JPS6029409A - Method for decarburizing and dephosphorizing molten steel - Google Patents

Abstract

PURPOSE:To decarburize and dephosphorize efficiently molten steel refined to specified carbon and oxygen contents by blowing a converter by adding a flux contg. sodium carbonate to the molten steel. CONSTITUTION:Molten steel refined to <=0.1% carbon content and >=200ppm oxygen content by blowing in a converter is efficiently decarburized and dephosphorized by adding a flux contg. sodium carbonate to the molten steel. Soda ash, sodium hydrogencarbonate or sodium potassium carbonate, especially soda ash is used as the sodium carbonate.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a decarburization/dephosphorization method for molten steel, and particularly to a decarburization/dephosphorization method for producing ultra-low quenching steel, which is highly practical from an industrial perspective.

Hot metal tapped from blast furnaces etc. contains a large amount of components such as C+S and PtSi, so these are refined by converter blowing in the subsequent process, and generally C50.2%,
S<0.03%, P<0.03%, Si<0.01
%, respectively, and are removed and used as steel materials (1).

However, the conditions under which steel is used have become increasingly severe in recent years; for example, in the Arctic Circle, it is used as a low-temperature structural material in harsh environments with extremely low temperatures, and as a marine structural material, it is highly corrosive. used in the environment. Therefore, steel materials used under such harsh conditions are required to have higher quality in order to exhibit the required material properties, and such demands are increasing more and more.

In response to such high quality, S is required as an impurity.

Possible methods include removing P, etc., and adding expensive alloying elements to achieve high alloying, but even when high alloying is desired, it is necessary to remove impurities in order to improve the yield. It is essential. Furthermore, a decrease in C is desired in order to prevent carbide precipitation as much as possible in order to improve corrosion resistance, weldability, and even toughness.

By the way, various methods for removing S, which are thought to particularly harm the toughness of steel, have been studied and examined, and some of these methods have already been implemented.Today, for example, hot metal pretreatment methods are used to remove S< Ultra-low sulfur steel of 0.003% has been put into practical use.

On the other hand, P can be removed by oxidation by adding compounds such as Ca or Na to the molten metal during hot metal pretreatment or converter blowing.
In this case, the removal of other elements, such as C+Si, was not always satisfactory because there was no strong removal means such as C+Si. Industrially, for example, when using a Ca-based flux using a Ca compound, P≧0.02
% is considered to be the limit, and in the case of Na-based flux using a Na compound, P≧0°01% is considered to be the limit.

In addition, in recent years, Na20-3i02 (sodium metasilicate)
A method for producing ultra-low sintering steel with P≦0.005% has been proposed in which dephosphorization is performed during ladle refining after converter blowing using system flux, but sodium metasilicate is extremely expensive; There are problems with the practical application of such methods.

Furthermore, with regard to C, reducing the C content to 0.01% or less requires a separate process with complicated operations such as vacuum treatment, but such treatment is not only expensive in itself; Manufacturing costs are extremely high due to the small processing capacity.

In this way, methods that use sodium metasilicate in particular for dephosphorization, and methods that require vacuum treatment for decarburization have high processing costs, so they cannot be used as general-purpose steel materials, for example. It is difficult to put this into practical use in mass-produced steel types.

Thus, an object of the present invention is to provide a decarburization and dephosphorization method that is cost-effective and sufficiently practical for the production of low-carbon, extremely low-temperature steel.

Another object of the present invention is to provide an easy and industrially practical decarburization/dephosphorization method as a pretreatment for molten steel of high alloy steel or low alloy steel.

By the way, even in the past, sodium-based flux, mainly Na2, has been used for desulfurization and dephosphorization in hot metal pretreatment.
Go 3 (soda ash) was used, and the dephosphorizing ability of soda ash was widely known.

However, what was conventionally known was the dephosphorization of hot metal, but nothing was known about molten steel at high temperatures exceeding 1,600°C, and it either sublimated unreacted or violently dephosphorized. The only consideration was whether molten steel would scatter due to the reaction. No industrial experiments or studies have been carried out. In fact, according to the results of preliminary experiments conducted by the present inventors, the boiling phenomenon of the /8 water and the scattering of sodium gas occurred violently, and dephosphorization was not achieved.

As a result of various studies to achieve the above objectives, the present inventors have found that when adding sodium carbonate such as soda ash to molten steel, the radical reaction can be suppressed by treating it under specific conditions. The present invention was completed based on the discovery that dephosphorization can be carried out efficiently and decarburization is unexpectedly promoted.

That is, first of all, the following chemical reaction between soda ash and molten steel can be considered: Na2CO3+2C-
2Na (g) +3CO... (1) Na2CO3
+Fe =2Na (g) +PeO+CO2...
(2) Na2CO3+ 415 P = Na'20 + 21
5 P205 +C... (3) Among these, the reactions of formulas (1) and (2) are reactions that result in a loss of Na content, but the reaction of formula (2) proceeds slowly.

