JPS6023166B2 - Melting method of low link chromium steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing low phosphorus chromium steel in which dephosphorization of high chromium molten metal, which has been difficult with conventional methods, is carried out on an industrial scale.

The high chromium molten metal targeted by the present invention refers to high chromium steel such as stainless steel and heat-resistant steel, crude molten steel in the process of melting it, or ferrochrome containing 4% to 70% Cr.
The molten metal contained therein will be explained below, mainly taking the stainless steel melting process as an example.

Stainless steel is generally made from iron sources such as hot metal and scrap.
Chromium sources such as ferrochrome, high chromium steel scrap,
And if necessary, it is melted by melting a nickel source and performing refining.

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 high-chromium molten metals. In other words, while ordinary hot metal or molten steel can be easily dephosphorized by oxygen smelting under high basicity slag, Rakuyo, which contains more than a few percent of easily oxidized elements such as silicon and chromium, can be dephosphorized. In addition to a large amount of components being oxidized and lost, the dephosphorization ability is significantly reduced both physically and chemically due to the oxides that have migrated into the slag, making it difficult to actually proceed with dephosphorization. Therefore, 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 solvent is used as a chromium source.

(b) Sufficiently reduce the phosphorus content on the iron source side, and then reduce the chromium source.

m Use metallic chromium together as an extremely low phosphorous chromium source.

However, due to the recent deterioration of the resource situation, 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 like 1.

On the other hand, various methods have been developed to strengthen dephosphorization on the iron source side (b), but under conditions where the P% on the chromium source side remains the same or increases, it is difficult to reduce the phosphorus content of only the iron source. However, the effect on the low phosphorus content of the resulting high chromium steel is small. In addition, m is the easiest method in terms of steel manufacturing, but since metal chromium is extremely expensive, it leads to a significant increase in the manufacturing cost of port steel. Recently, as mentioned above, due to changes in the resource situation, the P% of high-chromium steel is generally on the rise, while the number of high-grade stainless steels that require a P% lower than the conventional level is increasing. For this reason, 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.

There is already a method for dephosphorizing high-chromium molten metal using alkali metals such as Ca and their fluorides, but this method requires expensive flux and is difficult to implement in refractories, and requires electroslag dissolution. Since it is necessary to use remelting using a water-cover mold as in the above, it is mainly intended for manufacturing small quantities of high-quality products, and is difficult to apply to the mass production process targeted by the present invention.

The present inventors first dephosphorized a high-chromium molten metal by making the molten metal unsaturated with carbon and treating it with a flux containing calcium carbide and alkali metal fluorides as main components in a non-oxidizing atmosphere. invented (patent application 1984-29)
43 Tokusho 60-199143).

In this method, (CaC2) (Ca) 12 [C]
......[Metal calcium is released by the reaction of 11, 3(Ca)+2[P]→(Ca3P2)
......■ causes phosphorus in the melt to migrate into the slag.

Here, () indicates the component in the slag, and [] indicates the component in the metal. Further, flux means a dephosphorizing agent before being added to the molten metal, and slag means a product formed after the addition of the molten metal. The problems when implementing this method on an industrial scale are as follows.

[a} The role of the alkali metal fluoride in the dephosphorization flux is to allow metallic calcium and Ca3P2 to stably exist in the slag, and to lower the melting point and viscosity of the slag.

Generally, the latter purpose requires a higher amount of fluoride. However, as the amount of fluoride increases, the erosion of the oxide-based refractory lining becomes more severe, so the amount of fluoride should be kept to a minimum. In that case, since the slag has poor fluidity, strong slag must be applied in order for the reaction to proceed. Further, in order to proceed with the dephosphorization reaction, the atmosphere must be non-oxidizing in order to suppress oxidative consumption of metallic calcium. 'b- When the slag after the dephosphorization reaction is cooled in the atmosphere, Ca3
The reaction between P2 and atmospheric water vapor may generate toxic and odoriferous phosphine (P&). (Ca3P2)
10$ LO → (Ca○) 10 is wide... (3' To prevent this, it is necessary to oxidize the slag after the dephosphorization reaction has been completed to convert phosphorus into an oxide form that is difficult to react with water vapor. It is.

(Ca3P2) 1402 → 3 (Cao) + (P2Q).
・・・． ...{4) If oxidation of slag is performed in the presence of molten metal, the metallic calcium in the slag will be consumed and the reverse reaction of the '21 formula may occur, which may lead to rephosphorization.

Therefore, if the slag is separated from the molten metal and then subjected to oxidation treatment after the dephosphorization process is completed, there is no risk of rephosphorization, but if it is actually carried out on an industrial scale, some of the scattered slag will react with water vapor in the atmosphere. When the slag is separated from the molten metal, the slag solidifies and the reaction rate of the '4} formula becomes low, making it difficult to completely oxidize the slag. Therefore, when the slag is in contact with the molten metal and kept at high temperature,
It is desirable to carry out stabilization by oxidation using a method that can reduce the amount of rephosphorus as much as possible. As mentioned above, in order to dephosphorize high-chromium port water on an industrial scale, in the same reaction vessel,
And in the coexistence of slag and molten metal, '1} non-oxidizing atmosphere,
Slag, Molten Metal Goto Azusa'2} In an oxidizing atmosphere, slag is oxidized and stabilized without rephosphorization.

