JPH0892621A - Production of ultra-low nitrogen steel - Google Patents

Abstract

PURPOSE: To provide a process for producing an ultra-low nitrogen steel capable of stably obtaining molten metal of a low nitrogen concn. to the extent that such low concn. is hardly attainable by the existing converter refining methods. CONSTITUTION: When the max. oxygen feed rate throughout the entire period of blowing in converter blowing is decided as a reference value and if the oxygen feed rate at the time of the blowing decreases by >=30% of this reference value, the effective sectional area of a converter throat expressed by A(m<2> ) of the equation: A=AE-AL, where AE: the sectional area (m<2> ) of the aperture at the converter throat and AL: the cross sectional area (m<2> ) of a top blowing lance 5, is so controlled as to satisfy the equation: 0.01<A/([C]×q2 )<0.15, where [C]: the carbon concn. (wt.%) in the molten metal at the time of the blowing described above and q02 : the oxygen feed flow rate (Nm<3> /min) at the time of the blowing. As a result, the ultra-low nitrogen steel is produced by stable operation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for refining molten iron using a converter-type refining vessel, and particularly, to reduce the nitrogen content of steel materials as much as possible and to control the content easily. It relates to the refining method that can be carried out.

[0002]

2. Description of the Related Art As seen in the history of the development of steelmaking,
The converter method and open hearth method that use air containing a large amount of nitrogen gas as the blowing gas are pure oxygen converter method in which pure oxygen gas is blown as a blowing gas onto the molten metal at high speed and the blowing is completed in a short time. Was replaced by, and the nitrogen content of steel products was reduced. This has contributed to the improvement in quality and expansion of demand for thin plate products for which excellent processing performance is required for today's automobiles and cans.
However, due to the demand for further improvement of the product quality of these steel materials, further reduction of carbon and nitrogen and stabilization thereof have been required for the materials of the products.

The development of composite blowing technology in converter refining and secondary refining technology for degassing mainly of RH has greatly contributed to the reduction of carbon in steel materials. That is, in the converter, by strengthening the stirring by the bottom-blown gas,
It became possible to blow down to a low carbon concentration without increasing the oxidation loss of Fe. In this way, in addition to the decrease in the carbon concentration of the molten steel discharged from the converter, in the degassing step,
The decarburization function of the degasser is further improved by increasing the recirculation flow rate and blowing a large amount of gas, and an extremely low carbon concentration of 10 ppm or less has been realized in the integrated process of the converter and the degassing.

On the other hand, in response to the demand for low nitrogen content in steel materials, although measures to reduce nitrogen absorption in the degassing process and casting process, which have hitherto been a problem, have been studied, the effect is not sufficient. However, there is still a demand for complete improvement of the denitrification prevention technology and denitrification promotion technology in the combined blowing process and degassing process.

In the decarburization process in the converter, the nitrogen originally contained in the hot metal which is the initial molten metal tends to be denitrified from the molten metal with the progress of the blowing under the high carbon molten metal condition in the initial stage of the blowing. However, under the conditions of the medium carbon melt after the middle stage of the blowing, the denitrification does not proceed once, and then under the condition of the low carbon melt at the final stage of the blowing, it tends to absorb nitrogen. That is, in order to produce ultra-low nitrogen steel in the decarburization process of converter blowing, to promote denitrification in the initial stage of blowing and to suppress or prevent nitrification after the middle stage of blowing. Is extremely important. In particular, the technology for preventing nitrogen absorption is an essential condition for obtaining ultra-low nitrogen steel.

Regarding the mechanism of nitrogen absorption of molten metal in the final stage of blowing, it is conceivable that the gas phase atmosphere conditions above the surface of the molten metal in the converter may change during this period. That is, the carbon concentration in the molten metal is high from the initial stage of blowing to the first half of the middle period, and the blown oxygen gas reacts with carbon to produce a large amount of CO gas. Therefore, the gas component composition of the gas phase atmosphere is CO
Most of the gas or CO gas containing CO ₂ gas which has been further oxidized occupies most of the gas, and the nitrogen gas composition in the gas phase atmosphere can be kept very low. Therefore, the nitrogen in the molten metal is denitrified into the gas phase. Further, at this time, a high concentration of carbon in the molten metal, which increases the nitrogen activity in the molten metal, and a large amount of CO gas generated and strong stirring of the molten metal have an advantageous effect on the denitrification reaction. .

