JP2563721B2 - Decarburization refining method for molten steel containing chromium - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently decarburizing a chromium-containing molten steel by improving the decarburizing rate and suppressing the oxidation of [Cr] in the molten steel.

[0002]

2. Description of the Related Art Conventionally, 11 wt% such as stainless steel
Oxygen gas or oxygen gas from below and above the bath surface of the chromium-containing molten steel containing chromium as described above.
As a method of suppressing the oxidation of [Cr] in the molten steel and efficiently decarburizing in a composite blowing method in which a diluting gas is blown, for example, JP-A-55-158213 discloses oxygen under the bath surface. And decarburization by blowing an inert gas, and at the same time, an amount of oxygen corresponding to at least 0.2 times the oxygen amount is supplied from the bath surface, and by the heat of secondary combustion reaction due to the oxygen supplied from the bath surface, , A method for promoting decarburization is described.

Further, in Japanese Patent Laid-Open No. 59-166617, when decarburizing molten chromium-containing steel by a composite blowing method using oxygen for top blowing and mixed gas of oxygen and inert gas for bottom blowing, By controlling the height of the lance and the amount of top-blown oxygen according to the concentration of [C], the splash generated from the bath surface is suppressed, and the heat of secondary combustion reaction due to the top-blown oxygen promotes decarburization. The method is described.

Generally, the decarburization reaction of molten chromium-containing steel by complex blowing can be divided into a decarburization reaction by oxygen supplied from below the bath surface and a decarburization reaction by oxygen supplied from above the bath surface. The above-mentioned conventional techniques are all techniques for improving the decarburization reaction due to oxygen supplied from above the bath surface, and the correlation between the decarburization reaction due to oxygen supplied from both below and above the bath surface is Not paid attention.

The conditions for blowing oxygen supplied from below the bath surface and above the bath surface should have appropriate values depending on the [C] concentration in the molten steel. Since it cannot be controlled easily, there are large variations.

[0006]

In the decarburization refining of molten chromium-containing steel by the composite blowing method, the amount of inert gas used is reduced by maintaining the bottom blowing condition and the top blowing condition in the suitable ranges. It is intended to improve the speed, suppress the oxidation of chromium in molten steel, and efficiently decarburize, and at the same time, shorten the refining time.

[0007]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and decarburization in which oxygen gas or oxygen gas and a diluent gas are blown below and above the bath surface of molten steel containing chromium. In the refining method, in a region where the [C] concentration in the molten steel is 0.2 wt% or more and 0.7 wt% or less, oxygen gas and a dilution gas are provided below the bath surface, and only oxygen gas is provided on the bath surface by the formula (1) , (2), (3) and (4)
This is a decarburizing refining method for molten chromium-containing steel, which is characterized by blowing under conditions satisfying the formula.

0.5 ≦ A + B ≦ 1.5 (1) 1 ≦ A / B ≦ 5 (2) 0.20 ≦ C / (A + B) ≦ 0.50 (3) 0.05 ≦ L / L _O ≦ 0.2 (4) A: Oxygen gas flow rate per unit molten steel weight blown below the bath surface [Nm ³ / minT] B: Dilution per unit molten steel weight blown below the bath surface Gas flow rate [Nm ³ / minT] C; Oxygen gas flow rate per unit molten steel weight blown onto the bath surface [Nm ³ / minT] L; Depth of depression on the bath surface due to gas blown onto the bath surface [mm] L _{O 2} ; depth of molten steel in stationary bath state [mm] The present invention will be described in detail below.

The decarburizing refining method for molten chromium-containing steel of the present invention is shown in FIG.
Is a decarburization refining method by compound blowing as illustrated in
(A) shows a static bath state, (b) shows a gas blowing state,
In the figure, 1 is a top blowing lance, 2 is a bottom blowing double pipe tuyere, 3 is molten steel, 4 is slag, and oxygen is supplied from the top blowing lance 1.
Oxygen or oxygen and a diluent gas are blown from the inner tube of the bottom-blown double-tube tuyere 2. In the figure, 5 indicates the hot spot of the bath surface by top blowing.

