JPH08311519A - Steelmaking method using converter - Google Patents

Abstract

PURPOSE: To produce a high quality molten steel from molten iron and steel scrap by charging the molten iron, steel scrap and slag-making agent into a top/bottom blowing converter to execute phosphorization, decarburizing with oxygen blowing after discharging a large part of dephosphorized slag and desulfurizing under reducing condition in the furnace. CONSTITUTION: The molten iron and the steel scrap are charged into the molten slag of preceding charge remaining in the top/bottom blowing converter and N2 gas in blown from a tuyere in the furnace bottom so that stirring energy becomes 1.0kw/t to stir the molten iron, and the steel scrap is melted. Successively, basic flux of CaO, etc., and iron scrap as coolant are charged and the basicity of the molten slag is raised to 1.5-2.3 and molten iron temp. is made to the low temp. of 1300-1480 deg.C to remove P in the molten iron to 0.1-0.02%. After discharging >=75% of the dephosphorized slag by tilting the converter, the converter is straightly stood and the molten iron is decarburizingly refined by oxygen-blowing with the top-blowing lance and further, Al ash and CaO are added to dephosphorize the molten slag, and the molten iron is desulfurize-refined under the reducing slag. The molten steel dephosphorizingly, decarburizingly and desulfurizingly refined is tapped from the converter and raw material in the following heat is charged with the molten slag left behind to repeat the steelmaking refining.

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 performing dephosphorization / desulfurization / decarburization refining in a converter using hot metal.

[0002]

2. Description of the Related Art In recent years, the quality requirements for steel materials have become more severe as the utilization technology has become more sophisticated and diversified, and the need for producing high-purity steel is increasing more and more. In response to such demands for producing high-purity steel, hot metal pretreatment or secondary refining equipment has been expanded in the steelmaking process. Particularly, since phosphorus is efficiently dephosphorized in the hot metal stage having a low temperature level, it has become common to perform prior dephosphorization in the hot metal pretreatment step. As a method of performing prior dephosphorization in the hot metal pretreatment step, (1) a method of injecting a flux for dephosphorization into the hot metal in a torpedo car to perform predephosphorization (for example, JP-A-58-16007 and JP-A-62-62). -109908), (2) a method of performing pre-phosphorization by injecting or spraying a flux for dephosphorization on the hot metal in the ladle, or (3) using two converters while dephosphorizing And decarburization on the other hand (for example, JP-A-63-195209).
No. publication) is known.

However, according to the above-mentioned method, a relatively low ultimate phosphorus content level can be achieved in view of only the low phosphorus process capacity, but the processing time is long and the heat removal during the processing is large. It takes time to supply, and even if two converters are used, the hot metal is discharged after the treatment, and the temperature drop due to recharging into another converter is inevitable. Not a satisfying process. Furthermore, the recent use of total hot metal dephosphorization treatment further lowers the thermal tolerance in the converter process, reduces the degree of freedom in the raw materials used, and poses a serious problem from the viewpoint of active scrap recycling in future converters.

On the other hand, before the hot metal pretreatment method was developed, (LD Committee 10th Anniversary Commemorative Papers Japan BO
T Group LD Committee (S44) p. 235), there is a refining method called a double slag method in which dephosphorization pretreatment and decarburization refining are carried out in one converter. This is aimed at dephosphorization refining by soft blow refining in the first blow in the converter, and after dephosphorization, dephosphorization slag is discharged to the extent that hot metal does not flow out from the furnace port, and after degassing it is continuously decarburized and refined. Is a method of implementing.

Although the double slag method has a high heat tolerance, the converter at that time was a top-blown converter without bottom-blown stirring.
The dephosphorization reaction was difficult to reach equilibrium in a short time, and it was supplemented with an excess of quick lime. Therefore, in addition to the increase in the basic unit of flux, the CaO / SiO ₂ content in the slag is as high as about 3, and the fluidity of the slag is poor at low temperatures, so the first blow is stopped at a high temperature of 1400 ° C or higher, which is disadvantageous for the dephosphorization reaction. Waste was being removed. Therefore, there is no way to reduce the basic unit of flux, and there is also a drawback that melting loss of refractory in the converter inclined part increases. Even if the first blow is stopped at such a high temperature,
Due to the high CaO / SiO ₂ and lack of bottom-blown stirring, the slag slag state was also unstable, and only a slag ratio of about 50% could be achieved. As described above, the technique has a heavy load in terms of cost and refractory material, and is not a technique capable of enduring actual operation.

