JPH1112634A - Production of molten low nitrogen steel with arc furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing technique of a molten low nitrogen steel only with an arc furnace equipment which secures denitrificating reaction speed in the molten steel and clarifies the supplying condition of carbon and oxygen to be supplied into the molten steel and an N concn. limitation in carrier gas at the time of adding the carbon into the molten steel in order to obtain the molten low nitrogen steel by restraining the nitrogen absorption. SOLUTION: After melting down raw material of scrap, etc., O2 and C are supplied into molten iron. The purity of O2 is limited to >=99.5 vol.% and oxygen supplying speed Q02 and oxygen supplying time t* are limited to 0.55<=Q02 <=1.35 (Nm3 /min/ton) and t*>=-7.2Q02 +15 (min). In the case a value in the right side of the inequality: Cadd >=(12/112)Q02 .t*.W-CMD (kg) is the positive value, the C adding quantity Cadd satisfying the inequality is supplied into the molten iron. At this time, the N concn. in the carrier gas for C is regulated to <=3 vol.%. Wherein, W is molten quantity (ton) at the time of completing the melting of the raw material and CMD is the carbon weight (kg) contained in the molten iron at the completing the melting of the raw material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a process for producing steel by melting and refining iron source materials such as scrap, reduced iron and pig iron in an arc furnace, wherein high purity oxygen gas and carbon are added to the molten iron during the refining period. The present invention relates to a method for producing low nitrogen molten steel by supplying.

[0002]

2. Description of the Related Art Conventionally, the production of thin steel sheets, such as cold-rolled steel sheets and surface-treated steel sheets, from molten steel obtained by melting and refining scraps in an electric arc furnace is based on impurities such as Cu and Sn contained in a large amount in the scraps. Is not possible due to the problem that the steel remains on the steel material and adversely affects the material. However, in recent years, by increasing the mixing ratio of reduced iron or hot metal other than scrap, it has become possible to reduce the amount of impurity elements such as Cu and Sn in the charged raw material, so that the arc furnace steelmaking method There is also a movement to manufacture cold-rolled steel sheets for deep drawing, which could only be manufactured by the conventional blast furnace-converter steelmaking method.

On the other hand, in cold-rolled steel sheets for deep drawing, the nitrogen concentration is usually set at 40 p.
pm or less. In response to the above-mentioned situation, as a low-nitrogen steel manufacturing technique by a conventional arc furnace steelmaking method, for example, “Recent progress in arc furnace steelmaking” (Heisei 5
Published by the Iron and Steel Institute of Japan on October 7, 2006, p. 121) and “Strategy for Electric Steel Furnace Operation” (November 14, 1994)
Published by the Iron and Steel Institute of Japan, p. 112) (the above two cases are referred to as “prior art 1”).
A method is disclosed in which the concentration is increased to actively perform CO boiling accompanying the decarburization reaction in the refining period, or to promote denitrification (deN) by stirring the molten steel with Ar gas or the like. At present, the limit is to reduce the N concentration of molten steel to 50 to 60 ppm. Also, 75w of charged iron source material
The statement that molten steel having a nitrogen concentration of 40 ppm or less can be obtained by a high-reduction iron compounding operation in which at least t.% is directly reduced iron is described in “5th European Electric St.”
EelCongress Papers "(June 19, 1995, French Steel Federa)
sponsored by Tion, Les Editions de la
Revue de Metallurgie, p.
41) (referred to as “prior art 2”), but does not describe a detailed manufacturing method.

As described above, at present, no specific method has been proposed for melting low nitrogen steel using an arc furnace, for example, melting low nitrogen steel having a nitrogen concentration of 40 ppm or less.

[0005]

As described in Prior Art 1 described above, by improving the operation method of the arc furnace, the N concentration becomes 50 to 50%.
It is possible to produce a low-nitrogen steel material at a level of 60 ppm, and it is possible to produce a thin steel sheet having workability at the level of a hot-rolled steel sheet. The technology for manufacturing surface-treated steel sheets by improving the operation method of the arc furnace without adding special equipment or a raw material supply system has not been established.

