JP2683406B2 - Method of melting stainless steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing stainless steel, and discloses a process for efficiently producing stainless steel.

[Conventional technology]

The most typical process for smelting stainless steel is to use scrap or ferroalloy iron such as FeCr.FeNi as the main raw material to melt in an electric furnace, then decarburize and reduce refining with AOD or VOD, etc. In order to clean the molten steel and control the temperature by injecting Ar in a steel receiving pot, it is then placed in a continuous casting machine. That is, electric furnace-AOD (VOD)
-Ar bubbling-a continuous casting process.

However, this method has a drawback that it cannot be said to be a flexible process because the raw material source is all solid and the use of hot metal is prohibited.

As described in the literature (Iron and Steel (1985), vo.71.180), the stainless steel melting process in which the raw material source is made flexible is the hot metal in the upper and lower blowing converter without using an electric furnace. Is charged, and scrap or ferroalloy (FeCr or FeNi) is added to the specified components during decarburization blowing before and before blowing so that it becomes the component of stainless steel, and after the decarburization process, FeSi etc. There is a process in which ferroalloy is charged, a reduction step is performed, and then steel is tapped and continuously cast.

However, even with this method, a large amount of scrap or ferroalloy must be added, and it can be said that this is insufficient in terms of efficient melting of stainless steel using an inexpensive raw material.

There is a process using Cr ore as part of the stainless steel melting process. For example, literature (Iron and Steel (1985) .vo.
In 1.71.1072), hot metal is charged into AOD, then Cr ore and coke are added, so-called smelting reduction is carried out, and then slag is removed and ordinary decarburization refining is carried out.

Similarly, Japanese Patent Application Laid-Open No. 61-29191 discloses a technique in which ore is melted and reduced in the same furnace, slag is removed, and then decarburization and refining are performed.

In these methods, since smelting reduction and oxidative refining are carried out in the same furnace, even if slag is removed in the middle, it is not always sufficient to remove it, and a large amount of impurity sulfur remains after decarburization refining. The problem is that the final reduction-stage decarburization load to reduce the sulfur content of the product is significantly higher. That is, a precondition for using a large amount of a reducing agent such as FeSi and Al that satisfy the conditions for sufficient reduction of Cr ₂ O _{3 in} slag to effectively effect desulfurization, and ensuring the fluidity of slag that is essential for promoting desulfurization reaction. There was a drawback that the furnace refractory resulting from the melting was significantly melted.

Further, JP-A-60-9815 and JP-A-60-9814 disclose a method for producing a chromium-containing alloy by smelting and reducing Cr ore, but these are also related to the decarburization process of melting stainless steel. It is not mentioned and does not present a new solution to the above problem.

As mentioned above, it can be seen that there are many aspects to be improved and improved in the melting process of stainless steel using the smelting reduction process using inexpensive Cr ore.

 FIG. 2 is a flow sheet of a conventional process.

[Problems to be solved by the invention]

The present invention solves the problems that have not been sufficiently improved by these prior arts, and provides an effective stainless steel melting process.

That is, due to the large amount of expensive reducing agents such as FeSi existing in the reduction period after decarburization refining, which is the final process of stainless steel melting, instability of desulfurization, or slag with high fluidity during reduction treatment. The purpose is to solve all the problems such as significant melting loss of refractory furnace refractories.

Therefore, the smelting reduction method of chromium ore, which is a system organically combined with a process using an inexpensive raw material, provides a method for achieving the above object.

[Means for solving the problem]

The present invention, by charging molten iron into the top-bottom blowing converter and adding Cr ore and carbonaceous material while feeding acid from the top-blowing lance and bottom-blowing tuyere, achieves the smelting reduction of Cr ore, and a predetermined Cr When tapping at a high concentration, a desulfurizing agent such as CaO is put into a tap pot to desulfurize, and after slag is separated from this tap pot, the molten metal is charged into a decarburizing furnace that is another upper-bottom blowing converter. , Decarburization refining is carried out by feeding in oxygen-containing gas while appropriately inputting cold materials such as scrap, and ends when the predetermined decarburization target is reached, and steel is degassed without performing reduction refining. It is achieved by sending to the next process.

