JP3301683B2 - Desulfurization method of chromium-containing molten steel in decarburization refining - 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 desulfurizing chromium-containing molten steel in decarburization and refining, and to perform efficient desulfurization when decarburizing and refining chromium-containing molten steel.

[0002]

2. Description of the Related Art There are mainly two methods for desulfurizing molten chromium-containing steel as described below. One is to enhance reductive desulfurization simultaneously with the reduction of chromium oxide in the smelting reduction process, which is a process of producing chromium-containing crude molten steel (or hot metal) (for example, JP-A-2-232312). The other one is to simultaneously perform reductive desulfurization in the process of reducing and recovering chromium oxide in slag using a reducing agent such as FeSi or Al in the decarburization refining of chromium-containing molten steel after decarburization (for example, JP-A-63-140044, etc.). Also,
Similarly, there is a method of adding a flux to a medium carbon chromium-containing molten metal to keep CaO / SiO ₂ constant, and then adding a desulfurizing agent such as CaC ₂ .

[0003]

As described above, the most common method of desulfurizing chromium-containing molten steel is desulfurization accompanying the reduction of chromium oxide, but in such a method, chromium-containing crude steel is melt-reduced. Without the process of melting
Desulfurization is not sufficient in the smelting method without a reduction step, such as in the case of decarburizing and refining by adding only FeCr (FeCr method), or in the unreduced tapping method that does not use a reducing agent such as FeSi after decarburizing and refining. There was a problem.

[0004] Even in the case of performing reductive desulfurization, when NiS (Ni mat), which is an inexpensive high S-containing material, is used as an auxiliary material, the sulfur content is reduced to a satisfactory level by desulfurization by a slag-metal reaction. Could not be converted.
Further, during the decarburization refining, the method of adding desulfurization flux during the high carbon concentration period to perform desulfurization is based on the slag
Although S migrates into the slag due to the metal reaction, there is a problem that, up to the low-carbon period in which decarburization has progressed, return S from the slag to the metal occurs due to an increase in the oxygen concentration in the steel.

[0005] The present invention advantageously solves the above-mentioned problems, not only in the decarburization and refining of chromium-containing molten steel by an unreduced method without a reduction step, but also when a secondary material having a high S concentration is used. Another object of the present invention is to propose a method of desulfurizing chromium-containing molten steel in decarburization refining, which can strongly push desulfurization to obtain a sufficiently satisfactory low-S concentration chromium-containing molten steel.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in order to solve the above-mentioned problems. Was added to transfer S from the molten steel into the slag, and then it was found that vaporization and desulfurization was promoted by blowing oxygen gas onto the bath surface from the top blowing lance.
Further, since the effect of the vaporization and desulfurization is improved by increasing the flow rate of the top-blown oxygen gas and the addition amount of the de-S-flux, even when a secondary material having a high S concentration is used, a sufficiently satisfactory low-S steel is used. Was also found. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. In the decarburization refining of chromium-containing molten steel, which involves adding a carbon source and auxiliary raw materials and performing oxygen blowing into the chromium-containing hot metal charged in a vessel having a top and bottom blowing function for refining gas, In the decarburization refining, wherein high-carbon concentration period, high-base de-S flux is added to transfer S in the steel into the slag, and oxygen gas is blown from the top blowing lance to vaporize and desulfurize. Chromium-containing molten steel desulfurization method. 2. In the above (1), the carbon source is added so that the carbon in the molten steel is maintained at a saturated concentration from the start of the decarburization refining until the temperature of the molten steel reaches 1500 ° C. Chromium molten steel desulfurization method. 3. In the above 1 or 2, the highly basic de-S-flux is an alkali metal or alkaline earth metal oxide,
A method for desulfurizing molten chromium-containing steel in decarburization refining, which is at least one selected from carbonates and chlorides. 4. In the above 1, 2, or 3, a method for desulfurizing chromium-containing molten steel in decarburization refining in which the auxiliary material is a Cr or Ni material having a high S concentration. 5. In the above 1, 2, 3 or 4, the chromium-containing molten steel in the decarburization refining comprises adding a carbon source in an amount such that the C concentration in the steel is 1 wt% or more and the bath temperature is 1650 ° C or more at the beginning of the decarburization refining. Desulfurization method.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The desulfurization reaction by the slag-metal reaction is represented by the following formula [S] + O ^2- = (S ^2- ) + 1 / 2O ₂ , and the lower the oxygen potential, the more advantageous. Therefore, the conventional reductive desulfurization is performed in a state where oxygen in steel is reduced by carbon saturation or deoxidation by a reducing agent such as FeSi. During the decarburization refining, desulfurization easily progresses during the high carbon concentration period immediately after the introduction of the carbon source, that is, at the early stage of the decarburization refining. Therefore, it is considered effective to add a desulfurization flux in such a high carbon concentration period. However, S in the slag desulfurized in the high carbon region is restored when the oxygen potential increases at the end of decarburization. I will. Therefore, in the method of the present invention, S transferred into the slag by the S-free flux during the high carbon concentration period is vaporized into the gas phase by blowing oxygen gas.

