JPS6022046B2 - Process for producing low phosphorus high chromium 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 metal, which has been difficult with conventional methods.

The high chromium steel referred to in the present invention refers to steel containing 5 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 hot metal or scrap, a chromium source such as fluorochrome or high chromium steel scrap, and if necessary a nickel source, followed by refining. Manufactured.

Regarding decarburization and deflowing, which are normally carried out in refining,
Various methods have already been developed, and the objective can be achieved by selecting and combining them.

On the other hand, until now there has been no effective method for dephosphorizing high-chromium molten metal. That is, ordinary hot metal or molten steel can be easily dephosphorized by oxidation refining. Here, dephosphorization by oxidative refining is a method in which phosphorus in the moat is oxidized using an oxidizing agent such as oxygen gas or iron oxide, and is fixed with Ca○ and removed in the form of calcium phosphate.
If this is applied to sunlight that contains large amounts of easily oxidized elements such as silicon and chromium, these components will be oxidized and lost at the same time.

Particularly in the case of high chromium steels such as ferrochrome and stainless steel, the amount of chromium oxide in the slag increases and the dephosphorization ability of the slag is significantly reduced, making it difficult to actually proceed with dephosphorization. Therefore, in the past, the following methods have been used to melt stainless steel, which has a limited amount of windows.

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

(b) After sufficiently reducing the phosphorus content on the iron source side (for example, by chlorinating the dephosphorization of molten iron or twill steel, or selectively using low phosphorus scrap), reduce the 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, 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 as in (1).

On the other hand, various methods have been developed for (0) iron source neck phosphorylation, but if the P% of the chromium source remains unchanged, the P% of stainless steel that can be obtained by reducing the phosphorus content of only the iron source will decrease.
has its limits.

In addition, (m) is the easiest method in terms of steelmaking work, but
Since chromium metal is extremely expensive, the cost of producing molten steel will increase considerably.

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

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

The present inventors first dephosphorized high-chromium molten metal by making the molten metal unsaturated with carbon and treating it with a flux containing calcium carbide and fluorides of alkali metals as main components in a non-oxidizing atmosphere. The method was discovered (Tokukan Sho 50-29)
43 Mochigan Sho 50-99143).

In this method, (CaC2)2(Ca)12[C
] ・・・・・・・・・Metal calcium is liberated by the reaction of 01, and 3(Ca)+2[P](Ca3P2
)...・．． ...[2] causes the 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.

In order to carry out the dephosphorization reaction efficiently, especially with a reduced amount of flux used, it is necessary to advance the reaction of formula '11 to the right, and for this purpose, the higher the degree of carbon unsaturation of the molten metal, the better. desirable.

However, high carbon ferrite is currently used as the main chromium source for stainless steel twill.

It's chrome. Since high carbon ferrochrome is produced by melting and reducing chromium ore in the presence of excess coke, it contains carbon close to the saturated value, so it is disadvantageous to dephosphorize it as is by the above method. For decarburization, there are two methods: oxidation using oxygen gas or ore, and desilicification after increasing Si% and lowering carbon solubility. A significant increase in costs is inevitable. Therefore, in order to perform the dephosphorization treatment, it is necessary to select a time when the C% of the molten metal is most suitable for the dephosphorization treatment and at the same time the cost required to adjust to that C% is low.

[B] After the dephosphorization treatment, the flux and molten metal are not separated sufficiently, so if subsequent refining is performed, the liberated metallic calcium will be oxidized and consumed, and the reverse reaction of the '21 formula will proceed, leading to the risk of rephosphorization. be.

Therefore, it is necessary to rationally separate flux and ayayu.

As the C% of the molten metal increases with the dephosphorization reaction, if applied to the molten metal that has been decarburized to the C level of the finished product, the C% will increase.
% becomes high, and the final decarburization must be performed again.

In order to avoid C pickup associated with dephosphorization, metal calcium must be used as a Ca source, but this is extremely expensive and is a strong reducing agent, so it cannot be carried out in a refractory container, and water-cooled It is difficult to apply to mass production processes due to limitations such as the fact that it can only be carried out in a metal container such as a steel mold. 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 metal in the mass production process and making it possible to produce low phosphorus and high chromium steel. The first step is to melt and mix the chromium source and other ingredients to create a high-chromium molten metal, and then add a flux containing calcium carbide and an alkali metal halide, such as calcium fluoride, as main components to the molten metal in a non-oxidizing atmosphere. A low phosphorus, high chromium steel characterized by comprising a second step of dephosphorizing the molten metal by reaction, a third step of separating the molten metal and flux, and a fourth step of decarburizing the molten metal to a predetermined C%. This relates to a melting method.

