilted and the molten steel and slag are transferred from the tapping port to the ladle to tap the steel. At this time, as shown in Fig. 3, the molten steel is injected while the upper surface of the molten steel is covered with slag.
The contact area between the tapped molten steel and the air is reduced, thereby making it possible to prevent nitrogen in the air from being picked up by the molten steel as much as possible.

When the molten steel is transferred into the ladle as described above, slag having the above-mentioned composition floats on the molten steel in the ladle.

Then, while stirring the molten steel and slag in the ladle with argon gas, 70% of metal aluminum and 5 kg of sulfur were added from above where the slag was cut and the molten steel was exposed. Make adjustments.

At this time, since the slag having the above composition does not easily react with sulfur, it is possible to maintain a high sulfur addition yield.

By the way, in the above-mentioned examples, the carbon content of molten steel at the end of blowing, that is, before decarburization, molten steel after alloying element adjustment in an AOD furnace, and the final product was examined. The carbon content results are shown in characteristic line A in Figure 1. As shown in Figure 1, characteristic line A, the carbon content of the molten steel at the final decarburization stage, which is the final blowing stage, is 5 ppm, and the alloy The carbon content in the molten steel after adjusting the elements is 14 pl) m, and the carbon content in the final product is 301) pm.
Ultra-low carbon stainless steel was obtained.

In addition, the nitrogen content was investigated in molten steel after alloying element adjustment in an AOD furnace, molten steel after tapping, and the final product. The results are shown in characteristic line A in FIG. Characteristic 1 in Figure 4
As shown in i1A, the nitrogen content in the molten steel after adjusting the alloying elements is 44 ppm, the nitrogen content in the molten steel after tapping is 61 ppm, and the carbon content in the final product J3 is extremely high at 69 ppm. Therefore, a low nitrogen stainless steel was obtained.

Table 3 shows the aluminum addition yield rate and the sulfur addition yield rate found in the ladle in this example. Third
As shown in the table, the aluminum addition yield rate is 87%, and the sulfur addition yield rate is 65%, which is a good value. (88%, sulfur addition yield rate 67%). The reason is,
It is presumed that this is because the alumina-rich slag composition having the above-mentioned composition facilitates Ai reduction and less desulfurization. Here, the aluminum addition yield rate means the ratio of aluminum suspension in molten steel to the amount of metal aluminum added in the ladle. The addition yield rate means the ratio of sulfur in the molten steel to the amount of sulfur added in the ladle.

In addition, stainless steel I (AUM25)k, whose composition is basically the same as in the above example but only has a higher aluminum content of 3%, was subjected to the same refining method to reduce its nitrogen content. Examined. The results are shown by the characteristic line Δ in FIG. As shown in the characteristic line A in Fig. 6, it is 55 ppm after tapping, 391) l) m after adding lead, and 34 Dpm after adding metal aluminum.
The final product was 33 pl)m. Furthermore, normal AUM
A similar refining method was applied to steel types in the series, and the nitrogen content was investigated. The results are shown in characteristic line B in FIG. As shown in characteristic line B in Figure 6, after tapping, s7
ppm, 91 ppm after adding aluminum, 9 ippm after adding metal aluminum, and 931) pm in the final product. This shows that when stainless steel contains a large amount of aluminum, denitrification in the molten steel progresses and the nitrogen content in the molten steel decreases. This promotion of denitrification has a similar tendency for stainless steel 111 (AUM6), which has basically the same composition as in the above-mentioned embodiments, but has a higher aluminum content of 1%. It is inferred that this is due to the equilibrium reaction of AλN[S]-Affi+N. In addition, Fig. 7 shows 1900k.
It shows the equilibrium curve of (A pattern N=Ai+N) in , and shows that nitrogen in molten steel decreases as the amount of aluminum increases.

[Reference Example △] Furthermore, in order to understand decarburization caused by compositional fluctuations of the mixed gas injected into molten steel, as shown in Table 1, as Reference Example 1, the flow rate of oxygen in the mixed gas was 22ONm3/H. , the case where the flow rate of argon gas was 120 ONm3/H and the meteor ratio between the flow rate of oxygen and the flow rate of argon gas was (115,5) was tested. Furthermore, as reference example 2, the mixed gas oxygen meteor is 8ONm3/H, the argon gas meteor is 120ONm3/l-1, and the flow rate ratio of the oxygen flow rate to the argon gas flow m is (1/12>). A certain case was tested.Furthermore, as reference example 3, the flow rate of oxygen in the mixed gas was O, and the flow rate of argon gas was 120ONm.
3/H, and the case where Ml was not blown was tested.

In Reference Example 1, Reference Example 2, and Reference Example 3, as shown by characteristic line B in Fig. 1, the carbon content of the molten steel at the final stage of blowing is about isppm, and the carbon content of the molten steel after adjusting the alloying elements is is about 36 pp m, and the carbon content m in the final product is 52 pprl! The carbon content was higher than in the case of this example. In addition, in Reference Example 1, as shown by characteristic line B in FIG. 2, the molten steel temperature rose as the blowing time progressed, that is, when the blowing time reached 6 minutes, the molten steel temperature rose by about 10°C. In Reference Example 2, as shown by the characteristic line C in FIG. 2, the temperature of the molten steel decreases as the blowing progresses, that is, when the blowing time reaches 6 minutes, the molten steel ll! The temperature dropped by about 10 degrees Celsius. In Reference Example 3, as shown in the characteristic line in FIG.
Within minutes, the molten steel temperature dropped by about 30°C.

