JPH09170013A - Method for melting extra-low carbon and high manganese steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a melting method of an extra-low carbon and high manganese steel which can restrain a manganese loss quantity, oxidized loss quantity and vaporized loss quantity on a slag-metal interface. SOLUTION: At the time of melting the extra-low carbon and high manganese steel by using a vacuum degassing equipment and executing decarburizing treatment to molten high manganese steel, the carbon concn. in the molten steel before the decarburizing treatment is made to be <=0.06wt%, and while controlling dissolved oxygen concn. in the molten steel in the range of 200-400ppm during the decarburizing treatment, the vacuum decarburizing treatment is executed.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing ultra low carbon high manganese steel using a vacuum degassing facility.

[0002]

2. Description of the Related Art In recent years, in order to reduce the weight of automobiles, it has been required to develop a steel sheet for automobiles having high tensile strength and excellent workability. Therefore, ultra low carbon high manganese steel (the ultra low carbon high manganese steel in the present application has a carbon concentration of 0.01 wt%
Hereinafter, a steel having a manganese concentration of 0.7 wt% or more is referred to, and hereinafter, wt% is abbreviated as%).

Generally, manganese has a high vapor pressure and a strong affinity with oxygen. Therefore, when an ultra low carbon high manganese steel is melted by using a vacuum degassing equipment such as RH, Since decarburization is performed in a high vacuum, manganese easily loses vaporization, and it easily reacts with an oxygen source for decarburization (a gaseous oxygen source such as oxygen gas or a solid oxygen source such as iron oxide) to cause oxidation loss, resulting in manganese loss. The amount will increase.

For this reason, in the conventional ultra-low carbon-high manganese molten steel, since the evaporation loss amount and the oxidation loss amount are large, the manganese source has been added after the decarburizing treatment to perform melting. Since the manganese source at this time has reached the extremely low carbon range, inexpensive ferromanganese cannot be added due to the high carbon concentration, and the method of adding expensive metallic manganese is adopted, which increases the melting cost. There was a problem of doing.

In order to solve this problem, Japanese Patent Laid-Open No. 6-27
In Japanese Patent No. 1923 (hereinafter referred to as Prior Art 1), a mixed gas (diluting gas) of oxygen gas and an inert gas such as Ar gas is blown from an oxygen top blowing lance onto a molten steel surface in a vacuum chamber while degassing. The degree of vacuum in the vacuum tank during charcoal treatment is 50
00Pa or more, 40000Pa or less (38torr or more, 3
The carbon concentration is 300-
Decarburize to 500 ppm, then 10,000 Pa
A melting method of ultra low carbon high manganese steel is disclosed in which the vacuum decarburization treatment is performed to the ultra low carbon region while maintaining the temperature below (76 torr or less).

[0006] Normally, when melting ultra-low carbon steel with a manganese concentration of 0.5% or less, the degassing equipment has an exhaust capacity without maintaining a high vacuum tank pressure during vacuum decarburization. Leave it as low as possible (eg 1300Pa or less (10
less than torr)), and promote decarburization. On the other hand, the method of Prior Art 1 is 5000 Pa or more (38 torr or more)
By setting it to a high value, the evaporation loss of manganese under vacuum is suppressed, and the oxygen loss on the oxygen sprayed surface (so-called fire point) is suppressed by sending oxygen gas diluted with an inert gas. To do.

[0007]

DISCLOSURE OF THE INVENTION In addition to the above-mentioned oxidation loss and evaporation loss, the inventors have found that dissolved oxygen [O] in molten steel and manganese [ Attention was paid to manganese loss due to the reaction with [Mn]. Then, a test was performed in which the vacuum degassing conditions were variously changed, and the manganese loss amount was evaluated and examined.

[Mn] + [O] = (MnO) (1) where [Mn]; manganese in molten steel [O]; dissolved oxygen (MnO) in molten steel; manganese oxide in slag At the slag-metal interface, the reaction according to the equation (1)
This is an oxidation reaction in which manganese [Mn] in molten steel reacts with dissolved oxygen [O] to lose manganese, and is an equilibrium reaction that proceeds to the right as the manganese concentration or dissolved oxygen concentration in molten steel increases.

