JPH0353014A - Smelting method for extremely low-sulfur steel - Google Patents

Abstract

PURPOSE:To smelt the extremely low-sulfur steel at the low cost of refractories and desulfurizing agent by stopping the oxygen blow to a molten iron at a specific C level, adding the desulfurizing agent to the molten iron to make a desulfurization treatment, then discharging slag, further adding a slag making agent for dephosphorization to the molten iron and blowing oxygen thereto. CONSTITUTION:The molten iron which is previously subjected to a desiliconization treatment is desulfurized and dephosphorized in a torpedo ladle car 1 by blowing CaO, CaF2, iron oxide, etc., from an immersion lance into the molten iron. This molten iron is then charged through a molten iron ladle 3 into a top and bottom blown converter 4. Cold iron material and the slag making agent are added into the molten iron and oxygen is blown via a top blowing lance 5 to make desiliconization and decarburization blowing. The supply of the oxygen is stopped in the stage of 0.3 to 2.5% C in the molten iron in this oxygen blowing. The desulfurizing agent is added from the top blowing lance 5 into the molten iron to forcibly desulfurize the molten iron to about 20 to 50ppm. The converter 4 is thereafter tilted to discharge desulfurizing slag 7 and the slag making agent for dephosphorization is added to the molten iron. The oxygen is then blown to this molten iron. The extremely low-sulfur steel is smelted by preventing resulfurization and accelerating the desulfurization reaction; in addition, the refining is executed in a short period of time and the cost is reduced.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to the melting of ultra-low sulfur steel in a converter.

[Conventional technology]

Generally, when sulfur is contained as an impurity in steel, it causes a decrease in brittleness due to the eutectic interaction between the steel and iron sulfide, or a deterioration in weldability due to inhibition of spheroidization of carbides.

For this reason, in order to reduce sulfur in steel, for example, as shown in Japanese Patent Application Laid-Open No. 52-19109,
There is a method in which a lance is immersed in a pig iron mixer car and a desulfurizing agent is blown in together with a carrier gas, or as shown in JP-A-56-9308, a lance is immersed in a pig iron mixer car and a desulfurizing agent is blown in with a carrier gas, such as soda ash, fluorite, etc. Various methods have been proposed, such as injecting other desulfurization and dephosphorization agents to perform desulfurization and dephosphorization at the same time, some of which have been put into practical use and have achieved considerable results.

[Problem to be solved by the invention]

However, there are the following problems with methods in which a lance is immersed in a pig iron mixer and a desulfurization agent or a desulfurization agent is blown in, or a method in which desulfurization and dephosphorization are performed simultaneously.

First, desulfurization and dephosphorization can be carried out with fairly high efficiency, but in order to obtain extremely low sulfur and low phosphorus hot metal, it is necessary to use a large amount of treatment agent, extend the treatment time, and lower the hot metal temperature accordingly. becomes.

This drop in hot metal temperature hinders converter operation.

Furthermore, desulfurization at the hot metal stage generally requires time from desulfurization to charging into a converter, and during that time, resulfurization occurs. In addition, even if extremely low sulfur and low phosphorus pig iron is smelted in the preliminary treatment described above, charged into a converter, and blown with the addition of a cold iron source and auxiliary materials, etc. The so-called resulfurization phenomenon occurs in which the sulfur content in the molten steel increases from the slag remaining from pre-blowing, the refractory lining inside the furnace, coating materials, etc.

For these reasons, the present invention performs desulfurization and dephosphorization in each process in the same converter, prevents resulfurization, and promotes the desulfurization reaction in the converter, which was previously impossible. It is an object of the present invention to provide a method for producing ultra-low sulfur steel, which enables the production of ultra-low sulfur steel of the same level as the conventional method, and also reduces the cost of refractories and desulfurization agents compared to conventional methods.

[Means to solve the problem]

When melting ultra-low sulfur steel in a converter, the present invention uses converter blowing as the first step, and performs desiliconization and decarburization blowing until C in the molten iron reaches 0.3 to 2.5%, Thereafter, a desulfurization agent is added to the molten iron to perform a desulfurization treatment, and the slag after the desulfurization treatment is discharged from the furnace.Then, as a second step, a dephosphorization agent is added and acid blowing is carried out.

Here, in the first stage of desiliconization, decarburization, or initial blowing acid for decarburization, C in the molten iron is 0.・The reason for setting it at 3 to 2.5% is that if the C content is higher than 2.5%, the temperature of the molten iron will not rise sufficiently, and the desulfurization reaction will be less effective in accelerating the desulfurization reaction compared to desulfurization treatment outside the melting furnace. I can't get it.

On the other hand, when C is blown to a level lower than 0.3%, the molten iron is oxidized, the oxygen potential in the slag increases, or the oxygen concentration in the steel increases, resulting in a slag composition that is disadvantageous for desulfurization although the temperature is high. This results in a molten iron structure. Therefore, if you try to desulfurize at this level, a large amount of deoxidizing agent (AJ.
j! dross, etc.), and the unit consumption of quicklime also increases.

In other words, when C in molten iron is stopped blowing with acid at 0.3% or more,
Since the oxygen concentration in the molten iron and slag does not increase, the desulfurization efficiency in this state is extremely high. This desulfurization may be carried out by blowing an inert gas as a carrier gas from a top blowing lance, or by immersing it in the air and performing blowing and stirring. Alternatively, the desulfurizing agent may be blown in together with the carrier gas from the bottom blowing tuyere.

Quicklime and fluorspar are used as desulfurizing agents. A material made of one or more of metal Af, aluminum dross, etc. is used.

