JPS6086204A - Manufacture of low-carbon and low phosphor steel - Google Patents

Abstract

PURPOSE:To enable industrial manufacture of low-carbon and low-phosphor steel by specifying carbon and oxygen contents of molten pig iron containing a specific silica amount in a converter through injection of oxygen for refining without adding a slagging agent and carrying out decarbonization and dephosphorization with a flux containing sodium carbonate. CONSTITUTION:Prior to oxygen injection into a converter for refining, molten pig iron is preliminarily treated so that it may contain <=0.1% of Si and <=0.01% or S. Next, after removing slag sufficiently, oxygen is injected into a converter for refining with addition of only a protective agent for refractories and virtually without a slagging agent to make molten steel containing less than 0.1% of carbon and more than 200ppm of oxygen. Further flux containing sodium carbonate is added to carry out decorabonization and dephosphorization. Thus it is possible to obtain low carbon and low phosphor steel.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing low carbon and low phosphorus steel, which is industrially convenient and highly practical.

Hot metal obtained from blast furnaces etc. contains polycyclic components such as C, S, P, and Si, 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.

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 extremely low temperatures, and as a marine structural material in highly corrosive environments. used in 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.

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, with regard to S, which is thought to particularly impair the toughness of steel, various studies and examinations have been conducted to find ways to remove it.
Some of these methods have already been implemented, and today, ultra-low sulfur steel with S<0.003% has been put into practical use, for example, by hot metal pretreatment methods.

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, S, etc., was not always satisfactory because there was no strong removal agent available. Industrially,
For example, in the case of Ca-based flux using Ca compounds, 290,602% is considered to be the limit, and N
When using Na-based flux using a compound, P≧0
．． 0.1% is considered to be the limit.

In addition, in recent years, Na 20-3 is used during ladle refining after converter blowing.
A method for producing ultra-low hardening steel with P≦0.005% has been proposed in which dephosphorization is performed using i02 (sodium metasilicate)-based flux, but sodium metasilicate is extremely expensive1.
There are also problems in practical application of such a method from the economic point of view.

Furthermore, in order to reduce C to 50.01%, complex operations such as vacuum treatment are required as a pre-process (and expensive treatments are required; however, such treatments are not only expensive in themselves; (The manufacturing cost is 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, it is an object of the present invention to provide a method for producing low-carbon, low-t'lE steel that is reasonably practical and suitable for effective decarburization and dephosphorization methods. It is.

Another object of the present invention is to provide low-carbon, low-3'A steel that can be easily and industrially applied to a decarburization/dephosphorization method as a pretreatment for molten steel of high-alloy steel or low-alloy steel. An object of the present invention is to provide a manufacturing method.

By the way, even in the past, sodium-based flux, mainly Na2, has been used for desulfurization and dephosphorization in hot metal pretreatment.
Co 3 (soda ash) has been used, and the ability of such soda ash to dephosphorize hot metal has been widely known. However, what was conventionally known was the dephosphorization of hot metal, but nothing was known about molten steel, especially molten steel at high temperatures exceeding 1600°C. It was only considered that the molten steel would be scattered due to a violent reaction. No industrial experiments or inspections were conducted. In fact, the results of preliminary experiments conducted by the present inventors show that the boiling phenomenon of molten steel and the scattering of strium gas occurred violently, and dephosphorization was not achieved.

Under these circumstances, the inventors of the present invention have continued research for many years, and first developed a decarburization method by adding a flux containing thorium carbonate to molten steel with 0.1% carbon or less and 200 ppm or less oxygen. proposed a dephosphorization method (patent application 1986-135)
966) continued their research and found that there was no flux loss when the above-mentioned thorium carbonate-containing flux was added to 78 steel in the absence of slag.7!
It has been unexpectedly found that not only desalination but also decarburization is significantly promoted due to the synergistic effect of not requiring adjustment of basicity. On the other hand, when adding sodium carbonate salts such as soda ash to molten steel9, by treating them under specific conditions and adding them in parts, radical reactions can be suppressed and removed efficiently. The present invention was completed by discovering that charcoal and dephosphorization were unexpectedly promoted.

