JPH10259409A - Pressurized converter steelmaking method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To refine molten steel having a low over-oxidation degree with the high productivity and the high yield by setting the pressure in a combination- blown converter at higher than the atmosphere and changing top-blowing oxygen supplying speed, bottom-blown gas flow rate and the pressure in the furnace according to the change in carbon concn. in the molten steel. SOLUTION: The pressure (P: kg/cm<2> ) in the combination-blown converter is set at higher than the atmosphere over the whole or a partial period in the refining. Further, the top-blown supplying speed (F: Nm<3> /ton/min), the bottom- blown gas flow rate (Q: Nm<3> /ton/min) and the pressure in the furnace, are changed according to the change in the carbon concn. (C: %) in the molten steel. The presure P in the furnace in the range of <=1% carbon concn. C is controlled between the equation PA=0.8+5×C and PB=0+2×C. PA and PB can become <=1 but P is not made to <=0.9 kg/m<2> . By this method, the low carbon superclean steel can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a converter steelmaking method capable of blowing molten steel with high productivity, high yield, and low degree of peroxide.

[0002]

2. Description of the Related Art It is an ultimate object to blow molten steel with high productivity and high yield and low degree of peroxide in converter refining. The decarburization behavior in converter refining is as follows: In the region where the carbon concentration in the molten iron is high, the decarburization rate is controlled by the oxygen supply rate. Phase II is controlled by the mass transfer rate.

[0003] In order to improve the yield of molten steel, in addition to reducing the generation of dust and splash in phase I, in addition to suppressing the iron oxide loss to slag due to the peroxidation of molten steel in the low carbon region of phase II. There is a need to. When the molten steel is overoxidized, the (T.Fe) of the slag increases and the oxygen concentration in the molten steel also increases, so that a large amount of deoxidizing material is required, and a large amount of deoxidized products is generated. In addition, there is a problem that the cleanliness of the glass is significantly reduced.

[0004] In order to suppress the peroxidation in the stage II, in principle, it is considered to decrease the oxygen supply rate and increase the stirring power. However, there is a problem that a decrease in the oxygen supply rate is incompatible with an improvement in productivity because the refining time is extended. In addition, an increase in the bottom-blowing agitation force causes an increase in the agitation gas cost, and the agitation gas cost can be suppressed by suppressing the agitation force in phase I low and increasing only the II phase, but the same tuyere Since there is no technique for greatly changing the flow rate of the bottom blown gas, there is a problem that the erosion speed of the bottom blown tuyere brick increases.

[0005] On the other hand, there is known a technique for pressurizing the inside of a converter furnace for the purpose of increasing the oxygen supply rate and suppressing generation of dust. However, none of the techniques provide sufficient operating conditions as described below.

In Japanese Patent Publication No. 43-9982, an iron charge and a slag forming component are put into a top-blowing converter, and oxygen introduced from a lance located in the converter is directed downward. Flowing on the surface of the iron charge, causing a smelting reaction to remove carbon from the iron and produce a reactor gas,
Flowing the reactor gas from the converter into a gas collection device,
Providing pressure adjusting means for controlling the flow rate of the gas,
By maintaining a close relationship between the iron charge and the pressure regulating means to allow substantially all of the gas to pass through the pressure regulating means, wherein the pressure regulating means A method for refining iron is disclosed wherein a pressure of at least one atmosphere is provided in the furnace when refining with incoming oxygen.

This publication is characterized in that the carbon dioxide generation ratio (secondary combustion rate) is increased and the mass flow rate of the exhaust gas is reduced, so that the amount of dust is reduced. However, even in this case, there is no quantitative specification concerning the relationship between the oxygen supply speed and the bath surface collision energy of the top-blown oxygen jet and the pressure, which greatly affects the secondary combustion rate and the amount of dust generated. Since the basic conditions are greatly different from those of the bottom-blowing converter refining, it is impossible to operate the pressurized converter only by the invention.

Japanese Unexamined Patent Publication (Kokai) No. 2-205616 discloses a converter steelmaking method in which molten iron and, if necessary, scrap are refined to molten steel, the inside of the converter is pressurized to 0.5 kgf / cm ³ or more. The relationship between the total charge amount of hot metal and scrap W (t / ch) and the inner volume of the converter steel shell V (m ³ ) is expressed as: W> 0.8V or 0.8V ≧ W ≧ 0.5V And the acid transfer rate U (Nm ³ / min · t) into the furnace is U ≧ 3.
No. 7 discloses a high efficiency converter steelmaking method.