According to the present invention, in the compound blowing of chromium-containing molten steel, the [C] concentration in the molten steel is 0.2 wt% or more and 0.7 wt% or less.
%, The protective gas for preventing melting of tuyere is blown from the outer tube of the double tube tuyere 2, and the oxygen and dilution gas of a certain flow rate or more are blown from the inner tube to positively activate the molten steel. Agitate to make the flow on the bath surface vigorous. Further, by setting the O ₂ / Ar ratio of the gas blown from the inner tube to a suitable value, the oxidation of chromium in the molten steel is suppressed and the amount of the inert gas used is suppressed. On the other hand, the only gas blown onto the bath surface is oxygen, and the blown oxygen forms a high temperature region on the bath surface. Due to the high temperature action and the vigorous flow action, decarburization proceeds extremely efficiently on the bath surface. Further, since the fire point temperature of the oxygen gas blown onto the bath surface is generally 2000 ° C. or higher, the same decarburizing effect can be obtained without mixing the diluent gas.

Generally, when a gas is blown from below the bath surface using a double tube tuyere, a protective gas such as Ar or LPG is supplied from the outer tube to prevent the tuyere from melting as described above. Is blowing oxygen or oxygen and diluent gas for refining. When decarburizing molten chromium-containing steel only by bottom blowing, in order to suppress oxidation of chromium in the molten steel, the flow rate of oxygen flowing through the inner pipe is decreased and the dilution gas flow rate is increased as the [C] concentration decreases. The method is taken.

FIG. 2A shows an example of the conventional method, and FIG. 2B shows an example of the top / bottom blown gas blowing pattern of the present invention. In the conventional method, from the viewpoint of suppressing chromium oxidation as much as possible, O _{2 of the} bottom blown gas is used.
/ Ar ratio tends to be small, and A
In some cases, r dilution is performed. In the method of the present invention, the [C] concentration is 0.
Bottom blown O _{2 in} the range of 2 wt% to 0.7 wt%
It has been found that if the / Ar ratio is optimized, the required oxygen flow rate for bottom blowing can be secured, and the top blowing can suppress the oxidation of chromium without flowing a diluent gas. did.

FIG. 3 shows the relationship between the [C] concentration and the decarbonation efficiency when decarburizing SUS304 stainless steel is blown according to the blowing pattern of the method of the present invention shown in FIG. 2 (b). Note that the alternate long and short dash line in the figure shows the average value, and the amount of oxygen blown from the inner tube of the double tube tuyere below the bath surface as the bottom blowing gas is 0.5 Nm.
^{At 3} / minT, the O ₂ / Ar ratio [A / B] of oxygen and argon gas blown from the inner tube of the double tube tuyere is 1 or more and 5 or less, and the upper blown oxygen amount ratio [C / (A + B) ], And the dent ratio L / L _O of the bath surface due to top-blown oxygen was 0.1. The decarboxylation efficiency shows the ratio of oxygen used for decarburization to the blown oxygen, and the depression ratio of the bath surface due to the top blowing gas will be explained based on FIG. From FIG. 3, at a [C] concentration of 0.2 wt% or more, a high decarboxylation efficiency was obtained under the above conditions. On the other hand, if the [C] concentration is less than 0.2 wt%, the decarboxylation efficiency will drop sharply.

The decarboxylation efficiency is stable at a high level even in the range where the [C] concentration exceeds 0.7 wt%, but in this range, as shown in JP-A-58-77519. It is considered that bottom blowing is only oxygen.

According to the present invention, the amount of diluent gas used can be reduced and the amount of oxygen that can be supplied below the bath surface can be increased.

