JPH09249910A - Method for raising temperature of molten steel in rh degassing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for raising the temp. of molten steel in an RH degassing equipment restraining the development of MnO and FeO in slag with out deteriorating the cleanliness of the molten steel. SOLUTION: At the time of using [wt.% Mn] for Mn concn. in the molten steel 2 before raising the temp., α(kg/min) for quantity of Al flowing into a vacuum vessel 1 calculated from a product of Al concn. in the molten steel 9 and circulating flow rate, β(kg/min) for blowing oxygen quantity from a nozzle 5 and η or oxygen efficiency, the ratio of α and η×β during time raising the temp. is in the range of satisfying formula I. However, in the case the positional height of the nozzle is <=3m from the static molten steel 9 surface in a vacuum vessel 1 and in the case of dipping the nozzle 5 into the molten steel 9, the oxygen efficiency is η=1, and in the case the positional height of the nozzle 5 exceeds 3m, the oxygen efficiency η is shown in formula II. Formula I: α/η×β)>0.625×[wt% Mn]+0.625 Formula II: η= 0.12×[the positional height of the nozzle (m)] +1.36.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method of blowing oxygen gas into a vacuum chamber for RH degassing and burning Al in molten steel in the vacuum chamber to raise the temperature of the molten steel.

[0002]

2. Description of the Related Art There is a strong demand for improving the quality of steel, and therefore, after the steel is taken out of a converter, the number of steel types subjected to secondary refining such as desulfurization, decarburization and dehydrogenation is increasing. As the refining furnace used for this secondary refining, decarburization / dehydrogenation by vacuum treatment is possible, and desulfurization by flux by strong stirring in the vacuum tank is also possible, so RH degassing equipment is widely used. in use.

During the RH degassing process, the temperature of molten steel decreases due to heat transfer to the vacuum chamber and heat dissipation to the atmosphere.
Then, various treatments such as desulfurization, decarburization, and dehydrogenation are performed with the same heat, and the treatment time is extended, so that the molten steel temperature drop is further increased.

If the tapping temperature in the converter is increased in order to compensate for this temperature decrease, the refractory body of the converter furnace body is severely worn, resulting in an increase in the basic unit of the refractory body of the converter furnace body. . It is also possible to heat the inside of the ladle by applying electric energy such as arc heating, but in this case,
Furthermore, refining equipment with a heating device is required, which not only causes harmful effects due to increased equipment costs, but also lowers temperature of molten steel due to extension of residence time in ladle due to transfer of molten steel between refining furnaces, wear of refractory ladle, etc. Bring new evil.

Therefore, a technique for raising the temperature of molten steel in an RH degassing facility is disclosed in Japanese Patent Publication No. 56-50763 (hereinafter referred to as "Prior Art 1"). Prior art 1 introduces oxygen gas into the molten steel of a vacuum chamber,
This is a method of adding an exothermic agent such as Al or Si to the molten steel and utilizing the heat generated by the oxidation reaction of the exothermic agent and oxygen gas to raise the temperature of the molten steel. Also, a large amount of A produced by this oxidation reaction
If oxides such as l ₂ O ₃ remain in the molten steel, they adversely affect the product quality as non-metallic inclusions.
In the above, the adsorbent for the produced oxide is added to the vacuum tank at the same time as the oxygen gas, and is moved and arranged between the molten steel surface in the ladle and the slag by the subsequent molten steel flow to absorb the oxide. And, as the adsorbent, synthetic slag having CaO or SiO ₂ as a main component can be used.

[0006]

Problems to be Solved by the Invention Al, Si, and Mn
When oxygen gas is blown into or blown into molten steel containing Al, the affinity with oxygen becomes smaller in the order of Al, Si, Mn, and Fe, so Al and Si are easily oxidized, but Al and Si
Even if is present in the molten steel, Mn and Fe are simultaneously oxidized. Therefore, also in Prior Art 1, in addition to Al ₂ O ₃ and SiO ₂ , lower oxides of MnO and FeO are mixed as oxides to be generated.