On the other hand, according to the above-described prior art 2, molten steel having an N concentration of 40 ppm or less can be obtained using an arc furnace. This method is effective for producing low-nitrogen steel under conditions where the supply of direct reduced iron can be sufficiently provided.
However, there is a problem that special conditions must be provided such that a large amount of directly reduced iron having a mixing ratio of 75 wt.% Or more can be stably supplied.

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, it is an object of the present invention to ensure a high de-N reaction rate from molten steel in refining molten steel in an arc furnace and to suppress N absorption into molten steel by suppressing N absorption into molten steel. As conditions for obtaining molten steel, it is to clarify supply conditions of carbon and oxygen to be supplied to molten steel, and to limit N concentration in a carrier gas when carbon is added to molten steel. Without using a large amount of direct reduced iron and without adding a vacuum degassing refining process, a method for producing low-nitrogen molten steel using an arc furnace to obtain molten steel with a nitrogen concentration of 25 ppm or less only by improving the arc furnace operation method. To provide.

[0008]

Means for Solving the Problems The present inventors have made intensive studies to improve the operation method of the arc furnace from the above viewpoint.
In general, the nitrogen concentration of molten steel at the end of arc furnace refining [N] _Tap is the nitrogen concentration of molten steel at the start of refining, that is, the nitrogen concentration at the end of melting of scrap loaded in the furnace (during burn-down). Concentration [N] _MD , amount of N released from molten steel to CO bubbles generated by the reaction between oxygen gas blown into the furnace during the refining period and C in molten steel (removed N amount) Δ [N] _de , and refining Amount of nitrogen that has entered the furnace during the period
Amount) Δ [N] Sum of _ab : [N] _MD -Δ [N] _de + Δ [N]
_ab . In the present invention, the nitrogen concentration at the time of burn-down [N] _MD is given depending on the amount of N carried in from a raw material such as scrap. Therefore, in order to reduce the nitrogen concentration [N] _Tap of the molten steel at the end of refining as much as possible, a large amount of CO bubbles are generated during decarburization by blowing oxygen gas into the molten steel to promote denitrification. It is necessary to form a slag covering the steel to block air entering the molten steel and thereby suppress N absorption, and to prevent the absorption of N by preventing a bare metal portion from being formed on the surface of the molten steel. And N in the carrier gas of carbon blown into the furnace to secure the slag forming.
It is important to reduce the concentration as much as possible to eliminate the absorption of molten steel N from the carrier gas.

In the present invention, the present inventors have particularly focused on limiting the nitrogen concentration in the carrier gas for carbon injection and preventing the appearance of hot water during refining. Thus, the conditions for generating CO to promote denitrification, the conditions for ensuring sufficient slag forming, the limiting conditions for the nitrogen concentration in the carbon carrier gas, and the securing of the amount of slag for preventing the appearance of bare water during refining, Various tests were performed using an arc furnace on a scale of 60 to 200 tons, and the test results were analyzed.

As a result, it has been concluded that at the end of the refining of the arc furnace, in order to obtain a low-nitrogen molten steel having a nitrogen concentration of 25 ppm or less, it is necessary to simultaneously satisfy at least the following conditions.

In order to increase the above-mentioned de-N amount Δ [N] _de , it is necessary to feed acid at a de-N rate higher than a predetermined value for a predetermined time. That is, oxygen-flow-rate Qo ₂ is the following equation (4) into the furnace in the refining period, preferably ^{_{(5): Qo 2 ≧ 0.55 Nm 3 /}} min / ton ---------- (4) Blowing for a predetermined acid supply time (t ^* min) under the condition of Qo ₂ ≧ 0.67 Nm ³ / min / ton ----- (5) is necessary. The C concentration [C] ^* (wt.%) Necessary for obtaining a sufficient CO gas generation rate from the molten steel with respect to the acid supply rate Qo ₂ is secured in the molten steel. The condition of the above formula (4) or (5) is the de-N reaction of molten steel: 2 N = N
_{2 This} is a necessary condition for securing the reaction rate of (g).