On the other hand, from the latter half of the middle stage of blowing, the carbon concentration for increasing the nitrogen activity in the molten metal decreases,
Further, the concentration of oxygen, which is a surface-active element, in the molten metal becomes high, which is a disadvantageous condition for the denitrification reaction. Further, since the total amount of gas generated during blowing is reduced, the nitrogen gas concentration in the atmosphere becomes relatively high. In particular, in the latter half of blowing, the flow rate of oxygen is reduced to 1/2 or less of the peak decarburization period, so that the partial pressure of nitrogen in the gas phase is more than double that at the beginning of blowing by simple calculation. Therefore, under such conditions, it is better to prevent absorption of nitrogen than to expect denitrification from the molten metal.

Under these circumstances, in order to produce an extremely low nitrogen steel, JP-A-1-136922 discloses that after the peak period of gas generation due to decarburization, the inert gas such as Ar gas is passed. By injecting gas upward from the oxygen lance, it is possible to prevent air from entering the furnace through the gap between the converter and the exhaust gas hood and descending along the oxygen lance to prevent air from entering the furnace ( Hereinafter, the prior art 1) is disclosed. Further, in JP-A-2-301508, just before the end of the blowing of the converter, the amount of exhaust gas is secured by introducing an inert gas, nitrogen gas, air or the like into the converter exhaust gas recovery hood, A method for preventing the infiltration of air into the furnace (hereinafter referred to as Prior Art 2) is disclosed. Furthermore, the actual fair 4-
Japanese Patent Laid-Open No. 45-45 discloses that a converter exhaust gas circulation path is provided and a connecting pipe is provided in the exhaust gas exhaust path on the inlet side to circulate the generated gas at a time when the gas generation amount is small to secure the gas amount and A method for preventing intrusion into the furnace (hereinafter referred to as Prior Art 3) is disclosed.

[0009]

However, the prior art 1 is economical because a large amount of expensive inert gas is required in order to prevent air infiltration into the furnace due to the reduction of exhaust gas in the final stage of blowing. Not. Further, in Prior Art 2 and Prior Art 3, a gas consisting of an inert gas or circulated exhaust gas is introduced into the exhaust gas hood or the exhaust gas discharge path on the inlet side to reduce the exhaust gas reduction amount at the final stage of blowing. Since it is intended to prevent the infiltration of air into the furnace by compensation, the effect is not sufficient.

As described above, in each of the prior arts 1, 2 and 3, in order to prevent the infiltration of air into the furnace, a new gas is added to or circulated in the furnace or the exhaust gas hood. It is based on a method of increasing the amount of gas by adding exhaust gas and discloses a specific method of increasing the gas, but the above-mentioned problems have not been solved.

Therefore, the present inventors have studied a method of ensuring an appropriate value for the flow velocity of the generated gas at the furnace opening as a method for preventing air intrusion from the furnace opening. An exhaust gas hood is provided above and close to the converter so as to extend upward, and the gas generated during blowing is processed through this exhaust gas hood.
However, a part of the air sucked from the gap between the converter and the exhaust gas hood and entering the hood is affected by a complicated gas flow in the vicinity of the upper blowing lance, and the lance surface is affected. Entrained in the furnace with the downflow. Then, the nitrogen partial pressure of the gas phase atmosphere in the furnace is increased. In order to prevent the entrainment of this air into the furnace, it is effective to reduce the cross-sectional area of the furnace opening and secure the flow velocity of the gas generated from the furnace at the furnace opening at an appropriate value.

