JP2578046B2 - Decarburization refining method of chromium-containing molten steel - 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 decarburizing and refining chromium-containing molten steel, in which the decarburization rate is improved, and the [Cr] in the molten steel is suppressed from being oxidized to efficiently decarburize. It is.

[0002]

2. Description of the Related Art Conventionally, in a compound blowing method in which oxygen gas or an oxygen-containing gas (hereinafter simply referred to as oxygen) or oxygen and an inert gas are blown from below and above a bath of chromium-containing molten steel such as stainless steel. As a method of efficiently decarburizing, for example, Japanese Patent Application Laid-Open No. 55-158213
Degassing is performed by blowing oxygen and an inert gas under the bath surface, and at the same time, supplying an amount of oxygen equivalent to at least 0.2 times the amount of oxygen from the bath surface to thereby reduce CO generated from the bath surface. Positively performs the secondary combustion reaction to CO ₂ ,
It describes that the molten steel is heated by the heat of reaction to suppress the generation of chromium oxide and to reduce the amount of Si for reducing chromium oxide in slag.

Japanese Patent Application Laid-Open No. 61-266516 discloses a relational expression between a ratio (P) of top-blown oxygen reacting with molten steel, a lance height (L), and a velocity (V) of oxygen blown from the lance. The formula is presented and the top blowing oxygen is 150 ft.
No./s (45.7 m / s) to the speed of sound, it describes that a desired ratio of the top blown oxygen reacting with molten steel is obtained.

[0004] P = K-1629 (L / V) [2] Equation P: Desired ratio (%) of top-blown oxygen that reacts with molten steel K: Constant having a value of 56 to 72 L: Surface of molten steel Height of upper lance opening (ft) V: Velocity of oxygen blown from lance (ft / sec) In general, the secondary combustion reaction by top-blown oxygen occurs in a region where the speed of oxygen is lower than the sonic speed (free jet region). In view of the above, Japanese Patent Application Laid-Open No. 61-266516 aims at positively performing a secondary combustion reaction by top-blown oxygen.

[0005]

In each of the above-mentioned prior arts, a secondary combustion reaction by upward blowing oxygen is positively generated in order to improve the decarburization efficiency in the decarburization and refining method of chromium-containing molten steel, and the heat of the secondary combustion is increased. Thus, the temperature of the molten steel is raised to improve the decarburization efficiency. However, in order to raise the temperature of molten steel by the heat of secondary combustion, it is necessary to secure a necessary amount of oxygen for secondary combustion. For this reason, comparing the combined blowing with the bottom blowing with the same oxygen supply amount, the combined blowing consumes more oxygen for the secondary combustion, so the amount of oxygen directly reacting with the carbon in the molten steel , The decarburization and refining time is prolonged.

The problem to be solved by the present invention is to improve the decarburization efficiency by maintaining the bottom-blowing and top-blowing conditions in a suitable range in the decarburization refining of chromium-containing molten steel by the combined blowing method. An object of the present invention is to suppress the oxidation of chromium in molten steel to shorten the refining time and reduce the amount of Si for reducing chromium oxide.

[0007]

SUMMARY OF THE INVENTION The present invention has advantageously solved the above-mentioned problems, and the gist thereof is as follows. (1) When decarburizing the molten steel by blowing oxygen or oxygen and an inert gas under the bath surface of the chromium-containing molten steel, in the region where the [C] concentration in the molten steel is 0.4% or more, the top blowing lance is used. A method for decarburizing and refining chromium-containing molten steel, wherein oxygen or oxygen and an inert gas are blown into the bath surface of the molten steel using the following conditions. (1) The gas ejection speed from the nozzle of the upper blowing lance is equal to or higher than the sound speed. (2) The length h of the region where the speed of the gas ejected from the nozzle of the upper blowing lance is equal to or lower than the sound speed, and the minimum hole diameter of the upper blowing nozzle. preceding the ratio h / d _O of d _O 40 to 80 (2) top-blown molten steel temperature when the gas blowing start with lance may be higher than the equilibrium temperature of molten steel T which is determined by formula [1] 2. The method for decarburizing and refining chromium-containing molten steel according to 1.

T = 13800 / {8.76−Log ([% Cr] * _PCO / [% C])} [1] Formula T: Equilibrium molten steel temperature (K) [% Cr]: In molten steel Chromium concentration (% by weight) P _CO : CO gas partial pressure (atm) [% C]: Carbon concentration in molten steel (% by weight) (3) On the bath surface relative to the total oxygen amount blown below and above the bath surface 2. The method for decarburizing and refining chromium-containing molten steel according to the above item 1, wherein the ratio of the amount of oxygen blown into the steel is 20 to 70%.

