JP2001323314A - Method for desulfurizing stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently perform desulfurizing-refining by surely confirming the progressing state and the desulfurizing ability in the melting or the refining processes of a stainless steel. SOLUTION: In the processes for melting the stainless steel in an electric furnace and for refining in a refining furnace, (MnO) concentration in the slag is analyzed to grasp the progressing state and the desulfurizing capacity from this (MnO) concentration. Based on this grasped progressing state and desulfurizing capacity of the desulfurizing reaction, the stirring of the molten steel is performed and/or flux for desulfurizing is added.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a process for grasping the progress of desulfurization in the melting and / or refining process of stainless steel, and for allowing the desulfurization reaction to proceed accordingly.
The present invention relates to a method for efficiently performing a desulfurization treatment.

[0002]

2. Description of the Related Art In the manufacturing process of stainless steel containing Cr of 11 mass% or more, raw materials such as scrap and ferromagnetic iron are melted in an electric furnace to produce a coarse molten steel, and then a top and bottom blown converter, an AO
Refining is performed in refining furnaces such as D furnace and VOD furnace.
After adjusting the temperature and components in the ladle, continuous casting is performed, and intermediate products such as slabs and blooms are manufactured.

[S] contained in stainless steel is treated as an impurity, except for free-cutting stainless steel, which actively adds [S] to ensure machinability, and is treated as hot workability, corrosion resistance or mechanical properties. In general, to ensure the nature,
It is desulfurized to 100 ppm or less and 40 ppm or less for high-purity steel.
Recently, less than 10 ppm of stainless steel has also been produced.

[S] in stainless steel is inevitably brought in from raw materials such as scrap and ferromagnetic iron. In order to reduce the [S] concentration of the target product, it is necessary to use a melting step and a refining step. If the desulfurization capacity is insufficient with only an electric furnace and a refining furnace,
After melting in an electric furnace or refining in a refining furnace, a method of performing desulfurization treatment in a ladle has been adopted. In recent years, there has been a strong demand for lowering the target [S] concentration in order to improve product quality, while reducing raw material costs.
[S] The use ratio of inexpensive raw materials having a high concentration is increasing, and the importance of desulfurization treatment is increasing.

[0005] Desulfurization is mainly carried out by bringing [S] in molten steel into contact with molten slag, reacting it, and transferring it into the slag. Therefore, the means for improving the desulfurization ability is gas stirring. a method of enhancing the contact between the molten steel and slag, CaO -CaF ₂ system, with the addition of desulfurizing flux such as CaO -Al ₂ O ₃ system, basicity of the product slag _{[(CaO) / (SiO 2} )]
To increase the desulfurization ability of the slag itself.

With these methods, the desulfurization ability can be improved, but the progress of the desulfurization reaction, such as whether the desulfurization reaction can still proceed or is at a limit, cannot be grasped. As a result, the gas stirring time is excessively increased or the slag basicity is excessively increased, so that the production cost is increased.

As means for obtaining a high desulfurization ability stably by refining ordinary steel, for example, Japanese Patent Application Laid-Open No. 9-217110 and Japanese Patent Application Laid-Open No. 63-100122 disclose such means. In these methods, a high desulfurization ability is obtained by promoting the reduction reaction of slag and reducing the (FeO) and (MnO) concentrations in slag.
Japanese Patent No. 17110 discloses that a high desulfurization ability can be stably obtained by controlling the (FeO) + (MnO) concentration in the slag to 0.6% or less.

On the other hand, with respect to stainless steel, many methods have been disclosed for improving the desulfurization ability by adding a flux or strengthening the stirring.
No means for obtaining high desulfurization capacity is disclosed. Also,
When the method for obtaining a high desulfurization ability according to JP-A-9-217110 is applied to stainless steel, (Fe
No correlation between O) + (MnO) concentration and desulfurization ability was observed, and
In the desulfurization treatment in the smelting furnace and the desulfurization treatment in the ladle after the treatment in the smelting furnace, (FeO) + (MnO)
It has been confirmed that since the concentration satisfies 0.6% or less, it cannot be used as a means for determining the desulfurization ability.

[0009]

DISCLOSURE OF THE INVENTION The present invention has been found to solve such a problem. The present invention analyzes the (MnO) concentration in slag during desulfurization treatment, and determines the progress of the desulfurization reaction from the value. By grasping the state and providing and managing a "threshold" for obtaining a high desulfurization ability, the desulfurization treatment is performed efficiently and the production cost is reduced.

[0010]

A method for desulfurizing stainless steel according to the present invention which achieves the above objects has the following features (1) to (5). (1) A method of melting stainless steel in an electric furnace and performing refining in a refining furnace, wherein the desulfurization ability is adjusted according to the (MnO) concentration in the slag.

