JPH11279623A - Method for melting ferritic stainless steel containing high aluminum capable of suppressing erosion of refractory of refining container, and excellent in manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To melt a ferritic stainless steel generating no dent and containing high Al by a method where the erosion of a refractory of a refining container lined with a MgO refractory is suppressed. SOLUTION: In manufacturing a ferritic stainless steel containing high Al having the composition consisting of, by mass, <=0.03% C, <=1.0% Si, 15-26% Cr, 2-6% Al, 0-0.5% Ti (including no addition), containing 0.02-0.12% one or two or more kinds of rare earth elements as necessary, and the balance Fe with inevitable impurities using a refining container lined with a MgO refractory, a slag in contact with a molten steel is formed of CaO-MgO-Al2 O3 substantially containing no CaF2 , and the slag composition after regulating the steel composition and before the casting is controlled so as to satisfy the inequality of (CaO mass %)/Al2 O3 mass %) <=0.8.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a high Al-containing ferritic stainless steel in which the slag composition is controlled to suppress the erosion of MgO-based refractories in a smelting vessel. High A which hardly occurs and has good manufacturability
The present invention relates to a smelting method for obtaining an l-containing ferritic stainless steel.

[0002]

2. Description of the Related Art High Al-containing ferritic stainless steel is a material having excellent resistance to high-temperature oxidation, and has been conventionally used as an electric heating material, a high-temperature furnace member, a combustion exhaust system member, an automobile exhaust gas system component and the like. However, the steel type of this system generally tends to generate barbed flaws during hot rolling, and this often causes a significant decrease in yield, which is a problem. As an example, tubular bubbles are generated under the surface layer of the high Al-containing ferritic stainless steel slab obtained by the continuous casting method over a depth of 20 to 30 mm, and these become open flaws by slab grinding before hot rolling. When the slab is heated, the inside of the flaw is oxidized and appears as a scab on the hot rolling. The coil in which the flaw is generated is subjected to heavy grinding in the surface grinding process, and the product yield is significantly reduced. If the burrs are severe, the coil itself must be scrapped.

AOD is used as a mass production process for stainless steel.
And VOD are well known, and in recent years, the development of these processes has enabled various stainless steels to be economically mass-produced. In these processes, a basic refractory is generally used as a lining refractory in a smelting vessel in order to promote desulfurization and deoxidation and improve the quality of steel. For example, Magukuro brick (typical _{composition; MgO: 60%, Al 2} O 3: 10%, C
r ₂ O ₃ : 19%, FeO: 6%), Magdro brick (same as above;
Refractories containing MgO as a main component, such as 57% MgO and 39% CaO, are often used. These M
gO-based refractories have slag erosion resistance, spalling resistance,
It has excellent abrasion resistance and the like, and is very suitable for general stainless steel refining. However, in refining ferritic stainless steel containing high Al, it has been found that these refractories are susceptible to erosion, and that MgO in the refractories contributes to the generation of the above-mentioned "tubular bubbles". .

[0004] Usually, a considerable concentration of Mg is dissolved in the molten steel of a high-Al-containing ferritic stainless steel, and during the solidification during casting, supersaturated Mg is gasified to form bubbles in the cast material. Turned out to make. Mg in molten steel is increased by mainly reducing MgO in slag by Al and rare earth elements (Re) in molten steel as follows. 3 (MgO) +2 [Al] = (Al ₂ O ₃ ) +3 [Mg] 3 (MgO) +2 [Re] = (RE ₂ O ₃ ) +3 [Mg]

On the other hand, the MgO component in the slag is supplied from a slag-making agent or an MgO-based refractory. Also, from the viewpoint of preventing erosion of refractories, MgO is often intentionally present in the slag. For this reason, as long as the usual melting method using the MgO-based refractory container is followed, it is inevitable to some extent that the Mg concentration in the molten steel of the high Al content steel or the rare earth element added steel is increased.