(4) Depth L of bath surface is 70-500 m
m. The method for decarburizing and refining chromium-containing molten steel according to item 1 above, wherein (5) The method for decarburizing and refining chromium-containing molten steel as described in (1) above, wherein the shape of the nozzle hole of the upper blowing lance is a shape that widens forward. Hereinafter, the present invention will be described in detail.

The decarburizing and refining method of the chromium-containing molten steel of the present invention is a decarburizing and refining method by combined blowing as exemplified in FIG.
(A) shows a stationary bath state, (b) shows a gas blowing state, 1 in the figure is a top blowing lance, 2 is a bottom blown double tube tuyere,
Reference numeral 3 denotes molten steel, 4 denotes slag, and oxygen or oxygen and an inert gas are blown from the top blow lance 1 and oxygen or oxygen and an inert gas are blown from the inner tube of the bottom blown double tube tuyere 2 and from the outer tube. A protective gas for cooling the tuyere (such as Ar gas) is blown.
In the drawing, reference numeral 5 denotes a high-temperature spot caused by oxygen blowing from the upper blowing lance 1.

In general, the decarburization and refining operation of chromium-containing molten steel is carried out in the step of oxidizing and removing [C] in the molten steel by the decarburization reaction represented by the formulas [3] and [4] (oxidation stage) and in the oxidation stage. A reducing agent (eg, F
e-Si, Al) and slag-making material (for example, CaO, CaF ₂ )
And the step of reducing and recovering by the reaction represented by the equations [5] and [6] (reduction phase).

[0012] _{<oxide-life> 2 Cr + 3 / 2O 2} = (Cr 2 O 3) ··· [3] formula _{(Cr 2 O 3) +3 C} = 2 Cr + 3CO ··· [4] formula <-reduction phase> Fe-Si-use _{2 (Cr 2 O 3) +} 3Si = 4 Cr +3 (SiO 2) ··· [5] formula Al used at _{2 (Cr 2 O 3) +} 4Al = 4 Cr +2 (Al 2 O 3) ·・ [6] formula [4] so that a large amount of chromium oxide does not remain in the oxidation stage.
In order to accelerate the reaction of the equation, the molten steel temperature is increased (160
0 ° C. or higher), and oxygen blown from the top blow lance or the bottom blown double tube tuyere to lower the CO gas partial pressure (P _CO ) as the carbon concentration (hereinafter referred to as [C%]) in the molten steel decreases. It is common practice to increase the ratio of inert gas to inert gas (dilution ratio). On the other hand, the main parameters of the decarburization and refining cost of chromium-containing molten steel are inert gases such as Ar gas, reducing agents such as Fe-Si, and refractories, so that the amount of chromium oxide generated during the oxidation period should be minimized. In order to suppress the amount of inert gas such as Ar and the amount of refractory erosion that increase in proportion to the decarburization refining time, it is important to complete the decarburization refining in a short time.

The present invention provides a decarburization refining method for a chromium-containing molten steel having the above-mentioned characteristics by a combined blowing method, wherein a double pipe blade under a bath surface is provided in a region where the carbon concentration in the molten steel is 0.4% or more. By blowing a protective gas for preventing tuyere erosion from the outer tube of the mouth and oxygen or oxygen and an inert gas from the inner tube, the molten steel is actively stirred to activate the flow of the bath surface, By blowing oxygen or oxygen and inert gas onto the bath surface at high speed from the top blow lance, a high temperature fire point is generated on the bath surface, increasing the rate at which oxygen in the top blow oxygen gas reacts with carbon in the molten steel. By doing so, the decarburization efficiency is further improved, and the decarburization refining time is shortened and the reduction of the Si unit consumption (or Al unit consumption) for reduction is achieved.

In the combined blowing method, when oxygen is effectively blown onto the surface of molten steel to generate a high-fired portion, the decarburization reaction on the steel bath surface is a reaction at a high-fired temperature, and the decarburization reaction is remarkably accelerated. You. That is, the decarburization efficiency is improved by suppressing the amount of oxygen consumed in the secondary combustion and increasing the amount of oxygen that directly reacts with the molten steel to blow up oxygen so as to promote the formation of a hot spot. It is possible to do. In order to further promote the decarburization reaction at the hot spot, it is necessary to stir the molten steel and perform bottom-blowing under appropriate bottom-blowing gas conditions to effectively supply C in the molten steel to the hot spot. is necessary.