Thus, the present invention provides a process that does not require expensive treatments such as vacuum decarburization when producing low carbon and low phosphorous steel, and can efficiently perform decarburization at the same time as descaling, which has been a problem in the past. This provides an in-furnace decarburization/dephosphorization method, and its gist is that SiS2.
1%, preferably S≦0.01% and SiS 2.1%, pre-treat the hot metal, thoroughly remove the slag, and then add a refractory protectant as necessary.
i! in the converter without adding substantially any slag forming agent! ! Bare blowing with less than 0.1% carbon and more than 200 ppm of oxygen /
This is a method for producing low-carbon, low-range steel, which is characterized by performing SSi2, followed by adding a flux containing thorium carbonate to the self-melting steel in a converter to perform 1 (2 carbon removal 3A).

Furthermore, the present invention first provides SiS2.
1%, preferably S≦0.01% and SiS 2.1%, pre-treat the hot metal, thoroughly remove the slag, and then add a refractory protectant as necessary.
Without substantially adding any slag-forming agent, molten steel with 0.1% or less carbon and 20ppm or more oxygen is produced by oxygen blowing in a converter, and then a sodium J carbonate-containing flux is added to the molten steel in the converter. is added to decarburize and dephosphorize, resulting in zero carbon
．． 02% or less, and final dephosphorization is carried out by adding a sodium carbonate-containing flux to the tapped molten steel flow or the molten steel in the ladle during or after tapping the steel into the ladle.

Thus, according to the present invention, in order to reduce slag formation during converter blowing as much as possible, prior to converter blowing, preferably S≦0.01%, SiS2°1% After pre-treating the hot metal and thoroughly removing the slag, it is then heated to C50. %, oxygen≧200 ppm, and then a flux containing thorium carbonate is added to the molten steel in a converter to decarburize and dephosphorize it. The above-mentioned flux may be added to the molten steel by directly injecting it onto the surface of the molten steel immediately after oxygen absorption, but it may also be added to the molten steel by entraining it with a gas for stirring the molten steel and blowing it into the molten steel together with the gas. It is preferable to do so.

There have been various proposals for hot metal pretreatment, and the present invention is not limited to any particular one.

For example, regarding the desiliconization treatment, an oxygen source such as iron oxide may be added to the hot metal in a blast furnace gutter or a torpedo (pig-mixing car), and Si in the hot metal may be removed as 5i02. Furthermore, regarding desulfurization treatment, CaO or Na 2 CO3 is added as a desulfurization agent in the topedo or in the pouring ladle, mixed and stirred, S in the hot metal is transferred to land, and the slag is separated and removed by draining it. You may. As mentioned above, in the method according to the present invention, blowing in a converter is performed in a state in which substantially no slag is produced during blowing, or in a state where there is very little slag. At the same time, since no slag-forming agent (e.g. desulfurization/dephosphorization agent) is added during converter blowing, it is necessary to lower the S content in advance in a preferred embodiment.
2.1%, S≦0.01%.

Note that Ca-3i is generally treated in the hot metal pretreatment as described above, but Ca-3i is added to the molten steel after converter blowing.
(calcili) may be added to perform desulfurization at the same time as deoxidation. However, if Ca-3i contains other impurities (such as C), the molten steel composition after desulfurization will change, so Ca-3i
The purity of -3i must be increased, which may result in high cost.

By the way, since S in steel is not desulfurized in an oxidizing atmosphere, it is not removed from high oxygen-containing steel even by adding soda ash unless it is desulfurized by hot metal pretreatment in the present invention.