However, in the blowing of the converter, not only the gas generated from the inside of the furnace but also the fine point of molten iron called spitting or the fumes generated by the blowing of oxygen gas to the molten metal called fumes are generated. Evaporated iron (iron in the gas state) is collected as dust via the furnace opening. Therefore, if the cross-sectional area of the furnace opening is made too small, the superficial velocity of the gas at the furnace opening is increased, causing intense spitting and fume generation, increasing the amount of scattering outside the furnace, and in some cases the furnace opening. These scattered substances adhere to and accumulate on the furnace, causing clogging of the furnace opening and other factors, which also hinder stable operation.

Further, the amount of gas generated from the furnace is not determined by a simple stoichiometry such that the oxygen gas fed into the furnace becomes CO gas (C + O ₂ = 2CO) generated by reacting with carbon in the molten metal. It is also affected by the carbon concentration in the melt. As the carbon concentration in the molten metal decreases, the oxygen efficiency for decarburization decreases, it is also used for the oxidation of valuable metals such as iron, and the amount of generated gas decreases significantly in the low carbon region. It becomes difficult to secure a proper value.

Therefore, an object of the present invention is to solve the above-mentioned problems, and to obtain a molten metal having a low nitrogen concentration to the extent that it is difficult to achieve by the current converter refining method, which is an extremely low nitrogen steel. It is to provide a manufacturing method of.

[0015]

Means for Solving the Problems The inventors of the present invention have made extensive studies to solve the above-mentioned problems, and have obtained the following findings. In the blowing process in the converter, if decarburization progresses and the flow rate of oxygen is reduced, as described above, the amount of gas generated from the converter furnace port decreases and the air that enters the furnace through the furnace port decreases. The increase in quantity becomes a problem. Therefore, in order to manufacture ultra-low nitrogen steel, it is necessary to prevent nitrification of the molten metal by the infiltration air. The condition that air intrudes into the furnace from this furnace mouth is that the cross-sectional area of the converter mouth through which this generated gas actually passes when the amount of gas generated from the furnace decreases (hereinafter referred to as Area), to secure the flow velocity of the gas generated at the furnace opening, and to adjust the effective cross sectional area of the converter furnace opening according to the carbon concentration in the molten metal, to stably produce ultra-low nitrogen steel. It becomes possible to do.

The present invention was made on the basis of the above findings, and in the blowing of molten iron in which oxygen is fed from the upper blowing lance using a converter, the maximum amount of feeding oxygen during the entire blowing period is used as a standard. If the value, when the oxygen-flow amount during blowing is reduced by more than 30% of the reference value, the following equation _{(1): a = a E -A L --------} (1) Where A _{E is} the cross-sectional area of the converter furnace mouth opening (m ² ), A _{L is} the cross-sectional area of the upper blowing lance (m ² ) A (m ² ) of the converter furnace mouth effective cross-sectional area, Formula (2) below: 0.01 <A / ([C] × q _O2 ) <0.15 ------- (2) where [C]: carbon concentration in molten metal at the time of blowing ( wt.%
), Q _O2 : flow rate of _oxygen supply (Nm ³ / min) at the time of blowing, which is a characteristic of controlling so as to satisfy.

[0017]

According to the present invention, when the amount of gas generated from the furnace decreases, the cross-sectional area of the furnace opening through which the generated gas passes is substantially reduced. The flow rate of the evolved gas at does not decrease, thus preventing the infiltration of air into the furnace.

The inventors of the present invention have found out the conditions for preventing the above-mentioned air infiltration into the converter, in order to find out the flow rate of acid (q
_Pay attention to the product of _O2 ) and the carbon concentration in the molten metal ([C]), and the ratio of the converter furnace effective area (A) to this: A / ([C]
_XqO2 ) as a parameter, infiltration of air from the furnace opening,
The mechanism of nitrogen absorption by the molten metal during blowing and the amount of gas generated during blowing were investigated. Incidentally, converter furnace outlet effective area (A) is the following (1) _{equation: A = A E -A L --------} (1) where, A _E: disconnection of the converter furnace outlet opening Area (m ² ), _AL : It was defined by the cross-sectional area (m ² ) of the top blowing lance.