Conventionally, the secondary combustion reaction by top blowing oxygen is as follows:
Since it is said that this occurs when the surrounding CO gas is entrained in a region (free jet region) in which the velocity of the top-blown oxygen is lower than the sound speed of 330 m / sec, the free jet region is required to suppress the secondary combustion. Need to be controlled. When the flow rate of oxygen blown from the top blowing lance is the same, the free jet area becomes shorter as the lance height is lower and the area of the injected oxygen velocity is supersonic (jet core area). The proportion consumed in the secondary combustion of CO in oxygen decreases.

FIG. 2 shows a total acid flow rate of 4000 Nm ³ / Hr
The constant compound blowing was performed in the range of [% C] ≧ 0.5% in molten steel, and shows the change of dC / dO ₂ with respect to h / d _O. In the range of h / d _O = 40 to 80, dC / dO ₂ is greatly improved, and the smaller h / d _O is, the smaller d / d _{O 2} is.
C / dO ₂ is improved. Note that the lower limit of h / d _O is 40
The reason for this is that when h / d _O is small, the depression on the surface of the molten steel, which will be described later, becomes large, and the splash increases, which hinders the operation.

Here, h / d _O is an index indicating the length of the free jet area as shown in FIG. 3 and is given by the equations [7] and [8]. h / d _O = H / d _O −H _C / d _O Formula [7] H _C / d _O = 4.12 Pa-1.86 Formula [8] h: Free jet length [mm] H: Lance gap [mm] H _C : Jet core length (supersonic range length) [mm] Pa: Top blowing gas blowing pressure [kg / cm ² ] d ₀ : Top blowing lance nozzle hole diameter [mm] dC / DO ₂ is top blown or bottom blown oxygen 1
It indicates the amount of decarburization per Nm ^{3. The} larger this index is, the higher the decarburization efficiency is. FIG. 4 shows the relationship between the amount of oxygen consumed in the secondary combustion reaction in top-blown oxygen and h / d _{0 in} the case of performing combined blowing with various top-blown conditions using the same top-blown nozzle. It is shown. The smaller the _{value of} h / d _{0, the} more the amount of oxygen consumed for the secondary combustion is suppressed. From the above results, by controlling h / d ₀ to 40 to 80 to suppress secondary combustion, dC / dO ₂ is improved, and the decarburization refining time is shortened and the reduction of Si unit consumption for reduction is achieved. You.

FIG. 5 shows a total acid flow rate of 4000 Nm ³ / H.
r, the oxygen blowing speed of the upper blowing nozzle is higher than the sound speed, h / d ₀
= 75, combined blowing application area [% C] ≧ 0.5% Under constant conditions, blown onto the bath surface relative to the total acid supply blown under and above the bath surface given by equation [9] The relationship between the ratio of the oxygen amount (hereinafter referred to as the upper blowing ratio) and dC / dO ₂ is shown.

[0019] It dC / dO ₂ at a ratio blown top blowing oxygen flow × 100 / (the top-blown oxygen flow rate + bottom-blown oxygen flow rate) · [9] on expression 20% to 70% is increased, 40-60 %, The decarburization reaction efficiency at the hot spot is high due to the top blowing oxygen, and the molten steel is agitated to promote the decarburization reaction at the hot spot. It can be understood that the bottom blowing conditions for effectively supplying the gas to the high temperature fire point are effective.

FIG. 6 shows that during the combined blowing, where the application range of the combined blowing is variously changed under the constant conditions of h / d ₀ = 75 and the top blowing ratio = 30% (top blowing oxygen flow rate 1200 Nm ³ / Hr). Shows the decarburization efficiency. Since high decarburization efficiency can be maintained in the range of [% C] ≧ 0.4%, it is clear that the combined blowing area should be set to [% C] ≧ 0.4%. In addition, the length H _C of the jet core region is also affected by the shape of the nozzle from which the oxygen gas is ejected. If the nozzle shape is formed in consideration of the volume expansion of the gas, the length of the jet core region becomes longer. And the amount of secondary combustion oxygen decreases. FIG.
Indicates the nozzle shape of the lance used in the present invention. The nozzle shape is a widening shape (a so-called Laval nozzle) considering the volume expansion of the gas, and the gas ejection hole diameter 9 is larger than the minimum hole diameter (d ₀ ) 8 of the nozzle. Nozzle diameter 22.5mmφ, 1 hole, oxygen flow rate 2000Nm ³ / H
Table 1 shows the comparison between the length of the jet core area (H _C ) and the length of the free jet area (h) of the Laval nozzle and the straight nozzle when the lance height is 2000 mm. It can be seen that the Laval nozzle has a shorter free jet region length and a smaller amount of secondary combustion oxygen.