In view of the above problems, the present inventors have previously proposed the following steelmaking method. "The molten iron is charged into a converter having a bottom blowing function, the bottom blowing gas flow rate is controlled so that the stirring energy is 0.5 kW / t or more, and the CaO / SiO ₂ content in the slag after treatment is 0%. 0.7 or more and 2.5 or less, the amount of flux and the amount of cooling agent were controlled so that the hot metal temperature after treatment was 1200 ° C. or more and 1450 ° C. or less to dephosphorize the hot metal, suspend refining and tilt the furnace. Of 60% or more of the slag in the furnace, and then erecting the furnace for decarburization refining. ”(Japanese Patent Application No. 6-11027)

By this method, it becomes possible to integrate the pretreatment process, which has conventionally been directed to divisional refining for desiliconization and dephosphorization, into a converter process, which greatly improves the heat allowance and significantly reduces the cost. Reduction can be realized. Specifically, (1) the conventional hot metal dephosphorization process outside the converter can be integrated into the converter,
It will be possible to significantly reduce fixed costs. (2) Variable costs can also be reduced by reducing the basic unit of flux. (3) Operation according to need, such as improvement of scrap melting capacity, improvement of molten steel yield due to increase of iron ore reduction amount, reduction of flux cost by replacing quicklime with limestone, due to improvement of thermal tolerance caused by process consolidation You can enjoy the benefits. (4) The total amount of slag discharged from converter refining can be reduced to 2/3 or less of the conventional amount because the basic unit of flux used is reduced. It has become possible to obtain a remarkable effect.

However, in this method, sufficient desulfurization cannot be carried out in the same converter. Therefore, only the desulfurization step should be carried out in advance to a predetermined S level with a torpedo car or the like.
There was a problem that after tapping, it was necessary to carry out treatment with a hot pot or the like.

In addition, in the method, when the decarburizing slag is tapped with the demolition slag left in the furnace and the hot metal of the next charge is charged to recycle the decarburizing slag, the lime basic unit can be significantly reduced. It has become clear that However, if the next charge hot metal is charged immediately after tapping,
Iron oxide source (FeO, MnO, etc.) in decarburized slag and C in hot metal
Reacts violently to generate CO gas, and slag or hot metal violently jumps out from the inside of the furnace, which not only lowers the iron yield but also forces the operation to be interrupted.

Therefore, in the method, in order to avoid this, "the decarburizing slag produced during decarburizing and refining is left in the furnace, and the T.Fe concentration, the MnO concentration, and the slag in the slag satisfying the following formula (1) are satisfied. At the temperature, charge the hot metal of the next charge,
A converter smelting method for performing repeated dephosphorization and decarburization. ^{3.088 × 10 8 × [(%} T.Fe) + (% MnO)] 2 × exp (-91400 / (T S + T M +546) ≦ 0.1 ··· (1) T S: Datsusumikasu Temperature (℃) T _M : charging hot metal temperature (℃) "(Japanese Patent Application No. 6-11027) with a cooling agent such as CaCO ₃ or a deoxidizing agent such as coke or anthracite. By adding the mixture, the condition of formula (1) was created within the time of 5 to 10 minutes, and it became possible to significantly reduce the flux basic unit by recycling the slag. If hot metal is desired to be charged, the condition of the formula (1) cannot be satisfied, and there is no way to further shorten the operation time in the converter.

[0011]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above-mentioned problems, in which desulfurization treatment is efficiently performed in the same converter, and at the same time, immediately after tapping with the decarburizing slag left, The purpose is to allow charging hot metal to be charged.

[0012]

The gist of the present invention is to smelt hot metal using a top-blown converter to produce molten steel, and the first step is to blow hot-metal and scrap iron to the bottom. After charging into the converter, in the second step, while controlling the flow rate of the bottom-blown gas so that the stirring energy becomes 1.0 kW / t or more, the CaO / SiO ₂ after treatment is 1.5 or more and 2.3 or more.
Hereinafter, the amount of flux and the amount of cooling agent charged are controlled so that the molten steel temperature after the treatment is 1300 ° C. or more and 1450 ° C. or less, and dephosphorization treatment is performed to [P] 0.01 to 0.02%, and the third step As a result, the refining was suspended, the furnace was tilted to discharge 75% or more of the slag in the furnace, and as a fourth step, the furnace was erected and decarburized to a predetermined [C] concentration by adding flux and blowing acid. After that, Al ash powder or a mixed powder of Al ash and lime is blown into the molten steel to deoxidize the slag and deoxidize and desulfurize the generated molten steel, and immediately after tapping with the slag left as the fifth step, Charge the hot metal, the first
~ It is a converter steelmaking method characterized by repeating to 5th process.