In addition, the reaction of formula (1) causes a boiling phenomenon of molten steel due to the generation of -carbon oxide, and also generates white smoke mainly composed of Na (g), that is, sodium gas, and progresses violently. It was thought that soda ash treatment of molten steel would not be possible in the high temperature range. However, the present inventors suppressed the reaction of formula (2) by adjusting the C50 to 10% by blowing molten steel in a converter in advance, while suppressing the reaction of formula (2).
They have discovered that the reaction 1) can proceed gently.

Furthermore, regarding the reaction of formula (3) that governs dephosphorization, this reaction is dependent on the basicity (Na20/SiO2) in the slag.
), and it was found that the higher the basicity, the more rapidly the formula (3) progresses, and the higher the dephosphorization efficiency. In this case, at the end of converter blowing, almost all of the Si becomes slag and floats on the surface of the molten steel, so it is important to add Na2O in an amount equal to or more than 5i02 in the slag. In other words, Na2O/Si02≧1. Regarding the adjustment of basicity,
Even if sodium metasilicate is used as sodium oxide instead of sodium carbonate, the basicity of sodium metasilicate is Na20/5i02 = 1.0, so even if a large amount is added, the basicity will be 1. will not exceed. Therefore, it has been found that even when sodium silicate is used in part to suppress the generation of sodium gas, addition of another Na2O source, that is, addition of soda ash, is necessary to adjust the basicity.

Here, the gist of the present invention is characterized in that a flux containing sodium carbonate is added to molten steel refined by converter blowing to a concentration of 0.1% or less carbon and 200 ppm or more oxygen. , is a method for decarburizing and dephosphorizing molten steel.

Preferably, in the present invention, a flux containing sodium carbonate is added to molten steel obtained by converter blowing hot metal that has been subjected to desiliconization and/or desulfurization treatment.

Here, the decarburization/dephosphorization mechanism as described above in the present invention will be summarized as follows. In addition, in the following explanation, only an example in which soda ash (Na 2 GO 3 ) is used as a flux containing sodium carbonate is mentioned, but the present invention is limited thereto due to its nature. It is understood that this is not the case.

First, the C content in the molten steel is reduced to 0.1 by converter blowing in advance.
% or less, the reaction of formula (1) above is sufficiently suppressed, and excessive generation of CO gas and sodium gas is eliminated. The reaction proceeds relatively slowly. In a preferred embodiment, the carbon content is generally 0.05% or less, preferably 0.03% or less.

In this way, in the present invention, the progress of equation (1) is alleviated by lowering the absolute amount of carbon content, but unexpectedly, decarburization according to equation (1) is It has been found that it is possible to reduce the carbon content to levels previously possible only through special vacuum treatments.

On the other hand, the dephosphorization reaction proceeds according to formula (3), but in the present invention, as described above, the reaction of formula (1) is partially suppressed, so formula (3) is relatively promoted, and When sodium carbonate is added to reduce Na loss due to the reaction of formula (1), the basicity of the slag increases in accordance with the amount added, and the dephosphorization reaction also progresses accordingly. The synergistic effect of adding sodium carbonate on the dephosphorization reaction is remarkable.

In the present invention, in order to suppress the generation of sodium gas due to the reaction of formula (1), a part of this sodium carbonate may be replaced with, for example, sodium metasilicate. According to formula (3), some carbon is liberated, but
Most of this is oxidized to become CO gas, and the formula (1
) is removed by soda ash.

Here, the sodium carbonate used in the present invention includes Na 2 CO 3 (soda ash), Na 1 (Co
3 (sodium hydrogen carbonate), KNaCo 3 (sodium potassium carbonate), etc., but soda ash is most generally preferred. Furthermore, ores containing Na 2 Go 3 and Na11C03 as main components, such as trona ash and magazi ash, can be made into lumps or powder and used as they are as Fura Nox.

Next, according to the present invention, the amount of oxygen in molten steel, that is, the amount of dissolved oxygen, is limited to 200 ppm or more. Usually, the lower the amount of carbon in the molten steel, the more oxygen is contained in the molten steel after converter blowing, and even in the present invention,
When the dephosphorization reaction progresses according to equation (3), if the amount of oxygen in the molten steel is large, the oxygen potential increases, and at the same time the dephosphorization reaction progresses strongly, the 02 source generated from the Na-based flux is wasted and absorbed into the steel. You can prevent this from happening. Therefore, in the present invention, the higher the amount of oxygen in the steel, the more desirable. In the present invention, since the C50 is limited to 1%, the amount of oxygen contained at this time is 20.
The lower limit is 0 ppm. Preferably, the amount of oxygen in the steel is 30
It is 0 ppm, more preferably 500 ppm or more.

By the way, P blows and taps regular paper in a converter, and then blows it in the same converter with hot metal for ultra-low sintering steel. A so-called phosphor knob phenomenon in which P migrates into the interior is observed. However, according to the present invention, it has been found that an efficient dephosphorization reaction can be carried out despite such phosphorus pickup. Therefore, there is no need to provide a dedicated converter for ultra-low quenching steel, and there is no need to perform any special work prior to charging hot metal into the converter, which also allows for inexpensive operation.