[0006]

As means for preventing an increase in the Mg concentration in molten steel of a high Al-containing ferritic stainless steel, for example, Japanese Patent Application Laid-Open No. 7-116779 proposes the following method. CaO-CaF ₂ -Al ₂ O
How to use ₃ series slag. CaF ₂ is MgO in slag
It has the effect of significantly reducing solubility. A method of lining a refractory in a refining vessel with an alumina-based refractory. Thereby, elution of MgO from the refractory is avoided.

However, since the above method uses a slag containing CaF ₂ , the MgO-based refractory is significantly damaged. Therefore, when a refining vessel made of an MgO-based refractory is used, the cost for repairing the refractory increases, and the vessel itself tends to deteriorate. On the other hand, in the above method, the cost for lining with an alumina-based refractory is high, and preparing a special container every time the high-Al steel is melted causes various restrictions in operation. The present invention solves these drawbacks of the conventional method, and can provide a sound casting material of high Al-containing ferritic stainless steel free of tubular bubbles economically using a normal smelting vessel. The aim is to provide a method.

[0008]

In order to achieve the above object, the invention of claim 1 is characterized in that, in mass%, C ≦ 0.03%, Si ≦
1.0%, Cr: 15 to 26%, Al: 2 to 6%, Ti: 0 to 0.5
% (Including no additive), and the high Al-containing ferritic stainless steel consisting of the balance Fe and unavoidable impurities,
In producing using a refining vessel which is lined with gO-based refractories, and slag in contact with molten steel and CaF ₂ are substantially not contained CaO-MgO-Al ₂ O ₃ system, and the steel composition adjusted and casting before Slag composition in (% by mass CaO) /
(% By mass Al ₂ O ₃ ) ≦ 0.8, high aluminum with good manufacturability with reduced refractory erosion in refining vessels
This is a method of melting ferrite-based stainless steel.

Here, the "MgO-based refractory" is a refractory containing about 50% by mass or more of MgO, for example, the above-mentioned magcro brick or magdro brick.
CaF ₂ that "CaF ₂ is substantially free" means that the composition of the slag to not include CaF ₂ are controlled from the (attention is paid), mixed as unavoidable impurities Is generally allowed in a range of 1% by mass or less. “After steel composition adjustment / before casting” refers to a time when all refining and addition of component elements have been completed and casting is ready to start.

According to a second aspect of the present invention, in the first aspect of the invention, the slag composition after adjusting the steel composition and before casting is Ca
O: 25 to 40% by mass, MgO ≦ 15% by mass, (% by mass CaO)
/ (Mass% Al ₂ O ₃ ) ≦ 0.8 controls the composition and amount of the slag-making agent.

[0011] The invention according to claim 3 is that, in mass%, C ≦ 0.03
%, Si ≦ 1.0%, Cr: 15 to 26%, Al: 2 to 6%, T
i: 0 to 0.5% (including no addition), one or two or more rare earth elements: a high Al-containing ferritic stainless steel containing 0.02 to 0.12% in total, the balance being Fe and unavoidable impurities, In manufacturing using a refining vessel lined with MgO-based refractories, the slag in contact with molten steel is
A CaO—MgO—Al ₂ O ₃ system containing substantially no F ₂ , and the slag composition at the time of rare earth element addition is
This is a method for producing a high-Al-containing ferritic stainless steel which is controlled so that (mass% CaO) / (mass% Al ₂ O ₃ ) ≦ 0.8 and which suppresses refractory erosion of a refining vessel and has good manufacturability. .

Here, the rare earth elements are Y, La, Ce, N
d and the like. The term “rare earth element addition time” means immediately before the rare earth element is added to the molten steel. In the case of adding the rare earth element in a plurality of times, it is immediately before the first addition. The method of adding the rare earth element is not particularly limited. Generally, the addition of a rare earth element to molten steel is often performed after adjusting all other steel components and before casting.

According to a fourth aspect of the present invention, in the third aspect, the slag composition at the time of adding the rare earth element is Ca
O: 25 to 40% by mass, MgO ≦ 15% by mass, (% by mass CaO)
/ (Mass% Al ₂ O ₃ ) ≦ 0.8 controls the composition and amount of the slag-making agent.

