JP2011208170A - Method of producing manganese-containing low carbon steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To use manganese-based alloy iron containing carbon as a manganese source in a state in which manganese oxidation loss is suppressed in melting manganese-containing low carbon steel by vacuum decarburization refining.
An oxygen source is supplied to molten steel 3 in a vacuum tank 5 of an RH vacuum degassing apparatus 1 to perform decarburization refining under reduced pressure, and then the supply of the oxygen source to the molten steel is stopped. The undeoxidized molten steel is circulated through the vacuum tank and ladle 2 and decarburized under reduced pressure. After this decarburized refining, Al is added to the molten steel in the vacuum tank and deoxidized. The manganese-containing low carbon steel is smelted, and the manganese-based alloy iron containing carbon is contained in the vacuum tank during the decarburization refining under reduced pressure performed in a state where the supply of the oxygen source is stopped. It is added to molten steel, and carbon in the manganese-based alloy iron is oxidized and removed with dissolved oxygen in the undeoxidized molten steel.
[Selection] Figure 1

Description

  The present invention relates to a method for melting low-carbon steel containing manganese (hereinafter referred to as “manganese-containing low-carbon steel”) by subjecting molten steel to decarburization refining under reduced pressure in an RH vacuum degassing apparatus. In the decarburization and refining period, manganese-based alloy iron containing carbon as a manganese source is added to the molten steel in the vacuum tank at the time of recirculation decarburization and refining. The present invention relates to a method of oxidizing and removing carbon in manganese-based alloy iron with dissolved oxygen in molten steel, and melting manganese-containing low carbon steel with a small amount of expensive metallic manganese.

  In recent years, with the diversification of applications, steel materials are often used in a harsher environment, and higher performance of material properties is demanded more than ever. Under these circumstances, low carbon high manganese steel that has both high tensile strength and high workability has been developed for the purpose of reducing the weight of the structure, and it seems to be used as a steel plate for line pipes or steel plates for automobiles. It has become.

  In the steelmaking refining process, manganese sources used to adjust the manganese concentration in the molten steel include manganese ore, high carbon ferromanganese (carbon content: 7.5% by mass or less), medium carbon ferromanganese (carbon content: 2.0% by mass or less), low carbon ferromanganese (carbon content: 1.0% by mass or less), silicomanganese (carbon content: 2.0% by mass or less), metal manganese (carbon content: 0.01) Mass% or less) is common, and it becomes more expensive as the carbon content is lower, except for manganese ore. Therefore, for the purpose of reducing the manufacturing cost, a method for melting manganese-containing steel using manganese ore or high carbon ferromanganese, which is an inexpensive manganese source, has been proposed. In the present invention, high carbon ferromanganese, medium carbon ferromanganese, low carbon ferromanganese, and silicomanganese are collectively referred to as manganese-based alloy iron.

  For example, in Patent Document 1, high-carbon ferromanganese is introduced at the time of molten steel from the converter to the ladle to adjust the manganese component in the molten steel, and then oxygen gas is supplied to the molten steel in the vacuum degassing tank. A method has been proposed in which high manganese steel is produced by blowing up and decarburizing and refining the molten steel to oxidize and remove carbon in the molten steel.

  In Patent Document 2, when steel having a carbon concentration of 0.0050% by mass or less is melted in a degassing facility, the carbon content is 0.5 to 20% until 20% of vacuum decarburization refining has elapsed. There has been proposed a low-carbon steel melting method in which 9% by mass of manganese-based alloy iron is added and the manganese component in the molten steel is adjusted by oxidizing and removing carbon in the manganese-based alloy iron.

  Further, in Patent Document 3, ferromanganese is introduced for manganese adjustment at the beginning of the decarburization refining period of vacuum degassing refining when melting ultra-low carbon steel through converter refining and vacuum degassing refining. A melting method has been proposed. Also in this case, carbon in ferromanganese is oxidized and removed during decarburization refining in vacuum degassing refining.

