JP2864961B2 - Decarburization refining method for high Mn steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decarburizing and refining high Mn molten steel having [Mn] ≧ 3 wt%. Here, the high-Mn molten steel also includes [Cr] ≧ 1 wt% molten steel.

[0002]

2. Description of the Related Art In recent years, as the field of use of steel materials has been diversified, many new steel types have been developed. However, the Mn content is approximately 3% (hereinafter, unless otherwise specified,% is "% by weight"). Higher Mn steel is one of them.

[0003] High-Mn steel is austenitic in structure and is not only inexpensive than Ni-containing austenitic stainless steel, but also has advantages of high strength and low magnetic permeability. Applications for magnetically levitated railways, nuclear fusion devices, degaussing devices, electrical devices, etc. are expanding as non-magnetic steel, structural steel, and wear-resistant steel.

[0004] High Mn steels generally have high permeability, weldability and machinability deteriorate when the [C] is high. Therefore, expensive metal Mn is conventionally used as the Mn source, and inexpensive ferromanganese is based on the [C] standard. Used within the allowable range. In some cases,
[P] may be added as long as the specifications allow. Therefore, this method has a disadvantage in that a large amount of expensive metal Mn is used and the melting cost is high.

[0005] In recent years, the demand for reducing the [C] of high-Mn steels has become more severe, and there are many [C] standards of 0.1 to 0.2% or less, and the melting cost is becoming higher. Therefore, a more inexpensive technology, such as blending most of Mn as fairly inexpensive high-C ferromanganese and decarburizing the resulting high-Mn molten steel with a high C content to produce low-C high-Mn molten steel Is required.

[0006] Today, as a general decarburization refining method, a method of refining molten steel under vacuum is widely used. A typical example is VOD, which is known for its stainless steel manufacturing method.
(Vacuum Oxygen Decarburization, O ₂ top blowing under vacuum). This method was applicable when the [C] allowable for high Mn steel was high, but when manufacturing low C high Mn steel, [C] ≦
In the area of 0.2%, the following problems occur.

[0007] The decarburization rate is extremely slow, only Mn is oxidized and lost by the upper blowing O ₂ , the molten steel temperature rises due to the heat of oxidation of the Mn, and high MnO slag is generated by the oxidation of the Mn. The material is severely eroded.

Accordingly, the present inventors decarburized and refined high-Mn molten steel with [Mn] ≧ 3% and decarburized to [C] <0.1%.
A less expensive degassing and refining method for Mn steel has been developed as follows.

[0009] That is, in Japanese Patent No. 1539737 (Japanese Patent Publication No. 1-25370, Japanese Patent Application Laid-Open No. 58-113314), "In decarburization refining under vacuum, powdery decarburization refining is applied to the surface of molten steel. [C] 10 by spraying additives for refining, carrier gas, etc. at a speed that can penetrate molten steel sufficiently
The method of "melting ultra-low C steel of less than ppm" is disclosed. Examples of the additives include those mainly containing Cr oxide, Mn oxide, and Fe oxide.

[0010] However, the above method is subject to
[Mn] is 1.7% molten steel, and after powder blowing, [Mn]
n] = 1.12%, showing a significant loss of Mn. [Mn] Whether decarburization and refining can be performed efficiently for molten steel of ≧ 3%, Mn
He did not teach at all whether or not he could see the loss. Furthermore, there was no teaching about the optimum powder composition and the conditions for reduction and refining in the case of molten steel with [Mn] ≧ 3%.

Next, the present inventors disclosed in Japanese Patent Laid-Open No. 5-125428.
In the official gazette (Japanese Patent Application No. 3-288011), [Mn] ≧ 8% high Mn
When decarburizing and refining steel under reduced pressure, the powdery decarburizing and refining additive containing Mn oxide is sufficiently mixed with the refining gas and / or other carrier gas into the molten steel. Disclosed is a method for decarburizing and refining high Mn steel, characterized by spraying [Mn] ≧ 8% on the surface of molten steel at a speed at which it can penetrate.

