JPS6031885B2 - Dephosphorization method for high chromium molten steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for making low phosphorus, high chromium steel by making it possible to dephosphorize high chromium molten steel, which was difficult to do using conventional methods.

The high chromium steel referred to in the present invention refers to steel containing 4 to 40% Cr, such as stainless steel and heat-resistant steel. In addition, the target P% of copper varies depending on the composition, application, etc., but the meaning of "low phosphorus" is that it is obtained when virtually no dephosphorization can be performed after adding a chromium source, as in the conventional method. It refers to something lower than the P level. High chromium steel, especially stainless steel, is generally produced by melting and mixing an iron source such as molten iron or scrap, a chromium source such as ferrochrome or high chromium steel scrap, and if necessary a nickel source, followed by refining. Manufactured.

Regarding decarburization and desulfurization among the normally performed refining processes,
Various methods have already been developed, and the objective can be achieved by selecting and combining them.

On the other hand, there has been no effective method for dephosphorizing molten chromium. That is, ordinary hot metal or molten steel can be easily dephosphorized by oxidation refining. Dephosphorization by oxidation refining is a method of oxidizing phosphorus in the molten metal with an oxidizing agent such as oxygen gas or iron oxide, fixing it with Ca0, and removing it in the form of calcium phosphate. When applied to a molten metal containing large amounts of easily oxidized elements, those components will be oxidized and lost at the same time.

Particularly in the case of high chromium steels such as fluorochrome and stainless steel, the amount of chromium oxide in the slag increases and the dephosphorizing ability of the slag is significantly reduced, making it difficult to actually progress dephosphorization. Therefore, conventionally, the following methods have been used to produce stainless steel with limited phosphorus content.

(1) Low phosphorus ferrochrome produced by selectively using chromium ore with low phosphorus content, coke and a solvent is used as a chromium source.

(b) After sufficiently reducing the phosphorus content on the iron source side (for example, by strengthening the dephosphorization of hot metal or molten steel, or selectively using low-phosphorus scrap), add a chromium source.

(m) By using metallic chromium as a low phosphorus chromium source, the Cr% and P% of the molten steel are adjusted to suit the purpose.

However, recently, due to deteriorating resource conditions, the phosphorus content of chromium ore, coke, etc. has tended to increase, making it difficult to select and use high-quality low-phosphorus raw materials as in (1).

On the other hand, various methods have been developed for strengthening iron source dephosphorization in (D), but if the P% of the chromium source remains the same, the P% of stainless steel obtained by reducing the phosphorus content of only the iron source is There is a limit.

In addition, (m) is the easiest method in terms of steelmaking work, but
Since metallic chromium is extremely expensive, this results in a significant increase in the cost of producing molten steel.

Recently, as mentioned above, the P% of stainless steel has been on the rise due to resource considerations, and on the other hand, an increasing number of high-grade stainless steels are required to have a P% lower than the conventional level.

Therefore, there is a strong demand for the development of a method that enables dephosphorization of high-chromium molten metal and obtains low-phosphorus, high-chromium steel at low cost.

One way to dephosphorize high-chromium sunrises is to make the molten metal unsaturated with carbon and treat it with a flux containing calcium carbide and alkali metal fluorides as main components in a non-oxidizing atmosphere. -2943, patent application 1986-1999
143).

In this method, (CaC2)2(Ca)+2[C
]... By the reaction of [1), metallic calcium is liberated, and 3(Ca)+2[P](Ca3P2)...
...{21 causes phosphorus in the metal to be transferred to the flux. In order to implement the above method in a stainless steel melting process on an industrial scale, the following problems must be solved.

The progress of the dephosphorization reaction is essentially related to the C% of the molten steel, as shown in equation 2.

Therefore, in order to efficiently perform dephosphorization in the melting process of high chromium steel, appropriate values must be selected for the components of the molten steel before treatment. [Example] Phosphorus is contained in the slag after dephosphorization as Ca3P2, but when it is cooled in the atmosphere, it reacts with moisture and Ca3P2 Jugen LO → Father a
The reaction generates odorous, toxic gas, phosphine PH3.

Therefore, it is necessary to stabilize the slag. An important issue is how to incorporate this into the process. In view of the above circumstances, the present invention was obtained as a result of various studies from the viewpoint of dephosphorizing high chromium molten steel in the mass production process and making it possible to produce low phosphorus high chromium steel. Mix the iron wire and chromium source (wt% Bishu 6-8
The first step is to make high-chromium steel of o, and the second step is to dephosphorize the molten steel by reacting it with a flux containing calcium carbide and an alkaline earth metal halide as main components in a non-oxidizing atmosphere. This is a method for producing low phosphorus, high chromium steel, which is characterized in that the slag is subsequently oxidized to reduce the calcium carbide in the slag to 2% or less, and then the slag is separated.