  When melting low carbon steel containing manganese, such as low carbon high manganese steel, manganese ore is introduced into the converter during decarburization and refining of the hot metal in the converter, and manganese ore is reduced or converted from the converter. It is possible to increase the manganese concentration in the molten steel to a predetermined value by adding high carbon ferromanganese to the molten steel at the time of steel production or vacuum degassing refining. When molten steel with a high content is vacuum decarburized and refined, oxygen not only reacts with carbon in the molten steel, but also reacts with manganese, manganese loses oxidation and deteriorates the yield of manganese. It becomes very difficult to control the manganese concentration inside. In addition, vacuum decarburization refining means using a vacuum degassing facility such as an RH vacuum degassing apparatus to decarburize by adding an oxygen source such as oxygen gas to the molten steel under reduced pressure, or in an undeoxidized state. It is a refining method in which molten steel is decarburized by high vacuum treatment.

  Therefore, in order to avoid this problem, in the melting of manganese-containing low carbon steel such as low carbon high manganese steel, a method of adding a manganese source during degassing after vacuum decarburization refining has been performed. In this case, the low carbon high manganese steel has a low and narrow carbon concentration tolerance, so it is necessary to use a manganese source such as metal manganese with a low carbon content. Since it is expensive, it has been forced to increase the melting cost of manganese-containing low carbon steel such as low carbon high manganese steel.

JP-A-4-88114 JP-A-1-301815 Japanese Patent Laid-Open No. 2-47215

  The present invention has been made in view of the above circumstances, and its object is to suppress manganese oxidation loss when melting manganese-containing low carbon steel, including low carbon high manganese steel, by vacuum decarburization refining. An object of the present invention is to provide a method for melting manganese-containing low-carbon steel, in which manganese-based alloy iron containing carbon can be used as a manganese source.

  The method for melting manganese-containing low carbon steel according to the first invention for solving the above-mentioned problem is to supply oxygen source to the molten steel in the vacuum tank of the RH vacuum degassing apparatus to perform decarburization refining under reduced pressure. Next, the supply of the oxygen source to the molten steel is stopped, and the supply of the oxygen source to the molten steel is stopped, and the undeoxidized molten steel is circulated through the vacuum tank and the ladle to remove it under reduced pressure. After the decarburization and refining, a method of adding Al to the molten steel in the vacuum tank and deoxidizing the molten steel to melt the manganese-containing low carbon steel, the oxygen source At the time of decarburization and refining under reduced pressure performed in a state where supply is stopped, manganese-based alloy iron containing carbon is added to the molten steel in the vacuum tank, and the carbon in the manganese-based alloy iron is added to the undeoxidized molten steel. It is characterized by oxidation and removal with dissolved oxygen.

The method for melting manganese-containing low-carbon steel according to the second invention is the dissolved oxygen concentration in molten steel and the molten steel at the end of decarburization refining under reduced pressure performed by supplying the oxygen source in the first invention. The addition amount of the manganese-based alloy iron is adjusted within the range of the following formula (1) according to the medium carbon concentration.
[% O] ≧ ([% C]-[% C] _f + (W × η _C × 1/1000)) × 4/3… (1)
However, in the formula (1), [% O] is an oxygen concentration (mass%) in molten steel at the end of decarburization refining performed by supplying an oxygen source, and [% C] is performed by supplying an oxygen source. Carbon concentration (mass%) in molten steel at the end of decarburization refining, [C] _f is the target carbon concentration (mass%) in molten steel at the end of decarburization refining under reduced pressure, and W is a manganese-based carbon-containing carbon Addition amount (kg / t) per ton of molten steel of alloy iron, η _C is the carbon concentration (mass%) of manganese-based alloy iron containing carbon.

  According to the present invention, even when manganese-based alloy iron containing carbon such as high carbon ferromanganese is used as a manganese source, oxidation loss of manganese in vacuum decarburization refining can be suppressed, and at the same time a manganese-based alloy Carbon concentration pickup of molten steel due to carbon in iron can be suppressed, and manganese-containing low carbon steel can be melted at a lower cost than in the past.