According to this method, MnO _{2 in} the powder blown onto the surface of the molten steel is dispersed in the molten steel and reacts as follows. This is an endothermic reaction. MnO ₂ +2 [C] → [Mn] + 2CO (1) Here, decarburization can proceed effectively without Mn loss by powder containing Mn oxide as follows. It is disclosed that it depends on the action. The powder acts as a nucleus for generating decarburized gas bubbles and promotes the decarburization reaction. [MnO] at the decarburization site in the molten steel increases due to the Mn oxide in the powder introduced by top blowing. However, since the oxidation of Mn in molten steel is suppressed, the powder acts as a cooling material, and the Mn oxide causes an endothermic reaction shown in Equation (1), so that the molten steel is cooled and Mn is removed at the decarburization site in the molten steel. Evaporation is suppressed.

Although there is no problem in the case of a low Mn steel, in the case of a high Mn steel, when an oxide other than the Mn oxide (for example, Fe or another metal oxide) is added, the oxidation loss of Mn proceeds. ,
Slag increases and refractory erosion becomes severe. Therefore,
It is disclosed that in the case of high Mn steel, good effects cannot be obtained unless Mn oxide is selected as an oxidizing agent.

[0014]

However, when high carbon ferromanganese is used as the Mn source according to the method described in Japanese Patent Application Laid-Open No. 5-125428 ([C] after burn-through is 1.5%).
~ 2.5%) The following concerns remained.

[0015] On吹速of the Mn oxide powder, there is an upper limit value on the device, can not be fast decarburizing compared to O ₂ over blown, it took a long time to process. The upward blowing of the Mn oxide powder lowers the temperature of the molten steel and requires a long-time treatment as described above. Therefore, a means for raising the temperature of the molten steel was required.

That is, in the case of a furnace having a large amount of heat radiation, such as a small furnace of 5 to 20 ton class, application is difficult unless heating means such as electricity and high frequency is provided, and even if it is applied, it takes a long time to process. Cost me.

An object of the present invention is to make it possible to decarburize to a low-carbon region while minimizing Mn loss as much as possible.
It is an object of the present invention to provide a method for controlling the temperature of molten steel without using the above-mentioned heating means which is not directly involved in the decarburization reaction, and for decarburizing in a short time by improving the method disclosed in Japanese Patent Application Laid-Open No. H10-15064.

[0018]

Means for Solving the Problems The present inventors are able to decarburize to a low carbon region ([C] ≦ 0.1%) while suppressing the loss of Mn disclosed in the above-mentioned JP-A-5-125428. It has been found that the above problem can be solved by alternately combining a method and a method in which an oxidizing gas typified by conventional O ₂ top blowing is blown upward and the temperature is raised while performing high-speed decarburization treatment. .

Here, in the present invention, in the decarburization refining of a high Mn steel of [Mn] ≧ 3 wt% under reduced pressure, an oxidizing gas is sprayed on the surface of the molten steel to raise the temperature while decarburizing at a high speed. Process, a powdery decarburizing refining additive containing Mn oxide,
A second decarburization step in which the additive is sprayed onto the surface of the molten steel by a carrier gas at a speed at which the additive can sufficiently penetrate into the molten steel, and decarburization is performed while cooling the molten steel. 2 Control the molten steel temperature using the decarburization process as cooling means,
At least a second decarburization step is performed after the first decarburization step.

Further, after the decarburization refining is completed, an alloy containing Si and a compound containing CaO are added to the slag to reduce the Mn oxide in the slag, thereby reducing the Mn oxide in the slag. It is a decarburization refining method.

[0021]

[Action] About molten steel temperature control When the molten steel temperature is 1650 ° C or more, Mn evaporation proceeds, so Mn
If the slag containing MnO is present on the surface of molten steel, problems such as accelerated erosion of refractories are caused.

On the other hand, when the temperature of the molten steel is 1550 ° C. or lower, there is a problem that oxidation of Mn proceeds and the decarburization reaction itself tends to be delayed. If the temperature drops further, the problem of solidification of the molten steel occurs. Therefore, the molten steel temperature is desirably 1650 to 1550 ° C. When it is desired to cool the molten steel, it is effective to perform MnO ₂ top blowing as described above (second decarburization step) and decarburize the molten steel so that the molten steel temperature does not rise.