Low nitrogen concentration below a predetermined value due to refining
In order to obtain molten steel of N, the de-N reaction:N= N
_Two(G) is continued for a predetermined time or more, and the N removal amount Δ [N]_deTo
Must be greater than or equal to a predetermined value. Sending necessary for that
Acid time t^*(Min) is calculated by the following equation (2): t^*≧ -7.2Qo_Two+15 (min) ---------- It is necessary to satisfy (2). However, extend the acid supply time
As the decarbonization of molten steel increases, the carbon concentration at the time of
[C]_MDAnd target carbon concentration at the end of refining [C]_{Tap, aim}When
Difference: [C]_MD-[C] _{Tap, aim}Acid transfer according to the size of
The time limit is set.

The required C concentration [C] ^* described above
(Wt.%), The total amount of oxygen fed during the refining period should be equal to or greater than the carbon equivalent C ^* (kg), assuming that the oxygen reacts with the carbon in the molten steel to produce CO. Total carbon content It is necessary that C (kg) be present. That is, the following equation (6): It is necessary to satisfy C ≧ C ^* -------- (6). Here, T. C and C ^* are the following formulas (7) and (8), respectively: _{C = C MD + C add (} kg) ------------ (7) C * = (12 / 11.2) Qo 2 t * W (kg) -------- ---- (8) where C _MD : weight of carbon contained in molten steel at the time of meltdown (kg) C _add : weight of carbon to be added to molten steel during blowing (kg) W: weight of molten steel at the time of meltdown (Ton). From the expressions (6) and (7) and (8), the following expression (9): _CMD + _Cadd ≧ (12 / 11.2) Qo ₂ t ^* W ------------ (9) is obtained, from which the following formula (3) is obtained: C _add ≧ (12 / 11.2) Qo ₂ t ^* W-C _MD (kg) -------- (3) Can be Here, when the calculated value on the right side of Expression (3) is a positive value, in order to satisfy Expression (6), Expression (3)
Smelting must be performed while supplying carbon with a carbon addition amount C _add that satisfies the formula to the molten steel. However, when the calculated value on the right side of the equation (3) is a negative value, refining may be performed without supplying carbon to the molten iron. On the other hand, as the amount of C _add supplied to the molten steel increases _, the decarbonization amount up to the target carbon concentration [C] _{Tap, aim} at the end of refining must be increased. This is the de-N amount Δ
[N] It is advantageous in that _de increases, but the refining time is prolonged accordingly. Therefore, the amount of C _add is determined in consideration of the target operating conditions such as the balance between the target nitrogen concentration at the end of refining and the permissible required refining time.

As described above, oxygen gas is supplied into the furnace at the acid supply rate Qo ₂ of the equation (4), preferably the equation (5), and
It is necessary to continue the acid supply for the time t ^* while adding carbon of C _add to the molten steel so that the expressions (3) and (2) are simultaneously satisfied.

However, the oxygen gas sent into the furnace has a purity of 9%.
It is necessary that the oxygen gas is 9.5 vol.% Or more.
Normally, pure oxygen gas is produced from air, and thus contains nitrogen as a main impurity gas. This is because if the oxygen gas to be acidified contains 0.5 vol.% Or more of nitrogen, the nitrogen concentration in the molten steel does not become 25 ppm or less at the end of refining.

Also, the acid supply rate Qo in the refining period_TwoIs large
CO boiling has become more active and is effective in removing N from molten steel.
This is advantageous for obtaining low nitrogen molten steel,
Splash generation as gas generation rate increases
As the amount increases, the yield of molten steel decreases,
Operational problems arise due to deposition on the side walls and the like. The above problem
In the present invention, in order to prevent the generation of acid,_TwoTo
1.35 Nm^Three/ Min / ton or less. Follow
The acid transfer rate Qo in the refining period_TwoWith the following formula (1): 0.55 ≦ Qo_Two≤1.35 (Nm^Three/ Min / ton) ---------- Limited as (1). Note that FIG.
Speed Qo_TwoAnd acid supply time t ^*The area that should be satisfied
Indicated. In the figure, the acid feeding time t^*The upper limit of
As you can see, it is a kind of operating condition [C]_MD-[C]
_{Tap, aim}Is determined by

As a method of adding the carbon of the C _{add to} the molten steel, a method of directly charging coke or the like into a furnace,
Inject with carrier gas
There is a way. In the direct injection method, carbon floats on the slag, and does not contribute much to the decarburization reaction, which is not effective. Therefore, carbon is added to molten steel by a carrier gas using an injection lance. Aim at the tip of the lance pipe near the interface between the slag and the molten steel.
Here, it is particularly important to limit the nitrogen concentration in the carrier gas to a low level.