(2) In the method of melting stainless steel in an electric furnace, the molten steel is stirred and / or a desulfurizing flux is added so that the (MnO) concentration in the slag is 1.00 mass% or less. It is characterized by the following. (3) In a method in which stainless steel is melted in an electric furnace and desulfurized in a ladle, the (MnO) concentration in the slag is 0.50 mass%.
It is characterized in that the molten steel is stirred and / or a desulfurizing flux is added as described below. (4) In a method of melting stainless steel in an electric furnace and performing decarburization refining and reduction smelting in a smelting furnace, the slag (MnO)
It is characterized in that the molten steel is stirred and / or a desulfurizing flux is added so that the concentration becomes 0.30 mass% or less. (5) In a method in which stainless steel is melted in an electric furnace, decarburization refining and reduction smelting are performed in a refining furnace, and desulfurization treatment is performed in a ladle, the concentration of (MnO) in slag is set to 0.15 mass% or less. It is characterized in that the molten steel is stirred and / or a desulfurizing flux is added.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the process of melting stainless steel in an electric furnace and refining in a refining furnace, the present inventors dissolve the stainless steel in a crude molten steel in an electric furnace (hereinafter referred to as a melting step). In the ladle with slag and desulfurization in the ladle (hereinafter referred to as crude molten steel desulfurization step), and desulfurization step in the final reduction period after decarburization and refining in the refining furnace ( Hereinafter, referred to as a refining process), and a process in which molten steel after refining in a refining furnace is tapped into a ladle and desulfurized in the ladle (hereinafter, referred to as a molten steel desulfurization process). Also, the ratio of (S) concentration in slag and [S] concentration in molten steel, which is an index of desulfurization ability, and the so-called desulfurization distribution ratio (S) / [S] have very good correlation. I found something.

FIG. 1 shows the relationship between the (MnO) concentration in slag and the desulfurization distribution ratio (S) / [S] in the above-mentioned melting step and crude molten steel desulfurization step. FIG. 2 shows an AOD furnace as a refining furnace. In the slag (Mn
O) The relationship between the concentration and the desulfurization distribution ratio is shown. In the figure, white circles indicate values in the melting step, black circles indicate values in the coarse molten steel desulfurization step, white squares indicate values in the refining step, and black squares indicate values in the molten steel desulfurization step.

In FIG. 1 and FIG. 2, (Mn
Although the O) concentration and the value of the desulfurization partition ratio (S) / [S] are different, overall, as the (MnO) concentration decreases, the desulfurization partition ratio (S) /
There is a remarkable tendency to improve [S]. Using this relationship, by analyzing only the (MnO) concentration in the slag,
The desulfurization distribution ratio will be easily determined.

The melting step (open circles) and the crude molten steel desulfurization step
It can be seen that the (MnO) concentration decreases and the desulfurization distribution ratio improves in the order of (black circles), the refining process (white squares), and the molten steel desulfurization process (black squares).

Generally, the desulfurization reaction is represented by the following equation (1), and the reaction equilibrium constant K is represented by the following equation (2). (CaO) + [S] = (CaS) + [O] ... (1) K = a CaS a O / a CaO a S ... (2) where, a _CaS during slag activity of the (CaS), a _O is in molten steel
The activity of [O], a _CaO indicates the activity of (CaO) in slag, and a _S indicates the activity of [S] in molten steel.

When the above equation (2) is modified, the following equation (3) is obtained.
Derived. (S) / [S] ∝a_CaS/ A_S= Ka_CaO/ A_O ... (3) Regarding the right side of the above equation (3), K indicates that the molten steel temperature is constant.
Is a constant value if _CaO/ A_OIs the acid between the slag and the molten steel
Element distribution ratio, that is, the oxygen potential. FIG.
Melting and refining of stainless steel from the relationship between Fig. 2 and Fig. 2.
In the process, the (MnO) concentration corresponds to the oxygen potential
It was confirmed that it was an index to be performed.

Generally, in the stainless steel production process, the refining process has a higher stirring power of the molten steel and the molten steel temperature is higher than the melting process, so that the refining is performed with a higher slag basicity. Oxygen potential decreases. In addition, since both the crude molten steel desulfurization step and the molten steel desulfurization step are strongly affected by the previous step, the oxygen potential decreases in the order of the melting step, the crude molten steel desulfurization step, the refining step, and the molten steel desulfurization step. MnO) concentration is reduced, and the desulfurization partition ratio (S) / [S] is improved.

Assuming that the desulfurization distribution ratio (S) / [S] is L _S , the following equations (4) and (5) can be derived. L _S = (S) / [S] = α (MnO) (4) [S] = [S] _i / (W _m + W _s · L _S ) = [S] _i / (W _m + W _s · α) (MnO)) (5) where α is a constant depending on the (MnO) concentration in the slag, (MnO)
O) is the (MnO) concentration in the slag, [S] _i is the total [S] before desulfurization
The amount, W _m is the molten steel amount in the desulfurization process, W _s denotes the amount of slag in the desulfurization treatment.

