JP2001254116A - Method of producing low nitrogen steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing method of low nitrogen steel, by which the invasion of air into a furnace developed at the blowing end stage in a converter, can easily be prevented. SOLUTION: (1) In a decarburizing refining which top-blows oxygen and bottom-blows any one or more kinds among Ar, CO, CO2 and hydrocarbon gas, a ratio (Q/V) of the bottom-blown gas flowing rate Q (m3 (normal state)/s) and the volume V (m3) in the converter is regulated to >=0.002 (1/s). (2) Further, this ratio (Q/V) is regulated to <=0.02 (1/s). (3) The volume V in the converter is calculated with the following equation by using the volume Vo (m3) in an iron shell. V=0.48V0+5.15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for smelting low-nitrogen steel, and more particularly to a method for preventing nitrogen in air from being mixed into molten steel in an upper-bottom blow converter.

[0002]

2. Description of the Related Art For thick plates, it is required to increase the toughness of steel. Further, from the viewpoint of workability, thin sheets for processing are required to have low yield strength and high elongation. Further, it is also required to secure so-called aging resistance, which suppresses aging in which ductility deteriorates with the passage of time. In order to satisfy the above mechanical properties, it is required to reduce the nitrogen concentration in steel.

On the other hand, a method for producing steel by a normal blast furnace method is as follows. Conventionally, hot metal from a blast furnace is stored in a torpedo car or hot metal ladle and transferred to another hot metal processing vessel as it is or for pretreatment of hot metal such as dephosphorization, desulfurization or desiliconization. . In this hot metal pretreatment, various refining agents are added to the hot metal,
Gas or mechanical agitation is taking place.

[0004] After performing these hot metal pretreatments, the molten steel is charged into a converter and decarburized and refined with oxygen gas.
After the completion of converter blowing, the molten steel that has been roughly decarburized is tapped into a ladle, subjected to component adjustment and vacuum refining in a secondary refining device, and then turned into slabs in a continuous casting machine. The obtained slab is subjected to rolling, heat treatment, and the like according to each use. Nitrogen in molten steel decreases rapidly during converter blowing, but in the latter stage of blowing, the decarburization rate decreases and the flow rate of gas generated in the furnace becomes extremely small. As a result, the concentration of [N] in the molten steel at the end of blowing increased.

[0005]

As a countermeasure, Japanese Patent Laying-Open No. 8-92621 proposes a method for adjusting the cross-sectional area of the furnace port of a converter according to the [C] concentration in molten steel and the oxygen supply rate. ing.

However, it is extremely difficult to change the furnace port cross-sectional area disclosed in the publication during blowing, because of the equipment.
Poor feasibility. Therefore, an object of the present invention is to provide a method for melting low nitrogen steel of, for example, 20 ppm or less, which can easily prevent air from entering the furnace at the end of converter blowing.

[0007]

The present inventors have searched for conditions for preventing air from entering the converter and have obtained the following findings. (A) At the end of blowing, a large part of the oxygen supplied by converter blowing is reacted with molten steel, and FeO, Fe ₂ O ₃ , P ₂ O
_{5. Since} oxides such as MnO are formed and consumed, it becomes difficult to function as a gas for preventing air from entering. Therefore, it is necessary to add a gas species other than the oxygen gas that prevents the entry of air.

(B) Investigation was conducted in a small test furnace and converters of different sizes, and the critical gas flow rate for preventing air from entering was found to be the furnace volume V (volume in a bricked state) (m ³ ). It turned out to depend. That is, it was found that the larger the furnace volume, the larger the gas flow required to prevent air from entering.

Strictly speaking, the effective furnace volume varies depending on the amount of molten steel to be treated. However, since the molten steel volume is generally smaller than the furnace volume, it can be safely ignored. (C) Using the flow rate Q of the bottom blown Ar gas per furnace volume V as an index, the converter was blown and the [N] concentration (ppm) in the molten steel discharged into the ladle after blowing was compared.

FIG. 1 shows the test results. In addition, the spitting index in the figure is obtained by inserting a spitting sampling rod from the sublance hole into the upper part of the converter furnace mouth, measuring the amount of metal adhering to the spout sampling rod, and obtaining a ratio (Q / V) of 0.0005 (1/1). This is an index based on the amount of bullion under the condition of s).

As shown in the figure, the [N] concentration in the molten steel after tapping sharply decreased when the ratio (Q / V) exceeded 0.002 (1 / s) or more. This can be presumed to be because if the ratio (Q / V) is less than 0.002 (1 / s), the amount of gas for purging the furnace volume is small, and air entry into the furnace could not be prevented.

[0012] In addition to the Ar gas, CO ₂
A test using C ₃ H ₈ as a gas and a hydrocarbon gas was performed, and the same result as that in FIG. 1 was obtained. In addition, the ratio (Q /
When V) exceeded 0.02 (1 / s), the amount of spitting (scattered molten steel) increased due to an increase in the flow rate of the bottom blown gas. An increase in the amount of spitting is not preferable because iron yield decreases, and there is a problem in that productivity increases due to metal staking operations due to an increase in furnace slab ingots.