According to a fifth aspect of the present invention, in the first to fourth aspects of the present invention, in particular, the Mg concentration in the cast material is reduced to 0.014% by mass or less.

The invention according to claim 6 is the method according to claims 1 to 5, during refining.
In the invention of the above, the inert gas is blown into the molten steel at a supply rate of 0.4 to 20 Nl / min · T from a gas blowing device provided in the refining vessel to reduce the H (hydrogen) concentration in the molten steel,
The content of H in the cast material is set to 6 mass ppm or less.

Here, the "gas blowing device provided in the refining vessel" includes, for example, a porous plug, a tuyere, a nozzle and the like. "Nl / min.T" means 1T of molten steel
It is a gas supply rate in a standard state per (ton).

A seventh aspect of the present invention is the invention of the sixth aspect, in which the inert gas is Ar.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the results of investigation on the Mg content and the H content at the limit where no bubbles are generated in a slab of a high Al-containing ferritic stainless steel. From FIG.
Mg content is 0.014% by mass or less, and H content is 6% by mass.
It can be seen that no air bubbles are observed in the slab of less than ppm. Even when the Mg content and the H content slightly exceed these limit values, if the slab has few bubbles, the scab defects in hot rolling are greatly reduced, but in particular, Mg ≦ 0.014 mass,
In addition, when H ≦ 6 mass ppm, the problem of the scab defect peculiar to the present system steel does not occur.

FIG. 2 shows the relationship between the Al content and the Mg content in steel. In this experiment, a steel having a varied amount of Al in an Fe-18 to 21% by mass Cr steel was used for MgO.
Melt in a crucible, set the molten steel temperature to 1600 ° C., and float a CaO—MgO—Al ₂ O ₃ system slag in which (mass% CaO) / (mass% Al ₂ O ₃ ) is adjusted to 0.6 to 0.8 on the molten steel surface. After casting for a predetermined time in this state, casting is performed, and the components of the obtained ingot are analyzed. From FIG. 2, the Al content is 6% by mass.
In the following, it can be seen that Mg ≦ 0.014% by mass can be stably obtained.

FIG. 3 shows the relationship between the rare earth element (Re) content and the Mg content in steel. This experiment is
A steel in which the total content of rare earth elements in Fe-18 to 21% by mass Cr-5 to 6% by mass Al steel was changed variously was melted in a MgO crucible, and the ingot obtained as in the case of FIG. It is an analysis of the components. FIG. 3 shows that when the total content of rare earth elements is 0.12% by mass or less, Mg ≦ 0.014% by mass can be stably obtained.

FIG. 4 shows high Al dissolved in an MgO crucible.
Effect on the Mg content of ferritic stainless steel containing
CaO-MgO-Al ₂ O ₃ based slag (wt% CaO) /
5 is an experimental example in which the influence of (mass% Al ₂ O ₃ ) value was examined. In this experiment, Fe-20 mass% Cr-5 mass% Al steel was changed to MgO
30 kg was melted in a crucible, the temperature of the molten steel was set to 1600 ° C., and the (mass% CaO) / (mass% Al ₂ O ₃ ) value was variously changed.
aO-MgO-Al ₂ O ₃ slag is floated on the surface of molten steel,
La is added as a rare earth element, and molten steel at 0 min (immediately after addition), 3 min, and 6 min after La addition is sampled to determine the Mg concentration. In FIG. 4, C / A on the horizontal axis represents a value of (% by mass CaO) / (% by mass Al ₂ O ₃ ). FIG. 4 shows that when the (mass% CaO) / (mass% Al ₂ O ₃ ) value is 1.0 or less, the Mg concentration in the steel sharply decreases, and when the value is 0.8 or less, it tends to be stabilized at a low value. The reason why such a sharp decrease in the Mg concentration occurs is as follows: (% by mass CaO) / (% by mass A
(l ₂ O ₃ ) decreases, the solubility of MgO in the slag decreases and the viscosity of the slag increases, and these characteristics act to reduce the MgO reduction reaction in the slag at a certain critical point. It is presumed that it will be. In this experiment, an extremely low value of about 0.005% by mass or less of the Mg concentration in steel was obtained. This is due to the use of a high-purity raw material due to experimental reasons.