In the above equations (4) and (5),
[S] _i , W _m , and W _s are values that can be easily grasped during desulfurization treatment, and if L _S is known, the [S] concentration in the molten steel at that time can be known. Further, from the relationship between FIG. 1 and FIG. 2, if the (MnO) concentration in the slag is known from the analysis of the slag, the [S] concentration in the molten steel at that time can be known. If slag is analyzed by in-situ analysis using fluorescent X-rays, the shortest is about 1
Since the analytical value of the (MnO) concentration can be obtained in minutes, the desulfurization status can be grasped sequentially by grasping the (MnO) concentration.

FIG. 3 shows the stirring time from the start of the desulfurization treatment in the case where the desulfurization treatment was performed by adding gas stirring for each of the two charges of the melting step and the crude steel desulfurization step in the electric furnace, The relationship between (MnO) concentration is shown. In the figure, white circles indicate values in the melting step, and black circles indicate values in the crude molten steel desulfurization step. The (MnO) concentration in slag and the desulfurization distribution ratio (S)
It has been confirmed that the relationship of / [S] matches the relationship in FIG.

FIG. 3 shows that the melting step and the crude molten steel desulfurization step
There is a difference in the rate of decrease of the (MnO) concentration, but in the dissolution process, 1.
The concentration of (MnO) decreases rapidly with the passage of time until the mass falls below 00 mass% and in the crude molten steel desulfurization step until the mass falls below 0.50 mass%. However, even after long stirring, (MnO)
Concentration reduction is small, and instead gas costs increase,
This is not an effective desulfurization treatment because it causes the progress of erosion of the refractory. Therefore, to perform desulfurization efficiently,
(MnO) concentration in the slag, 1.00 mass% or less in the melting process,
In the crude molten steel desulfurization process, it is effective to make the content 0.50 mass% or less.

FIG. 4 shows that, after the melting step in the electric furnace, two recharges in the refining step and the molten steel desulfurization step when refining is performed using an AOD furnace as the refining furnace, CaO-20% Ca
And added per ton of the molten steel flux in the case of performing desulfurization treatment by adding flux F _2, in the slag (Mn
O) Shows the relationship between concentrations. In the figure, white squares indicate values in the refining process, and black squares indicate values in the molten steel desulfurization process. It has been confirmed that the relationship between the (MnO) concentration in the slag and the desulfurization distribution ratio (S) / [S] matches the relationship in FIG.

FIG. 4 shows that there is a difference in the reduction amount of the (MnO) concentration with respect to the added amount of flux between the refining process and the molten steel desulfurization process. Until turning off, the (MnO) concentration drops sharply with increasing flux addition. However, after that, even if a large amount of flux is added, the decrease in the (MnO) concentration is small, and on the contrary, the flux cost increases,
This is not an effective desulfurization treatment because it causes an increase in the cost of the stirring gas and the progress of melting of the refractory. Therefore, in order to perform desulfurization efficiently, the concentration of (MnO) in the slag should be 0.30 mass% or less in the refining process and 0.15 mass% in the molten steel desulfurization process.
% Is effective.

[0025]

[Example] Product [S] SU requiring a concentration of 20 ppm or less
Regarding the production of S304 (18 mass% Cr-8mass% Ni) stainless steel, in a production process consisting of an electric furnace, an AOD furnace and a continuous casting, in a crude molten steel desulfurization step between the electric furnace and the AOD furnace, and / or in an AOD furnace. The desulfurization treatment was carried out by including a molten steel desulfurization step after the refining of. Scrap in an electric furnace, an alloy of iron as a main raw material, dust, scale, etc. be added, amount of molten steel 58～60Ton, formulated to be in the range of [S] concentration 320 ～380Ppm, together, CaO, CaF _2, etc. Was dissolved by adding a slag-making agent.

After dissolution, the concentration of (MnO) in the slag is checked.
Gas agitation was performed for about 3 to 15 minutes until mass% or less was achieved, and then the crude molten steel was discharged to a ladle together with slag. When performing desulfurization treatment of crude molten steel, the ladle is tilted and drained until the amount of slag reaches a predetermined value, and then Ca
The O -CaF _2-based flux was blown together with the nitrogen gas. During the treatment, the analysis of the (MnO) concentration in the slag was performed sequentially, and 0.50 mass
%, The process was terminated.

