JPH0621288B2 - Dephosphorization method for molten metal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention comprises adding a rare earth metal (hereinafter abbreviated as “RE”) to a molten metal and stirring the mixture under a non-oxidizing atmosphere. The present invention relates to a method for removing phosphorus by levitating phosphorus as a phosphide of RE.

In particular, the present invention relates to a dephosphorization method for a molten metal containing an element such as Cr or Mn that is easily oxidized in a non-oxidizing atmosphere.

(Prior Art) As is well known, metal, such as phosphorus, contained in steel significantly deteriorates the quality and performance of steel such as brittleness and corrosion cracking. It has been a big target. However, as the conventional dephosphorization method, the use of oxidizing slag has been the mainstream.For low phosphorus high alloy steels, etc. Iron alloy iron has been added to molten steel melted by adding it at the end or from the beginning by blending scrap with low phosphorus, but low phosphorus alloy iron is expensive in itself and has a low phosphorus content. However, if the amount of iron alloy added increases, the phosphorus content of the molten steel increases.

Further, in refining molten metal containing elements that are easily oxidized, such as oxidation refining for high chromium steel, dechromization is prioritized over dephosphorization, and there are problems to be solved in the dephosphorization method using oxidizing slag. It was

On the other hand, some so-called reductive dephosphorization methods have been proposed in the past with respect to the dephosphorization method using the oxidizing slag, and if they are collectively classified based on the dephosphorization mechanism, they can be described as follows. it can.

(1) Method using alkaline earth metal and its halide (see, for example, Japanese Examined Patent Publication No. 51-20287): This method uses one or more of calcium, magnesium, strontium or barium fluoride. This is a method of using a slag as a main component, and adding to it one or more of calcium, magnesium, strontium, and barium in an amount of 0.1 to 60% of the total slag weight. That is, Ca-Ca
Phosphorus is removed in the form of Ca ₃ P ₂ using an F ₂ -based flux. Among these methods, among others, a method using calcium and its fluoride has been mainly studied, and it is known that the effect is great.

However, since this flux significantly corrodes the oxide refractory, there is a problem that its application is limited to the "ESR" process using a water-cooled copper mold or the like without such a problem.

(2) Method using CaC ₂ and halide of alkaline earth metal (eg, see JP-A-53-1619): This method is basically the same as the method (1) above, but CaC instead of Ca ₂ , the dephosphorization proceeds with Ca generated by the decomposition reaction of CaC ₂ represented by the following formula.

CaC ₂ Ca + 2 [C] As is clear from this, for example, when the molten steel [C] is high, the decomposition reaction of the above formula itself does not occur, and as a result, the dephosphorization reaction does not proceed. From this,
The target molten metal has a high degree of carbon unsaturation,
That is, it should not be high carbon.

(3) CaC ₂ method using only (e.g. see JP-A 55-134123): This method is essentially the same as the method (2). The difference is that the combined use of alkali metal halides is not necessary, but the limitation on the [C] content of the molten metal is the same as in the above method (2).

(4) Method using CaF ₂ and RE (Example: JP-A-55-97420
No.): This method was invented by the present inventors. That is, it was found as a result of investigations to develop a dephosphorization method similar to the method (1). Therefore, the processing method, as can be seen from the embodiment,
It is intended for the ESR method and cannot be agitated in the molten metal due to the layered solidification, and as described below, the generated phosphide does not have enough time to float, and as a result, as in the example, The effective dephosphorization reaction did not proceed unless the amount of flux relative to the amount of steel to be treated was increased.

As described above, conventionally, several methods are known as a dephosphorization method in a non-oxidizing atmosphere, including a method of adding RE in combination.

By the way, regarding addition of RE to molten steel by methods other than the dephosphorization method, magazine "Iron and Steel" vol. 62, No. 13, 1976, p.
p. 1653-1667, when RE in molten steel is added, R
E-sulfide and RE-oxysulfide preferentially precipitate, [S]
Of MnS during solidification (during hot rolling, it spreads in the rolling direction, so mechanical properties in the thickness and width directions (ductility)
It is known that it is possible to prevent the precipitation of].

However, if RE is too much with respect to [S], segregation occurs in the precipitated crystal zone of the ingot, which causes deterioration of mechanical properties. From this, [% RE] / [% S] is 2.7
It is said that it is necessary to limit the range to ~ 8.0, and more strictly to the range 3.0 to 6.0. By the way, recent [S]
Is approximately [S] ≦ 0.010% due to the progress of the desulfurization method. Therefore, the maximum amount of RE is [% RE] ≦ 0.
It will be 080%.

