JPH05247511A - Method for dephosphorizing molten iron - Google Patents

Abstract

PURPOSE:To provide a process in which dephosphorizing is executed in a short time and successively decarburization is executed by using the same converter. CONSTITUTION:At the time of producing a molten steel by refining molten iron as a first stage, the molten iron is charged into the converter and as a second stage, by executing flux addition and oxygen top-blowing, the dephosphorizing refining is executed so as to reduce the phosphorus content in the molten iron to a prescribed level and as a third stage, the converter is tilted to discharge slag produced at the second stage. Thereafter, the decarburization is executed in the same converter, and the molten steel is tapped leaving the slag in the converter, and in the second stage recycling the slag to the first stage in the molten steel producing method, CaO/SiO2 in the slag is made to be 1.1-2.5 and the top blowing oxygen feeding rate is made to be >=2.5Nm<3>/min/t.

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 hot metal in a converter.

[0002]

2. Description of the Related Art For minimizing the total cost of steelmaking and stable hot metal for low phosphorus steel, the conventional hot metal dephosphorization method is as follows: (1) A method for injecting a flux for dephosphorization into hot metal in a torpedo car for preliminary dephosphorization, (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 to perform dephosphorization on one side and dephosphorization on the other side. A method of performing charcoal (see, for example, JP-A-63-19521).
No. 0) is used.

However, the above (1), (2),
All of the methods (3) have the disadvantages that the hot metal needs to be transferred when the dephosphorization step moves to the decarburization step, the temperature must be lowered, and the energy loss is large. Also,
In the processes of (1) and (2), since a torpedo car and a hot metal ladle are used, if an attempt is made to increase the rate of top-blown acid feeding to shorten the processing time, foaming of the slag will cause operational problems. Therefore, process (1)
It was the current situation that the shortest treatment time of the dephosphorization step was about 20 minutes in the process and about 11 minutes in the process (2). Also, in the process (3), the acid transfer rate is 1.0 Nm ³ / min / t at most.
The operation time was about 10 minutes.

[0004] In the specification of Japanese Patent Application No. 2-181989, the present applicant concentrated the refining function over the conventional multi-steps in the converter to significantly reduce the energy loss of the hot metal and fixed costs before and after the converter. We have proposed a method that can significantly reduce (equipment costs, labor costs).

FIG. 2 shows this flow. As a first step, hot metal is charged into a converter, and as a second step, flux addition and oxygen top blowing are performed to perform dephosphorization / descaling refining. As a third step, the converter is tilted to remove the slag generated in the second step, and as a fourth step, decarburization is performed to a predetermined C content by adding flux and blowing O _2. As the fifth step, the slag produced in the fourth step is tapped while being left in the converter, the process is returned to the first step, and the steps up to the fifth step are repeated.

[0006]

If a process of continuously performing the dephosphorization and decarburization steps using the same converter is used, it is possible to reduce the energy loss during the transfer from the dephosphorization step to the decarburization step. Instead, a converter with a large freeboard on the top can be used. On the other hand, there is a need to shorten the dephosphorization treatment time by increasing the acid transfer rate. The present invention provides a method for dephosphorizing hot metal, which improves productivity by shortening the dephosphorization treatment time.

[0007]

In the present invention, when refining hot metal to produce molten steel, the hot metal is charged into a converter as a first step, and flux addition and oxygen top blowing are performed as a second step. Dephosphorization refining to reduce the phosphorus content to a specified level, tilt the converter as the third step to discharge the slag produced in the second step, and then perform the decarburization step in the same converter to remove the slag. In the second step of the molten steel manufacturing method, in which the slag is tapped while it is left in the converter, and the slag is recycled to the first step, CaO / SiO ₂ in the slag is 1.1 or more and 2.5 or less, and top blowing It is a method for dephosphorizing hot metal, characterized in that the acid transfer rate is 2.5 Nm ³ / min / t or more.

The present invention will be described in detail below. The present invention integrates the hot metal pretreatment and decarburization and operates in the same converter.
That is, as shown in FIG. 2, one or a plurality of bottom blowing tuyere for blowing dephosphorization and decarburizing flux to the bottom of the furnace, and a plurality of blowing gas for slag foaming to the facing furnace side of the tapping hole. Charge the hot metal into the upper and lower blowing converter equipped with individual tuyere,
A flux based on quicklime powder is blown from the above-mentioned bottom blowing tuyere using an inert gas such as N ₂ as a carrier gas.