The above parameters: A / ([C] × q _O2 )
Is the larger the product of the _{oxygen transfer} rate (qO2) and the carbon concentration ([C]) in the molten metal during the blowing of the denominator, the greater the amount of gas generated, and the numerator effective cross-sectional area (A) The smaller is, the generated gas must pass through the narrow furnace opening. Therefore, the smaller the parameter A / ([C] × q _O2 ) is, the faster the outflow rate of the gas generated at the furnace opening becomes, and the greater the effect of preventing air infiltration from the furnace opening into the furnace. However, there is also a disadvantage that the amount of spitting and fume generated increases as the exhaust gas flow velocity at the furnace opening increases.

Further, the effect of the decrease in the amount of gas generated during blowing on the nitrification of the molten metal was tested.

As a result, with regard to the amount of acid fed during converter blowing,
The maximum amount of acid transfer throughout the entire blowing period, that is, when the amount of acid transfer in high carbon region blowing at the beginning of blowing is taken as a reference value (referred to as the reference value in the present application), the amount of decrease in the amount of oxygen sent during blowing It was found that when the value of is less than 30% of the standard value, the molten metal does not absorb nitrogen.

On the other hand, even if the decrease in the amount of acid fed during blowing is 30% or more of the standard value, the parameter: A / ([C]
When the value of _xqO2 ) is less than 0.15, the molten metal does not substantially absorb nitrogen. However, when the value of the parameter is 0.01 or less, the occurrence of spitting and fume increases, the metal adheres to the furnace mouth severely, and normal operation is hindered. Therefore, when blowing in a state in which the amount of reduction in the amount of acid fed during blowing is 30% or more of the reference value,
T: The value of A / ([C] × q _O2 ) exceeds 0.01 to 0.1
The effective area of the converter throat: A must be controlled so that it falls within the range of less than 5.

[0023]

EXAMPLES The contents of the present invention will be described in detail below with reference to examples. [Example 1] In a 250 ton top-bottom blowing composite blowing converter, a slide gate was installed at the converter furnace opening so that the converter furnace opening effective area could be changed during blowing. FIG. 1 is a schematic vertical cross-sectional view showing the structure of a converter used in this embodiment, in which a slide gate is installed at the converter furnace mouth portion and the converter furnace mouth effective sectional area is variable. In the figure, 1 is a converter, 2 is molten steel, 3 is
Is a hood for collecting generated gas, 4 is a slide gate, 5 is a top blowing lance, and 6 is a bottom blowing tuyere. Table 1 shows the converter operating conditions and the nitrogen absorption investigation test conditions for the method Nos. 1 to 6 of the present invention and the comparative methods No. 1 to 7.

[0024]

[Table 1]

As for the operation flow, scrap was charged in advance in the furnace, and hot metal was charged therein, in accordance with the usual blowing in a converter. The scrap is a self-generated scrap generated in the steel mill, and about 5000 kg was used per charge. The hot metal used was dephosphorized and desulfurized in a pretreatment process in advance and used about 250 tons per charge. The converter is a composite blowing type, the bottom blowing is performed with Ar gas of about 50 Nm ³ / min, and the standard value of the top blowing oxygen amount is 1000 Nm ³ / m.
It was done in. The blowing time was about 14 minutes, but the amount of acid sent was about 400 after about 10 to 13 minutes after the start of blowing.
To 750 Nm ³ / min. In addition,
A charge that did not decrease was also implemented. At the same time, and the effective cross-sectional area BOF furnace outlet was reduced from about 12m ² to a predetermined value in a range of 1~12m ^2. In this way, the behavior of nitrogen concentration in molten steel was investigated when the amount of oxygen fed and the effective area of the converter furnace mouth were changed in the final stage of blowing, and the nitrogen absorption state was evaluated. That is,
Sampling the molten steel at the time of changing the amount of acid supply and at the end of blowing,
The carbon and nitrogen content was analyzed. After the completion of blowing, the condition of the furnace mouth was observed to evaluate the amount of metal attached.