The vaporization desulfurization is expressed by the following two equations (S ²⁻ ) + 1 / 2O ₂ = 1 / 2S ₂ + O ²⁻ (CaS) + 3 / 2O ₂ = SO ₂ + (CaO) Is done. Thermodynamically vaporized desulfurization reaction, the reaction system of the oxygen partial pressure P (O ₂₎ occurs at 10 ^-4 atm or more. However, in the decarburization refining initial is ^{_{P (O 2) <10 -10}} , never vaporized desulfurization occurs. However, when oxygen is blown onto the bath surface during the decarburization refining, the portion of the slag to which oxygen has been blown has a locally high oxygen potential, so that vaporization desulfurization proceeds. For this purpose, the higher the S concentration in the slag and the higher the bath temperature, the more advantageous.

[0010] In order to promote the vaporization desulfurization by increasing the S concentration in the slag, a flux having a high desulfurization ability is added during the high carbon concentration period to transfer S into the slag to increase the S concentration.
In this state, oxygen gas may be sprayed onto the bath surface. In such a vaporization desulfurization, S can be discharged out of the system, so that there is no occurrence of re-sulfuration even when a high oxygen potential state is reached at the end of decarburization as in the conventional case.

[0011] In There apply in this invention, when the addition of the carbon source, for the period from at least decarburization refining begin until the molten steel temperature is 1500 ° C., the carbon in the molten steel is added to maintain the saturation concentration Is preferred. This is because Cr oxidation is likely to occur in the low-temperature region where the molten steel temperature is 1500 ° C or less, and there is concern about the oxidation loss of Cr.In this regard, if there is excessive carbon in the molten steel, the excess carbon causes the oxidation loss. This is because Cr can be reduced and the temperature of molten steel can be effectively increased by a carbon oxidation reaction.

[0012] As a flux having a high desulfurization ability,
A highly basic de-S-flux consisting of at least one selected from the group consisting of oxides, carbonates and chlorides of alkali metals or alkaline earth metals is preferred. Among them, sodium carbonate, barium carbonate, sodium chloride, soda ash, etc. Are particularly advantageously adapted. All of these are high de-S (de-P) fluxes having an optical basicity of 0.8 or more. In addition, about the addition amount of such de-S flux,
It is preferable that the weight is about 10 to 15 kg per ton of molten steel.
In addition, auxiliary materials having a high S concentration are mainly composed of C such as FeCr and Met-Ni.
r and Ni sources, and carbon sources include coke, graphite,
Such as coal.

[0013] As described above, in shifting the S in the slag is advantageously higher the carbon concentration, and because the taking <br/> the vaporization desulfurization advantageous the higher the bath temperature, de S Prior to the addition of the flux, it is preferable to add a predetermined amount of a carbon source prior to the addition so that the C concentration in the steel is 1 wt% or more and the bath temperature is 1650 ° C or more. Incidentally, the reaction of the de-S flux, so changed by top-blown oxygen gas flow rate, in order to achieve good air hydrodesulfurization is top-blown oxygen gas flow rate of about 1.8~2.5 Nm ³ / min / t, L / L ₀ ≧ 0.08 where L: Depth of steel bath (according to Segawa et al.) L ₀ : Depth of steel bath It is preferable to perform the spraying under the condition.

FIGS. 1 (a), 1 (b) and 1 (c) show that when decarburization and refining are performed under normal conditions, and when a carbon source is added at the beginning of refining, the carbon source is added at the beginning of refining according to the present invention. together after addition of high-de S flux (soda ash), when carrying out the vaporization desulfurization, in steel C, and findings on temporal changes in S and bath temperature steels are shown organized. The flow rates of the top and bottom blown gases are as shown in FIGS. 2 (a) and 2 (b). As shown in FIG. 1 (a), when decarburization and refining were performed under normal conditions, after decarburization and refining, S in steel: 0.014 wt
% Could only be removed. In this regard,
When a carbon source was added at the beginning of refining, the de-S ability was improved by the extension of the high carbon concentration period.
The concentration was only about 0.008 wt% (Fig. (B)). On the other hand, after adding the carbon source, soda ash was
In the case of accelerated desulfurization, a chromium-containing molten steel with an extremely low S of 0.003 wt% was finally obtained (Fig. 3 (c)).
).

