JPS60218408A - Pretreatment of molten iron - Google Patents

Abstract

PURPOSE:To prevent rephosphorization during dephosphorization utilizing dephosphorization slag produced as a by-product by adding a lime-base flux to molten iron, by adding molten converter slag. CONSTITUTION:Molten iron is transferred from a blast furnace 1 to a desiliconizing stage 2, where it is desiliconized. To the desiliconized molten iron are successively added molten converter slag from a converter 4 and dephosphorization slag produced as a by-product in the following dephosphorizing stage 3. The dephosphorization slag may be blended with the molten converter slag and added to the desiliconized molten iron. They are transferred to the dephosphorizing stage 3, where a lime-base flux is added to dephosphorize the molten iron. The dephosphorized molten iron is transferred to the converter 4, where steel is manufactured.

Description

FIELD OF THE INVENTION The present invention relates to a method for pre-treating hot metal, and more particularly to a method for pre-treating hot metal by recycling dephosphorization slag obtained by adding lime-based flux to hot metal.

(Prior art) In recent years, the importance of preliminary treatment of hot metal has been widely recognized, and its use has shifted from the conventional perspective of simply utilizing slag as a waste resource. However, pretreatment of hot metal has been carried out to improve the properties of steel, especially in recent years to produce clean steel.

In the past, many types of hot metal pretreatment methods have been proposed, including the use of slag as a resource, and many have actually been adopted.
The method disclosed in No. 5-34693 is to perform dephosphorization using dephosphorization flux in a so-called torpedo car, charge only the hot metal into a converter, and then drain the dephosphorization slag left in the torpedo car. After this, the method repeats the pouring, desiliconization treatment, and after removing the desiliconization slag, the dephosphorization treatment is repeated.

According to this method, desiliconization, dephosphorization, and charging of hot metal to a converter are performed separately, and slag removal is performed at the end of each treatment operation.

On the other hand, this improved method is disclosed in JP-A-57-123911, in which hot metal that has been dephosphorized is taken out, raw hot metal is charged into a torpedo car with residual dephosphorization slag, and desiliconization is performed. This method involves reusing the dephosphorizing slag as a desiliconizing agent for desiliconizing hot metal. The basicity has been lowered from about 4 to about 2, which has the effect of making dephosphorized slag usable as roadbed material, etc. However, such dephosphorization slag is produced when the hot metal temperature is about 1300°C. On the other hand, desiliconized hot metal has a temperature of about 1500 to 1400°C, so when the above-mentioned dephosphorization slag is added to desiliconized hot metal, the P2O5 in the slag will rephosphorize into the hot metal. This is clear from, for example, the dephosphorization equilibrium equation below. The above publication states that there is no problem with rephosphorization if the basicity is 1.5 or higher, but if the hot metal temperature is 140
In a range where the temperature exceeds 0°C, rephosphorization into hot metal cannot be avoided.

Kp-(P2(>s)/CP)”・(Fe
d)'By the way, the dephosphorizing ability of converter slag is well known; for example, Japanese Patent Publication No. 55-30042 discloses an example of the use of a dephosphorizing agent mainly composed of converter slag and iron oxide. . Similarly, Japanese Patent Publication No. 55-24487 discloses an example in which converter slag is used for dephosphorization and desulfurization of hot metal. In this way, so-called converter slag is known to be used as a dephosphorizing agent in hot metal, but in order to prevent rephosphorization, converter slag is used alone or in a mixture with iron oxide to prevent rephosphorization. It is merely added and used as a.

(Objective of the Invention) An object of the present invention is to provide a hot metal pretreatment method that reuses dephosphorization slag obtained by adding lime-based flux to hot metal.

Another object of the present invention is to not only prevent rephosphorization but also promote dephosphorization in the preliminary treatment of hot metal in which dephosphorization slag obtained by adding lime-based flux to hot metal is reused. An object of the present invention is to provide a method for preliminary treatment of hot metal by desiliconization treatment.

Still another object of the present invention is that the hot metal temperature is 1400 to 15
It is an object of the present invention to provide a pretreatment method for desiliconized hot metal that can effectively prevent rephosphorization even at temperatures as high as 00°C.

(Summary of the Invention) As a result of various studies to achieve the above object, the present inventors found that converter slag is generally produced at a temperature of about 1,650°C, and therefore, the temperature of hot metal is about 1,500°C.
Based on the assumption that although it does not necessarily exhibit dephosphorizing ability for hot metal at ~1400°C, it does at least have the ability to inhibit rephosphorization, we conducted repeated research and added lime-based flux to hot metal. In the desiliconization treatment performed using dephosphorization slag (hereinafter referred to as lime-based flux dephosphorization slag) obtained by
The desiliconization slag obtained by charging the molten converter slag into a torpedo car and then mixing the dephosphorization slag and the converter slag not only has an excellent rephosphorization inhibiting ability but also The present invention was completed by discovering that it exhibits a slight dephosphorizing ability.

Here, the present invention provides (a step of performing a desiliconization treatment on al hot metal;
Each step of adding molten converter slag to the lime-based flux dephosphorization slag when adding the lime-based flux dephosphorization slag in step (b) above; This is a hot metal pretreatment method that includes.