The degree of MnO ₂ decomposition reaction and the reduction of molten steel temperature due to powder vary depending on local conditions such as furnace scale, refractory thickness, furnace shape, etc., but generally 1% decarburization amount [C]. On the other hand, it is -30 to -100 ° C.

The Mn loss in MnO ₂ top blowing varies depending on the stirring of the molten steel, the presence or absence of slag, and the properties of the slag, but is generally -2 to + 2% with respect to the decarburization amount [C] of 1%. .

On the other hand, when it is desired to heat the molten steel, it is effective to blow up the oxidizing gas (first decarburization step) and decarburize while raising the temperature (Mn also slightly burns Mn). Here, the oxidizing gas specifically indicates O ₂ , CO _2, or the like.

For example, in the case of the O ₂ top blowing method, the decarburization amount [C] is generally 1%, and the molten steel temperature is +100 to + 200 ° C. Mn loss is −2 to -4%.

Regarding the decarburization rate In the O ₂ top blowing method, the decarburization rate can be increased about 2 to 3 times as compared with the above Mn oxide top blowing, and the treatment can be performed in a short time. However, in the case of the O ₂ top blowing method, low carbonization of [C] ≦ 0.2% is difficult.
0.4%), decarburization mainly by Mn oxide top blowing,
At the same time, it is preferable to suppress Mn loss (due to oxidation or evaporation).

Therefore, in a preferred embodiment, O ₂ top blowing is performed in the initial stage of refining, decarburization is advanced in a short time, and O ₂ and Mn oxide top blowing is performed in an appropriate combination during refining to increase the temperature. At the end of refining, Mn oxide is blown up to control Mn loss while decarburizing to the low [C] range. Specifically, a method of performing Mn on-oxide sprayed after O ₂ on blown performs sprayed over the continuing O ₂ in the for, then the method of performing Mn on-oxide sprayed (the method of Example), the Next, a method of performing O ₂ upper blowing (when Mn loss is allowable) can be exemplified.

As for the type of powder to be blown upward for the decarburizing refining powder, a compound containing an oxide of Mn, for example, pure MnO ₂ or Mn ore is suitable. As described above, in the case of refining high Mn steel as in the present invention, it is not appropriate to add an oxide other than the Mn oxide.

If the supply rate of the powder is excessively increased, the slag containing Mn oxide deposited on the surface of the molten steel hinders the stirring of the molten steel and the stirring between the molten steel and the slag. Therefore, the powder supply rate is preferably 0.1 to 1.0 kg / min · ton (“ton” indicates per ton of molten steel).

The total supply amount of the powder varies depending on the oxidizing agent component and the [C] value before the powder is blown upward, but is preferably about 20 to 120 kg / ton. The spraying of powder for decarburization refining is, specifically,
For example, the molten steel was sprayed onto the surface of the molten steel at Mach 1 together with the carrier gas, but the velocity is not limited by this example as long as it is within a range capable of sufficiently penetrating into the molten steel. As a carrier gas, a commonly used gas such as Ar is used.

[0032] after completion of the decarburization refining for the reduction refining, for the recovery of Mn in the slag to the molten steel, adding an alloy containing Fe-Si, further melting point depression of the slag, containing CaO for basicity adjustment May be added to reduce MnO 2 as shown in formula (2). 2MnO + [Si] → SiO ₂ + [Mn] (2) The conditions of slag at the time of reductive refining are as follows.

In order to promote the reaction of the formula (2) between the molten steel and the slag, it is advantageous that the melting point of the slag is lower, and the range is (CaO) / (SiO ₂ ) = 1 to 2. preferable. In order to promote the reduction of MnO 2, it is considered desirable to lower the solubility of MnO 2. Since MnO 2 is a weak but basic oxide, it is desirable that the slag has a higher basicity. From that point, CaO was added.

If the stirring of the molten steel is insufficient, the movement of C in the molten steel (from the bulk in the molten steel to the decarburization site) is hindered in the low [C] range. It is considered that the reaction of the formula is delayed, and the decarburization rate is reduced. Therefore, molten steel stirring is important for maintaining a sufficient decarburization rate. Therefore, stirring enhancement such as bottom blowing may be performed in some cases. Next, the present invention will be described more specifically with reference to examples. However, this is merely an example and is not intended to unduly limit the present invention.