This publication describes that a high yield was obtained by suppressing the occurrence of slopping and spitting by applying pressure. However, there is no description of a method of operating under low carbon concentration conditions in stage II, which is the most important in terms of suppressing peroxidation and improving yield. JP-A-62-14
No. 2712 discloses that in a converter or a smelting reduction furnace, the pressure inside the furnace is set to a pressure higher than the atmospheric pressure, particularly, 2 to 5 kg / cm ² ,
A steelmaking and ironmaking method in a converter or a smelting reduction furnace, characterized in that the linear velocity of the secondary combustion gas is reduced, is disclosed.

In this publication, the rising velocity of the secondary combustion gas in the slag is reduced by pressurization, the heat exchange time between the gas and the slag is increased, and the heat transfer efficiency through the slag is improved. According to the invention, the furnace pressure is increased to 2 to 5 kg / cm ² , but according to the principle of the invention, the amount of slag that affects the heat exchange time between the gas and the slag that governs the heat transfer efficiency, There are no provisions concerning the amount of secondary combustion gas generated, the oxygen supply rate, the lance height, the cavity depth, and the like, and it is impossible to operate the pressurized converter with the invention alone. In particular, the embodiment of the present invention is a top-blown converter, in the case of an upper-bottom-blown converter in which slag forming is difficult due to strong stirring power, or in the case of blowing of hot metal pre-treated hot metal with a small amount of slag, The basic conditions are largely different from those of the present invention, and it is difficult to obtain the pressurizing operation conditions in the top and bottom blown converter from the present invention.

Japanese Unexamined Patent Publication (Kokai) No. 2-298209 discloses that an iron-containing cold material, a carbonaceous material, and oxygen are supplied to a melting converter in which seed water is present, and a required amount of seed metal in the melting converter and another refining process are performed. In the converter steelmaking method, the required amount of refining in the converter is used to obtain high-carbon molten iron, and the high-carbon molten iron is used as a raw material to perform oxygen blowing in a dedicated refining converter to obtain molten steel of the required component. A pressurized iron-containing cold material melting converter steelmaking method characterized by significantly reducing the amount of dust generated in a melting-only converter by controlling the pressure in the furnace according to the following equation is disclosed. Formula P ≧ 1.15 + 0.3 {[% C] −2.5} 2.5 ≦ [% C] ≦ 5 Symbol P: Pressure inside the converter exclusively for melting (atm) [% C]: Inside the converter exclusively for melting Liquid iron C content (% by weight).

This publication makes use of the fact that the energy when the top-blown oxygen jet due to pressurization collides with the bath surface and the volume of generated CO gas are reduced. The pressure is set high because CO is easily generated. However, the application of the above equation shows that C is 2.5
Since it is about 5%, it cannot be applied to converter refining for the purpose of decarburization. Further, the generation rate of dust largely depends not only on the pressure but also on the oxygen supply rate, and the oxygen supply rate is an important factor that governs the productivity of the converter for melting iron-containing cold material. Since there is no quantitative regulation on the relationship between the oxygen supply rate and the collision pressure between the bath surface of the top-blown oxygen jet and the pressure, and furthermore, the basic conditions are significantly different from converter refining for the purpose of decarburization. It is impossible to operate a pressurized converter only with the invention.

Further, none of the known examples discloses a method of operating in the low carbon region of stage II, which is the most important in terms of suppressing peroxidation and improving the yield.
Especially in the case of phase II, unless the conditions such as the top blowing oxygen supply rate, the stirring force by bottom blowing, and the furnace internal pressure are properly controlled,
It is impossible to suppress the peroxidation and improve the yield while improving the productivity.

Conventionally, ε defined by the equation is used as the stirring energy by bottom blowing (iron and steel, Vol. 67, 1981, p. 672 et seq.), And the uniform mixing time τ determined by the equation is used. The relationship between the BOC value and the decarburization characteristics of the converter is known (iron and steel, Vol. 68, 19
1982, p. 1946). ε = (371/60) · Q · T · {log (1+ (9.8 · ρ · H / P) · (10 ~ ⁴ )} ···· τ = 540 · (H / 0.125) ^2/3 · ρ ^{1 / 3} · ε BOC = {F / (1 / τ)} × [% C] where Q is the bottom blown gas flow rate (Nm ³ / ton / min), T is the molten steel temperature (K), ρ Is the molten steel density (g / cm ³ ), H is the bath depth (cm), P is the furnace pressure (kg / cm ² ), F is the top blowing oxygen supply rate (F: Nm ³ / t)
on / min) and [% C] indicate the carbon concentration.

In this relation, for example, when the bath depth is 1 to 2
m, the pressure inside the furnace is 1 kg / cm ² to 3 kg / cm ²
, The effect on ε and BOC was not significant, and it was estimated that there was no significant effect on metallurgical properties.