It is a schematic longitudinal cross-sectional view of the RH vacuum degassing apparatus used when implementing this invention. It is a figure which shows the relationship between the carbon concentration and decarburization time before decarburization refining.

  Hereinafter, the present invention will be specifically described. First, the background to the present invention will be described. In melting the manganese-containing low-carbon steel such as low-carbon high-manganese steel, the present inventors use manganese-based alloy iron containing carbon as a manganese source in a state in which the oxidation loss of manganese is suppressed. I studied and studied. The results of the examination and research are explained below.

  When supplying oxygen gas as an oxygen source to the molten steel in the vacuum tank of the RH vacuum degassing apparatus and vacuum decarburizing and refining the molten steel, in the molten steel not containing manganese or the molten steel having a low manganese content, the supplied oxygen gas is Since it only reacts with carbon except for dissolving in molten steel, the decarburization rate can be increased by increasing the supply rate of the oxygen source. However, when steel with a high manganese content is melted, the supplied oxygen gas reacts with manganese in addition to carbon. Therefore, in order to increase the decarburization rate and perform decarburization efficiently, oxygen and manganese Must be suppressed to promote the reaction between oxygen and carbon.

  In the case of adding manganese-based alloy iron containing carbon such as high carbon ferromanganese to molten steel being refined by an RH vacuum degassing apparatus, from the viewpoint of preventing oxidation of manganese-based alloy iron, conventionally, It is common to add to the surface of molten steel by dropping due to gravity (referred to as “top addition”). The added manganese-based alloy iron dissolves in the molten steel, and increases the manganese concentration and carbon concentration of the molten steel.

  As a result of repeatedly investigating the behavior of carbon and manganese during vacuum decarburization and refining with the addition of manganese-based alloy iron, the present inventors determined that the decarburization reaction and demanganese reaction We found that the behavior is different. That is, in vacuum decarburization refining (referred to as “acid feed decarburization refining”) performed by supplying an oxygen source such as oxygen gas to the molten steel in the vacuum tank, when the manganese concentration is high, decarburization reaction and demanganese reaction Both of these reactions proceed, but the oxygen source including oxygen gas is not supplied and the undeoxidized molten steel is circulated between the vacuum chamber and ladle, and dissolved in the molten steel under high vacuum in the vacuum chamber. It was found that when decarburization refining is carried out by oxidation reaction with oxygen (referred to as “reflux decarburization refining”), the decarburization reaction proceeds, but even when the manganese concentration is high, the demanganese reaction hardly proceeds. This is because, in the molten steel temperature range of about 1600 to 1650 ° C., carbon has a stronger affinity for oxygen than manganese, and only dissolved oxygen exists and oxygen present is not abundant. This is presumably because the reaction with carbon occurs preferentially. Here, the dissolved oxygen is oxygen dissolved in the molten steel without exhibiting the form of oxide.

  Therefore, when melting manganese-containing low-carbon steel, it first performs acid decarburization refining, then recirculation decarburization refining, and carbon such as high carbon ferromanganese is contained during this recirculation decarburization refining. By adding the manganese-based alloy iron to the molten steel in the vacuum chamber, the oxidation loss of manganese contained in the manganese-based alloy iron can be suppressed, and at the same time, the carbon content of the manganese-based alloy iron It has been found that oxidation removal can be promoted, manganese yield can be improved, and pickup of carbon concentration of molten steel by carbon in manganese-based alloy iron can be suppressed.

  The present invention has been made on the basis of the above examination and research results. An oxygen source is supplied to the molten steel in the vacuum tank of the RH vacuum degassing apparatus to perform decarburization refining under reduced pressure. With the supply to the molten steel stopped and the supply of the oxygen source to the molten steel stopped, the decarburized refining is performed under reduced pressure by circulating the undeoxidized molten steel through the vacuum tank and ladle. After refining, when adding Al to the molten steel in the vacuum tank and deoxidizing the molten steel to melt the manganese-containing low carbon steel, the above-described reduced pressure performed in a state where the supply of oxygen source is stopped When decarburizing and refining, carbon-containing manganese-based alloy iron is added to the molten steel in the vacuum chamber, and the carbon in the manganese-based alloy iron is oxidized and removed with dissolved oxygen in the undeoxidized molten steel. And