Thus, in the present invention, SiS2.1%, S≦0
．． By pre-treating the hot metal to 0.01%, decarburization proceeds without generating slag during oxygen blowing in the converter. However, if the residual Si in the hot metal turns into slag, refractory erosion progresses, so depending on the amount of Si in the hot metal, add CaO, M, O, etc. as a refractory protectant at 10kg/molten steel
It is also possible to protect the fire resistance inside the furnace by adding only a small amount of n or less.

Next, the molten steel in the converter is melted to C50.1% and oxygen≧200ppm, preferably C50.05% and oxygen≧400ppm, and has virtually no slag on the surface. On the other hand, a flux containing sodium carbonate such as soda ash (N82CO3) is added to perform decarburization and dephosphorization treatment.

By the way, as already mentioned, sodium-based fluxes, mainly Na2CO3, have been used for dephosphorizing hot metal, and the ability of Na2CO3 to dephosphorize hot metal is known. However, the dephosphorization ability of 78 steel at high temperatures is completely unknown, and it is thought that it will sublimate without reacting, or that it will react violently and cause molten steel to scatter, so experiments on an industrial scale have not been carried out. It wasn't. However, according to the findings of the present inventors, as the carbon content in molten steel decreases, the intensity of the reaction also decreases, and the dephosphorization reaction progresses.

First, the following is the main chemical reaction between soda ash and molten steel: Na2CO3,0002G = 2Na (g) +3CO...
...(1) Na2CO3+Fq = 2Na (g)
+FeO→-co2...(2) Na2CO3+415P=Na20 +215P205
(3) In these formulas, the reactions of formulas (1) and (2) are reactions that only consume Na, but the reaction of formula (2) proceeds slowly. In addition, the reaction of formula (1) causes the boiling phenomenon of molten steel due to the generation of -carbon oxide, and also causes Na(g),
In other words, white smoke consisting mainly of sodium gas is generated and progresses violently. For this reason, it was previously thought that molten steel could not be treated with soda ash, especially in high-temperature regions.

In general, the reaction of formula () generates an extremely large amount of dust, and the reaction itself is also violent; however, in terms of the relationship between carbon content and dust generation, by setting the C content of molten steel to C50.1%, the reaction can be improved. Dust can be collected using gas processing equipment such as OG, which is normally provided in the furnace, and actual operation is possible. Note that in order to increase the dephosphorization efficiency, the lower the molten steel carbon content, the better.

On the other hand, oxidized phosphorus, that is, P2O5, is fixed on land, and in this case, the basicity of the slag (Na20/Si○2) is important. According to the results of a series of experiments conducted by the present inventors, it is preferable to maintain the basicity of such slag at Na2O/5i02≧1.
The smaller the amount of 2, the easier it is to increase the basicity of the slag, the higher the dephosphorization efficiency, and the lower the added flux basic unit (the amount of flux required per '/8 steel weight). Therefore, according to the present invention, by using hot metal that has been sufficiently desulfurized and desanded, converter blowing is performed on this hot metal without adding a slag-forming agent, and sodium carbonate is added in a later step. The included flux increases decarburization and dephosphorization efficiency.

As described above, according to the present invention, if the carbon content of the molten steel is adjusted to C50.1% by converter blowing performed in advance,
While suppressing the reaction of formula (2), the reaction of formula (1) can also be allowed to proceed gently.

Here, if the carbon content is further reduced to C50.02%, the dephosphorization reaction of formula (3) proceeds extremely well. However, in normal converter blowing, C20,035% is the limit.
Even in composite blowing, in which various gases are injected into the molten steel at the bottom of the converter, C20,030% is the practical limit. Immediately after furnace blowing, preliminary dephosphorization is carried out, mainly to promote the decarburization reaction of formula (1), but also to advance the dephosphorization reaction of formula (3).
%, in order to sufficiently remove the slag, it is efficient to transfer the molten steel from the converter to a ladle, and then mainly proceed with the dephosphorization reaction of formula (3).