In the prior art 1, the generated oxide is slag
Is said to improve the cleanliness of molten steel.
When MnO or FeO is generated as an oxide to be formed,
Oxygen potential of slag absorbing these lower oxides
Rises. When the oxygen potential of slag rises,
Al or Si in molten steel and Mn in slag after RH degassing
Reaction with O and FeO occurs, and Al or Si in molten steel is acid
Alkalised (this oxidation is called "reoxidation")_TwoO_Three, S
iO_TwoAnd eventually Al in the molten steel_TwoO_ThreeAnd SiO_TwoIs new
Will be generated. And this reoxidation is
Casting after RH degassing occurs continuously after gas treatment
By the time enough Al_TwoO_ThreeAnd SiO _TwoAscent time
Since there is no_TwoO_Three, SiO_TwoRemains in molten steel
Existing as non-metallic inclusions, and therefore highly clean steel.
It cannot be manufactured.

The present invention has been made in view of the above problems,
The purpose is to use MnO and FeO in the slag.
R is suppressed and R is maintained without deteriorating the cleanliness of molten steel.
It is intended to provide a method for heating molten steel in an H 2 degassing facility.

[0009]

Means for Solving the Problems As a result of an investigation conducted by the present inventors in order to prevent the formation of lower oxides at the time of heating during RH degassing, the following facts have become clear. That is, Al
When the molten steel is heated to heat the molten steel and the amount of oxygen that burns the solute element in the molten steel or the molten steel (Fe) is insufficient when the amount of Al entering the vacuum tank due to recirculation is insufficient, combustion of Mn in the molten steel occurs. However, combustion of Fe also occurs subsequently. Then, the higher the Mn concentration in the molten steel, the larger the amount of Al that needs to enter the vacuum chamber. On the other hand, if the amount of oxygen is small, the rate of temperature rise is reduced, leading to extension of the temperature rise time. That is, by optimally controlling the ratio of the amount of Al and the amount of oxygen that enter the vacuum chamber in accordance with the Mn concentration in the molten steel, Al is preferentially oxidized without unnecessary extension of time and slag It is possible to suppress the increase in oxygen potential.

The present invention has been made on the basis of the above findings, and the method for heating molten steel according to the present invention is such that the molten steel containing Al is circulated in the vacuum tank of the RH degassing equipment, and the molten steel in the vacuum tank is supplied with a nozzle from a nozzle. In the method of heating molten steel in which oxygen gas is blown and blown to burn Al in molten steel to raise the molten steel temperature, the Mn concentration in the molten steel before heating is [wt% Mn]
And the amount of Al flowing into the vacuum chamber, which is calculated as the product of the Al concentration in molten steel and the annular flow rate, is α (kg / min), the amount of oxygen fed from the nozzle is β (kg / min), and the oxygen efficiency is Is defined as η, the ratio of α to η × β is within the range satisfying the expression (1) during the heating period. α / (η × β)> 0.625 × [wt% Mn] +0.625 (1)

However, when the nozzle position height is 3 m or less from the stationary molten steel surface of the vacuum chamber or when the nozzle is immersed in the molten steel, the oxygen efficiency is η = 1 and the nozzle position height exceeds 3 m from the stationary molten steel surface. In this case, the oxygen efficiency η is shown by the equation (2). η = −0.12 × [nozzle position height (m)] + 1.36 (2)

In RH degassing, oxygen gas is introduced into the vacuum chamber with a nozzle while the vacuum chamber is evacuated and depressurized.
Therefore, when the nozzle is not immersed in the molten steel, not all the oxygen gas introduced into the vacuum chamber reaches the surface of the molten steel to burn the solute element or the molten steel. The present inventors defined the amount of oxygen that reaches the surface of molten steel and burns the solute element or molten steel as the "effective oxygen amount", and also defines the ratio of the effective oxygen amount to the oxygen supply amount as "oxygen efficiency". In order to understand the amount of available oxygen during heating, the effect of nozzle height on the amount of available oxygen was investigated. Since it is easy to change the height of the nozzle position in the investigation, an upper blowing lance provided on the canopy of the vacuum chamber shown in FIG. 1 was used, and oxygen gas was blown to the molten steel from the nozzle attached to the tip of the upper blowing lance.

The reference point of the nozzle position height is the stationary molten steel surface in the vacuum tank (the stationary molten steel surface in the vacuum tank is the lathe head difference caused by the difference between the pressure in the vacuum tank and the atmospheric pressure). It is the calculated height position added to the molten steel surface), and the distance from this stationary molten steel surface was taken as the nozzle position height. FIG. 6 shows 25
In the molten steel of 0 tons, when the initial Al concentration was 0.2 wt%, the ring flow rate was 150 tons / min, and the amount of oxygen supply was 10 to 100 kg / min, the nozzle position height was changed to 8 m.
It is the figure which showed the result of having measured the amount of effective oxygen from the change of 1 concentration, calculated | required the oxygen efficiency ((eta)) from the ratio of the amount of effective oxygen, and the amount of oxygen transfer.