  In this case, if the amount of carbon brought into the molten steel by the manganese-based alloy iron increases, the amount of oxygen necessary for decarburization is insufficient with only dissolved oxygen, and it becomes impossible to decarburize to the target carbon concentration. Depending on the dissolved oxygen concentration in the molten steel and the carbon concentration in the molten steel at the end of the decarburizing and refining, that is, the acid feeding decarburization refining, the addition amount of manganese-based alloy iron is within the range of the following formula (1) It is preferable to adjust within.

That is, since the decarburization reaction of molten steel proceeds by the reaction of “C + O → CO”, the dissolved oxygen concentration in the molten steel at the end of decarburization refining performed by supplying an oxygen source is scheduled to be decarburized thereafter. It is preferable to adjust the addition amount of manganese-based alloy iron containing carbon so that it is more or equivalent in chemical equivalent than the carbon amount.
[% O] ≧ ([% C]-[% C] _f + (W × η _C × 1/1000)) × 4/3… (1)
However, in the formula (1), [% O] is an oxygen concentration (mass%) in molten steel at the end of decarburization refining performed by supplying an oxygen source, and [% C] is performed by supplying an oxygen source. Carbon concentration (mass%) in molten steel at the end of decarburization refining, [C] _f is the target carbon concentration (mass%) in molten steel at the end of decarburization refining under reduced pressure, and W is a manganese-based carbon-containing carbon Addition amount (kg / t) per ton of molten steel of alloy iron, η _C is the carbon concentration (mass%) of manganese-based alloy iron containing carbon.

According to JIS G 2301 and JIS G 2304, the carbon concentration of alloy iron defined as manganese-based alloy iron in the present invention is 7.5% by mass or less for high carbon ferromanganese and 2.0% by mass for medium carbon ferromanganese. Hereinafter, low carbon ferromanganese is 1.0 mass% or less, and silicomanganese is 2.0 mass% or less. Therefore, when calculating the formula (1), η _C is used from each alloy iron manufacturer. The chemical component values sent may be used, for example, 7.0% by mass for high carbon ferromanganese, 2.0% by mass for medium carbon ferromanganese, 1.0% by mass for low carbon ferromanganese, Silicomanganese may be 2.0% by mass. Incidentally, in the case of electrolytic manganese, η _C can be calculated as zero.

  Next, an embodiment of the present invention will be described.

  The hot metal discharged from the blast furnace is received in a hot metal transfer container such as a hot metal ladle or topped car, and transferred to a converter for decarburization and refining in the next process. Usually, hot metal pretreatment such as desulfurization treatment or dephosphorization treatment is applied to the hot metal during the conveyance, and in the present invention, no hot metal pretreatment is required in terms of the component specifications of the manganese-containing low carbon steel. Even in such a case, it is preferable to perform hot metal pretreatment, particularly dephosphorization treatment, in order to add manganese ore as an inexpensive manganese source into the converter and increase the yield of manganese ore in converter decarburization refining.

  In converter refining, manganese ore is added as a manganese source, and a small amount of quicklime is used as a slagging agent as required, and oxygen gas is blown up or bottom to decarburize and refining hot metal at atmospheric pressure. Do. In this case, after completion of decarburization refining, inexpensive manganese-based alloy iron such as high carbon ferromanganese may be added to the molten steel when steel is discharged from the converter to a molten steel transfer container such as a ladle. In addition, since the next process is vacuum decarburization refining with an RH vacuum degassing apparatus, at the time of steel extraction, Al and Si are not added to the molten steel, that is, the molten steel is not subjected to deoxidation with Al and Si. Is transported to the RH vacuum degassing apparatus in an undeoxidized state.

