JPH0925509A - Method for melting extra-low nitrogen chromium-containing steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an extra-low nitrogen Cr-containing steel in short time and at a low cost by adding Al into molten Cr-containing steel after roughly decarburizing in a reduced-pressure vessel, raising the temp., blowing H2 gas and thereafter, vacuum-decarburizing into an extra-low carbon range. SOLUTION: Aluminum is added into the molten Cr-containing steel in the reduced-pressure vessel after roughly decarburizing, and oxidized into Al2 O3 , and the temp. of the molten steel is raised to a prescribed temp. by this oxidation heat. In this temp. raising stage, H2 gas is blown into the molten steel, desirably at >=2.0Nm<3> /hr of flow rate per molten steel ton. By this method, fine H2 gas bubbles are acted as denitrification reaction site without rising the C concn. at the initial stage to effectively execute the denitrification. Thereafter, the dacarburization is executed to the desired extra-low carbon range by the ordinary vacuum decarburizing treatment. By this method, the decarburizing time is shortened, and the erosion of refractory and the labor can be reduced. Further, the extra-low carbon and extra-low nitrogen steel having <=about 30ppm C and <= about 100ppm N is economically and efficiently achieved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for smelting ultra-low nitrogen Cr-containing steel, and particularly proposes a method advantageous when used for producing ultra-low carbon steel by vacuum decarburizing treatment. It is a thing.

[0002]

2. Description of the Related Art It is known that in Cr-containing steel represented by stainless steel, the mechanical properties are improved by reducing the nitrogen concentration to 100 ppm or less. It is also known that in ferritic stainless steel, when the carbon concentration is 30 ppm or less, the rust resistance is dramatically improved and the toughness is also improved. As described above, in the Cr-containing steel, the quality can be greatly improved by reducing the carbon content and the nitrogen content to extremely low levels. Therefore, research and development are underway on ultra-low carbonization and ultra-low nitrogenization of such Cr-containing steel.

However, since Cr-containing steel has a higher equilibrium nitrogen concentration than ordinary steel, it is difficult to denitrify it to an extremely low nitrogen concentration by simply performing vacuum degassing treatment. Therefore very low nitrogen
The usual method for smelting Cr steel is to use a vacuum decarburizing process in which the initial C concentration of the molten steel contained in the decompression tank is made higher than a certain concentration and oxygen is blown into the decompression tank. A denitrification treatment method in which CO gas bubbles generated in large amounts in the initial stage are used as a reaction site for denitrification, that is, nitrogen in the steel is dissolved as nitrogen gas in the CO gas bubbles to remove the air from the bubbles and remove nitrogen. There is a treatment method to release to the outside.

In such a treatment method, although denitrification is performed together with decarburization, Cr-containing steels are required to have an extremely low nitrogen concentration in most cases in the case of steel types requiring an extremely low nitrogen concentration. Therefore, even if the concentration of either C or N is minimized after the treatment, desired characteristics cannot be obtained if the concentration of the other is not reduced so much. After all, the sum of carbon concentration and nitrogen concentration (C +
It is necessary to reduce both C and N by using N) as a parameter.

However, as described above, in order to denitrify the Cr-containing molten steel to an extremely low nitrogen concentration by the conventional method, it is necessary to raise the C concentration of the untreated molten steel to a certain level or higher. This means that long-term treatment is required to decarburize the molten steel to obtain ultra-low nitrogen to an ultra-low carbon concentration. This has had the adverse effect of increasing the number of processing utilities and increasing the cost significantly.

[0006]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems. In melting an extremely low nitrogen Cr-containing steel, an extremely low nitrogen concentration can be obtained without increasing the initial C concentration. Proposal of a method that enables denitrification up to the target area, thus shortening the processing time for decarburization to the target ultra-low carbon area, and reducing the melting loss of refractory materials and the utility costs The purpose is to do.

[0007]

According to the present invention, hydrogen gas is added to molten chromium-containing molten steel in a decompression tank after rough decarburization while Al is added to oxidize the molten steel to raise the temperature of the molten steel. It is a method for melting ultra-low nitrogen Cr-containing steel (first invention), which comprises decarburizing to a very low carbon region by vacuum decarburizing treatment.