At this time, the dephosphorization reaction rate can be increased by mixing the iron oxide powder with the quicklime powder or by forming the tuyere with a triple tube structure and blowing O ₂ gas through the same tuyere. Alternatively, the dephosphorization can be promoted by controlling the [Fe 0%] of the produced slag by spraying O ₂ gas from the top-blown lance and adding flux from the above by adding, blowing, or spraying flux. can do. When the P and C contents are reduced to the predetermined values, the furnace is tilted to the side of the unextruded steel (slag side) and only the slag is discharged.

Immediately after the slag is exhausted, the furnace is immediately erected and a small amount of auxiliary raw materials (refractory protection, quick lime for preventing P recovery, dolomite, iron ore, etc.) are put in to carry out normal upper-bottom blown decarburization refining. To do. After the blowing is stopped, the molten steel is tapped, but the slag is left in the furnace as it is and used as the molten iron pretreatment slag for the next charge.

By the way, if the dephosphorization and decarburization steps are continuously performed in the same converter, not only energy loss at the time of shifting from the dephosphorization step to the decarburization step can be reduced, but also a converter with a large upper freeboard. Since it can be used, it is possible to shorten the dephosphorization treatment time by increasing the acid transfer rate. However, until now, it was thought that when the rate of oxygen transfer was made considerably high, the efficiency of dephosphorization and oxygen reduction decreased, and the improvement of the dephosphorization rate could not be expected so much.

The dephosphorization oxygen efficiency referred to herein is generally expressed by the following equation. Dephosphorization oxygen efficiency = {Amount of oxygen used for dephosphorization (Nm ³ / t)
/ (Total blown oxygen amount (acid transfer rate × time) (Nm ³ / t) + oxygen amount in fed iron oxide (Nm ³ / t))} × 100 (%) However, according to the experiments by the present inventors, At a high acid transfer rate of more than 2.5 Nm ³ / min / t, the dephosphorization oxygen efficiency was not significantly decreased, the dephosphorization rate constant was improved, and the processing time of the dephosphorization process was shortened. It was confirmed to be possible.

[0013]

EXAMPLE A phosphorus removal experiment was carried out using an 8t test converter. Hot metal containing 4.5% C, 0.1% P and 0.3% Si and having an initial temperature of 1300 ° C. was smelted for 10 minutes. A predetermined amount of CaO was added as flux, and the acid transfer rate was 2.5.
˜3.6 Nm ³ / min / t. For comparison, an experiment was also conducted at an acid transfer rate of 1.1 Nm ³ / min / t. FIG. 1 shows changes with time in the [P] concentration in the hot metal in each case.
At this time, CaO / SiO ₂ was set to 1.1 to 1.5.
By setting the acid transfer rate to 2.5 Nm ³ / min / t or more, the processing time could be shortened by about 4 minutes as compared with the case where the acid transfer rate was 1.1 Nm ³ / min / t.

FIG. 2 shows the relationship between the oxygen transfer rate and the first-order phosphorus removal rate constant (K ' _p ) in the conventional process (1) in an actual machine,
It is shown together with the values of (2) and (3). CaO / S after refining
When iO ₂ was 0.6 to 1.1, only the dephosphorization rate constant equivalent to that of the conventional process (1) using torpedo or the conventional process (2) using ladle was obtained. However, Ca
It was confirmed that the dephosphorization rate constant about twice that of the conventional process (3) was obtained by setting O / SiO ₂ to 1.1 or more and 2.5 or less.

[0015]

According to the present invention, the processing time can be shortened and the productivity can be improved as compared with the conventional dephosphorization method.

[Brief description of drawings]

FIG. 1 is a chart showing changes with time of [P] concentration in hot metal.

FIG. 2 is a chart showing the relationship between the primary dephosphorization rate constant and the acid transfer rate.

FIG. 3 is a schematic explanatory diagram of a refining process using the same converter.

Claims (1)

[Claims] 1. When refining hot metal to produce molten steel, the hot metal is charged into a converter as a first step, and flux addition and oxygen top blowing are performed as a second step to perform dephosphorization refining. To the phosphorus content in the second step, tilt the converter as the third step to discharge the slag generated in the second step, then perform the decarburization step in the same converter, and leave the slag in the converter as it is. In the second step of the molten steel manufacturing method in which steel is recycled to the first step, CaO / SiO in the slag
_{2. The} method for dephosphorizing hot metal, characterized in that ₂ is 1.1 or more and 2.5 or less, and the rate of top-blown acid feeding is 2.5 Nm ³ / min / t or more.