JP2003239009A - Dephosphorization refining method of hot metal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dephosphorization refining method of hot metal in which the dephosphorization efficiency is enhanced to the upper limit while promoting slag formation of CaO by using a less costly dephosphorization refining agent. <P>SOLUTION: When performing dephosphorization refining of the hot metal through flux addition and oxygen top-blow by using a converter, a part of or the whole of the dephosphorization flux containing CaO is made into powder which is sprayed over the surface of the hot metal on the oxygen flow from a top-blow lance, and ≥10 mass % of the powder is the limestone powder containing calcium carbonate. The hot metal is charged in the converter, dephosphorization refining is performed through flux addition and oxygen top- blow, the phosphor content is reduced to a predetermined level, and the converter is tilted to discharge the generated slag. Thereafter, when a decarbonizing step is performed by the same converter, a part of or the whole of the flux for dephosphorization containing CaO is made into powder which is sprayed on the surface of the hot metal on the oxygen flow from the top-blow lance, and ≥10 mass % of the powder is the limestone powder containing calcium carbonate. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for dephosphorizing and refining hot metal in a converter, and more particularly to a method for dephosphorizing and refining in which the dephosphorization efficiency is maximized.

[0002]

2. Description of the Related Art Regarding the minimization of the total cost of steelmaking and the stable hot metal of low phosphorus steel, the conventional hot metal dephosphorization method is as follows.
(1) Method of pre-dephosphorization by injecting flux for dephosphorization into hot metal in torpedo car, (2) Pre-dephosphorization by injecting or spraying flux for dephosphorization on hot metal in ladle Or (3) using two converters, dephosphorization is carried out on the one hand and decarburization is carried out on the other hand (for example, JP-A-63-19521).
No. 0) is used.

However, when a hot metal carrier such as a torpedo car or a hot metal ladle is used, the capacity of the container is small and it is difficult to perform strong stirring refining. Especially, the dephosphorization reaction is far from equilibrium and the target dephosphorization amount is achieved. In order to do so, it is necessary to use more flux than necessary, and refining takes a long time. The dephosphorization refining time was about 20 minutes for the process (1) and about 11 minutes for the process (2). Also, when a large amount of oxygen is blown at high speed, CO is generated.
As the amount of gas increases, the slag foams excessively, causing a phenomenon called sloping, in which slag and metal ingots escape from the smelting container, which causes production obstacles and decreased iron yield.To avoid this, iron ore is used as an oxygen source. Since iron oxide such as iron and scale must be used, and the sensible heat of the hot metal is taken away by the reduction of iron oxide, the amount of scrap dissolved is limited. If a converter with a large vessel capacity is used, a large amount of oxygen can be supplied at high speed under conditions where mass transfer in molten iron is high with strong stirring, so dephosphorization can be performed in a short time and scrap consumption can be reduced. Also expand. For that reason, recently, many steel mills shift to a converter type dephosphorization process such as the process (3).

However, also in the process (3), since two converters including decarburization are required to produce molten steel from hot metal, the equipment cost is high, and the heat dissipation loss due to hot metal transfer operation is high. And the melting capacity of scrap decreases.
In order to solve this problem, the present inventors have previously disclosed Japanese Patent Laid-Open No.
As disclosed in JP-A-70626, a steelmaking process is proposed in which, after performing hot metal dephosphorization refining in a converter, the furnace is tilted to discharge the generated slag, and then a decarburization process is performed in the same converter.

In dephosphorization refining using a converter, lumped quick lime containing CaO as a main component is often used as a flux for dephosphorization, but the rate of molten slag called slag formation is slow, and the original slag is used. The dephosphorization refining is completed before fully enjoying the dephosphorization ability of. Although it is possible to promote the slag formation of quicklime by using a solvent medium such as fluorspar containing CaF ₂ as a main component, it is becoming impossible to use fluorspar due to environmental regulations such as F elution. In addition, CAMP-ISIJ,
vol. 4 (1991), p. 1153, powdered quicklime and calcium carbonate (CaC
There is also a process in which limestone containing O ₃ ) as the main component is blown into molten iron from the bottom tuyere to promote slag formation, but the effect is only the small particle size of the flux, and the melting point is about 2570 ° C.
There is a limit to the effect of promoting the slag formation of CaO.