Next, as the bottom blowing condition, equation (1) and
It is necessary to satisfy the formula (2). FIG. 4 shows the relationship between the bottom blowing gas flow rate and the decarbonation efficiency in bottom blowing only blowing. In addition, decarbonation efficiency is 0.2 w when the [C] concentration in molten steel is
In the region of t% or more and 0.7 wt% or less, the ratio of oxygen that has acted for decarburization in bottom-blown oxygen is shown. Also, before blowing [C]
The concentration is a value near 1.5 wt%, the flow rate of the protective gas from the outer tube of the double tube tuyere is 0.05 Nm ³ / minT, and the bottom blown O ₂ / Ar ratio is 2 to 4. Was in the range. From Figure 4, the bottom gas flow rate is 0.5 Nm ³ / minT.
As described above, the decarboxylation efficiency is stabilized at a high level at 1.5 Nm ³ / minT or less. Bottom blow gas flow rate is 0.5 Nm ³ / min
When it is less than T, the stirring power of the steel bath becomes small, the reaction interface area for the supplied oxygen to react with [C] becomes small, and the supplied oxygen is used for chromium oxidation and contained in the slag ( Cr ₂ O ₃ ). Further, when the bottom blown gas flow rate exceeds 1.5 Nm ³ / minT, splash and dust are increased and decarbonation efficiency is lowered.

FIG. 5 shows the relationship between the O ₂ / Ar ratio of the bottom-blown gas and the decarbonation efficiency in the bottom-blown only blowing. The decarbonation efficiency indicates the proportion of bottom-blown oxygen used for decarburization in the region where the [C] concentration in molten steel is 0.2 wt% or more and 0.7 wt% or less. Further, the [C] concentration before blowing is a value in the vicinity of 1.5 wt%, the total bottom blowing gas flow rate is constant at 1.0 Nm ³ / minT, and the O ₂ / Ar ratio of the bottom blowing gas is The training was carried out as a constant. From FIG. 5, when the O ₂ / Ar ratio of the bottom-blown gas exceeds 5, the decarboxylation efficiency is sharply reduced. This is because when O ₂ / Ar becomes large, the CO gas generated in the decarburization reaction becomes too large, and the CO partial pressure in the atmosphere becomes high, so the supplied oxygen is used for chromium oxidation rather than the decarburization reaction. It depends. Further, even if the O ₂ / Ar ratio of the bottom-blown gas is less than 1, the decarburization efficiency is stable at a high level, but when the O ₂ / Ar ratio is small, the amount of diluent gas used increases and the oxygen blowing rate is increased. Is smaller, which leads to extension of refining time, which is not preferable. Therefore, it is necessary to satisfy equations (1) and (2) as bottom blowing conditions.

FIG. 6 shows the relationship between the top blown and bottom blown oxygen ratio [C / (A + B)] and the decarburization rate ratio.

Regarding the decarburization rate ratio, the bottom blown oxygen amount was 0.8 Nm ³ / minT and the top blown oxygen amount ratio was 0 in the region where the [C] concentration in the molten steel was 0.2 wt% or more and 0.7 wt% or less. In this case, the average decarburization rate is 100 and indexed. The total oxygen flow rate is 0.8 Nm ³ / minT.
The values are constant when the O ₂ / Ar ratio [A / B] of the bottom-blown gas is 2, and the depression ratio L / L _O of the bath surface due to the top-blown oxygen is 0.1. From Fig. 6, if the value on the horizontal axis is 0.20 or more, 0.50
The value on the vertical axis stabilizes at a high level in the following cases. This is because when the top-blown oxygen amount ratio is small, the top-blown oxygen is mostly used in the secondary combustion reaction that reacts with the CO gas generated in the decarburization reaction by the bottom-blown oxygen to generate CO ₂ gas, and the decarburization reaction Is not used effectively. Further, when the ratio of the amount of top-blown oxygen is large, the amount of oxygen supplied to the top-blown flash point becomes excessive, and the supplied oxygen not only produces molten steel with a high oxygen concentration but also oxidizes [Cr]. Used in the slag (Cr ₂ O
₃ ) By increasing the concentration. Therefore, the ratio of the amount of top-blown oxygen must satisfy equation (3).