In the present invention, when the rare earth element is not added, the slag composition at the time of “after adjusting the steel composition / before casting” is defined. The molten steel temperature at this point is adjusted to an optimum temperature for casting, and is substantially constant for each steel type.
Therefore, by defining the slag composition at this point, the effect of temperature on the slag physical properties does not actually have to be a problem, and indeed, by optimizing the slag composition at this point, a sound without bubbles is obtained. A slab is obtained. In the steel composition within the range specified in the present invention, the molten steel temperature at this point is approximately 1600.
° C ± 15 ° C.

In the case of adding a rare earth element, the “time of adding the rare earth element”, that is, the slag composition immediately before the first rare earth element is added is defined. Since rare earth elements are very easily oxidized, in order to ensure the desired yield in steel,
It is usually added after adjusting other steel components and before casting.
As described above, the molten steel temperature at this time is almost constant, and in fact, a sound slab without bubbles can be obtained by optimizing the slag composition immediately before the addition of the rare earth element.

It is necessary that the slag composition at the above time point is (mass% CaO) / (mass% Al ₂ O ₃ ) ≦ 0.8, but specifically, CaO: 25 to 40 mass%, MgO
It is desirable to be ≦ 15% by mass. In the present invention, since the refining vessel uses an MgO-based refractory, the MgO concentration in the slag is affected by the MgO dissolved from the refractory. Further, the slag composition changes as the refining progresses. In order to adjust the slag composition at the above time within the specified range, it is preferable to control the composition and amount of the slag-making agent to be charged during refining, etc., while taking into account the dissolution of the refractory and the influence of the refining reaction.

By optimizing the slag composition as described above, the Mg content in the steel can be reduced. However, as described above, the reduction of the H content in the steel is also required in the present steel. It is very effective in preventing flaws. Therefore, it is desirable to actively remove H in the molten steel in the refining stage. As an effective method, blowing an inert gas into molten steel is effective. H in the molten steel is removed by diffusing into the bubbles of the blowing gas. As a result of the experiment, it was found that the H content in the steel can be reduced to 6 mass ppm or less by blowing the inert gas at a supply rate of 0.4 Nl / min · T or more from the gas blowing device provided in the refining vessel. . The removal rate of H is improved by increasing the supply rate of the inert gas, but if it is excessive, the splash of molten steel increases, and Al and the like in the molten steel are oxidized. Therefore, the supply speed of the inert gas is 20 N
It is desirable to be 1 / min · T or less. Any type of gas can be used as long as it is a gas that is effective in diffusing and diluting H. However, it is preferable to use an inert gas such as Ar to prevent oxidation of Al or the like.

Since the present steel has the property of easily absorbing H, it is necessary to pay attention to the moisture management of the raw materials and materials used.

Next, the constituent elements of the steel targeted in the present invention will be described. C: If the content is too large, the toughness of the steel material decreases, and abnormal oxidation tends to occur at high temperatures. Therefore, the content is set to 0.3 mass% or less.

Si: an element effective for high-temperature oxidation resistance, but if too much, the toughness of the steel material is reduced.
The content is as follows.

Cr is a basic component effective for high-temperature oxidation resistance, and has a content of 15% by mass or more in order to obtain excellent high-temperature oxidation resistance in combustion exhaust system members and other high-temperature applications. However, if too much, the toughness of the steel material will decrease.
The upper limit is mass%.

Al: Indispensable element for improving high-temperature oxidation resistance, its effect is exhibited when the content of Cr is 2% by mass or more in ferritic stainless steel. However, if the content is too large, the Mg concentration in the steel increases, and the manufacturability deteriorates significantly. Therefore, the upper limit is 6% by mass.

Ti: An element effective for fixing C and N. However, if it is excessive, it impairs high-temperature oxidation resistance. Therefore, when added, the content should be 0.5% by mass or less.