[0013]

The present invention will be described in detail below. The present invention, first, after charging the hot metal into a converter having a bottom blow or side blow function of gas and powder, add the flux by upward or bottom injection, blow oxygen gas from the top blowing lance, While stirring the gas from the bottom, the specified [P]
Perform dephosphorization refining to the concentration. After that, tilt the converter, discharge the slag used for dephosphorization and refining from the furnace mouth, raise the furnace again, add a small amount of flux for protecting the furnace body, and decarburize and refine by top blowing oxygen. To do.

After decarburizing to a predetermined [C] concentration, industrial waste Al ash powder or a mixed powder of Al ash powder and quick lime is blown into molten steel from bottom blowing or side blowing tuyere, and At the same time as decarburizing slag and deoxidation of molten steel with Al, the molten steel is desulfurized to a predetermined [S] concentration by top slag or blown lime while oxygen potential is lowered. At this time, if the slag after dephosphorization and refining is not sufficient, re-phosphorization from the slag occurs due to the decrease in the oxygen potential of the slag, and the [P] concentration in the produced molten steel becomes higher than the predetermined [P] concentration. turn into.

After the deoxidation of the molten steel and the slag and the desulfurization treatment of the molten steel are completed, the decarburized slag is tapped while being left in the furnace. Since the decarburizing slag remaining in the furnace has already been sufficiently deoxidized, even if the next charge hot metal is immediately charged, a vigorous reaction with the hot metal C does not occur and prompt slag recycling becomes possible.
After that, the converter refining is repeated.

The present inventors conducted various converter experiments, and first of all, required A for deoxidation of slag and metal and desulfurization of metal.
The amount of mixed powder of 1 ash or Al ash and CaO was examined. Since the slag temperature immediately after tapping is about 1600 ° C and the temperature of the hot metal charged is about 1350 ° C, even if the hot metal is charged immediately after tapping, it does not hinder the operation. When the Fe concentration and the MnO concentration are calculated from the equation (1), [(% T.Fe) + (% M
nO)] ≦ 8% is sufficient. Fig. 1 shows the amount of Al ash or 50% Al ash-50% Ca obtained from the converter experiment.
The relationship between the amount of O mixed powder and [(% T.Fe) + (% MnO)] is shown. T. in the slag The sum of Fe concentration and MnO concentration is 8
% To 12% or less with 1 ton of molten steel with Al ash
It was found that it is sufficient to blow 24 kg with a mixed powder of Al ash and CaO.

Further, FIG. 2 shows the amount of Al ash or the amount of 50% Al ash-50% CaO mixed powder obtained from the converter experiment and the [S] concentration after the treatment when the initial [S] concentration was 0.02%. Shows the relationship. Assuming that the target [S] concentration after treatment is 0.01%, Al ash is 10 kg per ton of molten steel, Al ash and Ca
It was found that it was sufficient to blow 7 kg of O mixed powder. Further, in the case of manufacturing low-sulfur steel having a [S] concentration of 0.008% or less after the target treatment, it is impossible to inject Al ash alone, and 12 kg of Al ash-CaO mixture per ton of molten steel. Need flour. Therefore, in order to perform both the deoxidizing treatment of molten steel and slag and the desulfurization treatment of molten steel to the level at which the operation is not impaired even if the molten iron is charged immediately after tapping, the post-treatment [ S] In the case of normal desulfurization treatment of about 0.01%, about 12 kg / t of Al ash alone was blown, and in the case of low-sulfur steel melting, about 24 kg / t of Al ash-CaO mixed powder was blown. desirable. Here, the mixing ratio of Al ash and CaO is not limited to 50% -50%, and when the desulfurization required amount is larger than the deoxidation required amount, the CaO mixing ratio is increased to the desulfurization required amount. In contrast, when the required amount of deoxidation is large, the treatment of deoxidation and desulfurization can be performed with a smaller amount of flux by increasing the mixing ratio of Al ash.