It is desirable to be able to perform one process in succession.

However, no processing method has hitherto been known that satisfies these functions. The present invention was obtained as a result of various studies to satisfy the above conditions. Carbon-unsaturated high-chromium Rakuyo is placed in a bag in a rollable reaction container equipped with a gas inlet, and then Non-oxidizing gas is blown into the molten metal to dephosphorize the molten metal, and a flux mainly composed of calcium carbide and an alkali metal halide, such as calcium fluoride, is added to dephosphorize the molten metal, and then the oxidizing gas is removed from the slag. This is a method of dephosphorizing high chromium molten metal, which is characterized by oxidizing the slag by blowing it into the layer or above it.

Hereinafter, it will be explained in detail based on Examples. First, carbon-unsaturated high-chromium Rakuyo is melted.

In the case of high chromium steel, a steelmaking furnace operated under atmospheric pressure is used to remove impurities such as carbon, phosphorus, and sulfur, as well as additive elements that easily combine with oxygen (AI, Ti, etc.). The molten metal with a similar composition is hereafter referred to as high-chromium crude molten steel. The raw materials for lacquer making may be those conventionally used, and when an electric furnace is used as the steelmaking furnace, scrap, ferrochrome, and the necessary raw materials are used. If a converter is used, hot metal and ferrochrome are used, and if necessary, a nickel source is used. It is desirable that the iron source before adding the chromium source be as low in phosphorus as possible, and especially when molten iron is used, it is desirable to precede dephosphorization by oxidative refining.

After melting the main raw material in a steelmaking furnace, depending on its carbon content, it is desirable to subsequently decarburize it by blowing acid or the like under atmospheric pressure within a range that does not cause excessive oxidation of chromium.

This is because the amount of calcium carbide that needs to be added in the next step increases when the degree of carbon unsaturation of the high chromium molten metal is low.

In the case of approximately 20% chromium molten metal, carbon is reduced to 0 when operated under atmospheric pressure.
．． If decarburized to less than 2%, chromium will cause excessive oxidation, so the optimal carbon content is in the range of 0.2 to 2.5%. This crude molten steel is tapped from the steelmaking furnace, and after removing as much oxidizing slag as possible, it is charged into a reaction vessel for the next process. The reaction vessel used here is one that is lined with an oxide refractory and has a gas inlet in the furnace body and is removable, such as an AOD furnace used for refining stainless steel.

The gas inlet is attached to the side or bottom of the furnace body, and can blow oxidizing gas such as Ar into the furnace.

Note that in order to reduce the intrusion of outside air into the furnace and to prevent the molten metal from spilling out of the furnace due to strong damage, it is desirable that the shape of the reaction vessel be such that the furnace opening is slightly constricted.

First, as shown in Fig. 1A, the high chromium crude molten steel obtained in the previous step is charged into this reaction vessel 1, and the gas injection port 2 is made to be below the molten metal surface, and a non-oxidizing gas, that is, Ar Any of the inert gases, reducing gases such as hydrocarbons, etc.
Alternatively, mixed gas is blown into the molten metal 3 to lighten the molten metal, and a dephosphorizing flux containing calcium carbide and calcium fluoride as main components is added to make the atmosphere inside the furnace non-oxidizing. gas, and some mixed oxygen by burning the dephosphorization flux),
Allow a reaction to occur with the molten metal.

Incidentally, flux may be blown in from the gas blowing port 2 together with a non-oxidizing gas.

The composition of the flux for dephosphorization is as follows.

Calcium carbide is an indispensable component for dephosphorization according to the formula (11,'2). Calcium carbide can be used as an easily available source of calcium carbide. On the other hand, the purpose of adding calcium fluoride is as mentioned above, and crabite is a source of calcium fluoride, but the commonly used crabite contains about 20% of impurities, mainly silica. There is. This silica content is '1
} Since the metallic calcium produced by the formula is easily oxidized, it is desirable to use refined crabite from which impurities have been removed by a mineral beneficiation method as a source of calcium fluoride. Weight ratio of calcium carbide and calcium fluoride in flux (CaC2
/CaF2) is optimally between 2 and 8.

The amount of flux added varies depending on the composition of the molten metal and the target dephosphorization rate, but C: 0.5% before treatment.
, Cr: 18%, Ni: 8%, P: 0.030%, and the target P after treatment is 0.015% (when the dephosphorization rate is 50%,
The amount of calcium carbide is about 5 to 20 kg/t-molten steel. When such a flux is added and the port water is forcibly attacked, at least a portion of the flux is melted to generate slag 4 containing metallic calcium, and the dephosphorization reaction proceeds according to the '21 formula.