Therefore, in another aspect of the present invention, a sodium carbonate-containing flux is once added to the molten steel in the converter to reduce the C5
After the concentration is reduced to 0.02%, thorium carbonate-containing flux is further added to the tapped molten steel flow or the molten steel in the ladle during or after tapping the steel into a ladle to perform final dephosphorization. At this time, since stirring of the molten steel greatly affects the processing time, it is most preferable to add flux to the molten steel flow being tapped from the converter into the ladle. However, it is naturally also possible to add the above-mentioned flux while stirring Ar gas or the like into the molten steel in the ladle.

According to the present invention, the amount of oxygen in the molten steel after converter blowing,
In other words, the amount of dissolved oxygen is limited to 200 ppm or more.

Normally, the lower the amount of oxygen in the molten steel, the more oxygen is contained in the molten steel after converter blowing.・
When the de-J-saving reaction progresses, if the amount of oxygen in the molten steel is large, the oxygen potential increases, and the decarburization and dephosphorization reactions progress strongly, and at the same time, the 02 source generated from the Na-based flux is not used to oxidize impurities. can be prevented from being absorbed into the steel. Therefore, in the present invention, the higher the amount of oxygen in the steel, the more severe it is.

In the present invention, the carbon content after converter blowing is C50.1%.
Therefore, the lower limit of the amount of oxygen contained at this time is 200 ppm. Preferably, the amount of oxygen in the steel is 300 ppm, more preferably 400 ppm or more.

By the way, P usually occurs when normal pig iron is blown and tapped in a converter, and then hot metal for ultra-low phosphorous copper is added to the same converter and blown, and the slag that adheres to the furnace walls etc. A so-called phosphorus pickup phenomenon, in which P is transferred into the hot metal, is observed. However, according to the present invention, it has been confirmed that an efficient dephosphorization reaction can be carried out despite such phosphorus pickup. Therefore, it is not necessary to provide a dedicated converter for ultra-low phosphorous copper, and there is no need to perform any special work prior to charging hot metal into the converter, which also enables inexpensive operation.

In the present invention, in order to suppress the generation of sodium gas due to the reaction of formula fil, a part of this sodium carbonate may be replaced, for example, by sodium metasilicate. According to formula (3), some carbon is liberated, but
This is mostly absorbed into the slag and removed by soda ash according to the reaction of equation (1).

Here, the soot and lithium carbonates used in the present invention include Na2Go3<soda ash), Na1
Examples include lCO3 (sodium hydrogen carbonate), KNaCO3 (sodium to potassium carbonate), but soda ash is particularly preferred; for example, an ore containing sodium sesquicarbonate that is a raw material for soda products such as trona ash and magazi ash. It may be used as a flux as it is in the form of lumps or powder.

The flux may be the above-mentioned thorium carbonate alone, but if additives for basicity adjustment such as CaO, M, and O are mixed, the sodium content required for terrestrial fixation of P2O5 can be reduced. Furthermore, it is more preferable because it prevents the molten steel surface from being exposed during the decarburization and descaling reaction and prevents temperature drop.

Claims (1)

[Claims] (1) Hot metal pre-treated to contain 0.1% or less silicon is oxygen blown in a converter without adding any slag forming agent to create a /8 steel with 0.1% or less carbon and 200ppm or more oxygen. Then, a flux containing thorium carbonate is added to the 18 steel in the converter to perform decarburization and dephosphorization.
Method for manufacturing low carbon/low range 114. (21i pig iron pre-treatment to reduce the silicon content to 1% or less is oxygen-blown in a converter without adding a slag-forming agent to produce molten steel with 0.1% or less carbon and 200ppm or more oxygen. Then, a sodium carbonate-containing flux is added to the molten steel in the converter to decarburize the carbon to 0.02% or less, and then a sodium carbonate-containing flux is further added to the molten steel during or after tapping into the ladle. A method for producing low-carbon, low-phosphorus steel, which is characterized by dephosphorizing the steel.