Thereafter, the slag in the ladle was forcibly discharged using a discharger, and then the crude molten steel was charged into an AOD furnace.
The AOD furnace, subjected to decarburization refining until a predetermined [C] concentration after decarburization completion, a reducing agent and CaO, a slag agent CaF _2, MgO, etc. were added thereto to carry out a reduction treatment. During the reduction, the (MnO) concentration in the slag was analyzed. When the slag reached 0.30 mass% or less, the treatment was terminated, and the molten steel was discharged to a ladle together with the slag.

The ladle was analyzed for the [S] concentration of the molten steel at the time of tapping, and when it reached 10 ppm or less, continuous casting was carried out as it was. For desulfurization of molten steel. After adjusting the amount of slag in a predetermined amount in the desulfurization process, it was blown CaO -CaF ₂ based flux with argon gas. During processing, sequentially, during slag (Mn
O) The concentration was analyzed, and when the concentration reached 0.15 mass% or less, the treatment was terminated, and thereafter, continuous casting was performed. In addition,
All of the charges subjected to molten steel desulfurization treatment have [S] concentration of 10pp
m was achieved.

Table 1 shows examples of the desulfurization treatment. Both the invention example and the comparative example were applied to 50 charges. In the invention example, one of the crude molten steel desulfurization step, the molten steel desulfurization step, or both are performed, and the melting step, the crude molten steel desulfurization step,
In slag (MnO) in refining process and molten steel desulfurization process
After confirming that the concentrations reached the respective predetermined concentrations or less, the process was shifted to the next step.

The comparative example is a method in which the desulfurization ability is not adjusted by the (MnO) concentration in the slag.
This is an example of a case where either one or both of the molten steel desulfurization steps were performed, but the treatment was performed without confirming that the (MnO) concentration in the slag reached a predetermined concentration in each step. As a result, in each step, there was a charge that did not achieve the predetermined (MnO) concentration.

[0031]

[Table 1]

The results are shown in Table 2. The refractory erosion index and the desulfurization cost index are values obtained by proportionally converting the invention example to 100. In the invention examples, the slab [S] concentration
In addition to achieving 20 ppm or less, there was no excessive refractory erosion and increase in desulfurization cost. However, in the comparative example, in the charge without the desulfurization treatment of crude molten steel, the charge with the slab [S] concentration exceeding 20 ppm was used. And excessive desulfurization treatment caused refractory erosion and increased desulfurization cost.

[0033]

[Table 2]

[0034]

According to the method of the present invention, the progress of desulfurization can be grasped in the steps of melting and refining stainless steel, so that desulfurization can be carried out without excess or deficiency. Further, since a "threshold" for obtaining high desulfurization ability can be provided, desulfurization treatment can be performed efficiently. As a result, extremely low sulfur steel can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between (MnO) concentration in slag and desulfurization distribution ratio (S) / [S] in a melting step and a crude molten steel desulfurization step.

FIG. 2 Slag in smelting process and molten steel desulfurization process
It is a figure which shows the relationship between (MnO) density | concentration and a desulfurization distribution ratio (S) / [S].

FIG. 3 is a diagram showing a relationship between a stirring time and a (MnO) concentration in slag in a melting step and a crude molten steel desulfurization step.

FIG. 4 is a diagram showing the relationship between the amount of flux added and the (MnO) concentration in slag in the refining process and the molten steel desulfurization process.

 ────────────────────────────────────────────────── ─── Continued from the front page (72) Inventor Masao Igarashi 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation Hikari Works F-term (reference) 4K013 AA02 BA05 CA01 CA02 CA09 CB04 CC01 CF12 CF13 DA03 DA05 DA10 DA13 EA03 EA09 FA05 4K014 CA04 CB01 CB03 CC07

Claims (5)

[Claims] 1. A method for dissolving stainless steel in an electric furnace and refining in a refining furnace, wherein the desulfurization ability is adjusted according to the (MnO) concentration in the slag. 2. A method for melting stainless steel in an electric furnace, wherein the molten steel is stirred and / or a desulfurization flux is added so that the (MnO) concentration in the slag is 1.00 mass% or less. A method for desulfurizing stainless steel, comprising: 3. A method in which stainless steel is melted in an electric furnace and desulfurized in a ladle, wherein the concentration of (MnO) in the slag is set at 0.
A method of desulfurizing stainless steel, comprising stirring molten steel and / or adding a desulfurization flux so as to be 50 mass% or less. 4. A method for melting stainless steel in an electric furnace and performing decarburization refining and reduction smelting in a refining furnace, wherein the molten steel is stirred so that the (MnO) concentration in the slag becomes 0.30 mass% or less. And / or a desulfurizing flux is added. 5. A method in which stainless steel is melted in an electric furnace, decarburization refining and reduction refining are performed in a refining furnace, and desulfurization treatment is performed in a ladle, so that the (MnO) concentration in the slag is 0.15 mass% or less. Stirring the molten steel and / or
A desulfurization method for stainless steel, comprising adding a desulfurization flux.