Hereinafter, the present invention will be explained in detail based on examples.

First, as the first step, using a steelmaking furnace operating under atmospheric pressure,
A molten metal with a Cr concentration close to that of the finished product is produced.

The smelting brick may be one normally used, and in the case of an electric furnace, it is mainly scrap, ferrochrome, and if necessary a nickel source, and in the case of a converter, it is mainly a mixture of hot metal, ferrochrome, and a nickel source, if necessary. be. However, it is desirable that the iron source be deoxidized before adding the chromium source, and especially when molten iron is used, it is necessary to perform dephosphorization by oxidative refining. The composition range of the molten metal to which the present invention is applied is one in which chromium is in the range of 4 to 40%, such as stainless steel and heat-resistant steel, as described above.

Since the iron source that has been dephosphorized and the normal chromium source are melt-mixed, the P% of the molten metal tends to increase with the chromium%. If Cr is 4% or less, the input of phosphorus from the chromium source is small, and dephosphorization by oxidative refining is possible, so the present invention is not applicable. On the other hand, although there is no technical restriction on the upper limit of chromium %, high chromium steels with a Cr content of 40% or less have not been put to practical use, so they are not subject to the present invention. After melting and mixing the main raw materials in a steelmaking furnace, it is desirable to subsequently perform decarburization treatment by blowing acid or the like within a range that does not cause excessive oxidation of chromium under operating conditions at atmospheric pressure.

The dephosphorization reaction in the present invention is CaC2 (in slag) → Ca (in slag) tenth order (in metal)...'1} father a (in slag) tenth order (in metal) →
Ca3P2 (in slag)...proceeds according to ■.

Therefore, the C% in the metal is essentially related to the progress of the reaction. In addition, since Cr has a large effect on the activity of C, the relationship between the Cr/C ratio and the dephosphorization efficiency in the next step is as shown in the figure, and in order to perform dephosphorization efficiently,
It is necessary that Cr/C is in the range of 6 to 80, and particularly preferably 7 to 35. In the figure, A is calcium carbide 20k9/t and refined fluorite 4k9/t, and B is calcium carbide 10k9/t and refined fluorite 2.5k9/t. If the Cr/C ratio is too small (that is, if C is too high), the amount of Ca produced by the '1} formula will decrease, and the dephosphorization efficiency will decrease. On the other hand, if the Cr/C ratio is too large (that is, if C is too low), the reaction of formula '1-' occurs quickly. If you look too much, a large amount of Ca will be generated,
Since the reaction of formula 2' is not completed in time, free Ca is likely to be lost by evaporation, resulting in a decrease in dephosphorization efficiency. At this intermediate Cr/C ratio, a stable high dephosphorization rate can be obtained. Note that if molten steel contains carbon up to the saturation value, f
Since the reaction of formula 1} does not proceed, dephosphorization naturally does not occur. Therefore, it goes without saying that it is also a necessary condition that the molten metal be carbon unsaturated. Cr targeted by the present invention
For molten steel of 40% or less, the carbon saturation condition is Cr/C of 5.
Since it is below the scale, the above-mentioned condition that Cr/C is 6 or more includes carbon unsaturation. Next, in the second step, the high chromium molten steel obtained in the first step is brought into contact with a flux containing calcium carbide and an alkaline earth metal halide (e.g. calcium fluoride) as main components in a non-oxidizing atmosphere. Perform dephosphorization reaction.

Among the flux composition, calcium carbide is {U, [2}
It is an indispensable component for dephosphorization according to the formula. Calcium carbide is an easily available source of calcium carbide.

Calcium carbide contains 10 to 30% calcium oxide as an impurity, but this does not pose a particular problem for the purpose of the present invention. The optimum blending ratio of calcium carbide in flux varies depending on the composition of molten steel (especially C%).
It is desirable to increase the force-bide blending ratio as the percentage increases. For example, if C is also 0.4%, it is 30-60%, and C is 2%.
%, the optimum range is 40 to 75%. Next, an alkali metal halide, such as calcined calcium, is necessary to lower the melting point of the flux and provide appropriate fluidity, as well as to allow the free metallic calcium produced by formula '1' to stably exist in the flux. . The larger the amount of calcium fluoride blended, the more advantageous it is for the reaction, but if it is too large, it will accelerate the erosion of the refractories in the reaction vessel, so the amount of calcium carbide should be 0.5 to 1. Use an appropriate amount of fangs. In addition,
Fluorite, which is commonly used as a source of calcium fluoride, is 20
% of mainly silica-based impurities.