As shown in FIG. 6, the oxygen efficiency did not change until the nozzle position height was 3 m and was 1.0, and when it exceeded 3 m, it decreased linearly. In the range where the nozzle position height exceeds 3 m, the oxygen efficiency is set to 1 of the nozzle position height by the least square method.
The following function was obtained to obtain the expression (2). Thus, in the present invention, the oxygen efficiency was set to 1.0 up to a nozzle position height of 3 m, and the oxygen efficiency was determined as a function of the nozzle position height in the range exceeding 3.0 m.

In the present invention, the oxygen efficiency is obtained from the position of the nozzle attached to the tip of the upper blowing lance installed on the canopy of the vacuum chamber. However, if the nozzle has a structure for blowing oxygen gas onto the molten steel surface, it is provided on the side wall of the vacuum chamber. Even with a different nozzle, the oxygen efficiency is the same as when it is blown from the nozzle attached to the tip of the upper blowing lance.

The oxygen efficiency (η) when the nozzle was immersed in molten steel was also investigated, and it was confirmed that the oxygen efficiency (η) was 1.0 when the nozzle was immersed. In this way, when the nozzle position is determined, the oxygen efficiency is determined.

The oxygen efficiency was determined at the nozzle position, the amount of acid supply (β) was changed, and the amount of MnO pickup in the slag before and after heating was investigated. The results are shown in FIGS. 2, 3 and 4. . Although these figures will be described in detail in Examples in the latter stage, the effective oxygen amount (α) with respect to the Al amount (α) flowing into the vacuum chamber calculated as the product of the Al concentration in molten steel and the annular flow rate at the end of heating ( η × β) ratio (α / (η × β)) on the horizontal axis,
The vertical axis shows the amount of MnO pickup in the slag before and after heating. Fig. 2 shows the Mn concentration in molten steel at 0.5 wt%, Fig. 3 shows the Mn concentration in molten steel at 1.0 wt%, and Fig. 4 shows the molten steel. Mn
The result is that the concentration is 1.5 wt%. In addition, Fe in the slag
Since O increases in proportion to the formation of MnO in the slag, MnO was representatively investigated as a lower oxide in the present invention.

From these figures, in order to suppress the amount of MnO pickup in slag to less than 1.0 wt%, Mn in molten steel should be reduced.
When the concentration is 0.5 wt%, α / (η × β) is 1.0 or more, when the Mn concentration in the molten steel is 1.0 wt%, it is 1.3 or more, and the Mn concentration in the molten steel is 1.5 wt%. In this case, 1.6 or more is required, and α / (η
It can be seen that it is necessary to increase xβ), that is, to increase the amount of Al flowing into the vacuum chamber or reduce the amount of available oxygen. If the MnO content in the slag is less than 1 wt%, the number of inclusions having a diameter of 0.020 mm or more in the cast piece is 3 pieces / cm ² or less and the cleanliness is high, as described in Examples. Was used as the standard.

FIG. 5 is a summary of the results of FIGS. In FIG. 5, the boundary line between the range in which the amount of MnO pickup in the slag is less than 1.0 wt% and the non-metallic inclusions are small and the quality is good, and the range in which the quality is poor due to the non-metallic inclusions is determined. As the boundary line, the following expression (3) is obtained as shown in FIG. 5, and the range of good quality is α / (η ×
β becomes a range larger than the boundary line of the expression (3), and the inequality of the expression (1) of the present invention can be obtained. α / (η × β) = 0.625 × [wt% Mn] +0.625 (3)

In this way, MnO and F in the slag are
A molten steel heating method that suppresses an increase in eO and does not deteriorate the cleanliness of molten steel can be obtained.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION A case where the embodiment of the present invention is applied to the RH degassing equipment shown in FIG. 1 will be described. In FIG. 1, 1 is a vacuum tank, 2 is an ascending side immersion pipe, 3 is a descending side immersion pipe, 4 is an upper blowing lance, 5 is a nozzle attached to the tip of the upper blowing lance 4, 6 is a recirculation gas blowing pipe, 7 is reflux gas, 8 is ladle, 9 is molten steel, 10 is slag, and 11 is oxygen gas.