[0021]

[Table 1]

When the blowing is performed under the above-described upper blowing conditions, the splash height generated by the upper blowing must be considered as an operational constraint. In general, the splash generation height under the upper blowing condition is represented by the molten steel dent depth L. That is, L shown in FIG. 1 (b) is an index representing the collision energy of the top-blown gas jet against the molten steel surface,
Since the splash height increases as L increases, it is an important index for performing upward blowing within a range that does not hinder operation due to the attachment of splash to the furnace opening. The inventor performed the operation under various blowing conditions. As a result, h / d ₀ satisfies the range of 40 to 80, and the range of L at which the adhesion of splash to the furnace port is not a problem is 70 to 500 mm.
Was found.

Here, L is an index representing the depth of depression of the molten steel surface due to the top blown gas, and is derived by the equations [10] and [11]. L = L _h * exp (−0.78H / L _h ) Expression [10] L _h = 63.0 * (Q _T / nd ₀ ) 2/3 Expression [11] L: Top blowing Depth depth of molten steel surface by gas [mm] H: Lance gap (distance between lance and molten steel surface) [mm] Q _T : Top blowing gas flow rate [Nm ³ / Hr] n: Number of top blowing lance nozzle holes d ₀ : Upper blowing lance nozzle hole diameter [mm] In addition, the decarburization reaction of chromium-containing molten steel is performed as shown in equation [1], where [C] and [Cr] concentrations in molten steel, molten steel temperature, and P _CO (blow under the molten steel surface) (Related to the oxygen content and the inert gas mixture ratio), it is necessary to appropriately set the upper blowing conditions in accordance with the above conditions also in the combined blowing. The inventor of the present invention has conducted various investigations on the molten steel temperature before the start of decarburization during combined blowing, and as a result, when the molten steel temperature before the start of decarburization is higher than the equilibrium molten steel temperature determined by the formula [1], furthermore, It has been found that the decarburization efficiency can be improved by combined blowing.

Table 2 shows that when the molten steel temperature before the start of decarburization is equal to or higher than the equilibrium molten steel temperature determined by the equation (1) (A) and below (B), the upper blowing lance nozzle is 22.5 mm.
φ, 1 hole, Laval nozzle, h / d ₀ = 75, L =
160, top blowing ratio = 30%, combined blowing area [% C]
The dC / dO ₂ and the reduction Si basic unit when ≧ 0.5% are shown.

When the temperature of the molten steel before the start of decarburization is equal to or higher than the equilibrium molten steel temperature, both dC / dO ₂ and the basic unit of reduction Si are improved.

[0026]

[Table 2]

[0027]

[Example] SUS304 stainless steel (18wt% Cr)
The decarburization refining of (−8 wt% Ni) 60T was carried out using the combined blowing furnace shown in FIG. 1 (b) with the blowing pattern shown in FIG. 8 (a). The carbon concentration in molten steel before the start of decarburization was 1.
7%, the molten steel temperature is 1525 ° C., the top-blown lance using a nozzle hole diameter 22.5Mmfai, the first hole nozzle, the total oxygen flow rate was 1.1Nm ³ / T / min constant. Combined blowing was performed up to [C] ≧ 0.5% in molten steel, and thereafter refining was performed in the same blowing pattern as bottom blowing.

Table 3 shows examples of the present invention and comparative examples. It can be seen that the decarburization efficiency (dC / dO ₂ ) and the reduction Si basic unit of the present invention example are improved as compared with the comparative example.

[0029]

[Table 3]

When the bottom blowing and the combined blowing of the present invention were performed under the same conditions as in Example A, the secondary combustion rate defined by the equation [12] and the rate of temperature rise of the molten steel were also compared. did. The molten steel heating rate was the same as that of the comparative example, and the secondary combustion rate was a very low value of 8% or less. Post combustion ratio (furnace _{gas) = 100CO 2% / (CO} 2
% + CO%) ... [12] In the prior art, as described in JP-A-55-158213, an oxygen-containing gas is blown from above the steel bath surface in combined blowing to generate from the bath surface. The CO gas is positively secondary-combusted and the temperature of molten steel is raised by the reaction heat of the formula [13] to improve the decarbonation efficiency.