Since this silica component tends to oxidize free metal calcium, it is not preferable that it exists in a large amount. Therefore, purified fluorite (Si02<5%) is recommended as a source of evolved calcium.
It is desirable to use The above two components are essential components for carrying out the dephosphorization reaction, and the others are mixed in with calcium carbide and fluorite, or are used as secondary components to adjust the melting point of the flux and suppress the reaction with refractories. These ingredients include calcium oxide, alumina,
Examples include magnesia and silica.

As mentioned above, in order to prevent free metal calcium from being consumed by oxidation, it is desirable to use a material that is difficult to reduce. In addition, in order to prevent oxidative consumption of metallic calcium due to mixed slag from the previous process or mixed oxygen gas, a deoxidizing agent such as Ca-Si, A, etc. may be added to the flux in advance. . The amount of flux added is C% and Cr% of the molten metal.
, the target dephosphorization rate (i.e., P% before treatment and target P% after treatment), and the method of contact between the molten metal and the flux (i.e., the degree of progress of the reaction), but C before treatment: 0.5
%, Cr: 18%, Ni: 8%, P: 0.030%, target P%: 0.015%, CaC2: 5-20k
9/t-molten steel (CaC2 purity in carbide approximately 80%,
In the case of CaF2: 20% combination, the flux amount is 10 to 40
k9/molten steel) is optimal.

The flux may be added in solid form either separately or as a mixture, but it is advantageous to accelerate the reaction if it can be prepared in advance by melting it in a separate furnace (such as a graphite-lined slag melting furnace). In order to efficiently proceed with the reaction of m, '2}, it is necessary to take care to suppress oxidation of metallic calcium due to mixed slag and atmosphere.

First, the oxidizing slag generated in the first step is separated as much as possible, and the second
It is necessary to reduce the amount of slag mixed into the process, but methods for separating slag include tilting the furnace in the case of electric furnaces to stir up the slag, and methods for separating slag in the case of converter furnaces. A device that can prevent outflow (for example, a slug ball, a sliding nozzle, etc.) may be used.

In addition, if the oxidizing slag cannot be sufficiently separated, the adverse effects can be reduced by adding a deoxidizing agent such as Ca-Si, CaC2, or A2 to the slag.

Next, regarding the atmosphere, it is important to suppress the intrusion of air as much as possible.

Since the oxygen gas that has entered the atmosphere reacts with calcium carbide and metallic calcium and is consumed, the atmospheric oxygen analysis value is not appropriate as a value that indicates the degree of atmospheric intrusion.
- On the other hand, since nitrogen gas has less reactivity than oxygen gas, by analyzing N2, the degree of atmospheric intrusion can be determined quite accurately. When the N2 content in the atmosphere is 50% or more, dephosphorization hardly progresses due to the adverse effects of the invading atmosphere. It is desirable to prevent atmospheric intrusion so that N2 is 10% or less.

Flux to efficiently carry out dephosphorization reaction after the above-mentioned flux and atmosphere conditions are satisfied.
There are the following methods for contacting the molten metal.

'a} A method in which flux is added when the molten moat flow from the steelmaking furnace is received in a reaction vessel that suppresses the entry of atmospheric air.

This method is advantageous in that it is not necessary to take any special measures to create a rope, since it can utilize the strong fly-pasting effect caused by the tapping flow.

In order to seal the atmosphere inside the reaction vessel from the atmosphere, there are methods such as once receiving the molten metal in a pony ladle and then draining it, or blowing inert gas or reducing gas into the reaction vessel.

In addition, it is desirable to increase the specific surface area of the falling sun in order to increase the reaction rate, but for this purpose, it is also possible to spray gas etc. to the tapping flow, or to make it into a spray as in the conventional tapping degassing method. The molten metal may be made into droplets by the gas generated by pouring Rakuyo through a ladle into a reaction vessel placed under reduced pressure. In either case, a portion of the flux may be placed in the reactor, and the remainder may be added as appropriate. {b} In a steelmaking furnace, flux is added to the hot water held in a ladle while suppressing the entry of atmospheric air.

Methods for removing the wound include injecting an inert gas such as Ar or a reducing gas into the wound.

The gas used for this has the effect of making the atmosphere non-oxidizing as well as suppressing the molten metal. In addition, blowing powdered flux into the molten metal along with the gas has a great effect of accelerating the reaction.