In the present invention, the molten converter slag may be charged at any time before receiving new hot metal or afterward until the dephosphorization process is completed.
It is preferable to remove the slag before the dephosphorization treatment. Further, it is preferable to discharge the slag before charging the molten converter slag.

Here, the lime-based flux refers to a hot metal dephosphorizing agent containing CaO as a main component. On the other hand, molten converter slag is slag in a molten state that is generated during dephosphorization treatment in a converter. Generally, molten converter slag is in the range of 1400-1650''c, but the present invention is not particularly limited by its composition. Any converter slag obtained under normal operating conditions can be utilized in the present invention.

By charging the charging ratio of molten converter slag to lime-based Franks dephosphorization slag at a rate higher than the calculated value calculated from the dephosphorization equilibrium equation, which is expected to prevent rephosphorization, the ability to inhibit rephosphorization can be effectively increased. It is possible to control it accurately and appropriately.

(Aspects of the Invention) According to one preferred embodiment of the present invention, slag after hot metal dephosphorization with lime-based flux is left in a hot metal transport container, for example, a torpedo car, and hot metal and slag that have been separately desiliconized are transferred. In a hot metal pretreatment method in which the basicity of desiliconized slag is adjusted by receiving it in a hot metal transport container, molten converter slag is introduced into the hot metal before or after receiving the desiliconized hot metal to effectively rephosphorize it. A pretreatment method for hot metal that prevents phosphorization or even performs light dephosphorization in some cases is required.

The invention will now be further explained in connection with the accompanying drawings, which show a flow diagram of the process according to the invention, in which hot metal from blast furnace stage 1 is first sent to desiliconization treatment stage 2. A desiliconization treatment is carried out, for example with a desiliconization agent such as iron oxide, and then the dephosphorization slag obtained in the subsequent dephosphorization treatment stage 3 is added thereto. At this time, the converter slag from the converter treatment stage 4 is sent in a molten state to this desiliconization treatment stage 2, and is added at an appropriate stage after the desiliconization treatment, or is mixed with the dephosphorization slag in advance. .

According to one aspect of the present invention, lime-based Franks phosphorus slag and molten converter slag are mixed in advance, and this is added to desiliconized hot metal. In this case, the above mixed slag may be added to the hot metal after removing the desiliconization slag. Note that in another embodiment, the molten converter slag may be added afterwards.

According to yet another aspect of the present invention, lime-based flux dephosphorization slag may be added first to desiliconized hot metal, and then molten converter slag may be added.

In that case as well, in one embodiment, the lime-based Franks dephosphorization slag and further the molten converter slag may be added after the desiliconization slag is removed.

Furthermore, according to another aspect, molten converter slag may be added to the desiliconized hot metal first, and in that case, the desiliconized slag may be removed and then the molten converter slag is added. good.

Thus, according to the present invention, not only the problem of rephosphorization that was observed in the conventional hot metal processing method using hot metal dephosphorization slag is completely overcome, but also a small amount of dephosphorization is performed in some cases, and an inexpensive method is used. This makes dephosphorization possible.

Next, the present invention will be explained by examples.

Decharred lime flux dephosphorization slag (CaO basic unit 19 kg/T) with the composition shown in Table 1 was obtained by charging the hot metal with m-calcination treatment, and the composition shown in Table 2. The molten converter slag (unit consumption: 30 kg/T) was received in a torpedo car, and then hot metal that had been desiliconized with iron oxide was charged. The composition of the hot metal at this time is shown in Table 3. The Si removal rate was 78%.

As a result, the basicity of the slag changed from 4 to 2.5.

Next, as a comparative example, the desiliconized hot metal was charged into the torpedo car to which the lime-based Franks dephosphorization slag was added without receiving the molten converter slag.

At this time, the P content of the blast furnace pig iron and torpedo car receiver was determined immediately after charging and at each time point after the dephosphorization treatment, and the state of dephosphorization and the degree of rephosphorization at each treatment stage were observed.

The above results are summarized in Table 4 along with each test condition. It can be seen that according to the present invention, no rephosphorization is observed, and that the dephosphorization is ultimately reduced to a significantly low degree.

[Brief explanation of drawings]

The accompanying drawings are flow diagrams of the method according to the present invention. 1: Blast furnace stage 2: Scaling treatment stage 3: Dephosphorization treatment stage 4: Converter treatment stage Applicant: Sumitomo Metal Industries, Ltd. Agent Patent Attorney Akira Hirose -

Claims (4)

[Claims] (1) (Step of performing desiliconization treatment on al hot metal; lb) A step of adding lime-based flux dephosphorization slag to the desiliconized hot metal obtained from step i8); and IcI lime-based flux dephosphorization slag in the above tb+ step A method for pre-treatment of hot metal comprising the steps of: adding molten converter slag to the lime-based flux dephosphorization slag when adding the flux dephosphorization slag. (2) The method according to claim 1, wherein the molten converter slag in step (C) is added before adding the lime-based flux dephosphorization slag to the desiliconized hot metal. (3) The method according to claim 2, wherein the molten converter slag is added after desiliconizing hot metal is slaged. (4) The method according to claim 1, wherein the molten converter slag in the C1 stage is added after the lime-based flux dephosphorization slag is added to the desiliconized hot metal.