Therefore, since a large amount of dissolved oxygen [O] is always present in the molten steel during the decarburization treatment, the manganese loss amount according to the equation (1) is equal to the above-mentioned oxidation loss amount and evaporation loss amount. It is expected to be so large that it cannot be ignored.
However, Prior Art 1 has no suggestion regarding manganese loss by the formula (1).

Further, the method of the prior art 1 focuses on suppressing the evaporation loss, and as described above, the pressure in the vacuum tank during the decarburization of acid is set high. Therefore, when melting is performed with the degree of vacuum in the tank being reduced to the maximum, in other words, compared with the case of melting normal ultra-low carbon steel with a manganese concentration of 0.5% or less, the entire decarburization is performed. The CO partial pressure in the vacuum chamber is high throughout.

Comparing these at the same carbon concentration level,
From the equilibrium theory of the decarburization reaction, which is defined by the following formula (2), the dissolved oxygen concentration [O] in the molten steel increases as the CO partial pressure increases. As a result, the method of Prior Art 1 can suppress the evaporation loss amount, but the manganese loss amount at the slag-metal interface according to the formula (1) is rather increased.

[C] + [O] = CO (g) (2) Here, [C]; carbon in molten steel [O]; dissolved oxygen in molten steel In addition, the method of Prior Art 1 , The decarburization reaction according to the formula (2) proceeds, but since the CO partial pressure is high, the reaction rate is low and the decarburization treatment time is long. And for the same reason,
Since the carbon concentration is difficult to decrease, the ultimate carbon concentration is 50p.
It stays around pm. At this extremely low carbon concentration level, an automobile steel sheet having insufficient workability and excellent workability cannot be obtained.

Further, in Prior Art 1, the carbon concentration at the end of the converter is set to a very high value of about 0.2% in order to reduce the oxidation loss in the converter refining to the maximum. It is the starting carbon concentration. For this reason, in the degassing equipment, since the carbon concentration is 0.2% to 400 ppm, the oxygen is decarburized, and then the vacuum degassing is performed to the extremely low carbon region. Become,
The above-mentioned oxidation loss amount and evaporation loss amount become extremely large.

The present invention has been made in order to solve the above-mentioned problems of the prior art, and suppresses the manganese loss amount according to the formula (1) and makes the carbon concentration before decarburization treatment an appropriate value. It is an object of the present invention to provide a melting method of ultra-low carbon high manganese steel that can suppress the amount of oxidation loss and the amount of evaporation loss by regulating the amount.

[0015]

The present invention is directed to decarburization of high-manganese molten steel using a vacuum degassing equipment to produce ultra-low carbon high-manganese steel, in which carbon in molten steel before decarburization is started. An ultra-low carbon high manganese steel characterized by vacuum decarburizing while controlling the concentration to 0.06 wt% or less and controlling the dissolved oxygen concentration in molten steel during decarburizing to 400 ppm or less and 200 ppm or more. It is a melting method.

The inventors of the present invention changed various degassing operating conditions in order to reduce the amount of oxidation loss and the amount of evaporation loss in the melting method of ultra-low carbon high manganese steel having a manganese concentration of 0.7% or more. The above test was conducted to investigate and examine the relationship between the carbon concentration before decarburization treatment and the amount of manganese loss.

As a result, as will be described in detail later, when the carbon concentration is 0.06% or less, the manganese loss amount shows a substantially constant value irrespective of the carbon concentration, but when it exceeds 0.06%, it rapidly increases. It was found that there is a tendency to increase. Therefore, in the present invention, the amount of oxidation loss and the amount of evaporation loss can be suppressed by regulating the carbon concentration within the range of 0.06% or less.

The present inventors also conducted and examined a test for investigating the manganese loss amount according to the formula (1) at the slag-metal interface in the ladle. As a result, manganese loss at the slag-metal interface can be suppressed by controlling the dissolved oxygen concentration in the molten steel during decarburization within the range of 400 ppm or less and 200 ppm or more.