  By using an inexpensive manganese source such as manganese ore or high carbon ferromanganese, the carbon concentration in the molten steel after steel is inevitably increased, but even when manganese-based alloy iron is added during steel production, Including, it is preferable to suppress the carbon concentration in the molten steel after steel output to 0.2% by mass or less. When the carbon concentration of the molten steel exceeds 0.2% by mass, it takes a long time for vacuum decarburization refining in the RH vacuum degassing apparatus in the next process, not only lowering the productivity of the RH vacuum degassing apparatus, but also vacuum decarburization. As temperature compensation by extending the refining time, it is necessary to increase the molten steel temperature at the time of steel output, which is not preferable because the manufacturing cost increases due to a decrease in iron yield and an increase in refractory wear due to this.

  Next, vacuum decarburization refining is performed on the molten steel in an RH vacuum degassing apparatus. FIG. 1 shows a schematic longitudinal sectional view of an RH vacuum degassing apparatus used when carrying out the present invention. In FIG. 1, 1 is a RH vacuum degassing device, 2 is a ladle, 3 is molten steel, 4 is a slag, 5 is a vacuum tank, 6 is an upper tank, 7 is a lower tank, 8 is a rising side dip tube, and 9 is a lowering Side dip pipe, 10 is a reflux gas blow pipe, 11 is a duct, 12 is a raw material inlet, 13 is an upper blow lance, and the vacuum tank 5 is composed of an upper tank 6 and a lower tank 7, and an upper blow The lance 13 can be moved up and down, and oxygen gas and a mixed gas of oxygen gas and inert gas such as Ar gas are sprayed from the upper blow lance 13 onto the molten metal surface of the molten steel 3 inside the vacuum chamber 5. It is supposed to be.

  In the RH vacuum degassing apparatus 1, the ladle 2 is raised by an elevating device (not shown), and the ascending side dip pipe 8 and the descending dip pipe 9 are immersed in the molten steel 3 in the ladle. Then, while circulating Ar gas is blown into the rising side dip tube 8 from the circulating gas blowing tube 10, the inside of the vacuum chamber 5 is evacuated by an exhaust device (not shown) connected to the duct 11. The inside of the tank 5 is depressurized. When the inside of the vacuum chamber 5 is depressurized, the molten steel 3 in the ladle rises along the rising side dip tube 8 together with Ar gas by the gas lift effect by Ar gas blown from the circulating gas blowing tube 10, and the vacuum chamber 5. Then, a flow returning to the ladle 2 via the descending side dip pipe 9 is formed, so-called recirculation, and RH vacuum degassing is performed.

If the recirculation of the molten steel 3 is formed, oxygen gas (industrial pure oxygen) or a mixed gas of oxygen gas and inert gas is sprayed from the top blowing lance 13 to the molten steel 3 in the vacuum tank, and the acid feed decarburization refining is performed. Start. Reaction between the carbon in the molten steel and the supplied oxygen gas (C + O → CO) occurs, and the carbon in the molten steel becomes CO gas and is discharged from the vacuum tank 5 through the duct 11 together with the exhaust gas. Acid feeding decarburization refining is performed. The carbon concentration in the molten steel decreases with the progress of this acid decarburization refining. In this case, part of the oxygen gas supplied is dissolved in the molten steel 3 to increase the dissolved oxygen concentration. In the present invention, when the carbon concentration in the molten steel is higher than the target carbon concentration [C] _f (= target carbon concentration after reflux decarburization refining) determined from the component specifications of the steel material, The gas supply is stopped and the acid sending decarburization refining is completed, and then the process moves to the reflux decarburization refining.

  Since the supply of oxygen gas from the top blowing lance 13 is eliminated, the atmospheric pressure in the vacuum chamber is lowered and the degree of vacuum is increased, and the molten steel 3 is in a non-deoxidized state. Exposure to the atmosphere causes a reaction (C + O → CO) between carbon in the molten steel and dissolved oxygen in the vacuum chamber, and the carbon in the molten steel becomes CO gas and is discharged from the vacuum chamber 5 together with the exhaust gas. The molten steel 3 is subjected to reflux decarburization refining.

