JP2896839B2 - Molten steel manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing molten steel in a converter.

[0002]

2. Description of the Related Art With regard to minimization of total cost of steel making and low phosphorus steel, (1)
A method for performing preliminary dephosphorization by injecting a dephosphorizing flux into the hot metal in a torpedo car, (2) A method for performing preliminary dephosphorization by injecting or spraying a hot metal in a ladle with a dephosphorizing flux, or (3)
A method of using two converters and performing dephosphorization on one side and decarburization on the other side (for example, JP-A-63-195210) is used.

However, the above (1), (2),
Any of the methods (3) has a drawback that when transferring from the dephosphorization step to the decarburization step, the hot metal needs to be transferred, the temperature must be reduced, and the energy loss is large. In order to solve this problem, Japanese Patent Application Laid-Open No. Hei 18-181989 discloses that a conventional multi-step refining function is integrated into a converter to greatly reduce the energy loss of hot metal and to fix steps before and after the converter. There have been proposed methods that can significantly reduce costs (equipment costs and labor costs).

FIG. 2 shows this flow. In the first step, hot metal is charged into a converter, and in the second step, desiliconization and dephosphorization refining are performed by adding a flux and blowing oxygen.
As a third step, the converter is tilted to discharge the slag generated in the second step, and then the fourth step is performed by adding flux and oxygen blowing in the same converter in the same step as the fourth step. As a fifth step, the slag generated in the fourth step is left in the converter as the fifth step, and the steel is returned to the first step, and the process returns to the first step, and the steps up to the fifth step are repeatedly performed. I do. In some cases, the slag generated in the fourth step may not be returned to the first step, but may be entirely discharged after tapping in the fifth step.

[0005]

If the above-described process of continuously performing the dephosphorization and decarburization steps using the same converter is used,
Energy loss during the transition from the dephosphorization step to the decarburization step can be reduced, and fixed costs (equipment costs and labor costs) can be significantly reduced. However, if the amount of slag discharged in the third step is small, the phosphorus removed in the slag in the second step returns to the molten steel in the fourth step again (hereinafter, this is referred to as re-phosphorus). It is necessary to dephosphorize again, and the amount of flux such as quicklime must be increased, which leads to an increase in cost. Moreover, since the slag having a high phosphorus concentration in the fourth step is reused in the second step, the dephosphorization load in the second step is increased and the cost is increased. If the amount of slag discharged in the third step is small, a problem arises in that the dephosphorization load increases and the cost increases.

According to the present invention, by optimizing the operating conditions in the second step of the molten steel production method, the amount of slag discharged in the third step is increased, the amount of reconstituted phosphorus in the fourth step is suppressed, and the unit of quicklime is reduced. It is an object of the present invention to provide a method for reducing the amount of light.

[0007]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and the gist of the present invention is to perform a dephosphorization treatment and a decarburization treatment in a single refining vessel to remove molten steel from hot metal. At the time of production, C in slag after dephosphorization
aO / SiO ₂ is 2.5 or less, the sum of iron oxide and manganese oxide concentrations (T.Fe + MnO) is 10% or more, MgO concentration is 10% or less, and the treatment end point temperature is 1
A method for producing molten steel, wherein the temperature is set to 320 ° C. or higher and 1400 ° C. or lower.

[0008]

The present invention will be described below in detail. In the present invention, hot metal pretreatment and decarburization are integrated and operated by the same converter. For example, as shown in FIG. 2, one or a plurality of tuyere tuyeres for blowing dephosphorization and decarburization flux into the furnace bottom, and gas blowing vanes for slag forming at the tapping hole and the belly facing the furnace. Hot metal is charged into an upper-bottom blow converter provided with a port, and a flux based on quicklime powder is blown from the above-described bottom-blow tuyere using an inert gas such as nitrogen as a carrier gas to perform a dephosphorization treatment.

At this time, the dephosphorization reaction rate can be increased by mixing iron oxide powder with quick lime powder or by forming a tuyere with a triple tube structure and blowing oxygen gas through the same tuyere. Alternatively, dephosphorization is promoted by blowing oxygen gas from the upper blowing lance and adding flux from above by adding, blowing, or spraying to control the iron oxide concentration of the generated slag. be able to.

At the time when the phosphorus content has decreased to a predetermined phosphorus content, the furnace is tilted to the counter-steel side (discharge side) to discharge only slag.
However, if the amount of slag discharged at this time is small, the phosphorus removed in the slag will return to phosphorus at the time of the next decarburization treatment, and it will be necessary to add extra quick lime, leading to an increase in cost. FIG. 1 shows the relationship between the waste rate (the amount of discharged slag / the amount of generated slag × 100) and the basic unit of quicklime. It can be seen that when the waste ratio is less than 60%, the quicklime basic unit sharply increases. Therefore, the waste rate must be at least 60% or more.

The amount of slag discharge decreases as the fluidity of the slag decreases. The fluidity is slag CaO / SiO ₂ ,
(T. Fe + MnO) concentration, MgO concentration, and temperature. This is because when CaO / SiO ₂ is high,
This is because when the (T. Fe + MnO) concentration is low, the melting point of the slag increases when the MgO concentration is high, and when the temperature is low, the solid phase precipitates in the slag, so that the apparent viscosity of the slag increases. Therefore, slag composition before slag discharge,
That is, CaO / SiO ₂ at the end of the dephosphorization treatment is 2.5 or less, and the sum of iron oxide and manganese oxide concentrations (T.Fe +
MnO) is set to 10% or more, the MgO concentration is set to 10% or less, and the temperature is set to 1320 ° C. or more, so that the fluidity of the slag can be increased and the waste rate can be set to 60% or more.
As for the temperature, the higher the temperature, the more advantageous it is, but the higher the temperature, the more the dephosphorization reaction is hindered.