On the other hand, an equation was used to calculate the cavity depth due to the top blown gas (Segawa Kiyoshi: "Iron Metallurgy Reaction Engineering", 1977, published by Nikkan Kogyo Shimbun). The effect of furnace pressure is not included. L ′ = Lh · exp (−0.78h / Lh) Lh = 63.0 (F ′ / nd) ^2/3 where L ′ is the cavity depth (mm) calculated by equation (4). , H is the distance between the lance and the steel bath surface (mm), F 'is the top blowing oxygen supply rate (Nm ³ / Hr), n is the number of nozzles, and d is the nozzle diameter (mm).

As the effect of the furnace pressure on the cavity depth, a behavior under reduced pressure has been reported (iron and steel, Vol. 63, 1977, p. 909 and thereafter). According to this, it is shown that the cavity is rapidly deepened by reducing the pressure, but this is a result at a pressure lower than the atmospheric pressure, and the behavior in a pressurized state is not mentioned at all. Dare to extrapolate the result under reduced pressure to pressurized,
The cavity depth will be very small.

[0018]

An object of the present invention is to reduce the yield of molten steel due to an increase in the amount of splash and dust generated and the occurrence of slopping when the oxygen supply rate is increased by the converter refining at normal atmospheric pressure. And the problem of increased non-blowing time, and Japanese Patent Application Laid-Open Nos. 2-205616 and 2-298
No. 209, Japanese Patent Application Laid-Open No. Sho 62-142712, and Japanese Patent Publication No. Sho 43-9982, which disclose pressurizing operation conditions in top and bottom blown converters having different basic conditions in pressurized converter technology. In addition, there is no disclosure of the operation method in the low-carbon region of stage II, which is the most important in terms of suppressing peroxidation and improving yield, and it is impossible to operate the pressurized converter. It is an object of the present invention to provide a converter refining method capable of solving molten steel with high productivity and high yield and capable of blowing molten steel having a low degree of peroxide.

[0019]

When the inside of a top-bottom blown converter is pressurized to perform decarburization operation, the inventors of the present invention have to supply oxygen to the top-blown converter according to changes in furnace pressure and carbon concentration. It has been found that it is necessary to regulate and control the feed rate and the flow rate of the bottom blown gas. The gist of the present invention resides in the following methods.

(1) In a top-bottom blow converter, the pressure inside the furnace (P: kg / cm ² ) is set to be higher than the atmospheric pressure over all or part of the period during blowing, and Oxygen supply rate (F: Nm ³ / ton / min) and bottom blowing gas flow rate (Q: Nm ³ / t)
on / min), and a pressure converter steelmaking method characterized in that the furnace pressure is changed according to the transition of the carbon concentration (C:%) in the steel bath.

(2) In (1), the carbon concentration in the steel bath;
A pressurized converter steelmaking method characterized by controlling the furnace pressure in a region where C is 1% or less; P is in a range of PA defined by the formula and PB defined by the formula. PA = 0.8 + 5 × C PB = 0 + 2 × C Here, PA and PB can be 1 or less in the mathematical formula.
Is not less than 0.9 kg / cm ² .

(3) In (2), the top-blowing oxygen supply rate in the region where the C is higher than 1% (F1: Nm ³ / ton / min) A pressure converter steelmaking method, wherein F is controlled so that β in the formula is in the range of −0.25 to 0.5. β = (F / F1) −C Here, F can be larger than F1 in the mathematical expression, but F is set to F1 or less.

(4) In (2), the bottom blowing gas flow rate in a region of 1% or less with respect to the bottom blowing gas flow amount in a region where C is higher than 1% (Q1: Nm ³ / ton / min); To the equation γ
Is controlled so as to fall within the range of -2 to 1. γ = (Q / Q1) −5 × (1-C)...

(5) In (1), the carbon concentration in the steel bath;
In the region where C is 1 to 0.1%, the furnace pressure; P, the top blowing oxygen supply rate; F, the bottom blowing gas flow rate;
Pressurized converter steelmaking method, characterized in that it is controlled to fall within the range of: δ = {(F × P) / Q} ^1/2 / C...

(6) In (1) to (5), the depth (L) of the cavity formed on the surface of the steel bath by the top-blown oxygen.
A pressure converter steelmaking method characterized in that the ratio (L / D) of the pressure and the bath diameter (D) is controlled to 0.15 to 0.35. Here, the furnace pressure is an absolute pressure (atmospheric pressure = 1 kg / cm ² ).

The carbon concentration during blowing can be estimated by decarbonation efficiency obtained empirically based on the total oxygen consumption of top blowing and bottom blowing, indirect estimation from intermediate sampling and exhaust gas analysis, or , A value obtained by continuous or semi-continuous direct analysis values by online analysis or on-site analysis.