Immediately after the transition to the reflux decarburization refining, manganese-based alloy iron containing carbon such as high carbon ferromanganese is added to the molten steel 3 in the vacuum chamber through the raw material inlet 12. The amount of manganese-based alloyed iron added is compared with the dissolved oxygen concentration, carbon concentration, and target carbon concentration after reflux decarburization refining ([C] _f ), 1) It is preferable to be within a range satisfying the formula. It is preferable that the addition time of manganese-based alloy iron is set as early as possible in the recirculation decarburization refining from the viewpoint of securing the decarburization time of carbon contained in the manganese-based alloy iron. The dissolved oxygen concentration in the molten steel at the end of the acid decarburization refining can be measured with an oxygen probe using an oxygen concentration cell, and the carbon concentration in the molten steel at the end of the acid decarburization refining is a sample taken from the molten steel. This can be understood by performing chemical analysis of the above, or by calculating from the carbon concentration in the molten steel and the oxygen gas supply amount before the start of the acid feed decarburization refining.

  The manganese-based alloy iron to be used may be any of high carbon ferromanganese, medium carbon ferromanganese, low carbon ferromanganese, and silicomanganese, but since high carbon ferromanganese is the cheapest, It is preferable to use high carbon ferromanganese as long as there is a margin in the amount of carbon to be brought in. However, in order to satisfy the formula (1), it is naturally possible to use other than high carbon ferromanganese.

The reflux decarburization refining was continued until the carbon concentration of the molten steel 3 became the target carbon concentration ([C] _f ) or less, and the carbon concentration of the molten steel 3 became a predetermined value less than the target carbon concentration ([C] _f ). Then, a strong deoxidizer such as Al is added to the molten steel 3 from the raw material inlet 12 to deoxidize the molten steel 3. By adding a strong deoxidizing agent such as Al, the dissolved oxygen concentration of the molten steel 3 rapidly decreases, and the reflux decarburization refining is completed.

  After recirculation decarburization and refining, the recirculation is continued for about several minutes, and if necessary, component modifiers such as Al, Si, Mn, Ni, Cr, Cu, Nb, and Ti are supplied from the raw material inlet 12 to the molten steel. 3 to adjust the components of the molten steel 3. Thereafter, the inside of the vacuum chamber 5 is returned to atmospheric pressure, RH vacuum degassing refining is completed, and manganese-containing low carbon steel is melted.

  As described above, according to the present invention, when molten steel is subjected to vacuum decarburization refining in an RH vacuum degassing apparatus to produce manganese-containing low carbon steel, carbon such as high carbon ferromanganese is contained. Even if manganese-based alloy iron is used as a manganese source, it is possible to suppress manganese oxidation loss in vacuum decarburization refining, and at the same time, to suppress carbon concentration pickup of molten steel due to carbon in manganese-based alloy iron. This makes it possible to produce manganese-containing low-carbon steel at a lower cost than in the past.

  In the above description, an example in which oxygen gas is used as an oxygen source in the acid-feed decarburization refining has been described, but iron oxide such as iron ore or mill scale can also be used as the oxygen source. What is necessary is just to throw iron oxide into the molten steel 3 in a vacuum chamber from the raw material inlet 12. Moreover, it is also possible to use oxygen gas and iron oxide together as an oxygen source.

  The hot metal discharged from the blast furnace is subjected to desulfurization treatment and dephosphorization pretreatment, and the molten steel is melted by decarburizing and refining the converter using this hot metal, and then the obtained molten steel is subjected to RH vacuum. Tests (test numbers 1 to 11) for melting manganese-containing low carbon steel by vacuum decarburization refining with a degasser were carried out. In test numbers 1 to 3, 6, and 8 to 11, manganese ore was added as a manganese source in the converter to increase the manganese concentration, and the obtained 350 tons of molten steel was put into a ladle with no deoxidation. . On the other hand, in test numbers 4, 5, and 7, ordinary decarburization refining was performed without adding manganese ore in the converter, and the obtained 350 tons of molten steel was put into a ladle with no deoxidation. Molten steel components at the time of steel removal are 0.03 to 0.06% by mass of carbon, 0.05% by mass or less of silicon, 0.1 to 0.8% by mass of manganese, 0.03% by mass or less of phosphorus, Sulfur was 0.003 mass% or less. The molten steel was conveyed to an RH vacuum degassing apparatus, and various conditions for vacuum decarburization refining were changed to produce manganese-containing low carbon steel. The dissolved oxygen concentration in the molten steel at the time of arrival at the RH vacuum degassing apparatus was 0.03 to 0.07% by mass.