Next, in order to keep the [P] concentration of the produced molten steel below a predetermined [P] concentration even if re-phosphorus is generated during decarburization refining, the necessary [P] concentration and required wastes after dephosphorization refining. I investigated the rate. Fig. 3 shows the relationship between the [P] concentration of the molten steel after the above-mentioned deoxidation and desulfurization treatment, the [P] concentration after the dephosphorization refining, and the slag ratio, which were obtained based on the 8t converter experiment. . The [P] concentration after deoxidation and desulfurization treatment is the target in the case of general-purpose steel grades.
The required slag removal rate to reach 02% or less is that the [P] concentration after dephosphorization and refining is 0.01%, 0.015%, 0.02%,
75%, 80%, 85 for 0.025%
% And 90%. Here, in any case, it is desirable that the rate of slag is infinitely close to 100%. According to the method of the present invention, it is impossible to reduce the [P] concentration after dephosphorization refining to less than 0.01%, and when the [P] concentration after dephosphorization refining is more than 0.025%, it is eliminated. Even if the slag ratio was infinitely close to 100%, the [P] concentration after deoxidation and desulfurization could not be reduced to 0.02% or less.

Further, the present inventors continued the 8t converter experiment in order to clarify the operating conditions at the time of dephosphorization and refining necessary to achieve the required [P] concentration and the required slag removal rate.
In order to carry out the dephosphorization treatment efficiently, it is important to proceed with the dephosphorization as close to the apparent equilibrium as possible. In the Japanese Patent Application No. 6-11027, the present inventors
It was shown that the dephosphorization reaction proceeds to an apparent equilibrium by ensuring the stirring energy by the bottom blowing gas to 1.0 kW / t or more. Here, the bottom blowing agitation power increases as the bottom blowing gas flow rate increases, but if the gas flow rate is too high, the hot metal blows through and the spitting increases significantly, so the bath depth of the hot metal and the diameter of the bottom blowing tuyere Therefore, the upper limit is the stirring energy that does not blow through the hot metal.

When the stirring energy is secured at 1.0 kW / t or more, the [P] concentration after the treatment is 0.01%, 0.
Post-treatment temperature and Ca in the required slag when dephosphorization and refining were carried out so as to reach 015%, 0.02%, and 0.025%
The O / SiO ₂ relationship is shown in FIG. It is shown that, for each temperature, if CaO / SiO ₂ having a value equal to or higher than the value indicated by the solid line is secured, it can be dephosphorized to a predetermined [P] concentration or lower.

The temperature after treatment, CaO / Si in the slag
The relationship between O ₂ and the rate of slag after dephosphorization refining is shown in FIG. From FIG. 4 and FIG. 5, the [P] concentration and the slag removal rate after dephosphorization and refining necessary to achieve the target [P] concentration in the molten steel after decarburization and refining to 0.02% or less are achieved. In order to
The range indicated by the thick line, that is, the temperature after the treatment is 1300
It was found that the conditions are that the temperature is not lower than 1450 ° C and not higher than 1450 ° C, and the CaO / SiO ₂ content in the slag is not lower than 1.5 and not higher than 2.3. It was also clarified that if the [P] concentration after dephosphorization and refining exceeds 0.02%, the required slag removal rate of 90% cannot be reached under any operating conditions.

From the above facts, the operating conditions during dephosphorization refining are as follows: bottom blowing agitation energy of 1.0 kW / t or more, post-treatment temperature of 1300 ° C. or more and 1450 ° C. or less, slag CaO /
SiO ₂ 1.5 to 2.3 and [P] 0.01 to
If dephosphorization treatment is performed up to 0.02% and 75% or more of the produced slag is discharged according to the [P] concentration after dephosphorization refining, even if deoxidation and desulfurization treatment are performed after decarburization treatment, the specified [ It has been clarified that predetermined desulfurization can be performed without re-phosphorization above the P] concentration, and that the operation hindrance due to hot metal charging immediately after tapping can be avoided.

[0023]

【Example】

(Example 1) In an 8t test converter having a bottom blowing function,
About 6 tons of hot metal containing 5% C, 0.1% P, 0.02% S and 0.3% Si was charged into the converter so that the stirring energy was 2.1 kW / t. Bottom-blown N ₂ gas to 20
The temperature after treatment was 1380 while controlling to 0 Nm ³ / h.
The dephosphorization treatment was carried out for about 8 minutes by adjusting the amount of introduced flux and the amount of introduced scrap so that CaO / SiO _{2 in} the slag after the treatment was 1.8 ° C. Top blowing acid velocity is 1
It was set to 000 Nm ³ / h.