The cavity depth L is calculated by the following equation. LG = Hc / (0.016 · L 0.5) -L ...... (10) Hc = f (P O / P OP) · M OP · (4.2 + 1.1M OP 2) · d f (X) = - 2.709X 4 + 17.71X ³ -40.99X ² + 40.29X-1
2.90 However (0.7 <X <2.1) f (X) = 0.109X 3 -1.432X 2 + 6.632X-6.35 where
(2.1 <X <2.5) L: Depth of molten iron cavity (mm) LG: Distance between lance tip and molten iron stationary molten metal surface (mm) P _O : Nozzle absolute secondary pressure (kgf / cm ² ) P _OP : Nozzle proper expansion absolute secondary pressure (kgf / cm ² ) M _OP : Mach number at proper expansion (-) d: Nozzle throat diameter (mm).

Here, the absolute secondary pressure P _O of the lance nozzle is the absolute pressure of the stagnation portion of the lance nozzle before throat.
Also, the proper expansion absolute secondary pressure P _OP of the lance nozzle is as follows.
It is calculated by equation (11).

S _e / S _t = 0.259 (P / P _OP ) ^−5/7 {1− (P / P _OP ) ^2/7 } ^−1/2 (11) S _e : at the lance nozzle outlet Area (mm ² ) _St : Area of lance nozzle throat (mm ² ) P: Absolute pressure of lance nozzle outlet atmosphere (kgf / cm ² ) P _OP : Absolute secondary pressure of lance nozzle proper expansion (kgf / cm ² ).

Here, the discharge Mach number M _OP at the time of proper expansion in the equation (10) is calculated by the following equation (12). M _OP = [5 · {(P _OP / P) ^2/7 -1}] ^1/2 …… (12) M _OP : Mach number at proper expansion (−) P: Absolute pressure of lance nozzle outlet atmosphere (kgf / cm ² ) P _OP : Lance nozzle proper expansion absolute secondary pressure (kgf / cm ² ).

The oxygen gas flow rate is calculated by the following equation (13). _{F O2 = 0.581 · S t ·} ε · P O ...... (13) S t: area of the lance nozzle throat section (mm ²⁾ P _O: lance nozzle absolute secondary pressure _{^{(kgf / cm 2) F O2}} : oxygen gas Flow rate (Nm ³ / h) ε: Flow coefficient (-) (usually in the range of 0.9 to 1.0).

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The pressurization conditions in the top and bottom blown converters are basically different between stage I and stage II. In stage II, the purpose is to suppress peroxidation while maintaining high productivity, and it is important to control the pressure, oxygen supply rate, and stirring power according to changes in carbon concentration. The decarburization rate (K;% C / min) in this region is represented by the following equation. K = dC / dt = (Ak / V) · (C-C _O ) where C is the carbon concentration, t is time, A is the reaction area, k
Is the mass transfer coefficient of carbon, V is the volume of molten iron, and _CO is the equilibrium carbon concentration. In order to increase K, it is necessary to increase A, k and decrease _CO. If oxygen is blown upward at a rate corresponding to the decarburization rate specified by K, in principle, molten iron is oxidized and molten steel is completely eliminated. The decarburization can proceed without causing oxygen absorption.

Operationally, in order to increase the moving speed of carbon, a bottom-blowing agitation force in accordance with the carbon concentration is provided, an oxygen supply speed corresponding to the agitation force is secured, and the decarburization reaction is performed efficiently. It is necessary to secure an upper blowing fire point (a high-temperature region formed when upper blowing oxygen collides with the bath surface) to allow the water to advance progressively. Here, bottom-blowing agitation increases the moving speed of carbon through the formation of a macroscopic circulating flow in the bath, and the area of the reaction interface due to the formation of slag and metal emulsion by the floating of the bottom-blown bubbles to the top-blown fire region. The upper blowing point causes a decrease in the equilibrium carbon concentration due to the formation of a high-temperature state, and an increase in the reaction interface area due to the formation of an emulsion of slag and metal by the upper blowing jet.

When pressure is applied, a decrease in the volume increase near the surface of the bottom-blown gas and an increase in the attenuation of the jet energy of the top-blown oxygen decrease the bottom-blown stirring power and the emulsion formation state. Therefore, these are quantitatively grasped as effects on the reaction rate, and the bottom stirring power, the jet energy of the top blown oxygen, the oxygen supply rate, and the furnace pressure are properly controlled in relation to the carbon concentration. There is a need to. In other words, in order to suppress the peroxidation of molten steel and obtain a high yield and high cleanliness steel while maintaining high productivity, the top-blown oxygen supply rate,
It is essential to change the bottom blown gas flow rate and the furnace pressure in accordance with the change of the carbon concentration in the steel bath.