LD is used as a means for promoting the slag formation of CaO.
-The AC method has been developed and applied to converter-type dephosphorization treatment. This is a method in which quicklime powder is mixed with oxygen for blowing and is sprayed on a steel bath surface together with an oxygen jet. In this method, oxygen blown to the surface of the steel bath oxidizes iron to form iron oxide and CaO that form low-melting-point calcium ferrite, so that the slag formation of CaO proceeds rapidly and the dephosphorization efficiency improves. It is said.
Also, although dephosphorization and decarburization are simultaneously blown in a converter, various slag-making agents are used as a method for preventing the speed reduction of the oxygen jet when powder is mixed into the oxygen stream from the inside of the lance. Japanese Patent Laid-Open No. 56-9311 discloses a technique in which a powder is ejected from a path different from that of oxygen and is accompanied by an oxygen stream outside the lance.

However, in these methods in which the powder flux is placed in the oxygen stream from the top-blown lance and sprayed onto the surface of the steel bath, calcium ferrite is produced at the high-temperature hot spot, and immediately the dephosphorization reaction occurs. Does not contribute to. It is thought that the efficiency of the dephosphorization reaction that proceeds at the slag / metal interface, which has a relatively low temperature, improves as the CaO slagging due to the formation of calcium ferrite migrates into the slag and the basicity of the entire slag increases. Has been. Therefore, there is a problem that the high dephosphorization ability originally possessed by calcium ferrite is not fully enjoyed. Further, in these technologies, quick lime used as a main dephosphorizing agent is produced by calcining limestone containing CaCO ₃ as a main component in a lime furnace, and therefore requires equipment and calcining fuel, labor costs, and is expensive. There was also a problem that it would be something like.

[0008]

SUMMARY OF THE INVENTION The present invention is to solve the above-mentioned problems and uses a dephosphorization refining agent that is low in cost, and promotes the slag formation of CaO, while maximizing the dephosphorization efficiency. A method for dephosphorizing and refining hot metal is provided.

[0009]

The subject matter of the present invention is as follows. (1) In a refining method in which hot metal is dephosphorized by adding flux and blowing oxygen in a converter, a part or all of the flux for dephosphorization containing lime is placed in the oxygen stream from the top blowing lance. A method for dephosphorizing and refining hot metal, characterized in that the powder is sprayed onto the surface, and 10% by mass or more of the powder is limestone powder containing calcium carbonate. (2) When refining the hot metal to produce molten steel, the hot metal is charged into a converter, and flux addition and oxygen top blowing are performed to perform dephosphorization refining to reduce the phosphorus content to a predetermined value, and the converter is used. When the decarburization process is performed in the same converter after discharging the slag generated by tilting the steel, part or all of the dephosphorization flux containing lime is placed in the oxygen stream from the top blowing lance and the surface of the hot metal is removed. 10 of the powder
A method for dephosphorizing and refining hot metal, which comprises using limestone powder containing calcium carbonate in an amount of at least mass%.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. First, the hot metal tapped from the blast furnace is charged into the converter as it is, or after being subjected to prior desulfurization or desiliconization if necessary. After that, while blowing oxygen from the upper blowing lance, P and Si in the hot metal are oxidatively refined. At this time, in order to perform high-speed dephosphorization refining, it is desirable to enhance the mass transfer inside the hot metal and the slag while blowing a stirring gas from the bottom blowing tuyere. During the treatment, an iron oxide source such as iron ore and scale as an auxiliary material for oxidation and a flux for dephosphorization containing CaO such as quick lime, dolomite, limestone and decarburizing slag are added. Since the slag formation does not proceed only with the quick lime of the above, a part or the whole amount of the flux for dephosphorization is made into powder, and the powder is fed into the oxygen gas pipe for refining with nitrogen gas as a carrier gas, and is fed into the oxygen stream. Then, it is sprayed on the surface of the hot metal through a top-blown lance to promote CaO slag formation. At this time, 10% by mass of the powder of the flux for dephosphorization which is sprayed onto the hot metal from the upper spray lance
The above is limestone powder containing calcium carbonate (CaCO ₃ ). The reason is as follows.

The present inventors have carried out various dephosphorization refining operations, and when the CaO amount in the dephosphorization refining flux per 1 ton of hot metal is almost the same, the dephosphorization efficiency is supplied from the upper blowing lance. The influence of the blending ratio of flux was investigated. FIG. 1 shows the relationship between the ratio of limestone powder in the flux for dephosphorization and refining sprayed on the hot metal from the top-blown lance and the [P] level after the treatment. 10 limestone powder
Up to a blending ratio of less than mass%, no influence was observed on the [P] level after the treatment, but when the blending ratio of limestone powder was 10 mass% or more, the effect of reducing the [P] level was recognized.
From the component analysis by local sampling of hot metal and slag, the mineral phase analysis, and the temperature measurement of the powder spraying part, it was found that this is the hot spot where CaCO ₃ which is the main component of limestone is sprayed with oxygen. It was clarified that this is because the temperature near the calcium ferrite that was generated due to thermal decomposition decreased, and it already contributed greatly to the dephosphorization reaction near the fire point.