The composition of the slag 10 having a higher Al ₂ O ₃ absorption capacity and a lower content of MnO and FeO are more convenient because a clean steel can be obtained. In order to lower MnO and FeO in the slag 10, the converter slag that flows in at the time of tapping the converter is removed and new slag is added, or
It is desirable to add a deoxidizing agent such as Al to the slag in the ladle. The newly added slag does not contain lower oxides of MnO and FeO.

The dipping pipes 2 and 3 are dipped in the molten steel 9 in the ladle 8, the inside of the vacuum chamber 1 is exhausted by an exhaust device (not shown), and the circulating gas is blown from the circulating gas blowing pipe 6 to the rising side. When blown into the pipe 2, the molten steel 9 rises in the ascending side immersion pipe 2 and flows into the vacuum chamber 1, and then flows back from the descending side immersion pipe 3 back to the ladle 8, forming a so-called reflux. Since it is difficult to actually measure the ring flow rate, the present invention calculates it using the empirical formula (4). Q = 11.4 × G ^1/3 × D ^4/3 × (ln (P ₁ / P ₂ )) ^1/3 ...... (4)

However, in the equation (4), Q is the reflux of molten steel.
Amount (ton / min), G is the reflux gas amount (Nl / min), D is the soak rate
Inside diameter of pickling pipe (m), P₁Is the gas injection point pressure (tor
r), P _TwoIs the vacuum chamber pressure (torr).

Samples for component analysis are taken from the molten steel 9 to grasp the Al concentration and Mn concentration in the molten steel before heating.

Al in the molten steel reacts with the oxygen gas 11 and generates heat to increase the temperature of the molten steel 9 and decrease it. Therefore, if Al is not added to the molten steel 9 during heating, Al in molten steel before heating
The concentration must have a calorific value of at least an equivalent amount when the molten steel 9 is heated by a predetermined temperature. Further, when Al is added during heating, the total calorific value is such that the total calorific value satisfies the same calorific value when the molten steel 9 is heated by a predetermined temperature, and the vacuum is a precondition for calculating the equation (1). The Al concentration in the molten steel must be such that the Al amount (α) flowing into the tank 1 can always be secured.

Further, when the Al concentration in the molten steel decreases, the heating time increases, so it is desirable to secure the Al concentration in the molten steel of 0.01 wt% or more at the end of the heating.

Ring flow rate calculated from equation (4) and Al in molten steel
From the concentration, the amount of Al (α) flowing into the vacuum chamber 1 by the recirculation is determined, and by substituting this Al amount (α) and the previously analyzed Mn concentration in the molten steel into the equation (1), the effective oxygen amount (η
The maximum value of xβ) can be calculated. From the maximum value of the amount of effective oxygen thus obtained, the nozzle position and the amount of acid supply can be determined. However, as described above, the amount of Al (α) flowing into the vacuum chamber 1 decreases as the temperature rises.
Therefore, there are the following two methods for obtaining the maximum value of the effective oxygen amount from the equation (1).

One method is to reduce the amount of oxygen fed into the vacuum chamber 1 as the temperature rises (α) and to reduce the amount of oxygen to be fed or the height of the nozzle to increase the effective oxygen amount (η).
It is a method of gradually decreasing stepwise or continuously during heating. At this time, the amount of Al flowing into the vacuum chamber 1 (α)
Is a sample for analysis taken from the molten steel during the heating
Although it can be accurately grasped by analyzing the concentration, it is also possible to preliminarily investigate the relationship between the total amount of available oxygen and the amount of decrease in Al, estimate the Al concentration in molten steel from the relationship, and grasp the Al amount (α). It is possible.

Another method is to grasp the amount of heat rise required to raise the temperature of the molten steel to a predetermined temperature, calculate the amount of decrease in Al corresponding to this amount of heat rise, and calculate the Al concentration in the molten steel before raising the temperature. To A
By subtracting the amount of decrease of l, the Al concentration in the molten steel at the end of the heating is predicted. Based on the predicted Al concentration in the molten steel at the end of heating, the amount of Al flowing into the vacuum chamber 1 by recirculation is calculated, and the maximum value of the effective oxygen amount is calculated by the formula (1) to determine the nozzle position. , And the amount of acid transfer. In this method, the effective oxygen amount is controlled to be constant on the precondition of the Al concentration in the final stage of heating, so that the Al flowing into the vacuum chamber 1
In the early stage of heating with a large amount, the effective oxygen amount is suppressed to be low, and the heating time is longer than that of the above-mentioned one method, but the analysis of Al in molten steel during the process becomes unnecessary.