That is, the present invention relates to a method for producing stainless steel, in which (i) a molten iron is added with a chromium-containing substance such as chromium ore, chromium oxide, chromium oxide slag or chromium-containing scrap and a carbonaceous material, and an oxidizing gas is supplied. Reduction step, (ii) a desulfurization step in which molten iron having reached a predetermined composition and temperature is discharged and a desulfurizing agent is added to it, (iii) decarburization refining in which this molten iron is continuously charged into a decarburizing furnace and an oxidizing gas is supplied. The feature is that the steps are sequentially performed.

In addition, Cr ₂ O ₃ slag, which is inevitably generated during decarburization refining, is discharged from the decarburization furnace, discharged, and then charged into the smelting reduction furnace to reduce the valuable component Cr with carbonaceous materials. It can be recovered and the present invention exerts a greater effect.

Furthermore, after tapping from the decarburizing furnace, part of the slag of the decarburizing furnace remains in the furnace, and this decarburizing furnace is charged with the hot metal that has undergone the smelting reduction process and the desulfurization process to perform decarburization refining. C in the molten metal at the initial stage of decarburization refining can reduce this slag and recover the Cr content.

[Action]

First, a specific method for solving the problems in the decarburization furnace, which is the main object of the present invention, will be described.

The reason that the conventional decarburization furnace needs a reduction period is the reduction and recovery of valuable metal Cr and the securing of desulfurization. For this reason, a relatively large amount of FeSi (which is cheaper than Cr) is used, a large amount of slag material such as CaO is used to secure the fluidity of slag,
In addition, there was a problem of significant melting loss of the refractory material of the furnace. All these difficulties should be eliminated by omitting the return period. In this case, it is completely unknown in the prior art where to play the important role that the conventional reduction period played.

With respect to this point, the inventors of the present invention have made extensive studies and have realized an ideal stainless steel melting process by using the following system configuration.

That is, by introducing a desulfurizing agent into the effluent stream after completion of smelting reduction, which has suitable desulfurization conditions, thorough desulfurization of the molten stainless steel is realized, and further, Cr-containing Cr ₂ unavoidably produced in a decarburization furnace. O ₃ slag is a process in which the main idea is to discharge the molten steel without performing reduction treatment, discharge it, return it to the smelting reduction furnace, and recover metal Cr using inexpensive carbonaceous material as a reducing agent.

 The present invention will be specifically described below.

After charging dephosphorized hot metal into a smelting reduction furnace and introducing a specified reducing agent, first, the temperature is raised by the combined use of acid feeding and carbonaceous material addition to the conditions necessary for the smelting reduction of Cr ore at 1500 to 1600 ° C. . After the temperature is raised, while feeding carbonaceous material, Cr ore and a slag forming agent, the acid transfer is continued to reduce Cr oxide by C. During this time, the hot metal temperature is 1500-1650 ℃, the hot metal carbon concentration is a value close to carbon saturation,
For example, about 5% by weight is secured.

During this time, a slag basicity CaO / SiO ₂ is also added as appropriate so as to ensure a maximum of about 2.5. After reaching a predetermined chromium concentration, the molten metal is discharged while preventing the slag from flowing out. At this point, the molten metal content is C-5% by weight, Cr-
The temperature is 10 to 20% by weight and the temperature is 1500 to 1600 ° C. The oxygen potential of this molten metal is remarkably low at Po ₂ to 10 ^-15 atm, and the temperature conditions necessary for ensuring the fluidity of the desulfurization agent are also suitable for the desulfurization reaction. Therefore, CaO with an appropriate sulfide capacity for this tap water
Sufficient desulfurization can be realized by adding a desulfurizing agent mainly containing

At this time, coexistence of smelting reduction furnace slag is also avoided, which is convenient for carrying out stable desulfurization. After the tapping is completed, the desulfurized slag on the Cr-containing molten metal in the ladle is mechanically removed. The removed slag is a highly basic substance and can be used as an auxiliary raw material for the smelting reduction furnace.

After that, the molten metal is charged into a decarburizing converter, and is decarburized while being blown with and sprayed with an oxidizing gas.

The molten metal at the time of charging the decarburizing furnace is C to 5% by weight, and the temperature is 1450 to 15
The temperature is 50 ° C, and decarburization refining is performed while introducing cold materials such as scraps to supplement the heat generated by the decarburization reaction. At the time of charging, S is sufficiently low, and desulfurization operation in a decarburization furnace is unnecessary,
Therefore, basically no solvent such as CaO is added.