FIG. 1 schematically shows an embodiment of the present invention.
Numerical values and other reasons for limitation in the constituent elements of the present invention are as follows. The reason why the present invention is limited to the operation in the top and bottom blown converter in claim 1 is that the bottom blown stirring power cannot be freely controlled in the top blown converter and the oxygen supply speed and the bottom blown stirring in the bottom blown converter. This is because the forces are generally proportional and cannot be controlled independently.

As the definition of the pressurized converter, the pressure in the furnace was set to be higher than the atmospheric pressure over the entire or partial period during blowing. The furnace pressure is preferably 1.2 kg / cm ² or more in order to obtain the effect of improving productivity by pressurization. If the pressure is too high, 5 kg / c because slag easily infiltrates refractory pores and shortens refractory life
m ² or less is desirable. In the case of stage II, the pressure is restored from the pressurized state according to the decrease in the carbon concentration, and at the time of blowing, or
The carbon concentration region near the blow stopped, the operation of at 0.9 kg / cm ² or more light under vacuum for sucking the atmospheric pressure or the exhaust gas, pressure, including that allowed to migrate to lower the pressure in continuous or stepwise It is defined as a converter.

Claims 2 to 5 define the operating conditions of the period II at the same time as claim 1. The carbon concentration range defining the operating conditions of the period II was C in a region lower than 1%.
As described above, the carbon concentration that transitions from stage I to stage II is 0.2
Although it changes within the range of ~ 0.5%, it is not enough to perform the blowing with suppressed peroxidation in stage II only by adjusting the blowing conditions after stage II. It is necessary to select appropriate blowing conditions. The inventors have found that the critical carbon concentration is 1% based on detailed experiments.

Claim 2 specifies the control of the furnace pressure P in accordance with the transition of the carbon concentration C. As shown in FIG. 2, P is defined by PA and PB is defined by the equation. Is controlled so as to fall within the range. PA = 0.8 + 5 × C PB = 2 × C Here, C is wt%, and PA and PB are (kg / .cm ² ). Matching does not matter.

The higher the pressure, the more suitable for a high oxygen supply rate for increasing productivity, but the lower the agitating power of the bottom blow and the energy of the jet of the top blow oxygen, the reaction interface area and the mass transfer coefficient of carbon are reduced. Decrease. An equation was obtained as a result of examining a quantitative optimal pressure change pattern from the relationship between the two.

In other words, the decarburization reaction by top-blown oxygen is a reaction between FeO generated at the flash point and carbon in the steel bath, and the FeO generated at the flash point is always independent of the carbon concentration and pressure. Since it is pure FeO, the reaction rate is determined only by the carbon concentration. Therefore, when carbon is high, the reaction rate is high, so that the nucleation rate of CO bubbles cannot follow, large CO bubbles are generated, and the splash of the bubbles due to the burst of the bubbles is large. Therefore, in order to suppress the splash, it is necessary to set the pressure to a high pressure when the carbon concentration is high. Conversely, if the pressure is increased in a state where the carbon concentration is lowered, the decarburization rate is reduced because the amount of _CO, which is the equilibrium carbon concentration, increases although the splash is small.

In other words, when the pressure is larger than PA, it means that the timing of the recompression is too late, and the equilibrium carbon concentration, _CO , increases, so that the decarburization rate decreases, and excess oxygen oxidizes the molten iron or the molten steel. It dissolves in the slag and causes an increase in (T.Fe) of the slag and oxygen concentration in the molten steel. If the pressure is smaller than PB, it means that the pressure recovery timing is too early,
Alternatively, since the pressure is restored in a state close to the stage I, a severe splash occurs. Further, when the pressure is restored in a state where the carbon concentration is high, since the carbon concentration in the molten steel is high, (TF
e), the CO gas is generated violently even with a small amount of (T.Fe), and there is also a problem that slopping becomes extremely easy.

A third aspect of the present invention specifies the control of the top blowing oxygen supply rate F according to the carbon concentration C in addition to the control of the furnace pressure P according to the transition of the carbon concentration C specified in the second aspect. In the region where C is higher than 1%, the upper blowing oxygen supply speed F1 in the region of 1% or less is changed from the upper blowing oxygen supply speed F2 in the region of 1% or less to the range where β in the formula is −0.25 to 0.5. Control so that β = (F / F1) −C...

That is, the higher the oxygen supply rate, the more suitable for increasing the productivity. However, the oxygen supply rate is determined by the reaction interface area A determined by the bottom-blowing agitating force, the jet energy of the top-blown oxygen, and the mass transfer coefficient k of carbon. If it is supplied in excess of the decarburization rate to be performed, the degree of peroxidation increases and the (T.Fe) of slag and the oxygen concentration of molten steel increase. It is clear from detailed experiments by the inventor that β needs to be controlled in the range of −0.25 to 0.5 as shown in FIG. 3 on the premise of the pressure control described in claim 2. It became. If β is less than -0.25, the peroxidation is suppressed because the decrease in the oxygen supply rate is too large, but the blowing acid time is greatly increased, which lowers the productivity and is larger than 0.5. In this case, the decrease in the oxygen supply rate is too small, so that peroxidation occurs and the (T · Fe) of the slag and the oxygen concentration of the molten steel increase.