The nozzle position and the amount of oxygen to be fed are determined as described above, and the oxygen gas 11 is introduced into the vacuum chamber 1 from the nozzle 5 through the upper blowing lance 4 and blown or blown onto the surface of the molten steel 9 to raise the gas according to the present invention. Start the heat. The heating is terminated at the time when the predetermined total amount of oxygen fed is reached, or when the temperature of the molten steel reaches a predetermined temperature even if the total amount of fed oxygen is not more than the predetermined amount.

Although FIG. 1 shows a structure in which the upper blowing lance 4 penetrates the canopy of the vacuum chamber 1, the upper blowing lance 4 does not.
No matter whether the nozzle 5 has a structure that penetrates the side wall or the structure in which the nozzle 5 is fixed to the side wall of the vacuum chamber 1, as long as it is a structure that blows / sprays oxygen gas toward the molten steel, The application does not hinder.

[0033]

[Example] Before the dehydrogenation treatment was performed on 250 tons of molten steel tapped from a converter into a ladle by the RH degassing shown in Fig. 1, the heating method of the present invention was carried out.

The recirculation flow rate was adjusted by changing the amount of Ar gas for recirculation flow blown into the immersion tube within the range of 2000 to 4000 Nl / min, and the recirculation flow rate was calculated based on the equation (4). still,
The inner diameter of the dip tube is 580 mm.

The oxide absorbing slag is burnt lime; 150
0 kg was added on the molten steel in the ladle together with 300 kg of Al as a deoxidizer at the time of tapping the converter. The content of MnO and FeO in the slag was 0 to 2 wt%.

A sample for analysis of molten steel and a sample for analysis of slag were taken before and after the start of heating and the Mn concentration in the molten steel and Al were analyzed.
The concentration and the MnO concentration in the slag were analyzed.

As for the amount of Al flowing into the vacuum chamber, the amount of heat rise required to raise the temperature of the molten steel to a predetermined temperature is grasped, the amount of decrease in Al corresponding to this amount of heat rise is calculated, and the amount of molten steel before raising the temperature is calculated. The amount of decrease in the amount of Al corresponding to the amount of heat rise is subtracted from the medium Al concentration to predict the Al concentration in the molten steel at the end of heating, and the product of the predicted Al concentration in molten steel at the end of heating and the ring flow rate. Decided as. Then, by substituting the amount of Al flowing into the vacuum chamber thus determined and the Mn concentration in the molten steel before heating to the formula (1), the maximum value of the effective oxygen concentration is determined, and the amount of oxygen supply and the nozzle position are determined. Then, acid was sent from the nozzle through the lance to raise the temperature. When the predetermined amount of total acid feeding amount was reached, the acid feeding was stopped and the heating was completed. The heating time is 8 to 12 minutes. After heating, dehydrogenation treatment was performed for about 15 minutes, and then cast by a continuous casting machine to obtain a slab.

Table 1 shows the Mn concentration in the molten steel before the heating, the Al concentration in the molten steel after the heating, the ring flow rate, the Al amount α flowing into the vacuum chamber determined by the above-mentioned method, the oxygen transfer amount β, Nozzle position, oxygen efficiency η obtained from nozzle position, ratio α / (η × β) between Al amount and effective oxygen amount, calculated value on the right side of equation (1), and MnO pickup amount in slag before and after heating And show the quality evaluation. The numbers in the nozzle position column of Table 1 represent the height from the stationary molten steel surface in the vacuum chamber, and the nozzle position "0" represents the case where the nozzle was immersed in the molten steel.

[0039]

[Table 1]

The quality was evaluated by examining inclusions in the cross section of the slab, and 3 inclusions / cm ² having a diameter of 0.020 mm or more were evaluated.
The following were evaluated as good quality, and the higher quality was evaluated as poor quality. The FeO pickup amount of the slag was almost half the MnO pickup amount.

Table 1 shows that when the value of α / (η × β) is out of the range of the present invention, that is, mainly with respect to the amount of Al flowing into the vacuum chamber at the end of heating, the amount of effective oxygen is The case where there were too many was described as a comparative example.