As is clear from the thermodynamics, as the decarburization progresses, the Cr oxidation reaction is more likely to proceed. Therefore, in accordance with thermodynamics, a known carbon-feeding pattern in which the ratio of inert gas such as Ar to oxygen is increased so as to reduce the partial pressure of CO gas in the furnace, the product carbon level such as C to 0.05% by weight is obtained. To proceed with the decarburization reaction. Although the Cr content of the molten steel after decarburization is slightly reduced, it does not increase the S concentration and meets the product specifications, so it is tapped in a molten steel ladle without the conventional reduction treatment. .

The slag generated in the decarburization reaction left in the furnace is Cr ₂ O ₃
It has a low fluidity and contains no major components, and does not cause melting loss of the furnace refractory due to the slag that has been found in the past. This slag contains valuable Cr content, and it is extremely preferable to recover it as metal, and the slag discharged by tilting the furnace after tapping is used as a charge before the hot metal charging of the smelting reduction furnace. It is returned to the smelting reduction furnace and the Cr content is recovered by smelting reduction treatment.

The Cr ₂ O ₃ slag contained in the decarburizing furnace, which remained without being cut off, was replaced by C in the molten metal during the initial stage of decarburizing and refining following the charging of the next charge of the decarburizing molten metal. Cr content is reduced and recovered. This is a process established because the charged molten metal has C to 5% by weight and a temperature of about 1500 ° C., which is sufficient for the reduction of Cr ₂ O ₃ . As a result, Cr, which has been used to recover expensive metals such as Si and Al as a reducing agent, is recovered by using an inexpensive reducing agent such as carbon in the molten reducing furnace or carbon in the decarburizing furnace charging melt. The economic effect in this respect will be great.

As described above, in addition to the advantage of the conventionally known smelting reduction furnace process that the chromium raw material can be obtained as an inexpensive chrome ore by organically connecting the chrome ore smelting reduction furnace and the decarburizing furnace according to the present invention. Then, various problems left unsolved in the decarburization furnace, namely, a). Use of large amounts of expensive Si and Al, b). Use a large amount of auxiliary materials such as CaO for desulfurization, c). Problems such as significant melting damage of decarburizing refractories were resolved at once.

〔Example〕

The flow of the embodiment of the present invention is shown in FIG. FIG. 2 is a comparative example, which is a conventional flow in which decarburization and reduction treatment are performed in the same furnace or another decarburization furnace after completion of smelting reduction.

In both the comparative example and the example, the smelting reduction furnace and the decarburization furnace are 100-ton molten steel scale top and bottom blowing converters.

Hot metal dephosphorization step 1 is common to both and is a normal tope car dephosphorization. The components and temperatures of the hot metal after dephosphorization are as follows.

C / 4.3% by weight, Si / <0.01% by weight, Mn / 0.012% by weight, S / 0.026% by weight, temperature to 1270 ° C. Hereinafter, the details of the embodiment of the present invention will be described first.

59 tons of the hot metal 1 is charged into the smelting reduction furnace 2. 2.5 tons of the decarburizing furnace slag 3 and 0.8 tons of the desulfurization slag 4 are charged.

 Here, those components are as follows.

Decarburization slag: CaO / 20%, MgO / 18%, SiO 2/5%, Cr 2 O 3/55% desulfurization _{slag: CaO / 60%, Al 2} O 3/15%, MgO / 15%, CaF ₂ /4%, SiO ₂ /6% Carbide 7 was fed with coke while sending acid, and after reaching 5 wt% of hot metal and temperature of 1550 ° C, temperature was increased from 1550 to 1550 while continuously casting Cr ore and coke. Melt reduction is performed by keeping at 1600 ℃.

During this period, the input amount of chromium ore: 42 tons Coke: 54 tons O ₂ : 31.200Nm ³ CaO: 9.3 tons Dolomite: 2.7 tons as an auxiliary material.

After the completion of smelting reduction, the molten metal content was C / 5.6% by weight, Si / <0.01% by weight, Mu / 0.21% by weight, P / 0.033% by weight, S / 0.012% by weight, Cr / 16.8% by weight, tapping amount: 82.2 tons Hot water discharge temperature: 1574 ℃ At the time of hot water discharge, 600 kg of 90% CaO-10% CaF ₂ desulfurization agent was added to the hot water flow, and the total amount of slag discharged from the furnace was approximately 10
Remove 00kg of slag with a scraper.