In the method of the present invention, it is advantageous to blow hydrogen gas into the molten steel at a flow rate of 2.0 Nm ³ / hr or more per ton of molten steel, and the decompression tank has a VOD method, a DH method or
It is desirable that the decompression tank be used in the RH method.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below as a decompression tank using
A case where a decompression tank of Method D is used will be specifically described. In decarburization of Cr-containing steel under reduced pressure, it has been conventionally known that denitrification proceeds when the initial C concentration is increased. As an example thereof, FIG. 1 shows the influence of the initial C concentration on the reached N concentration quoted from the Iron and Steel Handbook (edited by Japan Iron and Steel Institute, published by Maruzen Co., Ltd., 1978, Vol. 2, page 708).
From FIG. 1, it can be seen that the higher the initial C concentration, the lower the reached N concentration after the decarburization treatment.

This means that the CO bubbles generated in the decarburization reaction increase the denitrification reaction rate by increasing the gas-liquid interface area which is the denitrification reaction site. In other words, the CO gas bubbles It is considered that the nitrogen in the steel is dissolved as N ₂ gas in the inside to release the nitrogen out of the system together with the floating removal of the bubbles.

FIG. 2 shows the transition of the nitrogen concentration in the molten steel during the vacuum decarburization treatment of the Cr-containing molten steel carried out by the inventors of the present invention. From Fig. 2, it was found that denitrification progressed at the early stage of decarburization where many CO gas bubbles were generated, and denitrification hardly progressed when the C concentration was 0.1% or less.

[0012] Based on the above experimental results, the inventors further conducted intensive studies, and as a result, devised to use hydrogen gas bubbles instead of CO gas bubbles as a denitrification reaction site. . That is, when hydrogen gas is blown into the molten steel, the blown hydrogen gas easily dissolves in the molten steel due to its nature, and becomes numerous micro bubbles of hydrogen gas again near the surface of the steel bath. The gas-liquid interface area, which is the site, will increase dramatically. Since the nitrogen gas partial pressure is zero in the innumerable fine bubbles generated near the surface of the steel bath, the denitrification reaction proceeds as nitrogen gas is generated in the bubbles. In other words, even if there are no CO bubbles, by blowing hydrogen gas, fine hydrogen gas bubbles with a CO partial pressure of zero inside will be present near the bath surface. As a result, a pathway different from the conventional one is formed, and the denitrification reaction is promoted.

When hydrogen gas is blown from the bottom of the decompression tank, it is once dissolved in the molten steel under the static pressure of the molten steel, but this dissolved hydrogen reduces the static pressure of the molten steel near the bath surface.
It becomes hydrogen gas bubbles again, and countless fine bubbles are generated near the bath surface. Also, the blowing of hydrogen gas is
The example is not limited to such bottom blowing, but may be an example of another immersion lance or side blowing from a side wall tuyere.

Further, the decompression tank is not limited to the decompression tank used for the VOD method, but may be a decompression tank used for the RH method or the DH method, and the same effect can be obtained by other vacuum decarburization treatment methods.

By the way, it was revealed for the first time by extensive experiments and studies by the inventors that there are particularly suitable conditions for promoting the denitrification reaction with hydrogen gas as described above. That is, when melting Cr-containing steel,
Although high-temperature treatment is suitable for reducing the oxidation loss of Cr, it is usually avoided that the molten steel temperature decreases in the time from the rough decarburization in a converter to the start of vacuum decarburization treatment such as VOD. hard. Therefore, add Al immediately after the start of VOD treatment,
For example, by sending acid, this Al is oxidized and generates heat,
The temperature of molten steel is often raised by utilizing the heat generated by the oxidation.

[0016] Since Al burns preferentially during the temperature rising period of the molten steel due to the heat of oxidation of Al as described above, the decarburization reaction does not proceed. Therefore, if hydrogen gas is blown into the steel bath while Al is burning preferentially, only the denitrification reaction will proceed. This means that even if the initial C concentration is low, a low N concentration can be reached. Conversely, it means that it is not necessary to increase the initial C concentration in order to accelerate the denitrification reaction, and by this,
The vacuum decarburization / denitrification processing time can be shortened.

The flow rate of the hydrogen gas blown into the molten steel during the temperature rise of the molten steel as described above depends on the amount of dissolved hydrogen gas per ton of molten steel, the rate at which the dissolved hydrogen gasifies again into hydrogen bubbles on the bath surface, the molten steel. Although it depends on the flow rate and the static pressure of molten steel at the hydrogen gas injection position, various experiments and studies have found that a flow rate of 2.0 Nm ³ / hr or more per ton of molten steel is suitable. If the flow rate of hydrogen gas is less than that, the denitrification rate cannot be improved because it does not contribute much to denitrification, and the treatment time becomes long, so an industrially effective denitrification effect cannot be expected. On the other hand, the denitrification rate increases as the flow rate of hydrogen gas increases, but if the flow rate of hydrogen gas to be blown is too large, the splash will be scattered and the degree of vacuum will decrease. The upper limit of is appropriately regulated.