In the examples, since the Al concentration after heating was estimated and the amount of effective oxygen was determined, α was still obtained at the end of heating.
The value of / (η × β) is often higher than the calculated value on the right side of equation (1), but it does not become less than the calculated value on the right side of equation (1), and the pickup of MnO in the slag is also In the test of 17 times, it could be suppressed to a low value of 0.6 wt% at maximum.
The quality evaluations were all good.

In the comparative example, α / (η × β) at the end of heating
When the value of is close to the calculated value on the right side of the equation (1), the pickup amount of MnO in the slag is relatively small, but the pickup amount is still less than 1 wt%,
All the quality evaluations were bad, and cleanliness could not be secured.

In FIGS. 2, 3 and 4, the horizontal axis is α / (η × β) shown in Table 1 of the examples and comparative examples, and the vertical axis is the amount of MnO pickup in the slag during heating. Indicate. From these figures, in the example in which α / (η × β) was controlled within the range of the present invention, the MnO pickup amount was less than 1 wt% even in the test in which α / (η × β) was close to that of the comparative example. A clear difference can be seen between the example and the comparative example. This is because as long as it is within the scope of the present invention, Al in the molten steel preferentially burns to suppress the oxidation of Mn in the molten steel.

[0045]

EFFECTS OF THE INVENTION According to the present invention, M in molten steel is increased when the temperature is raised by the oxidation reaction between Al in molten steel and oxygen gas in RH degassing.
Depending on the n concentration, the ratio of the amount of Al entering the vacuum chamber and the amount of available oxygen is controlled to an optimum value, so Al in molten steel is preferentially oxidized and MnO and FeO pickup in slag are suppressed. As a result, it is possible to obtain a slab with high cleanliness.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of RH degassing equipment to which the present invention is applied.

FIG. 2 is a comparative example showing the effect of the ratio of the amount of available oxygen to the amount of Al entering the vacuum chamber on the amount of MnO pickup of slag in the example of the present invention when the Mn concentration in molten steel is 0.5 wt%. It is the figure shown with.

FIG. 3 is a comparative example showing the effect of the ratio of the amount of available oxygen to the amount of Al entering the vacuum chamber on the amount of MnO pickup of slag in the example of the present invention when the Mn concentration in molten steel is 1.0 wt%. It is the figure shown with.

FIG. 4 is a comparative example showing the effect of the ratio of the amount of available oxygen to the amount of Al entering the vacuum chamber on the amount of MnO pickup of slag in the example of the present invention when the Mn concentration in molten steel is 1.5 wt%. It is the figure shown with.

FIG. 5 is a diagram in which, according to the example of the present invention and the comparative example, a boundary line for obtaining good quality and poor quality is obtained due to a difference in pickup of MnO in slag during heating.

FIG. 6 is a diagram showing a result of investigation on influence of nozzle position height on oxygen efficiency.

[Explanation of symbols]

 1; vacuum tank 2; ascending side immersion pipe 3; descending side immersion pipe 4; upper blowing lance 5; nozzle 6; circulating gas blowing pipe 7; circulating gas 8; ladle 9; molten steel 10; slag 11; oxygen gas

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshio Takaoka 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Tsuyoshi Murai 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun (72) Inventor Hideto Takasugi 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd.

Claims (1)

[Claims] 1. A molten steel containing Al is circulated in a vacuum tank of an RH degassing facility, oxygen gas is blown and blown into the molten steel in the vacuum tank from a nozzle, and Al in the molten steel is burned to raise the molten steel temperature. In the method of heating molten steel, the Mn concentration in the molten steel before heating is [wt% Mn], and the amount of Al flowing into the vacuum chamber calculated as the product of the Al concentration in molten steel and the ring flow rate is α (kg / Min), the amount of oxygen sent from the nozzle is β (kg / min), and the oxygen efficiency is η, the ratio of α to η × β is within the range satisfying the formula (1) during the heating period. The method for heating molten steel in RH degassing, characterized in that α / (η × β)> 0.625 × [wt% Mn] +0.625 (1) However, when the nozzle position height is 3 m or less from the stationary molten steel surface of the vacuum tank and the nozzle was immersed in the molten steel. In this case, the oxygen efficiency η = 1, and when the nozzle position height exceeds 3 m from the stationary molten steel surface, the oxygen efficiency η is expressed by equation (2). η = −0.12 × [nozzle position height (m)] + 1.36 (2)