(Problems to be Solved by the Invention) Thus, the object of the present invention is to provide an efficient dephosphorization method which eliminates the above-mentioned drawbacks of the conventional method.

Another object of the present invention is to provide an excellent method for dephosphorization from molten steel metal using RE, which is held in a non-oxidizing atmosphere and contains elements such as chromium and manganese which are easily oxidized. is there.

Further, the object of the present invention is to perform deoxidation by adding RE to a molten metal containing an element that is easily oxidized such as chromium and manganese in a non-oxidizing atmosphere prior to oxidative refining in a series of refining processes. It is to provide a phosphorus method.

(Means for Solving the Problems) As described above, the present inventors have proposed a method of dephosphorizing in the ESR step using a flux composed of RE and CaF ₂ , but such a method is the ESR step. The existence of slag is essential because
It required the combined use of CaF ₂ . The ESR method is a method of melting the electrode (material to be melted) by Joule heat generated by electricity flowing in the molten steel slag.

Therefore, if it is not applied to the ESR method, it is not necessary to add CaF ₂ , and there should be no problem of furnace wall erosion.

However, as a result of continuing research after that, RE has a density close to that of Fe, so that its phosphide also slowly floats, and
It has been found that when the floating time is short, as in the case of laminated solidification such as the R method, the floating cannot be completed.

Here, according to the knowledge of the present inventors, RE is added alone to molten metal in a refractory container such as a ladle, not in the ESR step, to promote the floatation of the generated phosphide RE. Therefore, by adding "stirring" to the molten metal, it is possible to achieve better dephosphorization than the conventional method, and after completion of dephosphorization, remove the generated slag, and then perform oxidative refining. Even if a very large amount of RE is used to remain in the molten metal, it can be preferentially oxidized and removed, and if necessary, can be continuously subjected to treatments such as decarburization and desulfurization.

Therefore, the gist of the present invention is that the rare earth metal is added to the molten metal held in a non-oxidizing atmosphere in an amount such that the amount of the rare earth metal in the molten metal is 0.1% or more. This is a method for dephosphorizing molten metal, which comprises stirring the molten metal to remove the generated slag, and then oxidizing and refining the obtained molten metal.

Here, as the molten metal to be treated, any of Fe group, chromium group, and manganese group may be used, but considering the treatment cost, it is difficult to carry out by ordinary oxidative dephosphorization, that is, the affinity with oxygen is higher than that with phosphorus. Molten metals containing high elements are the main target.

Therefore, as a typical example, this molten metal is: Fe
Those containing chromium and / or manganese and / or silicon in an amount of, for example, 1% or more, for example, stainless steel melt, high manganese steel melt; ferromanganese;
And ferrochrome are equivalent.

The carbon content in the molten metal affects the dephosphorization according to the present invention. That is, when the carbon content becomes saturated, the dephosphorization rate decreases. Although the reason is not always clear, it is presumed that RE forms carbides and is not effectively used for phosphorus removal. Therefore,
As the carbon content of the molten metal, it is desirable to avoid the saturated state.

The treatment temperature is generally in the range of 1200 to 1700 ° C, preferably 1300 to 1600 ° C, and lower temperature is more advantageous for dephosphorization.

According to the method of the present invention, by adding a relatively large amount of RE, 0.1% or more of RE remains in the molten metal. Of course, if there is no problem even if this RE is contained, the dephosphorization method of the present invention may be used in the final treatment step of the molten metal. However, in general, when RE comes in,
Since there is a problem, it is desirable to carry out oxidative refining after removing the slag after this treatment. That is, RE that has entered the molten metal becomes an oxide in the oxidative refining process and is floated and removed. For example, in the manufacturing process of stainless steel, the dephosphorizing method of the present invention is applied at the stage of the crude molten steel before decarburizing with AOD or VOD, and then the slag containing phosphide floating on the surface of the molten metal is removed. Then, when the process proceeds to a normal decarburization process, RE in the molten metal is oxidized and removed in this process. In other words, the normal decarburization step and the RE oxidation removal step are linked.

As RE (rare earth metal), any of Ce, La, Nd, etc. may be used, but in terms of cost, misch metal (Ce 50%,
La 30%, Nd 15%, etc.) is advantageous. In addition, the amount of addition should be at least 0.1%, that is, 1 kg / t. On the other hand, the larger the amount, the better the dephosphorization, and there is no particular limitation. The following is considered the upper limit. In either case, it is important to add a relatively large amount so that the RE amount in the molten metal is 0.1% or more.