Thereafter, the furnace was tilted, and the intermediate slag was discharged in about 3 minutes. The rate of waste was 81%. After the intermediate waste is completed,
Immediately after the furnace is erected, the upper blowing acid velocity is 1500 Nm ³ /
Decarburization treatment was performed for about 9 minutes under h. After decarburization to a predetermined [C] concentration, top blowing acid was stopped and A
72 kg of 1 ash was blown into the steel for tapping. Immediately after tapping, the next hot metal was charged.

As a comparison, dephosphorization and decarburization refining were performed under the operating conditions shown in Japanese Patent Application No. 6-11027 using the same converter and hot metal under the same conditions, and immediately after tapping, the next charge hot metal was used. A charging test was also conducted.

Table 1 shows examples of the present invention, and Table 2 shows comparative examples of P and S in the metal and T in slag. The transition of the concentration of Fe and MnO is shown. In the case of the comparative example, the [P] concentration in the produced molten steel could be set to the predetermined 0.02% or less, but desulfurization could hardly be performed, whereas in the present invention example, both [P] and [S] were set to the predetermined values. I was able to reach the level. Further, in the comparative example, T. Fe and MnO
Since the sum of the above concentrations was over 8%, the molten steel and slag almost overflowed from the furnace due to the violent reaction during the hot metal charging.
Although it greatly hindered the subsequent operation, no outflow of molten steel or slag was observed in the example of the present invention even when hot metal was charged.

[0027]

[Table 1]

[0028]

[Table 2]

(Example 2) After dephosphorization refining, intermediate slag and decarburization refining under the same conditions as in Example 1, 50% Al ash-
145 kg of a mixed powder of 50% CaO was blown from the bottom.
After that, the steel was tapped and the next charge hot metal was immediately charged. Table 3 shows P, S in metal and T. in slag at that time. F
3 shows changes in the concentration of e and MnO. At the same time that it was possible to dephosphorize to a prescribed concentration, low-sulfur steel of [S] 0.007% could be melted. Also, T.I. Since the sum of the concentrations of Fe and MnO was 8% or less, no outflow of molten steel or slag was observed even when the hot metal was charged.

[0030]

[Table 3]

[0031]

According to the present invention, the desulfurization treatment and the decarburization treatment can be efficiently performed in addition to the dephosphorization treatment and the decarburization treatment to the predetermined concentration in the same converter. It is now possible to charge hot metal.

[Brief description of drawings]

FIG. 1 shows the amount of Al ash, the amount of Al ash-CaO mixed powder blown, and the T.V. in slag after desulfurization and deoxidation treatment. Graph showing the relationship between the Fe concentration and the sum of MnO concentrations

FIG. 2 is a graph showing the relationship between the injection amount of Al ash and Al ash-CaO mixed powder and the [S] concentration in molten steel after desulfurization and deoxidation treatment.

FIG. 3 is a graph showing the relationship between the intermediate slag ratio after dephosphorization refining and the [P] concentration in molten steel after desulfurization and deoxidation treatment.

FIG. 4 is a graph showing the relationship between the temperature after dephosphorization refining and the minimum required amount of CaO / SiO _{2 in} the slag after treatment at each target [P] concentration.

FIG. 5 is a graph showing the relationship between temperature, slag basicity and slag ratio after dephosphorization refining.

Claims (1)

[Claims] 1. When refining hot metal by using an upper-bottom blowing converter to produce molten steel, as a first step, the hot metal and scrap iron are charged into the upper-bottom blowing converter, and as a second step, stirring is performed. While controlling the bottom blown gas flow rate so that the energy becomes 1.0 kW / t or more, the CaO / SiO ₂ after the treatment is 1.5 or more and 2.3 or less, and the molten steel temperature after the treatment is 1300 ° C. or more and 1450 or less.
The amount of flux and the amount of cooling agent charged are controlled so that the temperature becomes less than or equal to ℃, and dephosphorization treatment is performed to [P] 0.01 to 0.02%. As the third step, refining is temporarily suspended and the furnace is tilted. 75% or more of the furnace slag is discharged, and as the fourth step,
Set the furnace upright and add the flux and sprinkling acid to the prescribed [C].
After decarburizing to a certain concentration, Al ash powder or a mixed powder of Al ash and lime is blown into the molten steel to deoxidize the slag and deoxidize and desulfurize the produced molten steel, and then tap the steel while leaving the slag as the fifth step. Immediately thereafter, the next charge hot metal is charged, and the above-described first to fifth steps are repeatedly carried out.