Rare earth elements: Y, C
elements such as e, La, and Nd. These are extremely effective in preventing abnormal oxidation, and all have almost equivalent actions in exhibiting the effect of preventing abnormal oxidation. For this reason,
They may be used alone or in combination. The effect of preventing abnormal oxidation is exhibited when the total content of rare earth elements is 0.02% by mass or more. However, an excessive amount causes an increase in the Mg concentration in the steel, and remarkably deteriorates the manufacturability. Therefore, the upper limit is 0.12% by mass.

Mg: an element for forming bubbles in the ingot, which causes scabs, it is desirable to suppress the content to 0.014% by mass or less.

H: This is also a bubble forming element in the ingot,
It is desirable to keep the content to 6 ppm by mass or less, since it causes scabs.

In addition to these elements, inevitable impurities such as Mn, Ni, P, S, Cu, Mo, N, O
And the like.

[0036]

[Example 1] Using a vacuum induction melting furnace, 30 kg of an Fe-Cr alloy having a predetermined composition was melted in an MgO crucible having a porous plug attached to the bottom thereof in an Ar atmosphere under an Ar atmosphere.
It was added aO-MgO-Al ₂ O ₃ slag 4.5 kg, holds the molten steel temperature to 1600 ° C., a predetermined amount of Al, was added Ti. When the rare earth element is added, a predetermined amount of the rare earth element is added 5 minutes after the addition of Al and Ti, and after holding for 3 minutes,
Cast. When no rare earth element was added, the casting was performed 5 minutes after the addition of Al and Ti. A predetermined amount of Ar gas was blown into the molten steel through a porous plug. The components of the obtained ingot were analyzed, and the presence or absence of air bubbles was examined in the longitudinal section of the ingot. Table 1 shows the results.

[0037]

[Table 1]

The steel composition and the slag composition within the range specified in the present invention did not produce any bubbles in the ingot (Nos. 1 to 7). On the other hand, (mass% Ca
(O) / (mass% Al ₂ O ₃ ) value exceeding 0.8, the Mg content exceeded 0.014 mass%, and generation of bubbles was observed (No.
9,10,14,15). Those having a rare earth element content of more than 0.12% by mass have a Mg content of 0.1 even if the slag composition is proper.
It exceeded 014% by mass, and generation of bubbles was observed (No. 11). A
The same was true for the case where the l content exceeded 6% by mass (No. 1).
2). No.13 only has an Ar gas supply rate of 0.4 N into molten steel
1 / min · T, but this is due to the H content of 6 mass p, despite other conditions being appropriate.
It was higher than pm, and the generation of bubbles was observed.

[Embodiment 2] In the VOD process, 20Cr-
70 tonnes of 5Al steel (with rare earth element added) and 18Cr-3Al steel (without rare earth element added) were melted. The refractory lining of the VOD pot is Magdro brick.
After adding the alloy, it was cast into a slab shape by a continuous casting method. For 20Cr-5Al steel, after steel composition adjustment / before casting, 1
In the 8Cr-3Al steel, the composition and amount of the slag-making agent were controlled so that the slag composition immediately before the addition of the rare earth element was within the range specified by the present invention. CaF ₂ was not added as a slag component. During refining by VOD, Ar gas of 200 Nl / min (2.9 Nl / min.T) was blown into the molten steel through a porous plug. The obtained slab was warmly ground to a depth of 5 mm from the surface, immediately sent to a heating furnace, hot-rolled, and examined for the presence of barbed flaws in the hot-rolled steel strip. In addition, the sample from which the slab was cut out was examined for the presence or absence of bubbles. Table 2 shows the results together with the slag composition.

[0040]

[Table 2]

In each of the charges, the slab had no air bubbles and no scorch was generated. In addition, the erosion of the refractory of the used pot was suppressed to about 2 to 3 times that of ordinary refining of stainless steel, and it was in a state where it could be reused sufficiently. In addition,
The same smelting as in Example 2 was attempted using slag to which 15% by mass of CaF ₂ was added. As a result, the magdro brick used for the lining of the pot was significantly damaged at the slag line, and was damaged to the extent that it could not be reused.