CO + 1 / 2O ₂ = CO ₂ ΔH = −67.7 kcal / mol Formula [13] This is because, when oxygen is blown upward under the combined blowing conditions of the present invention, secondary combustion occurs, but In spite of the fact that the temperature rise of molten steel due to the heat of secondary combustion reaction is not observed,
The improvement in the basic unit of Si for reduction indicates a difference from the prior art. In addition, when the flash point temperature generated on the bath surface by the upper blowing oxygen under the combined blowing conditions of the present invention was measured by a two-color method, it was found that the temperature was as high as 2400 to 2500 ° C. This means that under the combined blowing conditions of the present invention, a high-temperature fire spot was formed on the bath surface, and the chromium oxide generated by the top-blown oxygen and the bottom-blown oxygen in the high-temperature portion and [C] in the molten steel. It is considered that the reaction of the formula [4] is promoted, and it can be said that the promoting effect is stronger when the temperature before decarburization of the molten steel belongs to the preferential decarburization region.

[0032]

[Table 4]

[0033]

According to the present invention, in the decarburization and refining of chromium-containing molten steel, the decarburization efficiency and the decarburization rate can be improved with the same amount of supplied oxygen gas. Therefore, the reduction of the amount of Si for reducing chromium oxide, the improvement in the basic unit of the oxygen gas and the dilution gas, and the shortening of the decarburization smelting time, the reduction of the smelting cost such as the extension of the life of the smelting furnace and the improvement in productivity are brought about. .

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a combined blowing furnace for carrying out the present invention.

FIG. 2 is a diagram showing a relationship between h / d ₀ and dC / dO ₂ .

FIG. 3 is a diagram illustrating a spread state of gas ejected from an upper blowing lance.

FIG. 4 is a diagram showing the relationship between h / d ₀ and the amount of secondary combustion oxygen.

FIG. 5 is a diagram showing a relationship between an upper blowing ratio and dC / dO ₂ .

FIG. 6 is a diagram showing the relationship between the end stage [C] of combined blowing and the decarburization efficiency.

FIG. 7 is a cross-sectional view showing a nozzle shape of an upper blowing lance.

FIG. 8 is a diagram showing examples of blowing patterns of bottom blowing and compound blowing.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Top blowing lance 2 Bottom blowing double tube tuyere 3 Molten steel 4 Slag 5 Hot spot 6 Jet core area 7 Free jet area 8 Minimum nozzle diameter of nozzle 9 Gas injection hole diameter

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Ryuji Nakao 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Inside Nippon Steel Corporation Hikari Works (56) References JP-A-2-221318 (JP, A) JP JP-A-55-158213 (JP, A) JP-A-59-182909 (JP, A) JP-A-60-131908 (JP, A) JP-B-61-57886 (JP, B2) JP-B-53-24008 (JP, A) , B2)

Claims (5)

(57) [Claims] 1. When oxygen or oxygen and an inert gas are blown under a bath surface of a chromium-containing molten steel to decarburize the molten steel, the concentration of [C] in the molten steel is set to 0.4% or more. A method for decarburizing and refining chromium-containing molten steel, wherein oxygen or oxygen and an inert gas are blown into the bath surface of the molten steel using a blowing lance under the following conditions (1) and (2). (1) The gas ejection speed from the nozzle of the upper blowing lance is equal to or higher than the sound speed. (2) The length h of the region where the speed of the gas ejected from the nozzle of the upper blowing lance is equal to or lower than the sound speed, and the minimum hole diameter of the upper blowing nozzle. the ratio h / d _O of d _O 40 to 80 2. The chromium-containing molten steel according to claim 1, wherein the molten steel temperature at the start of gas injection using an upper blowing lance is higher than an equilibrium molten steel temperature T determined by the equation [1]. Charcoal refining method. T = 13800 / {8.76-Log ([% Cr] * _PCO / [% C])} Formula [1] T: Equilibrium molten steel temperature (K) [% Cr]: Chromium concentration in molten steel ( _PCO : CO gas partial pressure (atm) [% C]: Carbon concentration in molten steel (% by weight) 3. The chromium-containing molten steel according to claim 1, wherein the ratio of the amount of oxygen blown onto the bath surface to the total amount of oxygen blown below and above the bath surface is 20 to 70%. Decarburization refining method. 4. The method for decarburizing and refining chromium-containing molten steel according to claim 1, wherein the depth L of the bath surface dent is 70 to 500 mm. 5. The method for decarburizing and refining chromium-containing molten steel according to claim 1, wherein the shape of the nozzle hole of the upper blowing lance is a shape that widens forward.