The upper limit of the mixing ratio of limestone powder is not specified in terms of dephosphorization reaction efficiency. Rather, the total amount of limestone powder is advantageous for improving the dephosphorization efficiency, and also as the cost of the refining agent. Will be reduced. However, when the amount of heat in the decarburization process after dephosphorization is insufficient, the total amount of limestone used with decomposition endothermic reaction is regulated.

According to the present invention, when refining the hot metal to produce molten steel, the hot metal is charged into a converter as the first step, and the flux addition and oxygen top blowing are performed as the second step to dephosphorize the hot metal. It is applied in a molten steel manufacturing method where refining is performed to reduce the phosphorus content to a predetermined level, the converter is tilted as a third step to discharge the slag generated in the second step, and then a decarburization step is performed in the same converter. In that case, it exerts a greater effect.

In this molten steel manufacturing method, the dephosphorized slag produced in the second step is discharged by tilting the converter. However, when the slag discharge rate decreases, the amount of P introduced into the subsequent decarburization step is reduced. Because of the increase, "P disengagement" that does not reach a predetermined [P] level occurs when decarburization is stopped. In order to improve the discharge rate of slag, it is important to foam the slag to some extent in the second step. However,
If the slag foams excessively during dephosphorization and refining, a phenomenon called sloping that overflows the slag containing metal, occurs,
This will cause operational problems and reduce the iron yield. It is known that when the basicity of slag during refining becomes low due to the delay of CaO slag formation, the slag foams excessively due to an increase in the viscosity of the slag and a decrease in the surface tension. When the lance is placed in an oxygen stream and sprayed onto the hot metal, the slag foaming is suppressed by promoting the slag formation of CaO.

Further, the inventors of the present invention sprayed limestone powder onto the hot metal through the top blowing lance to cool the hot spot so that more oxygen supplied was used for the oxidation of P and at the hot spot. It was also found that the reduction rate of the generated FeO is also suppressed by the temperature decrease, and as a result, the decarburization rate is decreased, so that the CO gas generation amount, which is the main cause of foaming, is reduced and the slag foaming is suppressed. After the dephosphorization and refining was completed, it was revealed that when the spraying of limestone powder was stopped, the slag foamed appropriately and the discharge rate of slag in the third step could be maintained at a high level.

[0016]

EXAMPLES Example 1 Using a 350 ton converter, dephosphorization refining of 50-charge hot metal was carried out. 4.3-4.6
Mass% C, 0.09-0.10 mass% P, 0.4-
Initial temperature 1300 to 1430 containing 0.6% by mass of Si
320 to 350 t of hot metal and 30 to 60 t of scrap iron at a temperature of 0 ° C were charged into a converter and refined for 4 to 5 minutes. In addition to the iron ore 1 to 6 tons for temperature adjustment, lumped quick lime, quick dolomite and limestone powder were used as flux for dephosphorization. The total mass of CaO in the dephosphorization flux was twice the mass of SiO ₂ produced by the oxidation of Si in the hot metal, and the amount of limestone powder used was 1.5t to 3t. The ratio of limestone powder in the flux powder for dephosphorization in this example was in the range of 12% to 40%. Bulk quick lime and quick dolomite were added all at once from the charging chute immediately after the start of refining. The limestone powder was introduced into the oxygen gas pipe from a blow tank using nitrogen gas as a carrier gas, placed in an oxygen gas stream, and sprayed onto the hot metal at a rate of about 1000 kg / min through an upper blowing lance.
The top blowing acid rate was 58,000 Nm ³ / hour, and the bottom blowing was a double tube tuyere in which oxygen gas was blown together with carbon dioxide for cooling and propane gas at a total flow rate of 2000 Nm ³ / hour. The hot metal temperature after the treatment was adjusted to be in the range of 1350 to 1400 ° C.

(Comparative Example 1) Using the same converter and upper blowing lance as in Example 1, dephosphorization refining with 50 charges was carried out. In Comparative Example 1, the total mass of CaO in the flux for dephosphorization was set to be twice the mass of SiO ₂ generated by the oxidation of Si in the hot metal as in Example 1, but the mass sprayed from the upper spray lance was used. Did not use limestone powder, but used all quicklime powder. All other hot metal conditions and refining conditions were within the same ranges as in Example 1.