[0042]

As described above, according to the present invention, a high Al-containing ferritic stainless steel free of bark flaws can be easily and stably used by using a refining vessel lined with ordinary MgO-based refractories. It can now be manufactured.

[Brief description of the drawings]

FIG. 1 is a graph showing the influence of the Mg content and the H content in steel on the formation of bubbles in an ingot of a high Al-containing ferritic stainless steel.

FIG. 2 is a graph showing the relationship between the Al content and the Mg content in steel of a high Al content ferritic stainless steel.

FIG. 3 is a graph showing a relationship between a rare earth element content and a Mg content in steel of a high Al-containing ferritic stainless steel.

FIG. 4 M in steel of high Al content ferritic stainless steel
(% by mass CaO) / (% by mass A)
12 is a graph showing the effect of l ₂ O ₃ ) value.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/28 C22C 38/28 (72) Inventor Naoto Hiramatsu 4976 Nomura Minamicho, Shinnanyo-shi, Yamaguchi Prefecture In-house (72) Inventor Toshiaki Miyamoto 4976 Nomura Minamicho, Shinnanyo-shi, Yamaguchi Prefecture Inside Nisshin Steel Corporation

Claims (7)

[Claims] (1) In terms of mass%, C ≦ 0.03%, Si ≦ 1.0%,
A high Al-containing ferritic stainless steel containing Cr: 15 to 26%, Al: 2 to 6%, Ti: 0 to 0.5% (including no addition), the balance being Fe and unavoidable impurities, in in manufacturing using a lined been refined container, and the slag in contact with molten steel and CaF ₂ are substantially not contained CaO-MgO-Al ₂ O ₃ system, and slag composition before the steel components adjusted and casting A method for producing a high-Al-containing ferritic stainless steel having good productivity and suppressing refractory erosion of a refining vessel, which is controlled so that (mass% CaO) / (mass% Al ₂ O ₃ ) ≦ 0.8. 2. The slag composition after steel composition adjustment and before casting is such that CaO: 25 to 40% by mass, MgO ≦ 15% by mass, (% by mass CaO) / (% by mass Al ₂ O ₃ ) ≦ 0.8. And controlling the composition and the amount of the slag-making agent.
3. The method for producing a ferritic stainless steel having a high Al content according to item 1. 3. The method according to claim 1, wherein C ≦ 0.03%, Si ≦ 1.0%,
Cr: 15 to 26%, Al: 2 to 6%, Ti: 0 to 0.5% (including no addition), one or more of rare earth elements: including 0.02 to 0.12% in total, with the balance Fe and High Al content ferritic stainless steel, which is composed of unavoidable impurities,
In producing using a refining vessel which is lined with gO-based refractories, and slag in contact with molten steel and CaF ₂ are substantially not contained CaO-MgO-Al ₂ O ₃ system, and slag composition in addition timing rare earth elements Is (% by mass CaO) /
(% By mass Al ₂ O ₃ ) ≦ 0.8, high aluminum with good manufacturability with reduced refractory erosion in refining vessels
For producing ferrite-based stainless steels. 4. The slag composition at the time of adding the rare earth element is adjusted so that CaO: 25 to 40% by mass, MgO ≦ 15% by mass, (% by mass CaO) / (% by mass Al ₂ O ₃ ) ≦ 0.8. The method for melting a high Al-containing ferritic stainless steel according to claim 3, wherein the composition and the amount of the slag are controlled. 5. The method according to claim 1, wherein the Mg concentration in the cast material is 0.014% by mass or less. 6. During the refining, an inert gas is introduced into the molten steel from a gas blowing device provided in the refining vessel in an amount of 0.4 to 20 Nl / min.
The H content in molten steel is reduced by blowing at a supply rate of T to reduce the H content in the cast material to 6 ppm by mass or less.
3. The method for producing a ferritic stainless steel having a high Al content according to item 1. 7. The method for producing a high Al-containing ferritic stainless steel according to claim 6, wherein the inert gas is Ar.