Table 2 shows the parameters of each charge of the method of the present invention and the comparative method immediately after changing the amount of oxygen to be sent and at the end of blowing: A / ([C] × q _O2 ). Value, the nitrogen content (wt.%) Of molten steel and the amount of nitrogen absorption, and the state of adhesion of the metal at the furnace mouth.

[0027]

[Table 2]

It should be noted that in Table 2, A /
In the calculation of ([C] × q _O2 ), as [C], the carbon content of the molten steel at the time of changing the oxygen-sending amount, that is, the carbon content (wt.%) Of the molten steel sampled at the time of changing the oxygen-sending amount is changed. Using. The following items can be seen from Table 2. Parameter: A / ([C] ×
Depending on the value of q _O2 ), there is a difference in the amount of nitrogen absorption and the amount of metal attached to the furnace mouth. That is, as seen in the comparative method, when the value of the parameter: A / ([C] × q _O2 ) after changing the amount of acid fed is 0.15 or more, the amount of nitrogen absorption is large. However, in Comparative method No. 6, since there was no decrease in the amount of oxygen sent, no nitrogen absorption was observed. Further, in Comparative method No. 7, the decrease in the amount of acid fed was small, and thus the decrease in the amount of generated gas was also small.
Nitration was not observed even though the value of ta was 0.15 or more. In the comparative methods No. 5 and 6, the values of the above parameters at the end of blowing are proper values, but the value of the parameter A / ([C] × q _O2 ) at the time of changing the amount of _{oxygen transfer} is 0. Since the blowing conditions were 01 or less (0.079 and 0.0087, respectively), the metal adhesion to the furnace mouth was remarkable at that time. On the other hand, according to the method of the present invention, the parameter: A
Since the value of / ([C] × q _O2 ) was less than 0.15, the flow velocity of the gas generated at the furnace opening was kept high, so that nitrification did not occur. Further, since the value of the above parameter was more than 0.01, the adhesion of the metal in the furnace mouth did not occur.

[Embodiment 2] In a small-sized upper-bottom blowing composite blowing converter, a plug having a mechanism capable of moving up and down at the converter furnace opening so that the effective opening area of the furnace opening can be changed during blowing. Was installed. FIG. 2 is a schematic vertical cross-sectional view showing the structure of a converter used in this embodiment, in which a plug is installed at the converter furnace opening portion to make the effective opening area of the converter variable. In the figure, 1 is a converter, 2
Is a molten metal, 3 is a hood for recovering generated gas, 5 is a top blowing lance, 6 is a bottom blowing tuyere, and 7 is a plug. As shown in the figure, the plug 7 is installed concentrically with the upper blowing lance, and the effective opening area of the furnace opening can be changed by lowering the plug 7 to the upper end of the furnace opening. Plug 7
Had prepared several types in advance. The cross-sectional area of the furnace opening without a plug is 0.6 m ² , and the effective cross-sectional area of the furnace opening is 0.4 m ² , 0.3 m ² and 0.
Changed to 15m ² .

Table 3 shows the converter operating conditions and the nitrogen absorption investigation test conditions for the method Nos. 7 to 9 of the present invention and the comparative methods No. 8 and 9.

[0031]

[Table 3]

The operation flow was carried out in the same manner as in the usual blowing in a converter. After charging 6 tons of hot metal, the upper blowing lance was lowered and oxygen was fed. The converter is a compound blowing type, and about 2 Nm ³ / min of Ar gas is blown from three nozzles installed at the bottom of the furnace, and the standard value of the amount of oxygen fed from the top blowing lance is 18 Nm ³ / min.
It was set to min. Blowing time is about 17 minutes, 13 after starting acid transfer
Approximately 7 to 12 Nm ³ / m from the time when 16 minutes have passed
Reduced to within the range of in. At the same time, the plug is lowered to change the cross-sectional area of the furnace opening from 0.6 m ² without a plug to 0.
4m ^2, was reduced to throat effective area of 0.3 m ² or 0.15 m ^2. In this way, the behavior of nitrogen concentration in the molten metal when the amount of oxygen fed in the final stage of blowing and the effective area of the furnace opening were changed was investigated, and the nitrogen absorption state was evaluated. That is, immediately before reducing the amount of acid to be sent and at the end of blowing, the molten metal was sampled and analyzed for carbon and nitrogen contents. After the completion of blowing, the condition of the furnace mouth was observed to evaluate the amount of metal attached.