50 charges of Example 1 and 50 of Comparative Example 1
Table 1 shows the average value of the P concentration in the hot metal after refining and the number of occurrences of sloping during refining in the hot metal dephosphorization refining of charge. Regarding the average P concentration after refining, Example 1 is Comparative Example 1
It can be seen that it is reduced by about 0.006%. In addition, sloping occurred 6 times in Comparative Example 1, but never occurred in Example 1.

[0019]

[Table 1]

(Embodiment 2) After the same dephosphorization refining was carried out by using the same converter and upper blowing lance as in Embodiment 1,
The converter was tilted to discharge the dephosphorized slag, and then a decarburizing step was performed for 50 charges of refining. The hot metal and refining conditions in the dephosphorization refining process were within the same ranges as in Example 1. The mass of the discharged slag was measured by a weighing machine, and the discharge rate of the slag was calculated based on the generated slag amount calculated from the mass balance. Immediately after the dephosphorization slag was discharged, the converter was erected upright and a certain amount of lump quick lime was added, and the top blowing acid rate was 72000.
Decarburization refining was carried out at Nm ³ / h for about 13 minutes. The bottom gas flow rate was the same as during dephosphorization refining. The molten steel temperature after decarburization refining is 1630 to 1660 ° C, and [C] is 0.04 to 0.0
The amount of iron ore for cooling and the amount of massive quicklime for dephosphorization were adjusted so that 6% by mass and [P] were in the range of 0.014 to 0.018% by mass.

(Comparative Example 2) The same dephosphorization refining step, dephosphorization slag discharge step, and decarburization step refining steps as in Example 2 were carried out.
We carried out 0 charge. In the dephosphorization refining process, all the hot metal conditions and refining conditions were the same as in Example 2 except that the total amount of powder sprayed from the top blowing lance was quicklime powder.

50 charge of Example 2 and 50 charge of Comparative Example 2
Table 2 shows the average P concentration in the metal after dephosphorization refining and decarburization refining, the average emission rate of dephosphorization slag, and the average basic unit of massive quicklime used in decarburization refining during charge refining. . First, the average P concentration after dephosphorization and refining is
It can be seen that Example 2 is reduced by about 0.006% as compared with Comparative Example 2. Also, the discharge rate of dephosphorized slag was significantly improved in Example 2 as compared with Comparative Example 2, and as a result, during decarburization refining in which the [P] level after decarburization refining was controlled to be equivalent. It can be seen that the basic unit of quicklime in Example 2 can be reduced by about 5 kg / t in Example 2 as compared with Comparative Example 2.

[0023]

[Table 2]

[0024]

According to the present invention, it is possible to greatly improve the dephosphorization efficiency by using a flux which is cheaper in manufacturing cost and to manufacture stable low P steel without sloping.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a mixing ratio of limestone powder to a total amount of dephosphorization flux powder sprayed from an upper spray lance and a [P] concentration after dephosphorization refining.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kosuke Kume             No. 1 Nishinosu, Oita City, Oita Prefecture Made in New Japan             Oita Steel Works, Ltd. (72) Inventor, Kimitoshi Yonezawa             No. 1 Nishinosu, Oita City, Oita Prefecture Made in New Japan             Oita Steel Works, Ltd. F-term (reference) 4K014 AA03 AB03 AB12 AC12 AC17                       AE01                 4K070 AB06 AC02 AC14 AC20 BA05                       BA12 BB02 BC00 BC11

Claims (2)

[Claims] 1. A refining method in which hot metal is dephosphorized by adding flux and overblowing oxygen using a converter, and part or all of the dephosphorization flux containing lime is placed in an oxygen stream from an overblowing lance. The method for dephosphorizing and refining hot metal is characterized in that 10 mass% or more of the powder is limestone powder containing calcium carbonate, and the powder is sprayed onto the surface of the hot metal. 2. When refining the hot metal to produce molten steel,
The molten pig iron is charged into a converter, and flux addition and oxygen top blowing are performed to perform dephosphorization refining to reduce the phosphorus content to a predetermined level, and the slag produced by tilting the converter is discharged, and then the same rotation is performed. When performing a decarburization step with a furnace, a part or all of the dephosphorization flux containing lime is placed in an oxygen stream from a top blowing lance and sprayed onto the surface of the hot metal, and the powder is 10 mass% or more. A method for dephosphorizing and refining hot metal, characterized in that limestone powder containing calcium carbonate is used.