At this time, the molten metal content of S is 0.012% by weight to 0.003% by weight
The other temperature remains unchanged and the temperature reaches 1545 ° C.

This molten metal is charged into a decarburizing furnace. Pre-charged slag was not discharged to the decarburization furnace and remained 1.9 tons.

After the charging was completed, the temperature of the molten metal dropped to 1518 ° C, the components did not change, and oxygen gas was fed from the top blowing lance and bottom blowing tuyere,
C ~ 0.6 wt%, C ~ 0.25 wt% as a step, O ₂ /
Switched to mixed gas injection with Ar ratio 2/1, 1/3, C ~ 0.05
Decarburization is completed at the weight percentage.

During this period, 25 tons of SUS430 scrap was added for cooling,
The temperature at the end of refining was 1665 ° C, and the molten steel components were as follows.

C / 0.05% by weight Si / <0.01% by weight, Mn / 0.14% by weight, P / 0.031% by weight, S / 0.003% by weight, Cr / 15.9% by weight Immediately tapping, FeMn: 300kg, FeSi: 200kg, HC
Input FeCr: 100kg and send to secondary refining equipment.

4.5 tonnes of slag remained in the decarburization furnace, and about half of the slag flowed out after tilting, so it is recovered and returned as a pre-charge material in the smelting reduction furnace.

〔The invention's effect〕

In the above process, the reducing agent and the auxiliary material used in the decarburizing furnace process are only 100 kg each of FeMn, FeSi, and HCFeCr at the time of tapping, and the desulfurization flux additionally used in the present invention is 600 kg. It stays in a very small amount.

On the other hand, in the case of the comparative example, the materials used in the decarburization furnace are as large as CaO: 5 tons, MgO: 1 ton, FeSi: 1.66 tons, and the amount required is about 10 times the amount of the present invention.

Further, in the case of the comparative example, the reducing slag does not contain Cr and has a low basicity, so it is not worth returning and is discarded outside the system. On the other hand, in the example of the present invention, the waste slag outside the system is only the smelting reduction furnace slag that is common to the comparative example, which can be said to be a desirable process from the viewpoint of effective utilization of resources.

Further, the life of the decarburizing refractory of the present invention was remarkably improved in the present invention, and the great effect of postponing the furnace repair required every 600 charges for every 1500 charges was obtained.

This is an effect that can be understood from the fact that the composition of the decarburizing furnace slag is similar to that of a furnace refractory, for example, magnesia chrome brick.

[Brief description of the drawings]

 FIG. 1 is a flow sheet of an embodiment of the present invention, and FIG. 2 is a flow sheet of a conventional process. 1 ... Hot metal dephosphorization 2 ... Melt reduction process 3 ... Slag charging 4 ... Desulfurization slag 5 ... Carbon material 6 ... Cr ore 10 ... Desulfurization flux 11 ... Decarburization process

Front Page Continuation (72) Inventor Tetsuya Fujii 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Keizo Taoka 1 Kawasaki-cho, Chiba-shi Kawasaki Steel Co., Ltd. Chiba Steel In-house (56) References JP 62-60813 (JP, A) JP 62-60814 (JP, A) JP 61-166910 (JP, A) JP 1-215913 (JP, A)

Claims (3)

(57) [Claims] 1. A method for producing stainless steel, comprising: (i) a smelting reduction step in which a carbonaceous material and a chromium-containing substance are added to molten iron to supply an oxidizing gas; (ii) molten iron having a predetermined composition and a predetermined temperature. Take a bath
A stainless steel characterized by sequentially performing a desulfurization step of adding a desulfurizing agent thereto, and (iii) a decarburization refining step of charging the molten iron into a decarburization furnace and supplying an oxidizing gas to perform decarburization refining. Steel melting method. 2. The method according to claim 1, wherein after tapping the steel from the decarburizing furnace, the slag of the decarburizing furnace is taken out and used in the smelting reduction step. 3. The method according to claim 1, wherein after tapping from the decarburizing furnace, a part of the slag of the decarburizing furnace remains in the furnace, and the next decarburizing refining is performed using the decarburizing furnace. Or the method described in 2.