[0035]

【Example】

(Preliminary Experiment) FIG. 2 shows the results (graphs of changes in components and temperature) of a preliminary experiment conducted using the 10-ton furnace shown in FIG. FIG. 1B illustrates a lance used in the present invention. The lance of the present invention has a straight center hole 8 having a diameter of 5 mm.
And a Laval-type side hole 9 (3 mm inward angle 3 ° with a diameter of 2 mm)
Holes).

The initial molten steel component at the time of tapping the electric furnace is [C] =
2.1%, [Mn] = 18.4%, and the temperature before the refining treatment was 1520 ° C. First, the atmosphere was exhausted to 30 Torr through the duct 4. Further, the upper lance 6 is used to apply heat to the molten steel.
O _{2 was} blown on and decarburized.

When O ₂ is blown upward, 0.5 Nm ³ / min · ton (5 kg / cm ² ) of O ₂ gas from the side hole 9 and Ar for preventing clogging from the center hole 8.
0.1 Nm ³ / min · ton (3 kg / cm ² ) was blown upward. Further, the distance between the lower end of the upper blowing lance 6 and the surface of the molten steel 3 is 1100
mm.

In this O ₂ top blowing, Fe and Mn are oxidized and MnO-
FeO-based slag was slightly deposited on the molten steel surface. In addition, the temperature of molten steel could be raised to 1610 ° C by O ₂ top blowing for 20 minutes. Next, decarburization with Mn oxide powder was performed in an atmosphere of 20 Torr.

As the decarburizing agent powder 5 sprayed on the surface of the molten steel 3, Mn ore (T.Mn = 54.4%, T.Fe = 1.9%, Al ₂ O ₃
= 3.9%, SiO ₂ = 1.7%). Decarburizer powder 5
This was sprayed at a high speed of Mach 1 from a central hole 8 to molten steel 3 together with Ar as a carrier gas. Ar was blown out from the side hole 9 at Mach 3.8 to accelerate the decarburizer powder blown out from the center hole 8. The pressure of Ar of the carrier gas in the center hole 8 is 3 kg.
/ cm ^2, gas flow rate 0.2 ~0.4Nm ³ / min · ton, also of the side hole thereof 5 kg / cm ^2, was 0.4 ~0.5Nm ³ / min · ton.

On the other hand, the supply speed of the decarburizer powder is 0.8 kg / m
The total supply in in tons was 40 kg / ton. Further, the distance between the lower end of the upper blowing lance 6 and the surface of the molten steel 3 is 600
mm.

Further, Ar was blown from the furnace bottom through the porous plug 2 as a stirring gas of 1 to 2 Nl / min · ton. By this MnO ₂ top blowing for 50 minutes, the molten steel temperature dropped to 1570 ° C, and the Mn0-FeO slag that had just been deposited was
Slag was reduced by high [C] molten steel.

Thereafter, the O ₂ top blow and the MnO ₂ top blow were repeated again to achieve [C] ≦ 0.1% decarburization while controlling the molten steel temperature to around 1600 ° C. as shown in FIG. After the powder was blown up, Fe-Si and CaO were added. By this refining, Mn in the slag was recovered in the molten steel, and [Mn] in the molten steel increased by about 2%. In addition, (CaO) /
(SiO ₂ ) was about 1.5.

(Example) In this experiment, a 20-ton electric furnace and a 20-ton VOD furnace, which were larger than the preliminary experiment, were used, and the decarburization refining of the present invention (Example No. 1) was performed.
The decarburization refining was performed for only O ₂ upper blowing (Comparative Example No. 2) and MnO ₂ upper blowing only (Comparative Example No. 3). The structure and operation of the device followed the preliminary test.

As the Mn source, a high-carbon high-Mn molten steel ([C] {2.0-2.1%, [Mn]}) obtained by using a high-carbon ferromanganese alloy and melting it in the air by conventional means is used. (18-19%)
Decarburization and reduction refining were performed on 18 tons. In each case, Mach 1 Ar gas was used as a carrier gas. Table 1 shows the decarburization refining conditions and results.