As time passes, the slag composition changes due to tread loss of the refractory lining, etc., and as a result, phosphorus reaches a nearly constant value.

When the dephosphorization process is completed, as shown in Figure 1, the reaction vessel 1 is faced forward and the gas inlet 2 is positioned in the slag layer or above the slag layer 4 to remove as much of the molten metal as possible. The slag is oxidized by blowing in an oxidizing gas such as oxygen, Ar+oxygen, or air from the gas blowing port 2 in such a manner that no oxidation occurs.

A separate top blowing lance may be used for blowing in the oxidizing gas. Note that, if conditions are selected so that the molten metal does not scorch significantly, an oxidizing gas may be blown into the slag layer to promote the oxidation reaction.

At the same time, an advancing agent commonly used as a slag control agent may be added.

(CaC2) Sha2ichi (Ca.

) tenth. ．．・．． ．． ... [5' In this case, the above {4),
Simultaneously with the reaction of formula [5}, the reverse reaction of formula '1} and (2) described above occurs, and phosphorus in the slag becomes P205 via elemental phosphorus or suboxide. This intermediate form, elemental phosphorus or suboxide, has a high vapor pressure and is therefore easily vaporized. The vaporized elemental phosphorus or suboxide becomes P24 in the atmosphere, which is absorbed by water in the dust collection equipment.
Can be processed without adversely affecting outside the system. Further, other than the vaporized phosphorus, it is stabilized as P205 in the slag, and some of it returns to the molten metal during the oxidation process.

In order to promote gasification while suppressing the return of phosphorus to the twill steel, it is desirable to reduce the amount of takaazusa in the molten steel as much as possible. In addition, the average oxygen partial pressure P02 in the furnace is 0.5 to 2
00 skin Hg is desirable. This is because if P02 is too low, it will take a long time to oxidize the slag, while if P02 is too high, the proportion of phosphorus to be gasified will decrease, which is not preferable. After oxidizing the slag, the remaining phosphorus in the slag as Ca3P2, which is unstable against water vapor, is reduced to a value that does not pose an environmental problem (phosphorus as Ca3P2 is 0.0
2% or less).

Next, set up the furnace as shown in Figure 1A, and with the slag and port steel still coexisting, proceed as in the normal AOD furnace operation.
It is also possible to decarburize to a specified C% by blowing a mixed gas of inert gas and oxygen into the molten metal, and slag is not necessary. After oxidation to the level, the slag is separated, and then the molten steel is decarburized. Good too.

After decarburization, if necessary, the generated chromium oxide is reduced with ferrosilicon or the like to obtain a low chromium phosphate molten steel product. In addition, since desulfurization also progresses during the dephosphorization process, the sulfur content of the product can be reduced.

Example 1 18%Cr-8%Ni stainless crude molten steel melted in an electric furnace was tapped, received in a ladle, deoxidized, and then passed through a nozzle provided at the bottom of the ladle into an AOD furnace. Charge to.

The oxidizing slag mixed in from the electric furnace remains in the ladle. With the AOD furnace upright, Ar is blown through the gas inlet and 12k9 of calcium carbide (calcium carbide grade 80%) is heated. /t, Kasuisei Mitsuishi 4k9/
Add t to the molten metal and continue coagulating for 10 minutes. Next, take care of the furnace and blow pure 02 onto the slag layer with the gas injection port 10 degrees above the slag layer (for 10 minutes).
. Finish decarburization is performed under normal AOD operating conditions. The melt components at each stage are as follows. Example 2 The conditions were almost the same as in Example 1, but argon + oil mist was blown from the gas inlet during the dephosphorization process, and
Oxygen was blown into the slag layer during slag oxidation.

The moat components for each period are as follows. As described above, by carrying out the present invention, it is possible to stably dephosphorize high-chromium molten metal on an industrial scale without the need for generating toxic or odor gas, and regardless of resource conditions. This method has a great practical effect in that high chromium steel with a predetermined IJ content can be produced.

[Brief explanation of drawings]

FIG. 1 shows an embodiment of the present invention, where A shows the situation at the time when the dephosphorization process is being carried out, and FIG. 1 shows the situation at the time when the slag is being oxidized. 1: reaction vessel, 2: gas inlet, 3: molten metal, 4: slag layer. Figure 1

Claims (1)

[Claims] 1. A carbon-unsaturated, high-chromium molten metal is charged into a rollable reaction vessel with a gas inlet, and a non-oxidizing gas is first blown into the molten metal to mix calcium carbide and alkaline earth metals while stirring the molten metal. Dephosphorization is performed by adding flux containing halide as the main component, and then oxidizing gas is introduced into the slag layer.
Alternatively, a method for producing low phosphorus chromium steel characterized by oxidizing the slag by blowing into the upper part.