As a matter of course, the smaller the addition amount, the lower the dephosphorization rate, but in order to obtain an effective dephosphorization rate, as described above, the amount of RE in the molten metal (% by weight) is 0.1%. %
It is necessary to add the amount necessary for the above, and the amount of RE at this time is larger than the amount added for the purpose of adjusting the morphology of ordinary sulfides. Normally, RE <
It is 0.1%.

The processing container is preferably a ladle lined with ordinary refractory material. That is, it is not effective to add RE at the time of pouring the molten metal into the mold as in the case of adjusting the sulfide form. This is because, as described above, the phosphide produced is disadvantageous in floating when added in the template. In addition, RE
The addition means of is not particularly limited, and may be simply added.

In the method of the present invention, levitating RE phosphide
It is extremely important for promoting dephosphorization. Agitation is essential for this purpose. As a stirring method for that purpose, any method can be used as long as it can stir by bubbling with an inert gas such as Ar or induction stirring.

The atmosphere during the dephosphorization treatment needs to be a non-oxidizing atmosphere because the present invention is reduction dephosphorization. Usually, it is better to process in Ar atmosphere. Of course, a reducing atmosphere may be used.

The oxygen content of the molten metal before the treatment should be low. this is,
This is because when the oxygen is high, the added RE is consumed by oxidation. Therefore, if the molten metal has a high oxygen content, it is preferable to add a deoxidizing agent such as Al in advance and deoxidize it before performing the main treatment.

Further, according to the present invention, N removal progresses at the same time as phosphorus removal, as also specified in the following examples.
Therefore, the method of the present invention is extremely effective as a de-N method.

In the conventional dephosphorization method described above, particularly in the dephosphorization method using CaC ₂ , if the carbon content of the molten metal becomes too low, the decomposition reaction of CaC ₂ becomes too fast,
The Ca produced by decomposition evaporates before it contributes to dephosphorization, resulting in a decrease in the dephosphorization rate. However, in the dephosphorization method of the present invention, the boiling point of RE used as a dephosphorizing agent is higher than that of Ca (1440 ° C) (Ce 346
8 ℃, La 3470 ℃), so there is no such problem,
Dephosphorization is possible even when there is almost no carbon content [Example: (C) ≦ 0.01%].

Next, the present invention will be described in more detail by way of examples.

Example In an Ar atmosphere, 2 kg of a metal or alloy having the composition shown in Table 1 was melted in a MgO crucible, and then misch metal (Ce
40% of 50%, La 30%, Nd 15%, Pr 4%, etc.)
Then, induction stirring or Ar bubbling stirring was performed. The test conditions are summarized in Table 2. After processing for 20 minutes,
The composition of the refined molten metal was analyzed to determine the dephosphorization rate. In all cases, 0.1% or more of RE remained in the molten metal after the dephosphorization treatment. In this way, R remaining in the molten metal
After dephosphorization, E was able to be substantially removed by the decarburization operation by changing the non-oxidizing atmosphere to an oxidizing atmosphere and performing oxidative refining.

These results are summarized in Table 3. Test No. 1
The same test procedure is shown in Tables 3 to 3.

In the dephosphorization treatment of Test No. 1, the change in phosphorus content with respect to the dephosphorization treatment time is plotted and shown in the graph of the attached drawing (graph shown by a black circle).
You can see that it saturates in 10 minutes. Similarly, looking at nitrogen, as shown in the figure (the white triangle graph),
It can be seen that the amount of nitrogen is reduced to less than 1/3 in 10 minutes.

(Effect) As is clear from these results, according to the present invention,
In particular, the dephosphorization effect varies depending on the [C] level and the treatment temperature, but it is possible to dephosphorize a molten metal containing elements such as chromium, manganese, and silicon that are easily oxidized without substantial loss of these elements. .

[Brief description of drawings]

The accompanying drawings are graphs showing the dephosphorization effect of an example of the method of the present invention.

Claims (1)

[Claims] 1. A rare earth metal is added to a molten metal held in a non-oxidizing atmosphere in an amount such that the amount of rare earth metal in the molten metal is 0.1% or more, and the molten metal is stirred. A method for dephosphorizing molten metal, which comprises removing the generated slag and then oxidizing and refining the obtained molten metal.