As a result of extensive study by the inventors of the case where the hydrogen gas injection as described above is performed during decarburization, the hydrogen gas injection during decarburization causes an increase in splash generation amount in the high carbon concentration region. And to prevent it, CO
It will reduce the gas generation rate and will be disadvantageous for denitrification.
On the other hand, it was found that in the low carbon concentration range, the dissolved oxygen concentration in the molten steel increases, which is disadvantageous for denitrification.

From the above, the final stage of hydrogen gas blowing is at the start of the decarburization reaction, and even after all of the charged Al has been oxidized, while the degree of CO gas boiling due to the decarburization reaction is small. Blowing hydrogen gas can be expected to accelerate denitrification. Particularly when it is desired to obtain steel with a reduced nitrogen concentration, it is advantageous to positively set the time until the decarburization reaction starts by interrupting the acid feeding after the oxidation of Al.

When denitrification failure is found during decarburization and decarburization is in progress (low O in molten steel),
It is possible to denitrify by stopping the feeding of acid for decarburization and blowing hydrogen gas. Further, the atmosphere for blowing the hydrogen gas is preferably reduced pressure from the viewpoint of decreasing the equilibrium N ₂ concentration and increasing the expansion of H ₂ bubbles, but normal pressure may also be used.

[0021]

[Example] About 70 tons of 17% Cr steel roughly decarburized in a converter was received in a ladle. The composition of the molten steel in the ladle is shown in Table 1.

[0022]

[Table 1]

VOD treatment was performed on these molten steels.
The nitrogen concentration before treatment is 170-200ppm, the temperature before treatment starts is in the range of 1600 ± 20 ℃, and Al is 4.0-4.
5 kg was added, and oxygen gas was flown from the top blowing lance at a flow rate of 25 to 30 Nm ³ / hr t-steel for 15 to 20 minutes at a vacuum degree of 10 to 20 Torr, and the temperature was raised to about 50 ° C. Then, the usual decarburization treatment was performed.

In this treatment, in Examples 1 to 3 to which the present invention was applied, the initial C concentration was 0.51, 0.42,
It was 0.24%, and hydrogen gas was blown from the bottom blowing nozzle at a flow rate of 2.0, 3.5, 3.5 Nm ³ / hr t-steel in the temperature rising period utilizing Al oxidation heat generation. On the other hand, the comparative material was not blown with hydrogen gas. The initial C concentration of the comparative material is 0.55, 0.26%
Met. Example and comparative material treatment conditions and 0.1 ± 0.02
Table 2 shows the nitrogen concentration at the time of decarburization to% C. In addition, after denitrification, decarburization is continued to the extremely low carbon range,
0.003% was obtained, and the change in nitrogen concentration at that time was minute.

[0025]

[Table 2]

From the comparison of the achieved nitrogen concentrations of the example and the comparative material, it is understood that when the initial C concentration is the same, the reached nitrogen concentration is reduced by about 25% by blowing hydrogen gas.
In addition, in the comparative material with an initial C concentration of 0.26%, the reached nitrogen concentration is
The denitrification rate is 95 ppm, which indicates poor denitrification, whereas in the example of the present invention, denitrification progresses to 68 ppm, and it is clear that the decarburization time is shortened by about 25 minutes.

[0027]

EFFECTS OF THE INVENTION According to the present invention, the ultimate nitrogen concentration is reduced as compared with the conventional one, and Cr of extremely low nitrogen and extremely low carbon can be obtained in a short time.
The contained steel can be melted.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of initial C concentration on the reached N concentration in VOD.

FIG. 2 is a graph showing a transition of nitrogen concentration in molten steel during vacuum decarburization treatment of molten steel containing Cr.

Claims (3)

[Claims] 1. The molten chromium-containing steel in a decompression tank after rough decarburization contains Al
Is added to oxidize and the temperature of the molten steel is raised by this exothermic heat of oxidation, hydrogen gas is blown into the molten steel, and then decarburization is performed to a very low carbon range by vacuum decarburization treatment. Method for melting Cr-containing steel. 2. The method according to claim 1, wherein hydrogen gas is blown into the molten steel at a flow rate of 2.0 Nm ³ / hr or more per ton of molten steel. 3. The method according to claim 1, wherein the decompression tank is a decompression tank used in the VOD method, the DH method or the RH method.