FIG. 7 shows the depression ratio [L] of the bath surface due to the top-blown gas.
/ L _O ] and the decarboxylation efficiency are shown. The decarbonation efficiency is 0.2 wt% or more and 0.7 wt% or more of [C] concentration in molten steel.
The ratio of oxygen used for decarburization among the top-blown oxygen is shown in the following region. The [Si] concentration in the molten steel before blowing was 0.
It is 1 wt% or less, the [C] concentration is around 1.5 wt%, and the top-blown oxygen amount ratio is 0.4.

Steel bath depth L _{O in} the stationary bath state of FIG. 1 (a)
By supplying the top-blown and bottom-blown gases to the molten steel 3 of FIG. Here, the depth L of the dent on the bath surface due to the top-blown gas is derived from the equations (5) and (6).

L = L _h · exp (−0.78 _h / L _h ) (5) L _h = 63.0 · (Q _T / nd) ^2/3 (6) where L; above Depth of bath surface by blowing gas [mm] h; Top blowing lance gap [mm] Q _T ; Top blowing gas flow rate [Nm ³ / Hr] n; Number of top blowing lance holes d; Top blowing lance hole diameter [mm That is, by reducing the top blowing lance gap h and increasing the flow velocity of the gas to be blown, the depth L of the depression on the bath surface increases and L / L _O also increases.

From FIG. 7, L / L _O is 0.05 or more and 0.2
Within the following range, the value on the vertical axis is highly stable. This is L / L
_{When O} is small, the ratio of oxygen reaching the steel bath surface is small among the oxygen supplied by top blowing, and the oxygen is used in the secondary combustion reaction in the gas phase and is not effectively used in the decarburization reaction. . When L / L _O is large, most of the oxygen supplied by top blowing reaches the steel bath surface,
Since this oxygen is locally concentrated and supplied, it is used for the oxidation of [Cr] in the molten steel and is not effectively used for the decarburization reaction. Further, when the L / L _O is too large, the splash of the molten steel is increased, there is a problem that can not be performed stable operation. Therefore, the value of L / L _O must satisfy equation (4).

From the above, in the decarburization refining of the chromium-containing molten steel by the composite blowing method, in order to reduce the amount of the diluent gas used, suppress the oxidation of chromium in the molten steel, and perform the decarburization efficiently, [C] The conditions for gas injection into molten steel having a concentration of 0.2 wt% or more and 0.7 wt% or less are shown in the equation (1),
It is necessary to satisfy the conditions of Eqs. (2), (3), and (4). In the actual operation, it is necessary to set the conditions in correspondence with the molten steel temperature. However, the molten steel composition and the molten steel temperature are grasped at the time of charging the crude molten steel, and the [C] concentration up to 0.7 wt% is confirmed. Grasping is performed, and based on these, top blowing conditions and bottom blowing conditions are determined, and refining is performed. Adjustment of L / L _O is basically carried out by the adjustment of the top-blown lance gap. In addition, when the molten steel temperature is lower than the target temperature, it is possible to adjust by operating with a large amount of top-blown oxygen.

In the region where the concentration of [C] is less than 0.2 wt%, the blowing method is the same as the conventional method. In this case, the [C] concentration range to be sprayed upward must be determined in relation to the decarboxylation efficiency, but the [C] concentration is preferably 0.2 wt% or more in order to suppress the oxidation of chromium as much as possible.

[0026]

[Function] In the decarburization refining of molten chromium-containing steel, the injected oxygen once forms a solid solution in the molten steel, and then the decarburization reaction shown by the following formula is simultaneously performed with the oxidation of [Cr] in the molten steel shown by the formula (8). The reaction also proceeds.

[C] + [O] → CO (g) (7) 2 [Cr] +3 [O] → (Cr ₂ O ₃ ) (8) Equilibrium constants K ₇ , K of these reactions ₈ is (9), (1
It is expressed by equation (0).