The reason why the melting temperature is observed to decrease in Reference Examples 2 and 3 is presumed to be because there is less oxygen, so less oxygen is consumed for the oxidation of iron, chromium, etc. Also, in Reference Example 1, the temperature rise in the molten steel was observed because the carbon content is on the order of ppm, which is extremely small compared to metals such as iron and chromium. 2. Despite the increase compared to 3, the oxygen was not effectively used for decarburization and was consumed as a source of oxidation of iron, chromium, etc., and as a result, the temperature of the molten steel rose due to the heat of oxidation. It is presumed that there is. That is, in molten stainless steel with an ultra-low carbon content around 100 to 110001)l), when looking at the ratio of carbon to metals such as iron and chromium, the ratio of carbon to metals such as iron and chromium is Since carbon is extremely small, if the proportion of Ml in the mixed gas increases beyond the range of (1/7) to (1/9) mentioned above, the oxygen will mainly become an oxidation source for metals such as iron and chromium. However, if the proportion of oxygen in the mixed gas decreases from the above (1/7) to (1/9), the S content will decrease. Therefore, although oxidation of metals such as iron and chromium is reduced, decarburization reactions are also less likely to occur, and it is presumed that it will be similarly difficult to achieve ultra-low carbon.

[Reference Example B] By the way, in the conventional method, as shown in the conventional column of Table 2, CaO is 50%, 3iQ2 is 20%, Affi2 is
Slag with a composition of 8% 03 and 10% MgO was used, but after refining with that slag,
Regarding the nitrogen content of the molten steel when the molten steel is transferred from the AOD furnace to the ladle after removing slag from the top surface of the molten steel in the AOD furnace, as mentioned above, the AOD
The investigation was conducted on molten steel after alloying element adjustment in the furnace, molten steel after tapping, and the lowest $1 product. The results are shown in Figure 4.
Shown below.

As shown in characteristic line B in Fig. 4, the nitrogen content in the molten steel after adjusting the alloying elements is 43 pl)m, and the nitrogen content in the molten steel after tapping is 871)I)m, The nitrogen content in the final product was 93 ppm.

As is clear from the comparison between characteristic line A and characteristic line B in FIG. 4, in this example, the nitrogen content can be extremely reduced, and can be reduced by about 20 to 3 oppm in the final product. The reason for this is thought to be that when transferring the stainless steel from the ΔOD furnace to the ladle, the molten steel was covered with slag, which prevented nitrogen pickup as much as possible.

[Evaluation] As explained above, in this example, both nitrogen and carbon can be reduced. Therefore, in the final product, nitrogen is 69 ppm and carbon is 30 ppm, so the total amount of carbon and nitrogen is , 1ooppm in the final product
can be maintained, and can obtain ultra-low carbon, low nitrogen stainless steel products.

Further, as shown in Table 3, the addition yield rate of the Al Table 3 minilum in the ladle was 87%, and the addition yield rate of sulfur was able to maintain good values of 65%.

The reason for this is presumed to be due to the above-mentioned slag having a composition rich in alumina and having a low desulfurization ability. Therefore, it is possible to contribute to the development of stainless steel that contains trace amounts of both carbon and nitrogen, and also contains aluminum and aluminum, and has magnetic properties and free machinability.

[Advantageous Effects of the Invention 1] According to the present invention, it is possible to provide a stainless steel refining method that can maintain a high addition yield rate of aluminum added to molten steel in a ladle while suppressing the nitrogen content to a low level.

[Brief explanation of the drawing]

Figure 1 is a graph showing the carbon content of Examples and Reference Examples, Figure 2 is a graph showing temperature changes between Examples and Reference Examples, and Figure 3 is a graph showing molten steel in a ladle from an AOD furnace. FIG. 4 is a schematic diagram showing the state of transfer, and FIG. 4 is a graph showing the amount of nitrogen in Examples and Reference Examples. Figure 5 is a graph showing the relationship between basicity and desulfurization rate,
Figure 6 is a graph showing the nitrogen content of AUM steel types.
The figure is a graph showing the equilibrium relationship between aluminum and nitrogen.

Claims (1)

[Claims] (1) A slag-forming agent is added to high-temperature molten stainless steel stored in a furnace, and the content of CaO is 35 to 45% by weight, SiO
_2 1-5%, Al_2O_3 40-50%, Mg
producing a slag having a composition of 5 to 15% O and unavoidable impurities, and refining the molten steel in the furnace with the slag; tilting the furnace while the slag remains in the furnace; pouring the molten steel in the furnace together with the slag into a ladle and transferring it into the ladle; and applying an aluminum-containing agent with the slag floating on the upper surface of the molten steel in the ladle. and adjusting the aluminum content of the molten steel.