When the dissolved oxygen concentration exceeds 400 ppm,
The manganese loss amount according to the equation (1) increases. Also 200
If it is less than ppm, the amount of dissolved oxygen is not secured and the rapid decarburization reaction is not performed, so the vacuum decarburization time is extended and the evaporation loss amount is increased.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a situation in which ultra low carbon high manganese steel is melted in an RH degasser. Here, 1 is a vacuum tank, 2 is an alloy inlet, 3 is an oxygen top blowing lance, 4 is an ascending pipe, 5 is a descending pipe, 6 is slag, 7 is molten steel,
Reference numeral 8 is a ladle, and 9 is an Ar gas blowing tube for circulation.

Inexpensive manganese ore is put into the converter during refining and reduced in the converter to produce low-manganese molten steel with a manganese concentration of about 0.5%. Then, ferrous manganese-containing ferromanganese-containing ferromanganese is added at the time of tapping to adjust the molten steel composition before degassing to a high manganese molten steel with a carbon concentration of 0.06% or less and a manganese concentration of 0.7% or more. To do. This molten steel is subjected to an extremely low carbon treatment using a vacuum degassing equipment such as an RH degassing device.

In the vacuum degassing equipment, when the vacuum chamber 1 is evacuated and the degree of vacuum in the chamber reaches about 20,000 Pa (about 150 torr), the CO partial pressure in the vacuum chamber 1 becomes the equilibrium CO content of the formula (2). The pressure drops below the pressure and decarburization begins. Vacuuming is continued even after decarburization is started.

The degree of vacuum in the vacuum chamber 1 is determined by the amount of CO gas generated by the equation (2) and the capacity of the exhaust unit. In the present embodiment, the exhaust unit has a sufficiently large exhaust capacity, and after the decarburization is started, the degree of vacuum is reduced as short as possible to reduce the CO partial pressure in the vacuum tank to reduce the decarburization processing time. To reduce the amount of evaporation loss. Then, in the final stage of decarburization, a vacuum degree of several 100 Pa (several torr) or less is secured to obtain an ultra-low carbon steel having an ultimate carbon concentration of 20 ppm or less. In addition, oxidative loss on the sprayed surface is suppressed by carrying out decarburization with acid under such a condition that the degree of vacuum is rapidly lowered.

During the degassing process, a dissolved oxygen measuring instrument (not shown) having a solid oxygen probe at its tip is immersed in molten steel to measure the dissolved oxygen concentration, and the dissolved oxygen concentration is 400 ppm.
When the concentration exceeds 4, the deoxidizing material such as Al or Si is added on the molten steel in the vacuum tank so that the dissolved oxygen concentration during the decarburization treatment is 4
Control to below 00ppm.

When the decarburization reaction of the formula (2) proceeds and the dissolved oxygen falls below 200 ppm, the decarburization reaction rate decreases. In this case, oxygen gas is blown onto the molten steel from the oxygen top-blowing lance 3 to control the dissolved oxygen concentration to 200 ppm or more to achieve a rapid decarburization reaction.

When the decarburization treatment is carried out in this manner and the predetermined extremely low carbon concentration is reached, the decarburization treatment is terminated, and if necessary, the molten steel is circulated between the tank and the ladle, and the ferroalloy is charged. To make the components uniform and improve the cleanliness of molten steel.

[0027]

[Example] A test was conducted to melt 250 ton of undeoxidized molten steel from a converter in an RH degassing facility to produce an extremely low carbon and high manganese steel.

Table 1 shows the test conditions and test results.

[0029]

[Table 1]

The carbon concentration of undeoxidized molten steel is 0.03% to 0.
The test was conducted for 23 heats in total, with the range of 06% and the manganese concentration in the range of 0.7% to 1.3%. The values of dissolved oxygen during the decarburization treatment in Table 1 show typical values during the treatment. Test N
o.1 to No. 12 are examples, tests No. 13 to No. 17 are comparative examples in which the carbon concentration before degassing is outside the scope of the present invention, and tests No. 18 to No. 23 are dissolved oxygen concentrations Is a comparative example outside the scope of the present invention.

FIG. 2 illustrates the relationship between the carbon concentration in the molten steel before decarburizing treatment and the manganese loss amount from the results of No. 1 to No. 17. From Table 1 and FIG. 2, the carbon concentration before decarburization is 0.06% or less, and the dissolved oxygen concentration during decarburization is controlled to 400 ppm or less and 200 ppm or more. For example), decarburization time is 2
A very low carbon steel having a carbon concentration (achieved carbon concentration) after treatment as short as 0 minutes or less than 16 ppm was obtained, and the manganese loss amount was suppressed to 0.1% or less.