FIG. 2 shows the results of an analytical test of the nitrogen concentration of molten steel at the end of refining by varying the nitrogen concentration in the carrier gas when using Ar gas as the carrier gas. Experimental conditions in the figure, [C] _MD = 0.3 to
0.5 (wt.%), Purity of oxygen gas for refining = 99.5 (vo
l.%), Qo ₂ = 0.74 to 0.75 (Nm ³ / min /
ton), acid sending time after burn-through = 9.6 to 9.7 (mi)
n), C _add = 600-650 (kg), molten steel amount = 12
0 (ton), slag amount = 250 to 300 (kg / m ²⁾
-Molten steel surface area) and end point temperature = 1590-1600 ° C. As can be seen from the figure, the nitrogen concentration in the molten steel was 25%.
In order to reduce the concentration to less than ppm, it is necessary to reduce the nitrogen concentration in the carrier gas to 3 vol.% or less. Conventionally, in the operation of an arc furnace, when carbon is blown into the furnace in order to increase the amount of decarburized molten steel, air has generally been used as a carbon carrier gas. Therefore, absorption of N from the carrier gas is remarkable, and it is difficult to melt the low nitrogen steel.

In the present invention, in order to stabilize the nitrogen concentration at the end of refining to 25 ppm or less, it is desirable to carry out refining in addition to the above conditions and further adding the following and / or conditions.

In a normal operation, a target carbon concentration [C] _{Tap, aim} at the end of refining is given. Therefore, the higher the carbon concentration [C] _MD at the time of _{burn-through,} the larger the carbon concentration to be decarburized: [C] _MD- [C] _{Tap, aim} . vice versa,
The lower the carbon concentration [C] _MD at the time of _{burn-through,} the smaller the carbon concentration to be decarburized: [C] _MD- [C] _{Tap, aim} . However, the smaller the [C] _MD- [C] _{Tap, aim} is, the smaller the amount of CO generated by the required decarburization is.
[N] _{de is} also reduced, which is disadvantageous in the production of low nitrogen molten steel. Therefore, when [C] _MD- [C] _{Tap, aim} is small, the amount of carbon addition C _add during refining is determined to be more within the range represented by the above equation (3). Depending on the determined carbon amount C _{the add,} the oxygen-flow-rate Qo ₂ and oxygen-flow time t ^* of the range, on the assumption that meet their (1) and (2), the nitrogen concentration at the time of refining End Is stable and 25ppm
The more desirable conditions for the following are as follows.

The present inventors have set forth the above-mentioned conditions (1) to (4).
And the molten steel nitrogen concentration at the end of refining is 25 ppm or less.
Using the obtained arc furnace refining test heat,
Required decarburization amount until refining ends [C]_MD-[C]
_{Tap, aim}And the amount of acid transfer Qo between them_Twot^*Find out the relationship with
And the plot of FIG. 3 was obtained. From Fig. 3, at the end of refining
Target carbon concentration [C]_{Tap, aim}If given
Carbon concentration [C]_MDAnd the required decarburization amount [C]_MD
-[C]_{Tap, aim}Is calculated, and as this value increases,
Quantity Qo_Twot^*And a value greater than or equal to curve A in FIG.
Qo not to become_TwoAnd t ^*Is determined.

However, Qo ₂ and t ^* are limited within the region shown in FIG. The upper limit (illustrated by the dashed line Q) of this region (hatched portion) is determined by operating conditions such as the allowable refining time and the upper limit temperature of molten steel at the end of refining, etc. (in FIG. ₂ t ^* ). Therefore, FIG. 3 also has a corresponding upper limit of the acid supply amount Qo ₂ t ^* .