As shown in Example No. 1, according to the present invention, [C] finally becomes 0.1% or less, and
[Mn] was recovered at about 2%, and a low C high Mn steel was obtained. Also,
The processing temperature is about 1550-1650 without using heating means that is not directly involved in decarburization such as electricity, high-frequency induction, Al, Si heating, etc.
Decarburization was possible between ° C.

On the other hand, as shown in Comparative Example No. 2, with only O ₂ top blowing, decarburization stagnated at around 0.4% and Mn oxidation proceeded, and a low C high Mn steel could not be obtained. Moreover, the molten steel temperature is 18
Since the temperature exceeded 00 ° C and there was a concern of refractory melting, decarburization was stopped.

Also, as shown in Comparative Example No. 3, although only [C] can be reduced to 0.1% by only blowing over MnO ₂ , the temperature of molten steel is extremely lowered, and the molten steel at the bottom and wall of the ladle is solidified. And began to hinder operations.

In addition, from the viewpoint of the decarburization treatment time, Example No. 1
Comparing Comparative Example No. 3 with Comparative Example No. 3, in Example 1, O ₂ top blowing having a relatively high decarburization rate was used for 40 minutes.
Approximately 30% of the time was shortened from 200 minutes in Example 1 to 140 minutes in Example 1.

[0049]

[Table 1]

[0050]

According to the method of the present invention, the Mn loss can be minimized, the molten steel temperature can be controlled within a certain range without a heating means not directly involved in decarburization, and the [C]
Can decarbonize up to low coal area of 0.1% or less. Therefore, the method of the present invention is extremely useful for producing low-C high-Mn steel, which has been increasing in production in recent years, at low cost.

[Brief description of the drawings]

FIG. 1 (a) is an explanatory view of a furnace used in the method of the present invention and an embodiment thereof, and FIG. 1 (b) is an example of a bottom view of an upper blowing lance.

FIG. 2 is a graph showing the progress of decarburization of a high-Mn molten steel according to the method (preliminary experiment) of the present invention, and is a graph showing a relationship between a C concentration, a Mn concentration, and a molten steel temperature with respect to a processing time.

[Explanation of symbols]

 1: Decarburization refining vessel 2: Porous plug 3: Molten steel 4: Duct 5: Decarburization refining additive 6: Top blowing lance

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hideyoshi Yamaguchi 5-1-1109 Shimaya, Konohana-ku, Osaka-shi Sumitomo Metal Industries, Ltd. Steelworks (72) Inventor Ken Muroi 5-1-1, Shimaya, Konohana-ku, Osaka-shi No. 109 Sumitomo Metal Industries, Ltd. Steel Works (72) Inventor Toru Moriya 5-1-1, Shimaya, Konohana-ku, Osaka-shi Sumitomo Metal Industries, Ltd. Steel Works (56) References JP-A-5-125428 (JP, A) JP-A-1-92312 (JP, A) JP-A-63-293109 (JP, A) Materials and processes, Vol. 5, No. 1, p. 271 (1992) Materials and Processes, Vol. 5, No. 1, p. 276 (1992) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21C 7/00 C21C 7/04 C21C 7/068 C21C 7/10

Claims (2)

(57) [Claims] In a decarburization refining of a high Mn steel of [Mn] ≧ 3 wt% under reduced pressure, a first decarburization step of blowing an oxidizing gas onto a molten steel surface to raise the temperature while decarburizing at a high speed; A powdery decarburizing refining additive containing Mn oxide is sprayed onto the surface of the molten steel by a carrier gas at a speed at which the additive can sufficiently penetrate into the molten steel, and the decarburization is performed while cooling the molten steel. Controlling the molten steel temperature by using the first decarburizing step as a heating means and the second decarburizing step as a cooling means, and performing at least the second decarburizing step after the first decarburizing step. Decarburization and refining method for high Mn steel. 2. After completion of the decarburization refining according to claim 1, an alloy containing Si and a compound containing CaO are added to the slag to reduce Mn oxide in the slag. Refining method for high-Mn steel.