Immediately after the waste is finished, the furnace is erected immediately, and auxiliary raw materials (quick lime, dolomite, iron ore, Mn ore, etc. for refractory protection and rephosphorization prevention) are charged and ordinary top-bottom blowing decarburization refining is performed. . After blow-off, the molten steel is tapped, but the slag is left in the furnace as it is and used as a dephosphorization flux for the next charge.

[0013]

EXAMPLE 4.5% C, 0.1% P, 0.3% S
1350 ° C. hot metal containing i was charged into a 300-t converter with the pre-charged decarburized slag remaining, and while performing bottom-blowing stirring, quicklime and iron ore were added as dephosphorizing agents, and top-blown acid was added. After performing the dephosphorization treatment, a test for discharging the slag and then performing the decarburization treatment was performed. CaO / SiO ₂ , (T / Fe + MnO) concentration of slag at the end of dephosphorization treatment, MgO
Table 1 shows the concentration and the temperature.

[0014]

[Table 1]

Test No. 1 to 10 are CaO / SiO ₂
, 2.5% or less, the sum of iron oxide and manganese oxide concentrations (T. Fe + MnO) is 10% or more, the MgO concentration is 10% or less, and the temperature is 1320 ° C. or more. For comparison, test Nos. 11 to 12 are CaO / S
iO ₂ is 2.5 or less and (T.Fe + MnO) is 10
% And the MgO concentration is 10% or less but the temperature is less than 1320 ° C., the test No. 13-14 are Ca
O / SiO ₂ is 2.5 or less and (T.Fe + MnO)
Is 10% or more and the temperature is 1320 ° C. or more, but MgO
When the concentration was more than 10%, the test No. 15-16 are
CaO / SiO ₂ of 2.5 or less and MgO concentration of 10
% And the temperature is 1320 ° C. or higher, but (T. Fe +
MnO) is less than 10%, test No. 17-1
8 has (T.Fe + MnO) of 10% or more and MgO
The concentration is 10% or less and the temperature is 1320 ° C. or more.
This is the case where aO / SiO ₂ exceeds 2.5.

Table 2 shows the slag ratio and the basic unit of quicklime in each test. CaO / SiO ₂ is 2.5 or less, and the sum of iron oxide and manganese oxide concentrations (T.Fe + MnO) is 10
%, The MgO concentration is 10% or less, and the temperature is 13%.
When the temperature is set to 20 ° C. or higher, it can be seen that the waste rate is more stably set to 60% or higher than in the comparative example, and the quicklime basic unit can be reduced.

[0017]

[Table 2]

The present invention has been described above mainly by taking as an example a method of producing molten steel in which desiliconization, dephosphorization and decarburization are performed in the same converter and slag after decarburization is used for desiliconization and dephosphorization. However, the present invention is not limited to this, and does not deviate from the scope of the present invention when applied to a molten steel manufacturing method in which slag after decarburization treatment is discharged and desiliconization is not used for dephosphorization treatment. .

[0019]

According to the present invention, in the process of desiliconization, dephosphorization and decarburization in the same refining vessel, the amount of slag discharged after dephosphorization can be increased, and quicklime The unit can be reduced.

[Brief description of the drawings]

FIG. 1 Waste rate (discharged slag amount / generated slag amount × 10)
FIG. 2 is a diagram showing the relationship between the basic unit of quicklime and the basic unit of quicklime.

FIG. 2 is a schematic explanatory view of a refining process using the same converter.

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Noriyuki Shomitsu 12 Nakamachi, Muroran, Hokkaido Nippon Steel Corporation Muroran Works (72) Inventor Fumio Koizumi 12 Nakamachi, Muroran, Hokkaido Nippon Steel (56) References JP-A-5-140627 (JP, A) JP-A-5-247511 (JP, A) JP-A-5-302109 (JP, A) JP-A-7- 179921 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) C21C 5/28 C21C 1/02 110 C21C 1/04 101

Claims (2)

(57) [Claims] 1. When refining molten iron to produce molten steel, the molten iron is charged into a refining vessel as a first step, and dephosphorization refining or desiliconization and dephosphorization refining are performed in a second step to contain a predetermined amount of phosphorus. In the third step, the slag produced in the second step is discharged by tilting the smelting vessel as a third step, and then the second step is performed in a molten steel production method in which decarburization smelting is performed in the same smelting vessel as a fourth step. CaO / SiO _{2 in} the slag after the process is 2.
5 or less and the sum of iron oxide and manganese oxide concentrations (TF
e + MnO) is 10% or more, the MgO concentration is 10% or less, and the treatment end point temperature is 1320 ° C. or more and 140% or more.
A method for producing molten steel, wherein the temperature is set to 0 ° C. or lower. 2. When refining molten iron to produce molten steel, the molten iron is charged into a refining vessel as a first step, and a dephosphorization treatment, a desiliconization, and a dephosphorization refining are performed as a second step to contain a predetermined amount of phosphorus. The slag generated in the second step is discharged by tilting the smelting vessel as a third step, then decarburized in the same smelting vessel as a fourth step, and in the fourth step as a fifth step The steel is produced while leaving the generated slag in the refining vessel, and the process returns to the first step again. In the second step of the molten steel production method in which the above steps are repeatedly performed, CaO in the slag is removed.
/ SiO ₂ is 2.5 or less, the sum of iron oxide and manganese oxide concentrations (T.Fe + MnO) is 10% or more, and Mg
O concentration is 10% or less, and the processing end point temperature is 132
A method for producing molten steel, wherein the temperature is 0 ° C or higher and 1400 ° C or lower.