If the amount of slag covering the upper surface of the molten steel in the furnace is small, the thickness of the slag layer becomes non-uniform at the time of decarburization by blowing oxygen gas into the molten steel, and bare water may appear. In this case, the nitrogen gas in the air that has entered the furnace is absorbed by the molten steel, which hinders the promotion of denitrification of the molten steel, and the target carbon concentration cannot be obtained within the predetermined refining time, or the refining time is extended. However, the nitrogen concentration decreases only to a certain value. In order to prevent such a phenomenon from occurring, it is necessary to secure a slag amount of 150 to 200 kg / m ² or more on the molten steel upper surface. By doing so, it is possible to stably reduce the nitrogen concentration of the molten steel to 2
It can be reduced to 5 ppm or less. On the other hand, a large amount of slag is advantageous in that bare water does not appear,
A normally molten slag must be formed. Therefore, the upper limit of the slag amount cannot be uniformly determined by the value of the depth / diameter of the steel bath, but is usually 300 to 40.
It may be within the range of 0 kg / m ² .

As described above, the method for producing low-nitrogen molten steel using the arc furnace of the present invention is as follows. According to the first aspect of the present invention, at least one of iron source materials such as scrap, reduced iron and pig iron is melted in an arc furnace, and molten iron formed in the furnace after melting of the raw material is completed (burned down). To supply oxygen gas and carbon to smelt molten iron to produce low nitrogen molten steel. This smelting method is characterized in that the purity of the oxygen gas, the acid supply rate, the acid supply time, the amount of carbon added, and the nitrogen concentration in the carbon carrier gas are limited as follows. That is, the purity of the oxygen gas is 99.5 vol.% Or more, and the acid supply rate Qo ₂ and the acid supply time t ^* are expressed by the following formulas (1) and (2): 0.55 ≦ Qo ₂ ≦ 1.35 (Nm ³ / min / ton) ---------- (1) t ^* ≧ -7.2 Qo ₂ +15 (min) ------------ (2) is satisfied, and And the following formula (3): C _add ≧ (12 / 11.2) Qo ₂ t ^* W-C _MD (kg) --- (3) where W: molten iron at the end of melting of the raw material Amount (ton) C _MD : Weight of carbon contained in molten iron at the end of melting of raw material (k
When the carbon addition amount C _add calculated in g) is a positive value, refining is performed while blowing the carbon of the carbon addition amount C _add into the molten iron in the furnace with a carrier gas having a nitrogen concentration of 3 vol.% or less.

According to a second aspect of the present invention, in the method of the first aspect, the value of Qo ₂ t ^* , which is the amount of acid supply from the time of burn-through to the end of refining in the equation (3), Required decarburization of molten steel from the time of refining to the end of refining [C] _MD- [C]
_{Tap, aim} (However, [C] _MD : carbon concentration at burn-through,
[C] _{Tap, aim} : is characterized in that the respective values of the acid supply rate Qo ₂ and the acid supply time t ^* are determined so as to increase at a predetermined rate with an increase in the target carbon concentration at the end of refining. Things.

According to a third aspect of the present invention, there is provided the method according to the first or second aspect, wherein the upper surface of the molten iron is reduced by 15 mm when the molten iron is refined.
It is characterized in that it is covered with a slag in the range of 0 to 400 kg / m ² .

[0027]

Next, a preferred example of an embodiment of the present invention will be described. A predetermined steelmaking DC arc furnace is charged with steel scrap and cold iron as melting raw materials, further charged with initially charged coke and slag-making material, and melts the raw material while adding oxygen gas, and also on the surface of the molten metal. Form molten slag. After the burn-down, the temperature of the molten steel is measured and the carbon concentration is analyzed.
Next, in the refining period, the acid supply rate Qo ₂ (Nm ³ / min)
/ Ton) and blowing for t ^* (min) during the acid supply time. During this time, coke breeze was converted to carbon as a solid carbon source in terms of carbon content.
only _add (kg) was refining while watching blow. Here, the acid supply rate Qo ₂ , the acid supply time t ^*, and the carbon addition amount C _add are adjusted so as to satisfy the above equations (5), (2) and (3), respectively. After the refining, the temperature of the molten steel is measured and a predetermined component composition is analyzed. At the time of tapping into a ladle, a predetermined amount of iron alloy is added to adjust the component composition.