[0015]

【Example】

Example 1 Decarburization refining was carried out under the conditions shown in Table 1 using an upper-bottom blow converter. In the experiment, first, 10 kg / t of coke was added as a carbon source to the chromium-containing crude molten steel to start decarburization, and 10 kg / t of soda ash was added at a bath temperature of 1600 ° C as a guide. Next, blowing was interrupted with [C] = 1wt% as a guide, slag and metal sampling were performed by falling down the furnace, and then [C] = 0.1
The target was wt%. Table 2 summarizes the results of a study on the steel bath temperature, the amount of C in steel, the amount of S in steel and the amount of S in slag, the amount of S mixed in from auxiliary materials and the like, and the amount of S removed before and after the experiment. The amount of vaporization and desulfurization was determined from the S balance during the experiment as shown in FIG.

[0016]

[Table 1]

[0017]

[Table 2]

As is clear from Table 2, according to the present invention, when soda ash is added during the steel carbon concentration period to promote vaporization desulfurization, the S concentration is reduced to an extremely low S region of 0.005 wt%. Was completed.

Example 2 10 kg of a high sulfur content Ni mat (Ni-40% S) as an auxiliary material
/ t-Ni content was used. The experimental conditions were the same as in Example 1. Coke was added so that the temperature was initially raised to [% C]> 1,1650 ° C. or higher, and then 10 kg / t of soda ash was added. FIG. 4 shows the results of examining the changes over time of [% C], [% S], and (% S) during refining. As is clear from the figure, when the decarburization refining is performed according to the present invention, the S concentration can be finally reduced to 0.006 wt% despite the use of the secondary material having a high S content. Was.
According to the conventional reduction method, the desulfurization effect in the case of performing desulfurization simultaneously with the reduction of chromium oxide was also investigated. However, when the secondary material containing high S is used, the reduction can be reduced to only 0.018 wt%. could not.

[0020]

As described above, according to the present invention, desulfurization can be performed to a much lower level than in the prior art without extending the refining time during the decarburization refining of the chromium-containing molten steel. As a result, a chromium-containing molten steel having a low S concentration can be obtained even in the unreduced method. In addition, it becomes possible to use an auxiliary material having a high S concentration, which has not been able to be used heretofore, thereby effectively contributing to a reduction in manufacturing cost. Furthermore, by raising the temperature at the early stage of decarburization for desulfurization, chromium oxidation can be advantageously reduced thermodynamically and decarburization can proceed with high efficiency. In addition, since dephosphorization occurs at the same time as desulfurization, not only can the burden of pretreatment be reduced, but also the effect that there is no need to use expensive low-P raw materials is added.

[Brief description of the drawings]

FIG. 1 is a graph showing changes over time in C in steel, S in steel, and bath temperature during decarburization refining.

FIG. 2 is a graph showing the flow rates of top blown gas and bottom blown gas during decarburization refining.

FIG. 3 is a diagram showing an example of an S balance during an experiment.

[Fig. 4] [% C], [% S],
It is a graph which showed a time-dependent change of (% S).

Continuing from the front page (72) Inventor Bessho Nagayasu 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Prefecture Kawasaki Steel Engineering Co., Ltd. (72) Inventor Hiroshi Nishikawa 1-Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Inside steelworks (56) References JP-A-2-23212 (JP, A) JP-A-1-165709 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21C 5/28 C21C 7/064 C21C 7/076

Claims (5)

(57) [Claims] 1. A decarburizing and refining process for molten chromium-containing steel, which comprises adding a carbon source and an auxiliary material to a chromium-containing hot metal charged in a vessel having a top and bottom blowing function of a refining gas and performing oxygen blowing. In the high carbon concentration period in the early stage of refining, a highly basic de-S flux is added to transfer S in the steel into the slag, and oxygen gas is blown from the top blowing lance to vaporize and desulfurize. Of chromium-containing molten steel in decarburization refining. 2. The method according to claim 1, wherein the carbon source is added so that the carbon in the molten steel maintains a saturated concentration from the start of the decarburization refining until the temperature of the molten steel reaches 1500 ° C. Of chromium-containing molten steel in decarburization refining. 3. The decarburization refining according to claim 1, wherein the highly basic de-S flux is at least one selected from the group consisting of oxides, carbonates and chlorides of alkali metals or alkaline earth metals. Chromium-containing molten steel desulfurization method. 4. The method for desulfurizing a chromium-containing molten steel in decarburization refining according to claim 1, 2 or 3, wherein the auxiliary material is a Cr or Ni material having a high S concentration. 5. The steel according to claim 1, wherein the C concentration in the steel is 1 wt% or more and the bath temperature is 1650 ° C. in the initial stage of the decarburization refining.
A method for desulfurizing chromium-containing molten steel in decarburization refining comprising adding the above amount of carbon source.