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) Oxygen is blown upward in a top-bottom blow converter, and Ar, CO, C
In decarburization refining in which one or more of O ₂ and hydrocarbon gas are blown in the bottom, the ratio of the bottom blown gas flow rate Q (m ³ (standard state) / s) to the converter internal volume V (m ³ ) ( Q / V) is 0.002 (1 / s) or more. (2) The method for producing low-nitrogen steel according to (1), wherein the ratio (Q / V) is 0.02 (1 / s) or less. (3) The low-nitrogen steel according to (1) or (2), wherein the converter internal volume V is calculated by the following equation (1) using the steel shell internal volume Vo (m ³ ). Melting method.

V = 0.48 × Vo + 5.15 (1)

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method of the present invention, hot metal conveyed from a blast furnace is subjected to hot metal treatment such as desiliconization, dephosphorization, desulfurization or the like, and then the hot metal is charged into an upper-bottom blow converter. Perform blowing.

In the case of a converter, after charging raw materials such as hot metal and scrap into the converter, oxygen is supplied from a top blowing lance and / or a bottom blowing tuyere provided above the molten steel to perform blowing and bottom blowing. Any one or more of Ar, CO, CO ₂ and hydrocarbons are blown down from the tuyere.

Hydrocarbons include, for example, CH ₄ , C ₂ H ₆ ,
C ₃ H ₈ , C ₄ H _{10 and the} like can be used alone or as a mixture. Here, gas flow rates are Ar, CO, CO
_When each of the gases ₂ is used, the flow rate indicates the added flow rate as it is, and when a hydrocarbon is used, the flow rate after the hydrocarbon decomposition is indicated. That is, CH ₄ is doubled as H ₂ and C ₂ H
₆ converts the flow rate of 3 times the H _2.

The internal volume of the top and bottom blown converter with a refractory lining can be measured by a profile in the furnace using ultrasonic waves, microwaves or the like. However, taking a long time for the measurement causes a decrease in productivity. Therefore, the internal volume of the furnace from the internal volume Vo (m ³ ) of the refining furnace to the internal volume of the furnace V (m ³ )
We propose a simple estimation method. That is, regression analysis was performed on the basis of information on converters of various sizes to obtain a relationship represented by the following equation.

V = 0.48 × Vo + 5.15 --- (1) The steel inner volume Vo is defined as follows. That is, the iron shell inner volume Vo is the volume of a region surrounded by the outer iron plate of the refining furnace and the opening surface of the refining furnace.

[0020]

EXAMPLE A molten iron was charged into a 250 mass ton converter, a slag forming agent was added, oxygen was blown from a top blowing lance, and a stirring gas was flowed from a bottom blowing tuyere to obtain a [C] concentration of 0 in molten steel. .05
% Was blown. The charged hot metal was supplied from a blast furnace, received in a topped car, and used as it was or desiliconized, dephosphorized and desulfurized in a pretreatment station. The hot metal component charged to the converter is [Si]: <0.01
%, [P]: 0.012%, and [S]: 0.003%. The top-blown oxygen is blown all over using a water-cooled Laval lance, and the bottom-blown is a hydrocarbon (C
₃ H ₈ ), and Ar was used for the outer tube.

Table 1 shows the test results. Note that V (m^Three)
Is calculated using the above equation (1), and C _ThreeH₈Adjust the flow rate
To control the ratio (Q / V).

[0022]

[Table 1] As shown in the table, the ratio (Q / V) was 0.002 (1 /
In Comparative Examples less than s), the [N] concentration in the molten steel was as high as 25 ppm or more, and low-nitrogen steel could not be produced.

In order to smelt low-nitrogen steel having a [N] concentration of 20 ppm or less in molten steel, the ratio (Q / V) must be not less than 0.002 (1 / s) as shown in the examples of the present invention. It was necessary to do.

[0024]

According to the present invention, since air intrusion into the furnace at the end of the converter blowing can be easily prevented, low-nitrogen steel, for example, low-nitrogen steel of 20 ppm or less, preferably 10 ppm or less can be easily melted. .

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the [N] concentration in molten steel, the spitting index, and the ratio (Q / V).

Claims (3)

[Claims] 1. A top-blowing converter in which oxygen is blown upward, and Ar,
In the decarburization refining in which one or more of CO, CO ₂ and hydrocarbon gas are blown in the bottom, the flow rate of the bottom blown gas Q (m ³
(Standard state) / s) and the ratio between the rolling furnace volume V (m ³⁾ (Q /
V) is 0.002 (1 / s) or more, the method for melting low-nitrogen steel. 2. The ratio (Q / V) is 0.02 (1 / s).
The method for melting low-nitrogen steel according to claim 1, wherein: 3. The converter inner volume V is changed to a steel shell inner volume Vo (m
^{The method according} to claim 1 or 2, wherein the calculation is performed by the following equation (1) using ( ³ ). V = 0.48 × Vo + 5.15 (1)