By performing such treatment in a converter, compared to general desulfurization outside the furnace, the reaction can be accelerated by strong stirring achieved by blowing a large amount of gas, and the molten iron temperature can be raised by about 300°C due to the heat generated by decarburization of the molten iron. Since the oxygen concentration in the slag can be suppressed, desirable high-efficiency desulfurization is possible.

Furthermore, the amount of Af or aluminum dross used as a reducing agent for deoxidizing oxygen in molten iron and slag can be reduced. After the desulfurization treatment, the raw slag is removed by tilting slag, mechanical scraping, vacuum suction, or the like. This is because decarburization progresses due to the oxygen supplied during the second stage of dephosphorization and final decarburization, and when the C content becomes low, iron is oxidized and enters the slag, resulting in the desulfurization reaction CaO+S→CaS+
The equilibrium of ○ is shifted and the reverse reaction progresses, causing resulfurization.
Drain the desulfurization slag.

Next, on the second floor, quicklime and fluorite were placed in the furnace. A dephosphorizing agent consisting of one or more of dolomite, iron ore, etc. is added and acid blown. This blown acid performs dephosphorization and final decarburization to obtain molten steel with a predetermined fraction. The converter that can be applied here may be either a top blowing type or a top and bottom blowing type.

In this way, by performing the desulfurization treatment in the converter during the slag discharge, and combining it with the dephosphorization treatment after the slag is discharged, it is possible to increase the
It is possible to produce high-purity steel of QPPM or less, and at the same time, it is possible to produce high-purity steel with low phosphorus, and the overall processing cost can be reduced.

〔Example〕

FIG. 1 shows a processing flow according to an example of the present invention.

In the figure, Cab, (
: AFz, iron oxide, etc. are blown into the hot metal, which has been desiliconized in advance, to be desulfurized and dephosphorized. This hot metal is charged into a top-bottom blowing converter 4 through a hot metal ladle 3. Here, top blow lance 5
Oxygen is blown in from the furnace bottom, and some oxygen and cooling gas (C.H., C○, etc.) are blown in from the bottom tuyere 6 to carry out normal desiliconization and decarburization.

Regarding desiliconization, decarburization is mainly carried out. After this decarburization, the desulfurizing agent may be blown from the top blowing lance 5 (or the tuyere 6), or the lance 5 may be immersed in the bath and the desulfurizing agent may be blown into the bath.

Further, it is also preferable to flow a small amount of gas such as N2, ^r, CO2, etc. for tuyere protection from the tuyere 6, or rather to increase the amount to strengthen the stirring in the bath. As the desulfurizing agent, one or a combination of two or more of quicklime, fluorite, aluminum dross, metallic aluminum, etc., is injected. The representative group precepts and basic units are shown in Table 1.

Table 1 As mentioned above, by desulfurization treatment for about 15 to 20 minutes,
Strong desulfurization of 20 to 50 PPM is performed. After the desulfurization is completed, the converter 4 is tilted to scrape out the slag 7 and discharge it.

Furthermore, quicklime, a dephosphorizing agent, is placed inside the furnace. Dolomite, fluorite, etc. are added, and oxygen from the top blowing lance 5, some oxygen from the tuyere 6, and cooling gas (C.}l..C○
,〉 is blown in to complete dephosphorization and decarburization.

Through this step-by-step refining, not only extremely low sulfurization but also low phosphorus can be achieved, and high-purity steel can be easily obtained, which was previously impossible or required an increase in the load of secondary refining.

Desulfurization, which is a conventional reduction reaction, is carried out in the oxidation smelting furnace of a converter.
If a large amount of quicklime is used, only 30 to 50% desulfurization can be expected, and in extremely low sulfur steel, resulfurization occurs and the sulfur content in the steel increases.

However, in refining in which a desulfurizing agent is added to high-temperature molten iron after decarburizing blown acid in a converter, desulfurization occurs due to the high temperature of the molten iron, strong stirring, and maintaining a reducing atmosphere, and a significant improvement in the desulfurization reaction has not been achieved. Moreover, the consumption of desulfurizing agent and reducing agent can be reduced. This treatment also allows for ultra-low phosphorus, which is impossible with conventional methods. It has become possible to produce high-purity steel with low sulfur content.

Next, Table 2 shows the results of comparing the high-purity steel of the present invention with the conventional melting method using the same hot metal in a 175-ton converter.

Although this method uses about half the desulfurization agent consumption, (S)/
(S) can be greatly improved, so reaching (S) is also 20P
It was possible to achieve high purity by reducing even PM.

On the other hand, in the conventional method of decarburization refining in a normal converter, the unit consumption of the desulfurizing agent is high, and the attainment [S] is also high, making it difficult to melt and produce high-purity steel.

Table-2 This is the result using dephosphorized pig iron.

Note) Both are desulfurization outside the furnace. [Effects of the invention] As stated above, by using the method for producing ultra-low sulfur sulfur of the present invention, extremely low sulfur can be produced, which was conventionally impossible to produce by in-furnace resulfurization. In addition to making it possible to manufacture steel, it also enables the production of high-purity steel with low sulfur and low phosphorus. In addition, since smelting can be carried out in a short time, it is possible to significantly reduce the unit consumption, reduce costs by omitting secondary smelting processes, and eliminate the need to secure washing materials for the converter, making production planning easier. Excellent effects such as making it extremely easy to do the following can be expected.

[Brief explanation of drawings]

FIG. 1 shows an example of a processing flow according to the present invention.

Claims (1)

[Claims] 1. Oxygen blowing is performed by adding molten iron, cold iron material, and slag-forming agent, and when the C content in the molten iron is 0.3 to 2.5%, the supply of oxygen is stopped, and a desulfurization agent is added to the molten iron. 1. A method for producing ultra-low sulfur steel, which comprises performing a desulfurization treatment on the steel, then draining the desulfurization slag, and then adding a dephosphorization slag forming agent and blowing acid.