In the RH vacuum degassing apparatus, the flow rate of Ar gas for reflux is 1500 NL / min, the degree of vacuum in the vacuum tank during acid feeding decarburization refining is 6.7 to 40 kPa, and the amount of oxygen gas supplied from the top blowing lance is 2000 Nm ³ / h, The lance height (distance between the molten steel surface in the vacuum chamber and the tip of the lance) of the top blowing lance at the time of acid feeding was fixed at 6 m.

  Carbon-containing manganese-based alloy iron added during vacuum decarburization refining has a carbon content of about 7% by mass, a manganese content of about 75% by mass, and a high carbon ferromagnet having a particle size of 5 to 10 mm. Manganese was used. And, as a melting method, (1) a method of adding high carbon ferromanganese during acid feeding decarburization and refining in the initial stage of vacuum decarburizing and refining (Comparative Example), (2) high carbon ferromanganese feeding acid Method of adding and melting during reflux decarburization after completion of decarburization (example of the present invention), (3) without adding high carbon ferromanganese, deoxidizing with Al after vacuum decarburization and refining electrolytic manganese It was carried out by three kinds of methods, ie, a method of adding and melting (conventional example). Moreover, when the manganese concentration of molten steel cannot be ensured only by adding high carbon ferromanganese, electrolytic manganese was added after Al deoxidation after reflux decarburization refining. The addition rate of high carbon ferromanganese was 150 to 250 kg / min, and the addition amount of high carbon ferromanganese was constant at 1000 kg.

In each test, the acid feeding decarburization time was changed, and the dissolved oxygen concentration in the molten steel at the end of the acid feeding decarburization was changed. The target carbon concentration ([C] _f ) at the end of the reflux decarburization was 0.002% by mass. Table 1 shows the addition time of high carbon ferromanganese in each test operation, transition of molten steel components, decarburization refining time, decarburization amount, manganese yield, and the like. The decarburization amount shown in Table 1 is the difference between the molten steel component value before refining and the molten steel component value after refining in the RH vacuum degassing apparatus, and the carbon brought into the molten steel by the added high carbon ferromanganese. Is not considered.

  In Table 1, Test Nos. 1 to 7 are tests (examples of the present invention) in which high carbon ferromanganese was added during reflux decarburization refining, and Test Nos. 1 to 5 were high within the range satisfying the formula (1). Examples in which carbon ferromanganese was added, Test Nos. 6 and 7 were examples in which a higher amount of high carbon ferromanganese was added than in formula (1), and Tests Nos. 8 and 9 were conducted during acid feeding decarburization refining. Tests (Comparative Examples) and Nos. 10 and 11 in which high carbon ferromanganese was added were not added at the time of vacuum decarburization refining, and electrolytic manganese was added after Al deoxidation after vacuum decarburization refining. This is an added test (conventional example).

  As shown in Table 1, in the present invention examples of Test Nos. 1 to 7, the manganese yield of high carbon ferromanganese is 84 to 86%, which is equivalent to the yield of electrolytic manganese in the conventional examples of Test Nos. 10 and 11. Met. In other words, it was confirmed that manganese oxidation of high carbon ferromanganese did not occur during reflux decarburization refining. On the other hand, in the comparative examples of Test Nos. 8 and 9, the manganese yield of the high carbon ferromanganese was about 68 to 70%, and the manganese yield decreased by about 10% or more compared to the inventive example. This is because manganese was oxidized by the addition of high carbon ferromanganese during acid delivery.