Hereinafter, a detailed explanation will be given based on examples. First, as the first step, the finished product is made of Cr using a steelmaking furnace operating under atmospheric pressure.
Produces molten metal with a similar concentration. The raw materials for smelting may be those normally used, and in the case of an electric furnace, mainly scrap, ferrochrome, and a nickel source if necessary, and in the case of a converter, mainly a melt gun, ferrochrome, and a nickel source, if necessary. be. However, the iron source must be dephosphorized before adding the chromium source, and especially when hot metal is used, it is desirable 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 an iron source that has been dephosphorized and a normal chromium source are melt-mixed, the P% of the molten metal tends to increase as the chromium% increases. 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 steel with 40% Cr or more has not generally been put into practical use, so it is not subject to the present invention. Since the phosphorus content is particularly problematic in structural stainless steels, the present invention mainly targets austotenitic stainless steels containing 15 to 30% Cr and 5 to 25% Ni. It is. 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 temperature range that does not cause oxidation of chromium under operating conditions at atmospheric pressure.

Figure 1 shows the correlation between the C% of the molten metal and the required amount of flux.

The flux composition in this case is CaC250%, Ca0
15%, CaF 230%. In the case of 16 to 20% Cr, it is preferable because a falling sun with 25% or more of C requires a large amount of flux, which is disadvantageous in terms of cost, and also causes deterioration of refining workability in the second step and excessive drop in port steel temperature. do not have. In the present invention, the lower the molten steel C% at the end of the first step, the more advantageous it is, but in general, in operation under atmospheric pressure, decarburizing C to 0.2% or less causes excessive oxidation of chromium. The optimum carbon content after charcoal is 0.2 to 2%.

Incidentally, one method for making stainless steel is to obtain high chromium pig iron by mixing chromium ore and iron source and melting and reducing the mixture in a reduction furnace such as a blast furnace.

If the high chromium pig pig thus obtained is also carbon unsaturated by an appropriate method, the molten metal can be used for the second and subsequent processes of the present invention. Next, in the second step, the carbon-unsaturated, high-chromium molten metal obtained in the first step is brought into contact with a flux containing calcium carbide and an alkali metal halide, such as calcium fluoride, as main components in a non-oxidizing atmosphere. to perform the dephosphorization reaction. The composition of the flux is determined as follows. Calcium carbide is '
1}, is an indispensable component for dephosphorization according to the formula (2). Calcium carbide is an easily available source of calcium carbide.

Calcium carbide is accompanied by 10-30% calcium oxide as an impurity, but this is not a problem for the purpose of the present invention. The optimal blending ratio of calcium carbide in flux varies depending on the composition of molten steel (especially C%).
The higher the C%, the more it is necessary to increase the carbide blending ratio.
For example, in the case of C et al. 0.4%, 30 to 60%, C et al. 2%
In this case, 40 to 75% is optimal. Next, an alkali metal halide such as calcium fluoride is necessary to lower the melting point of the flux, provide appropriate fluidity, and allow the free metal calcium produced by formula '11 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. Use an appropriate amount of beep. In addition,
Satellite, which is commonly used as a source of evolved calcium, 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, it is desirable to use refined fluorite as the calcium fluoride source. 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 components include calcium oxide, alumina, magnesia, and silica. As mentioned above, in order to prevent free metallic 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 and mixed oxygen gas, deoxidizing agents such as Ca-Si, AI, etc. are added to the flux in advance.
etc. may be added. The amount of flux added varies depending on the C% and Cr% of Rakuyo, the target dephosphorization rate (i.e., P% before treatment and target P% after treatment), and the method of contact between the molten metal and flux (i.e., the degree of progress of the reaction). C: 0.5%, Cr: 18%, Ni: 8%, P: before treatment
0.030%, target P%: 0.015%, Ca
C2: 5-20k9/t-molten steel (CaC in carbide
In the case of a mixture of about 80% CaF2 pure and 40% CaF2, the optimum flux amount is about 10 to 40k9/molten steel. Solid fluxes may be added separately or in a mixture, but it is advantageous to accelerate the reaction if the flux 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 reactions {1) and (2), it is necessary to place emphasis on suppressing the oxidation of metallic calcium caused by the atmosphere with the mixed slag.

First, the oxidizing slag generated in the first process must be separated as much as possible to minimize its contamination in the second process, but in the case of an electric furnace, the method of separating the slag is to scrape out the slag by bothering the furnace. method, and in the case of a converter, equipment that can prevent slag from flowing into the tapping hole (e.g. slag ball,
A sliding nozzle, etc.) may be used.

In addition, if the oxidizing slag cannot be separated sufficiently, adding a deoxidizing agent such as Ca-Si, CaC2, or AI to the slag can reduce the negative effects. It is important to suppress it as much as possible.

Since the oxygen gas that has entered the atmosphere is consumed by the reaction of calcium carbide with metallic calcium, the oxygen analysis value of the atmosphere is not suitable 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.

The following methods are available for bringing flux and molten metal into contact in order to efficiently carry out the dephosphorization reaction, provided that the above-mentioned flux and atmosphere conditions are satisfied.

■ A method in which flux is added to the molten metal flowing from the steelmaking furnace into a reaction vessel that prevents air from entering.

This method is advantageous in that it is not necessary to take any special means for evacuation, since it can utilize the strong flywheel effect caused by the steel tapping flow.