Claim 4 specifies the control of the bottom blown gas flow rate Q according to the change in the carbon concentration C in addition to the control of the furnace pressure P according to the change in the carbon concentration C specified in claim 2. And controlling the bottom blowing gas flow rate Q1 in a region where C is higher than 1% so that Q2 in a region of 1% or less is such that γ in the expression is in the range of −2 to 1. .gamma. = (Q2 / Q1) -5.times. (1-C).

That is, the higher the bottom blowing agitation force, the higher the productivity because the decarburization rate specified by the mass transfer coefficient k of carbon is high. This causes a problem of shortened product life. Detailed experiments by the inventor have revealed that assuming the pressure control described in claim 2 as a premise, it is necessary to control γ in the range of −2 to 1, as shown in FIG.

When γ is smaller than -2, the increase in the bottom-blowing stirring power according to the decrease in the carbon concentration is too small.
Since the oxygen supply rate becomes excessive and peroxidation occurs, the (T.Fe) of the slag and the oxygen concentration of the molten steel increase. When γ is larger than 1, the stirring power in the low carbon concentration region becomes too strong, which causes problems such as an increase in bottom blown gas cost and a reduction in refractory life, and intense rocking of the steel bath, There is a problem that slag and molten iron are scattered outside the converter due to the swing.

According to a detailed study by the present inventors, a change in bottom blow stirring conditions caused by a change in furnace pressure has a greater effect on decarburization blowing in stage II than previously thought. It became clear that giving. In other words, it can be understood that in bottom-blown agitation, the deterioration of the decarburization characteristics caused by increasing the furnace pressure is much larger than the effect estimated from the indices of ε, τ, and BOC simply expressed by the following equation. did. This is because these indices calculate the work of bubble expansion due to the head difference between the bath surface and the furnace bottom where the gas is injected, whereas in reality the decarburization reaction takes place on the molten steel surface. This is because the stirring state mainly governs the decarburization characteristics.

The bubbles blown into the bath gradually expand as they ascend, but since the diameter of each bubble increases with the expansion, the bubble rising region is required to expand without being combined with the adjacent bubbles. It needs to spread sideways (Figure 5). When coalescing with an adjacent bubble, the bubble diameter is further increased, so that the levitation speed is accelerated, and the bubble rising area is not expanded, but the bubble diameter is further increased, and explosively reaches the surface. On the other hand, if the bubble rising area can be expanded,
Since the bubble diameter is maintained at a stable bubble diameter in proportion to the static pressure without merging with the adjacent bubbles, the rising speed is slow and the bubbles slowly float.

Whether the bubbles coalesce or the bubble rising region expands horizontally is determined by the relationship between the buoyancy energy and the surface tension energy. The present inventors obtained a change curve of the bubble diameter as shown in FIG. 6 by a basic experiment. In other words, one of the critical conditions or bubble rising region bubbles coalesce spreads horizontally, greatly affected by the surface near the static pressure, increasing the pressure inside the furnace than 1 kg / cm ^2, an explosion near the surface It became clear that the typical increase in bubble diameter disappeared. Thus, the explosive increase in the bubble diameter near the surface greatly contributes to the stirring of the molten steel surface, and the reaction interface area due to the formation of slag and metal emulsion due to the floating of the bottom-blown bubbles to the above-mentioned top-blown fire region. Has a significant effect on the increase in This explosive increase in the bubble diameter near the surface is difficult to predict from the calculation of ε, τ, and BOC, and is only possible by controlling γ shown in the present invention.

Claim 5 relates to the relationship among the three factors of the furnace pressure P3, the top-blown oxygen supply rate F3, and the bottom-blown gas flow rate Q3 in accordance with the transition of the carbon concentration C. This is a regulation that controls so that δ in the equation is in the range of 5 to 25. δ = {(F3 × P3) / Q3} ^1/2 / C...

As already described in detail, in the phase II operation in the pressurized converter, the carbon concentration C, the furnace pressure P,
Only by appropriately controlling the four factors of the top-blown oxygen supply rate F and the bottom-blown gas flow rate Q, high productivity, high yield, and high cleanliness by suppressing peroxidation can be achieved. Detailed experiments by the inventor have revealed that it is necessary to control δ in the range of 5 to 25 as shown in FIG. The phase II decarburization reaction is rate-controlled by carbon mass transfer as described above. This is because the reaction proceeds in the elementary process in which FeO generated by oxidation by top-blown oxygen is reduced by carbon in molten steel. Since the reduction is slower than the oxidation, the reaction is limited by the mass transfer rate of carbon that defines the reduction rate.