[0028] _{K 7 = P CO / a C} a O ...... (9) K 8 = a Cr2O3 / a Cr2 a O3 ...... (10) where the partial pressure of CO gas in P _CO atmosphere, a _C , A
_O, a _Cr is in the molten steel [C], shows the activity of [O], [Cr], a Cr2O3 shows the activity of the slag (Cr ₂ O _3).
Generally, a _Cr2O3 is set to 1, and a _C , a _O , and a _Cr
Is difficult to control, it is said that in order to preferentially advance the decarburization reaction of the formula, it is effective to increase the dilution gas amount and decrease _PCO .

However, use of an excessive amount of diluent gas is not preferable because it leads to an increase in cost and lowers the amount of oxygen. In the present invention, the quantitative relationship between the equations (9) and (10) was derived, and the proper dilution gas ratio shown in the equation (2) was derived.

Further, when oxygen is blown by the upper blowing, the secondary combustion reaction represented by the equation (11) proceeds in addition to the above reaction.

CO + 1 / 2O ₂ → CO ₂ (g) (11) This secondary combustion reaction is an exothermic reaction, and the temperature of the hot-spot portion 5 of the upper-blown oxygen in FIG. It is possible to raise the temperature and advantageously advance the decarburization reaction. However, the excessive progress of the secondary combustion reaction is disadvantageous because the oxygen supplied by the upper blowing is not used for the decarburization reaction.
Therefore, by controlling the secondary combustion reaction under the conditions shown in the equations (3) and (4), the decarburization reaction can be efficiently advanced.

In the decarburization reaction, the rate-determining process changes depending on the [C] concentration. On the low [C] concentration side, the rate-determining process is the transfer of [C], and by strengthening the stirring of the steel bath, the transfer of [C] is promoted and the decarburization rate is increased. Controlling the top blowing gas condition in this state does not lead to an improvement in the decarburization rate.

On the high [C] concentration side, the supply of oxygen to the steel bath is a rate-determining process, the effect of the diluent gas is small, and the supply of oxygen alone is sufficient. Further, the decarburization rate is improved by maintaining the oxygen supply condition to the steel bath within a suitable range.

[C] concentration 0.2 wt% or more 0.7 wt%
The following region is a region where these rate-determining processes change depending on the temperature and gas injection conditions, a constant stirring force is required, and the relationship of equation (1) is derived. Further, it is preferable to control the upper and lower blowing conditions at the same time, and effective decarburization can be achieved by simultaneously satisfying the above expressions (2), (3) and (4).

[0035]

Example: SUS304 stainless steel (8 wt% Ni-
18 wt% Cr) 60 ton processing, FIG.
In the embodiment shown in FIG. 2, the oxygen gas and Ar gas supply pattern shown in FIG. At the start of decarburization, the [C] concentration was set to 1.5 wt% and the upper blowing was performed only with oxygen gas. When the [C] concentration was 0.2 wt% or less, the upper blowing was stopped and the refining was performed only by bottom blowing. Table 1 shows examples of decarburizing and refining conditions in the region where the [C] concentration in molten steel is 0.2 wt% or more and 0.7 wt% or less.

The embodiment of the present invention is shown in (1) above.
Refining was carried out so as to satisfy the expressions (2), (3), and (4). No. of the comparative example. No. 6 of the comparative example is a method of blowing only by bottom blowing. No. 7 is recognized as a method generally practiced as a conventional method.
The method of No. 3 (FIG. 2 (a)) was followed. In addition, N of the comparative example
o. No. 8 is an example in which the bottom blown gas flow rate and the top blown oxygen amount ratio are outside the present invention, No. 9 is an example in which the top-blown oxygen content ratio is outside the scope of the present invention, and No. No. 10 is an example in which the bottom blown oxygen flow rate, L / L _O and the top blown oxygen amount ratio are outside the scope of the present invention. 11 is bottom blowing O ₂
The / Ar ratio is an example outside the present invention.