In the present invention, the method of adding flux containing sodium carbonate to molten steel is to add it to the molten steel in the converter as it is after the completion of converter blowing, and at the same time add it to the molten steel through the tuyere provided at the bottom of the converter. Either method can be used, such as stirring the molten steel by blowing inert gas into the steel, or adding the slag as described above in the ladle after tapping, but the less slag on the top of the molten steel, the better. Since the decarburization and dephosphorization efficiency is improved, it is preferable to add the slag in the ladle after removing the slag.

After decarburization and dephosphorization, it is sent to the ingot-forming process (or continuous casting process) as it is or after further processing such as adding alloying elements if necessary.

Next, the present invention will be further explained in connection with examples.

Hot metal from the SJLLL blast furnace was processed according to the following process order. At this time, the components of the hot metal and molten steel in each step were as shown in Table 1.

(i) After the hot metal was received in a torpedo, 23 kg of sintered ore containing iron oxide as a main component was added to the hot metal in an amount of 23 kg per ton of hot metal to perform desiliconization treatment.

(ii) Next, the hot metal is transferred from the topedo to a receiving ladle to remove slag, and then 4 kg of Na2C is added per ton of hot metal.
Desulfurization treatment was performed by blowing O3.

(iii) After removing the desulfurization slag, in order to perform the treatment according to the present invention, the hot metal is transferred to a 15-ton test converter having a structure that allows oxygen gas to be blown from the upper part and various gases to be blown directly from the bottom through the slats, In this converter, 15 kg of slag forming agent (CaO) was added per ton of hot metal, and converter blowing was performed by blowing oxygen gas from the upper part and CO2 gas from the tuyere at the lower part.

(iv) After performing converter blowing for a predetermined period of time, CO2 gas is blown into the molten steel from the lower part and while stirring the molten steel, 1 ton of molten steel is continuously heated from the upper part of the molten steel for 2 minutes.
0 kg of Na2Go3 was added.

Table 1 As is clear from the results shown in the above table, according to the present invention, despite the relatively simple method of adding soda ash, dephosphorization was achieved from 0.085% to 0.008%. Not only that, but also carbon was removed to 0.01%. Normally, a carbon content of 0.01% or less is a level that cannot be achieved without special vacuum treatment.

In this example, hot metal from a blast furnace was processed according to the following process order. At this time, the components of the hot metal and molten steel in each step were as shown in Table 2.

(i) After receiving the hot metal in the torpedo, 5 per ton of hot metal
．． Desulfurization treatment was carried out by adding 5 kg of Na 2 CO 3 into the hot metal.

(ii) Next, the hot metal was transferred from the torpedo to a receiving ladle to remove the slag, and then the hot metal was transferred to the same 15 ton test converter as used in Example 1 to carry out the treatment according to the present invention. Converter blowing was performed by adding 70 kg of slag forming agent (CaO) per ton of hot metal in the converter, and blowing oxygen gas from the upper part and CO2 gas from the lower part.

(iii) After performing converter blowing for a specified period of time, transfer the i9 steel to a ladle while cutting the slag, then blow a metered gas from the bottom of the ladle and continue stirring the molten steel, and pour 1 ton of molten steel from the top of the ladle. 3.5 kg of Na2Co per
3 was added.

Table 2 ■:) As is clear from the results summarized in Table 2, according to the present invention, despite the relatively simple method of adding soda ash, the Not only was dephosphorization achieved up to 0.01%, but also carbon was removed to 0.01%.

1 Based on 1 In this example, Example 1 was repeated, but in that case, sodium metasilicate was mixed into the molten steel dephosphorizing agent added to the molten steel after converter blowing, and the slag basicity was varied in various ways. At that time, the ratio of P2O5 in the slag to the P content in the molten steel was determined.

The results are shown graphically in the accompanying drawings.

As is clear from the illustrated results, the P2o's concentration in the slag, that is, (P2
．． O5) and (7) pH degree in i steel, that is, the ratio of [P], (P 20 s ) / (P) = 200 or more is required. Therefore, from this, the slag basicity is i, o
It can be seen that this is necessary.

The present invention has been described in detail above, but as is already clear to those skilled in the art, soda ash (Na 2 C03
) decarburization and dephosphorization of molten steel is a method of decarburizing and dephosphorizing molten steel.
By combining this with desiliconization), it is possible to easily and inexpensively manufacture low-carbon, ultra-low-temperature steel.

Thus, the present invention is an industrially excellent method that makes it possible to relatively easily produce excellent ultra-low quenching steel using an inexpensive treatment agent, and makes a great contribution to this field.

[Brief explanation of the drawing]

The attached drawing is a graph showing the relationship between the ratio of (P20s)/(P) and slag basicity in the method of the present invention. Applicant Sumitomo Metal Industries Co., Ltd. Representative Patent Attorney Akira Hirose - 0,20,40, 61,02461020 (Na20)
/ (SiO2)

Claims (1)

[Claims] A method for decarburizing and dephosphorizing molten steel, which is characterized by adding a flux containing sodium carbonate to molten steel that has been refined to 0.1% or less carbon and 200 ppm or more oxygen by converter blowing.