If the amount of flux to be added is large (if the temperature drop of the molten metal is large), it is desirable to use heating in combination.

Next, in the third step, oxidation is performed to stabilize the slag.

In order to prevent the phosphorus that has migrated to the slag from returning to the molten steel, it is desirable to separate the slag and molten steel after dephosphorization.
When cooled to below 00°C, as mentioned above, odor and toxic gases are generated, making the working environment worse. In the present invention, in order to solve this problem, the slag is not separated from the molten steel, but is oxidized as it is, and by optimizing the oxidation conditions, the amount of phosphorus that returns to the molten steel is minimized, and the phosphorus in the slag is It is characterized by promoting the migration of slag into the atmosphere in a gaseous state, and also fixing some of it in the slag as an oxide. To do this, first, the atmosphere is made oxidizing using blowing acid or the like, and CaC2 in the slag is oxidized as CaC20 or 21.

When the reaction of formula '3' progresses, the reverse reaction of formula '1' and (2) progresses, and phosphorus in the slag becomes F205 via elemental phosphorus or suboxide. This intermediate form, ie, elemental phosphorus or box oxide, has a high vapor pressure and is easily vaporized. The vaporized elemental phosphorus or suboxide is P in the atmosphere.
205, which is absorbed by water in the dust collection equipment,
, it can be processed without any adverse effects outside the system. In order to promote gasification while suppressing the return of phosphorus to the molten steel, it is desirable to minimize the amount of molten steel in the third step, unlike the second step. Further, it is desirable that the average oxygen and partial pressure Po2 in the furnace is 0.5 to 200 Torr. This is because if Po2 is too low, it will take a long time to oxidize the slag, whereas if Po2 is too high, the proportion of phosphorus to be gasified will decrease, which is not preferable. Also, oxides may be used to oxidize the slag. In particular, in order to increase the proportion of phosphorus to be gasified and reduce rephosphorus, it is desirable to reduce the pressure of the atmosphere.

After oxidizing the slag, the phosphorus in the slag that remains as Ca3P2, which is unstable to water vapor, is reduced to a value that does not pose an environmental problem (phosphorus as Ca3P2 is 0.02% or less).
In order to reduce the amount of CaC2 in the slag to 2% or less, the content of CaC2 in the slag must be reduced to 2% or less.

Once the slag is stabilized by oxidation, slag and molten steel can be further oxidized while they coexist to decarburize the molten steel to a predetermined C%. After that, the slag may be separated and the molten steel may then be decarburized. In any case, high chromium molten steel with a predetermined C% and sufficiently oxidation-stabilized slag are finally obtained, and subsequent treatments can be carried out as usual. Example After preliminary decarburization by melting scrap, high carbon ferrochrome and ferronickel in an electric furnace, 9E steel was tapped.

The molten steel is received by a steel plate with a porous plug embedded in it, and gas is blown into it from the porous plug using A. Place this ladle in an airtight container.
C2:80%) at 10k9/t, purified fluorite (CaF2:
96%) at 2.5k9/t and reacted for 7 minutes to perform dephosphorization. Next, the pressure inside the sealed container was reduced to 76 degrees H.
At each level of 300 g, 300 side Hg, and 50 side Hg, blowing of Ar gas from the polars plug was stopped and acid was blown for 5 minutes using a top blowing lance. CaC2 in the slag at this point
% was 0.5%, and phosphorus present as Ca3P2 in the slag was 0.005%. Next, while the slag and molten steel coexisted, gas injection from the porous plug was resumed to perform acid blowing and remove bracken. The table shows the analysis values and imbalance of molten steel components at each time point. In addition, the final omitted is 0.05
It's a sleeve.

In this way, by carrying out the present invention, high chromium steel can be dephosphorized, which was previously impossible in mass production.
Furthermore, the slag after dephosphorization can be rendered harmless.

Therefore, it can satisfy both industrial requirements and environmental issues.
It has great industrial value as it will enable practical dephosphorization of high chromium steel.

[Brief explanation of the drawing]

The drawing shows the influence of the Cr/C ratio on the dephosphorization efficiency at the end of the second step.

Claims (1)

[Claims] 1. The first step is to mix an iron source and a chromium source to produce high chromium molten steel with a (Cr)/C (wt% ratio) of 6 to 80, and then convert this molten steel into a mixture whose main components are calcium carbide and alkaline earth metal halides. The second step is to dephosphorize the slag by reacting it with the flux contained in the slag in a non-oxidizing atmosphere, and then to separate the slag after oxidizing the slag to reduce the amount of calcium carbide in the slag to 2% or less. Characteristic low phosphorus high chromium steel melting method.