  Further, comparing the time spent for the decarburization refining time, as shown in FIG. 2, in the present invention example of the test No. 1-7, as in the conventional examples of the test No. 10 and 11, before the decarburization refining. Although the decarburization and refining time tends to be longer in proportion to the carbon concentration in molten steel, the decarburization and refining time is relative in the comparative examples of tests No. 8 and 9, regardless of the carbon concentration in the molten steel before decarburization and refining. Tend to be longer. This is because in Test Nos. 8 and 9, carbon was brought in by high carbon ferromanganese and the amount of carbon to be decarburized increased. In Test No. 1-7, carbon is brought into the molten steel by high carbon ferromanganese, but in the case of Test No. 1-7, high carbon ferromanganese should be added in a state where the dissolved oxygen concentration in the molten steel is high. Therefore, it is considered that the decarburization reaction proceeds at a stretch and the decarburization refining time does not extend. In addition, FIG. 2 is a figure which shows the relationship between the carbon concentration in the molten steel before decarburization refining, and the decarburization time, and generally, the decarburization time becomes long, so that the carbon concentration before decarburization refining becomes high.

  The conventional examples of Test Nos. 10 and 11 and the present invention example are equivalent in that the yield of manganese in RH degassing refining is high and the decarburization refining time is short, but the present invention example uses electrolytic manganese. The economic effect that the amount can be significantly reduced and the manufacturing cost can be greatly reduced as compared with the conventional example using only expensive electrolytic manganese is exhibited.

In Test Nos. 6 and 7, the amount of carbon to be decarburized is stoichiometrically higher than the dissolved oxygen concentration in the molten steel before reflux decarburization refining, and the carbon in molten steel at the end of reflux decarburization refining. concentration was not able to achieve 0.002 wt% of the target carbon concentration ([C] _f), the present target carbon concentration ([C] _f) is set lower than steel standard steel product standards Was satisfied. However, it has been confirmed that it is preferable to operate under conditions that satisfy the formula (1) in order to effectively perform the reflux decarburization refining.

DESCRIPTION OF SYMBOLS 1 RH vacuum degassing apparatus 2 Ladle 3 Molten steel 4 Slag 5 Vacuum tank 6 Upper tank 7 Lower tank 8 Rising side immersion pipe 9 Lowering side immersion pipe 10 Recirculation gas blowing pipe 11 Duct 12 Raw material inlet 13 Upper blowing lance

Claims (2)

  The oxygen source is supplied to the molten steel in the vacuum tank of the RH vacuum degassing apparatus to perform decarburization refining under reduced pressure, and then the supply of the oxygen source to the molten steel is stopped, and the supply of the oxygen source to the molten steel is stopped. In the stopped state, the molten steel in the non-deoxidized state is circulated through the vacuum tank and ladle to perform decarburization refining under reduced pressure. After the decarburization refining is completed, Al is added to the molten steel in the vacuum tank. Is a method for melting manganese-containing low-carbon steel by deoxidation treatment, wherein the manganese-based alloy iron containing carbon at the time of decarburization refining under reduced pressure performed in a state where the supply of the oxygen source is stopped Is added to the molten steel in the vacuum tank, and carbon in the manganese-based alloy iron is oxidized and removed with dissolved oxygen in the undeoxidized molten steel. Depending on the dissolved oxygen concentration in the molten steel and the carbon concentration in the molten steel at the end of decarburization refining under reduced pressure by supplying the oxygen source, the amount of manganese-based alloy iron added is expressed by the following formula (1): The method for melting manganese-containing low carbon steel according to claim 1, wherein the method is adjusted within a range.
[% O] ≧ ([% C]-[% C] _f + (W × η _C × 1/1000)) × 4/3… (1)
However, in the formula (1), [% O] is an oxygen concentration (mass%) in molten steel at the end of decarburization refining performed by supplying an oxygen source, and [% C] is performed by supplying an oxygen source. Carbon concentration (mass%) in molten steel at the end of decarburization refining, [C] _f is the target carbon concentration (mass%) in molten steel at the end of decarburization refining under reduced pressure, and W is a manganese-based carbon-containing carbon Addition amount (kg / t) per ton of molten steel of alloy iron, η _C is the carbon concentration (mass%) of manganese-based alloy iron containing carbon.

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