On the other hand, the dissolved oxygen concentration during decarburization is 400
It is controlled to below ppm and above 200 ppm, but the carbon concentration before decarburization exceeds 0.06% No. 13 to No. 1
In 7 (comparative example), carbon concentration after treatment (reached carbon concentration)
Although an ultra low carbon steel of 13 ppm or less was obtained, the decarburization time was 23 minutes or more and the manganese loss amount was 0.1% or more, which is a large result.

FIG. 3 shows that the carbon concentration before decarburization is 0.06.
The relationship between the dissolved oxygen concentration and the manganese loss amount in a heat of not more than 10% is shown. As described above, in the examples of No. 1 to No. 12 in which the dissolved oxygen concentration during decarburization treatment is controlled to 400 ppm or less and 200 ppm or more, the manganese loss amount can be reduced to 0.1% or less.

However, the carbon concentration before decarburization is 0.06.
% Or less, but the dissolved oxygen concentration during decarburization is 400p
In No. 18 to No. 23 (comparative example) of pm or more, an extremely low carbon steel having a carbon concentration (reached carbon concentration) after treatment of 16 ppm or less was obtained, but the manganese loss amount was 0.13 to 0. ．
It increased to 26%, showing a tendency to rapidly increase with the dissolved oxygen concentration.

Further, as a result of introducing ferroalloy after decarburization treatment,
With respect to No. 5 to No. 12, which required time for homogenizing the components and required a reflux time of 10 minutes or more, the relationship between the pressure in the vacuum chamber and the amount of manganese loss during the molten steel reflux was investigated. The result is shown in FIG. From Fig. 4, the pressure in the vacuum chamber is 2000Pa.
By keeping at (15 torr) or more, the evaporation loss amount of manganese could be suppressed to 0.06% or less.

[0036]

EFFECTS OF THE INVENTION According to the method for melting ultra-low carbon and high manganese steel of the present invention, the carbon concentration before decarburization treatment is 0.06% or less, and the dissolved oxygen concentration during RH treatment is 400 ppm or less, 20% or less.
Since the vacuum decarburization treatment is performed while controlling to 0 ppm or more, the manganese loss amount according to the formula (1) at the slag-metal interface can be suppressed, and the oxidation loss amount and the evaporation loss amount can also be suppressed, so that the total manganese loss amount can be suppressed. .

[Brief description of the drawings]

FIG. 1 is a diagram showing a situation in which an extremely low carbon and high manganese steel is melted using an RH degasser.

FIG. 2 is a diagram showing a result of investigation of a relationship between a carbon concentration in molten steel before decarburization treatment and a manganese loss amount.

FIG. 3 is a diagram showing a result of investigation on a relationship between a dissolved oxygen concentration and a manganese loss amount.

FIG. 4 is a diagram showing a result of investigating the relationship between the degree of vacuum in the tank and the amount of manganese loss in the molten steel reflux.

[Explanation of symbols]

 1 Vacuum tank 2 Alloy input port 3 Oxygen top blowing lance 4 Rise pipe 5 Downcomer pipe 6 Slag 7 Ladle 8 Rise pipe 9 Ar gas blowing pipe for recirculation

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Manabu Tano 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Hidetoshi Matsuno 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside the Steel Pipe Co., Ltd. (72) Toshio Takaoka, Marunouchi 1-2-2, Chiyoda-ku, Tokyo Japan Steel Pipe Within (72) Inventor Tsuyoshi Murai, 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Co., Ltd. (72) Inventor Kanji Hiji 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nikko Steel Tube Co., Ltd.

Claims (1)

[Claims] 1. When decarburizing high-manganese molten steel using a vacuum degassing equipment to produce ultra-low carbon high-manganese steel, the carbon concentration in the molten steel before decarburizing is set to 0.06 wt% or less. A method for producing ultra-low carbon high manganese steel, which comprises performing vacuum decarburization while controlling the dissolved oxygen concentration in the molten steel during decarburization to be 400 ppm or less and 200 ppm or more.