As described above, in the refining period (5),
The knowledge that the expressions (2) and (3) are satisfied at the same time and that the N concentration in the carrier gas into which C is injected into the molten steel must be 3 vol. N pressure, N removal rate determined by the balance between the N removal rate and the N absorption rate, and
Focusing on the effect of the required time, it was obtained by analyzing the test results using an actual furnace. In this case, nitrogen in the molten steel is diffused and captured in CO gas bubbles generated by reacting with carbon in the molten steel due to acid supply, and escapes from the steel bath through a molten slag layer on the surface of the molten steel. At the same time, the molten slag
The bubbles enter the forming state and serve to block the steel bath from air entering the furnace. In the present invention,
Since oxygen gas having a purity of 99.5 vol.% Or more is used, the nitrogen concentration in the oxygen gas is less than 0.5 vol.%, And the nitrogen partial pressure of the gas in the forming slag is 0.0025 a.
tm or less is expected. Thus, for example, the equilibrium nitrogen concentration between the gas and molten steel at 1600 ° C. is 22.5
ppm, and the phenomenon of N absorption from the forming slag layer does not occur. For the above reasons, the denitrification of the molten steel proceeds, and at the end of refining, a low nitrogen molten steel of 25 ppm or less is obtained.

[0029]

Next, a method for producing a low nitrogen molten steel of the present invention will be described.
This will be described in more detail with reference to examples. Examples which are methods within the scope of the invention were tested according to the examples of the embodiments described above, and comparative examples outside the scope of the invention were tested where appropriate. The test method is as follows.

In a 120 ton DC arc melting furnace (electric capacity: 72 MVA), steel scrap: 90 wt.
10 wt.% And initially charged coke: 12 to 23 kg / charged t
On and slag-making material: 27 to 30 kg / charged ton were charged, and the charged raw material was heated and melted by arc heat and carbon combustion heat while adding oxygen gas. After burn-through, oxygen was supplied while blowing coke breeze (C purity: 85 wt.%) Into the steel bath. Oxygen was supplied from pure oxygen gas having a purity of 99.5 vol.% (Production method: deep cooling method), refined and de-N-treated to produce low nitrogen molten steel. However, in some of the comparative examples, the purity was 94 vol.%.
Pure oxygen was used. The amount of molten slag on the surface of the molten metal was 300 to 350 kg / m ² .

Tables 1 to 4 show Examples and Comparative Examples (Example 1).
Test conditions of Comparative Examples 1 to 7 and Comparative Examples 1 to 7) are shown. In the same table, the acid supply rate Qo ₂ , the required acid supply time t ^* determined by the equation (2), the actual value of the acid supply time, and the molten steel C concentration at the time of meltdown [C]
_MD and C weight in molten steel C _MD , required carbon addition amount C _add (C conversion value in coke) determined by equation (3), the actual value thereof, and N concentration in carrier gas (Ar gas) are shown.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

[Table 4]

Under the above smelting conditions, a low-carbon molten steel for a thin steel sheet was manufactured. Tables 1 to 4 also show the nitrogen concentration of molten steel at the end of refining. FIG. 4 shows a denitrification curve of molten steel as refining progresses after burn-down.

The test conditions of Comparative Examples 1 to 3 are the same as those of Examples 1 to
Compared to No. 7, the nitrogen concentration in the Ar carrier gas when coke was blown into the steel bath was as high as 3 vol.% Or more. As shown in FIG. 2, the nitrogen concentration in the molten steel increases as the N concentration in the Ar carrier gas increases.
In order to make the nitrogen concentration of the molten steel after (at the end of refining) 25 ppm, the nitrogen concentration in the Ar carrier gas should be 3 vol.%.
It must be: This is because the higher the nitrogen concentration in the carrier gas, the higher the nitrogen partial pressure of the gas in the forming slag, the higher the equilibrium nitrogen concentration in the molten steel,
It is considered that the de-N reaction did not proceed sufficiently.

The test conditions of Comparative Examples 4 and 5 are the same as in Example 1.
7, the amount of carbon added during refining is low outside the range of the present invention (for example, in Comparative Example 4, the required amount of carbon added: 797 k)
Less than 601 kg of carbon is added for g or more), which is only outside the scope of the invention. The nitrogen concentration of the molten steel after the acid supply time of 12 to 15 minutes (at the end of refining) has only decreased to 40 to 45 ppm. This is considered to be because the slag forming in the latter half of the refining period was insufficient due to the insufficient carbon concentration of the molten steel during the acid supply, and thus the absorption of N could not be sufficiently suppressed.