The area where the [C] concentration is less than 0.2 wt% is No. Other than 6, the upper blowing was performed to a [C] concentration of 0.05 wt%.

[0038]

[Table 1]

The results of implementation are shown in Table 2. In the table, the average decarboxylation efficiency is a value in the range of [C] concentration of 0.2 wt% or more and 0.7 wt% or less, decarburization rate index and refining time. 8
Is 100, and is proportionally converted by the supply oxygen gas flow rate, and the Ar gas usage is No. It is a value converted from the example of 7 as 100.

[0040]

[Table 2]

[0041]

According to the method of the present invention, in the decarburization refining of molten chromium-containing steel, the efficiency of decarbonation can be improved, and the decarburization rate can be improved with the same amount of oxygen gas supplied. Further, the amount of expensive Ar gas used is reduced and the refining time is shortened. As a result, the refining cost can be significantly reduced and the productivity can be improved.

[Brief description of drawings]

FIG. 1 is an explanatory view of an embodiment of the present invention, in which (a) is a static bath state and (b) is a gas blowing state.

FIG. 2 is a diagram showing an example of a gas supply pattern in the conventional method and the method of the present invention.

FIG. 3 is a diagram showing the reasons for limiting the carbon concentration and the O ₂ / Ar ratio of the bottom-blown gas in the method of the present invention.

FIG. 4 is a diagram showing the reason for limiting the bottom blown gas flow rate in the method of the present invention.

FIG. 5 is a diagram showing the reason for limiting the O ₂ / Ar ratio of the bottom-blown gas in the present invention.

FIG. 6 is a diagram showing the reason for limiting the ratio of the amount of top-blown oxygen in the method of the present invention.

FIG. 7 is a diagram showing the reason for limiting L / L _O by top blowing in the method of the present invention.

[Explanation of symbols]

 1 ... Top blow lance 2 ... Bottom blow tuyere 3 ... Molten steel 4 ... Slag 5 ... Top blow fire point

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hironori Takano 3434 Shimada, Hikari-shi, Nippon Steel Works, Nippon Steel Corporation (72) Inventor Toshihiro Kosuge 3434, Shimada, Hikari-shi Nippon Steel, Ltd. Hikari Steel Works (72) Inventor Hiroshi Hirata 20-1 Shintomi, Futtsu-shi Shin Nippon Steel Co., Ltd. Technology Development Headquarters (56) Reference JP-A-2-221318 (JP, A) JP-A-55-158213 (JP , A) JP 59-166617 (JP, A) JP 3-281720 (JP, A) JP 2-133510 (JP, A) JP 60-131908 (JP, A)

Claims (1)

(57) [Claims] 1. Below and above the bath surface of the molten chromium-containing steel,
In the decarburizing refining method in which oxygen gas or oxygen gas and diluent gas are blown, the [C] concentration in the molten steel is 0.2 wt%
In the region of 0.7 wt% or less, oxygen gas and dilution gas are provided below the bath surface, and only oxygen gas is provided above the bath surface (1).
Method for decarburizing and refining molten chromium-containing steel, characterized by blowing under conditions satisfying equations (2), (3) and (4) 0.5 ≦ A + B ≦ 1.5 ( 1) 1 ≦ A / B ≦ 5 (2) 0.20 ≦ C / (A + B) ≦ 0.50 (3) 0.05 ≦ L / L _O ≦ 0.2 (4) A An oxygen gas flow rate per unit molten steel weight [Nm ³ / minT] B blown below the bath surface; a dilution gas flow rate per unit molten steel weight [Nm ³ / minT] C below the bath surface C; blow above the bath surface Oxygen gas flow rate per unit molten steel weight [Nm ³ / minT] L; Depth of bath surface recessed by gas blown into the bath surface [mm] L _O ; Molten steel depth in stationary bath state [mm]