In order to seal the atmosphere inside the reactor chamber from the atmosphere,
There are methods such as once receiving the molten metal in a pony ladle and then draining it out (Fig. 2), and methods in which inert gas or reducing gas is blown into the reaction vessel.

In addition, it is desirable to increase the melt specific surface area in order to increase the reaction rate, but for this purpose, it is also possible to spray a gas or the like into the degassing flow, or to make it into a spray, as in the conventional degassing method. Alternatively, the molten metal may be poured into a reaction vessel placed under reduced pressure via a pony ladle, and the molten metal may be turned into droplets by the gas generated. In either case, part of the flux may be placed in the reaction vessel, and the remainder may be added as appropriate. 'b' is a method of adding flux to the molten metal held in a ladle while suppressing the entry of air into the molten metal.

Methods for this include blowing inert gas such as Ar or reducing gas into the moat.

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

If the amount of flux added is large (if the temperature drop of the molten metal is large, it is desirable to use heating together, but when using an arc as a heating source, even if a small amount of air enters the atmosphere, oxygen and Since nitrogen is activated and has a large influence on the reaction, it is effective to carry out the treatment while, for example, making the electrode hollow and supplying an inert gas such as Ar or a reducing gas to the arc portion through the hollow electrode.

After the dephosphorization reaction is carried out in the manner described above, the flux and the molten metal are subsequently separated in the third step.

After the completion of the second step, if the atmosphere becomes oxidizing, double phosphorus is likely to occur as described above, so it is desirable to perform the separation in a state where the flux is not exposed to the oxidizing atmosphere as much as possible. In addition to the conventional method, there are two ways to do this: attach a nozzle to the bottom of the reaction vessel, open the nozzle to allow the molten metal to flow out, and then close the nozzle just before the flux begins to flow out of the reaction vessel. It is preferable to use a method that allows flux to remain inside.

In addition to the C level obtained in the first step, the molten metal separated in this way is carburized by 0.1 to 0.8% in the second step due to the release of metallic calcium. C%
is higher than the predetermined product C%.

Therefore, in the fourth step, final decarburization is performed to a predetermined value. As a method, vacuum acid blowing method, Ar-02 bottom blowing method, etc., which are conventionally used for decarburizing high chromium steel, may be used.

Note that if the flux separated in the third step is left in the atmosphere, it will react with 20 and generate odorous gases (PH3, 2S, acetylene, etc.), so it must be stabilized by, for example, oxidation treatment. .

As described above, the present invention is a method* for most rationally applying dephosphorization using a flux containing calcium carbide and alkali metal halides as main components to a stainless steel melting process on an industrial scale. In particular, the key point is to solve the C% problem associated with using relatively inexpensive calcium carbide as a raw material to generate the metallic calcium necessary for refining, while making the most of the current smelting process. .

In addition, since desulfurization also progresses along with dephosphorization, it is also possible to omit the desulfurization step that was conventionally performed at the stage of molten ferrochrome, hot metal, or molten stainless steel.

Example '11 Ordinary steel scrap, stainless steel scrap, high carbon ferrochrome and ferronickel were melted in an electric furnace, primary decarburization was performed with blowing acid, and after exhaustion, steel was tapped.

At that time, as shown in Figure 2, the molten steel is placed in a pony ladle 5.
Powdered flux was added from the hopper 6, a mixed gas of Ar and propane (Ar:propane = 1:1) was supplied from the gas inlet 2, and the mixture was placed at the bottom of the ladle. Bubbling was performed through the porous plug 3. After tapping the steel, the sliding nozzle 1 was opened to receive the molten steel into another ladle, and the nozzle before the flux was discharged was closed to separate the molten steel and the flux.

The molten steel was transferred to a vacuum tank and decarburized using an oxygen blowing sickle. The composition and amount of the flux used and the analytical values of the molten steel components are as follows. {2) After decarburizing and dephosphorizing the melt gun in a converter,
After adding high carbon ferrochrome and decarburizing, the steel is tapped. The dephosphorization operation during tapping and subsequent steps are almost the same as in Example 1. The crude composition and amount of the flux used, as well as the analytical values of the molten steel components, are as follows.

[Brief explanation of the drawing]

Figure 1 shows the C% of the molten metal, the amount of flux used, and the amount of flux used when high chromium molten steel with Cr: 16 to 20% was dephosphorized using a calcium carbide-calcium fluoride calcium monoxide flux. FIG. 2 is a diagram showing the relationship between the ratios, and is a diagram showing an example of equipment for performing the dephosphorization treatment of the present invention on the hot water discharged from the steelmaking furnace. 1: Sliding nozzle, 2: Gas inlet, 3: Porous plug, 4: Lid, 5: Pony ladle, 6: Hopper, 7: Flux, 8: Molten steel. Figure 1 Figure 2

Claims (1)

[Claims] 1 The first step is to mix an iron source and a chromium source to make a high-chromium molten metal.
a second step in which the molten metal is dephosphorized by reacting it with a flux containing calcium carbide and alkaline earth metal halides as main components in a non-oxidizing atmosphere, and a third step in which the molten metal and the flux are separated. and a fourth step of decarburizing the molten metal to a predetermined C%.