The equation taking this elementary process into account is
The molecule (F × P) ^1/2 is an oxidation index considering pressure,
The denominator (Q ^1/2 × C) represents a reduction index in consideration of the carbon concentration. The fact that pressure is applied to the oxidation index has been clarified for the first time by the present inventors, and has the following meaning. In other words, when the pressure increases, the oxygen potential increases in proportion to the pressure because the partial pressure of the oxygen gas at the reaction interface increases even at the same oxygen supply rate.
This indicates that even if the inside of the furnace was pressurized with a gas other than oxygen, the partial pressure itself of the oxygen gas that reached the reaction interface was also high, a phenomenon that was not even considered until now. The operation of the pressurized converter becomes possible only with the introduction of this index.

When δ is less than 5, the peroxidation is suppressed because the reduction rate is too high than the oxidation rate, but the productivity is reduced because the blowing acid time is greatly increased. In this case, since the oxidation rate is too high than the reduction rate, peroxidation occurs and the (T.Fe) of the slag and the oxygen concentration of the molten steel increase.

The ratio (L / L) of the cavity depth L to the bath diameter D formed on the surface of the steel bath by the top-blown oxygen in claim 6.
Controlling D) to 0.15 to 0.35 also defines conditions for suppressing the peroxidation while improving the productivity in stage II. The depth of the cavity is one of the indicators of the energy of the jet of top-blown oxygen, but the top-blown oxygen jet creates a high-temperature fire point and gives strong downward energy to the steel bath surface. Have two effects.

That is, when (L / D) is less than 0.15, the energy of the top-blown oxygen jet is too small, so that the fire temperature decreases and the emulsion region also decreases, so that peroxidation occurs. On the other hand, when (L / D) is larger than 0.35, the energy of the top-blown oxygen jet is too strong, so that the occurrence of splash becomes severe and a problem in operation occurs. Further, since FeO generated at the flash point is suspended to a deep position in the steel bath, it receives a large static pressure, so that the reduction reaction does not easily proceed and the decarburization reaction rate is rather reduced.

A decrease in the decarbonation efficiency due to an increase in the furnace pressure due to the upward blowing cannot be predicted from the relationship with L ′ calculated by the conventional equation, and the cavity shown in the equations (10) to (13) cannot be predicted. After accurately evaluating the influence of the pressure in the pressurized state by the formula for calculating the tee depth L, it becomes possible only by controlling the L / D. Figure 8 shows the measured values of cavity depth under pressure and
It shows the relationship between L calculated by the formulas (10) to (13) and L ′ calculated by the formula, and L has a good correspondence with the actually measured value.

The behavior of the jet under pressure is such that the supersonic core becomes shorter because the gas density around the jet is large, and the spread of the jet becomes extremely large because the resistance around the jet is large. Therefore, the shape of the cavity formed by the top-blowing jet under pressure changes so large that it cannot be expected from changes due to vertical movement of the lance under atmospheric pressure, and an accurate value is obtained as shown in the present invention. Only after controlling, efficient refining becomes possible.

[0058]

EXAMPLES The test was carried out in a 5 ton scale test converter. The upper blowing lance has changed the throat diameter to 5-20mm 3 ~
A 6-hole Laval lance was used, and the bottom was blown using two double-tube tuyeres with the inner tube being oxygen and the outer tube being LPG, installed at the bottom of the furnace. The exhaust gas was guided to a dust collection system in a non-combusted state through a water-cooled hood fastened to a converter furnace opening, and the furnace pressure was adjusted by a pressure regulating valve provided in the middle. In the early stage of blowing, nitrogen gas was introduced and forced pressure was applied.
Self-pressurized with O ₂ . The temperature was measured by a sublance, and the carbon concentration was estimated from the intermediate sampling by the sublance, the amount of exhaust gas, and the composition of the exhaust gas. The situation of slopping and spitting was judged based on the image of the furnace monitoring camera.

The hot metal was smelted in a blast furnace and subjected to hot metal pretreatment. About 4.3% of C, about 0.12% of Si, about 0.25% of Mn, about 0.02% of P, Was about 0.015%, and the temperature before charging the converter was about 1300 ° C. The blow-off carbon concentration was about 0.05%, and the temperature was about 1650 ° C.

[0060]

[Table 1]

Table 1 shows the conditions and results of the examples and comparative examples. In the embodiment, the pressure, the carbon concentration, the oxygen supply rate, and the flow rate of the bottom blown gas are controlled according to the relationship shown in B, c, and c in FIGS. 0.
It is in an appropriate range of 20 to 0.30. As a result, converter blowing with a low yield of (T.Fe) or a high concentration of dissolved oxygen in the blow stopper was performed in a short time of only 6.1 minutes, and no dropping occurred.