Table 4 shows the parameters of each charge of the method of the present invention and the comparative method immediately after changing the amount of oxygen to be sent and at the end of blowing: A / ([C] × q _O2 ). Value, the nitrogen content (wt.%) And the nitrogen absorption amount (wt.%) Of the molten steel, and the blast metal adhesion state.

[0034]

[Table 4]

The following matters can be seen from Table 4. Parameters:
The difference in the amount of nitrogen absorption and the amount of adhesion of the base metal in the furnace mouth depends on the value of A / ([C] × q _O2 ), which means that the converter furnace volume is smaller than in Example 1 and the furnace mouth opening is small. Even if the reduction method of the area is changed, it is the same as the first embodiment. That is, as seen in the comparative method, the parameter after changing the amount of acid to be fed: A / ([C] × q _O2 ).
When the value of is 0.15 or more, the amount of nitrogen absorption is large. On the other hand, according to the method of the present invention, the value of the parameter A / ([C] × q _O2 ) is less than 0.15 even if the amount of oxygen to be sent is reduced to a large extent. , Substantial nitrification did not occur. Moreover, since the value of the above parameter was more than 0.01, adhesion of the metal at the furnace mouth did not occur.

FIG. 3 is a schematic vertical sectional view showing the structure of a converter in which another type of furnace port plug is installed to make the furnace port effective sectional area variable. Even if the effective area of the furnace opening was changed using the plug shown in the figure, the effect of the above-mentioned plug was not changed at all.

[0037]

According to the present invention, when the amount of oxygen fed is reduced at the end of molten iron blowing in a converter, the amount of gas generated from the furnace is inevitably markedly reduced. Nitrogen can be completely prevented, and the adhesion of metal to the furnace mouth can also be suppressed.Therefore, extremely low nitrogen steel can be produced by stable operation, which is a very useful industrial effect. Be done.

[Brief description of drawings]

FIG. 1 is a schematic vertical cross-sectional view showing the structure of a converter used in Example 1 in which a slide gate is installed at a converter furnace opening portion and an effective sectional area of the furnace opening is made variable.

FIG. 2 is a schematic vertical cross-sectional view showing the structure of a converter used in Example 2 in which a plug is installed in a converter furnace opening portion and a furnace opening effective sectional area is made variable.

FIG. 3 is a schematic vertical cross-sectional view showing the structure of another converter used in Example 2 in which a plug is installed in the converter furnace opening portion to make the effective opening area of the converter variable.

[Explanation of symbols]

 1 converter, 2 molten steel, 3 generated gas recovery hood 4 slide gate, 5 top blowing lance, 6 bottom blowing tuyere, 7 plug.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoshi Izawa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (1)

[Claims] 1. In the blowing of molten iron in which oxygen is fed from a top blowing lance using a converter, when the maximum amount of oxygen fed during the entire blowing period is used as a reference value, the amount of oxygen fed during blowing is the above-mentioned value. when reduced by more than 30% of the reference value, the following equation _{(1): a = a E -A L --------} (1) where, a _E: cross-sectional area of the converter furnace outlet opening (M ² ), _AL : The converter furnace port effective cross-sectional area represented by A (m ² ) of the cross-sectional area (m ² ) of the upper blowing lance is expressed by the following formula (2): 0.01 <A / ([ C] × q _O2 ) <0.15 ------- (2) where [C]: carbon concentration in the molten metal during the above-mentioned blowing (wt.%
), Q _O2 : Acid supply flow rate during blowing (Nm ³ / min), which is controlled so as to satisfy the following.