The test conditions of Comparative Examples 6 and 7 were the same as in Example 1.
As compared with the above, the purity of oxygen gas during refining is lower than the scope of the present invention (purity: 94 vol.% (PSA oxygen, nitrogen concentration: 3.5 vol.%), And a large amount of nitrogen in the balance as impurities. This is only outside the scope of the present invention. The nitrogen concentration of the molten steel after 15 to 17 minutes of the acid supply time (at the end of refining) has decreased only to 80 ppm. This is considered to be because the equilibrium nitrogen concentration of the molten steel was high due to the high nitrogen partial pressure of the gas during slag forming, and the deN reaction did not sufficiently proceed.

On the other hand, in all of Examples Nos. 1 to 7, the nitrogen concentration at the end of refining is reduced to 25 ppm or less.

As can be seen from the above-described embodiment, according to the present invention, a low-nitrogen molten steel having a nitrogen concentration of 25 ppm or less can be stably manufactured by an arc furnace.

[0042]

As described above, according to the present invention,
There is no need to prepare a special raw material supply system, such as using a high blended ratio of reduced iron as a melting raw material.If there is a supply system for iron source raw materials in an ordinary arc furnace operation, improvement of the arc furnace operation method It is possible to stably produce molten steel having a nitrogen concentration of 25 ppm or less. Therefore, N concentration 25pp
m, a cold-rolled steel sheet or a surface-treated steel sheet for deep drawing can be manufactured, and a method for manufacturing a low-nitrogen molten steel by an arc furnace can be provided, and an industrially useful effect is brought about.

[Brief description of the drawings]

FIG. 1 is a graph showing a range that an acid supply rate Qo ₂ and an acid supply time t ^* should satisfy in the method of the present invention.

FIG. 2 is a graph showing the effect of the nitrogen concentration in a carrier gas for carbon injection on molten steel on the nitrogen concentration in molten steel.

FIG. 3 Required decarburization amount [C] in the method of the present invention_MD−
[C]_{Tap, aim}And acidity Qo_Twot ^*Here is an example of the relationship with
It is a graph.

FIG. 4 is a graph showing the state of nitrogen in molten steel after meltdown in Examples and Comparative Examples.

Claims (3)

[Claims] At least one of an iron source material such as scrap, reduced iron and pig iron is melted in an arc furnace, and after the melting of the material is completed, oxygen gas and carbon are supplied to molten iron formed in the furnace. And refining the molten iron to produce a low-nitrogen molten steel, wherein the oxygen gas has a purity of 99.5 vol.% Or more, and the oxygen gas supply rate Qo ₂ and the acid supply time t ^* are as follows (1): And equation (2): 0.55 ≦ Qo ₂ ≦ 1.35 (Nm ³ / min / ton) ----- (1) t ^* ≧ −7.2Qo ₂ +15 (min) − ----------- Satisfies (2) and the following equation (3): C _add ≧ (12 / 11.2) Qo ₂ t ^* W-C _MD (kg) ---- ----- (3) where, W: molten iron amount at raw material dissolved ends (ton) C _MD: carbon weight (k contained in the raw material dissolution at the end of the molten iron
If the value on the right side of g) is a positive value, carbon having a carbon addition amount C _add that satisfies the expression (3) is supplied to the molten iron, and the carbon is supplied by a carrier gas having a nitrogen concentration of 3 vol.% or less. And refining while blowing into said molten iron. _2. The value of Qo ₂ t ^* , which is the amount of acid transferred from the time of meltdown to the end of refining in the above equation (3), is determined by the required amount of decarburization of molten steel from the time of meltdown to the end of refining [ C] _MD- [C]
_{Tap, aim} (However, [C] _MD : carbon concentration at burn-through,
2. The method for producing low-nitrogen molten steel by an arc furnace according to claim 1, wherein the ratio is increased at a predetermined rate as [C] _{Tap, aim} : the target carbon concentration at the end of refining). 3. The low-nitrogen furnace according to claim 1, wherein the refining of the molten iron is performed by covering an upper surface of the molten iron with a slag in a range of 150 to 400 kg / m ^2. Method for producing molten steel.