In the comparative example, the pressure, the carbon concentration, and the oxygen supply rate were controlled according to the relations A and a in FIGS. .3
Although it was in an appropriate range of 0, δ was 18 to 45. As a result, although high-speed blowing was performed, (T.Fe) and the dissolved oxygen concentration of the blowing stopper were high, the yield was low, and slopping occurred.

In the comparative example, the pressure, the carbon concentration, and the oxygen supply rate were controlled according to the relationship indicated by C and d in FIGS. .3
Although it was in an appropriate range of 0, δ was 2 to 10. As a result, the (T.Fe) and dissolved oxygen concentration of the blow stopper were low and the yield was high, but the oxygen supply time was long and the effect of increasing the productivity by pressurization could not be obtained.

[0064]

According to the present invention, it is possible to blow molten steel having a low degree of peroxidation with high productivity and a high yield and to produce a low-carbon high-purity steel by using a pressure converter. It became.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an embodiment of the present invention. The flue gas for introducing exhaust gas 8 is connected to a pressure regulator via a dust collector and a gas cooling device.

FIG. 2 shows carbon concentration C, furnace pressure P and (T ·
FIG. 7 is a view of an experimental result showing a relationship with Fe).

FIG. 3 is a diagram of an experimental result showing a relationship between a parameter β defined by an oxygen supply rate F and a carbon concentration C and (T · Fe) at the time of blowing off.

FIG. 4 is a diagram of an experimental result showing a relationship between a parameter γ defined by a bottom blown gas flow rate Q and a carbon concentration C, and (T · Fe) at the time of blowing stop.

FIG. 5 is a schematic diagram showing the behavior of bubbles blown into a bath.

FIG. 6 is a diagram of an experimental result (water model) showing the effect of the furnace pressure on the relationship between the depth of the bubbles blown into the bath from the bath surface and the bubble diameter.

FIG. 7 is a view of an experimental result showing a relationship between a parameter δ defined by a furnace pressure P, an oxygen supply rate F, a bottom blown gas flow rate Q, and a carbon concentration C, and (T · Fe) at the time of blowing off.

FIG. 8 is a diagram of an experimental result (water model) showing a comparison between a measured value and a calculated value of a cavity depth under pressure.

[Explanation of symbols]

1: converter steel shell, 2: refractory lining, 3: bottom blow tuyere, 4: molten iron, 5: oxygen, 6: top blow lance,
7: fastening device, 8: flue for introducing exhaust gas, 11: molten iron, 12: bubble rising area, 13: bubble.

Claims (6)

[Claims] In a top-bottom blow converter, the pressure inside the furnace (P: kg
/ cm ² ) is set to be higher than the atmospheric pressure over all or part of the blowing period, while the top blowing oxygen supply rate (F: Nm ³ / ton / min) and the bottom blowing gas flow rate (Q ： Nm ³ / ton
/ min), and the furnace pressure P to the carbon concentration in the steel bath (C: wt
%) According to the pressure converter steelmaking method. 2. The method according to claim 1, wherein the carbon concentration in the steel bath; the pressure in the furnace in a region where C is 1% or less;
A pressurized converter steelmaking method characterized by controlling so as to fall within a range between A and PB defined by the equation. PA = 0.8 + 5 × C PB = 2 × C ...... 3. The method according to claim 2, wherein the upper blowing oxygen supply rate (F1: Nm ³ / ton / min) in a region where C is higher than 1%,
Pressurized converter, wherein β in a formula expressed by a ratio with respect to an upper blowing oxygen supply rate and F2 in a range of 1% or less is in a range of −0.25 to 0.5. Steelmaking method. β = (F2 / F1) −C... 4. The method according to claim 2, wherein the bottom blown gas flow rate (Q1: Nm ³ / ton / min) in a region where C is higher than 1% and C is 1
%. A pressurized converter steelmaking method characterized by controlling γ in a formula expressed by a ratio to the bottom blown gas flow rate Q2 in a region of not more than% in a range of −2 to 1. γ = (Q2 / Q1) -5 × (1-C)... 5. The furnace pressure in the range where C is 1 to 0.1%; P3, top blowing oxygen supply rate; F3, bottom blowing gas flow rate; Pressure converter steelmaking method characterized in that the pressure is controlled to fall within the range of: δ = {(F3 × P3) / Q3} ^1/2 / C ... 6. A method according to claim 1, wherein the ratio (L / D) of the cavity depth (L: m) to the bath diameter (D: m) formed on the surface of the steel bath by the oxygen blown upward is 0.1. A pressure converter steelmaking method characterized by controlling the pressure to 15 to 0.35.